Microwave-Accelerated Aqueous Solvent Regeneration using Microwave Absorbers for Carbon Capture

Abstract

A method of regenerating spent aqueous solvent using microwave absorbers featuring: adding a microwave absorber to a spent CO.sub.2 solvent solution, forming a mixture of microwave absorber material dispersed in the spent CO.sub.2 solvent solution, wherein the spent CO.sub.2 solvent solution is an aqueous solution of spent CO.sub.2 sorbent, wherein spent CO.sub.2 sorbent is a CO.sub.2 sorbent with CO.sub.2 absorbed thereon; and exposing the mixture to microwaves, resulting in desorption of absorbed CO.sub.2 from the CO.sub.2 sorbent, regenerating CO.sub.2 sorbent. A system for regenerating spent aqueous solvent having: a mixture having: an aqueous solution of a spent CO.sub.2 sorbent, wherein the spent CO.sub.2 sorbent is a CO.sub.2 sorbent with CO.sub.2 absorbed thereon; and a microwave absorber material mixed in the aqueous solution of spent CO.sub.2 sorbent, wherein the microwave absorber material is an electrical insulator that is configured to be polarized by an applied electric field.

Claims

1. A method of regenerating spent aqueous solvent using microwave absorbers for carbon dioxide capture applications comprising the steps of: adding a microwave absorber material to a spent CO.sub.2 solvent solution, forming a mixture comprising the microwave absorber material dispersed in the spent CO.sub.2 solvent solution, wherein the spent CO.sub.2 solvent solution comprises an aqueous solution of spent CO.sub.2 sorbent, wherein spent CO.sub.2 sorbent comprises a CO.sub.2 sorbent with CO.sub.2 absorbed thereon; and exposing the mixture to microwave radiation, resulting in desorption of some amount of absorbed CO.sub.2 on the CO.sub.2 sorbent, regenerating fresh CO.sub.2 sorbent.

2. The method of claim 1 wherein the spent CO.sub.2 solvent solution comprises an aqueous solution of a CO.sub.2 sorbent, the CO.sub.2 sorbent comprising an alkanolamine selected from the group consisting of monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), methyldiethanoamine (MDEA), 2-amino-2-methyl-1-propanol (AMP), and combinations thereof.

3. The method of claim 1 wherein the microwave absorber comprises an electrical insulator material that is dielectrically polarized by the microwave radiation in the exposing step.

4. The method of claim 3 wherein the microwave absorber material comprises a nanoscale carbon-based material.

5. The method of claim 4 wherein the nanoscale carbon-based material comprises graphene.

6. The method of claim 1 wherein the microwave absorber material comprises between about 0.5 wt % to about 5 wt % of the mixture.

7. The method of claim 2 wherein the spent CO.sub.2 solvent solution comprises between about 25 wt % and about 45 wt % CO.sub.2 sorbent.

8. The method of claim 1 wherein exposing the mixture to microwave radiation heats the mixture to a temperature between about 60 C. and about 90 C.

9. The method of claim 1 wherein the desorption of some amount of absorbed CO.sub.2 from the spent CO.sub.2 sorbent occurs at an activation energy of desorption of between about 20 KJ.Math.mol.sup.1 and about 28 KJ.Math.mol.sup.1.

10. A system for regenerating spent aqueous solvent using microwave absorbers for carbon dioxide capture applications comprising: a mixture comprising: an aqueous solution of a spent CO.sub.2 sorbent, wherein the spent CO.sub.2 sorbent comprises a CO.sub.2 sorbent with CO.sub.2 absorbed thereon; and a microwave absorber material mixed in the aqueous solution of spent CO.sub.2 sorbent, wherein the microwave absorber material comprises an electrical insulator that is configured to be polarized by an applied electric field.

11. The system of claim 10 wherein the CO.sub.2 sorbent comprises an alkanolamine selected from the group consisting of monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), methyldiethanoamine (MDEA), 2-amino-2-methyl-1-propanol (AMP), and combinations thereof.

12. The system of claim 10 wherein the microwave absorber material comprises a nanoscale carbon-based material.

13. The system of claim 12 wherein the microwave absorber comprises graphene.

14. The system of claim 10 wherein the mixture comprises between about 0.5 wt % and about 5 wt % microwave absorber.

15. The system of claim 14 wherein the aqueous solution of spent CO.sub.2 sorbent comprises between about 25 wt % and about 45 wt % CO.sub.2 sorbent.

16. The system of claim 10 wherein the mixture is configured to desorb CO.sub.2 from the CO.sub.2 sorbent occurs at an activation energy of desorption of between about 20 KJ.Math.mol.sup.1 and about 28 KJ.Math.mol.sup.1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The invention together with the above and other objects and advantages will be best understood from the following detailed description of the preferred embodiment of the invention shown in the accompanying drawings, wherein:

[0019] FIG. 1 is a flowchart for a method of regenerating spent aqueous solvent using microwave absorbers, in accordance with the features of the present invention;

[0020] FIG. 2A is a cross-sectional SEM image of coal-based graphene, in accordance with the features of the present invention;

[0021] FIG. 2B is a cross-sectional TEM image of coal-based graphene, in accordance with the features of the present invention;

[0022] FIG. 2C is a XPS result of coal-based graphene, in accordance with the features of the present invention;

[0023] FIG. 2D is an AFM image of coal-based graphene, in accordance with the features of the present invention;

[0024] FIG. 2E is a horizontal cross-section profile indicated by the horizontal line in the AFM image of coal-based graphene in FIG. 2D, in accordance with the features of the present invention;

[0025] FIG. 2F is a vertical cross-section profile indicated by the vertical line in the AFM image of coal-based graphene in FIG. 2D, in accordance with the features of the present invention;

[0026] FIG. 3A is a thermal image of coal-based graphene after 3 seconds of 2 Watts microwave irradiation, in accordance with the features of the present invention;

[0027] FIG. 3B is a thermal image of MWCNT after 3 seconds of 2 Watts microwave irradiation, in accordance with the features of the present invention;

[0028] FIG. 3C is a thermal image of graphite after 3 seconds of 2 Watts microwave irradiation, in accordance with the features of the present invention;

[0029] FIG. 3D is a thermal image of bituminous coal after 3 seconds of 2 Watts microwave irradiation, in accordance with the features of the present invention;

[0030] FIG. 3E is a thermal image of biochar after 3 seconds of 2 Watts microwave irradiation, in accordance with the features of the present invention;

[0031] FIG. 4A is a thermal image of deionized water after 50 W of microwave irradiation for 13 seconds in a sealed tube able to hold pressure up to 40 psi, in accordance with the features of the present invention;

[0032] FIG. 4B is a thermal image of 30 wt % MEA solution after 50 W of microwave irradiation for 13 seconds in a sealed tube able to hold pressure up to 40 psi, in accordance with the features of the present invention;

[0033] FIG. 4C is a thermal image of 1 wt % graphene in 30 wt % MEA solution after 50 W of microwave irradiation for 13 seconds in a sealed tube able to hold pressure up to 40 psi, in accordance with the features of the present invention;

[0034] FIG. 5 is a graph of temperature profiles of 30 wt % MEA solution and of 30 wt % MEA solutions mixed with 1 wt % of five different carbon-based materials under 15 W of microwave irradiation in a sealed tube able to hold pressure up to 40 psi, in accordance with the features of the present invention;

[0035] FIG. 6 is a simplified schematic of a system for CO.sub.2 capture and regeneration testing, in accordance with the features of the present invention;

[0036] FIG. 7A is a graph of CO.sub.2 desorption curves of 30 g of 30 wt % MEA with 1 wt % graphene solution with under 50 W microwave heating, in accordance with the features of the present invention;

[0037] FIG. 7B is a graph of CO.sub.2 desorption curves of 30 g of 30 wt % MEA solution (without graphene) under 50 W microwave heating, in accordance with the features of the present invention;

[0038] FIG. 7C is a graph of CO.sub.2 desorption curves of 30 g of 30 wt % MEA solution (without graphene) under oil-bath heating, in accordance with the features of the present invention;

[0039] FIG. 8 is plot of CO.sub.2 desorption flux in relation to temperature for several MEA regeneration methods, in accordance with the features of the present invention;

[0040] FIG. 9 is an Arrhenius curve of graphene-MEA microwave regeneration, in accordance with the features of the present invention.

DETAILED DESCRIPTION

[0041] The foregoing summary, as well as the following detailed description of certain embodiments of the present invention, will be better understood when read in conjunction with the appended drawings.

[0042] The following detailed description should be read with reference to the drawings in which similar elements in different drawings are numbered the same. The drawings, which are not necessarily to scale, depict illustrative embodiments and are not intended to limit the scope of the invention.

[0043] As used herein, sorbent means a material that absorbs another substance, wherein a sorbent is not limited to a particular state of matter.

[0044] FIG. 1 depicts a flowchart for a method 1 of regenerating spent aqueous solvent using microwave absorbers for carbon dioxide capture applications. The method 1 begins by adding a microwave absorber material to a spent CO.sub.2 solvent solution 2, forming a mixture comprising the microwave absorber material dispersed in the spent CO.sub.2 solvent solution.

[0045] In the first step of method 1 shown in FIG. 1, a microwave absorber material is added to a spent CO.sub.2 solvent solution 2. In an embodiment, the spent CO.sub.2 solvent solution comprises an aqueous solution comprising a spent CO.sub.2 sorbent, wherein the spent CO.sub.2 sorbent comprises a CO.sub.2 sorbent with CO.sub.2 absorbed thereon. The CO.sub.2 sorbent comprises any molecule or compound capable of absorbing CO.sub.2 while said CO.sub.2 sorbent is dissolved in an aqueous solution. Exemplary CO.sub.2 sorbents include alkanolamines such as, but not limited to, monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), methyldiethanoamine (MDEA), piperazine (PZ), 2-amino-2-methyl-1-propanol (AMP), and combinations thereof. In a further embodiment, the spent CO.sub.2 solvent solution to which the microwave absorber material is added comprises an alkali metal hydroxide, alkali metal carbonate, and combinations thereof. Exemplary alkali metal hydroxides include, but are not limited to, potassium hydroxide and sodium hydroxide. Exemplary alkali metal carbonates include, but are not limited to, potassium carbonate and sodium carbonate.

[0046] In an embodiment, the spent CO.sub.2 solvent solution comprises a concentration of CO.sub.2 sorbent preferably between approximately 25 wt % and approximately 45 wt % and typically approximately 30 wt %.

[0047] A salient feature of the invention is the microwave absorber material that is added to the spent CO.sub.2 solvent solution. The microwave absorber material described herein comprises any electrical insulator capable of being dielectrically polarized by an applied electric field. Consequently, these dielectric materials can dissipate electromagnetic waves by converting them into thermal energy through dielectric loss and magnetic loss. Exemplary microwave absorber materials include ferrite-based materials, including black iron oxide, nickel ferrite, cobalt ferrite, and Awaruite, Group IVB-VIB transition metal carbides and nitrides nanoscale carbon-based materials, including porous carbon, single walled carbon nanotubes, multi walled carbon nanotubes, carbon fibers, and graphene, and combinations thereof. Typical forms of the absorber include, but are not limited to, powders, flakes, nanotubes, nanowires, fibers, and sheets.

[0048] In an embodiment, in the adding step 2, microwave absorber material is added to the spent CO.sub.2 solvent solution such that the resulting mixture comprises a concentration of microwave absorber in the mixture preferably between approximately 0.5 wt % and approximately 5 wt %, more preferably between approximately 1 wt % and approximately 2 wt %, and typically between approximately 1 wt % and 1.5 wt %.

[0049] In an embodiment, the concentration of the microwave absorber material in the mixture with the spent CO.sub.2 solvent solution has negligible effect on the pumping behavior of the microwave absorber material-aqueous solvent mixture (slurry) when used in regeneration, reboiler, and stripping column infrastructure found in amine-based carbon capture processes.

[0050] Notably, in an embodiment, the microwave absorber material described herein lacks a porous micro-structure and consequently does not affect the adsorption/desorption kinetics of CO.sub.2 after mixing with the aqueous solvent.

[0051] In an embodiment, the microwave absorber material described herein comprises graphene. In an embodiment, graphene is inexpensively synthesized from coal and coal by-products using a thermal molten salt process described in U.S. Pat. No. 11,535,518B1, the entirety of which is incorporated by reference herein. By using this method and the natural graphene like molecules contained in coal and coal by-products, the costs and technical challenges associated with making graphene are markedly reduced.

[0052] Returning to FIG. 1, the method continues with exposing the mixture, the spent CO.sub.2 solvent solution with the microwave absorber material dispersed therein, to microwave radiation 3, resulting in release of CO.sub.2 absorbed on the CO.sub.2 sorbent, thereby regenerating fresh CO.sub.2 sorbent. During the exposing step 3, the microwave absorber material absorbs microwaves and thereby increases in temperature, wherein the heated microwave absorber material is in thermal contact with and heats the spent CO.sub.2 sorbent, resulting in the release of at least some of the CO.sub.2 absorbed on the spent CO.sub.2 sorbent to form a regenerated aqueous sorbent 3.

[0053] In an embodiment, the spent CO.sub.2 sorbent has CO.sub.2 absorbed thereon after said CO.sub.2 solvent solution was used to scrub an effluent gas stream. A person having ordinary skill in the art will readily ascertain that said effluent gas stream can be a gas stream from any chemical or industrial process producing CO.sub.2.

[0054] In the exposing step 3, as described above, the microwaves heat the spent CO.sub.2 solvent solution of the spent CO.sub.2 solvent solution and microwave absorber mixture such that some portion of CO.sub.2 absorbed onto said spent CO.sub.2 sorbent is released. In an embodiment, the exposing step 3 heats the spent CO.sub.2 solvent solution to a regeneration temperature, wherein the regeneration temperature comprises the temperature wherein CO.sub.2 absorbed onto the spent CO.sub.2 sorbent begins to release, and wherein said regeneration temperature is the temperature of the bulk spent CO.sub.2 solution of the spent CO.sub.2 solvent solution and microwave absorber mixture. Preferably, said regeneration temperature is between approximately 60 C. and approximately 90 C., more preferably between approximately 75 C. and approximately 85 C., and typically between approximately 80 C. and approximately 85 C. A person having ordinary skill in the art will readily appreciate that these regeneration temperatures are exemplary and not meant to be limiting. In an embodiment, the regeneration temperature is any temperature suitable to cause release of CO.sub.2 absorbed onto spent CO.sub.2 sorbent.

[0055] A salient feature of the invention is that the exposing step 3 generates hot spots on the microwave absorber that is in contact with the spent CO.sub.2 sorbent, providing sufficient heat to cause release/desorption of CO.sub.2, without raising the bulk temperature of the CO.sub.2 solvent solution and microwave absorber to temperatures of 100 C. or more. This is in contrast to prior art methods requiring additional time and energy to heat the bulk solution to regeneration temperatures of 100 C. or more. With this feature, in an embodiment, the invention provides lower activation energies for CO.sub.2 release/desorption from spent CO.sub.2 sorbent, of approximately 25 KJ/mol, and lower regeneration temperatures, of approximately 60 C. to 90 C., than prior art methods, where the bulk solution was heated to at or above 100 C., the boiling point of water. Additionally, with this feature, in an embodiment, the invention increases the energy efficiency of the CO.sub.2 sorbent regeneration process, where desorption kinetics of microwave heating of the spent CO.sub.2 solvent solution and microwave absorber was approximately 100 times and approximately 10 times faster than thermal heating the spent CO.sub.2 solvent solution and microwave heating the spent CO.sub.2 solvent solution without microwave absorber, respectively. Likewise, in some embodiments, the regeneration method can be used to reduce the amounts of make-up water required for the solvent regeneration process, as the solvent regeneration temperature, at less than approximately 100 C. (the boiling point of water), and resulting evaporative losses are reduced. In an embodiment, the invention reduces evaporative losses between approximately 30% and approximately 80% compared to prior art methods. In some embodiments, the invention reduces evaporative losses 80% where the regeneration temperature is 60 C. and 30% where the regeneration temperature is 90 C. compared to methods heating the bulk solution to 100 C. or more. Further, the regeneration method can be used to reduce corrosion to regeneration infrastructure due to reduced regeneration temperature, as temperature and corrosion rates are directly proportional in the presence of amines. Still further, the reduced regeneration temperature enabled using the instant method reduces amine loss from the CO.sub.2 sorbent over prior art CO.sub.2 sorbent regeneration processes.

[0056] In some embodiments, in the exposing step 3 as described above, the mixture, the spent CO.sub.2 solvent solution containing the microwave absorber material, is exposed to microwave radiation preferably for approximately 10 minutes to approximately 60 minutes, and typically for approximately 15 minutes to approximately 20 minutes.

[0057] In some embodiments, in the exposing step 3 as described above, the microwave radiation power is approximately 15W to approximately 50W. In an alternative embodiment, the microwave radiation power is approximately 20 KW to approximately 200 kW.

[0058] In an embodiment, in the exposing step 3 as described above, CO.sub.2 absorbed on the spent CO.sub.2 sorbent is released and thereby regenerates fresh CO.sub.2 sorbent, such that approximately up to 200 mL of CO.sub.2 is released at 85 C., whereas approximately less than 1 mL of CO.sub.2 is released was released at 85 C. using thermal heating only.

[0059] In some embodiments, the regeneration method using graphene microwave absorbers disclosed herein can be used to reduce oxidation of the amine functional groups and thus increase the service life of the solvent, as the bulk solvent is not exposed to the high temperatures reached during conventional steam regeneration.

[0060] In some embodiments, the regeneration method using graphene microwave absorbers disclosed herein can be used to accelerate bulk solvent temperature rise much more quickly than using conventional heating methods, increasing energy efficiency. Likewise, in some embodiments, the regeneration method can be used to accelerate CO.sub.2 desorption rates compared to that of conventional heating methods, increasing energy efficiency.

[0061] In some embodiments, the graphene-amine solvent mixture is heated using microwave pulses instead of continuous microwave heating to prevent degradation of the amine solvent from graphene hot-spots, instantaneous high-temperature graphene particles in the mixture.

[0062] In some embodiments, the regeneration method using graphene microwave absorbers disclosed herein can be retrofitted into existing amine-based recapture processes. In an embodiment, a microwave source and waveguide can be added to an existing regeneration column or tank, allowing for the retrofitting of graphene microwave absorbers into existing amine-based recapture processes and replacing conventional steam stripping infrastructure for solvent regeneration.

Examples

[0063] Coal-based graphene was prepared using a thermal molten salt process.

[0064] Other carbon-based materials, including powdered activated carbon (AC) was purchased from Cabot Corporation, and graphite, single-wall carbon nanotube (SWCNT), and multi-wall carbon nanotube (MWCNT), were purchased from Sigma Aldrich.

[0065] Graphene was studied to study the effect of carbon-based microwave absorber on CO.sub.2 sorbent regeneration. The quality of coal-based graphene was assessed by scanning electron microscopy (SEM) (FIG. 2A), transmission electron microscopy (TEM) (FIG. 2B), atomic force microscopy (AFM) (FIG. 2D), and X-ray photoelectron spectroscopy (XPS) (FIG. 2C). Both SEM and TEM images of the graphene clearly showed that sheet type 2D morphology was obtained. AFM measurement revealed that the exfoliated graphene flakes had a thickness as small as 1.6 nm, as shown in FIG. 2E and FIG. 2F, indicating few layered graphene morphology was obtained. According to the elemental composition analysis by X-ray photoelectron spectroscopy (XPS), impurities, i.e., Cl and Si, were less than 1%. The major components of in-house graphene were carbon (C1s, 92.9 at %) and oxygen (01s, 4.5 at %).

[0066] Microwave absorption tests of nanoscale carbon-based materials, including graphene and multi-walled carbon nanotubes (MWCNT) versus bulk carbon materials, including graphite, bituminous coal, and biochar, after irradiation with low power microwaves (2 W) for a short period of time (3 seconds) were performed. Both graphene (FIG. 3A) and commercial MWCNTs (FIG. 3B) exhibited similar high surface temperatures (>150 C.), indicating their strong microwave absorption capability. In contrast, graphite showed a moderate microwave absorption (FIG. 3C), while coal (FIG. 3D) and biochar (FIG. 3E) showed no obvious microwave absorption under the same low power microwave irradiation conditions.

[0067] Microwave absorption tests of graphene in aqueous solutions were performed. A thermal camera was used to record the temperature profiles of aqueous solutions under microwave irradiation. Three solutions, including deionized water, an aqueous MEA (30%) solution, and an aqueous MEA (30%) solution with 1 wt % graphene, were irradiated with 50 W of microwaves for 13 seconds in sealed tubes able to hold pressure up to 40 psi. The temperature of water reached 46.5 C. (FIG. 4A), and the temperature of MEA solution reached 60.2 C. (FIG. 4B). This moderate temperature difference between the two solutions was likely due to the increased polarity by MEA. As shown in FIG. 3A, graphene exhibited strong microwave absorption and rapid temperature ramping (>150 C. over 3 seconds under 2 W). By adding 1 wt % of graphene into MEA solution (without mixing), as shown in FIG. 4C, the surface temperature of graphene reached 104 C. and bulk solution temperature was 66 C. within 13 s. During the microwave heating process, graphene was rapidly heated, and heat was concurrently dissipated to the bulk MEA solution, which resulted bulk temperature rise. Therefore, adding small amount of a microwave absorber, including graphene, can effectively absorb microwave energy and accelerate bulk solution temperature rise.

[0068] Microwave absorption tests for an aqueous 30 wt % MEA solution and MEA solutions mixed with 1 wt % of five different carbon-based materials, including powdered activated carbon, graphite, SWCNT, MWCNT, and coal-based graphene, were performed under 15 W of microwave irradiation in sealed tubes able to hold pressure up to 40 psi (FIG. 5). All said carbon-based nanoscale materials exhibited superior capability of microwave absorption than the aqueous MEA solution alone. Coal-based graphene showed similar final temperature to that of graphite. Despite the maximum temperatures of SWCNT and MWCNT solutions reached above 160 C., the ramping rate was slower than the other carbon materials. All of the carbon-based nanoscale materials show potential as effective microwave absorbers capable of accelerating aqueous MEA-based CO.sub.2 sorbent regeneration.

[0069] CO.sub.2 capture and regeneration testing were performed using the system shown in FIG. 6. About 50 ml of aqueous MEA (30%) solution was added in a round-bottom flask 53 and was then exposed to 14% CO.sub.2/N.sub.2 (50 mL/min) 51 through mass flow controllers 52 until saturated. The composition of the tail gas was analyzed by a mass spectrometer (OmniStar GSD 320 Gas Analyzer) 58 in communication with a computer 59. Once the MEA solution was fully saturated, the flask was transferred to a microwave reactor (CEM Discover) 54 in communication with an IR temperature sensor 55 for desorption experiments. The magnetron's 56 microwave power output was controlled at 50 W, unless specified.

[0070] CO.sub.2 desorption testing was performed. Only graphene was tested for CO.sub.2 desorption experiments, because its 2D structure has negligible effects on CO.sub.2 desorption kinetics. CO.sub.2 desorption tests were performed for aqueous amine (MEA) solutions with and without graphene (1 wt %) at temperatures of 65 C., 75 C., and 85 C. For each test, using the system shown in FIG. 6, about 50 ml of aqueous MEA (30 wt %) solution was saturated by exposing to 14% CO.sub.2/N.sub.2 (50 mL/min for several hours). During the microwave desorption, the amount of CO.sub.2 in effluent gas was sent through a cold trap 57, measured by an online mass spectrometer 58, and plotted against time elapsed irradiated under microwave radiation, as shown in FIG. 7A and FIG. 7B. For comparison, conventional thermal heating (TH) desorption tests at 65 C. and 85 C. using a hot plate with silicone oil were also measured as a baseline, as shown in FIG. 7C.

[0071] Under 50 W MW irradiation, average CO.sub.2 desorption rates of the 30 wt % MEA solution were 1.7 mL/min, 3.6 mL/min, and 9.1 mL/min at 65 C., 75 C., and 85 C., respectively (FIG. 7A). By adding 1 wt % of graphene into solution, average CO.sub.2 desorption rates immediately increased at least 35% up to 2.3 mL/min, 7.6 mL/min, and 12.6 mL/min at 65 C., 75 C. and 85 C., respectively (FIG. 7B). These boosted desorption rates in the presence of graphene were primary due to better microwave energy absorption and the existence of hot-spots, instantaneous high-temperature graphene particles in the MEA solution. In an embodiment, CO.sub.2 desorption is promoted at the interface between the graphene particle hot-spots and the MEA solution without having to raise the bulk MEA temperature to the high temperatures typically required for regeneration using conventional heating. Hence, as the bulk temperature of the MEA solution does not need to be heated to high temperatures to promote CO.sub.2 desorption, the addition of graphene microwave absorbers to the MEA solution reduces oxidation of the amine functional groups in the MEA solvent and consequently increases the stability and the service life of the MEA solvent.

[0072] In contrast, several hours were required to desorb the same amount of CO.sub.2 by using a conventional heating (FIG. 7C). The average desorption rate at 65 C. and 85 C. was 0.064 mL/min and 0.1 mL/min, respectively, which was significantly lower than those of microwave heating. Further, in an embodiment, when microwave energy was turned off, graphene immediately cooled down without external heat exchangers. This would make the microwave temperature swing process much simpler than current steam (thermal) regeneration processes which require large reboilers and heat exchangers for cooling the solvent.

[0073] In an embodiment, based on calculations, the CO.sub.2 desorption flux of MEA under thermal heating, microwave heating and microwave heating with graphene at 85 C. was 0.004 min.sup.1, 0.29 min.sup.1 and 0.5 min.sup.1, respectively, as shown in Table 1. Desorption flux measures the gas desorption kinetics from the same amount of solvent. In an embodiment, CO.sub.2 desorption flux was defined as the amount of CO.sub.2 transported per unit time across a unit volume normal to the direction of transport. Thus, the microwave desorption flux of MEA with graphene was over two orders of magnitude faster than the desorption rate of MEA using thermal heating. Stated differently, in order to release 1 mmol of CO.sub.2 from 100 mL of MEA (30%) solution at 85 C., it would take approximately 1 hour using thermal regeneration, whereas it would take approximately only half a minute for graphene-MEA solution using microwave heating.

TABLE-US-00001 TABLE 1 Desorption of CO.sub.2 using thermal heating (TH), microwave heating (MW) with and without graphene at 85 C. Time Desorption flux * (min .Math. mmol.sup.1) (mL .Math. min.sup.1 .Math. mL.sub.solvent.sup.1) TH @85 C. 56.3 0.004 MW @85 C. 0.77 0.29 MW W/graphene @85 C. 0.45 0.50 * 30% MEA solution Density (@85 C.) = 1.0 g/mL

[0074] Likewise, in an embodiment, a higher CO.sub.2 desorption flux was demonstrated for microwave heating graphene-MEA solution compared to other regeneration methods, as shown in FIG. 8.

[0075] In an embodiment, as CO.sub.2 desorption is recognized as a pseudo-first order reaction, the microwave-accelerated CO.sub.2 desorption rate constants for a graphene-MEA solution at 65 C., 75 C., and 85 C. were calculated and are shown in Table 2. Likewise, in an embodiment, according to the Arrhenius plot of FIG. 9, the apparent activation energy of desorption of was about 25.6 KJ.Math.mol.sup.1. This activation energy of desorption value for graphene-MEA solution is lower than previously reported values of about 35-60 KJmol.sup.1 for aqueous amine solutions regenerated using conventional steam heating.

TABLE-US-00002 TABLE 2 Reaction rate constant and activation energy of MEA/graphene regeneration using microwave heating (MW) k/s.sup.1 65 C. 75 C. 85 C. E.sub.a/kJ .Math. mol.sup.1 MW W/ 0.044 0.053 0.075 25.6 graphene

[0076] In an embodiment, the low activation energy of desorption demonstrated above implies that CO.sub.2 desorption can be performed at a low bulk temperature, i.e., about 85 C. using aqueous MEA solvent solution with graphene and microwave heating. There are marked advantages in performing CO.sub.2 sorbent regeneration below the boiling point of water, i.e., 100 C., including, but not limited to, lower energy penalty for the regeneration process, lower make-up water required to replace evaporative losses, reduced duty on re-boiler and heat exchangers, reduced size of the stripping column, and lower operation costs and capital costs.

[0077] Having described the basic concept of the embodiments, it will be apparent to those skilled in the art that the foregoing detailed disclosure is intended to be presented by way of example. Accordingly, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or alterations and various improvements of the subject matter described and claimed are considered to be within the scope of the spirited embodiments as recited in the appended claims. Additionally, the recited order of the elements or sequences, or the use of numbers, letters or other designations therefor, is not intended to limit the claimed processes to any order except as may be specified. All ranges disclosed herein also encompass any and all possible sub-ranges and combinations of sub-ranges thereof. Any listed range is easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, tenths, etc. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, etc. As will also be understood by one skilled in the art all language such as up to, at least, greater than, less than, and the like refer to ranges which are subsequently broken down into sub-ranges as discussed above. As utilized herein, the terms about, substantially, and other similar terms are intended to have a broad meaning in conjunction with the common and accepted usage by those having ordinary skill in the art to which the subject matter of this disclosure pertains. As utilized herein, the term approximately equal to shall carry the meaning of being within 15, 10, 5, 4, 3, 2, or 1 percent of the subject measurement, item, unit, or concentration, with preference given to the percent variance. It should be understood by those of skill in the art who review this disclosure that these terms are intended to allow a description of certain features described and claimed without restricting the scope of these features to the exact numerical ranges provided. Accordingly, the embodiments are limited only by the following claims and equivalents thereto. All publications and patent documents cited in this application are incorporated by reference in their entirety for all purposes to the same extent as if each individual publication or patent document were so individually denoted.

[0078] All numeric values are herein assumed to be modified by the term about, whether or not explicitly indicated. The term about generally refers to a range of numbers that one of skill in the art would consider equivalent to the recited value (e.g., having the same function or result). In many instances, the terms about may include numbers that are rounded to the nearest significant figure.

[0079] The recitation of numerical ranges by endpoints includes all numbers within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5).

[0080] One skilled in the art will also readily recognize that where members are grouped together in a common manner, such as in a Markush group, the present invention encompasses not only the entire group listed as a whole, but each member of the group individually and all possible subgroups of the main group. Accordingly, for all purposes, the present invention encompasses not only the main group, but also the main group absent one or more of the group members. The present invention also envisages the explicit exclusion of one or more of any of the group members in the claimed invention.