Volatility split sequencing

Simple distillation Relative volatility oc Use heuristics for split sequencing. Not feasible if a< LI. Check thermal stability of components. 

Solution. Approximate relative volatilities for all adjacent pairs except iCsInC are given in Table 1.6. The latter pair, with a normal boiling-point difference of 8.3°C, has an approximate relative volatility of 1.35 from Fig. 1.17. For this example, we have wide variations in both relative volatility and molar percentages in the process feed. The choice is heuristic 1, which dominates over heuristic 2 and leads to the sequence shown in Fig. 14.7, where the first split is between the pair with the highest relative volatility. This sequence also corresponds to the optimal arrangement. 

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. 

Sequencing of columns for separating multicomponent mixtures (a) perform the easiest separation first, that is, the one least demanding of trays and reflux, and leave the most difficult to the last (b) when neither relative volatility nor feed concentration vary widely, remove the components one by one as overhead products (c) when the adjacent ordered components in the feed vary widely in relative volatility, sequence the splits in the order of decreasing volatility (d) when the concentrations in the feed vary widely but the relative volatilities do not, remove the components in the order of decreasing concentration in the feed. 

Heuristics have been proposed for the selection of the Heuristic 1. Do D/E split last since this separation has the sequence for simple nonintegrated distillation columns1. smallest relative volatility. 

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. 

Four columns are needed to produce the desired products. Considering the Sharp Distillation Sequencing heuristics, heuristic (7) does not apply, as there is more than one product in this mixture. Fatty acids are moderately corrosive, but none is particularly more so than the others, so heuristic (2) does not apply. The most volatile product, the caproic and caprylic mixture, is a small (10 mol %) fraction of the feed, so heuristic (3) does not apply. The least volatile product, the oleic—stearic acids, is 27% of the feed, but is not nearly as large as the capric—lauric acid product, so heuristic (4) does not apply. The split between lauric and myristic acids is closest to equimolar (55 45) and is easy. Therefore, by heuristic (3) it should be performed first. The boiling point list implies that the distillate of the first column contains caproic, caprylic, capric, and lauric acids. This stream requires only one further separation, which by heuristic (/) is between the caproic—caprylic acids and capric—lauric acids. 

When the adjacent ordered components in the process feed vary widely in relative volatility, sequence the splits in the order of decreasing relative volatility. 

The reactor/separator/recycle structure is decided by considering the physical properties of the species found in the reactor effluent (Table 9.1). The catalyst and the organic phase are immiscible. Therefore, they can be separated by liquid-liquid splitting. The separation of the organic components by distillation seems easy. In a direct sequence, the inert and any light byproduct will be removed in the first column. The second column will separate the reactants, which have adjacent volatilities. Therefore, there will be only one recycle for both reactants. The third column will separate the product from the heavies. The reactor/separation/ recycle structure of the flowsheet is presented in Figure 9.2. 

When the concentrations in the feed vary widely but the relative volatilities do not, sequence the splits to remove components in the order of decreasing concentration in the feed. 

A simple column performs the separation of a single feed into two products. The simplest case is the separation of a ternary mixture ABC, with components ordered by decreasing volatility. Figure 7.24 shows the alternatives. In the direct sequence, the components are separated in the order of volatilities, firstly A and then B as overhead products. In the indirect sequence, the first split delivers the heaviest component C as bottoms from the first column, followed by the A-B separation in the second column. 

Solution. Table 7.26 lists the normal boiling points and the relative volatilities of components. The sequence starts by examining potential splits for the first column C-1. The results are as follows ... 

In Indirect sequence, the first split is much easier (35 trays), and the amount of entrainer is lower (0.08). In the second split, however, the situation is less favourable. Equilibrium diagram y-x shows difficult separation of pure acetone, because very low relative volatility with respect to benzene. However, the same high purity of 99.8% acetone can be obtained. It is important to note that the feed should be placed near to the reboiler, so that the second column is practically a stripper. The recycled entrainer benzene may contain a small amount acetone, which helps to get high purity products. The energetic consumption is slightly below the direct sequence, because low amount of entrainer. Hence, contrary to expectations, the indirect sequence has better indices, both as hardware and energetic consumption. 

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. 

Interchange (a) is not likely to cause any appreciable change in sequence cost when heuristics 1 and 2 are considered. The relative volatilities of the key components for the two splits are almost identical and neither of the two compounds removed (A and F) are present in appreciable amounts. Therefore, do not make this interchange. 

Sequence separation points in the order of decreasing relative volatility so that the most difficult splits are made in the absence of the other components. 

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