Direct air capture system

Abstract

An absorber unit 1, suitable for a direct air capture (DAC) system, said unit comprising: one or more contactors 12 for absorbing carbon dioxide from air stream 16; a mist eliminator 2 located downstr

Description

Technical field The invention relates to direct air capture (DAC) systems, and in particular to an absorber unit of such systems. Background Carbon capture and storage is expected to be a significant way to reduce the effects of global warming from the combustion of fossil fuels. 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. 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. There is a continued need to provide improved, lower weight, compact and more energy efficient carbon dioxide capture system. Summary According to a first aspect, there is provided an absorber unit for a direct air capture (DAC) system comprising: one or more contactors for absorbing carbon dioxide from an air stream; a mist eliminator located downstream of the one or more contactors and upstream of an outlet for venting the air stream; and a wetting element for wetting the mist eliminator with water, wherein the wetting element is arranged to maintain a water balance in the absorber unit. The wetting element is used to both wet the mist eliminator and to maintain the water balance in the system. The water balance may refer to one or more of a predetermined level of water, a predetermined level of absorbent liquid, and a predetermined sorbent concentration of the absorbent liquid. For example, maintaining the water balance may comprise maintaining a predetermined (correct) level of water in the absorbent liquid, which can be determined from a measured concentration of absorbent and a total liquid volume. The wetting element may comprise one or more sprayers for spraying the water that wets the mist eliminator. A sprayer of the one or more sprayers can be located downstream of the mist eliminator and be configured to spray water in an upstream direction onto the mist eliminator. Alternatively or in addition, a sprayer of the one or more sprayers may be located upstream of said mist eliminator and may be configured to spray water in an upstream direction so that the water is carried downstream by the air stream onto the mist eliminator. The wetting element may also be configured to provide a flow of water directly to the mist eliminator. The wetting element can be configured to receive both recycled water and top-up water. The absorber unit may comprise a control unit for setting a proportion of the top-up water provided by the wetting element. The proportion of top-up water can be a function of evaporation in the DAC system. The greater the evaporation, the greater the proportion of top-up water required to maintain the water balance in the system. Maintaining the water balance may comprise maintaining a substantially constant sorbent concentration in the absorber unit. Conversely, if there is very little evaporation, then a greater proportion of recycled water may be used to wet the mist eliminator. The control unit is preferably configured to control the wetting element to provide water at a rate that keeps the mist eliminator substantially saturated. The controller may use a continuous feed that is variable or an intermittent feed to wet the mist eliminator. For example, the controller may be configured to control an on / off valve to provide an intermittent flow with a determined averaged flow rate. Alternatively or in addition, the controller may be configured to control a regulated valve that can provide a variable valve opening. For example, the absorber unit may comprise one or more valves configured to provide water at an integrated rate over an hour that matches the demand rate to keep the mist eliminator substantially saturated. The one or more contactors may comprise at least two contactors arranged back-to-back so as to form a plenum between them, wherein the mist eliminator is located downstream of said plenum. The contactors may be horizontal cross-flow contactors that together form a vertical exit plenum between them. The one or more contactors may comprise at least two contactors stacked vertically when in use. The at least two contactors may comprise different amounts of packing. The absorber unit may further comprise an absorbent spray located upstream of the wetting element. The additional absorbent spray may be located in the plenum downstream of the one or more contactors and provide an additional stage of contacting before the air stream is vented. The absorber unit may further comprise an intermediate collector tray for collecting water from the mist eliminator, wherein the intermediate collector tray is located downstream of said absorbent spray, and wherein the absorber unit is configured to provide water to the wetting element from the intermediate collector tray. The intermediate collector tray may be located in a plenum between a plurality of stacked contactors of the absorber unit, and may be arranged below a first contactor and above a second contactor. The absorber unit may comprise a fan located between the mist eliminator and the outlet. The fan can cover a smaller area in a plane perpendicular to a flow velocity of the air stream than the mist eliminator. This can be beneficial as the mist eliminator may function better over a relatively narrow range of speeds. The mist eliminator design can then be optimised for a slower flow than that through the fan. The outlet may be arranged so that rainwater can enter the exit plenum. Any additional water can be taken into account in order to maintain the predetermined water level. The absorber unit can be configured to determine a water level at the base of the exit plenum and / or in the intermediate tray. The absorber unit can then be configured to adjust the amount or proportion of top-up water provided by the wetting element(s) accordingly. In some embodiments, rainwater thus collected can be used by the absorber unit if it meets appropriate purity levels. The absorber unit may comprise a collector tray for collecting water at a base of the absorber unit. The collector tray may collect droplets falling from the wetted mist eliminator. The absorber unit can be configured to provide water to the wetting element from the collector tray. At least a part of the water collected in the collector tray can be recycled and provided to the wetting element. The absorber unit may further comprise a settling tank arranged to receive water from the mist eliminator. Debris may settle at the bottom or may float to the top, and may be removed periodically. The cleaned water can be used to maintain the water level of the system and / or be recycled back to the wetting element. The absorber unit may further comprise one or more filters arranged to filter water from the mist eliminator. The filter(s) can be combined with the settling tank to further clean the water. A settling tank and / or filters may be particularly useful in embodiments where rainwater is used by the absorber unit, and may prevent uncontrolled dirt / debris from entering the absorbent liquid. The absorber unit can comprise one or more further wetting elements for wetting the mist eliminator with water, wherein the wetting element and the one or more further wetting elements are together arranged to maintain the water balance in the absorber unit. For example, the absorber unit may comprise a first wetting element located upstream of the mist eliminator and a second wetting element located downstream of the mist eliminator. The one or more further wetting elements may be identical to the wetting element. According to a second aspect, there is provided a method of operating an absorber unit in a direct air capture (DAC) system, the method comprising: determining a water balance of the absorber unit (e.g. determining the correct / optimal concentration of absorbent); determining an amount of top-up water required to maintain the water balance; and wetting a mist eliminator of the absorber unit with the amount of top-up water. The absorber unit may be the absorber unit of the first aspect described above. The method may further comprise wetting the mist eliminator with recycled water so that the total rate of water provided to the mist eliminator remains substantially constant and so that the total rate of water provided to the mist eliminator is sufficiently high to saturate the mist eliminator. Water may be recycling from a base of the absorber unit. If the amount of top-up water required decreases, then the amount of recycled water (which may comprise low levels of absorbent) can be increased so that the mist eliminator still receives sufficient water. Wetting the mist eliminator may comprise ejecting (e.g. spraying) water in an upstream direction, whereby the water is carried downstream onto the mist eliminator by an air stream (the air stream from which CO2 is captured). In other embodiments, wetting the mist eliminator can comprise spraying water directly onto the mist eliminator or providing a flow of water directly to the mist eliminator (without spraying). Determining the amount of top-up water required typically comprises measuring an absorbent concentration and a total amount of absorbent liquid in the absorber unit. The absorbent concentration will tend to increase, as solvent (water) evaporates over time. Brief description of drawings Figure 1 shows a schematic cross section of a part of an absorber unit; Figure 2 shows a schematic cross section of a part of an absorber unit; Figure 3 shows a schematic cross section of a part of an absorber unit with a smaller exit fan; Figure 4 shows a schematic cross section of a part of an absorber unit with a sprayer; Figure 5 shows a schematic cross section of a part of an absorber unit with two wetting elements; Figure 6 shows a schematic cross section of a part of an absorber unit with water recycling; Figure 7 shows a schematic cross section of a part of an absorber unit with a settling tank; Figure 8 shows a schematic cross section of an absorber unit; Figure 9 shows a schematic cross section of an absorber unit with sprayers for wetting the mist eliminator; Figure 10 shows a schematic cross section of an absorber unit with sprayers for wetting the mist eliminator; Figure 11 shows a schematic cross section of an absorber unit with two sections; Figure 12 shows a schematic cross section of an absorber unit with different absorber module designs; and Figure 13 shows a flow diagram of some steps of a method of operating an absorber unit. Detailed description Figure 1 shows a cross-sectional schematic diagram of a part of an absorber unit 1 of a DAC system. The absorber unit 1 comprises a mist eliminator 2 to capture moisture and reduce drift (i.e. loss of absorbent fluid in the system with the exit air stream). The mist eliminator 2 can comprise corrugated sheets or a layer of mesh to collect liquid droplets. Downstream from the mist eliminator 2, the air stream 3 is vented via outlet 4. A fan 5 draws the air stream 3 through the absorber unit 1. The absorber unit further comprises a wetting element 6 for wetting the mist eliminator 2 with water. The wetting element provides a flow of water directly to the mist eliminator. Wetting the mist eliminator 2 can increase its efficiency so that even more drift is captured. Some evaporation in a DAC system is inevitable, and top-up water needs to be added to the system in order to maintain the water balance. In this embodiment, top-up water is used by the wetting element 6 in order to add water to the system. For example, the water top-up required for water balancing the system is added to also wet the final mist eliminator 2 of the absorber unit 1. Water collected at the base 7 can be passed to the main absorbent volume. Hence, the system can compensate for solvent (water) evaporation in the system. The wetting element 6 may require more water to operate optimally than what is required to maintain the water balance. To solve this, the water provided by the wetting element may comprise both recycled water and top-up water. The proportion of top-up solution can be set so as to maintain a constant total volume of absorbent solution in the system. For example, if there is relatively high evaporation, then the proportion of top-up water can be increased to compensate. For sufficiently high evaporation and / or other sources of water loss, the wetting element 6 may be configured to use only top-up water. The absorber unit 1 may comprise a controller (not shown) for setting the proportion of top-up water. The controller may be comprised by the wetting element 6. The controller may use a continuous feed that is variable or an intermittent feed by controlling one or more valves to provide water at a determined average rate to both maintain the water balance and to keep the mist eliminator saturated. Even when low levels of top-up water are required, a water recycle approach in this water section allows the volume of water used for wetting for drift elimination to be increased so that even more drift is captured. For example, only the required portion of top up water is metered and left to pass into the main absorbent volume to maintain the water balance of the whole system. Drift collected during this process is returned with the water as water with very low absorbent concentration. The water which is recycled from the mist eliminator 2 may become less pure as it captures drift. The low concentration of absorbent may capture additional CO2. In addition, the water (both top-up and recycled) can capture dirt and debris in the system over time. A water settling device and / or water filtration device may be included to ensure dirt and debris is removed from the water that is recycled and water that is used to top-up the system. Relatively dense debris will settle at the bottom of a settling tank over time. Less dense debris will float on the surface of the settling tank. Relatively pure liquid free from dirt and debris can be taken from the settling tank and filters and strainers can help to ensure the liquid drawn off remains relatively clean. Over time the sediment at the base of the tank can be removed at regular maintenance intervals and the floating debris can be regularly skimmed off. Preferably, the contactors (also referred to as “absorber modules”) of the absorber unit 1 are of cross-flow type and arranged in arrays back to back with the air being sucked into a central plenum via the fan 5. The final mist eliminator 2 is arranged just upstream of the fan 5 and the top-up water is arranged to wet this final mist eliminator 2. Figure 2 shows a cross-sectional schematic diagram of a part of an 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. The absorber unit 1 comprises a mist eliminator 2 to capture moisture from the air stream 3, which is then vented via outlet 4 using a fan 5. The absorber unit 1 comprises a wetting element 6 comprising a sprayer for wetting the mist eliminator 2 with water. In this embodiment, the part of the absorber unit 1 is vertically arranged and may be referred to as an “absorber tower”. Top-up water can be added to wet the mist eliminator 2 at the exit of the absorber tower. Water collected in a collector tray 8 at the base is passed to the main absorbent volume. The wetting element 6 is located between the mist eliminator 2 and the fan 5. Water provided by the wetting element 6 is ejected by the sprayer in an upstream direction and onto the mist eliminator. At least a part of the water is top-up water added to the system in order to maintain the right level or concentration of absorbent solution. Figure 3 shows a cross-sectional schematic diagram of a part of an absorber unit 1 similar to that described in relation to Figure 2 above. In this embodiment, the mist eliminator covers a larger area than the outlet 4 (and exit fan 5). Mist eliminators often function best over a fairly narrow range of air flow speeds. The mist eliminator 2 in this embodiment may be optimised for a slower flow than that through the fan 5. Figure 4 shows a cross-sectional schematic diagram of a part of an absorber unit 1 of a DAC system. The absorber unit 1 comprises a mist eliminator 2 to capture moisture from the air stream 3, which is then vented via outlet 4 using a fan 5. The absorber unit 1 comprises a wetting element 6 for wetting the mist eliminator 2 with water. A collector tray 8 collects water from the mist eliminator 2 that can be added to the main absorbent volume. The wetting element 6 is configured to provide water directly to the mist eliminator 2 and also comprises a sprayer or nozzle 9 (e.g. a mist spray) located upstream of the mist eliminator. Water provided by the sprayer or nozzle 9 is carried downstream to the mist eliminator 2 by the air stream 3. Hence, the wetting element 6 is configured to wet the mist eliminator 2 both directly and indirectly. A mist spray of top-up water prior to the final mist eliminator 2 can further enhance drift capture efficiency through droplet impact and coalescence. The ratio of liquid flow into the upstream sprayers / nozzles 9 and directly onto the mist eliminator 2 can be controlled to increase / optimise drift reduction at different ambient conditions and for a minimum of pressure loss. Figure 5 shows a cross-sectional schematic diagram of a part of an absorber unit 1 similar to that described in relation to Figure 2 above. The absorber unit comprises a second wetting element 10, located upstream of the mist eliminator 2. Top-up water is added by the second wetting element 10 to wet the mist eliminator 2 and to provide droplets upstream of the mist eliminator 2 using a spray or nozzle. The mist eliminator 2 is at least partially wetted by the second wetting element 10. Water collected by a collector tray 8 at the base is passed to the main absorbent volume. The upstream sprayer(s) of the second wetting element may be directed downwards, sideways or upwards. Figure 6 shows a cross-sectional schematic diagram of a part of an absorber unit 1, which may be the absorber unit described in relation to Figure 1 above. The diagram shows how water (and small amounts of absorbent solution) from the mist eliminator 2 is collected at the base 7. A part of the collected water is fed back to the wetting element 6 and another part is added to the main absorbent volume to maintain the level or concentration of absorbent solution in the system. Hence, the absorber unit 1 is configured to split the collected water into two parts, wherein one part is used to maintain the level or concentration of absorbent solution in the system and the other part is provided to the wetting element 6. The amount / proportion of top-up water provided by the wetting element can be adjusted based on the amount of water that is fed back from the base 7. The absorber unit 1 may comprise one or more valves and flow meters to for this purpose. This embodiment may be particularly suited to absorbent formulations and weather conditions where top-up water requirements are relatively low. As the recycled water becomes low concentration absorbent, this section of the absorber unit 1 can capture a small additional amount of carbon dioxide from the air leaving the plant. The levels may be low, because the concentration of carbon dioxide in the exiting air stream 3 is already a fraction of that of the inlet air, and the absorbent is of low concentration in this section. Figure 7 shows a cross-sectional schematic diagram of a part of an absorber unit 1 similar to that described in relation to Figure 4 above. Top-up water is added to wet the mist eliminator 2 both directly and indirectly via an upstream sprayer / droplet generation device 9. Water collected at the base 7 is split into two parts, wherein a first part is provided to the main absorbent volume (equal to the top-up water demand of the system) and a second part, which is recycled to provide a larger wetting volume. The ratio between the first and second parts can be controlled and adjusted, for example based on changes in temperature and humidity and a propensity of the system to generate drift. The absorber unit 1 may comprise one or more valves and flow meters to for this purpose. Also, the ratio of liquid flow into the upstream sprayers / nozzles 9 and directly onto the mist eliminator 2 can be controlled to increase / optimise drift reduction at different ambient conditions for a minimum of pressure loss. The absorber unit 1 further comprises a filter and / or settling tank 11. The filter and / or settling tank 11 can help to ensure that the water added to main absorbent volume is free from dirt and debris, which could otherwise cause problems with the cleanliness of the main absorbent loop. In an embodiment, water from the settling tank 11 and / or filter(s) is recycled and provided to the wetting element 6. A potential benefit of this technology is that the top up water used to sweep up the drift, can be settled and filtered to remove dirt and debris that could make its way through the absorber modules or enter through the fan exit section because of gravity or weather conditions. This keeps the absorbent system clean and reduces the maintenance burden of the plant. Figure 8 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 12 (six contactors shown, also referred to as “absorber modules”) stacked three high and arranged back-to-back with outflows into a central exit plenum 13. Each contactor 12 is a cross-flow hybrid contactor comprising a sprayer 14 and packing 15. Other embodiments may comprise different types of contactors, e.g. comprising only sprayers 14 or only packing 15. The sprayer 14 can provide a fine mist of absorbent, which creates a large and efficient contacting area for pulling CO2 out of the air stream 16. The packing 15 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. An exit fan 5 is located before (upstream of) the outlet 4 of the exit plenum 13 to draw air out of the exit plenum 13. A final mist eliminator 2 is located before the fan 5 to capture moisture from the air stream 3 and reduce drift. The mist eliminator 2 can comprise corrugated sheets or a layer of mesh to collect liquid droplets, which can conjoin and fall back down the exit plenum 13. The mist eliminator 2 can be arranged over a larger flow area to reduce pressure losses. The absorber unit 1 further comprises a wetting element 6 for wetting the mist eliminator 2 with water. The wetting element provides a flow of water directly to the mist eliminator 2. Top-up water is used by the wetting element 6 in order to both wet the mist eliminator 2 and to add water to the system. Water collected in the collector tray 8 at the base 7 can be passed to the main absorbent volume and / or recycled back to the wetting element 6. Drips from the wetted mist eliminator 2 can create a thick slow rain in the plenum 13 with the drips acting as “sweeper drips” to effectively remove drift in the exit plenum 13 before the mist eliminator 2. The droplet size is big enough to ensure the droplet population falls down through the volume under gravity, rather than being picked up and lifted by the airflow. By creating a droplet population size that is neither too small to fly upwards 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. This improves the sweeper droplet effect for drift removal. Any drift exiting from the contactors 12 and venting into the central plenum 13 has an increased probability of interaction with the drops from the mist eliminator 2 to give sweeping removal of the drift. The mist eliminator 2 prior to the fan 5 acts together with the sweeping droplets to provide a low drift system. Figure 9 shows a schematic cross section of an absorber unit 1 similar to that described in relation to Figure 8 above. In this embodiment, the wetting element 6 comprises sprayers for ejecting water in a downstream direction onto the mist eliminator 2. Water from mist eliminator 2 can drip down through the plenum 13 as described in relation to Figure 8. Figure 10 shows a schematic cross section of an absorber unit 1 similar to that described in relation to Figure 8. In this embodiment, the wetting element 6 comprises one or more sprayers located downstream from the mist eliminator 2 (between the mist eliminator and the fan 5 at the outlet 4). The wetting element 6 ejects water upstream onto the mist eliminator 2. Again, water from mist eliminator 2 can drip down through the plenum 13 to further reduce drift and even increase CO2 absorption as weak absorbent solution is recycled by the wetting element. Figure 11 shows a schematic cross section of an absorber unit 1 similar to that described in relation to Figure 8 above. The absorber unit 1 comprises a second wetting element 10 for providing water. The first and second wetting elements 6 and 10 are located on either side of the mist eliminator 2. In this embodiment, the exit plenum 13 is into a top-up water drift elimination section at the top and an additional absorbent spray section at the bottom. The absorber unit 1 comprises an intermediate collector tray 17 located in the exit plenum 13 and arranged to collect a part of the water falling down from the mist eliminator 2 and from the wetting elements 6 and 10. In this case water collected at the intermediate collector tray 17 is split between that which is passed to the main absorbent volume (equal to the top up water demand from the system) and that which is recycled back to the wetting elements 6 and 10 to provide a larger wetting volume. Droplets in the spray / droplet generation device of the second wetting element 10 are ideally sized to act as sweeper droplets to collide and coalesce with drift and absorbent droplets leaving the absorber modules giving an enhanced drift elimination capability. If the droplets are sized such that the force balance on them is biased only slightly in the downwards direction, then they will travel down the plenum slowly (until they collide / coalesce at which point they accelerate downwards). The absorber unit further comprises an additional absorbent spray 18 located below the intermediate collector tray 17. The additional absorbent spray 18 makes further use of the additional volume provided by the exit plenum 13 for additional contacting. It may be advantageous that the water balance is maintained by allowing the desired flow of water to spill over to the lower volume. Alternatively, the spill over water can be taken to another part of the system. Figure 12 shows a schematic cross section of an absorber unit 1 similar to that described in relation to Figure 11 above. When the exit plenum 13 is divided into a top-up water drift elimination section at the top and an additional absorber spray section at the bottom, then it may be advantageous to provide two different absorber module designs depending on their relative position. The top contactors 12A comprise hybrid absorbers (spray / packed volumes) giving good levels of carbon dioxide absorption for low differential pressure and low levels of drift at the exit. The bottom contactors 12B can have less packing for better performance (mass transfer) and lower differential pressure. The additional absorbent sprays 18 (using sweeper drops) and subsequent top-up water sprays 6 and 10 (using sweeper drops again) allow higher levels of carbon dioxide capture for zero or near zero drift production, whilst also maintaining a low differential pressure system design. Figure 13 is a flow diagram of some steps of a method of operating an absorber unit, such as the absorber unit described in relation to any one of Figures 1 to 12 above. The method comprises determining a water balance of the absorber unit (S1), determining an amount of top-up water required to maintain the water balance (S2), and wetting a mist eliminator of the absorber unit with the amount of top-up water (S3). While specific embodiments of the invention have been described above, it will be appreciated that further embodiments are possible. The descriptions above are intended 5 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. Each feature disclosed or illustrated in the present specification may be incorporated in 10 the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.

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;a mist eliminator located downstream of the one or more contactors and upstream of an outlet for venting the air stream; anda wetting element for wetting the mist eliminator with water, wherein the wetting element is arranged to maintain a water balance in the absorber unit.

2. An absorber unit according to claim 1, wherein the wetting element comprises one or more sprayers for spraying the water that wets the mist eliminator.

3. An absorber unit according to claim 1 or 2, wherein a sprayer of the one or more sprayers is located downstream of said mist eliminator and is configured to spray water in an upstream direction onto the mist eliminator.

4. An absorber unit according to any one of the preceding claims, wherein a sprayer of the one or more sprayers is located upstream of said mist eliminator.

5. An absorber unit according to claim 4, wherein the sprayer is configured to spray water in an upstream direction so that the water is carried downstream by the air stream onto the mist eliminator.

6. An absorber unit according to any one of the preceding claims, wherein the wetting element is configured to provide a flow of water directly to the mist eliminator.

7. An absorber unit according to any one of the preceding claims, wherein the wetting element is configured to receive both recycled water and top-up water.

8. An absorber unit according to claim 7, further comprising a control unit for setting a proportion of the top-up water provided by the wetting element.

9. An absorber unit according to claim 8, wherein the proportion of top-up water is a function of evaporation in the DAC system.

10. An absorber unit according to claim 8 or 9, wherein the control unit is configured to control the wetting element to provide water at a rate that keeps the mist eliminator substantially saturated.

11. 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 plenum between them, and wherein the mist eliminator is located downstream of said plenum.

12. An absorber unit according to any one of the preceding claims, further comprising an absorbent spray located upstream of the wetting element.

13. An absorber unit according to any one of the preceding claims, further comprising an intermediate collector tray for collecting water from the mist eliminator, wherein the intermediate collector tray is located downstream of said absorbent spray, and wherein the absorber unit is configured to provide water to the wetting element from the intermediate collector tray.

14. An absorber unit according to any one of the preceding claims, wherein the one or more contactors comprises at least two contactors stacked vertically when in use, and wherein the at least two contactors comprise different amounts of packing.

15. An absorber unit according to any one of the preceding claims, further comprising a fan located between the mist eliminator and the outlet, wherein the fan covers a smaller area in a plane perpendicular to a flow velocity of the air stream than the mist eliminator.

16. An absorber unit according to any one of the preceding claims, further comprising a collector tray for collecting water at a base of the absorber unit.

17. An absorber unit according to claim 16, wherein the absorber unit is configured to provide water to the wetting element from the collector tray.

18. An absorber unit according to any one of the preceding claims, further comprising a settling tank arranged to receive water from the mist eliminator.

19. An absorber unit according to any one of the preceding claims, further comprising one or more filters arranged to filter water from the mist eliminator.

20. An absorber unit according to any one of the preceding claims, further comprising one or more further wetting elements for wetting the mist eliminator with water, wherein the wetting element and the one or more further wetting elements are together arranged to maintain the water balance in the absorber unit21. A method of operating an absorber unit in a direct air capture (DAC) system, the method comprising:determining a water balance of the absorber unit;determining an amount of top-up water required to maintain the water balance; andwetting a mist eliminator of the absorber unit with the amount of top-up water.

22. A method according to claim 21, further comprising wetting the mist eliminator with recycled water so that the total rate of water provided to the mist eliminator remains substantially constant and so that the total rate of water provided to the mist eliminator is sufficiently high to saturate the mist eliminator.

23. A method according to claim 21 or 22, further comprising, recycling water from a base of the absorber unit.

24. A method according to any one of claims 21 to 23, wherein wetting the mist eliminator comprises ejecting water in an upstream direction, whereby the water is carried downstream onto the mist eliminator by an air stream.

25. A method according to any one of claims 21 to 24, wherein determining the amount of top-up water required comprises measuring an absorbent concentration and a total amount of absorbent liquid in the absorber unit.IntellectualPropertyOfficeApplication GB2418556.3Search report under Section 17 of the Patents Act 1977Date search completed: 15 July 2025Claims searched: 1-25International classificationSubclass and subgroup Valid from B01D53 / 14 01 / 01 / 2006 B01D53 / 18 01 / 01 / 2006 B01D53 / 78 01 / 01 / 2006 B01D53 / 96 01 / 01 / 2006Field of searchWorldwide search of patent documents classified in the following areas of the IPC:B01DDatabases used in the preparation of this search report:SEARCH-PATENTDocuments considered to be relevantPatent literatureCategory Relevant Document of relevanceclaimsX 1-3, 6, 12, 15 &16 CN 106540519 B (CHINA SHENHUA ENERGY CO LTD), Figure 1 and related text. Non-patent literature Category Relevant claims Document of relevanceCategoriesLetter or DescriptionsymbolX Document indicating lack of novelty or inventive step.Y Document indicating lack of inventive step, if combined with anotherdocument of the same category.& Member of the same patent family. A Document indicating technological background. P Document published on or after the priority date but before the fling date of the present application. E Earlier application published on or after the filing date of the present application.

Images