Direct air capture system

Abstract

An absorber unit for a direct air capture (DAC) system comprises: one or more contactors for absorbing carbon dioxide from an air stream; an exit plenum arranged to receive the air stream from the one or more contactors, and connected to an outlet for venting the air stream; and a drip unit located downstream from the one or more inlets and upstream from the outlet in the exit plenum, wherein the drip unit is arranged to provide liquid droplets of absorbent that travel upstream in the exit plenum.

Description

[0001] Direct Air Capture System

[0002] Technical field

[0003] The invention relates to direct air capture (DAC) systems, and in particular to the absorber unit of such systems.

[0004] Background

[0005] Carbon capture and storage is expected to be a significant way to reduce the effects of global warming from the combustion of fossil fuels.

[0006] Capture of carbon dioxide (CO₂) may involve systems for extracting CO₂ from a CO₂ containing gas using an absorbent medium. Typically, this involves creating a gas flow over the absorbent medium under conditions where the medium will absorb CO₂ from the gas, and then altering the conditions so that the medium releases the absorbed CO₂ allowing it to be captured and stored. This process may be used to reduce atmospheric CO₂ to mitigate the anthropogenic emissions that are associated with global warming, or climate change. Direct Air Capture (DAC) is the capture of CO₂ from atmospheric air which, as the atmosphere contains less than 0.05% CO₂, involves processing large volumes of air.

[0007] Some Direct Air Capture systems use a liquid medium to absorb CO₂ in an absorber or liquid-air contactor. There is for example a sorbent containing solution, distributed within an absorber, subject to a high air flow necessary to process large quantities of air. The sorbent may be distributed by spray nozzles, which produces an aerosol of sorbent in solution with a high surface area. Alternatively, the air may be brought into contact with liquid films of sorbent solution propagated across solid surfaces, such as conventional structured packing / film fill.

[0008] There is a continued need to provide improved, lower weight, compact and more energy efficient carbon dioxide capture system.

[0009] 38256247-1 Summary

[0010] According to a first aspect there is provided an absorber unit for a direct air capture (DAC) system comprising:

[0011] one or more contactors for absorbing carbon dioxide from an air stream; an exit plenum arranged to receive the air stream from the one or more contactors, and connected to an outlet for venting the air stream; and

[0012] a drip unit located downstream from the one or more inlets and upstream from the outlet in the exit plenum, wherein the drip unit is arranged to provide liquid droplets of absorbent that travel upstream in the exit plenum.

[0013] The drip unit and exit plenum provide a third stage of contacting after the contactors / absorber modules, which can increase the carbon capture and also reduce drift.

[0014] In use, the exit plenum is typically arranged to be vertical, so that the air stream flows upwards through the exit plenum. The drip unit is preferably arranged so that the liquid droplets fall through as large volume of the air stream as possible. For example, the drip unit may be arranged so that the liquid droplets fall at least 80% of a height of the exit plenum and preferably at least 90% of the height of the exit plenum. More preferably, the drip unit is arranged so that the droplets fall the full height of the exit plenum. In some embodiments, the height of the exit plenum can be defined as the vertical distance from the bottom / base of a lowermost contactor of the one or more contactors to the top of an uppermost contactor of the one or more contactors.

[0015] The one or more contactors may comprise at least two contactors arranged back-to-back so as to form a central plenum between them, and wherein the central plenum forms at least a part of the exit plenum. The contactors may be so called cross-flow absorber modules that may be arranged back-to-back to form the plenum between them.

[0016] The absorber unit may also comprise at least two contactors that are vertically stacked, comprising a first contactor and a second contactor. For example, the absorber unit may comprise two pairs of two contactors arranged back-to-back and stacked two high, or two pairs of three contactors arranged back-to-back and stacked three high, etc. The

[0017] 38256247-1 drip unit may be particularly useful for an absorber unit comprising a plurality of stacked cross-flow contactors, as this can provide greater height for the droplet to fall through in the exit plenum.

[0018] In an example, the first contactor is arranged above the second contactor, wherein the first contactor is configured to have a lower drift compared to the second contactor. For example, the first (higher) contactor may comprise more packing. The second contactor may be configured to have a lower pressure drop and a greater mass transfer rate compared to the first contactor. For example, the second (lower) contactor may comprise less packing and may comprise more sprayers and / or one or more sprayers that are finer (producing a finer mist of absorbent solution) compared to those of the first contactor. Since the air stream from the lower absorber modules will travel a greater distance through the exit plenum before reaching the outlet, they may need less drift removal and / or carbon absorption compared to the relatively higher placed absorber modules. For three or more contactors stacked on top of each other, three or more different designs (e.g. different amounts of packing and / or different sprayers) may be used.

[0019] The drip unit may be configured to provide droplets having a predetermined size, wherein a droplet having the predetermined size experiences a net positive force in an upstream direction inside the exit plenum. The absorber unit may further comprise a control unit for setting the predetermined size of the liquid droplets. The control unit may be arranged to determine the predetermined size based on at least a force of gravity and a flow velocity of the air stream. The control unit may be configured to set the predetermined size of the liquid droplets by setting a pressure at the drip unit. For example, the control unit can set the pressure of liquid ejected from a sprayer of the drip unit. Alternatively or in addition, the control unit may be configured to change the size of an aperture of the drip unit through which the liquid droplets are provided. The aperture may be an opening in a nozzle.

[0020] The absorber unit may further comprise a liquid collector for collecting the liquid droplets and transferring liquid for further processing and / or recycling. For example, the liquid collector may comprise a tray located beneath the exit plenum. The drip unit can be configured to receive liquid from the regeneration stage of DAC system, from the contactors, and / or from the exit plenum.

[0021] 38256247-1 The absorber unit may comprise a mist eliminator located between the outlet and the drip unit. The mist eliminator can further reduce drift.

[0022] The drip unit can be arranged to provide liquid droplets by ejecting the liquid droplets in a downstream direction. For example, when vertically arranged, the drip unit may eject liquid upwards, whereupon gravity acts on the droplets to cause them to move downwards through the exit plenum.

[0023] The absorber unit may comprise a plurality of drip units (e.g. two or three) located at different heights within the exit plenum and by the outlet. For example, the drip unit may be a first drip unit, wherein the absorber unit further comprises a second drip unit located in the exit plenum upstream from the first drip unit, wherein the second drip unit is arranged to provide further liquid droplets. The second drip unit can be configured to provide the further liquid droplets having a smaller size than the liquid droplets provided by the first drip unit. In general, the further upstream in the exit plenum a drip unit is located, the smaller the size of the liquid droplets it provides may be, since the droplets are in a lower velocity air stream they can be smaller and still have a net upstream force and still fall. In other embodiments, the absorber unit may comprise three or more drip units arranged at different heights in and / or above the exit plenum. For embodiments with two or more drip units arranged at different heights in the exit plenum, different drip units may be pre-configured to provide droplets in a suitable size range for a given height. For example, each drip unit may comprise a plurality of apertures for releasing liquid, and the size (e.g. the diameter) of the apertures may increase with height. In addition, the absorber unit may be configured to adjust the droplet size (in situ) by adjusting the pressure at the drip unit.

[0024] According to a second aspect, there is provided a method of operating an absorber unit according to the first aspect. The method comprises determining and setting the predetermined size of the liquid droplets. The predetermined size can be set such that the droplet population moves in an upstream direction. Setting the size may comprise adjusting a pressure at the drip unit. For embodiments with two or more drip units arranged at different heights in the exit plenum, different drip units may be pre-configured to provide droplets in a suitable size range for a given height (e.g. by having different nozzle sizes), and then the size may be tuned by adjustment of the pressure at the drip

[0025] 38256247-1 unit. For example, each drip unit may comprise a plurality of apertures for releasing liquid, and the size (e.g. the diameter) of the apertures may increase with height.

[0026] Brief description of drawings

[0027] Figure 1 shows a schematic cross section of an absorber unit with a drip unit in exit plenum;

[0028] Figure 2 shows a schematic cross section of an absorber unit comprising a drip tray; Figure 3 shows a schematic cross section of an absorber unit comprising a pipework distribution;

[0029] Figure 4 shows a schematic cross section of an absorber unit comprising a second drip unit;

[0030] Figure 5 shows a schematic cross section of an absorber unit comprising different absorber modules; and

[0031] Figure 6A and 6B show schematic diagrams of the forces acting on a droplet from a drip unit.

[0032] Detailed description

[0033] Figure 1 shows a schematic cross section of an absorber unit 1 being a part of a direct air capture (DAC) system. The DAC system may comprise one or more further absorber units (not shown). The absorber unit 1 comprises a plurality of contactors 2 (six contactors shown, also referred to as “absorber modules” herein) stacked three high and arranged back-to-back with outflows into a central exit plenum 3. Each contactor 2 is a hybrid contactor comprising a sprayer 4 and packing 5. Other embodiments may comprise different types of contactors, e.g. comprising only sprayers 4 or only packing 5. The sprayer 4 can provide a fine mist of absorbent, which creates a large and efficient contacting area for pulling CO2 out of the air stream 6. The packing 5 may be configured to provide a solid surface over which a liquid film of sorbent solution is propagated to provide a large surface area of fluid in contact with the air stream 6 at any one time.

[0034] An exit fan 7 is located before (upstream of) the outlet 8 connected to the exit plenum 3 to draw air out of the exit plenum 3. A final mist eliminator 9 (also referred to as

[0035] 38256247-1 “demister”) is located before the fan 7 to capture moisture and reduce drift (i.e. loss of absorbent fluid in the system with the exit air stream 10). The mist eliminator 9 can comprise corrugated sheets or a layer of mesh to collect liquid droplets, which can conjoin and fall back down the exit plenum 3. The mist eliminator 9 can be arranged over a larger flow area to reduce pressure losses.

[0036] Under the mist eliminator 9 is a drip unit 11 (e.g. comprising one or more sprayers), which is arranged to deliver droplets of absorbent that fall downwards (upstream) in the exit plenum 3. This arrangement of a drip unit 11 in the absorber unit 1 provides a third stage of contacting in the plenum 3 after the contactor units 2. The otherwise unused space in the exit plenum 3 is thereby used to effectively increase the contacting area by dripping absorbent through the volume of the exit plenum 3 from a top end. The droplet size / weight is configured to cause the provided droplets to fall down (upstream) through the exit plenum 3 under gravity, rather than being picked up and lifted by the airflow 6. As a result of the contactors 2 being arranged back-to-back, forming a central plenum (the exit plenum 3), the drops from the drip unit 11 are sweeping both sections / sides of absorber modules.

[0037] The drip unit 11 can be arranged to provide droplets in different ways. For example, the drip unit may comprise one or more of a static pipe distributor, a spray head, a rotating arm. The drip unit can be configured to release drops downwards (upstream), sideways or upwards (downstream, e.g. near-parabolic trajectories). Releasing the drops upwards can further extend the effective contacting length in the exit plenum 3.

[0038] Liquid for the droplets can be fresh absorbent directly from a regeneration step or could be partially loaded absorbent from absorber units. For example, partially loaded absorbent from mist eliminator 9 can be collected at the bottom end 12 of the exit plenum and fed back to the drip unit 11. Alternatively or in addition, liquid for the droplets can be taken from the bottom of the contactor stages (directly underneath the sprayers 4 and packing 5).

[0039] By creating a droplet population size that is neither too small to become drift nor too large to drip down quickly (with little flow resistance), at least part of the droplet population drips down slowly with a small downwards net load. With a small net force downwards, the droplets fall more slowly through the exit plenum 3. This increases droplet effective

[0040] 38256247-1 area and reduces droplet speed to give higher contacting surface residence time. The droplet production is controllable based on local weather conditions and the need for additional contacting or drift removal. The droplet size can be controlled in situ by controlling / adjusting the delivery pressure at the drip unit 11. For example, when the drip unit comprises one or more sprayers, the pressure for ejecting liquid from the sprayers can be set and adjusted in order to provide droplets having a predetermined (optimal) size. Alternatively, the shape of the drip unit can be changed to change the droplet size. For example, a size of an aperture for releasing liquid may be adjustable. For example, the size of the opening of a spray nozzle may be set to provide droplets having the predetermined size.

[0041] An additional potential benefit of this arrangement is that the droplets can interact with at least some of the drift leaving the absorber units, which can provide for less drift leaving the top of the absorber block.

[0042] Using the exit plenum 3 as a further droplet contactor can increase the absorption of carbon dioxide so that the vented air 10 has a lower carbon dioxide concentration. The droplet size is such that the droplets themselves are not forced upwards as drift but instead fall downwards.

[0043] Any drift exiting from the contactors 2 and venting into the central plenum has an increased probability of interaction with the drops from the drip unit 11 to give sweeping removal of the drift. The mist eliminator 9 prior to the fan 7 acts together with the sweeping droplets from the drip unit 11 to provide a low drift system.

[0044] Figures 2 to 5 illustrate other embodiments of the absorber unit 1 of a DAC system. The same reference numerals have been used in different figures to denote equivalent or similar features to aid understanding and are not intended to limit the illustrated embodiments. Features with the same reference numeral in any figure may be as described in relation to Figure 1 above.

[0045] Figure 2 shows a schematic cross section of an absorber unit 1 of a DAC system similar to that of Figure 1. The absorber unit 1 comprises contactors 2, an exit plenum 3, sprayers 4 and packing 5 for capturing carbon dioxide from the air stream 6. The absorber unit 1 further comprises a common exit fan 7 at the outlet 8 after a mist

[0046] 38256247-1 eliminator 9 for drawing out air 10. The drip unit 11 provides droplets that fall down the exit plenum 3.

[0047] The absorber unit 1 further comprises a drip tray 13 for collecting absorbent from the drip unit 11. The collected liquid may be re-used. The liquid for the droplets from the drip unit 11 can comprise fresh absorbent (e.g. directly from the regeneration step of the DAC system) and / or can be partially loaded absorbent from the absorber unit 1.

[0048] Figure 3 shows a schematic cross section of an absorber unit 1 of a DAC system similar to that of Figure 1. The absorber unit 1 comprises contactors 2, an exit plenum 3, sprayers 4 and packing 5 for capturing carbon dioxide from the air stream 6. The absorber unit 1 further comprises a common exit fan 7 at the outlet 8 after a mist eliminator 9 for drawing out air 10. The drip unit 11 provides droplets that fall down the exit plenum 3, and are collected in a drip tray 13 at the bottom end 12. The drip unit 11 comprises a pipework distribution 14 with holes to provide an even coverage across the exit plenum 3.

[0049] Figure 4 shows a schematic cross section of an absorber unit 1 of a DAC system similar to that of Figure 1. The absorber unit 1 comprises contactors 2, an exit plenum 3, sprayers 4 and packing 5 for capturing carbon dioxide from the air stream 6. The absorber unit 1 further comprises a common exit fan 7 at the outlet 8 after a mist eliminator 9 for drawing out air 10. The drip unit 11 provides droplets that fall down the exit plenum 3, and are collected in a drip tray 13 at the bottom end 12.

[0050] A second drip unit 11 B is located further down (upstream) in the exit plenum 3. The second drip unit 11 B may be the same or different from the first drip unit 11. Releasing droplets at different levels within the exit plenum 3 can improve mass transfer and carbon dioxide capture as well as improving drift elimination. The (optimal) droplet size can be determined based on the level / height of the drip unit 11 in the exit plenum. The drip unit 11 in the top section can provide “sweeper drops” floating slowly down (on droplet-droplet impact they will accelerate downwards). A drip unit located at a lower level can be arranged to provide smaller droplets, which improve mass transfer and are more suited to lower airflow velocities.

[0051] 38256247-1 For embodiments with two or more drip units 11 arranged at different heights in the exit plenum 3, each drip unit 11 may be pre-configured to provide droplets having a size in a suitable size range for a given height. For example, the different drip units 11 can be configured to have different nozzle sizes. In addition, the absorber unit 1 may be configured to adjust the droplet size at a drip 11 unit by adjusting the pressure at that drip unit.

[0052] Figure 5 shows a schematic cross section of an absorber unit 1 of a DAC system similar to that of Figure 1. The absorber unit 1 comprises contactors 2A and 2B (eight shown), an exit plenum 3, sprayers 4 and packing 5 for capturing carbon dioxide from the air stream 6. The absorber unit 1 further comprises a common exit fan 7 at the outlet 8 after a mist eliminator 9 for drawing out air 10. The drip unit 11 provides droplets that fall down the exit plenum 3, and are collected in a drip tray 13 at the bottom end 12.

[0053] The absorber unit 1 comprises two different type of contactors 2A and 2B, wherein the higher (downstream) contactors 2A comprise more packing 5 compared to lower (upstream) contactors 2B. The lower contactors 2B may comprise more sprayers 4 and / or finer sprayers 4 compared to the higher contactors 2A.

[0054] More drift is removed from the lower modules than for the higher modules (due to the longer contacting area in the exit plenum 3), which can allow the contactor design to be optimised based on its relative height / position in the absorber unit 1. For the case where the modules comprise hybrid contactors with sprayers 4 and packing 5, modules at the top of the stack may have

[0055] i) slightly larger droplet size in the sprayers 4 to reduce their drift generation, ii) slightly longer or denser packing 5 to capture more of the drift prior to release into the exit plenum 3,

[0056] iii) the addition of a downstream mist eliminator to capture more of the drift,

[0057] iv) a combination of i), ii) or iii).

[0058] Conversely, modules at the bottom of the stack may use finer sprayers 4 or less packing 5, since the release of some drift at the base is more likely to be swept up by the sweeper drips from the drip units 11 in the exit plenum 3. The sweeper drips reduce the duty for the main mist eliminator 9 under the fan 7. The pressure loss of such a system is designed to be low and it can operate under different turbulence levels.

[0059] 38256247-1 Conventional packing materials are often in excess of 10s of Pa / m, which are significant for a DAC system given the amount of air that needs to be induced through the subsystem to achieve the required mass transfer duties. The energy requirement of the fan to drive air through the contactor is significant. Hence, being able to reduce the amount of packing 5 in lower contactors 2A can provide significant benefits.

[0060] Figure 6A and 6B show the forces acting on a droplet 20 from a drip unit (not shown) in the exit plenum and the net force 21 on that droplet 20 respectively. The droplet 20 experiences an upwards force 22 from the air stream 6 and a downwards gravitational force 23, which provides a net force 21. The size of the droplet 20 is configured so that the net force is downwards (upstream of the air stream 6).

[0061] Gravity acts on the droplet to keep it sweeping downwards and air resistance (drag) slows its velocity. If the force due to gravity is only very slightly larger than the air resistance force, then the droplet will float downwards slowly, only picking up pace as it gathers additional mass through impact and coalescence.

[0062] Consider a droplet of radius r. It has a volume of 4 / 3*π*r³ and a surface area of 4*π*r². The drag force acting on the droplet is approximately 1*p*u²*π*r²*C_D, where p is the air density, u the air velocity and C_D the drag coefficient. The drag coefficient is C_D and this is a function of Reynolds number (Re). The weight of the droplet is 4 / 3*π*r³*p_w*g, where p_w is the liquid density and g is standard gravity. Hence 4 / 3*π*r³*p_w*g > 1*p*u²*π*r²*C_D or 4 / 3*r*p_w*g > 1*p*u²*C_D if the droplet is to float downwards. This suggests that the droplet radius r > (3*p*u²*C_D) / (8*p_w*g) or droplet diameter d > (3*p*u²*C_D) / (4*p_w*g). In an air stream of 1m / s at 15degC, Re=rho*u*d / mu = 1.225kg / m³ * 1m / s *d / 17.9e-6 Pas.

[0063] Some example results from the above equations are provided for different diameters d of the droplet:

[0064] a) d = 100 pm, Re = 6.8, the drag coefficient for spheres gives a CD of 5.7. So this is expected to fly upwards (downstream).

[0065] b) d = 250 pm, Re = 17.1, CD approx. 3.2. The forces here are closely balanced, but the net force is still in the upwards direction.

[0066] 38256247-1 c) d = 300 m, Re = 20.53, CD approx. 2.9. The forces here are closely balanced but now biased to fall gently.

[0067] The air flow velocity increases with height, since the cross-sectional area of the plenum is similar but more absorber modules are contributing to the flow with height. This implies that the droplet size should be configured to be bigger further up the plenum. This effect is significant, since the droplet size for force balance increases with the velocity squared. The size of droplets being released at different levels, as illustrated in Figure 4, can be set to capitalize on this velocity effect.

[0068] Atmospheric conditions also affect the force balance on the droplets, and these can be taken into account when controlling droplet size in the plenum. Higher air densities, such as with lower air temperatures, can have slightly larger droplets. Consideration can also be given to the evaporation that occurs and that can shrink the droplet size. A larger bias may be required to give margin for this effect.

[0069] While specific embodiments of the invention have been described above, it will be appreciated that further embodiments are possible. The descriptions above are intended to be illustrative, not limiting. It will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

[0070] Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.

[0071] 38256247-1

Claims

CLAIMS:

1. An absorber unit for a direct air capture (DAC) system comprising:one or more contactors for absorbing carbon dioxide from an air stream; an exit plenum arranged to receive the air stream from the one or more contactors, and connected to an outlet for venting the air stream; anda drip unit located downstream from the one or more contactors and upstream from the outlet in the exit plenum, wherein the drip unit is arranged to provide liquid droplets of absorbent that travel upstream in the exit plenum.

2. An absorber unit according to claim 1, wherein the exit plenum is arranged to be vertical, so that the air stream flows upwards through the exit plenum, when in use.

3. An absorber unit according to claim 1 or 2, wherein the drip unit is arranged so that the liquid droplets fall through at least 80% of a height of the exit plenum and preferably at least 90% of the height of the exit plenum.

4. An absorber unit according to any one of the preceding claims, wherein the one or more contactors comprise at least two contactors arranged back-to-back so as to form a central plenum between them, and wherein the central plenum forms at least a part of the exit plenum.

5. An absorber unit according to any one of the preceding claims, wherein the one or more contactors comprise at least two contactors that are vertically stacked, comprising a first contactor and a second contactor.

6. An absorber unit according to claim 5, wherein the first contactor is arranged above the second contactor, and wherein the first contactor is configured to have a lower drift compared to the second contactor.

7. An absorber unit according to claim 5 or 6, wherein the first contactor is arranged above the second contactor, and wherein the second contactor is configured to have a lower pressure drop and a greater mass transfer rate compared to the first contactor.38256247-18. An absorber unit according to any one of claims 5 to 7, wherein the first contactor is arranged above the second contactor, and wherein the first contactor comprises more packing compared to the second contactor.

9. An absorber unit according to any one of claims 5 to 8, wherein the first contactor is arranged above the second contactor, and wherein the second contactor comprises one or more sprayers that are finer compared to the first contactor.

10. An absorber unit according to any one of the preceding claims, wherein the drip unit is configured to provide droplets having a predetermined size, wherein a droplet having the predetermined size experiences a net positive force in an upstream direction inside the exit plenum.

11. An absorber according to claim 10, further comprising a control unit for setting the predetermined size of the liquid droplets.

12. An absorber unit according to claim 11, wherein the control unit is arranged to determine the predetermined size based on at least a force of gravity and a flow velocity of the air stream.

13. An absorber unit according to claim 11 or 12, wherein the control unit is configured to set the predetermined size of the liquid droplets by setting a pressure at the drip unit.

14. An absorber unit according to any one of the preceding claims, further comprising a liquid collector for collecting the liquid droplets and transferring liquid for further processing and / or recycling.

15. An absorber unit according to any one of the preceding claims, further comprising a mist eliminator located between the outlet and the drip unit.

16. An absorber unit according to any one of the preceding claims, wherein the drip unit is arranged to provide liquid droplets by ejecting the liquid droplets in a downstream direction.38256247-1M& C PX221162W01417. An absorber unit according to any one of the preceding claims, wherein the drip unit is a first drip unit and the absorber unit further comprises a second drip unit located in the exit plenum upstream from the first drip unit, wherein the second drip unit is arranged to provide further liquid droplets.

18. An absorber unit according to claim 16, wherein the second drip unit is configured to provide the further liquid droplets having a smaller size than the liquid droplets provided by the first drip unit.

19. A method of operating an absorber unit according to any one of the preceding claims, the method comprising:determining and setting a predetermined size of the liquid droplets.

20. A method according to claim 19, wherein the step of setting comprises adjusting a pressure at the drip unit.38256247-1

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