Indirect 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... 

Figure 5.3 The direct eind indirect sequence for Example 5.2. 

Similarly, with the first column in the indirect sequence, the composition of component B first increases above the feed as the less volatile component C decreases. It reaches a maximum only to decrease as the more volatile component A increases. Again, the composition of component B reaches a peak only to be remixed. 

Similarly, Fig. 5.15a shows a thermally coupled indirect sequence. The condenser of the first column is replaced by a thermal coupling. The four column sections are again marked as 1, 2, 3, and 4 in Fig. 5.15a. In Fig. 5.156, the four column sections are arranged to form a side-stripper arrangement. ... 

Indirect sequence pairings should be replaced by side-strippers. 

Fig. 4. Column sequences and material balance lines for the (a) direct and (b) indirect sequences for separating nonazeotropic...

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. 

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.)...

The side-stripper can be modeled as two columns in the thermally coupled indirect sequence, as shown in Figure 11.13. The first column is modeled as a conventional column with a partial condenser and partial reboiler. The second column is modeled as a sidestream column with a liquid sidestream on stage above the feed stage4. The vapor entering the condenser and liquid leaving can be calculated from vapor-liquid equilibrium (see Chapter 4). This allows... 

Figure 11.13 A side-stripper can be modeled as a sequence of two simple columns in the indirect sequence.

Figure 11.16 The thermally coupled indirect sequence for crude oil distillation.

The majority of atmospheric crude oil distillation columns follow the configuration shown in Figure 11.17, which is basically the partially thermally coupled indirect sequence. 

For complex columns, tasks can be combined together to produce hybrid tasks, as shown in Figure 11.2319. A hybrid task can then be evolved for different complex column arrangements, depending on whether the hybrid involves a direct or indirect sequence combination. 

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 21.14 Thermally coupling the indirect sequence changes both the loads and levels.

When PhCHO is made to react in D20, no D is incorporated into the CH2 group of (51) showing that H(D) must be transferred (as the above mechanism requires) directly, from one aldehyde molecule to the other, and not by any indirect sequence involving the solvent. In very concentrated base, the rate equation may, e.g. with HCHO... 

Indirect reduction, 14 501 Indirect sequence heuristic, for simple distillation, 22 299... 

Published routes to pyrazino[2,3-3][l,4]thiazines, as reviewed in CHEC-II(1996) <1996CHEC-II(7)737>, involved the reaction of a 3-aminopyrazine-2-thiol with a biselectrophile and of a 3-chloropyrazin-2-amine with thioglycolate. In more recent work, the system is formed by condensation of a dichloropyrazine with a 2-aminothiophenol (Equation 182) <2002CPB922>. The same approach, using either 2-aminothiophenol or 2-aminoethanethiol, has been used to form oxadiazole-fused systems (Scheme 100) <1997CHE1352>. In studies toward the synthesis of ER-49890 151, benzannulated pyrazino[2,3-/ ][l,4]thiazines have also been accessed by an indirect sequence (Scheme 101)<2005H(65)403>. 

The six sequencing heuristics are formulated to reduce the separation load on downstream columns, favoring easier separations early and difficult separations in the absence of nonkey components. If only two products are to be derived from a mixture and all of the components in one product are more volatile than all of the components in the other product, then the next split should divide the mixture into the two products. The presence of hazardous or corrosive materials can gready increase costs, and such components should be removed as early as possible. The most plentiful product in a mixture should be removed (if it can be) with one separation and if the relative volatility is favorable. Direct sequences, ie, removing a light product as distillate, generally are favored over indirect sequences, ie, removing a heavy product as bottoms. If no product dominates the feed composition, then separations that yield approximately equimolar splits are favored. Only if no other heuristic applies should the easiest separation be performed next. 

---