Cold model study on mass-transfer enhancement at gas-liquid interfaces exposed to sound waves

Recent studies show that sonic or ultrasonic oscillations can provide an attractive tool in enhancing mass-transfer rates in fluid media. An especially significant enhancement can be obtained for the interface mass transfer due to the ability of acoustic energy to be transferred through homogeneous fluids with little energy loss and to be greatly dissipated at the interfaces. In pyrometallurgical processes, many chemical reactions proceed at the interface between gas and molten bath under gas- or liquid-phase mass-transfer control. In the present study, cold model experiments were performed to examine whether the sonic irradiation can be useful for the enhancement of such reactions. In the experiments, the rates of three gas-liquid absorption reactions were measured under different experimental conditions that include blowing of the gas onto the interface, exposing the interface to sound waves and agitating the bath with an impeller. The experimental results showed that the sound waves are able to enhance the rate of reaction if it is fully or partly controlled by the gas-phase mass transfer. Within the frequency range of 0.53 15 kHz, sound waves of higher frequencies were more effective in enhancing the gas-phase mass transfer. Besides, the enhancement effect was found to be larger under resonant-like conditions. Additional experiments revealed that sound waves impose oscillations on gas flowing above the free surface that imparts turbulent-like characteristics to the gas flow even if it is originally laminar. It is assumed that these acoustically imposed oscillations play the key role in enhancing the effective diffusion coefficient at the gas-liquid interface.