Optimal synthesis and design of extractive distillation systems for bioethanol separation: from simple to complex columns

Bioethanol has been considered as a green fuel and a valid alternative to reduce the dependence on fossil distillates. The development of an optimal separation process is considered as a key element in the design of an efficient process able to be cost effective and competitive. Despite many separation techniques are today available, distillation is up to now the frontrunner technology in most of the plants. In particular extractive distillation is considered as an energy efficient method to produce pure ethanol overcoming the purity limitations imposed by the water-ethanol azeotrope. The subspace of simple distillation configurations was generated considering the possibility to employ partial or total condensers to transfer the non-product mixtures between the columns, moreover different numbers of columns are considered. Once the most promising sequences are obtained, the complex columns are investigated. The complex column subspace is generated introducing one or more thermal couplings and considering the possibility to intensify the process combining the column sections performing the same separation task. All the configurations considered, simulated by means of Aspen Plus V 7.3, are compared considering the total condenser and reboiler duty as energy index. The capital costs and the solvent consumption are also taken into account in the final selection. Among all the complex configurations considered the two-column sequence can reduce the capital cost above 10% compared to the best simple column sequence. The performances for different solvents are also taken into account in the process alternatives comparison.