A process and plant

Abstract

A process of regenerating a metal alkali hydroxide, also known as a caustic metal alkali, for absorbing carbon dioxide gas using an electrolytic cell. The present disclosure also relates to an electrolytic cell and a plant including an electrolytic cell for regenerating a metal alkali hydroxide for absorbing carbon dioxide gas from a gas stream. In this process, metal alkali hydroxide contacts a gas containing carbon dioxide and is converted to carbonated metal alkali absorbent and precipitant, the carbonated metal alkali including precipitants is supplied to the electrolytic cell for regeneration, and regeneration involves re-producing the metal alkali hydroxide and producing carbon dioxide in the electrolytic cell.

Description

A PROCESS AND PLANTRELATED APPLICATIONS

[0001] The present application claims priority to Australian provisional application number 2024900843 filed on 28 March 2024. The full contents of provisional application are incorporated into this specification.FIELD OF THE INVENTION

[0002] The present disclosure relates to a process of regenerating a metal alkali hydroxide, also known as a caustic metal alkali, for absorbing carbon dioxide gas using an electrolytic cell. The present disclosure also relates to an electrolytic cell and a plant including an electrolytic cell for regenerating a metal alkali hydroxide for absorbing carbon dioxide gas from a gas stream.BACKGROUND

[0003] Examples of caustic metal alkali absorbents that have a high affinity for carbon dioxide include potassium hydroxide (KOH) and sodium hydroxide (NaOH). In order for these absorbents to be reused in a continuous cycle, loaded carbonated forms of the metal alkali may be subject to a range of heat treatments. A difficulty with heat treatments is that the temperatures required to volatilise carbon dioxide typically also triggers decomposition of the absorbents into oxygen and elemental alkali metal which are unable to be readily reused for absorbing carbon dioxide.

[0004] It is an object of the present disclosure to provide an alternative means for regenerating carbonated metal alkali so that it can be reused for absorbing carbon dioxide.SUMMARY

[0005] An embodiment relates to a process of regenerating metal alkali hydroxide for absorbing acid gas, including carbon dioxide gas, the process includes the steps of: contacting a solvent solution including a metal alkali absorbent with a gas to absorb carbon dioxide therefrom to produce a product gas that is leaner in carbon dioxide and a loaded solvent solution containing a carbonated metal alkali that includes precipitants; regenerating the carbonated metal alkali in an electrolytic cell by: generating hydrogen ions in an acidic compartment and hydroxide ions in a basic compartment, the compartments being separated by at least one permeable membrane;supplying at least part of the loaded solvent solution containing carbonated metal alkali, including precipitants thereof, to the electrolytic cell so that carbonated ions are converted to carbon dioxide in the acidic compartment and metal alkali ions are associated with hydroxide ions in the basic compartment; discharging a regenerated first solvent solution including causticized metal alkali from the basic compartment; and using the first solvent solution as at least part of the solvent solution in the contacting step.

[0006] One of the advantages of the process is that it has been realised that the carbonated metal alkali, including precipitants thereof, can be supplied to the acidic compartment and converted into causticized metal alkali.

[0007] Throughout this specification, the term “carbonated ions” embraces both carbonate ions (COs2-) and bicarbonate ions (HCOs-1). Similarly, the term “carbonated metal alkali" embraces metal alkali carbonate, such as but by no means limited to potassium carbonate, and metal alkali bicarbonate, such as but by no means limited to potassium bicarbonate. It will be appreciated that a metal alkali bicarbonate (XHCO3, where X is a metal alkali anion) will be in equilibrium with metal alkali carbonate (X2CO3) in the solvent solution.

[0008] That is to say, the term “carbonated metal alkali that includes precipitants” embraces metal alkali carbonate and metal alkali bicarbonate, in which at least the metal alkali bicarbonate forms the precipitants. Optionally, some of the precipitants may also include metal alkali carbonate. The metal alkali may be a potassium species, either as potassium carbonate and / or potassium hydroxide.

[0009] The metal alkali may also be any one or a combination of metal alkali, such as potassium, calcium and sodium. But suitably, the metal alkali may be potassium.

[0010] The solvent solution may be a concentrated potassium species, either as potassium hydroxide, potassium carbonate and / or potassium bicarbonate at a concentration range from 35 to 60wt% concentration. In one example, the solvent solution has a concentrated species at a concentration in the range from 45 to 50wt% concentration.

[0011] Throughout this specification, references to the “loaded absorbent solution”, "loaded absorbent", "loading of the absorbent", or variations thereof, refers to the molar proportion of the targeted species in the absorbent solution on a scale of 0 - 1 , where the loading a totally regenerated solution is zero and loading of a totally loaded solution is 1 . By way of example,when the targeted species is carbon dioxide and the absorbent is alkali carbonate, the loading of carbon dioxide incorporated into the alkali carbonate / alkali bicarbonate mixture, a loading of zero represents a solution containing only alkali carbonate and the loading of a solution containing only alkali bicarbonate is 1. In this example, the loading is equivalent to moles of carbon dioxide absorbed per mole of potassium carbonate. The targeted species may be any acid gas including CO2, SOx and NOX.

[0012] The loaded solvent solution, includingthe carbonated metal alkali and precipitants thereof, may have a loading ranging from 0.65 to 0.90. In one example, the loading may range from 0.75 to 0.80. The loaded solvent solution may have a temperature in the range from 20 to 70°C. The temperature of the loaded solvent solution may be the operating temperature of the contacting step.

[0013] The process may also include a cooling step in which the loaded solvent solution is cooled to a temperature ranging from 20 to 70°C. In one example, the cooling step may cool the loaded solvent solution to a temperature ranging from 20 to 40°C.

[0014] In the situation where the carbonated metal alkali includes potassium carbonate, the loaded solvent solution may have precipitants, including potassium bicarbonate, in the range from 5 to 40% wt / wt. In one example, the loaded solvent solution may have precipitants, including potassium bicarbonate, in the range from 10 to 30 % wt / wt.

[0015] The process may include a liquid / solid separating step in which the loaded solvent solution from the contacting step is separated into a solid phase including the precipitants of the carbonated metal alkali and a liquid phase. The solid phase may be supplied to the acidic compartment.

[0016] It will be appreciated that the liquid / solid separating step may be not completely efficient. That is to say, the solid phase may be wet and include some of the solvent solution. Similarly, the liquid phase may include, in addition to dissolved forms of the carbonated metal alkali, unseparated amounts of the precipitants.

[0017] In any event, the solid phase may, in one example, include at least 50%wt solids. In another example, the solid phase may include at least 75%wt solids, In yet another example, the solid phase may include at least 90%wt solids. The majority of the residual mass may be water. Optionally, the solid phase may be dewatered to produce a dry solid phase.

[0018] It will be appreciated that a portion of the loaded solvent solution may also bypass the liquid / solid separating step, and this portion may be fed to the electrolytic cell, with the solid phase.

[0019] The process may include: i) supplying at least part of the loaded solvent solution containing carbonated metal alkali to a thermal regenerating step and ii) regenerating the load solvent solution in the thermal regenerating step in which carbon dioxide is volatised from metal alkali bicarbonate to produce a second solvent solution containing metal alkali carbonate. The thermal regenerating step may include heating to volatise carbon dioxide.

[0020] The at least part of the loaded solvent solution supplied to the thermal regenerating step may be the liquid phase provided by the separating step. It is also possible that a portion of the loaded solvent solution may also bypass the liquid / solid separating step, and this portion may be fed to the thermal regenerating step.

[0021] The contacting step may include using the first solvent solution to produce a first product gas and the second solvent solution to produce a second product gas in which the first product gas has a lower carbon dioxide content than the second product gas.

[0022] The contacting step includes an auxiliary contacting step in which the first solvent solution absorbs carbon dioxide from the gas to produce the first product gas, in which the gas supplied to the auxiliary contacting step has an initial carbon dioxide concentration of greater than 400ppm and the first product gas has carbon dioxide down to ambient air levels or less. That is to say, to carbon dioxide levels down to 400ppm. In this instance, the contacting step may include the first solvent solution contacting the gas in an extension stage for absorbing carbon dioxide therefrom and produce the first produce gas. That is to say, the gas may have had carbon dioxide absorbed therefrom in preceding stages of the contacting step, when viewed in the direction of flow of the gas. The first product gas of the gas may have carbon dioxide at levels below 400ppm and at nominal concentrations, for example from Oppm to 200ppm, or 1 ppm to 200ppm. In another example, the first product gas may have carbon dioxide at nominal concentrations, for instance from Oppm to 100ppm, or 1ppm to 100pmm, or 2ppm to 100ppm,

[0023] The process can include controlling the rate of regeneration of the carbonated metal alkali in the electrolytic cell to in turn control the carbon dioxide content of the first product gas discharged from the absorbing step. For example, down to atmospheric levels that is about 400ppm. In this example, the electrolytic cell 22 may be required to regenerate from 0.1 to 10% of the total loading capacity of the loaded solvent solution. In another example, it may require the electrolytic cell to regenerate, from 1 to 7%, and suitably from 2 to 5% of the total loading capacity of the loaded solvent solution 18 in the electrolytic cell 22.

[0024] When the first product gas has carbon dioxide levels below 400ppm, and suitably in the range from 1 to 200ppm, in excess of 75% of the total carbon dioxide absorbed by the loaded solvent solution may form the precipitant and be supplied to the electrolytic cell for regenerating therein. That is to say, upto 25 % w / w of the loaded solvent solution is precipitant and supplied to the electrolytic cell for regeneration.

[0025] The term “process gas” embraces any industrial gas including a pre-combustion gas such as natural gas, a post-combustion gas such as a flue gas, a synthesis gas, also known a syngas and so forth. An example of a gas that is not a process gas is ambient air.

[0026] The contacting step may include a preliminary contacting step in which the solvent solution, including the second solvent solution, absorbs carbon dioxide from the gas to provide a second product gas.

[0027] The contacting step may include a preliminary contacting step in which the solvent solution, including at least part of the first solvent solution after the first solvent solution has been used in the auxiliary contacting step, and the second product gas is supplied to the auxiliary contacting step to absorb carbon dioxide therefore and convert the second product gas into the first product gas.

[0028] The second product gas may have a carbon dioxide concentration that is greater than 400ppm, and suitably the concentration of carbon dioxide of the second product gas is greater than 500 ppm, and suitably greater than 600ppm. In another example, the second product gas may have a carbon dioxide concentration from 410ppm to 2000ppm.

[0029] The second product gas may be used as a feed to the auxiliary contacting step.

[0030] The first solvent solution discharged from the extension stage of the contacting step may be used, in combination with, the second solvent solution in the preliminary stage of the contacting step.

[0031] The contacting step may include contacting the first solvent solution with the ambient air to absorb carbon dioxide therefrom. In this instance, the first solvent solution may contact air in a direct air contactor, known as a DAC unit, to produce treated air. In this instance, treated air may have carbon dioxide at levels below 400ppm. The treated air may have carbon dioxide at nominal concentrations, for example from Oppm to 200ppm, or 1ppm to 200pmm, or 2ppm to 200ppm. In another example, the first product gas may have carbon dioxide at nominal concentrations, for instance from Oppm to 100ppm, or 1ppm to 100pmm, or 2ppm to 100ppm.

[0032] The process may include discharging the first solvent solution from the air contacting step and supplying the first solvent solution to the auxiliary contact step for use therein.

[0033] In terms of the electrolytic cell, the acid compartment maybe an anodic compartment. Similarly, the basic compartment may be a cathodic compartment.

[0034] The permeable membrane may be at least one of a cation exchange membrane and / or an anion exchange membrane.

[0035] The permeable membrane may include a cation permeable membrane that allows the migration of metal alkali ions into the basic compartment to generate the causticized metal alkali, namely metal alkali hydroxide.

[0036] The step of supplying the carbonated metal alkali to the electrolytic cell may include supplying the carbonated metal alkali to the acidic compartment so that the carbonated metal alkali is converted into alkali metal ions and carbon dioxide, and the metal alkali ions diffuse to the basic compartment through the permeable membrane.

[0037] The permeable membrane may include an anion permeable membrane that allows the migration of the anions into the acid compartment.

[0038] The process may include inhibiting the transfer of carbonate or bicarbonate ions from the acid compartment to basic compartment. Suitably, the process avoids carbonate or bicarbonate ions transferring from the acid compartment to basic compartment.

[0039] The process may include operating the acidic compartment at a pH that enables the electrolytic cell to generate a carbon dioxide partial pressure sufficient for carbon dioxide production in the electrolytic cell.

[0040] The process may include operating the electrolytic cell with the acidic compartment to a pH in the range from 3 to 9, or suitably to a pH in the range from 5 to 8.

[0041] The process may include operating the electrolytic cell with the basic compartment to a pH in the range from 13 to 15, or suitably to a pH in the range of 14 to 14.5.

[0042] The process includes monitoring the pH in at least one of the acidic and / or basic compartments.

[0043] The process may include a controlling step to control the pH so that the pH is lower in the acidic compartment than the basic compartment.

[0044] The controlling step may include adjusting the electrical current applied to the electrolytic cell to adjust the rate at which hydrogen ions are generated in the acidic compartment.

[0045] The controlling step may include adjusting the flow rate of the carbonated metal alkali to the acidic compartment which can in turn indirectly change the rate at which hydrogen ions are consumed converting carbonated metal alkali to metal alkali ions.

[0046] The controlling step may include adding hydrogen ions to the acidic compartment. For instance, this may include adding hydrogen by electrolytic means.

[0047] The process may include controlling the temperature in the acidic and basic compartments to a temperature in a range from 20 to 140°C, more suitably in the range from 50 to 100°C, and ideally in the range from 70 to 90°C.

[0048] In one example, the step of generating hydrogen ions in an acidic compartment may include oxidising water in the acidic compartment to produce oxygen gas and hydrogen ions. The hydrogen ions can then react with the carbonated metal alkali to produce carbon dioxide and metal alkali ions. The process may include discharging a gas mixture including oxygen gas and carbon dioxide from the anodic / acidic compartment. The gas mixture may than be contacted with a solvent solution to reabsorb the carbon dioxide from the mixture. This may be done in a 1 or 2 step process, including possibly use of a pressure change, such as a flashing step, to produce two gas streams, one more highly enriched in carbon dioxide that the other. In one example, the gas mixture may form part of the gas with which the solvent contacts in the contacting step.

[0049] In one example, hydrogen ions may be reduced to hydrogen gas in the basic compartment (at the cathode). In another example, water is reduced to hydrogen gas in the basic compartment.

[0050] The gas mixture may also be used as a high purity oxygen source in combustion (oxy-fuel) to produce a flue gas stream substantially free of nitrogen and oxygen. This flue gas stream may then be subject to treatment and storage as desired.

[0051] In this example, the acidic and basic compartments include an anode and a cathode respectfully. The anode and cathodes may be non-sacrificial electrodes.

[0052] In another example, the process may include exchanging hydrogen ions from at least one electrode. The capacity to exchange hydrogen ions can be driven by voltage driven intercalation.

[0053] For instance, the acidic compartment and the basic compartment may include anode and cathode electrodes that can absorb and desorb hydrogen anions through voltage driven intercalation, thereby generating hydrogen ions in an acidic compartment and hydroxide ions in a basic compartment. That is to say, oxidation in the acidic compartment can result inhydrogen ions being desorbed from the anode, and reduction in the basic compartment can result in hydrogen ions being absorbed from the cathode and hydroxide ions being supplied. A possible benefit provided by this example is that oxygen gas does not need to be generated, which means that the carbon dioxide released in the acidic compartment does not need to be separated from oxygen.

[0054] Both the anode and cathode may have the hydrogen and / or hydrogen ions absorbing and desorbing capacity.

[0055] The process may include reversing the polarity of the electrolytic cell after an operating period.

[0056] In one example, reversing the polarity of the electrolytic cell may include changing the polarity of the electrical voltage applied to the electrodes so the cathode becomes a new anode, and the anode becomes a new cathode. The new anode would define a new acidic compartment and the new cathode would define a new basic compartment.

[0057] In another example, reversing the electrolytic cell may include physically swapping the cathode and anode in the electrolytic cell so as to reallocate the cathode in the former basic compartment, to provide a new anode in the new acidic compartment, and to reallocate the anode in the former basic compartment, to provide a new cathode in the new basic compartment. Physical swapping the electrodes in this manner avoids changing the polarity of the voltage supplied to the electrolytic cell.

[0058] In a further example, a reverse voltage may be applied to the cells during a ‘regeneration’ phase to return the electrodes to their original hydrogen ion loading.

[0059] In either example, the process may include supplying the carbonated metal alkali, including the precipitant, to the new acidic compartment, and discharging the metal alkali hydroxide from the new cathode compartment.

[0060] In another example, the process includes reducing hydrogen ions to produce a first hydrogen gas in the basic compartment, and oxidising hydrogen gas to produce hydrogen ions in the acidic compartment. A possible benefit of this example is that water is not oxidized in the acidic compartment which avoids the generation of oxygen gas. Instead, water and carbon dioxide only will be generated from the conversion of the carbonated metal alkali in the acidic compartment.

[0061] The process may include supplying the first hydrogen gas generated in the basic compartment to the acidic compartment for oxidation.

[0062] In another example, the process includes a disassociating step of disassociating water molecules into hydroxide ions and hydrogen ions from at least two bipolarised substrates driven by a voltage differential. The disassociation of the water molecules occurring on opposite sides of the at least two bipolarised substrates to provide the acidic compartment and the basic compartment. Specifically, the process may include a disassociating step in which water molecules are disassociated in hydroxide ions and hydrogen ions from opposite side of at least two bipolarised substrates, providing the acidic compartment and the basic compartment, and driven by voltage driven water disassociation

[0063] The bipolarised substrates may be located so that opposite sides of the bipolarised substrates, generating hydroxide ions and hydrogen ions respectively, are directed toward each other and are separated by the semi-permeable membrane therebetween.

[0064] The step of supplying the carbonated metal alkali includes supplying the carbonated metal alkali directly or indirectly into the acidic compartment.

[0065] The process includes arranging a series of the bipolar substrates that are arranged back-to-back in a parallel arrangement with the permeable membrane between each, and operating an anode electrode and a cathode electrode at opposite ends of the arrangement.

[0066] The permeable membrane may be a cation permeable membrane.

[0067] The process may also include locating an anion permeable membrane between the cation permeable membrane and the acidic compartment, thereby defining an intermediate compartment between the anion permeable membrane and the cathodic permeable membrane.

[0068] The step of supplying the carbonated metal alkali may include supplying the carbonated metal alkali into the intermediate compartment. In this instance, the metal alkali cation can diffuse from the intermediate compartment to the basic compartment and the carbonated ions, including carbonate and bicarbonate can diffuse from the intermediate compartment to the acidic compartment.

[0069] The bipolar substrates may be permeable or semi-permeable membranes.

[0070] A possible benefit of this example is that the bipolarised substrates use voltage differences at the anode and cathode electrodes to drive charge separation of water into hydroxide ions and hydrogen ions, which in turn can avoid the generation of oxygen gas and subsequent downstream separation of oxygen and carbon dioxide that is disassociated from the carbonate metal alkali.

[0071] The second solvent solution may be fed to the penultimate contacting stage of the contacting step in the flow direction of the gas.

[0072] The solvent solution of the electrolytic cell may be fed to the last contacting stage of the contacting step in the flow direction of the gas, and optionally contacts air to remove carbon dioxide therefrom prior to the last contacting stage.

[0073] One or both of the acidic or basic compartments may be actively circulated by pump or equivalent mechanism.

[0074] The process may include maintaining the acidic compartment at a pressurised state to prevent the formation of gaseous carbon dioxide (CO2).

[0075] The process may include passing electrolyte of the acidic compartment passes through a flash drum to remove soluble carbon dioxide (CO2) therefrom.

[0076] The process may include adding carbonated metal alkali to the basic compartment.

[0077] The carbonated metal alkali added to the basic compartment may be obtained from the acidic compartment.

[0078] The regenerating step may include converting metal alkali bicarbonate to metal alkali carbonate to produce a second regenerated solvent solution.

[0079] An embodiment relates to a plant that includes: an absorber that is configured to contact a solvent solution with a gas to absorb carbon dioxide therefrom and form a loaded solvent solution including carbonated metal alkali including precipitants and a lean gas; the electrolytic cell that receives carbonated metal alkali including precipitants of the solid phase from the separator and generates a first solvent solution containing the causticized metal alkali; and wherein the first solvent solution forms at least part of the solvent solution supplied to the absorber.

[0080] The plant may include a separator that separates the loaded solvent solution into a solid phase and a liquid phase, in which the solid phase includes the carbonated metal alkali including precipitants, and the electrolytic cell having an inlet that receives that the solid phase from the separator.

[0081] The plant may include a thermal regenerator having an inlet that receives the liquid phase from the separator, including metal alkali bicarbonate and metal alkali carbonate, and a heat source that volatilises carbon dioxide from the liquid phase to regenerate a second solvent solution including metal alkali carbonate.

[0082] The first solvent solution generated by the electrolysis cell may be used to absorb carbon dioxide from the gas to reduce the carbon dioxide therein down to, for example, 700 ppm, and in another example down to 600 ppm, and in another example down to 500 ppm, and in another example down to 400ppm, and in another example down to 300ppm, and in another example down to 200 ppm, and in another example below 200ppm. An advantage is that the first solvent can be used to reduce the carbon dioxide content in the gas to a concentration below the content of carbon dioxide in ambient air.

[0083] The metal alkali carbonate regenerated in the thermal regenerator can form a second solution, either by itself or when mixed with already CO2 contacted first solvent, that may be used to absorb carbon dioxide down to a concentration (ppm) of greater than the concentration achieved in the solvent solution generated by the electrolytic cell. For example, down to a concentration of 1 ,000 ppm, 800pmm, 700ppm, or 600ppm.

[0084] The absorber includes a main absorber that receives the process gas and an extension absorber that is arranged downstream of the main absorber in the direction of flow of the process gas through the absorber, and the extension absorber has a first inlet for the first solvent solution and the main absorber receives the first solvent solution discharged from the extension absorber and the second solvent solution

[0085] The absorber may be configured so that the second solvent solution is supplied to contacting stages up stream of the extension absorber in the direction of flow of the gas through the absorber.

[0086] The plant may include a direct air contact (DAC) unit which is configured to contact the first solvent solution with air to absorb carbon dioxide therefrom, and from which carbon dioxide depleted air, namely treated air can be released.

[0087] The plant may include a transfer line for transferring the first solvent solution from the direct air contact (DAC) unit to the extension absorber. The transfer line may also transfer to the first solvent solution to the main absorber.

[0088] The plant may include a cooler for cooling the loaded solvent solution to promote the formation of precipitants in the loaded solvent solution. The cooler may be located upstream of the separator in the direction of the flow of the loaded solvent solution.

[0089] The plant may include a line for conveying carbon dioxide and oxygen gas from the electrolytic cell to the gas fed to the absorber.

[0090] The thermal regenerator may include an outlet line for discharging carbon dioxide volatilised from the loaded solvent solution. The plant may include a manifold for combining carbon dioxide from the electrolytic cell and carbon dioxide from the regenerator.

[0091] The electrolytic cell may have a recirculation or stirring device for mixing the precipitants in the acidic compartment.

[0092] The electrolytic cell may be configured to inhibit the transfer of the carbonate or bicarbonate ions or particles from transferring from the acidic compartment to the basic compartment. The membrane will be permeable to potassium ions, allowing the potassium to transfer to the basic compartment.

[0093] An embodiment relates to a process of regenerating metal alkali hydroxide from a loaded solvent solution containing a carbonated metal alkali that includes precipitants, the process includes the steps of: generating hydrogen ions in an acidic compartment and hydroxide ions in a basic compartment of an electrolytic cell, the compartments being separated by at least one permeable membrane; supplying at least part of the loaded solvent solution containing carbonated metal alkali, including precipitants thereof, to the electrolytic cell so that carbonated ions are converted to carbon dioxide in the acidic compartment and metal alkali ions are associated with hydroxide ions in the basic compartment; discharging a regenerated solvent solution including causticized metal alkali from the basic compartment.

[0094] The process of paragraph

[0093] may include any one or a combination of the features described herein, including any one or a combination of the features in paragraphs

[0005] to

[0078] ,

[0095] An embodiment relates to an electrolytic cell generating metal alkali hydroxide for absorbing acid gas, including carbon dioxide gas, the electrolytic cell Includes: an acidic compartment associated with an anode and a basic compartment associated with a cathode, wherein hydrogen ions and hydroxide ions aregenerated in the compartments, and a permeable membrane is provided between the acidic and basic compartments; an inlet that supplies carbonated metal alkali including precipitants thereof to the electrolytic cell so that carbonated anions are converted to carbon dioxide in the acidic compartment and metal alkali ions are associated with hydroxide ions in the basic compartment; and an outlet that discharges a regenerated solvent solution including causticized metal alkali from the basic compartment.

[0096] The electrolytic cell may include an anode and a cathode to which a voltage difference is applied.

[0097] The electrolytic cell may have a recirculation or stirring device for mixing the precipitants in the acidic compartment, or a secondary mixing vessel.

[0098] The electrolytic cell is configured to inhibits the transfer of the carbonate or bicarbonate ions or particles from transferring from the acidic compartment to the basic compartment. The membrane will be permeable to potassium ions, allowing the potassium to transfer to the basic compartment.

[0099] The electrolytic cell may include a permeable membrane that inhibits transfer of carbonate or bicarbonate ions from the acid compartment to basic compartment, and permits that transfers of the metal alkali ions from the acid compartment to the basic compartment.

[0100] In one example, the acid compartment of the electrolytic cell may be operated at a pH may range from 3 to 9. In another example, the pH may range from 5 to 8.

[0101] The basic cell of the electrolytic cell may be operated at a pH may range from 13 to 15. In another example, the pH may be in the range from 14 to 14.5.

[0102] The electrolytic cell may include a controller for controlling the electrical current applied to the anode and cathode and to in turn, control the pH in the acidic and basic compartments.

[0103] The electrolytic cell may include a monitor for measuring the pH in the acidic and / or basic compartments, and the controller may adjust the electrical current based on an output of the monitor representing the pH in the respective cell.

[0104] In one example, the cathode and the anode are non-sacrificial electrodes.

[0105] In one example, water may be oxidised in the acidic compartment (at the anode) to generate oxygen gas and hydrogen ions. That is to say, a gas mixture including oxygen gasand carbon dioxide will be co-produced in the acidic compartment and discharged therefrom.Similarly, hydrogen ions are reduced to hydrogen gas in the basic compartment (at the cathode) to produce hydrogen gas and water can be disassociated to form hydroxide ions.

[0106] In another example, at least one electrode, namely an anode or cathode of the acidic compartment and the basic compartment has the capacity to exchange (absorb and desorb) with hydrogen ions.

[0107] The capacity to exchange hydrogen ions can be driven by voltage intercalation at the anode and cathode.

[0108] For instance, the acidic compartment and the basic compartment may include anode and cathode electrodes that can absorb and desorb hydrogen anions through voltage driven intercalation. That is to say, oxidation in the acidic compartment can result in hydrogen ions being desorbed from the anode, and reduction in the basic compartment can result in hydrogen ions being absorbed from the cathode. A possible benefit provided by this example is that oxygen gas does not need to be generated, which means that the carbon dioxide disassociated from the carbonated metal alkali in the acidic compartment does not need to be separated from oxygen.

[0109] Suitably, both the anode and cathode can absorb and desorb hydrogen ions. It will be appreciated that after an operating period, at least one of the following will occur: the electrode acting as the anode can become depleted of hydrogen ions, and / or the electrode acting as the cathode can become loaded with hydrogen ions after an operating period, and the cell may be configured to allow the anode and cathode to be swapped so that the anode can be operated as a new cathode, and the cathode can be operated an new anode.

[0110] Alternatively, the cell may be configured to allow the polarity of the electrolytic cell to be reversed and the acidic and basic compartments can be swapped.

[0111] In a further alternative, a reverse voltage may be applied to the cells during a ‘regeneration’ phase to return the electrodes to their original hydrogen ion loading.

[0112] In another example, hydrogen ions are reduced to a first hydrogen gas in the basic compartment, and a second hydrogen gas is supplied to the acidic compartment where is oxidized to hydrogen ions. A possible benefit of this example is that water is not oxidized in the acidic compartment which avoids the generation of oxygen gas. Instead, water and carbon dioxide only will result from the disassociation of the carbonated metal alkali in the acidic compartment.

[0113] The first hydrogen gas generated in the basic compartment may be the second gas supplied to the acidic compartment.

[0114] The cell may include a passageway for conveying the first hydrogen gas generated in the basic compartment to the acidic compartment.

[0115] In another example, the cell includes at least two bipolarised substrates and the permeable membrane is positioned between the bipolarised substrates, the bipolarised substrates disassociate water molecules into hydroxide ions and hydrogen ions from opposite sides of each substrate to provide the acidic compartment and the basic compartment when a voltage difference is applied, and the bipolarised substrates are arranged so that the side that generates the hydroxide ions of one of the bipolarised substrates faces toward the side of a different bipolarised substrate that generates the hydrogen ions of the other bipolarised substrate.

[0116] The cell may include an inlet that supplies the carbonated metal alkali directly or indirectly into the acidic compartment that is located between the side of the bipolarised substrate that produces hydrogen ions and the permeable membrane, and the cell may include an outlet for discharging the metal alkali hydroxide from the basic compartment that is provided between the side of the bipolarised substrate that produces hydroxide ions and the permeable membrane.

[0117] The cell may include a series of the bipolar substrates arranged back-to-back in parallel with at least one of the permeable membranes located between each bipolar substrate, and an anode electrode and a cathode electrode at opposite ends of the cell.

[0118] The permeable membrane may be a cation permeable membrane.

[0119] The cell may include an anion permeable membrane between the cation permeable membrane and the acidic compartment, thereby providing an intermediate compartment between the anion permeable membrane and the cathodic permeable membrane.

[0120] The cell may include an inlet for supplying the carbonated metal alkali into the intermediate compartment.

[0121] The bipolar substrates may be permeable or semi-permeable membranes.

[0122] An embodiment relates to a process of generating metal alkali hydroxide for absorbing acid gas, including carbon dioxide gas, the process includes the steps of:contacting a solvent solution including a metal alkali absorbent to absorb carbon dioxide from a gas to produce a product gas that is lean in carbon dioxide and a loaded solvent solution containing a carbonated metal alkali; regenerating the carbonated metal alkali in an electrolytic cell by: generating hydrogen ions in an anodic / acidic compartment and hydroxide ions in a cathodic / basic compartment, the compartments being separated by a permeable membrane; supplying the carbonated metal alkali thereof to the anodic / acidic compartment so the carbonated metal alkali is converted into alkali metal ions and carbon dioxide, and the metal alkali ions diffuse to the cathodic / basic compartment through the permeable membrane; discharging a regenerated solvent solution including causticized metal alkali from the cathodic / basic compartment; and using the regenerated solvent solution as at least part of the solvent solution in the contacting step.

[0123] It will be appreciated that features of the process described herein may be features of the electrolytic cell and / or the plant. Similarly feature of the electrolytic cell may be features of the process and / or the plant, and likewise, features of the plant may be features of the process and electrolytic cell.BRIEF DESCRIPTION OF THE DRAWINGS

[0124] These and other features, aspects, and characteristics of the present disclosure are described with reference to the Figures of certain embodiments, which are intended to schematically illustrate certain embodiments and not to limit the disclosure. The Figures may be summarised as follows.

[0125] Figure 1 is a flow diagram of a process and plant including an electrolytic cell for regenerating a metal alkali hydroxide, also known as a caustic metal alkali, for absorbing carbon dioxide gas. A mixture including carbon dioxide and oxygen gas can be produced by the cell.

[0126] Figure 2 is schematic illustration of the electrolytic cell according to one example for regenerating the metal alkali hydroxide that can be used in the process and plant shown in Figure 1.

[0127] Figure 3 is a flow diagram of a process and plant including an electrolytic cell for regenerating a metal alkali hydroxide for absorbing carbon dioxide gas, in which carbon dioxide gas is produced by the cell.

[0128] Figure 4 is schematic illustration of the electrolytic cell according to one example for regenerating the metal alkali hydroxide that can be used in the process and plant shown in Figure 3.

[0129] Figure 5 is schematic illustration of the electrolytic cell according to another example for regenerating the metal alkali hydroxide that can be used in the process and plant shown in Figure 3.

[0130] Figure 6 is schematic illustration of the electrolytic cell according to yet another example for regenerating the metal alkali hydroxide that can be used in the process and plant shown in Figure 3.DETAILED DESCRIPTION

[0131] Although certain examples are described herein, those of skill in the art will appreciate that the disclosure extends beyond the specifically disclosed examples and / or uses and obvious modifications and equivalents thereof. Thus, it is intended that the scope of the disclosure herein disclosed should not be limited by any particular examples described herein.

[0132] To maintain clarity of the Figures not all reference numerals are shown in each Figure. For instance, reference numerals relating to electrolytic cell 22 are shown in Figures 2, 4, 5 and 6.

[0133] Figures 1 and 3 illustrate a process 10 and plant 11 for regenerating carbonated metal alkali including metal alkali bicarbonate, into at least one and suitably both metal alkali hydroxide, that can be included in a first solvent solution 15, and metal alkali carbonate that can be included in a second solvent solution 16.

[0134] The plant 11 includes an absorber 12 comprising a main absorber 13 and an extension absorber 14, being arranged after the main absorber 13 in the direction of the follow of a process gas 17, such as a flue gas. Although Figures 1 and 3 illustrate the extension absorber 14 being structurally separate from the main absorber 13, it will be appreciated that the extension absorber 14 and the main absorber 13 can be arranged within a single structure or column, or any number of structures. The solvent solution flowing downwardly in the absorber 12, including the extension absorber 14 and the main absorber 13, contacts the process gas 17 to absorb carbon dioxide therefrom and form a loadedsolvent solution 18. Figures 1 and 3 show the loaded solvent solution 18 as being discharged from a lower portion of the main absorber 13. However, it will be appreciated that the loaded solvent solution 18 may be discharged from multiple stages within the main absorber 13. In addition, side streams of the solvent solution may also be recirculated about one or more stages of the main absorber 13 and / or extension absorber 14. Separate stages within the main absorber 13 and side streams are not shown in the Figures 1 and 3. The plant 11 also includes a separator 19 that separates the loaded solvent solution 18 into a solid phase 20 and a liquid phase 21, and an electrolytic cell 22 that receives the solid phase 20 (or a metal alkali carbonate derived from the solid phase 20) for regenerating 43 the solid phase 20 which includes metal alkali bicarbonate into metal alkali hydroxide which can forms a first solvent solution 15 or at least part thereof. The plant 11 also includes a thermal regenerator 23 that uses heat to volatilise carbon dioxide to regenerate metal alkali bicarbonate to metal alkali carbonate which forms a second solvent solution 16. The heat may be obtained from any suitable heat source and may be provided a reboiler located within the thermal regenerator 23, as show in Figures 1 and 3, or located outside of the thermal regenerator 23. A recuperative heat exchanger 24 is also provided for transferring heat from the second solvent solution 16 to the liquid phase 21 fed to the thermal regenerator 23. The thermal regenerator 23 produces a rich carbon dioxide gas 53 and water condensate 50 can be fed to the electrolytic cell 22. The carbon dioxide gas 49 can be compressed as required in compressor 51.

[0135] The absorber 12 and the thermal regenerator 23 can be operated in a manner as described in our two earlier International patent applications, namely PCT / AU2006 / 001177 (W02007019632) published on 22 February 2007and PCT / AU2011 / 000462 (WO 2011 / 130796) published on 27 October 2011. The full contents of these International applications are hereby incorporated into the present specification by cross reference.

[0136] The plant 11 also includes two optional units namely a direct air contactor (DAC) unit 25 for contacting the first solvent solution 15 containing metal alkali hydroxide that is regenerated in the electrolytic cell 22 with air, and a cooler unit 26 for cooling the loaded solvent solution 18 to enhance the precipitation of metal alkali bicarbonate therein.

[0137] The process gas 17 supplied to the absorber 12 may embrace any process gas containing carbon dioxide, including a pre-combustion gas such as natural gas, a postcombustion gas such as a flue gas, a synthesis gas also known a syngas, and so forth. A gas that is not a process gas, such as ambient air, may be treated in the DAC unit 25 which discharges treated air 44 that is depleted in carbon dioxide.

[0138] The process 10 includes feeding the process gas 17, such as flue gas, to the bottom of the main absorber section 13 and it, which flows upwardly therein. The process 10 also includes supplying the second solvent solution 16 to a preliminary contacting step 52 being performed in the main absorber 13, and supplying the first solvent solution 15 to an auxiliary contacting step 45 which is performed in the extension absorber 14. Optionally the contacting step 42 may include an air contacting step 48 performed in the direct air contactor (DAC) unit 25. In addition, the first solvent solution 15 discharged from the DAC unit 25 can be supplied to the main absorber 13. Similarly, the first absorbent solution 15 discharged from fthe extension absorber 14 may also be supplied to the main absorber 13. A portion of the first solvent solution 15 discharged from the DAC unit 25 may also be supplied directly to the extension absorber 14. The first solvent solution 15 discharged from the DAC unit 25 may be combined with the second solvent solution 16 and supplied to main absorber 13. Although not shown in the Figures, the process 10 may include supplying the first solvent solution 15 from the DAC unit 26 directly to the main absorber 13 if the extension absorber 14 is not present. Similarly, the first solvent solution 15 discharged from extension absorber 14 may be combined with the second solvent solution 16 and supplied to the main absorber 13. It will be appreciated that portions of the first solvent solution 15 discharged from the DAC unit 25 and / or the extension absorber 14 may also be directly supplied to the main absorber 13, while other portions of the first solvent solution 15 discharged from the DAC unit 25 and / or the extension absorber 14 may also be combined with the second solvent solution 16 and then supplied to the main absorber 13.

[0139] The process 10 includes supplying a loaded solvent solution 18 containing carbonated metal alkali, including precipitants thereof, to the electrolytic cell 22 in which carbonated ions can be converted to carbon dioxide in the acidic compartment and metal alkali ions are associated with hydroxide ions in the basic compartment to regenerate the first solvent solution 15 in the electrolytic cell 22. That is to say, the electrolytic cell 22 includes converting metal alkali bicarbonate, such as potassium bicarbonate, to metal alkali hydroxide, such as potassium hydroxide. The regenerated first solvent solution 15 includes metal alkali hydroxide, such as potassium hydroxide.

[0140] The process 10 can include contacting the first solvent solution 15 with process gas 17 in the auxiliary contacting step 45 performed in the extension absorber 14 and / or air in the air contacting step 48 performed in the DAC unit 26. In the auxiliary contacting step 45 and / or the air contacting step 48, metal alkali hydroxide, such as potassium hydroxide, reacts with carbon dioxide according to the following reaction:Reaction A 2 KOH + CO2-> K2CO3+ H2O

[0141] As mentioned above, the contacting step 42 of the process 10 can include an auxiliary contacting step 45 performed in the extension absorber 14 where the the first solvent solution 15 contacts with the process gas 17. As can be seen, the extension absorber 14 is arranged after the main absorber 13, in the flow direction of the process gas 17. The process gas 17 is discharged from extension absorber 14 as a first gas product 27 and may be emitted to atmosphere. The concentration of carbon dioxide of the process gas 17 can be reduce tdown to 400ppm, or less, and thereby produce a first product gas 27. For example, the carbon dioxide levels the first product gas 27 may approach nominal concentrations, for example, in the range from 0 to 200ppm in which case the product gas 27 will have zero or near zero carbon dioxide emissions. Reaction A mentioned above would be the predominant reaction by which the first solvent solution 17 absorbs carbon dioxide in the auxiliary contacting step 45 to produce the first product gas 27.

[0142] In addition, the contacting step 42 includes the air contacting step 48 in which the first solvent solution 15 contacts and absorbs carbon dioxide from air so that treated air 44 exiting the DAC unit 25 has a carbon dioxide level below 400 ppm. The concentration of carbon dioxide in the treated air 44 may approach nominal concentrations, for example, in the range from 0 to 200ppm. That is to say, the process 10 includes discharging a treated air 44 from the air contacting stage 48 performed in the DAC unit 25 in which the treated air is air depleted of carbon dioxide so as to provide zero, or close to zero carbon dioxide emissions form the DAC unit 25. Reaction A can absorber carbon dioxide below atmospheric levels as Reaction A proceeds at low carbon dioxide partial pressure.

[0143] The contacting step 42 also includes a preliminary contacting step 52 performed in the main absorber stage 13 in which the solvent solution comprises the second solvent solution 16 and the first solvent solution 15 discharged from the extension absorber 14 and / or the DAC unit 25 contact the process gas 17. As the auxiliary contacting step 45 is arranged downstream of the preliminary contacting step 52 in the flow direction of the process gas 17, the first solvent solution supplied to the preliminary contacting step has at least some of the potassium hydroxide therein converted to carbonate according to Reaction A. The solvent solution supplied to the preliminary contacting step 52, being largely potassium carbonate, is supplied to an upper region of the main absorber 13, and the process gas 17 is supplied to a bottom region of the main absorber 13. The solvent solution can absorb carbon dioxide gas, and other acid gasses, if present. The preliminary contacting step 52 produces a second gas product 46 that is discharged from the main absorber 13 and will have a higher carbon dioxide content than the first product gas 27.Typically the second product gas 46 will have a carbon dioxide content in the range from 400 to 700ppm. The second product gas 46 can then be treated with the first solvent solution 15, as described above, by supplying the second product gas to the auxiliary contacting stage 45 performed in the extension absorber 14 to further reduce the carbon dioxide content below 400ppm as described above, thereby producing the first gas product 27.

[0144] The process 10 includes combining the first and second solvent solutions 15 and 16, which may occur within the preliminary contacting step 52, and / or may occur prior to the preliminary contacting step 52. That is to say, the first and second solutions 15 and 16 may be supplied separately to the preliminary contacting step 52 (performed in the main absorber 12) in which they will be mixed, and / or the first and second solution 15 and 16 may be mixed prior to being supplied to the preliminary contacting step 52 (performed in the main absorber 12). The process includes discharging the first product gas 27 and a loaded solvent solution 18 from the contacting step. The loaded solvent solution 18 may have any suitable metal alkali carbonate, such as potassium, calcium or sodium. In the situation in which the species is potassium, the loaded solvent solution 18 may be include either potassium carbonate and / or potassium bicarbonate at a concentration between 35-60wt% concentration, and ideally between 45-50wt% concentration. In addition, the loaded solvent solution 18 may have a loading from 0.65 to 0.90, and ideally from 0.75 to 0.80.

[0145] The predominant reaction occurring in the contacting step 42 of the main absorber 13 is as follows:Reaction B K2CO3+ CO2+ H2O ^ 2 KHC03That is say, potassium carbonate absorbs carbon dioxide to form potassium bicarbonate.

[0146] The loaded solvent solution 18 includes metal alkali carbonate and bicarbonate. The metal alkali bicarbonate has low solubility and precipitates in the loaded solvent solution 18. The loaded solvent solution 18 is discharged from the absorber 12 and undergoes a separating step 47 in the separator 19 in which the loaded solvent solution 18 is separated into solid and liquid phases 20 and 21, with a portion of, or all of, the solid phase 20 being supplied to an acidic compartment(s) of the electrolytic cell 22. The liquid phase 21 is also supplied to the top of the thermal regenerator 23.

[0147] The solid phase 20 may be wet and may be transported as a slurry to the acidic compartment 28 via a solids inlet 34. Although not shown in Figures 1 and 3, the process 10 may include washing the solid phase 20 prior to it being supplied to the acidic compartment 28. It will be appreciated that the solid phase 20 will include entrained solution. Similarly, the liquid phase 21 may also include some precipitants, although ideally this would be insmall amounts. Furthermore, to manage solids addition to, and for various process and operational reasons within the electrolytic cell 22, one or more compartment recirculation streams may be incorporated with acidic compartment(s) 28 of the electrolytic cell 22. the recirculation streams are not show in the Figures.

[0148] The electrolytic cell 22 converts metal alkali bicarbonate, such as potassium bicarbonate, to metal alkali hydroxide, such as potassium hydroxide, accordance with the following reaction:Reaction C KHC03-> KOH + CO2

[0149] The contacting step 42 can include: i) Reaction A occurring in the auxiliary contacting step 45 (performed in absorber extension 14) and / or the air contacting step 48 (performed in the DAC unit 25) as described above and; ii) Reaction B occurring in the preliminary contacting step 52 (performed in the main absorber 13). If Reactions A and B go to completion, one mole of potassium bicarbonate processed through Reaction C can be absorbed to one-half mole of carbon dioxide absorbed / captured from air in the DAC unit 25. A benefit of the process is t hat Reaction C increases the capacity to capture carbon dioxide compared to when Reaction C is not present, and the regeneration relies solely on reversing Reaction B.

[0150] In reality, Reaction A in the DAC unit 25 does not go to completion on account of reducing chemical driving forces, and may for example, proceed to around 75-80% completion depending on operational conditions. The final 20 to 25% of the potassium hydroxide (KOH) of the first solvent solution 15 discharged from the DAC unit 25, that is, an air contacting step between air and first solvent solution 15 can have sufficient driving force for absorbing carbon dioxide from the process gas 17 having an initial carbon dioxide content of greater than in air, i.e., greater than 400ppm. In other words, the first solvent solution 15 discharged from the air contacting stage 48 performed in the DAC unit 25, can still have capacity to absorber carbon dioxide from the process gas 17, in either the auxiliary contacting stage 45 performed in the extension absorber 13 and / or the main absorber 13, so that the first product gas 27 has carbon dioxide down to atmospheric concentrations, and thus achieving net zero emissions from the source of the process gas 17.

[0151] The process 10 may include controlling the rate of regeneration of the carbonated metal alkali, i.e, metal alkali bicarbonate in the electrolytic cell 22 to in turn control the carbon dioxide content of the first product gas 27 released to atmosphere. For example, process can include controlling the carbon dioxide content of the first product gas 27 to approximate atmosphere levels, i.e, about 400ppm.

[0152] Controlling the rate of regeneration of the carbonate metal alkali including precipitants (such as potassium bicarbonate precipitants), in the electrolytic cell 22 can include controlling the flow rate of the solvent solution supplied to the contacting step 12 and the flow rate of the loaded solvent solution 18. Increasing the flow rate of the solvent solution to the contacting step 1 can increase the rate at which the precipitant is formed and in turn, increase the rate of regeneration of the carbonated metal alkali in the electrolytic cell 22.

[0153] For example, to control the carbon dioxide content of the first product gas 27 to approximate atmosphere levels, the process 10 can include controlling the flow rate of the first solvent solution 15 to the contacting step 42 so each one half mole of carbon dioxide absorbed from the process gas 17 in the auxiliary contacting step 45 in which potassium hydroxide is converted to potassium carbonate (ie. Reaction A in the extension absorber 13), and one half mole of carbon dioxide is absorbed from the process gas 17 in the preliminary contacting step 52 in which potassium carbonate is converted to potassium bicarbonate (i.e. , Reaction B in the main absorber 12). That is to say, the full capacity of the potassium hydroxide in Reaction A can be used and for each one half mole of carbon dioxide absorbed / captured in auxiliary contacting stage 45 ( preformed in the absorber extension 14), one-half mole of carbon dioxide must also be absorbed / captured in the preliminary contacting step 52 performed in the main absorber 13 (a 1 :1 ratio), and one mole of carbon dioxide must be released in the electrolytic cell 22 for each mole of potassium bicarbonate converted to potassium hydroxide in the electrolytic cell 22 according to Reaction C. In this example, the process 10 can achieve nett zero carbon dioxide emissions from the process gas 17 as the first product gas 27 will include carbon dioxide at atmospheric levels.

[0154] In the situation where the solvent solution is a metal alkali carbonate at a 35 to 60wt% concentration, and suitably 45 to 50wt% concentration, the separating step 47 may include separating the loaded solvent solution 18 to a solid phase 20 containing 0.1 to 5.0% w / w of the mass of the loaded solvent solution 18 that is supplied to the electrolytic cell 22 to achieve a first product gas 27 including carbon dioxide at atmospheric levels. Assuming that the solid phase 20 is essentially dewatered potassium bicarbonate, in one example the solid phase may be from 0.1 to 5.0%wt of the loaded solvent 18. In another example, the solid phase may be from 0.5 to 3.5%wt of the loaded solvent. In yet another example, the solid phase may be from 1 .0 to 2.5%wt of the loaded solvent 18.

[0155] In systems that do not include an electrolytic cell 22 for regeneration of the loaded solvent solution 18, typically 90 to 95% of the carbon dioxide is absorbed and captured from the process gas by metal alkali carbonate being converted to metal alkali bicarbonate. Toreduce the carbon dioxide content of the process gas 17 i.e. , the first product gas 27 down to atmospheric levels of about 400ppm, an additional 5 to 10% absorption and capture of carbon dioxide is required using metal alkali hydroxide, such as a potassium hydroxide, in the extension absorber. The thermal regenerator 23 would operate largely the same to the situation in which there was no electrolytic cell 22. However, it may require the electrolytic cell 22 to regenerate, for example, from 0.1 to 10% of the total loading capacity of the loaded bicarbonate stream to be regenerated in the electrolytic cell 22. In another example, it may require the electrolytic cell 22 to regenerate, for example, from 1 to 7% of the total loading capacity of the loaded solvent solution 18 to be regenerated in the electrolytic cell 22. In another example, it may require the electrolytic cell 22 to regenerate, for example, from 2 to 5% of the total loading capacity of the loaded solvent solution 18 to be regenerated in the electrolytic cell 22.

[0156] The process 10 can include controlling the rate of regeneration of the carbonated metal alkali in the electrolytic cell 22 to in turn control the carbon dioxide content of the first product gas 27 to below atmospheric levels i.e, below 400ppm. In this instance controlling the rate of regeneration of the carbonated metal alkali in the electrolytic cell 22 will include increasing the flow rate of the solvent solution supplied to the contacting step 42 compared to the flow rate of the solvent solution when the first product gas 27 has a concentration approximately equal to atmospheric levels. In order for the product gas 27 to have carbon dioxide levels below 400ppm, and in the range from 1 to 200ppm, in excess of 75% of the total carbon dioxide absorbed by the loaded solvent solution 18 may need to form the precipitant and supplied to the electrolytic cell 22, with the remainder undergoing regeneration in the thermal regenerator 23. In this situation, upto 25 % w / w of the loaded solvent solution 18 may be separated as a precipitant in the separating step and supplied to the electrolytic cell 22 for regeneration.

[0157] Increasing the flow rate or recirculation rate of the solvent solution will in turn increases the size of the DAC unit 25 and require an increase in capacity of the electrolytic cell 22. In addition, the increased flow rates will alter the equipment size for the contacting step, i.e., the absorber 12, and the thermal regenerator 23. Ultimately, capital expenditure and operating costs will dictate the extent carbon dioxide levels are reduced to below atmospheric levels.

[0158] The primary reaction occurring in the acidic compartment to regenerating the solid phase 20 in the electrolytic cell 22 is through acidification of the bicarbonate species, suitably potassium bicarbonate according to the following reaction:Reaction D KHCO3+ H+^ K++ co2+ H2OWhen potassium carbonate is used, the reaction proceeds as:Reaction E K2CO2+ 2 H+2 K++ CO2+ H2O

[0159] Figure 2 is an example in which the process carried out in the electrolytic cell 22 includes disassociating water into hydrogen ions in the acidic compartment 28, with the hydrogen ions hydrogenating the metal alkali bicarbonate so that it is converted metal alkali ions. Likewise, Figures 5 and 6 are examples in which the process in the electrolytic cell 22 includes disassociating water into hydrogen ions in the acidic compartment 28, with the hydrogen ions hydrogenating the metal alkali bicarbonate so that it is converted metal alkali ions. Figure 4 is an example in which the anode desorbs hydrogen ions and adsorb hydrogen ions respectively.

[0160] The electrolytic cell 22 also has a permeable membrane 32 through which the metal alkali ions pass into basic compartment 29. Disassociation of water in the basic compartment provides hydroxide ions and the metal alkali ion passing from the acidic compartment 28 to the basic compartment 29 forms metal alkali hydroxide which is discharged from the basic compartment from the first outlet 39 and forms the first solvent solution 15.

[0161] Although not illustrated in the Figures, the process 10 may also include monitoring the pH in the electrolytic cell 22 and controlling the electrical current supplied to the cell 22 to adjust pH in the acidic compartment 28 to be less than the pH in the basic compartment 29. If required, a make up acid solution may also be added to the acidic compartment 28. For example, to maintain sufficient conductivity in the anodic compartment, as well as to maintain pH buffering capacity, an additional electrolyte species may be used in this acidic compartment 28. This may involve a species with pH buffering capacity such as potassium phosphate.

[0162] In addition, the acidic compartment 28 may be stirred. Moreover, one or both of the acidic or basic compartments 28, 29 may be actively circulated by pump or equivalent mechanism. The process 10 may also monitoring the temperature of the compartments and / or controlling the temperature of one or more of the compartments 28 and 29 to a temperature in the range from 70 to 90°C.

[0163] An example of an electrolytic cell 22 that can be used in the plant 11 and process 10 of Figure 1 is illustrated in Figure 2. In particular, the disassociation of water in the acidic compartment 28 including anode 30 generates oxygen gas as shown in Figure 2, and also releases water and carbon dioxide. A gas mixture 35 comprising carbon dioxide and oxygen gas may be discharged via a gas outlet from the acidic compartment 28 and may becombined to the process gas 17 as illustrated in Figure 1. In the basic compartment 29, which includes cathode 31, the disassociation of water generates hydrogen gas and hydroxide ions, the latter reacting with the metal alkali ions to form the regenerated first solvent solution 15 including metal alkali hydroxide as described above with is discharge via outlet 39. Although not illustrated, the gas mixture 35 may also be used as a rich oxygen gas source in combustion.

[0164] The process 10 and plant 11 illustrated Figure 3 differs from Figure 1 in that the loaded solvent solution 18 discharged from the absorber 12 undergoes cooling prior to the liquid / solid phase separation step 47 in separator 19. Cooling the loaded solvent solution 18 further reduces the solubility of carbonated metal alkali, including carbonates and bicarbonates, which can increase the precipitant yield from the separating step. In addition, instead of the gas mixture 35 comprising carbon dioxide and oxygen gas being generated in the acidic compartment 28, Figure 3 illustrates a gas line 37 comprising dioxide gas that is combined with the rich carbon dioxide 53 being discharged from the regenerator 23.

[0165] Electrolytic cells 22 that can be used in the process 10 to provide the rich carbon dioxide gas, transported within gas line 37 in Figure 3, are exemplified in Figures 4 to 6.

[0166] In the case of Figure 4, the anode and cathode electrodes 30 and 31 of the electrolytic cell 22 have an ability to absorb and desorb hydrogen ions for an operating period that is dependent on the loading capacity of the electrodes 30 and 31 and driven by voltage intercalation applied to the electrodes 30 and 31. That is to say, oxidation in the acidic compartment 28 can result in hydrogen ions being generated in the acidic compartment by being desorbed from the anode 30, and reduction in the basic compartment 29 can result in hydrogen ions being absorbed by the cathode 31 and the generation of hydroxide ions . As mentioned above, a possible benefit is that oxygen gas is not generated, which means that the carbon dioxide released in the acidic compartment does not need to be separate from oxygen gas and can be combined with the carbon dioxide released from the regenerator 23.

[0167] The electrode may include materials capable of intercalation or other proton absorbing / desorbing reactions including but not limited to metal chalcogens, Prussian Blue analogues and MXene materials (including metal carbides). For example, metal oxides where the metal is from the transition metal group. For example, manganese oxide (MnC>2), molybdenum oxide (MoOs) and tungsten oxide (WO3).

[0168] Without wanting to be bound by theory, it is believed that hydrogen ions can be absorbed and desorbed from the electrodes 30 and 31 by several mechanisms. For example, hydrogen ions can be absorbed and desorbed as atomic hydrogen on the surfaceof the electrodes 30 and 31 , or within the core of the electrodes 30 and 31. This can be represented by the following.Reaction F H+(absorbed) H+ (bulk)

[0169] Absorbed hydrogen ions can also result in phase transformations in which metal hydride can be formed. This can be represented by the following.Reaction G H+ (bulk)MH

[0170] It is also possible that hydrogen molecules can be adsorb and desorb from the surface of the electrode E. This may be presented as follows.Reaction H E + H2E - H2

[0171] Hydrogen can also become dissociative in which hydrogen ions can be absorbed and desorbed as follows.Reaction I 2E - H22E-H)

[0172] It is also possible that the electrode may have an organic backbone.

[0173] To accommodate the loading capacity of the electrodes 30 and 31, the process 10 will include periodically reversing the polarity of the electrolytic cell 22, for instance this may occur after the operating period. During or after the operating period, at least one of the following will occur: the electrode acting as the anode 30 can become depleted of hydrogen ions, and / or the electrode acting as the cathode 31 can become loaded with hydrogen ions after the operating period.

[0174] In one example, reversing the polarity of the electrolytic cell 22 may include changing the polarity of the electrical voltage applied to the electrodes 30 and 31 so the cathode 31 becomes a new anode, and the anode 30 becomes a new cathode. The new anode would define a new acidic compartment 28 and the new cathode would define a new basic compartment 29.

[0175] In another example, reversing the polarity of the electrolytic cell 22 may include physically swapping the cathode 31 and anode 30 in the electrolytic cell 22 so as reallocate the cathode 31 in the former acidic compartment, now the new basic compartment, and the anode 30 in the former basic compartment, now the new acidic compartment. Physical swapping the electrodes 30 and 31 in this manner avoids changing the polarity of the voltage supplied to the electrolytic cell 22.

[0176] In a further example, a reverse voltage may be applied to the electrodes 30 and 31 during a ‘regeneration’ phase to return the electrodes to their original hydrogen ion loading.

[0177] As a result, the process 10 may include supplying the carbonated metal alkali, including the precipitant, to the new acidic compartment, and discharging the metal alkali hydroxide from the new cathode compartment.

[0178] Figure 5 illustrates an electrolytic cell 22, where the process 10 includes disassociating water in the basic compartment 29 which produces a hydrogen gas, and then supplying the hydrogen gas to the acidic compartment 28 via passageway 36. The process 10 then includes oxidising the hydrogen gas to produce hydrogen ions in the acidic compartment 28 which react with the metal alkali bicarbonate to produce metal alkali ions.

[0179] The permeable membrane 32 allows the transfer of metal alkali ions to the basic compartment 29 yet maintains the pH difference between the compartments 28 and 29.

[0180] The hydrogen gas supplied to the acidic compartment 28 may be released into the electrolyte adjacent to the anode 30. The anode 30 may also be porous, as represented in Figure 5, to allow the hydrogen gas to be delivered to the anode 30 and permeate through the anode 30. Any suitable cathode 31 may be used.

[0181] Figure 6 illustrates another electrolytic cell 22 in which a series of bipolar substrates 38, suitably membranes, are arranged back-to-back in a parallel arrangement with permeable membranes 32 and 40 between each. Although Figure 6 illustrates two bipolar membranes 38 defining a single unit within the electrolytic cell 22, the electrolytic cell 22 could include any number of units comprising bipolar substrates 38 arranged back-to-back in parallel as indicated by the Zig-Zag lines. For example, upto 100 or more bipolar units could be arranged in a single electrolytic cell 22 with a pair of anode and cathode electrodes 30 and 31 may be arranged at opposite ends of the electrolytic cell 22.

[0182] The step of disassociating water includes at least two bipolarised substrates 38 disassociating water molecules into hydroxide ions and hydrogen ions from opposite sides of each substrate to provide the acidic compartment 28 and the basic compartment 29 when a voltage difference is applied to the anode and cathode electrodes 30 and 31. The bipolarised substrates 38 are configured so that hydroxide ions and hydrogen ions are generated from the opposite sides of the bipolarised substrates 38. In addition, the side that generates the hydroxide ions of one of the bipolarised substrate 38 faces toward the side of the other bipolarised substrate 38 that generates the hydrogen ions and at least one permeable membrane 39, 40 between the bipolarising substrates 38.

[0183] Although not shown in Figure 6, this makes it possible for multiple pairs of acidic and basic compartments 28 and 29 to be arranged between each pair of electrodes 30 and 31.

[0184] As can be seen in Figure 6, the cell 22 may include an inlet 34 that supplies the carbonated metal alkali directly or indirectly into the acidic compartment 28 that is located between the side of the bipolarised substrate 38 that produces hydrogen ions and the permeable membrane 32. In addition, the cell 22 may include a second permeable membrane 40, suitably an anion exchange membrane 40 which defines an intermediate compartment 41 between the first permeable membrane 32, suitably a cation permeable membrane and the second permeable membrane 40. In this situation, the solids inlet 34 may feed to the solid phase 20 directly into the intermediate compartment 41 and the cell 22 may include a solution outlet 39 for discharging the metal alkali hydroxide from the basic compartment 29 that is provided between the side of the bipolarised substrate 38 that produces hydroxide ions and the permeable membrane 32.

[0185] In the case of Figure 6, the process 10 may include measuring the pH in one or more of the acidic compartments 28 and in one or more of the basic compartments 29. The process 10 may include controlling the pH by adjusting electrical current supplied to the electrodes 30 and 31 to maintain the require pH difference between the compartments 28 and 29, and indeed for the acidity and alkalinity of the acidic and basic compartments 30 and 31.

[0186] A benefit of the electrolytic cell shown in Figure 6 is that the bipolarised substrates 38 use voltage differences at the anode 30 and cathode 31 to drive charge separation of water into hydroxide ions and hydrogen ions, which in turn can avoid the generation of oxygen gas and subsequent downstream separation of oxygen gas and carbon dioxide.

[0187] Although not shown in the Figures, the process 10 and plant 11 may include maintaining the acidic compartment 28 at a pressurised state to prevent the formation of gaseous carbon dioxide (CO2). The process 10 and plant 11 may include passing electrolyte of the acidic compartment 28 through a flash drum to remove soluble carbon dioxide (CO2) therefrom.

[0188] The process 10 may also include controlling transfer of the carbonated metal alkali, including precipitant thereof, from the acid compartment 28 to the basic compartment 29. Suitably, the process 10 includes inhibiting, or ideally preventing, the transfer of the carbonated metal alkali, including precipitant thereof, from the acid compartment 28 to the basic compartment 29. In the case of Figures 2, 4, 5, and 6, this can be achieved by using permeable membranes 32 that are permeable to metal alkali such as potassium but impermeable to carbonate and bicarbonate using known membranes. Moreover, the process may include controlling the pH generate a carbon dioxide partial pressure sufficient for carbon dioxide generation in the electrolytic cell 22.

[0189] By minimising the transfer of carbonate or bicarbonate from the acid compartment 28 to basic compartment 29, the process 10 may include operating the acidic compartment 28 at a pH that enables the electrolytic cell 22 to generate a carbon dioxide partial pressure sufficient for carbon dioxide production in the electrolytic cell 22. By way of example, the process 10 may include operating the electrolytic cell 22 with the acidic compartment 28 to a pH in the range from 3 to 9, or suitably from 5 to 8. That is to say, the process 10 may also include controlling the pH of the acidic compartment 28 to a pH in the range from 3 to 9 or suitably in the range from 5 to 8. It will be appreciated that the temperature electrolytic cell 22 will also have an impact on the reactions occurring, and in turn, the pH ranges of the cell 22.

[0190] By way of example, the process 10 may include operating the electrolytic cell 22 with the basic compartment 29 to a pH in the range from 13 to 15 or more suitably from 14 to 14.5. That is to say, the process 10 may also include controlling the pH of the basic compartment 29 to a pH in the range from 13 to 15, or suitably from 14 to 14.5.

[0191] By minimising the transfer of carbonate or bicarbonate from the acid compartment 28 to basic compartment 29, several benefits can be provided, including the following. i) The pH in the acidic compartment 28 can be higher, and may for example, be in the range from 3 to 9, or suitably in the range from 5 to 8. Lower pHs would be required of transfer did occur. ii) The first solvent solution 15 discharged from the electrolytic cell 22 is less likely to be contain carbonate and / or bicarbonate ions. iii) The overall energy required to operate the electrolytic cell 22 is significantly less.

[0192] In addition, the process 10 and plant 11 may include adding carbonated metal alkali to the basic compartment. The carbonated metal alkali added to the basic compartment may be obtained from the acidic compartment.

[0193] Another benefit of the embodiments described herein is that the electron demand for regenerating bicarbonate in the electrolytic cell 22 is less than if carbonate was regenerated in the electrolytic cell 22. For instance, the stoichiometry of acidic proton production in electrolysis systems results in one electron being produced per proton generated in the anodic compartment 28. To demonstrate this, according to the embodiment in which hydrogen gas is pumped into the anodic compartment 28, the electrolytic half-cell equation is as follows.Reaction J H2-> 2H++ 2e~

[0194] When combined with the above carbonate reactions (Reactions D and E), this effect can be clearly observed with one electron being generated per carbon dioxide molecule released in the bicarbonate case, compared to two electrons per carbon dioxide molecule generated for carbonate:Reaction K H2+ 2 KHC022 K++ 2 e~ + 2 CO2+ 2 H2OReaction L H2+ K2CO2— * 2 K + 2 s + CO2+ H2O

[0195] If we assume faradic efficiency and voltage is constant, the electrical energy requirements represented by the demand for electrons is halved per unit of CO2 by converting bicarbonate according to reaction K instead of converting carbonate according to reaction L.

[0196] Those skilled in the art of the present invention will appreciate that variations and modification may be made to the embodiments described herein without departing from the spirit and scope of the invention. For example, although not shown in Figures 2, 4 and 5, the electrolytic cell shown in these Figures may also include a second permeable membrane, suitably an anion permeable membrane located between the acidic compartment 28 the first permeable membrane to define an intermediate compartment into which the solid phase is fed into the cell 22.

[0197] Throughout this specification, conditional language used herein, such as, among others, "can," "might," "may," “for example,” and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and / or states. Thus, such conditional language is not generally intended to imply that features, elements and / or states are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without author input or prompting, whether these features, elements and / or states are included or are to be performed in any particular embodiment. The terms “comprising,” “including,” “having,” and the like are synonymous and are used inclusively, in an open-ended fashion, and do not exclude additional elements, features, acts, operations, and so forth. Also, the term “or” is used in its inclusive sense (and not in its exclusive sense) so that when used, for example, to connect a list of elements, the term “or” means one, some, or all of the elementsin the list. Further, the term “each,” as used herein, in addition to having its ordinary meaning, can mean any subset of a set of elements to which the term “each” is applied.

[0198] Disjunctive language such as the phrase “at least one of X, Y and Z,” unless specifically stated otherwise, is to be understood with the context as used in general to convey that an item, term, etc. may be either X, Y, or Z, or a combination thereof. Thus, such conjunctive language is not generally intended to imply that certain embodiments require at least one of X, at least one of Y and at least one of Z to each be present.

[0199] Unless otherwise explicitly stated, articles such as “a” or “an” should generally be interpreted to include one or more described items.Reference Numeral Table

Claims

CLAIMS1 . A process of regenerating metal alkali hydroxide for absorbing acid gas, including carbon dioxide gas, the process includes the steps of: contacting a solvent solution including a metal alkali absorbent with a gas to absorb carbon dioxide therefrom to produce a product gas that is leaner in carbon dioxide and a loaded solvent solution containing a carbonated metal alkali that includes precipitants; regenerating the carbonated metal alkali in an electrolytic cell by: generating hydrogen ions in an acidic compartment and hydroxide ions in a basic compartment, the compartments being separated by at least one permeable membrane; supplying at least part of the loaded solvent solution containing carbonated metal alkali, including precipitants thereof, to the electrolytic cell so that carbonated ions are converted to carbon dioxide in the acidic compartment and metal alkali ions are associated with hydroxide ions in the basic compartment; discharging a regenerated first solvent solution including causticized metal alkali from the basic compartment; and using the first solvent solution as at least part of the solvent solution in the contacting step.

2. The process according to claim 1, wherein the metal alkali is potassium and the carbonated potassium species includes potassium carbonate, potassium hydroxide and the causticized metal alkali includes potassium hydroxide.

3. The process according to any one of the preceding claims, wherein the solvent solution includes the potassium species at a concentration range from 35 to 60wt%, and suitably at a concentration in the range from 45 to 50wt%.

4. The process according to any one of the preceding claims, wherein the loaded solvent solution including the carbonated metal alkali and precipitants thereof, has a loading ranging from 0.65 to 0.90, and suitably ranging from 0.75 to 0.80.

5. The process according to any one of the preceding claims, wherein the loaded solvent solution has precipitants ranging from 5 to 40% wt / wt, and suitably ranging from 10 to 30 % wt / wt.

6. The process according to any one of the preceding claims, wherein the process includes a cooling step in which the loaded solvent solution is cooled to a temperature ranging from 20 to 70°C after the contacting step, and suitably to a temperature ranging from 20 to 40°C.

7. The process according to any one of the preceding claims, wherein the process includes: i) supplying at least part of the loaded solvent solution containing carbonated metal alkali to a thermal regenerating step and ii) regenerating the load solvent solution in the thermal regenerating step in which carbon dioxide is volatised from metal alkali bicarbonate to produce a second solvent solution containing metal alkali carbonate.

8. The process according to any one of the preceding claims, wherein the process includes a liquid / solid separating step in which the loaded solvent solution from the contacting step is separated into a solid phase including the precipitants of the carbonated metal alkali and a liquid phase, in which the solid phase is supplied to the acidic compartment of the electrolytic cell.

9. The process according to claim 8, wherein the solid phase includes at least 50%wt solids, suitably at least 75%wt solids, and even more suitably at least 90%wt solids.

10. The process according to claim 8 or 9 when appended to claim 7, wherein at least part of the loaded solvent solution supplied to the thermal regenerating step is the liquid phase provided by the separating step.11 . The process according to any one of the preceding claims, wherein the contacting step includes using the first solvent solution to produce a first product gas.

12. The process according to any one of the preceding claims, wherein the process includes controlling the rate of regeneration of the carbonated metal alkali in the electrolytic cell to in turn control the carbon dioxide content of the first product gas discharged from the absorbing step.

13. The process according to any one of the preceding claims, wherein the contacting step includes an auxiliary contacting step in which the first solvent solution absorbs carbon dioxide from the gas to produce the first product gas, in which the gas supplied to the auxiliary contacting step has an initial carbon dioxide concentration of greater than 400ppm and the first product gas has carbon dioxide down to ambient air levels or less.

14. The process according to claim 13, wherein the electrolytic cell regenerates from 0.1of the total loading capacity of the loaded solvent solution, and suitably from 2 to 5% of the total loading capacity of the loaded solvent solution.

15. The process according to claim 13, wherein the first product gas has carbon dioxide at levels below 400ppm, and suitably from Oppm to 200ppm, or 1ppm to 200pmm, or suitably from Oppm to 100ppm, or 1 ppm to 100pmm,16. The process according to claim 15, wherein the electrolytic cell regenerates in excess of 75% of the total loading capacity of the loaded solvent solution.

17. The process according to claim 16, wherein upto 25 % w / w of the loaded solvent solution is precipitant and supplied to the electrolytic cell for regeneration.

18. The process according to any one of claims 13 to 17, wherein the contacting step includes a preliminary contacting step in which the solvent solution, including at least part of the first solvent solution after the first solvent solution has been used in the auxiliary contacting step, absorbs carbon dioxide from the gas to provide a second product gas.

19. The process according to any one of claims 13 to 18, wherein the second product gas is supplied to the auxiliary contacting step to absorb carbon dioxide therefore and convert the second product gas into the first product gas.

20. The process according to any one of claims 13 to 19, wherein the second product gas has a carbon dioxide concentration that is greater than 400ppm, and suitably the concentration of carbon dioxide of the second product gas is greater than 500 ppm, and suitably greater than 600ppm.21 . The process according to any one of claim 13 to 20, wherein the first solvent solution and the second solvent solution are contacted with the gas in the preliminary contacting step to absorb carbon dioxide.

22. The process according to any one of claims 13 to 21 , wherein the first solvent solution used in, and discharged from the auxiliary contacting step, and the second solvent solution, are contacted with the gas in the preliminary contacting step to absorb carbon dioxide from the gas.

23. The process according to any one of the preceding claims, wherein the gas is a process gas.

24. The process according to any one of the preceding claims, wherein the contacting step includes contacting the first solvent solution with the ambient air to absorb carbon dioxide therefrom in an air contacting step, the treated air has carbon dioxide atconcentrations below 400ppm, and for example from Oppm to 200ppm, or from 2ppm to 200ppm, and suitably from Oppm to 100ppm, or from 2ppm to 100ppm.

25. The process according to claim 24 when appended to any one of claims claim 13 to 23, wherein the process includes discharging the first solvent solution from the air contacting step and supplying the first solvent solution to the auxiliary contact step for use therein.

26. The process according to any one of the preceding claims, wherein the permeable membrane is at least one of a cation membrane and / or an anion membrane.

27. The process according to any one of the preceding claims, wherein the step of supplying the carbonated metal alkali to the electrolytic cell includes supplying the carbonated metal alkali to the acidic compartment so that the carbonated metal alkali is converted into alkali metal ions and carbon dioxide, and the metal alkali ions diffuse to the basic compartment through the permeable membrane.

28. The process according to any one of the preceding claims, wherein an intermediate compartment is defined between the anion permeable membrane and the cation permeable membrane, and the step of supplying the carbonated metal alkali includes suppling the carbonated metal alkali into the intermediate compartment.

29. The process according to claim 28, wherein carbonated anions diffuse from the intermediate compartment to the acidic compartment, and the metal alkali cation diffuse from the intermediate compartment to the basic compartment30. The process according to any one of the preceding claims, wherein the process includes inhibiting the transfer of carbonate or bicarbonate ions from the acid compartment to basic compartment, and suitably, the process avoids carbonate or bicarbonate ions transferring from the acid compartment to basic compartment.31 . The process according to any one of the preceding claims, wherein the process includes operating the acidic compartment at a pH that enables the electrolytic cell to generate a carbon dioxide partial pressure sufficient for carbon dioxide production in the electrolytic cell.

32. The process according to any one of the preceding claims, wherein the process includes operating the electrolytic cell with the acidic compartment to a pH in the range from 3 to 9, or suitably to a pH in the range from 5 to 8.

33. The process according to any one of the preceding claims, wherein the process includes operating the electrolytic cell with the basic compartment to a pH in the range from 13 to 15, or suitably to a pH in the range from 14 to 14.5.

34. The process according to any one of the preceding claims, wherein the process includes controlling the temperature in the acidic and basic compartments to a temperature in a range from 20 to 140°C, more suitably in the range from 50 to 100°C, and suitably in the range from 70 to 90°C.

35. The process according to any one of the preceding claims, wherein the process includes oxidising water in the acidic compartment to produce oxygen gas and hydrogen ions, the hydrogen ions reacting with the carbonated metal alkali to produce carbon dioxide and metal alkali ions, and discharging a gas mixture including oxygen gas and carbon dioxide from the acidic compartment.

36. The process according to any one of the preceding claims, wherein water is reduced to hydrogen gas in the basic compartment.

37. The process according to claim 36, wherein the gas mixture can be contacted with the solvent solution.

38. The process according to any one of claims 1 to 34, wherein the process includes exchanging hydrogen ions from at least one electrode.

39. The process according to any one of claims 1 to 34, wherein the acidic compartment and the basic compartment include anode and cathode electrodes that can absorb and desorb hydrogen ions.

40. The process according to claim 39, wherein oxidation in the acidic compartment can result in hydrogen ions being desorbed from the anode, and reduction in the basic compartment can result in hydrogen ions being absorbed from the cathode.41 . The process according to any one of claims 38 to 40, wherein the process includes reversing the polarity of the electrolytic cell after an operating period.

42. The process according to claim 41, wherein reversing the polarity of the electrolytic cell includes changing the polarity of the electrical voltage applied to the electrodes so the cathode becomes a new anode, and the anode becomes a new cathode.

43. The process according to claim 41, wherein reversing the electrolytic cell includes physically swapping the cathode and anode in the electrolytic cell so as reallocate the cathode in the former basic compartment, now the new anode in the new acidic compartment, and the anode in the former basic compartment, now the new cathode in the new basic compartment.

44. The process according to any one of claims 1 to 34, wherein the process includes reducing hydrogen ions to produce a first hydrogen gas in the basic compartment, and oxidising hydrogen gas to produce hydrogen ions in the acidic compartment.

45. The process according to claim 44, wherein the process includes supplying the first hydrogen gas generated in the basic compartment to the acidic compartment for oxidation.

46. The process according to any one of the preceding claims, wherein the acidic and basic compartments include an anode and a cathode respectively.

47. The process according to any one of claims 1 to 34, wherein the process includes a disassociating step of disassociating water molecules into hydroxide ions and hydrogen ions from at least two bipolarised substrates driven by a voltage differential, and the disassociation of the water molecules occurring on opposite sides of the at least two bipolarised substrates to provide the acidic compartment and the basic compartment.

48. The process according to claim 47, wherein the bipolarised substrates are located so that opposite sides of the bipolarised substrates, generating hydroxide ions and hydrogen ions respectively, are directed toward each other and are separated by the permeable membrane therebetween, and the step of supplying the carbonated metal alkali includes suppling the carbonated metal alkali directly or indirectly into the acidic compartment.

49. The process according to claim 47 or 48, wherein the process includes arranging a series of the bipolar substrates that are arranged back-to-back in a parallel arrangement with the permeable membrane between each, and operating an anode electrode and a cathode electrode at opposite ends of the arrangement.

50. The process according to any one of claims 47 to 49, wherein the bipolar substrates may be permeable or semi-permeable membranes.51 . The process according to any one of the preceding claims, wherein either one or both of the acidic or basic compartments are actively circulated by pump or equivalent mechanism.

52. The process according to any one of the preceding claims, wherein the acidic compartment is maintained in a pressurised state to prevent the formation of gaseous carbon dioxide.

53. A plant that includes: an absorber that is configured to contact a solvent solution with a gas to absorb carbon dioxide therefrom and form a loaded solvent solution including carbonated metal alkali including precipitants and a lean gas; the electrolytic cell that receives carbonated metal alkali including precipitants of the solid phase from the separator and generates a first solvent solution containing the causticized metal alkali; and wherein the first solvent solution forms at least part of the solvent solution supplied to the absorber.

54. The plant according to claim 53, wherein the plant includes a separator that separates the loaded solvent solution into a solid phase and a liquid phase, in which the solid phase includes the carbonated metal alkali including precipitants, and the electrolytic cell having an inlet that receives that the solid phase from the separator.

55. The plant according to claim 53 or 54, wherein the plant includes a thermal regenerator having an inlet that receives the liquid phase from the separator, including metal alkali bicarbonate and metal alkali carbonate, and a heat source that volatilises carbon dioxide from the liquid phase to regenerate a second solvent solution including metal alkali carbonate.

56. The plant according to claim 55, wherein the absorber includes a main absorber that receives the process gas and an extension absorber that is arranged downstream of the main absorber in the direction of flow of the process gas through the absorber, and the extension absorber has a first inlet for the first solvent solution and the main absorber receives the first solvent solution discharged from the extension absorber and the second solvent solution57. The plant according to claim 56, wherein the absorber is configured so that the second solvent solution is supplied to contacting stages up stream of the extension absorber in the direction of flow of the gas through the absorber.

58. The plant according to any one of claims 53 to 57, wherein the plant includes a direct air contact (DAC) unit which is configured to contact the first solvent solution with air to absorb carbon dioxide therefrom, and from which carbon dioxide depleted air, namely treated air can be released.

59. The plant according to any one of claims 53 to 58, wherein the plant includes a cooler for cooling the loaded solvent solution to promote the formation of precipitants in the loaded solvent solution.

60. The plant according to any one of claims 53 to 59, wherein the plant includes a line for conveying carbon dioxide and oxygen gas from the electrolytic cell to the gas fed to the absorber.61 . The plant according to any one of claims 53 to 50, wherein the thermal regenerator includes an outlet line for discharging carbon dioxide volatilised from the loaded solvent solution, and a manifold for combining carbon dioxide from the electrolytic cell and carbon dioxide from the regenerator.

62. The plant according to any one of claims 53 to 50, wherein the electrolytic cell has a recirculation or stirring device for mixing the precipitants in the acidic compartment.

63. The plant according to any one of claims 53 to 50, wherein the electrolytic cell is configured to inhibit the transfer of the carbonate ion, bicarbonate ions or precipitants from transferring from the acidic compartment to the basic compartment.

64. A process of regenerating metal alkali hydroxide from a loaded solvent solution containing a carbonated metal alkali that includes precipitants, the process includes the steps of: generating hydrogen ions in an acidic compartment and hydroxide ions in a basic compartment of an electrolytic cell, the compartments being separated by at least one permeable membrane; supplying at least part of the loaded solvent solution containing carbonated metal alkali, including precipitants thereof, to the electrolytic cell so that carbonated ions are converted to carbon dioxide in the acidic compartment and metal alkali ions are associated with hydroxide ions in the basic compartment; discharging a regenerated solvent solution including causticized metal alkali from the basic compartment.

65. The process according to claim 64, wherein the metal alkali is potassium and the carbonated potassium species includes potassium carbonate, potassium hydroxide and the causticized metal alkali includes potassium hydroxide.

66. The process according to claim 64 or 65, wherein the solvent solution includes the potassium species at a concentration range from 35 to 60wt%, and suitably at a concentration in the range from 45 to 50wt%.

67. The process according to any one of claims 64 to 65, wherein the loaded solvent solution including the carbonated metal alkali and precipitants thereof, has a loading ranging from 0.65 to 0.90, and suitably ranging from 0.75 to 0.80.

68. The process according to any one of claims 64 to 67, wherein the loaded solvent solution has precipitants ranging from 5 to 40% wt / wt, and suitably ranging from 10 to 30 % wt / wt.

69. The process according to any one of claim 64 to 68, wherein the process includes a liquid / solid separating step in which the loaded solvent solution from the contacting step is separated into a solid phase including the precipitants of the carbonated metal alkali and a liquid phase, in which the solid phase is supplied to the acidic compartment of the electrolytic cell.

70. The process according to claim 69, wherein the solid phase includes at least 50%wt solids, suitably at least 75%wt solids, and even more suitably at least 90%wt solids.71 . The process according to any one of claims 64 to 70, wherein the electrolytic cell regenerates from 0.1 to 10% of the total loading capacity of the loaded solvent solution, and suitably from 1 to 7% of the total loading capacity of the loaded solvent solution, and suitably from 2 to 5% of the total loading capacity of the loaded solvent solution.

72. The process according to any one of claims 64 to 70, wherein the electrolytic cell regenerates in excess of 75% of the total loading capacity of the loaded solvent solution.

73. The process according to claim 72, wherein upto 25 % w / w of the loaded solvent solution is precipitant and supplied to the electrolytic cell for regeneration.

74. The process according to any one of claims 64 to 73, wherein the step of supplying the loaded solvent solution including carbonated metal alkali to the electrolytic cell includes supplying the carbonated metal alkali to the acidic compartment so that the carbonated metal alkali is converted into alkali metal ions and carbon dioxide, and the metal alkali ions diffuse to the basic compartment through the permeable membrane.

75. The process according to any one of claim 64 to 74, wherein an intermediate compartment is defined between the anion permeable membrane and the cation permeable membrane, and the step of supplying the carbonated metal alkali includes suppling the carbonated metal alkali into the intermediate compartment.

76. The process according to claim 75, wherein carbonated anions diffuse from the intermediate compartment to the acidic compartment, and the metal alkali cation diffuse from the intermediate compartment to the basic compartment77. The process according to any one of claims 64 to 76, wherein the process includes inhibiting the transfer of carbonate or bicarbonate ions from the acid compartment to basic compartment, and suitably, the process avoids carbonate or bicarbonate ions transferring from the acid compartment to basic compartment.

78. The process according to any one of claims 64 to 77, wherein the process includes operating the acidic compartment at a pH that enables the electrolytic cell to generate a carbon dioxide partial pressure sufficient for carbon dioxide production in the electrolytic cell.

79. The process according to any one of claims 64 to 78, wherein the process includes operating the electrolytic cell with the acidic compartment to a pH in the range from 3 to 9, or suitably in the range from 5 to 8.

80. The process according to any one of claims 64 to 79, wherein the process includes operating the electrolytic cell with the basic compartment to a pH in the range from 13 to 15, or suitably from 14 to 14.5.81 . The process according to any one of claims 64 to 80, wherein the process includes controlling the temperature in the acidic and basic compartments to a temperature in a range from 20 to 140°C, more suitably in the range from 50 to 100°C, and ideally in the range from 70 to 90°C.

82. The process according to any one of claim 64 to 81, wherein the process includes oxidising water in the acidic compartment to produce oxygen gas and hydrogen ions, the hydrogen ions reacting with the carbonated metal alkali to produce carbon dioxide and metal alkali ions, and discharging a gas mixture including oxygen gas and carbon dioxide from the acidic compartment.

83. The process according to any one of claims 64 to 82, wherein water is reduced to hydrogen gas in the basic compartment.

84. The process according to claim 83, wherein the gas mixture can be contacted with the solvent solution.

85. The process according to any one of claims 64 to 84, wherein the process includes exchanging hydrogen ions from at least one electrode.

86. The process according to any one of claims 64 to 85, wherein the acidic compartment and the basic compartment include anode and cathode electrodes that can absorb and desorb hydrogen ions.

87. The process according to claim 86, wherein oxidation in the acidic compartment can result in hydrogen ions being desorbed from the anode, and reduction in the basic compartment can result in hydrogen ions being absorbed from the cathode.

88. The process according to any one of claims 86 to 87, wherein the process includes reversing the polarity of the electrolytic cell after an operating period.

89. The process according to claim 88, wherein reversing the polarity of the electrolytic cell includes changing the polarity of the electrical voltage applied to the electrodes so the cathode becomes a new anode, and the anode becomes a new cathode.

90. The process according to claim 89, wherein reversing the electrolytic cell includes physically swapping the cathode and anode in the electrolytic cell so as reallocate the cathode in the former basic compartment, now the new anode in the new acidic compartment, and the anode in the former basic compartment, now the new cathode in the new basic compartment.91 . The process according to any one of claims 67 to 82, wherein the process includes reducing hydrogen ions to produce a first hydrogen gas in the basic compartment, and oxidising hydrogen gas to produce hydrogen ions in the acidic compartment.

92. The process according to claim 91, wherein the process includes supplying the first hydrogen gas generated in the basic compartment to the acidic compartment for oxidation.

93. The process according to any one of claims 67 to 82, wherein the acidic and basic compartments include an anode and a cathode respectively.

94. The process according to any one of claims 67 to 82, wherein the process includes a disassociating step of disassociating water molecules into hydroxide ions and hydrogen ions from at least two bipolarised substrates driven by a voltage differential, and the disassociation of the water molecules occurring on opposite sides of the at least two bipolarised substrates to provide the acidic compartment and the basic compartment.

95. The process according to claim 94, wherein the bipolarised substrates are located so that opposite sides of the bipolarised substrates, generating hydroxide ions and hydrogen ions respectively, are directed toward each other and are separated by the permeable membrane therebetween, and the step of supplying the carbonated metal alkali includes suppling the carbonated metal alkali directly or indirectly into the acidic compartment.

96. The process according to claim 94 or 95, wherein the process includes arranging a series of the bipolar substrates that are arranged back-to-back in a parallel arrangement with the permeable membrane between each, and operating an anode electrode and a cathode electrode at opposite ends of the arrangement.

97. The process according to any one of claims 94 to 96 wherein the bipolar substrates may be permeable or semi-permeable membranes.

98. The process according to any one of claims 67 to 97, wherein either one or both of the acidic or basic compartments are actively circulated by pump or equivalent mechanism.

99. The process according to any one of claims 67 to 98, wherein the acidic compartment is maintained in a pressurised state to prevent the formation of gaseous carbon dioxide.

100. An electrolytic cell generating metal alkali hydroxide for absorbing acid gas, including carbon dioxide gas, the electrolytic cell Includes: an acidic compartment associated with an anode and a basic compartment associated with a cathode, wherein hydrogen ions and hydroxide ions are generated in the compartments, and a permeable membrane is provided between the acidic and basic compartments; an inlet that supplies carbonated metal alkali including precipitants thereof to the electrolytic cell so that carbonated anions are converted to carbon dioxide in the acidic compartment and metal alkali ions are associated with hydroxide ions in the basic compartment; and an outlet that discharges a regenerated solvent solution including causticized metal alkali from the basic compartment.101 . The electrolytic cell according to claim 100, wherein the electrolytic cell includes an anode in the acidic compartment and a cathode in the anodic compartment to which a voltage difference is applied.

102. The electrolytic cell according to claim 100 or 101 wherein the electrolytic cell has a recirculation or stirring device for mixing the precipitants in the acidic compartment.

103. The electrolytic cell according to any one of claims 100 to 102, wherein the electrolytic cell is configured to inhibit the transfer of the carbonate or bicarbonate ions or particles from transferring from the acidic compartment to the basic compartment.

104. The electrolytic cell according to any one of claims 100 to 103, wherein the electrolytic cell includes a permeable membrane that inhibits transfer of carbonate or bicarbonate ions from the acid compartment to basic compartment, and permits that transfers of the metal alkali ions from the acid compartment to the basic compartment.

105. The electrolytic cell according to any one of claims 100 to 104, wherein the acid compartment of the electrolytic cell may be operated at a pH in the range from 3 to 9, or suitably in the range from 5 to 8.

106. The electrolytic cell according to any one of claims 100 to 105, wherein the basic cell of the electrolytic cell may be operated at a pH in the range from 13 to 15, or suitably in the range from 14 to 14.5.

107. The electrolytic cell according to any one of claims 100 to 106, wherein water is oxidised in the acidic compartment (at the anode) to oxygen gas and hydrogen ions, and a gas mixture including oxygen gas and carbon dioxide is be co-produced in the acidic compartment and discharged therefrom.

108. The electrolytic cell according to any one of claims 100 to 107, wherein at least one electrode, namely an anode or cathode of the acidic compartment and the basic compartment has the capacity to exchange (absorb and desorb) with hydrogen ions.

109. The electrolytic cell according to claim 108, wherein the acidic compartment and the basic compartment include anode and cathode electrodes that can absorb and desorb hydrogen anions through a voltage driven process, and oxidation in the acidic compartment can result in hydrogen ions being desorbed from the anode, and reduction in the basic compartment can result in hydrogen ions being absorbed from the cathode.

110. The electrolytic cell according to claim 108 or 109, wherein both the anode and cathode can absorb and desorb hydrogen ions, and after an operating period, at least one of the following will occur: the electrode acting as the anode can become depleted of hydrogen ions, and / or the electrode acting as the cathode can become loaded with hydrogen ions after an operating period, and the cell is configured to allow the anode and cathode to be swapped so that the anode can be operated as a new cathode, and the cathode can be operated an new anode.

111. The electrolytic cell according to claim 108 to 110, wherein the cell is configured to allow the polarity of the electrolytic cell to be reversed and the acidic and basic compartments can be swapped.

112. The electrolytic cell according to claim 110 or 111 , wherein during a regeneration phase a small reverse voltage is placed across the cell to drive a reversal of the proton exchange to revert the electrodes to their starting condition.

113. The electrolytic cell according to any one of claims 100 to 107, wherein hydrogen ions are reduced to a first hydrogen gas in the basic compartment, and a second hydrogen gas is supplied to the acidic compartment where is oxidized to hydrogen ions.

114. The electrolytic cell according to any one of claims 100 to 107, wherein the cell includes a passageway for conveying the first hydrogen gas generated in the basic compartment to the acidic compartment.

115. The electrolytic cell according to any one of claims 100 to 107, wherein the cell includes at least two bipolarised substrates and the permeable membrane is positioned between the bipolarised substrates, the bipolarised substrates disassociate water molecules into hydroxide ions and hydrogen ions and direct them to opposite sides of the substrate to provide the acidic compartment and the basic compartment when a voltage difference is applied, and the bipolarised substrates are arranged so that the side that generates the hydroxide ions of one of the bipolarised substrates faces toward the side that generates the hydrogen ions of the other bipolarised substrate.

116. The electrolytic cell according to claim 115, wherein the cell includes an inlet that supplies the carbonated metal alkali directly or indirectly into the acidic compartment that is located between the side of the bipolarised substrate that produces hydrogen ions and the permeable membrane, and the cell includes an outlet for discharging the metal alkali hydroxide from the basic compartment that is provided between the side of the bipolarised substrate that produces hydroxide ions and the permeable membrane.

117. The electrolytic cell according to claims 115 or 116, wherein the cell includes a series of the bipolar substrates arranged back-to-back in parallel with at least one of the permeable membranes located between each bipolar substrate, and an anode electrode and a cathode electrode at opposite ends of the cell.

118. The electrolytic cell according to claims 115 or 117, wherein the permeable membrane is a cation permeable membrane.

119. The electrolytic cell according to claim 118, wherein the cell includes an anion permeable membrane between the cation permeable membrane and the acidic compartment, thereby providing an intermediate compartment between the anion permeable membrane and the cathodic permeable membrane.

120. The electrolytic cell according to claim 119, wherein the cell includes an inlet for supplying the carbonated metal alkali into the intermediate compartment.121 . The electrolytic cell according to any one of claims 115 to 120, wherein the bipolar substrates are permeable or semi-permeable membrane.

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