Figure 3.17 Conventional process.

and in the reflux drum (TD) of the two columns. Reflux-drum temperatures are constant at 320 K, so the relative volatilities at the top of the columns are equal to 2 (bottom two graphs, Fig. 3.17). However, in the base of the column the higher temperatures result in somewhat smaller relative volatilities as the a390 parameter decreases.

Reactive Distillation. Figure 3.18 and Table 3.8 give optimum design results for the reactive distillation process for a range of temperature-dependent relative volatilities. As the a390 parameter decreases, the optimum pressure decreases. This occurs because lower pressure helps the vapor-liquid equilibrium because it lowers temperatures and hence increases relative volatilities. However, a lower temperature is unfavorable for reaction because the reaction rates are too small. The result is a rapid increase in the required number of reactive trays.

Note that the optimum number of stripping trays is larger than the optimum number of rectifying trays. This is caused by the higher temperatures in the lower part of the column, which means lower relative volatilities.

Although cases are considered in which the reference a390 approaches and even drops below 1, the actual relative volatilities in the reactive column do not get too close to 1. If they did, the required separation would become impossible. Figure 3.19 illustrates this point. The average relative volatility through the reactive zone is 2 for the base case (a390 = 2), because it is constant. However, the average relative volatilities in the reactive zone become smaller as the value of a390 decreases. The decrease of the column temperature prevents the relative volatilities from becoming too small. As shown in the top right

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