Underutilization of loop reactors due to desorption

Abstract

Loop reactors or circulating bubble columns are considered to be promising multiphase contactors to conduct biochemical and chemical reactions due to flexibility in their design, their low power requirement and low fluid shear stresses in these reactors. A generalized model for external loop airlift reactors (EL-ALRs), accounting for axial variations in holdups of the gas and liquid phases and total pressure, is employed, to predict the performance of these reactors when used for slow liquid phase reactions. As the liquid travels upward in the riser, the decrease in the local pressure due to reduction in the static head of liquid reduces the saturation concentration of the solute (coreactant transferred from the gas phase) in the liquid phase. At certain locations in the riser, especially in the top portion of the riser, the liquid phase concentration of the solute may exceed the local saturation level resulting in desorption of the reactive solute. This undesired situation leads to underutilization of the EL-ALR as far as conversion of the absorbed reactant is concerned. The effects of design parameters, such as the height to diameter ratio, area ratio, and sparger location, and operational parameters, such as superficial feed gas velocity, composition of the feed gas, kinetic coefficient, and top pressure, on the performance of the EL-ALR are examined in considerable detail. Existence of optimum values of height-to-diameter ratio, sparger location in the downcomer, area ratio, and top pressure for minimization/obviation of desorption, maximization of net rate, and maximization of net rate per unit power consumption is established. The importance of proper selection of the key design and operating parameters for EL-ALRs for optimal performance of these is demonstrated.