Analysis of Thermally Coupled Distillation Sequences in Reaction-Separation-Recycle Systems and Its Influence in the Snowball Effects

It is well known that even the simplest chemical process involves at least one reaction unit and one separation process, usually interlinked through a recycle stream. The behavior of the reactor-separator-recycle system (RSR) has been studied with special interest because of the existence of snowball effects(1): a small change in a load translates into a large change in the recycle flow rate, which propagates around the whole system. Snowball phenomenon it is a steady state effect, but it can have important implications on dynamic behavior and in control structure selection as well. On the other hand, from economic and environmental considerations, there is a special incentive to develop more integrated processes (2) (for instance through energy and/or mass integration), but the extra recycles can become an additional source of control problems. Several authors (1,3) have focused on the study of snowball effects using simple separators as flash units or conventional distillation columns. This work explores the existence of snowball effects in steady state conditions under the implementation of thermally coupled distillation sequences. In particular, the implementation of coupled systems with side columns (the sequence with a side rectifier and the sequence with a side stripper) is studied. The effects of considering thermally coupled systems on snowball effects are compared to those generated by conventional distillation systems.

1. Luyben, W. L., (1994) Snowball Effects in Reactor/Separator Processes with Recycle. Ind. Eng. Chem. Res, 33, 299-305.

2. Agrawal, R. and Fidkowski, Z.T., (1999), New Thermally Coupled Schemes for Ternary Distillation, AIChE Journal, 45, 485-496.

3. Larsson, T., Govatsmark, M. S., Skogestad, S. y Yu, C. C.,(2003) Control Structure Selection for Reactor, Separator, and Recycle Processes. Ind. Eng. Chem. Res., 42, 1225-1234.