Changing Relative Volatilities of Reactants

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In this section we will explore the effects of the relative volatilities of the reactants on the optimum feed tray locations. By relative volatilities of reactants, we mean that the separation between the two reactants (A and B) becomes easier or more difficult while keeping the relative volatilities between adjacent reactants and products constant at 2. Two cases are studied: one is a more difficult separation (i.e., aA/aB = 3/2) and the other is an easier one (i.e., aA/aB = 6/2) compared to the base case (i.e., aA/aB = 4/2).

In the first case, the relative volatilities are aC = 6, aA = 3, aB = 2, and aD = 1. With the conventional feed arrangement (reactants fed at the two ends of the reactive zone), we have 33% more energy consumption (0.0428 kmol/s) compared to that of the base case. This shows that, similar to conventional distillation, difficult separation, even between reactants A and B, requires more energy. Moreover, Figure 18.6 shows that the composition of A is higher toward the lower reactive zone compared to the base case, and this leads to a decrease in product D composition, which subsequently requires a larger vapor rate to meet the specification. The optimization procedure predicts that the optimum feed trays are NF,A = 11 and NF,B = 13 (Fig. 18.6b), and this configuration produces a 15.2% energy savings (from 0.0428 to 0.0363 kmol/s) over the conventional feed arrangement (Table 18.2). Note that the percentage of energy savings is computed with respect to the conventional feed arrangement in each case. We immediately observe that the two feeds move closer to each other (only two trays apart), and a nonmonotonic reactant composition distribution can be seen (Fig. 18.6b). Similar to the base case (e.g., Fig. 18.5b), we also have an almost monotonic composition distribution in D. This results in a higher temperature in the lower section of the reactive trays and leads to a higher reaction rate and consequently higher conversion, as shown in Figure 18.6b.

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