(671e) Economic and Process Engineering Evaluations of Polymer-Grade Terephthalic Acid Production In A Spray Reactor

There is emerging interest in developing renewable plastics derived from feedstocks (such as renewable p-xylene) from plant-based biomass. In such cases, it is imperative that green conversion technologies are also employed. CEBC researchers have been investigating a spray process concept as a greener alternative to the conventional Mid-Century (MC) process for terephthalic acid (TPA) production from p-xylene. Compared to the MC process in which the air is dispersed into the stirred liquid phase containing p-xylene and the Co/Mn/Br based catalyst dissolved in acetic acid, the spray process uses a nozzle to disperse the reaction mixture as fine droplets into a gas phase dominated by saturated acetic acid vapor and also containing O₂ (in slight excess over the stoichiometric amount needed for p-xylene oxidation to TPA) and CO₂ (as carrier for O₂). The increased gas/liquid interfacial area was aimed at eliminating O₂ starvation in the liquid phase, thereby avoiding incomplete oxidation to achieve improved selectivity and purity of the TPA product. In the MC process, the crude TPA from the stirred reactor contains significant amount of impurities such as approximately 1400 ppmw 4-carboxybenzaldehyde (4-CBA) and 600 ppmw p-toluic acid due to the incomplete oxidation. To obtain polymer-grade TPA, a purification process by hydrogenation associated with a significant number of additional processing steps is required. Employing a semi-continuous spray process (p-xylene sprayed continuously but no continuous product withdrawal), it was confirmed by reliable analytical methods that the solid TPA product meets the polymer-grade purity specification of 25 ppmw 4-CBA or less, implying the hydrogenation step is not required. Comparative economic analyses show that the total capital investment and utility costs associated with the spray process are roughly half of that in the MC process, thus significantly reducing the cost of the polymer grade TPA product. Preliminary experimental results from continuous operation of the spray reactor show that p-xylene was being converted mostly to TPA and that the purest solid TPA product obtained in the reactor contains 3 ppmw 4-CBA. Based on reactor modeling, factors affecting the control of reactor temperature and open-loop stability of the reactor are also elucidated.