Optimization of a batch extractive distillation process with recycling off-cuts

Abstract

A multiple step solvent recovery distillation process is optimized where the components form several azeotropes, and the off-cuts are recycled. Both conventional and extractive batch distillation are studied. The recovery of methanol from an industrial waste solvent mixture (acetone–methanol–tetrahydrofuran–water–toluene) is rigorously simulated and optimized. The first fore-cut is incinerated; the second fore-cut, the after-cut and the hold-up are recycled to the next batch in order to decrease the loss of methanol. The optimization of both processes is performed by a genetic algorithm coupled with a professional flow-sheet simulator performing the dynamic simulation. The optimization of the batches is performed consecutively, with the objective function being the profit of the actual batch. The optimization variables are the reflux ratios of the steps (fore-cuts, main cut), duration of fore-cut withdrawals and for the batch extractive distillation the flow rate and duration of entrainer (water) feeding. The effects of the recycling of the off-cuts on the charge composition and on the optimal values of the parameters are studied, and the optimized batch distillation and batch extractive distillation processes are compared with each other and with the batch distillation process used in the plant. The extractive process gives a 67% higher profit than the optimized batch distillation, whose profit was in turn by 25% higher than without optimization. The environmental impact is also assessed by calculating the related CO2 emission and the waste water production. The specific CO2 emission (kg CO2/kg product) is by 20% lower for the extractive process than for the optimized and by 22% lower than for the non-optimized batch distillation.