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

Abstract

The gas holdup ( G ), 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 N 2 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 acidbenzaldehyde) 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 G 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%.

Mass transfer studies in shallow bubble column reactors

“…Some researchers found k L to be independent of U G [31][32][33][34]. Gopal and Sharma [35] found k L to decrease slightly with U G [12] while Lemoine et al [35] and Mena et al [36] found k L to increase with U G . Han and Al-Dahhan [19] found that k L increases with U G at low U G range but the increase in k L is not significant at high U G range.…”

Novel correlations for gas holdup in large-scale slurry bubble column reactors operating under elevated pressures and temperatures

“…Froude number = U G /(gd) 1 dense phase (small gas bubble phase) G gas phase L liquid phase P solid particle SL slurry trans transition from homogeneous to heterogeneous…”

“…(1) to predict the corresponding total gas holdup. It should be emphasized that BCRs and SBCRs operating in the churn-turbulent flow regime, small and large gas bubbles were reported to coexist [1,15,19,28]; Eq. (1) is valid for predicting the total holdup of all the gas bubbles present.…”

“…This is because most of these correlations were developed for aqueous, highly ionic systems in small-diameter reactors operating under atmospheric pressure and/or ambient temperature; the data were obtained with different gas spargers; and the majority of the solids were non-catalytic particles. Statistical correlations have also been proposed for predicting the hydrodynamic and mass transfer parameters in two-phase and three-phase reactors and although they have been shown to enjoy high confidence levels [1], they are system-dependent and accordingly their application to predict and/or extrapolate the behavior of other gas-liquid-solid systems could be misleading [1]. Thus, adequate correlations which can be used to predict the gas holdup in multiphase reactors operating under wide ranges of industrial conditions are needed.…”

Efficiency of non‐reactive isothermal bubble column based on mass transfer

“…[17] This verdict of the pressure effect on the k l values was corroborated by Lemoine. [44] The apparent decrease of k l is attributed to the change in bubble dynamics and hydrodynamics at 0.4 MPa. [45] At high pressure, smaller bubble sizes have lower slip velocity.…”