Reactive distillation can be an attractive technology for integrating reaction and separation, potentially reducing capital and operating costs. However, reactive distillation requires the reaction conditions to be compatible with the separation conditions, which may not be the most favorable for reaction kinetics or equilibrium. Others have shown that “pumparound” or “side car” reactors can be used to replace reactions internal to the distillation column,¹ and that it is possible to improve the overall system performance by manipulating the number, placement, or conditions of such external reactors.^2,3

With external reactors, the temperature and phase for the reactions can be manipulated independently of the separation. However, it is challenging to determine optimal or even feasible configurations with simulation alone due to extreme nonlinearity of the models and lack of robust convergence strategies. In this paper we describe a strategy for design of a distillation system including side car reactors.

The strategy begins with a reactive distillation design, and then using a series of simulations to move the reaction into a side car reactor. A key step is the use of equilibrium reactive distillation design techniques using equilibrium constants based on achievable conditions in an external reactor. From such a reactive distillation design, we can observe where conversion occurs in the column, and simulate a column with small or even single tray reaction zones, achieving the same result as the full reactive distillation. The final step is to remove reaction from the column and instead perform the same amount of conversion in an external reactor.

Examples will be given to illustrate the method.

References: 1. Baur, R. and R. Krishna, “Distillation column with reactive pump arounds: an alternative to reactive distillation,” Chem. Eng. and Processing,, 43, 435 (2004).

2. Kaymak, D. B. and W. L. Luyben, “Design of distillation columns with external side reactors,” Ind. Eng. Chem. Res., 43, 8049 (2004).

3. Kamath, R. S., Q. Zhiwhen, K. Sundmacher, P. Aghalayam, and S. M. Mahajani, “Comparison of Reactive Distillation with Process Alternatives for the Isobutene Dimerization Reaction,” Ind. Eng. Chem. Res., 45, 2707 (2006).