Study On The Separation Of Minimum Boiling Azeotropes By Extractive Distillation With Mixed Solvent

The separation of solvents is always involved in chemical industry, some solvents form azeotropes which can’t be separated by ordinary distillation. Extractive distillation is widly used to overcome this problem. Because single solvent can’t guarantee high selectivity and high solubility at the same time, extractive distillation with mixed solvent is developed, but a simple and efficient screening strategy for mixed solvent is not yet exists. Hexane-ethyl acetate mixture, cyclohexane-propanol mixture and cyclohexane-isopropanol mixture all can form binary minimum boiling azeotropes. The separation of these mixtures is studied in this thesis by the selection of mixed solvent, the simulation of extractive distillation process and the experiments of extractive distillation process.1) Firstly, two parameters were selected to measure the selectivity and solubility of single solvent. Secondly, a single solvent with high selectivity and another single solvent with high solubility were combined to form alternative mixed solvents. Thirdly, when the composition of mixed solvents changed within the whole range, the relative volatility between the two components of the system was calculated to determine the best mixed solvent and its composition. Finally, the pseudo-binary vapor-liquid equilibrium diagram of the system with mixed solvent was calculated and drawn to show the effect of mixed solvent. According to the screening strategy above, DMSO/DMAC was choosen as the binary mixed solvent to separate hexane-ethyl acetate mixture, DMSO/NMP was choosen as the binary mixed solvent to separate cyclohexane-propanol mixture, and DMSO/DMAC was choosen as the binary mixed solvent to separate cyclohexane-isopropanol mixture. The relative volatility between the two components and the pseudo-binary vapor-liquid equilibrium diagram of the system were evaluated by vapor-liquid equilibrium experiments to verify the feasibility of mixed solvent.2) With the selected mixed solvent, continuous extractive distillation process was simulated and optimized using Aspen Plus for each system. The process of continuous extractive distillation with two towers was simulated using RadFrac module. Effects of the mixed solvent feed stage, the mixture feed stage, the reflux ratio and the mass ratio of mixed solvent to mixture (solvent ratio) were investigated using sensitivity analysis module and the optimum operating conditions were obtained.3) With the selected mixed solvents and the determined operating conditions, experiments of extractive distillation for each sysem were conducted using an extractive distillation column. By changing the reflux ratio and the solvent ratio separately, effects of the reflux ratio and the solvent ratio were confirmed and the feasibility of extractive distillation with the mixed solvent was proved. All mixtures were separated successfully, and the purity of the products can get 98%wt or more.These three minimum azeotropes were successfully separated by extractive distillation with mixed solvent, which indicated that the screening strategy for mixed solvent and the simulation of extractive distillation process proposed in this thesis were accurate and feasible. These methods can be used in directing the research and design of extractive distillation process with mixed solvent.