PengRobinson EOS in FPROPS

This article is about planned development or proposed functionality. Comments welcome.

Work on this is went on as a part of GSoC 2010 (Project Ankit) and continues as part of GSoC 2011 (Richard Towers). See also FPROPS.

Comments and suggestions are welcome

Overview

The Peng-Robinson EOS is a cubic equation of state in that it contains volume terms to the third power. It is usually expressed to give Pressure in terms of Temperature and Molar Volume:

<math>

P =\frac{RT}{V_m-b}-\frac{a(T)}{V_m(V_m+b)+b(V_m-b)} </math> where

<math>\begin{align}

a(T)&=0.45724 \frac{R^2{T_c}^2}{P_c} \left(1+\kappa \left(1-\sqrt{\frac{T}{T_c}} \right) \right)^2 \\

\kappa&=0.37464+1.54226\omega - 0.26992\omega^2 \end{align} </math>

It is sometimes more convenient to express the equation as a cubic polynomial in terms of compressibility factor <math>Z=\frac{PV_m}{RT}</math>

<math>

\begin{align} Z^3+(-1-B)Z^2+(A-3B^2-2B)Z-AB+B^2+B^3=0 \end{align} </math> in which

<math>

\begin{align} A=\frac{aP}{(RT)^2} \\ B=\frac{bP}{RT} \end{align} </math>

Departure Functions

Departure functions represent the departure of the real properties from the ideal properties - i.e the properties of a fluid at zero pressure or infinite molar volume. The departure functions of the Peng-Robinson equation of state are as follows:

<math>

\begin{align} H_{m}-H_{m}^{\text{Ideal Gas}}=RT(Z-1)+\frac{T\left(\frac{da}{dT}\right)-a}{2\sqrt{2}b}\ln\left[\frac{Z+(1+\sqrt{2})B}{Z+(1-\sqrt{2})B}\right] \end{align} </math>

<math>

\begin{align} S_{m}-S_{m}^{\text{Ideal Gas}}=R\ln (Z-B)+\frac{\frac{da}{dT}}{2\sqrt{2}b}\ln\left[\frac{Z+(1+\sqrt{2})B}{Z+(1-\sqrt{2})B}\right] \end{align} </math> Clearly to evaluate these functions we need to be able to evaluate <math>\frac{da}{dT}</math>:

<math>

\begin{align} \frac{da}{dT}=-\frac{\left(0.45724T_{c}R^{2}\kappa\left(1-\sqrt{\frac{T}{T_{c}}}\right)\right)}{P_{c}\sqrt{\frac{T}{T_{c}}}} \end{align} </math>