Boiling split sequencing

A convincing application of two columns sequence is the split of the azeotrope ethanol (A) / water (B) with tetrahydrofurane (C), as proposed by Stichlmair (1999). Figure 9.19 depicts qualitatively the split sequencing. The entrainer is a low-boiler forming a minimum azeotrope with water (az nbp 64.2 C) below the boiling point of the original water-ethanol azeotrope (azj, nbp 78.2 °C). There is also an azeotrope tetrahydroftirane-ethanol (az, nbp 65.9 C), but this is not essential. Water and ethanol,... 

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. 

As a final step in the fractionation sequence, the hexane fraction is separated into a dimeihyibutanes concentrate as a net overhead product and an -hexane-rich bottoms stream to be recycled for the further isomerization of ihe n-hexane and methylpentancs. Wilh economically practical fractionation, Hie methylpentanes split between the overhead and bottoms of the deisohexanizer column. For the Cs fraction, the boiling points of the two isomers are far enough apart to make a relatively clean split economically feasible. For the C<, fraction, the greater number of... 

The increased splitting rate with the molybdenum-containing catalysts is expressed also in the higher content of low-boiling fractions in the gasoline. The experiments were made at different temperatures in order to compare the catalysts at similar conversions. However, comparison at the same temperature would show the same sequence in aromatics content. 

Figure 4.9(a) and (b) illustrate the system behavior at a total pressure of 15 atm and 8 atm, respectively. As can be seen from the location of the PSPS, this system has similar features as the ideal system example 1 which has an elhpse-shaped PSPS (see Fig. 4.2(a)), as discussed above. Due to the boiling sequence of the reaction components, the PSPS is fully located outside the physically relevant composition space and, as a consequence, no reactive azeotrope can appear. It is worth noting that inside the phase-splitting region, the PSPS of the real heterogeneous system and the PSPS of the pseudohomogeneous system are different However, this does not affect the feasible top and bottom products of a fully reactive distillation column. 

How should the required separation system be synthesized One common sequencing practice is to perform the easiest split first. Considering the boiling point distribution of the components and azeotropes and thinking in terms of equipment, an opportunistie first split is a distillation column in which all of the methyl acetate, all of the methanol, and as much water as azeotropes with the methyl acetate are taken as overhead. All of the acetic acid and the remaining water are taken as bottoms (Fig. 9). At first, this may not seem like much... 

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 ... 

Let s consider the separation of a binary minimum azeotrope AB with a medium boiling entrainer C (Fig. 9.12). Note that the AB azeotrope and the component B are nodes, while both A and C are saddles. The separation regions for the first split are delimited by direct and indirect sequences, respectively. If the boiling points of A and AB azeotrope are not too close, A can be obtained as distillate, even if it is a saddle Rooks et al. (1998) has given recently a consistent explanation the split is feasible when the concentration profiles of both rectification and stripping zones points to the same common saddle, in this case the component C. 

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. 

Figure 8.27. Phase equilibria map and sequences for distillation of a binary azeotropic mixture (1,2) with an intermediate boiling en-trainer (3) (a) indirect split 2,3 1 in the first column, (b) direct split 2 1,3 in the first column, and (c) preferred split 2,3 13 in the first column.

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