(182c) The Potential Of Osmotic Membrane Dehumidification

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.