(505d) Effect of Relative Volatility Ranking On the Design and Operation of Batch Reactive Distillation Systems

Batch reactive distillation (BREAD) is an
attractive process alternative which combines the advantages of reactive
distillation and the flexibility of batch processes. There are three basic
batch distillation column configurations: (1) Conventional batch distillation column
(CBD): Feeds are charged at the bottom and products are taken out at the top.
(2) Inverted batch distillation column (IBD): Feeds are charged at the top and
products are taken out at the bottom. (3) Middle-vessel column (MVC): Feeds are
charged in a middle-vessel of the column and products are withdrawn at the top
and the bottom.

Tung et al. [1] used an ideal system to
study the effect of relative volatility ranking on the design of column
configuration for continuous RD systems. A reversible reaction A+B <=> C+D is
considered and constitutes a quaternary system which has 24(4!) possible
rankings according to the relativity volatility among reactants and products.
They further grouped these 24 possibilities into 6 (24/2!/2!) distinct
categories since the two reactants and two products are interchangeable. In this
work, these 6 distinct volatility rankings are applied to the 3 basic BREAD column
configurations to study the effects of relative volatility ranking on BREAD
process design. The process design focuses on the choice of column
configuration and the optimal operating policy including when and where to
collect products and off-cuts and the corresponding reflux profile. The designs
are optimized based on the batch capacity (CAP), defined as the total quantity
of products meeting the purity specification produced divided by the total
batch time [2]. The results indicate that if one of the reaction products is
the lightest key, CBD or MVC performs the best. On the other hand, if one of
the reaction products is the heaviest key, IBD or MVC performs the best.

Furthermore, we investigate a new column
configuration: a modified MVC in which the location of the reaction vessel is not
exactly in the middle of the column. We
consider the general case where the reaction vessel can be connected to the
column at any point. If the reaction vessel is connected all the way at the
bottom, we recover the CBD process whereas if it is connected all the way at
the top, we recover the IBD. Finally, for each relative volatility ranking, we
propose the most suitable column configuration and identify the operating
policy with the highest CAP.   

References

1.
Tung, S. T. and C. C. Yu (2007). Effects of relative volatility ranking to the
design of reactive distillation. AIChE Journal 53(5): 1278-1297.

2.
Luyben, W. L. (1971). Some Practical Aspects of Optimal Batch Distillation
Design.  Industrial & Engineering Chemistry Process Design and
Development 10(1): 54-59.