Optimisation of solvent recovery in two batch distillation columns of different size

The recovery of acetone from an aqueous industrial waste solvent mixture containing a small amount of dichloromethane is studied. This separation can be performed in a single, large enough batch distillation column. When the amount of the waste solvent mixture increases and exceeds the processing capacity of this column, either the excess of solvent must be incinerated, or the processing capacity must be increased. The incineration is very disadvantageous from both economic and environmental points of view. It is investigated whether capacity can be efficiently increased by applying an additional, much smaller column, which is already available in the plant. Dichloromethane is removed in fore-cuts from both columns. The charge of the larger column (Column2) consists of two main cuts obtained from two consecutive batches of Column1. High purity acetone is obtained in the main cut of Column2. The optimisation of both the single- and two-column processes is performed by a genetic algorithm coupled with a professional flow-sheet simulator. The objective function minimised is the specific energy demand of the production of acetone. The effects of the composition of the fresh feed on the optimal values of the parameters and of the objective function are studied: the specific energy demand changes favourably on the increase of water and the decrease of dichloromethane concentration. A response surface method is used to successfully predict the optimal operating parameters in the space of compositions studied. Each optimised case is evaluated from economic and environmental points of view, as well. For an average charge composition, also by considering recycling of the residues of both columns, the weekly profit is by 12% higher and the CO₂ emission is by more than 20% lower compared to the single-column process. Moreover, the processing capacity is increased by about 30%. It is shown in our work that it is worth applying a second, possibly smaller batch distillation column available in the plant before a larger one in order to increase the processing capacity.