Hollow fiber reactor for carbon capture and method thereof

Abstract

The present invention relates to structures for and methods of carbon capture, particularly separation of gaseous carbon dioxide from a gas mixture by a cyclic adsorption and desorption process using a sorbent material for carbon dioxide adsorption. We describe a hollow fiber for adsorption and desorption of carbon dioxide, comprising: a cylindrical hollow fiber structure capable of carbon dioxide sorption, where a cross-section of the hollow fiber structure is substantially symmetrical; and a semi-permeable layer that is impermeable to a liquid heat transfer medium but permeable to a vapor state of the heat transfer medium, where the semi-permeable layer is disposed on a surface of the hollow fiber structure adjacent to a lumen of the hollow fiber. We also describe a method for cyclic adsorption and desorption of carbon dioxide by exposing the hollow fiber to ambient air. CO2 in the ambient air is adsorbed to the hollow fiber.

Description

[0001] HOLLOW FIBER REACTOR FOR CARBON CAPTURE AND METHOD THEREOF

[0002] Field of the Invention

[0003] The present invention relates to carbon capture. In particular, the present invention relates to structures for and methods of carbon capture. More particularly, the present invention relates to materials for separation of gaseous carbon dioxide from a gas mixture by a cyclic adsorption and desorption process using a sorbent material for carbon dioxide adsorption; and to such methods.

[0004] Background of the invention

[0005] Direct air capture (DAC) is a technique to remove or capture carbon dioxide from air in order to reduce its atmospheric concentration and thus to mitigate the effects of global warming and climate change. One of the most dominating technologies of capturing carbon dioxide from the air (or other carbon dioxide-containing gas mixture) is through carbon dioxide adsorption onto a sorbent material having an affinity for binding carbon dioxide. The process comprises an adsorption phase in which the gas mixture containing carbon dioxide contacts the carbon dioxide affine sorbent material at ambient atmospheric conditions so that at least part of the carbon dioxide is bound at the surface of the sorbent material until it is sufficiently enriched or saturated. Afterwards, in a second, desorption phase, the carbon dioxide enriched sorbent material is heated, set under vacuum subject to an electrical current, contacted with a humid steam, and / or a separate purge gas is streamed along the sorbent material which removes the carbon dioxide from the sorbent material. It may be appreciated that various other processes may be used to release carbon dioxide from the sorbent material. After separation from the purge gas carbon dioxide is obtained in a concentrated form. The carbon dioxide may then be utilised in some way, for example, for synthetic fuel production, green concrete production, carbonation of beverages, use in a greenhouse, sequestered, or transported to a storage facility for later use or integrated into a desalination process or integrated in a data center. Many methods and apparatus have been proposed for executing the DAC process.

[0006] Hollow fiber (hollow fiber) membranes are extensively used in the field of fluid separation and purification. Some of their important applications include gas separation, water purification, desalination of seawater, extracorporeal blood treatment. The membranes have an intricate porous surface, the parameters of which (specific BET (Brunauer-Emmett- Teller) surface area, pore size distribution and porosity) govern the efficacy of hollow fiber membranes. Tunability of these surface parameters allows tailoring of hollow fibers for specific applications. Pore structure distribution can classify the hollow fibers into microfiltration, ultrafiltration, nanofiltration, and reverse osmosis membranes. Bulk-scale production and manufacture of hollow fiber membranes and modules has led to the commercialization of this technology in several fields. Widespread applications in gas separation membranes, hollow fiber membrane research for selective gas adsorption cycles at a nascent stage of development.

[0007] The inside space of a hollow fiber, referred to as a lumen, allows for a flow of a fluid, that is separated from the exterior of the hollow fiber.

[0008] LIS8133308 describes a fiber-based contactor including aligned hollow fibers for the adsorption of carbon dioxide from a flue gas stream. The flue gas flows through the contactor across the surfaces of the hollow fibers during an adsorption phase, during which cool water may flow through a manifold for the hollow fibers and through the lumen of each fiber and warm water flows out of the hollow fiber and exits the contactor. During a desorption phase, steam flows through the lumen of the hollow fibers and spent steam flows out of the hollow fiber and exits the contactor. The hollow fibers of LIS8133308 include tortuous pathways in contact with sorbent and a barrier layer coated within the lumen of the hollow fiber for preventing fluid communication between the lumen and the pathways. As such, only heat is transferred from the steam in the lumen to the tortuous pathways and the sorbent. However, carbon dioxide released from the fiber sorbents during desorption may remain trapped in the tortuous pathways in concentrated or pure form, limiting the driving force for its desorption and slowing down the desorption kinetics.

[0009] US9316123 discloses a sorption unit including a hollow fiber absorbent using zeolite as the sorbent material. A flue gas flows over the hollow fiber. When carbon dioxide is adsorbed to the zeolite, heat is released. The heat of enthalpy of sorption is transferred to cold water to heat the cold water to preheated water feedwater. At a certain saturation of the sorbent (or other performance characteristics), the sorbent unit transitions into desorption mode. Steam / water enters the sorbent unit in the hollow centre core of the zeolite tube for heating the sorbent. US9316123 describes an impermeable barrier of, for example, latex or thin, waterproof barrier, that lines the walls of the hollow void and keeps the liquid or fluid in the hollow void from entering the zeolite core, thus potentially contaminating the zeolite. While it is contemplated to use nitrogen to sweep the carbon dioxide from the zeolite, US9316123 does not disclose allowing for steam to pass through the barrier so as to reduce the carbon dioxide content in the zeolite. Further, neither LIS8133308 nor US9316123 contemplate the capture of carbon dioxide from ambient air in a direct air capture process.

[0010] WO 2022 / 245790 describes a sorbent article having a sorbent region, a desorbing media region, and a selective barrier layer positioned between the two regions. It is disclosed that carbon dioxide could be adsorbed and desorbed in the sorbent region in a temperature swing adsorption I desorption direct air capture process. The desorbing media disclosed is steam, which is selectively allowed to permeate through the sorbent region to assist in desorption (either temperature or humidity swing). The selective barrier layer may be selectively permeable to heat from the steam, such that heat moves from the desorbing media region into the sorbent region to desorb CO2 from the sorbent material and may further be permeable to water vapor but selectively impermeable to liquid water. By doing this, the excess desorbing material (e.g., condensed water from the steam) can exit the sorbent article. WO 2022 / 245790 also describes using cooling media to cool the sorbent region following desorption. Polyethyleneimine is disclosed as an option for the sorbent material, as are amines. The barrier layer can be formed to a polymer such as polytetrafluoroethylene or polyethylene. WO 2022 / 245790 describes a module including multiple sorbent articles in parallel.

[0011] However, WO 2022 / 245790 does not disclose use of hollow fibers and also does not disclose using hot water as the desorbing media. Due to unequal distances from the lumen to the sorbent sites, the configuration disclosed in WO 2022 / 245790 suffers from an uneven heat supply to the sorbent. Similarly, the configuration of WO 2022 / 245790 has a substantially lower contact surface area to air per kg of sorbent relative to a hollow fiber configuration.

[0012] Furthermore, the sorbent article of WO 2022 / 245790 suffers from uneven temperature distribution along the length of the sorbent article in the direction of the desorbing media region thus limiting the length of the sorbent article. This is because the high pressure of steam at the inlet of the desorbing media region of WO 2022 / 245790 (i.e. due to high pressure drop in longer sorbent articles) would trigger more condensation at the inlet of the sorbent article as opposed to the exit.

[0013] Accordingly, there is a need for alternative articles and media for carbon capture. The present invention seeks to address the problems of the prior art. of the Invention

[0014] In its broadest sense, the present invention provides, in one aspect, a hollow fiber defining a lumen, wherein the hollow fiber includes a hollow fiber structure capable of sorption of carbon dioxide and a semi-permeable layer adjacent to the lumen of the hollow fiber that is impermeable to a liquid heat transfer medium but permeable to a vapor state of the heat transfer medium and a method for capturing carbon dioxide using the hollow fiber.

[0015] In one aspect, the present invention provides a hollow fiber for adsorption and desorption of carbon dioxide, comprising: (a) a cylindrical hollow fiber structure capable of carbon dioxide sorption, where a cross-section of the hollow fiber structure is substantially symmetrical; and (b) a semi-permeable layer that is impermeable to a liquid heat transfer medium but permeable to a vapor state of the heat transfer medium, where the semi- permeable layer is disposed on a surface of the hollow fiber structure adjacent to a lumen of the hollow fiber.

[0016] In certain embodiments, the semi-permeable layer includes non-polar polymers.

[0017] In some examples, the non-polar polymers are rubbery or glassy at ambient conditions.

[0018] In some embodiments, the semi-permeable layer comprises an ethylene propylene diene monomer rubber, a polychloroprene, a polystyrene, or a styrene-butadiene.

[0019] Optionally, the semi-permeable layer includes ethylene propylene diene monomer (EPDM) rubber.

[0020] Further optionally, the semi-permeable layer comprises an ethylene propylene diene monomer rubber formed by cross-linking ethylene propylene diene monomer with organic peroxides or inorganic peroxides, for example, benzoyl peroxide, dicumyl peroxide, tertbutylperoxybenzoate, or lauroyl peroxide, or combinations thereof.

[0021] In some embodiments, the hollow fiber structure includes a polymer matrix.

[0022] In some examples, the polymer matrix is functionalized with nucleophilic groups.

[0023] In some embodiments, the hollow fiber structure includes at least one filler. Optionally, the at least one filler is at least one filler selected from metal organic frameworks (MOFs), covalent organic frameworks (COFs), zeolites, ion-exchange resins, alumina, silica, activated carbon, and inorganic salts.

[0024] In certain embodiments, the at least one filler is functionalized with nucleophilic groups or an aminosilane.

[0025] In some examples, the nucleophilic groups are at least one nucleophilic group selected from amines, amide salts, alcohols, alkoxides, thiols, and metal alkyls.

[0026] In certain examples, the nucleophilic groups are amine groups.

[0027] In some examples, the amine groups comprise small molecule amines.

[0028] Optionally, the amine groups comprise small molecule amines selected from the group comprising ethylenediamine (EDA), 1,3-propylenediamine (PDA), meta-xylylenediamine (meta-XyDm), para-xylylenediamine (para-XyDm), triethylenetetramine (TETA), tetraethylenepentamine (TEPA), and pentaethylenehexamine (PEHA).

[0029] In some examples, the amine groups comprise polymeric amines; optionally polymeric amines selected from the group comprising branched polyethyleneimine and polyallylamine.

[0030] In some embodiments, the polymer matrix has pore sizes in the range of 20 - 60 nanometers.

[0031] In some embodiments, the semi-permeable layer has a water vapor permeability of 300 Barrer or less or 1000 Barrer or less.

[0032] In some embodiments, the semi-permeable layer has a thickness of between 50 - 500 micrometers.

[0033] In some embodiments, the semi-permeable layer comprises a plurality of layers. In some examples, at least some of plurality of layers have a different composition.

[0034] In some examples, the polymer matrix is immediately adjacent to the semi-permeable layer. In a further aspect, the present invention provides a method for cyclic adsorption and desorption of carbon dioxide, the method comprising: (a) exposing a hollow fiber to ambient air, where the hollow fiber includes a polymer matrix that is functionalized with nucleophilic groups and a semi-permeable layer coated on the lumen of the hollow fiber, whereby carbon dioxide in the ambient air is adsorbed to the hollow fiber; (b) introducing a first flow of liquid heat transfer medium at a first temperature to the lumen of the hollow fiber, where the first flow of heat transfer medium is introduced to the lumen substantially in the absence of steam, whereby the hollow fiber is heated and at least part of the adsorbed carbon dioxide is desorbed from the at least one of the plurality of hollow fibers, and where the semi- permeable layer substantially prevents a flow of liquid heat transfer medium from contacting the polymer matrix in liquid form and allows a flow of heat transfer medium vapor to fluidly communicate with the polymer matrix; and (c) introducing a second flow of heat transfer medium at a second temperature through the lumen of the hollow fiber, where the first temperature is higher than the second temperature, whereby the hollow fiber is cooled by the second flow of heat transfer medium and where the first and second flows of heat transfer medium are introduced to the lumen in a first direction and the hollow fiber is exposed to the ambient air in a second direction at an angle relative to the first direction.

[0035] In some embodiments, the heat transfer medium is water.

[0036] In certain embodiments, the water is introduced to the lumen substantially in the absence of steam.

[0037] In some examples, the first flow of heat transfer medium is at a temperature in the range of 45 tO 110 °C, or 70 to 110 °C.

[0038] In some examples, the second temperature is a temperature between 5 to 40 °C.

[0039] In preferred embodiments of the methods of the present invention, the hollow fiber is a hollow fiber as defined above.

[0040] In a yet further aspect, the present invention further provides a method of improving adsorption and desorption of carbon dioxide on a polymer matrix, the method further comprising functionalizing the polymer matrix with nucleophilic groups.

[0041] In certain embodiments, the nucleophilic groups are at least one nucleophilic group selected from amines, amide salts, alcohols, alkoxides, thiols, and metal alkyls. In some examples, the nucleophilic groups are amine groups.

[0042] In some examples, the nucleophilic groups are mono-, di-, tri-, tetra- or poly- amino functionalized alkoxy-amino-silanes.

[0043] Optionally, the amine groups comprise small molecule amines.

[0044] In some examples, the small molecule amines are selected from the group comprising ethylenediamine (EDA), 1 ,3-propylenediamine (PDA), meta-xylylenediamine (meta-XyDm), para-xylylenediamine (para-XyDm), triethylenetetramine (TETA), tetraethylenepentamine (TEPA), and pentaethylenehexamine (PEHA).

[0045] In some examples, the amine groups include polymeric amines; optionally polymeric amines selected from the group comprising branched polyethyleneimine and polyallylamine.

[0046] The above and other aspects of the invention will now be described in further detail, by way of example only, with reference to the following examples and the accompanying drawings, in which:

[0047] Figure 1 is a schematic perspective view of a section of an embodiment of a hollow fiber in accordance with the present invention;

[0048] Figure 2 shows scanning electron micrographs of two examples of hollow fibers of embodiments of the present invention;

[0049] Figure 3 is a schematic representation of an adsorption phase in use of an embodiment of a hollow fiber in accordance with the present invention;

[0050] Figure 4 is a schematic representation of a desorption phase in use of an embodiment of a hollow fiber in accordance with the present invention;

[0051] Figure 5 is a schematic representation of a cooling phase in use of an embodiment of a hollow fiber in accordance with the present invention; Figure 6 is a qualitative graph illustrating generation of a decreasing gradient of chemical potential of the heat transfer medium across the structure of an embodiment of a hollow fiber in accordance with the present invention; and

[0052] Figure 7 is a schematic representation of a direct air capture system including multiple reactors fluidically coupled to manage the flow and temperature of water for various operational phases of the reactors;

[0053] Figure 8 is a schematic showing two reactors operating in parallel, enabling simultaneous adsorption and desorption processes to optimize efficiency; and

[0054] Figure 9 is a temperature cycling graph illustrating flow and temperature management of water during the desorption and cooling phases in a system comprising multiple hollow fiber reactors.

[0055] Detailed Description

[0056] Example embodiments are described below in sufficient detail to enable those of ordinary skill in the art to embody and implement the apparatus, systems and processes described herein. It is to be understood that embodiments can be provided in many alternate forms and the invention should not be construed as limited to the specific embodiments and examples set forth herein but by the scope of the appended claims.

[0057] With respect to terminology of inexactitude, the terms “about” and “approximately” may be used, interchangeably, to refer to a measurement that includes the stated measurement and that also includes any measurements that are reasonably close to the stated measurement. Measurements that are reasonably close to the stated measurement deviate from the stated measurement by a reasonably small amount as understood and readily ascertained by individuals having ordinary skill in the relevant arts. Such deviations may be attributable to measurement error, differences in measurement and / or manufacturing equipment calibration, human error in reading and / or setting measurements, minor adjustments made to optimize performance and / or structural parameters in view of differences in measurements associated with other components, particular implementation scenarios, imprecise adjustment and / or manipulation of objects by a person or machine, and / or the like, for example. In the event it is determined that individuals having ordinary skill in the relevant arts would not readily ascertain values for such reasonably small differences, the terms “about” and “approximately” can be understood to mean plus or minus 10% of the stated value.

[0058] As used herein "hollow fiber" refers to a semi-permeable tubular element with an axial lumen. The hollow fiber has a lumen side and a shell side. The hollow fiber may have a cross-sectional shape that is cylindrical, but may have an alternative cross-sectional shape, which may be symmetrical or asymmetrical, providing that it defines a lumen within.

[0059] As used herein, “hollow fiber structure” refers to the structural component of the hollow fiber structure that is capable of carbon dioxide sorption without any lumen coating. It is to be understood that the hollow fiber structure may itself be capable of carbon dioxide capture or may have sites that may be functionalized to be capable of carbon dioxide capture.

[0060] As used herein, “additives” may refer to the reagents added to a dope mixture to tune the surface morphology of the hollow fiber during the spinning process via non-solvent induced phase separation. Examples of surface morphology include without limitation pore size and surface area of the hollow fiber.

[0061] As used herein, a “filler” may refer to an organic and inorganic matrix material added, dispersed through, or otherwise incorporated into a hollow fiber structure. Non-limiting examples of fillers include, but are not limited to, an ion exchange resin, such as a strongly basic anion exchange resin (e.g., DowexTM, MarathonTM A, di- and multi-amines, polyethyleneimine, or another suitable carbon dioxide adsorbing material, such as desiccant, carbon molecular sieve, carbon adsorbent, graphite, activated alumina, molecular sieve, aluminophosphate, silicoaluminophosphate, zeolite adsorbent, ion exchanged zeolite, hydrophilic zeolite, hydrophobic zeolite, modified zeolite, natural zeolites, faujasite, mordenite, metal-exchanged silico-aluminophosphate from Dow Chemical Company, etc.), zeolite, activated carbon, alumina, uni-polar resin, bi-polar resin, aromatic cross-linked polystyrenic matrix including but not limited to poly-styrene based ion-exchange resins and amine functionalized resins, brominated aromatic matrix, methacrylic ester copolymer, graphitic adsorbent, carbon fiber, carbon nanotube, nano-materials, metal salt adsorbent, perchlorate, oxalate, alkaline earth metal particle, metal oxides (zinc oxide, titanium dioxide, zirconium dioxide, magnesium oxide, silicon dioxide, iron oxide, aluminium oxide, copper oxide, cerium oxide, yttrium oxide, hafnium dioxide, beryllium oxide, chromium oxide), chemisorbent, amine, organo-metallic reactant, hydrotalcite, silicalite, zeolitic imidazolate framework, covalent organic framework (COF) and metal organic framework (MOF) adsorbent compounds, and combinations thereof. As used herein, “mixed matrix hollow fiber structures” refer to hollow fibers including filler materials. The filler materials (both aminated and non-aminated variants) may be dispersed throughout the hollow fiber structure during the fiber production process.

[0062] As used herein, “liquid heat transfer medium” may comprise water or any suitable liquid that takes part in heat transfer by serving as an intermediary in cooling one side of a process, transporting and storing thermal energy, and heating on another side of a process. For example, demineralized water, water dosed with a conditioning agent, which may be or comprise a corrosion inhibitor, an anti-frost additive, a biocide treatment, a boiling point adjuster, for example silicone oil, propylene glycol, ethylene glycol, polyethylene glycol (PEG), a salt, or other water miscible ingredient may be added to water or used alone. Other examples of chemical fluids that may be suitable include m-Xylene, ethyl benzoate, o- Xylene, decamethyltetrasiloxane (MD2M), and undecane. Alcohols that may be suitable include but are not limited to methanol, ethanol, t-butanol, 2-propanol, 1-propanol, 2-butanol, t-amyl alcohol, i-butanol, 1 butanol, i-amyl alcohol, 2 ethylbutanol, 2-ethylhexanol, other alcohol, mixtures thereof, and mixtures thereof with water. Hydrocarbons that may be suitable include but are not limited to heptane, octane, chlorobenzene, p-cymene, and tetralin.

[0063] As used herein, “coated hollow fiber” refers to a hollow fiber having an inner surface adjacent to a lumen of the hollow fiber having a semi-permeable layer that is impermeable to a liquid heat transfer medium but permeable to a vapor state of the heat transfer medium.

[0064] As used herein "nucleophile” is a chemical species that forms bonds by donating an electron pair. Examples of nucleophilic groups include amines, amide salts, alcohols, alkoxides, thiols, and metal alkyls.

[0065] As used herein, “unfunctionalized hollow fiber structure” refers to the structural component of a hollow fiber as prepared after spinning prior to incorporation of additional functional or reactive groups apart from those present in the polymer skeleton and I or the mixed matrix hollow fiber structure, on either the lumen side or the shell side of the hollow fiber structure.

[0066] As used herein, “functionalized hollow fiber structure” refers to a hollow fiber structure that may or may not contain fillers as obtained after reaction with nucleophilic molecules, such as amines. As used herein, “process heat” refers to the lower temperature heat remaining after the higher temperature heat has been used to generate electricity. Moreover, “process heat” may be provided from the use of sources of energy to produce products other than power or electrical generation. For example, primary processing such as chemical processing, production of cement, steel or aluminum, production of energy products like coal to liquid energy products, refining, may use heat to drive the primary processing, and the unused heat remaining after the primary processing or created during the primary processing would be the process heat of such processing.

[0067] As used herein, the term "ambient air" is defined herein as air at pressure, temperature, and carbon dioxide presence conditions that the hollow fiber capture unit of the present disclosure is exposed to when outside. Ambient air pressure conditions typically include pressures in the range from 0.8 to 1.1 bar absolute. Ambient air temperature conditions typically include temperatures in the range of -40 to 60 °C, more typically -30 to 45 °C Carbon dioxide conditions present in ambient air typically include concentrations ranging between 380 ppm and 1000 ppm.

[0068] Structure of the hollow fiber

[0069] An example hollow fiber 101 is shown in Figure 1. The hollow fiber 101 comprises a structure for capture of carbon dioxide sorption, indicated at 102, and a lumen, indicated at 103, extending axially therethrough. A lumen side 104 and a shell side 105 of the hollow fiber 1010 are indicated. The lumen side 104 may be considered an “interior” side and the shell side 105 an “exterior” side. Optionally, and in this illustrated example, the hollow fiber 101 comprises a semi-permeable barrier layer 106, shown radially disposed between the structure for capture of carbon dioxide sorption 102 and the lumen 103.

[0070] In some embodiments, the structure for capture of carbon dioxide sorption may include a polymer matrix functionalized with nucleophilic groups. Examples of polymer matrices that may be suitable for functionalization by nucleophilic groups include without limitation polyetherimide (PEI), polyvinylchloride (PVC), polyimides (Pls), poly(vinylbenzyl chloride), polybenzimidazole, and chloropolyphenyleneoxide.

[0071] In some embodiments, the structure for capture of carbon dioxide sorption may include a polymer and one or more fillers, for example, metal-organic frameworks (MOFs), zeolites, ion-exchange resins, aromatic cross-linked polystyrenic matrix including but not limited to poly-styrene based ion-exchange resins and amine functionalized resins, activated carbon, alumina, silica, inorganic nanoparticles, and any other suitable filler. Such structures may rely on polymers for structural properties rather than for sites where functionalization may occur. In these cases, functionalization may occur on the filler additive instead of the polymer matrix. As such, the polymer for structures with fillers may be different from structures that include a polymer matrix functionalized with nucleophilic groups. Polymers that may be suitable for structural support of fillers additives may include but not be limited to polypropylene (PP), polybenzimidazole (PBI), polyvinylenedifluoride (PVDF), polysulfone (PSU), polyethersulfone (PES), polyphenyleneoxide (PPO), polytetrafluoroethylene (PTFE), combinations thereof or any other suitable polymer may be used.

[0072] Nucleophilic groups that may be suitable for functionalization of the polymer matrix and I or the fillers additives include, without limitation, amines, amide salts, alcohols, alkoxides, thiols, and metal alkyls.

[0073] In one exemplary embodiment, the nucleophilic groups may include amine groups.

[0074] Polymeric amines may benefit from more sites for chemisorption of carbon dioxide; however, such polymeric amines may be branched and / or bulky, which may impede reaction conversion to the nucleophilic group due to pore blockages and potential for crosslinking. Subsequently, the surface underneath the aminated region of polymeric amines may have reduced access to free amines to allow further functionalization. Oppositely, small molecule amines may demonstrate faster diffusion coefficients compared to the polymeric amines, leading to a more complete functionalization. As such, functionalization using a mixture of two or more amines may be beneficial. For example, the polymer matrix and / or the fillers additives may be functionalized with a mixture of small molecule and polymeric amines. Small molecule amine groups may include ethylenediamine (EDA), 1 ,3-propylenediamine (PDA), meta-xylylenediamine (meta-XyDm), para-xylylenediamine (para-XyDm), triethylenetetramine (TETA), tetraethylenepentamine (TEPA) or pentaethylenehexamine (PEHA), any other suitable small molecule amine group, or combinations thereof. Polymeric amine groups may include branched polyethyleneimine, polyallylamine, polyaniline (PANI), any other suitable polymeric groups, or combinations thereof.

[0075] Furthermore, in some embodiments, hollow fibers may be coated, such that an inner surface adjacent to the lumen of the hollow fiber has a barrier layer. In one example, the barrier layer may be a semi-permeable layer that is impermeable to a liquid heat transfer medium but permeable to a vapor state of the heat transfer medium. For instance, the liquid heat transfer medium may be liquid water, and a vapor state of the heat transfer medium may be water vapor. As such, the semi-permeable layer may be formed of a material that substantially prevents the passage of the liquid heat transfer medium (e.g., liquid water, etc) but allows for the passage of a vapor state of the heat transfer medium (e.g., water vapor, etc).

[0076] The semi-permeable layer material may be chosen based on certain criteria. For example, the solvent used for the coating should not chemically or physically modify the hollow fiber microstructure, which would result in loss of chemical integrity and alteration of the surface morphology of the hollow fiber, respectively. Suitable materials include non-polar polymers, particularly non-polar polymers which are rubbery or glassy at ambient conditions. As examples, suitable materials may include ethylene propylene diene monomer (EPDM) rubber (ethylene propylene diene monomer cross-linked with organic peroxides such as Di- (4-methylbenzoyl)-peroxide, Dibenzoyl peroxide, 1 ,1-Di-(tert-butylperoxy)-3,3,5- trimethylcyclohexane, tert-Buty dicumyl peroxide, tert-butylperoxybenzoate, lauroyl peroxide, radical initiators or combinations thereof), polychloroprene, polystyrene, and styrenebutadiene. Other suitable materials will be apparent to the skilled person.

[0077] The lumen coating should have a water vapor permeability below 3000 Barrer (1 Barrer = 3.348 x 10-16 mol.m / (m2.s.Pa)). Coatings with water vapor permeability between 1000 - 3000 Barrer could also be used while thicker lumen coating would be required to inject the same amount of steam. Ideally polymeric coatings with water vapor permeability < 1000 Barrer would be the most suitable for the desired steam injection as they would not warrant thicker lumen coating which would increase the thermal weight of the corresponding HF.

[0078] In certain examples, the water vapor permeability of the semi-permeable layer is selected to be less than 100-1000 Barrer (1 Barrer = 3.348 x 10-16 mol.m / (m2.s.Pa)). A semi-permeable layer having a thickness of between 10 -100 micrometers has been determined to be particularly suitable in the processes of the present invention.

[0079] The lumen coating material should present durable adhesion to the underlying hollow fiber structure. The lumen material should also be able to withstand water and steam at 70 - 100 °C, preferably at 90 - 100 °C. Polymers having rubber-like characteristics are considered to be more appropriate for our requirements as they are less prone to defects and stress- induced cracking.

[0080] In another embodiment of the invention, polymer composite lumen coatings may be used to coat the inner lumen side of the hollow fiber structures. Polymer composites are multi-phase systems consisting of particles dispersed in a polymer matrix. If the dispersed particle has a dimension in the nano scale, then this system may be referred to as a polymer nanocomposite. Typically, addition of these composites in the polymer matrix may alter the effective diffusion pathway of the molecule as the addition of such particles to the polymer matrix or coating may lead to change in the tortuosity within the polymer matrix. These alterations to the material tortuosity may influence the diffusion path length, porosity, mechanical properties, pore structure and volume. Changes to the tortuosity may also exert control on the permeability characteristics of the polymer coating. The permeating molecule (eg. water in this invention) may encounter the composite materials embedded in the polymer matrix. These interactions with the composite materials may present additional resistance or hindrance to the pathway of the water molecule. Therefore the diffusion pathlength of the permeating water molecule may be significantly increased and thus may affect the permeability of the coating material. Since selective water vapor permeation is central to the controlled steam injection and generation aspect of our injection, addition of composites to the lumen coating may provide another tunable handle to tune the amount of steam generated during the desorption process. The diffusion of the water vapor molecule may be governed by the composite loading in the lumen coating as well as the lumen polymer and the thickness of the lumen coating. The composite lumen coating may be advantageous in creating a very thin lumen coating on the inner wall of the hollow fiber structure.

[0081] A very small composite incorporation (<5% by weight) may be sufficient to bring about reinforcement behaviour. This may be also accompanied by significant changes in the mechanical and permeability properties of the polymeric composite. Typically, for achieving control on the permeability and the mechanical properties, nano fillers such as layered silicates (clays such as bentonite, montmorillonite and sepiolite, talc, etc.), natural fibres (sisal, agave, cellulose, banana, etc.), particulates like TiC>2, FeTiCh, Fe3C>4, Cr2O3, SiC>2, CaCCh, ZnO, montmorillonites, bentonite, Laponite, oxide coated mica substrates (Sn, Sb, Fe, Ti oxides) etc., speciality materials like carbon nanotubes (CNT; or multi wall nano tubes- MWNT), graphene, fullerenes, etc. may be used. Dispersion of non-soluble polymeric microparticles in a given polymer matrix may also be used to prepare polymeric-composite coating materials. The composite particles may be loaded in the lumen coating solution with a concentration varying between 1 - 20 wt%.

[0082] Additionally, incorporation of inorganic composite particles such as silver-, copper-, zinc- based additives may result in a polymeric coatings with anti-microbial properties. Typically, silver-based additives are used as anti-bacterial and anti-fungal agents. Copper-based composites are often exploited due to their anti-microbial properties, while zinc-based composite particles are highly effective against fungi and are used in coatings and rubber products. Therefore, addition of such inorganic composite particles in the lumen coating might be useful to prevent microbial growth and scaling on the lumen side of the hollow fiber structures which may increase the durability and the longevity of the hollow fibre structures and may also warrant lower maintenance of the sorbent material.

[0083] The composite particles that may be used in this embodiment are not limited to nano fillers such as layered silicates (clays such as bentonite, montmorillonite and sepiolite, talc, etc.), natural fibres (sisal, agave, cellulose, banana, etc.), particulates like TiC>2, FeTiCh, Fe3C>4, Cr2C>3, SiC>2, CaCCh, ZnO, montmorillonites, bentonite, laponite, oxide coated mica substrates (Sn, Sb, Fe, Ti oxides) etc., speciality materials like carbon nanotubes (CNT; or multi wall nano tubes- MWNT), graphene, fullerenes, and inorganic composites of copper, silver, or zinc.

[0084] EXAMPLE 1. Ethylene Propylene Diene Monomer (EPDM) Lumen Coating

[0085] In one embodiment, lumen coating experiments were initiated with ethylene propylene diene monomer (EPDM) rubber. While EPDM is elastic in nature, it is stable in petroleum ether and does not impact the underlying hollow fiber structure. It is stable towards steam and water at elevated temperatures. EPDM solutions commercially available as Trilene® 65 were used in the presence of various thermally initiated crosslinkers. The unfunctionalized hollow fiber structures were placed in a stainless steel module and were coated in situ with the coating solution. The module with 30 cm length, 0.012 m2 inner surface area and 1 inch connections was equipped with 5 - 12 hollow fiber structures. The hollow fiber structures were epoxyed at the top and bottom to ensure that they are not mobile during the treatment. In our case, up to 5 cm from the top and bottom of the hollow fiber structures were epoxyed in the module. The effective length of the hollow fiber structure in the module varied between 20 - 23 cm. The coating solution (Trilene® 65 and crosslinkers) was passed against gravity through the hollow fiber structures module. Subsequently, the module was flipped upside down and the coating solution was passed from the other end of the module (again against gravity). Following this nitrogen gas was passed through the module to remove any excess EPDM solution. Finally, the module was cured in the vacuum oven at temperatures varying between 90 - 160 °C. Two - three coating attempts were applied to ensure a lumen coating of homogeneous thickness. In the demonstrated embodiment, a coating solution of Trilene® 65 and dicumyl peroxide (DCP crosslinker) was used, to provide EPDM crosslinking to obtain a durable lumen layer.

[0086] The viscosity of the lumen coating solution was optimized to prevent the pressure drop during the coating process, restrict asymmetric pore penetration, and obtain a homogeneous coating. Preliminary lumen coating experiments were carried out by varying the EPDM and crosslinker concentration in petroleum ether as set out in Table 1 below. Since the crosslinkers initiated thermal crosslinking of EPDM, curing conditioning at 160 °C for 5 - 18 hours was required.

[0087] Preliminary results obtained by using dicumyl peroxide and tert-butylperoxybenzoate (Luperlox® P (Lup P)) as crosslinkers resulted in complete pore penetration and the solution leached out to the outer side of the hollow fiber. A high curing temperature significantly decreased the viscosity of the lumen coating solution and resulted in full pore penetration.

[0088] Table 1 : Lumen coating composition for hollow fiber coating

[0089] Lauroyl peroxide (Lau) was chosen for crosslinking for further trials as the thermal treatment required milder conditions than the previously used analogues. The Lau-crosslinked coating also showed a better coating, without solution leaching or pore penetration. A CO2 / N2 selectivity of 5.6 was obtained. Scanning electron microscope (SEM) images of the EPDM coated hollow fiber structures are shown in Figure 2 and demonstrate the proof-of-concept of in-module lumen coating of the hollow fiber structure bundle. Lumen coating of varying thicknesses were obtained. In certain examples, the lumen coating had thickness varying between 50 - 500 pm. The hollow fiber structure lumen was coated multiple times to increase the lumen thickness.

[0090] In another embodiment, polymeric coatings comprising single systems i.e. devoid of crosslinker were also evaluated. Polymers with low water vapor permeability (< 1000 Barrer) and soluble in n-hexanes, toluene, xylenes, or petroleum ether were coated in a module. Lumen coatings of polychloroprene, polystyrene, styrene butadiene rubber, and polydimethylsiloxane (PDMS) were applied in the hollow fiber HF module consisting of 1 - 2 unfunctionalized HFs. Multiple lumen layer coatings were applied as per the requirements to increase the thickness of the lumen layer.

[0091] EXAMPLE 2. Polydimethylsiloxane (PDMS) Lumen Coating

[0092] In another embodiment, PDMS was used as a lumen coating for the hollow fiber structures. PDMS is widely used to heal the surface defects on the lumenbore side of the hollow fiber structures. Commercially available Sylgard® 184 (referred to as Sylgard) from Dow Chemicals was used as the PDMS elastomeric material for the coating applications. The solubility of PDMS in n-hexanes makes it feasible to be used as a coating material as hexanes would not affect the chemical and mechanical integrity of the hollow fiber structure. Sylgard solutions of concentrations varying between 5 - 60 wt% loading in hexanes were obtained with constant stirring at room temperature for 30 mins - 24 hours. Homogeneous solutions were obtained in all cases. The viscosities of the PDMS / hexanes solutions varied between 4 - 30 mPa.s. The PDMS solutions were prepared by mixing the Sylgard PDMS along with the hydrosilane crosslinker present in the Sylgard® 184 elastomeric kit. The ratio of PDMS and the crosslinker was maintained at 10:1 wt / wt% in all the PDMS / hexanes solutions.

[0093] PDMS solutions with 50 - 60 wt% PDMS content in hexanes were deemed to be highly suitable for coating the hollow fiber structures because of their suitable viscosities with acceptable pressure drop. The unfunctionalized hollow fiber structures were placed in a stainless steel or clear plastic tube module and were coated in situ with the coating solution. The module with 30 cm length, 0.012 m2 inner surface area and 1 inch connections was equipped with 1 - 14 hollow fiber structures. The hollow fiber structures were epoxyed at the top and bottom to ensure that they are not mobile during the treatment. In our case, up to 5 cm from the top and bottom of the hollow fiber structures were epoxyed in the module. The effective length of the hollow fiber structure in the module varied between 20 - 23 cm. The coating solution (Sylgard® 184 and crosslinkers in hexanes) was passed against gravity through the hollow fiber structures module. Subsequently, the module was flipped upside down and the coating solution was passed from the other end of the module (again against gravity). Following this nitrogen gas was passed through the module to remove any excess PDMS solution. Finally, the module was cured in the vacuum oven at temperatures varying between 40 - 90 °C with a constant N2 sweep gas flow. Two - three coating attempts were applied to ensure a lumen coating of homogeneous thickness.

[0094] In another embodiment, amine-functionalized hollow fiber structures were also coated with PDMS solution. The TETA- and polyethyleneimine-functionalized hollow fiber structures were constructed into the hollow fiber module containing 1 - 14 hollow fiber structures. A similar hollow fiber module creation process was used as described above. Then the coating solution (Sylgard® 184 and crosslinkers in hexanes) was passed against gravity through the hollow fiber structures module. Subsequently, the module was flipped upside down and the coating solution was passed from the other end of the module (again against gravity). Following this nitrogen gas was passed through the module to remove any excess PDMS solution. Finally, the module was cured in the vacuum oven at temperatures varying between 40 - 90 °C with a constant N2 sweep gas flow. Two - three coating attempts were applied to ensure a lumen coating of homogeneous thickness.

[0095] The integrity of the PDMS coatings were qualitatively investigated through gas permeation tests. The PDMS coated hollow fiber structures in the clear plastic modules were injected with water on the shell side by using a syringe. The shell was completely filled with water and taped to prevent flow of water through the pin hole created by the syringe. Nitrogen gas was passed through the lumen side of the hollow fiber with a pressure varying between 1 - 5 bar. The number of bubbles created on the shell surface of the fibers were visually investigated for qualitative screening of the PDMS coating. The PDMS coated hollow fibers resulted in significantly less bubbles as compared to the uncoated hollow fibers. This could be attributed to the presence of the PDMS lumen layer which significantly impedes the permeation of the nitrogen gas through the layer.

[0096] EXAMPLE 3. Ethylene Propylene Diene Monomer (EPDM) Lumen Coating with Silver Nanoparticles

[0097] In another embodiment, PDMS was used as a lumen coating for the hollow fiber structures. EPDM solutions commercially available as Trilene® 65 were used in one embodiment of the invention in the presence of Lauryl peroxide (Lau) as the thermally initiated crosslinker. EPDM with the crosslinker was charged with silver (Ag) nanoparticles with a composition of 20 / 3 / 1 wt / wt% EPDM / Lau / Ag nanoparticles balanced with petroleum ether used as a solvent. This solution mixture was used as the composite lumen coating solution.

[0098] The TETA-functionalized polyetherimide hollow fiber structures were placed in a stainless steel module and were coated in situ with the composite coating solution containing Ag nanoparticles. The module with 30 cm length, 0.012 m2inner surface area and 1 inch connections was equipped with 5 hollow fiber structures. The hollow fiber structures were epoxyed at the top and bottom to ensure that they are not mobile during the treatment. In our case, up to 5 cm from the top and bottom of the hollow fiber structures were epoxyed in the module. The effective length of the hollow fiber structure in the module varied between 20 cm. The coating solution (Trilene® 65, Lau, Ag nanoparticles dispersed in petroleum ether) was passed against gravity through the hollow fiber structures module. Subsequently, the module was flipped upside down and the coating solution was passed from the other end of the module (again against gravity). Following this nitrogen gas was passed through the module to remove any excess composite coating solution. Finally, the module was cured in the vacuum oven at temperatures varying between 100 °C. Two coating attempts were applied to ensure a lumen coating of homogeneous thickness. This demonstrated the preparation of hollow fiber structures with the composite lumen coating solutions.

[0099] Use of the hollow fibers of the invention

[0100] The method of use of the hollow fiber in carbon dioxide adsorption from a carbon dioxidecontaining fluid will now be outlined.

[0101] Figures 3, 4 and 5 illustrate an adsorption phase, a desorption phase and a cooling phase respectively of an example carbon capture process.

[0102] In Figure 3, the shell side 105 of hollow fiber 101 is shown exposed to a flow 1001 of a gas comprising carbon dioxide, in this specific example, air. The air comprising carbon dioxide contacts the hollow fiber 102, thus adsorbing the carbon dioxide to the hollow fiber and depleting the air surrounding the hollow fiber 101 of carbon dioxide. Thus, the hollow fiber 102 acts as a solid sorbent for capturing carbon dioxide from the gas. In this Figure, the gas comprising carbon dioxide is shown flowing, on the shell side 104 of the hollow fiber 101, in the direction indicated by arrow 1002, towards a first side 1003 of the hollow fiber 101 (as carbon dioxide laden gas) and flowing from a second, opposite side 1004 of the hollow fiber 101 (as carbon dioxide depleted gas); however, it is to be appreciated that the gas comprising carbon dioxide may flow across, along or around the shell side 104 of the hollow fiber 101 in any direction or directions.

[0103] Optionally, a liquid heat transfer medium (not shown) may be passed through the lumen to adjust the temperature of the hollow fiber as appropriate during the adsorption phase. Typically, adsorption occurs under ambient conditions. However, as exposure of the hollow fiber to elevated temperatures may be detrimental to the performance of the hollow fiber, the temperature of the hollow fiber may be reduced from ambient conditions during adsorption using a cold or cool water as the liquid heat transfer medium, such as water. For example, for particularly hot ambient conditions (e.g., temperatures above 40 degrees Celsius, etc.), it may be beneficial to reduce the temperature of the hollow fiber during adsorption so as to reduce oxidative degradation of the hollow fiber due to exposure to oxygen in the air at elevated temperatures.

[0104] In Figure 4, a flow 1101 of a liquid heat transfer medium, in this example comprising water, at a first, hot temperature is shown being fed into the lumen 103 of the hollow fiber 101. In some instances, the liquid heat transfer medium, such as water, may be fed into the lumen in absence of a vapor form of the heat transfer medium (i.e. water vapor). It will be appreciated that the process may generate steam naturally. However, steam generation is not a requirement of the process of the present invention.

[0105] The provision of liquid heat transfer medium substantially in the absence of the vapor form of the heat transfer medium has benefits. Taking water and steam as an example, water has a heat capacity twice that of steam, meaning that twice the mass flow rate of steam may be necessary to supply the same amount of heat assuming the same temperature drop when passing the hollow fiber. Further, steam flow through the lumen of the hollow fiber may cause pressure drop, resulting in the condensation of steam at the high pressure end of the hollow fiber being the inlet. As water is incompressible, the release of heat is independent of the water pressure, and thus the use of longer hollow fibers may be possible. Furthermore, if this steam does not condense then it will at least be more compressed than at the low pressure end of the hollow fiber resulting in different flow and heat transfer behaviour along the hollow fiber. Finally, as water allows for higher pressure drops, higher flow rates of water may be possible.

[0106] The thermal energy of the liquid heat transfer medium results in a temperature increase of the hollow fiber 101 and desorption of carbon dioxide and water from the hollow fiber 101. Single water molecules diffuse through the semi-permeable layer, leaving it as steam thereby reducing the concentration of carbon dioxide within the polymer matrix and allowing for improved desorption kinetics. This form of steam formation is called pervaporation. The desorption phase is typically carried out under vacuum.

[0107] Beneficially, mixing water with water miscible liquid heat transfer mediums having high boiling points, such as polyethylene glycol, may reduce the water permeation tendency. In this Figure, the liquid heat transfer medium is shown flowing, in the direction indicated by arrow 1102, along the lumen side 104 of the hollow fiber 101.

[0108] Beneficially, the hollow fiber may have a cross-sectional shape that is cylindrical with circular cross-sections that have substantially equal distances from the lumen to the sorbent sites, allowing for even heat supply to the sorbent.

[0109] In some embodiments of the invention, the heat demand for the desorption step was accounted for by integrating low grade heat. Typically, low grade heat sources can be integrated into the process to assist desorption. Heat sources at temperatures above 120 °C can be cooled down via a heat exchanger to the desired desorption temperature i.e. 80 - 110 °C. Process heat from industrial processes can act as sources of low grade heat. Process heat may be provided by other types of energy sources, such as, for example, fossil fuel, geothermal, nuclear, biomass, and other renewable energy sources. The term “process heat” as used herein refers to the lower temperature heat remaining after the higher temperature heat has been used to generate electricity. Moreover, “process heat” may be provided from the use of sources of energy to produce products other than power or electrical generation. For example, primary processing such as chemical processing, production of cement, steel or aluminum, production of energy products like coal to liquid energy products, refining, may use heat to drive the primary processing, and the unused heat remaining after the primary processing or created during the primary processing would be the process heat of such processing.

[0110] When low grade heat is available at temperatures below 90 °C, then a heat pump may be utilized to attain the desorption temperature conditions. Such type of heat can be obtained from cooling tower return streams at industrial processes. Low grade heat offered by dairy industries and breweries are typically within the range of 65 - 85 °C and can also be integrated into the process. Solar-heating for Industrial Processes (SHIP) offer heat sources ranging between 60 - 100 °C and could potentially offer a valuable solution to power carbon dioxide desorption in DAC processes. While SHIP integration might be intermittent as it depends on the weather patterns, it can be used to reduce the OPEX of the DAC process. In another embodiment of the invention, a two-step desorption profile was adopted. A thermal heating ramp of two isothermal steps has been selected to selectively remove desorbed or bound liquid water in the first step and carbon dioxide in the second step. The first step requires lower quality heat, being heat at a lower temperature level, than the second step, which requires higher quality heat, being heat at a higher temperature level. This operation allows potential OPEX savings if a heat pump is used, which requires less electrical energy to provide lower quality heat being at a lower temperature level.

[0111] In some embodiments, a thermal treatment of the fibers can be implemented after a certain number of cycles to improve the performance. This treatment consists of the heating of the fibers using a heating medium that flows inside the lumen side of the fibers to a temperature above the operating desorption temperature and below a temperature of 300C, preferably to a temperature between 200 and 250C. The elevated temperature is then maintained for a set amount of time, which can vary in the range from some seconds to several hours, typically between 10 seconds to 4 hours, preferably around 10 seconds, preferably around 60 seconds, preferably around 1 hour, or preferably around 4 hours. During the thermal treatment the hollow fibers are exposed to an inert atmosphere depleted of oxygen or carbon dioxide, for example, a pure nitrogen atmosphere or under vacuum. After the thermal treatment of the fibers the capturing performance of the unit is expected to significantly improve in a range between of 5 to -20%, preferably above 1%, preferably above 5%, or preferably about 10%, or preferably about 20% for various grafted amine-based sorbents with various oxide support materials.

[0112] In Figure 5, a flow 1201 of a liquid heat transfer medium, in this example comprising water, at a second, cooler temperature is shown being fed into the lumen 103 of the hollow fiber 101 , so as to cool the hollow fiber 101 in preparation for another adsorption phase. In this Figure, the liquid heat transfer medium is shown flowing, in the direction indicated by arrow 1202, along the lumen side 104 of the hollow fiber 101.

[0113] As described herein, the semi-permeable layer may be impermeable to the liquid heat transfer medium, such as liquid water, present in the lumen side of the hollow fibre, while allowing the transfer of individual heat transfer molecules. After passing through the semi- permeable layer, these molecules form a vapor phase. As such, the semi-permeable layer acts as a barrier to liquids while being permeable to vapors, and is therefore referred to as a pervaporation membrane in the field of membrane technology. In doing so, a concentration of carbon dioxide within the hollow fiber structure may be further reduced, thereby allowing for improved desorption kinetics for carbon dioxide.

[0114] For the heat transfer medium to be transported through the semi-permeable layer, the construction of the fiber allows for the generation of a decreasing gradient of the chemical potential of the heat transfer medium across the semi-permeable layer and the subsequent porous hollow fiber structure. This is illustrated schematically in Figure 5, which plots relative chemical potential of the heat transfer medium (y-axis) across different portions of the hollow fiber, from the semi-permeable layer, through the hollow fiber structure to the shell side of the fiber.

[0115] To increase the chemical potential gradient across the layers and thus the permeation rate of the heat transfer medium across said layers, a sweeping gas and / or vacuum can be applied to the shell side of the hollow fiber and / or the temperature of the heat transfer medium in the lumen side can be raised. To reduce the chemical potential gradient and thus the permeation rate of the heat transfer medium, the temperature of the heat transfer medium in the lumen side can be lowered and / or the shell side can be operated under lower vacuum or, even, atmospheric pressure. Another means to lower the heat transfer medium chemical potential in the lumen side and thus its permeation is by diluting the heat transfer medium with another liquid with a much higher boiling point.

[0116] Beneficially, mixing water with liquid heat transfer mediums having high boiling points, such as polyethylene glycol, may reduce the liquid water permeation tendency. In this Figure, the liquid heat transfer medium is shown flowing, in the direction indicated by arrow 1102, along the lumen side 104 of the hollow fiber 101.

[0117] The carbon dioxide-rich steam flow may be then processed to remove or capture the recovered carbon dioxide desorbed from the hollow fiber. The carbon dioxide may then be utilized in some way, for example, for synthetic fuel production or carbonation of beverages, use in a greenhouse, sequestration, etc.

[0118] The semi-permeable layer material was chosen based on certain criteria. First, the solvent used for the coating should not chemically or physically modify the HF microstructure, which would result in loss of chemical integrity and alteration of the surface morphology of the HF, respectively. The lumen coating should have a water vapor permeability below 5000 Barrer, (1 Barrer = 3.348 x 10'16mol.m / (m2.s.Pa)), preferably below 3000 Barrer. Coatings with water vapor permeability between 1000 - 3000 Barrer could also be used while thicker lumen coating would be required to inject the same amount of steam. Ideally polymeric coatings with water vapor permeability < 1000 Barrer would be the most suitable for the desired steam injection as they would not warrant thicker lumen coating which would increase the thermal weight of the corresponding HF. The lumen coating material should present durable adhesion to the underlying polyetherimide support. The lumen material should also be able to withstand water and steam at 90 - 100 °C. Polymers having rubberlike characteristics are considered to be more appropriate for our requirements as they are less prone to defects and stress-induced cracking.

[0119] The amine-functionalized HFs were further utilized as sorbent materials for CO2 capture from the ambient air. The amine-functionalized HF sorbent materials are suitable for selective capture or removal of CO2 from a gas stream. Since these HFs have high gravimetric surface area and pore sizes in the range of 20 - 60 nm, they are suitable for selective chemisorption of CO2 molecules. The CO2 capture performances of the HFs were studied in a tubular reactor built in-house. The reactor (0.5 x 0.035 x 11.8 in x in x in). The reactor was designed to hold a maximum of 20 HFs for the ads / des studies. The HFs were of a length of 20 cm and were potted with thermally cured epoxy prior to placement in the reactor. The resulting HF sorbent was pre-dried in the vacuum oven at 70 °C overnight to remove any pre-adsorbed CO2 and moisture.

[0120] During adsorption, the feed stream is passed through the reactor column at a rate of 1 L / min and pressure 1 bar. The feed stream is either a mixture of CO2 / N2 gases (with concentration ranging from 0 - 100% CO2 in N2, usually 400ppm) or an air stream generated by an air compressor. The humidity of the gas stream is controlled by passing the feed stream through a water humidifier while the humidities varied between 0 - 90% RH. After a certain duration the HF reactor bed got saturated i.e. breakthrough of the gas stream and that denoted the completion of the adsorption cycle. The adsorption cycle was between 0.5 - 3.0 hours while the typical adsorption duration was ~ 2.5 hours.

[0121] The captured CO2 molecules in the HF matrix were released during the desorption step where thermal energy was provided to facilitate desorption. Since exposure of the amine- functionalized sorbent to air at elevated temperatures is detrimental to the amines, the reactor was placed under vacuum swing conditions before heating the reactor. While the vacuum pressure inside the reactor varied between 20 - 200 mbar, typically 50 - 100 mbar vacuum pressure was used. In one embodiment of this invention, temperature vacuum swing assisted desorption was used to undertake desorption. Here, the HF reactor was heated using a heating and cooling jacket. The desorption temperature was chosen between 50 - 110 °C with an optimum temperature around 95 °C. A flush gas of N2 stream was used to enhance the desorption kinetics. The N2 flow rate of 0.1 - 0.2 L / min was used. The desorption was carried out for 1 - 12 hours but ideally for 2 hours.

[0122] In another embodiment of this invention, the HF reactor was connected to distinct air and water flow channels and steam-assisted desorption was carried out. Adsorption was achieved via the crossflow of the CO2 stream on the outer shell side and the desorption was carried out by passing hot water through the lumen of the HF as shown above. After the completion of adsorption, hot water (T = 80 - 100 °C) was passed through the connectors attached to the inner lumen side of the HF while the reactor was placed under vacuum. As the hot water passed through the lumen of the HF, sorbent heated up and, thus, initiated CO2 desorption. The steam permeable lumen coating allowed selective permeation of the steam generated from the hot water stream to pass through the porous matrix of the HF and escape out. During this process, the steam resulted in localized decrease of the CO2 partial pressure in the porous network which drove the desorption kinetics. The temperature of the hot water, flow rate, ID of the HF, and lumen layer thickness are optimized to achieve the steam flow rate of up to 10 kg steam / kg sorbent. Following the desorption, the HFs were cooled by passing cold water through the lumen of the HF. The HF module was cooled down to 25 - 35 °C before exposing them to iterative ads / des (adsorption / desorption) cycles.

[0123] In another embodiment of the invention, the heat demand for the desorption step was accounted for by integrating the waste heat generated by cooling towers. Typically, low grade heat sources can be integrated into the process to assist desorption. Heat sources at temperatures above 120 °C can be cooled down via a heat exchanger to the desired desorption temperature i.e. 80 - 110 °C. Process heat from industrial processes can act as sources of low grade heat. Process heat may be provided by other types of energy sources, such as, for example, fossil fuel, geothermal, nuclear, biomass, and other renewable energy sources. The term “process heat” as used herein refers to the lower temperature heat remaining after the higher temperature heat has been used to generate electricity. Moreover, “process heat” may be provided from the use of sources of energy to produce products other than power or electrical generation. For example, primary processing such as chemical processing, production of cement, steel or aluminum, production of energy products like coal to liquid energy products, refining, may use heat to drive the primary processing, and the unused heat remaining after the primary processing or created during the primary processing would be the process heat of such processing. When low grade heat is available at temperatures below 90 °C, then a heat pump may be utilized to attain the desorption temperature conditions. Such type of heat can be obtained from cooling towers at industrial processes. Low grade heat offered by dairy industries and breweries are typically within the range of 65 - 85 °C and can also be integrated into the process. Solar-heating for Industrial Processes (SHIP) offer heat sources ranging between 60 - 100 °C and could potentially offer a valuable solution to power CO2 desorption in DAC processes. While SHIP integration might be intermittent as it depends on the weather patterns, it can be used to reduce the OPEX of the DAC process.

[0124] In another embodiment of the invention, a two-step desorption profile was adopted. A thermal heating ramp of two isothermal steps was chosen to selectively desorb water in the first step and CO2 in the second step. A two-step desorption process results in obtaining a higher purity of the CO2 gas stream as compared to the single-step desorption process. Typically, the CO2 output stream is contaminated with water vapor during the desorption step in a single-step desorption process and required to be separated in the product or fluid separation tank. Therefore, a two-step desorption process would eliminate or minimize the CO2 contamination with water as well as bypass the product tank.

[0125] Results

[0126] Hollow fibers were prepared as discussed above, with a polyetherimide polymer matrix within a semi-permeable layer of EPDM rubber. The polymer matrix of one fiber was unfunctionalized to act as a control.

[0127] A carbon dioxide working capacity (CO2_cap) i.e. the difference between amount of adsorbed carbon dioxide (CO2_ads) and the amount of desorbed carbon dioxide (CO2_des) was determined for different variants of aminated hollow fibers. The CO2_cap varied between 0.05 - 0.35 mol / kg. The un-functionalized hollow fiber control (devoid of amine functionalization) demonstrated a CO2_cap of ~ 0.005 mol / kg.

[0128] It can be seen that amine functionalization by different types of amines, amine concentration, and reaction conditions may result in higher CO2_cap. An interdependence of the amine concentration, reaction temperature, type of amines, and reaction temperature on the CO2_cap was observed. The highest CO2_cap for the best-optimized amine-functionalised hollow fiber sorbent was 0.35 mol / kg. This demonstrates the successful synthesis of carbon dioxide capture active sorbents from the carrier material as compared to the untreated or unfunctionalized counterparts. Figure 7 shows a schematic representation of a direct air capture system 700 including multiple reactors fluidically coupled to manage the flow and temperature of water for various operational phases of the reactors.

[0129] As shown in Figure 7, the system comprises two primary components, a first reactor 702 and a second reactor 704, which are connected in series and are responsible for the heat exchange processes.

[0130] The arrangement of reactors in the system 700 is not limited to two reactors in series, as depicted in the figure. Instead, the system can incorporate more than two reactors configured in various layouts, such as parallel, series, or hybrid configurations, depending on the specific requirements for optimizing the flow and temperature of water. By tailoring the reactor arrangement, the system can accommodate varying process demands, such as different flow rates, temperature gradients, or operational cycles, thereby enhancing the overall energy efficiency and performance of the carbon capture process.

[0131] Furthermore, in the embodiments described herein, the reactors include hollow fibers for the adsorption and desorption of carbon dioxide; however, the system is not limited to this specific configuration. Any suitable reactor capable of benefiting from the described cyclic adsorption and desorption process may be employed. For instance, reactors utilizing alternative sorbent materials, geometries, or configurations that facilitate efficient heat transfer and gas separation can be integrated into the system.

[0132] The system includes a first inlet 706 configured to introduce water from a heating loop into a first reactor 702 and a first outlet 708 configured to remove water from the first reactor 702. Effluent water exiting the first reactor 702 is selectively routed either to a second inlet 710 of a second reactor 704 for preheating and / or heating, or back to the heating loop. The second reactor 704 discharges through a second outlet 712. Water leaving the first reactor 702 that is returned to the heating loop is directed via a hot water return 716 when at or near desorption temperature and via a cold water return 718 when at or near post-cooling temperature, thereby enabling temperature-based segregation of streams, improving sensible heat recovery, and reducing parasitic thermal losses across successive adsorptiondesorption cycles.

[0133] The system 700 includes a first valve 720 and a second valve 722 that regulate fluid communication between a first reactor 702, a second reactor 704, and a heating loop, thereby orchestrating the preheating, heating, and cooling phases of operation. By selectively directing hot effluent to the heating loop, diverting intermediate-temperature water to preheat a downstream reactor, and routing chilled water for cooling, the valves enable precise temperature staging. This coordinated valve control maintains stable process conditions within each reactor, maximizes reuse of sensible heat and reduces energy losses across cycles.

[0134] The processes of adsorption and desorption cycling in the hollow fiber reactor are facilitated through distinct phases of preheating, heating, and cooling, which enhance the efficiency of carbon dioxide capture and release. During the preheating phase, warm water between 35°C and 70°C, preferably above 45°C, is introduced into the lumen of the hollow fiber to elevate the temperature of the hollow fiber to an intermediate level, preparing the sorbent material for the subsequent desorption phase. In the heating phase, hot water at a higher temperature, typically between 70°C and 110°C, preferably above 100°C, preferably about 110°C, is circulated through the lumen to provide the thermal energy required to desorb the adsorbed carbon dioxide. The semi-permeable layer of the hollow fiber allows the vapor state of the heat transfer medium to permeate. Following desorption, the cooling phase is initiated by introducing a flow of cooler water, typically at a temperature between 5°C and 40°C, through the lumens of the reactors to lower the temperature of the hollow fiber. This cooling step restores the sorbent material to favorable conditions for the next adsorption cycle, supporting efficient and continuous operation of the carbon capture process.

[0135] During the desorption phase in the first reactor 702, hot liquid water at approximately 110 °C is introduced from the heating loop to the lumens of the hollow fibers of the first reactor 702 to release adsorbed carbon dioxide. Following this phase, the cooling process begins by introducing chilled water into the lumens of the first reactor 702, displacing the hot water. The narrow configuration of the hollow fiber lumen minimizes mixing between the hot and cold water, creating a temperature gradient in the effluent water stream. The first valve 720 directs the initial effluent liquid water to a heating loop via hot water return 716 for use in subsequent desorption cycles, for example. As the cooling phase progresses, the temperature of the injected water rises due to heat transfer from the hot hollow fibers, resulting in warm water at an intermediate temperature. The first valve 720 then directs the warm water to the second reactor 704 that is prepared for desorption, where the warm water preheats the reactor to an intermediate temperature. Once the second reactor 704 reaches this temperature, the hot liquid water from the heating loop at 110 °C is circulated through the second reactor 704 to complete the desorption process. Figure 8 is a schematic showing two reactors operating in parallel, enabling simultaneous adsorption and desorption processes to optimize efficiency. During operation, the first reactor 702 undergoes desorption, where hot liquid water is introduced into the lumen of the hollow fibers to release adsorbed carbon dioxide. The effluent water from the first reactor 702 is directed to the hot water return 716 for reuse in subsequent cycles. Concurrently, the second reactor undergoes the adsorption phase, where ambient air enters through the air inlet 818 and flows across the hollow fibers on the shell side. Carbon dioxide is adsorbed onto the sorbent material within the hollow fibers, and air depleted of carbon dioxide exits the system through the air outlet 820. This parallel configuration allows continuous operation, as one reactor captures carbon dioxide while the other regenerates the sorbent material within its hollow fibers.

[0136] Figure 9 is a temperature cycling graph 900 illustrating the flow and temperature management of water during the desorption and cooling phases in a system comprising multiple hollow fiber reactors. The graph demonstrates the transition of water temperature exiting the first reactor and corresponding operational phases.

[0137] The graph begins with hot liquid water at 110°C, which is directed to the heating loop after being used in the desorption phase of a reactor. This hot liquid water is then reused in subsequent desorption cycles, ensuring efficient energy recovery. The temperature of the liquid water remains constant at 110 °C during this phase, as indicated by the horizontal segment at 902.

[0138] Following the desorption phase, the cooling process is initiated by introducing chilled water into the lumen of the hollow fibers. This displaces the hot water and creates a temperature gradient in the effluent water stream, as indicated by the segment at 904. The graph shows a sharp decline in temperature as the hot water is replaced by cooler water, with the temperature dropping from 110 °C to 45 °C. This intermediate temperature water, referred to as "warm water," is then directed to a second reactor that is prepared for desorption.

[0139] The warm water at 45 °C (preheats the second reactor, which is ready for a two-step desorption process. The first step involves heating the reactor to 45 °C using the warm water, while the second step involves further heating the reactor to 100 °C using hot liquid water at 110 °C from the heating loop. This two-step desorption process is designed to optimize energy efficiency by utilizing the warm water for preheating, thereby reducing the energy required to reach the final desorption temperature. After the cooling phase, the water temperature continues to decrease, as shown by the downward slope of the graph at 906, until the temperature reaches 25 °C. The cooled water is then directed to the heating loop.

Claims

CLAIMS1. A hollow fiber for adsorption and desorption of carbon dioxide, comprising:(a) a cylindrical hollow fiber structure capable of carbon dioxide sorption, where a cross-section of the hollow fiber structure is substantially symmetrical; and(b) a semi-permeable layer that is impermeable to a liquid heat transfer medium but permeable to a vapor state of the heat transfer medium, where the semi- permeable layer is disposed on a surface of the hollow fiber structure adjacent to a lumen of the hollow fiber.

2. A hollow fiber as claimed in claim 1 wherein the semi-permeable layer includes nonpolar polymers, preferably non-polar polymers that are rubbery or glassy at ambient conditions.

3. A hollow fiber as claimed in claim 1 or claim 2 wherein the semi-permeable layer comprises an ethylene propylene diene monomer rubber, a polydimethylsilosane (PDMS), a polychloroprene, a polystyrene, or a styrene-butadiene.

4. A hollow fiber as claimed in claim 3 wherein the semi-permeable layer includes polydimethylsilosane (PDMS), ethylene propylene diene monomer (EPDM) rubber.

5. A hollow fiber as claimed in claim 4 wherein the semi-permeable layer comprises an ethylene propylene diene monomer rubber formed by cross-linking ethylene propylene diene monomer with radical initiators, benzoyl peroxides, dicumyl peroxide, tertbutylperoxybenzoate, or lauroyl peroxide, or combinations thereof.

6. A hollow fiber as claimed in any preceding claim wherein the semi-permeable layer includes at least two polymer coatings.

7. A hollow fiber as claimed in claim 6, wherein the at least two polymer coatings are coatings of the same material.

8. A hollow fiber as claimed in claim 6, wherein the at least two polymer coatings include a first coating and a second coating and wherein the first coating and second coating are coatings of different materials.

9. A hollow fiber as claimed in any preceding claim wherein the hollow fiber structure includes a polymer matrix.

10. A hollow fiber as claimed in claim 9, where the polymer matrix is functionalized with nucleophilic groups.

11. A hollow fiber as claimed in claim 9 or claim 10, where the hollow fiber structures includes at least one filler, optionally a filler selected from metal organic frameworks (MOFs), covalent organic frameworks (COFs), zeolites, ion-exchange resins, poly-styrene based ionexchange resins and amine functionalized resins, anion exchange resins, alumina, silica, activated carbon, and inorganic salts, wherein the at least one filler is functionalized with nucleophilic groups.

12. A hollow fiber as claimed in claim 11 wherein the nucleophilic groups are at least one type of nucleophilic group selected from amines, amide salts, alcohols, alkoxides, thiols, and metal alkyls.

13. A hollow fiber as claimed in claim 11 wherein the nucleophilic groups are mono-, di-, tri-, tetra- or polyamino-functionalized amino- alkoxy-silanes aminosilanes, aluminas, and silicas.

14. A hollow fiber as claimed in any one of claims 10 to 12 wherein the nucleophilic groups are amine groups.

15. A hollow fiber as claimed in claim 14 wherein the amine groups comprise small molecule amines.

16. A hollow fiber as claimed in claim 15 wherein the amine groups comprise small molecule amines selected from the group comprising ethylenediamine (EDA), 1 ,3- propylenediamine (PDA), meta-xylylenediamine (meta-XyDm), para-xylylenediamine (para- XyDm), triethylenetetramine (TETA), tetraethylenepentamine (TEPA), and pentaethylenehexamine (PEHA).

17. A hollow fiber as claimed in claim 14 wherein the amine groups comprise polymeric amines.

18. A hollow fiber as claimed in claim 17 wherein the polymeric amines are selected from the group comprising branched polyethyleneimine, polyvinylamine and polyallylamine.

19. A hollow fiber as claimed in any preceding claim wherein the semi-permeable layer has a water vapor permeability of 300 Barrer or less or 1000 Barrer or less.

20. A hollow fiber as claimed in any preceding claim wherein the semi-permeable layer has a thickness between 10 - 500 micrometers.

21. A hollow fiber as claimed in any preceding claim wherein the polymer matrix is immediately adjacent to the semi-permeable layer.

22. A method for cyclic adsorption and desorption of carbon dioxide, the method comprising:(a) exposing a hollow fiber to ambient air, where the hollow fiber includes a polymer matrix that is functionalized with nucleophilic groups and a semi-permeable layer coated on the lumen of the hollow fiber, whereby carbon dioxide in the ambient air is adsorbed to the hollow fiber;(b) introducing a first flow of liquid heat transfer medium at a first temperature to the lumen of the hollow fiber, where the first flow of heat transfer medium is introduced to the lumen substantially in the absence of steam, whereby the hollow fiber is heated and at least part of the adsorbed carbon dioxide is desorbed from the at least one of the plurality of hollow fibers, and where the semi-permeable layer substantially prevents a flow of liquid heat transfer medium from contacting the polymer matrix in liquid form and allows a flow of heat transfer medium vapor to fluidly communicate with the polymer matrix; and(c) introducing a second flow of heat transfer medium at a second temperature through the lumen of the hollow fiber, where the first temperature is higher than the second temperature, whereby the hollow fiber is cooled by the second flow of heat transfer medium and where the first and second flows of heat transfer medium are introduced to the lumen in a first direction and the hollow fiber is exposed to the ambient air in a second direction at an angle relative to the first direction.

23. A method as claimed in claim 22 further comprising introducing a third flow of heat transfer medium at a third temperature through the lumen of the hollow fiber, where the thirdtemperature is higher than the first temperature, whereby a performance of the hollow fiber is increased.

24. A method as claimed in claim 23, further comprising exposing the hollow fiber to an inert environment.

25. A method as claimed in claim 22 wherein the heat transfer medium is liquid water or liquid water mixed with one or more other heat transfer mediums.

26. A method as claimed in claim 25 wherein the liquid water or liquid water mixed with one or more other heat transfer mediums is introduced to the lumen substantially in the absence of steam.

27. A method as claimed in any one of claims 22 to 26 wherein the first flow of heat transfer medium is at a temperature in the range of 45 to 110 °C.

28. A method as claimed in any one of claims 22 to 26 wherein the second temperature is a temperature between 5 to 40 °C.

29. A method as claimed in any one of claims 22 to 27 wherein the hollow fiber is a hollow fiber as claimed in any one of claims 1 to 21.

30. A method of improving adsorption and desorption of carbon dioxide on a polymer matrix, the method further comprising functionalizing the polymer matrix with nucleophilic groups.

31. A method as claimed in claim 30 wherein the nucleophilic groups are at least one nucleophilic group selected from amines, amide salts, alcohols, alkoxides, thiols, and metal alkyls.

32. A method as claimed in claim 31 wherein the nucleophilic groups are amine groups.

33. A method as claimed in claim 32 wherein the amine groups comprise: i) small molecule amines; optionally small molecule amines selected from the group comprising ethylenediamine (EDA), 1 ,3-propylenediamine (PDA), meta- xylylenediamine (meta-XyDm), para-xylylenediamine (para-XyDm),triethylenetetramine (TETA), tetraethylenepentamine (TEPA), and pentaethylenehexamine (PEHA); and / or ii) polymeric amines; optionally polymeric amines are selected from the group comprising branched polyethyleneimine and polyallylamine.; and / or iii) aminosilanes; optionally aminosilanes are selected from the group comprising mono-, di-, tri-, tetra- or polyamino functionalized amino- alkoxy-silanes.

34. A method as claimed in claim 22 further comprising directing the liquid heat transfer medium from the hollow fiber to a lumen of a downstream hollow fiber, where the hollow fiber is a first hollow fiber and the downstream hollow fiber is a second hollow fiber and where the first hollow fiber is in series with the second hollow fiber.

35. A method as claimed in claim 34, where the liquid heat transfer medium from the hollow fiber is between 45 - 110 °C.

36. A method as claimed in claim 34 further comprising ceasing flow of the liquid heat transfer medium to the second hollow fiber when a temperature of the second hollow fiber reaches a predetermined temperature.

37. A method as claimed in claim 36 further comprising introducing hot liquid water from a heating loop to the lumen of the second hollow fiber when the temperature of the second hollow fiber reaches a predetermined temperature.

38. A method as claimed in claim 34, where the first hollow fiber is in a first reactor and the second hollow fiber is in a second reactor.

39. A method as claimed in claim 38, where the first reactor undergoes a desorption phase and where the second reactor undergoes an adsorption phase.

40. A method as claimed in claim 22, wherein the hollow fiber is a first hollow fiber and a second hollow fiber is disposed downstream of the first hollow fiber; and wherein the method further comprises exposing the second hollow fiber to ambient air at least partially during step (b).

41. A method as claimed in claim 22, wherein the hollow fiber is a first hollow fiber and a second hollow fiber is disposed upstream of the first hollow fiber; and wherein the methodfurther comprises exposing the second hollow fiber to ambient air at least partially during step (b).

42. A method as claimed in claim 22, where the hollow fiber is a first hollow fiber and a second hollow fiber is disposed fluidically parallel with the first hollow fiber, and the method further comprises exposing each hollow fiber to ambient air at least partially during step (b).