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 lowpressure 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.

Single reactive section External recycle Two reactive sections Process synthesis Process design a b s t r a c t The synthesis and design of reactive distillation columns with two reactive sections (RDC-TRS) are attempted for the separations of reacting mixtures with somewhat unfavorable ranking of relative volatilities (RM-SURRV, i.e., two reactants are the light and heaviest components and two products the lightest and heavy ones). With the deliberate arrangement of the two reactive sections, the disadvantages by the unfavorable relative volatility can be substantially alleviated, facilitating consequently the reaction operation and separation operation involved. The separations of a hypothetical quaternary reaction, the esterification of acetic acid with methanol, and the transesterification of butyl acetate with ethanol are employed to evaluate the RDC-TRS. The process is found considerably superior to the reactive distillation column with a single reactive section and this demonstrates its feasibility and effectiveness in the separations of RM-SURRV. The RDC-TRS is found to present comparable or even improved performance in comparison with the reactive distillation column with an external recycle from bottom to reactive section and this corroborates it a potentially competitive alternative for the separations of the RM-SURRV. The RDC-TRS is also highlighted for the other kind of RM-SURRV (i.e., two reactants are the lightest and heavy components and two products the light and heaviest ones).

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Because of the very close boiling points of cyclohexene and cyclohexane, their separation is extremely difficult with conventional distillation systems and reactive separation with water as reactive entrainer offers great economic incentives. In the current work, the synthesis and design of a flow-sheet with two reactive distillation columns (i.e., the hydration and dehydration reactive distillation columns) in series (FSTRDC) is first conducted subject to the minimization of total annual cost (TAC), and this leads to a process design with an excessive use of water. Although the process design facilitates the hydration of cyclohexene into cyclohexanol, it gives rise to a serious remixing effect in the hydration reactive distillation column and poses an unfavorable effect to the decomposition of cyclohexanol into cyclohexene and water in the dehydration reactive distillation column. For the suppression of these deficiencies, the hydration reactive distillation column was then modified to be heated directly with a vapor flow from the reboiler of the dehydration reactive distillation column, and this gives rise to a novel thermally coupled reactive distillation system (TCRDS). The mass and thermal coupling between these two reactive distillation columns involved eliminates completely the remixing effect and reduces greatly the excessive use of water and enables the TCRDS to require substantially less capital investment (CI), operating cost (OC), and TAC than the FSTRDC. The implementation of the TCRDS in terms of a dividing-wall distillation column is finally highlighted and found to lead to further reductions in the CI and TAC. These striking outcomes demonstrate the great significance of including effective mass and thermal coupling to the two reactive distillation columns in series, reactively separating close boiling binary mixtures.

Because of the intensive material and thermal coupling within dividing‐wall distillation columns (DWDCs), the so‐called black‐hole (BH) problem may occur and arouse consequently uncertainties to control simultaneously the primary component compositions in the top, intermediate, and bottom products and the composition ratio between the rest components in the intermediate product. The elimination of the BH problem is attempted through the adoption of feed splitting in this work, and a systematic procedure is derived to achieve the purpose. Because feed splitting augments the decision variables for process synthesis and design, it could be used effectively to refine the relationship between the prefractionator and main distillation column involved and lead to the elimination of the BH problem. The design and operation of a DWDC fractionating an equi‐molar mixture of ethanol, propanol, and butanol are examined to evaluate the systematic procedure proposed. Both open and closed‐loop controllability assessments demonstrate that feed splitting can serve as an effective means to eliminate the BH problem. The resultant DWDC with the elimination of the BH problem is even found to exhibit improved process dynamics and controllability when three specifications have been given, respectively, to the primary components of its three products. © 2015 Curtin University of Technology and John Wiley & Sons, Ltd.