Aging Of Crude Terephthalic Acid

PTA (Purified Terephthalic acid), one of the most important raw material of polyester, is produced by two steps: catalytic oxidation of PX (para-Xylene) to CTA (Crude Terephthalic acid) and latter purification. With the domestic technology development of PTA as background, this thesis focuses on the aging process of CTA. By experiments and modeling, some novel results in crystallization thermodynamics, aging kinetics of CTA particles and purification kinetics were obtained. The work included the following three parts:1. Solid-liquid equilibrium of TA system in simulated mother liquorThe solid-liquid equilibrium data of TA system is a kind of prerequisite thermodynamics data for the further research of TA production and purification. Solubilities of TA were experimental determined in the industrial operated temperature and solvent compositions. The effect of impurities in liquid phase was examined. The partition of main impurity 4-CBA (4-Carboxybenzaldehyde) between solid and liquid phase was studied respectively with and without oxygen. The results showed that it was very hard for 4-CBA to achieve solid-liquid equilibrium by molecule diffusion, however oxidation can decrease the concentration of 4-CBA in solid phase remarkably. Crystal structure affected the partition of 4-CBA between solid and liquid pahse.2. Aging kinetics of CTA particlesBased on a series of batch experiments, the evolution of CTA crystal size was carefully studied over a temperature range from 180°C to 230°C. According to the experimental results, the aging mechanism and kinetics of crystal size were developed. The results indicated that aging of CTA obeyed Ostwald ripening theory and the kinetics model can describe the crystal size evolution well. Aging of CTA at elevated temperature can improve the crystal size distribution remarkably and increase the crystal size. The velocity of aging increased with the increase of temperature.3. Aging kinetics of CTA purification The concentration of impurity 4-CBA in CTA crystals was carefully studied by batch experiments over a temperature range from 180°C to 230°C. Diffusion-surface reaction kinetics model of CTA purification was developed and the model parameters were experimentally determined. The results indicated that aging of CTA with oxygen can decline the concentration of 4-CBA efficiently in CTA crystals. The higher the temperature was, the faster decreasing of impurity 4-CBA. The diffusion-surface reaction model can illustrate the concentration of 4-CBA in CTA crystals.