Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Reexamination Certificate
2001-03-30
2002-12-24
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C096S004000, C096S011000
Reexamination Certificate
active
06497749
ABSTRACT:
TECHNICAL FIELD
This invention relates to conditioning a gas stream, such as air, and especially to the dehumidification of a gas stream.
BACKGROUND ART
Conditioning of a gas stream, such as air, generally involves the removal or addition of moisture and the increase or decrease of temperature to make the gas stream suitable for its intended environment. For air conditioning in warm weather, this typically involves dehumidifying and cooling the air to comfortable levels.
Current dehumidification technology is based on the conventional, refrigerant vapor compression cycle (hereinafter referred to as DX technology) or on desiccant substrate capture technology (hereinafter referred to as DS technology). DX technology requires cooling humid supply air, such as the air within a room and/or outside air, to the water vapor condensation point, with external heat rejection on the compression side. This usually requires the supply air to be cooled below comfortable temperatures and, thereafter, either reheated or mixed with warmer air to raise its temperature to an acceptable level before directing it into the space being dehumidified. 20-35% of the energy expended in cooling the high humidity air is utilized to remove the latent heat from the air (the heat of condensation associated with water vapor condensation). Cooling and dehumidification of the air are thus coupled. That makes it impossible to independently control comfort parameters, making the DX cycle less efficient, from an overall system perspective, than a technology that would allow independent control of sensible and latent heat.
In applications where the outside air has both high humidity and temperature and the functional use of the interior space generates high water vapor levels (e.g. populated convention halls, exercise rooms, school buildings, etc.), it may not be possible for the DX technology to maintain the air introduced into the interior space at the correct humidity and temperature for maintaining comfort. The air delivered is cool but “muggy”, since further cooling to remove additional water would result in the air being uncomfortably cool.
In stand-alone dehumidification using a conventional compression cycle, heat reject is in direct contact with the room air. As a consequence, the room air becomes more comfortable from a humidity side, but may be less comfortable (too warm) from a temperature parameter consideration. Again the comfort parameters are coupled.
DS systems are generally applied in central air, ducted systems. Water vapor is captured by capillary condensation on a solid phase substrate containing pores of the appropriate size (typically less than 100 Angstroms) to cause capillary condensation. The capture process is efficient and rapid. However, removal of the water vapor from the pores, wherein the intrinsic vapor pressure of the water is lowered in correspondence with the Kelvin equation, requires energy input. It also requires removing the substrate from the high humidity air stream and placing it in an exhaust, water reject stream, before adding the re-evaporation energy. Alternatively, the substrate may remain fixed and the treated air and exhaust streams flow directions interchanged as is done in a parallel bed, desiccant drier system.
In these DS systems, the re-evaporation energy is the latent heat of condensation plus the heat of adsorption of the water vapor in the substrate pore material. It is important to note that DS technology requires, in steady state operation, the addition of this energy at a rate equal to or greater than the latent heat of condensation of water in the desiccant substrate. That is, the water vapor reject power input must exceed the equivalent latent heat of condensation power. After water removal from the desiccant substrate the substrate must be re-cooled to the water capture temperature range of the substrate. As a consequence, some of the sensible heat of the subsequent cooling system (e.g. a DX cooling system) must be utilized in treating the DS substrate rather than for cooling the now dehumidified air.
The advantage of DS technology is that humidity levels in the outside air and/or recirculated air can be adjusted independently of the subsequent cooling step. The disadvantage is the requirement to move the substrate and treated air stream relative to each other for capture and rejection of the water vapor. This requires moving a large substrate through a sealed system, or, in a parallel bed DS system, requires complicated valving and valve cycling to move the humid air stream and an exhaust stream alternately across the desiccant beds. Again, application in typical stand alone, non-ducted room-type dehumidifiers is difficult if not impossible.
DISCLOSURE OF INVENTION
One object of the present invention is to provide an improved method and means for dehumidifying a gas stream.
Another object of the present invention is an efficient method and means for removing water from an air stream wherein the level of dehumidification is not interdependent with the temperature to which that stream may need to be ultimately cooled (for comfort or other purposes) before it is exhausted into the space being conditioned.
According to one embodiment of the method of the present invention, moisture is removed from a gas stream by bringing that stream into contact with the front surface of a hydrophilic capillary condenser layer that captures the water. An osmotic driving force, resulting from a water concentration gradient, transports the condensed water from the rear surface of the condensing layer through a semi-permeable membrane of collodion and into an osmotic fluid.
In apparatus used in the practice of the present invention, a porous wall is used to separate a moist gas stream from an osmotic fluid. The wall is comprised of a thin hydrophilic capillary condensing layer on the gas stream side with a collodion (nitrated cellulose) osmotic layer or membrane disposed on the surface facing the osmotic fluid. In one embodiment the osmotic fluid is a solute dissolved in water, wherein the solute has a high molecular weight and a high concentration. The collodion layer is a membrane permeable to water and not to high molecular weight molecules in solution. The choice of solute and any other additives making up the osmotic fluid will be determined by the transport properties through the membrane. In another embodiment the osmotic fluid is one that is miscible with water at all concentrations, such as glycerol. Here the fluid can be maintained at low water concentrations in order to maximize the osmotic flux. Typical membranes have permeabilities for glycerol which are about one thousand times less than for water. However, some reverse transport will likely occur.
In a preferred embodiment, a biocidal component may be added to the osmotic fluid. The biocidal component is selected to prevent microbial growth or biofouling on surfaces which would naturally occur in an aqueous environment and eventually block the membrane or pores. Examples of biocidal or bacteristatic additives that can exist in osmotic fluid as ionic species include silver and copper. In addition to these simple ionic antimicrobial agents, small concentration of larger molecules such as detergents, quaternary amines, or gluteraldehydes may be used. Gluteraldehyde is an example of a sterilant and disinfectant that is less corrosive than most other chemicals and does not damage plastics. Bleach (e.g. hypochlorous acid), for example, is antimicrobial but accelerates corrosion and would not be a preferred additive to the osmotic fluid.
Preferably the osmotic layer is in the form of a thin membrane adjacent to the surface of the capillary condenser layer. If the osmotic fluid is a solute dissolved in water, the membrane must have a material composition, thickness, pore size and porosity that must a) prevent the solute within the osmotic fluid from entering or blocking the pores of the membrane, and b) allow water to flow from the capillary condenser layer through the membrane and into the osmotic fluid as a result o
Blechner Jack N.
Jarvis Rebecca
Kesten Arthur S.
Satyapal Sunita
Tsai Chung-yi A.
Spitzer Robert H.
United Technologies Corporation
Wall Marjama & Bilinski LLP
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