(372e) Off-Cut Recycling in Batch Reactive Distillation Processes

Batch reactive distillation (BREAD) is an integrated process which combines the advantages of reactive distillation and the flexibility of batch processes. BREAD process performance can be improved by withdrawing an off-cut (slop cut) during the batch. In non-reactive batch distillation processes, it has been shown that off-cut withdrawal can reduce total batch time compared to operation without off-cut withdrawal, especially when the separation is difficult (Christensen and Jorgensen, 1987). However, very few authors have considered the collection of off-cut for BREAD processes (Venimadhaven et al., 1999; Wajge and Reklaitis, 1998), and we are not aware of any that consider the recycling of off-cut for BREAD processes.

Recently, we have also shown that for BREAD processes off-cut withdrawal improves the process in terms of the batch capacity (CAP) (Kao 2010; Kao and Ward, 2010). Usually, the off-cut that is collected is not discarded directly, instead it must be processed to recover valuable materials or treated to remove hazardous pollutants.

In this work, we consider an alternative which is to recycle off-cut to the next batch as a part of initial feed. This transforms the operation problem from single batch problem into a batch-to-batch problem. A canonical set of quaternary reversible reaction A + B ↔ C + D is used to investigate the design of batch campaigns with off-cut recycle. The pseudo-steady state concept proposed by Luyben (1988) for non- reactive batch distillation is applied to BREAD processes to simplify the optimization problem. A pseudo-steady state for a batch campaign means that for a fixed operating recipe the amount and composition of off-cut produced in the previous batch (which is included in the feed to the present batch) is identical to the amount and composition of off-cut produced in the present batch.

The optimal operating policy based on maximizing CAP is found for a process operating at pseudo-steady state, and the result is compared with the optimal operating policy for single batch operation when the off-cut is not recycled. The results show that the optimal CAP when off-cut is recycled is similar to the case when off-cut is collected but not recycled, and is significantly more than the case when off-cut is not used. Therefore, recycling off-cut may not only save the trouble processing the worthless off-cut, but also make the process more economical.

 References

Christensen FM, Jorgensen SB. Optimal-Control of Binary Batch Distillation with Recycled Waste Cut. Chem Eng J Bioch Eng. Feb 1987;34(2):57-64.

Kao YL. Effect of Relative Volatility Ranking On the Design and Operation of Batch Reactive Distillation Systems. Master Thesis. National Taiwan University, Taiwan. July, 2010.

Kao YL, Ward JD. Effect of Relative Volatility Ranking On the Design and Operation of Batch Reactive Distillation Systems. AIChE Annual Meeting. Salt Lake City, USA. Nov. 7-12, 2010.

Luyben WL. Multicomponent Batch Distillation .1. Ternary-Systems with Slop Recycle. Ind Eng Chem Res. Apr 1988;27(4):642-647.

Venimadhavan G, Malone MF, Doherty MF. A novel distillate policy for batch reactive distillation with application to the production of butyl acetate. Ind Eng Chem Res. Mar 1999;38(3):714-722.

Wajge RM, Reklaitis GV. An Optimal Campaign Structure for Multicomponent Batch Distillation with Reversible Reaction. Ind Eng Chem Res. May 1998;37(5): 1910-1916.