Distillation sequence nonkey components

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. 

It is interesting to reflect on the heuristics for sequencing of distillation columns in Chapter 11. Heuristics 2, 3 and 4 from Section 11.1 tend to minimize the flowrate of nonkey components. Heuristic 1 relates to special circumstances when there is a particularly difficult separation8. 

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. 

For Distillation, Favor Sequences that Remove Components One-by-Oneas Overhead Product. This advice is based on the need to condense or remove heat from overhead streams and add heat to bottom streams. Adding nonkey components to the overhead stream increases the cooling load on the overhead condenser and the heating load on the reboiler for a simple column. Removing components one-by-one in Older of decreasing volatility also minimizes the vapor flow in the column supporting a direct sequence. 

In practice, the deisobutanizer is usually placed first in the sequence. In Table 12.1, the bottoms for Case 1 then becomes the feed to the debutanizer, for which, if nC4 and iCs are selected as the key components, component separation specifications for the debutanizer are as indicated in Fig. 12.3 with preliminary estimates of the separation of nonkey components shown in parentheses. This separation has been treated by Bachelor. Because nC4 and Cg comprise 82.2 mole% of the feed and differ widely in volatility, the temperature difference between distillate and bottoms is likely to be large. Furthermore, the light-key split is rather sharp but the heavy-key split is not. As will be shown later, this case provides a relatively severe test of the empirical design procedure discussed in this section. 

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