EP2849872A1

EP2849872A1 - Co2 capture with carbonic anhydrase and tertiary amino solvents for enhanced flux ratio

Info

Publication number: EP2849872A1
Application number: EP13780585.9A
Authority: EP
Inventors: Geert F. Versteeg; Nathalie J.M.C. Penders; Sylvie Fradette; Julie Gingras; Jonathan A. CARLEY; Glenn R. Kelly; Normand Voyer; Peter W. J. DERKS; Olivera Ceperkovic
Original assignee: Co2 Solutions Inc
Current assignee: Co2 Solutions Inc
Priority date: 2012-04-24
Filing date: 2013-04-23
Publication date: 2015-03-25
Also published as: CN104602789A; EP2849872A4; WO2013159228A1

Abstract

Techniques for treating CO₂ containing gas include contacting the gas with an aqueous absorption solution including carbonic anhydrase as well as an absorption compound, which may be a tertiary amino compound for enzymatically enhanced flux of CO₂. The absorption compound may include MDEA, TEA, DEMEA, DMMEA, TIPA or DMgly, for example. The techniques may provide concentrations to enhance the enzymatic catalysis and inhibit viscosifying of the absorption solution or enzyme denaturing that would lower the overall CO₂ absorption rate. The absorption may be conducted at a temperature between about 0° C and about 80 °C, for example. Processes, uses and formulations are provided for enhanced CO₂ capture.

Description

CO2 CAPTURE WITH CARBONIC ANHYDRASE AND TERTIARY AMINO

SOLVENTS FOR ENHANCED FLUX RATIO

FIELD OF INVENTION

The present invention generally relates to the field of C0₂ capture and more particularly relates to C0₂ capture using carbonic anhydrase and an absorption compound.

BACKGROUND

There are various techniques for absorbing C0₂ from C0₂ containing gases. One technique involves using an absorption compound in combination with carbonic anhydrase enzyme.

The following patent documents relate to C0₂ absorption from C0₂ containing gases where carbonic anhydrase and an absorption compound may be used: US patent No. 7,740,689; US patent No. 8, 192,531 ; US patent application published under No. 20120129246; US patent application published under No. 20120122195; US patent application published under No. 20120129236; and international application published under No. WO 2012167388.

There are various challenges associated with removal of C0₂ from gases using enzymes or absorption compounds that may relate to efficiently obtaining high C0₂ absorption rates and efficient regeneration of the C0₂ loaded liquid.

SUM MARY OF INVENTION

Various techniques are provided for enzymatic C0₂ capture.

In some scenarios, there is provided a process for treating a C0₂ containing gas, comprising contacting the gas with an aqueous absorption solution comprising carbonic anhydrase and an amount of tertiary amino absorption compound sufficient to increase the enzymatically enhanced flux of C0₂ absorbed into the aqueous absorption solution by at least 6 times.

In some scenarios, the tertiary amino absorption compound comprises a tertiary alkanolamine and/or a tertiary amine. In some scenarios, the tertiary alkanolamine comprises MDEA, TEA, DEMEA, DMMEA or TIPA or a combination thereof.

In some scenarios, the tertiary amino absorption compound has a structure NR^Rs, wherein R is hydroxyethyl, isopropyl, methyl or ethyl, R₂ is methyl, ethyl, isopropyl or hydroxyethyl, and R₃ is methyl, ethyl, isopropyl or hydroxyethyl.

In some scenarios, the tertiary amino absorption compound has a concentration of at least 0.4 M, at least 1 M, at least 2 M, at least 3 M or at least 4 M.

In some scenarios, the tertiary amino absorption compound has a concentration between 0.4 M and 4 M, between 0.5 M and 3 M, between 0.75 M and 1.75 M or between 1 M and 2 M.

In some scenarios, the flux ratio between the enzymatically enhanced flux of C0₂ over the non-enzymatic flux of C0₂ is above 8 or above 10.

In some scenarios, the flux ratio between the enzymatically enhanced flux of C0₂ over the non-enzymatic flux of C0₂ is between 6 and 12.

In some scenarios, the carbonic anhydrase is provided free in the aqueous absorption solution as dissolved enzymes or as enzyme aggregates.

In some scenarios, the carbonic anhydrase is provided on or in particles that flow with the aqueous absorption solution, being entrapped in pores of the particles, covalently bonded to the particles, or otherwise immobilized with respect to the particles.

In some scenarios, the carbonic anhydrase is provided on or in packing material.

In some scenarios, the tertiary amino absorption compound and the carbonic anhydrase are provided in relative quantities between about 0.5 M per 0.2 g/L to about 2 M per 0.2 g/L, between about 1 M per 0.2 g/L to about 1.5 M per 0.2 g/L, in a range that may be determined from one or more of Figs 3 to 9.

In some scenarios, there is provided a process for treating a C0₂ containing gas comprising contacting the gas with an aqueous absorption solution comprising carbonic anhydrase and an amount of a slow absorption compound sufficient to increase the enzymatically enhanced flux of C0₂ absorbed into the aqueous absorption solution by at least 6 times.

In some scenarios, there is provided a process for treating a C0₂ containing gas comprising contacting the gas with an aqueous absorption solution comprising carbonic anhydrase and a tertiary amino absorption compound having the structure NR^F^, wherein Ri is hydroxyethyl, isopropyl, methyl or ethyl, R₂ is methyl, ethyl, isopropyl or hydroxyethyl, and R₃ is methyl, ethyl, isopropyl or hydroxyethyl. In some scenarios, the tertiary amino absorption compound is an alkanolamine.

In some scenarios, there is provided a process for treating a C0₂ containing gas comprising contacting the gas with an aqueous absorption solution comprising carbonic anhydrase and a tertiary amino absorption compound, wherein the concentrations of the carbonic anhydrase and tertiary amino absorption compound are selected to enhance the enzymatic catalysis and inhibit viscosifying of the absorption solution or enzyme denaturing that would lower the overall C0₂ absorption rate.

In some scenarios, there is provided a process for C0₂ capture, comprising:

supplying a C0₂ containing gas and an absorption solution as an ion lean solution into an absorption unit, wherein the ion lean solution has a lean C0₂ loading and comprises water and a tertiary amino compound selected from diethylmonoethanolamine (DEMEA), dimethylmonoethanolamine (DMMEA) and dimethylglycine (DMgly); contacting the C0₂ containing gas with the absorption solution in the presence of carbonic anhydrase or an analogue thereof, thereby producing a C0₂ depleted gas and an ion loaded solution that are released from the absorption unit, wherein the ion loaded solution has a rich C0₂ loading; supplying ion loaded solution to a desorption unit for producing a C0₂ stream and a regenerated solution; and recycling at least part of the regenerated solution as at least part of the ion lean solution supplied to the absorption unit.

In some scenarios, the rich C0₂ loading of the ion loaded solution is between about 0.05 and about 1. In some scenarios, the lean CO₂ loading of the ion lean solution is between about 0 and about 0.2

In some scenarios, absorption is conducted at a temperature between about 0ºC and about 80ºC.

In some scenarios, absorption is conducted at a temperature between about 40ºC and about 70ºC.

In some scenarios, absorption is conducted at a temperature between about 15ºC and 35ºC.

In some scenarios, absorption is conducted at a temperature about 25ºC.

In some scenarios, the tertiary amino compound has a concentration of at least 1 M in the absorption solution.

In some scenarios, the tertiary amino compound has a concentration of at least 2 M in the absorption solution.

In some scenarios, the tertiary amino compound has a concentration of at least 3 M. in the absorption solution.

In some scenarios, the tertiary amino compound has a concentration of at least 4 M in the absorption solution.

In some scenarios, the carbonic anhydrase or analogue thereof is provided as part of the absorption solution at a concentration of at least 100 mg/L.

In some scenarios, the carbonic anhydrase or analogue thereof is provided as part of the absorption solution at a concentration of at least 200 mg/L.

In some scenarios, the carbonic anhydrase or analogue thereof is provided as part of the absorption solution at a concentration of at least 400 mg/L.

In some scenarios, the carbonic anhydrase or analogue thereof is provided as part of the absorption solution at a concentration of at least 800 mg/L. In some scenarios, the tertiary amino and the carbonic anhydrase or analogue thereof are provided in concentrations sufficient to increase an overall forward reaction rate constant (k₀v) by at least about 250 s^-1 compared to a corresponding solution comprising N-methyl-diethanolamine (MDEA).

In some scenarios, the tertiary amino and the carbonic anhydrase or analogue thereof are provided in concentrations sufficient to increase an overall forward reaction rate constant (k₀v) by at least about 1250 s^"1 compared to a corresponding solution comprising N-methyl-diethanolamine (MDEA).

In some scenarios, the tertiary amino and the carbonic anhydrase or analogue thereof are provided in concentrations sufficient to increase an overall forward reaction rate constant (k₀v) by at least about 2500 s^"1 compared to a corresponding solution comprising N-methyl-diethanolamine (MDEA).

In some scenarios, the process also comprising selecting the tertiary amino compound in accordance with the pKa thereof.

In some scenarios, there is provided a process for absorbing C0₂ from a C0₂ containing gas, comprising contacting the C0₂ containing gas with an absorption solution in the presence of carbonic anhydrase or an analogue thereof, the absorption solution comprising water and a tertiary amino compound selected from diethylmonoethanolamine (DEMEA), dimethylmonoethanolamine (DMMEA) and dimethylglycine (DMgly).

In some scenarios, there is provided a method of enhancing enzymatic impact on C0₂ absorption, comprising conducting enzymatically catalysed C0₂ absorption into a solution comprising a tertiary amine compound selected from diethylmonoethanolamine (DEMEA), dimethylmonoethanolamine (DMMEA) and dimethylglycine (DMgly)

In some scenarios, there is provided a method of increasing C0₂ loading in a solution, comprising providing a tertiary amine compound selected from diethylmonoethanolamine (DEMEA), dimethylmonoethanolamine (DMMEA) and dimethylglycine (DMgly) in the solution and contacting the solution with a C0₂ containing gas in the presence of carbonic anhydrase or an analogue thereof. In some scenarios, there is provided a use of a tertiary amine compound selected from diethylmonoethanolamine (DEMEA), dimethylmonoethanolamine (DMMEA) and dimethylglycine (DMgly) for C0₂ absorption in the presence of carbonic anhydrase or an analogue thereof.

In some scenarios, there is provided a formulation for absorbing C0₂, comprising water, carbonic anhydrase or an analogue thereof, and a tertiary amine compound selected from diethylmonoethanolamine (DEMEA), dimethylmonoethanolamine (DMMEA) and dimethylglycine (DMgly).

In some scenarios, there is provided a formulation for absorbing C0₂, comprising water, carbonic anhydrase or an analogue thereof, and a tertiary amine compound having the formula R^NRa; wherein is selected from the group consisting of methyl, ethyl and propyl; R₂ is selected from the group consisting of methyl, ethyl and propyl; and R₃ is selected from the group consisting of 2-hydroxyethyl and carboxymethyl.

In some scenarios, Ri and R₂ are the same or different. In some scenarios, Ri and R₂ are selected from the group consisting of methyl and ethyl.

In some scenarios, the tertiary amine compound is selected form from diethylmonoethanolamine (DEMEA), dimethylmonoethanolamine (DMMEA), dimethylglycine (DMgly) and diethylglycine (DEgly).

In some scenarios, the tertiary amine compound has a pKa of at least 8.8, at least 9, at least 9.2, or at least 9.7.

In some scenarios, there is provided a process for desorbing C0₂ from an ion loaded solution, comprising:

Supplying the ion loaded solution to a desorption unit, wherein the ion loaded solution comprises water, bicarbonate and hydrogen ions and a tertiary amino compound selected from diethylmonoethanolamine (DEMEA), dimethylmonoethanolamine (DMMEA) and dimethylglycine (DMgly); providing carbonic anhydrase or an analogue thereof in the desorption unit in order to catalyze a dehydration reaction of the bicarbonate and hydrogen ions, thereby producing a C0₂ stream and a regenerated ion lean solution; and releasing the C0₂ stream and the regenerated ion lean solution from the desorption unit.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig 1 is a process block flow diagram.

Fig 2 is another process block flow diagram.

Fig 3 is a graph of C0₂ flux versus concentration for different compounds using enzyme or no enzyme.

Fig 4 is a graph of C0₂ flux ratio of enzyme over no enzyme, versus concentration for different compounds.

Fig 5 is a graph of overall kinetic rate constant as a function en enzyme concentration in MDEA solutions of 1 , 2, 3 and 4 M at 25 .

Fig. 6 is a graph of the overall kinetic rate constant as a function of enzyme concentration in TEA solutions of 1 , 2 and 4 M at 25

Fig 7 is a graph of the overall kinetic rate constant as a function of enzyme concentration in DMMEA solutions in 1 and 2 M at 25^.

Fig 8 is a graph of the overall kinetic reaction rate constant as a function of the enzyme concentration in TIPA solutions at 1 and 2 M at 25°C.

Fig 9 is a graph of the overall kinetic rate constant as a function of enzyme concentration in DEMEA solution at 0.5, 1 and 2 M at 25^.

Fig 10 is a graph of k_ov versus enzyme concentration with 1 M of DMMEA.

Fig 11 is a graph of k_ov versus enzyme concentration with 2 M of DMMEA.

Fig 12 is a graph of k_ov versus enzyme concentration with different concentrations of DMMEA.

Fig 13 is another graph of k_ov versus enzyme concentration with different compounds, TEA, MDEA and DMMEA. Fig 14 is a graph of k_ov versus pK_a with different concentrations of absorption compounds, combined with 100 mg/L of carbonic anhydrase.

Fig 15 is a graph of k_ov versus pK_a with different concentrations of absorption compounds, combined with 200 mg/L of carbonic anhydrase.

Fig 16 is a graph of k_ov versus pK_a with different concentrations of absorption compounds, combined with 400 mg/L of carbonic anhydrase.

Fig 17 is a graph of k_ov versus pK_a with different concentrations of absorption compounds, combined with 800 mg/L of carbonic anhydrase.

Fig 18 is a diagram of a stirred cell contactor.

Fig 19 is a diagram illustrating phase transfer.

Fig 20 is a process flow diagram of an example C0₂ capture system including absorption and desorption stages.

DETAILED DESCRIPTION

Various techniques are described for absorbing C0₂ from a C0₂ containing gas using an absorption solution in the presence of carbonic anhydrase or an analogue thereof.

Carbonic anhydrase is an enzyme known to catalyse C0₂ hydration through the following reaction:

The enzymatic C0₂ hydration rate increases with dissolved C0₂ concentration available in the liquid medium. The reaction limiting step can be related to the H⁺ release from the enzyme to the surrounding liquid medium. One way of accelerating this step is to have present in the liquid medium a compound that will capture this ion such as a base or a buffer solution.

At some concentration ranges of absorption compounds, such as tertiary amines or alkanolamines, the enzymatic impact is higher. At low concentrations, the enzymatic impact may be less pronounced, while at higher concentrations there may be viscosifying or enzyme denaturing effects that can decrease the effectiveness of the enzymatic process. In some scenarios, there is a concentration range enabling enhanced enzymatic impact on the C0₂ capture process. In some scenarios, the tertiary alkanolamine and/or tertiary amine may be selected and provided in a concentration that provides an enzymatic enhancement of the C0₂ capture process and does not detrimentally viscosify the absorption solution such that the mass transfer and C0₂ capture rate would be decreased. Tertiary compounds may be selected according to low viscosity characteristics at concentrations that provide enhanced enzymatic catalysis.

In some implementations, the present invention provides a carbonic anhydrase catalyzed C0₂ capture process utilizing an absorption solution including a tertiary amine or alkanolamine compound, where the concentration of the tertiary compound is provided in an optimal that enhances the enzymatic impact on absorption. In some scenarios, the concentration of the tertiary compound may be above 0.4 M. In some scenarios, the concentration of the tertiary compound may be between 0.4 M and 4 M. The concentration may be between 0.75 and 2.5 M, or between 1 M and 2 M.

In some implementations, the present invention provides a carbonic anhydrase catalyzed C0₂ capture process utilizing an absorption solution including a tertiary amine or alkanolamine compound, where the concentrations of the carbonic anhydrase and/or the tertiary compound are high enough to enhance the enzymatic catalysis while not too high to cause viscosifying that would lower the overall C0₂ absorption rate. For example, in some scenarios, the tertiary concentration may be from about 0.4 M to about 2 M. In some scenarios, the enzyme concentration may be sufficiently high to be at or near a maximum absorption rate per quantity of enzyme. For example, the enzyme concentration may be between 250 mg/L and 500 mg/L for an absorption solution including MDEA as a tertiary alkanolamine. The enzyme concentration may be in a concentration range having a high slope of absorption rate versus enzyme concentration relationship.

In some implementations, one or more of the appended Figs 3 to 9 may be employed for determining or adjusting the enzyme and/or absorption compound concentration for a C0₂ capture system, in order to provide enhanced enzymatic catalysis of the absorption reaction and/or increase the overall absorption of the system. Referring to Fig 1 , an example of the overall C0₂ capture system 10 includes a source 12 of C0₂ containing gas 14. The source may be a power plant, an aluminum smelter, refinery or another type of C0₂ producing operation.

The C0₂ containing gas 14 is supplied to an absorption unit 16, which is also fed with an aqueous absorption solution 18 for contacting the C0₂ containing gas 14.

In some implementations, the aqueous absorption solution 18 comprises carbonic anhydrase and an absorption compound, which may be a tertiary alkanolamine such as TEA and/or MDEA, but may also be other types of compounds which will be discussed further below. The carbonic anhydrase may be free in the aqueous absorption solution 18 as dissolved enzyme or aggregate particles of enzymes. The carbonic anhydrase may be on or in particles that are present in the aqueous absorption solution 18 and flow with it through the absorption unit 16. The carbonic anhydrase may be immobilized with respect to the particles using any method while keeping at least some of its activity. Some immobilization techniques include covalent bonding, entrapment, and so on. The carbonic anhydrase may be immobilized with respect to supports, which may be various structures such as packing material, within the absorption unit 16 so as to remain within the absorption unit 16 as the aqueous absorption solution 18 flows through it.

The absorption unit 16 may be various types, such as a packed reactor, a spray reactor or a bubble column type reactor. There may be one or more reactors that may be provided in series or in parallel.

In the absorption unit 16, the enzyme carbonic anhydrase catalyses the hydration reaction of C0₂ into bicarbonate and hydrogen ions and thus a C0₂ depleted gas 20 and an ion rich solution 22 are produced.

The ion rich solution 22 is then supplied to a desorption unit 26 to produce a C0₂ stream 28 and an ion depleted solution 30. Alternatively, the ion rich solution 22 may be supplied to another type of regeneration step such as mineral carbonation.

Referring now to Fig 2, the system 10 may also include a separation unit 32 arranged in between the absorption unit 16 and the desorption unit 26, for removing at least some and possibly all of the carbonic anhydrase in the event the enzyme is flowing with the ion rich solution 22, e.g. when the enzyme is free in solution or provided with respect to particles. The separation unit 32 produces an enzyme depleted stream 34 that may be supplied to the desorption unit 26 and an enzyme rich stream 36 that may be recycled, in whole or in part, to the absorption unit 16. The separation unit may also include one or more separators in series or parallel. The separators may be filters or other types of separators, depending on the removal characteristics for the enzymes and the form of the enzymes or particles.

The system may also include various other treatment units for preparing the ion rich solution for the desorption unit and/or for preparing the ion deplete unit for recycling into the absorption unit. There may be pH adjustment units or various monitoring units.

In some implementations, at least some carbonic anhydrase is provided in the desorption unit. The carbonic anhydrase may be provided within the input ion rich solution or added separately. The carbonic anhydrase may be tailored, designed, immobilised or otherwise delivered in order to withstand the conditions in the desorption unit.

The system may also include a measurement device for monitoring properties of various streams and adjusting operation of the absorption unit 16 to achieve desired properties. Adjusting could be done by various methods including modifying the liquid and/or gas flow rates, for example.

Referring to Fig 20, an overall C0₂ capture system and process 10a is shown and includes an absorption unit 12a and a desorption unit 14a. The absorption unit 12a may include an absorber reactor 16a which receives a C0₂-containing gas 18a that can come from a variety of sources. In one aspect, the C0₂-containing gas 18a is an effluent gas such as power plant flue gas, industrial exhaust gas, aluminum refining flue gas, aluminum smelting off-gas, steel production flue gas, chemical production flue gas, combustion gas from in-situ oil sands production etc. In another optional aspect, the C0₂-containing gas 18a includes or is a process gas stream such as raw or semi- processed natural gas, a hydrocarbon cracked gas (such as in ethylene production) or a carbon monoxide catalytic shift gas (such as in ammonia production). In another optional aspect, the C0₂-containing gas 18a is a naturally occurring gas such as ambient air. The absorber reactor 16a also receives an absorption solution 20a (which may also be referred to as a "C0₂-lean solution" herein). In the absorber reactor 16a, the conversion of C0₂ into bicarbonate and hydrogen ions takes place in the presence of carbonic anhydrase or an analog thereof, thereby producing a C0₂-depleted gas 22a and an ion-rich solution 24a. Preferably, the absorber reactor 16a is a direct-contact type reactor, such as a packed tower or spray scrubber or otherwise, allowing the gas and liquid phases to contact and mix together. The ion-rich solution 24a may be pumped by a pump 26a to downstream parts of the process, such as heat exchangers, desorption units, regeneration towers and the like. Part of the ion-rich solution 24a may be recycled back to the absorber reactor 16a via an ion-rich solution return line, which can improve mixing of the bottoms of the absorber reactor to avoid accumulation of precipitates and reactor deadzones, as the case may be. The absorber 16a may also have other recycle or return lines, as desired, depending on operating conditions and reactor design.

In some optional scenarios, as shown in Fig 20, the ion-rich solution 24a may then be fed to the desorption unit 14a, in which it can be regenerated and a C0₂ gas can be separated for sequestration, storage or various uses. The ion-rich solution 24a is preferably heated, which may be done by one or more heat exchanger 32a, to favor the desorption process. The heat exchanger may use heat contained in one or more downstream process streams in order to heat the ion-rich solution, e.g. ion-depleted solution 42a. The heated ion-rich solution 34a is fed into a desorption reactor 36a. In the desorption unit, carbonic anhydrase or analogs thereof may be present within the ion- rich solution 34a, allowing the carbonic anhydrase to flow with the ion-rich solution 34a while promoting the conversion of the bicarbonate ions into C0₂ gas 38a and generating an ion-depleted solution 40a. The carbonic anhydrase could also be fixed or immobilized within reactors or particles passing within and/or through the reactors. Alternatively, the enzymes could also be removed from the ion-rich stream prior to feeding it to the desorption reactor 36a. The process also includes releasing the C0₂ gas 38a and the ion-depleted solution 40a from the desorption unit 14a and, preferably, sending a recycled ion-depleted solution 42a to make up at least part of the absorption solution 20a. The ion-depleted solution 42a may be combined with a make-up stream 50a containing water, absorption compound and/or enzyme. The ion-depleted solution 42a is preferably cooled prior to re-injection into the absorption unit, which may be done by the heat exchanger 32a. The desorption reactor 36a may also include various recycle or return streams (not illustrated) as desired. The desorption unit 14a may also include one or more reboilers each of which takes a fraction of the liquid flowing through a corresponding one of the desorption reactors and heats it to generate steam that will create a driving force such that C0₂ will be further released from the solution. In some embodiments of the process, absorption is performed around Ο -δΟ , optionally 40'C- 70 , and desorption around 60 -180 , optionally 70 -150 . Optionally, absorption may be performed between ^5 and 35 to favor enz ymatic activity in some scenarios, although this may depend on the characteristics and stability of the given CA enzyme that may be used. In order to provide the carbonic anhydrase to the ion-rich solution 34a entering the desorption reactor 36a, there may be an enzyme feed stream 48a prior to the inlet into the desorption reactor 36a.

It should be noted that various types of absorption and desorption units may be used. For example, the absorption unit may be a packed tower, a spray reactor or a bubble column depending on the application and design considerations.

Regarding delivery of the enzyme to the process, in one optional aspect the enzyme is provided directly as part of a formulation or solution. There may also be enzyme provided in a reactor to react with incoming solutions and gases; for instance, the enzyme may be fixed to a solid non-porous packing material, on or in a porous packing material (which includes the scenario where the enzyme may be in a porous coating, such as a porous polymeric layer, provided a non-porous packing structure), on or in particles or as aggregates flowing with the absorption solution within a packed tower or another type of reactor. The carbonic anhydrase or analogue thereof may be in a free or soluble state in the formulation or immobilised on or in particles or as aggregates, chemically modified or stabilized, within the formulation. It should be noted that enzyme used in a free state may be in a pure form or may be in a mixture including impurities or additives such as other proteins, salts and other molecules coming from the enzyme production process. Immobilized enzyme free flowing in the solutions could be entrapped inside or fixed to a porous coating material that is provided around a support that is porous or non-porous. The enzymes may be immobilised directly onto the surface of a support (porous or non-porous) or may be present as cross linked enzyme aggregates (CLEAs) or cross linked enzyme crystals (CLECs). CLEA include precipitated enzyme molecules forming aggregates that are then cross-linked using chemical agents. The CLEA may or may not have a 'support' or 'core' made of another material which may or may not be magnetic. CLEC include enzyme crystals and cross linking agent and may also be associated with a 'support' or 'core' made of another material. When a support is used, it may be made of polymer, ceramic, metal(s), silica, solgel, chitosan, cellulose, alginate, polyacrylamide, magnetic particles and/or other materials known in the art to be suitable for immobilization or enzyme support. When the enzymes are immobilised or provided on particles, such as micro-particles, the particles are preferably sized and provided in a particle concentration such that they are pumpable with the solution throughout the process. When the enzymes are provided on micro-particles, the micro-particles may be sized in a number of ways.

In some implementations, the absorption compound may include a tertiary alkanolamine, such as Triethanolamine (TEA), N-Methyldiethanolamine (MDEA), Diethyl- monoethanolamine (DEMEA) and/or Dimethyl-monoethanolamine (DMMEA). Other tertiary amino compounds may also be used (and may or may not be alkanolamines having an alcohol group) such as the tertiary amine Triisopropylamine (TIPA). The tertiary alkanolamine may have the structure NR^Rs, where Ri is selected from hydroxyethyl, isopropyl, methyl or ethyl, R₂ is selected from methyl, ethyl, isopropyl or hydroxyethyl and R₃ is selected from methyl, ethyl, isopropyl or hydroxyethyl. Tertiary amines and alkanolamines may be seen as examples of slow absorption compounds since they do not react with C0₂ as primary and secondary amines and/or alkanolamines.

In some implementations, the absorption solution may include certain tertiary amino compounds, such as diethylmonoethanolamine (also known as diethylethanolamine and abbreviated as "DEMEA" or "DEEA"), dimethylmonoethanolamine (also known as dimethylethanolamine and abbreviated as "DMMEA" or "DMEA") and/or dimethylglycine (abbreviated as "DMG" or "DMgly").

As will be appreciated from the Examples section, in some C0₂ capture implementations, the effect of carbonic anhydrase on the acceleration of C0₂ absorption is more pronounced in the case of DMMEA, DEMEA and DMgly than in the cases with other tertiary alkanolamines, such as N-methyl-diethanolamine (abbreviated as "MDEA") and triethanolamine (abbreviated as "TEA"). As will be discussed and shown further below, DMMEA, DEMEA and DMgly also have lower pKa properties than MDEA and TEA. The catalyzing effect of carbonic anhydrase was observed to be generally dependent on the pKa of the alkanolamine in solution, increasing with increasing pKa as observed in the order DMMEA > MDEA > TEA, for example.

In some implementations, the tertiary amino compound has the formula R₄R₅NR₆, where R₄ is selected from the group consisting of methyl, ethyl and propyl; R₅ is selected from the group consisting of methyl, ethyl and propyl; and R₆ is selected from the group consisting of 2-hydroxyethyl and carboxym ethyl. R₄ and R₅ may be the same or different and in some examples may be selected from the group consisting of methyl and ethyl.

For example, other tertiary amino compounds that may be used include diethylglycine (abbreviated as "DEGly")

The absorption solution may also include further chemical additives in addition to the tertiary amino compound. For example, the absorption solution may further include a chemical additive selected from a primary amine, a secondary amine, an additional tertiary amine, a primary alkanolamine, a secondary alkanolamine, an additional tertiary alkanolamine, a primary amino acid, a secondary amino acid, an additional tertiary amino acid, or a carbonate compound, or a combination thereof. More particularly, the chemical additive may include at least one of the following: piperidine, piperazine, derivatives of piperidine or piperazine which are substituted by at least one alkanol group, monoethanolamine (MEA), 2-amino-2-methyl-1-propanol (AMP), 2-(2- aminoethylamino)ethanol (AEE), 2-amino-2-hydroxymethyl-1 ,3-propanediol (TRIS), N- methyldiethanolamine (MDEA), triisopropanolamine (TI PA), triethanolamine (TEA), dialkylether of polyalkylene glycols, dialkylether or dimethylether of polyethylene glycol, glycine, proline, arginine, histidine, lysine, aspartic acid, glutamic acid, methionine, serine, threonine, glutamine, cysteine, asparagine, valine, leucine, isoleucine, alanine, valine, tyrosine, tryptophan, phenylalanine, and derivatives thereof, taurine, N.cyclohexyl 1 ,3-propanediamine, N-secondary butyl glycine, N-methyl N-secondary butyl glycine, sarcosine, methyl taurine, methyl-a-aminopropionic acid, N-^-ethoxy)taurine, Ν-(β- aminoethyl)taurine, N-methyl alanine, 6-aminohexanoic acid and potassium or sodium salts thereof; potassium carbonate, sodium carbonate, ammonium carbonate, promoted potassium carbonate solutions and promoted sodium carbonate solutions or promoted ammonium carbonates, or combinations thereof. In the following section, cyclic capacity, C0₂ loading, carbonic anhydrase and absorption kinetics will be discussed in further detail.

The "theoretical cyclic capacity" and the "real cyclic capacity" are concepts that can be related to C0₂ capture processes. Theoretical cyclic capacity is the difference between the lean and rich C0₂ loadings of the absorption solution when chemical equilibrium is reached. Lean C0₂ loading is the loading of the C0₂-lean solution 20 entering the absorption unit 12 while the rich C0₂ loading is the loading of the ion-rich solution 24 leaving the absorption unit 12. Real cyclic capacity is the difference between the lean and rich C0₂ loadings of the absorption solution obtained under process conditions. Real cyclic capacity is lower than the theoretical cyclic capacity since chemical equilibrium conditions are not practically reached during process conditions, due to requiring a continued driving force for example.

Carbonic anhydrase is an efficient catalyst that enhances the reversible reaction of C0₂ to HC0₃ ^". Carbonic anhydrase is not just a single enzyme form, but a broad group of metalloproteins that exists in three genetically unrelated families of isoforms, α, β and γ. Carbonic anhydrase (CA) is present in and may be derived from animals, plants, algae, bacteria, etc. The human variant CA II, located in red blood cells, is the most studied and has a high catalytic turnover number. The carbonic anhydrase includes any analogue, fraction and variant thereof and may be alpha, gamma or beta type from human, bacterial, fungal or other organism origins, having thermostable or other stability properties, as long as the carbonic anhydrase can be provided to function in the C0₂ absorption and/or desorption processes to enzymatically catalyse the reaction:

The addition of carbonic anhydrase to an absorption solution having slow kinetics results in an increased C0₂ absorption rate and can help a system to reach a real cyclic capacity which is closer to the theoretical cyclic capacity. This is achieved because carbonic anhydrase increases the C0₂ reaction rate in the solution, leading to an increased C0₂ absorption rate into the solution and hence a higher C0₂ concentration in the solution which can also be expressed as C0₂ loading. Using carbonic anhydrase in an absorption unit, can enable a higher C0₂ loading of the absorption solution, in the case where equilibrium is not reached without the enzyme under the same operating conditions, and a corresponding increase in the real cyclic capacity.

"CO₂ loading" of an absorption solution means the CO₂ concentration in the solution in the forms of carbonate ions, bicarbonate ions and dissolved CO₂ per mole of absorption compound.

Some discussion and derivations regarding kinetics of CO₂ absorption will now be described. Regarding kinetics and reaction mechanisms, when CO₂ is absorbed, for example in an alkanolamine absorption solution, the following reactions occur simultaneously:

Reaction I: with primary or secondary alkanolamines

The corresponding reaction rate may be formulated as follows:

Reaction II: with tertiary alkanolamines

The corresponding reaction rate may be formulated as follows:

Reaction III: with hydroxide ions

The corresponding reaction rate may be formulated as follows:

Reaction IV: with water

The corresponding reaction rate may be formulated as follows:

The presence of carbonic anhydrase has an impact on the CO₂ reaction with water. Carbonic anhydrase catalyses this reaction and thus increases this reaction rate.

The overall forward reaction rate constant, k_ov, is determined by the contributions of each of these four reactions, whose kinetic rate expression is usually given as follows:

Under the process conditions, CO₂ will mainly react according to reaction II when no enzyme is present and according to reactions II and IV in presence of carbonic anhydrase.

It should be noted that carbonic anhydrase and analogues thereof may include naturally occurring, modified or evolved carbonic anhydrase enzymes; and analogues thereof may be variants or non-biological small molecules that are naturally occurring or synthesized to achieve or mimic the effect of the enzyme.

It is also noted that the patent documents referred to herein are incorporated herein by reference in their entirety.

EXAMPLES & EXPERIMENTATION

Impact of carbonic anhydrase in primary and tertiary alkanolamines

Tests were performed to compare the effect of carbonic anhydrase on C0₂ flux using with solutions including three different absorption compounds, namely TRIS, TEA and MDEA, at different concentrations. The enzyme concentration was constant at 0.2 g/L for all tests.

Fig 3 shows the results of some comparative tests. The tertiary alkanolamines, TEA and MDEA, provide higher C0₂ flux compared to TRIS at the higher concentrations of 1 M and 2 M. Fig 4 shows the flux ratio of the three compounds over the concentrations and the tertiary alkanolamines, TEA and MDEA, provide higher enzymatic C0₂ flux ratios compared to TRIS over the concentrations, the trend being particularly pronounced at the higher concentrations of 1 M and 2 M. The enzyme enhanced C0₂ flux for TEA is at least 6 times greater for all concentrations compared to no enzyme. The enzyme enhanced C0₂ flux for MDEA is at least 6 times greater for concentrations estimated above about 0.4 compared to no enzyme; and enzyme enhanced C0₂ flux for MDEA is around 10 or more times greater for concentrations of 1 or 2 M compared to no enzyme.

The data shown in Figs 3 and 4 were obtained from a comparative study including tests that were performed in a 160 mL-stirred cell reactor (Parr). An absorption solution, with a specific concentration of a C0₂ absorption compound (MDEA, Tris or TEA), was added into the stirred cell reactor. Carbonic anhydrase was added to reach an enzyme concentration of 0.2 g/L. C0₂ was injected into the stirred cell reactor to reach an initial pressure level of 10 psi. Temperature of the system was 25'C. Stirring conditions were adjusted to maintain a flat liquid-gas interface. The liquid and gas phases were stirred. C0₂ flux across the gas-liquid interface was determined and the data is presented in the Figs. The tertiary alkanolamines have a different response in a carbonic anhydrase enhanced C0₂ capture system at certain conditions, compared to the primary sterically hindered alkanolamine.

Tris is a sterically hindered primary alkanolamine.

With primary alkanolamines including Tris, the nitrogen reacts rapidly and directly with carbon dioxide to bring the carbon dioxide into solution according to the following reaction sequence:

For sterically hindered primary amines such as Tris, the carbamate reaction product (RNHCOO^") is then hydrolysed to bicarbonate (HC0₃ ^") as follows:

In forming a carbamate, the primary and secondary alkanolamines undergo a fast direct reaction with C0₂ which makes the rate of carbon dioxide absorption rapid.

MDEA, which is a tertiary alkanolamine, is a compound that does not react directly with C0₂, since the formation of the above described carbamate moiety is not possible.

The molecular structure of MDEA is as follows:

The role of MDEA is to capture the proton from C0₂ hydration reaction, whether such reaction is catalyzed or not. The overall reaction is as follows: This equilibrium has been studied in the literature, and it was shown that the rate constant of said equilibrium is much slower than the rate constant of the C0₂ hydration reaction with a primary alkanolamine.

In the presence of carbonic anhydrase, MDEA does not compete with C0₂ for reaction and therefore the impact of the enzyme on catalysis is maximized and improved relative to compounds that do compete for reaction with C0₂ such as Tris.

A similar reasoning can explain the higher C0₂ absorption flux in TEA than in Tris.

Furthermore, the enzymatic enhancement on the C0₂ flux may have an optimal range for tertiary alkanolamines, as evidenced in Fig 4, which shows that the MDEA and TEA curves both have a peak with a maximum at approximately 1.25 M and 1 M respectively, while the TRIS results show a steady decline in the C0₂ flux ratio of enzyme versus no enzyme.

For the conditions of these experiments and conditions similar and/or analogous thereto, one may provide the tertiary alkanolamines in a concentration range between about 0.4 M to about 2 M for C0₂ absorption, or between about 0.75 M and about 1.75 M, for example. At other operating conditions, such as enzyme concentration, pressure, temperature, relative liquid and gas flow rates, and so on, different absorption compound concentration ranges may be used.

Impact of the carbonic anhydrase concentration on the C0₂ reaction rate constant in tertiary amines

Tests were performed to evaluate the impact of the concentration of carbonic anhydrase on the C0₂ reaction rate in different tertiary amine solutions. Tests were conducted using a stirred cell. In a typical experiment a tertiary amine solution with desired concentration was prepared by dissolving a known amount of the amine in a known amount water together with a known amount of enzyme solution (human carbonic anhydrase (hCA II) or a thermostable variant of hCA II. Approximately 500 ml of the solution was transferred to the reactor, where inerts were removed by applying vacuum for a short time. Next, the solution was allowed to equilibrate at 298 K before its vapour pressure was recorded. Then C0₂ gas is introduced in the reactor at a known pressure, liquid agitation in started and C0₂ is absorbed into the solution. The C0₂ pressure is monitored and used to calculate the C0₂ absorption rate. Under the tests conditions, C0₂ absorption data are used to calculate the C0₂ reaction rate in the tertiary amine solution. This reaction rate is the sum of the reaction rate of the reaction between C0₂ and the tertiary amine and the reaction rate of C0₂ hydration reaction catalysed by carbonic anhydrase. Tests were conducted with tertiary amines MDEA, TEA, TIPA, DEMEA, DMMEA at different concentrations ranging from 0.5 to 4M depending on the tertiary amine tested. Enzyme concentrations ranging from 0.05 to 2.2 g/L.

Results are shown in Figs 5 to 9. For all solutions, it is clear that adding carbonic anhydrase into a tertiary alkanolamine/amine solution increases the overall C0₂ reaction rate in the solution as compared to the solution without enzyme (enzyme concentration = 0 mg/L).

Regarding k_ov, the C0₂ reaction in a MDEA solution is a pseudo-first order reaction where the overall reaction rate is governed by the following equation:

where R_∞2 is the C0₂ reaction rate in mol/L.s, k_ov is the overall pseudo-first order kinetic constant (s^"1) and C_Co2 is the C0₂ concentration in mol/L. The kinetic constant k_ov is defined as:

where C_MDEA is the MDEA concentration in mol/m³ and k₂ is the kinetic constant for the reaction of C0₂ in a MDEA solution.

In addition, referring to Fig. 6, it can be seen that at a certain higher concentrations of tertiary alkanolamine, e.g. 4 M, the rate of increase in k_ov can decrease with increasing enzyme concentration, while the enzymatic impact with lower concentration solutions, e.g. 1 or 2 M, is greater as enzyme concentration increases. In this regard, TEA is a more viscosifying compound in aqueous solutions than MDEA.

Further tests including DEMEA , DMMEA, DMgly solvents

Absorption experiments were performed in a thermostated stirred cell type reactor operated with a smooth and horizontal gas-liquid interface. The reactor was connected to two gas supply vessels filled with carbon dioxide (99.9 %, Hoekloos) or nitrous oxide (> 99 %, Hoekloos) from gas cylinders. Both the reactor and gas supply vessels were equipped with digital pressure transducers and PT-100 thermocouples. The measured signals were recorded in a computer. The pressure transducer connected to the stirred cell was a Druck PTX-520 pressure transducer (range 0 - 2 bars) and the gas supply vessels were equipped with Druck PTX-520 pressure transducers (range 0 - 100 bars). A schematic drawing of the experimental set-up is shown in Fig 18.

The following experiments were conducted for testing solutions including carbonic anhydrase and different absorption compounds at 25°C.

The following tertiary alkanolamines were tested in various additional experiments.

Tertiary alkanolamines pKa MW

TEA 7.7 150

TIPA 7.8 190

MDEA 8.6 120

DMMEA 9.2 90

DEMEA 9.7 120 The Figs 5 and 10 to 17, for instance, illustrate the impact of combining carbonic anhydrase with various tertiary amino absorption compounds.

For all tested conditions, and based on reactions II and IV, it is clear that using carbonic anhydrase in combination with the different tertiary amine solutions leads to an increase in the C0₂ reaction rate. Because of this, the absorption of C0₂ into the solution will be greater resulting in a solution with a higher C0₂ concentration or higher C0₂ loading when the enzyme is present as compared to a system operated under same conditions but with no enzyme. In addition, using a higher enzyme concentration enables reaching a higher C0₂ absorption rate, because of a faster reaction rate (higher k_ov), resulting in a solution with a higher C0₂ loading (given of course equilibrium is not reached).

This enhanced enzymatic effect on the reaction rate may be leveraged in various ways. For instance, it may be used to design smaller absorption units; to provide higher flow rates of absorption solution through a given absorption unit to achieve similar C0₂ loadings as a lower flow rate without enzymes; to increase the real cyclic capacity for an existing C0₂ absorption system; and so on.

Claims

1. A process for treating a C0₂ containing gas, comprising contacting the gas with an aqueous absorption solution comprising carbonic anhydrase and an amount of tertiary amino absorption compound sufficient to increase the enzymatically enhanced flux of C0₂ absorbed into the aqueous absorption solution by at least 6 times.

2. The process of claim 1 , wherein the tertiary amino absorption compound comprises a tertiary alkanolamine and/or a tertiary amine.

3. The process of claim 1 , wherein the tertiary alkanolamine comprises MDEA, TEA, DEMEA, DMMEA or TIPA or a combination thereof.

4. The process of claim 1 , wherein the tertiary amino absorption compound has a structure NR^Rs, wherein Ri is hydroxyethyl, isopropyl, methyl or ethyl, R₂ is methyl, ethyl, isopropyl or hydroxyethyl, and R₃ is methyl, ethyl, isopropyl or hydroxyethyl.

5. The process of claim 1 , wherein the tertiary amino absorption compound has a concentration of at least 0.4 M, at least 1 M, at least 2 M, at least 3 M or at least 4 M.

6. The process of claim 1 , wherein the tertiary amino absorption compound has a concentration between 0.4 M and 4 M, between 0.5 M and 3 M, between 0.75 M and 1.75 M or between 1 M and 2 M.

7. The process of claim 1 , wherein the flux ratio between the enzymatically enhanced flux of C0₂ over the non-enzymatic flux of C0₂ is above 8 or above 10.

8. The process of claim 1 , wherein the flux ratio between the enzymatically enhanced flux of C0₂ over the non-enzymatic flux of C0₂ is between 6 and 12.

9. The process of claim 1 , wherein the carbonic anhydrase is provided free in the aqueous absorption solution as dissolved enzymes or as enzyme aggregates.

10. The process of claim 1 , wherein the carbonic anhydrase is provided on or in particles that flow with the aqueous absorption solution, being entrapped in pores of the particles, covalently bonded to the particles, or otherwise immobilized with respect to the particles.

11. The process of claim 1 , wherein the carbonic anhydrase is provided on or in packing material.

12. The process of claim 1 , wherein the tertiary amino absorption compound and the carbonic anhydrase are provided in relative quantities between about 0.5 M per 0.2 g/L to about 2 M per 0.2 g/L, between about 1 M per 0.2 g/L to about 1.5 M per 0.2 g/L, in a range that may be determined from one or more of Figs 3 to 9.

13. A process for treating a C0₂ containing gas comprising contacting the gas with an aqueous absorption solution comprising carbonic anhydrase and an amount of a slow absorption compound sufficient to increase the enzymatically enhanced flux of C0₂ absorbed into the aqueous absorption solution by at least 6 times.

14. A process for treating a C0₂ containing gas comprising contacting the gas with an aqueous absorption solution comprising carbonic anhydrase and a tertiary amino absorption compound having the structure NF R2R₃, wherein is hydroxyethyl, isopropyl, methyl or ethyl, R₂ is methyl, ethyl, isopropyl or hydroxyethyl, and R₃ is methyl, ethyl, isopropyl or hydroxyethyl.

15. The process of claim 14, wherein the tertiary amino absorption compound is an alkanolamine.

16. A process for treating a C0₂ containing gas comprising contacting the gas with an aqueous absorption solution comprising carbonic anhydrase and a tertiary amino absorption compound, wherein the concentrations of the carbonic anhydrase and tertiary amino absorption compound are selected to enhance the enzymatic catalysis and inhibit viscosifying of the absorption solution or enzyme denaturing that would lower the overall C0₂ absorption rate.

17. A process for C0₂ capture, comprising:

supplying a C0₂ containing gas and an absorption solution as an ion lean solution into an absorption unit, wherein the ion lean solution has a lean C0₂ loading and comprises water and a tertiary amino compound selected from diethylmonoethanolamine (DEMEA), dimethylmonoethanolamine (DMMEA) and dimethylglycine (DMgly); contacting the CO₂ containing gas with the absorption solution in the presence of carbonic anhydrase or an analogue thereof, thereby producing a CO₂ depleted gas and an ion loaded solution that are released from the absorption unit, wherein the ion loaded solution has a rich CO₂ loading; supplying ion loaded solution to a desorption unit for producing a CO₂ stream and a regenerated solution; and recycling at least part of the regenerated solution as at least part of the ion lean solution supplied to the absorption unit.

18. The process of claim 17, wherein the rich CO₂ loading of the ion loaded solution is between about 0.05 and about 1.

19. The process of claim 17 or 18, wherein the lean CO₂ loading of the ion lean solution is between about 0 and about 0.2

20. The process of any one of claims 17 to 19, wherein absorption is conducted at a temperature between about 0ºC and about 80ºC.

21. The process of any one of claims 17 to 19, wherein absorption is conducted at a temperature between about 40ºC and about 70ºC.

22. The process of any one of claims 17 to 19, wherein absorption is conducted at a temperature between about 15ºC and 35ºC.

23. The process of any one of claims 17 to 19, wherein absorption is conducted at a temperature about 25ºC.

24. The process of any one of claims 17 to 23, wherein the tertiary amino compound has a concentration of at least 1 M in the absorption solution.

25. The process of any one of claims 17 to 23, wherein the tertiary amino compound has a concentration of at least 2 M in the absorption solution.

26. The process of any one of claims 17 to 23, wherein the tertiary amino compound has a concentration of at least 3 M. in the absorption solution.

27. The process of any one of claims 17 to 23, wherein the tertiary amino compound has a concentration of at least 4 M in the absorption solution.

28. The process of any one of claims 17 to 27, wherein the carbonic anhydrase or analogue thereof is provided as part of the absorption solution at a concentration of at least 100 mg/L.

29. The process of any one of claims 17 to 27, wherein the carbonic anhydrase or analogue thereof is provided as part of the absorption solution at a concentration of at least 200 mg/L.

30. The process of any one of claims 17 to 27, wherein the carbonic anhydrase or analogue thereof is provided as part of the absorption solution at a concentration of at least 400 mg/L.

31. The process of any one of claims 17 to 27, wherein the carbonic anhydrase or analogue thereof is provided as part of the absorption solution at a concentration of at least 800 mg/L.

32. The process of any one of claims 17 to 23, wherein the tertiary amino and the carbonic anhydrase or analogue thereof are provided in concentrations sufficient to increase an overall forward reaction rate constant (k₀v) by at least about 250 s^"1 compared to a corresponding solution comprising N-methyl-diethanolamine (MDEA).

33. The process of any one of claims 17 to 23, wherein the tertiary amino and the carbonic anhydrase or analogue thereof are provided in concentrations sufficient to increase an overall forward reaction rate constant (k₀v) by at least about 1250 s^"1 compared to a corresponding solution comprising N-methyl-diethanolamine (MDEA).

34. The process of any one of claims 17 to 23, wherein the tertiary amino and the carbonic anhydrase or analogue thereof are provided in concentrations sufficient to increase an overall forward reaction rate constant (k₀v) by at least about 2500 s^"1 compared to a corresponding solution comprising N-methyl-diethanolamine (MDEA).

35. The process of any one of claims 17 to 34, further comprising selecting the tertiary amino compound in accordance with the pKa thereof.

36. A process for absorbing C0₂ from a C0₂ containing gas, comprising contacting the C0₂ containing gas with an absorption solution in the presence of carbonic anhydrase or an analogue thereof, the absorption solution comprising water and a tertiary amino compound selected from diethylmonoethanolamine (DEMEA), dimethylmonoethanolamine (DMMEA) and dimethylglycine (DMgly).

37. A method of enhancing enzymatic impact on C0₂ absorption, comprising conducting enzymatically catalysed C0₂ absorption into a solution comprising a tertiary amine compound selected from diethylmonoethanolamine (DEMEA), dimethylmonoethanolamine (DMMEA) and dimethylglycine (DMgly)

38. A method of increasing C0₂ loading in a solution, comprising providing a tertiary amine compound selected from diethylmonoethanolamine (DEMEA), dimethylmonoethanolamine (DMMEA) and dimethylglycine (DMgly) in the solution and contacting the solution with a C0₂ containing gas in the presence of carbonic anhydrase or an analogue thereof.

39. Use of a tertiary amine compound selected from diethylmonoethanolamine (DEMEA), dimethylmonoethanolamine (DMMEA) and dimethylglycine (DMgly) for C0₂ absorption in the presence of carbonic anhydrase or an analogue thereof.

40. A formulation for absorbing C0₂, comprising water, carbonic anhydrase or an analogue thereof, and a tertiary amine compound selected from diethylmonoethanolamine (DEMEA), dimethylmonoethanolamine (DMMEA) and dimethylglycine (DMgly).

41. A formulation for absorbing C0₂, comprising water, carbonic anhydrase or an analogue thereof, and a tertiary amine compound having the formula R_!R₂NR₃; wherein is selected from the group consisting of methyl, ethyl and propyl; R₂ is selected from the group consisting of methyl, ethyl and propyl; and R₃ is selected from the group consisting of 2-hydroxyethyl and carboxymethyl.

42. The formulation of claim 41 , wherein Ri and R2 are the same or different.

43. The formulation of claim 41 or 42, wherein and R2 are selected from the group consisting of methyl and ethyl.

44. The formulation of any one of claims 41 to 43, wherein the tertiary amine compound is selected form from diethylmonoethanolamine (DEMEA), dimethylmonoethanolamine (DMMEA), dimethylglycine (DMgly) and diethylglycine (DEgly).

45. The formulation of any one of claims 41 to 44, wherein the tertiary amine compound has a pKa of at least 8.8.

46. The formulation of any one of claims 41 to 44, wherein the tertiary amine compound has a pKa of at least 9.

47. The formulation of any one of claims 41 to 44, wherein the tertiary amine compound has a pKa of at least 9.2.

48. The formulation of any one of claims 41 to 44, wherein the tertiary amine compound has a pKa of at least 9.7.

49. A process for desorbing C0₂ from an ion loaded solution, comprising: