Type III Alternating

Type IIIp LK 1 HHK = LLK 1 HK. When one of the products (C) is the lightest component, we have the following reaction:

LK HHK LLK HK (1716)

The boiling point ranking indicates that if we consume the heavy reactant (HHK component B) before it reaches the bottom of the reactive zone, the heavy product (HK component D) can be obtained from the column base. The scenario is simpler toward the top of the column because the light product (LLK component C) can be withdrawn from the top while the light reactant (LK component A) can be prevented from leaving the reactive zone. The popular methyl acetate production via acetic acid esterification (Chap. 7) example falls into this category.

methanol + acetic acid o methyl acetate + water

Because two products are withdrawn from the opposite ends of the column, the reactive zone is placed in the middle. This column configuration is shown in Figure 17.14. Table 17.3 gives the parameter values of this design.

This flowsheet has a TAC of $321,000, which is the second lowest in all six cases (126% of that of type Ip). The reactive distillation column has a total of 59 trays with 49 reactive trays, 8 stripping trays, and 2 rectifying trays. Two feeds (11 trays apart) are introduced into

Figure 17.14 Final design for type IIIp process (LK + HHK , LLK + HK).

Figure 17.14 Final design for type IIIp process (LK + HHK , LLK + HK).

the upper section of the reactive trays as shown in Figure 17.14. Both the reflux ratio and boilup ratio are slightly greater than 2.

Actually, this is a relatively simple reactive distillation column with moderate energy consumption, despite having a relatively large number of reactive trays. Having one reactant that is the HHK (reactant B) has its advantages and disadvantage. The HHK increases tray temperatures when we have significant amounts of this heavy reactant, which is advantageous for the reaction (see the temperature profile in Fig. 17.15b). The down side is that we have to react away almost all of the HHK in the reactive zone (otherwise it will end up in the column base), and this leads to a large number of reactive trays. This is clearly illustrated in Figure 17.15a, where we have a very small amount of conversion between tray 9 and tray 40. The purpose for this portion of the reactive trays is to consume the remaining heavy reactant (HHK component B). The composition of the LK reactant (component A) is kept fairly constant below the feed point to ensure the dominance of the forward reaction (Fig. 17.15a). The two products are further purified in the top and bottom rectifying and stripping sections of the reactive distillation column to meet the specifications.

Type IIIR: LLK 1 HK = LK 1 HHK. For this alternating type, when one of the reactants (A) is the lightest component, we have the following reaction:

The boiling point ranking suggests that if we consume the light reactant (LLK component A) toward the top of the column, the light product (LK component C) can be obtained from

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