Patent Application
20140106440
The invention relates to the field of enzyme catalyzed CO2 absorption and CO2 capture.
Warnings from the world's scientific community combined with greater public awareness and concern over the issue of global climate change has prompted increased momentum towards global regulations aimed at reducing man-made greenhouse gas (GHGs) emissions, most notably carbon dioxide (CO2). Ultimately, a significant cut in North American and global CO2 emissions will require reductions from large power generation and industrial point-sources of fossil fuel-based emissions. According to the International Energy Agency's (IEA) GHG Program, as of 2008 there were approximately 8,200 such point-sources worldwide generating 14.7 billion tons of CO2, representing nearly half of all global anthropogenic CO2 emissions. Carbon Capture and Sequestration (CCS) provides a solution to reducing emissions from these sources.
The CCS process involves selective removals of CO2 from a CO2-containing flue gas, and production of a highly concentrated CO2 gas stream which is then compressed and transported to a geologic sequestration site. This site may be a depleted oil field or a saline aquifer. Sequestration as mineral carbonates is an alternate way to sequester CO2 that is in the development phase. Captured CO2 can also be used for enhanced oil recovery, for injection into greenhouses, for chemical reactions and production, and for other useful applications.
Technologies for CO2 capture from post-combustion flue gases and other gas streams are based primarily on the use of an aqueous alkanolamine based solution which is circulated through two main distinct units: an absorption tower coupled to a desorption or stripping tower.
A significant barrier to adoption of carbon capture technology on large scale is cost of capture. Conventional CO2 capture with available technology, based primarily on the use of monoethanolamine (MEA), is an energy intensive process that involves heating the solvent to high temperature to strip the CO2 (and regenerate the solvent) for underground sequestration. The use of MEA involves an associated capture cost of approximately US $60 per ton of CO2 (IPCC), which represents approximately 80% of the total cost of carbon capture and sequestration (CCS), the remaining 20% being attributable to CO2 compression, pipelining, storage and monitoring. This large cost for the capture portion has, to present, made large scale CCS unviable; based on data from the IPCC, for instance, for a 700 megawatt (MW) pulverized coal power plant that produces 4 million metric tons of CO2 per year, the capital cost of MEA based CO2 capture equipment on a retrofit basis would be nearly $800 million and the annual operating cost and plant energy penalty would be nearly $240 million. As such, there is a need to reduce the costs of the process and develop new and innovative approaches to the problem.
In order to help address the high costs associated with traditional CCS systems, biocatalysts have been used for CO2 absorption applications. For example, CO2 hydration may be catalyzed by the enzyme carbonic anhydrase or an analog thereof as follows:
U.S. Pat. No. 7,740,689 describes a formulation and method for absorbing CO2 from a gas using a solution containing an absorption compound and carbonic anhydrase. In addition, international PCT patent application Nos. PCT/CA2010/001212, PCT/CA2010/001213 and PCT/CA2010/001214 describe using carbonic anhydrase in combination with absorption compounds to enhance CO2 capture.
The above patent and applications are incorporated herein by reference along with the following references: U.S. Pat. No. 6,908,507, U.S. Pat. No. 7,176,017, U.S. Pat. No. 6,524,843, U.S. Pat. No. 6,475,382, U.S. Pat. No. 6,946,288, U.S. Pat. No. 7,596,952, U.S. Pat. No. 7,514,056, U.S. Pat. No. 7,521,217, U.S. Patent Application No. 61/272,792 and U.S. Patent Application No. 61/344,869, which are all currently held by the Applicant. Various systems, reactors and processes described in the preceding references may be used in connection with various techniques described below.
In one aspect, the present invention relates to a method for increasing or maximizing a capture rate of CO2 from a CO2-containing gas into an absorption solution, the method including:
In an optional aspect of the method, the step of selecting the absorption solution may be performed such that the pKa maximize the capture rate of CO2 in presence of the selected enzyme or analog thereof.
In an optional aspect of the method, the overall pKa may be of at least 7, at least 7.5, at least 8.5 or at least 9.
In an optional aspect of the method, the method may include providing a concentration of the selected enzyme or analog thereof in the absorption solution in accordance with the pKa thereof.
In an optional aspect of the method, the selected enzyme may be a recombinant enzyme, a variant enzyme, a naturally occurring enzyme or any combination thereof. Optionally, the selected enzyme may be selected from archeal, bacterial or fungal source enzymes or any combination thereof. Optionally, the selected enzyme may be a carbonic anhydrase.
In an optional aspect of the method, the step of selecting the absorption solution may be performed in accordance with the following formula:
k
3
*=A+B pKa;
k
4
*=C+D pKa;
In an optional aspect of the method, the step of coordinating may include selecting the enzyme so as to increase or maximize k3* and reduce or minimize k4* at the pKa of the absorption solution.
In another aspect, the present invention relates to a method for controlling a reaction rate of the reaction CO2+H2O←→H++HCO3− in a reaction solution in presence of an enzyme or analog thereof, the method including controlling a pKa of the reaction solution as well as the concentration and type of the enzyme or analog thereof present in the reaction solution.
In an optional aspect of the method, the pKa of the reaction solution and the concentration and type of the enzyme or analog thereof may be controlled so as to maintain a generally constant k2* in a reactor.
In an optional aspect of the method, the controlling of the pKa and the concentration and type of enzyme is performed in accordance with the following formula:
k
3
*=A+B pKa;
k
4
*=C+D pKa;
In another aspect, the present invention relates to a method for controlling a reaction rate of the hydration reaction of CO2 into hydrogen ions and bicarbonate ions in an absorption solution in presence of an enzyme or analog thereof. The method includes controlling a pKa of the absorption solution as well as the concentration and type of the enzyme or analog thereof present in the absorption solution.
In an optional aspect of the method, the pKa of the absorption solution and the concentration and type of the enzyme or analog thereof may be controlled so as to maintain a generally constant k2* in a reactor.
In an optional aspect of the method, the controlling of the pKa and the concentration and type of enzyme may be performed in accordance with the following formula:
k
3
*=A+B pKa;
k
4
*=C+D pKa;
In another aspect, the present invention relates to a process for absorbing CO2 from a CO2-containing gas at an enzymatically catalyzed CO2 capture rate. The process includes:
In an optional aspect of the process, the pKa of the absorption solution may be at least 7.
In an optional aspect of the process, the pKa of the absorption solution may be at least 7.5.
In an optional aspect of the process, the pKa of the absorption solution may be at least 8.
In an optional aspect of the process, the pKa of the absorption solution may be between 9 and 10.5.
In an optional aspect of the process, the absorption reactor may have a size which is reduced according to the enhanced or maximized CO2 capture rate.
In another aspect, the present invention relates to a use of an absorption compound for absorbing CO2 at an enzymatically enhanced or maximized CO2 capture rate. The absorption compound has a pKa sufficient to increase or maximize the CO2 capture rate in presence of a selected enzyme or analog thereof.
In an optional aspect of the use, the carbonic anhydrase enzyme and the absorption solution may be coordinated in accordance with the following formula:
k
3
*=A+B pKa;
k
4
*=C+D pKa;
In another aspect, the present invention relates to an absorption solution for absorbing CO2 from a CO2-containing gas. The absorption solution includes:
In an optional aspect of the absorption solution, the carbonic anhydrase enzyme and the absorption solution may be coordinated in accordance with the following formula:
k
3
*=A+B pKa;
k
4
*=C+D pKa;
In another aspect, the present invention relates to a system for absorbing CO2 from a CO2-containing gas into an absorption solution. The system includes:
In an optional aspect of the system, the carbonic anhydrase enzyme and the absorption solution may be coordinated in accordance with the following formula:
k
3
*=A+B pKa;
k
4
*=C+D pKa;
In another aspect, the present invention relates to a process for absorbing CO2 from a CO2-containing gas into an absorption solution. The process includes:
In another aspect, the present invention relates to an enzyme enhanced CO2 capture method including:
In an optional aspect of the method, the absorption compound may be selected and provided in a concentration such that k′Am is negligible with respect to kH2O.
In an optional aspect of the method, the k′Am is up to 10%, up to 8%, up to 5%, up to 2%, or lower with respect to kH2O.
In an optional aspect of the method, the absorption compound may include at least one tertiary alkanolamine.
In an optional aspect of the method, the at least one tertiary alkanolamine may be selected from TEA, TIPA, MDEA, DMMEA and DEMEA.
In an optional aspect of the method, the absorption compound may include at least one carbonate.
In an optional aspect of the method, the absorption compound may include at least one alkanolamine, preferably a hindered alkanolamine.
In an optional aspect of the method, the absorption compound may include at least one aminoether, preferably a hindered aminoether.
In an optional aspect of the method, the absorption compound may have a pKa of at least 7, at least 7.5, at least 8.5 or at least 9.
In an optional aspect of the method, the absorption compound may be provided in a concentration of at least 0.5 M in the solution, at least 2 M in the solution, or at least 4 M in the solution.
In an optional aspect of the method, the carbonic anhydrase may be provided in a concentration of at least 50 mg/L in the solution, at least 100 mg/L in the solution, at least 200 mg/L, or at least 400 mg/L in the solution.
In an optional aspect of the method, the carbonic anhydrase may be provided in a concentration in the solution such that the k2* is below a plateau of k2* versus carbonic anhydrase concentration.
In an optional aspect of the method, the method may include producing an ion-rich solution loaded with the bicarbonate ions and the hydrogen ions. The method further may include supplying the ion-rich solution to a desorption stage for releasing the bicarbonate ions and the hydrogen ions in the form of gaseous CO2 and producing a regenerated ion-depleted solution.
In an optional aspect of the method, the method may include supplying the regenerated ion-depleted solution back as the solution for absorption of the CO2.
In another aspect, the present invention relates to an enzyme enhanced CO2 capture method including:
In an optional aspect of the method, the CO2 loading may range depends on the characteristics of the solution, for instance the concentration and type of absorption compound(s) used therein.
In another aspect, the present invention relates to an enzyme enhanced CO2 capture method including:
In an optional aspect of the method, the pKa may be used as a design guide related to turnover factor in order to design, construct and/or operate an absorption reactor employing carbonic anhydrase and an absorption compound.
In an optional aspect of the method, the absorption compound may include a protonable buffer compound.
In an optional aspect of the method, the absorption compound may include at least one tertiary alkanolamine.
In an optional aspect of the method, the absorption compound may have a pKa of at least 7, at least 7.5, at least 8.5 or at least 9.
In an optional aspect of the method, the at least one tertiary alkanolamine may be selected from TEA, TIPA, MDEA, DMMEA and DEMEA.
In an optional aspect of the method, the absorption compound may be selected for its pKa and its low regeneration energy and the absorption-desorption process may be designed accordingly.
In an optional aspect of the method, the method may be further combined with aspects and/or embodiments of methods described herein.
In an optional aspect of the method, the method may include absorption-desorption design and control based on functions of carbonic anhydrase and the absorption compound.
In another aspect, the present invention relates to a method of controlling an enzyme enhanced CO2 capture process including an absorption stage for absorbing CO2 from a CO2 containing gas and producing a CO2 loaded solution and a desorption stage for receiving the CO2 loaded solution and producing a separated CO2 stream and an ion-lean solution for reuse in the absorption stage. The method includes:
In an optional aspect of the method, the step of managing the concentration of the carbonic anhydrase in the solution may be performed to control the catalyzed CO2 hydration rate into the water of the solution.
In an optional aspect of the method, the absorption compound may include a protonable buffer compound.
In an optional aspect of the method, the absorption compound may include at least one tertiary alkanolamine.
In an optional aspect of the method, the absorption compound may includes at least one of TEA, TIPA, MDEA, DMMEA and DEMEA.
In another aspect, there is provided a method of controlling an enzyme enhanced CO2 capture process. The method includes:
In an optional aspect of the method, the step of managing the concentration of the carbonic anhydrase in the solution may be performed to control the catalyzed CO2 hydration rate into the water of the solution.
In an optional aspect of the method, the absorption compound may include a protonable buffer compound.
In an optional aspect of the method, the absorption compound may include at least one tertiary alkanolamine, hindered alkanolamine and/or hindered aminoether.
In an optional aspect of the method, the at least one tertiary alkanolamine may be selected from TEA, TIPA, MDEA, DMMEA and DEMEA.
In an optional aspect of the method, the CO2 capacity of the solution may be increased to reduce the overall volume of the solution required.
While the invention will be described in conjunction with example embodiments, it will be understood that it is not intended to limit the scope of the invention to such embodiments.
On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included in the present description and the appended claims.
It should be understood that any one of the above mentioned optional aspects of each process, method, use and absorption solution may be combined with any other of the aspects thereof, unless two aspects clearly cannot be combined due to their mutually exclusivity. For example, the various operational steps and/or structural elements of the process described herein-above, herein-below and/or in the appended Figures, may be combined with any of the general method or use descriptions appearing herein and/or in accordance with the appended claims.
Embodiments, examples and illustrations of some of the techniques described herein will be further understood in light of the following figures.
The present invention provides techniques for removing CO2 from a gas by contacting the gas with an absorption solution in the presence of an enzyme or an analog thereof. In some implementations, the absorption solution may contain one or more absorption compounds and the enzyme may include carbonic anhydrase. As will be explained further below, a variety of different types of carbonic anhydrase may be used and with various delivery techniques.
Relationships Between Enzyme, Solution pKa, Temperature, Reaction Kinetics
Working extensively with different types of carbonic anhydrase, it has been found that each carbonic anhydrase can have its own character with regard to the kinetics of catalyzing the hydration reaction of CO2 into hydrogen and bicarbonate ions. While carbonic anhydrases from various different sources and of various different characters provide enzymatic catalysis for enhanced CO2 capture, the variability between different carbonic anhydrase types can involve some challenges for the design and operation of CO2 capture systems. In addition, this variability between carbonic anhydrases can increase the dependency of a given CO2 capture system on a given enzyme type or enzyme production so that the CO2 capture system continues to function as desired under its designed operating conditions.
However, it has been found that in a CO2 capture system with an absorption solution, there is a relationship between the kinetics of the CO2 absorption, the carbonic anhydrase and the pKa (acid dissociation constant) of the absorption solution. The relationship as well as its discovery and derivation will be further described below. Pursuant to these findings, it is possible to design and/or operate a CO2 capture system that uses an absorption solution and an enzyme such as carbonic anhydrase, by coordinating the character of the enzyme with the pKa of the absorption solution, in order to enhance, maximize or control the CO2 capture kinetics.
In one instance, for example, the relationship may be summarized by the following equation:
k3* and k4* may be correlated with pKa of an absorption compound as follows:
k
3
*=A+B pKa;
and
k
4
*=C+D pKa;
In addition, k3* and k4* may be correlated with temperature of an absorption system as follows:
k
3*(T)=E×exp(F/T);
and
k
4*(T)=G×exp(H/T);
Using temperature and pKa correlations, an absorption system may be designed or operated to achieve a desired range of absorption kinetics while utilizing an efficient concentration and type of carbonic anhydrase.
In some implementations, the relationship between pKa and the enzyme may be used to design or operate a CO2 capture process, such as the one illustrated in
Referring to
In one optional scenario, the ion-rich solution 24 may be further processed, used or valorized, for example by reacting or contacting waste streams containing cations such as sodium, calcium and/or magnesium in order to precipitate a solid carbonate. The waste stream may be industrial wastes such as bauxite residue from aluminum refining, steel slag, related and/or other waste streams or mineral sources. The ion rich solution 24 may also be reutilized and/or combined with cations as a bicarbonate solid or slurry for such purposes as enhanced algae or other microbial farming. In this sense, the process may be a “once-through” absorption process whereby the ion-rich solution generated in the absorption process is not subjected to desorption to separate the CO2 gas but is rather used directly to utilize the ions therein to produce, for example, a neutralized mineral product.
In another optional scenario, as shown in
It should be noted that the carbonic anhydrase may be provided in a number of other ways. For instance, carbonic anhydrase may be provided to the absorption solution 20 which flows through the absorber reactor 16 and is not removed from the ion-rich solution 34 which is fed to the desorption reactor 36. In this scenario, the carbonic anhydrase is introduced into the overall CO2 capture process 10 via an absorption solution make-up stream 50, which is preferably mixed with the recycled ion-depleted solution 42. According to another optional aspect, the carbonic anhydrase may be added to the absorption or desorption units via multiple enzyme feed streams. Depending on operating conditions and the thermal stability of the carbonic anhydrase strain, fraction, variant or analog that is used in the process, the carbonic anhydrase may be introduced at a given point in the process and spent enzyme may be replaced at a given point in the process. For example, when free enzyme is used as a component of the absorption solution, the process may include periodic or continuous removal of denatured enzyme or reduced-activity enzyme, which may be done as part of an absorption solution reclaiming or make-up technique. It should also be mentioned that one or more of multiple absorption and desorption reactors may have enzyme flowing there-through, depending for example on the temperature within each reactor, so as to maximize enzyme activity and minimize enzyme denaturing. The enzyme may alternatively be allowed to flow through the entire system to flow through each one of the desorption reactors.
Carbonic anhydrase is a very efficient catalyst that enhances the reversible reaction of CO2 to HCO3−. Carbonic anhydrase is not just a single enzyme form, but a broad group of metalloproteins that exists in three genetically unrelated families of isoforms, α, β and γ. Carbonic anhydrase (CA) is present in and may be derived from animals, plants, algae, bacteria, etc. The human variant CA II, located in red blood cells, is the most studied and has a high catalytic turnover number. The carbonic anhydrase includes any analogue, fraction and variant thereof and may be alpha, gamma or beta type from human, bacterial, fungal or other organism origins, having thermostable or other stability properties, as long as the carbonic anhydrase can be provided to function in the CO2 capture or desorption processes to enzymatically catalyse the reaction:
One of the ways carbonic anhydrase enhances performance of CO2 capture solutions in the desorption unit is by reacting with dissolved bicarbonate ions and maintaining a maximum CO2 concentration gradient between gas and liquid phases to improve CO2 transfer rate from the liquid solution phase to the gas phase. Referring to
During enzyme catalyzed carbon dioxide absorption into absorption solutions, several reactions occur and may be summarized in the “wheel of reaction” represented in
Referring to
At low buffer concentrations (<10 mM), the intermolecular proton transfer, i.e. the second step of Reaction b, is rate limiting, while at high buffer concentration, the intra molecular proton transfer, i.e. the first step of Reaction b, is rate limiting. Since water is a very weak base and therefore a poor proton acceptor and OH− is not abundant at the pH at which the enzyme functions best, a dilute buffer solution is preferably used as proton acceptor in kinetic experiments. In some aspects of the present invention, the dilute buffer solution (millimolar range) is replaced by a more concentrated alkanolamine solution with concentrations that may be, in some aspects and example, up to about 4 M and a corresponding pH range of about 11 to about 11.6. It should be noted that in other aspects, the concentrations may be up to 10 M, for example, depending on the particular compound being used. For instance, in one aspect, the concentration is up to a concentration such that the increased viscosity of the resulting solution does not have a too negative effect on the process at the given process conditions.
The article by F. Larachi. “Kinetic model for the reversible hydration of carbon dioxide catalyzed by human carbonic anhydrase II”. Ind. Eng. Chem. Res., 49(19):9095-9104, 2010 (hereinafter referred to as “Larachi”) showed that CO2 hydration by hCA II is best described by a random pseudo quad quad iso ping pong catalytic (1-transitory complex) mechanism. In that mechanism, the first transitory complex (EZnOH−CO2EZnHCO3−) is left out of consideration and the intermolecular H+ transport (2nd part of Reaction b) is extended with an additional parallel reaction:
This mechanism results in a very complex and long kinetic rate expression and therefore Larachi is referred to and incorporated herein by reference.
Reactive absorption of CO2 from process gas streams has been an important part of many industrial processes. The conventional technology to capture CO2 on a large scale is an absorption-desorption process, in which aqueous solutions of alkanolamines (also referred to in industry as “amines”) are frequently used as solvents. Different alkanolamines can be used including primary, secondary or tertiary alkanolamines. The reaction mechanisms between primary/secondary and tertiary amines with CO2 are different. The reaction between CO2 and primary/secondary amines is significantly faster than the reaction between CO2 and tertiary amines. As a result of the faster reaction, the absorption column has smaller dimensions when primary/secondary amines are used. However, an advantage of tertiary amines is that the regeneration energy is significantly lower than the regeneration energy of primary and secondary amines. As a result of the lower regeneration energy of tertiary amines, the processing costs for stripping may be lower.
It would be advantageous to have a combination of both fast absorption and low regeneration energy. In one aspect, one may use carbonic anhydrase enhanced absorption with a low desorption energy compound, such as tertiary amines and carbonate based solutions, which facilitate lower energy requirements for desorption and lower temperatures, which can also reduce or avoid denaturing of the carbonic anhydrase and enable use of a smaller desorption tower. In another aspect, one may use a fast absorption compound, such as primary and/or secondary amines for enhanced absorption, with carbonic anhydrase enhanced desorption to lower the energy requirements for the primary/secondary amine solution regeneration.
In another aspect, the enzyme carbonic anhydrase is provided to flow with the solution throughout the process, to not only accelerate the transformation of CO2 to HCO3−, but also the reverse reaction, which is of major importance during the regeneration of the CO2 loaded solution (also referred to as the “carbonate loaded solution” or the “ion-rich solution” herein).
Regarding kinetics and reaction mechanisms, when CO2 is absorbed for example in an alkanolamine absorption solution, the following reactions occur simultaneously:
The corresponding reaction rate may be formulated as follows:
HCOC
CCO
The corresponding reaction rate may be formulated as follows:
RCO
The corresponding reaction rate may be formulated as follows:
RCO
The corresponding reaction rate may be formulated as follows:
RCO
The overall forward reaction rate constant, kOV, is determined by the contributions of each of these four reactions, whose kinetic rate expression is usually given as follows:
k
OV
=k
C
−k
OH
C
+H
H
O
k
OV
=k′
Am
+k′
OH
+k′
H
O
The forward reaction rate constants of the four reactions I, II, III and IV as reported in literature are listed in the following table.
Table 1 illustrates that in a 2 kmol·m−3 MDEA solution the contribution of Reaction IV can be neglected based on the reaction rate constant. The pH of a lean 2 kmol/m3 MDEA solution is approximately 11.4, giving a hydroxide ion concentration of 0.00286 kmol/m3; however as soon as the solution is slightly loaded the hydroxide ion concentration quickly decreases. Therefore, after initial loading, the contribution of Reaction III can also be neglected. As a result the overall forward reaction rate for the absorption of CO2 into an aqueous tertiary alkanolamine solution is fully determined by the rate of Reaction I and/or II, and therefore kOV≈k′Am.
The absorption solution includes at least one absorption compound which may serve as base. Optionally, the base may also be bicarbonate ions HCO3− formed in the different reactions of the overall absorption reaction mechanism (
Experiments on the mechanism of enzyme catalyzed carbon dioxide absorption into absorption solutions have shown that it is the overall hydration reaction of CO2 into bicarbonate ions and hydrogen ions which is catalyzed in presence of an enzyme.
The overall absorption reaction rate therefore strongly depends on the hydration reaction rate. The latter may even be considered as the overall absorption reaction rate. The overall reaction rate may be reduced to:
In aqueous (e.g. sodium) carbonate systems, carbon dioxide can react with:
1. hydroxide (Pinsent et. al., 1956; Pohorecki and Moniuk, 1988)
RCO2=kOH·COH·CCO2=k′OH·CCO2
2. water (Pinsent et. al., 1956; Kern, 1960)
RCO2=kH2O·CH2Ox·CCO2=k′H2O·CCO2
Regarding mass transfer considerations, the absorption of a gas A into a liquid is generally described by the following equation:
For a system consisting of a pure gas and assuming ideal gas behaviour and a freshly prepared and therefore lean liquid (CA,L=0), the above equation can be simplified to:
The chemical enhancement factor, EA, is a function of the so-called Hatta number. When the absorption occurs in the first order regime and Ha>2, the enhancement factor equals the Hatta number:
For reactions different from the simple first-order reaction, the process can be considered in the pseudo first order regime when next criterion is fulfilled:
2<Ha<<Einf
where Einf is the infinite enhancement factor. For irreversible reactions, the infinite enhancement factor is defined as follows:
In further optional aspects of the process, the ion-rich solution may contain from about 0.1 M to 10 M of bicarbonate ions. The carbonate loading of the solution will depend on the operating conditions, reactor design and the chemical compounds that are added. For instance, when potassium or sodium bicarbonate compounds are used in the absorption solution, the ion-rich solution may contain from about 0.2 M to 1.5 M of bicarbonate ions and when other compounds such as tertiary amines are used the ion-rich solution may contain from about 1 M to 10 M of bicarbonate ions. When the ion-rich solution is highly loaded with carbonate/bicarbonate ions, it may become much more viscous which can have a detrimental effect of mass transport within the solution. The presence of carbonic anhydrase flowing with the solution further enhances the mass transport along with the enzymatic reaction, thus improving the desorption unit and overall CO2 capture and regeneration process, for instance by supersaturating the solution with bubbles of gaseous CO2. In addition, temperatures in the desorption unit may range between about 60° C. and about 150° C., for example.
In one aspect of the present invention, it has been found that by using an absorption compound, such as a tertiary alkanolamine like MDEA, in combination with carbonic anhydrase, at certain conditions and parameters, the concentration of the absorption compound does not materially affect the absorption rate while the carbonic anhydrase significantly enhances the absorption of CO2 in aqueous solution. Therefore, the enzyme does not enhance the reaction of CO2 with the absorption compound, since the rate of this reaction is a function of the absorption compound concentration. Rather, the enzyme enhances the reaction of CO2 with water in the aqueous solution. In the presence of enzyme, this reaction is not only first order in CO2, but also first order in water. Thus, carbonic anhydrase may provide a solution for the efficient capture of CO2 from flue gases by significantly increasing the kinetics of its absorption into an aqueous solution containing a compound such as MDEA, a tertiary amine, which enables increased absorption capacity of bicarbonate and hydrogen ions and also requires relatively low regeneration energy for downstream desorption for example.
Various absorption experiments, calculations and derivations were performed, some of which will be described below, and relationships between variables of the CO2 capture system have been found.
Absorption experiments were performed in a thermostated stirred cell type reactor operated with a smooth and horizontal gas-liquid interface. The reactor was connected to two gas supply vessels filled with carbon dioxide (99.9%, Hoekloos) or nitrous oxide (>99%, Hoekloos) from gas cylinders. Both the reactor and gas supply vessels were equipped with digital pressure transducers and PT-100 thermocouples. The measured signals were recorded in a computer. The pressure transducer connected to the stirred cell was a Druck PTX-520 pressure transducer (range 0-2 bars) and the gas supply vessels were equipped with Druck PTX-520 pressure transducers (range 0-100 bars). A schematic drawing of the experimental set-up is shown in
In a typical experiment, an amine (e.g. MDEA) solution with desired concentration was prepared by dissolving a known amount of MDEA (99%, Aldrich) in a known amount of water together with a known amount of enzyme solution (human carbonic anhydrase (hCA II) or a thermostable variant of hCA II (‘5X’mutant, CO2 Solutions Inc.). Approximately 500 ml of the solution was transferred to the reactor, where inerts were removed by applying vacuum for a short time. Next, the solution was allowed to equilibrate at 298 K before its vapour pressure (Pvap) was recorded.
Regarding physical absorption, a predefined amount of N2O was fed to the reactor from the gas bomb. The stirrer in the reactor was switched on, while a flat gas-liquid interface was maintained in the reactor. The stirrer speed was adjusted to 100 rpm. The absorption rate was studied by measuring the pressure decrease as a function of time. After a certain time the stirrer speed was increased to approximately 1000 rpm to reach the equilibrium pressure (Peq) in the gas phase. The final temperature and pressure in the gas supply bomb was noted. From the initial and final conditions (T and P) in the gas supply system, the amount of gas added to the reactor was calculated. A mass balance over the gas and liquid phase for N2O in combination with an above equation yields the following:
The N2O partial pressure in the reactor was calculated by subtracting the lean liquid's vapour pressure, determined explicitly at the beginning of the experiment, from the recorded total pressure in the reactor. The liquid side mass transfer coefficient, kL, is determined from the straight line with a constant slope yielded by plotting the In-term on the left hand of the previous equation versus time. The distribution coefficient of N2O in aqueous MDEA can be calculated from the same experiment by the following:
Regarding reactive absorption, the method for the reactive absorption is analogous to the method for physical absorption, only now the gas is CO2 instead of N2O. A mass balance over the gas phase for CO2 in combination with some of the above equations yields the following:
The CO2 partial pressure in the reactor was calculated by subtracting the lean liquid's vapour pressure from the recorded total pressure in the reactor. Typically, a plot of the natural logarithm of the carbon dioxide partial pressure versus time will yield a straight line with a constant slope, from which the overall kinetic rate constant, kOV, can be determined, once the required physico-chemical constants are known. The diffusion coefficient of carbon dioxide in the solution is calculated with the N2O analogy from the diffusion coefficient of N2O in the solution and the diffusion coefficients of CO2 and N2O in water were calculated using the correlations given in the A. Jamal. “Absorption and Desorption of CO2 and CO in Alkanolamine Systems” PhD thesis, The University of British Colombia, Canada, 2002 (hereinafter referred to as “Jamal”).
The distribution coefficient of carbon dioxide is estimated using the N2O analogy:
The distribution coefficients of CO2 and N2O in water were calculated using the correlations given by Jamal. The physical solubility of N2O in aqueous MDEA was experimentally determined for experimental conditions relevant for the present study as described above.
To determine the influence of carbonic anhydrase on the physical solubility of nitrous oxide in aqueous MDEA solutions, measurements with and without carbonic anhydrase were performed. Two series of experiments were carried out at 298 K, MDEA concentration of 2 kmol/m3 and enzyme concentrations ranging from 0 to 1000 g/m3 for freshly prepared solutions and solutions with a CO2-loading of 1%. From the experimental data, it can be concluded that, within the experimental accuracy, the physical solubility of nitrous oxide is not influenced by the presence of carbonic anhydrase. The obtained distribution coefficient is well in line with data found in literature.
Regarding liquid side mass transfer coefficient (kL), it is determined for the same set of experiments. The experimental data show that for a fresh aqueous MDEA solution the enzyme concentration has some influence on kL; initially kL decreases and then increases with increasing enzyme concentration. However, as soon as the solution is slightly pre-loaded with CO2 (1%<α<5%) the presence of enzyme has no influence on kL.
In order to further validate the obtained overall reaction rate constants from experiments without enzyme, the results obtained in this study were compared to data from literature. Most correlations in literature are for the second order reaction rate constant for the amine. By multiplying this constant with the amine concentration as used in the experiment, the corresponding second order overall reaction rate constant is obtained. It was concluded that the results of the present experiments are well in line with data found in literature.
Experiments were performed on alkanolamine absorption solutions in presence of the enzyme carbonic anhydrase. Studied alkanolamines include diethylethanolamine (DEMEA), dimethylethanolamine (DMMEA), monoethanolamine (MEA), triethanolamine (TEA) and tri-isopropanolamine (TIPA).
Referring to
5
.2
.8
indicates data missing or illegible when filed
The following empirical Equation (1) may be used for illustrating the dependency between k2* and the enzyme concentration.
wherein k2* is the enzyme enhanced reaction rate constant in m3/mol/s;
The absorption reaction rate is therefore dependent on the enzyme concentration and a combined effect between the carbonic anhydrase and the absorption solution. The combined effect can be described and quantified by a pair of constants (k3*, k4*).
Constants k3* and k4* may be derived from experimental data with derivation methods, such as the least squares method or the linear regression method.
In some implementations, there is provided techniques for coordinating the acidity of the absorption solution with the character and concentration of the enzyme.
For identified (k3*, k4*) pairs, a relationship between the kinetic constants (k3*, k4*) and a pKa value of the absorbing compounds, such as an alkanolamine, has been found. More particularly, this relationship may be linear, as shown in
k
3
*=A
3
+B
3 pKa Equation (2)
k
4
*=A
4
+B
4 pKa Equation (3)
wherein (A3, B3) and (A4, B4) are pairs of coefficients characterizing the enzyme.
As mentioned above, the following describes the least squares method and the linear regression method for obtaining coefficients A and B.
Least Squares Method:
From the experimental data shown in
Referring to
k
3*=1.8597·10−2·pKa−0.11683
k
4*=−1.073·pKa+10.162
wherein A3=−0.11683 and B3=1.8597·10−2; and A4=10.162 and B4=−1.073.
Linear Regression Method:
From the experimental data shown in
Referring to
k
3*=0.014033·pKa−0.07042
k
4*=−2.441·pKa+23.941
wherein A3=−0.07042 and B3=1.4033·10−2; and A4=22.941 and B4=−2.441.
In view of the above, in some scenarios, for a given enzyme such as a given strain, variant or batch of carbonic anhydrase, one may obtain the enzyme acidic character constants, such as A and B, in order to coordinate the given enzyme with an absorption solution acidity in order to obtain CO2 capture kinetics.
In some implementations, one can achieve enhancing or maximizing the absorption reaction rate by selecting or controlling the acidity (pKa) of the absorption solution; the character of the enzyme; and/or the concentration of the enzyme.
In one example, the absorption compound may be selected based on its pKa in accordance with a particular enzyme's response characteristics to pKa. In another example, a carbonic anhydrase enzyme may be selected based on having a high A constant and low B constant. In another example, a mixture of multiple carbonic anhydrases may be used having different characters and A,B constants for a given absorption compound pKa.
In some scenarios, one may determine or approximate the kinetic constants (k3*, k4*, A, B, C, D) to facilitate selection of one or more absorption compounds and/or enzyme to be used in a CO2 capture system.
In some scenarios, one may determine or approximate the kinetic constants (k3*, k4*, A, B, C, D) to facilitate operation of a CO2 capture system that uses an absorption compound and an enzyme. An existing CO2 capture system, which may include absorption and desorption reactors and may be similar to the system shown in
In some scenarios, techniques described herein can allow the efficient design, operation or control of a CO2 capture system while avoiding guesswork and trial and error. For example, in a case where a new type of enzyme is to be used in a CO2 capture system, its different acidic response character may be accounted for by determining a desired pKa or acidity and a desired enzyme concentration according to the derived relationship to maintain a high or constant level of CO2 capture.
In some scenarios, multiple different carbonic anhydrase types having different characters may be selected for use with a certain absorption solution. For example, since the cost of absorption compounds can vary, it may be desirable to modify the composition of the absorption solution to provide a more cost effective system. Such modifications may reduce the acidity of the modified solution which, in turn, would modify the kinetic constants associated with the enzyme. One may therefore modify the enzyme type or add additional enzyme(s) of different type and character to correct for the modified absorption solution while maintaining suitable absorption kinetics.
In some scenarios, the coordinating of the pKa or acidic character of the absorption solution with the enzyme may be done by using experimental protocols, such as determining kinetic constants of the absorption reaction rate according to solving approaches for overdetermined systems in data fitting, such as the least squares method or linear regression method. The coordinating may also be done based on generated or pre-determined charts or graphs of kinetic constants versus pKa for different enzymes. The coordinating of the pKa or acidic character of the absorption solution and the enzyme may include selecting an enzyme and providing the enzyme in a concentration sufficient for accelerating the absorption reaction according to the pKa of the absorption solution.
Various different types of absorption compounds may be used. For example: amine solutions, alkanolamine solutions, aminoether solutions, carbonate solutions, amino acid solutions, and so on. In some optional aspects, the absorption solution may include a chemical compound for enhancing the CO2 capture process. For instance, the ion-rich solution may further contain at least one compound selected from the following: piperidine, piperazine, derivatives of piperidine or piperazine which are substituted by at least one alkanol group, monoethanolamine (MEA), 2-amino-2-methyl-1-propanol (AMP), 2-(2-aminoethylamino)ethanol (AEE), 2-amino-2-hydroxymethyl-1,3-propanediol (Tris), N-methyldiethanolamine (MDEA), dimethylmonoethanolamine (DMMEA), diethylmonoethanolamine (DEMEA), triisopropanolamine (TIPA), triethanolamine (TEA), DEA, DIPA, methyl monoethanolamine (MMEA), TIA, TBEE, HEP, AHPD, hindered diamine (HDA), bis-(tertiarybutylaminoethoxy)-ethane (BTEE), ethoxyethoxyethanoltertiarybutylamine (EEETB), bis-(tertiarybutylaminoethyl)ether, 1,2-bis-(tertiarybutylaminoethoxy)ethane or bis-(2-isopropylaminopropyl)ether, and the like, dialkylether of polyalkylene glycols, dialkylether or dimethylether of polyethylene glycol, amino acids including glycine, proline, arginine, histidine, lysine, aspartic acid, glutamic acid, methionine, serine, threonine, glutamine, cysteine, asparagine, valine, leucine, isoleucine, alanine, valine, tyrosine, tryptophan, phenylalanine, and derivatives such as taurine, N,cyclohexyl 1,3-propanediamine, N-secondary butyl glycine, N-methyl N-secondary butyl glycine, diethylglycine, dimethylglycine, sarcosine, methyl taurine, methyl-α-aminopropionic acid, N-(β-ethoxy)taurine, N-(β-aminoethyl)taurine, N-methyl alanine, 6-aminohexanoic acid and potassium or sodium salts of the amino acids, or mixtures thereof. The solution may include primary, secondary and/or tertiary alkanolamines. The solution may include hindered alkanolamine and/or hindered aminoether.
In another optional aspect, the solution may be a carbonate-based solution, such as potassium carbonate solution, sodium carbonate solution, ammonium carbonate solution, promoted potassium carbonate solutions, promoted sodium carbonate solutions or promoted ammonium carbonates; or mixtures thereof. These carbonate-based solutions may be promoted with one or more of the above-mentioned chemical compounds.
The enzyme may be provided in a concentration between about 0.05 kg/m3 and 2 kg/m3. Optionally, the enzyme may be provided in a concentration of at least 0.2 kg/m3.
Referring to
It should be understood that various techniques described herein are not limited to CO2 absorption but include CO2 desorption processes and related systems and solutions. Enhancement of the backward dehydration reaction kinetics should also be facilitated by the techniques described herein in a similar manner to enhancement of the forward hydration reaction.
It should also be understood that any one of the above mentioned aspects of each method, process, use and solution may be combined with any other of the aspects thereof, unless two aspects clearly cannot be combined due to their mutually exclusivity. For example, the various embodiments of the method for enhancing or maximizing a capture rate of CO2 described herein-above, herein-below, in the appended Figures and/or in the appended claims, may be combined with any of the process for absorbing CO2 from a CO2-containing gas, method for controlling the reaction rate of CO2 hydration, use of at least one absorption compound appearing herein and/or in accordance with the appended claims.
In other examples, CO2 absorption experiments were performed with a 0.3 M sodium carbonate solution containing 0, 400, 800, 1600 or 2400 g·m−3 of the enzyme carbonic anhydrase at 298, 313 or 333 K.
The anhydrous sodium carbonate used for the preparation of the aqueous solutions had a purity of >99% and it was used as supplied by Merck. The enzyme used was a thermostable carbonic anhydrase provided by Codexis inc. in a purified form. All solutions were prepared with demineralized water. The carbon dioxide (99.9%) was obtained from Air Liquide.
The diffusion coefficient of carbon dioxide is estimated from the solution's viscosity using the Stokes-Einstein relationship:
The results for the rate constants k3* and k4* derived from the experimental results are presented in Table 7.
From the results presented in Table 7,
Substituting equations (B) and (C) into equation:
gives the following correlation for the enzymatic rate constant:
With these equations the enzymatic rate constant kH2O* was estimated within an accuracy of 20% or 40%.
It should be understood that kH2O* calculated with equation (D) as mentioned above is equivalent to k2* characterizing the enzyme catalysed hydration reaction rate. kH2O* may be defined by the ratio of kH2O on CH2O. It should further be noted that kH2O may be referred to as k′H2O.
Additional Experiments and Results with Enzymatically Enhanced CO2 Capture with Carbonic Anhydrase
CO2 absorption experiments in MDEA were conducted with the following scope:
The following experiments were performed using CO2 and MDEA. Results are gathered in Table 8.
Results of the absorption experiments with MDEA of Table 8 are presented in
Comparing
When 4 M MDEA with 500 mg/I enzyme hCA II solution was drained from the reactor after the experiments, some degree of denaturation seemed to have occurred. This seems a likely explanation for the relatively small increase in k2 upon addition of CA.
The following table 9 shows enhancement factors for unloaded 2M MDEA solutions with different enzyme concentrations.
From Table 9, it can be drawn that E increases with increasing enzyme concentration in the solution. The kL slightly decreases by the presence of enzyme, while kOV increases significantly.
TEA is also a tertiary alkanolamine. It has a lower pKa than MDEA and hence a lower reactivity towards CO2. The molecular weight of TEA is slightly higher than that of MDEA, and hence the variation in water concentration is a little more pronounced in this set of experiments.
Absorption rate experiments were conducted at TEA concentrations of 1, 2 and 4 kmol/m3 and at enzyme concentrations up to a maximum of about 1600 mg/L.
The TEA concentrations and corresponding water concentrations are presented in the following table along with the values for the physico-chemical constant (m·√D) used to interpret the absorption rate experiments. Also, in this table, the second-order kinetic rate constants of the reaction between TEA and CO2-k2—are listed.
In relation to the mechanistic study, it can be said that at certain experimental conditions (i.e. CTEA=1.0 & 2.0 kmol/m3 and 50≦CM5X≦400 mg/L), the observed kOV seems not a function of TEA concentration and hence it may be concluded that the overall kinetic rate constant is (predominantly) determined by the contribution of the (catalyzed) reaction between water and CO2, and therefore kOV=kH2O. Also for these conditions, it can be said that the rate constant of the H2O—CO2 reaction is a function of the enzyme concentration and that the rate constant of the H2O—CO2 reaction seems not a function of TEA and water concentration. Outside these conditions, the overall rate constant seems to be decreasing with increasing TEA concentration. This may be the influence of the simultaneously decreasing water concentration having its effect on the H2O—CO2 reaction rate, but also enzyme denaturation effects cannot be ruled out at this point. In addition, the catalyzing effect of M5X seems to be dependent on the pKa of the alkanolamine in solution.
DMMEA is another tertiary alkanolamine and has a higher pKa than MDEA and hence a higher reactivity towards CO2. The molecular weight of DMMEA is relatively low, resulting in just a slight variation in water concentration in this set of experiments.
Absorption rate experiments were conducted at DMMEA concentrations of 1 and 2 kmol/m3 and at enzyme concentrations up to a maximum of about 1600 mg/L.
The DMMEA concentrations and corresponding water concentrations are presented in the following table along with the values for the physico-chemical constant (m·√D) used to interpret the absorption rate experiments. Also, in this table, the second-order kinetic rate constants of the reaction between DMMEA and CO2-k2—are listed.
In relation to the mechanistic study, it can be said that as the observed kOV is not a function of DMMEA concentration (in case CM5X≧50 mg/L), it can be concluded that the overall kinetic rate constant is (predominantly) determined by the contribution of the (catalyzed) reaction between water and carbon dioxide, and therefore kOV=kH2O. In addition, the rate constant of the H2O—CO2 reaction is a function of the enzyme concentration. Also, the rate constant of the H2O—CO2 reaction seems not a function of DMMEA and water concentration. It should be noted, however, that the water concentration was only slightly varied in this set of experiments. In addition, the catalyzing effect of M5X seems to be dependent on the pKa of the alkanolamine in solution: it increases with increasing pKa as observed in the order DMMEA>MDEA>TEA.
DEMEA is also tertiary alkanolamine and has an even higher pKa than DMMEA and hence a higher reactivity towards CO2. The molecular weight of DEMEA is comparable to MDEA.
Absorption rate experiments were conducted at DEMEA concentrations of 0.5, 1 and 2 kmol/m3 due to the possibility of enzyme denaturation in the presence of this amine.
The DEMEA concentrations and corresponding water concentrations are presented in the following table along with the values for the physico-chemical constant (m·√D) used to interpret the absorption rate experiments. Also, in this table, the second-order kinetic rate constants of the reaction between DEMEA and CO2-k2—are listed.
In relation to the mechanistic study, it can be said that the observed kOV seems to be a function of DEMEA concentration, with the exception of the experiments performed with 100 mg/L M5X enzyme in solution. This may either indicate towards a water-concentration dependence or towards enzyme denaturation effects in the solutions. In addition, the catalyzing effect of M5X is less in DEMEA than in DMMEA despite its higher pKa. The effect is higher, though, than in solutions with MDEA and TEA.
TIPA is another tertiary alkanolamine under study and it has a lower pKa than MDEA, comparable to TEA. TIPA has a lower reactivity towards CO2. The molecular weight of TIPA, however, is much larger than that of MDEA, and hence the variation in water concentration is more pronounced in this set of experiments.
Absorption rate experiments were conducted at TIPA concentrations of 1 and 2 kmol/m3 and at enzyme concentrations up to a maximum of about 800 mg/L.
The TIPA concentrations and corresponding water concentrations are presented in the following table along with the values for the physico-chemical constant (m·√D) used to interpret the absorption rate experiments. Also, in this table, the second-order kinetic rate constants of the reaction between TIPA and CO2-k2—are listed.
In relation to the mechanistic study, one can say that as the observed kOV is not a function of TIPA concentration (in case CM5X≧50 mg/L), it can be concluded that the overall kinetic rate constant is (predominantly) determined by the contribution of the (catalyzed) reaction between water and carbon dioxide, and therefore kOV=kH2O. In addition, the rate constant of the H2O—CO2 reaction is a function of the enzyme concentration and it levels off at higher enzyme concentration. Furthermore, the rate constant of the H2O—CO2 reaction seems not a function of TIPA and water concentration within the experimental conditions studied. In addition, the catalyzing effect of M5X seems to be dependent on the pKa of the alkanolamine in solution: it increases with increasing pKa as observed in the order DMMEA>MDEA>TIPA>TEA.
The main conclusions drawn from the experimental results presented herein-below are the following: first, in the presence of the (M5X) enzyme (≧50 mg/L), the overall kinetic rate constant is predominantly determined by the (enzyme catalyzed) reaction between carbon dioxide and water; the catalysis effect increases with increasing enzyme content (this effect, however, seems to level off at higher enzyme concentrations); and the catalysis effect increases with increasing pKa of the alkanolamine.
Another conclusion is further discussed in the following table and figures, in which the experimentally determined overall rate constants are listed as a function of pKa.
The effect of the amine concentration at a given, constant enzyme concentration was also studied. The results of these experiments are presented in
k
OV
=k
AmCAm+kOHCOH+k′H
Apparently, MDEA mainly acts as proton acceptor during the regeneration of the enzyme (see Reaction c). From these results, it can be concluded that the intermolecular H+ transport is not rate determining since the rate of this reaction is also dependent on the MDEA concentration. Therefore, it seems justified to conclude that Reactions I, II and IV occur in parallel and that the effect of the presence of the enzyme is taking place via Reaction IV. The experimentally determined values of kOV are corrected for Reaction I and II via:
k
OV
=k
OV
−k
AmCAm
where kAm is derived from the results obtained in this study, resulting in the following: kAm=0.0064 m3mol−1s−1.
These experiments on the mechanism of enzyme catalysed carbon dioxide absorption into aqueous tertiary alkanolamines show that the enzyme does not catalyze Reaction I or II, the reaction between CO2 and tertiary alkanolamine, since the overall reaction rate constant is not influenced by the amine concentration. The amine mainly acts as proton acceptor during the regeneration of the enzyme (Reaction b). Besides, this study also showed that Reactions I, II and IV, CO2 hydrogenation, occur parallel, enzyme enhances Reaction IV and that Reaction IV is not only 1st order in CO2, but also 1st order in H2O. The enzyme carbonic anhydrase significantly increases kinetics of the absorption of carbon dioxide in aqueous MDEA solutions. Thus, the combination of CA with aqueous MDEA may provide a solution for the efficient capture of carbon dioxide from e.g. flue gases, since MDEA requires less energy for regeneration than MEA, the current industry benchmark.
Absorption Rate in AMP (amino-2-methyl-1-propanol)
AMP is sterically hindered primary amine with a pKa higher than that of MDEA.
Tests were also conducted to determine the impact of the temperature on kov values in enzyme enhanced 2 M MDEA solutions.
Impact of carbonic anhydrase was also evaluated in 1.45 M potassium carbonate solution at different CO2 loadings and enzyme concentration. Results in
Impact of carbonic anhydrase was also evaluated in 0.5 M sodium carbonate solution at different CO2 loadings (0, 0.2 and 0.5) and enzyme concentrations (0, 0.1 and 1.0 g/L). Enzyme used is 5X developed by CO2 Solution inc. Results in
Regarding delivery of the enzyme to the process, in one optional aspect the enzyme is provided directly as part of a formulation or solution. There may also be enzyme provided in a reactor to react with incoming solutions and gases; for instance, the enzyme may be fixed to a solid non-porous packing material, on or in a porous packing material, on or in particles or as aggregates flowing with the absorption solution within a packed tower or another type of reactor. The carbonic anhydrase may be in a free or soluble state in the formulation or immobilised on or in particles or as aggregates, chemically modified or stabilized, within the formulation. It should be noted that enzyme used in a free state may be in a pure form or may be in a mixture including impurities or additives such as other proteins, salts and other molecules coming from the enzyme production process. Immobilized enzyme free flowing in the solutions could be entrapped inside or fixed to a porous coating material that is provided around a support that is porous or non-porous. The enzymes may be immobilised directly onto the surface of a support (porous or non porous) or may be present as cross linked enzyme aggregates (CLEAs) or cross linked enzyme crystals (CLECs). CLEA include precipitated enzyme molecules forming aggregates that are then cross-linked using chemical agents. The CLEA may or may not have a ‘support’ or ‘core’ made of another material which may or may not be magnetic. CLEC include enzyme crystals and cross linking agent and may also be associated with a ‘support’ or ‘core’ made of another material. When a support is used, it may be made of polymer, ceramic, metal(s), silica, solgel, chitosan, cellulose, alginate, polyacrylamide, magnetic particles and/or other materials known in the art to be suitable for immobilization or enzyme support. When the enzymes are immobilised or provided on particles, such as micro-particles, the particles are preferably sized and provided in a particle concentration such that they are pumpable with the solution throughout the process.
When the enzymes are provided on micro-particles, the micro-particles may be sized in a number of ways. The micro-particles may be sized to facilitate separation of the micro-particles from the ion-rich mixture. For instance, the micro-particles may be sized to have a diameter above about 1 μm or above about 5 μm. The micro-particles may also be sized to have a catalytic surface area including the biocatalysts having an activity density so as to provide an activity level equivalent to a corresponding activity level of soluble biocatalysts above about 0.05 g biocatalyst/L, optionally between about 0.05 g biocatalyst/L and about 2 g biocatalyst/L. Furthermore, the absorption solution and the CO2 form a reactive liquid film having a thickness and the micro-particles may be sized so as to be within an order of magnitude of the thickness of the reactive liquid film. The micro-particles may also be sized so as to be smaller than the thickness of the reactive liquid film. The thickness of the reactive liquid film may be about 10 μm. In another optional aspect, the micro-particles are sized between about 1 μm and about 100 μm. It should also be noted that precipitates may be formed in the ion-rich solution and the micro-particles may be sized to be larger or heavier than the precipitates or to be easily separable therefrom. In some optional aspects of the process, the particles may be sized so as to be nano-particles. The micro-particles may also be provided in the absorption solution at a maximum particle concentration of about 40% w/w. In some optional aspects, the maximum micro-particle concentration may be 35% w/w, 30% w/w, 25% w/w, 20% w/w, 15% w/w, 10% w/w, or 5% w/w. The micro-particles may be composed of support material(s) that is at least partially composed of nylon, cellulose, silica, silica gel, chitosan, polystyrene, polymethylmetacrylate, alginate, polyacrylamide, magnetic material, or a combination thereof. The support may preferably be composed of nylon or polystyrene. The density of the support material may be between about 0.6 g/ml and about 3 g/ml.
Enzymes may also be provided both fixed within the reactor (on a packing material, for example) and flowing with the formulation (as free enzymes, on particles and/or as CLEA or CLEC), and may be the same or different enzymes, including carbonic anhydrase.
In some aspects, the carbonic anhydrase enzymes may be provided as chemically modified and/or stabilized. More particularly, in one embodiment, chemically modified and stabilized carbonic anhydrase enzymes are obtained following chemical modifications of charged groups at their surface. Such modifications change the overall residual surface charge and the hydrophobicity/hydrophilicity balance of the enzymes. These modifications can be operated on an enzyme by altering polar charged groups at its surfaces and result result in significant changes in conformational stability, resistance to denaturating agents and solvents, thermostability, substrate selection, catalytic efficiency, and/or others.
It should also be noted that “carbonic anhydrase” includes analogues thereof and includes naturally occurring, modified, recombinant and/or synthetic enzymes including chemically modified enzymes, enzyme aggregates, cross-linked enzymes, enzyme particles, enzyme-polymer complexes, polypeptide fragments, enzyme-like chemicals such as small molecules mimicking the active site of carbonic anhydrase enzymes and any other functional analogue of the enzyme carbonic anhydrase.
In some aspects, the carbonic anhydrase enzymes may be thermo-morphic enzymes.
The following general notation has been used herein:
AGL surface area of G/L interface [m2]
CA concentration of A [mol·m−3]
DA diffusion coefficient of A [m2·s−1]
EA enhancement factor [-
JA flux of A [mol·m2s
k2 second order reaction rate constant [m2·mol−1s
k2* enzyme enhanced reaction rate constant [m3·mols−1]
k3* [m·mol
·s−1]
k1* [m3·g1]
kL liquid side mass transfer coefficient·s−1]
kOV overall reaction rate constant [s
mA G/L distribution coefficient of A [-
P pressure [Pa]
R gas constant [8.3] J·mol−
·K
]
RA reaction rate of A [mol·ms−1]
T temperature [K
V volume [m3]
The following subscripts have also been used herein:
Eq equilibrium
G gas phase
inf infinite
L liquid phase
vap vapor
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CA2012/050393 | 6/11/2012 | WO | 00 | 12/5/2013 |
| Number | Date | Country | |
|---|---|---|---|
| 61495834 | Jun 2011 | US |
