Equations for the Soave-Redlich-Kwong equation of state for multi-component systems

The multi-component version of the Soave-Redlich-Kwong (SRK) equation of state is the same as for the single-component version:

The parameters a and b, however, are obtained through mixing rules. With i and j denoting the component indices, we then have:

where

xi is the mole fraction of component i

Tc,i is the critical temperature of component i

Pc,i is the critical pressure of component i

kij is the binary interaction parameter for components i and j

The combination formula for calculation of a(T) with the introduction of the interaction parameter kij is intended to improve predictions for the mixture properties.

The standard component libraries provide binary interaction parameters kij for many pairs of library components. The software also contains correlations for values for systems with hydrocarbons and N2, O2, H2, H2S, CO2, mercaptans, and other sulfur compounds. Some kij values are also provided for hydrocarbon splits such as ethane-ethylene and propane-propylene. SRK approximates hydrogen phase behavior by modifying the acentric factor.

The interaction parameter kij is determined from experimental vapor-liquid equilibrium (VLE) data for the binary system with components i and j such that the phase equilibria behavior is matched.

The remaining equations depend on your Alpha Selection for your Fluid Type in AVEVA Process Simulation. You can find the Alpha Selection list on the Fluid Editor, in the Equilibrium Options section. It appears only when you select an equation of state in the System list in the System section.

If you select Acentric Factor Formulation in the Alpha Selection list, the remaining equations follow the same formulation as the SRK equation of state:

where

wi is the acentric factor for component i

If you select Alpha Databanks in the Alpha Selection list, the remaining equations follow the SRK formulation for ai, but the alpha formulation that the software uses is different for each component and depends on the alpha data in the data banks that you add to the Alpha Data Banks box.

where

ai(Tr,i,wi) depends on the data specified in the selected ALPHA data banks

If there is any alpha data missing from the selected ALPHA data banks, the software calculates the alpha value based on the acentric factor formulation.

Warning: If you use a custom ALPHA data bank, you should also use a binary interaction data bank that contains adjusted binary interaction parameters that correspond to the alpha data in your custom ALPHA data bank.

For SRK, the default Alpha Selection is Acentric Factor Formulation. We recommend that you use the default selection.

Regardless of the selection in the Alpha Selection list, this equation of state uses a specialized alpha formulation for water. See Special adjustments for water: Alpha formulation for the equations of state for more information.