Calculating Mixture Critical Temperature and Pressure using Equation of State?

Aug 2018
Hello Everyone,
I am trying to calculate critical Temperature and Pressure of binary and ternary mixtures using equation of state. In particular, I am using Peng-Robinson Equation of State. I combined this with Van-der Waals critical criterion to get my result, but am way off experimental data.

Vander Waals criteria states: dP/dv = 0 and d2P/dv2=0

My steps:
1. I solve the two partial derivative equations mentioned above to end up with a cubic equation in 'v' (molar volume). Solving this I get the critical molar volume
2. I put this back in either of the partial derivative conditions and calculate Temperature using iteration.
3. Now I know Tc and Vc. I put these values back in the equation of state to get my pressure.

My Tc values are still within range or close to experimental data but the Pc values are way off.

Attached Plot:
Attached plot is of a binary mixture of methane(C1) and propane(C3). Tc and Pc plots are attached vs mole fraction of C1


Sep 2014
Brasília, DF - Brazil
Can you describe in more details what you did?