The determination of a distillation curve is accomplished by approximating the system as a series of flash equilibrium stages. At each equilibrium stage, the Saturated Vapor Pressure of each pure component is evaluated based on a DIPPR correlation (xEquation 4–2), and the activity coefficient is calculated based on the UNIFAC group interaction correlation as explained in UNIFAC Activity Coefficients.

The ratio of the liquid to vapor is determined by iterating in a Rachford Rice algorithm [46]. Stages subsequent to the primary stage are fed with the composition of liquid leaving the previous stage, and temperature is slightly increased.

The validation of activity coefficient calculations is through the formation of Tx_y diagrams for several binary mixtures. The Tx_y diagram shows the bubble-point and dew-point temperatures as a function of the fraction of one component in a mixture of two components. The lower curve is the bubble-point curve, below which the mixture is liquid and the upper curve is the dew-point curve above which the mixture is vapor. The calculated profiles were tested and compared to known Tx_y profiles. The results of a binary mixture of 2,3-dimethylbutane and trichloromethyl are compared to experiment in Figure 4.1: Txy diagram for a system of 2,3-dimethylbutane and trichloromethyl, reproducing the positive azeotrope behavior. (Experimental data are shown as symbols; these data [] are compared to UNIFAC method calculations.), and the ethanol/water azeotrope is predicted as shown in Figure 4.2: Ethanol/water Txy diagram showing the positive azeotrope at ~0.9 mol fraction composition, calculated from UNIFAC correlations. Data from Perry et al. 1997 [].

Figure 4.1: Txy diagram for a system of 2,3-dimethylbutane and trichloromethyl, reproducing the positive azeotrope behavior. (Experimental data are shown as symbols; these data [46] are compared to UNIFAC method calculations.)

Figure 4.2: Ethanol/water Txy diagram showing the positive azeotrope at ~0.9 mol fraction composition, calculated from UNIFAC correlations. Data from Perry et al. 1997 [47].

The distillation curve is then determined through a series of stages, all at Vapor-Liquid Equilibrium. An example of the Ethanol Water distillation curve is show in Figure 4.3: Example distillation curves for different binary mixtures of ethanol and water. The initial conditions at the azeotrope of 0.9 produce a single stage where both species vaporize at the depressed temperature seen in Figure 4.2: Ethanol/water Txy diagram showing the positive azeotrope at ~0.9 mol fraction composition, calculated from UNIFAC correlations. Data from Perry et al. 1997 []., which has a variety of initial compositions at the inlet.

Figure 4.3: Example distillation curves for different binary mixtures of ethanol and water. The initial conditions at the azeotrope of 0.9 produce a single stage where both species vaporize at the depressed temperature seen in Figure 4.2: Ethanol/water Txy diagram showing the positive azeotrope at ~0.9 mol fraction composition, calculated from UNIFAC correlations. Data from Perry et al. 1997 [].