Tray Number Condenser

Figure 17.15 (a) Composition profile, feed locations, and fraction of total conversion (Ri/Rt) and (b) temperature profile for type IIIp process (LK + HHK , LLK + HK).

the column overhead. The scenario is simpler toward the bottoms of the column because the heavy reactant (HHK component D) can be separated easily from the heavy reactant (HK component B). Methyl acetate hydrolysis9 is a typical example.

methyl acetate + water LLK HK

methanol + acetic acid LK HHK

Note that the neat design is considered here instead of the excess reactant design often seen in methyl acetate hydrolysis. Because the light product (LK component C) and heavy

9M. F. Malone, R. S. Huss, and M. F. Doherty, Green chemical engineering aspects of reactive distillation, Environ. Sci. Techno. 37, 5325-5329 (2003).

Figure 17.16 Final design for process type IIIR (LLK + HK , LK + HHK).

Figure 17.16 Final design for process type IIIR (LLK + HK , LK + HHK).

product (HHK component D) are withdrawn from the top and bottoms of the column, the reactive zone is placed in the middle. Figure 17.16 gives the column configuration. Following the proposed design procedure, the final design is shown in Table 17.3 with detailed parameter values.

We might think that this is simply the mirror image of type IIIp. However, this flowsheet has a TAC of $726,000, which is more than twice that of type IIIp. The reactive distillation column has a total of 96 trays comprising 57 reactive trays, 4 stripping trays, and 35 rectifying trays. Two feeds are introduced into the top and bottoms of the reactive zone as shown in Figure 17.16. The reflux ratios and boilup ratios are much higher than those of type IIIp with values greater than 6. Actually, this is a capital intensive reactive distillation column with relatively high energy consumption.

Liquid-phase reactions with one reactant being the LLK (reactant A) pose a difficult design, especially for reactive distillation. The reasons are 1) we have to consume all of the LLK (reactant A) toward the top of the column and 2) we have to maintain a high concentration for the heavy reactant (HK component B) to ensure that the forward reaction is dominant. Thus, 57 reactive trays are used to react away most of the LLK (reactant A) as is clearly shown in Figure 17.17a, where we have a very small amount of conversion between tray 30 and tray 61. Leaving the top of the reactive zone of the column, we have almost equal molar amounts of LK ( product C) and HK (reactant B) with a trace amount of LLK (reactant A). A large number of rectifying trays (35 trays) are employed to return HK back to the reactive zone while having LLK and LK as the top product. The scenario toward the bottoms is much simpler because we are performing separation between HK (reactant B) and HHK (product C). Thus, only 4 stripping trays are required. The composition profiles produce a very interesting temperature profile (Fig. 17.17b) with two distinct plateaus.

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