TRANSPORT OF MULTICOMPONENT LIQUID SOLUTIONS OF NONELECTROLYTES IN PRESSURE-DRIVEN MEMBRANE SEPARATION PROCESSES

The pressure-driven membrane separation of dilute multicomponent solutions of nonelectrolytes is considered. Integrated performance equations are developed for the process under consideration, starting from the extended Stefan-Maxwell equations augmented to include viscous flow effects. Several simplifying assumptions, including Onsager reciprocity, are used. Assumptions concerning the microstructure of the membrane are not needed to derive the performance equations. The performance equations for binary systems are formally identical to previously-derived results, but suggest slightly different interpretations of the parameters describing separation performance.; To test the performance equations, the permeation of binary and ternary aqueous solutions of mannitol, 1,6-hexanediol, and 1-pentanol through asymmetric cellulose acetate membranes was studied over a wide range of pressures. Solute rejection was correlated with total permeate flux using the performance equations to obtain parameters which adequately describe the observed separation behavior. For a given membrane at a given flux, solute rejection from binary solution increased in the sequence 0.10 mole percent 1-pentanol < 0.08 mole percent 1,6-hexanediol < 0.05 mole percent mannitol < 0.20 mole percent mannitol. The trends observed for binary solutions are attributed to both frictional and partitioning effects. Significant multicomponent effects were also observed. Mannitol rejection increases in the presence of either 1,6-hexanediol or 1-pentanol. 1,6-Hexanediol rejection increases in the presence of mannitol, but decreases slightly in the presence of 1-pentanol. 1-Pentanol rejection is independent of the presence of 1,6-hexanediol, but decreases in the presence of mannitol. These effects could not be attributed to either frictional coupling of solute fluxes or multicomponent effects on partitioning. The observed permeation behavior is, however, consistent with the hypothesis that the membrane consists of interpenetrating microphases, and it is suggested that further research be conducted to verify that hypothesis.; It is concluded that the observed permeation behavior can be attributed to a combination of frictional and partitioning effects; that significant multicomponent effects can occur in the membrane separation of dilute solutions of nonelectrolytes; and that the observed multicomponent effects are the result of a mechanism other than frictional coupling.