Patent Application
20230321591
Cost effective CO2 capture processes are in high demand due to environmental concerns related to CO2 emissions, the need to remove CO2 for upgrading fuel streams such as CH4 and H2, and to meet the demands for CO2 utilization in enhanced oil recovery (EOR), etc. Continually looping solvent-based processes for CO2 capture offer advantages in design, operation, and efficiency. Traditionally, CO2 solvents have been categorized as either chemical solvents or physical solvents according to the strength of the interaction of CO2 with the solvent. The standard chemical solvent for low partial pressure CO2 capture is a solution of aqueous amine such as mono-ethanol amine. The high CO2 binding energy of chemical solvents allow CO2 capture at low partial pressures (<0.15 bar) of CO2; however, aqueous amines present several problems which add costs to the capture process. For example, the high-water content adds to regeneration energy demands due to the high heat capacity of water and latent heat of evaporation. Aqueous amines are caustic solutions which present corrosion problems.
The high temperatures required for solvent regeneration result in significant evaporation of water and amine along with degradation of the amine. Amine solutions are typically regenerated with steam stripping and any amine and/or water evaporated during the solvent regeneration step must be removed from the recovered CO2 prior to pipeline transport. All of these issues reduce the overall efficiency of the CO2 capture system. Many of these issues can be reduced or eliminated by using nonaqueous solvents with low vapor pressure and moderate CO2 binding energy.
Physical solvents are better suited for applications involving higher partial pressures (>8 bar) of CO2 since elevated pressures are needed to increase CO2 solubility due to the weaker CO2 interaction with the solvent. The lower binding energy of CO2 in physical solvents provides the advantage of a much lower energy demand for solvent regeneration since no heating or only mild heating is required; however, the lower CO2 solubility in such solvents often creates a need for solvent chilling, additional compression of the feed gas, and/or larger solvent volumes which ultimately reduces the efficiency of the CO2 capture process. The weaker CO2 interaction with physical solvents can also lead to reduced CO2 absorption selectivity with soluble gases such as methane. Currently employed common industrial physical solvents for CO2 such as Selexol (polyethylene glycol), NMP (N-methyl pyrrolidinone), Fluor solvent (propylene carbonate), and methanol (Rectisol) all suffer from high water solubility and or volatility issues. High water uptake in the solvent reduces CO2 capacity and increase pumping demands due to increased solvent mass. The dissolved water in the solvent leads to high humidity levels in the recovered CO2 which must be rectified prior to pipeline transport. Volatile solvents must be operated chilled or condensed out of the recovered CO2 stream and recycled. High water content in the solvent can lead to increased corrosion rates as well.
Many of the above-mentioned limitations on current chemical and physical solvents for CO2 removal are significantly reduced by the versatile family of beta-amino carboxylate (BAC) solvent disclosed herein. BAC solvents are designed to operate as non-aqueous solvents with low viscosities and low vapor pressures. The molecular structures of the solvents can be tuned to give ideal CO2 binding energies suited to the partial pressure range of CO2 in the treated gas stream. For low partial pressure CO2 capture applications such as biogas upgrading, land fill gas upgrading, and CO2 recovery from cement and steel production, non-aqueous BAC solvents with strong CO2 binding give high CO2 working capacities at 25° C. without the need for gas compression. Solvent regeneration can be accomplished with mild regeneration temperatures of 60° C. using a pure CO2 sweep gas at 1 bar with no need for steam stripping. Under these conditions, Selexol is completely ineffective and standard aqueous amines would require significantly higher regeneration temperatures and steam stripping of the solvent leading to a water saturated CO2 stream requiring additional drying.
The following discussion is copied from the American Biogas Council website: https://americanbiogascouncil.org/biogas-market-snapshot/
Sources: American Biogas Council, Biogas Opportunities Roadmap (USDA, EPA, DOE, 2014), EPA AgSTAR 2016, EPA LMOP 2017, Water Environment Federation “Enabling the Future” last updated Apr. 26, 2018: “Operational US Biogas Systems—The U.S. has over 2,200 sites producing biogas in all 50 states: 250 anaerobic digesters on farms, 1,269 water resource recovery facilities using an anaerobic digester (˜860 currently use the biogas they produce), 66 stand-alone systems that digest food waste, and 652 landfill gas projects. For comparison, Europe has over 10,000 operating digesters and some communities are essentially fossil fuel free because of them.
The potential for growth of the U.S. biogas industry is huge. We count 14,958 new sites ripe for development today: 8,574 dairy, poultry, and swine farms and 3,878 water resource recovery facilities (including ˜380 who are making biogas but not using it) could support new biogas systems, plus 2,036 food scrap-only systems and utilizing the gas at 415 landfills who are flaring their gas. If fully realized, according to an assessment conducted with the USDA, EPA and DOE as part of the Federal Biogas Opportunities Roadmap, plus data from ABC, these new biogas systems could produce 103 trillion kilowatt hours of electricity each year and reduce the emissions equivalent of removing 117 million passenger vehicles from the road. These new biogas systems would also catalyze an estimated $45 billion in capital deployment for construction activity which would result in approximately 374,000 short-term construction jobs to build the new systems and 25,000 permanent jobs to operate them. Indirect impacts along supply chains would be even greater.”
The invention comprises a family of unique hybrid CO2 solvents which contain both chemisorptive and physisorptive functional groups within the solvent molecule. The modular molecular structure of the solvents is purposely designed to modify the CO2 heat of absorption to obtain CO2 uptake over a wide pressure/temperature range (10-40° C./0.2-25 bar). Modifying the solvent molecular structure to provide stronger CO2 binding energies leads to significant enhancement in CO2 solubility over common commercial physical solvents such as Selexol at low partials pressures (0.2-3 bar). The non-aqueous BAC solvents also provide a particular enhancement in ease of regeneration over common commercial chemical CO2 solvents such as aqueous amines. By modifying the steric constraints near the amine sight to give a weaker CO2 binding energy, the solvents become better suited for high partial pressure CO2 capture. The inclusion of carboxylate functional groups, particularly ester groups, on the molecule also adds a significant boost to CO2 physical solubility since the ester function group is known to be one of the most effective functional groups for increasing CO2 affinity in a solvent.
The critical feature of the molecular structures of the solvents disclosed herein provides an alkyl-amide or alkyl-ester (known generally as an alkyl carboxylate) containing an alkylamine functional group located on the second carbon from the carbonyl carbon (commonly referred to as the beta “β” carbon). The ester or amide functional group in combination with an optimal degree of steric crowding around the amine nitrogen are tailored to modify the strength of CO2 binding in the solvent. These non-aqueous solvents are prepared to contain either one (mono-aminocarboxylate) or two (bis-aminocarboxylate) functional groups per molecule. The solvents have been shown or projected to possess very high CO2 solubilities and high CO2/H2, CO2/N2 and CO2/CH4 as well as good CO2/H2O absorption selectivities. As such, the solvents are well-suited for absorption of CO2 over a wide range of partial pressures from 0.25 to 25 bar and can be regenerated at 1-20 bar with a mild temperature ramp of 25-50° C. above the absorption temperature. Temperature ranges for absorption are solvent dependent and can be tailored to range from 10° C. to 45° C. (20-35° C. preferred). Regeneration temperatures are typically in the range of 35-80° C. (55-70° C. preferred). Many of the solvents can be operated at or above ambient temperature without the need for solvent chilling and regenerated using low grade waste heat. Working capacities for CO2 in the disclosed solvents are higher than standard commercial solvents even when operated at absorption temperatures 10-15° C. warmer than the standard solvents. The disclosed solvents also have the advantage of low viscosity, low vapor pressure, low environmental impact, and low-cost synthesis from commodity reagents.
In a first aspect a method of separating or removing CO2 from a CO2-containing fluid, comprising: providing a beta-amino carboxylate composition comprising at least 5 wt % of a beta-amino carboxylate or mixture of beta-amino carboxylates; combining the CO2-containing fluid and the beta-amino carboxylate or mixture of beta-amino carboxylates at a first temperature and pressure to form a reaction product of CO2 and the beta-amino carboxylate or mixture of beta-amino carboxylates; and exposing the reaction product of CO2 and the beta-amino carboxylate or mixture of beta-amino carboxylates to a second temperature and pressure to release the CO2 from the reaction product.
In a second aspect, the invention provides a method of separating or removing CO2 from a CO2-containing fluid, comprising: providing a beta-amino carboxylate composition comprising a beta-amino carboxylate or mixture of beta-amino carboxylates; combining the CO2-containing fluid and the beta-amino carboxylate or mixture of beta-amino carboxylates at a first temperature and pressure to form a reaction product of CO2 and the beta-amino carboxylate or mixture of beta-amino carboxylates; wherein the CO2-containing fluid comprises at least 10 mol % H2O; wherein the beta-amino carboxylate composition absorbs a higher percentage (or at least twice as high a percentage, or at least 5 times as high) of the CO2 than H2O from the CO2-containing fluid; and exposing the reaction product of CO2 and the beta-amino carboxylate or mixture of beta-amino carboxylates to a second temperature and pressure to release the CO2 from the reaction product.
In the present invention, a “beta-amino carboxylate” is defined as a compound having an amino group and beta-carboxyl group, where the beta carboxyl group is part of an ester or amide moiety. The reaction product can be the product of a chemical reaction, for example to form a carbamic acid or carbamate. Alternatively, the reaction product can be a complex with CO2 physisorbed to the beta-amino carboxylate.
In another aspect, the invention provides a beta-amino carboxylate composition comprising one or any combination of the compounds described herein; and comprising at least 5 wt % CO2 or at least 5 wt % of the reaction product of CO2 and one or any combination of the compounds described herein. In a further aspect, the invention provides a CO2 separation system comprising any of the compositions described herein.
In any of its aspects, the invention can be further characterized by one or any combination of the following: wherein the beta-amino carboxylate composition comprises less than 5% or less than 3% or less than 1% water by weight; wherein the beta-amino carboxylate composition comprises at least 50 wt % or at least 75 wt % or at least 90 wt % of a beta-amino carboxylate or mixture of beta-amino carboxylates; wherein the CO2-containing fluid is a CO2-containing fluid mixture comprising CO2 and at least one other gaseous species, and wherein the step of separating or removing CO2 comprises separating CO2 from the at least one other gaseous species in the CO2-containing fluid mixture; comprising pressure swing adsorption wherein the second pressure is less than the first pressure; comprising temperature swing adsorption wherein the second temperature is greater than the first temperature; further comprising a step of storing the released CO2 in an underground cavern; wherein the released CO2 could be used as a displacement gas in enhanced oil recovery systems; wherein the beta-amino carboxylate or mixture of beta-amino carboxylates comprises one or more of the compounds described in this specification; wherein the beta-amino carboxylate comprises one of the compounds described herein; wherein the beta-amino carboxylate composition comprises less than 5% or less than 3% or less than 1% water by weight; wherein the beta-amino carboxylate composition comprises less than 5% or less than 3% or less than 1% added water by volume (“added water” means water that is present in the BA composition prior to the combining step); wherein the beta-amino carboxylate composition comprises at least 50 wt % or at least 75 wt % or at least 90 wt % or at least 99 wt % of a beta-amino carboxylate or mixture of beta-amino carboxylates; wherein the beta-amino carboxylate composition comprises at least 10 wt % of a non-aqueous solvent; comprising a plurality of cycles of temperature swing absorption; comprising a plurality of cycles of pressure swing absorption; wherein the CO2 containing fluid comprises at least 1 mol % CO2 or at least 2 mol % CO2 or at least 5, or at least 10, or at least 20, or at least 50 mol % CO2; wherein the CO2 containing fluid comprises at least 5 mol % H2O, or at least 10 mol % H2O, and a CO2/H2O in a ratio of from 0.01 to 5, and wherein, in a single cycle, more CO2 is adsorbed than H2O; and/or wherein CO2 is preferentially absorbed relative to H2O; and/or wherein at least twice as much CO2 than H2O is adsorbed (in the present invention, the terms “adsorbed” and adsorbed” may be used interchangeably and refer to CO2 removed from the CO2 containing fluid into the BA-containing composition); wherein the CO2 containing fluid comprises 10 mol % H2O or less, or 5 mol % H2O or less, or 1 mol % H2O or less, and a CO2/H2O in a ratio greater than 0.5, and wherein, in a single cycle, more CO2 is adsorbed than H2O, and/or wherein CO2 is preferentially absorbed relative to H2O; and/or wherein at least twice as much CO2 than H2O is adsorbed; wherein the method is conducted in a temperature range of 0 to 50° C.; wherein the CO2 containing fluid comprises 5 to 50 mol % CO2.
In a further aspect, the invention provides beta-amino carboxylate composition comprising at least 5 wt % of one or more of the compounds described in the specification; and comprising at least 5 wt % CO2 or at least 5 wt % of the reaction product of CO2 and one or more of the compounds described in the specification. The invention also includes a beta-amino carboxylate composition comprising at least 2 wt % of one or more of the compounds described in the specification; and comprising at least 1 wt % CO2 or at least 2 wt % of the reaction product of CO2 and one or more of the compounds described in the specification; and further comprising at least 10 wt % (or at least 25 wt % or at least 50 wt %) of a non-aqueous solvent.
Some preferred non-aqueous solvents include commercial polyethylene glycol solvents such as Selexol®, or hydrophobic solvents such as tributyl phosphate, diethyl adipate, diisobutyl adipate, and diethyl sebacate. Appropriate solvents to use as blends with BAC would exhibit properties of low viscosity, moderate to high CO2 solubility, low CH4 solubility, and high boiling point. Hydrophobic solvents would be preferred in operations involving CO2 removal from high humidity gas streams. Thus, the non-aqueous solvents preferably have a lower neat (pure) viscosity than the CO2 loaded phase of the BAC solvent and a boiling point greater than 200° C. at standard conditions of 2° C. and 1 atm.
The data presented herein shows properties of the invention. The invention may be further characterized by having the one or any combination of the properties (or within ±30% or ±20% or ±10% or ±5%) of one or any combination of the properties described herein.
For precombustion applications, BAC solvents have been developed with enhanced CO2 binding and higher heats of CO2 absorption over Selexol. The physical properties of BAC solvents allow for similar volumes of captured CO2 at higher temperatures than used in the Selexol process and thus has the potential to reduce the cost of solvent chilling. In addition, the higher heat of CO2 absorption provides a possible route for thermal regeneration of the solvent at elevated CO2 pressures near or at the capture pressure. If employed, the higher CO2 recovery pressure could lead to a significant reduction in compression cost of the recovered CO2 prior to pipeline injection.
The highly tailorable solvents disclosed here satisfy the demand for low-cost CO2 solvents with intermediate CO2 binding energies for enhanced CO2 recovery spanning a wide range of industrial and biogenic processes involving a broad band of CO2 partial pressures such as biogas upgrading, landfill gas upgrading, and CO2 capture in cement production, steel manufacturing, ammonia production, and hydrogen production.
Examples of applications of the invention include: pre-combustion CO2 separation and capture at IGCC-CCS; CO2 removal during generation of H2 from reformed natural gas or from syngas; adjust CO/H2 ratio for coal & biomass to liquids; remove CO2 from syngas for coal & biomass to ammonia/fertilizer; natural gas sweetening; biogas upgrading; landfill gas upgrading; CO2 capture during cement manufacturing, steel manufacturing, and petroleum refining. The BAC solvents can be used in continually looping solvent systems for CO2 removal in the treatment of product gas streams such as CH4 or H2 or alternatively in the capture of CO2 from exhaust/biproduct streams in industrial processes, and the invention includes these systems. The solvents can be employed in any process wherein the control of CO2 concentrations is achieved by contact of the gas stream to be treated with a solvent and where the CO2 capture step is done near ambient temperatures (e.g., 40° C. or less). Thus, the gas stream to be treated could be exposed to the BAC solvent percolating through a packed column, or as an aspirated solvent, or bubbled through the solvent, or to a solvent in contact with or part of a gas separation membrane.
Advantages of BAC solvents over existing approaches include either higher pressure CO2 removal processes such as precombustion CO2 capture or H2 production and low CO2 concentration removal processes such as biogas/landfill gas upgrading and CO2 capture during cement manufacturing, etc. For precombustion, BAC solvents provide higher CO2 uptake at warmer temperatures than commercial physical solvents and reduce the need for solvent chilling. BAC solvents have higher heats of CO2 absorption and provide a more efficient route to high pressure recovery of CO2 compared to commercial physical solvents. BAC solvents show lower water solubility compared to Selexol and have significantly lower corrosion rates than water.
For Biogas upgrading: BAC solvents: absorb significantly more CO2 than Selexol or water; do not require compression of the biogas to be effective; have CO2 working capacities operating at ambient pressure which are 3-5× that of Selexol and 20× that of water operating at system pressures of 6-8 bar; and can achieve reduced system size/capital costs. Ester-based BAC solvents environmentally decompose into benign amino acids. BAC solvents can be synthesized from common reagents and tuned for desired physical properties. BAC solvents have CO2:CH4 and CO2:H2O absorption selectivities several times higher than Selexol which reduces methane losses. BAC solvents are less susceptible to algae and bacterial growth than water.
The current invention provides CO2 solubility in non-aqueous solvents by designing the molecular structure of the solvent to include two functional groups having favorable interactions with CO2. The first functional group incorporated is the carboxylate group. The carboxylate group, in particular the ester functional group, is among the best for CO2 physical solvents.1 This functional group will enhance the CO2 solubility in the higher-pressure region of the isotherm. The second functional group built into the solvent molecules is the amine functional group. Amines are known to react with CO2 to form carbamic acids. In aqueous systems with excess amine, the reaction often proceeds to the formation of an ammonium carbamate by deprotonation of the carbamic acid. Additional hydrolysis by water can result in the formation of ammonium bicarbonate/carbonates. The regeneration of CO2 is more energy intensive for bicarbonate/carbonates than it is for carbamic acids so our goal in the design of strong physical solvents for CO2 is to limit bicarbonate/carbonates formation and thus reduce the energy required for regeneration while maintaining a strong CO2 affinity.
The amine solvents are tailored to maximize CO2 uptake while minimizing the energy required for solvent regeneration in three ways. The first is to use the solvents as neat solvents or nonaqueous mixtures. Removing water from the CO2 capture solvent system (that is, to use non-aqueous solvents) increases the overall efficiency of the process. Water promotes corrosion, has a high heat capacity, low boiling point, and latent heat of evaporation which combine to create a large energy penalty in the process. Water also has a very low CO2 solubility. As such, dilution of the CO2 active solvent with water will only reduce the overall volumetric CO2 solubility. Increased water content in the solvent is “dead weight” and may also increase solvent viscosity. Both of these lead to increased pumping costs. Furthermore, the recovered CO2 will have a high moisture content which will need to be remediated prior to pipeline injection. The less water in the solvent, the better. The second strategy is to locate the amine functional group beta to the carboxylate group. The electronic withdrawing effects of a functional group beta to the amine location tends to reduce the basicity and hence reactivity of the amine group. The stronger the electronic withdrawing effect, the less basic and CO2-reactive the amine. The third approach is to sterically hinder the amine site to limit the strength of the interaction of CO2 with the amino nitrogen. The more sterically crowded the amine site, the less favorable the CO2 interaction is with the amine. The “hindered amine” and water lean techniques have been investigated by other researchers as a way to increase CO2 capture efficiency, however, those studies were often hindered by one or more undesirable attributes.25 For example, some studies include complex molecules which will be costly to scale up. Other studies on solvents involving branched alky amines suffer from costly complex molecules or involve highly volatile low molecular weight amines. Conversely, high molecular weight alkyl amine solvents are not favorable since high mole fractions of hydrocarbon in their structures have a negative effect on CO2 solubility due to the weak interaction of CO2 with methyl (—CH3) or methylene (—CH2—) units. Non-aqueous or water lean amine solvents often show significant increases in viscosity upon reaction with absorbed CO2.
The molecular weights of the solvents are designed to provide a balance for low vapor pressure, low viscosity, and high CO2 capacity. Physical properties along with the solvent hydrophilicity are tailored through the size and shape of the alkyl moieties. Additional alkyl hydrocarbon groups increase molecular weights and lower the vapor pressure while also reducing water affinity. These are positive effects for CO2 physical solvents. Addition of too many hydrocarbon units, however, will increase the viscosity and lower the volumetric CO2 solubility since the interaction of CO2 with hydrocarbons is much weaker than the interaction of CO2 with carboxylate groups and amines. Increased hydrocarbon mole fraction in the solvent may also increase the CH4 solubility of the solvent which is undesirable for applications involving CO2 removal from methane streams. An optimal range of hydrocarbons has been determined in which the overall molecular weight of the beta-amino carboxylate is in the range of 180-400 g/mol, with solvent densities in the range of 0.85-1.1 kg/L, viscosities at 25° C. from 2-32 cP and boiling points of at least 200° C. In some embodiments, the solvent has a molecular weight (or in the case of mixtures, a weight average molecular weight) of 350 or less or 300 Daltons or less, or in the range of 150 to 400 or 200 to 300 Daltons. BAC solvents will typically contain C1-C6 amino, amido, or ester groups. The alkyl groups can be linear, branched, or cyclic depending on the desired property of the solvent.
The syntheses of the solvents can be accomplished using established organic reactions as outlined in Scheme 2. The first step in the reaction sequence is the formation of the amide or ester derivative of acrylic acid, methacrylic acid, or crotonic acid. Many ester derivatives of acrylic acid, methacrylic acid, or crotonic acid are commercially available with the methyl, ethyl, and isobutyl esters very common and inexpensive. As such, the use of acrylic acid, methacrylic acid, or crotonic acid derivatives as a route to sterically hindered amines is a potentially more cost-effective route than preparing sterically hindered alkyl amines. Amide versions of the BAC compounds will typically require the reaction of an appropriate alkyl amine with the respective acid chloride. The next step is to react the amido or ester derivative of acrylic acid, methacrylic acid, or crotonic acid with an appropriate alkyl amine using a catalyzed Michael addition where the amine will add to the terminal alkene of the acrylate, methacrylate, or crotonate. Typical reaction times are one to a few days at temperatures ranging from 60-100° C., with lighter amines typically done in sealed reactors to minimize evaporative losses. Several catalysts which are effective for the Michael addition are known and include silica, alumina, and rare earth salts. The Michael addition can be done using a 1:1 ratio of amine to acrylate, methacrylate, or crotonate, a 2:1 ratio of amine to di-acrylate, di-methacrylate, or di-crotonate, or in a 1:2 ratio of ammonia to acrylate, methacrylate, or crotonate. The reactions are often done without the need for solvent. Many beta-amino carboxylates have been prepared and characterized in the course of this work. One skilled in the art would realize that mixed reactions are also possible for the 2:1 and 1:2 reactions in which two or more different amines could be used, or two or more different acrylate, methacrylate, or crotonate compounds could be used to give products with two or more functional groups in the final product. This might be valuable as a route to further refine physical properties of the solvents and CO2 solubilities. Physical mixtures of pure BAC solvents could also be prepared, or mixtures of BAC solvents with other organic solvents, or mixtures of BAC solvents with water, or mixtures of BAC solvents with water and other organic solvent, etc. in a nearly limitless number of ways as well. The Examples focus on the preparation and characterization as pure solvents, or in some mixtures with organic solvents including Selexol, diethyl sebacate, and tributyl phosphate, but anyone skilled in the art could easily expand the applications of these solvents through a variety of mixtures.
Structures of inventive BAC solvents include (but are not limited to) the following:
Space filling modeling showed the steric effects of methyl groups adjacent to the amine site on absorbed CO2 as it interacts with the amine N site. The steric crowding limits the CO2 binding strength of the amine site. The steric effect can be tailored to optimize the CO2 solubility in BAC solvents at the appropriate temperature and pressure conditions while also reducing the energy demands for solvent regeneration The exceptional CO2 solubilities resulting from their hybrid molecular structures along with an ability to tune the strength of CO2 interaction within the BAC family of solvents makes these solvents highly versatile and very-well suited to a wide range of CO2 capture processes. The CO2 absorption isotherm data shown in
The isotherms in
The incorporation of an amine functional group into the solvent adds a variable degree of chemisorptive qualities to the CO2 interaction with the solvent. The higher heat of absorption for CO2 in the BAC solvent instills a higher sensitivity of the CO2 solubility to changes in temperature. The effects of temperature on the shape of the CO2 absorption isotherms are demonstrated in
The BAC solvents have a unique advantage in possessing an inherent boost in CO2 capacity due to an enhanced interaction with the carboxylate and hindered amine sites. These two effects combine to increase both the low-pressure and higher-pressure CO2 solubility in the solvents. Thus, the solvents can be optimized for applications involving capture of CO2 at lower partial pressures such as encountered in biogas/landfill gas,8-22 cement production23-24 and steel production or for applications involving higher partial pressure of CO2 such as H2 and NH3 production.7 The benefits of these solvents will now be discussed in two contrasting applications as examples of their unique properties. The first demonstration will involve pre-combustion CO2 capture in a reforming process which generates H2 as fuel for a turbine in an Integrated Gasification Combined Cycle (IGCC) power plant where CO2 is at an elevated partial pressure of 20-25 bar at ˜50% concentration in H2. The second example will be for biogas methane upgrading which is typically performed at CO2 partial pressures near 2.5 bar at ˜35% concentration in methane.
Physical solvents are ideally suited to CO2 capture under precombustion conditions due to the higher partial pressure of CO2 involved (20-25 bar).6-7 The solvents are typically chilled to further maximize the CO2 solubility even though solvent chilling introduces energy penalties associated with the higher solvent viscosity and chiller operation. Operations at warmer temperatures with similar CO2 solvent capacity would thus be beneficial. As shown in
The higher heat of CO2 absorption in the EMA-NH-EMA compared to a typical physical solvent such as Selexol offers other potential advantages in system design for CO2 removal. With a higher heat of absorption, the CO2 uptake in the solvent is more effected by temperature swing. As such, the CO2 solubility in EMA-NH-EMA drops significantly more than in Selexol with a ramp in temperature. This temperature sensitivity provides a means for regeneration of the solvent using only a change in temperature without a need for a drop in pressure. Referring again to the isotherms shown in
High Molecular Weight Solvents with Reduced Volatility
A subgroup of BAC solvents was created which contain two beta-amino carboxylate groups per molecule. These solvents were prepared from diacrylate derivatives which have a terminal double bonds and internal ester groups (bottom right in Scheme 1). The bis-(amino) nature leads to solvents with higher molecular weights. The higher molecular weights of the solvents result in projected boiling points >300° C. and very low vapor pressures, but higher room temperature viscosities. The high-boiling solvents were designed for applications where CO2 capture, and solvent regeneration could be accomplished at elevated temperatures with little solvent loss due to evaporation. One potential benefit to this application window would be in the ability to regenerate the solvent at elevated pressures using a temperature ramp as discussed above with only negligible solvent loss due to evaporation. High-boiling solvents allow higher regeneration temperatures and thus higher working capacities for regeneration at elevated pressures. As discussed above in reference to the isotherm data in
Biogas methane is formed through the anaerobic digestion of municipal or agricultural waste.8-22 The gas generated in the digestor averages 35% CO2 with the balance mostly methane. (Note that landfill gas can have significantly higher concentrations of N2 which would also need to be removed from the product CH4 gas in a separate stage from the CO2 capture.) In most commercial solvent processes, capture of the sub-ambient pressure CO2 in the biogas is facilitated by pressurizing the system to 6-8 bar to give a CO2 partial pressure of approximately 2.5 bar. CO2 is then removed using a circulating solvent system which typically uses water, or in some cases Selexol. Compression of the biogas is required due to the low CO2 solubility in water and Selexol. Compression of the biogas adds to the energy demands of the process and increases the CH4 solubility in the CO2 removal solvent. As such, BAC solvents with significantly higher CO2 solubility at low partial pressures of CO2 could reduce or even remove the need for compression of the feed gas and thus reduce the energy demands of the treatment process. The benefits for BAC solvents in biogas upgrading can be summarized as follows: high CO2 removal capacity without pressurization of biogas; low pressure operation reduces CH4 co-absorption in solvent; high CO2 capacity reduces size of absorber; high purity of recovered CO2; and mild solvent regeneration conditions.
To demonstrate the effectiveness of BAC solvents at removing CO2 under ambient conditions, CO2 absorption tests were run at 25° C. using a flowing 30/70 mixture of CO2/N2 at a total pressure of 1 bar. Note that Selexol is ineffective under these conditions since the CO2 solubility at the regeneration condition (PCO2 1 bar, 60° C.) is slightly higher than the CO2 solubility at the capture condition (PCO2 300 mbar, 25° C.). The working capacity of Selexol is essentially 0 wt % when the feed gas is not compressed above ambient pressure. In contrast, pure BAC solvents 1AHEMA, 1AA-iBMA and 1AB-DECAM, and a blended BAC solvent TBP/1AB-DECAM (TBP=tributyl phosphate) consisting of 20 wt % TBP and 80 wt % 1AB-DECAM all show very high CO2 uptakes under these conditions. Working capacities (WC) are indicated on the chart using conservative estimates of the absorbed amounts of CO2 after a 20-minute time window corresponding to the approximate time required for CO2 saturation in Selexol. Note that even higher working capacities are possible with extended CO2 contact times with the solvent. The working capacities are also variable depending on choice of regeneration temperature. The ultimate choices of absorption and regeneration temperatures would be guided by system parameters including CO2 mass transfer rates and solvent evaporation rates. The results presented in
Solvent stability was evaluated for BAC solvents under simulated absorption/desorption cycling. The results of a cycling test for BAC solvent 1AB-DECAM is shown in
Additional testing was done to determine the effect of moist CO2 on the absorption and the ability to regenerate BAC solvent 1AH-EMA since the presence of significant amounts of water in amine/CO2 mixtures can lead to the formation of carbonates which require more energy input to regenerate CO2. As noted earlier, typical biogas processing involves a water removal step prior to the CO2 removal step so moisture levels in the biogas feed are expected to be well below the levels used in the 1AH-EMA evaluation. The tests were designed to include higher moisture levels to ensure the solvent is stable. The results of the water vapor absorption and wet CO2 absorption tests with 1AH-EMA are summarized in
The results shown in
Water Solubility.
Water solubility in CO2 solvents is an important property to consider. High water solubility can be beneficial in some applications where co-removal of water and CO2 from the gas being upgraded is desired. This approach is often applied in natural gas upgrading where hydrophilic solvents such as Selexol, NMP, and propylene carbonate are designed to aid in dehydration of the sour gas. In other applications, such as pre-combustion CO2 capture, co-adsorption of water with the captured CO2 is not desired since water is needed for the combustion system, water reduces the CO2 capacity of the solvent, increases the pumping cost of the solvent, and must be removed from the captured CO2 prior to injection into the transmission pipeline to the sequestration site. Thus, control of water solubility is important in CO2 solvent design depending on the targeted application. Water solubility in BAC solvents can be controlled to a significant degree depending on the nature of the carboxylate and amine functional groups. In general, esters are more hydrophobic than amide functionalized BAC solvents. For example, a humidification chamber study on ester solvent EMA-NH-EMA and amide solvent 1AB-DECAM showed that EMA-NH-EMA absorbed 25 wt % of water and 1AB-DECAM absorbed 54 wt % of water after 200 hours of continuous exposure to 90% relative humidity at room temperature. Note that these evaluations were done under extreme conditions. In an actual solvent looping process, the majority of the moisture would be removed from the feed gas prior to the CO2 removal step and contact times with the solvent would be much shorter with regeneration of the solvent performed after each absorption step.
Some carbon capture solvents containing amines undergo side reactions to form solid carbamates, bicarbonates or carbonates, particularly in the presence of water. The formation of these unwanted compounds increases the energy demand of the solvent regeneration step, reduces the capacity of the solvent by converting a portion of it irreversibly, and diminishes the amount of solvent available for reversible CO2 capture. The formation of solids can also increase solvent viscosity, clog solvent pumps, and foul system channels causing downtime. Samples of pure BAC solvents were studied by FTIR spectroscopy under both dry and wet N2 and CO2, repeatedly cycling between these conditions. The results showed that while evidence of both physisorbed and chemisorbed CO2 was seen in all BAC solvents, the amount of CO2 absorbed was not diminished in the presence of water nor were any irreversible side products observed; sample spectra always reverted to their original appearance after purging with dry N2. Results obtained for BAC solvent 1AH-EMA are shown in
For high pressure CO2 removal processes in pre-combustion applications, the commercial solvent of choice is Selexol. The absorption process takes advantage of the highly reversible nature of CO2 absorption in the physical solvent by using a pressure swing adsorption/desorption cycle along with solvent warming. The CO2 is captured from a high partial pressure stream consisting of ˜50/50 mixture of CO2 and H2 at a total pressure near 50 bar. The absorber operates at 10° C. to enhance the volumetric CO2 solubility and reduce the size of the absorption column. The absorbed CO2 is then released near 25° C. in a series of flash tanks which step down the pressure to about 1.5 bar. Once the CO2 is recovered, it needs to be cleaned and then recompressed to 1500-2200 psi to transport it via pipeline for geologic storage, enhanced oil recovery, or CO2 utilization.
Tailored BAC solvents are designed to improve the efficiency of the process by providing high CO2 capacity without the need for solvent chilling while also providing a means for reducing compression costs of the recovered CO2 by regenerating the solvent at or near the absorber pressure using a moderate temperature ramp. The contrast in CO2 uptake at different temperatures between BAC solvent EMA-NH-EMA and Selexol are shown in
A number of commercially important processes generate CO2 at concentrations intermediate between those involved with post-combustion CO2 capture and syngas processes. These processes are listed in Table 1 and include biogas/landfill gas CH4 upgrading, refineries, cement production and steel manufacturing. The CO2 concentrations in these applications span a typical range of 20-40%. While different CO2 removal technologies have been proposed for some of these processes, only biogas upgrading has seen significant deployment. The advantages of BAC solvent for biogas upgrading will be applicable to other applications listed in Table 1 since these processes all involve CO2 removal under similar concentrations, however the current discussion will focus on biogas upgrading since this is the most widely employed commercial need.
Current commercial biogas upgrading processes that involve continually looping solvent systems predominantly use water or Selexol as CO2 physical solvents. Both water and Selexol have relatively low CO2 solubility and thus require compression of the biogas stream to increase CO2 uptake in the solvent. Compression of the feed gas increases the energy demand of the system and increases co-adsorption of CH4 in the solvent. Replacing Selexol with BAC solvent 1AH-EMA will allow the system to operate at ambient pressure while providing a significant enhancement in CO2 working capacity and reduction in CH4 co-adsorption. The results shown in
The working capacity of Selexol is essentially 0 wt % when the feed gas is not compressed above ambient pressure. As a consequence, biogas upgrading systems that employ Selexol operate the absorber at elevated pressures in the range of 6-8 bar and regenerate the solvent at 40° C. at 1 bar. The working capacity for Selexol under these conditions can be estimated based on the CO2 Henry's constants for Selexol of 0.166 mol/(L*bar) at 20° C. and 0.107 mol/(L*bar) at 40° C. The theoretical maximum CO2 working capacity in a 30% CO2 stream at 8 bar total pressure would be 2.4 bar×0.166 mol/(L*bar)−1 bar×0.107 mol/(L*bar)=0.291 mol/L. In contrast, the theoretical CO2 working capacity for BAC solvent 1AH-EMA operating without compression of the feed gas with an absorber temperature of 25° C. and a solvent regeneration temperature of 45° C. is 0.42 mol/L. This is a 45% increase in working capacity using the same 20° C. temperature swing while removing the need for compression of the feed gas. The working capacity of 1AH-EMA can be improved significantly with a modest increase in the regeneration temperature. For example, increasing the absorption/desorption temperature swing from 20° C. to 30° C. more than doubles the working capacity from 0.42 mol/L to 1.01 mol/L. In contrast, increasing the desorption temperature for Selexol by 10° C. only shows a meager gain in working capacity from 0.29 mol/L to 0.31 mol/L. By simply increasing the temperature swing from 20° C. to 30° C., the working capacity of 1AH-EMA can be increased to 325% that of Selexol. This result is even more impressive because it does not require any compression of the feed gas.
Replacing Selexol in a biogas upgrading process with BAC solvent 1AH-EMA could theoretically increase CO2 working capacity by 300% or more while removing the need for biogas compression. Operating the system at 1 bar will also provide an addition benefit of significant reduction in co-adsorbed CH4 in the CO2 removal solvent. The absorption of CH4 in a solvent at conditions typical for biogas upgrading follows a linear correlation with pressure according to Henry's law, i.e., CH4 solubility=Pressure×(Henry's Constant for CH4). As such, the CH4 solubility in the CO2 removal solvent at 1 bar will be ⅙ the value at 6 bar or ⅛ the value at 8 bar. By operating the BAC biogas upgrading at 1 bar, the co-adsorption of CH4 will be reduced by nearly 85% from a Selexol system operating at 6 bar assuming similar CH4 henry's constants for the two solvents. In-house gravimetric measurements gave CH4 solubility in Selexol of 0.0086 mol/(L*bar) at 20° C. and CH4 solubility in 1AH-EMA of 0.016 mol/(L*bar) at 25° C. Based on these respective values, the CH4 solubility from a 30/70 mixed CO2/CH4 feed gas at a total pressure of 6 bar would be 0.036 mol/L in Selexol. The same gas stream at 1 bar total pressure would give a CH4 solubility in 1AH-EMA of 0.0112 mol/L. This equates to a 69% reduction in CH4 losses due to solvent absorption. The low CH4 uptake in 1AH-EMA will also lead to a significant enhancement in the purity of the recovered CO2. The ratio of absorbed CO2 to absorbed CH4 in 1AH-EMA at 25° C. for a gas stream consisting of 30% CO2 in CH4 at 1 bar is approximately 1.6/0.0112=143:1. Assuming complete release of co-adsorbed CH4 and 1 mol/L of CO2 during the regeneration will result in a recovered CO2 stream consisting of only 88% CO2/12% CH4 for the Selexol process versus 99% CO2/1% CH4 for the 1AH-EMA process.
The use of a non-aqueous BAC solvent such as 1AB-DECAM, 1AA-IBMA or 1AH-EMA in a CO2 removal application in processes such as biogas upgrading, landfill gas upgrading, and cement production where CO2 concentration in the feed gas is above 15% and operating near ambient conditions provides advantages over traditional aqueous amine solvents. These advantages include reduced corrosion rates and milder solvent regeneration conditions. BAC solvent such as 1AB-DECAM or 1AH-EMA can be regenerated at 1 bar under pure CO2 streams at mild temperatures of 45-60° C., whereas aqueous amine solvents typically require steam stripping at temperatures in the range of 80-160° C. The cooler desorption temperature and pure CO2 stripping sweep gas will result in significantly reduced solvent loss and reduced solvent decomposition while yielding a cleaner CO2 recovery without co-adsorbed water. The perceived benefits of water lean amine solvents have led to an increasing interest recently among research groups to develop new and improved CO2 capture solvents based on this strategy. The vast majority of these research efforts are aimed at applications involving post-combustion CO2 capture, but some of their results are indeed relevant to related applications targeted for BAC solvents such as biogas upgrading, landfill gas upgrading, and cement production. The majority of prior attempts at water lean amine solvents for CO2 capture have been hampered by significant increases in solvent viscosity in the CO2 loaded phase. The increase in viscosity can be several orders of magnitude which leads to major penalties in CO2 mass transfer rates and pumping costs. To evaluate the effect of CO2 loading on viscosity, three BAC solvents were tested after saturation with CO2 at 1 bar, 25° C. The results are shown in
The defining chemical component of the BAC solvent molecular structures comprises the beta-amino carboxylate group shown below:
The BAC carboxylate component may comprise either an ester or an amide. This core structural motif is critical for several reasons. The alkyl ester or dialkyl amide moiety controls the melting point and viscosity to ensure that the compound is liquid, stable, and has low volatility in the targeted temperature and pressure range of the application. Location of the amine site in a position beta to the carboxylate carbonyl carbon provides a unique electronic effect on amine reactivity and also provides a convenient route to synthesis using established high yield organic reactions, e.g., Michael addition, involving simple organic amines wherein the steric crowding of the amine site can be conveniently controlled. The physical properties and CO2 affinity of the solvent can be easily optimized for a particular application via the incorporation of different alkyl groups on the amide, ester, and/or amine site. The optimal size of the alkyl groups is from C1-C6 to maintain high volumetric CO2 uptake, low viscosity, and low melting point. To further illustrate the spirit of the invention, examples of BAC solvents which have been prepared and characterized are included in the appendix. The amine site functions best as a secondary amine. Two alkyl groups on the amide nitrogen are preferred to provide a melting point below room temperature.
The solvents are designed with the intention of using them as the primary CO2 capture component in a CO2 capture system. The total CO2 capacity of the solvent will be maximized when operated as a neat solvent. However, in certain applications, the rate of CO2 diffusion may be more critical than the overall CO2 capacity of the solvent. In these circumstances, it may be advantageous to blend the BAC solvent with one or more co-solvents. These co-solvents would have a lower neat viscosity than the CO2 loaded phase of the BAC solvent. The blend would then be optimized to provide the highest CO2 diffusion rate and highest CO2 absorption amount wherein one or more BAC solvents in the blend would be the main CO2 reactive component.
Examples of alternative versions of BAC solvents are shown above. Two derivatives of BAC solvents with similar molecular weights and steric hinderance around the amine sites. The structure on the left is prepared as described in this disclosure using a common crotonyl derivative and butyl amine, whereas the structure on the right is prepared from a specialty chemical and ethyl amine. An alternative BAC solvent (bottom) using an ether functionalized amine in place of an alkyl amine.
The structures of the BAC solvents can be designed to minimize water affinity. In certain applications where water absorption needs to be optimized, or in applications where water vapor absorption is of little concern, functionalization of the pendant alkyl groups may be preferred. As an example, the n-butyl group on the amine site could be replaced with an ether or ester group to provide an enhanced CO2 affinity of the solvent. The BAC core structure can be derived from functionalization of an acrylate, methacrylate, or crotonate core since each of these building blocks are commodity chemicals and readily available at reasonable costs. One skilled in the art could use a specialty chemical in which the methyl group of the methacrylate or crotonate is replaced with an ethyl, propyl, butyl, pentyl or hexyl group in order to prepare a modified BAC solvent with similar molecular weight and steric influence on the amine site.
This application claims the priority benefit of U.S. Provisional patent application Ser. No. 63/325,146 filed 29 Mar. 2022 which is incorporated herein as if reproduced in full below.
Government Rights: This invention was created utilizing funding from the Department of Energy, RSS contract number 89243318CFE000003. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63325146 | Mar 2022 | US |
