Rectification of Multicomponent Mixtures

Multicomponent mixtures are those containing more than two components in significant amounts. In commercial operations, they are encountered more generally than are binary mixtures, and as with binary mixtures, they can be treated in batch or continuous operations, in bubble-plate or packed towers. Since the continuous operation is much more amenable to mathematical analysis, owing to the steady conditions of concentration and operation, it will be considered first. 

In the foregoing discussion of multicomponent systems, it was assumed that the complete composition of the liquid at some position in the column was known as a starting point for the calculation. The determination of this complete composition as a starting point is often tfie most difficult part of the whole multicomponent design. This difficulty arises from the fact that there are a limited number of independent variables which will completely define the distillation process therefore, it is not possible to select arbitrarily the complete composition of a liquid or vapor at some position in the distillation system Thede rees of freedom involved in a j3istiHation system can be evaIuated(R law of conservation of matter, 

Operating pressure over each plate and in still and condenser.. 

Heat gain or loss to or from each plate and condenser. .  

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For preliminary studies of batch rectification of multicomponent mixtures, shortcut methods that assume constant molal overflow and negligible vapor and liquid holdup are useful. The method of Diwekar and Madhaven [Ind. Eng. Chem. Res., 30, 713 (1991)] can be used for constant reflux or constant overhead rate. The method of Sundaram and Evans [Ind. Eng. Chem. Res., 32, 511 (1993)] applies only to the case of constant remix, but is easy to apply. Both methods employ the Fenske-Uuderwood-GiUilaud (FUG) shortcut procedure at successive time steps. Thus, batch rectification is treated as a sequence of continuous, steady-state rectifications. 

Serafimov, L. A., Timofeev, V. S., Balashov, M. I. (1973a). Rectification of Multicomponent Mixtures. 3. Local Characteristics of the Trajectories Continuous Rectification Process at Finite Reflux Ratios. Acta Chimica Academiae Scien-tiarum Hungarical, 75,235-54. 

RECTIFICATION OF MULTICOMPONENT MIXTURES 253 On substituting the values of Eqs. 9-13 and 9-15 and rearranging,... 

RECTIFICATION OF MULTICOMPONENT MIXTURES 2G9 Table 9-4. Rematching Feed Plate from Below... 

Batch Rectification of Multicomponent Mixture at Total Reflux. As an example, consider the fractionation of an equimolal mixture of A, B, C, and D at total reflux with relative volatility ab = 2.0, ao = 4.0, and ad — 8.0. The column is equivalent to three theoretical plates and holdup of liquid will be neglected. 

Batch Rectification of Multicomponent Mixture at Finite Reflux Ratio. The... 

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