THE POTENTIAL OF OSMOTIC MEMBRANE DEHUMIDIFICATION
Arthur Kesten, Jeffrey McCutcheon, Ariel Girelli and Jack Blechner
An osmotic membrane dehumidifier can use a flexible, semi-permeable membrane to facilitate capillary condensation of water vapor and the transport of condensed water through the membrane into a salt solution by osmosis. Here a humid gas stream is brought into contact with a semi-permeable membrane, which separates the gas stream from an osmotic (e.g., salt) solution. Some of the pores of the membrane are small enough to permit capillary condensation. Liquid formed within these pores can connect with liquid formed in adjacent pores, collectively forming continuous paths of liquid. These ‘liquid bridges' extend across the thickness of the semi-permeable membrane and provide paths by which water can travel across the membrane. Because the membrane is so thin, water concentration gradients across the membrane can be large. This can provide a large driving force for water transport between the humid air and the osmotic fluid. The flexibility of the polymeric membrane allows for considerable design flexibility that enhances the potential for retrofit with any cooling system. An illustration of this two-step spontaneous process is given below.
Liquid desiccants are particularly effective as osmotic agents because water entering the desiccant solution is bound to the salt as water of hydration. This enhances the water concentration gradient across the membrane.
Laboratory testing of membranes and draw solutions under different environmental conditions is conducted using a cell comprised of two halves separated by a membrane. On the top half, the humid air is passed above the membrane. The draw solution is pumped through the bottom half of the cell; the draw solution is contained in a reservoir that is placed on a digital scale that measures mass to a hundredth of a gram. Humid air contacting the membrane condenses by capillary condensation in the pores of the membrane and the condensed water is drawn by osmosis into the draw solution. Mass changes versus time are recorded with the digital scale that is capable of measuring a maximum of 4,000 grams.
Osmotic dehumidification begins with condensation of water molecules, followed by the osmotic draw of the liquid water out of the pores into a draw solution. If the rate-limiting step is the osmotic de-swelling of the membrane, the highest osmotic pressure should be used to maximize flux. However, if capillary condensation is the limiting step, a threshold is reached beyond which increased osmotic pressure does not increase flux. To understand these limits, we tested three concentrations of magnesium chloride with a cellulose acetate membrane under identical conditions. The water flux varied directly with concentration suggesting that the limiting step for this membrane was the osmotic removal of liquid water as opposed to capillary condensation.
A separate experiment was constructed to compare the effectiveness of no membrane to our capillary condensation/osmotic dehydration system. Having no membrane in the system is the equivalent of exposing liquid desiccant directly to a humid air stream. Measured water removal rates for that system were less than one fifth of the rates for the membrane/draw solution tested here. And, of course, with no membrane between the humid air and the desiccant, there is always the potential for entraining desiccant in the air stream.
Cooling of the osmotic solution results in more effective dehumidification. Lower water vapor pressure leads to capillary condensation at lower relative humidity. For modest levels of cooling, the relative humidity exiting the Nanocap process will be around 50%.
Scale up of the process to larger surface areas of membrane can be accomplished with plate and frame systems, spiral wound devices and bundles of cylindrical fibers. Results with a plate and frame system demonstrate the effectiveness of a mathematical model as a basis for design of a larger system.
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