A. Light hydrocarbons

There are reasonably reliable charts available from which equilibrium constants for light hydrocarbons can be obtained. Perhaps the best, and most widely used, are those published by the Gas Processors Association (GPSA, 1972). Values from the charts, if properly used, will agree closely with the values obtained from a good equation of state. In addition, they are easily and quickly used and ideal for hand calculations.

If more accurate equilibrium constants are desired, or if equilibrium conditions are being predicted by computer, an equation of state is used for estimating the equilibrium constant values. The most widely used equations of state for light hydrocarbons are the Soave version of the Redlich-Kwong (SRK) (Redlich and Kwong, 1949; Soave, 1972; Erbar, 1973), the Starling version of the Benedict-Webb-Rubin (BWR) (Starling and Han, 1972), and the Peng-Robinson (PR) (Peng and Robinson, 1976). Each has advantages and disadvantages, proponents and opponents. More importantly, different computer solutions for each equation may yield equilibrium constants that are significantly different. Erbar and Maddox (1977) have made comparison calculations for simple equilibrium flash calculations that differed by as much as 20%. The same equation of state was used for all calculations the programming of the solution was different.

Equilibrium coefficients are calculated from the equation of state using one of the Equation 503.8, Equation 503.9, Equation 503.10 and Equation 503.11. All thermodynamic quantities may be related to each other by mathematical manipulation. The fugacity coefficient is related to pressure, volume, temperature, and compressibility factor by

\[\label{eq1} \ln \gamma_{g_i}=\dfrac1{\tilde{R}T}\int_{v}^{\infty}\left[\left(\dfrac{\alpha P}{\alpha n_i}\right)_{\!T,V, n_j}\!\!-\dfrac{\tilde{R}T}{V}\right]dV-\ln Z \tag{1}\]