Ternary Reactive Distillation Systems

The chemical system considered in previous chapters featured the classical quaternary two-reactant, two-product A + B , C + D reversible reaction. Some interesting phenomena were discussed. In particular, the effect of the number of reactive trays on energy consumption was demonstrated to be counterintuitive, that is, there is an optimum number of reactive trays that minimizes energy consumption.

In this chapter we explore a similar but somewhat different chemical system. The reaction is A + B , C, which is reversible, liquid phase, and exothermic. There is only one product instead of two. This seemingly small change in the chemistry alters the effects of several important design parameters of a reactive distillation column. Because there are three components, we label the system ternary.

This is a very important class of reactions. The majority of commercial reactive distillation columns deal with this type of reaction. The large volume gasoline additive chemicals produced by reactive distillation, such as MTBE, ETBE, and TAME, are this type. These three examples involve an alcohol and an iso-olefin (either C4 or C5). The reactants are methanol and isobutene for MTBE, ethanol and isobutene for ETBE, and methanol and isoamylenes for TAME.

Although there are only three components involved in the reaction, in many of the A + B , C systems there are more than three components in the column because the feed-streams contain other components. These components are "inert" from the standpoint of the reaction, but they are not inert from the standpoint of their effect on the vapor-liquid equilibrium in the column. These inert components are present in the olefin feedstreams that contain the reactive iso C4 and C5 olefins in the examples cited. The reason for their presence is the great difficulty in separating the desired iso-olefin from the other components. For example, in the MTBE and ETBE cases, isobutene is produced in a catalytic cracker in a refinery along with a number of other C4 components (isobutane, «-butane, and n-butene).

Reactive Distillation Design and Control. By William L. Luyben and Cheng-Ching Yu Copyright # 2008 John Wiley & Sons, Inc.

All of these C4 components have very similar boiling points. Their separation using distillation would be very energy intensive. Therefore, the mixture of all of these C4 components is fed to the reactive distillation column, and the chemically inert components are removed as a product stream from the column.

We first discuss the ternary system without inerts to gain some insights into how changing from a two-product system to a one-product system impacts reactive distillation design. Unlike the quaternary column with distillate and bottoms products, the ternary column without inerts has only one product stream leaving the column. The effects of several design variables are shown to be quite different in the ternary system than those we observed in the quaternary system.

Then we explore the ternary system with inerts present in one of the feedstreams. The reactive distillation column now has two streams leaving the column. One contains product C and the other contains the inerts.

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