Simulation studies of Extractive Divided Wall Distillation Columns

Abstract

Current environmental constraints require processes that are more energy efficient and lead to lesser greenhouse emissions. Distillation is the most working, but also the highly energy intensive separation method being used in chemical process industry (CPI). Therefore, the efficient design of the distillation process is essential so as to diminish the energy requirement without compromising with the product quality. Various techniques have been tried and tested,the foremost being process intensification. Divided wall column (DWC) is an innovative design in this direction where two or more simple columns are thermally coupled into one column by providing a vertical wall. Extractive Divided Wall Column (EDWC) is a newest form of process integration, which eliminates the use of solvent recovery column required in extractive distillation. EDWC are just an extension of Divided Wall Column (DWC) for separating mixtures forming azeotropes, whose separation by ordinary distillation is not possible. Simulation of an EDWC is a difficult task, as it involves several connecting streams making the problem highly non-linear. In the present work simulation studies of an EDWC were performed for the separation of water ethanol mixture. Water ethanol mixture form a minimum boiling azeotrope and its separation by ordinary distillation is not possible. The use of EDWC was proposed for the separation of this mixture and simulation studies were done using the rigorous Radfrac model available in ASEPN PLUS. Ethylene glycol was used as a solvent for separating water ethanol mixture. Conventionally, water ethanol mixture is separated by extractive distillation, requiring at least two columns, one for carrying out separation and other for solvent recovery. EDWC eliminates the use of solvent recovery column and performs the operation in just a single column. The structural and process parameters of an EDWC are generally optimized separately and there are no reports in the literature on the simultaneous optimization of the structural and operating parameters of an EDWC. Box–Behnken design (BBD) under response surface methodology (RSM) was used for the optimization of the structural and operational parameters and to evaluate the effects of these parameters and their interactions on the energy efficiency of an EDWC. The system has many structural variables and process variables including the location of the feed stage, the location of the side stream stage, and the number of stages in the prefractionator and in the main column, location of the divided wall, number of stages in the divided wall section, reflux rate, liquid spilt, vapour split and the solvent flow rate. These variables were used for the optimization of the product purities and the reboiler duty and CO2 emission. The main objective during optimization was to achieve maximum product purities of all the components and minimization of energy requirements and CO2 emission. It was found that the process variables are highly significant for the purity of distillate (ethanol)and energy efficiency of an EDWC. However the structural variable namely, the interaction of vapour split stage with number of stages in main column and the interaction of side product stage with vapour split stage were found to be significant for the purity of distillate (ethanol) and energy efficiency of an EDWC also. The energy requirements (reboiler duty) was 2369 kW and 1880 kW for the conventional extraction distillation and the EDWC respectively for the separation of water ethanol mixture, i.e. a saving of 20.6 % reboiler duty for an EDWC. 20% less CO2 emissions was found for the EDWC as comparison of conventional process.