Single-Stage Flash Calculations

After the ideal equilibrium compositions have been found, they are used to find improved values of the fugacity and activity coefficients. The process is continued to convergence. 

The compositions of vapor and liquid phases of a binary system at equilibrium sometimes can be related by a constant relative volatility which is defined as 

In terms of vaporization equilibrium ratios, ai2 = K1/K2 = yfPt VyzTf, 

Usually the relative volatility is not truly constant but is found to depend on the composition, for example. 

Other relations that have been proposed are 

Data for the system ethanol + butanol at 1 atm are taken from the collection of Kogan et al. (1966, 1038). The values of jc/(100—jc), y/(100 — y), and a are calculated and plotted. The plot on linear coordinates shows that relative volatility does not plot linearly with x, but from the linear log-log plot it appears that 

Equilibria, Pergamon, London, 1967). Other expressions can be deduced from Eq. (13.15) and some of the equations for activity coefficients, for instance, the Scatchard-Hildebrand of Table 13.2. Then 

Beyond a certain complexity these analytical relations between vapor and liquid compositions lose their utility. The simplest one, Eq. (13.14), is of value in the analysis of multistage separating equipment. When the relative volatility varies modestly from stage to stage, a geometric mean often is an adequate value to use. Applications are made later. Example 13.1 examines two ways of interpreting dependence of relative volatility on composition. 

Equilibria between the components of a binary mixture are expressed as a functional relation between the mol fractions of the usually more volatile component in the vapor and liquid phases, 

The definition of relative volatility, Eq. (13.14), is rearranged into this form  

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The above is a variation on the single-stage flash calculation for a vapor-liquid separation. 

Single-Stage Flash Calculations 375 Bubblepoint Temperature and Pressure 376 Dewpoint Temperature and Pressure 377 Flash at Fixed Temperature and Pressure 377 Flash at Fixed Enthalpy and Pressure 377 Equilibria with Ks Dependent on Composition 377... 

Rijkers, M. P. W. Heidemann, R. A. 1986. Convergence behavior of single-stage flash calculations. American Chemistry Society Symposium Series, 300, 476-493. 

Given the then each assumed value V/F (or L/f) has a unique solution for V". This is a variation on the single-stage flash calculation for a vapor-liquid separation. 

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