Design and control of a thermally coupled reactive distillation process synthesizing diethyl carbonate

Diethyl carbonate (DEC) is a versatile material due to its excellent chemical and physical properties. Several chemical routes have been reported for the preparation of DEC. However, they suffer the drawbacks of using poisonous gases or attaining a low yield of DEC. In this study, a promising route by the transesterification of propylene carbonate and ethanol is used to coproduce DEC and propylene glycol. The transesterification reaction offers an excellent green chemical process. However, the reaction is a typically equilibrium-limited one. Reactive distillation (RD) with excess reactant is adopted in the study to improve reaction conversion and obtain high-purity products in the DEC synthesis. A base-case design, consisting of a RD column and a DEC purification column with its overhead unreacted ethanol stream recycled back to the RD column, is developed and optimized by minimizing the total annual cost. An alternative thermally coupled RD process is also developed which results in substantially reduced energy consumption. Further, steady-state analysis is used to design the control strategies of the thermally coupled RD process. It has been demonstrated that the proposed temperature control + constant reflux-ratio control scheme is sufficient to maintain not only feed ratio of the reactants but also product purities at or around their designed values for the RD process with thermal coupling.