OSMOTIC DISTILLATION

Introduction
Osmosis....simply the movement of water down a concentration gradient.  Water moving from an area of high concentration to an area of lower concentration.  So how does osmotic distillation differ?  Rather than using concentration gradients as a driving force, osmotic distillation uses the differences in vapor pressures of the contacting liquid phases.  This allows actual separation of water from other components without the other other components "following" the water as it moves.   This is particularily useful in concentrating food and pharmaceutical products that are sensitive to elevated temperatures.
A liquid phase with one or more volatile components is separated from a salt solution by a non-wetting polymer membrane.  The membrane is typically made of non-polar polymers such as polyethylene, polypropylene, or PTFE.  An example of the geometry one of the fibers is shown in Table 1:

Table 1:  Hoechst Celanese Celgard fiber specifications

 This membrane functions as a vapor barrier between the liquid phases.  The salt solution is usually composed of sodium or calcium chloride.  Figure 1 shows the interface arrangement used in osmotic distillation.

Mechanism
Water moves across the osmotic distillation (OD) membrane by evaporating, diffusing through the pores, and condensing on the other side of the pores.  The liquid is prohibited from entering the pores by membrane design.  The membrane is designed such that the liquids cannot exceed the capillary forces required to enter the pores.   Design factors include surface tension, contact angle, capillary pressure, and pore radius.  The heat of vaporization is supplied by conduction or convection from the upstream liquid through the membrane.  The temperature gradient across the membrane is typically less than 2 ⁰C making the process nearly isothermal.

Exclusive Transport of Water
When OD is performed at low temperatures the vapor pressures of components (other than water) are substantially low (relative to water).  This reduces the driving force for those components to pass through the membrane.  As an example, we'll compare the vapor pressure of water with acetic acid at 30 ⁰C.  The vapor pressure of water at this temperature is 32 mm Hg while acetic acid has a vapor pressure of 20 mm Hg.  Some larger biological molecules found in pharmaceuticals would have an even lower vapor pressure at 30 ⁰C.  In addition to this mechanism, most of solutes in the water have lower solubilities in the salt solution than in water.   This also aids in keeping the solutes in place while they are being concentrated.

Setup
    Figure 2 below shows a typical setup used in osmotic distillation.

Summary
    Osmotic distillation provides a means of purifying heat sensitive substances including pharmaceuticals and biological products.  Increased costs involved with OD separation will not allow this technology to rival ultrafiltration and reverse osmosis when these processes provide the necessary purity.  There are however many applications where OD offers many advantages over other separation technologies.   This has produced significant interest in OD by pharmaceutical companies in particular.

References:
Hogan, Paul A., R. Philip Canning, Paul A. Peterson, Robert A. Johnson, Alan S. Michaels, "A New Option: Osmotic Distillation", Chemical Engineering Progress, July 1998