A spray reactor concept for catalytic oxidation of p-xylene to produce high-purity terephthalic acid

Abstract

A spray reactor in which the liquid phase containing dissolved p-xylene (pX) and the catalyst is dispersed as fine droplets by a nozzle into a continuous vapor phase containing the oxidant (O2) is shown to produce high-purity terephthalic acid (TPA) with less than 25ppm 4-carboxybenzaldehyde (4-CBA) in the solid TPA product. In sharp contrast, the solid TPA product obtained from a conventional stirred reactor similar to the configuration used in the conventional Mid-Century (MC) process contains nearly 1000ppm 4-CBA even though the reactor is operated at similar pressure and temperature (15bar and 200°C) but with the gas phase dispersed into the liquid phase. The dramatic improvement in TPA product quality during spray reactor operation is attributed to two main factors: the alleviation of interphase gas–liquid mass transfer limitations that facilitates more complete oxidation of the pX and the intermediate oxidation products to TPA, and reduced backmixing that enhances the oxidation rates. Theoretical calculations show that the time constants for O2 diffusion in typical spray droplets (50μm diameter) is 1–2 orders of magnitude lower than the kinetic rate constant confirming complete O2 penetration and saturation of the droplets. Further, the usage of CO2 as an inert gas and the dominance of acetic acid (>50mol.%) in the vapor phase under reaction conditions create an environment that falls outside of the flammability envelope. Mathematical modeling of the stirred reactor using MC process conditions accurately predicts the steady state temperatures observed in industrial reactors. The model also clearly divulges that the cooling provided by partial evaporation of the acetic acid solvent is vital to maintain stable steady state operation. By eliminating the need for a hydrogenation step, the spray reactor has the potential to not only reduce capital and operating costs but also provide environmental benefits.