Sorbent and method for carbon dioxide capture and recovery

Abstract

A functionalized carbon dioxide sorbent of a functionalized graphene oxide (fGO) substrate having substitution sites substituted with a functional group. The functional groups can be secondary or tertiary amines, a phosphate, a sulfonate or magnetite. The sorbent can have a binder intermixed with the fGO substrate, in the form of pellets, using a hydroxyethyl cellulose binder. A method using the functionalized sorbent provided captures a CO.sub.2 from a flue gas, by passing the flue gas containing moisture and a concentration of CO.sub.2 across the sorbent packed bed of the functionalized sorbent to adsorb selectively a portion of CO.sub.2 in the flue gas onto the fGO of the sorbent. The captured CO.sub.2 can be desorbed from the sorbent by exposure to a fluid at elevated temperature and/or reduced pressure conditions sufficient to desorb the CO.sub.2, and separating and concentrating the desorbed CO.sub.2 from the fluid. The functional moieties can be at least one of a primary and secondary amine, and a secondary function group of tertiary amines, phosphates, sulfonates and/or magnetite.

Claims

1. A method for capturing selectively carbon dioxide from a flue stream, the method comprising the steps of: a. providing a functionalized carbon dioxide sorbent that comprises a functionalized graphene oxide substrate having a plurality of substitution sites substituted with a plurality of functional groups, wherein the plurality of functional groups include (i) at least one of a primary amine and a secondary amine, and (ii) a tertiary amine; b. passing a flue gas containing moisture and a concentration of carbon dioxide (CO.sub.2) across the sorbent; and c. adsorbing selectively a portion of CO.sub.2 in the flue gas onto the fGO of the sorbent to reduce the concentration of the CO.sub.2 content of the flue gas, and generating a partially-saturated sorbent comprising adsorbed CO.sub.2.

2. The method according to claim 1 wherein the flue gas containing a concentration of carbon dioxide (CO.sub.2) is introduced into the packed bed of the sorbent at a temperature of about 40-80 C.

3. The method according to claim 2 wherein the temperature of the fluid exposed to the partially-saturated sorbent is about 70-125 C. to at least partially desorb CO.sub.2 from the partially-saturated sorbent.

4. The method according to claim 3, further including a step of exposing the partially-desorbed sorbent to a vacuum of about 0.1 bar or less to extract a further portion of CO.sub.2 remaining on the sorbent.

5. The method according to claim 1 wherein the sorbent is in the form of a pellet.

6. The method according to claim 5 wherein the sorbent pellet comprises a binder intermixed with the functionalized graphene substrate.

7. The method of claim 1, for releasing the captured carbon dioxide, further including the steps of: d. exposing the partially-saturated sorbent to a fluid at conditions of temperature and pressure sufficient, to desorb the adsorbed CO.sub.2 from the partially-saturated sorbent into the fluid; and e. separating and concentrating the desorbed CO.sub.2 from the fluid.

8. The method according to claim 1 wherein the plurality of functional groups further include one or more of a phosphate, a sulfonate, a nitrile, a hydroxyl, and magnetite.

9. The method according to claim 1 wherein the weight range of oxygen in the graphene oxide is from 5% to 50%.

10. A method for capturing selectively carbon dioxide from a flue stream, the method comprising the steps of: a. providing a packed bed of a functionalized carbon dioxide sorbent that comprises a functionalized graphene oxide (fGO) substrate having a plurality of substitution sites substituted with a plurality of functional groups, wherein the plurality of functional groups include (i) at least one of a primary amine and a secondary amine, and (ii) a tertiary amine; b. passing a flue gas containing moisture and a concentration of carbon dioxide (CO.sub.2) across the sorbent; and c. adsorbing selectively a portion of CO.sub.2 in the flue gas onto the fGO of the sorbent to reduce the concentration of the CO.sub.2 content of the flue gas, and generating a partially-saturated sorbent comprising adsorbed CO.sub.2.

11. The method according to claim 10 wherein the plurality of functional groups further include one or more of a phosphate, a sulfonate, a nitrile, a hydroxyl, and magnetite.

12. The method according to claim 11 wherein the flue gas containing a concentration of carbon dioxide (CO.sub.2) is introduced into the packed bed of the sorbent at a temperature of about 40-80 C.

13. The method according to claim 12 wherein the temperature of the fluid exposed to the partially-saturated sorbent is about 70-125 C. to at least partially desorb CO.sub.2 from the partially-saturated sorbent.

14. The method according to claim 13, further including a step of exposing the partially-desorbed sorbent to a vacuum of about 0.1 bar or less to extract a further portion of CO.sub.2 remaining on the sorbent.

15. The method according to claim 10 wherein the sorbent is in the form of a pellet.

16. The method according to claim 15 wherein the sorbent pellet comprises a binder intermixed with the functionalized graphene substrate.

17. The method of claim 10, for releasing the captured carbon dioxide, further including the steps of: d. exposing the partially-saturated sorbent to a fluid at conditions of temperature and pressure sufficient, to desorb the adsorbed CO.sub.2 from the partially-saturated sorbent into the fluid; and e. separating and concentrating the desorbed CO.sub.2 from the fluid.

18. The method according to claim 10 wherein the weight range of oxygen in the graphene oxide is from 5% to 50%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates the chemical structures of graphene oxide (GO).

(2) FIG. 2 illustrates the molar adsorption of CO.sub.2 on polyaniline-functionalized graphene oxide.

(3) FIG. 3 illustrates the molar adsorption of CO.sub.2 magnetite-(Fe.sub.3O.sub.4) substituted on multi-walled carbon nanotubes.

(4) FIG. 4 illustrates the chemical structure of a secondary amine.

(5) FIG. 5 illustrates various reactions, including (a) a reaction adding a nitrate to an aromatic ring via sulfuric acid; (b) a reaction adding a nitrile group to the ketone functional group in GO; (c) a reaction adding a sulfur to graphine oxide via sulfonation; (d) a phosphorylation reaction in which phosphoric acid reacts with a hydroxyl group; (e) the electronic density and the electrophilic carbon of CO.sub.2; and (f) the Arrhenius equation by which an sorbate-sorbent interaction follows.

DETAILED DESCRIPTION OF THE INVENTION

(6) Background on Graphene Oxide.

(7) The rationale for selecting graphene oxide (GO) as a start material is its chemical structure, shown in FIG. 1. GO can have three substitution groups at specific locations, with ether (COC) and hydroxyl (COH) groups on the basal plane and carboxyl groups (COOH) on the edges. Each functional group has a different rate of reactivity, which gives a degree of freedom to functionalize GO based on the strength of nucleophilic groups, for example, an amine, a sulfonate, a phosphate, and magnetite. These nucleophilic functional groups are selected due to their common usage in the specialty chemical industry, which can reduce the cost of the sorbent.

(8) Advanced Materials Design.

(9) A solid sorbent is used for carbon capture. By functionalizing graphene, specific gas species can be designed for adsorption over other gas species; that is, the functionalized graphene has high gas selectivity. In addition to high gas selectivity, the functionalizing of the GO material can be designed for rapid kinetics during adsorption of carbon dioxide, and low energy desorption of the captured carbon dioxide from the GO substrate and are selected to minimize operational costs.

(10) Process Design.

(11) The present invention provides a carbon-capture system that can be retrofitted to the flue gas emission facilities. This end of pipe deployment of carbon capture at post combustion affords minimal disruption in retrofitting the technology and requires the least amount of capex. In some embodiment, continuous operation is effected through a rotational carousel or packed bed system. An online and calibrated gas sensor provides feedback control to detect threshold CO.sub.2 concentrations. Capture cartridges are staged into the carousel and rotated into and out of operation positioning into the capture system. Once the CO.sub.2 threshold of the processed is exceeded, the cartridge is designated to be saturated. The saturated cartridge can be rotated out of, and a fresh cartridge is rotated into, the capture system. The saturated cartridge can be dismounted and a new or refreshed cartridge inserted into the carousel. To minimize design, project engineering, maintenance costs and wide serviceability, currently-available Commercially-Off-The-Shelf (COTS) components and systems are used.

(12) Carbon-Circular

(13) The gases selectively adsorbed into the saturated cartridges have high purity. The invention includes a process and system for desorption of the captured gases from the cartridge and volatilizing the released gases using mild heat, and condensing the volatilized gas for reuse and resale in enhanced oil recovery, refrigerant, supercritical solvent, or food and beverages markets. Once the captured CO.sub.2 on the cartridges have been desorbed, the refreshed cartridge can be returned to the capture system.

(14) Typically, the saturated cartridge comprising the fGO saturated with CO.sub.2 is heated to a temperature of 60-120 degrees C. to shift the equilibrium of the process to release the captured CO.sub.2, and capture and condense the released gas into liquid form.

(15) Once a functionalized GO has exceeded its regeneration cycle, the exhausted functionalized GO can be recovered and reprocessed (thermolysis) back to a graphene oxide that can be redeployed in cement production for cement strengthening, completing the lifecycle of returning carbon to the geosphere.

(16) Experiments

(17) The present invention provides a Direct Air Capture (DAC) technology or point-source carbon capture by utilizing a solid sorbent for capturing CO.sub.2, and using low-grade waste heat and/or low temperature to desorb the captured carbon and regenerate the sorbent, leveraging the excellent thermal conductivity of graphene, graphene oxide and reduced graphene oxide. Due to high electrical conductivity of graphene, highly efficient desorption can also be achieved through facile ultra-short time scale joule heating (see D. X. Luong, K. V. Bets, W. A. Algozeeb, M. G. Stanford, C. Kittrell, W. Y. Chen, R. V. Salvatierra, M. Q Ren, E. A. McHugh, P. A. Advincula, Z. Wang, M. Bhatt, H. Guo, V. Mancevski, R. Shahsavari, B. I. Yakobson, & J. M. Tour, Gram-scale bottom-up flash graphene synthesis, Nature, Jan. 27, 2020, the disclosure of which is incorporated by reference in its entirety) using solar-derived low-carbon electricity.

(18) In preferred embodiments of the carbon capture-desorption system, the techno-economic parameters are designed to provide CO.sub.2 desorption requirements of 0.96 GJ/ton at 60 C. Operationally, avoidance of fines in packed beds is important for continuous and safe operations. The powder form of functionalized graphene oxide will be extruded (<140 C.) to form pellets using hydroxyethyl cellulose as a binder. The length of pellets is targeted to be about 1 mm.

(19) A functionalized graphene oxide (fGO) is provided by introducing a uniform dispersion of NH.sub.2-moieties with GO at a controlled NH.sub.2-molarity, to provide a fGO with controlled physical adsorption and low energy desorption of CO.sub.2. The fGO will be achieved by mixing specific amine chemistries to anchor with graphene or graphene oxide in large volume liquid suspensions. Specifically, graphite oxide dispersed in water is sonicated with N-Hydroxysuccinimide and 1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide (Y. Liua, B. Sajjadib, W. Y. Chen, & R. Chatterjee, Ultrasound-assisted amine functionalized graphene oxide for enhanced CO.sub.2 adsorption, Fuel 247 (2019) pp. 10-18, the disclosure of which is incorporated by reference in its entirety) and mixed thoroughly. The GO will then be centrifuged out and rinsed with methanol and re-dispersed in methanol and ethylenediamine. The GO will then be centrifuged out, recovered and dried, and used to form pellets or coating. Adsorption can be evaluated using a thermogravimetric analysis as well as a packed bed reactor with a gas chromatograph/mass spectrometer at the outlet.

(20) The end result is fGO having a primary or secondary amine on GO. A secondary amine structure is illustrated in FIG. 4. If R2 is H, the functional group is a primary amine.

(21) A key in solid sorbent technology is the morphology and functionalization of graphene. The morphology in general should be high surface area, for example, twisted plane, number of graphene layers (fewer graphene layers correlating with a higher surface area per weight). Functionalization is predicated on the oxygen content of the graphene oxide. Also note that the mechanism for fGO's high carbon capacity is its high surface area, which affords multilayer adsorption and CO.sub.2 cluster. In other solid sorbents, the mechanism of adsorption is due to size exclusion principles and thus their capacity is limited to monolayer adsorption and non-clustering (See G. M. Meconi and R. Zangi, Adsorption-Induced Clustering of CO.sub.2 on Graphene, Physical Chemistry Chemical Physics, August 2020, the disclosure of which is incorporated by reference in its entirety).

(22) In a preferred embodiment, a polyaniline-functionalized GO (PANI-fGO), as described in A. K. Mishra and S. Ramaprabhu, Nanostructured polyaniline decorated graphene sheets for reversible CO.sub.2 capture, J. Mater. Chem., 2012, 22, 3708, the disclosure of which is incorporated by reference in its entirety (hereinafter Mishra) has a CO.sub.2 sorption capacity that is ten times that of typical CO.sub.2 sorbents, which are between 4-5 mmol CO.sub.2/gm of sorbent at similar pressures, as shown in FIG. 2 (FIG. 4 of Mishra). The effective mechanism in PANI-fGO is multilayer sorption due to the planar geometry of GO. In standard sorbents, their mechanism of adsorption is size exclusion in a monolayer adsorption. In another embodiments, magnetite-(Fe.sub.3O.sub.4) substituted on multi-walled carbon nanotubes (MWCNT) showed a similar 10 increase in adsorption over typical sorbents, as shown in FIG. 3 (FIG. 7 of Mishra).

(23) Production of Functionalized Graphene Oxide:

(24) There are several variants of fGO to maximize adsorption capacity and to minimize heat of desorption. They are 1) strength of the nucleophilic nitrogen in the amines, 2) length of the hydrocarbon chain to influence steric hindrance, and 3) the concentration of the carbonyl or hydroxyl groups (not the ether groups) on the GO. The first variant can be tuned by using primary) (1, secondary) (2 or tertiary) (3 amines. The strongest interaction is with the primary amines.

(25) The second variant, length of the hydrocarbon chain, also influences the strength of the electrophilic interaction from the carbon in CO.sub.2 to the amines. A shorter hydrocarbon chain increases steric hindrance, decreases interaction, and decreases heat of adsorption.

(26) The third variant, group concentration, addresses the specific capacity of CO.sub.2 adsorption. More functional groups directly increase the adsorption capacity which can be varied and optimized through the level of oxidation of graphene oxide.

(27) Using FIG. 1 to identify the individual molecular units, for example, benzene, ether, or carboxyl groups, of graphene oxide, one can conceive other nucleophilic groups attached to GO. For example, instead of amine groups, nitration is another pathway to add nucleophilic groups on the edges or on the basal plane. For example, a nitrate can be added to an aromatic ring via sulfuric acid to initiate an electrophile, illustrated in FIG. 5(a).

(28) Similarly, a nitrile group can be added to the ketone functional group in GO, as illustrated in the reaction shown in FIG. 5(b).

(29) In addition to the N (nucleophilic) of the amine groups, other elements in Groups 15 (Pnictogens) and 16 (Chalcogens) can serve as electron donors, and specifically phosphorus. Phosphorylation can treat the carbonyl groups on the GO to create phosphate functional groups. In Group 16, oxygen and sulfur can serve the function to donate electrons to the carbon in CO.sub.2. The oxygen is already there in GO in forms of ether, hydroxyl or carbonyl groups.

(30) Sulfur can be added to the GO via sulfonation, creating sulfate groups on the GO. Sulfonation is a common electrophilic aromatic substitution with H.sub.2SO.sub.4 (J. D. Roberts & M. C. Caserio, Basic Principles of Organic Chemistry 2nd Edition (1977), W. A. Benjamin, p. 1039), as shown in the reaction illustrated in FIG. 5(c)

(31) To apply the concept to GO, GO powder is added into fuming sulfuric acid to introduce the sulfonate groups on the basal plane and the edges of GO.

(32) Phosphorylation takes a different route. Since phosphoric acid is a weaker acid than sulfuric acid, it reacts with a hydroxyl group instead directly with a carbon-carbon bond, and proceeds as illustrated in the reaction shown in FIG. 5(d).

(33) In GO, the phosphate groups will react with the hydroxyl groups on the basal plane of GO.

(34) In summary, there are three pathways to functionalize GO that provide the strength of interaction, H.sub.ads, and the location of the functional groups (edges or basal plane). The implication is that the adsorption capacity is much higher due to the interaction of the basal plane and the edges of fGO. In addition, there are clustering effects and multilayer adsorption of CO.sub.2 on fGO further increase CO.sub.2 adsorption by an order of magnitude over size exclusion-based sorbents. See FIGS. 2 and 3.

(35) CO.sub.2 Capture Mechanism

(36) The carbon of CO.sub.2 is electrophilic and has an electronic density as shown in the diagram shown in FIG. 5(e).

(37) The functional groups named above are nucleophilic. The strength of interaction between the electrophilic carbon on the CO.sub.2 molecule and the functional groups on the fGO is the first order effect in determining the selectivity and desorption energetics of carbon capture. Because CO.sub.2 does not have a dipole, quadrupole interaction of the functional groups on GO and CO.sub.2 is the second order effect.

(38) Selectivity is a key attribute of the present invention as flue gases are a mixture of nitrogen gas (N.sub.2), CO.sub.2, carbon monoxide (CO), nitrous oxides (NOX), sulfurous oxides (SOx), and methane (CH.sub.4). For diesel exhaust gases, following a SOx scrubber, the volumetric CO.sub.2 concentration [CO.sub.2] is about 7% and the volumetric moisture concentration [H.sub.2O] is about 7%. The CO, NOx, SOx and H.sub.2O molecules would normally compete against CO.sub.2 for adsorption sites.

(39) There are four approaches to selectively separate these two reactants: 1) design the sorbent to be hydrophobic; 2) install a column, prior to the carbon capture unit, to adsorb moisture of the flue gas; 3) introduce OH groups on GO to selectively interact with H.sub.2O; or 4) leverage the reaction kinetics to favor CO.sub.2 versus H.sub.2O adsorption. Without being bound by any particular theory, it is believed that, on an adsorption versus time plot, CO.sub.2 adsorbs faster than H.sub.2O in the initial time phase. Over a longer time, H.sub.2O displaces the bound CO.sub.2 and achieves a higher amount of adsorption on a mass basis. The easiest pathways are the second or the third approaches. In the third approach, to introduce OH groups on GO to selectively interact with H.sub.2O, after the initial time phase as the [CO.sub.2]/[H.sub.2O] threshold has been achieved, the column from adsorption to desorption, and the absorption column is never permitted to proceed to later phases where the H.sub.2O would be permitted to displace the bound CO.sub.2.

(40) Regeneration.

(41) The present invention also provides for regeneration of the CO.sub.2-saturated fGO. The number of regeneration cycles is inversely proportional to the sorbent cost per capture cycle. Regeneration is based on optimizing the stability of the macro platelets of the GO, which is well-documented, with the degradation of functional groups over the thermal cycles. There are several standard thermomechanical industrial processes for regeneration-temperature swing, vacuum swing, or pressure swing. An optimal process will be based on the balance of the lowest capex and opex requirements at the requisite throughput.

(42) Adsorption

(43) In various embodiments, a form for the fGO comprises pellets disposed in a packed bed process. The use of a pellet form in a packed bed can provide a higher throughput through the packed beds, as commonly used in large scale catalytic chemical synthesis. The fGO powders are extruded, with binders such as hydroxypropyl cellulose, into 1-mm diameter cylinders. The pellet form is preferred to minimize dusting in operations.

(44) In preferred embodiments of the adsorption process, the flue gas is conditioned before passing through the packed bed. In various embodiments, the flue gas is cooled from a typically flue gas outlet temperature to an adsorb inlet temperature of about 40-80 C. The preferred temperature is toward the low end of the range to increase adsorption capacity of the fGO for CO.sub.2, which is a trade off with additional energy needed to cool the flue gas, typically being the result of combusted fuel, to the required low adsorption temperature.

(45) Typically, a fresh sorbent is a cylindrical pellet formed with a binder comprising hydroxyethyl cellulose and having a 1 mm diameter and a 5 mm length. The void fraction (porosity), , of the packed bed of sorbent is typically about 40%. A typical size of the bed column is 1 meter in diameter and 10 meters tall, with a superficial gas velocity of about 1 meter/sec. The outlet pressure is atmospheric.

(46) Desorption

(47) One or more of several process parameters can be used to rapidly regenerate the sorbent, which can include temperature change, temperature and vacuum changes, temperature and moisture changes, and electrical current changes. In general, energy is applied to dislodge a sorbate from the sorbent. The energies are in the form of heat (high temperature), mechanical (pressure or vacuum) or electrical current. The latter is not applicable to our process as it is not energy efficient in a gas-phase desorption process.

(48) It is well known that sorbate-sorbent interaction follows the Arrhenius equation (R. Nix, Kinetics of Adsorption, Ch.2.3, LibreText Chemistry, 2023) shown in FIG. 5(f). where H is the exothermic heat of adsorption. As temperature increases, the concentration of CO.sub.2 decreases on the sorbent. Hence, the first stage of desorption is through the application of heat. In various embodiments, an operating temperature during desorption is about 110 C., which is compatible with the temperature of waste heat generated in industrial sites. In some embodiments, the desorption is performed at 80 C. to desorb CO.sub.2 sufficiently. A second stage, via vacuum or pressure, is to extract a portion of CO.sub.2 remaining on the sorbent. In some embodiments, a vacuum pressure of about 0.1 bar is used during desorption of the remaining CO.sub.2 from the fGO sorbent.