This application claims the benefit of priority of U.S. Patent Application No. 63/118,926, filed Nov. 29, 2021, the entire contents of which are hereby incorporated by reference.
The present invention relates to an apparatus and method for capturing CO2 from air and concentrating it using adsorbents, particularly from source air having low temperature and/or low humidity.
The amount of CO2 in the atmosphere increases year after year which contributes to the rise of global surface temperatures. Direct air capture and sequestration is one method to reduce the amount of CO2 in the atmosphere thereby reducing global warming. The importance of direct air capture can be seen from the Intergovernmental Panel on Climate Change's “Special Report on Global Warming of 1.5° C.” which requires carbon negative technologies such as direct air capture in order to meet international climate goals. This has spurred many companies' pursuit of direct air capture technologies such as:
The problem with current direct air capture technologies is that they are too expensive, with costs surpassing the current U.S. DOE goal of 27-39 $/ton CO2. Therefore, improvements are required before direct air capture can become an economically viable method for reducing the amount of atmospheric CO2 for the purpose of greenhouse gas reduction.
CO2 can be captured from the air anywhere on the planet to reduce net CO2 emissions. However, from a technological standpoint, the location where direct air capture is carried out is very important for the economic viability of the technology. This is due to the dilute nature of CO2 in air. In air, 420 ppm of CO2 occupies only 0.76 g/m3 (at 25° C. and 1 atm) and therefore a substantial amount of air (1,300,000 m3 of air) would be required to capture 1 ton of CO2. Thus, it is uneconomical to significantly condition the air (that is, change its temperature, humidity, or pressure) when trying to capture the CO2 and therefore, the CO2 must be captured from the air at near ambient conditions.
In addition, limitations in the implementation of direct air capture technology occur when the utilized adsorbents are not suitable for use in low-temperature environments, thereby limiting where they can be utilized or requiring further energy input in order to maintain a suitable working temperature.
In addition to temperature limitations of existing direct air capture systems, conventional thinking in the field is that, since there is water in the air, adsorbents that are water unstable or preferentially sorb water over CO2 are not suitable for use in direct air capture. These adsorbents have therefore been ruled out as viable materials for direct air capture. For example, zeolites are typically hydrophilic and will preferentially adsorb water over CO2 when exposed to both, therefore they have been determined unsuitable adsorbents for processing ambient air. If materials such as these were to be used, the water content of the air must first be reduced before efficient CO2 separation can occur. Separating water from air (25° C., 60% humidity) is an energy intensive step and would require roughly six times more energy than separating CO2 from air using Carbon Engineering's process. This line of thinking has been why sorbents such as zeolites, that are water unstable or preferentially sorb water over CO2, have been ruled out as potential materials for direct air capture by many experts (see for example, Keith et al, Capturing CO2 from the atmosphere: rationale and process design considerations. Geo-engineering climate change: environmental necessity or Pandora's box. 2010:107-26; Shi et al. Sorbents for the direct capture of CO2 from ambient air. Angewandte Chemie International Edition. 2020 Apr. 27; 59(18):6984-7006).
Based upon these limitations, there exists a need for an apparatus and method for capturing and concentration CO2 from air for a low energy penalty, to provide a cost-effective manner of removing CO2 from the air. The novel selection of adsorbents and environmental conditions for implementing the direct air capture process provides for CO2 capture in an economically viable manner.
It is an object of the invention to provide an apparatus and method to capture and concentrate CO2 from the air in an efficient and cost-effective manner.
According to an aspect of the present invention, there is provided an apparatus to capture CO2 from the air comprising: an enclosure (having an interior volume), an adsorbent contained within the interior volume of the enclosure, a vacuum pump coupled to the enclosure, a source of input air coupled to the enclosure, and a heater capable of heating the interior volume of the enclosure. The interior volume of the enclosure can be selectively isolated from one or more components of the apparatus to enable selective control of the contents, pressure, and temperature of the interior volume of the enclosure. Prior to entering the enclosure, the input air has a temperature equal to or less than 0° C., and/or humidity of equal to or less than 5 g of H2O per kg of air.
According to another aspect of the present invention there is provided a method of capturing CO2 from the air comprising: flowing a source of input air having a temperature equal to or less than 0° C., and/or humidity of equal to or less than 5 g of H2O per kg of air into an interior volume of an enclosure containing a CO2 adsorbent material, heating the CO2 adsorbent material and applying a vacuum source to the interior volume of the enclosure to permit extraction of the CO2 from within the enclosure, and equilibrating the pressure of the enclosure by permitting an influx of air or gas until the interior volume of the enclosure returns to about atmospheric pressure.
In various aspects the CO2 capture apparatus further comprises a drying means to dry the input air.
In various aspects the CO2 adsorbent material comprises a zeolite, metal organic framework, covalent organic framework, silica, or alumina. CO2 from the air is captured through exposure to the CO2 adsorbent material, the heating of CO2 adsorbent material, and subsequent removal of the CO2 from the interior volume of the enclosure under vacuum.
Aspects of the invention provide various benefits, including that the apparatus and method require low input energy, allowing for the CO2 to be captured directly from the air at a low cost per given unit. Using input air with low temperature and/or low humidity enables a broad range of CO2 adsorbent material to be used. The method and apparatus allow for the capture and concentration of CO2 from the air to be performed in a low cost, cyclical, and continuous manner.
These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings wherein:
The accompanying drawings illustrate embodiments of the invention:
One or more embodiments of the invention will now be described by way of example. It will be apparent to persons skilled in the art that a number of variations and modifications can be made without departing from the scope of the invention as defined in the claims.
ambient air utilizing a temperature vacuum-swing adsorption TVSA cycle (1). This method relies upon dry input air, either from atmospheric conditions or after having been subject to a subsequent drying means such as condensation, crystallization, desiccation, adsorption, membranes, or other absorption method to extract atmospheric water. Input air preferably remains in a dried state below a dew point of −40° C. (at ambient or near ambient conditions), and in a further preferred embodiment remains in a dried state below a dew point(s) of −45° C., −50° C., −55° C., −60° C., −65° C., −70° C., −75° C., or −80° C. (each at ambient or near ambient conditions).
In a preferred embodiment, the TVSA cycle (1) generally comprises the following steps:
After the pressurization step (5), the cycle is repeated. By repeating the TVSA cycle (1), further concentration of CO2 can be achieved from the initial input air.
flows dry air over the adsorbent in a CO2 adsorbent bed (10), preferably by using a fan (11) located either upstream or downstream of the CO2 adsorbent bed (10). This dry input air contains CO2 concentrations equal to that of atmospheric levels (approximately 420 ppm as of 2021 with CO2 increasing rapidly year-over-year and being of greater concentration near CO2 emitting sources). If the input air contains pollutants above acceptable limits (based upon either worker safety or component compatibility), they will need to be removed prior to this step by using materials such as activated carbons or zeolites which do not interact significantly with CO2.
Dry input air, containing approximately 420 ppm of CO2, flows over the adsorbent which is located in the CO2 adsorbent bed (10). The CO2 is captured via adsorption onto the adsorbent's surface. The air flowing over the adsorbent would be at near ambient conditions of temperature and pressure of the input air. In a preferred embodiment, the input air has a temperature of about 0° C., −1° C., −2° C., −3° C., −4° C., −5° C., −6° C., −7° C., −8° C., −9° C., −10° C., −11° C., −12° C., −13° C., −14° C., −15° C., −16° C., −17° C., −18° C., −19° C., −20° C., −21° C., −22° C. −23° C., −24° C., −25° C., −26° C., −27° C., −28° C., −29° C., −30° C., −31° C., −32° C., −33° C., −34° C., −35° C., −36° C., −37° C., −38° C., −39° C., −40° C., −41° C., −42° C., −43° C., −44° C., −45° C., −46° C., −47° C., −48° C., −49° C., −50° C., −51° C., −52° C., −53° C., −54° C., −55° C., −56° C., −57° C., −58° C., −59° C., −60° C., −61° C., −62° C., −63° C., −64° C., −65° C., −66° C., −67° C., −68° C., −69° C., −70° C., −71° C., −72° C., −73° C., −74° C., −75° C., −76° C., −77° C., −78° C., −79° C., or −80° C., or any range or combination of those temperatures. It is further preferred embodiment, the method takes place in a climate with an annual mean temperature of about 0° C., −1° C., −2° C., −3° C., −4° C., −5° C., −6° C., −7° C., −8° C., −9° C., −10° C., −11° C., −12° C., −13° C., −14° C., −15° C., −16° C., −17° C., −18° C., −19° C., −20° C., −21° C., −22° C. −23° C., −24° C., −25° C., −26° C., −27° C., −28° C., −29° C., −30° C., −31° C., −32° C., −33° C., −34° C., −35° C., −36° C., −37° C., −38° C., −39° C., −40° C., −41° C., −42° C., −43° C., −44° C., −45° C., −46° C., −47° C., −48° C., −49° C., −50° C., −51° C., −52° C., −53° C., −54° C., −55° C., −56° C., −57° C., −58° C., −59° C., −60° C., −61° C., −62° C., −63° C., −64° C., −65° C., −66° C., −67° C., −68° C., −69° C., −70° C., −71° C., −72° C., −73° C., −74° C., −75° C., −76° C., −77° C., −78° C., −79° C., or −80° C. (or any range or combination thereof).
The adsorption step (2) can operate at atmospheric pressures between 30 and 120 kPa, and at CO2 concentrations between 10 to 10,000 ppm. Air exiting from the CO2 adsorbent bed (10) would contain significantly less CO2 than the input air up until the CO2 adsorbent bed (10) begins to reach its adsorption capacity. The adsorption step (2) proceeds until the adsorbent bed (10) reaches its target adsorption capacity. In a preferred embodiment, target adsorption capacity is measured from a feedback loop measuring the exiting CO2 concentration from the CO2 adsorbent bed (10). In another preferred embodiment, the target adsorption capacity is preconfigured via predictive modelling based upon based the adsorbent's characteristics, and the input air's flow rate and temperature.
In a preferred embodiment, the CO2 adsorbent bed (10) is designed to have a low
pressure drop across the bed (in the direction of air flow), and in a preferred embodiment the pressure drop across the CO2 adsorbent bed (10) would be below 2000 Pa. In a more preferred embodiment the pressure drop would be below 500 Pa. These low pressure drops can be achieved by using monolithic adsorbent structures, structured adsorbent packing, or packed beds filled with large pellets with the packed bed having a low length over diameter ratios.
In a preferred embodiment, the ratio of the length of the CO2 adsorbent bed (10) over the diameter of the CO2 adsorbent bed (10) is less than 10, less than 9, less than 8, less than 7, less than 6, less than 5, less than 4, less than 3, less than 2, or less than 1 for adsorbent pellets between 1 mm and 100 mm in diameter. In a more preferred embodiment, the diameter of the adsorbent pellets is 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, 10 mm, 11 mm, 12 mm, 13 mm, 14 mm, 15 mm, 16 mm, 17 mm, 18 mm, 19 mm, 20 mm, 21 mm, 22 mm, 23 mm, 24 mm or 25 mm (or any ranger or average of values therein), with length over diameter ratios less than 2, less than 1.9, less than 1.8, less than 1.7, less than 1.6, less than 1.5, less than 1.4, less than 1.3, less than 1.2, less than 1.1, less than 1, less than 0.9, less than 0.8, less than 0.7, less than 0.6, less than 0.5, less than 0.4, less than 0.3, less than 0.2, less than 0.1, and any ranges, combinations, or averages thereof.
The size and scale at which the invention can be carried out can be tailored to best account for the available space and desired throughput of the system. In a preferred embodiment, the CO2 adsorbent bed (10) is fully contained within a larger enclosure, such as a tank or container, and the remaining steps of the method are carried out by altering the pressure, temperature, or air sources within the enclosure.
In alternative embodiments, the enclosure is comprised of a series of tanks or containers linked together. In a further alternative embodiment, the enclosure containing the adsorbent bed (10) is configured as a cylindrical column or tube (or series of columns or tubes), which optimizes the interaction between the input air and the CO2 adsorbent material. In a further embodiment, the CO2 adsorbent bed (10) can be isolated via rotation of a rotating/moving conduit gate valve. In another embodiment, the CO2 adsorbent bed (10) can be rotated/moved in order to be isolated.
After isolating the CO2 adsorbent bed (10) from the input air, a vacuum source such as a vacuum pump (15) is connected to the CO2 adsorbent bed (10). In a preferred embodiment, the vacuum pump (15) is connected to the CO2 adsorbent bed (10) by a gate, valve, or baffle. The vacuum pump (15) reduces the pressure within the CO2 adsorbent bed (10) to below ambient pressures. The degree to which the pressure is reduced during this step determines the purity of the final CO2 product stream. The lower the pressure during the blowdown step (3), the higher the final purity of the CO2 product stream. In a preferred embodiment, the blowdown step (3) occurs at 0.0000001, 0.000001, 0.00001, 0.0001, 0.001, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, or 1 atm pressure, and any range or combination or average thereof. In a particularly preferred embodiment a high purity CO2 product stream can be obtained having a CO2 concentration above 90%, above 91%, above 92%, above 93%, above 94%, above 95%, above 96%, above 97%, above 97.5%, above 98%, above 98.5%, above 99%, above 99.5%, or above 99.9%. In a particularly preferred embodiment, the blowdown step (3) occurs at a pressure between 0-0.1 atm and achieves a CO2 concentration above 99%.
By reducing the pressure within the CO2 adsorbent bed (10), the weakly adsorbed components of the input air (predominantly composed of N2, O2, and Ar) are removed from the CO2 adsorbent bed (10) while keeping the bulk of the CO2 on the adsorbent. In a preferred embodiment, this stream of air, (being rich in N2, O2 and Ar), can be stored in a buffer tank for later use.
In an alternative embodiment, where the target concentration of the CO2 product stream is acceptable without the requirement of removing the N2, O2, and Ar, from the CO2 adsorbent bed (10) using a vacuum, the blowdown step (3) can be omitted and the method proceeds from the adsorption step (2) straight to the evacuation step (4).
The heater (16) can apply heat to the CO2 adsorbent bed (10) via any acceptable electrical, chemical, sensible, radiative, heat exchange, or other generally known heating means, and may heat the inner area of the CO2 adsorbent bed (10) via an immersion heater or heat exchanger, or alternatively heat the exterior or other portion of the enclosure, and thereby conduct, convey, or radiate heat to the CO2 adsorbent bed to indirectly heat the CO2 adsorbent material. In a preferred embodiment, the heater (16) utilizes heat from an alternative source, such as utilizing “waste” heat from a separate source to facilitated CO2 capture. In a preferred embodiment, the CO2 adsorbent bed (10) is heated using pressurized CO2 flowing between the heater (16) and CO2 adsorbent bed (10). In a particularly preferred embodiment, this heated CO2 is obtained from previous operation of the TVSA cycle (1).
In a preferred embodiment, a vacuum source is also used to remove the CO2 from the CO2 adsorbent bed (10). Once the bed is sufficiently heated, a vacuum pump (15) would be turned on to reduce the pressure within the CO2 adsorbent bed (10). In a preferred embodiment, the heating of the CO2 adsorbent bed (10) and the reduction of pressure can occur simultaneously. In a further preferred embodiment, the pressure of the CO2 adsorbent bed (10) is reduced to between 0 to 0.25 atm, to extract as much of the CO2 from the CO2 adsorbent bed (10) as possible. As the evacuation step (4) is occurring, a purified stream of CO2 exits the vacuum pump (15) and can be collected for further use. This CO2 stream can be as high as 99.999% pure. In a further embodiment, the CO2 can be used in a sequestering process, enabling long-term removal of CO2 from the atmosphere.
In a preferred embodiment, the TVSA cycle with desiccation and filtration generally
comprises the following steps:
After the waterbed regeneration step (17), the method is repeated with the adsorption step (6). By incorporating the waterbed regeneration step (17), significant energy savings are achieved by decreasing the energy required to regenerate the desiccant.
This TVSA cycle with desiccation and filtration relies upon many of the same principles discussed in respect of the previously-described TVSA cycle. As such, focus of this section will only be placed upon additional elements or particular areas of focus, and reference to the previous discussion of the adsorption, blowdown, evacuation, and pressurization steps of the TVSA cycle (1) are intended to be incorporated herein.
Input air passes first through a particulate filter (12) to remove any solids that may be present within the air stream that will accumulate in the system. These solids can be particulate matter, ice crystals, or any other materials that are greater than 1 μm which are airborne and may negatively impact the function of the adsorbent. The filtering of these particulates from the input air can be achieved using a known technology such as grates, electrostatic, or fiber filters. In a preferred embodiment, the filtering is done in-line with the remaining step in the method.
Filtered air is then passed through a water capture bed (13) comprising a desiccant that removes water from the air. This water capture bed (13) can be filled with any of several known desiccants such as silica gel, 3A (a zeolite that is also often described as a molecular sieve), activated carbons, aluminas, or certain metal organic frameworks (MOFs). In a preferred embodiment, the desiccant contained within the water capture bed (13) does not adsorb CO2 in significant quantities (for example, 0.1 mmolCO2/gdesiccant@0° C.) at a partial pressure of 0.0004 atm CO2 and removes water from the input air at near ambient conditions.
The resulting dried air then flows into the CO2 adsorbent bed (10) in a similar way to
the previously described adsorption step (2) from
In a preferred embodiment, this reverse flow of air for the pressurization step (9) and waterbed regeneration step (17) acts to ensure the apparatus is regenerated between cycles, while ensuring water is not able to enter the CO2 adsorbent bed (10). This permits the method to be cycled with minimal downtime and at reduced energy cost. During the evacuation step (8), the CO2 adsorbent bed (10) is heated to desorb the CO2. Much of this heat is retained by the CO2 adsorbent bed (10) (i.e. as sensible heat), and needs to be cooled before the method can be repeated. During the pressurization step (9) and the waterbed regeneration step (17), dry air flows into the CO2 adsorbent bed (10) at a slow rate so that it can be heated by this sensible heat of the CO2 adsorbent bed (10). This heated dry air can then be exhausted through the water capture bed (13). The desiccant is regenerated using the heat from this warm air. In appropriate circumstances, the heater (16) or an additional heater (not shown) can be operated during the waterbed regeneration step (17) to ensure that a sufficient supply of warm air is available to regenerate the water capture bed (13).
In another preferred embodiment, once the water capture bed (13) has been regenerated, the flow of air can be reversed and air from inside the CO2 adsorbent bed (10) can be allowed to exhaust through the water guard (14), acting to regenerate the water guard (14) for future use. In an alternative embodiment, the regeneration can be aided by the heater (16) or an additional heater placed (not shown) configured so as to further heat the flow of air between the water guard (14) and the CO2 capture bed (10).
In a preferred embodiment, care is taken to ensure that no water is in the input air for
the pressurization step (9) and waterbed regeneration step (17) by using dried air that has been stored from another step in the method or the exhaust air from the adsorption step (6) of a parallel method. By taking care to ensure no water is in the input air, the water guard (14) can be omitted from the design as no water is required to be removed prior to input air being fed into the CO2 capture bed (10).
In a preferred embodiment, the alternative flow of air through the various cycles is controlled by the fan (11), which in a particularly preferred embodiment is located at the most downstream portion of the system relative to the input air used in the adsorption step (6).
Under either method described above, modifications can be made to the implementation of the invention. For example, in a preferred embodiment there are multiple machines performing the method simultaneously, but at offset steps, such that dry air from the first method can be immediately used as input air in the second method. Alternatively, the steps need not be directly linked, such that the various steps are operated only during their most optimal time, such as overnight for the adsorption step (2 or 6) when cold temperatures are most desired, which is then held until the day when temperatures have increased to perform the heating steps of the evacuation step (4 or 8) when higher temperatures are desired. This would be expected to provide a further reduction in the cost per ton to capture and concentrate CO2, balanced only against the desired throughput.
In preferred embodiments, the methods described above are carried out in facilities located at, or in close proximity to, clean energy sources such as wind, solar, hydro, geothermal, nuclear energy generating stations, or other clean energy sources. Other than powering this method to capture and concentrate CO2, this method would enable otherwise excess “waste” energy not needed by the power grid (during times of low power usage) to be utilized in CO2 capture.
In alternatively preferred embodiments, the methods described above are carried out in connection with facilities that generate a suitable supply of input air, or are able to utilize the produced concentrated CO2 or other concentrated gasses obtained during the blowdown step (3 or 7) or evacuation step (4 or 8).
Materials that preferentially sorb water over CO2 have been used air pre-purification processes since the early 1980s. Air pre-purification units would utilize faujasite structured zeolites such as Na—X to capture CO2 from the air to reduce its concentration to less than the ppm level. Such methods work by first pressurizing the ambient air to high pressures (e.g. 50-150 psia) which serves to separate the water, then feeding the pressurized gas into an adsorbent bed for the removal of trace amounts of water and CO2. This dry air, free of CO2, is then fed into another unit for the production of N2 or O2. However, these materials have not been used for the capture and concentration of CO2, and had been deemed unsuitable due to their low affinity for CO2 in comparison to water.
However a wide variety of adsorbents, including those previous deemed unsuitable for use in the capture and concentration of CO2, are suitable for use in accordance with this invention. In a preferred embodiment, the adsorbent chosen for the CO2 adsorbent bed (10) should have at least an adsorption capacity for CO2 greater than 0.1, greater than 0.2, greater than 0.3, greater than 0.4, greater than 0.5, greater than 0.6, greater than 0.7, greater than 0.8, greater than 0.9, greater than 1.0, greater than 1.1, greater than 1.2, greater than 1.3, greater than 1.4, greater than 1.5, greater than 1.6, greater than 1.7, greater than 1.8, greater than 1.9, greater than 2.0, greater than 2.1, greater than 2.2, greater than 2.3, greater than 2.4, greater than 2.5, greater than 2.6, greater than 2.7, greater than 2.8, greater than 2.9, greater than 3.0, greater than 3.1, greater than 3.2, greater than 3.3, greater than 3.4, greater than 3.5, greater than 3.6, greater than 3.7, greater than 3.8, greater than 3.9, greater than 4.0, greater than 4.1, greater than 4.2, greater than 4.3, greater than 4.4, greater than 4.5, greater than 4.6, greater than 4.7, greater than 4.8, greater than 4.9, greater than 5.0 mmolCO2/g adsorbent at conditions of 0° C. and partial pressures of 0.0004 atm of CO2, including ranges, combinations, and averages thereof.
In another preferred embodiment the preferred CO2 adsorbent has a surface area greater than 100 m2/g. In a further preferred embodiment, the CO2 adsorbent has a pore structure that allows CO2 to diffuse through its structure at temperatures below 0° C.
In another preferred embodiment the adsorbent has an average heat of adsorption of CO2 less than 100 kJ/mol, less than 95 kJ/mol, less than 90 kJ/mol, less than 85 kJ/mol, less than 80 kJ/mol, less than 75 kJ/mol, less than 70 kJ/mol, less than 65 kJ/mol, less than 60 kJ/mol, less than 55 kJ/mol, less than 50 kJ/mol, less than 45 kJ/mol, less than 40 kJ/mol, less than 35 kJ/mol, less than 30 kJ/mol, or less than 25 kJ/mol, including any ranges, combinations, or averages thereof.
The heat of adsorption is important with regards to the energy required to desorb the CO2 with larger heats of adsorption requiring more energy for desorption of the CO2. The profile of the heat of adsorption with respects to loading, which can be seen in a Clausius-Clapeyron relationship, is also important. This relationship shows that the initial CO2 that is adsorbed releases more energy than subsequent CO2 adsorbed. Thus, the initial CO2 adsorbed would require more energy to desorb than subsequently adsorbed CO2 molecules. In a particularly preferred embodiment, the heat of adsorption of CO2 on the adsorbent would be as low as possible and constant over a range of loadings.
One beneficial aspect of the invention is that adsorbents that are commonly considered to be water unstable, or otherwise preferentially adsorb water over CO2, can be used to capture and concentrate CO2 due to the water removal prior to the separation of CO2. This allows many adsorbents, such as aluminas, zeolites, covalent organic frameworks (COF), and MOFs, to be used for DAC, contrary to accepted practices.
In a preferred embodiment, the CO2 adsorbent bed (10) is made up of zeolites having oxygen tetrahedral frameworks incorporating Si, Al, P, Ge, B, Mg, Zn, Ga, Co, or Be, (including the presence of two or more differing structures, or mixtures of different structures). In an alternatively preferred embodiment, the CO2 adsorbent bed (10) is made up of mixtures of CO2 adsorbent materials having non-framework species, or mixtures of framework and non-framework species.
In a further preferred embodiment, the zeolite frameworks include, but are not limited to, Linde Type A, faujasite, or chabazite, which all have large CO2 adsorption capacities at low CO2 partial pressures but adsorb water competitively over CO2. Faujasite structured zeolites, and in particular faujasite structured zeolites with a Si/Al ratio of below 2, are particularly preferable adsorbents for this separation.
Preferred zeolites can have a variety of counterbalancing cations in the metals group within them, such as alkali or alkaline earth metals, which change the strength of interaction with CO2 and therefore, the heat of adsorption of CO2. In a preferred embodiment, MOF's including, but not limited to, NbOFFIVE-1-Ni, SGU-29, Mg-MOF-74, SIFSIX-3-Cu, SIFSIX-2-Cu, Mg-dobpdc-mmen are also preferred adsorbents for this separation.
In a preferred embodiment, the CO2 adsorbent bed (10) can be composed of one or more types of adsorbents. In a further preferred embodiment, the adsorbents can be arranged to according to the flow of input air to first expose the air to adsorbent with a weaker CO2 interaction, then an adsorbent with a stronger CO2 interaction. In a further preferred embodiment, the same adsorbent in two configurations can be layered according to the direction of the flow. These two configurations can be a pellet/structure/packing with a higher pellet/structure/packing diffusion resistance, and a lower pellet/structure/packing diffusion resistance. These would be oriented with regard to the flow of air as to first have the higher pellet/structure/packing diffusion resistance, and then the lower pellet/structure/packing diffusion resistance
Another benefit of the invention is that it is can be designed to operate at low temperatures that favour separation, due to the amount of work to separate and concentrate CO2 operating as a function of the temperature at which the separation occurs. Specifically, the lower the system temperature of the invention, the lower the expected energy required to capture CO2 in accordance with the invention. Though not intended to be limited by any particular scientific principle, this phenomenon is believed to be governed by the second law of thermodynamics, and illustrated in
In a low temperature embodiment, the synergistic effects between the adsorption step (2 or 6) occurring at ambient temperatures below 0° C., and the adsorbents are highlighted leading to lower overall energy requirements for the method. This is due to the adsorbents' CO2 adsorption capacity being greater at colder temperatures. This can be seen in
At colder temperatures, the Henry's Law constant for CO2 is significantly higher than that of warmer temperatures (Li—X, 3.35 mmol/gatm@−60° C., 0.012 mmol/gatm@60° C.). This phenomenon is beneficial for the invention because more CO2 is adsorbed on the adsorbent per cycle thereby reducing sensible energy losses, leading to overall lower energy demands for the method.
Another benefit of the invention in such an embodiment is that, due to the use of cold (i.e. below 0° C.) input air, less water needs to be separated prior to capturing and concentrating CO2, thereby lowering the total energy requirements of the method. Since separating water requires a significant amount of energy, a particularly preferred embodiment of the invention utilizes a dry input air source, such as air from drier locations or air dried from an alternative means as a “waste” product.
In an alternative embodiment, the invention can utilize air with temperatures above 0° C., provided it contains a very low water content and otherwise behave in an equivalent manner. For example, the Atacama Desert, the Tibetan Plateau, and the Gobi Desert are known for being some of the driest places on the planet, and despite having an annual mean temperature between 0° C. and 20° C., and an annual mean absolute humidity between 0 and 5 g H2O/kg Air, would be particularly preferable input air source for the invention due to the low amount of water that must be separated before capturing CO2.
Adsorbents that co-adsorb both water and CO2 or preferentially adsorb CO2 also have similar synergisms with cold conditions as mentioned above. Separations are more favorable in cold conditions, the adsorbents perform better in the cold conditions, and they adsorb less water in comparison to CO2 in cold conditions. These three synergisms increase the performance of these adsorbents except that the air does not need to be dried before entering into the method. If appropriately cold and/or dried air is utilized in the invention, the air does not need to be dried before entering the method for the adsorption step (2 or 6), pressurization step (5 or 9), or waterbed regeneration step (17).
The embodiment exemplified by
Na—X, a low Si/Al ratio faujasite structured zeolite with Na+ as a cation, was used as the adsorbent in the CO2 adsorbent bed (10). Modelling used the following properties of Na—X, obtained either from Na—X analysis or otherwise accepted values: Cps,0 of 800 J/kg K, ks of 0.147 W/m K, rpore of 0.0000001 cm, τ of 5, ρS of 1826 kg/m3, εpellet of 0.38, ρP of 1132 kg/m3, εbed of 0.38, ρB of 778 kg/m3, and average Hads of 45 kJ/mol.
Silica gel was selected as the desiccant for use in the water capture bed (13), modelled using the following properties (as measured or taken from accepted values): Cps,0 of 870 J/kg K, ks of 0.151 W/m K, ρS of 1240 kg/m3, εsorbent of 0.348, ρB of 720 kg/m3, and Hads of 2980 kJ/kg.
This example assumes no solids or other contaminant that would affect the adsorbent are entering the method. Dry air from a parallel method is used in the pressurization step (9) of the TVSA cycle as well as for the waterbed regeneration step (17), and therefore, no water guard (14) is required for the exemplified method.
To model the interaction between the ambient air and the adsorbent during the adsorption step (6) of the method, the Rosen model and TD-Toth model were used and were validated experimentally. These experiments allowed for the quantification of the capture fractions and adsorption capacities at 95% inlet concentration within the model.
From the results of the Rosen Model,
This synergism can be seen from
This fast uptake rate can be seen in
The energy required to capture 1 ton of CO2 using the TVSA cycle from
The additional energy required to desiccate the air (E_W) to capture 1 ton of CO2 using the TVSA cycle from
To show the benefit of cold and dry conditions for
The map from
These locations provide examples to highlight the beneficial properties of the invention when operated using a cold input air source. To the extent an air source can be obtained in a cool state as a by-product from another operation (and not cooled specifically for CO2 capture), these beneficial properties could be similarly achieved.
Similarly, given the input variables of temperature and humidity are relevant to efficiency, areas such as the Atacama Desert, the Tibetan Plateau, and the Gobi Desert, which are known for being some of the driest places on the planet, would equally allow for beneficial implementation of the invention. Due to very low humidity in these locations, this method would be expected to require less than 6 MWh/ton CO2 of energy to operate.
Combining this method along with an inexpensive renewable energy source would allow for this method to be a viable solution for global warming because it would reduce the amount of CO2, a known greenhouse gas, within the atmosphere. Using wind power from a location such as Antarctica, known to be the windiest location on Earth, would allow for a cheap renewable energy source to run this method, further reducing the cost per ton of CO2 captured via this method. Assuming that wind energy can be produced at $6/MWh, which is the least expensive wind PPA in 2018 in the USA, operating costs to run this method can be as low as $6/ton CO2 which is significantly lower than DOE targets for CO2 capture and sequestration.
All references and publication referred to herein are hereby incorporated by reference in their entirety.
While I believe that the theories that I have presented give benefits to this method, I do not wish to be bound by any particular theory relating to how the invention works, nor should any calculation be taken as exactly true in all circumstances.
Certain currently preferred embodiments have been described by way of example. It will be apparent to persons skilled in the art that a number of variations and modifications can be made without departing from the scope of the claims.
Filing Document | Filing Date | Country | Kind |
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PCT/CA2021/051696 | 11/26/2021 | WO |
Number | Date | Country | |
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63118926 | Nov 2020 | US |