(84a) 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.