WO2024103124A1

WO2024103124A1 - Process for removing carbon dioxide from a gas stream

Info

Publication number: WO2024103124A1
Application number: PCT/AU2023/051170
Authority: WO
Inventors: Paul Feron; Ali Kiani; Robert Bennett; Graeme PUXTY
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 2022-11-18
Filing date: 2023-11-17
Publication date: 2024-05-23

Abstract

The invention provides a process for removing carbon dioxide from a gas stream comprising carbon dioxide, the process comprising contacting the gas stream with an aqueous absorbent solution comprising (i) an involatile amine absorbent, (ii) non- carbonate hygroscopic metal salt in an amount of at least 10 wt.% of the aqueous absorbent solution, and (iii) water, thereby absorbing carbon dioxide from the gas stream into the aqueous absorbent solution to produce a carbon dioxide-lean gas and a carbon dioxide-rich absorbent composition.

Description

Process for removing carbon dioxide from a gas stream

[1 ] The present application claims priority from Australian provisional patent application No. 2022903484 filed on 18 November 2022, the contents of which are to be understood to be incorporated into this specification by this reference.

Technical Field

[2] The invention relates to a process for removing carbon dioxide from a gas stream comprising carbon dioxide using an aqueous absorbent solution comprising an involatile amine absorbent and non-carbonate hygroscopic metal salt. The invention further relates to an aqueous absorbent solution for carbon dioxide capture and a system for removing carbon dioxide from a gas stream comprising carbon dioxide.

Background of Invention

[3] Emission of carbon dioxide (CO2) is considered the main cause of the greenhouse effect and global warming. In the Paris agreement the United Nations have set targets for allowable temperature increase that will require significant reduction of greenhouse gas emissions. One method of reducing atmospheric CO2 emissions is to capture CO2 from CO2-rich gas effluents produced by industries such as power stations, steel plants, cement kilns, calciners, biogas plants, natural gas processing, methane reforming and smelters, with subsequent geological storage. A leading technology for such applications uses amine-based aqueous absorbents to absorb CO2 from the gas stream at low temperature in an absorber and then to release CO2 at high temperature in a desorber. The CO2-lean absorbent is then recycled to the absorber, while the concentrated CO2 product is liquified by compression and cooling for injection into an underground reservoir. Various amines and alkanolamines have been investigated as reactive absorbents in liquid-based absorbents, with monoethanolamine most commonly considered a reference in industrial applications.

[4] Such approaches will remain an important tool as the world seeks to achieve a net zero emission future, particularly as a means to mitigate CO2 emissions from hard-to-decarbonize industries. However, a 2021 Intergovernmental Panel on Climate Change report has indicated that large scale removal of CO2 from the atmosphere is also needed to avoid excessive increases in global temperature compared to preindustrial times. The development of scalable negative emissions technologies, including efficient CO2 capture from air (direct air capture; DAC), is therefore of great interest.

[5] Amine-based aqueous absorbents can also be used in DAC applications. One challenge that arises is the loss of amine by vaporisation when large volumes of air are contacted with the absorbent in an unconfined environment. Amine losses can be limited by the addition of a separate water wash section to reabsorb the amine from the CO2-lean air, but this adds investment and operational costs to the process.

[6] A further issue with the use of amine-based aqueous absorbents in DAC applications is the loss of water from the process. Air generally has a relative humidity less than 100%. Therefore, water will evaporate from the absorbent into the air during the CO2 absorption step. The water losses are dependent on the temperature and relative humidity of the ambient air and will change throughout the day and season. At 25 °C, evaporative water losses are typically in the range of 0 (at 100% relative humidity) to more than 10 kg/kg CO2 removed (at 30% relative humidity and lower). Operation of the DAC system therefore requires supply of significant amounts of water. In dry areas water might not be available or only available at high cost. Loss of water may also lead to operational issues as the solutions might become more concentrated with a high risk of the formation of precipitates that could lead to blockages and performance reduction. The same issue typically does not arise in post-combustion applications where the inlet gas is water saturated. It would be desirable to develop DAC processes which mitigate or avoid water losses, or are even able to maintain a positive water balance by simultaneously absorbing water and CO2 from the air.

[7] While the foregoing discussion relates specifically to DAC processes, it will be appreciated that similar considerations apply in other applications where CO2 is absorbed from gas streams which are sub-saturated in water, such as environmental control in spacecrafts, mine shelters or submarines.

[8] There is therefore an ongoing need for processes for removing carbon dioxide from a gas stream which at least partially address one or more of the above- mentioned short-comings, or provide a useful alternative. [9] A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that the document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

Summary of Invention

[10] The inventors have discovered that water losses from amine-based aqueous absorbent solutions in DAC and other CO2 capture processes can be mitigated or negated by including a dissolved hygroscopic metal salt in the absorbent solution. It has been found that a neutral water balance, or even simultaneous CO2 and water capture, can be achieved with absorbents having hygroscopic metal salt concentrations well below the solubility limit. An involatile amine absorbent can thus be dissolved in the solution at practically useful concentrations, with the amine absorbent and its reaction products with CO2 remaining soluble during the capture process. The CO2 mass transfer rate during the absorption step remains acceptable despite the presence of significant quantities of the hygroscopic metal salt and the consequent increase in the viscosity of the absorbent solution.

[1 1 ] The invention thus provides a process for removing carbon dioxide from a gas stream comprising carbon dioxide. The process comprises contacting the gas stream with an aqueous absorbent solution comprising an amine absorbent, hygroscopic metal salt and water. Carbon dioxide is thereby absorbed from the gas stream into the aqueous absorbent solution to produce a carbon dioxide-lean gas and a carbon dioxide-rich absorbent composition.

[12] The amine absorbent may be an involatile amine absorbent, thus avoiding significant losses of the amine absorbent from the aqueous absorbent solution to the gas feed.

[13] The hygroscopic metal salt may be present in an amount sufficient to significantly reduce or entirely suppress the loss of water to the gas stream during the CO2 absorption step, such as at least 10 wt.% of the aqueous absorbent solution.

[14] The hygroscopic metal salt may be a non-carbonate hygroscopic metal salt. [15] The hygroscopic metal salt may have the property that air in equilibrium with a saturated aqueous solution consisting of the hygroscopic metal salt and water has a relative humidity of less than 30% at 30°C.

[16] In a first set of embodiments, the process comprises: contacting the gas stream with an aqueous absorbent solution comprising (i) an involatile amine absorbent, (ii) non-carbonate hygroscopic metal salt in an amount of at least 10 wt.% of the aqueous absorbent solution, and (iii) water, thereby absorbing carbon dioxide from the gas stream into the aqueous absorbent solution to produce a carbon dioxidelean gas and a carbon dioxide-rich absorbent composition.

[17] In some embodiments, the aqueous absorbent solution comprises the noncarbonate hygroscopic metal salt in an amount sufficient that net water desorption from the aqueous absorbent solution to the gas stream is zero or negative.

[18] In some embodiments, the gas stream further comprises water vapour. The gas stream may have a relative humidity of less than 80%, or less than 60%, such as less than 40%.

[19] In some embodiments, water is absorbed from the gas stream into the aqueous absorbent solution.

[20] In some embodiments, the non-carbonate hygroscopic metal salt is present in an amount of at least 15 wt.%, or at least 20 wt.%, such as at least 25 wt.%, of the aqueous absorbent solution.

[21 ] In some embodiments, the aqueous absorbent composition has the property that air in equilibrium with the aqueous absorbent composition has a relative humidity of less than 80%, or less than 60%, such as less than 40%, at 30°C.

[22] In some embodiments, the aqueous absorbent composition has the property that air in equilibrium with the aqueous absorbent composition has a relative humidity of less than 80%, or less than 60%, such as less than 40%, at the temperature of the carbon dioxide-lean gas when separated from the carbon dioxide-rich absorbent composition. [23] In some embodiments, the non-carbonate hygroscopic metal salt has the property that air in equilibrium with a saturated aqueous solution consisting of the noncarbonate hygroscopic metal salt and water has a relative humidity of less than 60%, or less than 50%, or less than 40%, such as less than 30%, at 30°C.

[24] In some embodiments, the non-carbonate hygroscopic metal salt comprises a cation selected from the group consisting of alkali metals, alkali earth metals and nickel. The cation may be selected from the group consisting of lithium, sodium, potassium, calcium, nickel and magnesium.

[25] In some embodiments the non-carbonate hygroscopic metal salt comprises an anion selected from the group consisting of halides, Ci-Ce alkyl or aryl carboxylates, nitrate and thiocyanate.

[26] In some embodiments, the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, lithium iodide, lithium nitrate, lithium thiocyanate, sodium bromide, sodium iodide, sodium thiocyanate, potassium fluoride, potassium formate, potassium acetate, potassium nitrite, potassium thiocyanate, magnesium chloride, magnesium bromide, magnesium iodide, magnesium nitrate, magnesium thiocyanate, calcium bromide, calcium iodide, calcium acetate, calcium nitrate, calcium thiocyanate, strontium iodide, strontium thiocyanate, barium iodide, chromium chloride, manganese chloride, manganese bromide, iron bromide, cobalt bromide, cobalt nitrate, nickel chloride, nickel bromide, copper nitrate, zinc chloride, zinc bromide, zinc iodide, zinc nitrate, cerium chloride, and combinations thereof.

[27] In some embodiments, the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, lithium iodide, lithium thiocyanate, potassium fluoride, potassium formate, potassium acetate, potassium thiocyanate, magnesium chloride, magnesium bromide, magnesium iodide, calcium bromide, calcium iodide, calcium acetate, calcium thiocyanate, nickel bromide, zinc chloride, zinc bromide, zinc iodide, and combinations thereof.

[28] In some embodiments, the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, potassium formate, and potassium acetate. [29] In some embodiments, the involatile amine absorbent is selected from the group consisting of amino acids or salts thereof, polyamines comprising both quaternised and neutral amine groups, high molecular weight amines and combinations thereof. In some embodiments, the involatile amine absorbent is an amino acid or salt thereof.

[30] In some embodiments, the amino acid is selected from the group consisting of taurine, sarcosine, alanine, glycine, lysine, dimethylglycine, proline, phenyl-alanine, glucosamine, arginine, methyl-taurine, cysteine, tryptophan, hydroxyproline, asparagine, tyrosine, histidine, glutamine, diglycine, serine, methionine and combinations thereof.

[31 ] In some embodiments, the involatile amine absorbent is present in the aqueous absorbent solution in an amount of between 0.1 mol/L and 6 mol/L, such as between 0.5 mol/L and 3 mol/L.

[32] In some embodiments, the water is present in the aqueous absorbent solution in an amount of at least 30 wt.%.

[33] In some embodiments, the aqueous absorbent solution further comprises (iv) a base. The base may be selected from a hydroxide, a carbonate, a phosphate, a further amine having a pKa greater than the involatile amine absorbent, and combinations thereof.

[34] In some embodiments, the gas stream is air. The air may be selected from ambient air, air from a confined environment and ventilation air.

[35] In some embodiments, the gas stream is contacted with the aqueous absorbent solution at a temperature of between -5°C and 35°C.

[36] In some embodiments, the process further comprises removing carbon dioxide from the carbon dioxide-rich absorbent composition to produce a carbon dioxide-lean absorbent composition, and recycling the carbon dioxide-lean absorbent composition to the aqueous absorbent solution. Carbon dioxide may be removed from the carbon dioxide-rich absorbent composition by heating the carbon dioxide-rich absorbent composition to desorb carbon dioxide. [37] In a second set of embodiments, the process comprises: contacting the gas stream with an aqueous absorbent solution comprising (i) an involatile amine absorbent, (ii) hygroscopic metal salt in an amount of at least 10 wt.% of the aqueous absorbent solution, and (iii) water, thereby absorbing carbon dioxide from the gas stream into the aqueous absorbent solution to produce a carbon dioxide-lean gas and a carbon dioxide-rich absorbent composition, wherein the hygroscopic metal salt has the property that air in equilibrium with a saturated aqueous solution consisting of the hygroscopic metal salt and water has a relative humidity of less than 30% at 30°C.

[38] The process according to the second set of embodiments may generally have features as disclosed above in the context of the first set of embodiments.

[39] The invention also provides an aqueous absorbent solution for carbon dioxide capture. The aqueous absorbent solution comprises an amine absorbent, hygroscopic metal salt and water.

[40] The amine absorbent may be an involatile amine absorbent, thus avoiding significant losses of the amine absorbent from the aqueous absorbent solution in use.

[41] The hygroscopic metal salt may be present in an amount sufficient to significantly reduce or entirely suppress the loss of water from the aqueous absorbent solution in use. The hygroscopic metal salt may be present in an amount of least 10 wt.% of the aqueous absorbent solution.

[42] The hygroscopic metal salt may be a non-carbonate hygroscopic metal salt.

[43] The hygroscopic metal salt may have the property that air in equilibrium with a saturated aqueous solution consisting of the hygroscopic metal salt and water has a relative humidity of less than 30% at 30°C.

[44] In a first set of embodiments, the aqueous absorbent solution for carbon dioxide capture comprises (i) an involatile amine absorbent, (ii) non-carbonate hygroscopic metal salt in an amount of at least 10 wt.% of the aqueous absorbent solution, and (iii) water. [45] In some embodiments, the non-carbonate hygroscopic metal salt is present in an amount of at least 15 wt.%, or at least 20 wt.%, such as at least 25 wt.%, of the aqueous absorbent solution.

[46] In some embodiments, the aqueous absorbent composition has the property that air in equilibrium with the aqueous absorbent composition has a relative humidity of less than 80%, or less than 60%, such as less than 40%, at 30°C.

[47] In some embodiments, the non-carbonate hygroscopic metal salt has the property that air in equilibrium with a saturated aqueous solution consisting of the non- carbonate hygroscopic metal salt and water has a relative humidity of less than 60%, or less than 50%, or less than 40%, such as less than 30%, at 30°C.

[48] In some embodiments, the non-carbonate hygroscopic metal salt comprises a cation selected from the group consisting of alkali metals, alkali earth metals and nickel. The cation may be selected from the group consisting of lithium, sodium, potassium, calcium, nickel and magnesium.

[49] In some embodiments, the non-carbonate hygroscopic metal salt comprises an anion selected from the group consisting of halides, Ci-Ce alkyl or aryl carboxylates, nitrate and thiocyanate.

[50] In some embodiments, the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, lithium iodide, lithium nitrate, lithium thiocyanate, sodium bromide, sodium iodide, sodium thiocyanate, potassium fluoride, potassium formate, potassium acetate, potassium nitrite, potassium thiocyanate, magnesium chloride, magnesium bromide, magnesium iodide, magnesium nitrate, magnesium thiocyanate, calcium bromide, calcium iodide, calcium acetate, calcium nitrate, calcium thiocyanate, strontium iodide, strontium thiocyanate, barium iodide, chromium chloride, manganese chloride, manganese bromide, iron bromide, cobalt bromide, cobalt nitrate, nickel chloride, nickel bromide, copper nitrate, zinc chloride, zinc bromide, zinc iodide, zinc nitrate, cerium chloride, and combinations thereof.

[51 ] In some embodiments, the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, lithium iodide, lithium thiocyanate, potassium fluoride, potassium formate, potassium acetate, potassium thiocyanate, magnesium chloride, magnesium bromide, magnesium iodide, calcium bromide, calcium iodide, calcium acetate, calcium thiocyanate, nickel bromide, zinc chloride, zinc bromide, zinc iodide, and combinations thereof.

[52] In some embodiments, the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, potassium formate, and potassium acetate.

[53] In some embodiments, the involatile amine absorbent is selected from the group consisting of amino acids or salts thereof, polyamines comprising both quaternised and neutral amine groups, high molecular weight amines and combinations thereof. In some embodiments, the involatile amine absorbent is an amino acid or salt thereof.

[54] In some embodiments, the amino acid is selected from the group consisting of taurine, sarcosine, alanine, glycine, lysine, dimethylglycine, proline, phenyl-alanine, glucosamine, arginine, methyl-taurine, cysteine, tryptophan, hydroxyproline, asparagine, tyrosine, histidine, glutamine, diglycine, serine, methionine and combinations thereof.

[55] In some embodiments, the involatile amine absorbent is present in the aqueous absorbent solution in an amount of between 0.1 mol/L and 6 mol/L, such as between 0.5 mol/L and 3 mol/L.

[56] In some embodiments, the water is present in the aqueous absorbent solution in an amount of at least 30 wt.%.

[57] In some embodiments, the aqueous absorbent solution further comprises (iv) a base. The base may be selected from a hydroxide, a carbonate, a phosphate, a further amine having a pKa greater than the involatile amine absorbent, and combinations thereof.

[58] In some embodiments, the aqueous absorbent solution further comprises absorbed carbon dioxide at a ratio of carbon dioxide to involatile amine absorbent (mol/mol) of at least 0.05, such as at least 0.1 . [59] In a second set of embodiments, the aqueous absorbent solution for carbon dioxide capture comprises (i) an involatile amine absorbent, (ii) hygroscopic metal salt in an amount of at least 10 wt.% of the aqueous absorbent solution, and (iii) water, wherein the hygroscopic metal salt has the property that air in equilibrium with a saturated aqueous solution consisting of the hygroscopic metal salt and water has a relative humidity of less than 30% at 30°C.

[60] The aqueous absorbent solution according to the second set of embodiments may generally have features as disclosed above in the context of the first set of embodiments.

[61 ] The invention also provides a system for removing carbon dioxide from a gas stream comprising carbon dioxide. The system comprises: an aqueous absorbent solution according to any embodiment disclosed herein; an absorption unit for contacting the aqueous absorbent solution with the gas stream, thereby absorbing carbon dioxide from the gas stream into the aqueous absorbent solution to produce a carbon dioxide-lean gas and a carbon dioxide-rich absorbent composition; and a regeneration unit for removing carbon dioxide from the carbon dioxide-rich absorbent composition, thereby producing a carbon dioxide-lean absorbent composition for recycling to the aqueous absorbent solution.

[62] In some embodiments, the system is a system for direct air capture. In some embodiments, the gas stream is selected from ambient air, air from a confined environment and ventilation air.

[63] Where the terms “comprise”, “comprises” and “comprising” are used in the specification (including the claims) they are to be interpreted as specifying the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof.

[64] Further aspects of the invention appear below in the detailed description of the invention.

Brief Description of Drawings

[65] Embodiments of the invention will herein be illustrated by way of example only with reference to the accompanying drawings in which: [66] Figure 1 schematically depicts a system for removing carbon dioxide from a gas stream according to some embodiments of the invention.

[67] Figure 2 is a graph showing the CO2 mass transfer coefficients into aqueous solutions comprising 2 mol/L taurate and/or potassium carboxylate hygroscopic salt, as measured in Example 2.

[68] Figure 3 is a graph showing the CO2 mass transfer coefficients into aqueous solutions comprising 0.5 mol/L taurate, or 0.5 mol/L taurate and lithium halide hygroscopic salt, as measured in Example 2.

[69] Figure 4 is a graph showing the temperature-dependent viscosity of aqueous solutions comprising taurate, or taurate and hygroscopic metal salt, as measured in Example 2.

[70] Figure 5 schematically depicts an apparatus for measuring the water vapour pressure above a hygroscopic salt solution, as used in Example 3.

[71 ] Figure 6 is a graph showing the relative humidity (ratio of water vapour pressure above the solution to water vapour pressure above pure water) of various aqueous solutions of hygroscopic metal salts, as a function of salt concentration and with comparison against the prediction of Raoult’s law, as determined in Example 3.

[72] Figure 7 is a graph showing the CO2 captured by, and the water lost from or gained by, a liquid absorbent solution under direct air capture conditions when using a 2M taurate in 44 wt.% potassium formate solution as the liquid absorbent solution and synthetic air of different humidities as the gas feed, as measured in Example 4.

[73] Figure 8 schematically depicts an absorption unit including the parameters used to model water loss or gain in a direct air capture process under different climatic conditions in Example 5.

[74] Figure 9 is a graph showing the expected water loss or gain of a direct air capture process using an amino acid-based absorbent, either with added hygroscopic metal salt (System 2) or without (System 1 ), as modelled for three different climatic conditions in Example 5. Detailed Description

[75] The present invention relates to a process for removing carbon dioxide from a gas stream comprising carbon dioxide. The process comprises contacting the gas stream with an aqueous absorbent solution comprising (i) an amine absorbent, (ii) hygroscopic metal salt, and (iii) water. Carbon dioxide is thus absorbed from the gas stream into the aqueous absorbent solution to produce a carbon dioxide-lean gas and a carbon dioxide-rich absorbent composition. In preferred embodiments, the process further comprises removing carbon dioxide from the carbon dioxide-rich absorbent composition to produce a carbon dioxide-lean absorbent composition, and recycling the carbon dioxide-lean absorbent composition to the aqueous absorbent solution. The aqueous absorbent solution is thus repeatedly cycled between carbon dioxide absorption and desorption process steps.

[76] The amine absorbent may be an involatile amine absorbent, thus avoiding significant losses of the amine absorbent from the aqueous absorbent solution to the gas feed.

[77] The hygroscopic metal salt may be present in an amount sufficient to significantly reduce or entirely suppress the loss of water to the gas stream during the CO2 absorption step, such as at least 10 wt.% of the aqueous absorbent solution.

[78] The hygroscopic metal salt may be a non-carbonate hygroscopic metal salt.

[79] The hygroscopic metal salt may have the property that air in equilibrium with a saturated aqueous solution consisting of the hygroscopic metal salt and water has a relative humidity of less than 30% at 30°C.

Gas stream

[80] The gas stream may in principle be any gas stream which contains carbon dioxide and which is susceptible to treatment with an aqueous amine absorbent to remove a portion of the carbon dioxide. In some embodiments, the carbon dioxide may be present in an amount of less than 10 wt.%, such as less than 1 wt.%.

[81 ] The gas stream may be air, thus containing dinitrogen and dioxygen as the major components. Typically, the carbon dioxide in the air may be present in an amount of between 350 ppm and 5000 ppm. The Short-Term Exposure Limit is equal to 30,000 ppm with concentrations above 40,000 ppm representing an Immediate Danger to Life or Health, and breathable air must be controlled to have carbon dioxide levels well below these limits. For example, the process may be useful to control the carbon dioxide levels of air in confined environments, such as in vehicles (e.g. spacecraft, submarines), mine shelters, ventilation air, or other locations where carbon dioxide may accumulate or reach undesirable levels. Alternatively, the air may be atmospheric air, e.g. for direct air capture applications, with an expected carbon dioxide content of between 400 and 450 ppm.

[82] The gas stream may contain water vapour. In principle, water may be lost from a conventional aqueous absorbent solution into the gas being treated even when the gas stream is initially at or near water saturation (i.e. 100% relative humidity). The gas may be heated during contact with the absorbent, thus increasing its capacity to absorb water from the absorbent solution. However, the risk and extent of unacceptable water loss becomes greater when the initial water content of the gas stream is low. Therefore, in some embodiments, the gas stream has a relative humidity of less than 80%, or less than 60%, less than 50%, less than 40%, such as less than 30%, at the temperature of the gas stream as supplied to the process. In DAC applications, for example, the methods disclosed herein are considered particularly useful for locations where atmospheric air commonly has a humidity of less than 80%, or less than 60%, less than 50%, less than 40%, such as less than 30% at the dry bulb temperature.

Aqueous absorbent solution

[83] The aqueous absorbent solution may comprise an involatile amine absorbent. The process disclosed herein is of particular interest in CO2 removal applications where there is a concern about the loss of volatile components from the aqueous absorbent solution to the gas feed. Therefore, to avoid unacceptable consumption of the amine absorbent in the process, involatile amines are required.

[84] As used herein, an involatile amine absorbent refers to a compound comprising at least one amine group which is susceptible to chemical reaction with CO2 in aqueous solution and which has a negligible vapour pressure under CO2 absorption conditions. For example, the involatile amine absorbent may have a vapour pressure of less than 0.1 Pa at 25°C, as a pure compound. Suitable amine absorbents are typically involatile because they are ionic under absorption conditions or because they have a sufficiently high molecular weight.

[85] In some embodiments, the involatile amine absorbent is fully dissolved in the aqueous absorbent solution during the absorption step. However, it is not excluded that the involatile amine absorbent may partially precipitate during or following absorption.

[86] In some embodiments, the involatile amine absorbent comprises at least one primary or secondary amine group. In some embodiments, the involatile amine absorbent is selected from the group consisting of amino acids or salts thereof, polyamines comprising both quaternised and neutral amine groups, high molecular weight amines and combinations thereof.

[87] Suitable polyamines comprising both quaternised and neutral amine groups may be polyamine compounds containing (i) a basic amine group, for example a secondary or tertiary amine, which exists in quaternised form (by protonation) in solution in the expected pH range during absorption, and (ii) a less basic amine group which is at least partially neutral in solution in the expected pH range during absorption, and thus available for reaction with CO2. One non-limiting example of such as polyamine compound is 1 ,4-pentanediamine. Other examples of diamines expected to quaternise under CO2 absorption conditions are disclosed in US patent 9,409,122. Optionally, the polyamine compound may be quaternised by the addition of acid (e.g. hydrochloric or sulfuric acid) to the solution prior to use in carbon dioxide absorption, preferably in sufficient amounts to fully quaternise the basic amine group. Suitable high molecular weight amines have a vapour pressure of less than 0.1 Pa at 25°C, as a pure compound.

[88] Amino acid absorbents are of particular interest. Amino acids generally exist as ionic species in aqueous solution at the pH ranges typical of CO2 capture due to ionisation of the acid functionality, and therefore have low volatility. Moreover, many simple amino acids provide good CO2 absorption kinetics and cyclic capacities. The anionic form of many amino acids reacts rapidly with CO2 in solution to form carbamate which may then partially hydrolyse into bicarbonate anions. This is shown in Scheme 1 for the case of taurine.

Scheme 1

[89] Suitable amino acids may include taurine, sarcosine, alanine, glycine, lysine, dimethylglycine, proline, phenyl-alanine, glucosamine, arginine, methyl-taurine, cysteine, tryptophan, hydroxyproline, asparagine, tyrosine, histidine, glutamine, diglycine, serine, methionine and the like. The amino acids may be provided in the aqueous absorbent solution as amino acid salts, for example a potassium or sodium salt.

[90] The involatile amine absorbent, for example an amino acid, may be present in the aqueous absorbent solution in any amount sufficient to capture CO2 while preferably avoiding precipitation of amine species in the process. In some embodiments, the involatile amine absorbent is present in an amount of at least 0.1 mol/L, or at least 0.3 mol/L, or between 0.1 mol/L and 6 mol/L, such as between 0.5 mol/L and 3 mol/L, for example between 1 .5 mol/L and 2.5 mol/L. The maximum amine concentration may be limited by the presence of the non-carbonate hygroscopic metal salt, for example when using a lithium halide hygroscopic salt. However, the inventors have found that aqueous absorbent solutions containing hygroscopic lithium halide salts can still dissolve amino acids in sufficient concentrations, such as about 0.5 mol/L or even higher, to absorb practically useful amounts of CO2 while avoiding water losses. Moreover, other suitable hygroscopic salts, such as potassium carboxylate salts, were found to impose no practical limitations on the solubility of amino acid absorbents. [91 ] The aqueous absorbent solution comprises a hygroscopic metal salt, preferably a non-carbonate hygroscopic metal salt, in an amount sufficient to significantly reduce or entirely suppress the loss of water to the gas stream during the CO2 absorption step. The aqueous absorbent solution therefore comprises the hygroscopic metal salt, preferably being a non-carbonate hygroscopic metal salt, in an amount of at least 10 wt.% of the aqueous absorbent solution, and optionally at least 15 wt.%, or at least 20 wt.%, such as at least 25 wt.%, of the aqueous absorbent solution. As used herein, the concentration of components of the aqueous absorbent solution, whether expressed in mol/L or wt.%, refers to the concentration in the aqueous absorbent solution in the absence of, or excluding the contribution from, absorbed CO2. It will be appreciated that the maximum amount of hygroscopic metal salt may be limited by its aqueous solubility and by the requirement for the aqueous absorbent solution to solubilise the involatile amine absorbent and its reaction products with CO2. Typically, the hygroscopic metal salt is present in amounts lower than the saturation concentration in pure water.

[92] The concentration of hygroscopic metal salt, preferably being a non- carbonate hygroscopic metal salt, may be selected to impart a desired hygroscopicity to the aqueous absorbent solution. While it may be desirable to entirely suppress water loss from the aqueous absorbent solution, this is not necessarily required. In practice, the benefits of reduced water losses may be balanced against other considerations including the impact of the hygroscopic metal salt on the CO2 capture properties and viscosity of the solution. In some implementations, an optimised process will thus still accommodate some level of water losses, albeit less than in the absence of the hygroscopic metal salt.

[93] In some embodiments, however, the aqueous absorbent solution comprises the hygroscopic metal salt in an amount sufficient that net water desorption from the aqueous absorbent solution to the gas stream is zero or negative when absorbing carbon dioxide from the gas stream into the aqueous absorbent solution. Thus, the ratio of water to hygroscopic metal salt in the carbon dioxide-rich absorbent composition is equal to or greater than the ratio in the aqueous absorbent solution.

[94] In some implementations, it may be desirable that the net water desorption from the aqueous absorbent solution to the gas stream is approximately zero. In other words, the aqueous absorbent solution neither loses nor gains substantial amounts of water in use. The hygroscopic metal salt may be present in an amount suitable to achieve this goal.

[95] In practice, however, it may not be possible to continuously achieve a continuous perfect water balance, particularly if there is variation in the relative humidity and/or temperature of the gas stream fed to the process (as would be expected for a DAC process). The absorbent composition and the process more generally may be designed to accommodate such variation. For example, the amount of hygroscopic metal salt may be selected such that there is net water desorption from the aqueous absorbent solution to the gas stream during some periods of operation (e.g. when the temperature is higher or the gas humidity is lower) and net water absorption to the aqueous absorbent solution from the gas stream during other periods of operation (e.g. when the temperature is lower or the gas humidity is higher). In a DAC process, such periods of operation may correspond to different parts of the diurnal cycle (e.g. day and night). Alternatively, the amount of hygroscopic metal salt may be selected such that the net water desorption from the aqueous absorbent solution to the gas stream is close to zero but remains either consistently positive or negative at all envisaged operating conditions. The composition of the aqueous absorbent solution may then be continuously or intermittently restored by controlled addition of water to or removal of water from the solution.

[96] In some implementations, it may be desirable that the net water desorption from the aqueous absorbent solution to the gas stream is negative. In other words, the aqueous absorbent solution absorbs both carbon dioxide and water from the gas stream. This may be useful in various applications such as in regulating air quality in confined locations, where both humidity and carbon dioxide need to be controlled, or in applications such as natural gas processing where simultaneous removal of both water and carbon dioxide from the gas stream is desirable. The hygroscopic metal salt may be present in the aqueous absorbent solution in an amount suitable to achieve this goal.

[97] The concentration of hygroscopic salt required to achieve a desired net water desorption from the aqueous absorbent solution to the gas stream (whether still positive but mitigated, approximately zero, or negative) may be determined, with the benefit of this disclosure, by conventional engineering principles. In particular, it will be appreciated that the net water desorption from the absorbent solution will be at or close to zero when the relative humidity of the gas stream (e.g. air) matches the relative humidity of a vapour phase (e.g. air) in equilibrium with the aqueous absorbent solution. Therefore, by measuring the relationship between the relative humidity of a representative gas in equilibrium with the amine-containing aqueous absorbent solution, as a function of its hygroscopic salt concentration, a suitable concentration of hygroscopic salt can be determined for any particular implementation.

[98] In some embodiments, the aqueous absorbent composition has a composition, and in particular a hygroscopic salt concentration, such that air in equilibrium with the aqueous absorbent composition has a relative humidity of less than 80%, or less than 60%, or less than 50%, or less than 40%, at 30°C (and 1 bara pressure). In some embodiments, the aqueous absorbent composition has a composition, and in particular a hygroscopic salt concentration, such that air in equilibrium with the aqueous absorbent composition has a relative humidity of less than 80%, or less than 60%, or less than 50%, or less than 40%, at the expected temperature (and pressure) of the carbon dioxide-lean gas when separated from the carbon dioxiderich absorbent composition. Such relative humidity values may be measured as a matter of routine under controlled laboratory conditions, for example using the method disclosed in Example 3.

[99] Hygroscopic metal salts have the property that, when dissolved in an aqueous solution, they cause the partial pressure of water in the gas phase in equilibrium with the solution to be significantly lower than that predicted by Raoult’s law. In principle, any metal salt having this property may be used in the aqueous absorbent composition, provided that the solution remains capable of carrying the involatile amine absorbent, absorbing carbon dioxide and typically also cycling repeatedly between absorption and desorption steps without excessive degradation. A wide range of hygroscopic metal salts have previously been used in saturated solutions to control moisture in gases, for example as disclosed in Winston et al, Ecology 1960 41 232- 237 and Rockland, Anal. Chem. 1960, 32, 10, 1375-1376). Such metal salts can generally be expected to be suitable for the methods disclosed herein. [100] The hygroscopic metal salt may be a non-carbonate hygroscopic metal salt. As used herein, a non-carbonate hygroscopic metal salt refers to a hygroscopic metal salt which is not a metal carbonate. Concentrated metal carbonate solutions are used in various CO2 capture applications, such as the Benfield process, with the carbonate absorbing CO2 by acid-base chemistry (e.g. K2CO3 + CO2 + H2O - 2 KHCO3). A disadvantage of this approach is the slow reaction rates, making carbonate solutions ineffective in applications such as DAC. In contrast, the present process uses a hygroscopic metal salt to suppress water loss from the absorbent, with an involatile amine absorbent as the primary CO2 absorbent compound. Non-carbonate hygroscopic metal salts suitable for the presently disclosed methods typically make little or no contribution to the CO2 absorption capacity of the aqueous absorbent solution.

[101] In the presently disclosed process, the non-carbonate hygroscopic metal salt is typically present in concentrations which are considerably lower than the solubility limit in water (corresponding to the concentration of saturated solutions). Nevertheless, the properties of saturated metal salt solutions may conveniently be used to characterise the suitability of the component metal salt for use in the process disclosed herein. In some embodiments, therefore, the non-carbonate hygroscopic metal salt has the property that that air in equilibrium with a saturated aqueous solution consisting of (i.e. containing only) the non-carbonate hygroscopic metal salt and water has a relative humidity of less than 60%, or less than 50%, or less than 40%, such as less than 30%, at 30°C (and 1 bara pressure). For example, air in equilibrium with saturated lithium chloride and potassium acetate solutions is reported to have a relative humidity of 1 1 .5% and 22% respectively. Again, this may be measured as a matter of routine under controlled laboratory conditions.

[102] The non-carbonate hygroscopic metal salt may comprise a single metal salt or multiple metal salts. In some embodiments, the non-carbonate hygroscopic metal salt consists essentially of a single metal salt. For example, one metal cation forms at least 90 mol% of its cationic content and one anion forms at least 90 mol% of its anionic content. In some embodiments, the non-carbonate hygroscopic metal salt consists of a single metal salt.

[103] The hygroscopic properties of a metal salt typically depend on an interaction between the cation and anion. Nevertheless, some common metal cations and anions are expected to provide suitable hygroscopic properties when paired with a suitable counterion. In some embodiments, the non-carbonate hygroscopic metal salt comprises an alkali metal cation, alkali earth metal cation or nickel, for example selected from lithium, sodium, potassium, calcium, nickel and magnesium. In some embodiments, the non-carbonate hygroscopic metal salt comprises an anion selected from halides, Ci-Ce alkyl or aryl carboxylates, nitrate and thiocyanate.

[104] In some embodiments, the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, lithium iodide, lithium nitrate, lithium thiocyanate, sodium bromide, sodium iodide, sodium thiocyanate, potassium fluoride, potassium formate, potassium acetate, potassium nitrite, potassium thiocyanate, magnesium chloride, magnesium bromide, magnesium iodide, magnesium nitrate, magnesium thiocyanate, calcium bromide, calcium iodide, calcium acetate, calcium nitrate, calcium thiocyanate, strontium iodide, strontium thiocyanate, barium iodide, chromium chloride, manganese chloride, manganese bromide, iron bromide, cobalt bromide, cobalt nitrate, nickel chloride, nickel bromide, copper nitrate, zinc chloride, zinc bromide, zinc iodide, zinc nitrate, and cerium chloride. A vapour phase (e.g. air) in equilibrium with saturated aqueous solutions of such salts is expected to have a relative humidity of less than about 60% at 30°C. The non-carbonate hygroscopic metal salt may also be a combination of such salts provided that good hygroscopic properties are retained.

[105] In some embodiments, the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, lithium iodide, lithium thiocyanate, potassium fluoride, potassium formate, potassium acetate, potassium thiocyanate, magnesium chloride, magnesium bromide, magnesium iodide, calcium bromide, calcium iodide, calcium acetate, calcium thiocyanate, nickel bromide, zinc chloride, zinc bromide, and zinc iodide. A vapour phase (e.g. air) in equilibrium with saturated aqueous solutions of such salts is expected to have a relative humidity of less than about 30% at 30°C. The non-carbonate hygroscopic metal salt may also be a combination of such salts provided that good hygroscopic properties are retained.

[106] In some embodiments, the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, potassium formate, and potassium acetate. The inventors have demonstrated by experiment and modelling that a neutral water balance can be maintained in direct air capture processes when using aqueous absorbent solutions containing less than 35 wt.% of such salts, across a range of climatic conditions.

[107] The aqueous absorbent solution is preferably an alkaline solution. For example, the pH of the aqueous absorbent solution may be between 8 and 12, such as between 9 and 11 , before contact with the gas stream to absorb carbon dioxide.

[108] In some embodiments, the aqueous absorbent solution further comprises a base. The base may be included to maintain the pH of the aqueous absorbent solution in a suitable range during absorption of CO2. Reaction of CO2 with a primary or secondary amine to form a carbamate releases a proton and consequently tends to reduce the pH of the solution. The proton may protonate a second involatile amine absorbent molecule (as seen in Scheme 1 ) or other base in the formulation. Including a further base to accept the released protons, ideally in preference to the involatile amine absorbent due to a higher pKa value, may provide a number of benefits, including an increased availability of the amine for CO2 capture. Moreover, efficient capture of the protons may mitigate or avoid undesirable interactions with the noncarbonate hygroscopic metal salt. For example, protonation of formate or acetate anions could produce formic or acetic acid, with the risk that such species may be lost from the aqueous absorbent solution into the gas stream.

[109] In some embodiments, the base has a pKa greater than the involatile amine absorbent, thus facilitating preferential absorption of protons released during CO2 absorption by the base instead of the involatile amine absorbent. In some embodiments, the base has a pKa higher than the anion of the non-carbonate hygroscopic metal salt, such as formate or acetate, thus facilitating preferential absorption of protons released during CO2 absorption by the base instead of the anion.

[1 10] The base may be selected from a hydroxide, a carbonate, a phosphate, a further amine having a pKa greater than the involatile amine absorbent, such as a tertiary amine or a sterically hindered amine, being a compound containing at least one primary or secondary amino group attached to either a secondary or tertiary carbon atom, and combinations thereof. For example, the base may be an alkali metal carbonate (e.g. K2CO3). Preferably, the base is itself involatile thus avoiding its loss from the aqueous absorbent solution into the gas stream.

[1 1 1] In some embodiments, the aqueous absorbent solution comprises the base in an amount of less than 10 wt.% of the aqueous absorbent solution, preferably in an amount of less than 5 wt.% of the aqueous absorbent solution, such as between 1 wt.% and 5 wt.%. The maximum molar concentration of base required is half the molar concentration of the involatile amine absorbent, assuming that all of the involatile amine absorbent reacts with CO2. In reality, CO2 capture in DAC applications is limited to about 0.3 mol CO2 per mol amine, so that base is only needed in an amount of 0.15 mol base per mol of involatile amine absorbent. Depending on the molecular weight of the base, addition of between 1 and 5 wt.% is typically sufficient to absorb all protons released by CO2 reaction with the involatile amine absorbent under capture conditions. In embodiments where the anion of the non-carbonate hygroscopic metal salt can be protonated, sufficient base is preferably added to maintain the pH above the range where protonation of the anion will occur. This avoids or minimises the formation of undesirable species such as formic acid or acetic acid.

[1 12] The aqueous absorbent solution comprises water, which may be present in an amount of at least 30 wt.%, or at least 40 wt.%, such as at least 50 wt.%.

[1 13] In some embodiments, the aqueous absorbent solution comprises: (i) an involatile amine absorbent, preferably an amino acid, in an amount of between 1 wt.% and 50 wt.% of the aqueous absorbent solution, (ii) non-carbonate hygroscopic metal salt in an amount of between 10 wt.% and 60 wt.% of the aqueous absorbent solution, (iii) water in an amount of between 30 wt.% and 80 wt.% of the aqueous absorbent solution, and optionally (iv) a base in an amount of up to 10 wt.% of the aqueous absorbent solution. In some embodiments, the aqueous absorbent solution comprises: (i) an amino acid in an amount of between 3 wt.% and 40 wt.% of the aqueous absorbent solution, (ii) non-carbonate hygroscopic metal salt in an amount of between 20 wt.% and 40 wt.% of the aqueous absorbent solution, (iii) water in an amount of between 30 wt.% and 70 wt.% of the aqueous absorbent solution, and optionally (iv) a base in an amount of up to 5 wt.% of the aqueous absorbent solution. [1 14] In some embodiments, the aqueous absorbent solution comprises absorbed carbon dioxide. Typically, the aqueous absorbent solution is cycled between absorption and desorption steps, with the carbon dioxide-lean absorbent composition produced in the desorption step being recycled to form at least a part of the aqueous absorbent solution for contact with the gas stream. Since only part of the absorbed carbon dioxide content is desorbed in the desorption step, the aqueous absorbent solution will contain carbon dioxide even immediately prior to contact with the gas stream.

[1 15] The viscosity of the aqueous absorbent solution is a relevant consideration in carbon dioxide capture processes since excessive viscosity may unacceptably reduce the kinetics of CO2 absorption and desorption. However, the inventors have found that the viscosity increase of amine absorbent solutions caused by functionally significant concentrations of non-carbonate hygroscopic metal salts is relatively minor, and it is expected that the resultant impact on mass transfer coefficient of CO2 absorption and desorption can be accommodated in a commercial-scale process. In some embodiments, the aqueous absorbent solution has a viscosity of less than 20 mPa.s, or less than 10 mPa.s, such as less than 5 mPa.s, at 30°C. The viscosity can be measured by conventional analytical methods including capillary flow viscometry and rotational rheometry. In some embodiments it is measured by rolling ball viscometry, for example using a Lovis 2000 ME rolling ball viscometer.

Absorbing carbon dioxide from the gas stream

[1 16] The process comprises a step of contacting the gas stream with the aqueous absorbent solution, thereby absorbing carbon dioxide from the gas stream into the aqueous absorbent solution to produce a carbon dioxide-lean gas and a carbon dioxide-rich absorbent composition.

[1 17] In some embodiments, the gas stream is contacted with the aqueous absorbent solution at a temperature of between -18°C and 50°C, such as between -5°C and 35°C or between 15 and 30°C. In DAC and environmental control applications, for example, the gas feed is typically air fed for contact with the aqueous absorbent solution at ambient temperature conditions. Notably, the presence of the non-carbonate hygroscopic metal salt is expected to lower the freezing point of the absorbent solution, potentially allowing CO2 capture to be conducted at sub-zero temperature. In some embodiments, the gas stream is contacted with the aqueous absorbent solution at a pressure of less than 5 bara, such as about 1 bara.

[1 18] In some embodiments, the gas (air) stream is contacted with the aqueous absorbent solution in an unconfined environment. Since the carbon dioxide-lean gas is thus released directly into the atmosphere, there is no opportunity to recover volatile components lost from the aqueous absorbent solution. The presently disclosed methods are thus particularly useful to avoid losses of water and amine from the process.

[1 19] The rate of CO2 absorption into the aqueous absorbent solution is an important consideration, particularly in applications (such as DAC) where the CO2 concentration in the gas stream is low. Therefore, the gas stream may be contacted with the aqueous absorbent solution in process equipment which provides a high surface area gas-liquid interface or provides significant mixing of gas and liquid to ensure high mass transfer. Mass transfer improvement and high surface area gas/liquid involves a trade-off with energy consumption for movement of fluids. For example, gas pressure drop should be minimised to avoid excessive energy consumption for the movement of gas through the contactor. Various gas-liquid contactor designs, such a spray towers, counter-current packed bed contactors, crossflow liquid film contactors and membrane contactors, have previously been proposed for DAC systems with this consideration in mind and it is envisaged that the presently disclosed process may use any such arrangements. The use of cooling tower equipment is considered to be particularly advantageous for CO2 capture from ambient air as the equipment is low cost and produced in large volumes. The high surface area needed to provide practically useful carbon dioxide uptake rates also renders the process susceptible to water losses, and the presently disclosed methods are thus useful to minimise or avoid this issue.

[120] In some embodiments, the net water desorption from the aqueous absorbent solution to the gas stream is approximately zero or negative, according to the principles already disclosed herein. In some embodiments, water is absorbed from the gas stream into the aqueous absorbent solution (i.e. the net water desorption from the aqueous absorbent solution to the gas stream is negative). [121] The carbon dioxide-rich absorbent composition, after separation from the carbon dioxide-lean gas, may contain absorbed carbon dioxide at a ratio of carbon dioxide to involatile amine absorbent (mol/mol) of at least 0.05, such as at least 0.1 .

Regenerating the aqueous absorbent solution

[122] After absorption of CO2 into the aqueous absorbent solution, the resultant carbon dioxide-rich absorbent composition is separated from the carbon dioxide-lean gas and is typically then sent for regeneration. The process may therefore include a step of removing carbon dioxide from the carbon dioxide-rich absorbent composition to produce a carbon dioxide-lean absorbent composition, and recycling the carbon dioxide-lean absorbent composition to the aqueous absorbent solution. In this manner, the aqueous absorbent solution may be repeatedly cycled between absorption and desorption process steps.

[123] The carbon dioxide may be removed from the carbon dioxide-rich absorbent composition by any suitable method, typically by desorption. Desorption may be achieved by heating the solution to temperatures between 80 °C and 160 °C, typically between 100 °C and 125 °C if the desorption is carried out at atmospheric pressure. Other methods of removing carbon dioxide, such as pressure reduction, reducing the solution pH and carbonate crystallisation (e.g. by addition of bis-iminoguanidines or other material which forms insoluble carbonates) are also contemplated. In each case, the CO2 removal regenerates the involatile amine absorbent (e.g. restores the amino acid in free amine form) in the aqueous absorbent solution.

[124] In embodiments where water is absorbed from the gas stream into the aqueous absorbent solution, the water may be removed together with the carbon dioxide in the desorption step, thus maintaining the overall water balance in the process.

[125] The desorbed carbon dioxide, which may be present in a high concentration CO2 stream, may then be disposed of in a manner consistent with the overall process goals. For example, in a DAC process, the desorbed carbon dioxide may be liquified by compression and cooling and injected into an underground reservoir for permanent sequestration. Alternatively, in applications where the goal is to control the carbon dioxide (and optionally also the water) content of air in a confined space, the desorbed carbon dioxide stream may simply be released outside of the confined space.

System for removing carbon dioxide from a gas stream comprising carbon dioxide

[126] The invention further relates to a system for removing carbon dioxide from a gas stream comprising carbon dioxide. The system comprises an aqueous absorbent solution as disclosed herein, an absorption unit for contacting the aqueous absorbent solution with the gas stream, thereby absorbing carbon dioxide from the gas stream into the aqueous absorbent solution to produce a carbon dioxide-lean gas and a carbon dioxide-rich absorbent composition, and a regeneration unit for removing carbon dioxide from the carbon dioxide-rich absorbent composition, thereby producing a carbon dioxide-lean absorbent composition for recycling to the aqueous absorbent solution.

[127] In some embodiments, the absorption unit includes a gas-liquid contactor selected from a spray tower, a counter-current packed bed contactor, a cross-flow liquid film contactor or a membrane contactor. The gas-liquid contactor in the absorption unit in DAC processes is preferably low cost and the use of cooling towers, either standard or bespoke designs, are advantageous in that respect. In some embodiments, the regeneration unit is a desorption unit for desorbing carbon dioxide from the carbon dioxide-rich absorbent composition, for example by temperature increase and/or pressure decrease. The desorption unit can be a counter-current packed bed, with steam used as the stripping gas, which may be produced in the reboiler. The desorption unit can operate at sub-atmospheric pressure to lower the temperature of the regeneration process. The desorption unit may also consist of one or more flash units in which the carbon dioxide-rich absorbent solution is heated and injected into a vessel at lower pressure upon which steam and CO2 are released.

[128] Depicted in Figure 1 is a system 100 for removing carbon dioxide from a gas stream according to some embodiments of the invention. In particular, system 100 may be a system for direct air capture. System 100 includes absorption unit 1 10, in the form of a gas-liquid contactor. Absorption unit 1 10 is configured to receive gas feed 1 12 (air in a DAC process) for contact with aqueous amine absorbent 114 in gas absorption contact region 116. Carbon dioxide is thus absorbed into the aqueous amine absorbent 1 14, and the resultant carbon dioxide-lean gas 1 18 separates from the carbon dioxiderich absorbent 120 and is emitted from absorption unit 110.

[129] In some embodiments, aqueous amine absorbent 1 14 is fed at the top of absorption unit 1 10 and flows as a thin film under influence of gravity over packing located in gas absorption contact region 1 16. This provides a high surface area interface between gas feed 1 12 and aqueous amine absorbent 1 14, facilitating the absorption of CO2 into the absorbent. Gas feed 1 12 may suitably flow upwards in a counter-current mode to the flow of absorbent or it may flow orthogonally to the flow of absorbent in a cross-current design. In some embodiments, gas absorption contact region 116 is an unconfined environment, thus operating at approximately atmospheric pressure (1 bara) and exposed to the atmosphere. Air may thus be blown, as gas feed 1 12, by fans through gas absorption contact region 1 16 for contact with aqueous amine absorbent 1 14, thereafter passing out of absorption unit 1 10 as carbon dioxide-lean gas 118 to the atmosphere.

[130] Aqueous amine absorbent 1 14 may have a composition according to any of the embodiments disclosed herein in the context of the process of the invention, and thus comprises non-carbonate hygroscopic metal salt in an amount of at least 10 wt.%. Advantageously, the hygroscopic metal salt reduces, and in some embodiments negates or even reverses, the loss of water from aqueous absorbent solution 1 14 into carbon dioxide-lean gas 1 18 which would occur in the absence of the hygroscopic metal salt.

[131] Optionally, carbon dioxide-rich absorbent composition 120 may be recirculated around absorption unit 1 10 via recirculation loop 122 to increase the total uptake of carbon dioxide prior to regeneration. In this case, only a slip stream of carbon dioxide-rich absorbent composition 120 is sent via desorber feed line 124 for regeneration. An advantage of this arrangement is that the recirculation assists to cool the inlet temperature of aqueous amine absorbent 1 14, to that the temperature during absorption approaches the wet bulb temperature. Therefore, the capacity of air 118 for carrying water is reduced. [132] System 100 includes desorption unit 130 for desorbing carbon dioxide from carbon dioxide-rich absorbent composition 120. The resultant carbon dioxide-lean absorbent composition 132 is recycled via desorber return line 134 to form part of aqueous absorbent solution 1 14 fed to absorption unit 110.

[133] Desorption unit 130 may be configured to heat carbon dioxide-rich absorbent composition 120, thus driving the desorption of carbon dioxide in a temperature swing process. Preferably, desorption unit 130 confines the absorbent thus allowing the desorbed carbon dioxide stream 136 to be directed to a desired location. In some embodiments, carbon dioxide-rich absorbent composition 120 is fed at the top of desorption unit 130 and flows under influence of gravity over packing located in gas desorption region 138. This provides a high surface area which facilitates the desorption of carbon dioxide, and optionally also water if required to balance the water content of the system, from solution. Steam 139 may be added to desorption unit 139 as a stripping gas and heat source, to facilitate desorption of carbon dioxide. Carbon dioxide-lean absorbent composition 132 then flows out of the bottom of desorption unit 130.

[134] In a temperature swing process, carbon dioxide-lean absorbent composition 132 flowing through desorber return line 134 and carbon dioxide-rich absorbent composition 120 flowing through desorber feed line 124 may both be passed through heat exchanger 140, allowing heat transfer from the lean to the rich solution and thus improved energy efficiency.

[135] During continuous operation, aqueous absorbent solution 114 fed to absorption unit 1 10 thus comprises recycled carbon dioxide-lean absorbent composition 132, optionally supplemented by absorbent recirculated around absorption unit 1 10 via recirculation loop 122 and, when needed, make-up absorbent solution fed via make-up line 142.

EXAMPLES

[136] The present invention is described with reference to the following examples. It is to be understood that the examples are illustrative of and not limiting to the invention described herein. Example 1 . Solubility studies

[137] The solubility limits of different amino acid salts, i.e. an equimolar mixture of amino acids and potassium hydroxide, in various hygroscopic salt solutions at ambient temperatures were assessed. The amino acid salts and hygroscopic salts were purchased from commercial suppliers, including Vosun Chemical Co., Ltd. Aqueous solutions of each hygroscopic salt were prepared first at the concentrations specified in Table 1. Then a small amount of an amino acid salt was added to the solutions and mixed until dissolved. Additional amounts of the amino acid salt were added periodically until the amino acid salt was no longer dissolved. The concentrations of amino acid salts at this point were noted as the maximum amount of amino acid salts that could be dissolved in such solutions, as indicated in Table 1 .

Table 1.

[138] The solubility of amino acids in the potassium carboxylate solutions was very high. More limited solubility was found in a lithium chloride and lithium bromide solutions, although concentrations below 0.5 mol/L remain useful for some CO2 absorption applications.

Example 2. Mass transfer performance

[139] A wetted wall column (WWC) was used to evaluate the CO2 absorption rate with a range of absorption liquid formulations. In this system, aqueous mixtures of an amino acid salt (potassium taurate) and different hygroscopic salts were tested, at a temperature of 25 °C and a liquid rate of 125 mL/min, while the gas rate varied from 3 L/min to 6 L/min. Based on the number of CO2 moles absorbed in the experiments and the CO2 partial pressure difference between the gas and equilibrium (referred to as driving force), mass transfer coefficients were determined. The method was similar to one described in the literature (Chiao-Chien Wei, Graeme Puxty, Paul Feron, 2014, Amino acid salts for CO2 capture at flue gas temperatures, Chemical Engineering Science 107, 218-226), as a standard method to determine mass transfer coefficients of CO2 into various absorption liquids.

[140] The CO2 overall mass transfer coefficients into the potassium carboxylate salt solutions are shown in Figure 2. The CO2 absorption rate was slightly reduced for the hygroscopic 2 M taurate solutions (containing 44 wt.% potassium formate or 42 wt.% potassium acetate, produced by dissolving the taurate salt in 58 wt.% potassium formate or potassium acetate solution), compared to 2M taurate solutions lacking hygroscopic salts. Nevertheless, the CO2 absorption rate was still acceptably high, and can potentially be increased by increasing the concentration of amino acid salt, which is possible as seen from Example 1 . The mass transfer coefficients for CO2 absorption into the solutions increased with gas velocities, indicating that CO2 mass transfer resistance in gas phase can also be important, whereas mass transfer resistance in gas phase is generally not considerable in CO2 capture from flue gases.

[141] The CO2 absorption rate into a hygroscopic 58 wt.% potassium formate solution, lacking any amine absorbent, was found to be much lower than the solutions containing the amino acid salt, thus demonstrating that the amino acid remains the primary C02-absorbent compound in the solutions. Acetate and formate solutions are slightly alkaline and will therefore absorb CO2 at a high rate than water alone.

[142] The CO2 overall mass transfer coefficients into the lithium halide salt solutions are shown in Figure 3. A lower concentration of amino acid (0.45-0.5 M) was used due to solubility limitations (see Example 1 ), so that the solutions contained 0.5 M taurate and 40 wt.% lithium bromide or 0.45 M taurate and 28 wt.% lithium chloride (produced by dissolving the taurate salt in 43 wt.% LiBr or 30 wt.% LiCI). Correspondingly lower CO2 absorption rates were thus obtained.

[143] The mass transfer rate of CO2 into the hygroscopic absorbent solutions may be affected by the increased viscosity of these solutions. The viscosity of hygroscopic absorbent solutions (0.5 M taurate in 28% LiCI or 2M taurate solutions in 44 wt.% potassium formate) and non-hygroscopic absorbent solutions (0.5 M and 2M taurate) was thus measured by a viscosimeter (Lovis 2000 ME microviscometer), and the results are shown in Figure 4. The viscosity of the absorbent solutions comprising substantial amounts of hygroscopic metal salts was higher than the viscosity than the equivalent amine solutions without the hygroscopic metal salt, but remained within an acceptable range. Both hygroscopic solutions had viscosities of below 5 mPa.s at 30°C. Moreover, the viscosity increase that was seen for the absorbent containing potassium formation had only limited effect on CO2 absorption rate, as seen in Figure 2.

Example 3. Water vapour liquid equilibrium

[144] An apparatus for measuring the water vapour pressure above a hygroscopic salt solution is schematically depicted in Figure 5. For each experiment, a solution 302 was placed in sealed jar 304, and pump 306 circulated air 308 from above the solution through humidity sensor 310 and back into the jar where it was bubbled through the solution. The humidity values were recorded electronically to humidity logging device 312. The whole system, including the pipes and relief valve 314, was placed in oven 316 to maintain a target temperature. After starting the pump, the system was left to equilibrate and reach a steady state condition. It is assumed that steady state was achieved once the measured humidity data varied no more than 1 % over a period of 30 minutes.

[145] The humidity of air was thus measured above solutions of the following hygroscopic salts - lithium chloride, lithium bromide, potassium formate and potassium acetate - at salt concentrations of between 0 and 72 wt.% at a temperature of 30°C. The results are shown in Figure 6, with comparison against the results predicted by Raoult’s law. The relative humidity of air in equilibrium with the solutions of lithium chloride, lithium bromide, potassium formate and potassium acetate reduced with increased concentration of the salts, with significant deviations from the values predicted by Raoult’s law due to the hygroscopicity of the salts.

[146] The relative humidity of air above an aqueous solution containing only potassium taurate, in an amount of 31 wt.%, was also determined. The result, also shown in Figure 6, is consistent with the predictions of Raoult’s law, confirming that the amino acid is not itself a hygroscopic salt. [147] Also shown in Figure 6 is the relative humidity of air above aqueous solutions containing various concentrations of potassium formate, as hygroscopic metal salt, together with 29 wt.% potassium taurate, as involatile amine absorbent (concentrations relative to the total weight of the solution). The results show that addition of the amino acid does not inhibit the hygroscopicity of potassium formate in solution. In fact, addition of potassium taurate caused a further drop in hygroscopicity relative to potassium formate solutions with the same potassium formate concentration.

Example 4. Water loss/gain in absorbent under direct air capture conditions

[148] A wetted wall column (WWC; as also used in Example 2), was used to evaluate the loss or gain of water from an aqueous absorbent solution comprising an involatile amine absorbent and non-carbonate hygroscopic metal salt under direct air capture operating conditions. Four experiments were conducted with a 2M taurate in 44 wt.% potassium formate solution (produced by dissolving taurate salt in 58 wt.% potassium formate solution). Inlet synthetic air (-420 ppm CO2 in nitrogen) with different humidities, 0, 43, 82, and 93% was fed to the system at a rate of 3.5 L/min and contacted with the absorption liquid at 125 mL/min. The experiments were conducted at ambient pressure and temperature. The results are shown in Figure 7 and Table 2.

Table 2.

^a Negative value indicates water loss from the absorbent solution.

[149] CO2 was absorbed from the air feed into the aqueous absorbent solution in all experiments. With very dry air feed (0.4% humidity), water was lost from the absorbent solution to the treated air. With an air feed with 43% humidity, water was still lost from the absorbent solution to the treated air, although the system was close to balanced. When using air feeds with 82.5 and 93% humidity, water was captured from the air feed into the absorbent solution. It can be expected that the water gain/loss from the absorbent would be zero if using an air feed with a humidity of approximately 50%.

[150] The results demonstrate, under direct air capture conditions using water- unsaturated air feeds: (i) the effectiveness of the involatile amine absorbent to capture CO2 despite the presence of the hygroscopic metal salt, and (ii) the effectiveness of the hydroscopic metal salt to suppress the loss of water from the absorbent solution, or even to simultaneously capture water and CO2.

Example 5. Water balance modelling in a typical DAC system

[151] Using the results in Examples 2 and 3, the loss/gain of water from the absorber of a typical DAC process as detailed in Kiani et al. (Techno-Economic Assessment for CO2 Capture From Air Using a Conventional Liquid-Based Absorption Process. Frontiers in Energy Research, 2020, 8) but operating at a scale of 200 t/a CO2 capture, was modelled for two absorbent solutions: 2 M taurate in water (System 1 ) and 2M taurate in 37 wt.% potassium formate aqueous solution (System 2). Operation of the process was modelled in three different climates: (A) London, (B) Chinchilla in Queensland, Australia and (C) Singapore. These three climates have been selected as they have distinctive temperature and humidity conditions, two of the most important factors affecting the extent of water loss/gain in a DAC system. The air temperature and humidity data for all these climates over a typical 24 hours was retrieved from online weather sources and used to model water loss/gain in this system.

[152] Figure 7 shows a schematic diagram of a cooling-tower based absorber considered here for CO2 capture from ambient air. From the water loss point of view, the main parameters are the inlet temperature (Ta n), outlet temperature (Ta.out) and humidity (%RHin) of the ambient inlet air, the temperature (TL, in) and composition of the absorption liquid and the air flow rate. As discussed in previous sections, the type and concentration of hygroscopic salts used in such systems determine the water vapour pressure which can be correlated to the humidity of the outlet air (%RH_0Ut). Using these parameters and Equation 1 , we used the psychrometric chart to determine the water loss at different conditions in this process.

Mloss/gain = V * (Wout ^— Win) (1 ) where Mioss/gain is water lost or gained during the process in kg/s, V is the air flow rate in m³/s, and Wout and Win are the mass of water per volume of air in inlet and outlet in kg/m³. Wout and Win are extracted from psychrometric chart using the temperature and humidy of inlet and outlet air streams.

[153] Figure 8 shows the tonne of water lost from or gained into the absorbent solution, per tonne of CO2 absorbed, in the DAC process. Significant water losses were found when using the System 1 absorbent in all three climates. By contrast, the use of the System 2 absorbent containing hygroscopic metal salts resulted in a net water gain.

[154] The concentrations of the hygroscopic salts required to obtain negligible water loss/gain was also calculated, and the results are shown in Table 3. It is noted that the concentrations of salts required for neutral water balance are significantly below the concentrations investigated in Example 2, so that the effects of the hygroscopic salts on the CO2 mass transfer coefficients and absorbent viscosity will be less than measured in that Example. In Example 2, we simply considered a hygroscopic salt concentration at which the water vapour pressure (outlet air humidity) is reduced by around 50% which is not necessarily required for the most climates.

Table 3.

[155] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.

Claims

1 . A process for removing carbon dioxide from a gas stream comprising carbon dioxide, the process comprising contacting the gas stream with an aqueous absorbent solution comprising (i) an involatile amine absorbent, (ii) non-carbonate hygroscopic metal salt in an amount of at least 10 wt.% of the aqueous absorbent solution, and (iii) water, thereby absorbing carbon dioxide from the gas stream into the aqueous absorbent solution to produce a carbon dioxide-lean gas and a carbon dioxide-rich absorbent composition.

2. The process according to claim 1 , wherein the aqueous absorbent solution comprises the non-carbonate hygroscopic metal salt in an amount sufficient that net water desorption from the aqueous absorbent solution to the gas stream is zero or negative.

3. The process according to claim 1 or claim 2, wherein the gas stream further comprises water vapour, wherein the gas stream has a relative humidity of less than 80%.

4. The process according to claim 3, wherein water is absorbed from the gas stream into the aqueous absorbent solution.

5. The process according to any one of claims 1 to 4, wherein the non-carbonate hygroscopic metal salt is present in an amount of at least 20 wt.% of the aqueous absorbent solution.

6. The process according to any one of claims 1 to 5, wherein air in equilibrium with the aqueous absorbent composition has a relative humidity of less than 80% at the temperature of the carbon dioxide-lean gas when separated from the carbon dioxide-rich absorbent composition.

7. The process according to any one of claims 1 to 6, wherein the hygroscopic metal salt has the property that air in equilibrium with a saturated aqueous solution consisting of the non-carbonate hygroscopic metal salt and water has a relative humidity of less than 60% at 30°C. The process according to any one of claims 1 to 7, wherein the hygroscopic metal salt has the property that air in equilibrium with a saturated aqueous solution consisting of the non-carbonate hygroscopic metal salt and water has a relative humidity of less than 30% at 30°C. The process according to any one of claims 1 to 8, wherein the non-carbonate hygroscopic metal salt comprises a cation selected from the group consisting of alkali metals, alkali earth metals and nickel, and wherein the non-carbonate hygroscopic metal salt comprises an anion selected from the group consisting of halides, Ci-Ce alkyl or aryl carboxylates, nitrate and thiocyanate. The process according to any one of claims 1 to 9, wherein the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, lithium iodide, lithium nitrate, lithium thiocyanate, sodium bromide, sodium iodide, sodium thiocyanate, potassium fluoride, potassium formate, potassium acetate, potassium nitrite, potassium thiocyanate, magnesium chloride, magnesium bromide, magnesium iodide, magnesium nitrate, magnesium thiocyanate, calcium bromide, calcium iodide, calcium acetate, calcium nitrate, calcium thiocyanate, strontium iodide, strontium thiocyanate, barium iodide, chromium chloride, manganese chloride, manganese bromide, iron bromide, cobalt bromide, cobalt nitrate, nickel chloride, nickel bromide, copper nitrate, zinc chloride, zinc bromide, zinc iodide, zinc nitrate, cerium chloride, and combinations thereof. The process according to any one of claims 1 to 10, wherein the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, lithium iodide, lithium thiocyanate, potassium fluoride, potassium formate, potassium acetate, potassium thiocyanate, magnesium chloride, magnesium bromide, magnesium iodide, calcium bromide, calcium iodide, calcium acetate, calcium thiocyanate, nickel bromide, zinc chloride, zinc bromide, zinc iodide, and combinations thereof. The process according to any one of claims 1 to 11 , wherein the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, potassium formate, and potassium acetate. The process according to any one of claims 1 to 12, wherein the involatile amine absorbent is selected from the group consisting of amino acids or salts thereof, polyamines comprising both quaternised and neutral amine groups, high molecular weight amines and combinations thereof. The process according to any one of claims 1 to 13, wherein the involatile amine absorbent is an amino acid or salt thereof, wherein the amino acid is selected from the group consisting of taurine, sarcosine, alanine, glycine, lysine, dimethylglycine, proline, phenyl-alanine, glucosamine, arginine, methyl-taurine, cysteine, tryptophan, hydroxyproline, asparagine, tyrosine, histidine, glutamine, diglycine, serine, methionine and combinations thereof. The process according to any one of claims 1 to 14, further comprising (iv) a base selected from a hydroxide, a carbonate, a phosphate, a further amine having a pKa greater than the involatile amine absorbent, and combinations thereof. The process according to any one of claims 1 to 15, wherein the gas stream is air. An aqueous absorbent solution for carbon dioxide capture, comprising (i) an involatile amine absorbent, (ii) non-carbonate hygroscopic metal salt in an amount of at least 10 wt.% of the aqueous absorbent solution, and (iii) water. The aqueous absorbent solution according to claim 17, wherein the non-carbonate hygroscopic metal salt is present in an amount of at least 20 wt.% of the aqueous absorbent solution. The aqueous absorbent solution according to claim 17 or claim 18, wherein air in equilibrium with the aqueous absorbent composition has a relative humidity of less than 80% at 30°C. The aqueous absorbent solution according to any one of claims 17 to 19, wherein the hygroscopic metal salt has the property that air in equilibrium with a saturated aqueous solution consisting of the non-carbonate hygroscopic metal salt and water has a relative humidity of less than 60% at 30°C. The aqueous absorbent solution according to any one of claims 17 to 20, wherein the hygroscopic metal salt has the property that air in equilibrium with a saturated aqueous solution consisting of the non-carbonate hygroscopic metal salt and water has a relative humidity of less than 30% at 30°C. The aqueous absorbent solution according to any one of claims 17 to 21 , wherein the non-carbonate hygroscopic metal salt comprises a cation selected from the group consisting of alkali metals, alkali earth metals and nickel, and wherein the non-carbonate hygroscopic metal salt comprises an anion selected from the group consisting of halides, Ci-Ce alkyl or aryl carboxylates, nitrate and thiocyanate. The aqueous absorbent solution according to any one of claims 17 to 22, wherein the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, lithium iodide, lithium nitrate, lithium thiocyanate, sodium bromide, sodium iodide, sodium thiocyanate, potassium fluoride, potassium formate, potassium acetate, potassium nitrite, potassium thiocyanate, magnesium chloride, magnesium bromide, magnesium iodide, magnesium nitrate, magnesium thiocyanate, calcium bromide, calcium iodide, calcium acetate, calcium nitrate, calcium thiocyanate, strontium iodide, strontium thiocyanate, barium iodide, chromium chloride, manganese chloride, manganese bromide, iron bromide, cobalt bromide, cobalt nitrate, nickel chloride, nickel bromide, copper nitrate, zinc chloride, zinc bromide, zinc iodide, zinc nitrate, cerium chloride, and combinations thereof. The aqueous absorbent solution according to any one of claims 17 to 23, wherein the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, lithium iodide, lithium thiocyanate, potassium fluoride, potassium formate, potassium acetate, potassium thiocyanate, magnesium chloride, magnesium bromide, magnesium iodide, calcium bromide, calcium iodide, calcium acetate, calcium thiocyanate, nickel bromide, zinc chloride, zinc bromide, zinc iodide, and combinations thereof. The aqueous absorbent solution according to any one of claims 17 to 24, wherein the non-carbonate hygroscopic metal salt is selected from the group consisting of lithium chloride, lithium bromide, potassium formate, and potassium acetate. The aqueous absorbent solution according to any one of claims 17 to 25, wherein the involatile amine absorbent is selected from the group consisting of amino acids or salts thereof, polyamines comprising both quaternised and neutral amine groups, high molecular weight amines and combinations thereof. The aqueous absorbent solution according to any one of claims 17 to 26, wherein the involatile amine absorbent is an amino acid or salt thereof, wherein the amino acid is selected from the group consisting of taurine, sarcosine, alanine, glycine, lysine, dimethylglycine, proline, phenyl-alanine, glucosamine, arginine, methyltaurine, cysteine, tryptophan, hydroxyproline, asparagine, tyrosine, histidine, glutamine, diglycine, serine, methionine and combinations thereof. The aqueous absorbent solution according to any one of claims 17 to 27, further comprising (iv) a base selected from a hydroxide, a carbonate, a phosphate, a further amine having a pKa greater than the involatile amine absorbent, and combinations thereof. The aqueous absorbent solution according to any one of claims 17 to 28, further comprising absorbed carbon dioxide at a ratio of carbon dioxide to involatile amine absorbent (mol/mol) of at least 0.05. A system for removing carbon dioxide from a gas stream comprising carbon dioxide, comprising: an aqueous absorbent solution according to any one of claims 17 to 29; an absorption unit for contacting the aqueous absorbent solution with the gas stream, thereby absorbing carbon dioxide from the gas stream into the aqueous absorbent solution to produce a carbon dioxide-lean gas and a carbon dioxide-rich absorbent composition; and a regeneration unit for removing carbon dioxide from the carbon dioxide-rich absorbent composition, thereby producing a carbon dioxide-lean absorbent composition for recycling to the aqueous absorbent solution.