The invention is in the field of CO2 capture. The invention is in particular directed to a method to recover CO2 from a feed fluid and a system for the method.

Carbon dioxide (CO2) is a naturally occurring compound. The amount of CO2 emission has significantly increased in the past decades due to i.a. industrial processes that burn fossil fuels. The increasing amount is considered to be one of the major causes for climate change.

It is accordingly widely acknowledged that it is desirable to minimize carbon dioxide emission, and move towards a more sustainable and circular economy (i.e. a system of closed loops in which renewable sources are used and in which the used materials lose their value as httle as possible). Several methods to achieve this goal have been presented, such as carbon taxes ( e.g . taxes for each ton CO2 emitted) and alternative energy sources such as solar, wind and hydro-powered sources. However, increased taxes may lead to resistance of i.a. the industry and the alternative energy sources generally depend on the amount of available sunlight, wind, water etc.

Another method to minimize the carbon dioxide emission is to capture and optionally regenerate the CO2 from e.g. the flue gasses from industry. Captured and/or regenerated carbon dioxide may i.a. be stored underground or may be sold for further use such as for enhanced oil recovery.

Conventional CO2 capture and regeneration include processes as described in US1783901 and typically comprise feeding a carbon dioxide containing gas to an absorber wherein an alkaline solvent, often an amine- based solvent, is provided. The CO2 may react with the solvent leaving a C0 ₂ -lean gas that can leave the absorber. The C0 ₂ -rich stream may be fed to the top of a regeneration column (e.g. a stripper) passing through e.g. a cross heat exchanger to recover part of the available thermal energy. Typically, a reboiler is present at the bottom of the regeneration column which can provide the required energy to reverse the reaction between the carbon dioxide and the solvent, thereby releasing (i.e. regenerating) carbon dioxide and vaporizing at least part of the solvent. The reboiler however requires additional energy. The gas produced in the reboiler may flow upwards in the regeneration column and may accordingly promote solvent regeneration. The vapor that may leave the top of the regeneration column (; i.a . water, volatile components) can be condensed and recycled back to the process. The regenerated solvent can also be recycled.

An additional drawback of these systems is that they require additional steps to further process the CO2 gas, such as compression before it can be e.g. stored and/or transported.

A further drawback of the above-described conventional CO2 capture processes is that they are generally not be suitable for gases that comprise molecular oxygen due to degradation of the solvent. Degradation of the solvent may pose problems such as emissions, corrosion and/or safety issues.

Alternatively, CO2 can be captured from the air (direct air capture, also referred to as DAC) or from seawater. The concentration of CO2 in seawater is in equihbrium with that in the atmosphere. Therefore, capturing CO2 from seawater induces natural uptake of CO2 from the atmosphere, thus leading to indirect air capture.

Processes for direct air capture are typically based on absorption in alkaline hquids, or adsorption on solids, with subsequent desorption.. An example, as described in US8119091, rehes on absorbing CO2 in an aqueous solution of sodium hydroxide. Other examples rely on using KOH solutions, forming K2CO3. The C0 ₂ -depleated air leaves the absorber (also referred to as contactor). After an ion exchange reaction with Ca(OH)2, CaCOe is formed. The CO2 release from CaCOe requires temperatures as high as 900 °C, which typically requires burning of fossil fuels, thus lowering the CO2 avoidance potential of said technology.

Another method for carbon capture is disclosed in EP2163294, in which carbon dioxide is separated from the solvent by bipolar membrane electrodialysis.. However, the bipolar membrane electrodialysis based separation of carbon dioxide from the solvent is typically energy-intensive and expensive.

US2020/0038803 discloses a system and a method to regenerate a carbon-rich amine solution produced in a carbon dioxide capture from a mixed gas. The system comprises a bipolar membrane electrodialysis apparatus and a carbon dioxide removal apparatus.

Another example is presented in EP2329875, that discloses a method comprising providing water and processing the water using electrochemical steps to generate an acidic and an alkahne solution. The acidic solution is neutralized, and the alkaline solution is used for capturing carbon dioxide. However disadvantageously, the reaction product formed by the reaction of carbon dioxide with the alkaline solution is disposed and neither the CO2 nor the alkaline solution is regenerated, leading to substantial waste streams.

Processes for capturing CO2 from seawater typically rely on pH swing of the media, by applying an electrical current in an electrochemical cell, which leads to the in-situ formation of acids or bases. When acids are formed, CO2 is stripped as a gas. Major drawbacks of this method are high energy demand and slow kinetics.

The CO2 produced by any of these capture processes is typically compressed so that it can be liquified (typically at pressures between 7 and 25 bar) for truck, rail or ship transport or transported in pipelines (typically at pressures above 30 bar). Due to the need for compression, producing CO2 at elevated pressures lowers the process costs and electricity demand. A drawback of electrochemical CO2 regeneration processes is that those are typically operated at atmospheric pressure.

It is an object of the present invention to provide an improved method for recovering carbon dioxide from a feed fluid that overcomes at least part of the above-mentioned drawbacks. The present inventor surprisingly found that a such a method can be obtained using a basic liquid and an acidic stripping agent that can be regenerated using electrical separation.

Figure 1 illustrates a schematic overview of the method according to the present invention.

Figure 2 illustrates a schematic overview of a preferred embodiment of the method according to the present invention.

Figure 3 illustrates a schematic overview of a preferred embodiment of the method according to the present invention.

Figure 4 illustrates a schematic overview of a system according to the present invention.

Figure 5 illustrates a schematic overview of a preferred embodiment of a system according to the present invention.

Figure 6 illustrates a schematic overview of a suitable electrical separator comprising electrodialysis.

Thus, in a first aspect the present invention is directed to a method for at least partially recovering CO2 from a feed fluid(l). The method is schematically illustrated in Figure 1 and comprises:

- providing the feed fluid (1) in an absorber (2) that comprises a basic liquid to absorb CO2 in the basic liquid to obtain a C0 ₂ -rich liquid stream (3) and a C0 ₂ -lean fluid stream (e.g. a C0 ₂ -lean liquid or C0 ₂ -lean gas stream) (4).

- leading the C0 ₂ -rich liquid stream (3) to a desorption vessel (5) that comprises an acidic stripping agent to obtain a C0 ₂ -rich gas stream (6) and a salt solution stream (7). - recovering the C0 ₂ -rich gas stream from the desorption vessel to obtain a recovered C0 ₂ -gas stream;

- leading the salt solution stream to an electrical separator (8) to subject the salt solution stream to electrical separation to obtain a regenerated acidic stripping agent stream (9) and a regenerated basic liquid stream (10).

Advantageously, each of the steps has a minimal complexity and highly specialized apparatuses may not be needed. Additionally, the individual steps typically present more possibilities for individually and separately choosing the conditions such as the pressure and temperature.

Any feed fluid may suffice as feed fluid as long as carbon dioxide is present. The feed fluid may be a feed gas, or a feed liquid. Suitable feed gasses may for instance further comprise molecular oxygen ( e.g . up to 21 vol%) and/or molecular nitrogen. Examples of suitable feed gasses include flue gasses from industrial processes, air (e.g. for direct air capture, also referred to as DAC), exhaust gases from engines, natural gas and/or biogas. The method may accordingly be used for any process that desires the capture of carbon dioxide, these processes may include, but are not limited to, natural gas sweetening, biogas upgrading and/or hydrogen production. Suitable feed liquids comprise water. Water is herein meant any water obtained from a water source that is in direct contact with the atmosphere, such that the water in the water source can dissolve CO2 from the atmosphere. Examples thereof include sea water which is particularly preferred as feed fluid.

The feed fluid, such as the feed gas, is provided in an absorber (herein also referred to as contactor) that comprises a basic liquid. The basic liquid is chosen such that it is capable of absorbing carbon dioxide. Preferably, the basic liquid selectively absorbs carbon dioxide. This may be beneficial to limit any contamination of the basic hquid that may complicate the regeneration thereof. Absorption of the CO2 may for instance be achieved by a reaction between CO2 and the basic liquid. Such a reaction is typically exothermic (i.e. energy is released), which may result in an increased temperature in the absorption vessel. It may be preferred that the feed fluid is provided in the absorber at a temperature between 20 and 80 °C, such as approximately 40 °C. In the absorber, the temperature may be between 20 and 80 °C, such as between 30 and 50 °C, such as approximately 40 °C.

The basic liquid preferably comprises an organic and/or inorganic liquid, preferably an inorganic liquid. In contrast to organic hquids such as monoethanolamine (MEA), methyldiethanolamine (MDEA) and piperazine (PZ), inorganic liquids generally allow for no degradation of the basic liquid if molecular oxygen is present in the feed fluid. Degradation is typically associated with lower process efficiency and issues such as emission, corrosion, foaming, equipment fouling and safety hazards. It is particularly preferred that the basic liquid comprises ammonia (NH3) and/or a metal hydroxide, for example alkali hydroxide such as NaOH and/or KOH or alkaline hydroxide. Basic liquids comprising amino acid salts, such as potassium taurate or potassium alanate may also be suitable. The concentration of ammonia, amino acid salt and/or the metal hydroxide in the basic liquid is preferably at least 1 mol/liter. The metal hydroxide may typically be present in approximately 10 wt% and the ammonia may typically be present in around 5 wt%.

The type of absorber or contactor is not particularly hmiting for a feed gas. In such cases, an absorber column may be suitable. In case of a feed liquid, it is typically not preferred to have the feed hquid and basic liquid in direct contact in order to prevent mixing of the liquids. In such cases, the basic liquid is preferably indirectly contacted with the C0 ₂ -rich water, preferably by using a membrane contactor.

A membrane contactor is a device that may allow for the indirect contact and component exchange between fluids. Typically, membrane contactors are used for gas-liquid contact, but may be extended to liquid- liquid contact. The contactor typically uses hollow fiber membranes, such that the fluid comprising the compound of interest ( e.g . CO2) is fed on one side of the membrane and the basis liquid on the other side of the membrane. Which side is preferred for the fluid and for the basic liquid may depend on i.a. the type of membrane, reaction conditions.

The membranes that are often used for membrane contactors are porous membranes that allow for a flux of components through the membrane. The compound of interest may migrate from one fluid to the other side of the membrane through the pores of the membrane. The rate of transfer is typically determined by the driving force, that depends on i.a. the partial pressure and fugacity of the compound of interest. Additionally, the basic liquid (i.e. fluid that receives the compound of interest) may determine the selectivity of the process. As more compound is removed from the basic liquid phase, the driving force is shifted towards more removal. As such, it can be imagined that it is preferred that the liquids do not mix and are separated by a membrane that serves as selective barrier.

In case the feed fluid comprises water (e.g. seawater), a hydrophobic porous membrane contactor is preferred, to avoid wetting of the pores and promote CO2 transfer from the water to the basic liquid. To avoid loss of basic liquid to the water, basic liquids with no vapor pressure are preferred, including basic liquids comprising alkali hydroxides such as NaOH and/or KOH, as well as amino acid salts, such as potassium taurate or potassium alanate.

Absorption of CO2 in the basic liquid results in a C0 ₂ -rich liquid stream and a C0 ₂ -lean fluid stream. Depending on the feed fluid, this may be a C0 ₂ -lean gas stream, or a C0 ₂ -lean liquid stream. The C0 ₂ -lean fluid stream may be recovered from the absorber. In case of a C0 ₂ -lean liquid stream, such as a C0 ₂ -lean seawater stream, this may be redirected to the water source, such as the sea. The CO2 removal rate is typically more than 90%, preferably more than 95%, more preferably more than 99% based on the total CO2 content in the feed gas stream.

The C0 ₂ -rich liquid stream is led to a desorption vessel. In the desorption vessel, CO2 is desorbed from the C0 ₂ -rich liquid stream. The pressure in the desorption vessel may be ambient pressure (i.e. approximately 1 bar). However, it may be preferred that the pressure in the desorption vessel is at least 2 bar, preferably at least 4 bar, more preferably at least 5 bar, even more preferably at least 6 bar, most preferably at least 7 bar. The pressure may depend on the feed fluid. In case of a feed gas, it may be preferred to have the pressure in the desorption vessel to be at least 2 bar and in case of a feed liquid at least 1 bar. An elevated pressure (i.e. above ambient pressure) may be beneficial as the recovered C0 ₂ -gas stream may accordingly be under pressure (typically at pressures up to 10 bar) and the CO2 can be directly liquefied or at least further processing steps may be minimized for the CO2 to be suitable for e.g. storage and/or transport. The pressure typically dictates the pressure at which the C0 ₂ -rich liquid stream is lead to the desorption vessel. For instance, the C0 ₂ -rich liquid stream may be pressurized before leading it to the desorption vessel to reach a similar pressure. Alternatively, if the pressure in the desorption vessel is at ambient pressure, the recovered C0 ₂ -gas stream may be further processed, by for instance leading it to a compressor which may be followed by liquifying. Accordingly, the method may further comprise liquefying and/or compressing the recovered C0 ₂ -gas stream.

The temperature of the C0 ₂ -rich liquid stream generally depends on the temperature at which it is obtained from the absorption vessel. Accordingly, the temperature is typically between 20 and 80 °C, preferably between 30 and 50 °C, more preferably the temperature is around 40 °C. Similarly, the temperature in the desorption vessel is typically dictated by the temperature of the incoming C0 ₂ -rich liquid stream. Advantageously, there is little to no need for input of thermal energy to the desorption vessel. The desorption vessel comprises an acidic stripping agent. The stripping agent may advantageously be used to obtain a C0 ₂ -rich gas stream and a salt solution stream. The stripping agent promotes the release of CO2 from the C0 ₂ -rich liquid stream. Additionally, the acidic stripping agent typically reacts with basic hquid present in the C0 ₂ -rich liquid stream to form a salt.

The acidic stripping agent preferably comprises an organic and/or inorganic acid solution, preferably said acid solution comprises an organic acid . The acidic stripping agent is preferably a relatively weak acid, with pKa preferably above 0, such as above pKa 2, as this minimizes the energy demand of the process. Organic acids are typically preferred over inorganic acids, such as strong inorganic acids (i.e. pKa < 0), such as H2SO4 and HC1 as the regeneration of organic acids may be energetically more favorable. Particularly, it is preferred that the organic acid is selected from the group consisting of lactic acid, citric acid, acetic acid, formic acid, fumaric acid, picolinic acid, propionic acid, pyruvic acid, succinic acid, tartaric acid, butyric acid, glyceric acid, mahc acid, salicylic acid and combinations thereof.

Leading the C0 ₂ -rich liquid stream to the desorption vessel comprising an acidic stripping agent may thus result in a C0 ₂ -rich gas stream and a salt solution stream. For example, if NaOH is used as basic liquid and lactic acid as acidic stripping agent the salt solution stream may contain sodium lactate.

The salt solution stream may be led to an electrical separator to subject the salt solution stream to electrical separation to obtain a regenerated acidic stripping agent stream and a regenerated basic liquid stream. Electrical separation is particularly beneficial as it generally only requires electrical energy and no thermal energy. For instance, the electrical input for regeneration of the acidic stripping agent and the basic liquid may be between 3-4 MJ _e ^ g CO2. The electrical separation preferably comprises an electro-membrane process as for instance detailed in the review article by Handojo et al. (RSC Adv., 2019, 9, 7854-7869). Preferably the electro membrane process comprises electrodialysis (ED), electrometathesis (EMT), electro-ion substitution (EIS), electro-electrodialysis (EED), electrodialysis with bipolar membranes (EDBM), and electro deionization (EDI) and/or a combination thereof. Electro-membrane processes may be beneficially used to limit i.a. concentration steps, use of hazardous solvents and high energy consumption.

Electro-membrane processes are typically performed in cells comprising one or more anionic exchange membranes and cation exchange membranes arranged between an anode and a cathode. The anionic exchange membranes are typically permeable to anionic species while substantially blocking the passage of cationic species and vice versa. Applying an electrical field results in the migration of ionic species and due to the membranes selective passage of ions may be possible. Accordingly, electro-membrane processes are generally based on the selective permeability of the ion-exchange membranes. For instance, a suitable electrical separator (8) may comprise electrodialysis wherein the separator typically comprises a cationic exchange membrane (203) is located between two anionic exchange membranes (202, 204) which are arranged between a anode (201) and an cathode (205) as illustrated in Figure 6. The salt solution stream (7) may be fed to the separator at the indicated locations. When an electric field is applied, ionic species tend to move and selectively pass through the ionic exchange membranes. Accordingly, after some time a regenerated basic liquid stream (10) and a regenerated acidic stripping agent stream (9) may be obtained at the indicated locations.

As illustrated in Figure 2, the regenerated acidic stripping agent stream (9) may advantageously be recycled to the desorption vessel (5). The regenerated acidic stripping agent stream may be concentrated before it is recycled back to the absorber. Additionally or alternatively, the regenerated basic liquid stream (10) can be recycled to the absorber (2). The water balance is typically crucial as it is often not desirable to dilute the basic liquid.

Preferably, the method is a continuous method.

It may also be preferred that the method further comprises leading the salt solution stream (7) to a flash tank (11) before leading the salt solution into the separator (8) as illustrated in Figure 3. For instance, if the desorption vessel is at an elevated pressure, there may be some CO2 dissolved in the salt solution stream. By leading this stream to a flash tank, the dissolved CO2 may be released to obtain a released C0 ₂ -gas stream (6b), which may be rejoined with the obtained CCk-gas stream.

The invention is further directed to a system (100) suitable for the method according to the present invention. The system is schematically illustrated in Figure 4. The system comprises an absorber (2) adapted to during use comprise a basic liquid. The absorber comprises a feed fluid inlet (21), such as a feed gas inlet, a basic liquid inlet (22), a C0 ₂ -lean fluid outlet

(23) (e.g. a C0 ₂ -lean gas outlet), and a C0 ₂ -rich liquid outlet (24). The system further comprises a desorption vessel (5) adapted to during use comprise an acidic stripping agent. The desorption vessel comprises a CO2- rich liquid inlet (51), a salt solution outlet (52), a C0 ₂ -rich gas outlet (53) and a stripping agent inlet (54). The system further comprises an electrical separator (8) comprising a salt solution inlet (81), a basic liquid outlet (82) and a stripping agent outlet (83). In the system the C0 ₂ -rich liquid outlet

(24) is in fluid connection with the C0 ₂ -rich liquid inlet (51), the salt solution outlet (52) is in fluid connection with the salt solution inlet (81), the stripping agent outlet (83) is in fluid connection with the stripping agent inlet (54) and the basic liquid outlet (82) is in fluid connection with the basic liquid inlet (22).

It may be appreciated that when the system is not installed and/or not operational, it may not contain the basic liquid and/or the acidic stripping agent as this allows for i.a. easier transportation and/or installation. If it is operational, the system thus comprises the basic liquid in the absorber and the acidic stripping agent in the desorption vessel.

It may be preferred that the feed fluid inlet (21) is located below the basic liquid inlet (22). Additionally or alternatively, in order for the system to be able to perform under a preferred pressure ( vide infra), the desorption vessel may be adapted to be under ambient pressure (i.e. approximately 1 bar), but may also be adapted to be under a pressure of at least 2 bar, preferably at least 4 bar, more preferably at least 5 bar, even more preferably at least 6 bar, most preferably at least 7 bar.

A particularly preferred embodiment is illustrated in Figure 5. In this preferred embodiment a fluid displacement unit, such as a fan (17), may be present to lead the feed fluid to the absorber (2). A fan may be particularly preferred for a feed gas. The system may further comprise pumps, such as centrifugal pumps (14, 16) and/or heat exchangers (13, 15). For instance, pump (14) for pumping the C0 ₂ -rich liquid stream may be located between the absorber (2) and the desorption vessel (5). A heat exchanger (15) may further be present between the pump and the desorption vessel. Similarly, a pump (16) may be present between the electrical separator (8) and the absorber (2) to pump the regenerated basic liquid. Further, a heat exchanger (13) may be located between the pump (16) and the absorber (2). Additionally, it may be preferred to have a stirring device (12), such as a mechanical stirring in the desorption vessel.

For the purpose of clarity and a concise description features are described herein as part of the same or separate embodiments, however, it will be appreciated that the scope of the invention may include embodiments having combinations of all or some of the features described.

The invention may further be illustrated by the following, non limiting examples. Example 1

Flue gas from a gas-fired turbine, after pre-treatment, comprising 4% CO2, 10% O2, 86% N2 (on molar dry basis) and water saturated enters the bottom of a packed absorber at 1.05 bar and 40 °C. In the absorber, the gas is contacted with the basic liquid comprising 10% NaOH in water (mass basis) entering at the top of the absorber. The CO2 is removed from the gas and reacts with the sodium hydroxide according to the following reaction:

NaOH + C0 ₂ ® NaHC0 ₃

The gas leaving the top of the absorber is lean in CO2 containing less than 10% of the amount of CO2 that entered the absorber. The C0 ₂ -rich liquid stream leaving at the bottom of the absorber is pumped to a desorption vessel operating at 7 bar where a solution of acetic acid (HAc) is added. In the desorption vessel the CO2 is released according to the following reaction:

NaHC0 ₃ + HAc ® NaAc + H ₂ 0 + C0 ₂

The CO2 released leaves at the top of the desorption vessel while the salt solution stream of sodium acetate is sent to the electrical separator. In the electrical separator, the sodium acetate is converted and separated in two regenerated streams of sodium hydroxide and acetic acid. The sodium hydroxide solution is sent back to the absorber while the acetic acid is returned to the desorption vessel. The high-pressure CO2 produced at the desorption vessel is sent to a cooler for liquefaction. The process is schematically illustrated in Fig. 5.