Osmotic distillation membranes

Membrane processes, in general, are very attractive for their simplicity and flexibility. They are capable of achieving separations at a molecular level. Membrane modules are often compact and easily scaleable. For clarification and concentration, microfiltration, ultrahltration, and reverse osmosis are the current methods of choice. RO has been widely used in the food industries as an attractive alternative to classical evaporahon the only hmitahon being its dependence on osmotic pressure, which practically limits concentration of fluid streams to 25°Bx-30°Bx. Hence, currently it is used more as a preconcentration step. In recent years, membrane processes, notably pervaporahon, membrane dishUahon and osmotic membrane distillation (OMD) [21], have been used either by themselves or in combinahon with other membrane processes to overcome the problems associated with thermal processes. 

Osmotic membrane distillation is one of the membrane distillation variants that operate at ambient temperature and atmospheric pressure. In OMD, a microporous hydrophobic membrane separates the two aqueous solutions, namely, feed and osmotic agent (OA) having different solute concentrations (osmotic pressure). The driving force in OMD is vapor pressure difference across the membrane. Water evaporates from the surface of the feed solution having higher vapor pressure, diffuses through the membrane in the form of vapor, and condenses on the surface of a solution with a lower vapor pressure. This results in the concentration of feed and dilution of osmotic agent solution. 

Celere, M. and Gostoli, C. The heat and mass transfer phenomena in osmotic membrane distillation. Desalination, 147, 133, 2002. 

Due to the water vapour partial pressure gradient, in both MD and osmotic membrane distillation (OMD), water vapour is transferred through the pores from one side of the membrane to the other. Both MD and OMD differ from other membrane techniques as the driving force for the process is the difference in total water pressure across the membrane itself. As such, in both MD and OMD the driving force is quite different from other well-known membrane separation processes using hydrophilic membranes, such as RO, driven by hydraulic pressure difference, dialysis (DA), driven by concentration difference, and ED, driven by electric potential difference. 

Ravindra Babu B., Rastogi N.K., Raghavarao K.S.M.S. (2008), Concentration and temperature polarization effects during osmotic membrane distillation, J. Membrane ScL, 322,146-153. 

Warczok X, Gierszewska M., Kujawski W, Guellet C. (2007), Application of osmotic membrane distillation for reconcentration of sugar solutions from osmotic dehydration, 6 ep. Purif. Technol, 57,425-429. 

Membrane distillation offers a number of advantages over alternative pressure-driven processes such as reverse osmosis. Because the process is driven by temperature gradients, low-grade waste heat can be used and expensive high-pressure pumps are not required. Membrane fluxes are comparable to reverse osmosis fluxes, so membrane areas are not excessive. Finally, the process is still effective with slightly reduced fluxes even for very concentrated solutions. This is an advantage over reverse osmosis, in which the feed solution osmotic pressure places a practical limit on the concentration of a salt in the feed solution to be processed. 

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. 

However, technological advances related to the development of new membranes operations and innovative strategies of process design, have partially overcome this limitation. Membrane distillation, for example, is not subject to osmotic-pressure limitation and can be therefore employed in integrated systems when high permeate recovery factors or retentate concentrations are requested. 

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... 

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. 

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. 

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

Alves, V.D. and CoeUioso, I.M. Orange juice concentration by osmotic evaporation and membrane distillation A comparative study, J. Food Eng., 74(1), 125, 2006. 

Membrane crystallizers, membrane emulsifiers, membrane strippers and scrubbers, membrane distillation systems, membrane extractors, etc. can be devised and integrated in the production lines together with the other existing membranes operations for advanced molecular separation, and chemical transformations conducted using selective membranes and membrane reactors, overcoming existing limits of the more traditional membrane processes (e.g., the osmotic effect of concentration by reverse osmosis). 

Cervellati, A. Zardi, G. Gostoli, C. Osmotic distillation developments in technology and modeling. Proceedings of Conference on Membrane Distillation, Osmotic Distillation and Membrane Contactors Cetraro, Italy, 1994 39-42. 

FIGURE 4.9 Schematic representation of the osmotic distillation/ membrane distillation. 

---