Dew-T calculations

At the moderate pressures where the gammdplii approach to VLB is appropriate, activity coefficients are assumedindependentof pressure. The need for iterationis evident, for example, in a BUBL P calculation of y and P, which requires values of, that are functions of P and y,. Simple iterative procedures, described in the following paragraphs, apply to BUBL P, DEW P, BUBL T, and DEW T calculations. 

DEW T Calculations In this calculation, neither T nor x,- is known. The iteration scheme is shown in detail by Fig. 14.4. As in the DEW P procedure, the set xi calculated within tlie inner loop is not constrained to sum to unity, and is tlierefore normalized by setting... 

Values of 4>, and are implicit in an equation of state, and with Eq. (14.29) allow calculation of mixture VLE. The same basic principle applies as for pnre-species VLE, but the calculations are more complex. With a fnnctionof T, P, and y,, and a function of T, P, and xi, Eq. (14.29)TepiesentsN relations among the 2N variables T, P,(N— l)yisand (N — 1) xis. Tims, specification of N of these variables, usually either T ox P and either the vapor- or liquid-pliase composition, allows solution for the remaining N variables by BUBL P, DEW P, BUBL T, and DEW T calculations. 

Dew T calculation. We solve Raoult s law for the liquid mol fractions and add the resulting equations ... 

These tie-lines correspond to the same overall composition z, = 0.3 (see the Txy graph in Figure 11-1). In the bubbler calculation, this composition refers to the liquid in the dew T calculation, it refers to the vapor. 

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