The Potential of Osmotic Membrane Dehumidification
Arthur S. Kesten1, Jeffrey R. McCutcheon2, Ariel Girelli3 and Jack N. Blechner1,
(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.
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.
Membrane Comparison
25
Mass Change (grams)
20
15
TFC Active Down
TFC Active Up
ES Active Down
10
ES Active Up
NW Active Down
5
NW Active Up
0
0
-5
200
400
600
800
1000
1200
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.
Flow Comparison with ES @25C
70
60
Mass change (grams)
50
40
1 LPM
5 LPM
30
Poly. (1 LPM)
20
Poly. (5 LPM)
10
0
0
-10
200
400
600
800
1000
1200
1400
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.
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:
Temperature Comparison at 5 LPM
120
y = 0.0693x + 0.0157
R² = 0.9999
y = 0.0561x + 0.0269
R² = 0.9998
Mass change (grams)
100
80
y = 0.0496x - 0.1945
R² = 0.9995
60
15C
20C
40
25C
y = 0.0219x - 0.3186
R² = 0.9997
20
30C
0
0
-20
200
400
600
800
1000
1200
1400
1600
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.