Column configurations of continuous heterogeneous extractive distillation
Résumé
Increasing regulations and constraints have renewed interest for more efficient distillation process in separating azeotropic mixtures and close boiling components. We investigate the feasibility of heterogeneous extractive distillation process in a continuous column considering several feed point strategies for the entrainer recycle stream and for the main azeotropic feed. Depending on these choices, the heterogeneous distillation column is composed of one, two or three column sections. A differential mass balance model enables to compute continuous liquid composition profiles of the rectifying, extractive and stripping sections. Unlike homogeneous extractive distillation, reflux policy composed by a single or both decanted liquid phases is considered as well as the external feeding influence on the composition of the top column liquid stream. Limiting operating conditions of key parameters like the entrainer/feed flowrate ratio and reflux ratio required to obtain a target top and bottom product compositions are obtained. For illustration, separation of acetonitrile – water mixture using butyl acetate as a heavy heterogeneous entrainer is selected. In this case, withdrawal of a saddle binary heteroazeotrope is the main difference of this process compared with the well known heterogeneous azeotropic distillation process where the top vapor product is the lower boiling point of the ternary system.
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