Effect of boundary layer mass transfer resistance in the pervaporation of organic solutes from dilute aqueous solutions

The separation of dilute aromatic solutes from their aqueous solutions is investigated using a membrane pervaporation process. Particular emphasis is laid on demonstrating the importance of liquid phase boundary resistance to solute transport. Identifying the controlling resistance is a critical factor influencing module design and process efficiency. The pervaporation process is modelled using a two resistance model incorporating both the membrane resistance and the liquid phase resistance. Both flat membrane and tubular membrane modules are employed in the current study and the boundary layer effect is investigated by studying the variation of the observed solute permeability with membrane thickness and the hydrodynamic conditions. For the flat membrane configuration, a phenomenological approach is used by expressing the flux in terms of the chemical potential driving force and a simple expression for the observed permeability as a function of membrane thickness is obtained. Selectivity relationships for various conditions are also established. An attempt is also made to reconcile the different pervaporation expressions employed by various investigators. For the tubular flow module, a more fundamental approach is employed by solving the continuity equation for the solute flowing through the membrane tubes. A numerical method based on the Crank-Nicholson technique is developed to solve the resulting partial differential equation governing solute composition variation through the membrane. The theoretical predictions correlate the experimental data reasonably well. The dimensionless Graetz number is found to be a useful parameter in terms of which the experimental and theoretical results are represented. Another important parameter that is identified is the dimensionless E parameter, which represents effects due to solute/membrane interactions, the extent of solute non-ideality in the aqueous phase and membrane module dimensions. It represents the relative importance of the liquid phase resistance in relation to the membrane resistance.;The central aim of the research study is to drive home the fact that for transport processes involving a liquid phase, the liquid phase boundary resistance is an important factor affecting the overall solute yield and process efficiency.