01b Hydrocarbon Phase Behaviour

7/28/2019 01b Hydrocarbon Phase Behaviour

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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




Phase Diagram - Water




Phase Envelope – multi-

component hydrocarbon

Cricondenbar




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




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




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





• 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





• 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.




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

http://images.google.com/imgres?imgurl=http://infosys.korea.ac.kr/research/kdb/hcprop/molimg/582.gif&imgrefurl=http://infosys.korea.ac.kr/research/kdb/hcprop/showprop.php%3Fcmpid%3D582&h=140&w=155&sz=1&hl=en&start=2&um=1&tbnid=wunPkGOwQHAtmM:&tbnh=88&tbnw=97&prev=/images%3Fq%3DCyclononane%26svnum%3D10%26um%3D1%26hl%3Den%26lr%3D%26sa%3DN%26as_qdr%3Dall




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.




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.




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.




Crude Oil Composition Schematic




HYSYS Component and EOS

Set Up




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




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


C o lour B ro wn

HYSYS is not

flawless




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


C o lour S traw




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




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




Test Separator and Burner




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.




• 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




• 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




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 %




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




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 %




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.




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.




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




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




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




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




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




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)




K Value Charts




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.




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).




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




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




Cubic Equation

Z

Gas root

Liquid root

z3 - z2 + (A - B- B2) z - AB

System in

equilibrium if

fugacities at roots

are equal.

http://en.wikipedia.org/wiki/File:Polynomialdeg3.svg
















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




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




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




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




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




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

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5

0 . 9

1

0 . 9

7

Z




Example




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




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




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




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.




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.




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




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





• 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





• 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




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.




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:




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




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.




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




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



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.

01b Hydrocarbon Phase Behaviour

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