Distillation of multicomponent mixtures

In industry many of the distillation processes involve the separation of more than two components. The general principles of design of multicomponent distillation towers are the same in many respects as those described for binary systems. There is one mass balance for each component in the multicomponent mixture. Enthalpy or heat balances are made which are similar to those for the binary case. Equilibrium data are used to calculate boiling points and dew points. The concepts of minimum reflux and total reflux as limiting cases are also used. 

The variables in the design of a distillation column are all interrelated, and there are only a certain number of these which may be fixed in the design. For a more detailed discussion of the specification of these variables, see Kwauk (K2). 

Shortcut calculation methods. In the remainder of this chapter, shortcut calculation methods for the approximate solution of multicomponent distillation are considered. These methods are quite useful to study a large number of cases rapidly to help orient the designer, to determine approximate optimum conditions, or to provide information for a cost estimate. Before discussing these methods, equilibrium relationships and calculation methods of bubble point, dew point, and flash vaporization for multicomponent systems are covered. 

For multicomponent systems which can be considered ideal, Raoult s law can be used to determine the composition of the vapor in equilibrium with the liquid. For example, for a system composed of four components. A, B, C, and D, 

In hydrocarbon systems, because of nonidealities, the equilibrium data are often represented by 

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Underwood, A. J. V., Fractional Distillation of Multicomponent Mixtures, Chemical Engineering Progress, 44, 603 (1948). 

Colburn, A. P. (1941) Trans. Am. Inst. Chem. Eng. 37, 805. The calculation of minimum reflux ratio in the distillation of multicomponent mixtures. 

Underwood AJV (1946) Fractional Distillation of Multicomponent Mixtures - Calculation of Minimum Reflux Ratio, 7 Inst Petrol, 32 614. 

Z. Colburn, A. P., The Calculation of Minimum Reflux Ratio in the Distillation of Multicomponent. Mixtures, Tra7is. AIChE, Vol. 37, p. 805, 1941. 

When operation is under vacuum, however, the performance of the reboiler is sensitive to changes in the liquid driving head, especially in the distillation of multicomponent mixtures. The optimum liquid level for vacuum service is midway between the tube sheets, with about 50 percent of the liquid vaporized per pass. For usual applications with saturated steam on the shell side the heat flux can be estimated from Figure 15.13, which is based on BWG 14 stainless-steel... 

Underwood, A.J. (1948) Fractional distillation of multicomponent mixtures. Chemical Engineering and Processing, 44, 603-614. 

Continuous distillation of multicomponent mixtures is also described by mass balances and phase equilibria ... 

Flash distillation of multicomponent mixture. For flash distillation, the process flow diagram is shown in Fig. 11.3-1. Defining/= VjF as the fraction of the feed vaporized... 

Underwood, A. J. V. (1946b). Fractional Distillation of Multicomponent Mixtures (Calculation of Minimum Reflux Ratio). J. Inst. Petrol, 32,614-26. Underwood, A. J. V. (1948). Fractional Distillation of Multicomponent Mixtures. Chem. Eng. Prog., 44,603-14. 

The state of the art for the batch distillation of multicomponent mixtures is even less satisfactory than for binary mixtures, and except for total reflux, no accurate and practical method is available even without liquid holdup in the column. This difficulty arises from the fact that, starting with a given liquid composition in the still, it is possible to calculate the equilibrium vapor leaving the still, but the composition of the overflow to the still from the first plate cannot be calculated without knowing the composition of the distillate leaving the system. In a binary system, it is possible to choose the composi-... 

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