The Simulation And Study On The Crystallization Process Of Crude Terephthalic Acid

Purified terephthalic acid (PTA) is the important feedstock in production ofpolyester. Usually in the first step of the process, crude terephthalic acid (CTA) isobtained via catalytic oxidation of liquid p-xylene with air at high temperature inacetic acid solvent through multistage crystallization as pressure drop andvaporization. CTA is then purified by hydrogenation resulting purified terephthalicacid. This paper focus attention on the research and analysis of the oxidation endscrystallization as pressure drop and vaporization. The basic thermodynamic andkinetic data of oxidation ends crystallization is presented, it is fulfilled that theprocess simulation of multistage crystallization as pressure drop and vaporization ofthe industrial oxidation ends and the optimization of process operating condition isput forward.It is necessary to design and develop a special experimental device for PXoxidation reaction so that in the crystallization kinetics test we can directly use hotoxidation ends produced in the laboratory that is close to the discharge of industrialoxidation reactor.The solubility of terephthalic acid in acetic acid at quasi industrial oxidation endscondition is measured after ascertaining the solubility measuring method of TAthrough experimental test. It is clear that the solubility of TA in acetic acid is small atlow temperature while relatively large at higher temperature. By using the lasermethod to measuring metastable zone of TA in acetic acid in oxidation ends conditionit is determined that the effect of metastable zone with initial concentration of TAtemperature drop rate and oxidation conversion. The calculate formulas of solubilityand metastable zone of TA are obtained from the experimental data regression.Through experimental research of crystallization thermodynamics it will providebasic data for the crystallization process simulation.By means of batch dynamic method the TA crystal nucleation and growth kineticdata were measured in the laboratory, with which the regressive parameter of crystalnucleation and growth kinetic model was obtained. The research results indicate thatin the course of TA crystallization the most important effective factor to crystalnucleation and growth is its supersolubility. As for crystal nucleation secondarynucleation is the main contribution while the effect of crystal suspension density tothe nucleation rate is quite small. According to the characteristic of TA crystallizationkinetics the crystal size distribution could be improved by choosing suitable operatingcondition.A mathematic model of continuous multistage crystallization was made based onthe thermodynamic and kinetic research of TA crystallization, with which theindustrial crystallizers were simulated and the calculated results were coincident withthe site measuring parameter, it thus verify the reliability of the model. By simulationthe influence to crystal size distribution of operating parameters were evaluated, andin this basis it was presented that the design parameter of TA multistage crystallizer aspressure drop and vaporization of a 500000t/a PTA unit.