(426f) A Spray Reactor for One-Step Polymer-Grade Terephthalic Acid (TPA) Production

Recently, we demonstrated a spray reactor as an alternative to the Mid-Century (MC) process for producing TPA. In the MC process, the air is dispersed into the stirred liquid phase containing p-xylene and the Co/Mn/Br based catalyst dissolved in acetic acid. At 200 °C and 20 bar, the solid TPA product is approximately 99.5% pure. The impurities are due to the partial oxidation products, most notably the 4-carboxybenzaldehyde (4-CBA). The 4-CBA is eliminated in a subsequent reactor and the TPA is thereby further concentrated to greater than 99.99% purity to obtain a polymer-grade product. In the spray reactor, a nozzle is used to disperse the liquid phase as a mist into the gas phase containing O₂. The increased gas/liquid interfacial area was aimed at eliminating O₂ starvation in the liquid phase, thereby avoiding incomplete oxidation that results in the undesirable impurities such as 4-CBA. Our first demonstration of the spray reactor produced a TPA product that was purer than that from the MC process but still had impurities requiring removal. Measurement of the axial temperatures showed significant gradients with temperatures less than 200 °C in certain zones. The spray reactor was redesigned to minimize axial temperature gradients. In the redesigned spray reactor, the temperature was maintained above 200 °C throughout the reactor, resulting in significant improvements in the resulting TPA yield (>98%) and solid TPA purity (>99.99%). These results along with the effects of spray reactor operating variables (p-xylene concentration, oxygen partial pressure, reaction temperature, catalyst concentration and spray rate) on the temperature profiles, product purity and acetic acid burn rate will be presented. A mathematical model which accounts for the gas/droplet mass transfer with simultaneous reactions and heat effects will be presented to show the effect of droplet size on oxygen penetration into the droplet and its effect on the TPA yield and solid TPA purity.