Research On Modeling And Optimization Of An Industrial Acetic Acid Dehydration System Via Heterogeneous Azeotropic Distillation

In the production of pure terephthalic acid, tiny amount of reactant p-xylene (PX) and by-product methyl acetate (MA) may enter into the acetic acid (HAc) dehydration system through the feed stream. In this work, considering PX and MA as feed impurities in the industrial HAc dehydration process via heterogeneous azeotropic distillation using n-propyl acetate (NPA) as entrainer, the binary parameters for the thermodynamic model of the quinary mixture were obtained by correlating the phase equilibrium data from experiment and literature. The phase equilibrium behavior and distillation character of the quinary system in the process were predicted and analyzed. Based upon the phase equilibrium thermodynamics, the models of the actual overloaded HAc dehydration process were developed and the process simulation was conducted. Then the sensitivity analyses of key operation parameters were conducted, and the operation optimization of each plant and the whole process was carried out. All these provide a new technical guidance for the operation optimization in HAc dehydration process. The main contents of this paper are summarized as follows:(1) The vapor-liquid equilibrium (VLE) of four bianry systems of PX+HAc, PX+MA、 PX+NPA and MA+NPA were measured under atmospheric pressure using cycle method with Ellis ebulliometer, in which the system of PX+HAc was employed to verify the reliability of our experimental approach, and the other three systems are not credible or can’t be found in literature. To confirm the quality of experimental data, the thermodynamic consistency tests of the VLE data of these four systems were conducted with Herington’s area test method, and the results shows all the four systems pass the tests.(2) According to the physical property of the quinary mixture of HAc+Water+NPA+PX+MA, HOC and UNIQUAC models were chose respectively to compute the strong nonideality of vapor and liquid phase caused by the association of HAc. The binary parameters of the quinary mixture in the UNIQUAC model were regressed with the equilibrium data from experiment and literature using Maximum Likelihood Method, and the estimated results of phase equilibrium agree well with the measured data. Based on these regressed parameters, the distillation characters of the quinary mixture in the industrial HAc dehydration columns were analyzed by triangle phase diagrams and residuce curve maps (RCM), which provides rigorous theory guidance and initial value prediction reference for the further modeling research.(3) The equilibrium stage steady-state mechanism model of heterogeneous azeotropic distillation was developed, and the simulation of the industrial HAc dehydration system including HAc dehydration column, PX pure column, decanter and NPA recovered column under standard design condintion was carried out with Aspen Plus. The simulation results agree well with the process data with the error being in±6%, which indicates the correctness of the thermodynamic model, the regressed binary parameters, the mechanism model and the simulated method used in the reseach. Then the simulated process model of the HAc dehydration process under actual overloaded condition was was conducted by amending the model of standard condition using the process data collected from acutual prodution, and the model can accurately describe process mechanism and operating characteristics of the actual overloaded process with the error of simulation results within±8%. Based on the model, the influences of key operation parameters to the energy consumption and separation effect were analysed. The results show that the greatest impact to the energy consumption of HAc desydration column are the reflux and the bottom water content; feed flow of saturated steam direcly influencs the separation effect of PX pure column; and the separation effect of NPA recycle column directly depends on the flow rate of saturated steam and side-draw.(4) Based on the strict calculation of mechanism model, the operation optimization analysis of each plant (HAc dehydration column, decanter and NPA recycle column) was conducted using RCMs, sensitivity analysis and software optimization module. Then the operation optimization of the whole dehytration process was carried out with the minimum total reboiler duty on the basis of best separating effect as optimal object, and the bottom water content and reflux of HAc dehydration column, temperature of decanter, component of side-draw and flowrate of top distillate of NPA recycle column were choosen to be the key operating variables. Under the optimized conditions, there is3.27t/hr saturated steam (about3.53million Yuan) can be saved for the single HAc dehytration column and2.56t/hr saturated steam (about2.76million Yuan) can be saved for the whole HAc dehytration process. Operating under the optimal conditions will enhance separating effect and reduce the energy consumption and thereby increase profit, which provids effective theoretical guidance for the optimizing operation of actual process.