The Gamma Phi Formulation of VLE

Modified Raonlt s law includes the activity coefficientto accountfor liquid-phase nonidealities, but is limited by the assumption of vapor-phase ideality. This is overcome by introdnction of tire vapor-pliase fugacity coefficient. For species i in a vapor mixtnre, Eq. (11.48) is written  

Equatioir (11.44) requires these two expressions to be eqnal wheirce, 

Superscripts v aird 1 are irot used here because of a presuirrptioir that refers to tire vapor phase and tliat y,- and /, are hquid-plraseproperties. Substitutiirgfor /, by Eq. (11.41) gives  

Since the Poyntingfactor (represented by the exponential)at low to moderate pressures differs from unity by only a few parts per thousand, its omission mtroduces negligible error, and this equation is often simplified  

Sy stematicapplication of Eqs. (14. l)and (14.2) depends on the availability of correlations of data from which values may be obtamedfor ,, and y, . The vapor pressure of a pure species is most commonly given by the Antome equation, Eq. (6.71)  

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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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