Intensification And Optimization Of Liquid-phase Oxidation Of P-Xylene

The liquid phase oxidation of p-xylene (PX) is the core of the whole purified terephthalic acid (PTA) production technology. With the maturity of oxidation technology and intensified competition of PTA industry, it has been become the new target to pursue the more efficient and more energy-saving PX oxidation technology. In this work, the intensification and optimization of PX oxidation process were studied by combining experiments with model simulation as follows:1. The main and side reaction kinetics of liquid-phase oxidation of PX was investigated by using both semi-continuous and continuous experiments at conditions of lower oxygen partial pressure (OPP). It was found that there exists a threshold for the influence of oxygen partial pressureon PX oxidation, which is between 17.1 and 22.8 kPa. The OPP is of substantial influence on the PX oxidation kinetics when it reduces from 22.8 kPa. Based on the free radical chain reaction mechanism, the simplified PX oxidation kinetic model under oxygen limited conditions with eight parameters was developed and these parameters were estimated with narrow confidence intervals through the semi-continuous experiments. The model identification results showed that the model established in this work prevails over others reported in literatures from the fitting effect, statistical tests, and residual analysis. The side reaction was also studied for the influence of OPP. Based on the reaction mechanism of CO2 formation, the side reaction kinetic model under oxygen limited conditions was established. The results showed the calculated values from model accords with the experimental values well.2. The promoting effect of CO2 on liquid phase oxidation of PX catalyzed by transition metals/bromide was investigated in both semi-continuous and continuous experiments under conditions of industrial interests. The results showed the PX oxidation reaction was obviously strengthened with the presence of CO2. There exists an optimal value for the yield of TPA with the increase of CO2 content. On the basis of free radical chain reaction mechanism of MC oxidation of alkyl aromatics, the kinetic model involving six parameters was developed of CO2-assisted PX oxidation. This model fits the experiments well with narrow confidence intervals of rate constants for conditions of various CO2 addition, temperatures, and catalysts. It was found that only the rate constant concerning chain initiation was dependent of different experimental conditions while other model parameters kept constant. It was confirmed that the promoting function of CO2 could be ascribed to the formation of active peroxocarbonates resulting from a synergistic interaction of CO2 and O2. The side reaction for CO2-assisted PX oxidation was investigated, which showed the CO2 not only accelerates the main reaction of PX oxidation, but also restrains the side reaction.3. The continuous oxidation experiments of oxygen-limited and CO2-assisted PX oxidation have been carried out to verify the kinetic model of PX oxidation under lower oxygen partial pressure, and the distribution of oxygen inside the industrial PX oxidation reactor and the multistage loop reactor (MSLR) were simulated. The CSTR (continuous stirred tank reactor) model of the oxidation reactor was established to simulate the industrial PX oxidation reactor, and it was found that simulation resultsfitted the outlet value of industrial reactor well. Meanwhile, the model was also used to simulate the The axial back mixing model (ABM) and stirred tank reactor partition model (STRP) were established to simulate the distribution of gas and liquid inside industrial reactor of PX oxidation. The calculation results from ABM model showed that the concentrations of liquid components inside reactor are almost same, and close to the uniform distribution. There exists the distribution of partial pressure and flow along the tower for O2、HAc and H2O. As for the STRP, the simulation results showed when the oxygen deficient region inside the reactor were more than 50%, the conversion of PX would be lower than 99.3%, and the yield of TPA be under 93%. The effect of liquid exchange fraction was very small. The model of MSLR was also established, and the simulation results showed that with increasing of the numbers of reaction units, the conversion of PX and yield of TPA were improved significantly. When the reaction units were more than 5, the p-TA and 4-CBA content can be reduced by up to 80% and 65%, respectively.4. The oxidation load distribution for two reactor in cascade was optimized by both experiments and model simulation. The results showed that the temperature of the first reactor has substantial influence on the whole outlet index of series reactor. With the increase of temperature in the first reactor, the impurity content in the reactor was reduced significantly. Whether the raising of reactor temperature, or that of the crystallizer temperature, the morphology of solid particles change little. When the total residence time of two reactor was fixed, extending the residence time of the first reactor was not helpful to reducing impurities, and meanwhile the reduction of impurities is obvious in the second reactor.