Effective Arrangement of an External Recycle in Reactive Distillation Columns

For reactive distillation columns separating reacting mixtures with somewhat unfavorable relative volatilities (e.g., the reactants are the light and heaviest components and the products are the lightest and heavy ones), a novel process configuration with an external recycle from the bottom to a certain stage of the reactive section is proposed in this article. The flow rate and location of the external recycle are important decision variables for process development and can help to enhance internal mass integration and internal energy integration between the reaction operation and the separation operation involved. The separations of one hypothetical ideal exothermic reaction mixture, A + B k(B)k(F) C + D, and two real nonideal reaction mixtures, the esterification of acetic acid with methanol and the transesterification of butyl acetate with ethanol, were chosen to evaluate the impact of such an external recycle, and parametric analysis was also conducted with respect to the relative volatility, chemical equilibrium constant, product purity, total amount of catalyst employed, and heat of reaction in terms of the separation of the ideal reacting mixture. Through strict comparison with conventional reactive distillation columns, it is demonstrated that the inclusion of such an external recycle is beneficial to system performance and should be taken into account in process synthesis and design. The potential applications of an external recycle to other reactive distillation columns separating reacting mixtures with somewhat unfavorable relative volatilities (e.g., the reactants are the lightest and heavy components and the products are the light and heaviest ones) are also discussed.