Patent Application
20140157985
The present invention generally relates membrane dehumidification systems, and more particularly to water permeable membrane systems for dehumidifying gases at ambient pressure. The present invention also relates to membrane systems for dehumidifying gases at ambient pressure that utilize a sweep gas to sweep a permeate side of a membrane.
Membrane systems for the dehumidification of gas streams have previously been proposed for natural gas (13) (14) (15) (16), ethanol, and compressed gas. In the context of compressed gas, the humid feed gas is generally at pressures greater than 100 psia (7 bars). However, in the context of membrane systems with the humid feed gas at standard atmospheric pressures (≈1 bar), the literature and research is limited. Bend Research (9) and Kneifel et al. (10) have previously looked at designing membrane modules for dehumidifying air that have proper mass transfer capabilities and minimum treated gas pressure drops. The Kneifel et al. (10) system used an aqueous salt flowing on the permeate side of the membrane to establish the driving force for the humidity mass transfer via absorption. El-Dessouky et al. (7) did a paper study and simulation of the energy savings of adding a membrane-based dehumidifier without a recycle sweep in the permeate, prior to sensible heat removal via evaporative cooling in an integration air conditioning system. El-Dessouky et al.'s conclusion was that such a design could lead to an 86% energy saving compared to using only a conventional coiling coil system (e.g., a vapor compression refrigeration cycle). These estimated savings indicate that there is a superior and unexpected potential for energy savings with the present invention.
The removal of humidity from air flowing in an air conditioning (AC) system saves the overall energy of the AC system. It also reduces the required capacity of a refrigeration plant, thus reducing operating cost, capital cost, and discharged fluorocarbons, which are greenhouse gases, into the atmosphere.
This need for humidity control has long been identified. For example “the relative humidity should not exceed 60% at any point in the occupied space . . . .” (ASHRAE Handbook of Fundamentals, 1972, Chapter 33 p 667). ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) was founded in 1894 and it handbook and standards are often cited in building codes. Additional reference from ASHRAE is Standard 55-66 (published in 1966). ASHRAE's Thermal Comfort Conditions specify comfort conditions and humidity control in further detail. While control of humidity by overcooling/reheating and by desiccant drying processes has long been applied and understood, both of these methods are energy intensive and hard to control. Therefore, the recommendation for humidity control, from the middle of the last century, is still not widely applied, further indicating an unmet need.
The process of removing water vapor from gases has a number of names; such as, dehumidification, dehydration, humidity controlled air conditioning, etc. It is an energy intensive and widely needed process in industrial manufacturing and air conditioning. For example, air conditioning represents 50% of a building's energy use and is critical for worker productivity, manufacturing quality, and health. For these critical outcomes, humidity control, not temperature control, is the primary function of the air conditioning.
For instance, building occupants are more comfortable if the humidity is controlled within a defined range. This range is generally 30% to 60% relative humidity, but varies slightly with dry bulb temperature and clothing (ASHRAE Standard #55). The economic impact of this comfort is increased worker productivity. Reducing humidity in an occupied space also leads to a cooler feeling. Some occupants overcome the humid feeling by reducing the dry-bulb temperature, but not the humidity, of the space. This is a compromise between feeling too cold rather than too humid, which is uncomfortable but currently widely accepted. Humidity is also an important consideration in manufacturing quality control, particularly when using hygroscopic materials, and avoiding corrosion on machined metal parts.
Worker health can be a significant concern. If humidity is too low, the drying of the mucus membranes reduces the body's immune system. If humidity is too high, environmental mold and mildew growth increase. Therefore, controlling humidity within a narrow range promotes good health practices in the workplace.
Current technologies, are energy intensive and lack precise control. Water phase change is common to the technologies currently in use, which change the temperature of a gas while removing humidity. This requires the application of a second process to return the gas temperature to the original or another desired temperature. These multiple step processes are energy intensive and difficult to control with precision. The two current technologies for air conditioning dehumidification are cooling coil and desiccant cycle.
Cooling Coil: In this process, a cold coil, which may be finned, is placed in the gas stream. The temperature of the coil must be slightly below the desired wet bulb temperature of the dehumidified gas. Humidity condenses out of the gas onto the cold surface of the coil. The gas stream leaving the coil is at the desired dew point temperature, and the dry bulb temperature is only slightly above the dew point temperature. The gas must then be heated to the desired dry bulb temperature. This reheating of the gas represents an additional energy penalty required to dehumidify the gas. A conventional Vapor Compression Refrigeration Cycle (VCC) could produce cool coils and could also supply reheat energy.
Desiccant Cycle: This is a three step process. In the first step, the desiccant, exposed to the humid gas, adsorbs the humidity from the gas. This is an exothermal step, so the gas heats up as the water vapor absorbs. Before this gas can be used it must then be cooled to the desired delivery temperature. In the second step, high grade heat regenerates the desiccant. Heating the desiccant increases the surface vapor pressure above the vapor pressure of the surrounding gas, and the moisture leaves the desiccant. The third step is to cool the desiccant so that its water vapor pressure will be below the vapor pressure in the processed gas. Energy is therefore used in both cooling the air after the desiccant step and in the regeneration step.
In general, the design of an air conditioning system provides the proper balance between sensible cooling and humidity control based on a “standard day” for a particular location. The system controls to a set-point temperature based on the humidity level of the “standard day.” There is no measurement of the humidity or control of the process based on humidity. The amount of dehumidification achieved is a function of the run time determined by the temperature controller and often little actual humidity control occurs. This is because the design conditions of the “standard day” occur for only a few hours of each year. During the remaining hours, the temperature and humidity vary with little relation to each other. Often the humidity can be higher than design criteria when the dry-bulb temperature is lower than design criteria. When this occurs the humidity within a space may rise significantly.
If humidity control is applied at all, three technologies are currently applied: reheat cycle, desiccant drying, and humidity exchanger. The most common of the currently applied technologies is the reheat cycle. This involves overcooling the air with VCC air conditioning, resulting in both a decrease in latent heat and sensible heat of the air mass. The air mass now being too cold (i.e. 12° C.) is reheated to the desired condition (i.e. 23° C.). The second technology uses a desiccant drying system. This process removes latent heat from the air mass but adds sensible heat to the air mass. The overheated air is then re-cooled to the desired condition. In either of these first two technologies, energy is wasted moving along the temperature scale for water vapor removal only to move back toward the original temperature of the air mass for comfort or health. The use of water phase change as the dehumidification mechanism dictates this movement along the temperature scale. Phase change is not the mechanism in the present invention.
The third technology is to use a humidity exchanger between the process air and waste air stream. This is usually a heat wheel coated with a desiccant, but a few plate type humidity exchangers are available and several liquid desiccant systems are also in use. This process does not serve as humidity control because it will not result in the removal of all the water vapor required. It is used as a pretreatment to humidity control and is effective in reducing the energy cost of humidity control in some climates but not all regions of the world.
Historically, a membrane process establishes a partial pressure difference across the membrane by operating the feed and permeate streams at different absolute pressures. For instance, compressed air systems use a feed stream that is ≧100 psig while operating the permeate stream at ambient pressures. Alternatively, the feed gas could be at ambient pressure and a vacuum pump may be attached to the permeate (7). For humidity controlled air conditioning systems the former is impractical and the later results in a very small driving force, ΔF. For example, if a vacuum permeate membrane system has a high water selectivity (Table 1) then the vacuum pump attached to the permeate stream must maintain an absolute pressure of less than 12 mmHg (0.232 psia or 29.45 in Hg of vac) for practical air conditioning results.
A number of membrane systems in the literature are variations on the existing technologies; namely, cooling coil and absorption. Membrane manifestations of the cooling coil, or “membrane condensers,” have been used for humidity control in microgravity environments (17) and proposed as a means to improve the efficiency of condenser (clothes) dryers (1). While these are novel applications of membrane technologies, these “membrane condensers” still dehumidify air by cooling it and give no practical advantage for air conditioning over standard cooling coil technology. Membrane manifestation of absorption may use aqueous salt solutions; such as LiCl (10). These membrane systems are variations on the existing desiccant cycles. The membrane serves only as a method of contacting the humid air with the desiccant.
Several patent documents specify a compressed or pressurized gas feed with a portion of the retentate used as a permeate sweep. The following are representative of these high pressure feed patents: U.S. Pat. No. 6,540,818, describing compressed air feed, retentate “reflux” as sweep; U.S. Pat. No. 5,259,869, describing pressurized feed, retentate sweep, no specification on permeate pressure; U.S. Pat. Nos. 4,793,830 and 4,687,578, describing feed “compressed to at least one atmosphere” with an ambient pressure retentate sweep. Thus, high pressure membrane systems exist for dehumidifying industrial gases (2). Some of these units recycle a portion of the produced gas to aid in establishing the driving force for the dehumidification process. These industrial membrane systems operate with a cross membrane pressure drop of greater than 6.5 atmospheres. However, known membrane systems for dehumidifying atmospheric pressure gases suffer from low driving forces across the membrane.
The following patents are also of note, as representations of the state of the art: U.S. Pat. No. 4,718,921, limited to hollow filaments made of aromatic imide polymer with retentate sweep, and preferably having pressurized gas feeds and ambient permeate pressures made in the retentate sweep claim; U.S. Pat. No. 4,900,448, limited to hollow fiber membranes and vacuum only with no vacuum retentate sweep; U.S. Pat. No. 5,681,368, limited to pressurized feed and vacuum permeate with no retentate sweep; U.S. Pat. No. 5,525,143, limited to hollow fiber membranes with internal module sweep gas generation; U.S. Pat. No. 4,783,201, using “sufficiently porous membranes” to create the retentate sweep via “leaking membranes.”
Proposals to use membrane systems for humidity control date back to before October 2000, when El-Dessouky et al. (7) published a study that a successfully designed membrane air drying system could result in 86.2% energy savings over commonly used conventional mechanical vapor compressor air conditioners. However, to date no such system is available in the market, indicating an unmet need with a significant commercial value potential.
Accordingly, there remains a long-felt but unmet need for systems that can dehumidify gases isothermally. There remains a need for cost-effective and efficient dehumidification of gases that are at about ambient pressure, such as gases in air conditioning units and dryers. There also remains a need for a membrane dehumidification unit that can remove water vapor from gases with a relatively low pressure drop (e.g., less than about 6.5 atm) across the membrane.
This Summary lists several embodiments of the presently invention, and in many cases lists variations and permutations of these embodiments. This Summary is merely exemplary of the numerous and varied embodiments. Mention of one or more representative features of a given embodiment is likewise exemplary. Such an embodiment can typically exist with or without the feature(s) mentioned, likewise, those features can be applied to other embodiments of the presently disclosed subject matter, whether listed in this Summary or not. To avoid excessive repetition, this Summary does not list or suggest all possible combinations of such features.
Embodiments of the present invention include apparatuses and methods for removing water vapor from a gas. Some embodiments of the present invention use a membrane process for humidity control that is substantially different than the technologies currently used for humidity control and that resolve the above-discussed unmet needs. In some embodiments the apparatus for removing water vapor from gas comprises a membrane, a membrane housing comprising a first pressure side and a second pressure side, with the membrane dividing the first pressure side from the second pressure side, a feed gas inlet directing a feed gas with a first humidity into the first pressure side and in contact with the membrane, a feed gas outlet on the first pressure side, a sweep gas inlet directing a sweep gas with a second humidity into the second pressure side and in contact with the membrane, a sweep gas outlet on the second pressure side allowing the sweep gas and a permeate to exit the membrane, a sweep gas flow regulator to direct the sweep gas into the second pressure side, and a pump that imparts a lower pressure in the second pressure side and directs the sweep gas through the second pressure side, wherein water vapor from the feed gas is drawn through the membrane into the second pressure side as the permeate.
In some embodiments the sweep gas flow regulator is an expansion valve, a throttling device, a valve, a capillary tube, or an orifice, and the orifice can be an opening in the membrane. The sweep gas flow regulator can be within the membrane housing and/or outside the membrane housing.
In some embodiments the apparatus further comprises a flow splitter to direct a re-directed portion of the feed gas exiting the first pressure side to the second pressure side as the sweep gas. The re-directed portion can be about 0.1% to about 99.9%, about 0.1% to about 50%, or about 0.1% to about 20% of the gas exiting the feed gas outlet, for example.
In some embodiments of the apparatus, a pressure in the second pressure side is lower than a pressure in the first pressure side. In some embodiments the feed gas enters the first pressure side at ambient pressure. Also, in some embodiments a pressure in the second pressure side is about 200 mmHg-absolute or less, about 100 mmHg-absolute or less, or about 50 mmHg-absolute or less.
Some embodiments of the present invention further comprise a water collection device to collect condensed water vapor from the feed gas, and the water collection device can be attached to the second pressure side of the membrane housing and/or be downstream of the sweep gas outlet.
The membrane in some embodiments is a spiral wound membrane, a tubular membrane, a hollow fiber membrane, a flat sheet membrane, a capillary membrane, or combinations thereof. The membrane can be a water permeable membrane, a semi-permeable membrane, or combinations thereof, and specific examples of membranes comprise polydimethylsiloxane, cellulose acetate, sulfonated polyethersulfone, polyethylene oxide, sulfonated poly(ether ether ketone), poly(vinylalcohol)-EDTMPA, [emim][Tf2N], [N(4)111][Tf2N], [emim][BF4], or combinations thereof.
Some embodiments of the present invention can achieve feed gas in the feed gas outlet having a dew point of about −42° C. to about 35° C., and can have dehumidification efficiencies of about 50% to about 600%.
Some embodiments further comprising a recycle loop that is in fluid communication with the feed gas inlet and the feed gas outlet or a recycle loop that is in fluid communication with the sweep gas outlet and the feed gas inlet. The recycle loop can connect a gas outlet of a water collection device to the feed gas inlet in some embodiments.
Some embodiments are part of a heating system, a ventilation system, an air conditioning system, a drying system, a liquid recovery system, or combinations thereof. For example, the feed gas in the feed gas outlet can enter a drying system. Also, a dryer gas from an outlet of the drying system is recycled to the feed gas inlet in some embodiments.
Embodiments of the present invention also comprise a method for manufacturing the above described embodiments of the present invention as well as variations thereof. Furthermore, other embodiments comprise methods for removing water vapor from gas comprising providing an embodiment of the present invention described above as well as variations thereof, delivering the feed gas to the feed gas inlet, vacuuming the second pressure side with the pump to provide the sweep gas to the second pressure side and drive water vapor through the membrane, and collecting a product.
In some embodied methods the product is the feed gas from the feed gas outlet, water vapor collected from a water collection device, or combinations thereof.
In some embodiments the feed gas that is to have water vapor removed is air, oxygen, nitrogen, methane, biomethane, ethane, ethylene, ethanol, butane, butanol, or combinations thereof. In some embodiments the sweep gas is a portion of the feed gas from the feed gas outlet, a preselected gas, or combinations thereof.
In some embodiments the feed gas enters the first pressure side at ambient pressure and the sweep gas comprises a portion of the feed gas.
Embodiments of the present invention also comprise a plurality of the above described embodiments of apparatuses as well as variations thereof, wherein the plurality of apparatuses are connected together to establish concurrent flow, countercurrent flow, cross-flow flow, or combinations thereof.
Further advantages of the presently-disclosed subject matter will become evident to those of ordinary skill in the art after a study of the description, figures, and non-limiting Examples in this document.
Embodiments of the present invention generally relate to a membrane-based dehumidifying system and methods for using and implementing the system. In some embodiments the membrane system can use a fraction of the dehumidified gas as “a dehumidifying working fluid” (e.g., sweep gas) that passes through a sweep gas flow regulator prior to reenter the membrane housing. Without being bound by theory or mechanism, the combination of gas expansion and low absolute pressure sweep gas establish a driving force strong enough to achieve dehumidification efficiencies, defined as the ratio of latent heat removed to the energy consumed, greater than about 200%. Notably, in some embodiment the driving force is sufficient such that gas at ambient pressure can be dehumidified, and therefore the pressure drop across the membrane is at most about 1 atm. The produced gas can have a lower humidity than the feed gas. Some embodiments of the present invention produce gases with dew points less than about 0° C.
In the following description, various embodiments of the present invention will be disclosed. For purposes of explanation, specific numbers and/or configurations are set forth in order to prove a thorough understanding of the embodiments. However, it will also be apparent to one skilled in the art that the embodiments may be practiced without one or more of the specific details, or with other approaches and/or components. In other instances, well-known structures and/or operations are not shown or described in detail to avoid obscuring the embodiments. Furthermore, it is understood that the embodiments shown in the figures are illustrative representations and are not necessarily drawn to scale.
References throughout this specification to “one embodiment,” “an embodiment,” and so forth mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, references to certain “embodiments” and so forth throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
Unless otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in this specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by the presently disclosed subject matter.
As used herein, the term “about,” when referring to a value or to an amount of mass, weight, time, volume, concentration or percentage is meant to encompass variations of in some embodiments ±20%, in some embodiments +10%, in some embodiments ±5%, in some embodiments ±1%, in some embodiments ±0.5%, and in some embodiments ±0.1% from the specified amount, as such variations are appropriate to perform the disclosed method.
Some embodiments of the present invention are a low energy system for direct humidity control. Such embodiments directly meet long-felt needs that are not met with current commercially available technologies. Embodiments of the present invention remove humidity from gases in ways that are thought to be unattainable with conventional cooling coil dehumidification; namely, for example, isothermal dehumidification and the production of gases with dew points <0° C.
Some embodiments of the present invention use a portion of the retentate expanded through a sweep gas flow regulator (e.g., controllable valve) to create the desired combination of vacuum pressure and sweep gas flow rate. These embodiments dehumidify the feed gas, which then becomes the retentate. Alternative embodiments use the resulting spent sweep gas to produce liquid water that may or may not be potable.
Some embodiments of the present invention can be used in conjunction with heating, ventilation, and/or air conditioning systems (HVAC). Traditionally, in residential and smaller structures temperature control instead of humidity control is the norm. Humidity control using current technologies adds both capital and energy cost because of the need to add a reheat or desiccant system. However, in the context of air conditioning systems, the decoupling of latent and sensible heats reduces energy cost of the entire air conditioning system by avoiding over cooling and then reheating of the processed air. Lowering the moisture content of air within a building may also contribute to energy conservation. Low humidity buildings “feel” cooler and direct humidity control can eliminate the need to cool buildings for occupant comfort. Thus, some embodiments of the present invention that are used in conjunction with HVAC systems can reduce the net amount of energy required to make conditions within a structure comfortable. In addition, this may lead to increased public health by reducing the growth of bacteria, mold and mildew. Proper levels of humidity can also boost the body's immune function.
Accordingly, some embodiments of the present invention can be used for direct humidity control and decoupling of humidity control from air temperature control. Benefits of the invention include, for example, increased use of humidity control versus temperature control, smaller refrigerant plants leading to a decrease in the environmental impact of hydrofluorocarbon (HFC) refrigerant gases, positive economic impacts, and reduced costs.
The present invention does not use hydrofluorocarbons (HFCs) and may result in smaller cooling units containing smaller volumes of HFC working fluids since the cooling units will have reduced heat loads (e.g., reduced latent heat leaving only sensible heat loads). HFCs are strong greenhouse gases; therefore, the invention may benefit the public and reduce greenhouse gases in two ways: reduced energy use and reduced production of HFCs.
Aside from air conditioning, some embodiments of the present invention are directed to systems and processes to increase the energy efficiency of drying systems. Drying systems include clothes dryers, dryers used for pharmaceutical manufacturing, and the like. Some embodiments can isothermally dehumidify the exit gases from a dryer, and this dehumidified exit gas can then be recycled to the dryer so as to achieve direct recycling of the sensible heat to the dryer.
Specific examples of economic impacts of embodiments of the present invention, due to its ability to reduce humidity, include reduced bedding and linen replacement for hotels, decreased cleaning and maintenance of equipment and facilities, avoidance of extreme cases in which high humidity leads to the loss of buildings, and better humidity control for pharmaceutical manufacturing and packaging operations, which is also important for quality control during production and for shelf life during storage and packaging.
One superior and unexpected result of embodiments of the present invention is the size of the driving force for removing humidity from the feed gases. The dew point temperatures of the permeate may indicate the size of this driving force, and in some embodiments the permeate may have a dew point below the freezing point of water. In certain embodiments of the present invention, the permeate dew point was as low as minus 42° C. The conventional cooling coils used in the air conditioning industry physically may not reach driving forces for humidity removal that are this large, since ice formation on the coils sets the minimum dew point for a conventional coil at around 0° C. In addition, the invention may produce dehumidified gases as a product with dew points <0° C. Extremely dry product gases, with dew points <0° C., are typically impossible for convention cooling coils.
Some embodiments achieve dew points of about −42° C., about −40° C., about −35° C., about −30° C., about −25° C., about −20° C., about −15° C., about −10° C., about −5° C., about 0° C., about 5° C., about 10° C., about 15° C., about 20° C., about 25° C., about 30° C., about 35° C., or any value therebetween. Of course dew points may also be adjusted above or below this range to meet the needs of particular circumstances.
To person having ordinary skill in the art, the driving forces of the present invention that achieve product gases with sub-zero dew points and isothermal dehumidification would be superior and unexpected results. A person having ordinary skill in the art of membranes, looking at the similar results for existing high pressure gas drying units, would also find it to be superior and unexpected that embodiments of the membrane system work with low atmospheric pressure feeds, compared to the 100 psig or greater feeds required for known high pressure gas units.
Embodiments of the present invention include a technology that dehumidifies gases with low energy use that could garner significant market share from existing atmospheric pressure technologies (e.g. cooling coils and desiccants). As discussed above, one industry is the air conditioning industry. Also as discussed above, some embodiments of the present invention are applicable to the drying of solids; such as processing pharmaceuticals and drying clothes. Because clothes dryers currently account for about 5.8% of household energy use in a process recognized as being energy inefficient (1), those of ordinary skill will recognize the energy and cost advantages that may be achieved with certain embodiments of the present invention. Furthermore, some embodiments of the claimed subject matter are also capable of recovering gaseous water as a liquid, and such liquid water may be potable.
By using certain membranes, embodiments of the present invention dehumidify gases by creating a vapor pressure difference across such membranes. This removes water vapor from gas without changing the temperature of the gas. Thus, some embodiments of the present invention dehumidify gases isothermally. This one step process is less energy intensive and more controllable than certain previously known methods. The driving force, measured as the effective dew point temperatures of the “sweep gas,” can be below the freezing point of water.
Also, because some embodiments of the present invention are able to dehumidify gases that are at ambient pressure, the pressure difference across the membrane is at most about 1 atm, corresponding to the difference between the near vacuum on the permeate side of the system and the atmospheric pressure present on the retentate side
Adsorption (desiccants) and absorption (aqueous salts) exploit a phase change from vapor to a solid or liquid matrix. In contrast to phase change, other properties such as membrane permeability or molecular size can be exploited in the embodiments of the membrane-based separation system of the present invention. The ideal energy cost of separation by phase change (condensation, adsorption, or absorption) is approximately the water's heat-of-vaporization or the latent heat, while the energy cost of a membrane-based separation is only the cost of maintaining a partial pressure difference across the membrane.
Membrane-based gas dehumidification can have technical, energy, and economical advantages over other dehumidification technologies, such as absorption, adsorption, and refrigeration depending on the application (4). The US Department of Energy has previously recognized the low energy cost of membrane separations by including them in road maps for separation research (3). The advantages of simple installation, ease of operation, and low process cost have allowed successful applications to dehumidify high pressure compressed air (4). Table 1 contains polymers typically used for gas dehumidification along with some other novel membranes. Table 1 also contains the permeability or permeance along with selectivities. Permeability and permeance are measurements of the partial pressure normalized rate of water vapor transport through the membrane. Selectivity is the normalized rate of water transport divided by the gas transport through the same membrane, and is a measure of humidity separation using the referenced membrane.
However, the term “membrane”, as used herein, refers to any membrane that is selective for a substance that is desired to be removed from a feed gas. Thus, the term membrane is not limited to the membranes in Table 1. However, membranes can include, but are not limited to, room termperature ionic liquid membranes (RTIL), polymer membranes, water permeable membranes, and semi-permeable membranes. Furthermore, the membrane can be, but is not limited to, a flat membrane (plate and frame), a spiral wound membrane, a tubular membrane, a hollow fiber membrane, a capillary membrane, or combinations thereof. Each of these geometries has advantages. A geometry with a low pressure drop from the feed to the retentate may be advantageous in certain embodiments of the present invention.
Notably, the rate of transport through a membrane, including those listed above, is generically defined by the equation:
Q/A=j=(K/δ)ΔF=LiΔF (1)
where: j=Q/A is the flux of the transport species (Q=quantity transported, A=surface area of the membrane, K is the permeability coefficient of the membrane material, δ is the membrane thickness, Li=K/δ is the membrane permeance or the inverse of the resistance to flux and ΔF is the driving force or the difference in the transporting species' chemical potential across the membrane. There are many ways of reporting this chemical potential difference; however, the most practical means for water vapor transport is partial pressure. Those of skill in the art may utilize equation 1 to achieve desired mass transfer in an embodiment of the present invention.
Looking now to
The term “feed gas”, “bulk gas”, and the like, as used herein, refer to any gas mixture from which a substance can be removed by the membrane. In certain embodiments the substance to be removed is water, and more specifically, water vapor. In one embodiment, the apparatus is optimized to remove water from air at atmospheric pressure and temperature. Feed gases in other embodiments also include, but are not be limited to, methane, biomethane, ethane, ethylene, ethanol, butane, butene, butanol, and combinations thereof.
To aid mass transfer, a vacuum pump may lower the pressure of the permeate side 13 of the membrane housing 5 below the pressure in the retentate side 9 of the membrane housing 5. The gas, water, and other substances that do not pass through the membrane may exit the retentate side 9 of the membrane housing 5. The feed gas exiting the retentate side 9 may have a lower specific humidity compared to the feed gas that enters the retentate side 9.
Also as shown in
In some embodiments, the sweep gas entering the permeate side 13 of the membrane housing 5 aids mass transfer by “sweeping” away permeate from the permeate side 13 of the membrane 7. In some embodiments this is achieved by using a sweep gas having a lower humidity than the permeate. By sweeping the permeate with a sweep gas, the apparatus 1 may achieve higher driving forces across the membranes and avoid high concentrations of the substance that the membrane is selective for (e.g., water) from building up on the permeate side 13 of the membrane 7.
The term “flow splitter”, as used herein, generally refers to any device or object that can split the flow of a fluid into two or more streams. In some embodiments the flow splitter is a T-junction that splits an incoming stream into two outgoing streams. Furthermore, the “re-directed portion of the feed gas exiting the first pressure side” can be any amount of the feed gas that exits the first pressure side. For instance, the re-directed portion can comprise anywhere from 0.1% to 99.9% of the feed gas exiting the first pressure side.
In some embodiments the re-directed portion of the feed gas exiting the first pressure side comprises about 0.1%, about 2.5%, about 5.0%, about 7.5%, about 10.0%, about 12.5%, about 15.0%, about 17.5%, about 20.0%, about 25%, about 30%, about 35%, about 40%, about 50%, about 60%, about 70%, about 80%, about 90%, about 99.9%, or any value therebetween of the feed gas exiting the first pressure side.
As discussed above, the permeate side 13 can operate at a vacuum pressure. As used herein, the terms “lower pressure”, “vacuum pressure”, “vacuum”, and the like generally refer to a pressure that is lower than a pressure in a first pressure side (e.g., retentate side) of a membrane housing. In some embodiments the vacuum pressure is any pressure below ambient pressure. In some embodiments vacuum pressure is about 50 mmHg-absolute, 100 mmHg-absolute, or 200 mmHg-absolute.
In some embodiments, a vacuum pressure is about 50 mmHg-absolute, about 100 mmHg-absolute, about 150 mmHg-absolute, about 200 mmHg-absolute, about 250 mmHg-absolute, about 300 mmHg-absolute, about 350 mmHg-absolute, about 400 mmHg-absolute, about 450 mmHg-absolute, about 500 mmHg-absolute, about 550 mmHg-absolute, about 600 mmHg-absolute, about 650 mmHg-absolute, about 700 mmHg-absolute, about 750 mmHg-absolute, about 800 mmHg-absolute, about 850 mmHg-absolute, about 900 mmHg-absolute, about 950 mmHg-absolute, about 1000 mmHg-absolute, or any value therebetween.
The term “ambient pressure”, as used herein, generally refers to a pressure that is equal to about the pressure in the atmosphere in which an apparatus 1 is located. Accordingly, in some applications the ambient pressure will be approximately 1 atm. However, ambient pressure may deviate due to atmospheric conditions, altitude, and the like. Furthermore, in some embodiments a feed gas is fed to the first pressure side 9 with a pump, fan, or the like, that can cause the pressure in the first pressure side 9 to be slightly greater than that in the surrounding atmosphere. Lastly, the ambient pressure in the first pressure side 9 can deviate in the first pressure side 9 because of pressure drops caused within the membrane housing 5.
As used herein, the term “sweep gas flow regulator” generally refers to any device that can control the flow of sweep gas into the permeate side and also allows a vacuum pressure to be created in the permeate side. Examples of sweep gas flow regulators include expansion valves, throttling devices, needle valves, other valve designs, capillary tubes, orifices, and the like. The sweep gas flow regulator may be located either inside (not shown), on (not shown), or outside the membrane housing. For instance, the sweep gas flow regulator may be located on a sweep gas flow inlet (
A sweep gas flow regulator on the membrane is one example of an internal regulator. In some embodiments the internal sweep gas flow regulator is a leak or orifice on the membrane. For some embodiments comprising an internal sweep gas flow regulator, the sweep gas inlet can also be internal and may or may not be the same element as the sweep gas flow regulator.
The terms “pump”, “vacuum pump”, and the like, as used herein, generally refer to any device that modulates gas pressure. In some embodiments the pump imparts low pressure or a vacuum in a structure. Those of skill in the art will be able to determine the appropriate pump to achieve desired pressures in specific embodiments, and will appreciate that pumps are not to be limited in structure, design, and the like, but instead merely need to displace a fluid by any means to modulate pressure. The pumps may be selected from any known pump that may achieve the results desired in terms of efficiency, water removal, capacity, and the like. Examples of pumps include, but are not limited to, reciprocating pumps, rotary pumps, screw pumps, peristaltic pumps, compressors, and centrifugal pumps. Pumps can function to, among other things, keep the permeate side at a lower pressure relative to the retentate side, which drives mass transfer across the membrane, and aids the sweep gas in sweeping the membrane.
The arrangement shown in
The term “water collection device”, as used herein, generally refers to any device that can collect water from a fluid that comprises water in gas, liquid, and/or solid form. In some embodiments the water collection device is a known cooling coil system that condenses water that is in the fluid in the sweep gas outlet. The water collection device can also be a device that comprises a membrane to separate water from the fluid in the sweep gas outlet. Any other suitable device may be utilized as a water collection device so long as it separates water from the fluid in the sweep gas outlet and can recover this water as a liquid.
The embodiment shown in
Accordingly, the embodiments shown in
The embodiment also comprises a water recovery unit 25 that is located downstream from the pump 21 as well as a gas recycle 29 that recycles gas from the water collection device 25 back to the feed gas inlet 3, which also minimizes energy losses that would be caused by releasing heated gases. Accordingly, the depicted embodiment removes some or all the water from the substrate in the dryer drum 37 as liquid water in the water collection device 25, and the gases that are heated by the heater 35 are not released, which minimizes energy losses.
Further embodiments of the present invention comprise methods of utilizing the above described embodiments as well as variations thereof for removing vapor water from a gas. Some embodied methods comprise providing an apparatus for removing water vapor from gas, delivering a feed gas to the feed gas inlet of the apparatus, vacuuming a second pressure side of the apparatus with a pump to provide the sweep gas to the second pressure side and dryer water vapor through a membrane of the apparatus, and collecting a product.
As used herein, the term “providing” generally refers to, but is not limited to, making, using, lending, offering, selling, licensing, or leasing an embodied apparatus. Accordingly, the entity providing the apparatus may or may not actively participate in the removal of water vapor from a gas. Furthermore, as used herein, the term “delivering” generally refers to placing a gas in such a position that it enters the feed gas inlet of an apparatus. For example, delivering may be an active process where the feed gas inlet has a negative pressure and therefore draws the gas into the feed gas inlet. In other embodiments feed gas is delivered by a fan, pump, compressor, or the like to the feed gas inlet. “Vacuuming”, as used herein, is used to refer to the activation of a pump of an apparatus, which in turn imparts a low or vacuum pressure in the second pressure side of an apparatus and thereby moves a sweep gas through the second pressure side and/or drives mass transfer across the membrane of an apparatus.
Lastly, the term “collecting”, as used herein, refers to the physical collection, use, manufacture, or the like of a “product”. For example, collecting a product can comprise venting feed gas from a feed gas outlet into a building or structure so that the air within the structure is less humid that it would otherwise be. Collecting a product can also comprise collecting condensed water vapor from a water collection device and using it for drinking or non-drinking purposes. Collecting a product can also comprise using the feed gas from a feed gas outlet for various processes, such as drying clothes with a dryer, pharmaceutical process, defrosting windows, and so forth. Accordingly, those of skill in the art will appreciate that one or more different products may be collected from embodiments of the present invention for various different purposes.
To illustrate the effectiveness of embodiments of the present invention used for drying processes,
Another embodiment of the present invention is a membrane module that may be constructed unlike membrane modules currently in general use. Hollow fiber modules with the feed gas passing through the interior of the hollow fiber may have too high of a pressure drop. Fortunately, significant progress has been made in designing membrane modules with minimum feed gas pressure drop. Newbold et al. (9) in 1996 and Kneifel et al. (10) in 2006 published designs of membrane modules that may meet current needs. In the case of the Kneifel et al., designed air flow velocities of 4 meters/sec produced back pressures of less than 0.001 bars. This result is still useable for the present invention's purposes even though the membrane module used aqueous salts as an absorption-based dehumidification working fluid on the permeate side of the membrane.
In other embodiments, sweep flow rate and permeate pressure may not be totally independent of each other, and may work together to establish the necessary driving force to remove the desired level of humidity from the feed. The energy cost of this system may be dependent on sweep flow rate and/or permeate pressure. Decreasing the permeate pressure or increasing the sweep rate both may lead to larger vacuum pump energy demands via the following isothermal relationship:
Work=NRT*ln(760/Pp)/efficiency (2)
where N=number of moles pumped by the vacuum pump, which is the sum of the fraction of feed recycled as the permeate sweep plus the moles of water fluxed through the membrane, R=ideal gas constant, T=absolute temperature of the process. Pp=permeate absolute pressure in mmHg, and efficiency=vacuum pump isothermal efficiency. Note that in the energy relationship formula, the total moles in the sweep, N, has a direct relationship, while the permeate pressure, Pp, has a logarithmic relationship. Therefore, the sweep rate may be the more sensitive factor in reducing the energy cost.
Using [emim][BF4]-membranes listed in Table 1, embodiments of the present invention were established to test the concept of the invention. These experiments underestimated the invention's performance (conservative data) because the membrane module used (
As discussed herein, embodiments of the present invention remove water vapor from gases using a water selective membrane. The driving force for water flux through the membranes can come from expanding a small portion of the retentate gas into the permeate space of the membrane module that is maintained at a lower absolute pressure than the feed/retentate side pressure. The combination of gas expansion and low absolute pressure sweep gas may establish a driving force strong enough to achieve dehumidification efficiencies >200%. In some embodiments dehumidification efficiency is about 200% to about 600% or even greater. Of course, dehumidification efficiencies of less than 100% are competitive with current technologies and may be desired in certain embodiments. In some embodiments the efficiency is 50%-100%. Dehumidification efficiency may also be adjusted to be about 1% to about 50%. Thus, the retentate gas humidity may be significantly reduced compared to the feed gas.
In the context of air conditioning systems, the invention could remove latent heat from the air prior to cooling via a conventional refrigeration vapor compression cycle (VCC) or evaporative cooling. Air conditioning systems using embodiments of the present invention for latent heat removal can use less energy overall than current VCC alone systems. The decoupling of latent and sensible heats may reduce energy cost of the entire air conditioning system by avoiding over cooling (followed by reheating) of the processed air.
Some embodiments of the present invention can remove humidity with a small sweep rate and obtainable permeate pressures. Combining the removal of humidity (
Some embodiments of the invention are low energy systems for direct humidity control air conditioning. Therefore, embodiments of the present invention directly meet needs previously defined in the literature and engineering guidelines that are not met with current commercially available technologies. Embodiments of the present invention remove humidity from gases in ways that can not be achieved with certain conventional cooling coil dehumidification; namely, isothermal dehumidification and the production of gases with dew points <0° C. Both of these results may be superior and unexpected to those routinely engaged in air conditioning engineering.
Other non-limiting examples of applications for low energy dehumidification could also include defrosting car windows without the need to run the air conditioner, thus saving gas. Also, considering the permeate as the product, this invention may produce drinking water in remote locations, and may therefore be proper for humanitarian or military applications.
The disclosed embodiments of the present invention are further illustrated by the following non-limiting examples.
In this Example a system was designed to analyze the effectiveness of particular embodiments of the present invention for removing water vapor from certain feed gases.
Looking to
Upon exiting the 300-mL vessel 109, the humidified gas entered the stainless steel dual chambered membrane module 5. The membrane module 5 was sealed from the atmosphere by the compression of two O-rings. As shown in
Three sensor ports 106 were in the experimental set-up to measure conditions of the Feed 3, Retentate 11, and Permeate 17 streams. The downstream ports were used to determine exit conditions of the retentate and permeate. All of the sensor ports 106 had calibrated Honeywell HIH-3610 Series relative humidity sensors. In addition the feed and retentate ports 106 had National Semiconductor LM34 temperature sensors. The retentate and permeate ports 106 had Omega PX139 pressure sensors.
Also, upon exiting the membrane module 5 the gas passed through a permeate port 106, a vacuum pressure controller 108, and a pump 21.
It was noted that the sweep flow rate and permeate pressure may not be totally independent of each other. Such is the case where, for example, the needed driving force across the membrane requires the permeate to be at “room neutral” (dew point of 13° C.). With zero recycle sweep the vacuum pump will need to operate at below 50 mmHg. However an absolute permeate pressure of 100 mmHg will produce the desired driving force with recycle sweeps as small as 5%. Surprisingly, some of the effective permeate dew points generated are below 0° C.; with zero recycle sweep this may require a permeate absolute pressure <5 mmHg. The test module was single pass co-current flow. For the specific embodiment, performance can be superior using counter-current flow in certain circumstances.
As discussed below, the data obtained from embodiments of the present invention illustrate the connection between sweep flow and permeate absolute pressure. The permeate absolute pressures covered were the vacuum pressures obtainable by either a rotary water sealed pump (absolute values >100 mmHg) or, for the lower tested vacuums, a reciprocating vacuum pump (12). Both of these types of pumps are commonly used in commercial applications. Other embodiments may use other pump designs. In summary, the permeate absolute pressures reported are 200 mmHg, 100 mmHg, 50 mmHg, and 5 mmHg.
This Example explains some of the superior and unexpected results observed in connection with the embodiment discussed in Example 1.
Alternatively, the data in
This Example discusses the “dehumidification efficiencies” observed using the embodiment of Example 1.
Specifically,
To calculate the dehumidification efficiency per kilogram of dry air (kg-DA) produced by the unit, we first calculate the latent heat removed per kg-DA,
Latent Heat Removed=ΔH*λ (4)
where ΔH=absolute humidity change from feed to retentate (g/kg-DA) and λ=latent heat at the air stream temperature (kJ/g). The work required is equation 2, isothermal compressor work, scaled for the reduction in produced dehumidified gas by the fraction of the feed rate used in the permeate sweep,
In eq. 5, it was assumed an isothermal compressor efficiency of 60% and accounted for both the moles of gas split from the retentate for the sweep and the moles of water fluxing through the membrane to calculate the total moles fed to the vacuum pump, N. Since condensation may be unnecessary to remove water vapor in a membrane unit, the latent heat carried by the water vapor may be larger than the vacuum pump work used to facilitate the permeation of the water vapor through the membrane. Therefore, combining eqs. 3 through 5 may produce dehumidification efficiencies greater that 200% (
Examples 1 to 3 discuss observations made for a single pass embodiment. However, two units may be run in series with the sweep gas from the last unit flowing counter-current as the sweep for the first unit, for example.
In this Example the performance of embodiments of the present invention for high temperature drying applications were analyzed. Embodiments were tested at elevated temperatures (50° C.) and showed similar absolute humidity removal to those reported in
In this Example the effective permeate dew points obtained with the embodiments of the above Examples were analyzed. While the following does not directly speak to application performance, it helps contrast embodiments of the invention against conventional cooling coils used for building dehumidification and may provide further evidence of a surprising result. The driving force for dehumidification using cooling coils may be the establishment of a temperature on the coils below the dew point of the air being dehumidified. The driving force for membranes may also be related to the dew point of the gas on the permeate side of the membrane. Tables 2-4 show this effective dew point driving force for the various feed conditions discussed. Many of these permeate dew points are below the freezing point of water. This sub-zero effective coil temperature may be a surprising and unexpected result for someone used to working with conventional dehumidification technologies.
The invention thus being described, and as discussed above, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the Specification, including the disclosed embodiments, tests, data, and examples, be considered as exemplary only, and not intended to limit the scope and spirit of the invention.
ASHRAE: American Society of Heating, Refrigeration, and Air Conditioning Engineers
Dry Bulb Temperature: The temperature of air measured directly by a thermometer.
Expansion Valve: Any throttling device, such as, but not limited to, a valve, capillary tube, throttle, or an orifice.
g/kg-DA: A humidity unit notation meaning grams of water vapor per kg of dry air.
Humidity: A measure of the amount of water vapor in the air stream.
Humidity Control: A process to actively control both the sensible heat and the latent heat of a space or air mass to a range of dry bulb and wet bulb temperatures. Both of these temperatures are measured and the process adjusted to achieve both desired ranges.
Latent Heat: The energy added to or removed from an air mass by increasing or decreasing the humidity in the air mass.
Relative Humidity: Quantifies the amount of water vapor in the air as a percentage of the maximum amount of water vapor air can hold at the Dry Bulb Temperature of the air.
Sensible Heat: The energy added to or removed from an air mass to change the Dry Bulb Temperature
VCC: Vapor Compression (refrigeration) Cycle
Throughout this application various publications are referenced. All such references, including those listed below, are incorporated herein by reference.
This application claims priority from U.S. Provisional Patent Application No. 61/481,979, filed May 3, 2011, the entire disclosure of which is incorporated herein by this reference.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US12/36368 | 5/3/2012 | WO | 00 | 1/9/2014 |
| Number | Date | Country | |
|---|---|---|---|
| 61481979 | May 2011 | US |
