A review of mass transfer to interfaces

The theories for mass transfer to rigid or mobile interfaces are reviewed and compared with selected experimental data. Except for transfer to particles in stagnant fluids, there is no case where the rate depends on the first power of the diffusivity, as implied by the film theory. The transfer coefficients for single rigid spheres, for particles in packed beds, and for the gas film in packed absorbers vary with about the 2/3 power of the diffusivity, as predicted by the boundary layer theory, which works fairly well for these cases because transfer to the wake region is relatively small. The penetration theory holds exactly for many devices which expose liquid surfaces for a short and known contact time, and, judged by a 0.5 exponent for the diffusivity, the penetration theory probably holds for the liquid phase in absorbers and for both phases in bubble‐cap towers. Theory and data for transfer to mobile interfaces show the coefficients to vary with D 0.5 to D 3/5 , the limiting solutions for high and low interface velocity corresponding to the penetration theory and the boundary layer theory. Mass transfer in turbulent fluids is an unsteady‐state process, and the transfer coefficient varies with the 0.5‐0.8 power of the diffusivity even with very high turbulence.