Temperature Control of Heterogeneous Reactive Distillation

In this work, we explore the temperature control of heterogeneous reactive distillation. By heterogeneous reactive distillation, we mean a two-liquid phase exists in the reflux drum and a decanter is used to separate the aqueous product from the organic reflux. Process characteristics of n-butyl propionate are explored and a systematic procedure is proposed for the design of butyl propionate reactive distillation. The control structure design procedure consists of the following steps: (I) setting the control objective, (2) selection of manipulated variables, (3) determination of temperature-control trays, (4) finding controller settings for regulatory control, and (5) providing feedforward compensation for production rate variation. Because specifications on the bottoms product have to be met and stoichiometric balance has to be maintained, we have a 2 X 2 control problem with two obvious inputs: reboiler duty and feed ratio. The reactive distillation exhibits unique temperature sensitivities and the nonsquare relative gain successfully identifies temperature-control trays, which results in an almost one-way decoupled system. Therefore, decentralized PI controllers are used at the regulatory level. As a result of the unique feature of a kinetically controlled distillation column, maintaining constant tray temperatures does not imply the same quality specification. Feedforward temperature compensation is necessary to maintain the desired product composition. The proposed design procedure is further extended to butyl acetate reactive distillation, where a similar process behavior is observed, and good control performance can be achieved using simple temperature control.