Reactive distillation columns incorporate both phase separation and chemical reaction. They have economic advantages in some systems over conventional reactor, separation, and/or recycle flowsheets, particularly for reversible reactions in which chemical equilibrium constraints limit conversion in a conventional reactor. Because both reaction and separation occur in a single vessel operating at some pressure, the temperatures of reaction and separation are not independent. Therefore, reactive distillation is limited to systems in which the temperatures conducive to reaction are compatible with temperatures conducive for vapor-liquid separation.

Pressure in conventional distillation design is usually set by a minimum temperature in the reflux drum (so that cooling water can be used) or a maximum temperature in the reboiler (to prevent fouling or thermal decomposition). Establishing the optimum pressure in a reactive distillation column is more complex because of the interplay between reaction and phase separation. Most VLE relationships show an increase in volatility with decreasing temperature. On the other hand, reaction rates decrease with decreasing temperature. If the reaction is exothermic, the chemical equilibrium constant increases with decreasing temperature. So low operating pressure or temperature, which facilitates the phase separation, may require lots of catalyst or liquid tray holdup to compensate for the low reaction rates.

In conventional distillation design, tray holdup has no effect on steady-state composition. In reactive distillation, tray holdup (or amount of catalyst) has a profound effect on conversion, product composition, and column composition profiles. So, in addition to the normal design parameters of reflux ratio, number of trays, feed tray location, and

Distillation Design and Control Using Aspen™ Simulation, By William L. Luyben Copyright © 2006 John Wiley & Sons, Inc.

pressure, reactive distillation columns have the additional design parameter of tray holdup. If there are two reactant feedstreams, an additional design parameter is the location of the second feed.

Reactive distillation is usually applied to systems in which the relative volatilities of the reactants and products are such that the products can be fairly easily removed from the reaction mixture while keeping the reactants inside the column. For example, consider the classical reactive distillation system with reactants A and B reacting to form products C and D in a reversible reaction.

For reactive distillation to be effective, the volatilities of the products C and D should be greater or less than the volatilities of the reactants A and B. Suppose the volatilities are

Reactant A would be fed into the lower section of a reactive column and rise upward. Reactant B would be fed into the upper section and flow downward. As the components react, product C would be distilled out the top of the column, and product D would be withdrawn out the bottom. The reactants can be retained inside the column by vapor boilup and reflux while the products are removed. Figure 9.1 illustrates this ideal case.

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