Getting Done with Input ofCharacterized Compositions

Getting Done with Input of CharacterizedCompositionsIt is generally to be recommended to enter fluid compositions into PVTsim as either Plus or No Pluscompositions for which types of compositions PVTsim will carry out a fluid characterization and establishall the component parameters needed in the subsequent PVT simulations.

If the fluid composition originates from another simulation program it will already be characterized. Thecomposition data may for example consist of

Mol % Tc Pc Mw Cpen

Comp 1 - - - - - -

Comp 2 - - - - - -

.

.

.

This type of data may be entered into PVTsim by changing the Fluid type to Characterized.

When entering a characterized composition into PVTsim certain rules must be observed in order not to getuseless or inconsistent results.

Choice of EOS

Make sure there is consistency between the EOS currently selected in PVTsim and the EOS used when thefluid was generated. PVTsim supports the following EOS's, all of which may be used with or withoutPeneloux volume correction.

SRK (Soave-Redlich-Kwong).

PR (Peng-Robinson 1976).

PR78 (Peng-Robinson 1978).

These EOS’s are outlined in the sections SRK Equation and PR/PR-78 Equation of the PVTsim MethodDocumentation.

Getting the most accurate Densities

If density is of importance, the EOS chosen must be used with Peneloux volume correction. The Penelouxvolume correction is outlined in the sections SRK with Volume Correction and PR/PR-78 with VolumeCorrection of the PVTsim Method Documentation. The volume correction may either be assumed to beconstant or a linear function of temperature.

To take advantage of the volume correction, the following parameters are required for allcomponents:

EOS variation Parameters required

SRK Peneloux Cpen SRKPR Peneloux Cpen PRPR78 Peneloux Cpen PRSRK Peneloux (T) Cpen SRK and CpenT SRKPR Peneloux (T) Cpen PR and CpenT PRPR78 Peneloux (T) Cpen PR and CpenT PR

If CpenT is not defined (input field is blank) a value of 0.0 will be used.

Input of characterized pseudo-components

To make it possible to enter a characterized composition containing pseudo-components, a number ofstandard pseudo-components are made available in the component database. With the Fluid Type set toCharacterized, press the <Add Comps> button. In the field Number of Pseudos enter the number of pseudosto add, and pseudo-components of the names CHCmp_1, CHCmp_2 and so on are added. Fill in thecomponent properties for the actual pseudo-components. Be aware that the default molecular weights of100 need to be changed.

Completion of Component Properties

To support the whole variety of simulation options in PVTsim, more component properties will generallybe needed than that of the characterized fluid in the above example. To calculate for example enthalpy (H)and heat capacity (CP), 4 ideal gas CP coefficients are needed for each component. For defined componentsas for example methane (C1), the needed properties are drawn automatically from the PVTsim purecomponent database. This cannot be done for pseudo-components, for which missing component propertiesmay instead be estimated from other component properties, using a so-called Complete option. The

Complete facility is described in the section Missing Properties of the PVTsim Method Documentation. Thesection Component Properties of the PVTsim Method Documentation contains references to sectionsdescribing the correlations used to estimate the component properties.

Completion of properties will not modify already entered component properties but only fill in gaps. Thenumber of component properties that can be estimated depends on which component properties have beenentered. Liquid densities are for example needed to estimate CP coefficients and Peneloux parameters, whyit is highly recommended to enter liquid densities if available. Peneloux parameters may be used as inputinstead of densities, which enables the Complete option to estimate densities from Peneloux parameters.

Getting Consistency

To get consistent amounts and compositions the following must be the same

EOS

Critical temperature (Tc)

Critical temperature (Pc)

Acentric factor ()

Omega A (a)

Omega B (b)

Mixing rule and interaction parameters.

To get consistent phase properties the parameters in the below tables will also have to be identical.

Parameter DensityLBC

viscosityCSP

viscosityThermal

conductivitySurfacetension

Cpen x x x

CpenT x x x

Vcrit x

CP coefficients

LBC coefficients x

Mw (number average) x x x x

Mw (weight average) x

Parachor x

Re-characterization as an Alternative

An alternative to entering the fluid as a characterized composition is to back-calculate the composition to aplus fraction composition and have PVTsim re-characterize the fluid. Formulas for back-calculatingmolecular weight and density are.

Mw(n) Plus = (Mw(n)*Mol%)/ (Mol%).

Volume Plus = ((Mw(n)/Density)*Mol%)/ (Mol%).

Density Plus = Mw(n) Plus/ Volume Plus.

The summation should run from the carbon number where the plus fraction shall start and on.

It may not produce results that are consistent with the characterized composition, but is often an attractivealternative if consistency with other programs is not a major issue.