SummaryMembrane distillation can only be applied on liquid mixtures which do not wet, a microporous hydrophobic membrane. Solutions of inorganic material in water have such high values of surface tension ( y,_ 2 72 X lo-" N/m) that the non-wetting condition is fulfilled for a number of hydrophobic membranes. As soon as orgamc solutes are present m the solution, the surface tension ;>I> will be lowered, and if the concentration of organic material becomes too high, wetting of the membrane will occur. By means of theoretical considerations a critical solute concentration or surface tension at which a homogeneous smooth material will be wetted (0 < 90' ) can be calculated. For (micro)porous membranes no such theoretical relation can be derived. Therefore, a simple experimental method is described to measure the maximum allowable concentration for a (micro)porous membrane. On the basis of these measurements, the maximum allowable concentration under process conditions can be determined

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Ethanol-water concentration profiles in cellulose acetate membranes were measured under steady-state pervaporation conditions. Knowledge of these profiles leads to a better understanding of the diffusion process during pervaporation. The concentration profiles were determined by a film-stack method, using three to six layers. It is shown that permeation of ethanol-water mixtures proceeds in a coupled way and that crossterm diffusion coefficients need to be considered. Furthermore, the occurrence of sorption resistances at the feed/membrane interface can be established from these experiments.

SummaryA new process design for pervaporatlon 1s described m which a composite membrane, conslstmg of a selective hydrophlhc toplayer and a mlcroporous hydrophobic sublayer, IS used [ 1 ] The feed mixture IS brought into contact wth the hydrophlhc layer At the permeate side of the membrane a permeate-absorbing liquid 1s brought into contact with the porous sublayer The driving force for this process IS caused by the thermal gradient that exists between the warm feed side of the membrane and the cold permeate side The restnctmg condltlon of this process design 1s that the liquid at the downstream side does not penetrate mto the pores of the hydrophobic membraneIn this design, the equipment generally needed m conventional pervaporatlon processes to produce a reduced pa&al pressure at the permeate side and to condense the permeatmg vapour 1s no longer necessaryThe performance of the membranes IS comparable to those m a conventional pervaporatlon process