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 L a, were measured for a range of superficial gas velocities. Application of vibrations to the liquid phase leads to enhancement in ε and k L a 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.