The Potential of Osmotic Membrane Dehumidification

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1 The Potential of Osmotic Membrane Dehumidification Arthur S. Kesten 1, Jeffrey R. McCutcheon 2, Ariel Girelli 3 and Jack N. Blechner 1, (1)Nanocap Technologies LLC, Longboat Key, FL, (2)Department of Chemical and Biomolecular Engineering, University of Connecticut, Storrs, CT, (3)Biomedical Engineering, University of Connecticut, Glastonbury, CT 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.

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3 Comparative Performance of Several Membranes Previous experiments had demonstrated that membranes designed for forward osmosis applications perform more effectively than membranes designed to handle high pressure gradients. HTI fabricates both cellulose acetate and thin film composite membranes for forward osmosis. Two cellulose acetate and one thin film composite membrane available commercially were tested, first with the active side of the membrane facing the humid air stream and then with that side of the membrane facing the draw solution. In all cases, concentrated (5M) magnesium chloride was used as the draw solution. With the active side of the membrane facing the humid air, concentration polarization in the draw solution penetrating into the membrane support structure can inhibit transport of condensed water. With the active side of the membrane facing the draw solution, transport of humid air through the membrane support can slow down transport to the active surface where capillary condensation occurs. As seen below, one of the cellulose acetate membranes with active side facing the humid air performed best over the course of a day of testing.

4 25 Membrane Comparison 20 Mass Change (grams) TFC Active Down TFC Active Up ES Active Down ES Active Up NW Active Down NW Active Up Time (minutes) Effect of Air Flow The above tests were performed at a modest volumetric air flow rate of 1 liter per minute. Air flow was varied in subsequent tests, first to find the effective lower limit of relative humidity that can be achieved using membrane dehumidification and then to measure enhanced performance when air flow is raised. It was found that at 25C, osmotic dehumidification with the HTI membranes can reduce relative humidity to 50%; relative humidity can be reduced to around 40% at a draw solution temperature of 20C and 33% at 15C. Raising the air flow rate from 1 liter per minute to 5 liters per minute has an appreciable effect on the rate of transport from the humid air stream to the membrane and a significant impact on the water flux through the membrane.

5 70 Flow Comparison with Mass change (grams) LPM 5 LPM Poly. (1 LPM) Poly. (5 LPM) Time (minutes) Here the water flux reaches about 0.4 liters/square meter-hr at 25C. The flux will continue to rise with air flow rate until it becomes controlled by the resistance of the membrane and osmotic transport into the draw solution. What Happens with No Membrane? 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.

6 Cooling of the Osmotic Solution Results in More Effective Dehumidification Under typical air cooling/dehumidification applications, the draw solution can be cooled to temperatures as low as 15C to enhance the dehumidification process as well as provide appropriate outlet temperature levels. Reducing solution temperature raises the relative humidity of the air and results in capillary condensation in larger pores. The larger pores are better connected to other pores and enhance the rate of transport of water through the membrane. The impact of temperature is plotted below:

7 120 Temperature Comparison at 5 LPM Mass change (grams) y = x R² = y = x R² = y = x R² = y = x R² = C 20C 25C 30C Time (minutes) At 15C, the mass flux is 0.55 liters/square meter-hr. The flux is reduced significantly at 30C because the higher the temperature, the smaller the pore size where capillary condensation will occur. Condensed water in small pores will have a much harder time finding a way to get across the thickness of the membrane.