Equations for the Peng-Robinson equation of state

The Peng-Robinson (PR) equation of state (EOS) is based on the cubic equation of state:

For the PR EOS, u = 2 and w = -1:

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 constant for components i and j

The combination formula for calculation of a(Tr) with the introduction of the term kij was intended to improve prediction of the mixture properties.

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 Soave-Redlich-Kwong (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 PR 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 the PR equation of state, the default Alpha Selection is Acentric Factor Formulation. We recommend that you use the default selection.

For both selections in the Alpha Section list, when wi > 0.39849, the software uses the following alpha formulation instead:

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.