(107d) Reactive Distillation as a Multipurpose Reactor for Controlling Selectivity In Multiple Reaction Systems

Reactive distillation (RD), in which the reaction and separation are integrated in a single unit operation, has been established in process technology. The instantaneous product removal from the reaction zone through distillation leads to an increased yield of chemical equilibrium-limited reactions, such as etherification and esterification. The majority of published papers on RD investigate chemical systems comprising only one main reaction. These papers have demonstrated the possibility of RD to increase reactant conversion as well as the purity of the desired product. In contrast, selectivity plays a secondary role in such chemical systems. In our opinion, the potential of RD has not been fully tapped. Our work focuses on multiple reaction systems, from which more than a single desired product is formed. We want to prove that RD enables the process engineer to flexibly vary the selectivity of each product while still having high reactant conversions in such chemical systems. The reactive distillation column can act as a multipurpose reactor, which consequently extends the current field of application for RD. No reports on the simultaneous control of conversion and selectivity in such a system have been published so far, and no experimental results on a pilot-scale for such a chemical system could be found.

A promising example is the reversible consecutive second-order transesterification of dimethyl carbonate (DMC) with ethanol via ethyl methyl carbonate (EMC) to produce diethyl carbonate (DEC). The transesterification of DMC was homogeneously catalysed by sodium ethoxide, and the reactive distillation experiments were performed in a pilot-scale RD-column with a diameter of 0.05 m that was equipped with Sulzer BX packing elements. The total packing height was 5.4 m, and the total feed rate was 4 kg/h. As temperature and concentration along the RD-column are measured, the experimental setup provides a detailed composition profile. The experiments demonstrated the potential of RD to act as a multipurpose reactor for multiple reaction systems producing more than one desired product. In the same column, selectivity for EMC higher than 80% and, after changing the operating conditions, selectivity for DEC higher than 80% could be achieved.

Additionally, the experiments provided reliable data for the validation of the applied non-equilibrium stage model for RD. The validated process model could be used for an intensive theoretical analysis of the reactive separation process. The non-equilibrium stage model is implemented in the simulation ASPEN Custom Modeller^® environment, which considers reaction kinetics, multi-component mass and heat transfer rates as well as column hydrodynamics. As the homogeneously catalysed transesterification was carried out in a packed column, the reactive distillation model also takes into account the chemical reaction in the liquid hold-up of the internal column distributors. Consequently, the validated process model was used to determine the influence of operational parameters on conversion and selectivity.