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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Product and Discharge Specifications1

Upstream Process

Engineering Course

2. Product and Discharge

Specifications

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Vapour PressureThe vapor pressure of a liquid, is the pressure exerted by its vapor when the liquid

and vapor are in dynamic equilibrium. Vapor pressure is the pressure of a vapor in

equilibrium with its non-vapor phases All solids and liquids have a tendency to

evaporate to a gaseous form, and all gases have a tendency to condense back. At

any given temperature, for a particular substance, there is a partial pressure at

which the gas of that substance is in dynamic equilibrium with its liquid or solid

forms. This is the vapor pressure of that substance at that temperature. Inmeteorology, the term vapor pressure is used to mean the partial pressure of water

vapor in the atmosphere, even if it is not equilibrium, and the equilibrium vapor

pressure is specified as such. Vapor pressure is an indication of a liquid's

evaporation rate. It relates to the tendency of molecules and atoms to escape from

a liquid or a solid. A substance with a high vapor pressure at normal temperatures

is often referred to as volatile. The higher the vapor pressure of a material at a

given temperature, the lower the boiling point.

The vapor pressure of any substance increases non-linearly with temperature

according to the Clausius-Clapeyron relation. The boiling point of a liquid is the

temperature where the vapor pressure equals the ambient atmospheric pressure.

At the boiling temperature, the vapor pressure becomes sufficient to overcome

atmospheric pressure and lift the liquid to form bubbles inside the bulk of the substance.

Water Vapour Pressure

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Crude Oil Product Specifications

• Crude oil is usually exported to market by tanker or by pipeline. Road and

rail shipments are sometimes used for smaller volumes.

• For safe handling of crude, the vapour pressure and maximum delivery

temperature are specified.

• For storage in tanks and transport by ship/road/rail, the True Vapour Pressure (TVP) must be less than 1 bara for tank pressure protection

integrity and to prevent vapour loss in transit.

• Pipeline TVP is set in conjunction with the operating parameters of the

system. TVP will be less than the lowest system pressure to prevent

vapour breakout.

• The TVP of the export oil is controlled by the exporting plant operatingconditions, e.g. pressure and temperature of the final stage of separation.

• Water and salt content also need to meet specified values to suit

downstream processing requirements.

The lighter ends of the

crude (shaded area) areremoved to meet the TVP

specification of the export

system

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Crude Oil Product Specifications

• For pipeline transmission other variables require to be controlled. These are

fixed by the sales requirement, the following are typical values

– Water content: 2-5 wt %

– Salt content: 70 - 200 mg per litre• Note, higher water contents reduce the pipeline or storage capacity and crude

sales value

• For tanker transport, a more stringent water specification is often specified:

– BS&W (Basic Sediment and Water) Content: 0.5 vol% maximum

• Corrosive components are often specified – H2S, CO2

• Pour point and/or viscosity may be considered for pipeline capacity and

storage problems but in general facilities are designed to accept the product

rather than vice versa

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Acidity The total acid number shall be no greater than 0.30 mg of potassium hydroxide per gram of Stabilised Crude Oil derived fromShippers Pipeline Liquids.

Emulsions Shippers’ Pipeline Liquids and those of Other Users when mixed should not formemulsions which are stable at temperaturesat or above 36oC and pressures at or above one bar absolute.

Metals Vanadium plus nickel shall not exceed 5ppm by weight in Shippers Pipeline Liquids.

Salt Content Maximum of 1500 milligrams per litre of sodium, calcium and magnesium chloridesin solution in Shippers Pipeline Liquids.

Entry Pressure Minimum of 125 barg at delivery in theForties System at either Unity or FortiesCharlie.

Oxygenates Shippers Pipeline Liquids shall not containoxygenates.

Alcohols Shippers Pipeline Liquids sha ll not contain

alcohols.

General No chemical additives or processingmaterial shall be injected into Shippers’ Pipeline Liquids either directly or throughprocessing without prior consultation andagreement with BP. All chemical additivesmust comply with the Offshore ChemicalNotification Scheme (OCNS) [Departmentof Trade and Industry] and no endocrinedisrupter are permitted.

Shippers’ Pipeline Liquids to be free of undesirable matter (including, withoutlimitation, radioactive materials).

Carbon Dioxide Maximum of 0.2 mole % of carbon dioxidein Shippers Pipeline Liquids.

Nitrogen Maximum of 0.2 mole % of nitrogen inShippers Pipeline Liquids

Carbonyl Sulphide Maximum of 0.02 ppm by weight inShippers Pipeline Liquids.

Hydrogen Sulphide Maximum of 0.1 ppm by weight as sulphur in Shippers Pipeline Liquids

Mercaptans Maximum of 0.1 ppm by weight as sulphur of volatile mercaptons in Shippers PipelineLiquids, which separate into Raw Gasunder Kinneil operating conditions.

Mercury Maximum of 0.35 ng/g as volatile organicor inorganic mercury in Shippers PipelineLiquids, which separate into Raw Gasunder Kinneil operating conditions.

Sediment and Water Maximum of 2% by volume provided thatShippers’ Pipeline Liquids are essentiallyfree of sediment. Produced water shall be

made compatible (at the cost of theShippers’ Owners) with water produced byother Forties System Users.

True Vapour Pressure Maximum of 125 psig at 60oF, asmeasured in the Shippers Pipeline Liquids.

Viscosity Maximum of 15 centiStokes at 4oC in theShippers Pipeline Liquids

Forties Pipeline Specifications

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RVP Specification

TVP cannot be measured directly, so instead

an experimental method measures the Reid

Vapour Pressure (RVP). The TVP is then

calculated using a correction factor.

RVP is determined experimentally as follows.

The sample is placed in a standard cell –

one fifth oil four fifths air. The RVP is

the pressure of the vapour in the cell at

100 ºF

Typical RVP spec is 0.7 bara (10 psia) at 38 ºC.

Nomograph for estimating the RVP of crude oil

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Gas Sales Specifications

• In order to condition gas for sales distribution a range of quality specificationsrequire to be achieved. These vary from country to country, common featuresare hydrocarbon and water dewpoint, temperature, pressure and compositione.g. H2S and CO2

• Water dewpoint limits are required to avoid corrosion and hydrate formation,and depend on typical ambient conditions. May be stated as a dewpoint (e.g. -10 deg C at 69 barg) or as a water content (e.g 2lbs/ mmscf).

• Temperature - A maximum temperature at the delivery point may be specified,usually around 30-50 ºC

• Pressure - The maximum gas pressure will be decided by the design pressure of the system and the allowable back pressure on other system entrants. Nominalgas pressure is the normal entry pressure to the pipeline, typically 70 - 140 Bar

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Gas Sales

• Hydrocarbon Dewpoint Control

– To prevent hydrocarbon condensation, with the

consequences of the pipeline flowing two-phase,

the pipeline operator often sets a limit on the gas

cricondenbar. The significance of the cricondenbar

is evident from the phase envelope - providedsystem pressures are higher than the cricondenbar

then a single phase will always exist irrespective

of temperature. A typical cricondenbar

specification is 105-110 bara maximum.

– An alternative to cricondenbar control a

hydrocarbon dewpoint or a liquid loading

maximum value may be given

• Solids: Free of particulates in amounts

detrimental to transmission and

utilisation equipment

Cricondenbar

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Gas Sales

• The sales gas specification will be subject to a pricing agreement

which is likely to include the following:

– Gross Calorific Value (GCV) or Higher Heating Value (HHV)

• The total heat produced by combustion of the fuel

– Net Calorific Value (NCV) or Lower Heating Value (LHV)

• The total heat produced by combustion of the fuel minus the latent heatcontained in the water vapour discharged as fuel gas, NCV represents the

available heat

– Wobbe Index (WI)

• Wobbe Index is used to compare fuel quality for different gases it

characterises flame stability

• WI is the ratio of GCV to the square root of the gas s.g. - units are MJ/Sm3

• This may be estimated from the molecular weight of the gas and corrected

for N2 and CO2

– Impurities

• Inert gases, usually N2/Ar, are removed to improve the gross calorific

value of the gas

– Sulphur Content

• Sulphur content is controlled for safety reasons, to prevent pipeline

corrosion and improve the sales value of the gas

Gas

GCV of dry

gas at 15 ºC

& 101.325

kPa (MJ/m3

)Methane 37.69

Ethane 66.03

Propane 93.97

i-Butane 121.43

n-Butane 121.78

i-Pentane 149.32

n-Pentane 149.65

Hexane 177.56Heptane 205.43

CO2 0

H2S 23.79

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Sales Gas Specifications

Units Continental Sales Gas UK Sales Gas (Transco) US

Gross Calorific Value MJ/Nm3 38 - 43.6 38.9 - 44.6 35 - 45

Wobbe Index MJ/Nm3 46.6 - 52.1 48.2 - 51.2

Hydrocarbon Dew Point@ 2 - 70 bara ºC -3.0 -1.0 45 ºF @ 400 psig

Water Dew Point

@ 69 bara ºC -8.0 -10.0 7lb/MMSCF

Impurities

Oxygen mol % 0.50 0.001 0.2

Carbon Dioxide mol % 2.50 2.0 - 4.0 2

Nitrogen mol % 1.00 7.00 1 - 2

Hydrogen Sulphide ppm 5.00 1.00 4.00

Total Sulphur ppm 15.00 15.00Mercury mg/Nm3 5.00

Delivery Temperature 5 - 30 ºC 1 - 38 ºC 120 ºF (max)

Delivery Pressure Bara 51.00 69.00

Sales Gas Specifications

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Water Content of Natural Gas

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Wobbe Index

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Gas Transportation

• For some applications gas may be transported from a remote facility to a

conditioning terminal - St Fergus - prior to gas sales. In this instance the gas

need only be partially processed for transportation between the remote site and

the terminal.

• As a general rule – ‘Don’t do offshore what you can do onshore’

• The degree of processing will be application specific but is likely to include

• Water dewpointing

• Hydrocarbon dewpointing

• Acid gas treatment if appropriate

• An alternative to water dewpointing is continuous addition of hydrate

suppressant and corrosion inhibitor. This is used in some SNS fields where

water loading is small and continuous inhibition is practical.

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Commercial Propane Commercial Butane Commercial B-P Mixtures

Composition

Predominantly

propanes and/or

propylenes

Predominantly

butanes and/or

butylenes

Predominantly mixtures

of butanes and/or

butylenes with propane

and/or propylene

Vapour Pressure

@ 100 ºF, max. 208 70 208@ 37.8 ºC, max. 1434 483 1434

Volatile Residue

temperature @ 95% evaporation,

ºF, max -37 36 36

ºC, max -38.3 2.2 2.2

butane and heavier, liquid vol %, max. 2.5 - -

pentane and heavier, liquid vol %, max. - 2 2

Residual Matter

residue on evaporation of 100 cm3, max 0.05 cm3 - -

oil stain observation pass (*) - -Corrosion, copper strip, max. No. 1 No. 1 No. 1

Total sulphur, mg/kg 185 140 140

Moisure content pass - -

Free water content - none none

Product Characteristics

Product Designation

LPG Specification

(*) An acceptable product shall not yield a persistent oil ring when 0.3 cm3 of solvent residue mixture

is added to a filter paper in 0.1 increments and examined in daylight as described in ASTM D-2158

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Quality Tests for LPG

• The standard test for corrosivity (H2S) is the Copper Corrosion Test (ASTM D-1838)

– A polished copper strip is immersed in the sample for 1 hour at 38ºC

– The test strip is then rated according to the following standards:

• No. 1 Slight tarnish (light to dark orange)

• No. 2 Moderate tarnish (red, lavender, brassy gold)

• No. 3 Dark tarnish (magenta, red, green)

• No. 4 Corrosion (black, dark grey, brown)

• There are several methods of determining the acceptable levels of moisture in propane

– The Cobalt Bromide Test• The cobalt bromide is supported on white cotton wadding and exposed to a stream of propane vapour, chilled to 0 ºC

• The colour of cobalt bromide changes from green to lavender at about 30% relative humidity indicating wet gas

– The Valve-Freeze Method

• A specially constructed and calibrated orifice designed to simulate expansion of propane through a pressure

regulator

• A liquid sample is passed through the valve at a pre-set flowrate and the time taken for the valve to freeze and

interrupt flow determines whether or not the propane is commercially ‘dry’

– The Bureau of Mines Dew Point Test

• A simple field test designed to use calculate moisture content of natural gases

• Not recommended as the accuracy is dependant on temperature and pressure which are difficult to control

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Valuation of Crude Oil

• When evaluating the value of a crude produced from a new field, it will be compared to the ‘benchmark’

crude, which for the North Sea is usually Brent Blend

• This takes into account the impact on the refinery of processing and any unusual qualities in the crude, for

example product yields and qualities

Blend ( or Grade) US$/bbl Fri. 31/01/03

US$/bbl Fri. 22/09/06

OPEC Basket 30.58 57.55

Dubai Fateh 29.13 57.94

Bonny Light 33.07 62.15

N. Sea Brent 32.50 60.49

Urals/ Mediterranean 31.45 57.48

W. Texas Intermediate 33.53 54.46

World Average - 56.38

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Valuation of Crude Oil

• When evaluating the value of a crude produced from a new field, it will be compared to

the ‘benchmark’ crude, which for the North Sea is usually Brent Blend

• This takes into account the impact on the refinery of processing and any unusual

qualities in the crude, for example product yields and qualities

• Any new crude is likely to be de-valued in order for a refinery to take the risk of

processing an unknown feed, this discount may be in the range of $0.10 to $1.00 per

barrel

• As the refineries experience handling the crude and market acceptance grows, these

discounts may be moderated

• The method of loading the crude oil can also affect its initial market value, the

perceived or actual risks of offshore loading such as delays due to the weather could

de-value a crude compared with pipeline delivery which is assumed to be more reliable

• Pre-production assay samples of all the wells to be produced are blended in the

appropriate production ratios to generate data for the valuation analysis

• Crude is separated into three key distillation cuts, Naphtha, Middle Distillate and

Residue as a basis for valuation.

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Crude Yields

• Naphtha

– Light naphtha (C5 - 95ºC)

– Medium Naphtha (95 - 150ºC)

– Heavy Naphtha (150 - 180ºC)

• Middle Distillate

– Kerosene (180 - 260ºC)

– Gas Oil (260 - 327ºC)

– Heavy Distillate (327 - 370ºC)

• Residue

– Vacuum Gasoil (350 - 565ºC)

– Vacuum Residue (565+ ºC)

• Approximate values March 2006follows (US$ per metric tonne):

– C4 $435-490/ te

– Naphtha $506-514 / te

– Middle Distillate $530-540/ te

• Gas Oil Specific Gravity

– The European oil retail market isvolume based

– To ensure consistency, cargo's are

sold using the internationally agreedspecific gravity of 0.845

– For example, if the sales price is$300/tonne and the actual gravity is0.85, the actual sales price is:

0.845/0.850 300 = $298.24

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Value Adjustment

• Specific crude values are adjusted to take account of variations in the

physical properties and compositional properties of the distillation cuts

• Naphtha C5-165 ºC

– no correction

• Gas Oil 165-350 ºC

– s.g. adjustments made

• Vacuum Gas Oil 350 - 550 ºC

– s.g., sulphur and viscosity adjustments

• Fuel Oil 550+ ºC

– s.g., sulphur and viscosity adjustments

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Brent Blend

• The properties of the North Sea benchmark crude are as follows:

– Gravity ºAPI 38.5

– Sulphur, wt % 0.36

– Pour Point, ºC 0

– Acid Number, mgKOH/g 0.05

• Crude Distillation Yields (wt %)

– C1 - C4 2.7

– Naphtha (C5 - 180ºC) 26.1

– Middle Distillate (180 - 370 ºC) 34.4

– Residue (370+ ºC) 36.9

• Middle Distillate Properties

– Kerosene smoke point, mm 22.8

– Gasoil cetane index 51.7

– Gasoil density @ 15ºC 0.848

– Gasoil sulphur content, wt% 0.22

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Crude Oil Contaminants/Adjustments

Quality

Typical Limit Attracting

Penalties Basis of Penalty

Acidity 0.3 mg KOH/gm

Blending, shipping and

working capital costs

Sulphur Continuous

Logarithmic relationship of fueloil prices at different sulphur

levels

Metals 3 ppm (Ni + V)

Catalyst costs in conversion

process

Con Carbon 5.7 wt% Loss of throughput

Nitrogen 1000 ppm in Vac. Gas Oil Yield Loss

Gas Oil S.G. Continuous Weight/volume relationship

Fuel Oil Viscosity ContinuousMarket prices for fuel oil andgas oil

Conradson Carbon - A measurement of hydrocarbon mixtures tendency to leave carbon deposits (coke) when burned as fuel or subjected to intense

heat in a processing unit such as a catalytic cracker. The ConCarbon test involves destructive distillation -subjection to high temperature which causes

cracking, coking, and drives off any volatile hydrocarbons produced--and weighing the residue which remains. A somewhat similar test, Ramsbottom

carbon, also measures mixtures tendency to form coke. For reasons of laboratory convenience, analysts ordinarily restrict the Ramsbottom method to

hydrocarbons which flow 90 C. To obtain a useful indication of carbon residue formation by light distillates, such as high-speed diesel, the industry

often measures coke formation by the last 10 percent of the material to boil. This technique goes by names such as "ConCarbon residue on 10 percent

bottoms

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Metal Contaminants

• Metals in crude oil (specifically; nickel, vanadium and sodium) are considered for three

distinct reasons

• Catalyst Poison

– In fluid catalytic cracking, nickel and vanadium act as a catalyst poison resulting in an

increased yield of hydrogen and coke at the expense of more valuable products – The refiner can deal with this problem by either increasing the catalyst replacement rates or

blending feedstocks

– The absolute limits on metals are very site specific and range from 1-2 ppm for a conventional

VGO (Vacuum Gas Oil) cracker to 60 ppm for a state of the art residue cracker

• Residue Specification

– Anode grade coke for use in aluminium smelting commands a premium, the metals

specification is stricter of these grades of coke and consequently requires a feedstock (560ºC +residue) of less than 400 ppm nickel plus vanadium

• Fuel Oil Specification

– Residual fuel oil has both a sodium and vanadium specification which refiners must meet

– For most of Europe this specification is 300 ppm vanadium, 150 ppm sodium

• Refiners will generally blend feedstocks to achieve this specification

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