Zeotropic behavior

The impact of inaccurate -model parameters can be very serious. The parameters have a major influence on the investment and operating costs (number of stages, reflux ratio), The influence of the -model parameters on the results is especially large if the separation factor is close to unity. Poor parameters can either lead to the calculation of nonexisting azeotrojjes in zeotropic systems (see Section 11.1) or the calculation of zeotropic behavior in azeotropic systems. Poor parameters can also lead to a miscibility gap which does not exist." In the case of positive deviation from Raoult s law a separation problem often occurs at the top of the column, where the high boiler has to be removed, since at the top of a distillation column the most unfortunate separation factors are obtained. 

Figure 13.10 on the next page summarizes the general appearance of some relatively simple temperature-composition phase diagrams of binary systems. If the system does not form an azeotrope (zeotropic behavior), the equilibrated gas phase is richer in one component than the liquid phase at all liquid compositions, and the liquid mixture can be separated into its two components by fractional distillation. The gas in equilibrium with an azeotropic mixture, however, is not enriched in either component. Fractional distillation of a system with an azeotrope leads to separation into one pure component and the azeotropic mixture. 

Figure 3.10 shows typical RCM for nonideal mixtures involving azeotropes. For the mixture ace tone/heptane /benzene (plot a) there is only one distillation field. The problem seems similar to a zeotropic system, except for the fact that the minimum boiler is a binary azeotrope and not a pure component. With the mixture acetone/chloroform/toluene (plot b) there is one distillation boundary linking the high-boiler with the low-boiler azeotrope. Consequently, there are two distillation regions. Similar behavior shows the plot c, with two azeotropes. The mixture acetone/chloroform/methanol (plotd) has four azeotropes (3 binaries and 1 ternary) displaying a behavior with four distillation regions. 

To understand the mutual behavior of the components depending on the degree of the mixture s nonideahty caused by the difference in the components molecular properties, it is better to use graphs yi - x, T - xi,T - yi,Ki - xi, and K2 - xi (Fig. 1.2). In Fig. 1.2, the degree of nonideality increases from atoh.a is an ideal mixture, h is a nonideal mixture with an inflection on the curve y - xi (a and b are zeotropic mixtures), c is a mixture with a so-caUed tangential azeotrope (curve yi - xi touches the diagonal in the point xi = 1), d is an azeotropic mixture with minimum temperature, c is a mixture with a so-called inner tangential azeotrope, / is a mixture with two azeotropes, g is a heteroazeotropic mixture, and h is an azeotropic mixture with two liquid phases. Azeotrope is a binary or multicomponent mixture composition for which the values of phase equihbrium coefficients for all components are equal to one ... 

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