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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 1
Upstream Process
Engineering Course
1b. Hydrocarbon Phase Behaviour
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 2
Phase Diagram - Water
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 3
Phase Envelope – multi-
component hydrocarbon
Cricondenbar
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 4
Reservoir Barrel
METHANE &
ETHANE
CARBON DIOXIDE
HYDROGEN SULPHIDE
SAND
WATER
LOWEST
BOILING POINT
HIGHEST
BOILING POINT
gases
naphtha
gasoline
kerosene
diesel oils
Lubricatingoils
Fuel oil
residue
Propane and butane gas for lighter fuel& camping stoves
Chemicals for medicines, plastics, paints,cosmetics & clothing materials
Petrol for vehicles
Jet fuel and paraffin
Diesel fuel
Machine oil, waxes and polishes
Fuel for ships and central heating
Bitumen for road surfaces and roofingmaterials
SUBSTANCE USES
Other components which
may require
treatment/consideration
are;
•Hydrogen Cyanide (HCN)
•Carbonyl Sulphide (COS)
•Carbon Disulphide (CS2)
•Mercaptans (RSH)
• Nitrogen (N2)
•Sulphur Dioxide (SO2)
•Mercury
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 5
Reservoir Fluid Types
• Four general types of reservoir
fluids
– Black Oil - Forties
– Volatile Oil - Gyda
– Condensate - Bruce
– Gas - SNS Villages
Typical P-T diagrams showing the position of
the critical point for all four reservoir fluid types
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 6
Phase Equilibrium Definitions
• Phase Diagram
– A record of the effects of temperature, pressure and composition on the kinds and numbers of
phases that can exist in equilibrium with each other
• Bubble Point
– The point at which the first infinitesimally small vapour bubble appears in a liquid system. The
bubble point curve on a phase diagram represents 0% vapour
• Dew Point
– The point at which the first infinitesimally small droplet of condensation forms in a gaseous
system. The dew point curve on a phase diagram represents 0% liquid
• Phase Envelope
– The area on a pressure-temperature phase diagram for a mixture enclosed by the bubble anddew point curves. This area represents the set of conditions for the mixture were vapour and
liquid phases co-exist in equilibrium.
• Cricondenbar
– The maximum pressure at which vapour and liquid can co-exist in equilibrium
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Phase Equilibrium Definitions
• Cricondentherm
– The maximum temperature at which vapour and liquid can co-exist in equilibrium
• Critical Point
– At the critical point, liquid and vapour phases of a fluid have identical physical properties
• Quality Lines
– Lines through the two-phase region showing a constant percentage of liquid and vapour
• Retrograde
– The name given to phase behaviour above the critical temperature and pressure were vapour
and liquid phases coexist and the amount of vaporisation or condensation changes with pressure
and temperature in the opposite direction to normal behaviour.• Equation of State
– An equation which describes the relationship between pressure, temperature and molar volume
of any homogenous fluid at equilibrium
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 8
Phase Equilibrium Definitions
• Fugacity
– A thermodynamic concept arising from the consideration of the change in Gibbs free energy
with changes in pressure and temperature for non-ideal mixtures and substances. Three types of
fugacity can be defined: pure component fugacity, mixture fugacity , partial fugacity . Fugacitymust have the same units as pressure by definition but will only be equal to pressure under ideal
conditions (e.g. low pressure gases).
• Binary Interaction Coefficient
– A constant which accounts for the deviation from ideality for component pairs in mixtures.
These are specific to one equation of state as they are calculated by the regression of measured
data. The coefficients used in a liquid activity method are essentially regression constants used
to calculate the liquid activity coefficient• Liquid activity coefficient
– The ratio of the partial fugacity of a component in a mixture to its pure component fugacity at
the same physical conditions, divided by its mole fraction in the mixture.
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 9
Alkanes (Parrafins).
Methane (CH4)
Ethane
Propane
Butane
……….
Octane (C8H18)
Aromatics -
Benzene
Napthenates -
one or more
cyclic structures
Cycloparaffins -
one or more
cyclic structures
Oil Components
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 10
Oil Components
Paraffins
The paraffinic series of hydrocarbon compounds have the general
formula CnH2n+2 and can be either straight chains (normal) or
branched chains (isomers) of carbon atoms. Paraffinic
hydrocarbons are very commonly referred to as alkanes.
The lighter, straight-chain paraffin molecules are found in natural
gas as well as in petroleum refinery byproduct gases and low
boiling point liquids. Examples of straight-chain molecules are
methane, ethane, propane, and butane (gases containing from one
to four carbon atoms), and pentane and hexane (liquids with five
to six carbon atoms).
The branched-chain (isomer) paraffins are usually found in
heavier fractions of crude oil and have higher octane numbers
than normal paraffins. These compounds are saturated
hydrocarbons.
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 11
Oil Components
Aromatics
Aromatics are unsaturated ring-type (cyclic) compounds which react readily
because they have carbon atoms that are deficient in hydrogen. All aromatics
have at least one benzene ring (a single-ring compound characterized by three
double bonds alternating with three single bonds between six carbon atoms) as
part of their molecular structure.
Naphthalenes are fused double-ring aromatic compounds. The most complex
aromatics are the polycyclic aromatic hydrocarbons (PAH), also referred to as
polynuclear hydrocarbons, with three or more fused aromatic rings and they
typically are found in the heavier fractions of petroleum crude oil.
Naphthenes
Naphthenes are saturated hydrocarbon compounds having the general formula
of CnH2n, arranged in the form of closed rings (cyclic) and found in allfractions of petroleum crude oil except the very lightest.
Naphthenes are also commonly referred to as cycloparaffins or cycloalkanes.
Some examples of typical naphthenes are depicted in the adjacent Figure 3.
Single-ring naphthenes (monocycloparaffins) with five and six carbon atoms
predominate, with two-ring naphthenes (dicycloparaffins) found in the heavier
ends of the naphtha fraction of petroleum crude oil.
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 12
Oil Components
Naphthenes
Naphthenes are saturated hydrocarbon compounds having the general
formula of CnH2n, arranged in the form of closed rings (cyclic) and
found in all fractions of petroleum crude oil except the very lightest.
Naphthenes are also commonly referred to as cycloparaffins or
cycloalkanes. Some examples of typical naphthenes are depicted in
the adjacent Figure.
Single-ring naphthenes (monocycloparaffins) with five and six carbon
atoms predominate, with two-ring naphthenes (dicycloparaffins)
found in the heavier ends of the naphtha fraction of petroleum crude
oil.
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 13
Crude Oil Composition Schematic
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 14
HYSYS Component and EOS
Set Up
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 15
Black Oil
C o m p one nt Mo l %
Methane (CH 4) 48 .83
Ethane (C 2H 6 ) 2 .75Propane (C 3 H8 ) 1 .93
Butanes (C 4 H1 0 ) 1 .6
Pe ntanes (C 5 H 1 2 ) 1 .15
Hexane (C 6 H1 4 ) 1 .59
C 7 + 42 .15
Mo lecular W e ight o f C 7 + 2 2 5
G O R (sc f /bb l) 6 2 5
O il G ravity (º A P I) 3 4 .3
C o lo ur B lack
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 16
Volatile Oil
C o m po nent Mo l %
Methane (CH4 ) 6 4 .36
Ethane (C 2H 6 ) 7 .5 2
Propane (C 3 H8 ) 4 .7 4
Butanes (C 4H1 0 ) 4 .1 2
Pe ntanes (C 5H1 2 ) 2 .9 7
Hexane (C 6H1 4 ) 1 .3 8
C 7 + 1 4 .91
Mo le cular W e ig ht o f C 7 + 1 8 1
G O R (sc f /b b l) 2 0 0 0
O il G ravity (º A P I) 5 0 .1
C o lour B ro wn
HYSYS is not
flawless
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 17
Gas Condensate
C o m po nent Mo l %
Methane (CH4 ) 8 7 .07
Ethane (C 2H 6 ) 4 .3 9
Propane (C 3 H8 ) 2 .2 9
Butanes (C 4H1 0 ) 1 .7 4
Pe ntanes (C 5H1 2 ) 0 .8 3
Hexane (C 6H1 4 ) 0 .6
C 7 + 3 .8
Mo le cular W e ig ht o f C 7 + 1 1 2
G O R (sc f /b b l) 18 2 0 0
O il G ravity (º A P I) 6 0 .8
C o lour S traw
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 18
Dry Gas
Component Mol %
Methane (CH4) 95.85Ethane (C2H6) 2.67
Propane (C3H8) 0.34
Butanes (C4H10) 0.52
Pentanes (C5H12) 0.08
Hexane (C6H14) 0.12
C7+ 0.42
Molecular Weight of C7+ 157GOR (scf/bbl) 105000
Oil Gravity (º API) 125
Colour White
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Fluid Sampling
• Drill Stem Tests
– Carried out on appraisal wells to provide sub-surface
information and data for facilities design
– Laboratory and field measurements made
– Basis for verifying simulation models
• Extended Well Test
– Testing over many months
– Sub-surface uncertainty reduced
– Opportunities to undertake extensive facilities testing
– Environmental implications require to be evaluated
• Topsides Samples
– Regular samples are taken to maintain product quality
and ensure levels of chemical treatment are sufficient
– Pressurised samples are taken of crude at normal
operating conditions for laboratory analysis and
atmospheric samples
• Bottom Hole Samples
– Capture pressurised reservoir
sample e.g asphaltene analysis
– QA for drilling fluid
contamination
– For fast track development
sometimes only sample
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 20
Test Separator and Burner
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 21
Fluid Modelling – Black Oil
Blackoil model refers to a multi-phase fluid model used in the oil and gas
industry. This type of model predicts fluid properties from some key known
properties – gas and oil density, gas oil ratio (GOR). Empirical correlationsdetermine phase splits and physical properties at specific pressures and
temperatures .
The blackoil model assumes the liquid at stock tank conditions remians in the
liquid phase at all pressures and temperatures. The gas can exist as free gas or
dissolved gas. This type of model is often used for reservoir simulation and also
used in the early well and pipeline multi-phase flow models.
A range of methods and correlations are available for black oil modelling.
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 22
• Compositional fluid modelling is a method for describing a stream based upon its
pure components. Equilibrium phase splits and phase properties are determined by
blending the properties of the stream constituents. Typically an equation of state is
used to predict phase behaviour and physical properties. The equation of state will be used to predict – VLE (vapour liquid equilibria), gas and liquid enthalpies, gas
and liquid densities, gas and liquid viscosities, surface tension and thermal
properties – e.g. thermal conductivity.
• It is not practical to model every component of a reservoir fluid due to the large
numbers of components that are present. Generally, for acceptable phase
behaviour prediction, it is sufficient to specify the mole fractions of the main lightend paraffins, typically C1 to C10 for black oils. If dealing with gas condensate
systems, however, more rigorous compositional data may be required, particularly
if modelling retrograde behaviour. Heavier components are handled as pseudo or
hypothetical components.
Fluid Modelling -
Compositional
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 23
• For the heavier components in a reservoir fluid mix one or more pseudo-
components (or hypothetical components) can be used.
• Pseudo-Components are a mixture of many components with different properties
but modelled as one component with generalised physical properties.• Pseudo-Components are generally defined either by the critical properties or by
average molecular weight, specific gravity and normal boiling point. For most
computer simulations a minimum of two of these must be specified. Acentricity
factors and binary interaction parameters can also be entered; most simulation
packages will estimate these if not supplied by the user. The more information
that is given on a pseudo component, however, the more accurate the phase prediction can be.
• The pseudo component facility can also be used to enter data into a simulation for
a component not contained in the component library of the simulation package
being used.
Pseudo-Components
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 24
Heavy End Fluid Characterisation
• Prediction of heavy end physical properties is
important to good prediction of phase
behaviour; incomplete or inaccurate data can
radically affect phase calculations. For
example the prediction of bubble and dew
points.• Standard fractional distillation tests can be
used to produce boiling point curves and
physical properties so that the heavy ends can
be split into a number of pseudo components.
• Some simulation packages have the facility to
take in distillation test data directly and to
produce the pseudo-componentsautomatically.
• Some ‘tuning’ may be required to match the
properties of the modelled reservoir fluids to
well test data.
Stabilised Crude Boiling Curve
0
10
20
30
4050
60
70
80
90
9 0 1 2 0
1 5 0
1 8 0
2 1 0
2 4 0
2 7 0
3 0 0
3 3 0
3 6 0
3 9 0
4 4 0
5 0 0
TBP Cut Deg C
T o t a l D i s t i l l a t e W t %
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 25
Pseudo-Components
Component Analysis
0
10
2030
40
50
C O 2
C 2
n C 4 C
6 C 9
C 1 2
C 1 5
C 1 8
C 2 1
C 2 4
C 2 7
C 3 0
C 3 3
C 3 6
+
E t h y l b
e n z e n e
Component
M o l
%
43 Components – simplify for
simulation
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 26
Pseudo-Components
C7 plus Components
0
1
2
34
5
6
7
8
C 7
C 9
C 1 1
C 1 3
C 1 5
C 1 7
C 1 9
C 2 1
C 2 3
C 2 5
C 2 7
C 2 9
C 3 1
C 3 3
C 3 5
B e n z e n
e
E t h y l b
e n z e
n e
Component
M o l %
Pseudo Component Simplification
02468
10121416
P s e u d
o C 7
P s e u d
o C 8
P s e u d
o C 9
P s e u d
o C 1 3
P s e u d
o C 1 7
P s e u d
o C 1 9
P s e u d
o C 2 3
P s e u d
o C 2 7
P s e u d
o C 3 1
P s e u d
o C 3 7
P s e u d
o C 4 5
P s e u d
o C 6 2
M o l %
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 27
Fluid Modelling - Summary
• The choice of thermodynamic model is critical when evaluating phase behaviour
• Fluid phase behaviour and properties can be predicted using a either a‘Black Oil’ model
or a ‘Compositional’ model. Compositional models are now custom and practice.
• Black Oil models are generally used for multiphase modelling in flowlines and
wellbores. They predict phase behaviour and bulk properties for varying conditions byrelating the fluid volumetric properties at the surface (e.g. Bo, GOR, Oil Gravity and Gas
Gravity) to the down-hole conditions. No detailed knowledge of fluid composition is
required. Black oil models are not recommended for preparing heat and mass balances.
With the development of equations of state, black oil models are now seldom used
• Compositional models are used for rigorous heat and mass balance work. They use a
property package and a composition specified in terms of individual components which
are available in the package database or ‘component library’. If required, pseudo-
components can be specified for non-library components or for boiling point cuts. Binary
interaction parameters for components pairs can be used to give improved VLE property
predictions.
• Modern simulation packages generally have several property packages available for
selection including both equations of state and activity models.
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 28
Property Package Selection
• Selection of the property package must take into account the components and the
operating conditions
• For most purposes, Peng Robinson (PR), Soave-Redlich-Kwong (SRK), or a modified
version of these methods, will provide sufficiently accurate modelling for oil and gas
field applications.• The PR and SRK equations of state give accurate modelling for systems containing up
to 5% N2, CO2 or H2S. For systems with greater than 5% N2, CO2 or H2S these
equations of state are still recommended if the system does not include free water. It
may be advisable, however, to utilise user defined binary interaction parameters if
available. This will depend upon the simulation/flash package being used and the quality
of the data in its components library. It is recommended to consult the user guide (or
help desk) for the particular package being used.
• Polar compounds can be problematic for EOS methods - methanol, glycol and water -
empirical correction factors are often required to improve accuracy. Most simulation
packages offer specialised property packages to handle such systems which utilise a
combination of an EOS to predict vapour phase fugacity coefficients and an activity
coefficient model for the liquid phase.
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 29
Equation of State - Fugacity
• Fugacity is a conceptual term which is related to Gibb’s free energy, temperature,
volume and pressure, the units of fugacity are the same as pressure
• Fugacity is a measure of the tendency of a substance to prefer one phase (liquid, solid,
gas) over another. At a fixed temperature and pressure, water (for example) will have a
different calculated fugacity for each phase. The phase with the lowest fugacity willthermodynamically be the most favorable: the one that minimizes Gibbs free energy.
Fugacity, therefore, is a useful engineering tool for predicting the phase state of multi-
component mixtures at various temperatures and pressures without doing the actual lab
test. And besides predicting the preferred solid, liquid, or vapor phase, fugacity also
applies to solid-solution equilibria.
• For equilibrium liquid and gas phase fugacities are equal.
• Although fugacity has the same units as pressure it will only be equal to pressure under
ideal conditions
• Calculating fugacity is an essential element of Equations of State
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 30
Equilibrium Ratios
• A simple fugacity model utilises a vapour-liquid equilibrium constant (K) . K is defined
as the mole fraction of any component in the vapour phase divided by the mole fraction
of the same component in the liquid phase
K i = yi / xi
• An equation of state can be used to calculate accurate K values when the vapour and
liquid phases co-exist in equilibrium
• For equilibrium, the fugacity of a component in the vapour phase must be equal to the
fugacity of the component in the liquid phase, when this criterion is met:
K i = fiL / fiv
• Use of Equilibrium Ratios is key to the preparation of oil and gas processing flow
calculations and heat and mass balances
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 31
Application of K Values
• The basic application of K values is the calculation of dew points, bubble points and the
vapour-liquid behaviour inside the phase envelope (flash calculations)
• Bubble Point Calculation
S K i xi = S yi = 1.0
– Assume a temperature for the known pressure (or assume pressure if temperature is known) – Find K i at pressure and temperature known and assumed
– Multiply K i by the corresponding xi (eqn. 1)
– If summation of values is 1.0, then pressure and temperature is correct, if not repeat until S K i xi
= 1.0, within accuracy limits
• Dew Point Calculation
S (yi/K i) = S xi = 1.0 – The steps involved in a dew point calculation are the same as for the bubble point calculation, using eqn. 2 in
place of eqn. 1
• A dew point calculation is less exact than a bubble point calculation, especially for lean
gases containing a small amount of heavy ends
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 32
Flash Calculation
• Flash calculations determine the amount of vapour and liquid in a two phase
system
– F = V + L (Overall)
– F zi = V yi + L xi (For each component)
– Where• F = mols of total feed
• V = mols of gas leaving system for F mols of feed
• L = mols of liquid leaving system for F mols of feed
• zi = mols of component ‘i’ in the feed stream per mol total feed
• yi = mol fraction of component ‘i’ in the gas stream (V)
• xi
= mol fraction of component ‘i’ in the liquid stream (L)
– Let F = 1 and substitute yi = K i xi , then
and)( i
ii
VK L
z x
)(i
ii
K
V L
z y
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 33
Flash Calculation Procedure
– For a specific pressure and temperature K i can bedetermined
– Flash calculation solution is iterative
– Guess V and calculate L
– Calculate all yi all xi
– Test Sxi or Syi = 1.0 then a solution has been obtained
– If not re-initialse L or V
0.1)(
i
i
i
VK L
z x 0.1
)(
i
i
i
K
V L
z y
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 34
K Values
K values can be estimated from the following – (Wilson, A
Modified Redlich-Kwong EOS, AICHE National Meeting.
P - absolute pressurePci – component i critical pressure in same units as P
T – absolute temperature
Tr – reduced temperature in same units as T
wi - Acentric factor component i (discussed later)
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Upstream Process Engineering Course Prepared by Genesis Oil and Gas Consultants Ltd Hydrocarbon Phase Behaviour 35
K Value Charts
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Equations of State
In physics and thermodynamics, an equation of state is a relation between state variables. More specifically, an equation of state is a
thermodynamic equation describing the state of matter under a given set of physical conditions.
In 1873, J. D. van der Waals introduced the first equation of state derived by the assumption of a finite volume occupied by the constituent
molecules. His new formula revolutionized the study of equations of state, and was most famously continued via the Redlich – Kwong equation of
state and the Soave modification of Redlich-Kwong.
The Van der Waals equation of state may be written:
where Vm is molar volume, and a and b are substance-specific constants. They can be calculated from the critical properties pc,Tc and Vc (noting
that Vc is the molar volume at the critical point) as:
Also written as:
Proposed in 1873, the van der Waals equation of state was one of the first to perform markedly better than the ideal gas law. In this landmark
equation a is called the attraction parameter and b the repulsion parameter or the effective molecular volume. While the equation is definitely
superior to the ideal gas law and does predict the formation of a liquid phase, the agreement with experimental data is limited for conditions
where the liquid forms. While the van der Waals equation is commonly referenced in text-books and papers for historical reasons, it is now
obsolete. Other modern equations of only slightly greater complexity are much more accurate.
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Soave-Redlich-Kwong
• SRK is robust and well proven, ideal for use in oil & gas and refinery
applications. It is suitable for cryogenic conditions and pressures up to 300
bara.
• For the highest accuracy binary interaction parameters for component pairs
should be used. Correlations can be used to generate binary interaction parameters for pseudo-components.
• Results for mixtures of hydrogen and hydrocarbons are good; those for
aromatics less so but these can be improved using appropriate binary
interaction parameters.
• Can handle up to 25% H2S with acceptable degree of accuracy if binaryinteraction parameters are used.
• Generally not considered suitable where accuracy is required for systems with
polar compounds.
• Liquid compressibility predictions are accurate enough for fugacity
calculations but not for accurate liquid densities (can be 10 - 20% low).
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Soave-Redlich-Kwong
• Phase behaviour in the critical region can be predicted although calculations
are somewhat unstable at the critical point itself.
• SRK is not considered to be accurate for heavy end VLE prediction and
vacuum systems
• The variables ‘a’ and ‘b’ are derived from the Van-der-Waals equation, which
was based on a model where molecules were represented by hard spheres
which behaved in a classical and predictable fashion
• The parameter ‘b’ represents the hard-sphere volume of the molecules
• The parameter ‘a’ represents the intermolecular attraction
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The temperature dependant term a/T of the
Redlich-Kwong equation was replaced by a
function a(T, w) involving the temperature
and acentric factor by Soave
Fugacity Coefficient
ln f z - 1 - ln (z - B) - A/B ln (1 + B/z)
Mixtures aa = SSyiy j(aa)ij
b = Syi bi
A = SSyiy jAij
B = SyiBi
(aa)ij = (1 - k ij)[(aai)(aa) j]0.5
Soave-Redlich-Kwong
Standard Form
Polynomial Compressibility Form
z3 - z2 + (A - B- B2) z - AB = 0
Parameters
A = (aaP)/(R 2T2) = 0.42748aPr /Tr 2
B = (b P)/(RT) = 0.08664Pr /Tr
a = WaR 2Tc2/Pc = 0.42748R 2Tc
2/Pc
b = W bRTc/Pc = 0.08664RTc/Pc
b)V(V
aα
b)(V
RTP
2
5.0
172.0574.148.0
where)1(1α
w w
m
Tr m
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Cubic Equation
Z
Gas root
Liquid root
z3 - z2 + (A - B- B2) z - AB
System in
equilibrium if
fugacities at roots
are equal.
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Peng-Robinson
• Peng-Robinson is robust and well proven, ideal for use in oil & gas and
refinery applications. It is generally similar in performance and applicability to
SRK except as follows.
• It is generally accurate over a wider range of conditions.
• It is more accurate around the critical point.
• Liquid phase density prediction is more accurate.
• Improved PR packages can be used for heavy end VLE prediction and vacuum
distillation
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Peng-Robinson
Validity is roughly the same as S-
R-K, but is more accurate near the
critical point
Mixtures
aa = SSyiy j (aa)ij
b = Syi bi
(aa)ij = (1 - k ij) [(aa )i(aa ) j]0.5
A = SSyiy jAij
B = SyiB
iAij = (1 - k ij)(AiA j)
0.5
k ii = 0
Standard Form
Polynomial Compressibility Form
z3 - (1- B)z2 + (A - 3B2 - 2B)z - (AB - B2 - B3) = 0
Parameters
a= 0.45724 [(R 2Tc2)/Pc]
b = 0.07780 RTc/Pc
a [1 (0.37464 + 1.54226w - 0.26992w2) (1-Tr 0.5)]2
A = (aaP)/(R 2T2) = 0.45724aPr /Tr 2
B = (bP)/(RT) = 0.07780Pr /Tr
Fugacity Coefficient
ln f = z - 1 - ln (z - B) - [A/2(2B)0.5] ln [(z+2.414B)/(z-0.414B)]
) b-2bVV(
aα
b)(V
RTP
22
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Notation
• Definitions
a Attraction parameter
b Residual volume parameter
W SRK Parameter (numerical co-
efficient)
A Derived parameter B Derived parameter
z = PV/RT, compressibility factor
ai = ^ f i/ f 0 , activity of a species in a
mixture^ f i Partial fugacity of species i
k ij
Binary Interaction Parameter
(values of kij for around 100 pairs are
available)
aij Cross-parameter
• Greek Symbols
a Parameter
aij Volatility of species i relative to that of
species j
f = f i/P, Fugacity Coefficient
w Acentric Factor
r Molar Density
g ai/xi, Activity Coefficient of species i
• Subscripts
c Value of any variable at the critical
point
r Reduced Value pc Pseudo-critical value
• Superscripts
0 property of a standard state, e.g. f 0
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Acentric Factor
• The acentric factor describes the
change in the intermolecular attraction
component, a with temperature
• In the S-R-K equation of state, the
variation of factor a of the component
i with temperature is given by:
– ai = aci ai (T)
– where aci is the value of a derived
from the critical values
– aci = 0.427 R 2 Tci2 / Pci
– and ai (T) = (1+mi (1-Tri0.5))2
– The variable mi is a function of theacentric factor wi
– wi = 0.480 + 1.574 wi - 0.172 wi2
Com pound A c entric Fac tor
N itrogen 0.039
Carbon D iox ide 0.239
M ethane 0.011
E thane 0.099
B utane 0.119
Hex ane 0.299
O c tane 0.398
Dec ane 0.489
• The table shows typical acentric factors,the value increases with the size of the
molecule and polarity
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Binary Interaction Coefficients
• The cubic Equations of State were originally developed for pure substances
and have been subsequently adapted for mixtures
• This adaptation introduced a measure of the polar and other interactions
between pairs of dissimilar molecules
• For a component in a mixture:
– where zi and z j are mol fractions of components i and j respectfully
– and
– where k ij is the binary interaction coefficient
• Binary Interaction Coefficients represent a flexible way of modelling the ideal
EOS to match the non-ideal reality of many mixtures
ij ji ji a z za
)1()( ij jiij k aaa
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ExampleCalculation of the saturation pressure of n-Pentane at 100 ºC using an Equation of State
• At equilibrium, the fugacity of the liquid and vapour phases is equal
• The experimentally determined vapour pressure is 5.86 atm
• Critical properties of n-Pentane are:
Tc = 469.7 K, Pc = 33.25 atm, w = 0.251
• Using S-R-K :
• Calculate A & B
• Substitute values of A & B into the polynomial compressibility equation:
• Solve to find Z for liquid and vapour phases
• Calculate the fugacity of the vapour and liquid phases:
ln f z - 1 - ln (z - B) - A/B ln (1 + B/z)
fV f V/PV and fL f L/PV
• At equilibrium, f V = f L therefore fV = fL
2172.0574.148.0where)1(1 w w a mTr m
) ) ) 094.16.469
3731251.0172.0251.0574.148.01 2
a
) ) P T Tc P P A c 0224.0//42747.02 a
) ) P T Tc P P B c 0033.0//08664.0
0)( 223 AB Z B B A Z Z
P V a p ou r L iqu id V a po u r L iq u id R at io
4 0 .8 9 43 0 .0 1 84 3 .66 5 5 .19 6
5 0 .8 6 47 0 .0 2 29 4 .4 77 4 5 .22 8 7 0 .8 5 6 3
6 0 .8 3 1 0 .0 2 75 5 .2 48 2 5 .25 1 2 0 .9 9 9 4
7 0 .7 9 55 0 .0 3 21 5 .9 77 5 5 .28 0 7 1 .1 3 2
z f
By interpolation of the above table, the predicted vapour pressure is 6.01 atm
-0.1
-0.05
0
0.05
0.1
0.15
0 . 0
1
0 . 0
7
0 . 1
3
0 . 1
9
0 . 2
5
0 . 3
1
0 . 3
7
0 . 4
3
0 . 4
9
0 . 5
5
0 . 6
1
0 . 6
7
0 . 7
3
0 . 7
9
0 . 8
5
0 . 9
1
0 . 9
7
Z
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Example
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E.o.S Comparison: Peng Robinson v
Zudkevitch Joffee
9 barg
54 C
1.1 barg
53 C
50 mbd stabilised crude
PR ZJ
MW 18.53 18.43
Q (mmscfd) 15.0 14.6
C1 (mol%) 88.57 88.94
C2 (mol%) 7.17 6.87
C3 (mol%) 0.13 0.12
PR ZJ
MW 22.09 21.54Q (mmscfd) 0.9 1.2
C1 (mol%) 74.75 76.34
C2 (mol%) 16.33 15.80
C3 (mol%) 0.45 0.41Reservoir
Fluids
GOR PR ZJ
1st Stage (scf/bbl) 299 292
2nd Stage (scf/bbl) 19 24
Overall (scf/bbl) 318 316
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Phase Behaviour – Pipeline Gas
Components Gas A
C1 86.5473
C2 7.30256
C3 2.01471
IC4 0.475166
NC4 0.566198
IC5 0.213075
NC5 0.230081
C6 0.120042
C7 0.100035
C8 0.060021
N2 0.880308
CO2 1.49052
SUM 100
Molar weight 19.1
HC dewpoint 23.5 °C @(Cricondentherm) 40.1 bara
HC dewpoint 101.2 bara
(Cricondenbar) @ -16.4
°C
Gullfaks Fluid from SOW
0
10
20
30
40
50
60
70
80
90
100
110
120
130
-40 -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35
Temperature (C)
P r e s s u r e
( b a r ) BWRS
PR
RKS
Cricondentherm
Cricondenbar
Application of BWRS erroneous – Pipeline two-phase
Minimum
Pipeline
Operating
Pressure
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High Pressure Compressor Simulation
• At pressure above ca 250 Bar P-R and SRK
may not accurately represent the system
thermodynamics• Other EOS options should be considered –
– Lee Kessler Plocker - LKP
– P-R with Lee Kessler Plocker Enthalpy – PR-LK
– Corresponding States Method - Infochem
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Limitations of Phase Behaviour Prediction
• All phase behaviour methods assume an equilibrium is reached, this is an idealised
situation and in reality equilibrium is not always achieved.
• EOS can not be used to predict water/hydrocarbon separation
• Phase behaviour prediction deals with mass transfer effects and therefore does not take
into account bulk fluid flow. In practice there is almost inevitably some bulk transfer of fluids between the phases: emulsions may be formed due to high shear rates across
control valves and the addition of corrosion inhibitor which holds water in the oil phase,
oil will carry over into the oil phase, liquid droplets carry over into the gas phase.
• Prediction methods are generally only valid over a particular range of conditions and for
particular components.
• EOS do not accurately predict phase behaviour at the critical point.
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Polar Fluids
• For the majority of calculations, simple treatment of hydrocarbon/water behaviour is
sufficient, however for systems containing large amounts of polar fluids (water,
methanol or glycol) more rigorous treatment is required.
• For polar systems it is generally recommended to use an activity model to predict liquid
phase behaviour. Although work is being undertaken to extent the validity of EOS
methods into the liquid phase (e.g. Peng-Robinson-Stryjek-Vera [PRSV])
• For water/oil mixtures the accuracy of calculations depends on the availability and
accuracy of the binary interaction data available, at low pressures NRTL will give
accurate results providing all the data required is available.
• For methanol/water mixtures, NRTL provides accurate results at pressures below 1000
kPa.
• Many of the commercially available simulators provide methods for such systems.
Expert advice is usually available from their technical support.
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Property Packages in Hysys
• Hysys, or any other process simulation package (i.e.
Unisim, ProII), requires a thermodynamic property
package to be specified before a simulation can be run
• The screenshot to the left shows all the property
packages available in Hysys
• The most appropriate thermodynamic package for agiven simulation is the one that gives the closest match
to reality. This is a function of the components included
in the model
• It is possible to use multiple property packages within a
model. This allows better modelling of different
systems inside a single model (i.e. hydrocarbon
processing and glycol regeneration system)
• If using multiple property packages care must be taken
to ensure consistency of the components included in
each different basis
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Equations of State (EOS)
• All the EOS models within Hysys are based on three EOS :
• Peng Robinson (PR)
• Soave-Redlich-Kwong (SRK)
• Twu-Sim-Tassone (TST)
• Within Hysys PR has been enhanced for low temperature cryogenic systems, high
pressure – high temperature systems and can handle heavy oils, glycols, He, H2, N2,
CO2, H2S, CO2 and methanol. If there are significant quantities of these components an
alternative model should be considered
• PRSV is a further improvement on PR as it can handle moderately non-ideal systems.
However the compromise is increased computational time
• SRK provides comparable results to PR in many cases. However, limitations include
• SRK not reliable for non-ideal systems and should not be used if methanol or
glycol are present
• SRK is applicable over a smaller temperature and pressure range compared to PR
• The Hysys manual presents details of each EOS model available
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Property Package Selection
• For a conventional oil, gas and petrochemical application the Peng Robinson (PR)
property package is generally preferred in Hysys
• Within Hysys the available property packages are grouped under the types detailed in the
table below
• Non-ideal components include alcohols, acids, glycols and other non-ideal or polar
chemicals. The more non-ideal components present, the less ideal the mixture
• There is a trade off between accuracy of property package and simulation run-time
Property Package Type Typical Application
Equations of State, Ideal hydrocarbon systems
Activity Models, Polar or non-ideal systems
Chao Seader Models Systems containing mainly liquid or vapour H2O or high H2 contents
Vapour Pressure ModelsHeavy hydrocarbon systems at low pressures (increasing light hydrocarbons
results in decreasing accuracy)
Miscellaneous Type Acid gas systems, steam systems, non-volatile oils, aqueous electrolyte
systems
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Property Package Selection
• The table below summarises recommended property packages for various systems commonly encountered in oil and
gas/ petrochemical simulations
• If unsure, the Hysys manual should be consulted prior to selecting a property package for a simulation
• Where4 appropriate, validation of simulation against real data should be carried out to check that the most
appropriate property package has been selected
Type of System Recommended Property Methods
TEG Dehydration PR
Sour Water Sour PR
Cryogenic Gas Processing PR, PRSV, TST
Air Separation PR, PRSV, TST
Atm Crude Towers PR, PR Options, GS, TSTVacuum Towers
PR, PR Options, GS (<10 mm Hg), Braun K10,
Esso K, TST
Ethylene Towers Lee Kesler Plocker
High H2 Systems PR, ZJ, GS, TST
Reservoir Systems PR, PR Options, TST
Steam Systems Steam Package, CS or GS
Hydrate Inhibition PR
Chemical systems Activity Models, PRSV
HF Alkylation PRSV, NRTL*
TEG Dehydration with Aromatics PR*Hydrocarbon systems where H2O solubility in HC is
importantKabadi Danner
Systems with select gases and light hydrocarbons MBWR*Recommended that Hyprotech is consulted for details of accuracy
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Activity Models
Although equation of state models have proven to be very reliable in predicting properties of most
hydrocarbon based fluids over a large range of operating conditions, their application has been limited to
primarily non-polar or slightly polar components.
Polar or non-ideal chemical systems have traditionally been handled using dual model approaches. In
this approach, an equation of state is used for predicting the vapour fugacity coefficients (normally ideal
gas assumption or the Redlich Kwong, Peng-Robinson or SRK equations of state, although a Virialequation of state is available for specific applications) and an activity coefficient model is used for the
liquid phase. Although there is considerable research being conducted to extend equation of state
applications into the chemical arena (e.g., the PRSV equation), the state of the art of property predictions
for chemical systems is still governed mainly by Activity Models.
Activity Models are much more empirical in nature when compared to the property predictions(equations of state) typically used in the hydrocarbon industry. For example, they cannot be used as
reliably as the equations of state for generalised application or extrapolating into untested operating
conditions. Their tuning parameters should be fitted against a representative sample of experimental data
and their application should be limited to moderate pressures. Consequently, more caution should be
exercised when selecting these models for your simulation.
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Acivity Models
The phase separation or equilibrium ratio Ki for component i, defined in terms of the vapour phase fugacity
coefficient and the liquid phase activity coefficient is calculated from the following expression:
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Activity Models
• Activity Models are suitable for highly non-ideal systems and are empirical in nature.
They are based on experimental data which is generated over a specific range and
therefore can not be used reliably for generalised applications
• The best results are produced when an Activity Model is applied in the operating region
for which the interaction parameters apply. Caution should therefore be taken when
selecting an Activity Model for a simulation• The table below shows when the different Activity Models can be used
Application Margules van Laar Wilson NRTL UNIQUAC
Binary Systems A A A A A
Multicomponent Systems LA LA A A A
Azeotropic Systems A A A A A
Liquid-Liquid Equilibria A A N/A A ADilute Systems ? ? A A A
Self-Associating Systems ? ? A A A
Polymers N/A N/A N/A N/A A
Extrapolation ? ? G G G
A = Applicable; N/A = Not Applicable;? = Questionable; G = Good; LA = Limited Application
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NRTL
The NRTL (Non-Random-Two-Liquid)
equation was proposed by Renon and Prausnitz
in 1968, is an extension of the original Wilson
equation. It uses statistical mechanics and the
liquid cell theory to represent the liquid
structure.
The NRTL equation in contains five adjustable
parameters (temperature dependent and
independent) for fitting per binary pair.
The NRTL equation in HYSYS has the form
shown.
The five adjustable parameters for the NRTLequation in HYSYS are the aij, aji, bij, bji, and
aij terms. The equation will use parameter
values stored in HYSYS or any user supplied
value for further fitting the equation to a given
set of data.
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INFOCHEM
Infochem has two main software packages, Multiflash and FloWax.
Multiflash, a multiphase equilibrium package, is supplied as an interactive
Windows program and as a DLL for linking to other Windows software or
packages written in Visual Basic, Java or C++. Optional add-on modules areavailable which allow you to put together the package most appropriate to your
needs. FloWax, also supplied as an interactive Windows program, predicts wax
deposition in pipelines.
The standard Multiflash package includes equation of state, activity coefficientand transport property models, a full range of flashes, a phase boundary tracer, a
246 pure component databank, petroleum fraction correlations and a petroleum
fluid characterisation facility.
SPR CRUDE OIL COMPREHENSIVE ANALYSIS
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Crude Assays
• An Assay is a detailed fluid analysis of
crude oil, sampled under strictly
controlled conditions
• Assay reports include the quantities and properties of distillates used for crude
valuation purposes, the quantity of
waxes, heavy metals, salt and sulphur
and physical properties of the fluid such
as viscosity
• Assay samples are taken monthly for the purposes of crude valuation purposes
and annually for crudes being exported
via a pipeline shared with other fields
Sample ID BAYOU CHOCTAW SWEET Date of Assay 18/09/2000
Crude
Specific Gravity, 60/60° F 0.8447 Ni, ppm 3.50 RVP, psi @ 100° F 4.62
API Gravity 36.0 V, ppm 5.49 Acid number, mg KOH/g 0.084
Sulfur, Wt. % 0.36 Fe, ppm 0.844 Mercaptan Sulfur, ppm 7.021
Nitrogen, Wt. % 0.114 H2S Sulfur, ppm 0
Micro Car. Res., W t. % 2.22 O rg. Cl, ppm 0.3 Viscosity: 77° F 6.874 cStPour Point, °F 31 UOP "K" 11.94 100° F 4.623 cSt
Fraction Gas 1 2 3 4 5 6 Residuum Residuum
C2 - C5 - 175° - 250° - 375° - 530° - 650° -
Cut Temp. C4 175° F 250° F 375° F 530° F 650° F 1050° F 650° F+ 1050° F+
Vol. % 1.7 7.3 8.1 14.2 16.3 10.0 31.8 42.4 10.7
Vol. Sum % 1.7 9.0 17.1 31.3 47.6 57.6 89.3 100.0 100.0
Wt. % 1.2 5.8 7.1 13.1 15.9 10.1 34.3 47.0 12.7
Wt. Sum % 1.2 7.0 14.1 27.1 43.0 53.1 87.4 100.0 100.0
Specific Gravity, 60/60° F 0.6730 0.7396 0.7763 0.8240 0.8526 0.9116 0.9349 1.004
API Gravity 78.8 59.8 50.8 40.2 34.5 23.7 19.9 9.4
Sulfur, Wt. % 0.0043 0.0040 0.0123 0.07 0.21 0.57 0.69 1.04
Molecular Weight 97 111 136 184 246 425
Hydrogen, Wt. % 15.89 14.65 na 12.99 10.61
Mercaptan Sulfur, ppm 14.6 10.1 22.5 17.4
H2S Sulfur, ppm 0.03 0.8 0.7 0.02
Organic Cl, ppm 2.1 0.5 0.5 0.6
Research Octane Number* 68.4 61.1 42.3
Motor Octane Number* 66.5 58.6 40.0
Flash Point, ° F 77 171 246 303
Aniline Point, ° F 122.4 144.1 164.3 193.3
Acid Number, mg KOH/g 0.03 0.10
Cetane Index 47.1 53.2
Diesel Index 62.2 58.0 56.6
Naphthalenes, Vol. % 4.42 8.20
Smoke point, mm 20.3 16.8
Nitrogen, Wt. % 0.0015 0.006 0.108 0.240 0.603
Viscosity, cSt 77° F 2.473
100° F 1.951 4.795
130° F 3.312 37.03 95.3
180° F 14.22 28.28 5671
210° F 1722
275° F 249.3
Freezing Point, °F -28.54
Cloud Point, °F 24.0 106
Pour Point, °F 19.9 102 75
Ni, ppm 7.539 26.2
V, ppm 11.81 41.0
Fe, ppm 3.856 13.87
Micro Car. Res., Wt. % 5.07 18.17
* = calculated from gas chromatographic data
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Partial Pressure
Partial pressure is a property often used in Process Engineering .
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which
the gas would have if it alone occupied the volume.
In chemistry, the partial pressure of a gas in a mixture of gases is defined as above. The
partial pressure of a gas dissolved in a liquid is the partial pressure of that gas which
would be generated in a gas phase in equilibrium with the liquid at the same
temperature. The partial pressure of a gas is a measure of thermodynamic activity of
the gas's molecules. Gases will always flow from a region of higher partial pressure to
one of lower pressure; the larger this difference, the faster the flow.
Vapor pressure is the pressure of a vapor in equilibrium with its non-vapor phases (i.e.,
liquid or solid). Most often the term is used to describe a liquid's tendency to evaporate.
It is a measure of the tendency of molecules and atoms to escape from a liquid or a
solid. A liquid's boiling point corresponds to the point where its vapor pressure is equal
to the surrounding atmospheric pressure.
Gases dissolve, diffuse, and react according to their partial pressures, and not
necessarily according to their concentrations in a gas mixture.
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