K Values, VLE, and Flash Calculations

K Values, VLE, and Flash Calculations A measure of the distribution of a chemical species between liquid and vapor phases is the K value, defined as the equilibrium ratio  

It has no thermodynamic content, but may make for computational convenience through elimination of one set of mole fractions in favor of the other. It does characterize lightness of a constituent species. A light species, with K 1, tends to concentrate in the vapor phase whereas a heavy species, with K 1, tends to concentrate in the liquid phase. 

When Raoult s law applies, Eq. (4-326) reduces to K, = P( /P. For modified Raoult s law, K, = yP/ /P. With K) = y,/x, these are alternative expressions of Raoult s law and modified Raoult s law. Were Raoult s valid, K values could be correlated as functions of just T and P. However, Eq. (4-326) shows that they are in general functions of T, P Xi), and y,, making convenient and accurate correlation impossible. Those correlations that do exist are approximate and severely limited in application. The nomographs for K values of light hydrocarbons as functions of T and P, prepared by DePriester Eng. 

Ser No. 7, 49 1-43 (1953)], do allow for an average effect of composition, but their essential basis is Raoult s law. 

Thus for bubble point calculations, where the X are known, the problem is to find the set of K values that satisfies Eq. (4-327), whereas for dew point calculations, where the y, are known, the problem is to find the set of K values that satisfies Eq. (4-328). 

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The treatment of flash calculations in Sec. 10.6 led to calculations based on Raoult s law and K-value correlations. Use of the gamma/phi formulation of VLE makes the calculations somewhat more complex, but the primary equation is unchanged ... 

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