Apparatus for dehumidifying gas and methods of use

An apparatus for dehumidifying gas is provided which converts humid gas into dehumidified gas using a hydrophilic membrane that includes a superabsorbent polymer. A sub-dew point cooling tower, sub-dew point evaporative cooler and sub-dew point water harvesting system which utilize the apparatus for dehumidifying gas are also provided.

FIELD OF THE INVENTION

This invention is directed to an apparatus for dehumidifying gas for the purpose of enhanced cooling and a sub-dew point cooling tower, indirect regenerative evaporative cooler, and water harvesting system that embody the same.

BACKGROUND OF THE INVENTION

Dehumidifying devices haven used in a variety of application including air conditioners, cooling towers and the like. One kind of dehumidifying apparatus includes a hydrophilic membrane. When humid gas is passed across the hydrophilic membrane, the membrane absorbs moisture from the humid gas, yielding dehumidified gas. The dehumidified gas can then be used for the various cooling applications.

Dehumidification of the gas depresses its dew point temperature. The amount of dehumidification, and thus the depression of dew point, are dependent upon the flow rate of the humid gas and the absorption capacity and permeability of the hydrophilic membrane. If the absorption capacity and rate of the hydrophilic membrane are increased, then the dehumidification of the humid air stream and/or its flow rate can also be increased. There is a need or desire for dehumidification apparatus which provides better water absorption and dehumidification of humid air for enhanced cooling.

SUMMARY OF THE INVENTION

The present invention is directed to an apparatus for dehumidifying gas. The apparatus includes an inlet for receiving humid gas; an internal hydrophilic membrane including a superabsorbent polymer; a flow path for passing the humid gas across the internal hydrophilic membrane, wherein the membrane absorbs moisture from the humid gas to yield dehumidified gas; and an outlet for the dehumidified gas. The superabsorbent polymer provides the hydrophilic membrane with a high absorbent capacity and rate, enabling greater dehumidification and dew point reduction of the gas and/or higher flow rate for the gas.

The invention is also directed to a sub-dew point cooling tower, a sub-dew point evaporative cooler and a sub-dew point water harvesting system, all of which embody the apparatus for dehumidifying gas. Because of the improved dew point depression of the dehumidified gas, its capacity for cooling is significantly improved.

The foregoing and other features and advantages of the invention will become further apparent from the following detailed description of the invention, read in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1schematically illustrates an apparatus10for dehumidifying gas. The apparatus10includes an enclosure12having an inlet14for receiving a stream16of humid gas (such as air) and an outlet18for discharging a stream17(confluent with the stream16) of dehumidified gas. A flow path22carries the stream16of humid gas across an upper surface23of an internal hydrophilic membrane24which, in the embodiment shown, extends from one side to the other of enclosure12. As the stream16passes across the hydrophilic membrane24the humid gas is dehumidified, progressively converting the stream16into the confluent stream17of dehumidified gas. The dehumidified gas passes through the outlet18for further processing and use.

The hydrophilic membrane24absorbs moisture from the humid gas and ultimately reaches a steady state water content and a steady state absorption/desorption rate. Water passes through the hydrophilic membrane24and is discharged through the lower surface25into a water basin26located in the enclosure12below the hydrophilic membrane24, typically at the bottom of enclosure12. When the hydrophilic membrane24reaches the steady state absorption/desorption rate, the rate of water absorption at the upper surface23equals the rate of water release from the lower surface25. As the basin26collects water, the water is discharged through an outlet27and forms an exit water stream28, which can be processed and used further.

The internal hydrophilic membrane24includes a superabsorbent polymer. The hydrophilic membrane24typically includes a hydrophilic base material upon which the superabsorbent polymer is disposed, so that the combination of hydrophilic base material and superabsorbent polymer forms the hydrophilic membrane.

Superabsorbent polymers are polymers that can absorb very large amounts of water relative to their own mass. Superabsorbent polymers can absorb water in an amount from about 40 to about 800 times their own weight, commonly from about 100 to about 500 times their own weight. The superabsorbent polymer can be selected from natural, synthetic, and modified natural polymers. Examples of natural and modified natural superabsorbent polymers include without limitation hydrolyzed acrylonitrile-grafted starch, acrylic acid grafted starch, methyl cellulose, chitosan, carboxymethyl cellulose, hydroxypropyl cellulose, and natural gums such as alginates, xantham gum, locust bean gum and the like. Examples of synthetic superabsorbent polymers include without limitation alkali metal and ammonium salts of polyacrylic acid, polymethacrylic acid, polyacrylamides, polyvinyl ethers, hydrolyzed maleic anhydride copolymers with vinyl ethers and alpha olefins, polyvinyl pyrrolidone, polyvinyl morpholinone, polyvinyl alcohol or basic or chloride and hydroxide salts of polyvinyl amine, polyquaternary ammonium polyamine, hydrolyzed polyamide, and mixtures and copolymers thereof. These superabsorbent polymers can be crosslinked or partially crosslinked to optimize their absorption rate and capacity, or their contribution to the absorption rate and capacity of the hydrophilic membrane24.

The superabsorbent polymer can include a zeolite. Zeolites are microporous aluminosilicate materials having a porous structure that can accommodate a wide variety of cations. Examples of mineral zeolites include analcime, chabazite, clinoptilolite, heuldanite, natrolite, phillipsite, and stilbite. Zeolites can be combined with superabsorbent polymers to form composites having both the highly absorbent properties of a superabsorbent and the ion exchange properties of a zeolite. The ion exchange properties help eliminate odors, bacteria and other unwanted substances from the air conditioner water used for evaporative cooling. For example, the zeolites can be combined with any of the foregoing synthetic superabsorbent polymers, suitably during synthesis and/or cross linking of the superabsorbent polymer, using known methods.

The hydrophilic base material can be selected from a wide variety of hydrophilic materials having wicking properties irrespective of the superabsorbent polymer, that are enhanced by the superabsorbent polymer. Suitable hydrophilic base materials include without limitation Manila paper, filter paper, and other cellulose materials. Cloth-like fabrics made from cotton and the like can also be used as the hydrophilic base material. The superabsorbent polymer can be applied to the hydrophilic base material by dipping, soaking, painting (brush coating), spray coating or the like. For example, the hydrophilic base material can be dipped or soaked in a solution containing the superabsorbent polymer. One exemplary solution includes isopropyl alcohol and water in a ratio of about 30-70 parts by weight isopropyl alcohol to about 30-70 parts by weight water, based on a combined 100 parts by weight isopropyl alcohol and water. A particularly suitable ratio is about 33 parts by weight isopropyl alcohol to about 67 parts by weight water. Other solvents and solvent combinations may also be employed.

The hydrophilic base material can have a thickness of about 100 to about 500 microns, suitably about 300 to about 400 microns. The superabsorbent polymer can have a dry coating thickness of about 5 to about 500 microns, suitably about 20 to about 100 microns. The overall hydrophilic membrane24can have a dry thickness of about 1 to about 50 microns, suitably about 10 to about 25 microns. The loading of the superabsorbent polymer onto the hydrophilic base material can range from about 0.3 to about 10 mg/cm2, suitably about 0.5 to about 5 mg/cm2, or about 1 to about 2 mg/cm2.

The superabsorbent polymer can be crosslinked or partially crosslinked to optimize its contribution to the absorption properties of the hydrophilic membrane. Crosslinking can be accomplished using a suitable crosslinking agent, and can occur before or after (suitably after) the superabsorbent polymer is applied to the hydrophilic base material. A wide variety of known crosslinking agents may be employed, including without limitation methylene bisacrylamides; monofunctional aldehydes; 1,4-butanedioldiacrylate; ammonium persulfate; polyols; functionalized polyvinyl alcohols; alkylene carbonates; oxazolidone compounds; and the like. Crosslinking can be initiated using heat, radiation, and other known techniques. In one embodiment, the crosslinking is performed by heat treating the superabsorbent-coated hydrophilic base material at 80° C. in an oven for one hour, followed by pressing the superabsorbent-coated hydrophilic base material at 80° C. for 5 min and 10,000 psi, using the crosslinking agent. The amount of crosslinking agent may vary, and can range from about 2% to about 40% based on the weight of the superabsorbent polymer, suitably about 10% to about 30% based on the weight of the superabsorbent polymer.

The hydrophilic membrane24can be supported by a porous screen, netting, or other suitable porous material (not shown) to preserve the integrity of the hydrophilic membrane24without compromising its ability to desorb water from its lower surface25. The type and thickness of the porous support material may vary depending upon the weight, thickness and area of the hydrophilic membrane24, as well as its structural integrity.

FIG. 2illustrates a sub-dew point cooling tower100which, in the embodiment shown, utilizes two of the apparatus10for dehumidifying gas. A cooling tower is intended for reducing the temperature of a water stream by transferring heat from the water to the atmosphere. The heat is transferred to the atmosphere by convective and evaporative heat transfer between water on the one hand, and air, flue gas or exhaust gas on the other hand. The main purpose of a cooling tower is to conserve water by using it as a recirculating coolant. Common applications for cooling towers include providing cooling for electric power generation, manufacturing, air conditioning. Often, the limiting factor of efficient operation and performance is temperature of water from the cooling tower. Alternatively, in power generation and building cooling, the hotter condensate return results in a greater energy penalty, reducing a cost-effectiveness of the plant operation. As such, colder water provided by the cooling tower reduces energy consumption to power turbines, compressors, water chillers, absorption machines, and other generating and process equipment, increasing heat transfer efficiency at lower cost. Greater cycle efficiency implies reduced water and fuel usage.

The main objective of the sub-dew point cooling tower (FIG. 2) is to utilize the apparatus10for dehumidifying gas to deliver counter-flow cooled water at temperatures either a) at or below ambient wet bulb temperature of the humid gas (air/flue gas, etc.) stream entering the apparatus10, or b) below ambient dew point temperatures while achieving plume abatement. The sub-dew point cooling tower accomplishes these objectives by increasing the evaporative cooling potential of ambient air by dehumidifying the incoming gas streams16to depress their dew point temperatures. This increases the cooling potential of the dehumidified gas streams17as they enter the cooling tower100by also depressing their ambient wet bulb temperatures. This permits cooling of the incoming warm water stream102in the cooling tower100to below the wet bulb temperature, or even below the dew point temperature of the incoming gas streams16depending on ambient conditions and how aggressively the gas streams16are dehumidified.

It is advantageous to not only dehumidify, but also to cool each dehumidified gas stream16before it enters the cooling tower100. In the embodiment ofFIG. 2, this is accomplished by diverting some of the cooled water104from the cooling tower back into the water basin26of each apparatus10, via an inlet water stream30extending from the cooling tower100to each apparatus10. The cooled water in the basin26provides a heat sink for cooling the interior of the enclosure12, including the hydrophilic membrane24and the gas streams16,17passing across it. The cooled dehumidified gas enters the cooling tower100via the inlets106in its lower half. Optionally, some of the cooled, dehumidified gas can be diverted via streams19to enter the cooling tower100via the inlets108near its top.

The cooled dehumidified gas passes through the cooling tower100and exits via exhaust streams110located at the top of cooling tower100. The counter flow stream102of warm water enters near the top of cooling tower100through water inlets112. The stream102of warm water includes a first sub-stream102aof water from an original source and a second sub-stream102bof water exiting the apparatus10via streams28, the sub-streams converging into stream102before entering the cooling tower. After entering the cooling tower100, the counter-flow water stream102passes through a heat exchanger114, is cooled by the cooled, dehumidified air from inlets106and is dispensed into a spray116that is further cooled as the spray falls and is collected as cooled water104in water basin118. The heat exchanger114separates the cooled dehumidified gas from the counter flow stream of water and provides flow paths for the dehumidified gas and water through the heat exchanger, enabling cooling of the counter-flow stream of water using the dehumidified gas. An exhaust fan (not shown) can be provided near the top of cooling tower100for controlling the flow rate of the dehumidified gas. Part of the cooled water from water basin118exits thorough outlet120as product stream122. Part of the cooled water exits through outlet124and is directed back into the apparatus10via streams30to aid in cooling the hydrophilic membranes24and the gas stream16,17.

In order to derive optimum cooling performance from the dehumidified gas streams17entering the cooling tower100, it is desirable to cool the gas streams17to just above their dew point, and below the dew point and wet bulb temperature of the humid gas streams16. This cooling can be accomplished by directing some of the cooled water104from basin118into the apparatus10as described above. Additionally, the dehumidified gas streams17can be further cooled using heat exchangers and other devices (not shown), either with the aid of cooled water104or other cooling fluids.

Another objective of the sub-dew point cooling tower (FIG. 2) is to utilize the apparatus10for producing water from humid gas thus reducing or eliminating make-up water for cooling tower and essentially reducing water consumption.

FIGS. 3-5illustrate first, second and third embodiments of a sub-dew point evaporative cooler200that utilize the apparatus10for dehumidifying gas. In addition to apparatus10, the sub-dew point evaporative cooler200includes an evaporative cooler inlet202for receiving the dehumidified gas from the apparatus10, an evaporative cooler membrane220having an upper wet surface and a lower dry surface, a first flow path217for passing the dehumidified gas across the dry surface and a second flow path227for passing at least a portion of the dehumidified gas across the wet surface. The stream17of dehumidified gas leaves the apparatus10and enters the evaporative cooler200through an inlet202, and pushes through evaporative cooler200along a flow path shown as stream217across the lower (dry) surface of evaporative cooler membrane220that extends the length of evaporative cooler200and bisects the evaporative cooler200into upper and lower sections. Water from the water basin26of apparatus10exits via water stream28which converges with a stream29of make-up water (if required) to form a combined water stream30. The water stream30can be heated using a heat source32to facilitate subsequent evaporation. The water stream30then enters the evaporative cooler200above the internal membrane220and wets the upper surface of internal membrane220. The stream217of dehumidified gas, after passing across the lower dry surface of membrane220, splits into an upper stream227and a lower stream225. The upper stream227of dehumidified gas passes around the membrane220and across the wet upper surface in a direction opposite the movement of water230.

As the upper stream227of dehumidified gas passes across the wet upper surface of membrane220, it evaporates water from the surface, resulting in evaporative cooling of the membrane220, and humidification of the gas stream227to form a humidified gas stream229, which exits the evaporative cooler200as exhaust. This cooling of the membrane220further cools the dehumidified gas stream217as it passes across the lower dry surface. The lower stream225from the cooled, dehumidified gas stream227passes through an internal space230of a building232and cools the internal space. The dehumidified gas stream225then leaves the internal space230and splits into two streams225aand225b. Stream225are-enters the evaporative cooler200, and re-converges with the gas stream227above the wet surface of the internal membrane220before exiting as part of exhaust stream229. Stream225bis an exhaust stream which can be used to cool the water in basin26as explained below.

The internal membrane220suitably has an upper hydrophilic layer defining the wet upper surface and a lower hydrophobic layer defining the dry lower surface. The upper hydrophilic layer can include a base layer and a superabsorbent polymer as described above for the internal hydrophilic membrane24of the apparatus10, and using any of the above-described materials. The upper hydrophilic layer wicks water supplied from the water stream30across the internal membrane220. The rate and efficiency of wicking are determined by the hydrophilic materials and are aided by the superabsorbent polymer. The lower hydrophobic layer keeps the lower surface for the internal membrane220dry. The hydrophobic layer can be formed of a wide variety of materials, including without limitation polypropylene and polyethylene homopolymers and copolymers, polytetrafluoroethylenes, polyesters, polycarbonates, titanium foam, nickel foam and combinations thereof.

The apparatus10inFIG. 3can be equipped with a heat exchanger40which cools the water in basin26. This in turn cools the interior of the apparatus10to increase its dehumidification rate. The coolant for the heat exchanger40can be provided by routing exhaust stream225bthrough the heat exchanger40as shown. Alternatively or additionally, a vacuum generating device may be provided below the second surface25of membrane24to create a positive vapor pressure gradient resulting in a decrease in vapor pressure between the first surface23and the second surface25of membrane24. Alternatively or additionally, a circulating liquid desiccant may be provided below the second surface25of membrane24to create a positive vapor pressure gradient resulting in a decrease in vapor pressure between the first surface23and the second surface25of membrane24.

FIG. 4illustrates a second embodiment of a sub-dew point evaporative cooler201. In the embodiment ofFIG. 4, as inFIG. 3, the stream217of dehumidified gas passes along the lower dry surface of internal membrane220and then splits into an upper stream227and a lower stream225. The upper stream227traverses the membrane220and passes across the wet upper surface of the membrane220in a direction opposite the water stream30. This causes evaporation of the water from the membrane220, humidification of the gas stream227, and evaporative cooling of the membrane220. The humidified gas exits the evaporative cooler as exhaust stream229.

The stream217of dehumidified gas cools as it passes across the dry lower surface of the evaporatively cooled membrane220. The lower stream225carries the cooled dehumidified gas into the internal space230of building232, causing cooling of the internal space. After leaving the internal space230, instead of joining the stream227, the stream225of dehumidified gas splits into a first stream225athat recycles back to the dry lower surface of membrane220and a second stream225bwhich is an exhaust stream. If the recycled gas stream225arequires dehumidification as well as cooling, then it may alternatively be diverted to the humid gas stream16entering the apparatus10for dehumidifying gas, and may be dehumidified before re-entering the sub-dew point evaporative cooler201. The exhaust stream225bis channeled through the heat exchanger40of apparatus10and cools the water in basin26, in turn cooling the apparatus10to increase its dehumidification rate as described above.

Except as described above, the sub-dew point evaporative cooler201ofFIG. 4is similar to the sub-dew point evaporative cooler200ofFIG. 3. The sub-dew point evaporative cooler202ofFIG. 5offers a further modification wherein a second internal membrane221having a dry hydrophobic upper surface and a wet hydrophilic lower surface is positioned below the first internal membrane220, such that the dehumidified gas stream217passes between the membranes220and221for a more efficient cooling. The water stream30splits into an upper water steam30afeeding the wet upper surface of membrane220and a lower water stream30bfeeding the wet lower surface of membrane221.

The dehumidified gas stream217is cooled to yield cooled dehumidified gas stream225, which cools the internal space230of building232. After leaving the internal space230, the gas stream225splits into a first stream225aand a second (exhaust) stream225b. The gas stream225apasses across the wet lower surface of the second internal membrane221, resulting in evaporation of water and evaporative cooling of the second internal membrane221, and is then exhausted as gas stream231. Alternatively, some of the gas stream225amay be diverted to the humid gas stream16entering the apparatus10for dehumidifying gas, and may be dehumidified before re-entering the sub-dew point evaporative cooler202as part of dehumidified gas stream217. The exhaust stream225bis channeled through the heat exchanger40of apparatus10and cools the water in basin26, in turn cooling the apparatus10to increase its dehumidification rate as described above. Alternatively or additionally, a vacuum circulating device and/or circulating liquid desiccant as described with respect toFIG. 4may be provided to create a positive vapor pressure gradient resulting in a decrease in vapor pressure between the first surface23and the second surface25of membrane24.

The use of apparatus10for dehumidifying gas in each of the evaporative coolers ofFIGS. 3-5enables the dehumidified gas streams17and217, and hence the cooled dehumidified gas stream225, to have dew points and wet bulb temperatures below the dew point and wet bulb temperature of the humid gas stream16. This in turn enables the cooled dehumidified gas stream225to have an actual temperature below the dew point and wet bulb temperature of the humid gas stream16, thus enabling greater and more efficient cooling of the internal space230.

FIG. 6illustrates a sub-dew point membrane water harvesting system300that utilizes the apparatus10for dehumidifying gas. The water harvesting system utilizes both the dehumidified gas (air) stream17and the water from the water basin26to yield potable water. Water from basin26exits apparatus10through outlet27and follows a water flow path28that carries the water to an evaporation channel302. Before entering evaporation channel302, the water can be heated from a heat source304. The water enters the evaporation channel304from the top, and flows downward as shown.

Dehumidified gas (air) leaves the apparatus10via dehumidified gas stream17and outlet18. Some of the dehumidified air is diverted to exhaust stream20. The remainder of the dehumidified air enters the evaporation channel302at its bottom, via inlet channel308, and flows upward in the evaporation channel302along flow path310, against the flow if incoming water. The flow path310can be driven with the aid of a suction blower312or another suitable device.

The dehumidified air evaporates considerable water along the flow path310through evaporation channel302, and becomes humidified and saturated. Excess water is discharged through outlet stream309at the bottom of evaporation channel302and is recirculated to the water basin26of apparatus10, suitably via heat exchanger38, to cool the water in water basin26relative to the humid gas stream16before being ultimately discharged at stream21. This is beneficial because water vapor on the lower side of membrane24will be saturated at steady state, and will generally have a higher partial pressure than the humid gas in the flow path22. The membrane24can thus have a positive pressure gradient between its upper surface23and lower surface25provided that the apparatus10is primed with non-saturated air above the membrane24and/or colder water in the basin26so that the vapor pressure decreases across the membrane24. The coldest part of the cycle exists near the bottom of evaporation channel302where the dehumidified gas stream17has been humidified to its wet bulb temperature. As the humid inlet gas stream16is dehumidifies across flow path22to the dehumidified gas stream17, the evaporative cooling potential near the bottom of channel302decreases further, driving the cold water discharge temperature at309down, further cooling the water in basin26, further reducing the saturated partial pressure of vapor below the membrane24, further driving dehumidification across the membrane24, further reducing the exiting wet bulb temperature of stream17, and so on until the system limits of the virtuous cycle are realized.

The saturated air from evaporation channel302then passes downward through condensation channel314along flow path10, where it is cooled, resulting in condensation of water The condensed, potable water320collects in the bottom of condensation channel314and can be discharged through potable water outlet stream322. The spent air is then discharged just above the collected potable water320, through exhaust stream324.

The sub-dew point membrane water harvesting system is highly efficient because the dehumidified air stream17has a lower dew point and higher evaporation capacity than the inlet humid air stream16. This enables more rapid evaporation of water, and greater evaporating of water in the evaporative channel302.

While the embodiments of the invention described herein are presently preferred, various modifications and improvements can be made without departing from the spirit and scope of the invention. The scope of the invention is indicated by the appended claims, and all changes that fall within the meaning and range of equivalents are intended to be embraced therein.