Study On Gas-Liquid Dispersion For P-Xylene Oxidation Reactors

Purified terephthalic acid (PTA), one of the main raw materials of polyester, is produced by p-xylene (PX) liquid phase oxidation. Column reactor and stirred reactor are two kinds of typical reactors for producing PTA. Bubble size distribution and local gas holdup are two most important parameters of gas-liquid dispersion; they directly affect the gas-liquid mass transfer and reaction, and play an important role in scale-up and design of reactors. Based on these technology and equipment, this thesis made an engineering study on local gas-liquid dispersion of the two kinds of typical reactors.1. Study on bubble size distribution and local gas holdup in bubble column.The bubble size distribution and local gas holdup in a bubble column with inner diameter 0.38m were measured with dual conductivities probes. Flow pattern in bubble column can be divided into transient flow regime and fully developed flow regime, according to the ratio of height above gas sparger to diameter equals to about 2.5. Local gas holdup increased with the axial height in the transient flow regime, but remained constant with axial height change in fully developed flow regime. Local gas holdup increased with the increase of gas velocity, but fluctuated in the vicinity of wall. Radial profile of gas holdup was parabolic shape. Bubble size increased with the axial height and decreased with the increase of gas velocity and radial position.2. Study on bubble size distribution and local gas holdup in stirred reactor.Bubble size distribution and local gas holdup were studied comprehensively in typical stirred reactor with 6DT-4PTU combination, including under various stirring speed and gas velocities. With the increase of stirring speed, bubble size decreased, and local gas holdup increased. With the increase of gas velocities, bubble size remained constant and local gas holdup increased in districts of upper impeller, upper cyclic and surface area, and remained constant in other districts. Average bubble size of 6DT-4PTU combination was correlated satisfactorily.Gas-liquid dispersion performance of another six different impeller combinations was studied. Lower impeller changes had little effect on local gas holdup, and gas-liquid dispersion was decided by upper impellers. For impeller combinations studied, 6PCBDT-4PTU was recommended comparatively, in particular, it was preferred at small bubble size, larger interfacial area and lower stirred power (thevalue of Ψ was the greatest) under the same operational conditions. Finally, average bubble size of 6PCBDT-4PTU combination was correlated satisfactorily.3. Simulation oxygen distribution of non-ideal typical PX oxidation reactorsUsing the network-of-zones model, oxygen distribution in two typical reactors was studied. PX oxidation reaction was controlled by reaction in lower part of reactors, and effected heavily by gas-liquid mass transfer in upper part of reactors.