Sequencing, of columns

The number n of top and bottom products from a battery of n 1 columns can be made in several different ways. In a direct method, the most volatile components are removed one-by-one as overheads in successive columns with the heaviest product as the bottoms of the last column. The number of possible ways of separating components goes up sharply with the number of products, from two arrangements with three products to more than 1(K) with seven products. Table 13.5 identifies the five possible arrangements for 

Separation of Four Components ABCD by Three Columns 

Ovhd Btms Ovhd Btms Ovhd Btms 

The literature of optimum sequencing of columns is referenced by King (1980, pp. 711-720) and Henley and Seader (1981, pp. 527-555). For preliminary selection of near optimal sequences, several rules can be stated as guides, although some conflicts may arise between recommendations based on the individual rules. Any recommended cases then may need economic evaluations. 

Perform the easiest separation first, that is, the one least demanding of trays and reflux, and leave the most difficult to the last. 

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Distillation. There is a large inventory of boiling liquid, sometimes under pressure, in a distillation column, both in the base and held up in the column. If a sequence of columns is involved, then, as discussed in Chap. 5, the sequence can be chosen to minimize the inventory of hazardous material. If all materials are equally hazardous, then choosing the sequence that tends to minimize the flow rate of nonkey components also will tend to minimize the inventory. Use of the dividing-wall column shown in Fig. 5.17c will reduce considerably the inventory relative to two simple columns. Dividing-wall columns are inherently safer than conventional arrangements because they lower not only the inventory but also the number of items of equipment and hence lower the potential for leaks. 

Note The sequence of columns in binary runs 1,2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096 and so on, each column being two times greater than the previous one. Compare this with the decimal system where each column is 10 times greater than the previous one. 

Sequencing of columns for separating multicomponent mixtures (a) perform the easiest separation first, that is, the one least demanding of trays and reflux, and leave the most difficult to the last (b) when neither relative volatility nor feed concentration vary widely, remove the components one by one as overhead products (c) when the adjacent ordered components in the feed vary widely in relative volatility, sequence the splits in the order of decreasing volatility (d) when the concentrations in the feed vary widely but the relative volatilities do not, remove the components in the order of decreasing concentration in the feed. 

Procedures for the sequencing of columns are also available in the commercial process simulator programs for example, DISTIL in Hyprotech s suite of programs (see Chapter 4, Table 4.1). 

Much of the benefit of countercurrent operation, without the problems associated with circulation of the adsorbent, can be achieved by using a multiple-column fixed-bed system with an appropriate sequence of column switching, designed to simulate a counterflow system. The general scheme is illustrated in Fig. 13. Such systems are widely used in wastewater treatment,... 

FIGURE 13 Schematic diagram showing the sequence of column interchange in a periodic countercurrent separation process. 

Doherty, M. F., and Caldarola, G. A. Design and Synthesis of Homogeneous Azeotropic Distillations. 3. The Sequencing of Columns for Azeotropic and Extractive Distillations, Ind. Eng. Chem. Fundam. 24, 474—485 (1985). 

Considerations 393 Sequencing of Columns 393 Number of Free Variables 3P5... 

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