Hydrodynamic and Mass-Transfer Characteristics in Organic Liquid Mixtures in a Large-Scale Bubble Column Reactor for the Toluene Oxidation Process

The gas holdup (εG), Sauter mean bubble diameter (d S), and bubble size distribution, as well as the volumetric mass-transfer coefficient (k L a), gas−liquid interfacial area (a), and mass-transfer coefficient (k L), of N2 and air were measured in a large-scale (0.316-m inside diameter, 2.8-m height) bubble column reactor (BCR) in toluene and its mixtures (toluene−benzoic acid−benzaldehyde) under a wide range of pressure (0.2−0.8 MPa) and superficial gas velocities (0.06−0.14 m/s). The k L a data were determined using the transient physical gas absorption technique, whereas the bubble size distributions as well as the Sauter mean bubble diameters were obtained using the dynamic gas disengagement technique. The gas holdup values were measured employing the manometric method with two differential pressure cells located at different positions in the BCR. From the experimental gas holdup, Sauter mean bubble diameter, and k L a results, a and k L were extracted under various operating conditions. The central composite statistical design and analysis technique was used to study the effect of operating conditions on the hydrodynamic and mass-transfer parameters. Under the operating conditions used, the k L a, a, and εG values were found to increase with increasing gas superficial velocity and pressure, whereas the d S and k L values appeared to decrease with pressure and increase with superficial gas velocity. The effect of the nature of the gas on the hydrodynamic and mass-transfer parameters was found to be insignificant, whereas the effect of addition of benzaldehyde and benzoic acid to pure toluene, aimed at mimicking the actual continuous liquid-phase toluene oxidation process, appeared to have a strong impact on both parameters because of froth formation. The experimental data were also compared with the available literature, and statistical correlations were developed to predict the hydrodynamic and mass-transfer experimental parameters with confidence levels greater than 95%.