Hydrodynamics of gas-lift reactors with a fast, liquid-phase reaction

The reactive absorption of CO₂ into concentrated KOH solutions was studied in an external-loop, gas-lift reactor. Three different inlet gas compositions were used: air, 50-50 vol% air-CO₂, and pure CO₂. The downcomer liquid velocity and the axial profile of the cross-sectionally averaged gas holdup in the riser were measured. The reaction is so fast that the CO₂ is consumed appreciably along the riser, and this causes a significant reduction in the liquid circulation relative to a system with no reaction. A one-dimensional, pseudo-steady-state model has been developed to describe the interactions of hydrodynamics, mass transfer, and chemical reaction for the bubbly flow regime in the riser. The model considers mass transfer from the gas to the liquid phase and its enhancement due to the chemical reaction, and is based on the spatially averaged equations of continuity, momentum, and macroscopic mechanical energy. The rate of liquid circulation, and the axial variation of gas holdup, gas composition, pressure, and gas and liquid velocity, are predicted. The gas/liquid mass transfer coefficient and the bubble radius at the sparger, neither of which was known a priori, were used to minimize the error of the data with respect to the model.