Distillation sequencing prefractionators

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. 

This remixing that occurs in both sequences of simple distillation columns is a source of inefficiency in the separation. By contrast, consider the prefractionator arrangement shown in Figure 11.9. In the prefractionator, a crude split is performed so that Component B is distributed between the top and bottom of the column. The upper section of the prefractionator separates AB from C, whilst the lower section separates BC from A. Thus, both sections remove only one component from the product of that column section and this is also true for all four sections of the main column. In this way, the remixing effects that are a feature of both simple column sequences are avoided4. 

By complex column, we mean a distillation set-up that can handle a mixture of at least three components, and deliver more than two products. The arrangement consists of a main distillation column surrounded by additional devices, as prefractionator, heaters, inter-coolers, side-strippers and side-rectifiers. Figure 7.25 presents seven design alternatives for separating a ternary mixture ABC. Note that direct and indirect two-column sequences are included in this list, since they may be seen as reference alternatives. The operation of different types may be summarised as follows ... 

From the above discussion it can be observed that the initial diluted feed can be sent to the water-column and not to the ethanol recovery column (azeotropic distillation). In consequence, this alternative has only two columns, and has been occasionally presented as innovative. Doherty Malone (2001) discusses in detail all the alternatives discovered along the time using benzene as entrainer. The twO-column sequence is not the most economical because of large entrainer recycle and expensive columns. It seems that the two-column sequence with prefractionator, in total three units, offers the best compromise between investment and operation (solvent recycle) costs. 

Besides sequences of simple columns, some types of distillation complexes, each of which can replace two adjacent simple columns, were examined in work (Ghnos Malone, 1988). The following complex columns and distillation complexes were examined column with side output above the feed cross-section, column with side rectifier, column with side stripping flowsheet with prefractionator, Petlyuk column top and side flows from the first column into the second one (Fig. 8.3a),... 

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