Intensification of biocatalytic gas absorption

Abstract

Intensification techniques are described for enhancing biocatalytic CO.sub.2 absorption operations, and may include the use of a rotating packed bed, a rotating disc reactor, a zig-zag reactor or other reactors that utilize process intensification. Carbonic anhydrase can be deployed in the high intensity reactor free in solution, immobilized with respect to particles that flow with the liquid, and/or immobilized to internals, such as packing, that are fixed within the high intensity reactor.

Claims

1. A biocatalytic process for treating a CO.sub.2 containing gas, comprising: supplying CO.sub.2 containing gas into a rotating packed bed (RPB) comprising a reaction chamber; supplying an absorption solution into the RPB; contacting the CO.sub.2 containing gas and the absorption solution within the reaction chamber, in the presence of carbonic anhydrase, under fluid acceleration conditions of at least 50 m/s.sup.2 to convert dissolved CO.sub.2 into bicarbonate and hydrogen ions and to form a CO.sub.2 depleted gas and an ion enriched solution free of bicarbonate precipitates, wherein the carbonic anhydrase is free in the absorption solution in that the carbonic anhydrase is not immobilized on or in particles flowing with the absorption solution and the carbonic anhydrase flows with the absorption solution; and withdrawing the CO.sub.2 depleted gas and an ion enriched solution from the RPB.

2. The biocatalytic process of claim 1, comprising supplying the ion enriched solution to a regeneration unit to produce a regenerated liquid stream and a CO.sub.2 gas stream.

3. The biocatalytic process of claim 2, wherein all of the ion enriched solution is supplied directly to the regeneration unit.

4. The biocatalytic process of claim 3, wherein the ion enriched solution passes through a heat exchanger prior to being introduced into the regeneration unit.

5. The biocatalytic process of claim 1, wherein the absorption solution further comprises a defoamer comprising a silicon based compound and the defoamer is provided in a concentration of at least 50 mg/L.

6. The biocatalytic process of claim 5, wherein the carbonic anhydrase is present in an enzyme concentration between about 0.1 g/L and about 2 g/L.

7. The biocatalytic process of claim 5, wherein the carbonic anhydrase is present in an enzyme concentration between about 0.2 g/L and about 1.5 g/L.

8. The biocatalytic process of claim 5, wherein the carbonic anhydrase is present in an enzyme concentration between about 0.5 g/L and about 1 g/L.

9. The biocatalytic process of claim 1, wherein the absorption solution comprises a carbonate compound.

10. The biocatalytic process of claim 9, wherein the carbonate compound comprises a monovalent metal ion.

11. The biocatalytic process of claim 10, wherein the carbonate compound comprises sodium carbonate.

12. The biocatalytic process of claim 10, wherein the carbonate compound comprises potassium carbonate.

13. The biocatalytic process claim 1, wherein the contacting of the CO.sub.2 containing gas and the absorption solution is performed in one pass through the reaction chamber.

14. The biocatalytic process of claim 1, wherein packing material in the reaction chamber comprises metal foam.

15. The biocatalytic process of claim 1, wherein packing material in the reaction chamber has between 80% and 95% porosity.

16. The biocatalytic process of claim 1, wherein the rotating packed bed is operated to provide fluid acceleration of at least 335 m/s.sup.2.

17. The biocatalytic process of claim 1, wherein the rotating packed bed has a radius of 0.1 m to 0.2 m and is operated with a rotational speed between 450 and 1200 rotations per minute.

18. The biocatalytic process of claim 1, wherein the absorption solution further comprises a defoamer.

19. The biocatalytic process of claim 1, wherein the RPB is operated at a liquid to gas (L/G) ratio that is between about 30 and about 300 on a w/w basis.

20. The biocatalytic process of claim 1, further comprising providing heat to the RPB from low-grade heat via a heat transfer fluid.

21. A biocatalytic process for treating a CO.sub.2 containing gas, comprising: supplying CO.sub.2 containing gas into a rotating packed bed (RPB) comprising a reaction chamber; supplying an absorption solution into the RPB; contacting the CO.sub.2 containing gas and the absorption solution within the reaction chamber, in the presence of carbonic anhydrase, under fluid acceleration conditions of at least 50 m/s.sup.2 to convert dissolved CO.sub.2 into bicarbonate and hydrogen ions and to form a CO.sub.2 depleted gas and an ion enriched solution free of bicarbonate precipitates, wherein the carbonic anhydrase is free in the absorption solution in that the carbonic anhydrase is not immobilized on or in particles flowing with the absorption solution and the carbonic anhydrase flows with the absorption solution; and withdrawing the CO.sub.2 depleted gas and an ion enriched solution from the RPB, wherein the absorption solution comprises a defoamer.

22. The biocatalytic process of claim 21, wherein the defoamer is present in the absorption solution at a concentration of 50 to 300 mg/L.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic representation of an absorption and desorption system including a rotating packed bed absorber.

(2) FIG. 2 is a schematic representation of a rotating discs contactor.

(3) FIG. 3 is a schematic representation of a zigzag bed contactor.

(4) FIG. 4 is a schematic representation of a second absorption and desorption system including a rotating packed bed absorber.

(5) FIG. 5 is a schematic representation of a system where multiple high intensity contactors can be used in series for CO.sub.2 absorption.

(6) FIG. 6 is a schematic representation of a system where multiple high intensity contactors are used in parallel to remove CO.sub.2 from a CO.sub.2 containing gas.

(7) FIG. 7 is a schematic representation of an absorption/desorption system where multiple high intensity contactors are used in series and where a stream of semi-lean liquid phase is sent back to one of the high intensity contactors.

(8) FIG. 8 is a graph of Acceleration of the CO.sub.2 absorption rates in a packed column using a 1M K.sub.2CO.sub.3 absorption solution in combination with 4 different carbonic anhydrase concentrations.

(9) FIG. 9 is a graph of Acceleration of the CO.sub.2 absorption rates in a packed column using a 1.45M K.sub.2CO.sub.3 absorption solution in combination with 4 different carbonic anhydrase concentrations.

(10) FIG. 10 is a graph of Acceleration of the CO.sub.2 absorption rates in a rotating packed bed using a 1.45 M K.sub.2CO.sub.3 in combination with 3 different enzyme concentrations at L/G of 30, 149 and 297 (w/w). The rotational speed of the contactor is 450 rpm.

(11) FIG. 11 is a graph showing Acceleration of the CO.sub.2 absorption rates in a rotating packed bed using a 1.45 M K.sub.2CO.sub.3 at an enzyme concentration of 0 and 4 g/L, for L/G of 30, 149 and 297 (w/w) and rotational speed of the contactor of 450, 1000 and 1500 rpm.

(12) FIG. 12 is a graph showing a comparison of the CO.sub.2 absorption performance obtained using a 5M MEA solution (lean CO.sub.2 loading of 0.28 mol C/mol MEA) and using a 1.45 M K.sub.2CO.sub.3 solution (lean CO.sub.2 loading of 0.62 mol C/mol potassium ions) containing 4 or 5.7 g/L of carbonic anhydrase enzyme for different L/G. The rotational speed of the contactor is 450 rpm.

(13) FIG. 13 is a graph showing the percentage of CO.sub.2 capture obtained in a rotating packed bed using a 1.45 M K.sub.2CO.sub.3 at a CO.sub.2 loading of 0.7 mol C/mol K.sup.+ at enzyme concentrations of 0 and 1 g/L at different L/G. The rotational speed of the contactor was adjusted at 450 rpm.

(14) FIG. 14 is a graph showing the percentage of CO.sub.2 capture obtained in a rotating packed bed using a 1.45 M K.sub.2CO.sub.3 solution at a CO.sub.2 loading of 0.7 mol C per mol K.sup.+. The RPB is operated at an L/G of 125 g/g and a rotational speed of 450 rpm. Enzyme concentrations are 0, 0.5 and 1 g/L.

DETAILED DESCRIPTION

(15) Various techniques are described for enhancing gas component capture operations, such as CO.sub.2 absorption. While the techniques will be described in detail with respect to the absorption and desorption of CO.sub.2 in particular, using carbonic anhydrase enzyme for biocatalytic enhancements, it should be understood that the techniques can also apply to other catalytic processes where a liquid stream and a gas stream containing a gas component are supplied to an intensification reactor, such as a rotating packed bed, such that mass transfer limitations between the gas and liquid phases are reduced to facilitate enhanced catalytic impact on the process of transferring the gas component from the gas phase to the liquid phase.

(16) Intensification of Biocatalytically Enhanced CO.sub.2 Capture

(17) Referring to FIG. 1, a biocatalytic system 10 for removing a gas component, such as CO.sub.2, from a gas stream 12 by absorption is illustrated. The CO.sub.2 containing gas 12 is supplied to a high intensity reactor, such as a rotating reactor 14. The rotating reactor 14 can be a rotating packed bed reactor, a rotating disc reactor, a rotating zig zag bed or another type of reactor that uses rotation to increase mass transfer rate. The rotating reactor 14 can be a rotating packed bed reactor including a gas inlet 16 for receiving the CO.sub.2 containing gas 12, a liquid inlet 18 for receiving an absorption solution 20, a reaction chamber 22 including packing material, discs or a zigzag bed, a gas outlet 24 for withdrawing a CO.sub.2 depleted gas 26, and a liquid outlet 28 for withdrawing an ion enriched solution 30. The rotating reactor 14 can also include a rotation mechanism 32 including a motor 36 and a drive shaft 34 operatively connecting the motor to the reaction chamber for providing the torque for rotating the reaction chamber around a rotational axis.

(18) The ion enriched solution 30 can then be supplied to a regeneration unit 38 for regenerating the solution by desorption or mineralisation, to produce a regenerated solution 40. In the case of desorption, a CO.sub.2 enriched gas stream 42 is produced, whereas in the case of mineralization a solid mineral stream (e.g., solid carbonates) is produced. The regenerated solution 40 can then be recycled in whole or in part to the absorption stage, which in FIG. 1 includes the rotating reactor 14. The regeneration unit can have various constructions and may take the form of various types of reactors, such as a conventional packed column or a high intensity reactor, such as a rotating packed bed (RPB). Carbonic anhydrase may be present in the desorption unit, for example immobilized with respect to internals of the desorption unit or micro-particles flowing with the ion rich solution, or free in the solution. The high intensity desorption unit can be operated in conjunction with the high intensity absorption reactor such that the temperature, pressure, pH, and solvent concentration conditions do not substantially denature the carbonic anhydrase.

(19) Another possible process configuration is shown in FIG. 4. According to this configuration, the ion enriched solution 30 is divided in two streams (30a and 44). Stream 30a is supplied to a regeneration unit 38 for regenerating the solution by desorption or mineralisation as described above and stream 44 is sent back to the high intensity reactor. In the case the solution is fed to a regeneration unit, the regenerated solution 40 is sent back to the rotating contactor where it is combined with stream 44 (which is part of the recirculating loop around the RPB), prior to being fed back to the biocatalytic unit 10. The CO.sub.2 loading of steam 20 depends on the CO.sub.2 loading of stream 40 and on the ratio of the flowrates of stream 44 to 40. For example, the ratio of the flowrates of stream 44 to stream 40 can be between 0/1 and 10/1, 1/10 and 5/1, 1/2 and 2/1 or 3/4 and 4/3, for example. The flowrate ratios can also change and can be controlled during operating in order to favour certain process conditions. In some scenarios, the composition of certain streams (e.g., ion enriched solution 30, absorption solution 20, and/or streams 40, 44/30a, etc.) can be monitored and the flowrate ratio can be modified depending on the compositions in order to effect greater regeneration or recycle more ion enriched solution directly back into the absorption reactor. In some implementations, the stream 30 is split such that more solution is sent to desorption than is recycled back without desorption. In addition, the step of splitting the stream 30 can produce two or more streams each having substantially similar compositions, although stream 30 can be separated so that the two streams have different compositions, e.g., stream 44 or 30a could have higher enzyme concentration or one of the streams may contain no enzyme. In some scenarios, the stream 30 can be split into more than two streams, and each of the resulting streams can be supplied to a different unit of the overall process, e.g., regeneration unit, heat exchanger, solids separator, back to one or more high intensity absorbers that are in series or parallel, and so on, and the flowrate ratios of the various streams can be controlled based on desired process parameters.

(20) In another process configuration where CO.sub.2 absorption is achieved by operating multiple rotating contactors in series and counter-currently, as illustrated in FIG. 5, the CO.sub.2 containing gas 12 can be fed to a first rotating contactor 14a to contact the absorption solution 20b. The treated gas 26a, exiting from the first contactor 14a, is then fed to a second contactor 14b and the gas exiting the second contactor 26b is then fed to a third contactor 14c. In a similar manner, the absorption solution 20 is fed to the third rotating contactor 14c and the solution 20a exiting the third rotating contactor 14c is then fed to the second contactor 14b, to further remove CO.sub.2 from the gas 26a and the absorption solution expelled from the second rotating contactor 20b is then sent to the first rotating contactor 14a. The treated gas 26 can then be sent out to the atmosphere whereas the CO.sub.2 rich absorption solution 30 can be sent to the regeneration unit 38 as shown, for example, in FIG. 1. An alternative arrangement to the one illustrated in FIG. 5 is that the gas and liquid streams are fed co-currently such that the liquid stream with the highest ion content is contacted with the gas stream with the highest CO.sub.2 content, and the liquid stream with the liquid stream with the lowest ion content is contacted with the gas stream with the lowest CO.sub.2 content.

(21) In another configuration where CO.sub.2 absorption is achieved by operating multiple rotating contactors in parallel with a counter-current operation, as illustrated in FIG. 6, the CO.sub.2 containing gas 12 is split into three gas streams 12a, 12b and 12c which are respectively fed to multiple (e.g., three) rotating contactors 14a, 14b and 14c. In a similar manner, the absorption solution stream 20 is split into multiple (e.g., three) liquid streams 20a, 20b and 20c and are respectively fed to the rotating contactors 14a, 14b and 14c. In each of the three rotating contactors the absorption solution contacts the CO.sub.2 containing gas and absorbs CO.sub.2. The treated gas streams 26a, 26b and 26c are then released out of the process. The CO.sub.2 rich absorption solutions 30a, 30b and 30c are then sent to a regeneration unit 38 as shown, for example, in FIG. 1. The streams 30a, 30b and 30c can be combined (30) prior to being fed to the regeneration unit. In an optional configuration, the entire gas flow would be fed to the first contactor 14a, and then the gas outlet would be fed to the second contactor 14b and then to the third contactor 14c. So the gas would flow though the units in series. In this optional configuration, the liquid flow would be split and fed to the units in parallel as described previously. In an additional optional configuration, the gas could be fed to the units in parallel and the liquid would flow through the units in series.

(22) In a configuration where multiple rotating contactors in series are used to remove CO.sub.2 from a CO.sub.2 containing gas 12 as shown in FIG. 7, a process variation is that a stream of partly regenerated absorption solution 40a is sent back to at least one of the rotating contactors. This recycle can be done for one or more of any of the contactors.

(23) Techniques described herein can facilitate increasing the impact of biocatalysts in a gas absorption process. For instance, in operations where a liquid phase is contacted with a gas phase to absorb a component of interest, biocatalysts may be provided free or immobilized on particles that are carried with the liquid phase. In order to increase the kinetic impact of the biocatalyst, the contact between the gas phase and the liquid phase (containing the biocatalyst) takes place in a high intensity gas-liquid contactor to intensify the mass transfer of the gaseous component of interest toward the liquid phase, employing process intensification principles. A significant option for intensifying the mass transfer in a gas-liquid contactor is to use a rotating contactor, which may include a cyclone/vortex or a rotor, under enhanced acceleration conditions. This enhanced acceleration facilitates formation of thinner films, smaller bubbles and droplets, and increased flooding velocities for counter-current systems. In some implementations, the enhanced acceleration can result in an increase in the gas-liquid mass transfer by a factor of 10 to 100 compared to conventional techniques.

(24) Intensification Reactors and Techniques

(25) Various types of intensification techniques may be used in conjunction with carbonic anhydrase for enhanced CO.sub.2 capture. The techniques can include intensification equipment and/or intensification methods. Rotating reactors, such as rotating packed beds, rotating disc contactors, rotating zigzag bed reactors, multi-rotor zigzag rotating contactor and/or rotating contactor with split packing can be used. In addition, other types of high intensity reactors can be used in connection with some implementations of the techniques described herein, such as gas-liquid jet reactors, swirling gas-liquid contactors, or contactors as described in US patent application published as No. 2010/0320294.

(26) Process intensification techniques typically rely on the intensification of various different process parameters with a view of accelerating the process and reducing the size of equipment required for unit operations. Some potential intensification parameters, such as elevated pressures and temperatures, have been leveraged to accelerate unit operations by enhancing the kinetics of various mass transfer and reaction phenomena in the process. However, the intensification of some process parameters can lead to detrimental effects on some biocatalytic processes that employ biocatalysts that can denature at elevated temperature conditions for example. Nevertheless, some process intensification techniques focus on “fluid dynamic” intensification parameters, such as reducing the liquid film thickness flowing over packing material by leveraging rotational force to drive the liquid instead of reliance on gravitational force. Contactors that leverage fluid dynamic intensification parameters can therefore increase mass transfer rates to take advantage of biocatalytic reaction kinetics while avoiding detrimentally impacting the biocatalysts. In this regard, the term “high intensity” reactor or “high intensity” contactor used herein refers to units that leverage fluid dynamic intensification parameters, rather than parameters such as high temperature that could have detrimental effects on the biocatalyst, to enhance mass transfer rates.

(27) Referring to FIGS. 1, 2, 3 and 4, a Higee (or high gravity) rotating contactor 14 may include rotating discs, rotating packing or rotating zigzag bed. The rotating disc contactor is also known as a spinning disc contactor, and the rotating packed contactor is also known as a Higee contactor or rotating packed bed (RPB). Various different configurations and constructions of rotating discs or RPBs, can be used. For both high intensity contactor types, the method includes feeding a gas phase, containing a gas species of interest to absorb, to the contactor. The gas phase is fed via an outer part of the contactor. The liquid phase that will absorb the gaseous species of interest is fed via an inner part of the contactor. Because of the centrifugal force coming from the rotation of the reaction chamber housing the packing or the discs, the liquid will flow outwardly through the packing or form a film on the surface of the discs or of the zigzag bed. The rotational speed can be adjusted to generate an important centrifugal force of several “g” in such a way as to minimize the liquid film thickness and maximize the contact surface area between the gas and the liquid phases, to enhance removal of the gas species of interest.

(28) Regarding high intensity reactors that have discs, packing material or zigzag bed, the process may be operated using biocatalytic particles that flow with the solution through the reactor and/or the process may be operated such that bicarbonate precipitates form in the solution and are carried out of the reactor. In scenarios where the particles and/or precipitates are nanosized, the reactor may be an RPB or a spinning disc reactor. In scenarios where the particles and/or precipitates are micron sized or larger, a spinning disc reactor may be preferred for handling such larger solid particulates.

(29) Regarding high intensity reactors that have packing material provided in the reaction chamber(s), the packing that is used can have certain characteristics to benefit the mass transfer and biocatalytic impact on CO.sub.2 absorption. In some implementations, the packing material can be a reticulated packing material, which can be composed of metal for example. The reticulated packing material can have large surface area per unit volume and/or enable high voidage characteristics. For example, the specific surface area of the packing material can be between about 500 ft.sup.2/ft.sup.3 to about 1000 ft.sup.2/ft.sup.3, optionally between about 700 ft.sup.2/ft.sup.3 to about 800 ft.sup.2/ft.sup.3, still optionally about 750 ft.sup.2/ft.sup.3. The voidage of the packing material can be above about 80%, above about 85%, above about 90%, or between about 85% and about 95%, for example. Other examples of packing are composite layers of gauze or expanded metal, wound layers of fibrous material, or structured and random packing commonly used in a packed column.

(30) Some combinations of carbonic anhydrase biocatalyst, with absorption compounds of interest, for CO.sub.2 absorption in a packed column as part of a CO.sub.2 capture process have resulted in CO.sub.2 capture rates, installation footprint and energy requirement comparable to conventional chemical solvent based processes. Some advantages of such enzyme enhanced processes is that the solutions being used (e.g., carbonate based solutions) are less reactive than conventional primary alkanolamines, more stable, and present less environmental issues. One finding with respect to the impact of the carbonic anhydrase enzyme in processes based on the use of a packed column as an absorber, is that the enzyme impact appears to be limited by the CO.sub.2 mass transfer rate provided in this conventional absorber type. Therefore, fluid dynamic intensification techniques combined with carbonic anhydrase can enhance the impact of the enzyme on the CO.sub.2 capture system and thus further benefit from both the intensification and biocatalytic effects.

(31) Absorption Solutions and Compounds

(32) In some implementations, the techniques can generally include removing a selected gas component from a gas phase, using a liquid phase that is contacted with the gas phase. The contact between the gas and the liquid will result in the absorption of the selected gas component by the liquid phase. This liquid phase can be selected and formulated based on its ability to absorb and store the absorbed selected gas component, and may therefore include one or more absorption compounds. The liquid phase composition can be formulated specifically to efficiently absorb the gas component of interest. The liquid phase may include water and other absorption compounds species that will absorb and react with the absorbed gaseous species. The liquid can also include other compounds such as de-foaming compounds. In some cases, the liquid phase can also contain the biocatalysts which are carried with the liquid through the reactor.

(33) In some implementations, the absorption solution can be formulated to include one or more absorption compounds in addition to water to facilitate CO.sub.2 absorption. In some implementations, the absorption compound can have a slow reaction rate with CO.sub.2, but provide high cyclic capacity, no carbamate formation and/or lower energy requirement for their regeneration as compared to primary alkanolamines commonly used in post-combustion CO.sub.2 capture process and/or enable the use of low grade heat available on the implementation site since regeneration conditions are at temperature and pressure conditions enabling such low grade heat utilization. “Low-grade heat” should be understood to include low- and mid-temperature heat that cannot be converted efficiently by conventional methods. The particular thermal conditions of low-grade heat or industrial waste heat are industry dependent For example, the low-grade heat can be waste heat from a process stream that is at a temperature level below about 1000; in the food and beverage industry the low-grade heat temperature level can be about 80° C. Low-grade heat can be provided by a heat transfer fluid having a temperature below 100° C., below 90° C., below 80° C., be low 70° C. or below 60° C., or between any two of the aforementioned values. The heat transfer fluid may include water, heat transfer oils, freon or phase-changing fluids, for example. The absorption compounds of interest can include, for example, slow reactive compounds. In some implementations, the absorption compound includes tertiary amines, tertiary alkanolamines, tertiary amino-acids, tertiary amino-acid salts, carbonates or a mixture thereof.

(34) In some implementations, the biocatalyst is used in conjunction with an absorption compound which may include primary, secondary and/or tertiary amines (including alkanolamines); primary, secondary and/or tertiary amino acids; and/or carbonates. The absorption compound may more particularly include amines (e.g. piperidine, piperazine and derivatives thereof which are substituted by at least one alkanol group), alkanolamines (e.g. monoethanolamine (MEA), 2-amino-2-methyl-i-propanol (AMP), 2-(2-aminoethylamino)ethanol (AEE), 2-amino-2-hydroxymethyl-i,3-propanediol (Tris), N-methyldiethanolamine (MDEA), dimethylmonoethanolamine (DMMEA), diethylmonoethanolamine (DEMEA), triisopropanolamine (TIPA) and triethanolamine), dialkylether of polyalkylene glycols (e.g. dialkylether or dimethylether of polyethylene glycol); amino acids which may include potassium or sodium salts of amino acids, glycine, proline, arginine, histidine, lysine, aspartic acid, glutamic acid, methionine, serine, threonine, glutamine, cysteine, asparagine, leucine, isoleucine, alanine, valine, tyrosine, tryptophan, phenylalanine, and derivatives such as taurine, N,cyclohexyl 1,3-propanediamine, N secondary butyl glycine, N-methyl N-secondary butyl glycine, diethylglycine, dimethylglycine, sarcosine, methyl taurine, methyl-a-aminopropionic acid, N-(β-ethoxy)taurine, N-(β-aminoethyl)taurine, N-methyl alanine, 6-aminohexanoic acid; and which may include potassium carbonate, sodium carbonate, ammonium carbonate, promoted potassium carbonate solutions and promoted sodium carbonate solutions or promoted ammonium carbonates; or mixtures thereof. Absorption compounds can be added to the solution to aid in the CO.sub.2 absorption and to combine with the catalytic effects of the carbonic anhydrase.

(35) Biocatalysts and Delivery Methods

(36) The biocatalysts considered for CO.sub.2 capture operations is the enzyme carbonic anhydrase. This enzyme is one of the fastest known enzymes, and catalyses the interconversion of CO.sub.2 and bicarbonate according to the following reaction:

(37) ##STR00001##

(38) Carbonic anhydrase is not just a single enzyme form, but includes a broad group of metalloproteins that exists in genetically unrelated families of isoforms, α, β, γ, δ and ε. Different classes, isoforms and variants of carbonic anhydrase have been used in order to catalyze the hydration reaction of CO.sub.2 into bicarbonate and hydrogen ions and the bicarbonate dehydration reaction into CO.sub.2 and water. Under optimum conditions, the catalyzed turnover rate of the hydration reaction can reach 1×10.sup.6 molecules/second.

(39) In some implementations, the biocatalyst can be immobilized directly onto the surface of the packing material via chemical fixation of the biocatalyst. In some implementations, the biocatalyst or an aggregate of the biocatalysts, such as CLEAs or CLECS, can be used in the high intensity reactor. In some other implementations, particles with the biocatalyst at their surface or entrapped inside the particles can be used.

(40) In terms of particle delivery methods, the biocatalysts can be immobilized or otherwise delivered via particles that are carried with the absorption solution through the reaction chamber. In a conventional packed reactor, there is reliance on gravity as a driving force for establishing the liquid film that flows over the packing material. In the high intensity reactors, the biocatalytic particles may be provided to have a size and concentration in the absorption solution to flow with the liquid and to be smaller than the liquid film flowing on the surfaces of the packing material, which may be reticulated packing material, composite layers of gauze or expanded metal, would layers of fibrous material, structured packing or random packing, as described above. The biocatalytic particles may also have other characteristics to remain distributed in the absorption solution in a generally uniform manner under the rotational force. In some implementations, the density of the biocatalytic particles is provided to be low enough such that the particles are carried with the liquid upon the substantial acceleration of the liquid within the rotating reactor. In addition, the particles can be sized in accordance with the thin liquid film, and may be for example at least an order of magnitude smaller than the thickness of the liquid film.

(41) In some implementations, the biocatalyst can be immobilized with respect to the internals (e.g., packing material, discs, zigzag bed, etc.) in the high intensity reactor. For RPB reactors, the biocatalyst can be immobilized on the packing material using various techniques, such as entrapment, covalent bonding, and so on. In some scenarios, the biocatalyst can be immobilized in an immobilization material that is provided on the packing material as a coating, and may be spray coated onto the packing material. The immobilization material may include polysulfone and/or polysulfone grafted with polyethylene glycol and/or any one or a combination of polymeric materials described in U.S. Pat. No. 7,998,714. The immobilization material may include micellar and/or inverted micellar polymeric materials, such as micellar polysiloxane material and/or micellar modified polysiloxane materials described in PCT patent application No. WO 2012/122404 A2. In some implementations, the immobilization material may include chitosan, polyacrylamide and/or alginate.

(42) In some scenarios, the biocatalyst present in the packed reactor can lose activity over time, and techniques for replenishing activity of the biocatalytic reactor may be employed. Various activity replenishment techniques can be used depending on the type of the reactor and the delivery method of the biocatalyst. Some activity replenishment techniques are described, for example, in U.S. patent application Ser. No. 14/401,609. In the case of smaller sized high intensity reactors, such as RPBs, activity replenishment can be facilitated for various reasons. In some implementations, the packing material including immobilized biocatalysts can be more easily removed and replaced. In some implementations, the packing material can be reactivated in situ within the reaction chamber by supplying one or more biocatalyst activation solution into the reaction chamber to contact the packing material. For example, such in situ reactivation can include a series of solutions to pre-treat, clean, functionalize, etc., and eventually provide the immobilized enzyme onto the packing material. Since the volume of the high intensity reaction chamber is significantly smaller than conventional packed columns, for example, solutions requirements for in situ reactivation can be reduced and reactivation can be generally facilitated.

(43) Biocatalyst can be provided at various concentrations in the high intensity reactor, in part depending on the delivery method of the biocatalyst.

(44) In some scenarios, the biocatalyst is provided free in the absorption solution supplied to the high intensity reactor, at an elevated concentration. In this context, “elevated concentration” means that the concentration of the biocatalyst is greater than the maximum concentration of the same biocatalyst under similar conditions in a conventional reactor, where such maximum concentration corresponds to a plateau of biocatalytic impact on the reaction.

(45) In some scenarios, in the case the absorption solution comprising a carbonate compound, such as sodium or potassium carbonate, the absorption of CO.sub.2 by the solution results in a dissolved concentration of bicarbonate ions that is sufficient to cause the precipitation of carbonated solids, such as sodium or potassium bicarbonate. The precipitated solids thus formed can be in suspension in the ion-rich solution expelled from the contactor. In some scenarios, the process is conducted so that the ion enriched solution will be free of precipitates, such as bicarbonate precipitates.

(46) In addition, the biocatalyst can be provided in the high intensity reactor at a concentration (which may be an elevated concentration) that is below an upper concentration limit corresponding to a plateau of biocatalytic impact in the high intensity reactor. At certain high concentrations of biocatalyst there will be a plateau of biocatalytic impact on the hydration reaction. For example, at certain high concentrations of biocatalyst the absorption solution may become more susceptible to foaming and/or may have a high viscosity that would begin to limit mass transfer in the high intensity reactor. By keeping the biocatalyst concentration below such a plateau enables more efficient use of biocatalyst in the system.

(47) In some scenarios, the biocatalyst can be provided at other concentrations depending on various factors, such as operating conditions, biocatalyst delivery method, type of biocatalyst, type of high intensity reactor, type of input gases and liquids, and so on. For example, carbonic anhydrase concentrations can range from 0.5 g/L to 10 g/L, 1 g/L to 8 g/L, 2 g/L to 6 g/L, or 3 g/L to 5 g/L, or from 0.1 g/L to 10 g/L, 0.2 g/L to 8 g/L, 0.5 g/L to 6 g/L, or 1 g/L to 5 g/L. In addition, the biocatalyst concentration can be maintained to be relatively constant, or may be modified over time which may be accomplished by in-line addition of biocatalyst to the absorption solution.

(48) Process Additives and Operation

(49) In some implementations, the absorption solution can include additives that may be in addition to one or more absorption compounds and/or biocatalysts. In some scenarios, the additives can include a “defoamer”. In the present application, the term “defoamer” includes foam-reducer compounds that reduce foam once it is formed and/or anti-foam compounds that inhibit foam formation. Defoamers can be used in various scenarios where the biocatalyst and/or the process operating conditions are such that the absorption solution tends to have foam production. The presence of foam can negatively affect gas-liquid mass transfer and therefore can reduce performance of the CO.sub.2 absorption. For example, higher concentrations of biocatalyst (e.g., carbonic anhydrase) can increase the tendency of foam production, which was observed during experiments. In some scenarios, both foam-reducer compounds and anti-foam compounds are used in conjunction with systems or processes as described herein.

(50) Various different types of defoamers can be used, depending on the given application of the process and operating conditions. The anti-foam or foam-reducer may comprise oil, hydrophobic solid particles or a mixture of both. Nonpolar oils (mineral oils, silicone oils) and polar oils (fatty alcohols and acids, alkylamines, alkylamides, tributyl phosphate, tioethers) can be used. The solid particles could be inorganic (silica, AL.sub.2O.sub.3, TiO.sub.2), wax or polymeric. The defoamer (i.e., anti-foam or foam-reducer) compound can include an oil-in-water emulsion, water-in-oil emulsion, polyol based compounds which may be in the form of a polyol based dispersion, silicon based compounds which may be in the form of a non-ionic silicon emulsion, or silica particle suspension, or a combination thereof.

(51) The defoamer can be provided in various concentrations, for example a concentration of at least 10 mg/L or at least 50 mg/L based on the volume of the absorption solution, a concentration of at least 200 mg/L based on the volume of the absorption solution. Or a concentration of between 50 and 300 mg/L or between 100 and 300 mg/L, based on the volume of the absorption solution.

(52) In addition, the high intensity reactor can be operated such that the intensification includes subjecting the fluids to high forces, e.g., high centrifugal forces, which are provided above certain thresholds for an enhanced biocatalytic process. For example, in a rotating high intensity reactor (e.g., RPB) the centrifugal force can be provided above a threshold relative to gravity (g). This can be reflected by the centrifugal acceleration as per the following equation:
A.sub.c=V.sup.2R where A.sub.c is centrifugal acceleration, V is the rotational velocity, and R is the relevant radius.

(53) For example, in an RPB having a radius of 0.15 m, operated at 1200 RPM (120 rad/s), the centrifugal acceleration is 2381 m/s.sup.2 (243 g) Operating at 450 RPM would provide a centrifugal acceleration of about 303 m/s.sup.2 (31 g).

(54) In some implementations, the high intensity reactor is operated to provide acceleration conditions of at least 25 m/s.sup.2, 50 m/s.sup.2, 100 m/s.sup.2, 150 m/s.sup.2, 200 m/s.sup.2, 250 m/s.sup.2, 300 m/s.sup.2, 350 m/s.sup.2, 400 m/s.sup.2, 450 m/s.sup.2, 500 m/s.sup.2, 600 m/s.sup.2, 700 m/s.sup.2, 800 m/s.sup.2, 900 m/s.sup.2, 1000 m/s.sup.2, 1500 m/s.sup.2, 2000 m/s.sup.2, 2500 m/s.sup.2, or 3000 m/s.sup.2, or acceleration conditions between any two of the above values or other values disclosed herein. Furthermore, due to the proportional relation of A.sub.c to V.sup.2, the rotational velocity can be adjusted to provide a significant impact on the acceleration and corresponding impact on the hydrodynamics of the system to enhance biocatalytic effects.

(55) It should also be noted that various aspects of the processes and/or systems for removing CO.sub.2 from a gas can also be applied to the removal of a gas component from a mixed gas stream and employing a catalyst (e.g., biocatalyst such as an enzyme) in a high intensity reactor. Examples, aspects and implementations described herein for CO.sub.2 capture and using carbonic anhydrase can be adapted using, for example, other biocatalysts having high turnover rates for a given reaction in order to covert a dissolved gas component into an ionic compound in the absorption solution.

Examples & Experimentation

Experimentation Series 1

(56) A CO.sub.2 absorption test series was conducted using a carbonic anhydrase as a biocatalyst in combination with a 1 M potassium carbonate solution (K.sub.2CO.sub.3); the lean CO.sub.2 loading of the solution was 0.81 mol carbon/mol potassium ions. An antifoam agent, AF-204 (Sigma Aldrich) which is a polyol-based dispersion, was added to the carbonate solution at a concentration of 200 mg/L. The CO.sub.2 concentration in the gas phase was 8% (v/v) dry basis. The absorber consists in a packed column containing 16 mm polypropylene Pall rings as a packing to provide the gas/liquid contact surface area. The column has a diameter of 0.175 m and a height of 6.85 m. The L/G ratio was 7 (w/w). Tests were conducted at 30° C. temperature. The impact of carbonic anhydrase on the CO.sub.2 transfer rates was evaluated at 4 different enzyme concentrations.

(57) Results reported in FIG. 8 show that increasing the enzyme concentration translates into an acceleration of the CO.sub.2 mass transfer rate. However, the impact of the enzyme is greater at lower concentration. This seems to indicate that the CO.sub.2 mass transfer from the gas phase to the liquid phase may be limiting the enzyme impact at higher enzyme concentration.

Experimentation Series 2

(58) Another CO.sub.2 absorption test series was conducted using a carbonic anhydrase as a biocatalyst in combination with a 1.45M potassium carbonate solution (K.sub.2CO.sub.3); the lean CO.sub.2 loading of the solution was 0.73 mol carbon/mol potassium ions. An antifoam agent, AF-204 (Sigma Aldrich) which is a polyol-based dispersion, was added to the carbonate solution at a concentration of 200 mg/L. The CO.sub.2 concentration in the gas phase was 10% (v/v) dry basis. The absorber consists in a packed column containing 4.57 m of Metal Mellapak M250Y packing and 3.05 m IMTP Metal 25 packing for a total packing height of 7.62 m. The column has a diameter of 0.254 m. The L/G ratio was 10 (w/w). Tests were conducted at a 30° C. temperature. The impact of carbonic anhydrase on the CO.sub.2 capture efficiency was evaluated at 4 different enzyme concentrations.

(59) Results are reported in FIG. 9. Data show that increasing the enzyme concentration translates into an acceleration of the CO.sub.2 mass transfer for enzyme concentration up to 1 g/L. This clearly indicates that the CO.sub.2 mass transfer from the gas phase to the liquid phase is limiting the enzyme impact at enzyme concentrations around 1 g/L and higher.

Experimentation Series 3

(60) CO.sub.2 absorption tests were conducted using a carbonic anhydrase as a biocatalyst in combination with a 1.45M potassium carbonate solution (K.sub.2CO.sub.3) in a rotating packed bed; the CO.sub.2 loading of the solution was adjusted to 0.62 mol carbon/mol potassium ions. An antifoam agent, AF-204 (Sigma Aldrich) which is a polyol-based dispersion, was added to the carbonate solution at a concentration of 200 mg/L. The CO.sub.2 concentration in the gas phase was 9.5% (v/v) dry basis. The packing consisted of steel foam with 90% porosity. The dimensions of the packing are the following: height 2.54 cm, outer diameter: 29.85 cm and inner diameter: 8.89 cm. The L/G ratios were 30, 149 and 297 (w/w). Tests were conducted at room temperature for 4 enzyme concentrations. The packed bed rotational speed was adjusted at 450 rpm. Results are shown in FIG. 12. It can be observed that for this absorber configuration, the increase of the enzyme concentration results in an increase in the acceleration of the CO.sub.2 capture rate as compared to a solution without enzyme up to an enzyme concentration value close to 5.6 g/L. The observed decrease in performance observed for an enzyme concentration of 8 g/L was correlated to the presence of foam in the system. Under these particular conditions, the anti-foam concentration was not sufficient to avoid solution foaming. As a consequence of the presence of foam, the mass transfer rate was decreased as the gas/liquid surface was decrease because of the presence of foam. Moreover, a comparison with results presented in FIG. 11, shows that a rotating packed bed reactor enables an increase in the CO.sub.2 mass transfer rate as compared to a packed column as the impact of enzyme is still significant for concentration of enzymes higher than 1 g/L, value where mass transfer becomes limiting in a packed column.

Experimentation Series 4

(61) Additional tests were performed in the same unit as described in Experimentation series 3. For these tests, a 1.45 M potassium carbonate solution (K.sub.2CO.sub.3) having a CO.sub.2 loading adjusted to 0.62 mol carbon/mol potassium ions was used. An antifoam agent, AF-204 (Sigma Aldrich) which is a polyol-based dispersion, was added to the carbonate solution at a concentration of 200 mg/L. The tests included measuring the CO.sub.2 absorption rate, at different rotational speeds (450, 1000 and 1500 rpm) of the RPB, and at different L/G ratios (30, 149 and 297 (w/w)). Two solutions were tested, the first solution did not contain enzyme whereas the second had an enzyme concentration of 4 g/L. Results are shown in FIG. 11.

(62) Regarding the results obtained for the solution not containing enzyme, it can be observed that the CO.sub.2 absorption rate increases with the rotational speed up to 1000 rpm for an L/G of 297 (w/w) and then the Acceleration stays at a plateau. However, for the 4 g/L enzyme solution, the rotational speed has an impact at lower L/G whereas at higher L/G ratios the maximum CO.sub.2 absorption rate at the tested process conditions is already reached at 450 rpm. This indicates that the optimal rotational speed depends on the L/G of the system and also on the presence of the enzyme. Acceleration is reported as the CO.sub.2 absorption rate divided by the CO.sub.2 absorption rate obtained for the solution not containing the enzyme at L/G 297 (w/w) and a rotational speed of 450 rpm.

Experimentation Series 5

(63) In order to compare the performance of the rotating packed bed to the performance obtained in the packed columns described in Examples 1 and 2, specific CO.sub.2 absorption rates per unit packing volume were calculated for each system. RPB performance considered for comparison was obtained at an enzyme concentration of 4 g/L, rotational speed of 450 rpm and a L/G of 296 (w/w). Results are shown in Table 1.

(64) TABLE-US-00001 TABLE 1 Ratio of the performance of RPB vs. packed columns Specific CO.sub.2 absorption rates (mg CO.sub.2 m.sup.−3s.sup.−1).sub.RPB (mg CO.sub.2m.sup.−3s.sup.−1).sub.PaCo RPB/packed column (Example 1) 54000/2400 = 22 RPB/packed column (Example 2) 98000/5000 = 20

(65) These data clearly show that using a rotating packed bed increases mass transfer intensity as the absorption rates are 20 times higher than in a packed column for a same volume of packing. It is a clear indication that there is a synergy in using CA containing absorption solution with a rotating packed bed for CO.sub.2 capture.

Experimentation Series 6

(66) For the sake of comparison and benchmarking, the performance obtained using carbonic anhydrase in combination with a potassium carbonate solution, 5M MEA solutions were also tested in the rotating packed bed described in Experimentation series 3 under the same L/G. Tests were conducted at 40° C. The CO.sub.2 loading of the MEA solution was adjusted to 0.28 mol C/mol MEA, which is typical of values encountered in industrial MEA-based CO.sub.2 capture processes. Results are shown in FIG. 12 together with some of the data previously report on FIG. 11. Acceleration values are calculated as the ratio of the CO.sub.2 absorption efficiency of a given solution to the CO.sub.2 capture efficiency observed with a 1.45 M K.sub.2CO.sub.3 solution at a lean CO.sub.2 loading of 0.62 mol C/mol potassium ions under same L/G conditions and at room temperature.

(67) It can be first surprisingly observed that MEA absorption rates under the tests conditions are only 1.5× higher than the corresponding absorption rates obtained in a 1.45 M K.sub.2CO.sub.3 (loading 0.62 mol/mol) at room temperature. A second surprising observation is that the addition of carbonic anhydrase to the potassium carbonate solution leads to a significant increase in the acceleration of CO.sub.2 absorption rates, the increase being more important when the enzyme concentration is higher. The acceleration is about 3.5× more important using 4 g/L enzyme and 5× more important using 5.7 g/L enzyme. These results were surprising notably since previous work using a packed column indicated that the packed column height should be higher when the enzyme was used in combination with potassium carbonate as compared to a MEA-based system for a same CO.sub.2 capture efficiency as the L/G for the absorber. This demonstrates that a rotating packed bed, a high intensity contactor, enables enhanced impact of carbonic anhydrase in the CO.sub.2 absorption process. This also clearly indicates that using carbonic anhydrase in combination with an absorption solution of interest (as described above) in a rotating packed bed is an advantageous option to reduce equipment size, installation footprint and process energy requirements in CO.sub.2 absorption processes.

(68) Some of the advantages related to process intensification of biocatalytically enhanced absorption operations over conventional technology can include equipment size reduction, higher kinetics, capital cost reduction, raw material cost reduction, increased process flexibility and maintenance, and enhanced environmental impact.

Experimentation Series 7

(69) In addition to CO.sub.2 absorption tests described in Experimentations 3 and 4, further tests were conducted using the same carbonic anhydrase and same rotating packed bed equipment. These tests were aimed at obtaining additional data in intermediate values of L/Gs and at lower values of enzyme concentration, liquid and gas flowrates. Another objective was to validate the impact of rotational speed of the bed. The absorption solution consisted 1.45M potassium carbonate solution (K.sub.2CO.sub.3); the CO.sub.2 loading of the solution was adjusted to 0.70 mol carbon/mol potassium ions. An antifoam agent, Suppressor 3592 (Hydrite Chemical) a polyol-based dispersion, was added to the carbonate solution at a concentration of 200 mg/L. The CO.sub.2 concentration in the gas phase was 9.5% (v/v) dry basis. The L/G ratios were 50, 70, 85, 100, 125, 140 (w/w). The gas flowrate was 60 liters per minute. Tests were conducted at room temperature for 3 enzyme concentrations: 0, 0.5 and 1 g/L. The packed bed rotational speed was adjusted at 450, 600, 750 and 900, 1050 and 1200 rpm. The results are reported as a percentage of CO.sub.2 capture in the gas phase. Results are shown in FIGS. 13 and 14. Results on FIG. 13 indicate that for L/G lower or equal to 125, at an enzyme concentration of 1 g/L, the percent of CO.sub.2 capture slightly depends on the L/G of the system. And the maximum CO.sub.2 capture rate is the maximum achievable value as the liquid phase at the outlet is in equilibrium with the gas phase. Results on FIG. 14 show the impact of increasing the enzyme concentration on the performance of the RPB for an L/G of 125. Results indicate in this case that the maximum performance is reached at 1 g/L enzyme, which corresponds to the high enzyme concentration value or elevated concentration as defined above, under the tests conditions.