The Study Of Process Intensification And Control For Separating Azeotropic Mixture By Special Distillation

Azeotropic mixtures are often formed in the production process of chemical industry,petroleum,medicine industries and so on which cannot be fully separated by the conventional distillation.Special distillation technology like extractive distillation is one of the most common methods in the separation of such azeotropic mixtures.However,the energy consumption of special distillation is high and the thermodynamic efficiency is low.In order to improve the energy efficiency and save the total annual cost of the investment,more and more attention has been paid to the process intensification design of special distillation based on the in-depth thermodynamic analysis,global coordination and optimization and other means.The mixture of ethyl acetate and ethanol is often generated in the production of ethyl acetate by esterification of acetic acid and ethanol.However,the binary azeotrope of the ethanol and ethyl acetate under atmospheric pressure leads to the impossibility of fully separation by conventional distillation process.Based on the study of extractive distillation process for separating ethanol and ethyl acetate,this work investigated the energy-efficient side-stream extractive distillation.In order to overcome the time-consuming weakness of the sequential iteration optimization method,the global optimization based on the improved genetic algorithm was finally applied in this work.The optimization of genetic algorithm is similar to the process of biological evolution in which the optimization problem of distillation process by genetic algorithm software is carried on according to the law of the survival of the fittest in biological evolution.During the multi-generation evolution process,operators such as crossover and mutation are used to ensure the diversity of suitable solutions.Finally,operators such as evaluation and selection are used to eliminate the solution with lower fitness function,so as to retain the optimal solution.The algorithm was improved on the basis of non-dominated sorting and weak mutation setting which has the significant superiority of random optimization,a designer’s preference and reducing overlapping solutions.In this work,the capital cost(CAP)and energy consumption cost(ENR)were used as the genetic algorithm objective functions.Moreover,the connection between Aspen Plus and genetic algorithm program was realized by Active X control and the genetic algorithm optimization program was developed into an software with a simple interface.By using the software,optimal operating parameters of different distillation processes with small CAP and ENR were eventually obtained.Through the comparison of the optimization results of two processes(i.e.side-stream extractive distillation and heat-integrated side-stream extractive distillation),the heat-integrated side-stream extractive distillation has shown the lowest total annual cost(TAC).Compared with the extractive distillation in the published work,the TAC of heat-integrated side-stream extractive distillation is reduced by 7.78%,and the carbon dioxide emission is reduced by 9.28%.Then,we further studied the dynamic control based on the optimal process for the separation of ethyl acetate and ethanol.Through the analysis of the dynamic performance under the feed disturbances of ± 10% flowrate and composition,the proposed control scheme of combining composition and temperature controllers has a good anti-interference ability for such complex side-stream extractive distillation process.This control scheme can also provide a good guidance for industrial application of similar side-stream extractive distillation processes.In addition,the separation of ternary azeotropic mixture tetrahydrofuran-methanol-water has already been studied by our group,in which the triple-column heat-integrated pressure-swing extractive distillation exhibited the most superiority in economic and environmental evaluation.Therefore,we further investigated the dynamic control of the complex distillation process to verify the feasibility and reliability of the optimal distillation process in the practical industry.An effective control structure with a composition controller was proposed.By comparing the IAE values,it was observed that the composition control scheme with a low level selector has the best dynamic performance.Considering the high cost of the composition controller,we finally explored the temperature control structure with a high level selector.As shown in the product purity graphs during the dynamic anti-disturbance performance,the temperature control strategy can also deal with ± 20%disturbances of feed flow and composition.These two control schemes can provide a decision reference for the industrial application of the complex triple-column heat-integrated pressure-swing extractive distillation.