Patent Application
20240342685
Carbon capture and storage (CCS) is considered a critical component of negative emission technologies (NETs) for achieving economy-wide carbon neutrality to mitigate climate change and limit global temperature increase. Burning fuels (e.g., coal, methane, oil, and biomass) by combustion produces a flue gas containing a mixture of gaseous (primarily N2, CO2, and H2O) and particulate matter (PM) components as byproducts. Removal of CO2 can be undertaken after the standard pollution controls. Yet, the separation of CO2 from flue gas via CO2 capture processes is challenging because a high volume of gas must be treated, the CO2 is dilute, the flue gas is at atmospheric pressure, trace impurities can degrade capture media, and the captured CO2 must be compressed.
In principle, the gas separation technologies which are currently used in the chemical industry, such as absorption in chemical solvents, can be adapted for post-combustion capture (PCC) of CO2. A 30 wt % aqueous solution of monoethanolamine (MEA) is considered the benchmark solvent for regenerative chemical absorption-based PCC. The process relies on aqueous phase chemistry between an organic base (amines) and acid gas (CO2), and is by far the most heavily studied, technologically mature, and economically viable approach.
A major drawback of amine-based systems is that they are energy intensive and thus significantly increase operational costs. The benchmark for new and emerging technologies in terms of cost, energy penalty, CO2 capture efficiency, and physicochemical properties is MEA. CO2-loaded MEA solutions are also corrosive to process infrastructure and degrade rapidly. Inevitably, some of these will be lost to the atmosphere where they react to form toxic compounds. In addition, amines contain nitrogen so require nitrogen fixation to manufacture, which is very energy intensive, countering the whole point of carbon capture. MEA also requires high solvent circulation rates, which leads to large equipment sizes and high energy consumption, making the technology capital and operating expenditures unacceptably high. There is a need for cost-effective alternative solvents to reduce the energy penalty and costs of CO2 capture with absorbents for wide-scale commercial deployment of PCC-based CCS technologies.
The present disclosure is predicated on the discovery that bioinspired carbon capture compounds can replace amines in CCS processes and systems. The present inventors built upon a regenerative chemical absorption carbon capture system utilized by a unique group of plants that have adapted to arid climates using a process called Crassulacean Acid Metabolism (CAM) where, to conserve water, the plants uptake CO2 by opening their stoma at night, rather than the day, and then immediately react it with another compound, phosphoenolpyruvate (PEP), to store the CO2 overnight. The reaction is reversed in the morning when the sun rises, releasing the CO2 to participate in photosynthesis. For the present disclosure, PEP serves as a template for designing similar molecules for use as CO2 solvents with thermochemical properties that address one or more of the drawbacks of MEA, facilitating CCS technologies, including in PCC absorber-stripper systems.
Embodiments of the present disclosure describe PEP variants possessing CO2 capture activity. The PEP variants described herein can be included in compositions for use as carbon capture solvents and devices/systems configured to capture, and optionally to release, CO2. Methods of capturing CO2, including contacting a CO2-containing gas stream (e.g., flue gas) with a PEP variant or composition thereof, and optionally releasing the captured CO2, are also described herein.
Accordingly, a first aspect of the present disclosure includes an absorbent compound for capturing carbon dioxide (CO2) having a chemical structure represented by Formula I:
In one or more embodiments of the first aspect, R is CH2 and R′ includes an alkene, an alkyne, or an aryl group, or R is a methyl group and R′ is a carboxylate group. X can be a phosphono or sulfo group, or a conjugated base thereof. R′ can be a carboxyl group or a carboxylate group and R is a hydrogen, a hydroxyl group, an oxo group, a carboxyl group or a carboxylate group, optionally the compound is phosphoenol-buten(3,4)one, phosphoenol-3-methylene-but-1-ynone, phosphoenol-keto-ethenyl-benzene, or phosphoenol-2-oxo-hex(4,5)enoate. R can be (CHaYb)d, where a is 2, 1 or 0, b is 1 or 0, and b+a<2, Y is selected from OH, CHOH, C(═O)OH, O, CH2, CHCH3; and d is from 1 to 6; optionally the compound is 2-hydroxy-2(phosphonooxy)acetic acid, 2-(phosphonooxy)acetic acid, 2-(phosphonooxy)malonic acid, 2-oxo-2-(phosphonooxy)acetic acid or (E)-3-hydroxy-2-(phosphonooxy)acrylic acid.
A second aspect of the present disclosure features a CO2 capture composition comprising the absorbent compound of any one of the embodiments of the first aspect, or any combination of embodiments of the first aspect and a carrier, vehicle, diluent or solid support. The composition can include about 0.1% to 90% or about 0.5% to 60% the absorbent compound of Formula I, by weight. The composition can include at least 25% the absorbent compound, by weight, such as about 30% to about 60% by weight, about 35% to about 55% by weight, or about 40% to about 50% by weight. The liquid carrier, vehicle or diluent can be selected from the group consisting of water, saline, buffered solutions, hydroalcoholic mixtures, and alcohols.
In a third aspect, the present disclosure features a method of capturing carbon dioxide (CO2) from a gas stream containing CO2, the method comprising: contacting a CO2-containing gas stream with an absorbent, the absorbent reversibly reacting with CO2 in a reaction to capture CO2 from the gas stream, wherein the absorbent has a chemical structure represented by Formula I:
The method of the third aspect can further include releasing the captured CO2 from the absorbent via a stripping process. Releasing the captured CO2 can be performed by reversing the reaction of the absorbent and CO2. Releasing the captured CO2 can be performed in the absence of enzymes. The stripping process can be performed by a stripper and operating the stripper can includes at least one of: increasing an operating temperature inside the stripper, decreasing the operating temperature inside the stripper, increasing an operating pressure inside the stripper, and decreasing the operating pressure inside the stripper. The method can further include increasing a temperature of the CO2-containing absorbent, after the capturing step and prior to the releasing step, using heat from the absorbent after the absorbent exits the stripper.
In a fourth aspect, the present disclosure features a device for capturing carbon dioxide (CO2) from a gas stream containing CO2, the device comprising: an absorbent; and an absorber comprising:
In some embodiments of the fourth aspect, R is CH2 and R′ is an alkene, an alkyne, or a aryl group, or R is a methyl group and R′ is a carboxylate group. X can be a phosphono or sulfo group, or a conjugated base thereof, R′ can be a carboxyl group or a carboxylate group and R can be a hydrogen, a hydroxyl group, an oxo group, a carboxyl group or a carboxylate group. In some cases, the absorbent includes phosphoenol-buten(3,4)one, phosphoenol-3-methylene-but-1-ynone, phosphoenol-keto-ethenyl-benzene, or phosphoenol-2-oxo-hex(4,5)enoate. R can be (CHaYb)d, where a is 2, 1 or 0, b is 1 or 0, and b+a<2, Y is selected from OH, CHOH C(═O)OH, O, CH2, CHCH3; and d is from 1 to 6, with the proviso that if R is CH2, R′ is not carboxylate. The absorbent can include 2-hydroxy-2(phosphonooxy)acetic acid, 2-(phosphonooxy)acetic acid, 2-(phosphonooxy)malonic acid, 2-oxo-2-(phosphonooxy)acetic acid or (E)-3-hydroxy-2-(phosphonooxy)acrylic acid. The absorbent composition can include a carrier, vehicle, or diluent selected from the group consisting of water, saline, buffered solutions, hydroalcoholic mixtures, and alcohols.
In a fifth aspect, the present disclosure features a system comprising the device of any embodiment or combination of embodiments of the fourth aspect, and a stripper configured to receive steam and the CO2-containing absorbent to reverse the reaction between CO2 and the absorbent thereby releasing CO2 from the absorbent, wherein the absorbent can be recirculated back to the absorber for re-use. The system can further include a heat exchanger configured to circulate the absorbent exiting the stripper and the CO2-containing absorbent exiting the absorber, wherein the absorbent exiting the stripper rejects heat to the CO2-containing absorbent. In some cases, operation of the stripper includes at least one of: increasing an operating temperature inside the stripper, decreasing the operating temperature inside the stripper, increasing an operating pressure inside the stripper, and decreasing the operating pressure inside the stripper.
The details of one or more examples are set forth in the description below. Other features, objects, and advantages will be apparent from the description and from the claims.
This written disclosure describes illustrative embodiments that are non-limiting and non-exhaustive. In the drawings, which are not necessarily drawn to scale, like numerals describe substantially similar components throughout the several views.
Reference is made to illustrative embodiments that are depicted in the figures, in which:
Embodiments of the present disclosure describe PEP variants possessing CO2 capture activity, for use as carbon capture solvents and devices/systems configured to capture, and optionally to release, CO2 and methods of capturing CO2.
The terms recited below have been defined as described below. All other terms and phrases in this disclosure shall be construed according to their ordinary meaning as understood by one of skill in the art.
As used herein, “about” or “approximately,” when used in connection with a numerical variable, generally refers to the value of the variable and to all values of the variable that are within the experimental error (e.g., within the 95% confidence interval for the mean) or within ±10% of the indicated value, whichever is greater.
As used herein, “heating” refers to increasing a temperature. For example, heating may refer to exposing or subjecting any environment, apparatus, object, material, etc. to a temperature that is greater than a current or previous temperature. In some cases, the current or previous temperature is the temperature of one or more of the environment, apparatus, object, material, etc.
The term “PEP variant” refers to an absorbent compound for non-enzymatic carbon capture based on phosphoenolpyruvate, encompassed by Formula I below, and is used interchangeably with “absorbent” in the present disclosure.
When used in the context of a chemical group, “hydrogen” means —H; “hydroxy” means —OH; “oxo” means ═O; “halo” means independently —F, —Cl, —Br or —I; “nitro” means —NO2; “cyano” means —CN; “isocyanate” means —N═C═O; “azido” means —N3; in a monovalent context “phosphate” means —OP(O)(OH)2 or a deprotonated form thereof; in a divalent context “phosphate” means —OP(O)(OH)O— or a deprotonated form thereof; “mercapto” means —SH; “thio” means ═S; “thioether” means ═S—; “sulfonyl” means —S(O)2— (see below for definitions of groups containing the term sulfonyl, e.g., alkylsulfonyl); and “sulfinyl” means —S(O)— (see below for definitions of groups containing the term sulfinyl, e.g., alkylsulfinyl).
In the context of chemical formulas, the symbol “-” means a single bond, means a double bond. The symbol “” represents a single bond or a double bond.
For the groups and classes below, the following parenthetical subscripts further define the group/class as follows: “(Cn)” defines the exact number (n) of carbon atoms in the group/class. “(C≤n)” defines the maximum number (n) of carbon atoms that can be in the group/class, with the minimum number as small as possible for the group in question, e.g., it is understood that the minimum number of carbon atoms in the group “alkenyl(c≤8)” or the class “alkene(c≤8)” is two. For example, “alkoxy(c≤10)” designates those alkoxy groups having from 1 to 10 carbon atoms (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10, or any range derivable therein (e.g., 3 to 10 carbon atoms). (Cn-n′) defines both the minimum (n) and maximum number (n′) of carbon atoms in the group. Similarly, “alkyl(C2-C10)” designates those alkyl groups having from 2 to 10 carbon atoms (e.g., 2, 3, 4, 5, 6, 7, 8, 9, or 10, or any range derivable therein (e.g., 3 to 10 carbon atoms)).
As used herein, “substituted” refers to all permissible substituents of the compounds described herein. In the broadest sense, the permissible substituents include acyclic and cyclic, branched and unbranched, carbocyclic and heterocyclic, aromatic and nonaromatic substituents of organic compounds. Non-limiting examples of substituents include halogens, hydroxyl groups, or any other organic groupings containing any number of carbon atoms, preferably 1-14 carbon atoms, and optionally include one or more heteroatoms such as oxygen, sulfur, or nitrogen grouping in linear, branched, or cyclic structural formats. Representative substituents include alkyl, substituted alkyl, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl, phenyl, substituted phenyl, aryl, substituted aryl, heteroaryl, substituted heteroaryl, halo, hydroxyl, alkoxy, substituted alkoxy, phenoxy, substituted phenoxy, aroxy, substituted aroxy, alkylthio, substituted alkylthio, phenylthio, substituted phenylthio, arylthio, substituted arylthio, cyano, isocyano, substituted isocyano, carbonyl, substituted carbonyl, carboxyl, substituted carboxyl, sulfonyl, substituted sulfonyl, sulfonic acid, phosphoryl, substituted phosphoryl, phosphonyl, substituted phosphonyl, polyaryl, substituted polyaryl, C3-C20 cyclic, substituted C3-C20 cyclic, heterocyclic, and substituted heterocyclic. As discussed herein, heteroatoms such as nitrogen may have hydrogen substituents and/or any permissible substituents of organic compounds described herein which satisfy the valences of the heteroatoms. It is understood that “substitution” or “substituted” includes the implicit proviso that such substitution is in accordance with permitted valence of the substituted atom and the substituent, and that the substitution results in a stable compound, i.e., a compound that does not spontaneously undergo transformation such as by rearrangement, cyclization, elimination, etc.
As used herein, “heteroatom” means an atom of any element other than carbon or hydrogen. Preferred heteroatoms are boron, nitrogen, oxygen, phosphorus, sulfur, and selenium. Other heteroatoms include silicon and arsenic.
As used herein, the term “halide” designates —F, —Cl, —Br, or —I.
The term “aliphatic” when used without the “substituted” modifier signifies that the compound/group so modified is an acyclic or cyclic, but non-aromatic hydrocarbon compound or group. In aliphatic compounds/groups, the carbon atoms can be joined together in straight chains, branched chains, or non-aromatic rings (alicyclic). Aliphatic compounds/groups can be saturated, that is joined by single bonds (alkanes/alkyl), or unsaturated, with one or more double bonds (alkenes/alkenyl) or with one or more triple bonds (alkynes/alkynyl). When the term “aliphatic” is used without the “substituted” modifier only carbon and hydrogen atoms are present. When the term is used with the “substituted” modifier one or more hydrogen atom has been independently replaced by —OH, —F, —Cl, —Br, —I, —NO2, —CO2H, —CO2CH3, —CN, —SH, —OCH3, —OCH2CH3, —C(O)CH3, or —OC(O)CH3.
As used herein, “alkyl” refers to the radical of saturated aliphatic groups (i.e., an alkane with one hydrogen atom removed), including straight-chain alkyl groups, branched chain alkyl groups, cycloalkyl (alicyclic) groups, alkyl-substituted cycloalkyl groups, and cycloalkyl-substituted alkyl groups. A straight chain or branched chain alkyl has 30 or fewer carbon atoms in its backbone (e.g., C1-C30 for straight chains, and C3-C30 for branched chains), preferably 20 or fewer, more preferably 15 or fewer, most preferably 10 or fewer. Likewise, preferred cycloalkyls have 3-10 carbon atoms in their ring structure, and more preferably have 5, 6, or 7 carbons in the ring structure. The groups —CH3 (Me), —CH2CH3 (Et), —CH2CH2CH3 (n-Pr), —CH(CH3)2(iso-Pr), —CH(CH2)2(cyclopropyl), —CH2CH2CH2CH3 (n-Bu), —CH(CH3)CH2CH3 (sec-butyl), —CH2CH(CH3)2(iso-butyl), —C(CH3)3(tert-butyl), —CH2C(CH3)3(neo-pentyl), cyclobutyl, cyclopentyl, cyclohexyl, and cyclohexylmethyl are non-limiting examples of alkyl groups.
The term “alkyl” is intended to include both “unsubstituted alkyls” and “substituted alkyls” having one or more substituents replacing a hydrogen on one or more carbons of the hydrocarbon backbone. Exemplary substituents include, without limitation, halogen, hydroxyl, carbonyl (such as a carboxyl, alkoxycarbonyl, formyl, or an acyl), thiocarbonyl (such as a thioester, a thioacetate, or a thioformate), alkoxyl, phosphoryl, phosphate, phosphonate, phosphinate, cyano, nitro, azido, sulfhydryl, alkylthio, sulfate, sulfonate, sulfamoyl, sulfonamide, sulfonyl, heterocyclyl, aralkyl or an aromatic or heteroaromatic moiety.
As used herein “heteroalkyl” refers to straight or branched chain, or cyclic carbon containing radicals, or combinations thereof, containing at least one heteroatom. Suitable heteroatoms include, but are not limited to, O, N, Si, P, Se, B, and S, wherein the phosphorous and sulfur atoms are optionally oxidized, and the nitrogen heteroatom is optionally quaternized. Heteroalkyls can be substituted as defined above for alkyl groups.
The term “alkanediyl” when used without the “substituted” modifier refers to a divalent saturated aliphatic group, with one or two saturated carbon atom(s) as the point(s) of attachment, a linear or branched, cyclo, cyclic or acyclic structure, no carbon-carbon double or triple bonds, and no atoms other than carbon and hydrogen. The groups, —CH2— (methylene), —CH2CH2—, —CH2C(CH3)2CH2—, and CH2CH2CH2— are non-limiting examples of alkanediyl groups.
The term “alkylidene” when used without the “substituted” modifier refers to the divalent group —CRR′ in which R and R′ are independently hydrogen, alkyl, or R and R′ are taken together to represent an alkanediyl having at least two carbon atoms. Non-limiting examples of alkylidene groups include: ═CH2, ═CH(CH2CH3), and ═C(CH3)2. When any of these terms is used with the “substituted” modifier one or more hydrogen atom has been independently replaced by —OH, —F, —Cl, —Br, —I, —NO2, —CO2H, —CO2CH3, —CN, —SH, —OCH3, —OCH2CH3, —C(O)CH3, or −OC(O)CH3. The following groups are non-limiting examples of substituted alkyl groups: —CH2OH, —CH2Cl, —CF3, —CH2CN, —CH2C(O)OH, —CH2C(O)OCH3, —CH2C(O)CH3, —CH2OCH3, —CH2OC(O)CH3, and —CH2CH2Cl. The term “fluoroalkyl” is a subset of substituted alkyl, in which one or more hydrogen has been substituted with a fluoro group and no other atoms aside from carbon, hydrogen and fluorine are present. The groups, —CH2F, —CF3, and —CH2CF3 are non-limiting examples of fluoroalkyl groups. An “alkane” refers to the compound H—R, wherein R is alkyl.
The term “alkenyl” when used without the “substituted” modifier refers to a monovalent unsaturated aliphatic group with a carbon atom as the point of attachment, a linear or branched, cyclo, cyclic or acyclic structure, at least one nonaromatic carbon-carbon double bond, no carbon-carbon triple bonds, and no atoms other than carbon and hydrogen. Non-limiting examples of alkenyl groups include: —CH═CH2 (vinyl), —CH═CHCH3, —CH═CHCH2CH3, —CH2CH═CH2 (allyl), —CH2CH═CHCH3, and —CH═CH—C6H5. The term “alkenediyl” when used without the “substituted” modifier refers to a divalent unsaturated aliphatic group, with two carbon atoms as points of attachment, a linear or branched, cyclo, cyclic or acyclic structure, at least one nonaromatic carbon-carbon double bond, no carbon-carbon triple bonds, and no atoms other than carbon and hydrogen (e.g., —CH═CH—, —CH═C(CH3)CH2—, and —CH═CHCH2—) are non-limiting examples of alkenediyl groups. When these terms are used with the “substituted” modifier one or more hydrogen atom has been independently replaced by —OH, —F, —Cl, —Br, —I, —NH2, —NO2, —CO2H, —CO2CH3, —CN, —SH, —OCH3, —OCH2CH3, —C(O)CH3, or —OC(O)CH3. The groups, —CH═CHF, —CH═CHCl and —CH═CHBr, are non-limiting examples of substituted alkenyl groups. An “alkene” refers to the compound H—R, wherein R is alkenyl.
The term “alkynyl” when used without the “substituted” modifier refers to a monovalent unsaturated aliphatic group with a carbon atom as the point of attachment, a linear or branched, cyclo, cyclic or acyclic structure, at least one carbon-carbon triple bond, and no atoms other than carbon and hydrogen. As used herein, the term alkynyl does not preclude the presence of one or more non-aromatic carbon-carbon double bonds. The groups, —C≡CH, —C≡CCH3, and —CH2C≡CCH3, are non-limiting examples of alkynyl groups. The term “alkynediyl” when used without the “substituted” modifier refers to a divalent unsaturated aliphatic group, with two carbon atoms as points of attachment, a linear or branched, cyclo, cyclic or acyclic structure, at least one carbon-carbon triple bond, and no atoms other than carbon and hydrogen. When these terms are used with the “substituted” modifier one or more hydrogen atom has been independently replaced by —OH, —F, —Cl, —Br, —I, —NO2, —CO2H, —CO2CH3, —CN, —SH, —OCH3, —OCH2CH3, —C(O)CH3, or —OC(O)CH3. An “alkyne” refers to the compound H—R, wherein R is alkynyl.
The term “aryl” when used without the “substituted” modifier refers to a monovalent unsaturated aromatic group with an aromatic carbon atom as the point of attachment, said carbon atom forming part of a one or more six-membered aromatic ring structure, wherein the ring atoms are all carbon, and wherein the group consists of no atoms other than carbon and hydrogen. If more than one ring is present, the rings may be fused or not fused. As used herein, the term does not preclude the presence of one or more alkyl group (carbon number limitation permitting) attached to the first aromatic ring or any additional aromatic ring present. Non-limiting examples of aryl groups include phenyl (Ph), methylphenyl, (dimethyl)phenyl, —C6H4—CH2CH3 (ethylphenyl), naphthyl, and the monovalent group derived from biphenyl. The term “arenediyl” when used without the “substituted” modifier refers to a divalent aromatic group, with two aromatic carbon atoms as points of attachment, said carbon atoms forming part of one or more six-membered aromatic ring structure(s) wherein the ring atoms are all carbon, and wherein the monovalent group consists of no atoms other than carbon and hydrogen. As used herein, the term does not preclude the presence of one or more alkyl group (carbon number limitation permitting) attached to the first aromatic ring or any additional aromatic ring present. If more than one ring is present, the rings may be fused or unfused. When these terms are used with the “substituted” modifier one or more hydrogen atom has been independently replaced by —OH, —F, —Cl, —Br, —I, —NO2, —CO2H, —CO2CH3, —CN, —SH, —OCH3, —OCH2CH3, —C(O)CH3, or —OC(O)CH3. An “arene” refers to the compound H—R, wherein R is aryl.
The term “aralkyl” when used without the “substituted” modifier refers to the monovalent group-alkanediyl-aryl, in which the terms alkanediyl and aryl are each used in a manner consistent with the definitions provided above. Non-limiting examples of aralkyls are: phenylmethyl (benzyl, Bn) and 2-phenyl-ethyl. When the term is used with the “substituted” modifier one or more hydrogen atom from the alkanediyl and/or the aryl has been independently replaced by —OH, —F, —Cl, —Br, —I, —NO2, —CO2H, —CO2CH3, —CN, —SH, —OCH3, —OCH2CH3, —C(O)CH3, or —OC(O)CH3. Non-limiting examples of substituted aralkyls are: (3-chlorophenyl)-methyl and 2-chloro-2-phenyl-eth-1-yl.
The term “heteroaryl” when used without the “substituted” modifier refers to a monovalent aromatic group with an aromatic carbon atom or nitrogen atom as the point of attachment, said carbon atom or nitrogen atom forming part of an aromatic ring structure wherein at least one of the ring atoms is nitrogen, oxygen or sulfur, and wherein the group consists of no atoms other than carbon, hydrogen, aromatic nitrogen, aromatic oxygen and aromatic sulfur. As used herein, the term does not preclude the presence of one or more alkyl group (carbon number limitation permitting) attached to the aromatic ring or any additional aromatic ring present. Non-limiting examples of heteroaryl groups include furanyl, imidazolyl, indolyl, indazolyl, methylpyridyl, oxazolyl, pyridyl, pyrrolyl, pyrimidyl, pyrazinyl, quinolyl, quinazolyl, quinoxalinyl, thienyl, and triazinyl. The term “heteroarenediyl” when used without the “substituted” modifier refers to an divalent aromatic group, with two aromatic carbon atoms, two aromatic nitrogen atoms, or one aromatic carbon atom and one aromatic nitrogen atom as the two points of attachment, said atoms forming part of one or more aromatic ring structure(s) wherein at least one of the ring atoms is nitrogen, oxygen or sulfur, and wherein the divalent group consists of no atoms other than carbon, hydrogen, aromatic nitrogen, aromatic oxygen and aromatic sulfur. As used herein, the term does not preclude the presence of one or more alkyl group (carbon number limitation permitting) attached to the first aromatic ring or any additional aromatic ring present. If more than one ring is present, the rings may be fused or unfused. When these terms are used with the “substituted” modifier one or more hydrogen atom has been independently replaced by —OH, —F, —Cl, —Br, —I, —NO2, —CO2H, —CO2CH3, —CN, —SH, —OCH3, —OCH2CH3, —C(O)CH3, or —OC(O)CH3.
The term “acyl” when used without the “substituted” modifier refers to the group —C(O)R, in which R is a hydrogen, alkyl, aryl, aralkyl or heteroaryl, as those terms are defined above. The groups, —CHO, —C(O)CH3 (acetyl, Ac), —C(O)CH2CH3, —C(O)CH2CH2CH3, —C(O)CH(CH3)2, —C(O)CH(CH2)2, —C(O)C6H5, —C(O)C6H4—CH3, —C(O)CH2C6H5, —C(O)(imidazolyl) are non-limiting examples of acyl groups. A “thioacyl” is defined in an analogous manner, except that the oxygen atom of the group —C(O)R has been replaced with a sulfur atom, —C(S)R. When either of these terms are used with the “substituted” modifier one or more hydrogen atom has been independently replaced by —OH, —F, —Cl, —Br, —I, —NO2, —CO2H, —CO2CH3, —CN, —SH, —OCH3, —OCH2CH3, —C(O)CH3, or —OC(O)CH3. The groups, —C(O)CH2CF3, —CO2H (carboxyl), —CO2CH3 (methylcarboxyl), and —CO2CH2CH3 are non-limiting examples of substituted acyl groups.
The term “alkoxy” when used without the “substituted” modifier refers to the group —OR, in which R is an alkyl, as that term is defined above. Non-limiting examples of alkoxy groups include: —OCH3, —OCH2CH3, —OCH2CH2CH3, —OCH(CH3)2, —OCH(CH2)2, —O-cyclopentyl, and —O-cyclohexyl. The terms “alkenyloxy”, “alkynyloxy”, “aryloxy”, “aralkoxy”, “heteroaryloxy”, and “acyloxy”, when used without the “substituted” modifier, refers to groups, defined as —OR, in which R is alkenyl, alkynyl, aryl, aralkyl, heteroaryl, and acyl, respectively. Similarly, the term “alkylthio” when used without the “substituted” modifier refers to the group —SR, in which R is an alkyl, as that term is defined above. When any of these terms are used with the “substituted” modifier one or more hydrogen atom has been independently replaced by —OH, —F, —Cl, —Br, —I, —NO2, —CO2H, —CO2CH3, —CN, —SH, —OCH3, —OCH2CH3, —C(O)CH3, or —OC(O)CH3. The term “alcohol” corresponds to an alkane, as defined above, wherein at least one of the hydrogen atoms has been replaced with a hydroxy group.
The term “reversible” or variants thereof (e.g., “reversibly”) with respect to a chemical reaction means that one or more steps of the reaction can proceed in either direction (backward or forward) depending on the reaction conditions.
Embodiments of the present disclosure describe absorbent compounds for capturing carbon dioxide (CO2) based on phosphoenolpyruvate (PEP) having chemical structure represented by Formula I:
The compounds of Formula I (also referred to as PEP variants herein) can be capable of forming acid and/or base salts by virtue of the presence of amino and/or carboxyl groups or groups similar thereto. Acceptable base addition salts can be prepared from inorganic and organic bases. Salts derived from inorganic bases, include by way of example only, sodium, potassium, lithium, ammonium, calcium and magnesium salts.
Exemplary PEP variants according to Formula I include embodiments wherein R is CH2 and R′ includes an alkene, an alkyne, or an aryl group, or R is a methyl group and R′ is a carboxylate group (e.g., a saturated or unsaturated fatty acid). In some cases, X is a —[PO3H]− or —[PO3]2− group, or sulfonato (—[SO3]−) group, R′ is a carboxyl group or a carboxylate group and R is a hydrogen, a hydroxyl group, an oxo group, a carboxyl group or a carboxylate group, or and salts thereof. In some cases, the compound of Formula I is a PEP variant as described in
In one or more embodiments, R is (CHaYb)d, where a is 2, 1 or 0, b is 1 or 0, and b+a<2, Y is selected from OH, CHOH C(═O)OH, O, CH2, CHCH3; and d is from 1 to 6, with the proviso that if R is CH2, R′ is not carboxylate. For example, a PEP variant of Formula I can be:
Methods for the chemical synthesis of exemplary PEP variants are described herein. These and other methods known to the skilled artisan can be modified and/or adapted to facilitate synthesis of additional compounds within the scope of Formula I.
A PEP variant of the present disclosure may be characterized by one or more of the following properties: a reaction enthalpy within an optimal range derived from process modeling of a Carbon Capture System (e.g., standard absorber/stripper architectures of a Post-Combustion Capture (PCC) system) and advantageous solution properties. In particular, the PEP variants advantageously exhibit a reduced tendency to cause corrosion, improved solubility, viscosity, and/or diffusivity, improved energetics and/or kinetics of reaction with the bicarbonate ion, improved reaction stoichiometry, or providing a net reduction in energy penalty relative to a particular benchmark (e.g., an amine-based absorbent). These properties can be evaluated using the methods known in the art or as described in the Examples.
For example, the suitability of a PEP variant for non-enzymatic carbon capture can be evaluated by modeling. The Electric Power Research Institute (EPRI; Palo Alto, CA) maintains and operates Aspen Plus process models based on the absorber/stripper configuration shown in
These models can be used to quantify the energy penalty of a PEP variant for carbon capture chemistry relative to the MEA benchmark. For example, an absorber VLE curve for MEA (11% mole fraction) shows that the system is in thermodynamic equilibrium at CO2 partial pressures significantly below that of supercritical pulverized coal power plant (SCPC) and combined cycle natural gas power plant (CCNG) flue gas concentrations (1.43 and 1.86 mol/kg for SCPC and CCNG, respectively). This equates to 71% and 84% cyclic efficiency (i.e., 71% and 84% of CO2 reacting with MEA at absorber conditions for SCPC and CCNG, respectively, is desorbed and released for compression at stripper conditions). The remaining 29% and 16% remain bound in solution during repeated absorber-stripper cycling. Therefore, in some cases, a PEP variant of the present disclosure exhibits improved cyclic capacity and/or efficiency relative to MEA at one or more concentrations (e.g., % mole fractions). The improvement can be evaluated at the temperatures of the absorber (40° C.) and stripper (120° C.). For example, a PEP variant of the present disclosure can exhibit greater than 71% cyclic efficiency for SCPC, such as greater than about 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5%, 99.9%, or about 100% cyclic efficiency for SCPC, greater than 84% cyclic efficiency for CCNG, such as greater than about 85%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5%, 99.9%, or about 100% cyclic efficiency for CCNG.
A PEP variant of the present disclosure can also be characterized by improved reaction stoichiometry, relative to MEA. For example, a PEP variant of Formula I can be half that of MEA, i.e., each molecule of the PEP variant can absorb one CO2 molecule while it takes two MEA molecules per CO2.
Embodiments of the present disclosure describe compositions comprising the absorbent compounds of Formula I for CO2 chemical absorption. The compositions facilitate selective reaction with CO2 (or bicarbonate) and the PEP variant of Formula I. In one or more embodiments, the composition is capable of reversibly reacting with CO2 (or bicarbonate). For example, a generalized reaction is based on a four-step process encompassing (1) dissolution of CO2 into the aqueous phases via Henry's law, (2) hydrolysis and ionization of aqueous CO2, (3) dissociation of the bicarbonate ion and (4) reaction with the absorbent compound.
One non-limiting mechanism for CO2 chemical absorption by a capture composition of the present disclosure is shown in
The compositions of the present disclosure can be in the form of solutions, suspensions, dispersions, slurries, powders, or particles, or other forms suitable for use in an intended Carbon Capture System. In one or more embodiments, the CO2 capture composition is a fluid composition (e.g., a solution, suspension, or slurry) comprising at least one PEP variant of Formula I and a liquid vehicle, carrier or diluent. Liquid vehicles/carriers/diluents of the present disclosure include, but are not limited to, water, saline, buffered solutions, hydroalcoholic mixtures, and alcohols. The alcohol can be selected from ethanol, methanol, isopropyl alcohol, and combinations thereof. In some cases, the fluid composition includes a co-solvent system with both a protic and aprotic solvent (e.g., an organic solvent) to improve the pumpability of the CO2 capture composition. Protic solvents can be selected from water, formic acid (HCO2H), n-butanol (CH3CH2CH2CH2OH), isopropanol ((CH3)2CH(OH)), nitromethane (CH3NO2), ethanol (CH3CH2OH), methanol (CH3OH), and acetic acid (CH3CO2H), or the like.
The concentration of PEP variant in the fluid composition can vary based on its intended application. Generally, the CO2 capture composition will contain about 0.1% to 90% or about 0.5% to 60%, by weight of a PEP variant of Formula I, the remainder being carrier, vehicle, and/or diluent. In some cases, the CO2 capture composition includes at least 25% by weight PEP variant and up to about 75% by weight PEP variant, such as about 30% to about 60% by weight, about 35% to about 55% by weight, or about 40% to about 50% by weight. The CO2 composition can include about 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, 60%, 61%, 62%, 63% by weight, or more PEP variant according to Formula I. At least a portion, e.g., 50% or more of the PEP variant can be dissolved in the liquid vehicle/carrier.
In some cases, the PEP variant according to Formula I is present in or on a solid support, such as a porous oxide support, porous framework (e.g., Metal-Organic Framework (MOF) or Covalent-Organic Framework (COF)) or incorporated into a resin or polymer, such as an intrinsically microporous polymer. The solid support can be a monolith, a particulate, a powder, a film, membrane (free standing or supported), rolled sheet, or a fiber (e.g., a hollow fiber or electrospun), suitable for use in Carbon Capture Systems.
Embodiments of the present disclosure describe methods of using the PEP variant for carbon capture from a feed stream. The PEP variant according to Formula I can be used in place of an amine-based solvent to capture CO2 from the CO2-containing feed stream (gas). In an example, the feed stream is flue gas.
The method can include contacting the gas with the PEP variant composition (absorbent or solvent) to capture CO2 contained in the feed gas. In an example, the absorbent/solvent can be water. The PEP variant composition or solvent can be present as an aqueous solution at an ambient temperature (outdoor air temperature) for direct air capture (DAC). An example ambient temperature range is from −40° C. to 40° C., another example ambient range is from 10° C. to 30° C. The ambient temperature is the initial temperature at contact of the PEP variant composition with the feed gas. The temperature can subsequently rise given that the capture reaction is exothermic. For flue gas, the capture temperature can be higher—for example, 40° C. A range for flue gas capture can be between 30° C. and 100° C. The pressure for gas capture can be in the range of from 1 to 50 bar and can commonly be achieved at pressures of around 1 bar. In pressurized systems, capture of gas can occur at pressures in the range of from 10 to 30 bar, and in an example, around 20 bar. It is recognized that the operating temperature and pressure for capture can vary depending, in part, on the source of the feed gas.
Although the focus of the present disclosure is on carbon capture, the method can optionally include releasing the absorbed CO2 from the PEP variant/absorbent and recovering or regenerating the absorbent. The releasing step can include a change in temperature and/or pressure. For amine-based absorbents, the release temperature can be around 120° C. For the PEP variant described herein, the release temperature can be between 80° C. and 150° C. The release pressure can be between 1 and 150 bar and can commonly be achieved at pressures around 1 to 2.5 bar. In pressurized systems, release can occur at pressures in the range of from 5 to 30 bar, and in an example, around 20 bar.
Note that a change (or swing) in the temperature or pressure can facilitate release of CO2. In the releasing step, there can be an increase in temperature or a decrease in pressure, relative to the capture step. In other examples, a decrease in temperature or an increase in pressure can be used to facilitate releasing CO2. Temperature and pressure swings are commonly used for absorbers and strippers.
Upon releasing CO2, the CO2 can be compressed. In an example, liquefied CO2 can be pumped into wells for long term storage.
The PEP variant can capture CO2 in a single step that can be reversible without the use of enzymes. The PEP variant can capture CO2 in a single phosphate-bicarbonate exchange, without changing the organic portion of the PEP variant. Thus, in the regeneration/release step, the exchange is reversed, and the PEP variant is returned to its original structure.
Embodiments of the present disclosure describe devices and systems for using the PEP variant for carbon capture.
The regenerator 14 can facilitate the release of CO2 from the absorbent/solvent such that the absorbent can be recycled to the absorber 12 for reuse. The regenerator 14 can include a stripper and a reboiler. The CO2 containing absorbent can release (desorb) the CO2 using a gas, such as steam. Operating conditions of the regenerator 14 can depend, in part, on at least the factors provided above in regard to the absorber 12 in terms of the feed gas and absorbent. Exemplary ranges for operating temperatures and pressures for CO2 release are provided above under the method.
The system 10 can also include a heat exchanger 16 to exchange heat between the CO2-containing absorbent exiting the absorber 12 and the regenerated absorbent exiting the regenerator 14, before the regenerated absorbent is recirculated back into the absorber 12. Specifically, the regenerated absorbent can transfer heat to the CO2-containing absorbent to increase the temperature of the CO2-containing absorbent to operating temperatures of the stripper/regenerator 14.
Embodiments of the present disclosure can include the absorber 12 only. The absorber 12 is used to capture CO2 and remove CO2 from the feed gas. The captured CO2/CO2-containing absorbent can then be disposed of or otherwise treated.
The methods and devices/systems described herein can be used in a variety of applications that can benefit from carbon capture from a feed gas. Such applications can include, but are not limited to, petrochemical, electrical power production, cement, metalworks, and carbon capture and storage (CCS), including fossil-fuel CCS and bioenergy with CCS (BECCS).
The following Examples are intended to illustrate the above invention and should not be construed as to narrow its scope. One skilled in the art will readily recognize that the Examiners suggest many other ways in which the invention could be practiced. It should be understood that numerous variations and modifications may be made while remaining within the scope of the invention.
These Examples are for illustrative purposes and are not meant to be limiting on the scope of the appended embodiments. The numerical values set forth in the specific examples are reported as precisely as possible; yet errors necessarily result from the standard deviation found in their respective testing measurements.
An initial assessment of the suitability of PEP and similar compounds (i.e., PEP variants) as PCC solvents was performed using group contribution methods to estimate the reaction enthalpies to determine if they fall within the optimal range derived from process modeling of standard absorber/stripper architectures discussed above. For compounds found to be in the optimal enthalpy range, nonreactive molecular dynamics simulations were performed to quantify their aqueous solution properties, like solubility, viscosity, and diffusivity. Furthermore, reactive molecular dynamics was used in conjunction with quantum mechanical calculations to simulate the energetics and kinetics of reaction with the bicarbonate ion. The assessment identified compounds with the largest confluence of optimal properties. These were fed back through the process model to evaluate the net reduction in energy penalty relative to the amine benchmark.
It is well understood that intermolecular forces are determined by the properties of the atoms and bonds making up the molecules and that these, in turn, determine macroscopic properties. This is the basis of group contribution methods wherein weighting factors are assigned to the atoms, atomic groups and bonds constituting a given molecule and these values algebraically summed to estimate certain properties of the molecule. The most accurate methods at the limit of estimation also allow for nearest neighbor and next nearest neighbor interactions of the atoms and functional groups comprising the molecule of interest to factor into the overall sum. The method of Benson is an example and is the basis of the American Society for Testing and Materials Chemical Thermodynamic and Energy Release Evaluation (CHEETAH) program distributed by the National Institute of Standards and Technology and widely used in industry. In the current work, CHEETAH was used to estimate the enthalpies of formation ΔHf for the various reactants and products in the reaction of phosphoenol compounds with bicarbonate according to the generalized and balanced reaction equation given below:
The reaction enthalpy ΔHrxn was then be calculated as the difference in the stoichiometric sum of formation energies between the products and reactants according to:
These calculations were repeated as the molecular structure of PEP was varied to determine the sign, magnitude, and trends in the reaction enthalpy of CO2 capture. The phosphoenol backbone depicted below was held constant while the variables R and R′ were changed by adding functional groups ranging from a simple hydrogen atom to increasingly more complex chemical structures like alkenes, alkynes, aryls and ketoacids. The nomenclature in what follows simply hyphenates the phosphoenol backbone with the added functional group, e.g., phosphoenol-alkenes, where R and R′ could be any number of different alkenes and alkene derivatives, as described above. For PEP, R is a carboxylate anion CHO2− while R′ is a methyl group CH2.
Starting with PEP—which belongs to the phosphoenol-ketoacid group where ketoacids consist of both a carboxylic acid (O═R—O−) and ketone (R—C═O) group —
Similar data are shown in
First approach estimates of reaction enthalpies via group contribution for reaction of phosphoenol compounds with bicarbonate revealed (i) core gateway structures that can be built upon with any number of different atoms and/or functional groups to customize molecular properties without changing reaction enthalpy and (ii) that these reaction enthalpies are within the optimal range for use as solvents in absorption-based PCC.
These investigations were carried out using non-reactive molecular dynamic (MD) simulations. The General Assisted Model Building with Energy Refinement (AMBER) Force Field (gAff) was used to parameterize molecules for the simulation because it is applicable to modeling interactions of small organic compounds, including PEP. The simulations were performed using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS), which is a molecular dynamics program from Sandia National Laboratories.
All simulations were established by organizing the necessary molecules periodically in a specified simulation box. Each system was run a total of six times with the orientations of the molecules randomized in each version. The molecules were situated approximately 6 Å away from each other to avoid collisions during set up. At the onset of each simulation, the energy of the system was minimized, thus reducing the size of the system. After this minimization, a Maxwell Boltzmann distribution was utilized to randomly assign kinetic energies to the molecules based on temperature. The system was then allowed to equilibrate for one nanosecond. Measurements were taken over a 10-nanosecond period following equilibration.
All simulations were run at typical absorber conditions of 40° C. and 1 atm. All bonds and angles were treated as harmonic, and water molecules were treated as rigid bodies to prevent angular vibrations. These simplifications were included because this investigation was focused on solution properties as a whole and the exact form of molecular deformations is therefore negligible. Table 1 describes the concentration of PEP/water mixtures studied by MD.
Non-reactive MD is unable to simulate the dissociation of an acid, therefore it is necessary to place the appropriate number of “pre-dissociated” ions in the simulation during setup. The runtime of a few nanoseconds is often insufficient for free protons to move away from their conjugate bases, as this process requires water molecules to interpose themselves between the charges. Therefore, hydronium ions (H3O+) were randomly distributed across the system during setup and a set of independent simulations were performed with different proton distributions. Since only the first dissociation constant (Ka) for PEP is available in the literature, only singly charged PEP molecules were considered.
C
3
H
5
O
6
P+H
2
O→C
3
H
4
O
6
P
−
+H
3
O
+
It was assumed that it was significantly more difficult for a PEP ion to lose its second or third hydrogen, as that would result in multiple like charges on the same molecule. Even if triply charged PEP would more accurately represent the state in solution than the singly charged state, results are conservative since the triply charged state is likely to be more soluble. Less than half of the PEP molecules in the solution were dissociated at modeled conditions. However, for the simulations including bicarbonate, a much higher pH was assumed, and essentially all PEP molecules were dissociated. It seems likely that under these conditions, doubly and possibly triply dissociated PEP ions would have been present, which decreases bond dissociation energies and decomposition barriers as shown later via reactive MD simulations.
The solubility of PEP is key to its energy-efficient use for PCC. The solubility was estimated by constructing a potential energy versus concentration curve in addition to the entropy of solution effects, which were approximated as an ideal solution by:
where χ is the mole fraction of PEP in solution. The energy of mixing is:
Additionally, pure water simulations were performed to calculate the difference in energy caused by simply changing the number of water molecules. This allows for the direct comparison of energy between various simulations.
Benchmark materials were used to check the accuracy of the simulation method. Acetic and formic acids were chosen as the initial benchmarks due to readily available literature values on their solution enthalpy. The enthalpy of mixing was calculated in a similar manner to the free energy of mixing, using the default LAMMPS enthalpy calculation. Simulations were run for 3, 4, and 5 mol/L of both materials. Comparative results between literature and calculated values are shown in Table 2; all values are reported at kJ/mol. Although the solubility trends are well reproduced, the values are not quantitative. This was expected for the simple approach taken here, which nonetheless proved sufficient.
MD simulations with 30% (by weight) aqueous monoethanolamine (MEA) solution (500 MEA molecules, 3955 water) were also performed to directly compare predicted performance measures of PEP against the current industry standard. These simulations were performed under the same conditions and with the same forcefield as the PEP simulations.
The system's radial distribution functions (RDFs), which are statistical mechanics descriptions of how density varies as a function of distance from a reference point, determine solution structure. The water position was defined by the oxygen atom; the inset of
The diffusivity of PEP in solution D was found via the Stokes-Einstein equation.
Mean squared displacement data were obtained every 5 picoseconds from trajectories. Diffusivity is important in the PCC process because it influences the kinetics of the PEP-bicarbonate reaction, which essentially determines the speed of CO2 absorption. An additional simulation was performed to find the diffusivity of bicarbonate in an equilibrium PEP-water solution. For this simulation, the optimum PEP concentration was used (48.23 wt %), and the pH was set to 1.5 to allow PEP to fully dissociate and enough bicarbonate to form in solution to obtain meaningful statistics. In a realistic system, the number of PEP ions would be the same but most counter ions would be sodium (Na+) instead of H3O+. However, the simulation is nonreactive, meaning molecules cannot react, so there is no significant difference since PEP concentration governs the results.
The shear viscosity of a liquid can be computed through several different atomistic simulation methods. The Green-Kubo relation was used to calculate the viscosity of the specified systems because it can be used on any equilibrated liquid and is easily implemented in LAMMPS. A running average of the viscosity was periodically revised every 3 picoseconds using this technique. In this method, the shear viscosity is obtained from the integral over time of the pressure tensor autocorrelation function:
where V is the volume of the system, kB is the Boltzmann constant, T is temperature, Pxy denotes the element xy of the pressure tensor, and the angle bracket refers to the ensemble average. Viscosity plays an important role in pumping costs for moving aqueous PEP solutions between the absorber and stripper. To contrast aqueous PEP solutions with the current amine-based benchmark, a simulation of 30 wt % MEA was run to find its viscosity.
A solubility curve for aqueous PEP solutions is shown in
As the PEP concentration of the solution increases, no significant structural changes were observed in the radial distribution functions (RDF) of water around PEP, as shown in
The mean square displacement over time was used to calculate the diffusion coefficient of PEP over a range of concentrations.
Bicarbonate transport was also included in the simulations. The bicarbonate concentration was calculated according to:
Values for constants were taken from the literature. The equilibrium constant Kc1 had a value of 2.5×10−4, and the Henry's law constant for CO2 is approximately 0.034 mol/L atm at neutral pH value. The mole fraction of CO2 was 0.205 based on the assumption that all oxygen in the air had been converted to CO2 in the combustion process.
Due to the concentration of HCO3− being very low at low pH value (explicit pH was 1.5 for the PEP simulations), it was necessary to significantly raise the pH of these simulations to have a useful concentration of bicarbonate at a reasonable simulation size. It was expected that all PEP molecules would likely have at least one dissociated proton and a significant amount would have more. But due to the limitations mentioned previously, only singly charged ions could be simulated; simulation parameters are shown in Table 3.
PEP appears to be a suitable replacement for MEA in PCC based on its properties in aqueous solution. However, variations on the PEP structure obtained by removing or adding different atoms and functional groups were tested for even better PCC solvent alternatives. An assessment of exemplary PEP variants is given here.
The first PEP variant (C2H5O6P) is depicted in
Substituting one of the hydrogen atoms bound to carbon atom C2 with a hydroxyl group, as shown in
Replacing a hydrogen atom on C2 with a carboxylic acid group (O═C—OH) instead of hydroxyl group increases the energy of dilution even more than the previous two variants, as shown in
For the fourth PEP variant (C2H3O7P), the hydrogen atom attached to C2 is substituted with a double bonded oxygen (oxy-group).
The final PEP variant (C3H5O7P) is shown in
The chemical potential-concentration curve shows that the expected solubility of PEP at 20° C. is 9.8 mol %. This is slightly lower than the 11 mol % of the standard MEA solution. However, the stoichiometric ratio of MEA to CO2 during absorption is 2:1 as shown below while that of the PEP-CO2 reaction is 1:1. Therefore, the absorption capacity of the PEP solvent for CO2 is almost doubled compared to most primary and secondary amines, which all follow the same general reaction stoichiometric ratio shown below:
CO2+NHR2+NR3↔NR2CO2−+NR3H+
where NR3 can be any primary, secondary or tertiary amine and NHR2 is either a primary or secondary amine. Higher temperatures present in absorber systems may further improve the solubility of PEP, especially given the curve is very shallow up to 55 wt % (11 mol %).
The viscosity of the PEP solution in the 45 to 55 wt % (9.8 mol %) range was estimated to be roughly triple that of the standard MEA solution, with a real viscosity of about 3 mPa*s. Based on the viscosity alone, a rough calculation shows that the same pipe infrastructure would require about 6% more power to pump PEP solution than MEA solution. Fortunately, the amount of power required to pump MEA is already a negligibly small part of the PCC energy cost.
Hydration shells, viscosity, and self-diffusion coefficient trends for PEP showed no notable behavior change over the examined range of concentrations. It is therefore unlikely that any sort of relevant phase or structure change is taking place.
In total, the observations made here show the promise of alternative PCC solvents based on phosphoenol compounds to increase the desired solvent properties relative to benchmark amines such as MEA.
The above results suggest that PEP or PEP variants can be reasonably expected be good alternative CO2 solvents for lower regenerative energy. The reaction mechanism of PEP and PEP variants with CO2 was accomplished by reactive MD simulations, which use empirical reactive force fields that allow bond formation and dissociation. The reactive force field (ReaxFF) uses an interatomic potential which implicitly describes chemical bonding using a bond-order formulism without explicit quantum mechanical (QM) calculations and thus can handle the dynamics of larger systems than pure QM simulation. In terms of molecular size, PEP and PEP variants are closest to glycine.
This Example presents an optimized ReaxFF for aqueous PEP solutions with CO2. Force fields were computationally trained against an extensive data set including density functional theory (DFT) calculations of reaction pathways of the dissociation of PEP3−, bond dissociation energies, and structures and partial charges of all molecules and ions. The parametrization process utilized a parallelized search algorithm developed at the University of California, Davis.
Quantum chemistry-based methods including DFT are powerful tools to describe chemical reactions on the atomistic scale. However, these types of calculations are very computationally expensive and thus limit the time and length scales. Alternatively, empirical force field methods like classical molecular dynamics (MD) simulation can study the system's dynamic evolution for comparatively long times (nanoseconds) with thousands of atoms but loses the reaction information since the atomic connectivity is predefined. Therefore, to describe the reaction step of PEP with HCO3− (
The ReaxFF force field P/N/C/O/H/Na was previously published for CO2 capture using the ionic liquid tetrabutylphosphonium glycinate. The CO2 related parameters were optimized, and it contains the important element P and the counter ion element Na to neutralize the charged system in the MD simulations. However, the important P—O interactions were not specifically developed, and the H/O parameters were from a 1st generation water force field for which the constant pressure water density is known to be significantly smaller than the experimental value. Furthermore, the H2O, H3O+, and OH− diffusion constants are more than an order of magnitude off. To improve the accuracy for an aqueous system, all the water related parameters from the P/N/C/O/H/Na force field were replaced with those from the 2nd generation water force field that corrects the 1st generation water parameters. Also, many of the C/P atom parameters and O/P interaction parameters were reoptimized to better describe the PEP-HCO3− reaction. Table 4 shows which parameters were chosen and why, including their physical importance to the current model system.
The optimization process consisted of two parts: training set generation and parallel search algorithm. The new parameter values are accepted so that they reduce the error between training set features and ReaxFF fitted features.
The training set had four sections: charge, geometry, energy and reaction. The charge and geometry sections collect the partial charges, bond lengths, and angles of all the neutral molecules related to PEP-HCO3− reaction step and their charged states. Since the LAMMPS modeling software being used requires a neutral system, H3O+ were inserted as counter ions. The bond section collected the bond dissociation energy scans for P—O and O—H bonds in neutral PEP and carboxyphosphate.
The reaction section had the reaction energy of the PEP-HCO3− reaction and the activation energy of the splitting of PEP3− into PO32− and the enolate form of pyruvate since the transition state structure of this reaction step is not stable in vacuum. Therefore, this activation energy is not appropriate to include in the training set.
After reoptimizing parameters, as discussed above, the new ReaxFF force field accurately reproduced the partial charges of CO2, the neutral and charged bicarbonate, PEP, carboxyphosphate, enolate, oxaloacetate, and the transition state relative to DFT values. Bond dissociation energy scans in
DFT was used to explore the bond lengths and bond dissociation energies of neutral PEP and PEP variants. As discussed previously, PEP variants are obtained by adding and removing atoms and functional groups from the original PEP molecule to vary the reaction energetics. Using the same two variants shown in
Simulations were similarly performed on the triply charged PEP and PEP variants to evaluate the energy barrier for decomposition into the phosphate and enolate ions, a critical step in the reaction mechanism. Results are shown in
The optimized ReaxFF force field reproduced atomic partial charges, geometries, and bond energies of the target reaction exceptionally well. The bond and angle errors between ReaxFF and DFT calculations were all less than 6%. More importantly, the reaction energy and barrier calculated from ReaxFF shows that the optimized force field accurately described the thermodynamics and kinetics of the first step in the PEP-HCO3− reaction pathway. Water density calculated using this force field and at 300K and latm with isothermal-isobaric ensemble (NPT) is 1.005±0.015 g/cc. Furthermore, first results from simulations of bond dissociation energies and decomposition energy barriers for PEP and PEP variants offer proof of concept for the ability to modify the phosphoenol structure to optimize compound properties and performance for PCC.
State-of-the-art process models of various absorber/stripper architectures in first-generation post-combustion capture (PCC) systems are used to quantify the total thermal energy demand of the CO2 absorption-desorption process, termed the regeneration energy Qreg. As mentioned previously, Qreg comprises three highly interdependent thermal energy terms, including (1) the sensible heat Qsen required to heat the aqueous carbon capture solvent, (2) the latent heat of vaporization Qlat for producing steam, and (3) the heat of reaction Qrxn for desorption of CO2.
The most common objective of PCC process modeling is to minimize Qreg by varying either process parameters, like solvent flow rate, stripper pressure and reboiler temperature, or solvent parameters, like CO2 vapor-liquid equilibrium (VLE), reaction enthalpy ΔHrxn and mass transfer properties. A typical absorber/stripper configuration is shown in a system 100 in
Counterpropagating flows of CO2-rich flue gas (1(g)), entering from the bottom of the absorber 102, and CO2-lean aqueous solutions of the carbon capture compound (1(liq,c)), entering from the top of the absorber 102, mix and CO2 is transferred from the flue gas to the solution via dissolution into the aqueous phase, dissociation to the bicarbonate ion and reversible reaction with the carbon capture compound, typically referred to as the solvent. The CO2-rich solvent (2(liq, c)) collected at the bottom of the absorber 102 is pumped through a heat exchanger 106 and thermal energy of the hot CO2-lean solvent on the return cycle (4(liq,h)) from the stripper is transferred to the CO2-rich solvent which is then sent to the top stage of the stripper 104 (3(liq,h)).
The gas not absorbed by the solvent exits the absorber 102 (2(g)) and can be released to the atmosphere; such gas can include N2, H2O, etc.
Counterpropagating flows of steam from the reboiler 108 at the bottom of the stripper 104 and CO2-rich solvent from the top (3(liq,h)) mix and CO2 is transferred or released from the aqueous phase to the vapor phase via a thermally driven reverse of the carbon capture reaction. Gaseous CO2 then exits the top (3(g)) of the stripper 104 to a compressor 110; the compressed gas can then be stored (for example, geologic storage). The CO2-lean solvent collects at the stripper bottom (4(liq,h)) and is recycled through the heat exchanger 106, transferring its thermal energy to the CO2-rich solvent, and then fed back to the top of absorber 102.
Process modeling studies to date have shown that operational variables—solvent flow rate and stripper pressure and temperature—are constrained in their degrees of freedom to influence the regeneration energy. One variable must be set to satisfy operational targets (e.g., 90% capture efficiency) while the others are limited by the chemical stability of the carbon capture compound. Therefore, it is widely recognized that the solvent is the most critical variable in minimizing Qreg. Much effort has been made to quantify optimal solvent properties, but these studies have exclusively focused on various amines and amine mixtures. There has been no effort to study alternative, non-amine carbon capture chemistry. In the current work, well established state-of-the-art process models within the Aspen Plus architecture were used to study phosphoenol-based CO2 capture chemistry starting with the base PEP molecule.
This example presents a second ReaxFF model developed to describe the CAM reactions involving PEP and the atomic interactions in the P/C/O/H system. The ReaxFF force field parameters were fitted against quantum mechanical (QM) training data for partial charges, molecular structures, bond dissociation energies, reaction energies and activation energies. Second generation water parameters were combined with P/C parameters for more accurate water description and P's electrostatic parameters were specially treated to correct P/O interactions. This developed P/C/O/H ReaxFF model was a re-parametrization of the ReaxFF model in Example 2. This model was able to reproduce the training set for structures and energetics of the molecules and reactions involved in the CAM process, and accurately describe the aqueous bicarbonate and PEP systems. Molecular dynamics simulation using this ReaxFF model depicts how bicarbonate reacts with PEP and in solution and determines the impact of local structure on reactions necessary to perform carbon capture using PEP, which enables the potential design of PEP variant as the optimal carbon capture absorbent.
The training set is generally the same as that described above under Example 2. The transition state refers to the reaction where the PO32− group dissociates from the charged PEP3−, which is shown in
The atomic partial charge errors between the ReaxFF simulations, under this model, and QM calculations are negligible, and the bond and angle errors are all less than 6% and 8%, respectively.
Bond dissociation energy scans are shown in
O—P, C—O, and O—H bonds at the active sites for both neutral PEP and neutral carboxyphosphate were scanned from very short to equilibrium distances and then to very large distances without relaxing the whole structure, and the energies along the scans were acquired. To account for the multiple spin states as the molecules break, both singlet and triplet scans were calculated, and the lower energies were taken. Some energy data points at far distances were removed to reduce computational cost. The structures corresponding to each DFT data point along the scans were then fed to ReaxFF MD simulations to calculate ReaxFF energies. The fitted ReaxFF energies generally match the DFT energies, especially at the regions near the equilibrium distances, and the performance of the force field for the model under this example improved, compared to the original ReaxFF model of Example 2.
Diffusion coefficients and radial distribution functions (RDFs) were also calculated and compared with literature values (38) using non-reactive MD simulations to characterize HCO3−, and PEP3− in water. Diffusion coefficients were obtained by fitting to the mean square displacement (MSD) over the time interval where MSD increased linearly. The diffusion coefficient of HCO3− at 298 K in published literature was 1.17×10−9 m2s−1, and the diffusion coefficient of HCO3− at 298 K in this ReaxFF fitted system was 0.67×10−9 m2s−1. Because both calculations were performed on one HCO3− in water, a statistical error was expected, and the discrepancy between the two diffusion coefficients were acceptable particularly as this ReaxFF model was not optimized for dynamics.
The RDF of HCO3− was calculated both in the published literature and in this ReaxFF system. The first peak of the RDF between the carbon in HCO3− and the oxygen in H2O was around 3.65 Å in the ReaxFF system whereas between carbon in HCO3− and hydrogen in H2O around 2.57 Å in the ReaxFF fitted system. Both matched with quantitatively. In addition, the RDFs of H2CO3, PEP3− and PEP were also calculated. Diffusion coefficients and RDFs compared between the non-reactive MD simulations in the published literature and this ReaxFF model show that the model not only correctly describes vacuum properties but also solution structure and dynamics. See
After optimization and initial validation (see above) long simulations were performed for 30 wt % PEP in water with bicarbonate using this ReaxFF model ff.P/C/O/H. Depending on the initial configuration, the reaction can sometimes proceed easily. The local structure is important for reaction progress. In one case, as shown in
The correlation between the partial charges and the reaction was also analyzed. For the system where two PEP-bicarbonate pairs reacted, the partial charge and the coordination number of P in the reacting PEPs were recorded over 25 ps, as shown in
In another run where only one PEP-bicarbonate pairs reacted, the partial charge and coordinate number of that P were recorded, and those of the P in 10 randomly chosen non-reacting PEPs were also recorded for comparison. See
In summary, this ReaxFF model ff.P/C/O/H for biomimetic carbon capture potentially uses PEP as a substitute for MEA. During the force field parametrization, it was found that the phosphorous' EEM parameters have some intrinsic problems that needed to be treated. This ReaxFF model was able to correctly predict vacuum properties of the molecules and reaction related to the target carbon capture chemistries, as well as solution properties. This ReaxFF model also correctly predicted the target reactions during MD simulations, where bicarbonates react with PEPs in solution to form the intermediate species and then dissociate into the carboxyphosphate and enolate form of pyruvate. It was also observed that if the PEP reacts with its neighboring bicarbonate, the partial charge of its P tends to be lowered and is negatively correlated with its coordination number. Thus, this ReaxFF model can determine the impact of local structure on reactions necessary to perform carbon capture using PEP. This will enable selection of optimal reaction conditions and design of new PEP variants that are more reactive with bicarbonate, more stable after reaction, and possibly more energy efficient.
The foregoing description of various preferred embodiments of the disclosure have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise embodiments, and obviously many modifications and variations are possible in light of the above teaching. The example embodiments, as described above, were chosen and described in order to best explain the principles of the disclosure and its practical application to thereby enable others skilled in the art to best utilize the disclosure in various embodiments and with various modifications as are suited to the particular use contemplated.
Although the description above contains much specificity, these specific examples should not be construed as limiting the scope of the disclosure, but as merely providing illustrations of some of the presently preferred embodiments of this disclosure. It is also contemplated that various combinations or sub-combinations of the specific features and aspects of the embodiments may be made and still fall within the scope of this disclosure. It should be understood that various features and aspects of the disclosed embodiments can be combined with or substituted for one another in order to form various embodiments. Thus, it is intended that the scope of at least some of the present disclosure should not be limited by the particular disclosed embodiments described above.
The scope of this disclosure should be determined by the appended claims and their legal equivalents. Therefore, it will be appreciated that the scope of the present disclosure fully encompasses other embodiments which may be recognized as equivalent to those skilled in the art. All structural, chemical, and functional equivalents to the elements of the above-described preferred embodiment that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims.
Reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the present disclosure, for it to be encompassed by the present claims. No element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2022/040229 | 8/12/2022 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63232807 | Aug 2021 | US |
