Operating Parameters and Selectivity in Batch Reactive Distillation

Batch reactive distillation (BRD), combines the flexibility of batch process with advantages of reactive distillation, and is an important process development tool for reactive distillation.

This paper is focused on selectivity improvements for improved raw material consumption and lower waste production, emphasizing the impact of operating parameters on selectivity in BRD.  We describe the impact in terms of the appropriate dimensionless groups: the reflux or reboil ratio, along with a certain type of Damköhler number (Da), which gives the ratio of the process time to the reaction time.

We show that selectivity improvements in BRD can be limited for high values of Da and that selectivity is enhanced at the expense of conversion as Da is decreased.  This conversion loss can be mitigated by increasing the reflux ratio. Consequently, for systems operated at low Da, there is an optimum value of reflux ratio that provides the maximum yield.

We show sample results for a hypothetical case of serial isomerization reactions and also a two-reactant system (synthesis of ethylene glycol), in which distillation may have a negative impact on selectivity by causing separation of the reactants. For the latter example, the reboil ratio in a batch stripper has a crucial impact on selectivity. In fact, too low a reboil ratio will cause even lower selectivity than found in a conventional batch reactor. Understanding the impact of these parameters allow us to explore the tradeoffs involved in choosing among alternative operating strategies.