Fischer-Tropsch Refining (DE KLERK:FISCHER-TROPSCH O-BK) || Refining Technology Selection

301

Part VRefining Technology

Fischer–Tropsch Refining, First Edition. Arno de Klerk. 2011 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2011 by Wiley-VCH Verlag GmbH & Co. KGaA.

303

16Refining Technology Selection

16.1Introduction

The refinery designs associated with industrial Fischer–Tropsch facilities (Chapters 6–12)highlighted the need to look at the molecules in syncrude and determine the refining strategyaccordingly. In instances where a crude oil refining approach was employed in the design of aFischer–Tropsch refinery, the designs were inefficient. Inattention to detail in the selection ofcatalysts and/or refining technologies also degraded performance.

Two important change drivers affected successive Fischer–Tropsch refinery designs, as well asmodifications to operational refineries. Foremost were the changes in transportation fuel speci-fications (Chapters 13–15). As the requirements for producing on-specification motor-gasoline,jet fuel, and diesel fuel changed, refining strategies had to be adapted to deliver products ofacceptable composition and quality. The second change driver was refining of chemicals. Theextraction of chemicals as higher valued products than fuels became a common thread in manyof the industrial Fischer–Tropsch facilities.

Central to refinery design and the ability to produce the desired products (fuels and/orchemicals) is the selection of appropriate refining technology. By doing so, Fischer–Tropschrefining can be more efficient than crude oil refining [1]. Some guidelines can be derived fromthe preceding chapters:

1) Carbon-number-based refining works better for Fischer–Tropsch syncrude than refiningbased on boiling range. This has its origin in the molecular properties of the syncrude,where specific carbon number ranges are compatible with specific refining technologies.Carbon-number-based refining [2], managing the molecules [3], or technology selection tomatch molecules [4] are all different ways of saying that one has to refine syncrude based onmolecular properties and not on the boiling range.

2) Boiling range broadening results from deoxygenation of syncrude. This is one of the maindetractors from boiling-range-based refining.

3) Catalyst selection for the refining of Fischer–Tropsch syncrude [5] is as important asselecting the conversion technology. The reactive nature of Fischer–Tropsch syncrude, dueto its significant alkene and oxygenate content, requires milder catalysts. Four catalyst classeshave been identified as being especially important in Fischer–Tropsch refining [6]: alumina,solid phosphoric acid (SPA), nonacidic Pt/L-zeolite, and mildly acidic Pt/SiO2 –Al2O3.

Fischer–Tropsch Refining, First Edition. Arno de Klerk. 2011 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2011 by Wiley-VCH Verlag GmbH & Co. KGaA.

304 16 Refining Technology Selection

These are not the only catalysts that can be used, but represent the catalyst types that,in conjunction with appropriate hydrotreating catalysts, allow the design of an efficientstand-alone Fischer–Tropsch refinery capable of producing on-specification transportationfuels (Figure 16.1).

4) The cost of a Fischer–Tropsch refinery is typically less than 15% of the total capital cost of aFischer–Tropsch-based facility. The capital needed to produce the syncrude exceeds the costof refining by far. Carbon-efficient refining consequently makes economic and environmentalsense and it requires the inclusion of conversion technologies for the aqueous product andC1 –C4 material in the refinery design.

5) Learn the lessons from the past [7, 8]. Fischer–Tropsch refiners have a much smallerexperience base to draw on than crude oil refiners. It is important not to repeat the mistakespreviously made.

6) By its nature, Fischer–Tropsch syncrude contains molecules that have value as chemicals[9–13]. Through proper refining, there are also opportunities to synthetically produce

Dehydration(alumina)

C2

Hydroisomerization(Pt/SiO2–Al2O3)

Hydroisomerization(Pt/SiO2–Al2O3)

Naphtha reforming(nonacidic Pt/L)

Hydrotreater

Oligomerization(SPA)

Aromatic alkylation(SPA)

Alkene hydration(SPA)

Jet fuel

Diesel fuel

Motor-gasoline

Motor-gasoline

Jet fuel

Motor-gasoline

Jet fuel

Motor-gasoline

Motor-gasoline(ethanol)

Hydrotreater

Hydrotreater

C5

C6–C8

C9 +

C4

Fischer–Tropschgas loop

C3

Benzene

C2

C3 +

Aqueousproduct

Light oilproduct

Syngas

Figure 16.1 Stand-alone high-temperature Fischer–Tropsch(HTFT) refinery design, illustrating the use of the four keycatalyst types needed to produce on-specification transporta-tion fuels.

16.2 Hydrotreating 305

chemicals. By selecting appropriate refining technologies, products can be produced in anefficient manner that can be applied as fuels or chemicals.

Refinery technology selection is discussed in terms of the conversion processes employed. Thisserves as an introduction to the chapters that describe selected conversion processes importantto high-temperature Fischer–Tropsch (HTFT) and low-temperature Fischer–Tropsch (LTFT)refining in more detail (Chapters 17–23). The conversion processes that have been selected fordetailed discussion are those that provide significant opportunity to Fischer–Tropsch refining.Detailed accounts of catalyst and technology selection for the refining of Fischer–Tropschsyncrude can be found in the literature [4, 5].

The conversion processes typically associated with crude oil refining have been briefly discussedin Chapter 2. If one were to include commodity petrochemical refining, the number of conversionprocesses grows somewhat. The list of conversion processes that are potentially relevant toFischer–Tropsch refining is more extensive. There are two reasons for this. The first is theabundance of alkenes and oxygenates, which allows refining pathways not equally accessible ina crude oil refinery. The second is the chemical nature of the syncrude, which invites the use oftechnologies not normally associated with mainstream crude oil refining.

There is unfortunately not an obvious classification system for conversion processes, andtreatises on refining tend to discuss them in a sequential manner [14–18]. In order togive some structure to the discussion on the application of different conversion processesto Fischer–Tropsch syncrude, the processes were organized into five categories, despite the in-herent overlap between some: hydrotreating, addition and removal of oxygen, alkene conversion,alkane conversion, and residue conversion processes.

16.2Hydrotreating

Hydrotreating is a very basic upgrading step and involves the use of hydrogen to removeheteroatoms, increase the H:C ratio of the feed material, and partially convert functional groupsby selective hydrogen addition. The reactor technology and catalyst/s are selected in line with theintended application [5, 19, 20].

In a refinery, hydrotreating is employed as a feed pretreatment step, or as a product polishingstep. When it is used for feed pretreatment, the objective is to remove contaminants in the feedthat will adversely affect downstream processes, or to transform material in such a way that itwill be amenable to downstream processing. When it is used as a product polishing step, theobjective is to adjust the quality of the final product in line with product specifications.

It is customary to refer to the nature of the hydrotreating action as if it is a separate process,although these processes occur in parallel:

1) Hydrogenation (HYD) of alkenes. This is a very important hydrotreating function inFischer–Tropsch refining (Section 16.2.1).

2) Hydrodearomatization (HDA) [21]. The polynuclear aromatic content of LTFT and HTFTmaterial is well below diesel fuel specifications (Table 15.4), and deep HDA to remove suchcompounds is not important in Fischer–Tropsch refining. The HDA of benzene is one wayof dealing with refinery benzene, but is not preferred when alternatives exist [22, 23].


3) Hydrodemetallization (HDM) [24, 25]. The metals and their levels present in syncrudesdepend mainly on the Fischer–Tropsch synthesis (Section 4.5). Metals may also be presentas corrosion products or produced in conversion processes. The metals are generally presentas metal carboxylates, and HDM catalysts are ineffective in their removal [26].

4) Hydrodesulfurization (HDS) [27, 28]. This is the most important hydrotreating function incrude oil refineries, but Fischer–Tropsch syncrude is sulfur-free.

5) Hydrodenitrogenation (HDN) [29–31]. The importance of HDN in a crude oil refinerydepends on the nitrogen content of the crude oils being processed. Fischer–Tropschsyncrude contains no nitrogen compounds.

6) Hydrodeoxygenation (HDO). Complete oxygen removal and the selective partial HYD of oxy-genates are important hydrotreating functions in Fischer–Tropsch refining (Section 16.2.2).

16.2.1Hydrogenation of Alkenes

The oxygenate content and nature of the oxygenates in the feed material that must behydrogenated will influence the HYD catalyst selection. Carboxylic acids are aggressive toreduced base metal catalysts [32]. The carboxylic acids leach the active metal when the operatingtemperature is below the metal carboxylate decomposition temperature. Noble metals havea higher leaching resistance, but are far more active and not practical for bulk alkene HYD.Noble metal HYD catalysts are useful for selective partial HYD applications. When carboxylicacids are present in a stream that requires bulk alkene HYD, upstream acid removal should beconsidered. This is not always practical, and in cases where the hydrotreating involves alkenesand oxygenates, sulfided base metal catalysts can be considered (Section 16.2.2).

Applications that involve alkene HYD in a Fischer–Tropsch refinery are the following:

1) Partial HYD of alkenes in order to avoid downstream processing problems. When the feedmaterial contains dienes, the removal of the dienes by selective HYD is often required toavoid the formation of gums (heavy products from diene oligomerization). Diene removalcan be combined with double bond isomerization to increase the octane number if theproduct becomes a blending component for motor-gasoline. Some metals are efficient forpartial HYD and double bond isomerization [33]. The reported order of isomerization activityof reduced metals is Pd > Ni > Rh, Ru > Os, Ir, Pt [34]. When double bond isomerizationtakes place with partial HYD, it is not only beneficial for downstream processing but alsoserves as an upgrading step (Section 16.4.1).

2) Partial HYD of alkenes in order to comply with final product requirements. Motor-gasolinespecifications may limit the amount of alkenes that can be included in the final product(Section 13.2). Because of the alkene-rich nature of syncrude, this may require the HYDof some alkenes. Ideally, one would like to selectively hydrogenate the alkene isomers thatwill result in the least octane number loss. Unfortunately, the most branched alkenes whichhave the highest hydrogenated octane number are also the most difficult to hydrogenate. Insingle carbon number cuts, it is best to fully hydrogenate with a bypass, rather than partiallyhydrogenate the total stream (Figure 16.2) [35]. The same strategy does not hold true formixed carbon number streams, because lighter alkenes have higher hydrogenated octanenumbers than isostructural heavier alkenes. One may also exploit unsulfided and sulfidedHYD to increase catalyst lifetime and improve selectivity [35, 36].

16.2 Hydrotreating 307

20

40

60

80

100

20 40 60 80 100

Temperature (°C)

Con

vers

ion

(%)

1-Octene

2,4,4-Trimethylpentenes

Partial (90%)hydrogenation

Completehydrogenation

10% bypass

RON = 42.6

RON = 44.5

50 : 50feed

mixture

Figure 16.2 Single carbon number alkene hydrogenationconfigurations illustrated by the hydrogenation of an equimo-lar mixture of 1-octene and mixed 2,4,4-trimethylpentenesover a Pd/C catalyst.

3) Complete HYD of alkenes in order to avoid downstream catalyst deactivation. This is typicallya feed pretreatment step that is considered before a process that employs a bifunctionalcatalyst, with both metal and acid functionality. This is an easy way to ensure downstreamstability, but it does not exploit the syncrude properties to its full potential. In some cases,the heat of alkene HYD can be exploited while controlling the alkene partial pressure andavoiding catalyst deactivation by thoughtful design [37].

4) Complete HYD of alkenes to comply with final product requirements. HYD of Fischer–Tropsch waxes and paraffinic solvents are typical applications [38]. It is also required in theproduction of iso-paraffinic kerosene (IPK) for synthetic jet fuel production (Section 14.2.1).

16.2.2Hydrodeoxygenation

HDO catalysis has been reviewed by Furimsky [39, 40]. As in the case of alkene HYD, catalystselection is constrained by the presence of carboxylic acids. The heavier carboxylic acids are not asaggressive as the short-chain carboxylic acids, and accounts of unsulfided base metal–containingcatalysts that were used for carboxylic acid hydrotreatment can be found in the literature [41–43].Nevertheless, the use of unsulfided base metal catalysts with carboxylic acid–containing feedruns the risk of catalyst deactivation by leaching. Noble metal catalysts are typically not consideredfor bulk HDO and HYD of syncrude because noble metals are too active. Catalysts that are tooactive make heat management difficult and can easily result in excessive temperature excursions.

Sulfided base metal catalysts are quite resistant to acid leaching and have been used for bulkHDO and HYD of Fischer–Tropsch syncrude [26, 44, 45]. The main disadvantage of sulfided


catalysts for syncrude HYD is that it requires the addition of sulfur in the form of a sulfidingagent to an otherwise sulfur-free feed. Bulk hydrotreating of syncrude find application in thefollowing:

1) Feed pretreatment by partial oxygenate conversion. The most common Fischer–Tropschapplication is carbonyl to alcohol HYD. It can be used to simplify aqueous product refiningby converting the aldehydes and ketones into alcohols (for example, Section 10.4.2). It is alsoextensively used in the production of chemicals from syncrude.

2) Feed pretreatment by HDO for downstream refinery processes that are sensitive to oxy-genates. A typical application is hydrotreating before catalytic naphtha reforming. Onemay also consider hydrotreating as a pretreatment step before hydrocracking. It has beenshown that oxygenates influence hydrocracking selectivity [46], and the yield advantage ofhydrotreating before mild hydrocracking to produce diesel fuel from HTFT syncrude hasbeen demonstrated [47].

3) HYD to comply with product requirements. Such HYD is typically performed on straight-runsyncrude that will be included in final products. For example, distillate hydrotreating toproduce diesel fuel from HTFT syncrude. Controlling the level of HYD is quite important,since properties like diesel fuel lubricity can be retained by proper catalyst selection andHDO control [5].

16.3Addition and Removal of Oxygen

16.3.1Dehydration

Alcohols are the dominant oxygenate class in LTFT syncrude and also one of the main oxygenateclasses in HTFT syncrude. In order to simplify refining, carbonyl compounds can be selectivelyhydrogenated to alcohols, thereby increasing the alcohol concentration in Fischer–Tropschsyncrude even further. Alcohol dehydration is an important syncrude refining technology and isdiscussed in detail later (Chapter 17).

Dehydration takes place over an acidic catalyst. Catalyst selection and operating conditionsdetermine the extent of dehydration. Mild dehydration leads to etherification (Section 16.3.2),whereas more severe dehydration produces alkenes. It is an equilibrium-limited reaction, and thereverse reaction, namely, alkene hydration (Section 16.3.3) to produce alcohols is also possible.A number of dehydration applications have been noted:

1) Complete dehydration of alcohols from the Fischer–Tropsch aqueous product has beensuggested as a way to simplify aqueous product refining [48]. By converting the oxygenatesvia alcohols into alkenes, the alkenes can be co-refined with the rest of the alkenes in thesyncrude.

2) Dehydration of specific n-1-alcohols to produce n-1-alkenes as chemicals [49].3) Low-temperature alkane conversion into alkenes [50]. The alkanes are activated by autox-

idation (Section 16.3.5), which is followed by partial HYD to alcohols and subsequentdehydration to alkenes.

16.3 Addition and Removal of Oxygen 309

4) Partial dehydration of alcohols to produce fuel ethers for both motor-gasoline and diesel fuel.The etherification of specifically the heavier n-1-alcohols produces linear ethers that are gooddiesel fuel blending components [51].

16.3.2Etherification

Fuel ethers can be prepared from the reaction of alcohols with alkenes, or by the reactionof alcohols with alcohols by partial dehydration. Since alcohols and alkenes are abundant insyncrude, etherification has a natural feed advantage. In the case of the etherification of alkeneswith alcohols, the C=C of the alkene must be on a tertiary carbon. The branched alkenes insyncrude is less than the n-alkenes, and skeletal isomerization (Section 16.4.3) can be employedto increase the degree of branching of the alkenes. Etherification is an important syncruderefining technology and it is discussed in more detail in Chapter 17.

Etherification is equilibrium limited and is catalyzed by an acidic catalyst. In crude oilrefining, the etherification of alcohols and alkenes found widespread use in motor-gasoline, butencountered complications of a political nature (Section 2.4.4). Nevertheless, fuel ethers are stillin use as octane improvers for motor-gasoline. Etherification of alkenes with alcohols has twoindustrial Fischer–Tropsch applications:

1) The purification of n-1-alkenes as chemicals from HTFT syncrude employs etherification[52]. Methanol reacts with the alkene isomers that are close boiling to 1-pentene and 1-hexeneto produce ethers that can be removed by conventional distillation.

2) Conventional etherification of branched alkenes with alcohols is used to produce fuel ethersas octane improvers for motor-gasoline [53]. This is usually performed in conjunction withalkene skeletal isomerization (Section 16.4.3) to improve etherification yield.

16.3.3Hydration

The hydration of alkenes to alcohols is usually performed to produce alcohols for chemical use. Ina Fischer–Tropsch refinery, hydration has one very specific application, namely, the conversionof ethene into a liquid product, ethanol, when an HTFT facility is remote from a petrochemicalmarket. The problem is analogous to that in gas-to-liquid conversion, where the otherwise usefulnatural gas cannot be exploited as an energy carrier unless it is converted into a liquid product(Section 3.2.1).

Alkene hydration is equilibrium limited and the equilibrium does not favor hydration, butdehydration. It is an acid-catalyzed reaction. It fits well within a Fischer–Tropsch refinery,because the hydration technology can benefit from the infrastructure in the aqueous productrefinery to process and purify the raw hydration product. Although the application of hydrationis very refinery-specific, it is important and is discussed in more detail later (Chapter 17).


R OH

O

′R OH

R O

O

′R

H2O

+ H+

− H+

− H+

+ H+

R

OH+

OH

O H′R

ROH+

OH

O H′R

Figure 16.3 Acid-catalyzed esterification of a carboxylic acid with an alcohol.

16.3.4Esterification

Carboxylic acids and alcohols can react with each other to produce esters (Figure 16.3). Thisreaction is not used in crude oil refining, but it is extensively used in the refining of biomass. Thedirect transesterification of plant oils and animal fats with methanol produces fatty acid methylesters (FAME) and 1,2,3-propantriol (glycerol) [54]. These esters can be used as renewable dieselfuel additive and are allowed as blending components in conventional diesel fuels (Table 15.1).Although analogous esters can be produced from syncrude, the concentration of heavy carboxylicacids in syncrude is low.

The esterification of carboxylic acids with alcohols is an equilibrium-limited reaction that iscatalyzed by acids. (Direct transesterification of bio-derived oils and fats with methanol is acid-or base catalyzed.) The potential application of esterification in a Fischer–Tropsch refinery is notto produce fuel blending components per se, but rather as a means to refine carboxylic acids:

1) The removal of carboxylic acids in syncrude by converting the acids into esters has beeninvestigated [55]. As a feed pretreatment step, the conversion of acids into a neutraloxygenates enables the use of acid-sensitive downstream technology and unsulfided basemetal hydrotreating catalysts.

2) Gas-phase esterification of carboxylic acids in the gas loop can potentially reduce the amountof carboxylic acids that are dissolved in the reaction water. Once the carboxylic acids aredissolved and end up in the Fischer–Tropsch aqueous product, further refining and recoverybecome difficult. The possibility of realizing this in practice is unfortunately low, mainly dueto the low equilibrium conversion to esters [56], competitive adsorption, and high risk of sidereactions.

3) HDO of carboxylic acids can be facilitated by converting the acids into methyl esters.

16.3.5Carbonyl Aromatization

Carbonyl aromatization is not a commercial refining technology. Aldehydes and ketones canrepeatedly undergo aldol condensation and dehydration. When this process is acid catalyzed, thetrimer unit can self-condense to produce an aromatic compound (Figure 16.4). Ethanal yieldsbenzene [57], whereas the C3 and heavier carbonyl compounds yield alkyl aromatics. For example,propanal produces 1,3,5-trimethylbenzene (mesitylene) [58], as does propanone (acetone) [59]. Inmixtures of carbonyl compounds, the products will not necessarily be radially symmetric.

The aromatization of carbonyl compounds is a potentially useful reaction to increase aromaticsproduction from the Fischer–Tropsch aqueous product [60].

16.3 Addition and Removal of Oxygen 311

R

O + H+

− H+

+ H+

− H+

+ H+

− H+

+ H+

− H+

H+

OH

R

O

RR

O

+

R

O

R

H2O

+ H2O+

R

O OH

R

O

RR

+R

O

RR

O

RR

R

O

RR

H2O+

R

R R

Figure 16.4 Carbonyl aromatization by repeated aldol condensation and dehydration.

16.3.6Hydroformylation

In a Fischer–Tropsch refinery, all the reagents necessary for hydroformylation are readilyavailable, namely, alkenes, CO, and H2. The hydroformylation reaction (Equation 16.1) is relatedto Fischer–Tropsch synthesis, and it involves chain growth by the addition of CO.

R–CH=CH2 + CO + H2 → R–CH2 –CH2 –CHO (16.1)

The reaction is conducted in the liquid phase with a homogeneous catalyst that is based oneither Co or on Rh [61]. There is a trade-off involved in selecting the hydroformylating metal.When employing a Co-based catalyst, a high operating pressure (>10 MPa) is required. Whenan Rh-based catalyst is employed, which is about two orders of magnitude more reactive thanCo-based catalysts, a lower pressure (∼1.5 MPa) is needed, but the catalyst is considerably moreexpensive [62].

Hydroformylation technology is generally too expensive to be considered for fuel refining andits extensive use by Sasol (Section 9.5) is for the production of chemicals.

16.3.7Autoxidation

The reaction of oxygen in air with hydrocarbons takes place at ambient conditions, but at suchconditions it is a slow reaction. This is called autoxidation and this is the reaction that causesfuel degradation during storage (Sections 14.3.7 and 15.3.8). However, autoxidation can also bebeneficial and it has found application in the production of chemicals from Fischer–Tropschwaxes (Sections 6.3.3 and 8.5.3). The application of autoxidation for chemical production isdiscussed in more depth later (Section 23.3).

Autoxidation is a free radical process and requires no catalyst, although a catalyst can beused to increase the reaction rate. It is typically conducted at moderate conditions, <180 ◦C andlow pressure [50, 63]. It should not be confused with autoxidation at more severe conditionsthat is used for asphalt hardening [64]. Although autoxidation is not extensively used in crudeoil refining at present, in future it has considerable potential for oxidative desulfurization


and oxidative denitrogenation [65–70]. In addition to oxidative heteroatom removal crude oilrefineries may share some of the syncrude refining applications:

1) Lubricity and cetane number of fuels can be improved by autoxidation. The hydroperoxidesare thermally labile and are cetane improvers. The oxygenates that are formed by oxygenincorporation are surface-active and improve boundary layer lubricity.

2) The low-temperature activation of alkanes to produce alkenes has already been mentioned[50].

3) Chemical applications where the products from syncrude autoxidation are useful oxygenates,such as oxidized waxes.

4) Autoxidation under more severe conditions can be used to oxidatively degrade heavy organicwaste streams to produce lighter oxygenates. Such oxygenates are biologically more easy todegrade, thereby reducing the environmental footprint of the process. If anaerobic digestionis employed, some of the carbon may even be recovered as methane, which can be convertedinto syngas again. Anaerobic digestion to produce methane is industrially employed inconjunction with HTFT refining by PetroSA (Chapter 10).

16.4Alkene Conversion

16.4.1Double Bond Isomerization

When highly olefinic motor-gasoline was allowed as commercial fuel, the easiest way to achievea good quality motor-gasoline from syncrude was by double bond isomerization. The syncrudeis rich in n-1-alkenes, which have the lowest octane numbers of all alkenes (Table 13.5). Byisomerizing the 1-alkenes to internal alkenes, a large gain in octane number is achieved. Thisstrategy works very well across all carbon numbers, with large gains in octane number that ispossible from a very simple conversion process.

Double bond isomerization can be catalyzed by acids, bases, free radicals, and metals inthe presence of hydrogen (Figure 16.5). Potential catalysts have been listed in the extensivereview of Dunning [71]. Newer literature includes studies on catalyst classes such as acidicresins [72], acidic zeolites [73], basics zeolites [74], and mixed oxides [75]. Although double bondisomerization is often regarded as a facile reaction, this is not always the case. There must besufficient thermodynamic driving force. Isomerization of 1-alkenes to internal alkenes may bea thermodynamically favorable reaction, but the surface intermediate that causes isomerizationto take place must also favor isomerization. As the carbon chain length of the alkene becomeslonger, steric effects may decrease the driving force for isomerization of the adsorbed speciesand may even promote isomerization to the terminal position.

In the past, double bond isomerization was extensively used in Fischer–Tropsch refining(Chapters 6–9). Bauxite treatment, or clay treatment over alumina-based catalysts, was aneffective way to improve motor-gasoline quality. Unfortunately, double bond isomerization nolonger has the same appeal as it had in the past.

There are three inherent drawbacks to double bond isomerization. The first is that the octanenumbers that can be achieved by just double bond isomerization is limited despite the significant

16.4 Alkene Conversion 313

(a) R R

R

R +

(d) R R

R

R

R

Ni Ni Ni

H

Ni

R

Ni Ni

(b) R R− −

(c) R R RR

H

+ H+

− H+

− H+

+ H+

− H+

+ H+

+ H+

− H+

+ H•

− H•

− H•

+ H•

+ H–Ni

− H–Ni

− H–Ni

+ H–Ni

Figure 16.5 Double bond isomerization catalyzed by (a)acids, (b) bases, (c) free radicals, and (d) metals in thepresence of hydrogen.

gain in octane number. The cost per octane number gain is very low, making it an attractiveconversion process, but the ultimate octane number that can be achieved makes it impractical forC7 and heavier alkenes. The second drawback is related to the nature of the conversion. Doublebond isomerization upgrades only the alkenes, and the gain is proportionally less as the alkanecontent of the syncrude increases. It also produces an olefinic product, and the octane numbergain will be completely lost if the alkene is hydrogenated; no structural change takes place andn-1-alkenes are still linear after double bond isomerization. The third drawback is related to theblending value of the material for motor-gasoline. Motor-gasoline specifications no longer allowa high alkene content or the addition of tetraethyl lead, and double bond isomerization producesa highly olefinic material of only moderate quality compared to the octane number requirementsof final on-specification motor-gasoline.

For the reasons mentioned, the prospect for future application of double bond isomerizationin a Fischer–Tropsch refinery is slim, despite the fact that it is a proven technology for syncrude[4]. Instances where it may still find applications are as follows:

1) Upgrading of a straight-run syncrude stream that is earmarked for direct blending intomotor-gasoline. Since such a feed will in any case be blended directly, any improvement inoctane number is valuable and the drawbacks mentioned do not apply.

2) Pretreatment of feed before a process that is sensitive to the double bond position. Forexample, 3-methyl-1-butene is not reactive for etherification, but the double bond isomers2-methyl-1-butene and 2-methyl-2-butene are. Some gain can also be found in isomerizing1-butene to 2-butenes before aliphatic alkylation using HF as catalyst (Section 16.4.5).

3) Conversion in combination with metathesis (Section 16.4.2) to shift the carbon numberdistribution by disproportionation of two carbon chain lengths that are presenting refiningdifficulties.

4) Enabling separation of compounds for chemical use. For example, purification of 1-hexenefrom Fischer–Tropsch syncrude by distillation requires the reactive removal of close boiling


isomers. Double bond isomerization is often sufficient to change the boiling point to enableseparation by distillation.

16.4.2Metathesis

Alkene disproportionation or metathesis (Figure 16.6) is employed in chemical production toproduce propene from ethene and 2-butene, different 1-alkenes, and specialized polymer products[76]. It is of potential interest in a Fischer–Tropsch refinery, because many of the commercialmetathesis technologies employ ethene for disproportionation. It presents an alternative refiningpathway for stand-alone Fischer–Tropsch facilities far from petrochemical consumers and,like hydration (Section 16.3.3), is able to convert ethene into heavier transportable products.Metathesis is also capable of changing the carbon number distribution around its mean whileretaining the same average molecular mass in the product as in the feed.

The most commonly used heterogeneous catalysts for metathesis are Re2O7 (Meta-4 process ofIFP/Axens), MoO3 (SHOP, Shell Higher Olefins Process), and WO3 (OCT, Olefins ConversionTechnology of ABB Lummus). These catalyst systems require regular regeneration. Someapplications of metathesis in a Fischer–Tropsch refinery that can be envisioned are given below:

1) Light and heavy fractions that do not have convenient refining pathways can be dispro-portionated to a product of intermediate carbon number. In this way, the carbon numberdistribution can be shifted to a distribution that is more useful for refining. Because of thehigh 1-alkene content, this type of application may require a double bond isomerization step.

2) The production of high-quality polyalphaolefin (PAO) lubricating oil from n-1-olefins canbe increased by metathesis of internal alkenes with ethene. This is especially useful for theproduction of PAO lubricating oil from alkanes via autoxidation [50], which yields mainlyinternal alkenes on dehydration.

3) Chemical applications can use metathesis technologies.

16.4.3Skeletal Isomerization

Skeletal isomerization is the process whereby the skeletal structure of an alkene is rearrangedso that the product is more branched than the feed. Increasing the degree of branchingof alkenes has a number of benefits. Foremost is the increase in synthetic value of thebranched alkenes for fuel production. Etherification of alkenes with alcohols (Section 16.3.2)requires branched alkenes, which can be provided from a skeletal isomerization unit. Duringoligomerization (Section 16.4.4), branched alkenes also produce more branched dimers andoligomers, which are especially useful if the material has to be hydrogenated. Alkylate-equivalent

R R

′R ′R ′R ′R ′R ′R

RR R R

+ +

Figure 16.6 Alkene disproportionation (metathesis).


high-octane paraffinic motor-gasoline can be produced by selective dimerization and HYD ofmethylpropene (isobutene) [77, 78], and is an environmentally friendly alternative to aliphaticalkylation (Section 16.4.5). Oligomerization of skeletally isomerized alkenes therefore yieldshigher octane number hydrogenated motor-gasoline and the branched kerosene range materialhas a low freezing point making it suitable for jet fuel.

Industrial processes for skeletal isomerization have been developed mainly for the conversionof n-butenes to isobutene [79, 80] and n-pentenes to methylbutenes [80, 81]. Hexenes canin principle also be skeletally isomerized [82] because hexenes are as resistant to cracking aspentenes, but they are less often used for etherification and oligomerization. Heptene and heavieralkenes are difficult to skeletally isomerize without significant cracking taking place.

Skeletal isomerization is acid catalyzed and it is difficult to prevent side reactions. Alkenes arereactive molecules and, once the alkenes have been skeletally isomerized, other acid-catalyzedside reactions are accelerated. Skeletal isomerization is an integral part of many conversionprocesses, such as hydroisomerization, hydrocracking, and catalytic cracking. In fact, skeletalisomerization is one of the key reactions in acid-catalyzed hydrocarbon conversion and it isdiscussed in detail later (Chapter 18).

In a Fischer–Tropsch refinery, skeletal isomerization of alkenes finds application in thefollowing instances:

1) Skeletal isomerization in combination with etherification to produce high-octane fuel ethersfor motor-gasoline.

2) Skeletal isomerization in combination with selective dimerization and HYD to producehigh-octane motor-gasoline. However, alkylate-equivalent material can also be producedwithout a separate skeletal isomerization unit [83].

16.4.4Oligomerization

Oligomerization is used to refer to one or more consecutive alkene addition reactions andincludes dimerization, which involves only a single addition.

Short-chain alkenes can be oligomerized to produce longer chain alkenes. In this way, thecarbon number distribution can be shifted to heavier products. Oligomerization is employed insome crude oil refineries, but alkene availability restricts its widespread use. Alkene availabilityis not constraining in a Fischer–Tropsch refinery, and oligomerization has been identified as akey refining technology for Fischer–Tropsch syncrude [7, 8]. By careful matching of the feed,catalyst, and operating conditions, a variety of high-quality products can be produced, whichrange from motor-gasoline to lubricating oils. Oligomerization is discussed in depth later on(Chapter 19).

Oligomerization as is mostly applied in refining context is an acid-catalyzed and very exothermicreaction. Metal-catalyzed oligomerization can also be performed, but it is better suited for chemicalapplications. There are many potential applications of oligomerization in Fischer–Tropschrefining:

1) High-octane olefinic motor-gasoline can be produced by oligomerization of a wide range ofalkenes. This type of conversion is the application of oligomerization most often found incrude oil refineries.


2) It can produce alkylate-equivalent high-octane paraffinic motor-gasoline by selective alkeneoligomerization followed by HYD. Butenes are usually the feed material for this type ofconversion, also called indirect alkylation, and the alkylate can be produced in two differentways. One way is to skeletally isomerize the butene to produce isobutene, which can then beselectively dimerized to trimethyl pentenes over an acidic resin or SPA catalyst [77, 78]. Theother way is to dimerize the 1-butene-rich syncrude directly over SPA at low temperature toproduce mainly trimethylpentenes [83, 84].

3) It can produce IPK for synthetic jet fuel. Oligomerization naturally introduces branching inthe product, which gives the hydrogenated kerosene fraction very good cold-flow properties.

4) By selecting an appropriate oligomerization technology that produces more linear distillaterange material [85–87], the oligomers can be hydrogenated to yield a high cetane numberdistillate.

5) Lubricating base oils can be prepared by oligomerization of n-1-alkene-rich fractions.6) Various chemicals can be produced by oligomerization: among others, the alkene feed for

hydroformylation (Section 16.3.6) to produce plasticizer and detergent alcohols, and alkenesfor benzene alkylation (Section 16.4.6) to produce detergents.

16.4.5Aliphatic Alkylation

Aliphatic alkylation involves the condensation reaction of isobutane with alkenes to produce aheavier, highly branched alkane mixture called alkylate. Alkylate is one of the main high-octaneparaffinic motor-gasoline blending components in crude oil refineries (Table 2.5). As technology,this is its only purpose.

It is a homogeneous acid-catalyzed process employing either hydrofluoric acid (HF) or sulfuricacid (H2SO4) as catalyst [88]. The products that are obtained are sensitive to the alkene feed andoperating conditions, but either catalyst is capable of producing a high-octane paraffinic product(Table 16.1) [89, 90].

Table 16.1 Octane number of alkylate produced by HF andH2SO4 alkylation of isobutane with different alkenes.

Alkene feed H2SO4 alkylation HF alkylation

RON MON RON MON

Propene 89 87.1 91–92 89.5–901-Butene 97.8 93.9 94.4 91.62-Butenes 97.8 93.9 97.8 94.6Isobutene 93.2 90.3 95.9 93.4n-Pentenes 91 88 82.5a –Methylbutenes 91.2 88.8 – –Pentenes (mixed) – – 90–91.5 89–90

aAverage value for pentenes.


+ HF(a) Initiation

F −

+

F −

+ + +F −

+

(c) AlkylationF −

+ +

F −

+

F −

+ + +F −

+

(b) Hydride transfer

(d) Propagation byhydride transfer

Figure 16.7 Aliphatic alkylation mechanism, which involveshydride transfer from isobutane to another carbocation inorder to make isobutane the primary carbocation source foralkylation.

Although aliphatic alkylation has been grouped under the alkene conversion technologies, it isalso an alkane conversion technology. High-quality alkylate can be produced only if the isobutanebecomes the carbocation source for alkylation. In order to ensure that the isobutane is activatedby hydride transfer to another carbocation (Figure 16.7), aliphatic alkylation is conducted with alarge excess of isobutane at low temperature. Typical operating conditions for H2SO4-catalyzedalkylation is 0–10 ◦C at an isobutane:alkene ratio of 5–8, and for HF-catalyzed alkylation it is10–40 ◦C at an isobutane:alkene ratio of 10–15 [89]. The large excess of isobutane also limits thereaction of alkenes with each other, as well as alkene-related side reactions.

The development of a solid-acid-catalyzed process for aliphatic alkylation has been ongoingfor decades. Strong acidity is required to activate the isobutane and to promote hydride transferfrom the isobutane to the carbocation-covered catalyst surface, but strong acidity easily leadsto rapid catalyst deactivation [91, 92]. Progress is being made with solid-acid-catalyzed aliphaticalkylation, but it has yet to be adopted by industry.

In most refineries, the motor-gasoline becomes off-specification without alkylate. Adopting anew technology in such a sensitive area is a risk, and liquid-acid-catalyzed processes remain thepreferred aliphatic alkylation technologies despite their large environmental footprint.

Unless there is an additional source of isobutane, aliphatic alkylation is not well matched withsyncrude, which contains too much alkenes and too little alkanes in the C4 fraction. Technologiesfor alkylate productions were evaluated, and it was concluded that ‘‘indirect alkylation’’ byoligomerization (Section 16.4.4) and alkene HYD was preferred over aliphatic alkylation [83].This is a significant difference between syncrude and crude oil refining, where aliphatic alkylationis key to the latter.

16.4.6Aromatic Alkylation

The addition of an alkene to an aromatic is called aromatic alkylation. In the petrochemicalindustry, aromatic alkylation is widely used for the production of commodity chemicals such


as ethyl benzene and isopropyl benzene (cumene) [93–95]. It is not a process often found infuel refineries, although it was suggested as an effective refining technology to lower refinerybenzene levels [22, 23]. The maximum benzene content in motor-gasoline is regulated. The mainadvantage of employing aromatic alkylation to reduce the refinery benzene level is that it doesnot destroy the octane value of benzene, but retains it in the motor-gasoline.

Aromatic alkylation is an acid-catalyzed process. In petrochemical production, acid-catalyzedside reactions of the alkene, such as oligomerization, is limited by operating at a high aro-matic:alkene ratio. When this process is used within a fuel refinery, it is not necessary tofollow the same operating philosophy, since alkene oligomers (polymer gasoline, Table 2.5) is awell-known motor-gasoline blending component. Depending on alkene availability in the refin-ery, one may even combine alkene oligomerization and benzene alkylation in a single process[96]. Aromatic alkylation is an important refining technology for syncrude and is discussed indetail in Chapter 20.

There are a number of potential applications for aromatic alkylation in Fischer–Tropschrefining, and it is a complementary technology to nonacidic Pt/L-zeolite naphtha reforming(Section 16.5.3):

1) Refinery benzene can be alkylated in a stand-alone unit, or by co-feeding the benzene with analkene feed to an oligomerization unit. The former is preferred when aromatic alkylation isused in conjunction with nonacidic Pt/L-zeolite naphtha reforming, which produces benzenespecifically for alkylation. A stand-alone unit also provides the refinery with the flexibility toproduce fuels or chemicals from the benzene. Benzene alkylation in an oligomerization unitis useful when the benzene level in the naphtha fraction is <5%.

2) Fully synthetic jet fuel production can be achieved in a single unit by combining aromaticalkylation and alkene oligomerization [97].

3) Linear alkyl benzenes (LABs) can be produced by aromatic alkylation with longer chainalkenes. The LAB product is a chemical, but it is also a high cetane number, high-densitycompound that in a pinch can be used to meet diesel fuel specifications [98].

4) It is possible to produce alkyl phenols. Although phenol is present only in a very lowconcentration in syncrude, there are pathways to phenol in Fischer–Tropsch refining.Phenol is industrially produced from cumene, which is also produced by aromatic alkylation.In coal-to-liquids facilities, phenol may be obtained from the gas liquor and coal tar naphthaproduced during coal gasification (Chapter 3). The hindered phenols are commonly usedantioxidants, and many of the hindered alkyl phenols display good antioxidant properties[99–101]. The alkylated phenols can also be employed as high-octane motor-gasoline blendingcomponents [102].

5) When a Fischer–Tropsch refinery has an associated coal tar refinery, the alkylation ofaromatic coal tar naphtha holds many quality benefits [103]. Central to the derived benefitis the ability to exploit the alkenes, which would otherwise be hydrotreated to produce lowoctane number alkanes.

6) Aromatic alkylation can be employed to shift the boiling range distribution in the refinery byconverting naphtha range aromatics and alkenes into distillate. Depending on the nature ofthe alkene, such heavier alkyl aromatics have good jet fuel properties and acceptable dieselfuel properties.

7) The refining of ethene in a stand-alone refinery far from petrochemical markets has beenpointed out before. Aromatic alkylation of ethene is a useful refining pathway to convert the

16.5 Alkane Conversion 319

ethene into a liquid product. Mono-alkylation to yield ethyl benzene can be used for chemicalor motor-gasoline production, whereas multiple alkylation is useful for jet fuel production.

8) Some synthetic lubricating oils can be prepared with an aromatic core and long-chain alkylgroups. Such oils are prepared by aromatic alkylation.

16.5Alkane Conversion

16.5.1Hydroisomerization

Hydroisomerization is an analogous process to skeletal isomerization, but for alkanes. Thealkane product is more branched than the feed. The conversion of butane into isobutane andthe conversion of n-alkane-rich light straight-run (LSR) naphtha into an isomerized naphtha(isomerate) are two processes often encountered in crude oil refining. Isobutane is used as feedmaterial for aliphatic alkylation (Section 16.4.5) and isomerate is a final motor-gasoline blendingcomponent (Table 2.5). Heavier material can also be isomerized, and crude oil processes likecatalytic dewaxing is essentially a hydroisomerization process.

The main difference between skeletal isomerization and hydroisomerization is that the latterrequires a bifunctional catalyst with metal and acid sites. The metal sites are required todehydrogenate the alkanes to alkenes, which can then be skeletally isomerized on the acid sites.The isomerized alkenes can then be hydrogenated to alkanes. By doing so, the risk of sidereactions occurring is reduced, because the amount of alkenes present at any given time is low.As in the case of skeletal isomerization, this is a fundamental step in the catalytic conversion ofalkanes and is discussed in more depth later (Chapter 18).

Hydroisomerization is an equilibrium-limited conversion. The equilibrium limitation canbe overcome by separating and recycling the less isomerized material. There are varioustechnologies for doing so [104]. The quality of the final product is determined by the overalldegree of isomerization. For once-through operation, the conversion process determines theoverall conversion, but with recycle the conversion step is of secondary importance.

The use of hydroisomerization in a Fischer–Tropsch refinery is quite similar to that in a crudeoil refinery and the applications depend on the carbon number of the feed:

1) Isobutane production for aliphatic alkylation is usually considered only if there is a significantsource of n-butanes in the refinery to match the butenes.

2) Hydroisomerization of C5 –C6 naphtha produces a good quality paraffinic motor-gasolineblending component. When a single carbon number feed is used, recycling of less isomerizedmaterial becomes easier. It has also been shown that hydroisomerization technology can beemployed without pretreatment with straight-run Fischer–Tropsch naphtha that is rich inn-alkenes [37].

3) The cold-flow properties of distillate range material can be improved by mild hydroisomer-ization. With longer chain feed materials, there is always the risk of cracking, which is whymildly acidic catalysts perform so much better for the hydroisomerization of heavier n-alkanefeed materials than their more acidic counterparts. Jet fuel requires sufficient isomerizationof the kerosene to meet the freezing point specification, and there are not really any adverse


effects of a high degree of isomerization. In diesel fuel there is a trade-off between cold-flowproperties and cetane number, and the degree of hydroisomerization must be controlled.

4) Lubricating base oils can be prepared from Fischer–Tropsch waxes by hydroisomerizationto improve the cold-flow properties of the otherwise solid material [105, 106].

16.5.2Hydrocracking

Hydrocracking is strictly speaking a residue conversion technology, but due to its central rolein LTFT wax upgrading it is better classified as an alkane conversion technology in the presentcontext.

In a crude oil refinery, hydrocracking has three functions. It is a hydrogen addition technologythat removes heteroatoms, increases the H:C ratio of the product relative to the feed, andperforms some HDA. Additionally it improves the cold-flow properties, and decreases average theboiling point of the material. Like hydroisomerization (Section 16.5.1), it requires a bifunctionalcatalyst, and hydrocracking is always accompanied by hydroisomerization. Because hydrocrackingcatalysts contain acid sites, hydrocracking is often preceded by a hydrotreater to remove most ofthe nitrogen bases that can undermine hydrocracker performance [107].

The hydrocracking of HTFT residue is analogous to crude oil hydrocracking in many respects,except for the absence of sulfur. The amount of HTFT residue is small and it contains somemetal carboxylates that reduce the run length of fixed bed hydrocracking by pressure drop, ratherthan activity decline [26].

Hydrocracking of Fischer–Tropsch waxes is very different to typical crude oil hydrocracking(Table 16.2) [108, 109]. It is near isothermal, is not prone to the formation of carbonaceousdeposits, and can be conducted at less severe conditions. Hydrocracking with mildly acidic unsul-fided Pt/SiO2 –Al2O3 catalysts enabled distillate selectivities of 75–80% at 70% wax conversion[110–112], which exceed that obtainable by sulfided base metal hydrocracking catalysts typicallyused for crude oil. More recently, Calemma and coworkers reported distillate selectivities around

Table 16.2 Typical processing conditions for conventionalcrude oil hydrocrackers, mild crude oil hydrocrackers, andFischer–Tropsch wax hydrocrackers.

Description Conventional Mild LTFT waxhydrocracking hydrocracking hydrocracking

Operating conditionsTemperature (◦C) 350–430 380–440 325–375Pressure (MPa) 10–20 5–8 3.5–7Space velocity (h−1) 0.2–2 0.2–2 0.5–3H2:feed ratio (m3·m−3) 800–2000 400–800 500–1800

Conversion propertiesCracking conversion (%) 70–100 20–40 20–100H2 consumption (mass% of feed) 1.4–4 0.5–1 <1Heat release Exothermic Exothermic Near isothermal

16.5 Alkane Conversion 321

85% at close to complete conversion using a Pt/SiO2 –Al2O3 catalyst [113]. Hydrocracking ofFischer–Tropsch syncrude is discussed in detail in Chapter 21.

The main applications of hydrocracking in a Fischer–Tropsch refinery are as follows:

1) Jet fuel production by hydrocracking and hydroisomerization of distillate range and heaviermaterial to isomerized paraffinic kerosene range material.

2) Production of distillate from C23 and heavier material. The distillate can then be used as ahigh cetane number, low-density blending component in diesel fuel;

3) Manufacturing of lubricant base oil by mild hydrocracking and isomerization of heavierthan distillate syncrude fractions. Lubricant base oils can be produced from LTFT syncrude[105, 106], as well as from HTFT syncrude [114].

16.5.3Naphtha Reforming and Aromatization

Naphtha reforming has been a key refining unit in crude oil refineries for many decades (Sections2.4.2–2.4.4). Initially, thermal naphtha reforming processes were employed [115], which werelater superseded by catalytic naphtha reforming processes. In both cases the main products werea high-octane aromatic motor-gasoline and hydrogen.

Catalytic naphtha reforming is a type of aromatization process. Catalytic naphtha reformingdiffers from the broader class of aromatization processes by its feed requirements. Aromatizationprocesses are capable of producing aromatics from a wide range of feed materials, which includeC5 and lighter hydrocarbons, whereas naphtha reforming is restricted to the aromatizationof C6 and heavier hydrocarbons. The catalyst types that are used for naphtha reforming andaromatization are different, the latter requiring the ability to promote chain growth.

Hydrogen production by catalytic naphtha reforming is of paramount importance incrude oil refineries, but less so in Fischer–Tropsch refineries where H2 is available fromthe Fischer–Tropsch gas loop. Standard catalytic naphtha reforming use chlorinated,platinum-promoted alumina (Pt/Cl−/Al2O3) catalysts [116]. Fischer–Tropsch naphtha performspoorly with Pt/Cl−/Al2O3-catalyzed reforming on account of its high linear hydrocarbon content[4]. Conversely, Fischer–Tropsch syncrude performs very well with nonacidic Pt/L-zeolitereforming for the same reason. Catalytic naphtha reforming and the differences between thetechnologies are discussed in detail later (Chapter 22).

Aromatization processes are normally found in petrochemical complexes and not in fuelrefineries. During aromatization, the alkene partial pressure is much higher than during catalyticreforming and catalyst regeneration is more frequent. Although most aromatization technologieswere developed to convert propane and butanes into aromatics, heavier feed materials andalkene-rich feed materials can also be converted.

There are a number of applications for catalytic naphtha reforming and aromatization in aFischer–Tropsch refinery:

1) The refinery C5 and heavier liquid yield can be improved by the aromatization of C3 –C4

alkanes, which would otherwise end up as liquid petroleum gas (LPG) [2].2) Aromatics can be produced from low octane number syncrude fractions that are difficult to

upgrade by catalytic naphtha reforming. If the syncrude fractions contain oxygenates, it is


best to consider deoxygenation before aromatization; otherwise, there is a risk of prematurecatalyst deactivation [117].

3) Typical reformate production by catalytic naphtha reforming provides a high octane numberaromatic motor-gasoline blending component. The selection of the feed material and reform-ing technology is important. When a large amount of benzene is coproduced, the benzenemust have an upgrading pathway, for example, by aromatic alkylation (Section 16.4.6).

4) Benzene, toluene, and xylenes (BTXs) can be produced as commodity chemicals. Suchproduction is especially efficient using nonacidic Pt/L-zeolite reforming of hydrotreatedC6 –C8 Fischer–Tropsch naphtha. Aromatization technology can also be employed for thispurpose, but then the feed selection may be different.

16.5.4Dehydrogenation

Dehydrogenation converts alkanes into their corresponding alkenes. The HYD of alkenes(Section 16.2.1) is a reversible metal-catalyzed reaction and, under high hydrogen partialpressure, typical for hydrotreating, the equilibrium favors HYD. When alkanes are brought intocontact with a metal-promoted catalyst (e.g., Pt/Al2O3) at high temperature, low pressure, andlow partial pressure of H2, dehydrogenation occurs (Equation 16.2)

R–CH2 –CH2 –R′ � R–CH=CH–R′ + H2 (16.2)

The operating temperature for metal-catalyzed dehydrogenation depends on the feed material.Heavier alkanes are dehydrogenated at around 450 ◦C [118], but lighter alkanes (C3 –C5) requirea temperature above 500 ◦C [119]. Dehydrogenation of light alkanes can also be promoted bythe use of O2, which increases the thermodynamic driving force by removing H2 as water(Equation 16.3).

R–CH2 –CH2 –R′ + 12 O2 → R–CH=CH–R′ + H2O (16.3)

Oxidative dehydrogenation takes place at high temperature and short contact time, and theprocess benefits from using a metal catalyst, such as Pt/Al2O3 [120]. Unlike the exclusivelymetal-catalyzed reaction, oxidative dehydrogenation takes place by a free radical mechanism andthe catalyst serves as a combustion promoter rather than a dehydrogenation catalyst. A lowertemperature pathway for the dehydrogenation of alkanes to alkenes that is based on indirectoxidative dehydrogenation has been suggested [50], but it is not clear whether this pathway ismore economical.

A number of applications of dehydrogenation in a Fischer–Tropsch refinery can be envisioned:

1) Isobutene can be produced from butane by hydroisomerization in conjunction with dehy-drogenation. Hydroisomerization of butane is more selective than skeletal isomerization ofbutene and this production route has a selectivity advantage. Syncrude does not containmuch butane, but butane availability may be increased from other sources, including refiningprocesses such as hydrocracking.

2) In locations where there is no market for LPG, the propane and butane can be dehydrogenatedand refined with the propene and butenes present in syncrude.

16.6 Residue Conversion 323

3) Dehydrogenation can in principle be used to convert all alkanes that do not have a usefulrefining pathway into alkenes, which generally have more potential refining pathways.

4) Petrochemical refineries can make use of dehydrogenation to increase the amount of etheneand propene in the refinery. In the case of HTFT synthesis, ethane is as abundant asethane, and better economy of scale for downstream ethene processing can be achieved bydehydrogenation. In this application, dehydrogenation is an alternative technology to steamcracking (Section 16.6.3).

5) Longer chain n-alkanes can be dehydrogenated for chemical applications that can employlinear internal alkenes, such as LAB production or the production of detergent alco-hols [118]. The linear internal alkenes may also be double bond isomerized to producen-1-alkenes [121].

16.6Residue Conversion

16.6.1Catalytic Cracking

Fluid catalytic cracking (FCC) is widely used in crude oil refining to upgrade residue fractions.Residue fractions are cracked to produce lighter products, but that is not the only purposeof FCC. The product has a higher H:C ratio than the feed, and the lighter cracking productscontain a large amount of alkenes. Furthermore, the naphtha fraction has a reasonable octanenumber on account of its aromatic and alkene content (Table 2.5). Most of the alkenes neededfor motor-gasoline production come from the FCC unit, making it a very important unit in amodern crude oil refinery.

The feed properties influence the yield and quality of the product and, although it is a residueupgrading technology, it benefits from better quality feed. Hydrotreating the feed to an FCC unitcan significantly reduce the heteroatom content in the final products, without adversely affectingthe alkene yield from the unit [17]. The best type of feed material for FCC is a feed rich incycloalkanes.

An aromatic feed with high heteroatom and metal content will increase catalyst consumptionand decrease yield [122]. Very heavy and aromatic feed materials are better upgraded by coking(Section 16.6.4).

Catalytic cracking is an acid-catalyzed process. In a typical FCC, the cracking process is started bybringing the feed into contact with the very hot catalyst from the regenerator. The initial crackingreactions are thermal, but subsequent reactions are dominated by acid-catalyzed cracking. It is acarbon rejection process, and heavier aromatic compounds are deposited as coke on the catalyst.The catalyst is rapidly deactivated, and catalyst activity is restored by combusting the coke in thecatalyst regenerator. Catalyst regeneration also provides the heat that drives the process, therebybeneficially employing the energy value of the carbon-rich deposits on the catalyst.

Fischer–Tropsch syncrude is a comparatively clean feed for FCC, which can in principle beapplied to convert heavy syncrude fractions into lighter olefinic products. It does not make senseto employ FCC as a carbon rejection technology with syncrude, because syncrude is alreadyhydrogen rich.


A number of studies have been published on FCC of Fischer–Tropsch wax [123–128]. Ithas been reported that an LTFT refinery making use of FCC as opposed to hydrocracking(Section 16.5.2) for wax conversion is more economical [127]. Catalytic cracking has also beenapplied with HTFT naphtha (Section 9.5.5), but with less success. Short-chain alkanes are quiteresistant to acid-catalyzed cracking. Catalytic cracking is discussed in detail later (Chapter 21).

Some applications of FCC in Fischer–Tropsch refining are as follows:

1) Increasing the alkene content in an LTFT refinery by FCC of wax. This can facilitatemotor-gasoline production, as well as enable other opportunities requiring more alkenes inthe syncrude.

2) Converting heavier syncrude fractions from either HTFT or LTFT syncrude into lighterproducts to increase chemical and/or fuel production based on alkenes and aromatics.

3) The partial deoxygenation of syncrude as a feed pretreatment step before further refining. Thiswould also modify the syncrude properties. It is analogous in concept to bauxite treatment,but more in line with suggestions to combine Fischer–Tropsch synthesis with acid-catalyzedconversion to improve the straight-run motor-gasoline properties of the syncrude [129–132].

16.6.2Visbreaking

Historically, crude oil residues were mainly upgraded by thermal processes and, even afterthe development of catalytic cracking (Section 16.6.1), thermal conversion processes were stillextensively used. The original objective of the technology was to avoid waste (cutter stock) byconverting the heavy material into fuel oil [133]. Over time, this objective has changed somewhat,with increasing emphasis being placed on incremental naphtha and distillate production at theexpense of fuel oil yield.

The mildest form of thermal upgrading is visbreaking, which is a mild thermal crackingof residue. The residue is heated to 430–490 ◦C, and the product is separated into light gas,naphtha, distillate, and residue fractions. The furnace exit temperature is set by the nature of thefeed and the desired conversion level. The alkene content of the lighter products is fairly high.When atmospheric residue is employed as feed, visbreaking is a simple technology to obtainthe maximum amount of naphtha and distillate while complying with residual fuel oil stabilityand viscosity specifications. When vacuum residue is employed as feed, the aim is to reduce theviscosity of the residue sufficiently so that it can be used as a fuel oil [115, 134].

Lower viscosity residue for fuel oil applications is the main product from visbreaking, andthe conversion specifically targets the thermal cracking of long-chain alkanes in the residue toachieve this objective [17]. As such, it is a refining technology that has no real application in aFischer–Tropsch refinery.

16.6.3Thermal Cracking

Thermal cracking of crude-oil-derived residues has been effectively replaced by catalytic cracking(Section 16.6.1), which is a more efficient cracking technology. Thermal cracking (steam cracking)

16.6 Residue Conversion 325

Table 16.3 Cracking of Fischer–Tropsch wax. A comparisonbetween thermal cracking and hydrocracking over a sulfidedNiMo/SiO2 –Al2O3 catalyst.

Description Thermal cracking Hydrocracking

With H2 No H2

Yield of C1 –C4 (mass%) 2 1 3Yield of C5-370 ◦C (mass%) 20 21 28Naphtha (C5 –C9):Distillate (C10 –C22) – a 1 : 9 1 : 7Operating conditionsTemperature (◦C) 442 476 370Pressure (MPa) 2 0 3.5LHSV (h−1) 1 8 1H2:wax (m3·m−3, normal) 250 0 1500

aNaphtha to distillate ratio not reported, but alkene content listed by source as 39% n-1-alkenes and 3% other alkeneisomers.

at severe conditions is still applied for the conversion of light alkane gases and naphtha intoethene and propene for petrochemical applications.

Alkane-rich feed materials are in general good thermal cracking feedstocks, and the crackingbehavior of the alkanes are well documented in the classic work by Egloff [135]. The thermalcracking of heavier alkanes yields a product that is rich in n-1-alkenes and resembles anoxygenate-free HTFT syncrude. Fischer–Tropsch waxes were in the past upgraded by thermalcracking at mild conditions (Sections 6.4.3 and 8.4.2). Fuel production by thermal cracking ofLTFT waxes was evaluated, and it was found to be less efficient than hydrocracking although itrequired no hydrogen co-feed and had a lower gas make (Table 16.3) [136]. Thermal cracking isdiscussed in more detail in Chapter 21.

Thermal cracking is better suited than the other cracking technologies for the production ofchemicals and chemical precursors from syncrude. The following petrochemical applications arenoted:

1) It was demonstrated that LTFT naphtha is a good feed material for steam cracking [137]. Thishas been suggested as an upgrading pathway for the naphtha from partial Fischer–Tropschrefining (Section 12.4.1).

2) Light alkanes (C2 –C4) can be used as steam cracking feedstock for petrochemical production.3) Wax and slack wax cracking can be a source of n-1-alkenes for applications such as PAO

lubricant and detergent production [138, 139].4) Mild cracking may be used to change wax properties for chemical applications [136].5) The partial thermal cracking of Fischer–Tropsch wax on its own and in combination with

polyethylene waste plastic can be employed to produce a base stock for hydroisomerizationto lubricating base oil [140].

6) Thermal cracking can be used to lower the viscosity and improve the cold-flow propertiesof LTFT syncrude to enable transportation by pipeline. This is a typical upgrader-typeapplication, which enables the syncrude to be sold as a synthetic crude oil for refiningelsewhere.


16.6.4Coking

When thermal cracking of a residue is initiated and the product is then kept at a temperaturearound 450–500 ◦C for a prolonged period, it produces hydrogen-enriched light products and anaromatic coke. Coking is a carbon rejection technology and the objective is twofold. Products inthe fuel boiling range is produced with a higher H:C ratio than the feed. At the same time, carbonand heteroatoms are rejected as coke, thereby reducing the H2 requirement of the refinery. Thecoke itself may also be a valuable product. The quality and potential applications for which thecoke may be suited depend on the quality of the residue [141]. It is therefore not possible to converta poor quality residue by coking into hydrogen-enriched light products and good quality coke.

The coking propensity of a feed is measured in terms of its Conradson carbon residue, asdetermined by the ASTM D 189 standard test method [142]. With the exception of HTFT residue,most Fischer–Tropsch syncrude fractions have a low coking propensity. This is understandable,because syncrude has a H:C ratio around 2, which makes carbon rejection very difficult. Cokinghas little application in a Fischer–Tropsch refinery, although it may be applied in coal-to-liquidsfacilities that coproduce coal liquids during low-temperature gasification.

16.7Fischer–Tropsch Refining Technology Selection

Refinery technology selection goes hand in hand with refinery design. Any of the refinerytechnologies that were discussed can be made to work with Fischer–Tropsch syncrude. Theoperative construct is ‘‘made to work.’’ If we apply a Green chemistry and engineering approach,refining technologies can be preselected on the basis of their goodness of fit with syncrude andtheir overall environmental footprint. Selection criteria have been developed for the evaluationof refining technologies for use with Fischer–Tropsch syncrude (Table 16.4) [4]:

1) Fischer–Tropsch syncrude compatibility: There are two aspects that govern compatibilitybetween syncrude and a refining technology. The first is the amount of feed pretreatmentthat is required to make the technology work. The second is that the chemistry is efficientfor the conversion of the molecules in the feed. This is often catalyst dependent, and aspecific refining technology may have different syncrude compatibility issues when differentcatalysts are used. A three-point scale is employed: ‘‘Good’’ indicates that syncrude has anadvantage compared to crude oil when used with this technology and catalyst combination.‘‘Average’’ means that syncrude is equally well suited to this technology as crude oil and thatthe same effort in feed pretreatment is required. ‘‘Poor’’ indicates that syncrude requiressignificantly more feed pretreatment than crude oil to make it compatible, or that thepurpose of the technology is not well aligned with syncrude composition. For example, it hasbeen indicated that FCC performs well with LTFT wax as feed, but it is a carbon rejectiontechnology and carbon rejection does not make sense in the context of wax refining. If youwant a light alkene-rich syncrude, select HTFT synthesis, it is likely to be more efficientthan converting an LTFT syncrude into an HTFT syncrude in the refinery.

2) Waste generation: Waste generation is an indication of the extent of waste that is generatedby the technology in relation to a typical refining process. A three-point scale is employed:‘‘Low’’ refers to processes that generate the same or less waste than the norm, where one

16.7 Fischer–Tropsch Refining Technology Selection 327

Table 16.4 Refinery technologies evaluated for processing ofFischer–Tropsch syncrude in terms of compatibility, environ-mental impact, and recommended use.

Refining technology Catalyst Syncrudecompatibility

Environmental impact

Waste Chemicals Energy use

HydrotreatingAlkene hydrogenation Sulfided Average Low DMDSa Low

Unsulfided Good Low None LowHydrodeoxygenation Sulfided Average Low DMDS Moderate

Unsulfided Average Low None LowAddition and removal of oxygen

Alcohol dehydration Alumina Good Low None Moderateb

Etherification (alcohol + alcohol) Acidic resin Average Low None LowEtherification (alcohol + alkene) Acidic resin Average Low None LowAlkene hydration H3PO4/support Average Moderate H3PO4 ModerateEsterification Solid acid Average Low None LowCarbonyl aromatization Solid acid Average Low None LowHydroformylation Rh/Co-complex Good Low Catalyst LowAutoxidation Thermal Good Low None Low

Alkene conversionDouble bond isomerization Alumina Good Low None HighMetathesis Metal oxide Average Low None Moderatec

Butene skeletal isomerization Ferrierite Average Lowd None HighPentene skeletal isomerization Alumina Good Lowd None High

Acidic mol sieve Averagee Low None ModerateAlkene di-/oligomerization Acidic resin Average Low None Low

H-ZSM-5 Good Low None ModerateMFS/TON Average Low None LowASA Good Low None LowSPA Good Low None LowHomogeneous Average Moderate Catalyst/NaOH LowThermal Good Low None High

Aliphatic alkylation HF Poorf,g Moderate HF LowH2SO4 Poorf,g High H2SO4 Low

Aromatic alkylation SPA Good Low None LowZeolites Average Low None Moderate

Alkane conversionButane hydroisomerization Pt/Cl– /Al2O3 Average Low Chloro-alkane ModerateC5 –C6 hydroisomerization Pt/Cl– /Al2O3 Poore Low Chloro-alkane Low

Pt/metal oxide Poorg Low None LowPt/mordenite Average Low None Low

Hydrocracking Sulfided Average Low DMDS HighUnsulfided Good Low None High

Catalytic reforming Pt/Cl– /Al2O3 Poorg,h Lowd Chloro-alkane HighNonacidic Pt/L Good Low None High

Aromatization M/H-ZSM-5 Average Low None High

(continued overleaf )


Table 16.5 (continued )

Refining technology Catalyst Syncrudecompatibility

Environmental impact

Waste Chemicals Energy use

Residue conversionCatalytic cracking Zeolite Poori Low None HighVisbreaking Thermal Poor Low None HighThermal cracking Thermal Good Low None HighCoking Thermal Poor Low None High

aDMDS = dimethyl disulfide (CH3–S–S–CH3)bDepends on the feed: ethanol dehydration (high), other alcohols (moderate).cDepends on metal oxide catalyst: Re2O7 (low), MoO3 (moderate), and WO3 (moderate/high).dSide product formation is 10% or more.eDepends on feed origin: oxygenates adsorb on catalyst to decrease its operating window.f Depends on butane availability, usually butane:butene �1.gOxygenates and water must be removed from feed to limit catalyst deactivation.hHigh linear hydrocarbon content and low cycloalkane and aromatic content (N + 2A < 30).iWorks well with wax, but attempts carbon rejection with a hydrogen-rich feed.

anticipates solid waste from catalyst change and some by-products during conversion. Mostprocesses fall within this category. ‘‘Moderate’’ indicates that the technology produces wastein excess of the norm. ‘‘High’’ is reserved for the waste management which in relation tothe technology is significant. One would select processes with ‘‘moderate’’ or ‘‘high’’ wastegeneration only if they provide a conversion that is essential to the refinery design.

3) Chemical addition: Some refining processes rely on the addition of chemicals to work. Oftenonly low levels of addition are required. It is a judgment call whether the additional hazardand the associated increase in environmental footprint are acceptable or not.

4) Energy requirements: The carbon efficiency of Fischer–Tropsch-based processes is relatednot only to the efficiency of converting raw material carbon into final products, but also tothe overall carbon balance over the system. Any net energy flow has an equivalent carboncost, irrespective of how the energy was generated or consumed. A three-point scale isemployed to rank energy use related to operating temperature to ‘‘low,’’ ‘‘moderate,’’ and‘‘high,’’ where ‘‘high’’ is for processes requiring temperatures >350 ◦C. Care should betaken in interpreting these designators, since a high-temperature process with good heatmanagement can easily be more efficient than a low-temperature process due to the qualityof the heat (higher temperature is more useful) and the net energy flow.

The qualitative assessment provided in Table 16.4 is a guideline but is not a substitute forsound engineering judgment.

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