Distillation Sequencing Using Simple Columns

Consider first the design of distillation systems comprising only simple columns. These simple columns employ  

If there is a three-component mixture and simple columns are employed, then the decision is between two sequences, as illustrated in Fig. 5.1. The sequence shown in Fig. 5.1a is called the direct sequence, in which the lightest component is taken overhead in each column. The indirect sequence, shown in Fig. 5.16, takes the heaviest component as the bottom product in each column. There may be 

Much work has been carried out to find methods for the synthesis of distillation sequences of simple columns that do not involve heat integration. However, heat integration may have a significant 

TABLE 5.1 The Number of Possible Distillation Sequences Using Simple Columns 

This appears to be a complex problem requiring simultaneous solution of the sequence together with heat integration. 

For a three-component mixture to be split into three relatively pure products, there are only two alternative 

there may be many ways in which the separation can be carried out to produce the same products. The problem is that there may be significant differences in the capital and operating costs between different distillation sequences that can produce the same products. In addition, heat integration may have a significant effect on operating costs. Heat integration of distillation will be considered later in Chapter 21. 

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Table 11.1 Number of possible distillation sequences using simple columns.

Consider now ways in which the best arrangement of a distillation sequence can be determined more systematically. Given the possibilities for changing the sequence of simple columns or the introduction of prefractionators, side-strippers, side-rectifiers and fully thermally coupled arrangements, the problem is complex with many structural options. The problem can be addressed using the optimization of a superstructure. As discussed in Chapter 1, this approach starts by setting up a grand flowsheet in which all structural features for an optimal solution are embedded. 

Table 7.20 The number of separation sequences using simple distillation columns...

Such an approach was used in the works (Petlyuk Platonov, 1965 Petlyuk, Platonov, Slavinskii, 1965) and in more detail in Agrawal and Fidkowski (1998, 1999). In the latter works the comparison not only of sequences of simple columns, but also of distillation complexes, was made for three-component mixtures. Capital costs can differ considerably for such separation flowsheets, but in this case it is not important because energy expenditures greatly exceed capital ones. 

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. 

When multicomponent mixtures are to be separated into three or more products, sequences of simple distillation columns of the type shown in Fig. 13-1 are commonly used. For example, if aternaiy mixture is to be separated into three relatively pure products, either of the two sequences in Fig. 13-4 can be used. In the direct sequence, shown in Fig. 13-4, all products but the heaviest are removed one by one as distillates. The reverse is true for the indirect sequence, shown in Fig. 13-4 7. The number of possible sequences of simple distillation columns increases rapidly with the number of products. Thus, although only the 2 sequences shown in Fig. 13-4 are possible for a mixture separated into 3 products, 14 different sequences, one of which is shown in Fig. 13-5, can be synthesized when 5 products are to be obtained. 

Table 11.10 presents some heuristics for using complex distillation columns to separate a ternary mixture into its pure component products. On the basis of these heuristics and those for simple columns, suggest two sequences containing complex columns that can be used to separate the mixture described in Table 11.9 into relatively pure products. 

Distillation sequencing. The separation of homogeneous nonazeotropic mixtures using distillation usually offers the degree of freedom to choose the distillation sequence. The choice between different sequences can be made on the basis of total vapor load, energy consumption, refrigeration shaft power for low-temperature systems, or total cost. However, there is often little to choose between the best few sequences in terms of such measures of system performance if simple distillation columns are used. 

The procedure explained bellow, known equally as the list processing method, allows the designer the identification of feasible sequences in separating ideal or slightly nonideal zeotropic mixtures using simple distillation columns. Table 7.25 presents the list of more specific heuristics that can be used for sequencing, supplementary to those in Table 7.21. The first heuristic indicates that the most difficult problem should be isolated and pushed the last in the sequence. The second heuristic favours the separation... 

Due to the tremendous costs associated to distillative separations, many alternate schemes to the simple column shown above have been proposed over the past several years both to improve on some of its inherent costs. Traditionally, when purifying a multicomponent mixture, an entire series of distillation columns are used in series, and the way in which these columns are sequenced may make a tremendous difference in the eventual process costs. However, due to the large energy requirements of even the most optimal sequence, more complex column arrangements have been proposed and subsequently utilized. These arrangements include thermally coupled columns such as side rectifiers and strippers, the fully thermally coupled columns (often referred to as the Petlyuk and Kaibel columns). 

Figure 9.2.1. Number of possible column sequences in multicomponent distillation of nonazeotropic mixture using simple distillation columns. Ternary mixture of species A, B and C (a) direct sequence (b) indirect sequence.

Probably the most common method used for sequence selection for simple distillation columns is heuristic. Many heuristics have been proposed, but they can be summarized by the following four ... 

Very little process synthesis literature has dealt successfully with the problem of selection between alternative methods of separation. Indeed, most process synthesis literature has dealt with the selection of the flowsheet sequences Tor a single sepamtion method with only simple, sharp separations between components of ndjacent selectivity, without recycle. Furthermore, most of this literature has used only distillation in illustrative epplications. In practice, much use is made of recycle, nonsharp eplits. and complex columns (in distillation) and, of coorse. distillation is not always the separation merited of choice. 

A second alternative, called indirect sequence , is depicted in the Figure 9.7-right. In the first split the pure high-boiler is recovered completely as bottoms, followed by the separation of the light and medium components. Note that the two alternatives make use of simple distillation columns, with one feed and two products. A suitable design can fiilfil any type of feed composition and purity specifications. 

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