HYDRODYNAMIC AND MASS TRANSFER CHARACTERISTICS OF COCURRENT DOWNFLOW BUBBLE COLUMN

The downflow bubble column is a new concept for contacting gas and liquid/slurry, as it provides a novel means for creating a long term contacting between the gas phase and liquid/slurry phases. The increased gas phase residence time in the downflow bubble column makes it suitable for reactions which occur in the diffusion regime or which require high gas phase conversions, or wherein the ratio of the throughputs of the liquid to the gas phase is greater than unity. Some examples are Fischer-Tropsch synthesis, chlorination reactions, effluent treatment etc.; The aim of the present investigation was to study the hydrodynamic and mass transfer characteristics in a multiphase downflow bubble column. The experiments were performed in a 0.075 m diameter and 2.65 m tall glass column. The hydrodynamic characteristics studied were phase holdups and gas phase backmixing. The phase holdups were studied using the hydrostatic head technique coupled with gravimetric analysis. The backmixing characteristics of the gas phase were determined by measuring the axial oxygen concentration profiles of the gas phase and using a two parameter dispersion model. The mass transfer characteristics studied were interfacial area (a) and volumetric mass transfer coefficient (k(,L)a) using the chemical method of sulfite oxidation.; The effects of a wide range of physical parameters such as gas and liquid velocities, surface tension, electrolyte solutions, solid size, concentration and nature of the solids were investigated. The gas and liquid velocities were varied from (0.001-0.025) m/s and (0.20-0.36) m/s respectively. The surface tension was varied from (0.073-0.036) N/m, using dilute alcohols and Triton-114 solutions. Three different solid particles, namely coal, shale and glass beads of sizes (30-265) (mu)m were utilized. The solid concentration was varied from (4-15) % w/w solids.; The experimental findings revealed that the values of the design parameters ((epsilon)(,g), a, k(,L)a), as also the effects of dilute alcohols and electrolytes on these parameters were very different from those observed in conventional (upflow) bubble columns. The data collected with solids provided some insight into the influence of solids on these parameters. Lastly, measurements of gas phase dispersion showed that a large degree of backmixing prevails in the gas phase. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMI...