Calculation of Activity Coefficients and Gas-Phase Composition

Parameters Bo, Bi, and B2 entering Eqs. (8.8) and (8.9) for the activity coefficients were obtained [13] based on the limited set of low-temperature T 378 K) experimental data on the total pressure of binary water-HP system for several values (x,T). As pages 345 and 346 provide the approximate expressions for P x,T) that are applicable at temperatures ranging from 330-370 K to T Tc, it becomes possible to determine Bo, Bi, and B2 more precisely for the extended temperature range. For this purpose, Eq. (8.7) combined with Eqs. (8.3) and 

Equations (8.8) and (8.9) in combination with Eqs. (8.17) or (8.18) can be used for calculating 7hp and Then, one can determine the equilibrium composition of the gas phase in the water-HP system by applying the following formulae  

Alternatively, for the gas phase obeying Dalton s law, y can be obtained directly (i.e., without calculating 7hp and 7 ) using the Duhem s equation [6]  

If the total pressure P is known as a function of y (along the isotherm), then Eq. (8.21) provides algebraic or transcendental dependence j/(a ) at a given T. Usually, total pressure is known as a function of solution composition, x. By 

To find a unique integral curve of Eq. (8.22), it is necessary to specify a point on the curve. For this purpose, one can choose a singular point at the edge of interval 0 A 1, where the molar fraction of one component in solution and in gas phase is zero. It can be shown that the singular point A = 0, T = 0 is of the saddle type, and Eq. (8.22) should be integrated from this point. 

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