Novel Operation Mode And Related Application Fundamentals Of Batch Extractive Distillation

Batch extractive distillation (BED) may have both provide the advantages ofbatch distillation and extractive distillation, i.e. more flexibility, multi-product with asingle column and low capital cost, etc. It is very suitable for the separation of closeboiling-point or azeotropic mixtures in fine chemicals and pharmaceutical industriesso that increasing attention is currently directed towards BED's related studies. In thispaper, the solvent selection, the pilot-plant experiment and the simulation of regularBED and the operation mode of BED with two reboilers are well studied bytheoretical analysis and experimental method.As far as the solvent selection for extractive distillation is concerned, a practicalmethod of selecting solvent based on the combination of the modified UNIFAC modeland experimental method is proposed. The solvent selection for the separation of sixazeotropic mixtures including ethanol-water by extractive distillation is well made tovalidate the reliability of the new method. The results show that the method can avoidthe faults of less accuracy when using only theoretical model or too large amount ofwork when using only experimental method. At the same time, it is very suitable forthe selection of one solvent and mixed solvents and shows that this is a rapid, highefficient and accurate method of selecting solvent in the application of extractivedistillation.Due to few reports on the pilot-plant experiment and the industrial applicationof BED, one pilot-plant experiment for the separation of ethanol and water usingglycol as solvent in regular BED was in particular discussed. The effect of some keyoperation parameters such as the reflux ratio, the flow rate and the temperature of thesolvent on the separation processes such as the purity of the distillate, the recoveryyield of the solvent, etc, was also discussed.For studying the process of BED theoretically, the nonlinear stiff differentialequations of the equilibrium mode describling BED are well solved by the modifiedtwo-piont implicit integration method, the convergence and stability of the calculatedmethod is very good. Moreover the accuracy of the calculation is higher. Thesimulated results and the above experimental data show good agreement. The effect ofsome parameters such as feed stage location of the solvent, the heat duty of thereboiler and the liquid hold-up on the separating properties of the processes was wellstudied. The separation process of acetone and methanol using water as solvent is wellsimulated and the results prove the accuracy and commonality of the model.As for the disadvantages of regular BED such as the large reboiler's voumleneeded and the complexity of controlling the operation of the process due topermanently varying parameters, a novel operation mode of BED with two reboilersis proposed and the feasibility of the operation mode is in particular analyzed. In themode, the liquid at the bottom of the column does not flow into the feed reboiler butinto the solvent reboiler by the baffle board in the bottom of the column. Then theliquid except the solvent is vaporized into the feed reboiler by simple distillationwhile the solvent-rich liquid stays in the solvent reboiler. At the same time, thepilot-plant experiment for the separation of ethanol and water using glycol as thesolvent in new mode was well studied. The experimental data were compared withthose under the same operation condition in regular BED. The distillate with a purityof +99.82% may still be obtained from the top of the column. The results show thatthere are, in new mode, more advantages such as more stability of the temperature inthe reboiler, easier control of the process, more flexibility of the operation, higheryield of the distillate and recovery of the solvent, etc.