Special adjustments for water: Alpha formulation for the equations of state

Typically, equations of state (EOSs) handle hydrocarbons well but struggle with polar components for water. Therefore, for systems that use an EOS, we modify the alpha formulation for water to better predict its activity coefficient.

We find that this modification results in more accurate predictions for the activity coefficient and, by extension, the vapor pressure for water.

Soave-Redlich-Kwong (SRK)

In the SRK equation of state, the standard alpha formulation is:

where

a is the alpha value that directly relates to the activity coefficient

w is the acentric factor

Tr is the reduced temperature

Instead of using the acentric factor, we use the following alpha formulation for water:

Peng-Robinson (PR)

In the PR equation of state, the standard alpha formulation is:

Instead of using the acentric factor, we use the following alpha formulation for water: