Effect of hydrodynamic instabilities on solute transport in a membrane system

The effect of the gravitational instability of the concentration boundary layers (CBLs) on the substance transport through a membrane is reported. The solute transfer is caused by the diffusion through the membrane and enhanced by the convective mass transport due the hydrodynamic vertical flow in near-membrane regions. The study has been carried out with binary mixtures—aqueous ethanol solution using laser interferometric technique. Measuring the concentration profiles, we have found that the transition from diffusive to diffusive–convective transport of solute at CBLs is related with stable or unstable behaviour of the CBLs. Their Rayleigh number determines the stability of these layers. The method to determine the critical concentration Rayleigh number is based on equation, which includes the diffusive permeability of the membrane, ωm, solution parameters (density, ρ, kinematics viscosity, ν, diffusion coefficient, D) and critical value of the concentration boundary layers thickness, δcrit.