Improving the Performance of Extractive Dividing-Wall Columns with Intermediate Heating

Abstract

In spite of the fact that using extractive dividing-wall columns (EDWCs) lead to a considerably reduced reboiler heat duty in comparison with conventional extractive distillation column (CEDC) flowsheets, the involvement of a heavy entrainer frequently requires a relatively high-pressure steam for process heating and incurs consequently adverse steady-state economics. To address the potential deficiency, we proposed, in the present study, to facilitate the EDWC with intermediate heating because the great boiling-point difference between the components in the azeotropic mixture and entrainer results in steep temperature profiles and allows effectively recovering sensible heat from the recycled entrainer and using relatively low-pressure steam. When the extractive separation operation dominates the EDWC, intermediate heating should be arranged in the left side of the dividing wall and feed preheating is frequently the most favorable option. On the other hand, when the entrainer recovery operation dominates the EDWC, intermediate heating should be arranged below the lower end of the dividing wall, i.e., in the stripping section, and an intermediate reboiler is the favorable option. Under both circumstances, the heat recovery from the recycled entrainer should be considered prior to the use of available utilities, thereby permitting great improvement in the steady-state economics. The strategy features simplicity in principle and requires a relatively small number of trial and error searches. It is evaluated in terms of extractive separations of two binary azeotropic mixtures: dimethyl carbonate and methanol (with aniline as the entrainer) and acetone and methanol (with dimethyl sulfoxide as the entrainer). It is found that intermediate heating could substantially enhance the performance of the EDWC with the resultant steady-state economics overwhelmingly above that of the CEDC flowsheet. Even compared with the CEDC flowsheet reinforced with intermediate heating, the EDWC is still likely to yield comparable steady-state economics. These outcomes indicate that intermediate heating should be taken into account in the synthesis and design of the EDWC and the EDWC with intermediate heating should be regarded as a potential option for the extractive separations of binary azeotropic mixtures.