Study On Process Intensification Of Separation Process Of Low-Carbon Mixed Alcohols

In the coal chemical industry with the Fischer-Tropsch synthesis（F-T）reaction as the core reaction,whether it is coal-to-oil or direct coal-to-alcohol,the process will produce C_1-5low-carbon mixed alcohols.Low-carbon alcohols can be used as clean fuel alcohols,gasoline additives and important mono-alcohol chemical products.However,when the C_1-5low-carbon mixed alcohol is generated,a large amount of water is also generated.Except methanol does not form azeotrope with water,all other alcohols form azeotrope with water.The problem of alcohol-water azeotrope is doomed to the disadvantages of high energy consumption,low recovery rate and complex process of the traditional separation process of low-carbon mixed alcohol.Therefore,in this study,the separation process of low-carbon mixed alcohol is the main line,and the process is intensified for the purpose of reducing energy consumption,improving yield and simplifying the process.Then,three process intensification processes of dividing wall column（DWC）intensification process,DWC-salting-out-distillation process and double-DWC-salting-out-pervaporation process were respectively proposed,and the intensification process was studied by means of combining simulation and experiment.In the intensification process of the DWC,the process of the first division section of the traditional separation process of low-carbon alcohol is intensified by using the technology of the DWC.And in view of the difficulty of optimizing the DWC,taking the minimum heat duty of the reboiler as the objective function and the column separation requirement as the constraint condition,firstly,the sequential quadratic programming method（SQP）combined with the sensitivity analysis was used to optimize the DWC by single factor.And then get its local optimal parameters.Then,based on the local optimal parameters of the DWC,the response surface coupled particle swarm optimization（RSM-PSO）is used to optimize the DWC globally.The simulation results show that the energy consumption of the partition wall column obtained by using the RSM-PSO coupling algorithm can be further reduced by 4.01%compared with the single factor optimization method,and the energy consumption of the DWC intensification process can be reduced by 21.86%compared with the traditional low-carbon alcohol process.In order to further improve the yield of low-carbon mixed alcohol and reduce energy consumption,the salting-out technology is used to strengthen the process of the DWC strengthening process,and the mixed alcohol solution collected from the side of the DWC is dehydrated by the salting-out method.Firstly,experiments were carried out.Potassium carbonate and its saturated salt solution,potassium pyrophosphate and its saturated salt solution were selected as salting-out agents,and the effects of potassium carbonate and potassium pyrophosphate salt concentrations on the separation effect of mixed alcohols at 298.15 K were respectively discussed.And the Othmer-Tobias equation was used to test the experimental results.Then,the effect of salt on the separation of mixed alcohols when saturated at different temperatures（298.15 K-354.95 K）was studied.In addition,the effects of two saturated salt solutions on the separation of mixed alcohols at different salting-out ratios（0.5-3）and at different temperatures（298.15 K,354.95 K）were also discussed.The experimental results show that both potassium carbonate and potassium pyrophosphate are good salting-out agents,and have good separation and dehydration effects on mixed alcohols.Finally,354.95 K saturated potassium carbonate solution was selected as the salting-out agent in the process simulation when the salting-out ratio was 2.5.The obtained DWC-salting-out-distillation process reduced the energy consumption by 42.29%compared with the traditional low-carbon alcohol process.Finally,in order to meet the proposed purpose of process enhancement,pervaporation and DWC technology are used to intensify the downstream process of the DWC-salting-out-distillation process,and the excess water after salting out is removed by pervaporation technology and sent to the second DWC for distillation to obtain mono-alcohol products.The pervaporation unit is modeled according to the pervaporation experimental data,and the separated material composition and the required membrane area are obtained.The simulation results show that after the separation by the pervaporation unit,the mixed alcohol contains only 0.22%water,and the membrane area at this time is 1 207.42 m²,the designed double-DWC-salting-out-pervaporation process has the characteristics of low energy consumption,high recovery rate and simple process,which can meet the requirements of target.This process reduces energy consumption by55.07%compared with the traditional process of low-carbon alcohol.In addition,for the preliminary separation of low-carbon alcohols in Fischer-Tropsch synthesis water,In this study,four alternative preliminary separation processes,direct two-column distillation,pervaporation-two-column distillation,direct DWC distillation,and pervaporation-DWC distillation,were designed first.According to the pervaporation experimental data,the pervaporation process model was constructed and simulated in Aspen Plus,and the optimal process parameters and simulation results of the distillation process were obtained through sensitivity analysis.The energy consumption and effective energy loss of the four processes were compared.The results show that the pervaporation-DWC distillation process has obvious energy-saving advantages.Compared with direct two-column distillation,the energy consumption could be reduced by 15.85%,and the effective energy loss could be reduced by 45.74%.After pre-concentration by the pervaporation membrane,the concentration of the solution could enter the appropriate separation concentration range of the DWC,so as to give full play to the advantages of the DWC.Since the energy required for pervaporation could be provided by low-grade heat sources such as waste heat,the loss of effective energy could be significantly reduced in the coal chemical industry with sufficient waste heat.For this process,when the price of pervaporation membrane is lower than 438 yuan/m²,the coupled process of pervaporation-DWC distillation would show higher economy.