Multicomponent distillation general considerations

The problem of determining the stage and reflux requirements for multicomponent distillations is much more complex than for binary mixtures. With a multicomponent mixture, fixing one component composition does not uniquely determine the other component compositions and the stage temperature. Also when the feed contains more than two components it is not possible to specify the complete composition of the top and bottom products independently. The separation between the top and bottom products is specified by setting limits on two key components, between which it is desired to make the separation. 

The complexity of multicomponent distillation calculations can be appreciated by considering a typical problem. The normal procedure is to solve the MESH equations (Section 11.3.1) stage-by-stage, from the top and bottom of the column toward the feed point. For such a calculation to be exact, the compositions obtained from both the bottom-up and top-down calculations must mesh at the feed point and match the feed composition. But the calculated compositions will depend on the compositions assumed for the top and bottom products at the commencement of the calculations. Though it is possible to 

---

In the following part of this section, we provide simple mathematical descriptions of a few common features of two-phase/two-region countercurrent devices, specifically some general considerations on equations of change, operating lines and multicomponent separation capability. Sections 8.1.2, 8.1.3, 8.1.4, 8.1.5 and 8.1.6 cover two-phase systems of gas-Uquid absorption, distillation, solvent extraction, melt crystallization and adsorption/SMB. Sections 8.1.7, 8.1.8 and 8.1.9 consider the countercurrent membrane processes of dialysis (and electrodialysis), liquid membrane separation and gas permeation. Tbe subsequent sections cover very briefly the processes in gas centrifuge and thermal diffusion. 

Many of the distillations of industry involve more than two components. While the principles established for binary solutions generally apply to such distillations, new problems of design are introduced which require special consideration. An important principle to be emphasized is that a single fractionator cannot separate more than one component in reasonably pure form from a multicomponent solution, and that a total of C - 1 fractionators will be required for complete separation of a system of C components. Consider, for example, the continuous separation of a ternary solution consisting of components A, B, and C, whose relative volatilities are in that order (A most volatile). In order to obtain the three substances in substantially pure form, the following two-column scheme can be used. The first column is used to separate C as a residue from the rest of the solution. This residue is necessarily contaminated with a small amount of B and an even smaller amount of A. The distillate, which is necessarily contaminated with a small amount of C, is then fractionated in the second column to give nearly pure A and B. 

---