Improvements to absorber for direct air capture comprising spray nozzles and packings

Abstract

An absorber for the capture of carbon dioxide (CO2) from a CO2 containing gas stream, the system comprising: an absorber volume to contact the CO2 containing gas stream with a sorbent, the absorber comprising, a first portion equipped with spray nozzles and a second portion which contains packing material arranged to be wetted by sorbent, wherein the first portion and second portion each have a respective duty which together contribute to a required absorber duty defined as a rate of capture of CO2.

Description

[0001] IMPROVEMENTS TO ABSORBER FOR DIRECT AIR CAPTURE COMPRISING SPRAY NOZZLES AND PACKINGS

[0002] Technical Field

[0003] The present disclosure concerns an absorber for a direct air CO2 capture process. The invention also relates to a direct air CO2 capture unit, and to a method of capture of carbon dioxide (CO2) from a CO2 containing gas stream, such as ambient air.

[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] The high energy use and other costs of running a direct air capture system may be improved by improving the design of the absorber. There are significant energy costs from, inter alia, the power needed for the fan to move CO2 through the absorber, power needed to pump sorbent through the absorber and in the preparation of, e.g. packing materials present in the absorber.

[0009] Summary of Invention

[0010] In an aspect of this disclosure, an absorber for the capture of carbon dioxide (CO2) from a CO2 containing gas stream is provided, the absorber comprising: an absorber volume to contact the CO2 containing gas stream with a sorbent, the absorber volume having a length dimension parallel to the direction of travel of the gas stream through the absorber from an inlet to an exit when in use; the absorber comprising at least a first portion nearest to the inlet and a second portion further from the inlet, wherein the first portion is equipped with spray nozzles configured to deliver the sorbent as an aerosol into the gas stream where it can absorb CO2 from the gas stream while droplets of the aerosol are suspended in the gas stream, and the second portion contains packing material arranged to be wetted by sorbent, wherein the first portion and second portion each have a respective duty which together contribute to a required absorber duty defined as a rate of capture of CO2. Preferably the absorber has an inlet configured to deliver a CO2 containing gas stream containing less than 1 % volume CO2, to said first portion of the absorber.

[0011] The duty of the first portion and the duty of the second portion may be selected to minimise the pressure drop across the absorber while providing the required absorber duty. The duty of the first portion and the duty of the second portion is preferably selected to minimise overall CO2 emissions intensity, wherein CO2 emissions intensity takes into account the CO2 emissions and optionally other Greenhouse Gas (GHG) emissions and their CO2 equivalents (C02e) derived from manufacture and operation of the absorber over its lifetime. Preferably the CO2 emissions intensity is derived from the energy required for fan power and pump power, the mass of materials used and the supply chain lifecycle emissions associated with materials production and supply and the supply chain lifecycle emissions associated with replacing sorbent lost. The first portion may be configured to provide at least 50%, and more preferably at least 60% of the absorber duty. The first portion may be configured to, e.g. provide between 30% and 90% of the absorber duty.

[0012] More preferably the first portion is configured to provide between 60% and 98% and still more preferably between 70% and 98% of the absorber duty. As shown in the modelling described herein, it is believed that this minimises the CO2 emissions intensity of the absorber. In length terms this may be equivalent to the first portion being configured to provide between 50% to 90% of the absorber length, more preferably between 55% and 80% of the absorber length and even more preferably between 60% and 80% of the absorber length.

[0013] The second portion may be configured to provide between 10% and 70%. More preferably the second portion may be configured to provide between 2% and 40% and still and more preferably between 2% and 30% of the absorber duty. In length terms this may be equivalent to the second portion being configured to provide between 10% to 50% of the absorber length, more preferably between 20% and 45% of the absorber length and even more preferably between 20% and 40% of the absorber length

[0014] The second portion may contain packing material that is wetted by sorbent, preferably fresh sorbent. In other words, the sorbent supplied to the second portion of the absorber is not simply sorbent carried to the second portion from the first portion. Rather it is solvent supplied to the packing material of the second portion.

[0015] The second portion may additionally be configured to capture drift from the first portion.

[0016] There may also be a drift eliminator downstream of the second portion.

[0017] The absorber may have a third portion between the first portion and the second portion, the third portion containing no packing or packing material arranged differently to the packing in the second portion. The third portion may be arranged to be only wetted by sorbent that has been carried from the first portion as an aerosol suspension. In this latter case, the third portion preferably contains packing material.

[0018] The first portion may contain no packing material. That is, the first portion may be a purely spray type contactor.

[0019] Preferably there is no portion containing packing material prior to said first portion. Preferably all the portions equipped with spray nozzles configured to deliver the sorbent as an aerosol into the gas stream are nearest to the inlet of the absorber. Thus preferably the gas (e.g. air) stream is initially sprayed with sorbent as an aerosol.

[0020] Preferably all the portions containing packing material arranged to be wetted by sorbent are nearest to the outlet of the absorber. Preferably all the portions equipped with spray nozzles configured to deliver the sorbent as an aerosol into the gas stream precede all the portions containing packing material arranged to be wetted by sorbent are nearest to the outlet of the absorber. Thus preferably the gas (e.g. air) stream contacts wetted packing material after it has passed through the portions equipped with spray nozzles.

[0021] The CO2 containing gas stream may flow perpendicular to the sorbent flow. The CO2 containing gas stream may flow in a horizontal direction or a vertical direction, or in intermediate direction.

[0022] The CO2 containing gas stream preferably contains less than 1 % volume CO2. The gas stream may be, for example, exhaust gas from Heating Ventilation and Air Conditioning (HVAC) equipment, exhaust gas from habitable spaces, or ventilation exhausts from biological processes where respiration is occurring (e.g. fermentation vessel exhausts) . Preferably the CO2 containing gas stream may be ambient air. Preferably the absorber has an inlet configured to deliver the gas stream (e.g. ambient air from the atmosphere) to said first portion of the absorber. Unlike absorbers designed to treat flue or exhaust gas, preferably the absorber is not connected to an upstream washing unit.

[0023] Preferably the absorber is connected to a desorber for recovery of said CO2 and sorbent.

[0024] There is also provided a direct air CO2 capture unit comprising the absorber as hereinbefore described.

[0025] There is also provided a method of capture of carbon dioxide (CO2) from a CO2 containing gas stream, the method comprising: providing an absorber for contacting sorbent with the CO2 containing gas stream, supplying sorbent as an aerosol to one portion of the absorber using spray nozzles, and supplying sorbent to a second portion of the absorber onto packing.

[0026] A preferred method further comprises providing a CO2 containing gas stream containing less than 1 % volume CO2, preferably ambient air.

[0027] The duty of the first portion and the duty of the second portion may be selected to minimise the pressure drop across the absorber while providing the required absorber duty. The duty of the first portion and the duty of the second portion is preferably selected to minimise overall CO2 emissions intensity, wherein CO2 emissions intensity takes into account the CO2 emissions derived from manufacture and operation of the absorber over its lifetime. Preferably the CO2 emissions intensity is derived from the energy required for fan power and pump power, the mass of materials used and the supply chain lifecycle emissions associated with materials production and supply and the supply chain lifecycle emissions associated with replacing sorbent lost.

[0028] The first portion may be configured to provide between 30% and 90% of the absorber duty. More preferably the first portion may be configured to provide between 60% and 98% and still more preferably between 70% and 98% of the absorber duty. As shown in the modelling described herein, it is believed that this minimises the CO2 emissions intensity of the absorber.

[0029] The second portion may be configured to provide between 10% and 70% of the absorber duty. More preferably the second portion may be configured to provide between 2% and 40% and still and more preferably between 2% and 30% of the absorber duty.

[0030] Alternatively the second portion may be configured to provide between 10% and 50% of the absorber duty.

[0031] The sorbent supplied to said second portion of the absorber is preferably fresh sorbent. In other words, the sorbent supplied to the second portion of the absorber is not simply sorbent carried to the second portion from the first portion. Rather it is solvent supplied to the packing material of the second portion.

[0032] The method may further comprise capturing drift from the first portion in said second portion.

[0033] The method may further comprise capturing drift in a drift eliminator downstream of the second portion.

[0034] The method may further comprise capturing sorbent that has been carried from the first portion as an aerosol suspension on a third portion, wherein said third portion is between the first portion and the second portion, and the third portion contains packing material arranged differently to the packing in the second portion.

[0035] In some preferred methods the first portion contains no packing material. In some further preferred methods, there is no portion containing packing material prior to said first portion. In still further preferred methods, all the portions equipped with spray nozzles configured to deliver the sorbent as an aerosol into the gas stream are nearest to the inlet of the absorber. In other preferred methods all the portions containing packing material arranged to be wetted by sorbent are nearest to the outlet of the absorber. In further preferred methods, all the portions equipped with spray nozzles configured to deliver the sorbent as an aerosol into the gas stream precede all the portions containing packing material arranged to be wetted by sorbent are nearest to the outlet of the absorber.

[0036] In some preferred methods the CO2 containing gas stream flows perpendicular to the sorbent flow.

[0037] In some preferred methods the CO2 containing gas stream flows in a horizontal direction.

[0038] The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore except where mutually exclusive any feature described herein may be applied to any aspect and / or combined with any other feature described herein.

[0039] Brief Description of Drawings

[0040] Embodiments will now be described by way of example only, with reference to the Figures, in which:

[0041] Fig. 1 is a schematic of a prior art carbon capture system;

[0042] Fig. 2 is a schematic of an absorber for a carbon capture system.

[0043] Fig. 3 is an illustration of performance of a packed absorber for a carbon capture system.

[0044] Fig. 4 is an illustration of performance of a hybrid absorber for a carbon capture system.

[0045] Fig. 5 is a graph showing relative carbon dioxide equivalent emissions intensity for capturing a given amount of CO2 for various first and second portion arrangements. Detailed Description

[0046] With reference to Fig. 1 , which shows a system for carbon capture from a CO2 containing gas, absorber 10 receives a CO2 containing gas stream from the inlet 20 to exit 30. A sorbent stream flows through the absorber, a lean stream 40 enters the absorber, contacts the CO2 containing gas and becomes a rich stream 50. Sorbent may be recirculated within the absorber as a recirculation stream 110, which increases the effective residence time of each portion of the lean stream of sorbent in the absorber. The gas stream may flow horizontally as shown, or may flow vertically from bottom to top of the absorber in counterflow to the sorbent stream, or from top to bottom in parallel with the sorbent flow.

[0047] The rich stream is typically passed through heat exchanger 100 to recover some heat from the lean stream returning from the desorber 60. Desorber 60 receives the rich stream and heats it up to a temperature where the CO2 will be released from the sorbent, typically using heating means 70 which may also generate steam to form vapour bubbles into which the desorbed CO2 can diffuse, leaving a lean stream of sorbent to return to the absorber to repeat the process.

[0048] The vapour and desorbed CO2 exit the desorber at 80, where a condenser 90 is usually used to cool the mixture causing the vapour to condense leaving a purer CO2 product stream.

[0049] The component or components that bring the sorbent solution into contact with the gas stream (e.g. air), are referred to here as the contactors, being one of the sub-components that make up the composite parts of the Absorber sub-system. The common design of these contactors is provision of a solid surface over which a liquid film of sorbent solution can be propagated to provide a large surface area of fluid in contact with the gas (e.g. air) stream at any one time. This type of component is conventionally referred to as “packing”, of which there are a myriad of generic and proprietary forms utilised extensively for both mass transfer and heat transfer unit operations within industry (e.g. Wet cooling towers, stripper columns). Fig. 2 shows an absorber 10, which may be used in a system like the one in Fig.1 , with a vessel for storing sorbent 210 positioned above the absorber, and an optional distribution mechanism 220 for distributing sorbent across the length and width of the absorber when it is released from the vessel 210. Vessel 210 comprises a release mechanism 240 for controlling the release of sorbent, which may be continuous or released intermittently in pulses either directly into the absorber or via the distribution tray 220. Sorbent supply 260 delivers sorbent into the vessel, which passes through the absorber downwards due to gravity, where it may be collected in a sump 270 after spending a residence time in contact with the gas (e.g. air) flow between inlet 20 and exit 30. From the sump, the sorbent may be recirculated through the absorber via sorbent supply 260, while a portion of the sorbent leaves the absorber as rich flow to the desorber 60. Lean sorbent from the desorber may be mixed with the recirculated sorbent in sorbent supply 260, or it may enter the absorber via a different stream.

[0050] The absorber may include packing 250 filling or partially filling the absorber 10. The packing may comprise layers or stacked pieces to distribute the sorbent within the absorber and increase the effective surface area of sorbent exposed to the gas (e.g. air) flow between inlet 20 and exit 30. The absorber may also include nozzles to distribute the sorbent as a spray or droplets within the absorber. These nozzles may deliver sorbent onto the packing, or create a spray or mist with a high effective surface area directly in the gas (e.g. air) flow within the absorber.

[0051] The release mechanism 240 may be configured to release a portion of the sorbent into the absorber, optionally via the distribution tray 220 or directly into nozzles, when the level of sorbent in vessel 210 exceeds a threshold depth 230. The release mechanism 240 may form part of vessel 210 or it may be a separate module fitted to vessel 210, either inside or outside the vessel. The release mechanism 240 may be electro-mechanical, such as variable flow valves or may be a passive mechanism that operates without the use of external power and does not rely on external control signals or sensors. The release mechanism 240 may be driven by gravity and the mass of sorbent, using buoyancy, siphons and / or tilting mechanisms. There may optionally or alternatively be a continuous supply of sorbent 260’ into the absorber. For example, there may be a continuous spray of sorbent, for example as a mist or trickle of sorbent which may be into open space or onto packing, and a second intermittent supply of pulses of sorbent delivered by the release mechanism. The release mechanism may be configured to deliver sorbent at different rates to different parts of the absorber, for example supplying spray nozzles in one portion of the absorber to produce mist of sorbent, and supplying other nozzles to wet a packed portion of the absorber.

[0052] Fig.3 illustrates the CO2 concentration and pressure drop in the gas as it passes through a packed absorber 300. The gas flow is indicated by the arrow from bottom to top on the page from inlet 20 to exit 30, the absorber may in fact be oriented horizontally or vertically.

[0053] Chart 320 illustrates how the CO2 concentration in the gas reduces as the gas passes though the absorber, reaching a minimum at the exit. The curve is approximately exponential, as the rate of absorption decreases as the concentration decreases.

[0054] Chart 330 illustrates the pressure drop through the packing 250, which is approximately linear as there will be roughly equal pressure drop per unit length assuming homogenous packing is used throughout the length, at the typical gas flow rates used in a carbon capture absorber.

[0055] Wetted surface type contactors can have high mass transfer efficiencies due to having Specific Surface Areas in the 100s of m2per m3, depleting the gas stream (e.g. air) of the solute gas as it transits through the component. Thus, the depth of the component relative to the gas (e.g. air) stream direction of travel sets the overall mass transfer duty of the sub-system. It is set to meet the overall design requirement of the system. This is not constant through the depth of the bed, as the rate at which the solute is captured is partly controlled by the concentration of solute gas at that given point in its transit. Thus, mass transfer can be said to be higher in earlier sections of the component, reducing as the gas (e.g. air) penetrates further. Therefore, the relationship of contactor depth to mass transfer duty is not linear given a consistent sorbent solution loading, so is subject to optimisation against the pressure drop incurred by the component. As any component placed within gas (e.g. air) stream will cause blockage and aerodynamic perturbances, to said gas (e.g. air) flow, causing pressure losses within the flow path. These pressure losses need to be overcome by any method of actively inducing gas (e.g. air) through the sub-system, which will incur an energy efficiency penalty on the system, which is undesirable and needs to be minimised and traded against the mass transfer. 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 sub-system to achieve the required mass transfer duties. The energy requirement of the fan to drive air through the contactor is significant.

[0056] An alternative to a wetted surface to provide contact between the two streams for the purpose of mass transfer is the usage of droplets. The primary assumption is that they are seeded or introduced into the gas (e.g. air) stream via spray / aerosol methods. However again, multiple methods are possible, to which this invention is not restricted, but could the provision of a spray or an array of multiple spray nozzles to achieve this. The method chosen would be able to achieve high specific surface areas comparable to that of the packings mentioned earlier. And if the components required to seed the gas (e.g. air) are mounted on the bounding walls that define the primary gas (e.g. air) flow path, then pressure drop per metre would not be significantly noticeable above the inherent duct pressure losses due to the presence of the bounding walls. If seeded was required to be performed by components protruding into the gas (e.g. air) flow, this penalty would increase but would be reduced compared to a unit depth of packing.

[0057] However, droplets have several shortcomings:

[0058] • The surface area of the droplets is not fixed, being susceptible to inter droplet collision and coalescence, and collision with the bounding walls of the gas stream (e.g. air) path. This can be validated experimentally in proposed DAC flow conditions. Furthermore, evaporation of the droplet solvent (H2O) into the gas (e.g. air) stream will also cause the droplet shape to change. This evaporation is not unique to a spray based system however.

[0059] • The transit / residence time of the droplet may be less than that of an equivalent volume of fluid in a surface film on a wetted surface.

[0060] • These are both parameters for optimisation through design and operation.

[0061] Therefore, the depth of bed for the same duty of solute gas capture for a droplet based system, is likely to be much deeper than an equivalent conventional packed bed type system. However, the pressure drop reduction is attractive, and when combined with benefits in CAPEX (amount of material, transportation and assembly reduced) further promotes this contactor type.

[0062] This drawback in contacting ability per unit depth, will be further inflated as the gas (e.g. air) becomes depleted of the solute gas, as mentioned before, So the X metres of spray contactor required to perform the latter fractions of absorption duty will become significantly larger compared to that of a surface type system, with the impacts of extra duct lengths and sorbent solution flow eroding the benefit provided by low pressure drop. Therefore, this invention proposes a hybrid system where a combination of both droplet contacting methods and surface film contact methods are used in combination.

[0063] Primarily it is envisaged that the gas (e.g. air) flow through the contactor sequentially, utilising the low pressure drop droplet system first to achieve the majority of the reduced mass transfer duty, then the remaining fraction is achieved with a high efficiency surface film type contactor. There is no restriction in numbers of these contactor “beds” and their configurations, being suitable for optimisation. However, the likely configuration would be instances of spray contactors, followed by instances of surface contacting beds. A generic example to compare against Fig 3, is shown in Fig 4 to illustrate the functional behaviour through a hybrid contactor type arrangement. Fig.4 illustrates the CO2 concentration and pressure drop in the gas as it passes through a hybrid absorber 400. The gas flow is indicated by the arrow from bottom to top on the page from inlet 20 to exit 30, the absorber may in fact be oriented horizontally or vertically.

[0064] This hybrid absorber has a first portion 440 where there is little or no packing, and absorption takes place in a droplet based system. Spray nozzles (not shown) deliver sorbent solution into the first portion as an aerosol or mist. Second portion 250 is a packed section where the packing is wetted as described above. There is an optional third portion 450 that may be simply a break between the first and second portions to allow the mist or droplets to settle, or it may be a section of packing without a separate wetting mechanism, that is wetted by the droplets from the first portion that are entrained by the gas flow.

[0065] Chart 340 illustrates how the CO2 concentration in the gas reduces as the gas passes though the absorber, reaching a minimum at the exit. The curve has three sections. The graph sections are aligned with the respective absorber portions. Each section is approximately exponential, as the rate of absorption decreases as the concentration decreases. However the curve of the first section approaches an asymptotic limit more quickly as the performance of the mist / droplets reduces more quickly with length than the packed portion. Without wishing to be bound by theory, this maybe because the droplets begin to coalesce, reducing the rate of diffusion of gas into the sorbent droplets.

[0066] Chart 330 illustrates the pressure drop through the packing 250, which again has three sections aligned with the portions of the absorber 400. Each section is approximately linear as there will be roughly equal pressure drop per unit length, however the pressure drop through the spray portion 440 of the absorber is much lower than through the packed portion 250.

[0067] By selecting a length for the spray portion and the packed portion, an optimum performance for pressure drop can be achieved, each portion of the absorber contributing to a total absorber duty. This overcomes the performance limitations of a purely spray absorber, as the packed section with a thin film of sorbent and longer residence time absorbs more CO2 than would have been achievable with a purely spray absorber of a similar size. The overall pressure drop and hence fan duty of the absorber is reduced compared to a fully packed absorber.

[0068] It has been found that providing a packed portion that delivers between 10% and 70% of the overall absorber duty achieves the lowest pressure drop compared to a fully packed absorber of the same duty. More preferably a second, packed, portion provides between 2% and 40% and still and more preferably between 2% and 30% of the absorber duty.

[0069] Furthermore, there is an additional physical minimum suggested for the required depth of wetted surface type contactor if placed as the final contacting component, in a hybrid type system. This is suggested to ensure function for both drift elimination as well as secondary mass transfer of impinged droplets. That being 5-10% of the depth required of wetted surface type packing to achieve the full absorption duty in a non-hybrid system.

[0070] The fraction of duty to be provided by the packed portion may vary due to the relative costs of sorbent loss, fan power and absorption performance, depending on the environment in which the plant is operated and the type of sorbent used. Thus the minimum duty fraction of the packed portion may be at least 2%. For example the minimum may be 10%, at least 20%, at least 30%, at least 40%. For example, the minimum in length terms may be equivalent to the first portion being at least 10%, at least 20%, at least 30%, at least 40%, preferably no more than 50% of the absorber length.

[0071] The benefits realised by introducing a spray section of the packing will reduce if the majority of the absorber is filled with packing. Therefore the maximum fraction of duty to obtain a benefit may be less than 90%, less than 80%, less than 70% or less than 60% or less than 50%. For example, the maximum fraction may be less than 98%, less than 90%, less than 80%, less than 70 % or less than 60%. For example the maximum in length terms may be equivalent to the fraction being less than 90%, less than 80%, less than 70%, less than 60% of the absorber length.

[0072] An optimal fraction can be obtained by modelling absorber performance for the intended environmental conditions using a chosen sorbent solution. The duty of the first portion and the duty of the second portion is preferably selected to minimise overall CO2 emissions intensity, wherein CO2 emissions intensity takes into account the CO2 emissions derived from manufacture and operation of the absorber over its lifetime. Preferably the CO2 emissions intensity is derived from the energy required for fan power and pump power, the mass of materials used and the supply chain lifecycle emissions associated with materials production and supply and the supply chain lifecycle emissions associated with replacing sorbent lost.

[0073] Taking into account sorbent loss, and the carbon dioxide equivalent emissions associated with the lifecycle operations including energy, had a surprising effect on the modelling. Taking this into account increases the optimised duty for the second portion to around 2-30%, the corresponding optimised duty for the first portion being around 70-98%. This was unexpected, given that conventional modelling predicts the duty of the second portion should generally be minimised.

[0074] Overall the hybrid absorber will require fewer components than a packed absorber, making it lighter and less resource intensive to build. Because the packed portion captures any drift from the mist portion, it avoids the high sorbent loss normally found in purely spray absorbers. A smaller drift eliminator may be installed at the exit from the absorber to catch any droplets entrained into the gas stream (e.g. air flow) from the packed portion, but this will be smaller than would be required in a purely spray absorber.

[0075] The invention reduces the amount of surface film contacting sub-components within the absorber gas (e.g. air) stream and sub-systems, providing further benefits including: • Reduced Pressure Drop.

[0076] • Reduced Operational Wetted Weight / Liquid Hold-up.

[0077] • Simplifies Maintenance Activities, due to improved access and reducing in fouling surfaces.

[0078] • Reduced Volumes of Surface Film Contactor Components - Further benefits through reducing volume of components to be purchased, transported, and assembled.

[0079] It is also highlighted that the invention is applicable to any arrangement of Absorber Module, with respect to the flow of the gas (e.g. air) and the sorbent solution relative to each other. Including, but not limited to, Vertical Counterflow Systems and Horizontal Cross-flow systems.

[0080] Compared to a fully spray absorber, the invention:

[0081] Provides high-efficiency mass transfer surfaces to overome worsening kinetics due to progressive depletion of sorbent in gas (e.g. air) stream.

[0082] Provides a drift elimination function, due to promoting collision and coalescence of droplets in gas (e.g. air) stream and collision with the contactor surfaces, through the tortuous path the latter provides. Furthermore, this portion of contactor could perform the combined function of mass transfer and drift elimination, either replacing the requirement for a dedicated drift eliminator in its entirety or at least reduces the duty it would need to perform. Thus, being beneficial in terms of pressure loss and the lifecycle cost of components.

[0083] The droplets colliding with the contactor provide a passive wetting of the contactor surfaces, increasing absorption of the solute per pass of sorbent solution through the absorber system and reduces the amount of sorbent solution that needs to be moved around the system to purely wet the surface film-based contactor components.

[0084] Further benefits of the surface-based contactors in proximity to a droplet-based contactor previously described, include both the occurrence of passive wetting and supplementary mass transfer from the droplets impinging onto the surface. This adds to both the overall mass transfer, reduces the amount of sorbent solution required to be transferred around the system purely for wetting of the contactor surfaces (improved capture per unit pass of sorbent through the module) and drift elimination. The latter is provisioned as a surface-based contactor does provide blockage and a tortuous path for the gas (e.g. air) flow, whose form is also beneficial for removing drift as well as mass transfer in most instances.

[0085] Within vertical counter-flow systems, the gas (e.g. air) flow could be seeded with droplets in a vertical counter flow manner (e.g. a fountain), this would provide increase residence times in the flow if the droplets behaved ballistically (with gravity), either returning to the sump, or wetting surfaces for further mass transfer beneath the injection point. Whilst the smaller droplets would travel with the gas (e.g. air) flow, impinging on the contacting surfaces above, removing their potential to drift, adding to the wetting of said surface, adding additional mass transfer and also providing the ability to coalesce and drip from the surface in large droplets that behave ballistically, returning to the sump or wetted surface contactors below.

[0086] Droplet production can be produced by a wide range of spray nozzle types, forms, arrays, and arrangements. They may be stationary or could be actively or passively locomoted for more optimal sub-system performance. They could also be intermittently operated for further efficiency and operational benefits.

[0087] Sorbent properties:

[0088] Sorbents for carbon capture generally have a changing equilibrium between the carbonate / carbamate forms and being in solution with CO2 that depends on temperature, concentration and other factors. Sorbents are carried in a solvent, for example water, which may contain further additives that can act as catalysts, modify the solution physical properties, reduce degradation or other desirable properties.

[0089] In general, where the absorber is an air to liquid contactor in a DAC system, the temperature of the sorbent and the temperature of the air within the absorber will be very close, as the sorbent is distributed in a thin film or droplets to achieve a high surface area and contact time, and the air flow is relatively high. In an operating model of operating a CO2 capture system, other factors than absorbing efficiency may need to be taken into account, such as energy efficiency, sorbent degradation rate, loss of sorbent, maintenance costs etc. For any CO2 capture system the threshold for rate of absorbing may be set taking into account all the other factors, to give the best overall operation of the system.

[0090] Sorbents may include alkaline absorbents such as hydroxides or organic sorbents. Alkaline sorbents may include potassium hydroxide or calcium hydroxide.

[0091] Organic sorbents may include amines, amino acids. Amines may include Ethanolamine (2-aminoethanol, monoethanolamine, ETA, or MEA).

[0092] Preferred sorbents include amino acids or alkali salt solutions of amino acids. The amino acids may be selected from the group consisting of alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, ornithine, phenylalanine, proline, sarcosine, selenocysteine, serine, taurine, threonine, tryptophan, tyrosine, or valine. The amino acids may be in a derivatised form, e.g. the amine group may be alkylated, e.g. as a methyl amine or diethyl amine.

[0093] Preferred alkali component of the amino acid salts is potassium or sodium. Examples of amino acid salts include, sodium glycinate, and potassium lysinate.

[0094] Amino acids are preferred because they are understood to have lower heat requirements for desorption, have less degradation than amines, and are less hazardous in use than many of the alternatives. They are often also less volatile so that less sorbent evaporates from the solution.

[0095] EXAMPLE

[0096] A fundamental tenet of a commercially successful Direct Air Capture unit is that it captures more carbon dioxide from the gas stream (e.g. ambient air) than is utilised during its manufacture and operation over its lifespan. This also holds true for the absorber sub-system described herein, in which gas-liquid contacting devices are installed to capture the solute (CO2) from the gas stream such as ambient air.

[0097] As described herein, the method of the present invention uses two methods of capture, namely packed sections, with their efficient mass transfer rates but higher pressure drops per metre in the gas phase, and spray nozzles with poor mass transfer rates but low pressure drops. The invention proposes a combination of both approaches, with the purpose of trading the benefits and penalties against each other by generally placing spray nozzles upstream of packed sections. The arrangement and duties of spray nozzles to packing that achieves CO2 capture for the minimum carbon emissions intensity is at the crux of this invention.

[0098] Conventional wisdom would point towards having a fully packed section (0% spray nozzles) to maximise mass transfer, which would come at a penalty of pressure drop and huge CO2 emissions intensity due to the fan power needed. In contrast, a spray nozzles only arrangement would give an order of magnitude lower pressure drop, but to achieve the same total mass transfer duty would need to be of excessive length due to the reduction in mass transfer efficiency afforded by the contacting method. In this conventional approach, in order to deduce an optimum ratio of spray nozzles-to-packing, chemical process engineering methods are employed, typically a thermodynamic model, into which dimensional information, mass transfer coefficients / models, chemical composition, flow rates of gas and liquid sorbent are provided as inputs. From this, resultant pressure drops and mass transfer rates or solute into the liquid phase can be derived, providing two of the key parameters for the optimisation for contactor of a given absorption duty. When this is done, the model suggests a 100% spray system should be used.

[0099] However, and as noted above, in a DAC system it is critical that the absorber and the method should capture more CO2 than is used to make and operate the system. Thus in the modelling used to develop the absorber and method of the present invention, a number of additional parameters were used. These included the CO2 emissions associated with: (i) the energy needed to pump sorbent solution through the system, (ii) the CO2 emissions associated with the energy need to induce the air through the system via fans, (iii) the mass of materials used and the supply chain lifecycle emissions associated with materials production and supply, and (iv) and the supply chain lifecycle emissions associated with replacing sorbent lost in the contact sections. For the materials used element, a CO2 intensity was assumed based on the mass of each contacting section (e.g. kgCChe / kgmaterial), thus representing the CO2 expenditure of the components of the sub-system. The sorbent loss was predicted based on the form of the contact sections, and a total mass summated for the assumed operation life of the DAC plant. This is again converted to a CO2 expenditure with a suitable conversion rate (e.g. kgCChe / kgsorbentsolution). To bring this all together, the total energy usage calculated above for moving gas and liquid round the absorber, is totalled again for an assumed operational lifespan of the plant via a suitable conversion rate (e.g. kgCChe / kWh). This ultimately gives a total CO2 emissions intensity for the hybrid contactor over its operational life, which can then be used to derive the optimum configuration for the absorber. Contrary to conventional wisdom and modelling, the CO2 emissions intensity was found to reach a minimum at around 70-80% spray nozzles in terms of length as shown in Figure 5. This corresponds to a first portion having a duty of 70-98%. Correspondingly the CO2 emissions intensity was found to reach a minimum at around 20-30% packing in terms of length, i.e. a second portion duty of 2-30%.

[0100] When the model was stress tested, e.g. by varying the absorbent losses by 100 fold, the same conclusion held true.

[0101] It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.

Claims

Claims1 . An absorber for the capture of carbon dioxide (CO2) from a CO2 containing gas stream, the absorber comprising: an absorber volume to contact the CO2 containing gas stream with a sorbent, the absorber volume having a length dimension parallel to the direction of travel of the gas stream through the absorber from an inlet to an exit when in use; the absorber preferably having an inlet configured to deliver a CO2 containing gas stream containing less than 1 % volume CO2, to said first portion of the absorber, the absorber comprising at least a first portion nearest to the inlet and a second portion further from the inlet, wherein the first portion is equipped with spray nozzles configured to deliver the sorbent as an aerosol into the gas stream where it can absorb CO2 from the gas stream while droplets of the aerosol are suspended in the gas stream, and the second portion contains packing material arranged to be wetted by sorbent, wherein the first portion and second portion each have a respective duty which together contribute to a required absorber duty defined as a rate of capture of CO2.

2. The absorber of claim 1 , wherein the duty of the first portion and the duty of the second portion are selected to minimise the pressure drop across the absorber while providing the required absorber duty.

3. The absorber of claim 1 or claim 2, wherein the second portion is configured to provide between 2 and 40% and still more preferably between 2% and 30% of the absorber duty.

4. The absorber of any previous claim, wherein the first portion is configured to provide between 50 to 98% and still more preferably between 70% and 98% of the absorber duty.

5. The absorber of any previous claim, wherein the second portion contains packing material that is wetted by sorbent, preferably fresh sorbent.

6. The absorber of any previous claim, wherein the second portion is configured to capture drift from the first portion.

7. The absorber of any previous claim, further comprising a drift eliminator downstream of the second portion.

8. The absorber of any previous claim, further comprising a third portion between the first portion and the second portion, the third portion containing no packing, or packing material arranged differently to the packing in the second portion.

9. The absorber of claim 8, wherein the third portion contains packing material and is arranged to be only wetted by sorbent that has been carried from the first portion as an aerosol suspension.

10. The absorber of any previous claim, wherein the first portion contains no packing material.11 . The absorber of any previous claim, wherein there is no portion containing packing material prior to said first portion.

12. The absorber of any previous claim, wherein all the portions equipped with spray nozzles configured to deliver the sorbent as an aerosol into the gas stream are nearest to the inlet of the absorber.

13. The absorber of any previous claim, wherein all the portions containing packing material arranged to be wetted by sorbent are nearest to the outlet of the absorber.

14. The absorber of any previous claim, wherein all the portions equipped with spray nozzles configured to deliver the sorbent as an aerosol into the gas stream precede all the portions containing packing material arranged to be wetted by sorbent are nearest to the outlet of the absorber.

15. The absorber of any previous claim wherein the CO2 containing gas stream flows perpendicular to the sorbent flow.

16. The absorber of any previous claim wherein the CO2 containing gas stream flows in a horizontal direction.

17. The absorber of any previous claim wherein the CO2 containing gas stream is ambient air.

18. The absorber of any previous claim, wherein the absorber is not connected to an upstream washing unit.

19. The absorber of any previous claim, wherein said absorber is connected to a desorber for recovery of said CO2 and sorbent.

20. A direct air CO2 capture unit comprising the absorber of any one of claims 1 to 19.

21. A method of capture of carbon dioxide (CO2) from a CO2 containing gas stream, the method comprising: providing an absorber for contacting sorbent with the CO2 containing gas stream, supplying sorbent as an aerosol to one portion of the absorber using spray nozzles, and supplying sorbent to a second portion of the absorber onto packing, wherein the first portion and second portion each have a respective duty which together contribute to a required absorber duty defined as a rate of capture of CO2.

22. The method of claim 21 , further comprising providing a CO2 containing gas stream containing less than 1 % volume CO2, preferably ambient air.

23. The method of claim 21 or 22, wherein the duty of the first portion and the duty of the second portion are selected to minimise the pressure drop across the absorber while providing the required absorber duty.

24. The method of any one of claims 21 to 23, wherein the second portion is configured to provide between 2 and 40% and still more preferably between 2% and 30% of the absorber duty.

25. The method of any one of claims 21 to 24, wherein the first portion is configured to provide between 50 to 98% and still more preferably between 70% and 98% of the absorber duty.

26. The method of any one of claims 21 to 25, wherein the sorbent supplied to said second portion of the absorber is fresh sorbent.

27. The method of any one of claims 21 to 26, further comprising capturing drift from the first portion in said second portion.

28. The method of any one of claims 21 to 27, further comprising capturing drift in a drift eliminator downstream of the second portion.

29. The method of any one of claims 21 to 28, comprising capturing sorbent that has been carried from the first portion as an aerosol suspension on a third portion, wherein said third portion is between the first portion and the second portion, and the third portion contains packing material arranged differently to the packing in the second portion.

30. The method of any one of claims 21 to 29, wherein the first portion contains no packing material.

31. The method of any one of claims 21 to 30, wherein there is no portion containing packing material prior to said first portion32. The method of any one of claims 21 to 31 , wherein all the portions equipped with spray nozzles configured to deliver the sorbent as an aerosol into the gas stream are nearest to the inlet of the absorber.

33. The method of any one of claims 21 to 32, wherein all the portions containing packing material arranged to be wetted by sorbent are nearest to the outlet of the absorber.

34. The method of any one of claims 21 to 33, wherein all the portions equipped with spray nozzles configured to deliver the sorbent as an aerosol into the gas stream precede all the portions containing packing material arranged to be wetted by sorbent are nearest to the outlet of the absorber.

35. The method of any one of claims 21 to 34, wherein the CO2 containing gas stream flows perpendicular to the sorbent flow.

36. The method of any one of claims 21 to 35, wherein the CO2 containing gas stream flows in a horizontal direction.

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