Residual Functions for the SRK and PR Equations of State

SRK

PR

residual property

enthalpy

entropy

Gibbs free energy

vary P

comparison with Aspen Plus

pressure P

10

300

350

400

450

500

-1500

-1000

-500

0

temperature (K)

residual enthalpy (J/mol)

In the gas phase, deviation from ideal behavior is taken into account by introducing residual functions, of the form

M

R

=M-M

ig

. As pressure approaches zero, the gas behaves like an ideal gas, so that

M

R

=M-M

ig

→0

.

This Demonstration plots the gas-phase residual properties for the gas propylene versus temperature (in Kelvin) using either the Soave–Redlich–Kwong (SRK) or the Peng–Robinson (PR) equation of state (EoS) [1, 2]. Use the slider to set the value of the pressure

P

(in bar) and choose among three thermodynamic properties:

H

(enthalpy in J/mol),

S

(entropy in J/mol.K) or

G

(Gibbs free energy in J/mol). At

P=10bar

, we get an excellent agreement between the results of our calculations (red curve for the SRK EoS, blue for PR EoS) and their counterparts obtained with the Aspen Plus process simulator [3] (green squares).