Osmotic distillation — a separation process in which a liquid mixture containing a volatile component is contacted with a microporous, non-liquid-wettable membrane whose opposite surface is exposed to a second liquid phase capable of absorbing that component — is nearing commercialization for the concentration of beverages and other liquid foodstuffs, and is under evaluation for the concentration of aqueous solutions of thermally labile pharmaceutical products and biologicals. Its primary advantage lies in its ability to concentrate solutes to very high levels at low temperature and pressure, with minimal thermal or mechanical damage to or loss of those solutes. The process also can enable the selective removal of a single volatile solute from an aqueous solution (for instance, ethanol from wine and other ferments) using water as the extracting solvent. Low-alcohol-content beverages have been produced in this manner with minimal losses of volatile flavor and fragrance components. Osmotic distillation (OD) promises to become an attractive complement or alternative to other athermal or lowtemperature separations techniques such as ultrafiltration (UF), reverse osmosis (RO), pervaporation, and vacuum freeze drying.
OD, which is also called “isothermal membrane distillation,” is a membrane transport process in which a liquid phase (most commonly an aqueous solution) containing one or more volatile components is allowed to contact one surface of a microporous membrane whose pores are not wetted by the liquid, while the opposing surface is in contact with a second nonwetting liquid phase (also usually an aqueous solution) in which the volatile components are soluble or miscible. The membrane thereby functions as a vapor gap between the two liquid phases, across which any volatile component is free to migrate by either convection or diffusion. The driving potential for such transport is the difference in vapor pressure of each component over each of the contacting liquid phases. The mechanism is illustrated schematically in Figures 1 and 2. If the sole or primary volatile component in solution is the solvent, then evaporation of solvent from the solution of higher vapor pressure into that of lower vapor pressure will result in concentration of the former and dilution of the latter. Thus, the rate of transport of solvent from one liquid phase to the other will increase as the solvent vapor pressure over the receiving phase is reduced. If the solvent vapor pressure over the liquid being concentrated drops to a value equal to that over the receiving phase, no further transport will occur.