Improving mass transfer in gas-liquid dispersions by vibration excitement

Abstract

The objective of our work is to demonstrate that significant enhancement of gas-liquid mass transfer in bubble columns can be achieved by subjecting the liquid phase to low-frequency vibrations. A special device, called a vibration exciter, is mounted at the bottom of the bubble column. The vibration is transmitted to the liquid phase by means of a piston. Both the amplitude of the vibration, and its frequency can be adjusted quite accurately. We show that application of low-frequency vibrations, in the 40-120 Hz range, to the liquid phase of an air-water bubble column causes significantly smaller bubbles to be generated at the nozzle. In experiments with a single capillary nozzle the bubble size is reduced by 40-50% depending on the gas flow rate. Using a 12-capillary nozzle arrangement, the gas holdup, ε, and the volumetric mass transfer coefficient, k_La, were measured for a range of superficial gas velocities. Application of vibrations to the liquid phase leads to enhancement in ε and k_La values of up to 400%. A careful study of the influence of vibration frequency, vibration amplitude and column height reveals that subtle resonance phenomena are at play. It is concluded that application of low-frequency vibration has the potential of improving the gas-liquid contacting in bubble columns.