System And Process For Capture Of Acid Gasses At Elevated-pressure From Gaseous Process Streams

Abstract

A system, method, and material that enables the pressure-activated reversible chemical capture of acid gasses such as CO.sub.2 from gas volumes such as streams, flows or any other volume. Once the acid gas is chemically captured, the resulting product typically a zwitterionic salt, can be subjected to a reduced pressure whereupon the resulting product will release the captures acid gas and the capture material will be regenerated. The invention includes this process as well as the materials and systems for carrying out and enabling this process.

Description

FIELD OF THE INVENTION

[0002] The present invention relates generally to organic solvents that perform (pressure activated) chemically selective capture of acid gases from gaseous product streams in the absence of water. More particularly, the invention is a system and process for capture of CO.sub.2 at elevated pressure from gaseous process streams.

BACKGROUND OF THE INVENTION

[0003] Acid gases such as carbon dioxide have been implicated as major and rapidly expanding contributors to climate change over the last decade. As such, significant effort has been applied to the capture and sequestration of carbon dioxide (CO.sub.2). CO.sub.2 capture from pre-combustion, post-combustion, and flue gas sources, as well as contained human living space environments (e.g., submarines). Many of these existing systems utilize aqueous solutions containing primary, secondary or tertiary alkanolamines such as monoethanolamine (MEA) or methyl diethanolamine (MDEA) that chemically react with CO.sub.2 and water to form thermally stable bicarbonate salts. However, aqueous solutions containing these capture agents have a low capture capacity (.about.7 wt %) and thus readily reach saturation. Additionally, these aqueous solutions are generally corrosive to steel and other common materials of construction. This corrosivity limits the alkanolamine concentration in water and requires the use of corrosion inhibitors. The limited alkanolamine concentration requires higher circulation rates and more energy expenditure for acid gas capture than would otherwise be necessary.

[0004] Physical absorbents are also commonly used as CO.sub.2 capture agents, but are known to have a low selectivity for CO.sub.2 unless CO.sub.2 pressures are very high and the gas stream has a large amount of CO.sub.2. These physical sorbents are often times irreversible or regenerable only after significant thermal or chemical treatment. Non-amine based capture agents including, e.g., polyethylene glycol (e.g., Selexol.RTM.), cryogenic methanol (e.g., Rectisol.RTM.), and N-methylpyrrolidone (e.g., Purisol.RTM.) also capture CO.sub.2 via physical adsorption by dissolution into the liquid. However, these sorbents typically suffer from low weight capture capacities (<10 wt %) and are typically used at total gas pressures near 600 psig (41.2 atm). See Fundamentals of Natural Gas Processing, Arthur Kidnay & William Parrish, CRC Press, Boca Raton, Fla. pages 100-104, 110-113. Accordingly, new approaches are needed that solve CO.sub.2 selectivity and capacity issues associated with conventional capture agents and adsorbent technologies. The present invention meets these needs.

SUMMARY OF THE INVENTION

[0005] The present invention provides a system, method and materials that enable the pressure-activated reversible chemical capture of acid gasses such as CO.sub.2 from gas volumes such as streams, flows or any other volume. Surprisingly, treating a dry gas stream using neat alkanolamines greatly increases the capture capacity of the amine and reduces the energy required for regeneration. In the case of CO.sub.2, contact with the resulting product is typically but not always limited to a zwitterionic salt, that can be subjected to a reduced pressure whereupon the resulting zwitterions decompose and release the captured CO.sub.2 thereby regenerating the alkanol to its original active state. Surprisingly the zwitterionic salt like analogous ionic liquids has a disproportionately high solubility for CO.sub.2 compared to aqueous solutions of alkanolamines, thus reducing the amount of these compounds need to capture a given quantity of CO.sub.2. This invention includes this process as well as the materials and systems for carrying out and enabling this process.

[0006] In one embodiment the process involves contacting a gaseous volume containing CO.sub.2 with at least one CO.sub.2 binding organic compound containing a neat (water free) tertiary alkanolamine that chemically binds CO.sub.2 to form a zwitterionic product at a pressure greater than ambient pressure, preferably greater than 100 psig; and removing the zwitterionic product from the gaseous stream or volume. If desired the zwitterionic product can then be subjected to a reduction in pressure to release the chemically bound CO.sub.2 and regenerate the CO.sub.2 binding organic compound.

[0007] The equipment used for the gas liquid contact to absorb the acid gases from the bulk gas stream is the same as that used conventionally for gas liquid contacting as is familiar to those skilled in the art. Examples are gas/liquid counterflow absorption vessels containing an arrangement of trays, packing material, spray nozzles, and liquid distributors. Other examples are concurrent contactors such as Venturi scrubbers, spray towers; compact devices such as Higee contactors, or emulsifiers. Thus, this invention can incorporate any systems that can be used for efficient gas liquid contact. Similarly, for fluid regeneration and separation of acid gas from the capture agent, pressure letdown valves, flash tanks, centrifugal devices, mist eliminators, and similar equipment used for separation of the acid gas from the liquid capture agent can be used.

[0008] Depending upon the exact desires of the user a variety of modifications and alterations to this general embodiment may be had. In one embodiment the binding organic compound is a liquid, selected from the group consisting of: N,N-Dimethylethanolamine (DMEA); N,N-Diethylethanolamine (DEEA), N,N-Diisopropylethanolamine (DIPEA); 2-(dimethylamino)-2-methyl-1-propanol (2-DMAM-PrOH); and combinations thereof. In some applications the zwitterionic product is an alkylcarbonate.

[0009] In another embodiment binding CO.sub.2 in said gaseous volume or stream includes using at least one CO.sub.2 binding organic compound containing a primary or secondary alcohol and a neat tertiary amine that when mixed, chemically binds CO.sub.2 in said volume or stream to form a zwitterionic alkylcarbonate product that removes the CO.sub.2 from the volume or stream.

[0010] A system for performing these methods includes at least one acid gas binding organic compound (in the absence of water) that forms a zwitterionic product when contacted by an acid gas at a pressure above ambient that chemically binds and removes the acid gas from the volume. In one embodiment of the invention the acid gas binding organic liquid is an amine, preferably an alkanolamine. Examples of acid gas binding organic compounds include tertiary alkanolamines like N,N-Dimethylethanolamine (DMEA); N,N-Diethylethanolamine (DEEA), N,N-Diisopropylethanolamine (DIPEA); 2-(dimethylamino)-2-methyl-1-propanol (2-DMAM-PrOH); or other types of alkanolamines. These materials form zwitterionic products such as salts and alkylcarbonates. In addition to these materials various alcohols and or polar/aprotic non-aqueous solvents such as but not limited to dimethylsulfoxide, dimethylformamide, acetone, may also be included.

[0011] A preferred example of the material utilized as the reversible acid gas capture agent has the structure: Wherein n is any carbon-based chain and R and R.sub.2 are any carbon-based chain or carbon containing alcohol. The reversible acid gas capture agent reverts between a non-ionic form in the absence of CO.sub.2 to an ionic alkylcarbonate in the presence of CO.sub.2 under elevated pressures. The general reaction is shown below:

##STR00001##

[0012] Various advantages and novel features of the present invention are described herein and will become further readily apparent to those skilled in this art from the following detailed description. In the preceding and following descriptions we have shown and described only one preferred embodiment of the invention, by way of illustration of the best mode contemplated for carrying out the invention. As will be realized, the invention is capable of modification in various respects without departing from the invention. Accordingly, the drawings and description of the preferred embodiment set forth hereafter are to be regarded as illustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIGS. 1a-1d show exemplary materials for use in system and process of the present invention.

[0014] FIG. 2 is a single component system showing chemical reaction between an exemplary tertiary alkanolamine and CO.sub.2 at elevated CO.sub.2 pressure.

[0015] FIG. 3 is a .sup.13C NMR spectrum of a DMEA solution showing formation of a zwitterionic DMEA-CO.sub.2 alkylcarbonate species.

[0016] FIG. 4 shows the chemical and physical uptake of CO.sub.2 in neat DMEA solution as a function of pressure.

[0017] FIG. 5 plots conductance values in DMEA solution as a function of CO.sub.2 pressure.

[0018] FIG. 6 plots the conductance of DMEA as a function of CO.sub.2 under repeated contact at pressures from 0 to 180 psi.

[0019] FIG. 7 shows a two component system involving reaction of an exemplary tertiary amine with a primary alcohol and CO.sub.2 at elevated CO.sub.2 pressure.

DETAILED DESCRIPTION

[0020] The following description includes the preferred best mode of one embodiment of the present invention. It will be clear from this description of the invention that the invention is not limited to these illustrated embodiments but that the invention also includes a variety of modifications and embodiments thereto. Therefore the present description should be seen as illustrative and not limiting. While the invention is susceptible of various modifications and alternative constructions, it should be understood, that there is no intention to limit the invention to the specific form disclosed, but, on the contrary, the invention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention as defined in the claims.

[0021] In one embodiment of the present invention a material, system and process for pressure reversible selective chemical binding of CO.sub.2 is described. This invention allows the CO.sub.2 to be chemically bound at a pressure greater than ambient (STP) conditions and to be released by lowering the pressure. This pressure swing release enables the capture material to be regenerated to future use in a much more simplistic way than in other applications that currently exist in the prior art. In one exemplary embodiment neat alkanolamines are utilized to form a low molecular weight hybrid (chemical and physical) CO.sub.2 scrubber that chemically captures CO.sub.2 and regenerates the capture agent using a pressure-swing, providing an attractive gas capture system from the vantage point of chemical selectivity, weight capacity, and non-thermal regeneration. The chemical selectivity provided by the invention for capture of CO.sub.2 is coupled with the ease and energy savings provided by pressure reversal for release and recovery of CO.sub.2.

[0022] The invention could be utilized in a variety of applications including natural gas sweetening (decontamination) and other CO.sub.2 scrubbing processes. Because CO.sub.2 scrubbing processes from natural gas operate under elevated pressures, e.g., from about 300 psi to about 1,000 psi. the ability to absorb CO.sub.2 at these elevated pressures combined with facile release at standard temperature and pressure (STP) constitutes a model technique for capture and recovery of CO.sub.2 from such sources. Further, the ability to release CO.sub.2 under pressure saves money for compression costs for sequestration. The invention provides the first pressure-reversible zwitterionic liquid that can provide direct replacement for conventional CO.sub.2 capture processes. Two embodiments of the invention are described hereafter.

[0023] In one embodiment, organic CO.sub.2 binding liquids containing neat tertiary alkanolamines include both amine and alcohol functionalities in a single structural moiety (i.e., single component systems). Single organic CO.sub.2 binding liquid systems are preferred over dual component systems described hereafter containing an amine and an alcohol as separate compounds due to their lower vapor pressures, which are better suited to industrial applications. However in other applications other configurations may be desired and appropriately created.

[0024] FIG. 1(a)-1(d) shows exemplary structure of tertiary alkanolamines that capture CO.sub.2 at elevated pressures. Exemplary tertiary alkanolamines include, but are not limited to, e.g., N,N-Dimethylethanolamine (DMEA), N,N-Diethylethanolamine (DEEA), N,N-Diisopropylethanolamine (DIPEA) and 2-(dimethylamino)-2-methyl-1-propanol (2-DMAM-PrOH). These liquid compounds are available commercially. Neat N,N-Dimethylethanolamine (DMEA) shows marked CO.sub.2 capture capacity and is expected to be an efficient CO.sub.2 capture agent for industrial applications.

[0025] FIG. 2 shows the reaction scheme of a single component system involving the chemical reaction between an exemplary tertiary alkanolamine (DMEA) with CO.sub.2 at elevated CO.sub.2 pressure in the absence of water. Ethanolamines tested in conjunction with the invention were purified via distillation and dried/stored over 3 .ANG. molecular sieves to remove water. To investigate the STP binding efficiency of ethanolamines, neat solutions of each were bubbled with CO.sub.2 for 1 hour. .sup.13C-NMR and conductivity experiments allow for quantitative and qualitative measure of DMEA absorption of CO.sub.2, both chemical and physical, as well as regeneration of the DMEA from the bound form (DMEA-CO.sub.2) upon simple depressurization.

[0026] FIG. 3 shows a typical .sup.13C-NMR spectrum of a DMEA solution showing formation of a zwitterionic DMEA-CO.sub.2 alkylcarbonate species, evidenced by peaks positioned at 125 ppm and between 156 and 158 ppm, respectively, which are attributed to: 1) dissolved CO.sub.2 and 2) a zwitterionic alkylcarbonate DMEA-CO.sub.2 moiety, respectively. The .sup.13C NMR spectrum of this solution shows peaks at 125 ppm and 156 ppm, which are attributed to dissolved CO.sub.2 and the zwitterionic alkylcarbonate DMEA-CO.sub.2 (shown in FIG. 2), respectively. Under STP conditions, none of these materials in the absence of water absorbed CO.sub.2, physically or chemically at standard temperature and pressure, as determined by gravimetric uptake and/or .sup.1H/.sup.13C NMR spectroscopy. At elevated pressures (100-500 psi), however, DMEA successfully captures CO.sub.2 via two modes simultaneously: chemical binding as the zwitterion, DMEA-CO.sub.2, and physical absorption.

[0027] FIG. 4 shows the chemical and physical wt % of CO.sub.2 uptake in neat DMEA solution as a function of pressure. TABLE 1 lists calculated values for chemical carboxylation and physical absorption as a function of pressure.

TABLE-US-00001 TABLE 1 Carbon dioxide uptake in neat DMEA at various pressures. CO.sub.2 Chemical Physical pressure absorption.sup.a absorption.sup.a,b Total Material (psi)/ (MPa) X.sub.CO2.sup.c wt. % X.sub.CO2.sup.c wt. % mole % wt. % DMEA 100/0.69 0.169 7.7% 0.060 2.9% 22.9% 10.6% 200/1.38 0.218 9.7% 0.145 6.7% 36.3% 16.4% 300/2.07 0.244 10.7% 0.204 9.1% 44.8% 19.9% 500/3.45 0.260 11.4% 0.191 8.6% 45.1% 20.0% .sup.acalculated from .sup.13C NMR integrations of pressurized reactions; values are the average to two experiments. .sup.bmoles or grams of physically absorbed CO.sub.2 divided by the sum of absorbed CO.sub.2, DMEA and DMEA-CO.sub.2. .sup.cmole fraction.

[0028] Chemical carboxylation was calculated by integration of the relative --CH.sub.2O-- carbons of DMEA-CO.sub.2 and DMEA, respectively. In the figure, formation of the DMEA-CO.sub.2 moiety increases as a function of applied gas pressure. Results in TABLE 1 show that DMEA chemically captures up to 7.7 wt. % carbon dioxide at pressures as low as 100 psi and 9.7, 10.7 and 11.4 wt. % at 200, 300 and 500 psi respectively. Physically absorbed CO.sub.2 also increases with increased gas pressure, exhibiting 2.9 wt. % at 100 psi to 6.7, 9.1 and 8.6 wt. % at 200, 300 and 500 psi, respectively. Because carbon dioxide shows relatively high solubility in ionic liquids and zwitterionic liquids, increasing the ionic nature of the DMEA/DMEA-CO.sub.2 moieties in solution at higher pressures may facilitate physical CO.sub.2 absorption.

[0029] As shown in TABLE 1 and in FIG. 4, at 100 psi the amount of physically absorbed CO.sub.2 is approximately one-third that of the chemically absorbed CO.sub.2. At elevated pressures, the same ratio (physical absorbed?) is two-thirds or higher. The combined chemical/physical CO.sub.2 capacity of DMEA is 10.6 wt. % at 100 psi followed by a significant jump to 16.4, 19.9 and 20 wt. % at 200, 300 and 500 psi, respectively. For reference, CO.sub.2 capacities were compared with CO.sub.2 capacities from conventional capture agents including, e.g., dimethyl (poly)ethylene glycol DEPEG, because of the similarity of DEPEG to SELEXOL.RTM.. TABLE 2 compares capture capacities for uptake of CO.sub.2.

TABLE-US-00002 TABLE 2 Comparison of the CO.sub.2 uptake capacities of DMEA and DEPEG, a Selexol .TM. derivative. CO.sub.2 Total CO.sub.2 Absorption pressure DMEA DEPEG.sup.a (psi) X.sub.CO2.sup.b wt. % X.sub.CO2.sup.b wt. %.sup.c 100 0.229 10.6% 0.18 3% 200 0.363 16.4% 0.29 5% 300 0.448 19.9% 0.37 7% 500 0.451 20.0% 0.55 13% .sup.aData taken/calculated from Gainar et al. (Fluid Phase Equilibr., 1995, 109, 281). .sup.bmole fraction. .sup.cEstimated from average molecular weight of mixture.

[0030] As shown in the table, at lower pressures (.ltoreq.300 psi), DMEA absorbs appreciably more CO.sub.2 than DEPEG per mole of solvent while at 500 psi DMEA shows evidence of an absorbance plateau. DMEA exhibits a substantial capacity advantage for CO.sub.2 over DEPEG (1.5.times. to 3.5.times.) at lower pressures. Thus mole capacities of DMEA rival those of DEPEG, a Selexol.RTM. derivative, at pressures .ltoreq.300 psi, while the weight capacities of DMEA is higher than those of DEPEG up to 500 psig. While results show DMEA is limited to .about.20 wt. % CO.sub.2 uptake, this feature adds to the utility of the material. As the zwitterionic salt remains dissolved in the DMEA solution (.about.3:1 ratio of DMEA:DMEA-CO.sub.2 at a chemical mole fraction of 0.26), overall solution viscosity remains relatively low such that the mixture can be pumped through capillary tubes with diameters as small as 300 .mu.m. Further, when DMEA/DMEA-CO.sub.2 solutions are depressurized, rapid decarboxylation occurs and the mixture cleanly and easily reverts to DMEA. This is evident by the disappearance of the alkylcarbonate and dissolved CO.sub.2 peaks and persistence of the DMEA signals in .sup.13C NMR spectroscopy. Thus, DMEA represents a CO.sub.2 sorbent which effectively absorbs CO.sub.2 both chemically and physically under pressure and successfully decarboxylates at STP to yield DMEA, avoiding the need for costly thermal regeneration.

[0031] Of the chemical and physical absorption of CO.sub.2 into DMEA, only chemical reaction leads to a solution whose conductance is significantly altered. Effect of chemical CO.sub.2 addition on solvent polarity was measured by the conductivity of DMEA over a CO.sub.2 atmosphere at various pressures. Anhydrous DMEA showed a conductance of 3 .mu.S/cm when introduced to a high-pressure conductance cell. The cell was pressurized with CO.sub.2 at 15 psi increments and the solution conductance was recorded.

[0032] FIG. 5 plots conductance values in a DMEA solution as a function of CO.sub.2 pressure. Diffusion and/or chemical addition of CO.sub.2 into DMEA proved slow at low pressures; at 15 psi, vigorous stirring for 18 hours was required to reach chemical equilibrium (i.e., established by an unvarying conductance). However, equilibrium was attained more rapidly at higher pressures, an observation attributed to physical CO.sub.2 saturation reached at lower pressures. At a pressure of from 45 psi to 60 psi, about 3.5 hours was required to reach equilibrium; less than 30 minutes was needed to reach equilibrium at pressures of 150 psi, 165 psi, and 180 psi. Conductance of the solution rose from 3 .mu.S/cm to 890 .mu.S/cm at CO.sub.2 pressures from 0 psi to 180 psi. At each pressure increase, temperature of the cell briefly increased by 3.degree. C. to 6.degree. C., indicating the chemical fixation of CO.sub.2 by DMEA is slightly exothermic or the heat of dissolution is exothermic. The significant increase in conductance as a function of pressure confirms that the interaction of DMEA with CO.sub.2 involves a chemical reaction to form an ionic species and not simply a physical dissolution.

[0033] The ability to regenerate DMEA was also tested. Anhydrous DMEA was carboxylated with CO.sub.2 at a pressure of 180 psi and depressurized over five cycles. FIG. 6 plots the conductance of DMEA for the repeated contacts with CO.sub.2 at pressures ranging from 0 to 180 psi. As shown in the figure, conductance of the DMEA:DMEA-CO.sub.2 solution for each cycle repeatedly reaches 890 .mu.S/cm (.+-.4 .mu.S/cm) at 180 psi and falls to 42 .mu.S/cm (.+-.3 .mu.S/cm) at 0 psi. A small residual conductance (42 .mu.S/cm) observed at the end of each cycle was reduced to 7 .mu.S/cm following an N.sub.2 purge of the solution, confirming that complete chemical decarboxylation can easy be achieved. These conductance measurements yield no discernable deterioration in the chemical CO.sub.2-binding uptake capacity of DMEA from repeated carboxylation/decarboxylation cycles. While this experiment does not unambiguously verify repeatable physical carbon dioxide uptake by DMEA, we surmise that gaseous dissolution remains unchanged after numerous cycles based on the chemical uptake repeatability. TABLE 3 compares carboxylation properties of DMEA, DEEA, DIPEA, and 2-DMAM-PrOH, respectively.

TABLE-US-00003 TABLE 3 Carbon dioxide uptake by anhydrous alkanolamines at elevated pressures (25.degree. C.). CO.sub.2 Chemical.sup.a Physical.sup.a,c Total Select .sup.13CNMR Material (psi). X.sub.CO2.sup.b wt. % X.sub.CO2.sup.b Wt. % wt. % signals (ppm).sup.d DMEA 300 0.244 10.7% 0.204 9.1% 19.8% 62.3 (--CH.sub.2OCO.sub.2--), 156.23 (--OC(O)O--) DEEA 300 0.209 7.2% 0.223 8.7% 15.9% 62.7 (--CH.sub.2OCO.sub.2--), 158.5 (--OC(O)O--) DIPEA 300 0.140 4% 0.430 12% .sup. 16% 66.1 (--CH.sub.2OCO.sub.2--).sub., 159.1 (--OC(O)O--) 2-DMAM-PrOH 300 --.sup.e --.sup.e --.sup.e --.sup.e --.sup.e 70.0 (--CH.sub.2OCO.sub.2--), 158.5 (--OC(O)O--) .sup.acalculated from .sup.13C-NMR integrations of pressurized reactions; values are the average to two experiments. .sup.bmole fraction. .sup.cmoles/grams of physically absorbed CO.sub.2 divided by the sum of absorbed CO.sub.2, chemically bound alkanolamine and free alkanolamine. .sup.dreferenced to dissolved CO.sub.2 set at 125 ppm except for 2-MDMA-PrOH. .sup.ecould not be determined; see text for details.

[0034] As shown in TABLE 3, DMEA shows significant CO.sub.2 uptake at pressures from 100 psi to 500 psi. Other alkanolamines listed in the table (e.g., DEEA, DIPEA and 2-DMAM-PrOH) exhibit distinctly different carboxylation properties under similar conditions.

[0035] Increasing the functionalization of the amine moiety with electron-donating substituents that increase basicity was found to decrease the chemical binding capacity for CO.sub.2. For example, results show chemical binding of CO.sub.2 decreases from DMEA to DEEA to DIPEA by from about 4 to 5 mole % each. While chemical binding of CO.sub.2 is observed for 2-DMAM-PrOH, CO.sub.2 pressurization of this low melting point (mp=19-20.degree. C.) liquid results in partial solidification that precludes accurate measurement of CO.sub.2 uptake capacities via NMR spectroscopy. To account for the decreasing chemical binding trend for CO.sub.2 of DMEA>DEEA>DIPEA, relative polarity effects of these solvents were considered. As described herein, chemical binding of CO.sub.2 by DMEA, DEEA, or DIPEA results in the formation of highly polar zwitterions, whereas the organic solvents themselves have relatively low polarities. Stabilization of the polar, highly charged zwitterions is thus impacted by the intrinsic polarity of the solvent medium. TABLE 4 lists absorption maxima for Reichardt's dye used as a molecular probe (given its acute absorption maximum sensitivity to small polarity changes) to assess polarity in the DMEA, DEEA, and DIPEA media measured using UV-VIS spectroscopy, along with absorption maxima in other common organic solvents.

TABLE-US-00004 TABLE 4 Absorption maxima of Reichardt's dye in alkanolamines DMEA, DEEA and DIPEA and select common organic solvents. Solvent TOL CH2Cl2 DIPEA CH3CN DEEA i-PrOH DMEA EtOH MeOH Reichardt's 806 692 624 617 605 592 580 550 516 dye, .lamda..sub.max (nm) -------------------- increasing polarity --------------------.fwdarw.

[0036] DMEA (with the smallest N-substituents) is the most polar of the selected alkanolamines, followed by DEEA, and then DIPEA, whose bulkier aliphatic N-substituents decrease the polarity of the solvent. The increasing chemical binding capacity for CO.sub.2 of DMEA>DEEA>DIPEA is attributed to more effective stabilization of the corresponding zwitterionic alkylcarbonate associated with the increasing solvent polarity. This stabilizing polarity effect overshadows the basicity effect of the alkanolamines, highlighting an important principle for these types of liquids with regard to chemical CO.sub.2 binding capacity. For physical uptake of CO.sub.2, the opposite trend is observed, with DIPEA>DEEA>DMEA. CO.sub.2 shows greater physical solubility in aliphatic, non-polar organic solvents than in polar media. Here the increased physical absorption of CO.sub.2 in DIPEA over the more polar DMEA and DEEA is attributed to the affinity of dissolved CO.sub.2 for non-polar organic solvents.

[0037] In another embodiment of the invention, a two component system for CO.sub.2 capture involves a tertiary amine (e.g., triethylamine) paired with a primary or a secondary alcohol at elevated pressures (above STP) to form ammonium alkylcarbonate ionic liquids, as shown in Equations [1] and [2]:

CO.sub.2+Base+ROHBaseH.sup.+ROCO.sub.2.sup.- [1]

K.sub.eq=[BaseH.sup.+][ROCO.sub.2.sup.-]/P.sub.CO2[Base][ROH][ 2]

[0038] Tertiary amines show little-to-no binding of CO.sub.2 in combination with alcohols at STP. Thus, captured CO.sub.2 can be easily stripped by depressurizing the system.

[0039] FIG. 7 shows the carboxylation of methanol with tertiary amines and other bases at elevated CO.sub.2 pressures, e.g., near 10 atm. TABLE 5 shows the reactivity of methanol with several exemplary tertiary amines and other bases at the elevated CO.sub.2 pressure of 10 atm.

TABLE-US-00005 TABLE 5 Carbonation of methanol with various tertiary amines IR (C.dbd.O) .sup.13C NMR pKa R.sub.3 Conversion.sup.a cm.sup.-1 ppm (DMSO) Triethylamine 75% 1654 52.3.sup.b 9.0 160.1.sup.c Diisopropylethylamine 92% 1647 51.1 18.6 158.8 (MeCN) DABCO.sup.d 64% 1650 49.1 8.9 160.0 DMAP.sup.e 64% 1651 52.4 160.5 DBU.sup.f 98% 1642 52.2 24 159.8 (MeCN) 2,6-Lutidine No reaction ~4 Pyridine No reaction 3.4 .sup.aConversion based on amine as determined by in situ .sup.13C NMR of 2M base in methanol under 10 atm CO.sub.2. .sup.bResonance of methyl carbon in CH.sub.3OC(O)O.sup.-. .sup.cResonance of carbonate carbon in CH.sub.3OC(O)O.sup.-. .sup.d1,4-diazabicyclo[2.2.2]octane. .sup.e4-(dimethylamino)pyridine. .sup.f1,8-Diazabicyclo[5.4.0]undec-7-ene.

[0040] Tertiary amines produced ammonium methylcarbonate salts at a high conversion (.about.159 ppm .sup.13C NMR). The alkylcarbonate peak is indicative of the chemical binding of CO.sub.2 (as compares with physical dissolution, which involves a CO.sub.2 peak at 125 ppm). At the pressures used in this study, there was also substantial dissolved CO.sub.2 observed in the .sup.13C NMR spectra. The strongest bases such as DBU and Diisopropylethylamine (Hunig's base) showed the highest conversion, followed by TEA, DABCO and DMAP. Hunig's base, which has the same basicity as TEA, has 17% more bound CO.sub.2. DABCO and DMAP bind less CO.sub.2 likely due to steric bulk. Lutidine and Pyridine showed no reactivity to form alkyl carbonates at this pressure most likely due to their much-reduced basicity compared to tertiary amines. Ammonium alkylcarbonate salts listed in TABLE 5 decompose back to the corresponding amine, methanol, and CO.sub.2 upon return of CO.sub.2 pressure to atmospheric conditions, however stronger bases such as DBU need thermal regeneration and do not decarboxylate upon reduction in pressure.

[0041] Carboxylations were also performed in MeCN solvent rather than methanol. TABLE 6 lists carbonation results of various alcohols with TEA and CO.sub.2 in MeCN.

TABLE-US-00006 TABLE 6 Carboxylation of various alcohols with triethylamine and CO.sub.2 R Conversion.sup.a CH.sub.3-- 75% CH.sub.3CH.sub.2-- 75% 1-octanol 25% ##STR00002## 65%.sup.b ##STR00003## 35%.sup.c i-Propanol 25% t-Butanol No Reaction Phenyl No Reaction a) Conversion based on NEt.sub.3. b) Initial product is ClCH.sub.2CH.sub.2OCO.sub.2-. This product cleanly cyclizes to ethylene carbonate. c) Reaction was performed in CH.sub.3CN. Only one hydroxyl is carbonated.

[0042] Primary and secondary alcohols readily convert to corresponding alkyl carbonates, whereas tertiary alcohols do not. Results are attributed to steric crowding of the alcohol. Data also show the degree of carboxylation of the alcohol decreases as the alcohol chain length increases and subsequently becomes less polar. The decrease in polarity is attributed to the lack of a polar solvent that can stabilize the transition states of the molecules during the carboxylation process. For example, trifluoroethanol (considered to have a steric bulk equivalent with that of ethanol, but with a much lower pKa) is unreactive toward CO.sub.2. Phenol doesn't carboxylate under these conditions, which is likely due to it being too acidic to bind CO.sub.2. Data in TABLES 5 and 6 suggest short linear alcohols and tertiary amines are preferred combinations for a high-pressure CO.sub.2 capture solvent system, but is not limited thereto.

[0043] A first challenge in designing neat trialkylamine and alcohol blends to perform capture in the absence of solvent is to use alcohols and bases that are non-volatile and to form liquid ammonium alkylcarbonates (not solids as in the case of methanol) that are cheap. We set out to find a non-volatile tertiary amine and alcohol that would mitigate material loss and improve costs. The amine and alcohol were bifunctionalized to make them less volatile. As CO.sub.2 is introduced over an organic molecule, it causes volumetric expansion and a decrease in polarity as the mole fraction of CO.sub.2 increases. CO.sub.2 binding requires a highly polar medium to stabilize the polar transition states and the zwitterionic alkylcarbonate. Polarity data measured for alkanolamines demonstrates that a CO.sub.2 pressure near 150 psi decreases the polarity. A drop in polarity promotes dissolution of CO.sub.2 into the alkanolamine, not the desired chemical binding. DMEA however is sufficiently polar to stabilize these polar species and subsequently is a good candidate to react with CO.sub.2 at a low pressure (i.e., 150 psi) condition.

[0044] Decomposition of the alkylcarbonate salts by depressurization is highly advantageous for high-pressure CO.sub.2 gas capture as the pressure swing avoids use of an energy intensive thermal solvent regeneration cycle. The demonstrated pressure desorption of the chemically bound CO.sub.2 from ammonium alkylcarbonates parallels the energy requirements for the release of physically absorbed CO.sub.2 by industrial materials such as SELEXOL.RTM. and RECTISOL.RTM.. These high-pressure anhydrous alkanolamines can potentially be superior to physical sorbents because they contain the economical pressure swing yet they contain a highly chemically selective CO.sub.2 capture.

[0045] The following example provides a further understanding of the invention. Dissolved CO.sub.2 appears at 125 ppm in .sup.13C NMR spectra in multiple organic solvents while the alkylcarbonate (R--O--CO.sub.2.sup.-) peak appears at 158 ppm. The carbonyl signal is attributed to the alkylcarbonate, as carbamates do not form for the tertiary amine. While primary and secondary amines can and do react with carbon dioxide to yield carbamates, tertiary amines do not react directly with CO.sub.2, although bicarbonate salts can form in the presence of water. .sup.13C NMR chemical shifts confirm that chemical CO.sub.2 binding to an alkanolamine (in the absence of water) proceeds via the alcohol moiety of DMEA as opposed to the tertiary amine. The appearance of a resolved --CH.sub.2O-- methylene signal at 63 ppm, downfield from the alcohol methylene of DMEA, is indicative of the effects of .beta.-carboxylation as opposed to N-carboxylation. To investigate the extent to which DMEA will absorb CO.sub.2, neat solutions of DMEA were pressurized at 100, 200, 300 and 500 psi of CO.sub.2 for 18 hours, loaded into a PEEK NMR tube and analyzed. Carboxylation experiments at higher pressures were not performed as CO.sub.2 begins to liquefy above 500 psi. Under these conditions CO.sub.2 is likely to phase separate from the DMEA/DMEA-CO.sub.2 mixture and hamper NMR interpretation. Both DMEA-CO.sub.2 and dissolved CO.sub.2 were observed at all pressures and the relative quantities of each calculated from .sup.13C NMR integrations. Extent of both chemical carboxylation and physical absorption as a function of pressure were calculated (see TABLE 1 and FIG. 4) using the relative integration of the --CH.sub.2O-- carbons of DMEA-CO.sub.2 and DMEA. Physical absorption was found by comparing the relative integration of the carbonyl carbons of CO.sub.2 and DMEA-CO.sub.2.

[0046] Neat solutions of alkyl ethanolamines (DMEA, DEEA, and DIPEA) absorb and chemically bind carbon dioxide at elevated pressures by formation of alkylcarbonates. Through both chemical binding and physical absorption DMEA captures up to 45 mole % (20 wt. %) carbon dioxide, while DEEA captures up to 43 mole % (16 wt. %) and DIPEA captures up to 57 mole % (16 wt. %) carbon dioxide (300 psi). The increasing chemical uptake capacity trend of DMEA>DEEA>DIPEA is attributed to solvent polarity effects while the physical CO.sub.2 absorption trend of DIPEA>DEEA>DMEA is explained by the affinity of carbon dioxide for non-polar organic media. DMEA shows the greatest wt. % uptake of carbon dioxide and chemically binds CO.sub.2 under pressure more effectively than the other tertiary ethanolamines to form the thermodynamically unstable zwitterionic alkylcarbonate salt DMEA-CO.sub.2. DMEA captures up to 45 mole % (20 wt. %) of CO.sub.2 at 500 psi via combined chemical binding and physical absorption. Carbon dioxide weight capacities of DMEA rival those of DEPEG, a SELEXOL.RTM. derivative, at pressures 300 psi. DMEA-CO.sub.2, DEEA-CO.sub.2 and DIPEA-CO.sub.2 are characterized by high-pressure .sup.13C NMR and give rise to .sup.13C resonances analogous to previously studied zwitterionic alkylcarbonates. The zwitterion DMEA-CO.sub.2 regenerates CO.sub.2 and DMEA upon depressurization. This natural decarboxylation when pressure is release is advantageous for high-pressure gas-capture systems as the sorbent can be regenerated by an economical pressure swing as opposed to a more costly thermal swing. Repeated CO.sub.2 absorption/release experiments show no decline in the chemical binding CO.sub.2 uptake capacity of DMEA over 5 cycles.

[0047] Tertiary amines combined with alcohols chemically and selectively bind CO.sub.2 under mild pressures to form thermodynamically unstable alkylcarbonate salts. The carboxylations of numerous amine and alcohol pairs can be tracked in situ using IR and NMR spectroscopy. Alkanolamines also capture CO.sub.2 under elevated pressures as zwitterionic alkylcarbonate salts. The degree of alcohol carboxylation is limited by the polarity of the solvent as well as the basicity of the amine. Ammonium alkylcarbonate salts decarboxylate into their corresponding alcohols and amines unless under pressures near 10 ATM. This natural decarboxylation when pressure is released is advantageous for high-pressure gas-capture systems because the sorbent can be regenerated by an economical pressure swing instead of the more costly thermal swing.

[0048] While preferred embodiments of the present invention have been shown and described, it will be apparent to those of ordinary skill in the art that many changes and modifications may be made with various material combinations without departing from the invention in its true scope and broader aspects. The appended claims are therefore intended to cover all such changes and modifications as fall within the spirit and scope of the invention.

Claims

1. A method for removing CO.sub.2 from a gaseous volume comprising the steps of: contacting a dry gaseous volume containing CO.sub.2 with at least one CO.sub.2 binding organic compound containing a neat (anhydrous) tertiary alkanolamine that chemically binds CO.sub.2 to form a zwitterionic product at a pressure greater than 100 psi and removing said zwitterionic alkylcarbonate product from said gaseous stream or volume.

2. The method of claim 1 further comprising the step of depressurizing said zwitterionic alkylcarbonate to release said chemically bound CO.sub.2 and regenerate said CO.sub.2 binding organic compound.

3. The method of claim 1, wherein said binding organic compound is a liquid.

4. The method of claim 1, wherein said tertiary alkanolamine is selected from the group consisting of: N,N-Dimethylethanolamine (DMEA); N,N-Diethylethanolamine (DEEA), N,N-Diisopropylethanolamine (DIPEA); 2-(dimethylamino)-2-methyl-1-propanol (2-DMAM-PrOH); and combinations thereof.

5. The method of claim 1, wherein the zwitterionic product is an alkylcarbonate.

6. A method for chemically binding and removing CO.sub.2 from a gaseous volume comprising the step of: binding CO.sub.2 in said gaseous volume using at least one CO.sub.2 binding organic compound containing a neat tertiary amine that when mixed with a primary or secondary alcohol chemically binds CO.sub.2 in said volume to form a zwitterionic alkylcarbonate product that removes the CO.sub.2 from said volume or stream.

7. A system for removal of CO.sub.2 from a gaseous volume or stream, characterized by: at least one CO.sub.2 binding organic compound comprising a neat tertiary alkanolamine that forms a zwitterionic product when contacted by CO.sub.2 at a pressure above ambient that chemically binds and removes CO.sub.2 from said volume.

8. The system of claim 7, wherein the total system pressure is 100 psi.

9. The system of claim 7, wherein the tertiary amine is a tertiary ethanolamine selected from the group consisting of: N,N-Dimethylethanolamine (DMEA); N,N-Diethylethanolamine (DEEA), N,N-Diisopropylethanolamine (DIPEA); 2-(dimethylamino)-2-methyl-1-propanol (2-DMAM-PrOH); and combinations thereof.

10. The system of claim 7, wherein said binding organic compound is a liquid.

11. The system of claim 7, wherein said binding organic compound further comprises a polar/aprotic non-aqueous solvent of up to about 50 wt % selected from the group consisting of: dimethylsulfoxide, dimethylformamide, acetone, and combinations thereof.

12. The system of claim 7, wherein the zwitterionic product is an alkylcarbonate.

13. A pressure swing reversible acid gas capture agent having the structure: wherein n is any carbon based chain and R is a carbon based chain or a carbon containing alcohol in the absence of water. ##STR00004##

14. A Method for removing acid gasses from a gaseous volume characterized by the step of: contacting a gaseous volume containing an acid gas with at least one acid gas binding organic compound that chemically binds with the acid gas to form a zwitterionic product at a pressure greater than 100 psi; removing said zwitterionic product from said gaseous stream or volume; and exposing said zwitterionic product to a lower pressure to release said acid gas and regenerate said acid gas binding organic compound.