Prototype centrifugal natural gas cleaner CONFIDENTIAL · Nomenclature A area [m2] CD drag force...

Prototype centrifugal

natural gas cleaner

CONFIDENTIAL

E. A. J. van de WateringReport number WPC 2007.06

Committee:Prof. dr. M. Golombok (chairman)Prof. dr. ir. J. J. H. BrouwersDr. ir. H. P. van KemenadeDr. R. van der VaartIr. F. G. A. HomburgIr. G. P. Willems (supervisor)

Eindhoven University of TechnologyDepartment of Mechanical EngineeringDivision TFE, Process TechnologyJune, 2007

Summary

Gas has to be subtracted from gas fields. Most of the known gas fields are already beingproduced or will be in the near future. However when gas fields reach the end of their life-cycle, quality of the gas decreases. Furthermore a substantial part of the fields that arenot yet in production, cannot be recovered with existing technologies. Contamination levelsin these fields exceed CO2 and H2S amounts of 15% and 5% respectively. This includesapproximately 16% of all the known global natural gas reserves. However, with a new breakthrough technology these gas fields can be produced. The basic idea is that the contaminatednatural gas mixture is brought to such a temperature and pressure, the contaminants willcondense and form a mist of fine droplets. These droplets are so small that the RotatingParticle Separator is necessary to provide a good separation at high throughputs. In thisreport the design of a RPS is given that can handle 20 MMscf/d.

A RPS can cope with high throughputs because droplets only have to travel a small radialdistance, thereby reducing the residence time. The small radial migration distances are causedby a multitude of axially positioned channels that rotate around a shaft. The channels arecombined in a so-called filter element. The droplets experience a centrifugal force field andare forced to the walls of the channels. On the walls a liquid film forms that breaks-up inlarger droplets at the end of the filter element. These droplets are again collected in thepost-separator.

Equations and theories are necessary to determine the dimensions of the filter element andother components of the RPS. With a turbulent separation efficiency curve and an expecteddroplet distribution with an average of 1.5 µm, a key design parameter, the dp,50%, can be setto 0.5 µm. The dp,50% indicates the particle diameter that is separated with a 50% probability.A production limitation on the length and a restraint on the Reynolds number in the channels,lead to a filter element with an outer radius of 0.2 m and a length of 0.2 m. Typical requiredrotational speeds lie between 1100 and 2400 RPM. The filter element could theoretically reacha rotational speed of 4000 RPM by the swirling flow. The dimensions of the filter elementare the leading design dimensions. All other dimensions are influenced by them.

Before the mixture enters the filter element it is already brought in rotation. In this area,called the pre-separator, coarse sand particles from 10 µm and up are separated to preventclogged channels of the filter element. Other fouling substances like salt, asphaltenes andwax particles can still be present in the gas flow. However quantities are difficult to estimatedue to pre-treatment of the gas mixture upstream. A cleaning system for the channels of thefilter element might be necessary.

The prototype will be positioned in the Euroloop in Botlek (The Netherlands). TheRPS needs to withstand an aggressive corrosive environment with high pressures and lowtemperatures which evolve from the condensation process. Furthermore the prototype needsto comply with all standards used by Shell. The rotating filter element is situated in a housing

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that is constructed from a standard 24” 600 CL pressure tube, capable of handling pressuresup to 100 bar. A material suited for the aggressive conditions and low temperatures is astainless steel from either the austenitic or super-austenitic family.

The housing contains two liquid collection rings to drain-off the liquid CO2. The shaft inthe housing will be supported by two bearings. One bearing will accommodate a support fora radial force and thrust force, the other will only provide a radial support. Different typesare considered. From the options, ceramic plain bearings and active magnetic bearings havethe best credentials.

To reach some of the required rotational speeds, necessary for sufficient separation, a drivesystem is needed. A standard electric motor will be used, which is linked to the shaft by amagnetic coupling. The motor needs to provide 6.5 KW of power. However for the highestflow rates the electric motor has to prevent the filter element from spinning up to too highrotational speeds. The electric motor will serve as a dynamic brake. An 30 KW electric motoris necessary.

Droplet break-off at the end of the filter element is investigated by both theory andexperimental studies. Two mechanisms can be defined; droplets caused by the centrifugalforce and droplets caused by the shear force. When shear force droplets are smaller thancentrifugal force droplets they will be used. However if they are smaller than the channel wallthickness, centrifugal droplets will be set as leading droplet diameter to determine the lengthof the post-separator.

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Samenvatting

Gas wordt gewonnen uit aardgas velden. De meeste, van de reeds bekende gasvelden, zijnal in productie of zullen dat in de nabije toekomst komen. Echter wanneer een gasveldopraakt gaat de kwaliteit van het gas achteruit. Ook wordt een substantieel deel van degas velden niet geproduceerd omdat de huidige technologieen dit niet toelaten. Veroorzakerhiervan is vervuiling van velden door CO2 en H2S, met hoeveelheden die de 15% en 5%respectievelijk, overschrijden. Dit omvat ongeveer 16% van de wereld aardgas reserves. Eennieuwe technologie biedt de mogelijkheid om deze velden wel te produceren. Het idee is, omvervuild aardgas naar een bepaalde druk en temperatuur te brengen waardoor de vervuilendesubstanties condenseren en een mist van fijne druppels vormen. Deze druppels zijn zo kleindat de Roterende Deeltjes Scheider moet zorgen voor een goede scheiding bij hoge doorzetten.In dit verslag zal het ontwerp van een 20 MMscf/d RDS worden gegeven.

De RDS kan hoge doorzetten halen omdat de druppels slechts een kleine radiale afstandhoeven af te leggen, wat de verblijftijd reduceert. De kleine radiale migratie afstand wordtmogelijk gemaakt door een array aan axiaal gepositioneerde kanalen die roteren om een geza-menlijke as. De kanalen zijn gebundeld in een zogenaamd filter element. Druppels in dekanalen ondervinden een centrifugaal kracht en worden naar de wand gedwongen. Aan dewanden vormen zich vloeistof filmpjes die aan het einde van het filter element opbreken ingrotere druppels. Deze druppels worden weer opgevangen in de na-separator.

Vergelijkingen en theorieen zijn nodig om de dimensies van het filter element en anderecomponenten te bepalen. Met een turbulente scheidingsefficientie curve en een verwachtedruppel verdeling met een gemiddelde van 1.5 µm, kan een leidende ontwerp parameter, dedp,50%, van 0.5 µm worden bepaald. De dp,50% geeft de druppel diameter aan die met 50% kanswordt afgevangen. Verder zorgen een productielimitatie en een begrenzing van het Reynoldsgetal in the kanalen, voor een filter element met een buitenstraal van 0.2 m en een lengte van0.2 m. Typische rotatie snelheden liggen tussen de 1100 en 2400 RPM. Het filter element kandoor de inkomende roterende stroming zelfs een theoretisch snelheid van 4000 RPM halen.De bepaalde dimensies voor het filter element vormen de leidende dimensies voor de rest vanhet ontwerp.

Voordat het mengsel het filter element in stroomt, is het al in rotatie. Dit gebied voor hetfilter element heet de voor-separator en in dit gebied worden o.a. zand deeltjes van 10 µm engroter afgevangen, zodat verstopping van de kanalen wordt voorkomen. Andere vervuilendedeeltjes zoals zout, wax en asphaltenes kunnen nog wel in de gas stroom aanwezig zijn.Hoeveelheden zijn echter lastig te voorspellen doordat er upstream al een vorm van reinigingplaats vindt. Een schoonmaak systeem voor de kanalen zou dus eventueel nodig kunnen zijn.

Het prototype zal in de Euroloop in Botlek worden geplaatst. De RDS moet in staatzijn om in een agressieve omgeving met hoge drukken en lage temperaturen te kunnen func-tioneren. Deze drukken en temperaturen worden veroorzaakt door het condensatie proces.

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Verder zal het prototype moeten voldoen aan alle standaarden en regels die binnen Shell ge-bruikt worden. Het roterende filter element is geplaatst in een standaard 24” 600 CL drukpijpdie drukken van 100 bar kan weerstaan. Een materiaal dat de corrosieve omgeving met lagetemperaturen aan kan zou een austenitisch of super-austenitisch roestvast staal kunnen zijn.

De behuizing bevat twee vloeistof verzamel ringen die het vloeibaar CO2 afvoeren. Deas wordt in de behuizing ondersteund door twee lagers. Een lager zal zowel een radiale alsaxiale kracht opvangen, het andere lager zal alleen radiale ondersteuning leveren. Er zijnverschillende type lagers overwogen. Keramische glijlagers en magnetisch lagers hebben beidegeschikte eigenschappen.

Om de vereiste rotatiesnelheden, die nodig zijn voor een goede scheiding, te behalenis een vorm van aandrijving nodig. Een standaard elektromotor die via een magnetischekoppeling aan de as gekoppeld is, zal worden gebruikt. De motor zal 6.5 KW aandrijfvermogennodig hebben. Echter voor de hoogste volumestromen wordt de elektromotor gebruikt om tevoorkomen dat het filter element opspint naar te hoge rotatiesnelheden. De elektromotor zaldienen als dynamische rem. Een motor van 30 KW is nodig.

Het afbreken van druppels aan het einde van het filter element is onderzocht met behulpvan theorieen en experimenten. Twee mechanismen kunnen worden gedefinieerd; druppelsdie afbreken als gevolg van de centrifugaal kracht en druppels die afbreken door de afschuifkracht. Als afschuif kracht druppels kleiner zijn dan centrifugaal kracht druppels zullen dezeworden gebruikt. Echter wanneer de afschuif kracht druppels kleiner zijn dan de wand diktevan de kanalen van het filter element, zullen centrifugaal kracht druppels worden gebruiktvoor de bepaling van de lengte van de na-separator.

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Nomenclature

A area [m2]CD drag force coefficient [-]CM torque coefficient [-]D diameter [m]d diameter [m]dc channel height [m]dp,50% particle collected with 50% probability for a uniform vapour

velocity[m]

dp average particle diameter [m]E Youngs modulus [Pa]f friction coefficient [-]F force [N]g gravitational acceleration [m/s2]Gθ loss of angular momentum [Nm]h height between concentric ring [m]J moment of inertia [kgm2]K loss factor due to appendages and fittings [-]L length [m]m mass [kg]m mass flow rate [kg/s]n number of moles [-]P particle distribution [-]P pressure [Pa]∆P pressure loss [Pa]Q volume flow rate [m3/s]r radial position [m]r recovery [-]R radius [m]R50% 50% cut radius [m]Re Reynolds number [-]s gap size [m]T temperature [K]t channel wall thickness [m]

v

v velocity [m/s]vo liquid feed velocity [m/s]x mol fraction [-]x ratio of pressure drop over complete RPS and channels [-]x∗ ratio of pipelength to hydraulic diameter [-]z axial distance [m]

Greek symbols

β empirical swirl factor [-]δ film thickness [m]δ inner to outer RPS filter diameter ratio [-]ǫ specific energy [J/kg]ǫred reduction of the effective cross sectional area due to finite

wall thickness[-]

η efficiency [-]µ dynamic viscosity [Pa s]ν kinematic viscosity [m2/s]Ω angular velocity [rad/s]ξ pressure loss factor at entrance channel/gap [-]ρ mass density [kg/m3]σ surface tension [N/m]σg geometric standard deviation [-]τ residence time [-]τs interfacial shear stress [Pa]

Superscripts and subscripts

ax axialbear bearingc cleanch channeldyn dynamicf liquidf feedfe filter elementg vapourh hydraulicgap gapi innerinlet inletN normalo outerout outletp particlepost post-separator

vi

pre pre-separatorr radialRPS Rotating Particle SeparatorRVS stainless steelshaft shaftshear shearstat staticsys systemt tangentialtot totalunbal unbalancew wasteθ rotational0 initial

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Contents

Summary i

Samenvatting iii

Nomenclature v

1 Introduction 1

1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Goal and outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 Rotational Particle Separator in C3-separation 3

2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2 Condensed Contaminant Centrifugal Separation . . . . . . . . . . . . . . . . . 3

2.2.1 Mixture characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2.2 Droplet formation and growth . . . . . . . . . . . . . . . . . . . . . . . 6

2.3 RPS working principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.4 Centrifugal separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.5 Angular momentum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.6 Gap leak flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3 Conceptual design 13

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.2 Pre-separator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.2.1 Separation process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.3 Filter element . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15


3.3.2 Swirl energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.3.3 Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.3.4 Creeping film in filter element . . . . . . . . . . . . . . . . . . . . . . . 21

3.3.5 Channel cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.4 Post-separator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.4.1 Droplet break-off process . . . . . . . . . . . . . . . . . . . . . . . . . 24


4 Large scale prototype 27

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

4.2 Design criteria and process . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

ix

4.3 Design parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.3.1 Filter element . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.3.2 Pre-separator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

4.3.3 Post-separator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

4.3.4 Liquid outlet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

4.4 Geometric configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

4.4.1 Static housing design . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

4.4.2 Construction material . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

4.4.3 Structural strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

4.4.4 Shaft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4.4.5 Cleaning system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4.5 Support systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

4.5.1 Bearings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

4.5.2 Electrical drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

4.5.3 Magnetic coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

5 Experiments 47

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

5.2 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

5.3 Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

5.4 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

6 Conclusion 51

6.1 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

Bibliography 55

A Equipement dimension calculation 57

A.1 Gap leak flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

A.2 Flow rates and properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

A.3 Filter element . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

A.4 Pre-separator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

A.5 Post-separator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

A.6 Liquid outlets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

A.7 Shaft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

A.8 Contamination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

A.9 Construction material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

A.9.1 Different corrosion mechanisms . . . . . . . . . . . . . . . . . . . . . . 72

A.9.2 Different material families . . . . . . . . . . . . . . . . . . . . . . . . . 74

A.9.3 Information on stainless steels . . . . . . . . . . . . . . . . . . . . . . . 75

A.10 Structural strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

B Preliminary drawings and components 81

B.1 Sealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

B.2 Bearings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

B.3 Magnetic coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

B.4 Design codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

x

B.5 Drawings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

C Experiments 93C.1 Axial RPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93C.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

Acknowledgements 96

Chapter 1

Introduction

1.1 Background

Natural gas is becoming more and more a commodity in the global energy consumption. Newtechnologies like the conversion from gas to liquid, contribute to this. From an environmentalpoint of view the use of natural gas also has its benefits because it is the cleanest of all fossilfuels. Composed primarily of methane, the main products of the combustion of natural gasare carbon dioxide and water vapour. Coal and oil contain a higher carbon ratio and highernitrogen and sulfur contents.

The latest estimate (January, 2007) on the world’s gas reserves by the Oil & Gas Jour-nal, indicates that an amount of 6.2·103 trillion (1012) standard cubic feet of natural gas ispresent in gas fields that can be produced with existing technologies. Most of these gas fieldsare already being produced or will be in the near future. However when gas fields reach theend of their life-cycle, quality of the gas decreases. Furthermore a substantial part of thefields, which are not included in that number, cannot be recovered with existing technolo-gies. Contaminants in the natural gas like CO2 and H2S cause these problems. If CO2 andH2S amounts are >15% and >5% respectively [22], existing technologies are not capable ofcleaning the natural gas. This includes approximately 16% of all the known global naturalgas reserves (2.1·104 trillion standard cubic feet [27]). A huge economical advantage can beobtained, if these fields could be recovered. A new technology, called Condensed ContaminateCentrifugal Separation or C3-Sep, offers the possibility to do just this. With the new breakthrough technology the contaminated natural gas mixture is brought to such a temperatureand pressure, that contaminants will condense and form a mist of fine droplets. These dropletsare so small that the Rotating Particle Separator, invented by Brouwers [1], has to be usedto provide a good separation, capable of handling high throughputs.

Van Wissen [27] was the first one to further develop the technology into a working pro-totype. The prototype of van Wissen is designed to clean 50 kscf/d of natural gas. Forindustrial use, a larger capacity is required. In this thesis the foundation is laid for an versionof an up-scaled prototype. The prototype will be situated in a new test facility. The testfacility is currently under construction in the Botlek region near Rotterdam.

2 Introduction

1.2 Goal and outline

In this thesis a prototype is designed that is capable of cleaning large amounts of highlycontaminated natural gas. In the above it became clear that methane, or natural gas, containslarge quantities of CO2 and H2S. Contamination levels that will be tested on the prototype,contain between 20% and 71% of CO2 and between 0.001% and 33.2% of highly toxic H2S.Furthermore high throughputs have to be handled. Typical throughputs for developed gaswells can be up to 350 MMscf/d (106 standard cubic feet). This is the first time the C3-Sepprocess will be applied in a large scale application. Therefore, to start with, a 20 MMscf/dinstead of a 350 MMscf/d prototype will form the basis for design. The prototype can beused for further testing and developing of this new technology.

The prototype is to be tested in the Euroloop in Botlek (The Netherlands). The testloopoffers the possibility to simulate an aggressive environment with low temperatures and highpressures. Each factor causes its own restraint on the design. Therefore regulations, directivesand design codes, that are set for the testloop, have to be applied on the design. The aim ofthis thesis is to provide a so-called ”basis for design” that is compliant with standards andregulations used at Shell. Subsequently Shell can use the ”basis for design” as a guideline formore detailed design drawings.

In the first chapter the working principle of both the C3-Sep process and a Rotating Par-ticle Separator (RPS) for gas/liquid separation is explained. After that, physical phenomenaon centrifugal separation and some general design issues are discussed. These general designissues return for each RPS design. The knowledge gathered in the second chapter will beused in the third chapter. This chapter focusses more on the specific design of the prototypeand describes some components and potential problems that could occur. Methods to dimen-sion the RPS are explained and with an overall efficiency curve the most important designparameter, dp,50% can be obtained.

In chapter four the design is finalized with the help of chapter three. Dimensions of thekey components are given. After that, the housing and shaft are designed and tested on e.g.structural strength. Different materials and applicability considerations are presented. Finallythe support systems, which incorporate drive, coupling and bearings, are selected. During thedesign process different directives and regulations are implemented in the design. Experimentsare performed in chapter five to obtain information on droplet break-off mechanisms in theRPS. Results and a discussion is given at the end. Finally, conclusions and recommendationsare given in chapter six. The recommendations indicate points, components or problems thatrequire further attention.

Chapter 2

Rotational Particle Separator in

C3-separation

2.1 Introduction

Designing a Rotating Particle Separator has been done in the past. Theories and methodsto do this are e.g. given by Mondt [18]. In this thesis a new kind of up-scaled RPS will bedesigned that has to work in conditions which were first encountered by van Wissen. Howeverthe general definitions and design issues that are of importance are still the same. In thischapter first a general outline of the process will be given. The idea behind C3-Sep will beexplained in the coming sections. When it is clear what type of mixture and particle sizes haveto be separated by the RPS, the general working principle and leading centrifugal equationswill be described. Furthermore there are always some design issues that return with eachRPS design. Therefore an introduction on required drive power and heath production aregiven in the section on angular momentum. The leak flow, that often occurs between a staticand rotating part, will also be explained. This knowledge will be used to further specify thisparticular RPS design in the next chapter.

2.2 Condensed Contaminant Centrifugal Separation

In the introduction a first glimpse was given in the so-called Condensed Contaminated Cen-trifugal Separation process or C3-Sep process. As mentioned, approximately 16% of thenatural gas fields that are currently known, are highly polluted (>10% CO2 and >5% H2S).The C3-Sep technique offers the possibility to efficiently recover these resources. The C3-Sepprocess is rather simple and consists out of two separate steps that will be described in thecoming parts.

2.2.1 Mixture characteristics

As the ”Condensed Contaminant” part of the C3-Sep already states, the pollutants, or con-taminants, will be condensed. This is done by cooling the CH4/CO2 mixture to a low tem-perature. For reasons of simplicity, only the condensation and separation of CO2 is treatedin this thesis because this is the main aim of this RPS design. However the highly toxic H2S,will be taken into account during the rest of the designing process. The condensed contam-

4 Rotational Particle Separator in C3-separation

inants will form a mist of small droplets. Later in this section it will become clear that thedroplets are expected to be so small that current separating techniques are not capable ofseparating these droplets, at high flow rates, efficiently. However van Wissen [27] stated thatthe Rotating Particle Separator, or RPS, invented by Brouwers [1] can be used to separatethe large quantities of small droplets with high efficiencies.

Contaminated gas

Compressor Expander

Rotating Particle Separator

Compressor

Liquid CO2

Clean gas

Induction tube

.

Figure 2.1: Schematic of C3-Sep process

The condensation process of the mixture will now be described more elaborate. The gasmixture that comes from the upstream pre-treatment plant, has to be cooled. This will bedone by expansion of the gas from a high pressure. The pressure of the pre-treatment plantis to low to expand from. Pressures up to 130 bar are needed. Therefore the C3-Sep loopwill contain its own compressor to ensure the right pressures. A schematic of the loop can beseen in figure (2.1). After the mixture has been pressurized, the schematic shows an expanderneeded to form the mist of droplets. The expander can be of a turbine type or it could bea less efficient Joule Thompson valve. Energy added to the mixture during the compressionstep can be regained when a turbine is used, thereby increasing the process efficiency. This isnot possible when a Joule Thompson valve is used. Furthermore a less efficient cooling curve,in comparison to a turbine which works isentropic, leads to less enriched CO2 droplets [7].

In figure (2.2) a phase diagram can be seen [27]. The amount of liquid CO2 that willform depends on the mixture composition. This phase diagram e.g. shows a 50/50 CH4/CO2

mixture. Expanding from the right start pressure to around 27 bar, makes the temperaturedrop to -50 0C. Figure (2.2) shows that a combination of both liquid and vapour will form.The vapour phase will be enriched in CH4 and depleted in CO2. For the liquid phase this isjust the other way around; the liquid is enriched in CO2 and depleted in CH4. So althougha great part of the two mixture substances take on a different phase, there is no completeseparation of CO2 and CH4 of each other. For other mixture compositions different amountsof CH4 and CO2 will dissolve in one another. It is most preferable that a clean vapour withthe highest concentration of CH4 will be obtained. The concentration of CH4 in the clean

2.2 Condensed Contaminant Centrifugal Separation 5

−100 −80 −60 −40 −20 00

20

40

60

80

100

T [oC]

p [b

ar]

L + S L

V + L

V + L + S V

Figure 2.2: Phase diagram of a 50/50 CH4/CO2 mixture. Vapour is indicated V, L is liquidand solid CO2 is indicated with an S [27].

product vapour stream can be described as [7]

xc =nCH4,c

nc(2.1)

where xc denotes the mol fraction CH4 in the clean product stream and nCH4,c denotesthe number of moles of methane in the product stream with a total number of moles nc.Furthermore we wish to recover the highest amount of CH4 possible in the clean productstream in relation to the number of moles CH4 that were delivered in the feed stream. Inother words, it is preferred that the number of moles of CH4 in the product stream matchesthe number of moles CH4 in the feed stream as good as possible. This preference or recoverycan be described by [7]

r =xcQc

xfQf(2.2)

where xc denotes the mol fraction CH4 in the clean product gas and Qc [kmol/s] the flow rateof clean product gas. xf is the mol fraction CH4 in the feed flow Qf [kmol/s]. It is assumedthat all the liquid that was formed during the expansion step will be separated by the RPS.The recovery r can be increased by optimizing the pressures and temperatures from whereto expand from and to [27]. Realistic values which take economical and physical factors intoaccount have to be chosen. According to van Wissen a maximal enrichment of xc ∼ 0.88at low CO2 content feed streams and an enrichment of xf = 0.3 to xc ≃ 0.6 for high CO2

content feed streams can be obtained in one step. The flow of liquid waste stream and vapourproduct stream for this RPS design will be quantified in chapter 4 by analytical calculationsand numerical flash vessel simulations.


2.2.2 Droplet formation and growth

After the expansion step, formation of droplets takes place in the induction section. The pipesection enables the droplets to grow. Although in this thesis no models on droplet formationand growth are worked out, a short outline will be given on the responsible mechanisms asalso described in van Wissen’s dissertation.

Droplet formation can be caused by two mechanisms; homogeneous nucleation and hetero-geneous condensation [27]. Homogeneous nucleation involves the formation of a large amountof very small droplets, so-called nuclei, by condensing CO2 vapour. Heterogeneous condensa-tion takes place by diffusion of CO2 vapour to some nuclei where it subsequently condenseson these nuclei. If the first method is dominant a large amount of very small particles arecreated. If the latter mechanism is dominant fewer but bigger droplets will form.

Another mechanism which results in droplet growth is coagulation. Here droplets willcollide with each other forming new bigger droplets thereby decreasing the number of droplets.Evaluation of these models lead to a theoretical worst case droplet size for different mixturecompositions (see figure (2.3)).

Figure 2.3: Droplet size versus the residence time using the coagulation theory [27].

In industry high throughputs are required to ensure feasible processes. Therefore residencetimes are generally in the order of 0.1 to 1 second. Assuming droplets are formed withhomogeneous nucleation and heterogeneous condensation, coagulation leads, according tofigure (2.3), to a mean droplet diameter of around 1.5 µm for a 50/50 CH4/CO2 mixture.The droplets will now be fed to the rotating particle separator that is described in the nextsection.

2.3 RPS working principle 7

2.3 RPS working principle

The small droplets of CO2 that form after expansion of the contaminated gas mixture, willnow reach the main design issue of this thesis; the Rotating Particle Separator. A sketch ofthe Rotating Particle Separator, or RPS, is given in figure (2.4). The RPS can be subdivided

Figure 2.4: Working principle RPS [27].

in three main parts; the pre-separator, the filter element and the post-separator. The firstand the latter can best be described as axial cyclones. The RPS distinguishes itself froma cyclone by the droplet size that can be collected. An industrial cyclone typically collectsparticles of 15 µm and up, where the RPS, due to the filter element, is able to collect 1µm particles. The mixture will enter the RPS and a swirling flow will be imposed to theliquid-gas mixture. This swirling flow can either be enforced by a swirl generator, which givesthe flow a tangential velocity profile by blades, or by a tangential inlet. When a swirlingflow has been established it enters the pre-separator. In the pre-separator the larger liquidCO2 droplets and other particles will be forced to the wall of the static housing. Howeverthe smaller droplets will not migrate to this wall and will enter the rotating filter element.This element consists out of an array of small parallel channels that rotate around the shaft.Because the height of the channels is small, generally 1 mm, a droplet or particle only has tomigrate a small distance before it will hit the channel’s wall. Therefore less residence time isrequired to separate the small droplets, making the RPS a very efficient bulk separator. Onthe outer radial walls of the channels a film of liquid CO2 will form. At the end of the filterelement, the film will break-up into larger droplets, as illustrated in figure (2.4). Because themixture is still rotating when it exits the filter element, the now considerably larger dropletsof CO2 will again migrate to the outer wall of the RPS. This part of the RPS is the so-calledpost-separator. The liquid CO2 can now be collected and the clean natural gas flow will leavethe Rotating Particle Separator. The outlet needs to regain as much of the mixture pressureprior its entering in the RPS.


2.4 Centrifugal separation

To get a better understanding on the centrifugal separation process the principle of centrifugalseparation will be explained more elaborate. As told in the previous section the RPS can besubdivided in a pre- and a post-separator and the actual filter element. Although all thesesections seem to differ from each other, the basic centrifugal principle stays the same.

The CO2 droplets are assumed to be spherical with a diameter dp. Furthermore theparticles will have a constant density ρf [kg/m3]. A Stokes-law will be assumed because theReynolds number of the moving particle is small (Rep <1). This Reynolds number can bedescribed as

Rep =ρgvpdc

µg(2.3)

The radial migration velocity of the particle vp,r directed to the outer wall, can be determinedby balancing the centrifugal force with the buoyancy and the drag force. The particle velocitycan therefore be described as [8]

vp,r =dr

dt=

(ρf − ρg)d2pv

2t

18µgr(2.4)

Where ρg [kg/m3] is the density of the carrier fluid, vt [m/s] the tangential or swirl velocity,µg [Pa·s] the dynamic viscosity of the carrier fluid and r [m] the radial position of the particle.For determining the radial migration of the particle, equation (2.4) has to be integrated. Thisleads to

r(τ)2 − r(0)2 =2(ρf − ρg)d

2p

18µg

∫ τ

0v2t dt (2.5)

The particle will travel from the inner radius r(0) to an outer radius r(τ) during a certaintime. This residence time, τ , is defined as the time it takes a particle to travel a certaindistance L [m]. The distance is, in this case, the length of the separator. When there is aconstant axial flow through the separator it’s possible to define the residence time as

τ =L

vax(2.6)

where vax [m/s] is the axial velocity. With this knowledge it is possible to give a theoreticalminimum particle diameter that can be separated. The minimum particle that will be sepa-rated has to travel the radial distance in the given time period τ . Assuming the entrance flowto the separator is uniform, the particles are distributed evenly over the separator flow area.To determine a particle diameter that will be caught of with a 50% probability, the flow areahas to be cut in half (see figure (2.5)) [27]. In this figure the radius Ro indicates the outercollection wall and Ri is the inner radius that is limited by e.g. a shaft. It can also be seenthat at a certain radius R50% two equal flow areas are defined. Using Ao = Ai the cut radiusR50% can be written as

R50% =

√

R2o +R2

i

2(2.7)

2.5 Angular momentum 9

Ro

R50%

Ri

Ai

Ao

Figure 2.5: 50% Radius at which Ao = Ai

To determine the particle diameter that will be separated with a probability of 50%, theso-called dp,50%, two boundaries, r(0) = R50% and r(τ) = Ro can be established. Substituting

equation (2.6) in equation (2.5) and using the definition for R50% and vax = dzdt gives

dp,50% =

√

18µgvax(R2o −R2

i )

2(ρf − ρg)∫ L0 v2

t dz(2.8)

The tangential velocity vt differs per type of separator. As indicated earlier, the RPSconsists of a pre- and post-separator and the actual filter element. The tangential velocity ofthe mixture is the driving parameter that enforces the centrifugal force. Depending on thekind of separator this tangential mixture velocity is created differently. In the next chapter thegeneral separation equation will be adapted to fit the characteristics of the different separatorsin the RPS. Also the separation efficiency will be discussed.

2.5 Angular momentum

The filter element has to be brought in rotation. The tangential inlet or swirl generator canbe used to drive the filter element. The swirling flow has a momentum Gθinlet [Nm] thatprovides this drive power [18]. An equilibrium between the angular momentum required todrive the filter element and the momentum that is induced by the flow can be established.At this equilibrium state, the filter element will have a constant rotating velocity.

Gθinlet = Gθfe(Ω) +Gθgap(Ω) +Gθbearings(Ω) +Gθpre (2.9)

Losses of angular momentum will occur due to friction between the rotating filter element andthe static housing, Gθgap and friction effects in the bearings, Gθbearings. These momentumsare important factors to determine the heath generation in the RPS. This has to be limitedin order to not disturb the thermodynamic processes. Gθfe indicates the generated angularmomentum at the exit of the rotating filter element.

A general definition for the angular momentum Gθ can be described as [18]

Gθ =

∫

AρgvaxvtrdA (2.10)

In this definition vax [m/s]is the axial mixture velocity, vt is the tangential velocity component.


Assuming a stationary flow at the entrance, no change of angular momentum will occur.Furthermore the entrance dimensions have to be known in order to determine the initialtangential velocity vt,0 at a given mass flow rate, mg. The momentum of the CH4/CO2

mixture at the entrance will be looked at from the center of the RPS thereby creating themoment of momentum.

Gθinlet = mgvt,0Rm (2.11)

where Rm is the length of the arm at which the momentum seizes. Rewriting equation 2.11with mg = ρgQc = ρgvt,0Ainlet and Rm = Ro −Rinlet

Gθinlet = πρgv2t0R

2inlet(Ro −Rinlet) (2.12)

where Rinlet [m] is the radius of the tangential inlet to the RPS.The loss of angular momentum in the gap is caused by the friction of the fluid between

the static housing and the rotating filter element. The rotation of the filter element in thestatic housing can be seen as a rotating cylinder in a static housing. According to Schlichting[23], the momentum loss can thus be described as:

Gθgap =1

2πCMρgΩ

2R4oL (2.13)

Ro is the outer radius of the filter element and L the combined length of the filter elementand post-separator. CM [−] denotes a torque coefficient [23]. This torque coefficient dependson the gap size sgap. When the gap size increases the angular momentum decreases therebyreducing the required drive power.

The loss of angular momentum in the bearings differs per type of bearing. Magneticbearings are not in physical contact with the shaft so almost no loss of momentum occurs.However roller bearings or sliding bearings cause loss of angular momentum. As told earlierthis loss will be transformed to heath.

The pre-separator can be seen as a hydraulically smooth tube. The axial decay of theangular momentum is caused by wall friction.

10logGθ,pre

Gθ0= −0.01605x∗

0.8

(2.14)

In equation 2.16 x∗ denotes the ratio between the length of the pre-separator and the hydraulicdiameter of the tube. Furthermore Gθ0 represents the initial angular momentum which equalsGθinlet.

Gθpre = Gθinlet(1 − 10−0.01605x∗0.8

) (2.15)

Gθ,pre is the actual angular momentum.When the gas flow leaves the rotating filter element, there is additional loss in angular

momentum. The gas behind the filter element has to be brought in rotation. It depends onthe outgoing axial velocity profile how big this angular momentum loss will be. Assuming aturbulent flow that leaves the filter element and a constant axial velocity profile, the angularmomentum can be described as:

Gθfe =1

2(1 − ǫred)vax,feρgΩR

4o(1 − δ4) (2.16)

2.6 Gap leak flow 11

2.6 Gap leak flow

The RPS contains a rotating filter element in a static housing. Therefore there will always bea gap between the housing and the filter element through which a leak flow can occur. Theflow has to go through the filter element to guarantee an efficient separation. However thebigger the gap size sgap, the more flow will leak past the gap and separation performanceswill be more difficult to predict. A smaller gap size has its disadvantages. When the gapsize is reduced the angular momentum (see previous section) will increase thereby increasingrequired drive power. An other limitation is the fact that some clearance is needed in caselarge vibrations, thermal gradients, production limitations and shaft deflections will occur. Itis therefore interesting to know what the leak flow will be when a certain gap size is chosen.

To determine the leak flow, it is assumed that there is a uniform pressure drop over thefilter element. Therefore the pressure drop in the gap is equal to the cumulative pressuredrop in both the filter element and the post separator.

∆pgap = ∆pch + ∆ppost (2.17)

The pressure drop in the post separator will be small compared to the pressure drop in thefilter element. This is caused by an hydraulic diameter of the post separator that is muchlarger then the channel diameter Dh,post >> dc. Therefore equation 2.17 becomes:

∆pgap = ∆pch (2.18)

Re-writing equation 2.18 leads to

(fgapLgap

Dh,gap+ ξgap)

1

2ρg(

Qgap

Agap)2 = (f

Lfe

dc+ ξfe)

1

2ρg(

Qfe

Afe)2 (2.19)

where Qfe [m3/s] is the flow rate through the filter element and Qgap indicates the flowrate through the gap. By rewriting the flow velocity in the filter element and the gap asvax,fe =

Qfe

Afeand vax,gap =

Qgap

Agaprespectively, assuming that Qc = Qfe + Qgap and the

entrance effects in the beginning of the gap and the filter element are equal, ξgap = ξfe = ξequation (2.19) becomes

(fgapLgap

Dh,gap+ ξ)

1

2ρf (

Qgap

Agap)2 = (f

Lfe

dc+ ξ)

1

2ρf (

Qc −Qgap

Afe)2 (2.20)

Substituting the hydraulic diameter of the annular gap by Dh,gap = 2sgap the volume flowthrough the gap Qgap can now be written as a function of the gap size sgap.

Qgap(sgap) =

√

ζ1ζ2(sgap)Qc

1 +√

ζ1ζ2(sgap)Qc

(2.21)

with ζ1 = A2fe(f

Lfe

dc+ ξ) and ζ2(sgap) = A2

gap(fgapLgap

2sgap+ ξ). As can be seen, the friction

coefficients, f [−] and fgap [−], are dependant on the Reynolds number and therefore thefluid velocities, who are on their turn dependant on the flow area’s Agap and Afe. To obtain acorrect value for the friction coefficients a convergence method was used. Reynolds numberswere chosen and thereby a value for the friction coefficients. By recalculating the velocitiesnew Reynolds numbers could be determined. Repeating this action will cause the Reynoldsnumbers to converge to a certain value so correct friction factors can be obtained. Results ofthe leak flow can be found in appendix (A.1).

Chapter 3

Conceptual design

3.1 Introduction

The previous chapter discussed the idea behind C3-Sep, the condensation process and somegeneral design issues. This chapter will further use this knowledge to specify and describe thedifferent parts of the RPS.

First the way to determine the dimensions of the pre-separator and some of its designrequirements will be explained. Two different methods to establish the dimensions of thefilter element will then be described. The first methods makes use of the old parameters likeRo, Lfe, Qc and Ω. The swirl method uses three parameters; Qc, τ and ǫ in combination withthe pressure drop over the RPS. The separation efficiency of the filter element and the way todetermine the minimum particle diameter that has to be separated, will be elaborated in thesubsequent sections. Furthermore physical processes that occur in the filter element will beexplained like e.g. channel contamination by particles and the liquid film build-up. Finallythe dimensions of the post-separator and the break-off droplet diameter will be treated.

3.2 Pre-separator

In section (2.4) a general definition for the dp,50% was given. This equation will now be usedto determine the minimum droplet diameter dp,pre that will be caught off with a probabilityof 50% in the pre-separator. In addition to the mixture of natural gas and liquid CO2, themixture could also contain contaminants in the form of coarse particles and large droplets(see appendix (A.8)). The particles can cause blockage of the channels of the filter elementthereby decreasing the overall collection efficiency [18]. To prevent damage, these particleshave to be separated in the pre-separator.

The pre-separator (see figure (3.1)) can be seen as an axial cyclone. There is a densitydifference between the carrier fluid and the heavier particles that will be subjected to acentrifugal force. The particles will move to the outer wall of the pre-separator.

The mixture will enter the RPS from the side of the housing through a small entrance.Therefore an initial tangential velocity is enforced on the mixture creating a centrifugal force.When first entering the RPS, the tangential velocity profile behaves like a free vortex (vt =1/r). After a certain axial distance this velocity profile will transform into a solid bodyrotation (vt =∝ r) [27]. A solid body rotation will be assumed to derive an expression for theparticle diameter.

14 Conceptual design

3.2.1 Separation process

Looking at equation (2.8) a few parameters have to be redefined. In the first chapter itbecame clear that the filter element is mounted on a shaft. The inner radius Ri will thus besubstituted by Rshaft [m]. Furthermore the tangential velocity of the swirling flow has to bedefined. The inlet of the RPS plays a key rol. When a certain total flow rate, Q, is appliedto the RPS and an inlet radius Rinlet [m] has been established, the inlet velocity becomes theinitial swirl velocity: vinlet = vt0 (see figure (3.1)). The swirling flow decays in axial direction

Rshaft

Rpre

vinlet = vt0

Ω

Rinlet

Figure 3.1: Inlet representation with vinlet = vt0

due to wall friction. Experimental results of axial decay of swirl flows in a tube, show thatsuch a flow can be described as [27]

vt = vt0 exp

(

− z

2Rpreβ

)

(3.1)

where z [m] is the axial distance, Rpre the radius of the pre-separator, β an empirical factorthat will be chosen at β = 0.05 [27] and vt0 is the initial tangential velocity. However thedecay of the swirling flow is small. For half a meter of tube the axial decay in this tube isless than 5% and can be neglected. The tangential velocity vt in equation (2.8) can thereforebe substituted by vt0. The residence time τ required for a particle to move from R50% to theouter wall Rpre, depends on the traveled distance Lpre and the axial velocity vax,pre. Because

a shaft is present the cut radius R50% can be described as R50% =

√

R2o+R2

shaft

2 . Substitutingthis and equation (3.1) in equation (2.8) gives

dp,pre =

√

√

√

√

9µg(R2pre −R2

shaft)

2(ρf − ρg)v2t0Rpre

vax,preβ(

1 − exp(

−LpreβRpre

)) (3.2)

3.3 Filter element 15

3.3 Filter element

The characteristic part of the RPS is the rotating filter element. This part of the RPS makesit possible to collect small particles at high flow rates. The rotational motion of the filterelement is enforced by the momentum of the swirling mixture and/or an electric motor. Todetermine the particle diameter that will be collected with a probability of 50% the generalequation (2.8) will be used.


In a filter element the channels are stacked on top of one another. The channels have asinusoidal shape. However for this coming derivation for the dp,50%, the sinusoidal channelsare simplified to concentric rings. The radial migration velocity of a particle is given byequation (2.4). It can be seen that the radial velocity of the particle depends on the radialposition r of the particle. The radial distance, or channel height dc, between the concentricrings is very small, in the order of 1 mm, it is therefore assumed that small changes in theradial position of the particle does not effect its migration velocity.

Now it is possible, with the help of figure (2.5), to determine a 50% cut radius. Assuminga uniform entrance flow to the RPS leads to an homogene distribution of particles. Lookingat the complete filter element the radius where the channels start is indicated as Ri and Ro

defines the outer radius. The ratio between the inner and outer radius is given by

δ =Ri

Ro(3.3)

The ratio δ [−] is chosen to be 12 [4]. The cut radius R50% can, with Ao = Ai, be defined as

R50% =Ro

2

√

1 + δ2 (3.4)

With a uniform entrance flow an axial velocity can be defined, that is constant over the radius.

vax,fe =Qc

πR2o(1 − δ2)(1 − ǫred)

(3.5)

For convenience it is assumed that the total flow rate Q [m3/s] passes through the filterelement. ǫred [−] is the reduction of the effective cross-sectional area of the filter element dueto the wall thickness of the channels. Brouwers [1] shows that a good approximation of thereduction factor is ǫred = 0.1. Furthermore the tangential velocity is given by vt = Ωr, whereΩ [rad/s] is the radial velocity of the filter element. Substituting this and equations (3.4) and(3.5) in equation (2.8) gives

dp,50% =

√

9√

2µgdcQc

(ρf − ρg)(1 + δ2)1/2(1 − δ2)(1 − ǫred)πΩ2LfeR3o

(3.6)


3.3.2 Swirl energy

In equation (3.6) the dp,50% is described by four key parameters; Ro, Lfe, Q and Ω. Theseparameters can be changed and varied in such a way that a good size for the filter elementcan be established. This leads to a dimension determination of the filter element by a trail-and-error method. It would therefore also be interesting to define the particle that willbe separated with a 50% probability, by looking at a different set of parameters. Theseparameters, or independent process parameters as they are called [28], make it possible todetermine dimensions of the RPS by using the energy consumption of the RPS due to thepressure drop. It is assumed that this pressure drop is a loss of energy that is used to drivethe filter element. So by using this method, the dimensions of the RPS will be determined insuch a way, that the RPS is self sufficient in its drive power needs. When choosing the dp,50%

that has to be separated and at a give pressure drop, the dimensions of the RPS will, withthis model, be obtained accordingly. Drive power fed to the RPS by an elektromotor willtherefore only be needed to help the filter element accelerate faster but not adding additionalrevolutions. There are three independent process parameters: Q, τ and ǫ. Q is the flow rateand at a certain flow Q, the residence time τ [s] gives an indication of the investment costs.ǫ [KJ/kg] gives a good indication on the operating costs.

As mentioned above, the method to determine the dp,50% is based on the loss of energydue to the total pressure drop over the RPS. This pressure drop consists out of two terms

∆pRPS = ∆Pt + ∆Pch (3.7)

Where ∆Pt is the tangential pressure drop caused by the swirling flow and ∆Pch is thepressure drop due to the flow of the mixture through the channels of the filter element. Thepressure drops or energy losses are a measure for the rotational speed of the filter element.The swirling flow is initiated at the entrance, or inlet of the RPS (see figure (3.2)). It is

L

Ro

Swirl generator, inlet

De-swirler, outlet

Figure 3.2: Schematic drawing of the rotational particle separator

assumed that the kinetic energy of this swirl is partially recovered at the end of the RPS atthe outlet. Therefore the irreversible tangential pressure drop caused by the swirling flow canbe described as

∆pt =1

2ρgv

2t0 (3.8)

The tangential pressure drop will be chosen at the average radius r = R50% (see section (2.4)).


With the tangential velocity vt = Ωr known, this leads to

∆pt =1

4ρg(ΩRo)

2(1 + δ2) (3.9)

The pressure drop in the small channels of the filter element can be described by usingequation (3.10)

∆pch =1

2ρgv

2ax,fef

Lfe

dc(3.10)

where the friction factor f in the channels [−] can be described by the equation of Blasius.This equation holds for Reax < 105 [27].

f = 0.316Re−0.25ax (3.11)

It is now possible to define the parameters Ro, Lfe, Qc and Ω as function of the threeindependent process parameters Qc, τ and ǫ. This can be done by using equation (3.9),(3.10) and the equation for the axial velocity (3.5) where the area reduction factor ǫred willnot be considered [27]. The parameter Qc will stay the same. τ , which is the residence timeof the fluid in the filter element, can be described as the length of the element divided by theaxial velocity. The new expressions for Ro, Lfe and Ω can be found in appendix (A.3). Asshown, those expressions are still dependant on either the pressure drop caused by swirl ∆pt

or by the pressure drop over the channels ∆pch. Using these equations and substituting themin equation (3.6) gives a new expression for the dp50%.

d2p,50% =

9µgd5

6c (1 + δ2)

1

2 f1

6

25

3 (ρf − ρg)π1

2 (1 − δ2)1

2

ρ7

6g

∆p1

6

ch∆pt

Q1

2c τ

−5

6 (3.12)

In this equation the specific energy parameter ǫ isn’t implemented yet. The specific energydepends on the total pressure drop over the RPS ∆pRPS

ǫ =∆pRPS

ρg(3.13)

It is however not known yet which pressure drop is more dominant; ∆pch or ∆pt. To find theratio between these pressure drops it is assumed that the ratio x depends on

x =∆pch

∆pRPS(3.14)

Rewriting equation (3.13) such that d2p,50% scales with the total pressure drop and the ratio

x, leads to [27]

d2p,50% ∼ 1

∆p1

6

ch∆pt

=1

(∆pRPS)7

6x1

6 (1 − x)(3.15)

A minimum ratio, for which ∆pch and ∆pt are each contributing to the total pressure drop∆pRPS and thereby to the minimal particle diameter dp,50%, can now be established. Differ-entiation and minimization leads to a minimum ratio of x =1/7. The ratio is a design choice


and can be varied when dimension results are not satisfying. This eventually leads to thenext two expressions

∆pch =1

7∆pRPS (3.16)

∆pt =6

7∆pRPS (3.17)

In the above is stated that the specific energy ǫ is given by ǫ = ∆pRPS/ρg. So ∆pch and ∆pt

can now be replaced by substituting equations (3.16) and (3.17) in the expressions for Ro,Lfe and Ω in appendix (A.3). In the next chapter this model will be used to compare resultsfound with the more conventional equations from the previous sections.

3.3.3 Efficiency

The diameter of the particles that reach the collection wall of a single channel with a prob-ability of 50%, is a diameter that indicates that 50% of the particles with that diameteractually reach the wall. The other 50% are not able to do so. Smaller particles than theones with a diameter of dp,50% can also reach the channel wall but with a lower probability.In general, the percentage of particles of a certain diameter which will be collected at thewall, will decrease with the degree by which these particles are smaller than dp,50% [2]. Sothe collection efficiency is an important parameter that enables us to get a better insight inthe separation proces. It becomes possible to determine a theoretical RPS efficiency when acertain particle distribution is known.

The introduction stated that large throughputs have to be handled by the RPS. Thecombined flow area of the channels of the filter element cannot be too big, to ensure acompact design. An axial version of the RPS has the preference over a tangential versionbecause laminair flow conditions impose a too severe restriction on the design [12]. Thusa more turbulent flow will be present in the channels of the filter element. Kuerten [12]determined, with a model using a DNS method, a particle collection efficiency curve for acircular channel with flow conditions in the transition area from laminair to turbulent, withan axial Reynolds number of

Reax =ρgvax,fedc

µg= 5300 (3.18)

The rotational Reynolds number can be defined as

ReΩ =ρgΩd

2c

µg= 980 (3.19)

and is an important condition because rotation is an additional destabilizing factor when itcomes to the particle collection [18]. The deduced efficiency curve is for a single circularchannel. As mentioned in section (3.3.1), in reality the channels have a sinusoidal shape.Kuerten [12] calculated efficiencies for circular channels in a turbulent transition region. Thisseparation efficiency will be used.

As mentioned above, the turbulent efficiency curve is valid for a single channel. If an idealentrance flow ( vax,fe ∝ r) is delivered to the filter element, this curve is also the collectionefficiency curve for the whole filter element [2]. However this is not the case because a uniform


entrance flow is assumed. The filter efficiency can thus be determined by solving the followingintegral

ηfe =1

Qc

∫ 2π

θ=0

∫ Ro

r=δRo

η(x, r)vax,fedA (3.20)

where dA is the area of a thin concentric ring dr, thus giving dA = rdrdθ. Furthermore thefilter element is axi-symmetric. η(x, r) indicates the efficiency per concentric ring. Solvingthis integral numerically gives a efficiency curve that can be seen in figure (3.3).

Dimensionless particle diameter dp/dp,50% [-]

Effi

cien

cy[-]

Total filter efficiencySingle channel efficiency at δ RoSingle channel efficiency at Ro

0 0.5 1 1.5 2 2.5 30

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Figure 3.3: Efficiency of total filter element with circular channels

The efficiency of one channel at the inner radius δRo of the filter element is given ascomparison. On the x-axis the dimensionless particle diameter is given. This is done bynormalizing a distribution of particle diameters with a certain mean particle diameter dp, toa dp,50% that is recalculated per concentric ring. On the y-axis the efficiency can be seen.The efficiency of the total filter element in comparison to the efficiency of one channel at theinner radius is higher. However the efficiency curve of the filter element lies lower than theefficiency curve of a row of channels on the outer wall of the element.

The collection efficiency curve is given for one particle distribution with a mean diameterdp. As became clear in the introduction, it is not yet quite known, what the mean particlediameter of the distribution will be. The efficiency curve therefore doesn’t give a completeoverview of the RPS’s possible operating area. It is interesting to know what the overallefficiency of the RPS will be when a range of particle distribution with a varying meanparticle diameter is fed to the filter element. The overall efficiency curve that will originatemakes it possible to determine a value for the dp,50% which has a more solid basis to workwith.

The overall or total collection efficiency can be described as the integral over a range of


particle diameters from zero to infinity.

ηtot =

∫

∞

0ηfePddp (3.21)

In equation (3.21) ηfe is the particle collection efficiency curve determined by equation (3.20).P indicates a function for the particle distribution. Although the shape of this distributionis yet not known, for simplicity reasons the shape of the particle distribution will be taken asa log-normal distribution (see equation 3.21) [10].

P (dp/dp) =1√

2πdp log(σg)exp

(

log(dp/dp)2

2 log(σg)2

)

(3.22)

According to Brouwers [4] this distribution can be seen regularly in natural processes wheresmall particles are formed or are present. It can also be said that although the mean diametercan vary, the geometric standard deviation or σg can be set to values between 2 and 2.5.

As can be seen in equation (3.22), the general equation for the particle distribution canbe normalized by x = dp/dp. This makes it possible to multiply it by the efficiency curveηfe(x) and leads to

ηtot =

∫

∞

0ηfe(x)

(

xdp

dp,50%

)

P (x)dx = g

(

dp

dp,50%

)

(3.23)

Again the integral for the total particle collection efficiency can be solved numerically. Thetotal overall efficiency curve is plotted in figure (3.4).

dp / dp50%

Tot

alpar

ticl

eco

llec

tion

effici

ency

[−]

0 0.5 1 1.5 2 2.5 3 3.5 40

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Figure 3.4: Total efficiency of filter element

In figure (3.4) the y-axis indicates the total particle collection efficiency. On the x-axisthe normalized particle diameter x = dp/dp,50% is stated. As shown, the total efficiency curve


becomes less steep when the dimensionless particle diameter x increases. From a certain x,where the total collection efficiency is approximately 98%, almost no increase occurs anymore.It can therefore be stated that after a certain value for x, the gain in efficiency is negligible andit is not feasible to chose a smaller dp,50%. Section (2.2) showed that a 100% process efficiencycannot be obtained. It is therefore not feasible to aim for a 100% separation performancebecause this prototype is a bulk separator. The dp,50%, that is coupled to the dimensions ofthe RPS’s filter element, has to be chosen in such a way that, with the smallest filter elementdimensions as possible a collection efficiency of around 98% will be obtained. So althoughthe exact particle distribution is not known yet, the dp,50% has to be around 1/3 of the meanparticle diameter dp in order to not oversize the RPS design to much.

3.3.4 Creeping film in filter element

The droplets of CO2 that are separated in the filter element will collide with the channelwalls of the filter element. This could be of influence to the separation performance of theRPS because the channel height is reduced. The liquid loads in the channels of the filterelement are also higher than for previous RPS designs. The liquid load can, in this situation,be defined as the amount of liquid that has to be drained per time unit. Because the liquidloads are high it is useful to know what their influence will be. Separators used in the offshoreindustry experience re-entrainment of liquid back into the vapour flow. This phenomena iscaused e.g. by large film thicknesses that causes a wavy film to develop. From the filmdroplet break-off occurs which has to be prevented. In case of the RPS the shear forceson the liquid film caused by the gas flow are large. Willems [26] worked out a theoreticalmodel to determine the height of the film thickness for a vertical rotating channel with thegravitation force directed downwards. Willems assumed a vapour flow that counteracts thegravitational direction. In our case the flow direction and thus the shear force are in the samedirection as the gravitation force, leading to a situation depicted in figure (3.5(a)). Althoughthis assumption is slightly different from the article Willems [26] produced, the creeping filmtheory doesn’t change significantly. In this thesis not the whole theory will be explained butonly the most important assumptions and results.

Using the equation of mass conservation and momentum conservation

~v

dt+ (~v · ∇)~v = − 1

ρf∇P + ν∇2~v + ~g (3.24)

where P in [pa] is the pressure in the liquid, ν [m2/s] is the kinematic viscosity and ~g thegravitational acceleration vector. Assuming a pressure drop over the channel, the flow velocityof the film can be solved by integrating equation (3.24).

u =1

2ν[y2 − 2yδfe]

[

Ω2rdδfe

dx− g − 2τs

ρf (Rδfe−R1)

]

+τs

2νρfy (3.25)

where δfe [m], in this case, indicates the thickness of the film and Ω2r [m/s2] the centrifugalacceleration in the negative y-direction. Furthermore τs [pa] indicates the interfacial shearstress from the vapour flow on the liquid film and channel wall. As can be seen in equation(3.25)the the gravitational part and shear stress part are in the same direction. Integration ofthe velocity over the film thickness leads to a two dimensional volume flow rate. Furthermorea centrifugal acceleration term is still present. Neglecting it is possible because it is small


compared to the shear force. This is due to the fact that a fully developed flat film doesn’tencounter the full influence of the centrifugal force, which is only present when sharp gradientsin the film surface are present. The centrifugal force tends to flatten and stabilize the filmdue to the high hydrostatic pressure difference.

U(x) =g

3νδ3fe

[

1 +2τs

ρfg(h− δfe)

]

+τs

2νρfδ2fe = v0x (3.26)

In equation (3.26) h [m] indicates the height of the channel and v0 [m/s] is a feed term whichsays something on how the film will be fed by the liquid CO2 droplets that will rain in thefilm over the length L of the channel.

Willems now normalized equation (3.26) to determine the film thickness δfe. Assuming

δfe =δfe

φ , h = hφ , x = x

Lfe, ψ = 3νv0

g and φ = 3τs0

2gρfwith τs0 = 1

2fρgv2g,0 where f is a friction

coefficient and vg0 [m/s] the initial vapour velocity when entering the channel, this leads to

δ3fe +

(

h

h− δfe

)2(4δfe

3(h− δfe)+ 1

)

δ2fe =ψLfex

φ3(3.27)

Solving this equation for h = 1.0, 3.0 and 6.0 which are good approximations for our case, givesa result plotted in figure (3.5). In the coming chapter this result will be used to determinethe influence of the film thickness in the design of the 20 MMscf/d RPS.

y

x

δfe(x)

g τ0

L

(a) Schematic of a liq-uid film in a channel

δfe =δfeφ

ψLfex

φ3

δ3fe + ( h

h−δfe)2(4

3

δfeh−δfe

+ 1)δ2fe =

ψLfex

φ3

h = 1h = 3h = 6

10−1 100

10−2

10−1

100

101

(b) Normalized film thickness δfe for one dimensionlesschannel height h =1.0, 3.0 and 6.0

Figure 3.5: Dimensionless film thickness

3.4 Post-separator 23

3.3.5 Channel cleaning

Not only the high liquid loads influence the separation process but also fouling could causeproblems. The RPS will be used in an downstream application. In upstream applicationscontaminants in natural gas could cause problems like clogging of the small channels of thefilter element, thereby decreasing the efficiency. Particles that cause blockage are e.g. sand orsalt particles and have to be removed from the channels. It is however necessary to determinewhat size of particles cause the filter channels to block and if they can be removed naturallyor by a cleaning device. The size of the particle will play an important role. A more detailedexplanation on the contaminants present in the process of the upstream facility that can reachthe RPS will be given in the next chapter

The size of particles that will be dragged with the flow of liquid CO2 depends on a balanceof forces. A fouling particle will be forced to the wall of the filter element by a centrifugalforce Fc caused by the rotation of the filter element. The Archimedes effect is responsible fora buoyancy force Fb that works in opposite direction. The sum of these two forces determinesa normal force FN . By definition FN is a measure for the friction force Fw.

Counteracting the friction force Fw will be the drag force induced by the flowing film ofliquid CO2. The flow of liquid CO2, that is enforced by a pressure difference between thetwo sites of the filter element, works on the particle. This induces a force Fd that causesthe particle to be dragged with the flow. The flow of liquid CO2 around the particles alsocauses a shear force that counts up to the drag force. A maximum size of particles that willbe dragged with the flow out of the filter element can now be described by balancing theseforces. Equation (A.28) can now be deduced [4].

dp,f 6

5dc

(

∆Pch

Lfe+ ρgg

)

Ω2Ro∆ρ(3.28)

An elaboration on this derivation can be found in appendix (A.8).

3.4 Post-separator

The last step of the separation process takes place at the end of the rotating channels ofthe filter element. To separate the small liquid CO2 droplets from the natural gas flow, therotating filter element will cause the droplets to coagulate into a liquid film. When this filmleaves the filter element at the end of the channels, it breaks-up into bigger droplets thatare more easy to separate at a collection wall. The last part of the separation proces cantherefore be subdivided in three steps. First the size of the droplets that break-off from theliquid CO2 film at the end of the small channels has to be determined. From there the lengthof the post-separator can be calculated. Eventually the droplets that are collected on therotating annulus of the post-separator will form a new film that will be led to the final liquidcollection rings, from where the liquid CO2 leaves the RPS.


3.4.1 Droplet break-off process

As stated above, the droplet break-off process at the end of a channel is important to determinethe length of the post-separator. Determining the key mechanisms that cause droplet break-off is therefore crucial. Looking at the end of the rotating filter element two main forces thatact on a droplet, can be found. On the one hand, there will be a shear force, Fshear. Thisforce is caused by the flow of gas over the liquid film and will, at the end of a channel, teardroplets from the liquid film. The other major force is the centrifugal force, Fc, which isdirected perpendicular to the channel walls. If one of both forces exceeds the surface tensionforce of the liquid, Fσ, and inertia effect are neglected, a droplet will break-off. A schematicrepresentation of both forces and other dimensions can be found in appendix (A.5).

The three forces are caused by different mechanism. The surface tension force can bedescribed by

Fσ = πdp,postσ (3.29)

where dp,post [m] is the droplet diameter in the post-separator and σ [N/m] is the surfacetension of in this case, liquid CO2. The centrifugal force is dominated by the centrifugalacceleration term. Here also a spherical droplet is assumed for reasons of simplicity. Thisleads to

Fc = mpΩ2r =

1

6πd3

p,postρfΩ2r (3.30)

The force will act on a droplet at a certain radius r between Ri ≤ r ≤ Ro. The shear forceworks on half the droplet’s area and can be seen as a pressure difference, 1

2ρgv2ax,fe, between

the two sides of a droplet (see also figure (A.3)).

Fshear =1

2ρgv

2ax,feπd

2p,post (3.31)

If the the shear force will be the leading force for determining the droplet size in the post-separator, the minimum droplet diameter can be defined by

dp,post =2σ

ρgv2ax,fe

(3.32)

However this is only valid if a droplet has a diameter larger then the wall thickness. In realityonly then the shear force will act fully on CO2 droplet. If this is not the case, the centrifugalforce could equal the surface tension force sooner. The centrifugal force will then be thedominating droplet break-off force. The worst case order estimation of the droplet diameterin the post-separator can therefore be given by

dp,post =

√

6σ

ρfΩ2Ro(3.33)

In order to establish the right design droplet diameter, both methods have to be used tocalculate their potential droplet size. If the droplets determined by the shear force are smallerthan the ones with the centrifugal force, these have to be used. However if the shear forcedroplets are smaller than the channel walls, which are typically 0.1 mm, the centrifugal forcewill be the leading force. To prove this theory is correct, measurements on the droplet sizewith a water/air mixture were preformed using a high speed camera. These experiments andtheir results will be described in chapter 5.

3.4 Post-separator 25


As told in the previous section the film of liquid CO2 will break-up in small droplets whenexiting the channel structure of the filter element. Because of the rotation of the filter elementthere is still a swirl and thus a centrifugal force field present that will force the droplets tomigrate outwards. Depending on the size of the droplets after break-off the length of thepost-separator, Lpost can be determined. Assuming a solid body rotation of the natural gasmixture, the mixture will rotate with a tangential velocity that equals the rotating velocityof the filter element. When exiting the small channels of the filter element, a force balanceon a particle can be formulated. On a CO2 droplet a centrifugal force and two counter actingforces, the buoyancy force Fb and the drag force Fd, are working. The drag force is dependenton a drag force coefficient CD and the frontal area of a particle. Rewriting this force balanceand assuming the acceleration phase for the moving particle is very short, the velocity can betreated as constant in respect to time (though not in respect to position). The radial velocityof the particle vp,r can be described as

vp,r(r) =

√

2(ρf − ρg)mpΩ2r

CDρgApρf(3.34)

When the particle is assumed to be spherical, the mass mp and frontal area Ap are mp =16πd

3pρf and Ap = 1

4πd2p respectively, the radial settling velocity vp,r becomes

vp,r(r) =

√

4(ρf − ρg)dpΩ2r

3CDρg(3.35)

With the settling velocity of the CO2 droplet known, the time τpost the particle needs totravel from inner radius Ri to the outer radius Ro can be determined. With other words thisspecific parameter indicates the time it takes to reach the wall of the post-separator. Becausethis is the largest distance to travel this will, in combination with the axial velocity, be thelength determining parameter.

Since the acceleration increases as the particle moves away from the axis of rotation, sodoes the force. As a consequence, the settling velocity is never achieved. However, the radial”drift velocity” of a particle is relatively constant so the settling velocity becomes:

dr

dt= vp,r(r) =

√

4(ρf − ρg)dp,postΩ2r

3CDρg(3.36)

Solving this equation by separation of variables and Ri = δRo the residence time of theparticle in the post-separator can be established.

τpost =

√

3CDρgRo(1 −√δ)2

(ρf − ρg)dp,postΩ2(3.37)

The length of the post-separator can now be described with equation 3.38.

Lpost = τpost · vax,post (3.38)

To obtain a correct value for the drag coefficient CD, the Reynolds number of the flow aroundthe particle has to be determined.

Rep =ρgvp,rdp,post

µg(3.39)


Where vp,r is the relative velocity of the particle with respect to the flow. The radial velocitycomponent to the outer wall of the separator, will be the dominating factor. The drag coef-ficient CD can then be described as [18]:

Rep < 1 : → CD = 24Re

1 < Rep < 103 : → CD = 24Re(1 + 1

6Re2/3)

103 < Rep : → CD∼= 0.44

As can be seen, the drag coefficient, CD depends on the Reynolds number. To obtain acorrect value of CD a convergence method was used. A Reynolds number was chosen andthereby a value for CD. Using equation (3.36) a mean radial velocity was calculated. With thisvelocity a new Reynolds number can be calculated. By repeating this action, the Reynoldsnumber will converge to the correct value, thereby obtaining a value for the drag coefficient.

Chapter 4

Large scale prototype

4.1 Introduction

The prototype will be designed with the equations and knowledge discussed in the previoussections. First the operating conditions will be set in order to determine the dimensions ofthe filter element, pre-separator and post-separator. These dimensions will be the leadingdimensions for this prototype. With the established dimensions, different design choices willbe made. The geometric configuration of the RPS, enables us to make a choice on the layoutof the housing, with its liquid collection device and sealing. Furthermore the best constructionmaterial needs to be chosen. Potential problems with the structural strength of the housingare then evaluated in the next section. With the design of the static housing finished theshaft will be designed. An evaluation on the need of a cleaning system for the channels ofthe filter element, will also be given. Finally choices on bearings and drive systems are made.These systems are so-called support systems and are vital for the operation of the RPS.

4.2 Design criteria and process

To design the RPS the operating area and operating conditions need to be known. Most of theprevious RPS designs worked in conditions equal or close to atmospheric and temperaturesvarying from room temperature and up. This RPS design differs from these conditions due tothe condensation temperatures and pressures necessary to form the liquid CO2 droplets. Thesepressures and temperatures depend on the mixture compositions that have to be purified. It istherefore crucial to know, what kind of mixture compositions need to be dealt with. Becausethis RPS design is a prototype, many different mixture compositions will be offered to theRPS. The different mixture compositions influence the separation performance by e.g. varyingvapour flow rates and densities. Testing the prototype with the different mixture compositionsis therefore useful to obtain data on the the C3-Sep working principle. Furthermore, with therange of test compositions known, the thermodynamic design requirements can be set.

The prototype is to be tested in a newly designed testloop called the Euroloop situatedin Botlek (The Netherlands). Flow rates of 20 MMscf/d can be fed to RPS there. The pilottestloop offers the possibility to deliver different mixture compositions to match real gas wellcompositions, of gas wells located around the world, as good as possible. The compositionsthat will be tested can be seen in figure (4.1).

28 Large scale prototype

20% CO2 30% CO2 40-60% CO2 70% CO2

0%

20%

40%

60%

80%

100%

33.2% H2S 25.6% H2S

Figure 4.1: Six different compositions that will be tested

As shown in this figure, there are six mixture compositions. Four of these compositionscontain a lot of CO2 and CH4. Conventional techniques are, as told in the general introduc-tion, not capable of cleaning gas mixtures that contain 15% CO2 or more. Therefore testcompositions start at 20% CO2 up to 70 mol% CO2 contamination. Figure (4.1) not onlyshows compositions that contain CO2 and CH4. Other substances like e.g. butane (C3H8)and heptane (C7H16) are also present in small quantities. These by-products are valuable butare lost with the waste flow of liquid CO2. They do however, have an influence on the physicalmixture properties like densities and viscosities of the vapour and liquid phase. Propertiesof the different phases can be found in appendix (A.1). The properties are given per differ-ent recovery rate. More on the recoveries will be told in the upcoming part. Furthermorea small amount of H2S is present in these four mixtures. H2S, is a colorless, highly toxic,flammable gas. Small amounts of H2S (>10 mg/m3) are already lethal. The gas quicklydeadens the sense of smell, so when leakage of this gas occurs, potential casualties may fall.The four mixtures only contain a little amount of H2S but mixture compositions 5 and 6,contain large amounts (>25%). In the future the C3-Sep technology offers the possibility toclean natural gas from H2S by condensation at even lower temperatures of around -900C [27].Although H2S cleaning will not be the primary objective of this RPS design, choices on con-struction materials, support systems, etc. will take H2S cleaning at these low temperaturesinto account.

The next step is to determine the vapour and liquid flow rates after condensation. Thiswas done with the help of numerical flash vessel simulations. Van Wissen determined theoptimal pressures and temperatures after the expansion process for an ideal CH4/CO2 mixtureat three different recovery rates; 0.90, 0.95 and 0.98. Although no ideal mixture is fed to thisprototype, the values form a good basis for further optimizing the condensation pressures andtemperatures for our six compositions. For now the values at different recovery rates of vanWissen will be used. The vapour and gas flow rates for a 25 MMscf/d gas mixture are givenin table (A.2). For safety, a flow rate of 25 MMscf/d instead of 20 MMscf/d is chosen toprevent overloading of the RPS. The testloop may not always offer the 20 MMscf/d designflow rate because higher peak flow rates could be present in the system. For the remainder ofthis thesis, 20 MMscf/d will be stated as design flow rate but all calculations are performedat 25 MMscf/d. Table (A.2) shows the different compositions and their vapour and liquidflow rates at the three given recoveries. It is shown that for a 20 MMscf/d gas mixture flow,flow rates per case vary a lot for different recoveries. E.g. vapour flow rates of case 4 run

4.3 Design parameters 29

from 0.181 to 0.49 m3/s for recoveries varying from 0.90 to 0.98. Furthermore the pressuresand temperatures after expansion are stated. A few extreme values for flow rates, pressuresand temperatures can be picked from table (A.2). These values are our boundaries of designand will be specified in this chapter when they are of influence as a design criterion.

The design of this prototype will now be subdivided in three different steps. In the firstsection the dimensioning of the apparatus will take place. With the given flow rates and adp,50% set, dimensions of the filter element can be established. The size of the filter elementis the leading design parameter of the RPS. All other parameters are influenced by thesedimensions. So the pre-separator and the post-separator are limited by the dimensions of thefilter element. The sizing of the fluid outlet, which is important to provide sufficient drainageof the liquid CO2, also takes place in this section. The second part of the design providesthe geometric configuration of the housing (section (4.4)). In the third part, section (4.5),the support systems are incorporated. With support systems, one can think of the drivesystem required to drive the filter element and the coupling of this drive system with thefilter element.

During the design process a few general design rules have to be taken into account. Adesign that will be used in the oil and gas industry needs to be simple and most importantly,it needs to work. This means that the number of redundant parts like auxiliary equipement,valves etc. need to be kept down to a minimum. A completely self-sufficient apparatus wouldbe the ideal design. Therefore, the aim for the design process is a stand alone apparatuscompliant with the ”Design and Engineering Practice” of Shell. The so-called DEP’s arerules and experiences acquired on the design, construction, operation and maintenance ofprocessing units and facilities.

4.3 Design parameters

The main design parameters; those of the filter element will be determined first, after whichthe dimensions of the pre-separator and the post-separator will be derived. Furthermore theperformance of the filter element will be calculated for the six different mixture compositions.After the dimensioning of the centrifugal parts, the sizing of the liquid drainage system willbe done.

4.3.1 Filter element

Dimensioning the filter element will be done by a combination of two methods. Those meth-ods, which were described in chapter 3, are both based on the probability that 50% of theparticles with a certain diameter, dp,50%, will reach the collections walls of the channels ofthe filter element. In section (2.2.2) was stated that the mean droplet diameter of the dropletdistribution, will be in the order of 1.5 µm. Using the mean diameter in cooperation withthe overall total RPS efficiency curve (section (3.3.3)), enables us to determine the requireddp,50%. The efficiency section stated, that in order to not over-design the RPS, the dp,50% hasto be a factor three smaller than the mean droplet diameter. This leads to a dp,50% of 0.5µm. A theoretical separation efficiency of 98% will be achieved.

The droplets will be fed to the RPS by the vapour flow. In the previous section it wasstated that for different cases and per chosen recovery, different flow rates occur. The extremevalues are stated in table (A.2). It shows, for the first case with only 20% CO2 present, avapour flow rate Qc at a recovery of 90% of 0.062 m3/s. This relatively low flow rate is due


to the high expansion pressure of 43 bar caused by the high CO2 content. Case four shows,at a recovery of 98%, a flow rate of around 0.5 m3/s due to the low expansion pressure. Sofor a flow of 20 MMscf/d, the vapour flow rates of the six cases vary from 0.062 to 0.5 m3/s.

With the range of flows given, the dimensioning can be done by using the new methodof van Wissen, which makes use of the swirl energy (see section (3.3.2)). Although the swirlmethod will give different dimensions for each flow rate, it can give a good indication ofpotential dimensions given. It will show that from a certain flow rate on, dimensions will notchange significantly anymore. Separation performance will only increase via the rotationalspeed.

It has to be noticed that not only the flow rates vary per case but also the mixture prop-erties (see table (A.1)). For convenience, the first size indication will happen for only onefixed value for the vapour and liquid densities. To match each case as good as possible, thedensities of all the compositions are averaged. This leads to a liquid density of ρf ≈ 1000kg/m3 and a vapour density of ρg ≈ 30 kg/m3. In the coming part, the effect of the differentdensities will be checked before finalizing the dimensioning.

Swirl method

The swirl method makes use of the pressure drop over the complete RPS, ∆pRPS , therebymaking the filter element naturally driven. As told before, it is assumed that one seventh ofthis total pressure drop is caused in the channels and six seventh is caused by the tangentialpressure drop. The tangential pressure drop over the RPS is described by equation (3.8) andas can be seen, its sole contributor is the initial tangential velocity, vt0. It will become clearin the next section that this RPS design will be one with an tangential inlet. This means thatthe initial tangential velocity depends on the flow area of the inlet, Rinlet, thereby linking itdirectly to the pressure drop over the RPS.

∆pt =1

2ρg

(

Qc

πR2inlet

)2

(4.1)

For convenience it is assumed that no losses will occur due to friction forces in e.g. the bearingsand the gap between the housing and the element. This means that the filter element willtake on a rotational speed similar to the initial tangential velocity created at the inlet of theRPS. For reasons of structural strength, the circumferential velocity of the filter element islimited to 100 m/s [4]. So with a static entrance and a given maximum flow rate of 0.5 m3/s,the inlet is limited to a radius of Rinlet=0.04 m. The equations for the radius, length androtational speed, stated in appendix (A.3), can now be used to determine the dimensions ofthe filter element. Results are plotted in figure (4.2).


Dim

ensi

on[m

]

Length [m]Radius [m]Ω [RPM]

Rot

atio

nal

spee

d[RPM

]

Vapour flow rate [m3/s]0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.50.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

x103

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Figure 4.2: Length, radius and rotational speed versus the volume flow rate

The figure shows the vapour flow rate on the x-axis. The left of the two y-axis indicatethe size of the radius and length while the right one shows the rotational speed. At low flowrates the initial tangential velocity is low, giving a relatively long filter element of half a meterand a radius of around 0.15 m. When the flow rate increases the initial tangential velocityand thus the rotational speed increases, thereby reducing the size of the filter element. Ata flow rate of 0.5 m3/s, the rotational speed is around 10000 RPM. For a radius R=0.08 mthis equals the predicted circumferential velocity of 100 m/s. One can further see, that untilflow rates of approximately 0.2 to 0.25 m3/s, the dimensions of the filter element will varya lot. However for higher flows the length and radius will not change significantly anymorecompared to the change of these dimensions at the lower flow rates. Dimensions that covermost of the flow rates, can be a length of 0.25 m and an outer radius of 0.1 m with δ = 1/2.This means that no complete natural drive will be present at the lower flow rates leading toa collection of a dp,50% larger than 0.5 µm. The swirl method uses, besides the flow rate,two other parameters; τ and ǫ. These parameters give a good indication if the dimensionsfor the filter element are realistic or not. For those values to be realistic the residence timeneeds to be in the order of τ=0.1 s and the specific energy needs to be in the order of ǫ=2KJ/kg [27]. For a flow rate of 0.5 m3/s through a filter element with an outer radius of 0.1m, the residence time is very low, τ ≈0.005 s. This means, very high axial Reynolds numbers(O(105)). These high Reynolds numbers will influence the separation performance.


Conventional method

The swirl method doesn’t take any physical limitations into account. However these limi-tations can not be disregarded. One of those physical limitation is a production limitation.Many filter elements were already constructed for previous RPS designs. At Duis Engineering,which is responsible for the manufacturing of these elements, a lot of production experience ispresent. A maximum length of the element of 0.2 m, can be produced with current machinery.This value will thus be used as the length of the filter element; Lfe = 0.2 m. The height of thechannels will be 1 mm with the current production techniques. Furthermore a limitation willbe set on the axial Reynolds number. For a good separation behavior, the Reynolds numberswant to be kept low. This is the first time a RPS is designed for such high flow rates andhigh liquid loads. The Reynolds number will therefore be set in the order of Reax=O(104)at the maximum flow rate. This will give a radius of the filter element, Ro=0.2 m, that isalmost twice as big as the one determined with figure (4.2). δ=1/2 leads to an inner radiusof Ri=0.1 m. Axial Reynolds numbers will now vary from 5000 to 11000 for the different flowrates and mixture properties. The dimensions enable us to specify the required rotationalspeed for each composition and flow rate, with the conventional method (see equation (3.6)).Results can be seen in figure (4.3).

Vapour flow rate [m3/s]

Rot

atio

nal

spee

d[RPM

]

Case 1Case 2Case 3Case 4Case 5Case 6Swirl

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.50

0.5

1.0

1.5

2.0

2.5

3.0

3.5

x103

Figure 4.3: Necessary rotational speed for different cases with R=0.2 and Lfe=0.2

Figure (4.3) shows the necessary rotational speeds vary from 1000 to 2400 RPM for thesix different cases. Furthermore the theoretical achieved rotational speed of the filter element,caused by the incoming tangential flow is given. Previous RPS designs, the ones which aredriven with a swirl generator, are designed to work at one flow rate and obtain the requiredrotational speed to separate the preferred dp,50%, at this flow rate. This RPS needs to betested for a whole range of flow rates. With the given dimension of the filter element, it willnot be self-sufficient for all these flow rates. It is shown that for some flow rates not enoughswirl momentum is generated, at the inlet, to make the filter element rotate fast enough.


This is especially the case for compositions 1, 2, 5 and 6. As table (A.1)) shows, the densitydifference for cases 1, 2 and 6 are around 30% lower than the averaged densities used inthe swirl energy method. This means a higher rotational speed is necessary to separate thepreferred dp,50% of 0.5 µm. On the other side, some flow rates, especially the higher ones(case 3 and 4), will make the filter element rotate faster than required. With a static inletchosen, this phenomena can’t be prevented. However angular momentum losses are present.These losses, which are stirred up by the bearings and the gap between the housing andthe rotational element, cause the filter element to rotate slower than the RPS’s initial inletvelocity. The filter element will be naturally driven for a flow of 0.2 m3/s and up (see figure(4.3)). A maximal circumferential velocity of around 80 m/s will occur at Qc=0.5 m3/s. Theadditional required drive power for flows below 0.2 m3/s will be discussed in section (4.5).

With the size of the filter element known, the pressure drop over the complete prototypecan also be determined. Equations (3.9) and (3.10) lead to a maximal pressure drop of∆pRPS,max=0.18 bar at the required rotational speeds. All data on the dimensions andperformance of the filter element, is summed-up in table (4.1).

Table 4.1: Dimensions of the filter element

Parameter Value Unit

dp,50% 0.5 µm

dc 1.0 mm

Lfe 0.2 m

Ro 0.2 m

Ri 0.1 m

Qc 0.062 < Qc < 0.50 m3/s

Ω 1100 < Ω < 4000 RPM

∆PRPS 0.05 < ∆PRPS < 0.47 bar

Reax 5000 <Reax<11000 -

The relatively large radius is not only an advantage for the axial Reynolds number butalso for the liquid handling capabilities of the filter element. Extreme values of the liquid flowcan be found in appendix (A.2). For convenience it is assumed that all the droplets will beseparated in the filter element and no droplets are caught of in the pre-separator. This leadsto a liquid CO2 flow varying from 1.7 l/s to 10.3 l/s for case 6. The last one equals a liquidload of 11.7%. As stated in chapter 3, it is interesting to know what the maximal thicknessof the film in the filter element will be. With the knowledge gathered in section (3.3.4) it ispossible to determine a theoretical worst case film thickness. Calculating the dimensionlessnumbers: φ=1.56·10−4, ψ=5.56·10−11 and vo=6.06·10−4. Using these in equation (3.27) andlooking in figure (3.5) at h=6, gives a film thickness of δ=0.16 mm. For composition 6, thechannels of the filter element will be blocked for around 15%. Nevertheless, this is a worstcase estimate and in reality this will be less due to the separation performance of the pre-separator. For the other cases, the liquid film thickness will be even smaller. Therefore thefilm thickness and thus the liquid loads shouldn’t be a problem in this prototype.


4.3.2 Pre-separator

It was already mentioned in sections (3.2) and (3.3.5), that the gas mixture also containscontaminants that can cause fouling. Appendix (A.8) shows problems can be caused by sand,salt, wax and asphaltenes [6]. It is preferable to remove as much as possible of these particlesand substances in the pre-separator. The diameter of the particles like e.g. for sand variesfrom 10 µm to 1 mm and for salt from 3 µm to 2 mm. These particles could potentially blockthe channels of the filter element thereby reduce the separation efficiency.

Initially it was stated that the length of the pre-separator can be established with equation(3.2). However, it will become clear in the coming sections, that the outer radius of the pre-separator is large. When the mixture enters the RPS by the 80 mm inlet, it will rotate ina relatively large space. A velocity profile other then a solid body rotation could form. Asolid body rotation is essential for equation (3.2) to work properly. To ensure a well definedsolid body velocity field, blades will be installed on the shaft in the pre-separator area (seefigure (B.9)). The blades will rotate with a speed equal to the rotational speed of the filterelement. In the above it became clear that until flows of around 0.22 m3/s the electric drivesystem will provide the required rotational speed, after that the swirl velocity will take over.Furthermore the particle diameters that need to be collected are in such a size range, that aStokes flow cannot be assumed unconditionally. The situation of the pre-separator is, due tothe blades, equal to that of the post-separator. To determine the length of the pre-separator,equation (3.38) will now be used.

The densities of salt and sand are 2160 kg/m3 and 1700 kg/m3 [15], respectively. Thedensities of asphaltenes and wax are around 1000 kg/m3 [5]. Although the lowest particledensities are those of the latter two, they will not be used to determine the dimensions of thepre-separator. Asphaltenes and wax particles form at -50 0C, but the particle sizes are difficultto predict [19]. Salt particles exist from diameters of 3 µm and up. These particles couldpotentially block the channels. In the coming section on the cleaning system, it will becomeclear that for certain flow rates, particles will be dragged from the channel by the liquid CO2

flow. For now sand particles will be used to establish the length of the pre-separator. Particlesizes of 10 µm and up have to be separated by the pre-separator.

The inner radius of the pre-separator is set to 0.10 m and the outer radius equals half theinner diameter of a standard 24” pressure tube, 0.28 m (see next sections). Although it isstated above, that from a flow rate of around 0.22 m3/s, the swirl velocity will speed the shaftup to 4000 RPM, this swirl speed will not be used to determine the length. The theoreticaldetermined rotational speed by swirl, doesn’t take friction effects in e.g. the bearings intoaccount. The actual rotational speed by the swirling flow is therefore lower. However therotational speeds necessary to provide sufficient separation per mixture composition has tobe reached at all costs, if not by swirl then by the electric drive system. These rotationalspeeds will thus be taken to determine the length. The length of the pre-separator is set to0.32 m, which is the highest value for the length to be found (see figure (A.2)).


4.3.3 Post-separator

After the dimensioning of the filter element, two dimensions for the post-separator are alreadyestablished; the inner radius Ri and the outer radius Ro. Now the length of the post-separatorneeds to be determined. The break-off principle (see section (3.4.1)) states that the drivingforce will be the centrifugal force when the droplets are smaller than the channel wall thickness.To check if this applies to our case, both the droplet sizes, by shear force and centrifugal force,are determined (see figure (A.4)). The shear force (indicated in red) and centrifugal forcedroplets (indicated in black) are calculated for the six cases at different flow rates. Each casehas its own surface tension coefficient. The centrifugal force depends solely on the rotationalspeed, therefore at different flow rates the required rotational speeds (see figure (4.3)) wereused. Also the centrifugal droplet diameter caused by the swirling flow, is depicted.

At all vapour flow rates, the smallest droplets are created by the centrifugal force. Dropletdiameters vary from a 100 to 48 µm. The worst case droplet diameters are used to determinethe length of the post-separator (see figure (A.5)). A maximal length of 0.05 m can be found.Even though the droplets created by the swirling flow are the smallest, it takes more effort toseparate droplets formed in case 4. This is caused by the lower rotational speed and therebythe smaller centrifugal force field. The length of the post-separator will be set to twice therequired length; Lpost=0.1 m. Again a blade construction will be used to stimulate a solidbody rotation. The blades ensure a well defined centrifugal force field. The distance betweenthe blades on the outer radius, has to equal half the length of the separator [4]. In this case17 blades are necessary (see figure(B.10)).

4.3.4 Liquid outlet

To determine how big the diameter of the liquid outlet tubes need to be, it is assumed that eachtube will lead to a separate collection vessel. The pressure difference between the collectionrings and vessel is set to 0.1 bar. For convenience, this difference is caused by a hydrostaticpressure difference between the outlets and the collection vessels. So the vessels are situateda meter under the outlets. The maximal flow rate of liquid CO2 through the tubes is 10.3 l/sfor composition 6 (see table (A.2)). For safety, both outlets need to be able to handle the full10.3 l/s. With the equations given in appendix (A.6) and the alternative Moody diagram [8],an outlet diameter of around 50 mm can be established. This diameter doesn’t take pressurelosses in fittings and appendages into account, which will increase the required flow diameter.It will therefore become clear in the next section, that two outlet tubes of 4” are chosen.These are large enough to provide sufficient drainage with the possibility for the gas to flowback from the liquid collection vessel to the RPS.


4.4 Geometric configuration

The given dimensions enable us to design the housing, or casing, of this prototype. Designof the shaft, considerations on construction material and strength calculations will follow. Inthe end a decision will be made on the necessity of a cleaning system for the filter element.

First three different possible geometric configurations of the RPS, are considered. Theconfigurations were applied in previous designs. These designs all proved their value in dif-ferent situations and conditions. The first two configurations were already briefly mentionedin section (3.3.3).

Versions

A tangential RPS, is a version in which the filter element is placed in a radial cyclone. Themixture will enter this radial cyclone tangentially and will swirl down, separating the largerparticles on its way. The direction of the mixture flow turns upwards at the bottom of thecyclone. In the top of the cyclone, the filter element is mounted, ensuring the last separationstep for the smallest particles. The flows through this filter element are mostly laminair,imposing large design restraints on the filter element (see section (3.3.3)). This means thatthe filter element for this design would be even larger than the current one.

The axial RPS is a geometric configuration where a swirl is induced on the mixture bya swirl generator. An axial cyclone will then cause the first separation step. After the firstcyclone the mixture will enter the filter element for the separation of the smallest particles.An advantage of this design should be the possibility to position it in-line. The pipe-linecan be directly attached to the RPS if the pipe-line diameter matches the size of the axialRPS. In our case the filter element is relatively big (Ro=0.2 m), causing a mismatch betweenthe pipe-line, which will probably be a 10” standard pressure tube [4], and the housing ofthe RPS. Furthermore a swirl generator can’t provide the required rotational speed for allflow rates (see figure (4.3)). This means that an electro motor has to be attached to theshaft somewhere, making a complete in-line version impossible. The only way to overcomethis problem is by making use of a so-called canned motor. More on that in section (4.5.2).Experiments of chapter 5 are conducted with an axial RPS, which was designed and testedby Mondt.

The last RPS is a combination of the previous two. Here a tangential inlet is combinedwith an axial cyclone. A schematic of this one was already shown in figure (3.2). A swirl isgenerated by the velocity of the incoming tangential flow. From there it works the same as anaxial RPS. The mixture will leave the housing via a tangential outlet. The advantage of thisversion, is that it is still pretty compact. The sides of the housing are free of any obstructions,to provide coupling possibilities of the motor to the shaft. Furthermore the filter element canbe accessed via two sides. The flow in the RPS is well defined and is constantly swirling withminimal interference. This type will therefore be used as the basic version of this prototype.In the future, when the C3-Sep process has proven itself in the field, it becomes more inter-esting to chose an axial inline RPS, which is naturally driven for a specified flow rate of acertain gas well.

4.4 Geometric configuration 37

Vertical/horizontal

With a geometric configuration set, only one general design decision has to be made. Itis possible to position the RPS either vertical or horizontal. The horizontal version offers thepossibility to reduce height of the mounting frame on which the prototype will be situated.Furthermore it is easy to acces the prototype from the sides. However the liquid collection inthe prototype could cause problems as will be explained in chapter 5.

A vertically positioned unit has a few more advantages. The first advantage is one froma constructional point of view. The clearance between the rotating element and the statichousing is very small. This means the shaft has to be lifted from the housing with great accu-racy without touching the housing, a thing which is difficult when the prototype is positionedhorizontal. There the housing has to be placed vertical first, to make lifting possible. Thesecond advantages, is a beter controllable liquid drain. The liquid droplets that are forced tothe walls, will flow down via these walls due to gravity. This makes it easier to collect theliquid at a low situated point in the housing. Given the advantages a vertically positionedprototype is chosen.

Now one extra design choice has to be made on the flow direction in the RPS. Themixture can either enter the RPS at the bottom and leave at the top, thereby flowing inopposite direction of the gravitational force, or the mixture could enter the RPS at the topand flow down, to exit the RPS there. The main disadvantage of the first flow direction isthe possibility that liquid in the channels will flow back [18], thereby blocking the channels.Due to the high liquid loads this is possible, creating pulsated bursts of liquid out of thechannels by the pressurized vapour flow. This leads to reduced separating efficiencies. Ifthe flow direction is from top to bottom, gravity will help to force the liquid film out of thechannels. Although droplets that break-off at the end of the channels may re-entrain in theflow, a vertical setup with a top inlet and bottom outlet will be designed.

4.4.1 Static housing design

Casing

The dimensions of the housing are partially determined by the dimensions of the filter el-ement, pre- and post-separator. The outer radius of the filter element of 0.2 m, is a restraintfor the inner diameter of the housing. The combined length of the three separators, which isalready around 0.6 m, is a restraint for the length of the housing. Furthermore the housingneeds to resist the pressure caused by the compression pressure. The easiest most compactspace to position the three separators in, is a hollow cylinder with a tangential in- and outlet.Instead of a custom made cylinder one could also use a standard pressure tube. The mainadvantage of doing this, is that such tubes are already available as standard parts, therebyreducing the manufacturing costs of this prototype. Another advantage is that standardpressure tubes are classified for a certain pressure in the ASME standard. The tubes aresubdivided in certain classes, depending on the pressures (in pounds) they can withstand.Different classes are e.g. 150 CL, 300 CL, 600 CL, 900 CL, 1500 CL, 2500 CL. In the Eu-roloop standard tubing of a certain class will be used. Although exact data on these pipingclasses are yet not known, for this design it is assumed that the 600 class (CL) will suite ourpressure requirements. Standard 600 CL tubes are able to withstand pressures up to a 100


bar. The size closest to our diameter restraint, is a standard 24” 600 CL pressure tube. Thisis a commonly used size, creating another benefit; the availability of standard fittings, flangesand accessories. Furthermore it can be delivered in different kinds of stainless steels, whichcan comply to standards set on corrosion by e.g. NACE (see section (B.4)). 600 CL standardpressure tubes are also available in smaller sizes for the in- and outlets. When this prototypewill actually be build it has to comply with the codes and standards used in the test loop[22]. If the 600 CL is not sufficient, an upgrade to a higher class is possible.

The housing, or casing, will need several outlets and one mixture inlet. The gas mixtureinlet and the clean gas outlet, need to attach to the standard pipe-line used in the Euroloop.It is assumed that 10” 600 CL tubes will suite this requirement. A restriction to Rinlet=0.04m, will be made in the 10” tube of the inlet, to achieve the necessary tangential inlet velocity.If the 10” tubes are not correct they can be substituted by other sizes.

A preliminary design drawing of the housing (see figure (4.4)) was made. As told, a 24”600 CL pressure tube was used as housing. The housing is bigger than the diameter of thefilter element, leading to a larger outer radius of the pre-separator. The space between thehousing and the filter element will be filled by the liquid collection device. A 10” tube thatwill function as clean gas outlet can be seen. It is tangentially connected to the main casing.The inlet will be positioned at the top. Standard 600 CL slip-on flanges and blinds are usedon each side. Slip-on flanges instead of more commonly used weld-neck flanges are chosen,to reduce distance between the bearings; the smaller the distance, the less severe vibrationswill occur. The blinds need to be custom made in order to house the bearings. The bearingcompartments need to be properly aligned. This will be done by positioning rings on theblinds and on the casing itself. Furthermore there is a hole in the top blind. This enables acoupling to attach the shaft to an electric motor.

More dimensions are given in appendix (B.5) to give an insight in the volume and lengths ofthe prototype. In the coming parts, components of this prototype are described more detailed.

Liquid collection device

The liquid collection device will consist of two inner collection rings. It is chosen to useinner collection rings as channels to reduce the number outlets and pressurized tubes on theoutside of the pressure housing. The outlets will decrease the structural strength. The innercollection rings in this design, are shown more detailed in figure (B.5). As stated in section(4.3.4), the outlets lead to a collection vessel(s) for the liquid CO2. 4” standard 600 CLpressure tube will be used.

The post-separator of the rotating filter element, is partly positioned in the post-separatorcollection ring. This needs to ensure that as much of the liquid CO2 film, which will flow downvia the wall of the rotating post-separator, will be collected. Only a minimum of the purifiednatural gas will now leak in the collection ring. The gap between the rotating post-separatorand the collection ring is 13 mm. This is based on the expected maximal film thickness in thepost-separator (see section (A.5)). Because the post separator is positioned in the collectionring, the complete shaft can only be removed via the top of the housing.

Furthermore the rings act as filling between the housing and the rotating element. Thiswill reduce the leak flow, which was described in the second chapter. An extra block be-tween the two rings helps to achieve this. A space is present between the pre-separator ringand the block, to reduce loss of angular momentum by friction. Another advantage of usingrings and a block, is the easiness of machining. A standard 24” pressure tube has a high


machining tolerance. Now the only machining that needs to be done, is on the outside wallof pre-separator ring and the block. This makes it possible to achieve a high machining pre-cision When the filter element is constructed with a high accuracy it will not rub the blockand ring. A gap size of 2 mm will therefore be chosen. The gap size is also large enough toaccommodate the combined deflections of the bearings. A leak flow of less than 5 % will occur.

Sealing

The housing has to be hermetically sealed to prevent leaking of the gas mixture, specificallythe toxic H2S, to the outside world. Sealings or gaskets are needed in three places; the bottomblind, upper blind and the coupling. Shell mentions sealing options in DEP 31.22.10.32-Gen.[21]. The two options which can be applied are so-called spiral wound gaskets and cam-profilegaskets. In appendix (B.1) examples of both gaskets can be seen. Both are available at Eriksor Econosto, in different sizes and materials, normalized to ASME B16.47 flanges in differentclasses (upto 1500 CL) [32], [31]. Spiral wound gaskets are the preferred type of gaskets forflange sizes up to 24”, preferably graphite filled and provided with a compression stop viainner and outer rings. For larger flange sizes cam-profile gaskets have proven to be reliable.A final decision on this will be made by Shell.

4.4.2 Construction material

The construction material will be used in a highly corrosive and aggressive environment.Section (4.2) mentioned that this prototype must be able to clean natural gas from H2S,meaning the whole prototype needs to withstand temperatures of -900C. Furthermore theCH4/CO2 mixture is contaminated with highly corrosive and toxic substances like chlorides,CO2 (20-71 mol%) and H2S (0.01-33.2 mol%). This may lead to different forms of corrosionlike pitting corrosion, sulphide stress cracking and crevice corrosion. More detailed explana-tions on different corrosion mechanisms can be found in appendix (A.9.1). Furthermore worstcase partial pressures of CO2 and H2S (if an ideal gas is assumed) are respectively PH2S=9.3bar and PCO2

=16 bar. According to the Shell DEP 39.01.10.11-Gen. [21], this design canbe categorized in domain three; severe sour service. However temperatures are very low andbecause e.g. sulphide stress cracking will mainly occur in a temperature range of 60 to 1000C, the corrosion effects will be less severe [24]. Mechanical properties of the different types ofstainless steel are influenced by temperature. Parameters like yield strength, ultimate tensilestrength, elongation and KCV impact strength can get higher or lower at different temper-atures [13]. When choosing a construction material all these factors have to be taken intoaccount.

In the oil and gas industry stainless steels are successfully applied in numerous applicationsthat have to be corrosive resistant and strong. Stainless steel will therefore be the most logicalchoice as construction material for the RPS. Different types of stainless steel are categorizedby their crystal structures. There are seven basic families of stainless steels to say: ferritic,austenitic, precipitation hardenable, superferritic, martensitic, duplex (ferritic-austenitic) andsuper-austenitic. Each family has its own characteristics (see appendix (A.9.2)) and in familiesdifferent types of stainless steels can be distinguished [24].

Not all the families are equally corrosive resistant. Appendix (A.9.1) states that stainlesssteels with a ferretic, martensitic, precipitation hardening and superferritic crystal structuredon’t match with the given operating conditions. Three material families; austenitic, super-


austenitic and duplex stainless steels are less sensitive to the corrosion mechanisms stated inthe above. A selection between the three families can now be made. Table (A.4) states thatduplex steels cannot be used at temperatures lower then -70 0C. This is due to its brittlenature at lower temperatures. Duplex will therefore not be used. Super-austenitic steels arebetter corrosion resistant compared to austenitic steels. Materials like 254 SMO and AISI904L, can both be applied in the given conditions (table (A.7)). They are however difficultto machine and expensive. Super-austenitic steels are 4.8 times more expensive than normalcarbon steel. In comparison, austenitic steels are 3.5 times more expensive. Austenitic steelsalso have good corrosion resistant properties at low temperatures. Normal AISI 316 steel couldbe a good substitute for the two super-austenitic steels. Of the austenitic steels, AISI 316 isthe most corrosion resistant type (see table (A.6)). AISI 316 is also available as standard 600CL pressure tube with matching flanges etc. Shell has the final say on the material choice.The choice has to comply with standards given in e.g. NACE (see also section (B.4)).

4.4.3 Structural strength

To obtain data on the structural integrity of the housing and blinds, a static analysis wasperformed with the finite element package Ansys. The data obtained gives an indication onpossible problem points in the design and will only be used as a helpful tool. Design codes,regulation and testing will, in the end, determine if the designed object can be applied in thefield.

When we look at the housing, problems can occur due to holes in the 24” pressure tube.An evaluation was done on possible deformations. The pressure in the housing is set to 100bar. The pressure acts on all the inside surfaces of the tube. An elasticity modulus of 210GPa and a maximum tensile strength of 275 MPa, are defined. The housing is held in placeby assuming the flanges are fixed constraints on the outside of the housing. Calculations showthat the flanges will have a neglectable deformation (max. 0.02 mm) at a pressure of 100 barand will retain their shape (see figure (A.9)).

Results on the equivalent stresses in the housing, are shown in figure (A.10). On pointswere the equivalent stresses equal the maximum tensile strength, failure can occur. It isclearly shown that the clean gas outlet forms a problem. The equivalent stresses on top ofthe outlet are larger than the tensile strength. This problem is prevented at the inlet dueto the restriction of the flow area. A similar solution, to prevent too large deformations andstresses, can be applied at the outlet. An other solution could be to take a higher classstandard 24” pressure tube or position reenforcement rings near the gas outlet. Furthermorea force equivalent to the maximal radial bearing force was applied to the housing. Theforce caused no deformation, because it is very small compared to the 100 bar pressure onthe housing. It can therefore be concluded that radial forces of the rotating shaft have noinfluence on the deformation of the housing.

The blinds are subjected to a pressure and force to see if the bearing housings are staticallydeterminate in the axial and radial direction. Again the flanges are used as fixed constraints.A pressure of 100 bar is applied on one side of the blind. Furthermore, the maximal radialbearing force is applied on the bearing housing and positioning ring. The result (figure (A.12))shows a maximal axial deflection of 0.1 mm and radial deflection of 0.2·10−3 mm. No problemswill occur at the bearing housings. The single radial bearing can handle the axial deflectionsof the blinds.


4.4.4 Shaft

The shaft consists out of two parts. One inner shaft and an outer shaft (figure (B.7)). Theinner diameter of the filter element is 0.2 m. This means a shaft with such an outer diameterneeds to be constructed. A solid shaft would weigh too much. Weight is reduced by taking ahollow shaft. The weight of the shaft is reduced to 211 kg. This value also incorporates theweight of the pre-separator blades, filter element and post-separator blades. The componentsare all exchangeable parts and can be easily replaced when necessary.

The inner shaft needs to provide rotational strength. A static analysis with the finiteelement package Marc Mentat was performed. A limited space between the static housingand the filter element, only allows a minor deflection of the shaft. The shaft will be supportedby two bearings. One bearing will accommodate an axial and radial support, the other willonly provide a radial support (see section (4.5.1)). The shaft is positioned vertically, meaningthe radial force will be the only force that causes a deflection. This force is positioned in themiddle of the shaft. The distance between the two bearings is 1 m. This leads to a maximaldeflection for the maximal radial force of 3.5 KN of 0.036 mm.

To see if eigenvibrations cause any trouble, a simple dynamic analysis was done on theinner shaft, using equation (A.17) [3]. The first eigenvibration can be set to around 15000RPM. With the outer shaft attached over the inner shaft, the first eigenfrequency will be evenhigher, because extra stiffness is added. Eigenfrequencies will not cause any trouble. Theextra stiffness also causes a decrease in the static deflection.

Although it is said for the bearings, that a 100 gr unbalance is present on the outer radiusof the filter element [9], this will only be for short periods of time. The RPS is a liquidseparator meaning the unbalance will be caused by non-uniform liquid loads. Under normaloperating conditions these will not occur. The unbalance of the shaft and its components,are recorded in the ISO 1940 standard [16]. Different balance grades can be found here.This design can be placed in either G2.5 or G6.3 grade, depending on the required rotationaccuracy. The complete length of the shaft is 1.7 m. This can be varied if necessary when e.g.the length of the housing increases by changing piping classes. Furthermore extra machiningneeds to be done on the shaft to create supports for the bearings and the couplings.

4.4.5 Cleaning system

A great part of potential fouling substances were already indicated in the above. Foulingsubstances like wax, asphaltenes, salt and sand, might be present in the mixture. The amountsin which they will be present, are not known. A large part of the substances has been removedin the upstream pre-treatment plant, however 100% removal is difficult [6]. The pre-separatorwill remove sand particles of 10 µm and up. Although sand particles may bounce via the wallof the pre-separator in the filter element [22], the bouncing effect is minimized by the presenceof a liquid CO2 film at the wall. It is therefore assumed that sand will not be present andcannot cause blocked channels. For salt, which has a higher density, particles of 8.4 µm andlarger, will be removed. Salt particles between 3 µm and 8.4 µm could cause problems whenthey enter the filter element. In section (3.3.5) a general equation was given on the naturalcleaning of the filter element. The equation doesn’t take boundary layer forming of the liquidfilm in the channels into account. Maximal particle diameters that will be dragged with theflow, will in reality, be smaller. Boundary layers can, at the end of the filter element for thehighest liquid flow rate, be 0.1 mm.


Results on the maximal fouling particles, in this case salt particles, that will be draggedwith the liquid flow out of the channels, are given in figure (A.7). The diameters weredetermined at the worst case radius; the outer radius, and the required rotational speeds toseparate the dp,50% of 0.5 µm. Because of the high rotational speeds, centrifugal forces arelarge, which leads to only very small particles being dragged with the flow. Bigger particleswill stick to the wall. All mixture compositions indicate that some salt particles will not bedragged with the liquid CO2 flow from the channels. Because salt doesn’t dissolve in liquidCO2, it can cause clogging problems of the filter element. However it depends on the amountsof salt if real problems will occur. Cleaning of the channels can easily be done by forcingwater through the filter element in which salt will dissolve.

It is not clear if wax and asphaltenes particles will cause any problems because exactamounts are not known. If problems are caused, some cleaning fluids, in which asphaltenesand wax will dissolve, need to be forced through the filter element once in a while.

4.5 Support systems

In this section the support system will be discussed. The support system incorporates thebearings, the drive system and the coupling between the drive system and the rotating shaft.These components are essential for the design. First the bearing choice will be discussed. Thelast two sections discuss the drive system and coupling.

4.5.1 Bearings

Bearings are needed to support the rotating shaft. To select the most suitable bearings, forcesand rotational speeds need to be known. Section (4.4.4) stated that a radial and radial/thrustbearing are necessary to handle the forces. The radial force on the bearings, is induced bythe 100 gr unbalance [9] on the outer radius of the filter element. Although it is said inprevious sections that the theoretical determined rotational speed of 4000 RPM (see figure(4.3)) might not be reached, it will be used to select the bearings because it gives a worstcase operating condition. The speed creates an outward directed radial force in the middleof the shaft of 3.5 KN. Each radial bearing has to support half this radial force. The axialforce, directed parallel to the shaft, consists of a static and dynamic force. The static force iscaused by the weight of the shaft, which is 211 kg, and leads to Fax,stat= 2.1 KN. This forcealso serves as prestress force. A dynamic axial force, caused by the pressure drop over thefilter element and the complete RPS leads to Fax,dyn=470 N. The total axial thrust force is2.57 KN. The radial/thrust bearing will be positioned in the upper bearing compartiment andthe radial bearing in the lower bearing compartment. The radial/thrust bearing in the uppercompartment makes it possible to remove the complete shaft and blind from the housing inone piece.

With the force requirements, operating conditions and shaft dimensions set, bearings canbe selected. Different bearing types were considered; active magnetic bearings (AMB’s), gasbearings, ceramic plain bearings, normal roller bearings and hybrid bearings. Each bearinghas its pro’s and con’s. To simplify the selection process, some selection criterions wereformulated. Results can be found in table (B.2). With this table a few bearing types can berejected instantly.

Roller bearings are not capable of reaching rotational speeds up to 4000 RPM for the givenshaft diameters [36]. Furthermore lubrication at low temperatures forms a problem because

4.5 Support systems 43

lubricants of Shell (aeroshell fluid 41: >-54 0C), Exxon (mobilgrease 28 & mobiltemp SHC32:>-50 0C) and Scheaffler (Braycote 601 EF: >-73 0C) are not able to withstand such lowtemperatures [35], [34].

Gas bearings need a very small gap size, or clearance, between the stator and rotor(0.01 mm). This causes problems when thermal gradients are present. Expansion of theconstruction material could cause contact between the rotor on the shaft and the statorhousing. Furthermore the shaft ”floats” on a layer of gas which cannot always provide the rightstiffness [33]. Extra auxiliary equipement is needed to pump the gas through the bearings. Itis preferable to keep auxiliary equipement down to a minimum.

Three options remain; hybrid roller bearings, ceramic plain bearings and AMB’s. AMB’shave the best overall performance characteristics for the given conditions. AMB’s provide acontactless levitation of the shaft with magnetic forces. No friction occurs and high rotationalspeeds can be achieved. The coils of the AMB’s can be canned in a stainless steel casingmaking them corrosion resistant [33]. The high initial costs form the only problem; thisdesign is a prototype and it is probably not feasible to use AMB’s. When the prototype hasproved itself in the field it is very attractive to implement them.

Hybrid roller bearings and ceramic plain bearings can be medium lubricated. Both bear-ings contain ceramic parts that can be lubricated with liquid CO2. Both are capable ofoperating at low temperatures, can handle the forces and required rotational speeds. Al-though hybrid bearings are cheaper, they are less reliable than ceramic plain bearings whichare more robust [30].

Ceramic plain bearings can be constructed from silicon carbide, which is one of the mosthardest materials present [30]. It does experience almost no thermal expansion. The casingaround the ceramic can be made of stainless steel (see figure (B.3)) and a special fitting inthe RPS’s housing, enables it to absorb thermal expansions of the bearing compartment,housing and shaft. The plain bearings are capable of supporting a force of 2 N/mm2. Withan axial force of 2.57 KN, an annulus of rthrust,in = rshaft1,out=57 mm and rthrust,out=60 mmcan already support the shaft in axial direction. The radial bearings will be even smaller.Although ceramic plain bearings are able to run dry for short periods of time, liquid CO2

needs to be fed to the bearings from e.g. the liquid collection rings. Dry-running will occurat start-up of the RPS when there is no liquid CO2 present and could cause the only problemwhen using ceramic plain bearings. When CO2 is fed to the bearings almost no friction ispresent thus almost no heath production.

4.5.2 Electrical drive

The most simple way to drive the RPS is by an electric motor. The motor needs to be coupledto the shaft. Gas is under no condition allowed to leak via the coupling. Two types of electricmotors can be used; a so-called canned motor, which is incorporated in the pressure housing,or a normal external electric motor. The canned motor is placed inside the pressure housingso the coils are surrounded by the corrosive substances. The inside placing causes an otherproblem; heath generated in the RPS will be fully absorbed by the mixture. This leads toa change of the thermodynamic mixture characteristics. Furthermore its difficult to reachthe motor and it is expensive to replace [6]. A normal externally placed electric motor istherefore more preferable. The normal electric motor has to be connected to the shaft bya dynamic coupling (more on that in the next section). The main disadvantage is that acoupling is required. The use of conventional electric motors, simple maintenance and easy


control, makes this option the best one to apply in this design.Sizing the electric motor depends on two possible operating conditions. The first operating

condition requires additional drive power to reach the necessary rotational speeds at vapourflow rates below 0.22 m3/s. Neglecting the swirl energy for this region gives a maximalrequired drive power of 6.5 KW and 38 Nm of torque. A maximal rotational speed of 1950RPM needs to be reached for case 6. The second condition, for flow rates higher then 0.22m3/s, might require an electric motor to be used as a brake system. In this case, the rotationalspeed is positive with a negative torque, causing a phenomenon called dynamic braking [17].At a vapour flow of 0.5 m3/s, the difference in delivered torque by the swirling flow (236 Nm,4000 RPM) and required torque to rotate 2400 RPM (148 Nm), has to be counteracted bythe electric motor. An electric motor of at least 22 KW is needed to dissipate the generatedpower. For our design an electric motor of 30 KW would do.

4.5.3 Magnetic coupling

The conventional external electric motor needs to be coupled to the shaft. This can be doneby a coupling with a dynamic sealing or a magnetic coupling. A dynamic sealing needs a lotof maintenance because wear, in time, makes the sealing leak. This has to be prevented atall costs because H2S is already lethal in small amounts. A coupling which hermetically sealsthe housing from the surroundings would be the best option. Magnetic couplings are capableof doing just that. A simple example of a magnetic coupling can be seen in figure (B.4). Thecan (usually a metal can) is positioned between the outer ”master” magnetic rotor (attachedto the electric motor) and the ”slave” magnet (attached to the RPS). The can transmits thetorque and at the same time functions as a seal.

Magnetic couplings can be used at very low temperatures and casings operating at pres-sures higher then 25 bar can be specially designed [29]. The main problem of magneticcouplings are so-called Eddy currents. These currents generate heath. The heath is producedby fluctuating magnetic flux lines when the master slave rotates around the static contain-ment shell. Burgmann offers different kinds of magnetic couplings. Cans, or containmentshells, made of ceramics (SC or ND) or metal (AISI 316) can be ordered. Eddy currentscan partially be prevented when using non conductive materials like ceramics. The ceramiccontainment shells can cope with pressures up to 1000 bar and temperatures varying from-125 0C to 250 0C [29].

For dimensioning the coupling, static break-away torques and rotational speeds are nec-essary. It can be said that the ratio between the moments of inertia has to be kept low inorder to minimize start-up break-away torques (see section (B.3)). With JRPS ≈0.26 kgm2

and Jelec ≈0.20 kgm2 [37], this ratio is 0.56. When using a ceramic containment shell, heathproduction in the coupling will not be higher then 1.0 KW at 3000 RPM with a torquetransmission of 208 Nm.

4.5 Support systems 45

Figure 4.4: Preliminary design of the rotational particle separator

Chapter 5

Experiments

5.1 Introduction

Experiments were performed to verify wether theoretical determined droplet sizes (see section((3.4.1)) equal experimental droplet sizes. This is crucial when the length of the post-separatorneeds to be determined. The prototype works with a CH4/CO2 mixture. The testloopworks with an air/water mixture. Properties of both liquid and gas differ from each other.Furthermore pressures, temperatures, rotational speeds and liquid loads vary. Although notall the operating conditions can be simulated in the testloop, the measurements can give abetter understanding on droplet break-off principles in the Rotating Particle Separators.

In this chapter the experimental setup will be described first. Subsequently the experi-ments that were performed are described. Finally results are discussed and a conclusion onthe driving droplet break-off principle will be given.

5.2 Experimental setup

The test setup is situated in the lab facilities of the Thermo Fluid Engineering division. Thesetup incorporates the axial RPS, designed by Mondt [18]. This RPS is driven with a swirlgenerator and the rotational speed can only be controlled by a change in the volume flowrate. A scheme of the testloop is depicted in figure (5.1).

As shown in this figure the gas, which is normal pressurized air (8 bar), will enter the loopat the inlet. The amount of air can be controlled by a valve. The valve is a SMC pressureregulator model AW 4000 [20]. By controlling this valve the rotational speed can be regulated.The rotational speed of the filter element is measured with a Turck inductive sensor, type Bi1-EG05-AP6X. In the RPS housing a hole is made in which the inductive sensor is mounted.The sensor is positioned within one millimeter of the outer wall of the filter element. Aninductive sensor detects changes in the magnetic field, which is caused by the presence orabsence of a conducting metal. A small slot in the outer wall of the filter element makesthe magnetic field decrease. The frequency with which this happens is the rotational speed[20]. After the valve, the air will enter flow meter. The flow meter measures the amountof air that flows through the system. Two 900 turns lead the mixture to the pipe sectionthat is attached to the RPS. A schematic of the RPS is shown in figure (C.1). The differentcomponents of the RPS are indicated. An extra component, the injection nozzle, is added tothe design. This injection nozzle is placed close to the channels of the filter element at the

48 Experiments

pre-separator side and injects normal water in the testloop. Because the distance betweenthe filter element and the nozzle is so small, the water will flow from the nozzle directly intothe channels. This prevents the large droplets of water, which leave the nozzle, from beingcollected in the pre-separator area. The nozzle is connected to a rotameter to measure theflow of water, inserted in the RPS.

Water will leave the filter element at the back. The droplets that originate are pho-tographed. This is done with a high speed camera and a high power focused light bundle.The lamp and camera are positioned behind the RPS and are protected from the flow by atransparant plexiglass screen. The screen enables us to get good visual acces to the post-separator area. The high speed camera is a pco.1200 hs from the CooKe corporation. Thesystem features an excellent resolution (1280 × 1024 pixel) and low noise. The availableexposure times, range from 1 µs (50 ns optional) to 5 s. This digital camera is perfectlysuited for our application because of the high rotational speeds of the filter element. A lensis mounted on the camera to provide enough focus capacity to clearly visualize the end of thechannels. Light is provided by a Dedotec tungsten lighthead which is able to bundle the lightthrough the limited space at the end of the filter element. This light is especially designed tomeet the requirements of high speed photography.

RPS Rotational Particle Separator

Flow meter

Injection point

Inductive sensor Camera

Light

IP

IS

ISC

L

C

L

IP

Valve

Inlet

outlet

Figure 5.1: Schematic of the experimental setup

5.3 Experiments 49

5.3 Experiments

To simulate the working conditions of the prototype, high liquid loads are necessary. Liquidload can, in this case, be defined as the ratio between the liquid flow and gas flow. Typicalliquid volume loads in the prototype are between 0.4% and 11.7%. As will become clear inthe results, these liquid loads cannot be reached in the axial RPS design of Mondt. With agas flow of 0.35 m3/s and a liquid flow rate of 0.067 l/s, a maximal liquid load of 0.02% isobtained. Although this is a factor 20 lower than the minimal liquid loads of the prototype,the photographs of the droplet diameter at the end of the filter element can still be used todetermine the droplet break-off principle.

A gas flow of 0.35 m3/s leads to a rotational speed of 1400 RPM. In order to produce goodphotographs, the exposure time of the high speed camera is set to 10 µs. At a radius of 0.09m, the filter element will move 0.1 mm during the exposure time. This equals the channel wallthickness. Lower exposure times lead to underexposed photographs with no visible results.A set of photographs is taken with the channels of the filter element in focus. Another set ofphotographs is taken with the rotating post-separator in focus.

5.4 Results and discussion

As stated above, the maximal liquid load is 0.02%. This is caused by the horizontal positionof the RPS. The liquid collection holes of the post-separator are positioned in such a waythat not all the water will flow out of the post-separator area. Water accumulates in thearea between the liquid outlets. If too much water is fed to the RPS, large droplets break-offfrom the liquid accumulation and re-entrain in the gas flow. Flooding of the testloop occurs.Photographs of the liquid accumulation can be found in appendix (C.2).

Measurements on droplet sizes are performed. Some results are depicted in figures (5.2)and (5.3). As can be seen, the wall thickness of the channels is indicated with t, which is 0.1mm. Furthermore a few droplets are encircled. The size of the droplets can be compared tothe wall thickness. Figure (5.2) shows that all the encircled droplets have sizes in the samerange of this thickness, the largest being around four times t.

The measured droplet diameters can be compared to a theoretical determined diameter.The surface tensions coefficient of water is 0.027 N/m. With the given rotational speed andgas flow rate, the droplet that originates by either the centrifugal force or the shear force (seesection (3.4.1)) is 0.4 mm or 1.2 mm resp. The centrifugal force droplets are the smallestones and therefore this break-off mechanism is the driving mechanism. The droplet diametercalculated with the centrifugal force equation equals the measured ones, thereby confirmingthe adopted theory. Figure (5.3) shows a droplet at the moment of break-off. Again thedroplet size equals the wall thickness. More results can be found in appendix (C.2).

Section (3.4.1) also stated, that if the droplet created by shear force is smaller thanthe channel wall thickness, droplet diameters created by centrifugal force will be taken todetermine the length of the post-separator. If the droplet created by the centrifugal forceis bigger than the wall thickness, the droplet equals the wall thickness. To confirm thistheory, a situation has to be created to verify this. However a setup is needed where therotational speed of the filter element can be regulated independent of the gas flow rate. Itis not possible to do this with the current setup. So the length of the post-separator of theprototype is determined in such a way that both break-off principles are anticipated on.

50 Experiments

t

Figure 5.2: Droplet break-off at 0.067 l/s and a gas flow rate of 0.35 m3/s

t

Figure 5.3: Droplet at break-off point

Chapter 6

Conclusion

This report discussed the design process and design of an up-scaled prototype of a RotatingParticle Separator used to clean natural gas. The aim of the project was to design a RPScapable of handling large volume flow rates of contaminated natural gas. With a throughputof 20 MMscf/d, vapour flow rates vary from 0.062 to 0.5 m3/s. The natural gas mixturecontains droplets with an average diameter in the order of 1.5 µm. The setup will be testedin the Euroloop in Botlek (The Netherlands). The design given in this thesis can serve as abasis for design for the yet to be build RPS.

The first part of the report described the C3-Sep principle, the working principle of a RPSfor gas/liquid separation and some general design issues that return for each RPS design.C3-Sep technology offers the possibility to recover gas fields that at the moment cannotbe produced. General design restraints on temperature and pressure result from the C3-Sepprocess. Furthermore it was shown that the RPS is a bulk separator. Enrichments of maximal88% can be achieved. If cleaner natural gas is required, conventional techniques must be used.

Equations and theories are necessary to determine the dimensions of the filter elementand other key components of the RPS. From the turbulent separation efficiency curve, anoverall efficiency curve that uses average particle diameters and the particle diameter that isseparated with a 50% probability, a dp,50% of 0.5 µm is obtained. A production limitation onthe length and a restraint on the Reynolds number in the channels, lead to a filter elementwith an outer radius of 0.2 m and a length of 0.2 m. The size of the filter element is alsobeneficial to large liquid loads in the channels. The maximal worst case liquid film thicknessin channels does not exceed 15% of the channel height. Typical required rotational speedslie between 1100 and 2400 RPM. However, because of the incoming swirling flow, theoreticalspeeds of 4000 RPM can be achieved. The speed doesn’t take friction effects in e.g. bearingsinto account.

The housing of the RPS is constructed from a standard 24” 600 CL pressure tube. Amaterial suited for the aggressive conditions and low temperatures is AISI 316 stainless steel.A structural strength problem occurs at the clean vapour outlet. Reenforcement is necessaryto cope with a 100 bar internal pressure. The shaft has a small static deflection of 0.036mm and a high first natural frequency. Furthermore, a channel cleaning system to preventblocked channels, might be necessary. The pre-separator separates all the sand particles of 10µm and up. The corresponding length is 0.32 m. Salt particles (min. diameter of 3 µm) cancause blocked channels because only particles from 8.4 µm and up, are completely caught ofin the pre-separator. The influence of fouling substances like asphaltenes and wax, on channel

52 Conclusion

clogging is not clear because no quantitative data is present.

For the support system, different bearings were considered. Although magnetic bearingshave the best credentials, they are expensive. This makes ceramic plain bearings a goodalternative. However lubrication is necessary by e.g. liquid CO2 to reduce wear and internalheath production. According to Ceratec only a small amount of liquid is necessary to resistthese problems. The filter element requires drive power to reach sufficient rotational speedsat the lower vapour flow rates. A 6.5 KW, 38 Nm electric motor can provide this. Howeverat higher flow rates, the motor will be used as a dynamic brake. An electric motor of 30 KWwill therefore be necessary. The motor is linked to the shaft by a magnetic coupling. Thecoupling can either be constructed from stainless steel or ceramics. The main advantage ofa ceramic can is the reduction of Eddy currents, which lead to internal heath production. Amagnetic coupling with a ceramic cap, reduces this production to no more then 1 KW at 3000RPM with a torque transmission of 208 Nm.

Droplet break-off at the end of the filter element was investigated by both theory andexperimental studies. It proved that particles with a minimal size of 48 µm originate at theend of the channels. When shear force droplets are smaller than centrifugal force dropletsthey will be used. However if they are smaller than the channel wall thickness, centrifugaldroplets will be set as leading droplet diameter. Dimensioning of the post-separator leads toa length of 0.1 m. The experiment endorses the theories when applied to the air/water RPSof Mondt. Photographed droplets were both experimentally and theoretically, in the order ofthe channel wall thickness.

6.1 Recommendations

Different design issues still require further research. The whole thermodynamic process isbased on optimal condensation points for CH4/CO2 mixtures described in the dissertationof van Wissen [27]. The six compositions that will be tested in the loop will contain othersubstances and contaminants which lead to different optimal condensation points. This leadsto new quantities of liquid and vapour which have to be determined.

From a constructional point of view some different items need further attention. The issueon liquid collection still posses some challenges. Firstly, it has to be decided how the liquidCO2 will be drained from the RPS. Liquid will flow only because a hydrostatic pressuredifference is assumed between the RPS and the collection vessel(s). Because no technicaldetails are present of the collection vessel(s), it is difficult to design the RPS liquid outlet.

Secondly, liquid is collected in the two collection rings due to the centrifugal force in thepre- and post-separator. The rings are positioned at the side of the rotating filter element tosave space and handle the large amounts of liquid. The pre-separator ring is partially coveredand only a small gap between the cover and the wall, leads to the inside of the ring. Thenatural gas has to flow from the larger tube/housing diameter through the smaller rotatingfilter element. It is not clear what the effect of the cover on the swirling flow is.

Liquid collection in the post-separator also needs some further attention because the post-separator rotates in the second collection ring. Liquid has to flow through this gap. It is notclear, partially due to the lack of clarity on the liquid collection vessel(s), what for effectthis has on the flow. Re-entrainment of liquid CO2 in the purified natural gas flow, has tobe prevented at all costs. A solution could be to design some kind of u-trap that providesa liquid seal between the post-separator area and the collection ring. Experiments on this

6.1 Recommendations 53

subject will be conducted with the axial RPS test section described in chapter 5. The setupwill be placed vertically.

Support systems, which are the electric motor, magnetic coupling and bearings, needfurther attention. If e.g. ceramic plain bearings are chosen, liquid CO2 has to be fed tothe bearings to provide sufficient lubrication. Liquid CO2 can be fed to the topside bearingcompartment, by making use of the pressure difference over the RPS. Liquid CO2 from thepre-separator collection ring can be used. The lower bearing compartment receives its CO2

from the post-separator collection ring. Exact design drawings of the bearing compartmentsand coupling need to be obtained from manufacturers. Furthermore the control/operatingsystem of the RPS needs to be designed. Not only the internal support but also the outersupport system acquires further attention. The RPS has to be mounted on a so-called skid.The way this is done and the positioning of mounting points need to be set.

The thesis can be used as a basis for design. However more work needs to be done tomake the design more compliant with Shell’s DEP’s and other directives and design codes.E.g. safety is a subject which needs more attention. Further talks on these subjects withShell Global Solutions and other parties, are necessary.

The prototype will be used to test the C3-Sep principle. Choices on measurement equip-ment and their implementation in the design, have to be made. For measurement equipement,one can think of flow meters at the mixture inlet, vapour outlet and the liquid CO2 outlets.Furthermore a rotational speed sensor, pressure sensors and temperature sensors are neces-sary to monitor the performance of the prototype. Some kind of measuring device, like amastersizer, to determine the incoming droplet size distribution is also necessary.

Heath production in the RPS wasn’t mentioned in the main text of this thesis. However aquick order estimate shows that internal heath production is not likely to cause any problems.Some sources of internal heath production are present due to friction effects and the magneticcoupling. Heath production caused by friction between the rotating filter element and statichousing can vary per flow rate from 1 to 9 KW. Exact data on heath production by thebearings is not known. Heath production in the magnetic coupling is set to around 1 KWat 3000 RPM for a coupling with a ceramic containment shell. Estimating the temperaturerise in the prototype can be done with q = mcp∆T . For an internal temperature rise ofonly one degree Kelvin, at vapour flow rates varying from 0.062 to 0.5 m3/s and a cp ≈1500[J/(Kg K)] (estimate made by using data of case 3), an energy input of 5.7 KW to 17 KW isnecessary. It was just stated that friction effects and the magnetic coupling cause an internalheath production of 1 to 10 KW for the same vapour flow rates. No changes will thereforeoccur in the thermodynamic mixture properties.

Bibliography

[1] Brouwers, J.J.H., 1996. Rotational Particle Separator: A new method for separatingfine particles and mists from gases, Chem. Eng. Technol. 19(1996) 1-10

[2] Brouwers, J.J.H., 1997. Particle collection efficiency of the rotational particle separa-tor, Powder Technology 92(1997) 89-99

[3] Brouwers, J.J.H., 2007. Stochastic processes in mechanical engineering, lecture notesEindhoven University of Technology

[4] Brouwers, J.J.H., 2007. Private communications

[5] Diallo, M.S., Cagin, T., Faulon, J.L., Goddard, III W.A., 2000. Thermodynamicproperties of asphaltenes: a predictive approach based on computer assited structuerselucidationand atomistic simulations, Asphaltenes and Asphalts, 2. Developments inPetroleum Science, 40 B

[6] Disselhof D., 2007. C3-Sep equipment feasibility investigation, Shell Global SolutionsInternational BV, The Hague

[7] Golombok, M., 2006. Design of C3-sep lab unit, Shell Exploration and Production,Rijswijk

[8] van Esch B.P.M., van Kemenade H.P., 2003. Process installations in industry, lecturenotes Eindhoven University of Technology

[9] Hendriks, A.J.A.M., 2000. Oil-water separator, MSc. Thesis, Eindhoven University ofTechnology

[10] Hinze, W.C., 1982. Aerosol technology, US, John Wiley & Sons, Inc.

[11] Kraker, B., 2000. Rotordynamics, Eindhoven University of Technology, MechanicalEngineering Departement

[12] Kuerten, J.G.M., van Esch B.P.M., van Kemenade H.P., Brouwers J.J.H., 2006. Theeffect of turbulence on the efficiency of the rotational particle separator, Conferenceon Modeling Fluid Flow 2006

[13] Lacombe, P., Baroux, B., Beranger, G., 1993. Stainless steels, Paris: Les Editions dePhysique Les Ulis

[14] Levy, A., 1995. Solid particle erosion and erosion-corrosion of materials, ASM Inter-national.

56 BIBLIOGRAPHY

[15] Liebrand, H.T.J., 2006. The design of an oil-water separator based on the RPS-principle, MSc. Thesis, Eindhoven University of Technology

[16] Matthews, C., 2002. Engineers’ guide to rotating equipement, London, Professionalengineering publishing limited

[17] Mohan, N., 2003. Electric drives ’an integrative approach’, MNPERE, Minneapolis

[18] Mondt, E., 2005. Compact centrifugal separator of dispersed phases, PhD. thesis, Eind-hoven University of Technology, Eindhoven

[19] Rahmania N.H.G., Dabrosb, T., Masliyaha, J.H. Evolution of asphaltene floc sizedistribution in organic solvents under shear, Chem. Eng. Sci., 59, (2004)685 697

[20] Roefs, E.F., 2006. Droplet/particle separation experiments with the Rotational ParticleSeparator, MSc. Thesis, Eindhoven University of Technology

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[22] Shell Global Solutions International, 2007. Private communications

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[26] Willems, G.P., 2007. Creeping film model for vertical rotating channels including shearstress, Eindhoven University of Technology, article

[27] Wissen, van, R.J.E., 2006. Centrifugal separation for cleaning well gas streams: fromconcept to prototype, PhD. thesis, Eindhoven University of Technology, Eindhoven

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[29] Burgmann Dichtungswerke GmbH & Co. KG, 2007. www.burgmann.com

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[36] SKF bearings, 2007. www.skf.com

[37] WEG, Exportadora S.A., 2007. www.weg.com.br

Appendix A

Equipement dimension calculation

A.1 Gap leak flow

Implementing equation (2.21) in matlab, gives the volume flow rate through the gap as apercentage of the total vapour flow rate. The result can be seen in figure (A.1). The leak flow

Gap size [mm]

Vol

um

eflow

[%]

0 1 2 3 4 5 6 7 8 9 100

5

10

15

20

25

Figure A.1: Percentual leak flow through the gap for a varying gap sizes

Qgap gets larger when the gap size increases. When the gap size reaches a value of 4.5·10−3

m, the leak flow will be around 10% of the total flow Qc. This result can be used to determinehow big the leak flow will be for different gap sizes.

58 Equipement dimension calculation

A.2 Flow rates and properties

For the different mixture compositions different values for the densities and viscosities can befound. These values will be used in the rest of this thesis.

Table A.1: Mixture properties for different compositions and recoveries

Compositions Recovery[%] ρg[kg/m3] ρf [kg/m

3] µg[Pa · s] µf [Pa · s] σ[mN/m]

Case 1 90 74.0 627 1.08·10−5 1.03·10−4 4.11595 61.2 701 1.05·10−5 1.25·10−4 7.47198 45.4 757 1.02·10−5 1.56·10−4 15.17

Case 2 90 61.8 758 1.06·10−5 1.29·10−4 6.45895 46.1 844 1.02·10−5 1.53·10−4 11.3998 35.0 871 1.02·10−5 1.72·10−4 18.27

Case 3 90 31.8 1070 1.05·10−5 1.67·10−4 12.7895 24.0 1100 1.07·10−5 1.71·10−4 15.0798 21.7 1100 1.08·10−5 1.73·10−4 16.09

Case 4 90 22.6 1130 1.06·10−5 1.67·10−4 14.1795 20.4 1140 1.07·10−5 1.67·10−4 14.6498 18.7 1150 1.09·10−5 1.65·10−4 14.88

Case 5 90 37.0 909 9.68·10−6 3.13·10−4 19.4595 28.1 924 9.59·10−6 3.27·10−4 21.8698 21.5 931 9.57·10−6 3.38·10−4 23.76

Case 6 90 27.7 664 9.31·10−6 1.99·10−4 27.0495 22.5 679 9.25·10−6 2.12·10−4 31.6298 15.3 698 9.11·10−6 2.36·10−4 39.17

A.2 Flow rates and properties 59

Different mixture compositions cause different flow rates of the liquid and vapour phase.Furthermore values are given for the expansion pressures and temperatures. These valueshave to be used when designing the RPS.

Table A.2: Flow rates for different mixture compositions and recoveries

Compositions Recovery [%] Qc [m3/s] Qw [l/s] Psys [Bar] Tsys [0C]

Case 1 90 0.062 4.7 43 -6495 0.087 3.1 38 -6298 0.137 1.7 30 -60

Case 2 90 0.073 4.8 38 -6295 0.117 3.3 30 -6198 0.186 1.9 23 -59

Case 3 90 0.145 5.5 20 -5895 0.271 3.6 14 -5798 0.372 2.2 12 -56

Case 4 90 0.181 6.9 13 -5795 0.266 5.7 11 -5698 0.499 2.2 9 -54

Case 5 90 0.099 5.0 28 -6195 0.142 4.6 22 -5998 0.205 4.1 17 -57

Case 6 90 0.088 10.3 22 -5995 0.130 9.4 18 -5798 0.203 7.9 12 -56


A.3 Filter element

First the dimensions of the filter element are given. These dimensions are based on the swirltheory of van Wissen [28]. The dimensions are still written as a function of either the tan-gential pressure drop and the channel pressure drop. Furthermore use was made of the axialvelocity through the filter element. Rewriting and substituting these equations in one anotherwill lead to the coming dimensions

Volume:

V = Qτ = πR2(1 − δ2)L (A.1)

Length:

L =

(

2∆Pchτ2dc

ρgf

)

1

3

(A.2)

Radius:

R = 2−1

6π−1

2 (1 − δ2)−1

2d−

1

6c f

1

6Q1

2 τ1

6 ρ1

6g ∆P

−1

6

ch (A.3)

Revolutions:

Ω = 27

6π1

2 (1 − δ2)1

2 (1 + δ)−1

2d1

6c f

−1

6Q−1

2 τ−1

6 ρ−

4

6g ∆P

1

2

ch∆P1

6

t (A.4)

Tangential velocity

vt,0 =

(

4∆Pt

ρg(1 + δ2)

)1

2

(A.5)

Axial velocity

vax =

(

2∆Pchdcτ3

2

ρgf

)1

3

(A.6)

A.4 Pre-separator 61

A.4 Pre-separator

The length was determined by using the required rotational speed that is necessary for theseparation of 0.5 µm droplets in the filter element. This means that until flow rates of 0.22m3/s, the electric drive system will provide the rotational speed. For higher flow rates theswirl energy will make the filter element and thus the pre-separator rotate faster. Howeverthis speed will not be used because effects of e.g. bearing friction will lower the speed. Therotational speeds necessary for the separation of 0.5 µm droplets in the filter element needto be reached at all costs. These speeds will thus be used to determine the length of thepre-separator. The length caused by the swirl velocity is also plotted and is 0.07 m smaller.If in the constructed prototype the swirl speed is reached, separation performances in thepre-separator will go up.

PSfrag


Len

gth

pre

-sep

.[m

]


0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.50

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Figure A.2: Vapour flow rate versus length of pre-separator. The length is determined for asand particle with dp,pre=10 µm.


A.5 Post-separator

0Schematic droplet break-off process

A schematic droplet break-off process can be seen. Different forces and parameters are indi-cated

Fshear

Fc

vax,fe

dp,post

τ

t

Figure A.3: Schematic representation of droplet break-off mechanisms

Droplet sizes and length

The sizes of the droplets that break-off at the end of the filter element are plotted for the sixdifferent mixture compositions.


Pos

t-se

p.

dro

ple

tdia

met

er[m

]


0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.510−5

10−4

10−3

10−2

Figure A.4: Droplet diameter for different cases and break-off mechanisms (centrifugal forcedroplets is black and shear force droplets in red) at R0=0.2 m

A.5 Post-separator 63

Droplet diameters created by the shear force are indicated in red. Droplet diameterscreated by the centrifugal force required at the different flow rates to separate a dp,50% of 0.5µm, are indicated in black. Furthermore the centrifugal force droplet diameter, created if theswirling flow velocity would determine the rotational speed, is also indicated in black. All thecentrifugal force droplet diameters are determined on the outer radius of the filter element.On that radius, the created droplets are the smallest due to the highest centrifugal forces.

As stated earlier the theoretical rotational speed generated by the swirling flow will, inreality, be smaller due to friction losses. However, droplets created by swirl, are the smallestfor vapour flow rates between 0.22 and 0.5 m3/s. For the lower flow rates, the centrifugal forcedroplets are also the smallest. These worst case droplet sizes will thus be used to determinethe length of the post-separator. The length is plotted in the next figure.


Len

gth

pos

t-se

p.

[m]


0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.50.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

Figure A.5: Vapour flow rate versus length of post-separator. The length is determined forthe worst case centrifugal droplets

The maximal length to be found is 0.05 m. This will be set as the length of the post-separator.


Film thickness in the post-separator

The film thickness is needed to determine the gap between the rotating post-separator andthe liquid collection ring. The amount of liquid that will reach the post-separator is hard topredict because the liquid CO2 droplet distribution is yet not known. A rough estimate ofthe film thickness in the post-separator will therefore be given.

The representation of the film flow at the wall of the post-separator can be comparedto the film flow in the channels of the filter element (see figure (3.5(a))). From the generalequation of motion a definition for a falling film can be found.

v∂v

∂y= ν

(

∂2v

∂x2

)

+ g (A.7)

For convenience, the viscosity effects of the liquid CO2 are neglected. Assuming for x a filmlayer thickness in the order of 1 mm, a length y of 0.1 m and a kinematic viscosity of 1·10−7

Pa, makes the viscosity term a factor 10 smaller than the gravity force. The velocity of thefilm can thus be described by

vf,post(x) =√

2gx+ vax,post (A.8)

where vf,post(x) is the film velocity at a certain position x on the wall. vax,post is the velocity ofthe vapour flow in the post-separator. The liquid film will have this velocity as start velocity.For determining the maximal film thickness δpost, the maximal liquid volume flow will beused.

vf,post(x = Lpost) =Qw

π(R2o − (Ro − δpost)2)

(A.9)

Composition 6 has a liquid CO2 flow of 10.3 l/s. This leads, at the end of the post-separator(Lpost=0.10 m) with a post-separator vapour flow velocity of 1 m/s, to a film thicknessof δpost ≈3.3 mm. A film of liquid CO2 with this thickness will flow, at the end of thepost-separator, with a velocity of 2.4 m/s. The minimal gap size between the rotating post-separator and the liquid collection ring has to be at least this distance. Viscosity effects makethe film velocity go down. The thickness will therefore increase. To anticipate on this effect,the gap size will be set to 13 mm, which is four times the maximal film thickness.

A.6 Liquid outlets 65

A.6 Liquid outlets

Designing the liquid outlets will be done with the modified Bernoulli equation [8]. The normalBernoulli equation is given by

∆Ptot,out =

(

fLout

Dout+∑

K

)

1

2ρfv

2out (A.10)

Because we want to know a good diameter for the liquid outlet tube, the velocity throughthe tube has to be known.

vout =4Qw

πD2out

(A.11)

As can be seen, the velocity also depends on the diameter, which is not known. Furthermorethe friction factor f depends on the velocity and thus the diameter of the pipe. Rewritingeverything as a function of the Reynolds number enables us to use the alternative Moodydiagram.

∆Ptot,out =

(

fLout

Dout+∑

K

)

1

2

µ2fRe

2out

ρfD2out

(A.12)

Neglecting K by assuming no appendages and fittings are present in the pipe system gives

∆Ptot,out =

(

π3Loutµ5f

128ρ4fQ

3w

)

fRe5out (A.13)

Composition 6 has values for the density, dynamic viscosity and flow rate of ρf=664 kg/m3,µf=1.99·10−4 Pa·s and Qw=0.0103 m3/s respectively. Furthermore the total pressure drop∆ptot,out is assumed to be caused by the hydrostatic pressure difference. The length for thetube is set to 1.2 m. The friction factor f can, with the alternative Moody diagram, be set tof=0.05. Substituting all the data in the Reynolds equation

Reout =4ρfQw

µfπDout(A.14)

gives a diameter D=0.05 m. This is the required size to make sure 10.3 li/s per outlet can bedrained-off.


A.7 Shaft

Material properties are defined in the next table. Furthermore a schematic of the shaft is given

L

x

z

EI

Rigid support

(a) Schematic of theshaft with two supportpoints created by thebearings

Mass

The weight of the shaft can be determined by summing up the weight of the different com-ponents. Shaft 1 is the inner shaft and shaft 2 the outer.

mshaft1 = ρrvsLshaft1π(r2shaft1,out − r2shaft1,in)

mshaft1 = ρrvsLshaft2π(r2shaft2,out − r2shaft2,in)

mfe = πR2o(1 − δ2)Lfeρfe + Lfeρrvsπ(r2fe,out − r2fe,in)

mpre = ρrvsLpreπ(r2pre,out − r2pre,in) +Npretblade,pre(R(1 − δ))Lpreρrvs

mpost = ρrvsLpostπ(r2post,out − r2post,in) +Nposttblade,post(Ro(1 − δ))Lpostρrvs

The length of the shafts and the inner and outer radiuses are given in figure (B.7). Theradiuses of the pre-and post-separator are the same as for the filter element; rfe,in = 0.085 mand rfe,out= 0.1 m. Furthermore 10 blades are present for the pre-separator and 17 for thepost-separator. They have a width, t, of 2.5 mm and a height equal to the difference betweenthe inner and outer radius of the filter element.

Moment of inertia

The moment of inertia can also be determined.

Jshaft1 = 1/8mshaft1(r2shaft1,out − r2shaft1,in)

Jshaft2 = 1/8mshaft2(r2shaft2,out − r2shaft2,in)

Jfe = 1/8mfeR2o(1 + δ2)

Jpre,post,fe = 1/8mpre,post,fe(r2pre,post,fe,out − r2pre,post,fe,in)

This is of interest when choosing the coupling and electric drive system.

A.7 Shaft 67

Eigenvibrations

The most simple dynamic model for a rotor-bearing system consists of a mass-spring sys-tem with one degree of freedom [11]. A schematic of the shaft (see figure (A.6(a))) showstwo bearings, which are assumed to have a very high stiffness. z is the direction in the axiallength of the shaft and x indicates the horizontal deflection. When the effective stiffness isdominated by the shaft itself, the eigenvibrations can be deduced from Newton’s second law[3]. Substituting the Bernoulli Euler relation, M = EI ∂2x

∂z2 , in Newton’s second law, leads to

EI∂4x

∂z4− ∂

∂z

(

Tr(z)∂x

∂z

)

+m∂2x

∂t2+ d

∂x

∂t= F (z, t) (A.15)

where

x = horizontal deflectionz = Vertical length of the shaftt = timeE = E-modulusI = moment of stiffnessTr = Tensionm = mass per unit lengthd = damping constant per unit lengthF = external force per unit length

Simplifying equation (A.15) can be done by assuming no tension forces are present and dis-regarding external forces (see also section (4.4.4)).

EI∂4x

∂z4+m

∂2x

∂t2= 0 (A.16)

With boundary conditions at z=0 of

x(0, t) = 0 and EI ∂2x∂z2 = 0

and at z=L:

x(L, t) = 0 and EI ∂2x∂z2 = 0

enables us to solve equation (A.16) by separation of variables. No adding an excitation forceto this solution leads to the function to determine the eigenvibrations [3]

ωn =π2n2

L2

√

EI

m(A.17)

where n indicates the nth mode of the natural frequency. For this design we are interestedin the first mode, so n=1. Although equation (A.17) indicates the eigenfrequency of a staticbeam, the frequency can be coupled to the rotational speed of the shaft [11]. This is however avery simple first estimate to see if rotordynamics cause any problems for this design. With anE-modulus of 210 GPa, an I of 7.84·10−6 m4, L=1 m and m=65 kg/m, the first eigenvibrationcan be set at 15000 RPM. This is almost four times the maximal theoretical rotational speedand even six times the required rotational speed of 2400 RPM.


A.8 Contamination

0Different fouling mechanisms

Table A.3: Fouling problems and their removal [6]

Contaminants infeed gas

Upstream re-moval?

Risk Probability of foul-ing of filter elements

Sand particle size:10-20 µm to 1 mm

De-sanding Erosion/wear resis-tant

Possible blocking ofchannels

Asphaltenes (heavytail from gas)

Removal by acids-washing with naph-tha (refineries-cokebarrel)

Paste-layer form-ing, asphaltenescoagulate at −50 oC

Possible, 100% up-stream removal isdifficult

Aromatics (liquid) Liquid knockout Attack o-rings(PTFE)

NON

Mercury (liquid be-comes problem)

Mercury removal bed Attacks aluminum NON

Glycol (liquid) Liquid knock-out Attack o-rings NON

Chlorides (vapourphase)

Chlorides in vapphase at high T;Otherwise chloridedissolves in H2O

Chlorides in com-bination with H2Olead to stress-cracking corrosion,general corrosion

NON, gas is dried(molsieve dryers typ-ically dry to waterdewpoint of -90C)

H20 De-hydration Corrosion/hydrates NON

Condensates-liquids Liquid knock-out Potential loss ofvalue of CO2 wastestream

NON

Wax Liquid knockout -wax is removed withcondensates

Wax coagulates at -50 0C

Possible, 100% up-stream removal isdifficult

Salts (chlorides) par-ticle size: 3 µm to 2mm

Filtration Deposition and ero-sion

Possible blocking ofchannels

0

A.8 Contamination 69

CO2 NO CO2 in combinationwith H2O leadsto stress-crackingcorrosion, generalcorrosion

NON

H2S NO H2S in combinationwith H2O leadsto stress-crackingcorrosion, generalcorrosion

NON

Forces acting on a fouling particle

A simple model of a particle in a film of fluid, in this case CO2 liquid, can be formulated.Different forces will act on this particle as can be seen in figure A.6. A centrifugal force

y

x

Fc

FN

Fb

Fw

Fd

Figure A.6: Forces acting on particle in a channel of a filter element

caused by the rotation of the filter element Fc = mΩ2r and a buoyancy force caused by theArchimedes effect Fb =

mρgac

ρp, induce a normal force FN

FN = Fc − Fb (A.18)

Assuming worst case conditions with a maximal centrifugal force at r = Ro, equation A.18becomes

FN = mΩ2Ro −mΩ2Rρf

ρp(A.19)

When the fouling particle is assumed to be spherical with a mass of m = 16πd

3p,fρp (A.19)

becomes

FN =1

6πd3

p,fΩ2Ro(ρp − ρf ) (A.20)


This normal force FN contributes to a friction force of the particle with the wall of the channel

Fw = cwFN (A.21)

where cw is a friction coefficient which, in this case, will equal 0.3. The particle will bedragged by a flow of liquid CO2. The force Fd that is induced by the flow, depends on thefrontal area of the particle Ap,f = 1

2πd2p,f . Furthermore the shear tension τ can be deduced

from the general equation of motion.

0 = −∂p∂z

+1

r

∂

∂r(rτry) + ρfg (A.22)

Assuming that −∂p∂z = ∆Pch

L and integrating equation (A.22) gives a shear tension τry of

τ =∆Pchdc

4L+

1

4ρfgdc (A.23)

As can be seen, a gravity component is present because the RPS will be placed vertically.This terms helps to get fouling particles out of the channels of the filter element. Furthermoreit is assumed that the pressure force of the fluid will be twice as big. Therefore Fd becomes

Fd = 2Ap,fτ (A.24)

Fd =1

4πd2

p,fdc

(

∆Pch

L+ ρfg

)

(A.25)

The maximal size of the particles that will be dragged with the flow can now be determinedby combining all the forces

Fd > Fw (A.26)

Gives

1

4πd2

p,fdc

(

∆Pch

L+ ρfg

)

> 0.31

6πd3

p,fΩ2Ro(ρp − ρf ) (A.27)

Leads to the final particle size of

dp,f 6

5dc

(

∆Pch

L + ρfg)

Ω2Ro∆ρ(A.28)

The maximal particle diameter that will be dragged with the liquid flow out of the channel,depends on both the pressure drop over the channels and the rotational speed.

A.8 Contamination 71

0Fouling particle diameter

The maximal diameters of particles that will be dragged with the flow are given in the nextfigure.


Fou

ling

par

ticl

edia

met

er[µm

]

Case 1Case 2Case 3Case 4Case 5Case 6

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.52

4

6

8

10

12

14

Figure A.7: Maximal particle diameter that will be dragged with the flow for different casesat Ro=0.2

The maximal particle diameter is a worst case diameter. At a radius of Ro=0.2 thecentrifugal force is big, leading to a large force with which particles are forced against thewall. The drag force of the liquid is not large enough to move the particles. At smallerradiuses the centrifugal force is less and particles will be dragged with the flow more easily.At the different flow rates the rotational speeds required to separate the dp,50% of 0.5 µm isused.

The density of salt was used for this calculation. It is shown that for some mixturecompositions the particles that will be dragged with the flow, are no larger then 3 µm.Clogging of the channels could cause problems. At some flow rates the particles will bedragged with the liquid, because rotational speeds are low.


A.9 Construction material

A.9.1 Different corrosion mechanisms

Even a high-quality alloy can corrode under certain conditions. The modes of corrosion canbe exotic and their immediate results are less visible than rust. They often escape notice andcan cause problems. The different forms of corrosion can be seen in figure A.8 and are furtherdescribed below [24].

No

corrosion

General Galvanic Erosion Crevice

Pitting stress

cracking

Inter-

granular

Leaching

Figure A.8: Different corrosion mechanism

Uniform corrosion is a form of corrosion where there is an even rate of metal loss overan exposed surface. The loss of material is expressed in terms of metal thickness lossper time unit and, when regular inspections take place, can be very predictable. Theloss of material with uniform corrosion most of the time is due to chemical attacks ordissolution of the metallic component into metallic ions.

Galvanic corrosion takes place when two different metallic materials are electrically con-nected and placed inside a conductive solution. The resulting electrochemical potentialthen leads to formation of an electric current that leads to electrolytic dissolving of theless noble material. It will also reduce the tendency for the more noble material todissolve. Galvanic corrosion can be prevented by electrical insulation of the materials,keeping the parts dry, keeping the size of the less-noble material significantly larger thanthe more noble ones or making use of a sacrificially material. This sacrificial materialis less noble then the other two metals thereby reacting first.

Pitting corrosion can be associated with uniform corrosion mechanism but is characterizedby highly localized losses of metal. In worst case circumstances, almost all of the surfacewill be protected, but tiny local fluctuations will degrade the oxide film. Corrosion atthese points will be greatly amplified, and can cause corrosion pits of several types,depending upon conditions. While the corrosion pits only nucleate under fairly extremecircumstances, they can continue to grow even when conditions return to normal, sincethe interior of a pit is naturally deprived of oxygen. In extreme cases, the sharp tipsof extremely long and narrow pits can cause stress concentration to the point thatotherwise tough alloys can shatter, or a thin film pierced by an invisibly small hole canhide a thumb sized pit from view. These problems are especially dangerous because theyare difficult to detect before a part or structure fails. Pitting remains among the most

A.9 Construction material 73

common and damaging forms of corrosion in stainless alloys, but it can be prevented byensuring that the material is exposed to oxygen (for example, by eliminating crevices)and protected from chlorides wherever possible.

Crevice corrosion occurs when the environment within the crevice gets more aggressivewith time. No movement of the corrodent in the crevice can take place and small changesdue to localized corrosion may become magnified. No replenishing of corrodent takesplace by the bulk solution deteriorating the environment. The mechanism of crevicecorrosion is similar to pitting corrosion, though it happens at lower temperatures.

Selective leaching is a process whereby a specific element is removed from an alloy due toan electrochemical interaction with the environment. The result is a porous and usuallybrittle variant of the original component. Examples of this kind of corrosion can befound when dezincification of brass alloys take place.

Intergranular corrosion is an internal corrosion mechanism. Individual grains within ametallic material that touch each other sometimes tend to react. Steel in such conditionis called sensitized. A special case of intergranular corrosion is called ’weld decay’ or’knifeline attack’(KLA). Due to the elevated temperatures of welding the stainless steelcan be sensitized very locally along the weld. The chromium depletion creates a galvaniccouple with the well-protected alloy nearby in highly corrosive environments. It ispossible to stabilize the steel to avoid this effect and make it welding-friendly. Additionof titanium, niobium and/or tantalum serves this purpose; titanium carbide, niobiumcarbide and tantalum carbide form preferentially to chromium carbide, protecting thegrains from chromium depletion. Use of extra-low carbon steels is another method andmodern steel production usually ensures a carbon content of < 0.03% at which levelintergranular corrosion is not a problem.

Stress corrosion cracking cracking can be a severe form of stainless steel corrosion. Itforms when the material is subjected to tensile stress and some corrosive environments,especially chloride-rich environments at higher temperatures. The stresses can be theresult of service loads, the type of assembly or residual stresses from fabrication. Thislimits the usefulness of stainless steels of the 300 series (304, 316) for containing waterwith higher than few ppm content of chlorides at temperatures above 50 0C. In moreaggressive conditions, higher alloyed austenitic stainless steels or duplex stainless steelsmay be selected.

Sulphide stress cracking is an important failure mode in the oil industry, where the steelcomes into contact with liquids or gases with considerable hydrogen sulfide content,e.g., sour gas. It is influenced by the tensile stress and is worsened in the presenceof chloride ions. Very high levels of hydrogen sulfide apparently inhibit the corrosion.Rising temperature increases the influence of chloride ions, but decreases the effect ofsulfide, due to its increased mobility through the lattice; the most critical temperaturerange for sulphide stress cracking is between 60-100 0C.

Erosion corrosion , or fretting corrosion, takes place when mechanical wear occurs due tofriction of e.g. two metallic materials with each other. The protective surface film getsdamaged or is removed completely thereby enhancing corrosion by the corrodent.


A.9.2 Different material families

Stainless steels are classified by their crystalline structures. In the enumeration below differentfamilies are described. Using these descriptions make it possible to cancel out certain membersof the stainless steel family [24].

Austenitic stainless steels comprise over 70% of total stainless steel production. They con-tain a maximum of 0.15% carbon, a minimum of 16% chromium and sufficient nickeland/or manganese to retain an austenitic structure at all temperatures from the cryo-genic region to the melting point of the alloy. The AISI designation system identifiedthe most common of these alloys with numbers beginning with 300 and resulted in the”300 series” stainless.

Superaustenitic stainless steels, such as alloy AL-6XN, 904L and 254SMO (UNS S31254),have a great resistance to chloride pitting and crevice corrosion due to high molybdenumcontents (> 6%) and nitrogen additions. The higher nickel content ensures betterresistance to stress-corrosion cracking over the ”300 series”. The higher alloy contentof these steels means they are expensive and similar performance can be achieved byusing duplex steels at much lower cost.

Ferritic stainless steels are highly corrosion resistant, but less durable than austenitic grades.They contain between 10.5% and 27% chromium and very little nickel. Most compo-sitions include molybdenum; some, aluminium or titanium. Common ferritic gradesinclude 18Cr-2Mo, 26Cr-1Mo, 29Cr-4Mo, and 29Cr-4Mo-2Ni. The AISI designationsystem identified the most common of these alloys with numbers beginning with 400and resulted in the ”400 series” stainless.

Martensitic stainless steels are not as corrosion resistant as the other two classes, but haveincreased strength and are highly machineable. Further hardening can be done byheat treatment. Martensitic stainless steel contains chromium (12−14%), molybdenum(0.2 − 1%), no nickel, and about 0.1 − 1% carbon (giving it more hardness but makingthe material more brittle). It is also known as AISI ”400 series” steel.

Precipitation-hardening stainless steel category can be divided in three alloy series: marten-sitic, austenitic an semiaustenitic. By using a different temperature cycle an increaseof mechanical properties is achieved. The material has higher strengths but corrosionresistance performances are a bit lower. The most common, 17-4PH, uses about 17%chromium and 4% nickel.

Duplex stainless steels contain roughly equal amounts of austenite and ferrite, the aimbeing to produce a 50:50 mix. In most commercial alloys the mix is more of a 40:60mixture. Duplex steel have improved strength over austenitic stainless steels and alsoimproved resistance to localized corrosion particularly pitting, crevice corrosion andstress corrosion cracking. They are characterized by high chromium (19 − 28%) andmolybdenum (up to 5%) and lower nickel contents than austenitic stainless steels.

Superferritic stainless steels contain high amounts of chromium ( 26%) and molybdenum(1%) making it extremely chloride stress corrosion resistant. Nowadays also general andlocalized pitting resistance is good. Superferritic materials have the disadvantage to bebrittle. Recent superferritic steels are Sea-Cure (S43635) and 29-4C (S44735).


A.9.3 Information on stainless steels

A lot of different types of steel exist. In this appendix a few characteristic of the differentsteel families and types are given. First the operating conditions and service temperatureswill be stated. Subsequently an indication on costs will be given. In the end two tables oncommonly used stainless steels are given.

Service temperature of different families

Table A.4: Minimum service temperature of metals[21]

Metal Min. design Impact Other requirementstemp [0C] tested

Carbon and lowalloy steels

Carbon steel can be used in piping systems withminimum design temperature down to 0 0C.

Carbon steel (LT 0) 0 For lower design temperatures (> -50 0C),LT 20 -20 X impact testing shall be performed at -46 0CLT 30 -30 X with an average impact value of 27 Joules.LT 40 -40 XLT 50 -50 XLT 80 (1.5 % Ni) -80 X Impact testing for well completion shallLT 100 (3.5 % Ni) -100 X be carried out at 10 0C or the minimumLT 120 (5 % Ni) -120 X design temperature if this is lower.

Martensitic stain-less steels

Use of 13 % Cr at temperatures below -30 0Crequires special evaluation.

13 % Cr -3013 % Cr 4 % Ni -50 X13 % Cr 4 % Ni -100 Xdouble tempered

Austenitic stain-less steels

Impact testing of austenitic stainless steel weld-ments is not considered necessary above -105 0C.

-105-196 X

Duplex stainlesssteels

Impact testing shall be performed at -50 0C. For25 % Cr minimum design temperature of -30 0C

22 % Cr -50 X applies to fabrication welds, while -50 0C appliesto piping components.

25 % Cr -50 X Note: Expro have pre-qualified duplex to -700C, which is lower than the limit currently setby DEP 30.10.02.31-Gen.


Costs

Different steels are compared to one another on a cost basis. The costs of the steels arenormalized to regular carbon steels.

Table A.5: Cost estimation [21]

Material cost factor (weight basis)

Carbon steel 1

13Cr for downhole production tubing 1.1 - 1.8

Weldable martensitic stainless steels/Super 13Cr 2.2 - 3.5

AISI 316 (solid or clad) 3.5 - 4.5

22Cr Duplex 4.0 - 4.6

AISI 904L 4.2 - 4.8

25Cr Duplex 5.0 - 5.6

6Mo 5.0 - 5.6

Alloy 825 clad 5.2 - 5.5

Alloy 625 clad 5.4 - 6.0

Alloy 825 solid 7.5 - 8.5

Alloy 625 8.5 - 10.0


Commonly used steels

Table A.6: Commonly used stainless steels

Series Types Application area

200 Austenitic chromium-nickel-manganese alloys

300 Austenitic chromium-nickel alloys

301 Highly ductile, for formed products. Also hardens rapidly during me-chanical working

303 Free machining version of 304 via addition of sulfur304 The most common; the classic 18/8 stainless steel316 The next most common; for food and surgical stainless steel uses; Al-

loy addition of molybdenum prevents specific forms of corrosion. Alsoknown as ”marine grade” stainless steel due to its increased ability toresist saltwater corrosion compared to type 304. SS316 is often used forbuilding nuclear reprocessing plants

400 Ferritic and martensitic chromium alloys

408 Heat-resistant; poor corrosion resistance; 11% chromium, 8% nickel409 Cheapest type; used for automobile exhausts; ferritic (iron/chromium

only)410 Martensitic (high-strength iron/chromium)416420 ”Cutlery Grade” martensitic; similar to the Brearley’s original ”rustless

steel”. Also known as ”surgical steel”430 Decorative, e.g., for automotive trim; ferritic440 A higher grade of cutlery steel, with more carbon in it, which allows for

much better edge retention when the steel is heat treated properly. Itcan be hardened to Rockwell 58 hardness, Also known as ”razor bladesteel”. Available in three grades 440A, 440B, 440C (more common) and440F (free machinable)

500 Heat resisting chromium alloys

600 Martensitic precipitation hardening alloys

630 Most common PH stainless, better known as 17-4; 17% chromium, 4%nickel


Table A.7: Commonly used stainless steels

EN-standard EN-standard steel name ASTM/AISI UNSsteel no. DIN steel type

Ferritic and martensitic

1.4016 X6Cr17 430

1.4512 X6CrTi12 409

Austenitic

1.4310 X10CrNi18-8 301

1.4318 X2CrNiN18-7 301LN

1.4307 X2CrNi18-9 304L S30403

1.4306 X2CrNi19-11 304L S30403

1.4311 X2CrNiN18-10 304LN S30453

1.4301 X5CrNi18-10 304 S30400

1.4948 X6CrNi18-11 304H S30409

1.4303 X5CrNi18-12 305

1.4541 X6CrNiTi18-10 321

1.4878 X12CrNiTi18-9 321H S32109

1.4404 X2CrNiMo17-12-2 316L S31603

1.4401 X5CrNiMo17-12-2 316 S31600

1.4406 X2CrNiMoN17-12-2 316LN S31653

1.4432 X2CrNiMo17-12-3 316L S31603

1.4435 X2CrNiMo18-14-3 316L S31603

1.4436 X3CrNiMo17-13-3 316 S31600

1.4571 X6CrNiMoTi17-12-2 316Ti S31635

1.4429 X2CrNiMoN17-13-3 316LN S31653

1.4438 X2CrNiMo18-15-4 317L S31703

Super austenitic

1.4539 X1NiCrMoCu25-20-5 904L N08904

1.4547 X1CrNiMoCuN20-18-7 254 SMO S31254

Duplex

1.4462 22Cr-12.5Ni-Mn-2.25Mo-Cb-VN 22 Cr S32205

1.4362 0.02C-0.10N-23Cr-4.8Ni-0.3Mo 23 Cr S32304

1.4362 0.02C-0.27N-25Cr-7Ni-4Mo 25 Cr S32750

A.10 Structural strength 79

A.10 Structural strength

00

Figure A.9: Deformation of flange

Figure A.10: Equivalent stresses on the housing


Figure A.11: Deformation of housing

Figure A.12: Deformation of the blind

Appendix B

Preliminary drawings and

components

B.1 Sealing

Two different sealings are described here. The way they function and their availability at gas-ket manufacturers is described. Shell has to make a choice on the exact sizes and types neededfor this design. They will have to base this on regulations and standards used in the Euroloop.

Spiral wound gaskets

The first gasket shown in figure (B.1) is the most common profile of spiral wound gasketand used extensively in ANSI B16.5 flanges. The gasket will be positioned between the flangeand the blind. The gaskets consist of a metal guide ring (or sometimes referred to as a cen-tering ring) and a spiral wound sealing element. This profile is normally used in raised andflat faced flanged. The second gasket has an inner ring and an outer guide. The outer guide

Spiral winding with outer guide

Spiral winding with outer guide and inner ring

Figure B.1: Schematic of two types of spiral wound gaskets [32]

functions as a positioning mechanism and the inner ring makes sure the spirals won’t collapseinward when pressure is applied. The outer ring is often made of carbon steel (painted orzinc plated to prevent corrosion) but can be made of alloys for higher and lower temperaturesand more severe medium applications. Therefore gaskets of the AISI 316 stainless steels arecommonly used. Different possibilities are offered by Eriks.

82 Preliminary drawings and components

Cam-profile gaskets

A schematic of a cam-profile gasket is depicted in figure (B.2). Cam-profile gaskets con-

Figure B.2: Schematic cam-profile gaskets [31]

sist of a metal core, generally stainless steel, with concentric grooves on either side. A sealinglayer is usually applied on both sides and depending on the service duty the material for thislayer can be graphite, PTFE (Teflon), asbestos free gasket sheeting material or metal (e.g.aluminium or silver). Cam-profiles can be used without sealing layers to provide an excellentseal but there is a risk of flange surface damage especially at high seating stresses. The sealinglayers protect the flange surfaces from damage in addition to providing an effective seal.

B.2 Bearings

The radial force is described with a 100 gr unbalance situated at the outer radius of the filterelement. The shaft rotates with 4000 RPM. This speed is the worst case rotational speed incase the swirl energy provides sufficient drive power.

Frad = munbalΩR2o (B.1)

The static axial force in completely caused by the weigth of the shaft and its components.The total weigth is 197 kg leading to

Fax,stat = mtotg (B.2)

The dynamic axial force is caused by the pressure drop over the channels of the filter element(see equation (3.10)) and the complete pressure drop over the RPS.

With the forces known a selection of different bearings can take place. Use will be madeof table (B.2).

B.2 Bearings 83

Table B.1: Bearing selection table

Criterions Roller Hybrid roller Ceramic gliding AMB’s Gasbearings bearings bearings bearings

Economical

Initial costs [EUR] 102 103 104 105 104

Lifetime expectancy - - + + ++ ++

Low maintenance - - + + ++ ++

Technical

Simplicity ++ ++ ++ - - -

Lubrication - - + + ++ ++

Auxiliary equipement ++ ++ ++ - - -

Reliability - - - + ++ +

Corrosion resistance - - ++ ++ ++ ++

Thermal gradient + + + ++ - -(expansion)

Large shaft diameters - - + ++ ++

Clearance [mm] 0.01 0.01 0.1 0.2-0.4 0.01

Rotational speed 1900 6000 5000 105 105

Load + + ++ ++ +

Figure B.3: Ceramic gliding bearings


B.3 Magnetic coupling

Can

Master magnetic rotor

Slave magnetic rotor

Figure B.4: Simple magnetic coupling [29]

The ratio between the moment of inertia of the RPS, JRPS , and the moment of inertia ofthe entire drive chain with electro motor Jelec, needs to be small.

Load =JRPS

JRPS + Jelec(B.3)

The bigger this ratio, the more Eddy currents will be created. Since the generated Eddycurrent losses and thus the efficiency of the coupling depend on the installed magnet volume,it makes sense to minimize the ratio between the moments of inertia as far as possible. Thiswill require a low coupling torque in the start-up phase. For this purpose it is possible , e.g.to mount additional rotating masses on the motor side in order to raise the moment of inertiato the required value. During operation the coupling also forms a sort of overload protection.

B.4 Design codes 85

B.4 Design codes

Pressure vessel code provides, together with the directives related to simple pressure ves-sels (87/404/EC), transportable pressure equipment (99/36/EC) and Aerosol Dispensers(75/324/EEC), for an adequate legislative framework on European level for equipmentsubject to a pressure hazard. Pressure vessel codes for specific location need to beapplied like e.g. ”Stoomwezen”, ASME and BS5500

NACE is an organization for industrial corrosion control. The main areas of activities arecathodic protection, coatings for industry and material selection for specific chemicalresistance. NACE publishes the journals, Materials Performance (MP) and Corrosion.NACE also publishes recommended standard practice and testing articles for use byindustry and other Corrosion societies. The standards set by NACE will be used todetermine the right construction materials.

DEP’s are based on the experience acquired during their involvement with the design, con-struction, operation and maintenance of processing units and facilities, and they aresupplemented with the experience of Group Operating companies. Where appropriatethey are based on, or reference is made to, international, regional, national and indus-try standards. The objective is to set the recommended standard for good design andengineering practice applied by Group companies operating an oil refinery, gas handlinginstallation, chemical plant, oil and gas production facility, or any other such facility,and thereby to achieve maximum technical and economic benefit from standardization[21]. The design has to comply with all DEP’s (Materials, LT DEP, amendment onBS5500).

ATEX , or ATmospheres EXplosives, is a European standard on explosion hazards andmust be applied on places where the danger of explosions is present. This design has tocomply with these directives because CH4 anf H2S are present.

IEC is an international standards organization dealing with electrical, electronic and relatedtechnologies. Some of its standards are developed jointly with ISO. The standard neededfor this design is IEC 60079. This part states general demands for the overall designand testing of electric material. These standard are applied for situations where gasexplosions could occur.

”Machine richtlijn” are directives for designing machines and relegate to other directives.Sometimes more then one directive can be applied on the design.


B.5 Drawings

Drawings of the different components of this design were made. These are more of schematicdrawings with only a limited amount of dimensions given. Certain design choices are notmade yet and need to be discussed with Shell and contractors. These meetings will lead todetailed drawings with tolerances etc. For now these schematics/drawings give a good sizeindication.

Figure B.5: Liquid collection rings

B.5 Drawings 87

Figure B.6: Housing


Figure B.7: Inner and outer shaft

B.5 Drawings 89

Figure B.8: filter element


Figure B.9: Pre-separator blades

B.5 Drawings 91

Figure B.10: Post-separator blades


Figure B.11: Blind with bearing housing

Appendix C

Experiments

C.1 Axial RPS

Nozzle

Figure C.1: RPS with swirl generator designed by Mondt

94 Experiments

C.2 Results

0

Water accumulation in post-separator collection ring

End of rotating

post-separator

Figure C.2: Edge between rotating post-separator and liquid collection area

Figure (C.2) shows the edge of the rotating post-separator. Water flows over the edgein the liquid collection area. The swirling flow that is still present, drives the water up thewall of the liquid collection area, preventing it from flowing back in the liquid outlet. Waterpiles-up and eventually large droplets are teared up from this accumulation and re-entrain inthe air flow.

C.2 Results 95

Droplet sizes

t

Figure C.3: Droplet break-off at 0.067 l/s and a gas flow rate of 0.35 m3/s

Figure C.4: Droplet hitting the wall of rotating post-separator

Acknowledgements

First of all I would like to thank my supervisor Guy Willems for his help and guidance incompleting my graduation project. The almost daily meetings with him proved very helpfulfor the progress of my project. Especially I would like to thank professor Michael Golombokfor helping me with preparations for my presentation (and application) for Shell. FurthermoreI like to thank Erik van Kemenade for answering my practical and constructional questions.The help of professor Brouwers was also very useful for designing the prototype

Although only a small part of my work contained experiments, I would like to thankMark Willekens for the short but intensive collaboration. Finally I would like to thank myroommates and my colleague graduate students for the pleasant and stimulating working at-mosphere.

Eric van de Watering

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