Llk Lk Hk

In terms of reactants and products, the boiling point ranking is just the opposite of type IIp [Eq. (17.15) vs. Eq. (17.14)]. Now the two reactants are lighter than the two products. Thus, these two reactants are more concentrated toward the upper part of the reactive distillation column, and the two products can be withdrawn from the bottom of the column. A typical example is the acetalization reaction for the synthesis of ethylal.8 The reaction can be expressed as formaldehyde + ethanol ethylal + water LLK LK HK HHK

Thus, the upper section of the column should be made reactive while the separation is carried out in the stripping section. This gives the reactive distillation configuration shown in Figure 17.12. The reaction takes place in the reactive distillation column, and the bottoms product is then fed to a simple distillation column to separate the two products. Because the two unreacted reactants (LLK and LK) are lighter than the two products (HK and HHK), the conversion in the reactive distillation column is set to 97.5% such that both products, HK in particular, can meet the 95% specification. The product compositions are 95% C (HK), along with two light reactants, and 97.5% D (HHK), along with 2.5% HK (C). Following the design procedure, the final design is shown in Figure 17.12. Table 17.3 gives the parameter values of this design.

In terms of topology, this process looks like the mirror image of type IIp in which the reactants and two products switch ends of the columns. However, the flowsheet gives a TAC of $1,096,000, which is 150% of that of type IIp. Moreover, the TAC of the reactive distillation column is $742,000, which is twice the cost of the reactive distillation for type IIp (TAC = $323,000). The reason for that is actually quite simple. The reaction takes place in the low temperature zone (upper section of the column). This leads to a reactive distillation with 24 reactive trays plus a reactive condenser and 3 stripping trays. As

Figure 17.12 Final design for type IIR process (LLK + LK , HK + HHK).

VS= 102.4 mol/s steam

VS=41.0 mol/s

r n

h ^ steam

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