Direct sequence

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... 

Heuristic 2 Sequences that remove the lightest components alone one by one in column overheads should be favored. In other words, favor the direct sequence. 

Consider the sequence of simple columns shown in Fig. 5.12. In the direct sequence shown in Fig. 5.12, the composition of component B in the first column increases below the feed as the more volatile component A decreases. However, moving further down the column, the composition of component B decreases again as the composition of the less volatile component C increases. Thus the composition of component B reaches a peak only to be remixed. ... 

Figure 5.12 Composition profiles for the middle product in the columns of the direct sequence show remixing effects. (From Triantafyllou and Smith, Trans. IChemE, part A, 70 118, 1992 reproduced by permission of the Institution of Chemical Engineers.)...

First consider thermal coupling of the simple sequences from Fig. 5.1. Figure 5.14a shows a thermally coupled direct sequence. The reboiler of the first column is replaced by a thermal coupling. Liquid from the bottom of the first column is transferred to the second as before, but now the vapor required by the first column is supplied by the second column instead of by a reboiler on the first column. The four column sections are marked as 1, 2, 3, and 4 in Fig. 5.14a. In... 

Direct sequence pairings should be replaced by side-rectifiers. 

A potentially general method of identifying a probe is, first, to purify a protein of interest by chromatography (qv) or electrophoresis. Then a partial amino acid sequence of the protein is deterrnined chemically (see Amino acids). The amino acid sequence is used to predict likely short DNA sequences which direct the synthesis of the protein sequence. Because the genetic code uses redundant codons to direct the synthesis of some amino acids, the predicted probe is unlikely to be unique. The least redundant sequence of 25—30 nucleotides is synthesized chemically as a mixture. The mixed probe is used to screen the Hbrary and the identified clones further screened, either with another probe reverse-translated from the known amino acid sequence or by directly sequencing the clones. Whereas not all recombinant clones encode the protein of interest, reiterative screening allows identification of the correct DNA recombinant. 

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.

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. 

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

Mixtures containing small amounts of both lights and heavies are common. Typically, such mixtures are separated in two columns by direct sequence (Dimian and Kersten, 1997). 

A simple model for side-rectifiers suitable for shortcut calculation is shown in Figure 11.12. The side-rectifier can be modeled as two columns in the thermally coupled direct sequence. The first column is a conventional column with a condenser and partial reboiler. The second column is modeled as a sidestream column, with a vapor sidestream one stage below the feed stage4. The liquid entering the reboiler and vapor leaving can be calculated from vapor-liquid equilibrium (see Chapter 4). The vapor and liquid streams at the bottom of the first column can then be matched with the feed and sidestream of the second column to allow the calculations for the second column to be carried out. 

Figure 11.12 A side-rectifier can be modeled as a sequence of two simple columns in the direct sequence.

In addition to these issues regarding constraints for simple columns, there is also the issue of the introduction of complex columns into the sequence. Figure 21.13a illustrates the thermal characteristics of a direct sequence of two simple columns. Once the two columns are thermally coupled, as illustrated in Figure 21.13b, the overall heat load is reduced. However, all of the heat must be supplied at the highest temperature for the system. Thus there is a trade-off in which the load is reduced, but the levels required to supply the heat become more extreme. The corresponding case for the indirect sequence is shown in Figure 21.14. As the indirect sequence is thermally coupled, the heat load is reduced, but now all of the heat must be rejected at the lowest temperature. Thus, there is a benefit of reduced load but a disadvantage of heat rejection at more extreme levels. The same problem occurs with... 

Figure 21.13 Thermally coupling the direct sequence changes both the loads and levels.

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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