Osmotic distillation

Cath, T.Y., Adams, D. and Childress, A.E. (2005) Membrane contactor processes for wastewater reclamation in space II. Combined direct osmosis, osmotic distillation, and membrane distillation for treatment of metabolic wastewater. Journal of Membrane Science, 257 (1—2), 111-119. 

The driving force, that is, a partial pressure gradient, is obtained by a temperature gradient between the feed and the distillate side. In the osmotic distillation (OD), a partial pressure gradient is activated by a difference in concentration. As a consequence, OD can proceed at ambient temperature and flavor and fragrance compounds can be more conveniently preserved, than in thermally activated concentration processes. 

In these systems, the interface between two phases is located at the high-throughput membrane porous matrix level. Physicochemical, structural and geometrical properties of porous meso- and microporous membranes are exploited to facilitate mass transfer between two contacting immiscible phases, e.g., gas-liquid, vapor-liquid, liquid-liquid, liquid-supercritical fluid, etc., without dispersing one phase in the other (except for membrane emulsification, where two phases are contacted and then dispersed drop by drop one into another under precise controlled conditions). Separation depends primarily on phase equilibrium. Membrane-based absorbers and strippers, extractors and back extractors, supported gas membrane-based processes and osmotic distillation are examples of such processes that have already been in some cases commercialized. Membrane distillation, membrane... 

In particular, liquid-liquid extractions, wastewater treatments, gas absorption and stripping, membrane, and osmotic distillation, are the processes more studied. For example, the VOCs removal, the extraction of aroma compounds and metal ions, the concentration of aqueous solutions, the acid-gases removal, the bubble-free oxygenation/ozonation, have been successfully carried out by using membrane contactors [1, 2]. 

The design of the first commercial modules has allowed the commercial application of membrane contactors for some specific operations. This is the case of the Membrana-Charlotte Company (USA) that developed the LiquiCel modules, equipped with polypropylene hollow fibers, for the water deoxygenation for the semiconductor industry. LiquiCel modules have been also applied to the bubble-free carbonation of Pepsi, in the bottling plant of West Virginia [18], and to the concentrations of fruit and vegetable juices in an osmotic distillation pilot plant at Melbourne [19]. Other commercial applications of LiquiCel are the dissolved-gases removal from water, the decarbonation and nitrogenation in breweries, and the ammonia removal from wastewater [20]. 

Peterson, P.A., Schneider, J. and Sengupta, A. (1998) Proceedings of the Workshop on Membrane Distillation, Osmotic Distillation and Membrane Contactors , Cetraro (CS) Italy, July 2 1. 

Osmotic distillation also removes the solvent from a solution through a microporous membrane that is not wetted by the liquid phase. Unlike membrane distillation, which uses a thermal gradient to manipulate the activity of the solvent on the two sides of the membrane, an activity gradient in osmotic distillation is created by using a brine or other concentrated solution in which the activity of the solvent is depressed. Solvent transport occurs at a rate proportional to the local activity gradient. Since the process operates essentially isothermally, heat-sensitive solutions may be concentrated quickly without an adverse effect. Commercially, osmotic distillation has been used to de-water fruit juices and liquid foods. In principle, pharmaceuticals and other delicate solutes may also be processed in this way. 

In certain cases it is desirable to selectively remove a volatile solute from a solution that contains other, less volatile, solutes as well as the solvent. Some examples are the reduction of ethanol content from alcoholic beverages or from dilute alcoholic extracts of aromatic flavors and fragrances from plant sources such as fruits or flowers. Conventional pervaporation would facilitate removal of water from such mixtures while retaining ethanol and the higher molecular weight organics that comprise the characteristic aroma and flavor profile of the products of interest. On the other hand, membrane distillation or osmotic distillation cannot retain the volatile components at all. 

Johnson, R., Sun, J.C., and Sun, J. 2002. A pervaporation-microfiltration-osmotic distillation hybrid process for concentration of ethanol-water extracts of the Echinacea plant. J. Membr. Sci. 209, 221-232. 

Membrane processes termed as osmotic distillation or membrane distillation could be shown to be applications of membrane contactor technology also. Both of these processes are based on gas membranes. Osmotic distillation, sometimes called osmotic evaporation, involves transfer of water vapor across a gas-fiUed membrane, the process is driven by a difference in water vapor pressure maintained across the membrane [58-59] by separate aqueous hquids. Membrane distillation is a process where water vapor transfer is driven solely by a temperature difference across the gas-fiUed membrane [60-61]. Water evaporates from a hot aqueous phase and condenses on a cooler surface. This process may be useful in desalinating water or producing pure water if a good natural source of warm water is available, such as in a geothermal process. 

Membrane contactors can be used in osmotic distillation process to transfer water vapor, as discussed in Section 2.8. Such a process has been investigated as a means of concentrating fruit juice [58] using concentrated brine as the receiving phase for water vapor. A photograph of a pilot system is shown in Figure 2.15. 

Hogan PA, Canning RP, Peterson PA, Johnson RA, and Michaels AS, Osmotic distillation A novel atheimal membrane-separation process for the food, pharmaceutical, and chemical industries. Chemical Engineering Progress 93, August 1997. 

Diffusion is the main mechanism involved in the mass transfer during osmotic distillation and the resistance to mass transfer comes from both membrane stmcture and presence of air trapped within the membrane pores. While the former resistance can be described by Knudsen diffusion Equation 19.26, the latter is described by molecular diffusion Equation 19.27. 

Osmotic distillation 60-70 Very good 1-3 Yes High Moderate Low Under development... 

Lefebvre, M.S.M. Method of performing osmotic distillation. United States Patent No. 4,781,873, 1988. 

Courel, A.M., et al. Modelling of water transport in osmotic distillation using asymmetric membrane, J. Membr. Sci., 173, 107, 2000. 

Courel, M. The problem of membrane characterization for the process of osmotic distillation. Desalination, 140, 15, 2001. 

Lefebvre, M.S.M. Osmotic distillation process and semipermeable barriers therefor. United States Patent, 5,098,566, 1998. 

Sheng, J., Johnson, R.A., and Lefebvre, M.S. Mass and heat transfer mechanisms in the osmotic distillation process. Desalination, 80, 113, 1991. 

Celere, M. and Gostoli, C. Osmotic distillation with propylene glycol, glycerol and glycerol-salt mixtures, J. Membr. Sci., 229, 159, 2004. 

Godino, M.P., et al. Coupled phenomena membrane distillation and osmotic distillation through a porous hydrophobic membrane, Sep. Sci. Technol, 30(6), 993, 1995. 

Durham, R.J. and Nguyen, M.H. Hydrophobic membrane evaluation and cleaning for osmotic distillation of tomato puree, J. Membr. Sci., 87(1-2), 181, 1994. 

Thompson, D. The application of osmotic distillation for the wine industries, Aus. Grape growers. Wine makers, 11, 11, 1991. 

Ali, F., et al. Transfer of volatiles through PTFE membrane during osmotic distillation. Proceedings of the ICOM. Toulouse, France,... 

Xu, J.B. Alginate-coated microporous PTFE membrane for use in osmotic distillation of oily foods, J. Membr. Sci., 240, 81, 2004. 

Other methods that will not be discussed here, but also can be used to increase concentration (some with and some without concomitant evaporation) are reverse osmosis, ion exchange, dialysis, electrodialysis, osmotic distillation, and applications that involve fluidized beds, cooling towers, or evaporation of aerosols. 

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