Combined carbon capture and soda ash process

Abstract

The invention provides an enzymatic carbon dioxide capture process, where carbon dioxide is converted to, and precipitated as, sodium bicarbonate.

Description

[0001] COMBINED CARBON CAPTURE AND SODA ASH PROCESS

[0002] Reference to a sequence listing

[0003] This application contains a Sequence Listing in computer readable form, which is incorporated herein by reference.

[0004] FIELD OF THE INVENTION

[0005] The present invention relates to enzymatic carbon dioxide treatment processes, where the carbon dioxide is captured and precipitated as sodium bicarbonate.

[0006] BACKGROUND

[0007] Carbonic anhydrase enzymes are well-known for use as catalysts in enzymatic carbon capture processes, as shown in for example WO2012 / 025577. In such processes, gaseous carbon dioxide is captured in a carbonic anhydrase containing liquid medium, which often also contains high amounts of carbonates and bicarbonates. The carbonic anhydrase acts as a catalyst to establish a chemical equilibrium between carbon dioxide, carbonate and bicarbonate.

[0008] The carbon dioxide is captured in an absorber unit, where the liquid medium is contacted with the gaseous carbon dioxide. Subsequently, the liquid medium is often subjected to treatment in a high-temperature stripper unit to release the captured carbon dioxide and regenerate the liquid medium.

[0009] The high temperature treatment in the stripper unit is highly challenging for the enzyme stability. Even thermostable carbonic anhydrases will be affected by extended use of high temperatures and will need to be replenished during continuous operation of such carbon capture processes. Thus, a process that avoids use of a high temperature stripper is highly desirous when enzymes are used in carbon capture processes.

[0010] SUMMARY OF THE INVENTION

[0011] The present invention provides, in a first aspect, an enzymatic process for treating a carbon dioxide containing gas, comprising:

[0012] (a) supplying the carbon dioxide containing gas to an absorber;

[0013] (b) supplying an aqueous absorption solution to the absorber, where the aqueous absorption solution comprises 10-50% w / w of sodium carbonate, has a pH in the range of pH 9.5-13, and comprises 0.0005 g / L to 5 g / L of a carbonic anhydrase;

[0014] (c) contacting the carbon dioxide containing gas with the aqueous absorption solution in the absorber at a temperature in the range of 30-80°C to provide a bicarbonate-rich solution and a carbon dioxide depleted gas,

[0015] (d) optionally removing the bicarbonate-rich solution from the absorber; and (e) reducing the temperature of the bicarbonate-rich solution to at least 20°C below the temperature in the absorber to precipitate sodium bicarbonate, and to provide a bicarbonate- depleted solution.

[0016] In an embodiment, the process further comprises:

[0017] (f) adjusting the pH of the bicarbonate-depleted solution to the pH of the aqueous absorption solution with sodium hydroxide to provide a regenerated bicarbonate-depleted solution; and optionally also

[0018] (g) using the regenerated bicarbonate-depleted solution to prepare the aqueous absorption solution.

[0019] In another aspect, the invention provides a system for treatment of a carbon dioxide containing gas, according to the invention, comprising:

[0020] (a) an absorber unit, comprising: i) an inlet for the carbon dioxide containing gas, ii) an outlet for the carbon dioxide depleted gas, iii) an inlet for the aqueous absorption solution, and iv) an outlet for the bicarbonate-rich solution; and

[0021] (b) a precipitation unit, comprising: v) an inlet for the bicarbonate-rich solution, vi) an outlet for the precipitated sodium bicarbonate, and vii) an outlet for the bicarbonate-depleted solution, or an outlet for the regenerated bicarbonate-depleted solution.

[0022] In an embodiment, the precipitation unit comprises an outlet for the regenerated bicarbonate-depleted solution, and further comprises an inlet for a sodium hydroxide solution.

[0023] Other aspects and embodiments of the invention are apparent from the description and examples.

[0024] Unless otherwise indicated, or if it is apparent from the context that something else is meant, all percentages are percentage by weight (% w / w).

[0025] Sequences

[0026] SEQ ID NO: 1: Amino acid sequence of a carbonic anhydrase from Thermovibrio ammonificans

[0027] SEQ ID NO: 2: Amino acid sequence of a variant carbonic anhydrase of SEQ ID NO: 1

[0028] SEQ ID NO: 3: Amino acid sequence of a carbonic anhydrase from Persephonella marina

[0029] SEQ ID NO: 4: Amino acid sequence of a variant carbonic anhydrase of SEQ ID NO: 3

[0030] Definitions

[0031] The term “carbonic anhydrase activity” or “CA activity” is defined herein as an EC 4.2.1.1 activity which catalyzes the conversion between carbon dioxide and bicarbonate [CO2 + H2O HCOa' + H+], For purposes of the present invention, CA activity is determined according to the procedure described in Example 1 . One unit of CA activity is defined after Wilbur [1 II = (1 / tc)- (1 / tu) x 1000] where II is units and tcand turepresent the time in seconds for the catalyzed and uncatalyzed reaction, respectively (Wilbur, 1948, J. Biol. Chem. 176: 147-154). The polypeptides of the present invention are considered to have CA activity if they have at least 20%; preferably at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, or at least 95%; and most preferably at least 100% of the CA activity of the polypeptide consisting of the amino acid sequence of SEQ ID NO: 1 or SEQ ID NO: 3.

[0032] The term “CO2-containing medium” is used to describe any material which contains at least 0.01 % CO2; preferably at least 0.1% CO2, at least 1 % CO2, at least 5% CO2, at least 10% CO2, or at least 20% CO2; and most preferably at least 50% CO2. Preferably the CO2-containing medium has a temperature between 5°C and 110°C; more preferably between 10°C and 100°C, between 20°C and 95°C, between 30 °C and 90°C, between 40°C and 85°C, between 50°C and 80°C, or between 55°C and 75°C; and most preferably between 60°C and 70°C at any pressure. CO2-containing media are in particular gaseous phases (including gas mixtures), liquids or multiphase mixtures. A CO2-containing gaseous phase is for example raw natural gas obtainable from oil wells, gas wells, and condensate wells, syngas generated by the gasification of a carbon containing fuel (e.g., methane) to a gaseous product comprising CO and H2, or emission streams from combustion processes, e.g., from carbon based electric generation power plants, or from flue gas stacks from such plants, industrial furnaces, stoves, ovens, or fireplaces or from airplane or car exhausts. A CO2-containing gaseous phase may alternatively be ambient air (including hot (above 40°C) air, e.g., desert air), or from respiratory processes in mammals (such as the CCh-containing gas phase in an artificial lung), living plants and other CO2 emitting species, in particular from green-houses. A CCh-containing gas phase may also be off-gas, from aerobic or anaerobic fermentation, such as brewing, fermentation to produce useful products such as ethanol, or the production of biogas. Such fermentation processes can occur at elevated temperatures if they are facilitated by thermophilic microorganisms, which are for example encountered in the production of biogas. A CCh-containing gaseous phase may alternatively be a gaseous phase enriched in CO2 for the purpose of use or storage. The abovedescribed gaseous phases may also occur as multiphase mixtures, where the gas co-exists with a certain degree of fluids (e.g., water or other solvents) and / or solid materials (e.g., ash or other particles). CCh-containing liquids are any solution or fluid, in particular aqueous liquids, containing measurable amounts of CO2, preferably at one of the levels mentioned above at any pressure. CCh-containing liquids may be obtained by passing a CCh-containing gas or solid (e.g., dry ice or soluble carbonate containing salt) into the liquid. CCh-containing fluids may also be compressed CO2 liquid (that contains contaminants, such as dry-cleaning fluid), supercritical CO2, or CO2 solvent liquids, like ionic liquids. A CCh-containing liquid may also be referred to as a “carrier liquid”. A CCh-containing liquid may also include compounds capable of improving the CC>2-containing capacity of the liquid, such as HCCh' (KHCO3 or NaHCCh), COa2' (NaaCCh or K2CO3), HPO42’ (K2HPO4 or Na2HPO4) or MDEA or Tris or NH3.

[0033] The term “CO2 extraction” is to be understood as a reduction of carbon from a CO2- containing medium. Such an extraction may be performed from one medium to another, e.g., gas to liquid, liquid to gas, gas to liquid to gas, liquid to liquid or liquid to solid, but the extraction may also be the conversion of CO2 to bicarbonate, carbonate or carbonic acid within the same medium or the conversion of bicarbonate to CO2 within the same medium. The term CO2 capture is also used to indicate extraction of CO2 from one medium to another or conversion of CO2 to bicarbonate / carbonate or conversion of bicarbonate / carbonate to CO2.

[0034] The term “thermostable” carbonic anhydrase, indicates that the enzyme is functional or active ( / .e., can perform catalysis) at an elevated temperature, i.e., above 50°C; preferably above 55°C, above 60°C, above 65°C, above 70°C, above 75°C, above 80°C, above 85°C, or above 90°C; and most preferably above 100°C. In a preferred embodiment the carbonic anhydrase displays optimum activity at one of the temperatures indicated above, i.e., the enzyme’s temperature optimum is at one of the temperatures indicated above. The temperature stability of the carbonic anhydrase can be increased to some extent by way of formulation, e.g., by combination with stabilizing chemicals or by immobilization of the enzyme or by chemical modification, e.g. cross-linking, to preserve the enzyme in its active three dimensional shape. In order for an enzyme to be considered as thermostable it remains active for at least 15 minutes; preferably for at least 2 hours, for at least 24 hours, for at least 7 days, for at least 10 days, for at least 14 days, or for at least 30 days; and most preferably for at least 50 days at the elevated temperature. Generally, the level of activity is measured using the assay described in Example 2 after incubation for the given time in 1 M NaHCCh buffer at pH 8 at the given elevated temperature. The activity may be compared with the enzyme activity prior to the temperature elevation, thereby obtaining the residual activity of the enzyme after the heat treatment. Preferably, the residual activity is at least 50% after the given time at the elevated temperature, more preferably at least 60%, at least 70%, at least 80%, or at least 90%, and most preferably the level of residual activity is at least equal to or unchanged after the given time at the elevated temperature.

[0035] For purposes of the present invention, the sequence identity between two amino acid sequences is determined as the output of “longest identity” using the Needleman-Wunsch algorithm (Needleman and Wunsch, 1970, J. Mol. Biol. 48: 443-453) as implemented in the Needle program of the EMBOSS package (EMBOSS: The European Molecular Biology Open Software Suite, Rice et al., 2000, Trends Genet. 16: 276-277), preferably version 6.6.0 or later. The parameters used are a gap open penalty of 10, a gap extension penalty of 0.5, and the EBLOSUM62 (EMBOSS version of BLOSUM62) substitution matrix. In order for the Needle program to report the longest identity, the -nobrief option must be specified in the command line. The output of Needle labeled “longest identity” is calculated as follows: (Identical Residues x 100) / (Length of Alignment - Total Number of Gaps in Alignment)

[0036] DETAILED DESCRIPTION

[0037] The present invention concerns an enzymatic process for treatment of, or carbon dioxide extraction from, a carbon dioxide containing gas. The invention is based on the water solubility properties of sodium bicarbonate, which ranges from about 7% w / w at 0°C to about 17% w / w at 60°C in pure water, and more at higher temperatures. The sodium bicarbonate solubility versus temperature is not a linear relationship, but it is surprisingly close in the temperature range from O to 100°C.

[0038] The absorption rate of carbon dioxide is reduced with increasing temperatures. Therefore, high temperatures are usually not desired for faster mass transfers of carbon dioxide into solution. However, with the addition of a thermostable carbonic anhydrase, the absorption kinetics can be greatly improved at the high temperature conditions needed to take advantage of the increased solubility of sodium bicarbonate.

[0039] When a warm concentrated aqueous solution of sodium bicarbonate (a bicarbonate-rich solution) is cooled to a temperature below its saturation temperature, solid sodium bicarbonate will precipitate, and the solution will contain less solubilized sodium bicarbonate (a bicarbonate- depleted solution). If the solution is subsequently heated to a higher temperature again, the solution will have a regenerated capacity to dissolve more sodium bicarbonate. If the temperature difference between the warm and the cooled bicarbonate solutions is high, it is possible to precipitate a large amount of sodium bicarbonate.

[0040] Sodium bicarbonate can be converted to sodium carbonate with sodium hydroxide: NaHCO3+ NaOH Na2CO3+ H2O

[0041] - and sodium carbonate can be used to capture carbon dioxide:

[0042] Na2CO3+ CO2+ H2O 2NaHCO3

[0043] - thus, the overall net reaction of the invention is to add carbon dioxide and sodium hydroxide and remove precipitated sodium bicarbonate:

[0044] CO2+ NaOH NaHCO3

[0045] The pH dependent equilibrium between carbon dioxide, bicarbonate and carbonate is well-known, but existing carbon capture processes have a strong preference for using potassium carbonate / bicarbonate because of a higher water solubility. Such processes extracts carbon dioxide from the absorption solution by using a high temperature treatment (a stripper unit), where the low solubility of carbon dioxide at high temperatures will drive the reaction: 2NaHCO3Na2CO3+ CO2+ H2O

[0046] Such high temperatures are, however, detrimental to the stability of carbonic anhydrase enzymes, which are used in enzymatic carbon capture processes. The present invention avoids the use of such high temperatures and significantly improves the enzyme stability and extends the service life of the carbonic anhydrase enzymes. Further, the precipitated sodium bicarbonate is not just a waste product but provides additional value as a component in many industrial uses. If needed, the sodium bicarbonate can even be thermally decomposed (calcination) to provide pure carbon dioxide and sodium carbonate (soda ash).

[0047] The carbon dioxide absorption and the sodium bicarbonate precipitation can be carried out in the same unit by having different temperature zones. Carbon capture processes are typically carried out in tall towers. Such a tower may have a high temperature zone suited for carbon dioxide absorption in the upper half of the tower, and a low temperature zone suited for sodium bicarbonate precipitation in the lower half of the tower. The high temperature zone would then be the absorber unit, and the low temperature zone would be the precipitation unit.

[0048] Thus, the invention provides a simple and efficient enzymatic process for treating a carbon dioxide containing gas, such as a flue gas, biogas, natural gas, or an excipient gas from cement production, comprising:

[0049] (a) supplying the carbon dioxide containing gas to an absorber;

[0050] (b) supplying an aqueous absorption solution to the absorber, where the aqueous absorption solution comprises 10-50% w / w of sodium carbonate, has a pH in the range of pH 9.5-13, and comprises 0.0005 g / L to 5 g / L of a carbonic anhydrase;

[0051] (c) contacting the carbon dioxide containing gas with the aqueous absorption solution in the absorber at a temperature in the range of 30-80°C to provide a bicarbonate-rich solution and a carbon dioxide depleted gas,

[0052] (d) optionally removing the bicarbonate-rich solution from the absorber; and

[0053] (e) reducing the temperature of the bicarbonate-rich solution to at least 20°C below the temperature in the absorber to precipitate sodium bicarbonate, and to provide a bicarbonate- depleted solution.

[0054] In an embodiment, the process further comprises:

[0055] (f) adjusting the pH of the bicarbonate-depleted solution to the pH of the aqueous absorption solution with sodium hydroxide to provide a regenerated bicarbonate-depleted solution; and optionally also

[0056] (g) using the regenerated bicarbonate-depleted solution to prepare the aqueous absorption solution.

[0057] In an embodiment, the aqueous absorption solution in step (b) comprises 10-40% w / w, preferably 10-30% w / w, more preferably 10-20% w / w, of sodium carbonate.

[0058] In an embodiment, the aqueous absorption solution in step (b) is saturated with sodium carbonate.

[0059] In an embodiment, the aqueous absorption solution in step (b) has a pH in the range of pH 9.5-12.5, preferably a pH in the range of pH 9.5-12, pH 9.5-11.5, or pH 9.5-11.0. In an embodiment, the carbon dioxide containing gas is contacted with the aqueous absorption solution in the absorber in step (c) at a temperature in the range of 40-80°C, preferably in the range of 40-70°C.

[0060] In an embodiment, the temperature of the bicarbonate-rich solution in step (e) is reduced to at least 25°C, preferably at least 30°C, below the temperature in the absorber.

[0061] Typically, the absorber is a tall and narrow construction, like a tower, where the aqueous absorption solution is sprayed in small droplets from the top of the construction, and by the force of gravity, moves in countercurrent with the carbon dioxide containing gas to the bottom of the construction.

[0062] The precipitation of sodium bicarbonate may take place in a separate precipitation unit, but it may also take place in a combined absorber / precipitation unit or reactor, as described below. In any case, the precipitated sodium bicarbonate can be isolated by solid-liquid separation techniques well known in the art, such as filtration, centrifugation, or hydrocyclones.

[0063] The invention also provides a system for treatment of a carbon dioxide containing gas, according to the invention, comprising:

[0064] (a) an absorber unit, comprising: i) an inlet for the carbon dioxide containing gas, ii) an outlet for the carbon dioxide depleted gas, iii) an inlet for the aqueous absorption solution, and iv) an outlet for the bicarbonate-rich solution; and

[0065] (b) a precipitation unit, comprising: v) an inlet for the bicarbonate-rich solution, vi) an outlet for the precipitated sodium bicarbonate, and vii) an outlet for the bicarbonate-depleted solution, or an outlet for the regenerated bicarbonate-depleted solution.

[0066] In an embodiment, the precipitation unit comprises an outlet for the regenerated bicarbonate-depleted solution, and further comprises an inlet for sodium hydroxide, such as a sodium hydroxide solution.

[0067] The absorber unit and the precipitation unit may be combined in the same reactor, for example a tower or a similar construction, as described above. In that case the system comprises: i) an inlet for the carbon dioxide containing gas, ii) an outlet for the carbon dioxide depleted gas, and iii) an outlet for the precipitated sodium bicarbonate.

[0068] The system may also comprise an inlet for sodium hydroxide, such as a sodium hydroxide solution, but the pH adjustment with sodium hydroxide may also take place in external loop outside the precipitation unit or the reactor. The system may further comprise a means for contacting the carbon dioxide containing gas with the aqueous absorption solution. Such means could be a device for spraying the absorption solution from the top of the absorber / reactor, for example a nozzle, shower head, or similar device, capable of producing small droplets of the absorption solution.

[0069] Typically, the flow of the carbon dioxide containing gas is upward in the absorber / reactor, while the flow of the absorption solution is downward. After the first contact between the carbon dioxide containing gas and the absorption solution, the absorption solution may even be pumped back to the means for contacting the carbon dioxide containing gas, in order to increase the contact time and improve the absorption of carbon dioxide.

[0070] Carbonic anhydrase

[0071] The carbonic anhydrases used in the process of the invention are catalytic proteins, and the term “active carbonic anhydrase protein” is defined herein as the amount of catalytic protein, which exhibits carbonic anhydrase activity. This can be determined using an activity based analytical enzyme assay (see Example 1). In such assays, the carbonic anhydrase typically catalyzes a reaction generating a colored compound or a color change. The amount of the colored compound can be measured and correlated to the concentration of the active enzyme protein. This technique is well-known in the art.

[0072] The aqueous absorption solution used in the process of the invention comprises 0.0005 g / L to 5 g / L of active carbonic anhydrase protein. Preferably the aqueous absorption solution comprises 0.0005 g / L to 5 g / L of active carbonic anhydrase protein, more preferably the aqueous absorption solution comprises 0.0005 g / L to 2 g / L, 0.001 g / L to 5 g / L, 0.001 g / L to 2 g / L, 0.001 g / L to 1 g / L, 0.005 g / L to 2 g / L, 0.005 g / L to 1 g / L, 0.01 g / L to 2 g / L, or 0.01 g / L to 1 g / L of active carbonic anhydrase protein.

[0073] Carbonic anhydrase enzymes used in the process of the invention may be any carbonic anhydrase of enzyme class EC 4.2.1.1. Preferably, the carbonic anhydrase is a thermostable carbonic anhydrase; for example, a carbonic anhydrase having at least 50% residual activity after incubation for 15 minutes at 80°C.

[0074] In an embodiment, the carbonic anhydrase has an amino acid sequence identity to SEQ ID NO: 1 , SEQ ID NO: 2, SEQ ID NO: 3, or SEQ ID NO: 4, of at least 60%, e.g., at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100%. In one aspect, the carbonic anhydrase amino acid sequence differs by up to 10 amino acids, e.g., 1 , 2, 3, 4, 5, 6, 7, 8, 9, or 10, from the amino acid sequence of SEQ ID NO: 1, SEQ ID NO: 2, SEQ ID NO: 3, or SEQ ID NO: 4.

[0075] In a preferred embodiment, the carbonic anhydrase is the Thermovibrio ammonificans carbonic anhydrase shown as SEQ ID NO: 1 or the variant thereof shown as SEQ ID NO: 2 (both are described in WO 2016 / 029316); or the Persephonella marina carbonic anhydrase shown as SEQ ID NO: 3 (described in WO 2012 / 025577) or the variant thereof shown as SEQ ID NO: 4 (described in WO 2024 / 118901).

[0076] Amino acid alterations of carbonic anhydrases, as described above, may be of a minor nature, that is conservative amino acid substitutions or insertions that do not significantly affect the folding and / or activity of the protein; small deletions, typically of 1-30 amino acids; small amino- or carboxyl-terminal extensions, such as an amino-terminal methionine residue; a small linker peptide of up to 20-25 residues; or a small extension that facilitates purification by changing net charge or another function, such as a poly-histidine tract, an antigenic epitope or a binding module.

[0077] Essential amino acids in a polypeptide can be identified according to procedures known in the art, such as site-directed mutagenesis or alanine-scanning mutagenesis (Cunningham and Wells, 1989, Science 244: 1081-1085). In the latter technique, single alanine mutations are introduced at every residue in the molecule, and the resultant molecules are tested for enzyme activity to identify amino acid residues that are critical to the activity of the molecule. See also, Hilton et al., 1996, J. Biol. Chem. 271: 4699-4708. The active site of the enzyme or other biological interaction can also be determined by physical analysis of structure, as determined by such techniques as nuclear magnetic resonance, crystallography, electron diffraction, or photoaffinity labeling, in conjunction with mutation of putative contact site amino acids. See, for example, de Vos et al., 1992, Science 255: 306-312; Smith et al., 1992, J. Mol. Biol. 224: 899- 904; Wlodaver et al., 1992, FEBS Lett. 309: 59-64. The identity of essential amino acids can also be inferred from an alignment with a related polypeptide, and / or be inferred from sequence homology and conserved catalytic machinery with a related polypeptide or within a polypeptide or protein family with polypeptides / proteins descending from a common ancestor, typically having similar three-dimensional structures, functions, and significant sequence similarity.

[0078] Single or multiple amino acid substitutions, deletions, and / or insertions can be made and tested using known methods of mutagenesis, recombination, and / or shuffling, followed by a relevant screening procedure, such as those disclosed by Reidhaar-Olson and Sauer, 1988, Science 241 : 53-57; Bowie and Sauer, 1989, Proc. Natl. Acad. Sci. USA 86: 2152-2156; WO 95 / 17413; or WO 95 / 22625. Other methods that can be used include error-prone PCR, CRISPR gene editing, phage display (e.g., Lowman et al., 1991, Biochemistry 30: 10832-10837; US 5,223,409; WO 92 / 06204), and region-directed mutagenesis (Derbyshire et al., 1986, Gene 46: 145; Ner et a / ., 1988, DNA 7: 127).

[0079] Further embodiments of the invention include:

[0080] Embodiment 1. An enzymatic process for treating a carbon dioxide containing gas, comprising:

[0081] (a) supplying the carbon dioxide containing gas to an absorber; (b) supplying an aqueous absorption solution to the absorber, where the aqueous absorption solution comprises 10-50% w / w of sodium carbonate, has a pH in the range of pH 9.5-13, and comprises 0.0005 g / L to 5 g / L of a carbonic anhydrase;

[0082] (c) contacting the carbon dioxide containing gas with the aqueous absorption solution in the absorber at a temperature in the range of 30-80°C to provide a bicarbonate-rich solution and a carbon dioxide depleted gas,

[0083] (d) optionally removing the bicarbonate-rich solution from the absorber; and

[0084] (e) reducing the temperature of the bicarbonate-rich solution to at least 20°C below the temperature in the absorber to precipitate sodium bicarbonate, and to provide a bicarbonate- depleted solution.

[0085] Embodiment 2. The process of the preceding embodiment, which further comprises:

[0086] (f) adjusting the pH of the bicarbonate-depleted solution to the pH of the aqueous absorption solution with sodium hydroxide to provide a regenerated bicarbonate-depleted solution; and optionally also

[0087] (g) using the regenerated bicarbonate-depleted solution to prepare the aqueous absorption solution.

[0088] Embodiment 3. The process of any of the preceding embodiments, where the aqueous absorption solution in step (b) is saturated with sodium carbonate.

[0089] Embodiment 4. The process of any of the preceding embodiments, where the aqueous absorption solution in step (b) comprises 10-40% w / w of sodium carbonate.

[0090] Embodiment 5. The process of any of the preceding embodiments, where the aqueous absorption solution in step (b) comprises 10-30% w / w of sodium carbonate.

[0091] Embodiment 6. The process of any of the preceding embodiments, where the aqueous absorption solution in step (b) comprises 10-20% w / w of sodium carbonate.

[0092] Embodiment 7. The process of any of the preceding embodiments, where the aqueous absorption solution in step (b) has a pH in the range of pH 9.5-12.5, such as pH 10-12.5.

[0093] Embodiment 8. The process of any of the preceding embodiments, where the aqueous absorption solution in step (b) has a pH in the range of pH 9.5-12, such as pH 10-12.

[0094] Embodiment 9. The process of any of the preceding embodiments, where the aqueous absorption solution in step (b) has a pH in the range of pH 9.5-11.5, such as pH 10-11.5.

[0095] Embodiment 10. The process of any of the preceding embodiments, where the aqueous absorption solution in step (b) has a pH in the range of 9.5-11.0, such as pH 10-11.

[0096] Embodiment 11. The process of any of the preceding claims, where the aqueous absorption solution in step (b) comprises 0.0005 g / L to 2 g / L of a carbonic anhydrase.

[0097] Embodiment 12. The process of any of the preceding claims, where the aqueous absorption solution in step (b) comprises 0.001 g / L to 2 g / L of a carbonic anhydrase.

[0098] Embodiment 13. The process of any of the preceding claims, where the aqueous absorption solution in step (b) comprises 0.001 g / L to 1 g / L of a carbonic anhydrase. Embodiment 14. The process of any of the preceding claims, where the aqueous absorption solution in step (b) comprises 0.005 g / L to 1 g / L of a carbonic anhydrase.

[0099] Embodiment 15. The process of any of the preceding claims, where the aqueous absorption solution in step (b) comprises 0.01 g / L to 5 g / L, 0.01 g / L to 2 g / L, or 0.01 g / L to 1 g / L of a carbonic anhydrase.

[0100] Embodiment 16. The process of any of the preceding embodiments, where the carbon dioxide containing gas is contacted with the aqueous absorption solution in the absorber in step (c) at a temperature in the range of 40-80°C.

[0101] Embodiment 17. The process of any of the preceding embodiments, where the carbon dioxide containing gas is contacted with the aqueous absorption solution in the absorber in step (c) at a temperature in the range of 40-70°C.

[0102] Embodiment 18. The process of any of the preceding embodiments, where the temperature of the bicarbonate-rich solution in step (e) is reduced to at least 25°C below the temperature in the absorber.

[0103] Embodiment 19. The process of any of the preceding embodiments, where the temperature of the bicarbonate-rich solution in step (e) is reduced to at least 30°C below the temperature in the absorber.

[0104] Embodiment 20. The process of any of the preceding embodiments, where the carbon dioxide containing gas is a flue gas, biogas, natural gas, or an excipient gas from cement or glass or steel production.

[0105] Embodiment 21. The process of any of the preceding embodiments, wherein the carbonic anhydrase is a thermostable carbonic anhydrase.

[0106] Embodiment 22. The process of any of the preceding embodiments, wherein the carbonic anhydrase is a thermostable carbonic anhydrase having at least 50% residual activity after incubation for 15 minutes at 80°C.

[0107] Embodiment 23. The process of any of the preceding embodiments, wherein the carbonic anhydrase has at least 60% amino acid sequence identity to any of SEQ ID NO: 1 , SEQ ID NO: 2, SEQ ID NO: 3, and SEQ ID NO: 4.

[0108] Embodiment 24. The process of any of the preceding embodiments, wherein the carbonic anhydrase has at least 70% amino acid sequence identity to any of SEQ ID NO: 1 , SEQ ID NO: 2, SEQ ID NO: 3, and SEQ ID NO: 4.

[0109] Embodiment 25. The process of any of the preceding embodiments, wherein the carbonic anhydrase has at least 80% amino acid sequence identity to any of SEQ ID NO: 1 , SEQ ID NO: 2, SEQ ID NO: 3, and SEQ ID NO: 4.

[0110] Embodiment 26. The process of any of the preceding embodiments, wherein the carbonic anhydrase has at least 90% amino acid sequence identity to any of SEQ ID NO: 1 , SEQ ID NO: 2, SEQ ID NO: 3, and SEQ ID NO: 4. Embodiment 27. The process of any of the preceding embodiments, wherein the carbonic anhydrase has at least 95% amino acid sequence identity to any of SEQ ID NO: 1 , SEQ ID NO: 2, SEQ ID NO: 3, and SEQ ID NO: 4.

[0111] Embodiment 28. The process of any of the preceding embodiments, wherein the carbonic anhydrase has up to 30 alterations (substitutions, deletions, and / or insertions), preferably up to 25, up to 20, up to 15, up to 10, up to 9, up to 8, up to 7, up to 6, up to 5, up to 4, up to 3, up to 2, or up to 1 alteration(s), as compared to the amino acid sequence of SEQ ID NO: 1, SEQ ID NO: 2, SEQ ID NO: 3, or SEQ ID NO: 4.

[0112] Embodiment 29. The process of any of the preceding embodiments, wherein the carbonic anhydrase has up to 30 substitutions, preferably up to 25, up to 20, up to 15, up to 10, up to 9, up to 8, up to 7, up to 6, up to 5, up to 4, up to 3, up to 2, or up to 1 alteration(s), as compared to the amino acid sequence of SEQ ID NO: 1 , SEQ ID NO: 2, SEQ ID NO: 3, or SEQ ID NO: 4.

[0113] Embodiment 30. The process of any of the preceding embodiments, wherein the carbonic anhydrase has at least 60% amino acid sequence identity to SEQ ID NO: 1 or SEQ ID NO: 3.

[0114] Embodiment 31. The process of any of the preceding embodiments, wherein the carbonic anhydrase has at least 70% amino acid sequence identity to SEQ ID NO: 1 or SEQ ID NO: 3.

[0115] Embodiment 32. The process of any of the preceding embodiments, wherein the carbonic anhydrase has at least 80% amino acid sequence identity to SEQ ID NO: 1 or SEQ ID NO: 3.

[0116] Embodiment 33. The process of any of the preceding embodiments, wherein the carbonic anhydrase has at least 90% amino acid sequence identity to SEQ ID NO: 1 or SEQ ID NO: 3.

[0117] Embodiment 34. The process of any of the preceding embodiments, wherein the carbonic anhydrase has at least 95% amino acid sequence identity to SEQ ID NO: 1 or SEQ ID NO: 3.

[0118] Embodiment 35. The process of any of the preceding embodiments, wherein the carbonic anhydrase has up to 30 alterations (substitutions, deletions, and / or insertions), preferably up to 25, up to 20, up to 15, up to 10, up to 9, up to 8, up to 7, up to 6, up to 5, up to 4, up to 3, up to 2, or up to 1 alteration(s), as compared to the amino acid sequence of SEQ ID NO: 1 or SEQ ID NO: 3.

[0119] Embodiment 36. The process of any of the preceding embodiments, wherein the carbonic anhydrase has up to 30 substitutions, preferably up to 25, up to 20, up to 15, up to 10, up to 9, up to 8, up to 7, up to 6, up to 5, up to 4, up to 3, up to 2, or up to 1 alteration(s), as compared to the amino acid sequence of SEQ ID NO: 1 or SEQ ID NO: 3.

[0120] Embodiment 37. A system for treatment of a carbon dioxide containing gas, according to any of the preceding embodiments, comprising

[0121] (a) an absorber unit, comprising: i) an inlet for the carbon dioxide containing gas, ii) an outlet for the carbon dioxide depleted gas, iii) an inlet for the aqueous absorption solution, and iv) an outlet for the bicarbonate-rich solution; and (b) a precipitation unit, comprising: v) an inlet for the bicarbonate-rich solution, vi) an outlet for the precipitated sodium bicarbonate, and vii) an outlet for the bicarbonate-depleted solution, or an outlet for the regenerated bicarbonate-depleted solution.

[0122] Embodiment 38. The system of the preceding embodiment, where the precipitation unit comprises an outlet for the regenerated bicarbonate-depleted solution.

[0123] Embodiment 39. The system of the preceding embodiment, where the precipitation unit further comprises an inlet for sodium hydroxide, such as a sodium hydroxide solution.

[0124] Embodiment 40. The system of any of embodiments 22-24, which further comprises a means for contacting the carbon dioxide containing gas with the aqueous absorption solution.

[0125] Embodiment 41. The system of the preceding embodiment, where the means is capable of producing droplets of the absorption solution, such as a nozzle or a shower head.

[0126] Embodiment 42. The system of the preceding embodiment, where the absorption solution is recirculated to the means with a pump.

[0127] Embodiment 43. The system of any of embodiments 22-27, where the flow of the carbon dioxide containing gas is upward and the flow of the absorption solution is downward.

[0128] Embodiment 44. A combined system for treatment of a carbon dioxide containing gas, according to embodiments 1 to 21 , comprising: i) an inlet for the carbon dioxide containing gas, ii) an outlet for the carbon dioxide depleted gas, iii) an outlet for the precipitated sodium bicarbonate, and optionally iv) an inlet for sodium hydroxide, such as a sodium hydroxide solution.

[0129] Embodiment 45. The combined system of the preceding embodiment, which further comprises a means for contacting the carbon dioxide containing gas with the aqueous absorption solution.

[0130] Embodiment 46. The combined system of the preceding embodiment, where the means is capable of producing small droplets of the absorption solution, such as a nozzle or a shower head.

[0131] Embodiment 47. The combined system of the preceding embodiment, where the absorption solution is recirculated to the means with a pump.

[0132] Embodiment 48. The combined system of embodiments 29-32, where the flow of the carbon dioxide containing gas is upward and the flow of the absorption solution is downward.

[0133] EXAMPLES

[0134] Chemicals were commercial products of at least reagent grade.

[0135] EXAMPLE 1 Carbonic anhydrase activity

[0136] The procedure for measuring carbonic anhydrase activity was described by Wilbur, 1948, J. Biol. Chem. 176: 147-154. The setup is based on the pH change of an assay mixture due to the formation of bicarbonate from carbon dioxide as given in equation 1 : CO2+ H2O HCOT + H+

[0137] The activity assay used in this study was derived from the procedure of Chirica et al., 2001 , Biochim. Biophys. Acta 1544(1-2): 55-63. A solution containing approximately 60 to 70 mM CO2 was prepared by bubbling CO2 at a flow rate of 100 ml / min into 100 ml distilled water using the tip of a syringe approximately 30 minutes prior to the assay. The CO2 solution was chilled in an ice water-bath at 0-4°C. To test for the presence of carbonic anhydrase, 2 ml of 25 mM Tris-HCI solution adjusted to pH 8.3 with 25 mM HCI (containing sufficient bromothymol blue to give a distinct and visible blue color) were added to two 13x100 mm test tubes chilled in 4°C water-bath. To one tube, 10 microliters of the enzyme containing solution was added, and an equivalent amount of deionized water was added to the second tube to serve as a control. 2 ml of CO2 solution was added very quickly and smoothly to the bottom of each tube. Simultaneously with the addition of the CO2 solution, a stopwatch was started. The time required for the solution to change from blue to yellow was recorded (transition point of bromothymol blue is pH 6-7.6). The production of hydrogen ions during the CO2 hydration reaction lowers the pH of the solution until the color transition point of the bromothymol blue is reached. The time required for the color change is inversely related to the quantity of carbonic anhydrase present in the sample. The tubes must remain immersed in the ice bath for the duration of the assay for results to be reproducible. Typically, the uncatalyzed reaction (the control) takes 40 to 150 seconds for the color change to occur, whereas the enzyme catalyzed reaction is complete between 5 and 20 seconds, depending upon the amount of enzyme protein in the enzyme solution added. Detecting the color change is somewhat subjective but the error for triple measurements was in the range of 0 to 1 sec difference for the catalyzed reaction. One unit is defined after Wilbur [1 II = (1 / tc)-(1 / tu) x 1000] where II is units and tc and turepresent the time in seconds for the catalyzed and uncatalyzed reaction, respectively (Wilbur, 1948, J. Biol. Chem. 176: 147-154). These units are also termed Wilbur-Anderson units (WAU).

[0138] The specific activity of the carbonic anhydrase of SEQ ID NO: 3 was determined to about 2 WAU / (mg / L), and the specific activity of the carbonic anhydrase of SEQ ID NO: 4 was determined to 1.1 WAU / (mg / L).

[0139] EXAMPLE 2

[0140] Carbon dioxide capture and precipitation of sodium bicarbonate

[0141] A carbonate buffer was prepared (28.8 g / L sodium bicarbonate and 100 g / L sodium carbonate) and 120 g was added to a tall 250 mL glass beaker. 300 pL enzyme solution (the carbonic anhydrase of SEQ ID NO: 4) was added, and the beaker was covered with parafilm and heated to 65°C in a water bath.

[0142] After heating, the beaker was removed from the water bath and fitted with a heating jacket and a pH meter. The temperature was set to 60°C. An overhead stirrer was attached, and a carbon dioxide inlet tube was placed below the stirrer.

[0143] Stirring was started at 800 rpm and the carbon dioxide flow was started at 1 L / min of 100% carbon dioxide. pH and temperature data logging were also started. The initial pH was 9.75.

[0144] The carbon dioxide flow was monitored throughout the experiment and adjusted if required.

[0145] After 10 minutes, the pH reached pH 8.31, and the gas and pH logging were stopped. The heating jacket, overhead stirrer, and pH probe were removed. The beaker was covered with parafilm, and the solution was allowed to cool to 20°C in an ice bath. After cooling, the beaker was kept at 20°C for 45 minutes to allow precipitation of sodium bicarbonate.

[0146] After precipitation, the solid sodium bicarbonate was separated by filtration. The filtrate was transparent without any visible precipitate.

[0147] Finally, the solid precipitated sodium bicarbonate was dried at 200°C until constant weight using a moisture analyzer. After drying, the sodium bicarbonate was decomposed to sodium carbonate: 2NaHCOs — > Na2COs + CO2 + H2O

[0148] The dry weight of the sample was measured to 3.208 g of sodium carbonate. The experimental details are summarized in Table 1.

[0149] Table 1. Treatment of a gas containing 100% carbon dioxide

[0150] The experiment was repeated without addition of enzyme. After 10 minutes and subsequent cooling, no precipitate was formed.

[0151] Before reaction with carbon dioxide, the absorbance at 280 nm was measured using a UV-Vis spectrometer. This is an indirect measure of the amount of enzyme. After precipitation, the absorbance of the filtrate was also measured.

[0152] The difference between the two measurements was only 0.009, which is negligible and indicates the carbonic anhydrase remains in solution after filtration.

[0153] EXAMPLE 3

[0154] Regeneration of sodium bicarbonate depleted solution The carbon dioxide capture process of Example 1 was repeated. After precipitation, filtration and drying, the process yielded 3.853 g sodium carbonate.

[0155] The filtrate (sodium bicarbonate depleted solution) was collected, and 3.83 mL of 50% NaOH was added to regenerate the reaction mixture.

[0156] The carbon dioxide capture process was repeated using the now regenerated bicarbonate-depleted solution. After completion of all steps, the process yielded 3.555 g sodium carbonate.

[0157] This demonstrates excellent recycling of the carbon dioxide absorption solution.

[0158] EXAMPLE 4

[0159] Absorption of carbon dioxide from a diluted CO2 gas

[0160] The carbon dioxide capture process of Example 1 was repeated twice using 1.5 L / min of 15% carbon dioxide (85% atmospheric air) and two different enzyme concentrations, 0.033 g / L and 0.1 g / L.

[0161] Table 2. Treatment of a gas containing 15% carbon dioxide

[0162] As shown in Table 2, a diluted CO2 gas could be treated in this simple experimental setup. The reaction rate was lower when a lower enzyme concentration was used.

Claims

CLAIMS1. An enzymatic process for treating a carbon dioxide containing gas, comprising:(a) supplying the carbon dioxide containing gas to an absorber;(b) supplying an aqueous absorption solution to the absorber, where the aqueous absorption solution comprises 10-50% w / w of sodium carbonate, has a pH in the range of pH 9.5-13, and comprises 0.0005 g / L to 5 g / L of a carbonic anhydrase;(c) contacting the carbon dioxide containing gas with the aqueous absorption solution in the absorber at a temperature in the range of 30-80°C to provide a bicarbonate-rich solution and a carbon dioxide depleted gas,(d) optionally removing the bicarbonate-rich solution from the absorber; and(e) reducing the temperature of the bicarbonate-rich solution to at least 20°C below the temperature in the absorber to precipitate sodium bicarbonate, and to provide a bicarbonate- depleted solution.

2. The process of the preceding claim, which further comprises:(f) adjusting the pH of the bicarbonate-depleted solution to the pH of the aqueous absorption solution with sodium hydroxide to provide a regenerated bicarbonate-depleted solution.

3. The process of the preceding claim, which further comprises:(g) using the regenerated bicarbonate-depleted solution to prepare the aqueous absorption solution.

4. The process of any of the preceding claims, where the aqueous absorption solution in step (b) comprises 10-40% w / w, preferably 10-30% w / w, or 10-20% w / w of sodium carbonate.

5. The process of any of the preceding claims, where the aqueous absorption solution in step (b) has a pH in the range of pH 9.5-12.5, preferably pH 9.5-12, or pH 9.5-11.5.

6. The process of any of the preceding claims, where the aqueous absorption solution in step (b) comprises 0.001 g / L to 5 g / L, preferably 0.01 g / L to 5 g / L of a carbonic anhydrase; or 0.0005 to 2 g / L, preferably 0.001 g / L to 2 g / L, or 0.01 g / L to 2 g / L of a carbonic anhydrase.

7. The process of any of the preceding claims, where in step (c), the carbon dioxide containing gas is contacted with the aqueous absorption solution in the absorber at a temperature in the range of 40-80°C, preferably in the range of 40-70°C.

8. The process of any of the preceding claims, where in step (e), the temperature of the bicarbonate-rich solution is reduced to at least 25°C, preferably at least 30°C, below the temperature in the absorber.

9. The process of any of the preceding claims, where the carbonic anhydrase is a thermostable carbonic anhydrase; preferably having at least 50% residual activity after incubation for 15 minutes at 80°C.

10. The process of any of the preceding claims, where the carbonic anhydrase has at least 80% amino acid sequence identity to any of SEQ ID NO: 1 , SEQ ID NO: 2, SEQ ID NO: 3, and SEQ ID NO: 4; preferably at least 90% or 95% amino acid sequence identity to any of SEQ ID NO: 1, SEQ ID NO: 2, SEQ ID NO: 3, and SEQ ID NO: 4.

11. The process of any of the preceding claims, where the carbon dioxide containing gas is a flue gas, a biogas, a natural gas, or an excipient from cement or glass production.

12. A system for treatment of a carbon dioxide containing gas, according to any of claims 1-3, comprising:(a) an absorber unit, comprising: i) an inlet for the carbon dioxide containing gas, ii) an outlet for the carbon dioxide depleted gas, iii) an inlet for the aqueous absorption solution, and iv) an outlet for the bicarbonate-rich solution; and(b) a precipitation unit, comprising: v) an inlet for the bicarbonate-rich solution, vi) an outlet for the precipitated sodium bicarbonate, and vii) an outlet for the bicarbonate-depleted solution, or an outlet for the regenerated bicarbonate-depleted solution.

13. The system of the preceding claim, where the absorber unit further comprises means for contacting the carbon dioxide containing gas with the aqueous absorption solution.

14. The system of the preceding claim, where the means is capable of producing droplets of the aqueous absorption solution, such as comprising a nozzle or a shower head.

15. The system of any of claims 12-14, where the absorber unit and the precipitation unit are combined into one reactor which comprises i), ii), iii), vi), and an inlet for sodium hydroxide.

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