Distillation sequencing direct sequence

The most volatile product (myristic acid) is a small fraction of the feed, whereas the least volatile product (oleic—stearic acids) is most of the feed, and the palmitic—oleic acid split has a good relative volatility. The palmitic—oleic acid split therefore is selected by heuristic (4) for the third column. This would also be the separation suggested by heuristic (5). After splitting myristic and palmitic acid, the final distillation sequence is pictured in Figure 1. Detailed simulations of the separation flow sheet confirm that the capital cost of this design is about 7% less than the straightforward direct sequence. 

Fig. 1. Fatty acid distillation sequence (a) sequence generated by ranked heuristics and (b) more expensive direct sequence.

FIG. 13-4 Distillation sequences for the separation of three components, a) Direct sequence, (h) Indirect sequence. 

The problem of distillation sequencing was discussed in Chapter 11, where the distillation columns in the sequence were operated on a stand-alone basis using utilities for the reboilers and condensers. Following the approach in Chapter 11, the best few nonintegrated distillation sequences would be found. These sequences would then be heat integrated as discussed above. Figure 21.8 shows how heat integration can be applied within a two-column direct... 

Remove final products one by one as distillates (the direct sequence). 

Table 2 shows the lAE values obtained for each composition control loop of the distillation sequences under analysis. It is observed that the Petlyuk column offers the best dynamic behavior, which is reflected in the lowest values of lAE, for the control of the three product streams. The dynamic response of each control loop when the Petlyuk column was considered is displayed in Figure 2, where a comparison can be made to the response obtained with the widely-used direct sequence. One may notice in particular how the direct sequence is unable to control the composition of the intermediate component, while the Petlyuk column provides a smooth response, with a relatively short settling time. It is interesting to notice that for this mixture with an ESI = 1, and a low content of the intermediate component in the feed, the Petlyuk column offers the highest energy savings and also shows the best dynamic performance from the five distillation sequences under consideration. 

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. 

Sometimes ihe intermediate imine is isolated, but generally it is nol and may even be inferior to direct alkylation (5following sequence for it was desired to acetylate ihe alkylated product as formed, A solution of 50 mmol of an aromatic aldehyde (I) and 50 mmol of aminoaceialdehyde dimethylacetal (2), refluxed 1.5 h in toluene under nitrogen, gave after distillation nearly quantitative yields of the SchifTs base 3 (5d). 

Figure 11.1 The direct and indirect sequences of simple distillation columns for a three-product separation (From Smith R and Linnhoff B, 1998, Trans IChemE ChERD, 66, 195, reproduced by permission of the Institution of Chemical Engineers.)...

Example 21.1 Two distillation columns have been sequenced to be in the direct sequence (see Figure 21.8). Opportunities for heat integration between the two columns are to be explored. The operating pressures of the two columns need to be chosen to allow heat recovery. Data for Column 1 and Column 2 at various pressures are given in Tables 21.1 and 21.2. 

A direct sequence of two distillation columns produces three products A, B and C. The feed condition and operating pressures are to be chosen to maximize heat recovery opportunities. To simplify the calculations, assume that condenser duties do not change when changing from saturated liquid to saturated vapor feed. This will not be true in practice, but simplifies the exercise. Assume also that the reboiler duty for saturated liquid feed is the sum of the reboiler duty for saturated vapor feed plus the heat duty to vaporize the feed. Data for the two columns are given in Tables 21.7 and 21.8. 

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