An experimental study on the time dependence of diffusive mass transfer of single oxygen bubbles

In the vast majority of gas–liquid engineering applications, the liquid phase contains a range of dissolved gaseous species. These dissolved process gases transfer from the liquid phase to the gaseous phase countercurrent to the typically desired mass transfer of gas to liquid. In process design, the resulting change in the composition of the gaseous phase is usually neglected, although a temporal change in the composition of the gaseous phase can directly influence the mass transfer performance over time. The current fundamental study quantifies the mass transfer performance of oxygen bubbles to liquid phases saturated with another gas. For this purpose, the oxygen mass transfer from a bubble to degassed, helium-, nitrogen-, argon- and carbon dioxide-saturated water is studied. Light Sheet Fluorescence Microscopy is used as imaging system for Planar Laser-induced Fluorescence measurements of dissolved oxygen concentration fields, delivering local instantaneous Sherwood numbers, diffusion coefficients and mass transfer coefficients. For the first time, the study showcases that the mass transfer performance from a gaseous dispersed to a liquid continuous phase is independent of time if mass transfer occurs in one direction only. If mass transfer occurs in both directions, the mass transfer performance of the dispersed phase is significantly lower and its time dependence higher to liquids containing gaseous species with high solubilities, such as carbon dioxide in water. Furthermore, the results suggest that applying intrinsic values, such as diffusion or mass transfer coefficients, obtained for binary systems in multicomponent systems can lead to high uncertainty.