Hydrophobic Alkyl-Ester Physical Solvents for CO2 Removal from H2 Produced from Synthesis Gas

Abstract

One or more embodiments relate to a method for removing CO.sub.2 from a gaseous stream containing CO.sub.2 having the steps of contacting the gaseous stream containing CO.sub.2 with a solvent at a first temperature and a first pressure to dissolve said CO.sub.2 in said solvent, where the solvent is made up of at least one ester, and where said at least one ester has two or more alkyl-ester functional groups on a central hydrocarbon chain.

Claims

1. A method for removing CO.sub.2 from a gaseous stream containing CO.sub.2 comprising: contacting the gaseous stream containing CO.sub.2 with a solvent at a first temperature and a first pressure to dissolve said CO.sub.2 in said solvent, wherein the solvent comprises at least one ester, and wherein said at least one ester comprises two or more alkyl-ester functional groups on a central hydrocarbon chain.

2. The method of claim 1 wherein the at least one ester is selected from the group of esters consisting of: ##STR00001## and combinations thereof, wherein 2≤n≤10, and wherein R.sub.1, R.sub.2, and R.sub.3 are alkyl groups having between and including one and six carbons.

3. The method of claim 1 wherein the solvent is a diester selected from the group consisting of diethyl sebacate, diisobutyl adipate, dibutyl succinate, diethyl adipate, diethyl succinate, diethyl glutarate, and combinations thereof.

4. The method of claim 1 wherein the first temperature is between approximately −15° C. and approximately 100° C. and, and wherein the first pressure is between approximately 5 bar and approximately 100 bar.

5. The method of claim 1 wherein the gaseous stream containing CO.sub.2 comprises products from syngas-generating process selected from the group of gasses consisting of CO.sub.2, CO, H.sub.2, and H.sub.2O.

6. The method of claim 1 wherein the gaseous stream containing CO.sub.2 also comprises H.sub.2 and H.sub.2O.

7. The method of claim 6 wherein the solvent has a water solubility of between approximately 0.2 mol/L of H.sub.2O and approximately 2 mol/L of H.sub.2O at 25° C.

8. The method of claim 6 wherein the solvent has a CO.sub.2/H.sub.2 selectivity between approximately 60 and approximately 90 at 25° C.

9. The method of claim 1 further comprising regenerating the solvent by at least one of reducing the pressure of the solvent to a second pressure and heating the solvent to a second temperature, wherein regenerating the solvent results in CO.sub.2 leaving the solvent.

10. The method of claim 9 wherein the solvent is heated using waste heat.

11. The method of claim 9 wherein the second temperature is between approximately 10° C. and approximately 180° C., and wherein the second pressure is between approximately 1 bar to approximately 25 bar.

12. The method of claim 1 wherein the solvent has a corrosion rate of less than around 0.005 mm/year when a carbon steel or stainless steel coupon is immersed in the solvent at a temperature of about 21° C. and in the presence of CO.sub.2 at a pressure of 100 psig.

13. The method of claim 1 wherein the gaseous stream contains CO.sub.2 at a partial pressure between approximately 15 bar and approximately 30 bar.

14. The method of claim 1 wherein the solvent is a physical solvent, and wherein the contacting step results in CO.sub.2 dissolved in the solvent without bonds formed between the CO.sub.2 and solvent.

15. The method of claim 1 wherein the solvent is hydrophobic.

16. The method of claim 1 wherein the solvent has a CO.sub.2 solubility between approximately 6.5 mol CO.sub.2 per liter of solvent and approximately 9.0 mol CO.sub.2 per liter of solvent.

17. The method of claim 1 wherein the solvent has a viscosity between approximately 1 cP and approximately 15 cP during the contacting step.

18. The method of claim 1 wherein the solvent has a CO.sub.2/H.sub.2 selectivity greater than 50.

19. The method of claim 18 wherein the solvent has a CO.sub.2/H.sub.2 selectivity between approximately 60 and approximately 90.

20. The method of claim 1 wherein the solvent has a boiling point between approximately 180° C. and approximately 300° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The invention together with the above and other objects and advantages will be best understood from the following detailed description of the preferred embodiment of the invention shown in the accompanying drawings, wherein:

[0021] FIG. 1 is a table of properties taken from the literature for commercially used CO.sub.2 solvents, in accordance with the features of the present invention;

[0022] FIG. 2 is a simplified schematic of a system for contacting a gaseous stream containing CO.sub.2 from a source with a physical CO.sub.2 solvent, in accordance with the features of the present invention;

[0023] FIG. 3 depicts the generic structures for hydrophobic esters for use as solvents for CO.sub.2 in a gaseous stream, in accordance with the features of the present invention;

[0024] FIG. 4 is a flow chart of a method for using hydrophobic esters as a solvent for CO.sub.2 capture from a gaseous stream, in accordance with the features of the present invention;

[0025] FIG. 5 depicts the structures of several hydrophobic esters suitable as solvents for CO.sub.2 in a gaseous stream, in accordance with the features of the present invention;

[0026] FIG. 6 depicts the structures of particular diesters tested for suitability as solvents for CO.sub.2 in a gaseous stream, in accordance with the features of the present invention;

[0027] FIG. 7A is a table of working capacities at 10° C. for various diester CO.sub.2 solvents described herein as well as two commercial solvents, in accordance with the features of the present invention;

[0028] FIG. 7B is a plot showing Henry's Law constants listed in the literature for various commercial CO.sub.2 solvents, in accordance with the features of the present invention;

[0029] FIG. 7C is a plot showing Henry's Law constants at 25° C. for various diester CO.sub.2 solvents described herein, in accordance with the features of the present invention;

[0030] FIG. 8 is a plot showing the volumetric CO.sub.2 uptake of diethyl adipate (CASSH-4) as a function of pressure at various temperatures as well as the volumetric CO.sub.2 uptake of diethyl succinate (CASSH-5) measured at 0° C., as well as diethyl glutarate (CASSH-6), and DEPG both measured at 10° C., in accordance with the features of the present invention;

[0031] FIG. 9 is a plot of Henry's Law constants at 25° C. as a function of moles of ester functional group per liter of the diesters shown in FIG. 5, in accordance with the features of the present invention;

[0032] FIG. 10 is a table summarizing physical properties and CO.sub.2 solubility parameters for ester CO.sub.2 solvents as compared to commercial solvents, in accordance with the features of the present invention

[0033] FIG. 11A is a plot of the saturated water capacity at 25° C. for various CO.sub.2 solvents, in accordance with the features of the present invention;

[0034] FIG. 11B is a table of water solubility values for moderately hydrophilic CO.sub.2 solvents at 25° C., in accordance with the features of the present invention;

[0035] FIG. 11C is a table of H.sub.2O capacity values for various diester CO.sub.2 solvents, in accordance with the features of the present invention;

[0036] 12A is a plot of gravimetric water absorption from humidified N.sub.2 at 25° C., in accordance with the features of the present invention;

[0037] FIG. 12B is a plot of mass change over time in solvent as feed gas is switched at time zero from dry CO.sub.2 to humified CO.sub.2 at 1 bar and 10° C., in accordance with the features of the present invention;

[0038] FIG. 13 is a plot of weight change of DEPG stored in humidified N.sub.2 over 30 hours, in accordance with the feature of the present invention;

[0039] FIG. 14 is a plot of low temperature isotherms for CO.sub.2 dissolved in various diester solvents as a function of pressure at 0° C., in accordance with the features of the present invention;

[0040] FIG. 15A depicts the generic structures of multi-ester (more than two) CO.sub.2 solvents, with FIG. 15B showing R groups for the generic structures of FIG. 15A, wherein TMP is an acronym for trimethylopropane esters, and wherein TME is an acronym for trimethylolethane esters, in accordance with the features of the present invention;

[0041] FIG. 15C is a table of molecular weights for various ester CO.sub.2 solvents, in accordance with the features of the present invention;

[0042] FIG. 16 is a plot of Henry's Law constants at 40° C. as a function of molar concentration of ester group for various ester CO.sub.2 solvents, in accordance with the features of the present invention;

[0043] FIG. 17A is a plot of water uptake over time at various relative humidities at 25° C. for DEPG, in accordance with the features of the present invention;

[0044] FIG. 17B is a plot of water uptake as a function of relative humidity for various ester CO.sub.2 solvent and a commercial CO.sub.2 solvent, in accordance with the features of the present invention;

[0045] FIG. 18 is a plot of CO.sub.2 absorption as a function of pressure at 40° C. for various ester CO.sub.2 solvents and various commercial CO.sub.2 solvents, in accordance with the features of the present invention;

[0046] FIG. 19 is a table showing corrosion rates measured for carbon and stainless steels in various solvents, in accordance with the features of the present invention; and

[0047] FIG. 20 is a plot showing relative corrosion rates of C1020 carbon steel in the presence of various solvents, in accordance with the features of the present invention.

DETAILED DESCRIPTION

[0048] The foregoing summary, as well as the following detailed description of certain embodiments of the present invention, will be better understood when read in conjunction with the appended drawings.

[0049] FIG. 2 depicts a simplified schematic of a system 10 to be used with the present invention. As shown in FIG. 2 the system 10 comprises a gaseous stream 12 eluting from a source 14, wherein the gaseous stream 12 flows from the source 14 (flow direction indicated with arrows) and into an absorber vessel 16 containing a solvent 18. Said source 14 is in reversible fluid communication with and upstream from said absorber vessel 16. In an embodiment, the gaseous stream 12 flows into the absorber vessel 16 through the solvent 18. The absorber vessel 16 is in reversible fluid communication with and upstream from a combustion module 20.

[0050] In an embodiment, the gaseous stream 12 comprises any stream of gasses containing CO.sub.2. Exemplary gaseous streams 12 comprise products from a syngas-generating process eluting from the source 14. In an embodiment, the products from a syngas-generating process comprise H.sub.2, CO.sub.2, and H.sub.2O and represent a syngas stream (combination of H.sub.2 and CO) that has been through a water gas shift reaction. Where the gaseous stream comprises products from a syngas-generating process, the components can be present in any proportion, with typical compositions comprising between approximately 10 mol % and approximately 70 mol % H.sub.2, and between approximately 20 mol % and approximately 80 mol % CO.sub.2, where the stream is saturated with water. Where the syngas products have not gone through the water gas shift reaction CO will be present, with CO.sub.2 and H.sub.2O present in lower amounts than in post-WGS reaction streams.

[0051] In an embodiment, the source 14 is any source of CO.sub.2, syngas, or combinations thereof. Exemplary sources include solid fuel gasification plants (IGCC) with the solid fuel being coal, biomass, or solid waste partially oxidized using either air or pure oxygen. Further exemplary sources include natural gas steam methane reforming SMR plants, and/or petroleum refineries reforming gaseous or liquid fuels into hydrogen and carbon dioxide.

[0052] A salient feature of the invention is the solvent 18 used to contact the gaseous stream 12. In an embodiment, the solvent is a physical solvent comprising one or more esters wherein the esters comprise at least two alkyl-ester functional groups on a central hydrocarbon chain. Specifically, the solvent 18 used in the instant invention is an ester or combination of esters selected from esters represented by the general formulae for suitable diesters 40, tri-esters 42, or tetra-esters 44 shown in FIG. 3, wherein 2≤n≤10, and wherein R.sub.1, R.sub.2, and R.sub.3 are alkyl groups having between and including one and six carbons. Suitable and exemplary R groups include methyl, ethyl, propyl, iso-propyl, n-butyl, sec-butyl, or iso-butyl. In an embodiment, whether the solvent 18 is a diester 40, tri-ester 42, or tetra-ester 44, all R groups are the same. In another embodiment, all the R groups are different. In still yet another embodiment, at least two of the R groups of the solvent 18 are the same, wherein, another R group is different than the at least two that are the same.

[0053] Embodiments of the invention use neat, pure compounds as solvent 18 (i.e. one ester at a time). In other embodiments one or more esters are combined, the combination used as the solvent 18. In still other embodiments, one or more ester compounds are combined with minor additives to create the solvent 18, wherein the additives do not affect the bulk properties of the solvent, wherein such additives include anti-microbial agents, anti-scaling agents, anti-foaming agents, anti-corrosion additives, and combinations thereof. In yet another embodiment, the solvent 18 comprises at least one of the esters shown in FIG. 2 combined with another CO.sub.2 solvent such as a hydrophobic Poly(ethylene glycol) (PEG)/polydimethylsiloxane (PDMS) solvent, a hydrophobic ionic liquid solvent, another hydrophobic and low pressure solvent, and combinations thereof.

[0054] FIG. 4 is a flow chart of a method 50 of using the solvents 18 described herein to remove CO.sub.2 from a gaseous stream 12 by solvating said CO.sub.2 while solvating little to no water. The method 50 begins by contacting 52 the gaseous stream 12 with solvent 18, wherein contacting the gaseous stream 52 with solvent comprises bubbling said gaseous stream 12 through said solvent 18, and wherein the contacting step 52 comprises dissolving the CO.sub.2 previously contained in the gaseous stream 12 into the solvent 18. The contacting step 52 is performed at a first temperature and pressure. The contacting step 52 removes CO.sub.2 from the gaseous stream and leaves a stream of H.sub.2 that may then be combusted. As the solvent 18 used in the method 50 is a physical solvent, said solvent dissolves CO.sub.2 during the contacting step 52 without forming bonds between the solvent 18 and CO.sub.2.

[0055] After the contacting step 52, the method continues by regenerating 54 the solvent 18 by heating the solvent containing dissolved CO.sub.2, reducing the pressure within the absorber vessel containing the solvent containing dissolved CO.sub.2, or a combination thereof. The regenerating step 54 of the invented method is performed at a second temperature and pressure.

[0056] A salient feature of the present invention is the ability of the contacting step 52 of the invented method to be performed at the first temperature and pressure. Said first temperature and pressure are higher than prior art processes that require cooler temperatures of −40° C. to 10° C. Suitable first temperatures are between approximately −15° C. and approximately 100° C., with typical first temperatures between approximately 20° C. and approximately 50° C. Suitable first pressures are between approximately 5 bar to approximately 100 bar, with typical first pressures between approximately 10 bar to approximately 60 bar. In an embodiment, the first temperature is above approximately room temperature, i.e. above approximately 20° C. As described herein, the first temperature is the initial temperature of the solvent prior to initial contact with the gaseous stream. As described herein, the first pressure is the initial pressure of the gaseous stream prior to initial contact with the solvent.

[0057] In an embodiment, during the regeneration step 54, the invented method utilizes a lower second temperature and pressure than prior art methods. In an exemplary embodiment, the suitable second temperatures are between approximately 10° C. to approximately 180° C., with typical second temperatures between approximately 40° C. and approximately 120° C. Suitable second pressures are between approximately 1 bar to approximately 25 bar, with typical second pressures between approximately 5 bar to approximately 15 bar. Given the relatively mild temperatures and pressures used in the regeneration step of the invented method, waste heat or low grade heat is suitable for heating the solvent containing dissolved CO.sub.2 to regenerate said solvent. As described herein, the second temperature is the temperature to which the solvent is heated to remove CO.sub.2. As described herein, the second pressure is the pressure of CO.sub.2 leaving the solvent during the regenerating step.

[0058] In order to facilitate use of said higher first temperature and pressure, suitable solvents are needed. The inventors have identified several specific solvents within the generic structures shown in FIG. 3 that are exemplary and suitable solvents for use in the present method. Six exemplary diester solvents are shown in FIG. 5 and are identified by the monikers CASSH-1 60, CASSH-2 62, CASSH-3 64, CASSH-4 66, CASSH-5 68, and CASSH-6 70 herein. Various physical properties of CASSH-1-CASSH-6 are shown below in TABLE 1 along with the names of the CASSH solvents. In TABLE 1, a * denotes a value at 25° C.

TABLE-US-00001 TABLE 1 MW, Density, Viscosity, Solvent Name g/mol Bp, ° C. g/mL* cP* CASSH-1 Diethyl 258.35 312 0.963 5.2 sebacate CASSH-2 Diisobutyl 258.35 293 0.954 5.2 adipate CASSH-3 Dibutyl 230.30 275 0.977 4.1 succinate CASSH-4 Diethyl 202.25 251 1.009 3.1 adipate CASSH-5 Diethyl 174.19 218 1.047 2.6 succinate CASSH-6 Diethyl 188.22 237 1.022 2.6 glutarate

[0059] Another salient feature of the invented method is the ability to separate CO.sub.2 from a gaseous stream that also contains H.sub.2O while absorbing little to no water. In an embodiment, the invented method solvates CO.sub.2 from a gaseous stream also containing H.sub.2O to concentrations equal to or greater than prior art methods. In order to facilitate this feature, the solvents used herein are hydrophobic where current commercial solvents are hydrophilic. In an exemplary embodiment, the invented method solvates between approximately 6.5 mol CO.sub.2 per liter of solvent and approximately 9.0 mole CO.sub.2 per liter of solvent at a solvent temperature of 10° C. and a CO.sub.2 partial pressure of 25 bar or between approximately 4.0 mol CO.sub.2 per liter of solvent and approximately 5.5 mol CO.sub.2 per liter of solvent at a solvent temperature of 25° C. and a CO.sub.2 partial pressure of 25 bar. In an embodiment, the solvent 18 of the instant method has a water solubility of between approximately 0.2 mol/L of H.sub.2O and approximately 2 mol/L of H.sub.2O at 25° C.

[0060] Yet another salient feature of the invented method is the CO.sub.2/H.sub.2 selectivity of the method. As the invented method separates CO.sub.2 from pre-combustion syngas streams containing at least H.sub.2 and CO.sub.2 while minimizing the solvation of the H.sub.2, said selectivity ratio is an important feature. In an exemplary embodiment, the CO.sub.2/H.sub.2 of the invented method is greater than approximately 50 and is typically between approximately 60 and approximately 90.

[0061] Still another salient feature of the invented method is the low vapor pressure of the solvents utilized. The solvents used in this method are designed specifically to minimize the volatility of said solvents to minimize the cost and difficulty associated with replenishing the solvents and to limit the necessity of separating solvent from gas streams that have been stripped of CO.sub.2. In an exemplary embodiment, the invented method features solvents having a low vapor pressure with boiling points between approximately 180° C. to over 300° C. for diesters with higher boiling points for tri- and tetra-esters.

[0062] Still yet another salient feature of the invention is the ability to perform the contacting step 52 of the invented method at elevated temperatures and pressures. Prior art methods require significant cooling and depressurizing of gaseous streams containing CO.sub.2 before contacting said streams with their CO.sub.2 solvent. Conversely, the invented method is suitable for separating CO.sub.2 from gaseous streams having any partial pressure of CO.sub.2 with typical CO.sub.2 partial pressures between approximately 15 bar and approximately 30 bar, and H.sub.2 partial pressures between approximately 5 bar and approximately 35 bar. The gaseous stream may also be saturated with water vapor, with relative humidity varying approximately between 50 and 100%.)

[0063] Another salient feature of the invention is the makeup of the solvents used to solvate CO.sub.2. The solvents used in the invented method are comprised of only C, H, and O atoms. Prior art solvents or sorbents require the use of silica, various nitrogen compounds/groups, and/or various fluorine compounds/groups. Any of these non-carbon, hydrogen, or oxygen substances have to be separated from gas streams that will go through a turbine or said turbine will be damaged. As the instant invention does not use any of these non-carbon, hydrogen, or oxygen substances, no step to remove contaminants is needed after CO.sub.2 capture and before the CO.sub.2 scrubbed gaseous stream is put through a turbine.

[0064] Still yet another salient feature of the invention is the low corrosive nature of the solvents used. Surprisingly and unexpectedly, empirical testing has demonstrated that the solvents used in the invented method have a corrosion rate of less than around 0.5 microns/year (0.005 mm/year) when a carbon steel or stainless-steel coupon is immersed in the solvent at a temperature of about 21° C. and in the presence of CO.sub.2 at a pressure of 100 psig. In an embodiment, the corrosion caused by use of the instant solvent on carbon or stainless steel is negligible or immeasurable.

[0065] Still yet another salient feature of the invention is the low vapor pressure and viscosity of the solvents during performance of the invented method. In an embodiment, the solvents have a vapor pressure between approximately 0.001 Pa and approximately 10 Pa during performance of the instant method. Additionally, the solvents have a viscosity between approximately 1 cP and approximately 15 cP during performance of the invented method.

[0066] In an embodiment, the properties of the solvent 18 can be tailored to the conditions of the process utilizing the invented method. For example, low molecular weight, low viscosity solvents such as CASSH-1 to CASSH-6 are particularly suitable for lower temperature applications in the −15° C. to 30° C. range and higher molecular weight (i.e. higher than CASSH-1 to CASSH-6), moderate viscosity solvents are suitable for higher temperature operations in the 30° C. to 70° C. range. Water tolerance can also be tailored to the solvent with water solubilities spanning a range of 0.5-2.5 wt % depending on solvent. The diester 40, tri-ester 42, and tetra ester 44 solvents shown in FIG. 3 and described herein can be tailored to desired application by changing the number of CH2 units between the ester groups and/or by changing the length and branching of the terminal alkoxy groups on the esters. These modifications affect volumetric CO.sub.2 absorption capacity, solvent melting points, boiling points, vapor pressure, viscosities, and water solubility and as such can be optimized for the demands of the intended application. As an example, increasing the size of the alkyl groups comprising the R groups of the solvent molecules shown in FIG. 3 and described above increases hydrophobicity.

[0067] Surprisingly and unexpectedly, the invented method achieves commensurate or superior CO.sub.2 solubility and CO.sub.2/H.sub.2 selectivity compared with state-of-the-art methods while utilizing hydrophobic rather than hydrophilic solvents. As a result, the current method performs as well or better than prior art methods by dissolving superior or commensurate amounts of CO.sub.2 in a stream containing CO.sub.2, H.sub.2, and H.sub.2O wherein the present method uptakes little to no water and has extremely low corrosion rates. Prior art methods utilize hydrophilic solvents that dissolve copious amounts of water along with CO.sub.2 and therefore have many disadvantages detailed above.

Solvent Preparation Detail

[0068] All diesters described herein are commercially available and were purchased from either TCI or Sigma-Aldrich. The tri-ester solvents described herein were derived from commercially available triols such as trimethyol ethane or trimethylol propane and then converted in-house to the corresponding esters via an esterification reaction, wherein the triol is reacted with the appropriate anhydride, preferably acetic, iso-butyric, or n-butyric anhydride. The tetra-esters, also known as citrates, described herein were prepared from either commercially available trialkyl citrates or citric acid which was then functionalized at the three acid sites and the one alcohol site. A procedure to generate the ester solvents disclosed herein is described by Chauhan et al., “Indium Triflate: An Efficient Catalyst for Acylation Reactions,” Synlett, No. 11, 1999, pp. 1743-1744, the entirety of which is incorporated by reference herein.

Solvent Performance Detail

[0069] As discussed herein, the instant method utilizes various ester solvents to dissolve CO.sub.2 from a gaseous stream also containing H.sub.2O. A series of experiments was performed to characterize the performance of the invented method using the above-described solvents.

[0070] Gravimetric CO.sub.2 adsorption measurements for solvents were obtained using a Hiden IGA-003 microbalance (Warrington, England). Samples (30-60 mg) were loaded into an open glass container which was suspended from a microbalance assembly inside of a pressure cell. The solvents were outgassed at reduced pressure (10-50 mbar) at 25° C. in flowing N.sub.2 for a minimum of 1 hr prior to introduction of pure to make adsorption measurements.

[0071] Isotherms were measured in a thermostated sample chamber at increasing pressure steps under flowing gas regulated by a mass flow controller and a back pressure regulator. The samples were held at the target pressures until >90% of the equilibrium CO.sub.2 absorption was established (typically 30-90 min). The final CO.sub.2 absorption equilibrium loading was then calculated by an asymptotic fit to the sample weight versus time curve. Buoyancy corrections were applied to the final equilibrium weights using known densities of the samples and all components in the sample and counter weight chambers using gas densities calculated with The National Institute of Standards and Technology's software titled “Reference Fluid Thermodynamic and Transport Properties.” Evaporative losses (determined by the difference in starting weight and final weight at 1 bar) over the course of the measurements were insignificant in relation to the mass increase due to CO.sub.2 adsorption for all of the solvents of interest at the conditions reported herein.

[0072] Karl Fischer titrations were performed according to the procedure described in ASTM E203—16 Standard Test Method for Water Using Volumetric Karl Fischer Titration, the entirety of which is incorporated by reference herein.

[0073] Henry's law constants were determined by calculating the slope of absorption isotherms at low pressure. The Henry's law constants were used to calculate working CO.sub.2 capacities.

[0074] The relative corrosion rates of C1020 carbon were studied under the following conditions: 21° C. and 100 psig CO.sub.2, 21° C. and 400 psig CO.sub.2, 40° C. and 100 psig CO.sub.2, and 40° C. and 400 psig CO.sub.2. The corrosion rates were determined gravimetrically according to ASTM G1-03, Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens, the entirety of which is incorporated by reference herein.

Diester Performance Detail

[0075] Diethyl sebacate “CASSH-1” was initially identified as a potential high performing CO.sub.2 solvent as described above. Subsequent experimental characterization of CASSH-1 verified its high CO.sub.2 uptake, low H.sub.2 uptake, and low moisture affinity. Guided by these encouraging results, research was expanded and more systematic experimental screening tests for CO.sub.2 solubility in 18 commercially available diester “CASSH” solvents were completed to establish a correlation between molecular structure and volumetric CO.sub.2 uptake. The structures of the 18 diesters that were characterized are shown in FIG. 6.

[0076] Through the testing of the 18 diesters shown in FIG. 6, it was determined that the six diester CASSH solvents have CO.sub.2 solubilities which are competitive with current commercial solvents while also having physical properties suitable for applications in pre-combustion CO.sub.2 capture applications. The 25° C. Henry's constants and CO.sub.2 working capacities for capture at 20 bar/10° C. with regeneration at 1.5 bar/25° C. for the six solvents, CASSH-1 through CASSH-6 are compared with current commercial solvents in FIGS. 7A-C. The data shown in FIGS. 7A-C demonstrate comparable or superior CO.sub.2 capture properties of the diester solvents to the current commercial solvents tested.

[0077] The lower molecular weight solvents CASSH-4, CASSH-5, and CASSH-6 are ideally suited as a CO.sub.2 solvent under chilled gas conditions between approximately −15° C. and approximately 10° C. where CO.sub.2 loading capacity is maximized, and solvent vaporization is negligible. The relatively high boiling points (217° C.-245° C.) of the solvents make them suitable for regeneration near room temperature with minimal evaporation loss. All the CASSH solvents described herein have CO.sub.2 heats of absorption in the range of 14-17 kJ/mol.

[0078] FIG. 8 shows the CO.sub.2 absorption isotherms for CASSH-4 over a range of temperatures between approximately 0° C. to approximately 40° C. The data shown in FIG. 8 exhibit significant enhancement in CO.sub.2 solubility with decreasing temperature as expected for physical solvents. The plotted data also include absorption data for CASSH-5 and CASSH-6 and the two main commercial solvents PC and DEPG at 10° C., a temperature commonly modeled for commercial DEPG systems. All three of the CASSH solvents show improved CO.sub.2 uptake over the commercial solvents. The CO.sub.2 solubility for CASSH-4 at 15° C. matches the DEPG at 10° C. showing the potential to operate at a warmer temperature with similar CO.sub.2 capacity. As systems using DEPG to dissolve CO.sub.2 require solvent temperatures of 10° C., this would reduce the need to chill the solvent by 5° C. if CASSH-4 replaced DEPG in the same system.

[0079] Analysis of the CO.sub.2 solubility data for the solvents shown in FIG. 6 showed a strong correlation between volumetric CO.sub.2 capacity and the concentration of ester groups within a volume of solvent. This correlation is shown in FIG. 9. The data of FIG. 9 shows that the CO.sub.2 uptake of an alkyl diester can be closely estimated from the density and molecular weight of the solvent. As shown by the data in FIG. 9, lower molecular weight solvents with higher densities maximize the concentrations of ester functional groups yielding a higher molar uptake of CO.sub.2 per volume of solvent. The effects of structural variations were also studied by testing “inverted diesters”, which have the ester groups flipped to have the alkoxy oxygen bonded to the internal hydrocarbon chain (structures labeled 80 in FIG. 6) and also derivatives with hindered rotation of the ester groups due to inclusion of either a double bond or ring structure in the molecule (structures labeled 82 in FIG. 6). The hindered rotation in the molecule was shown by the data to be highly detrimental to CO.sub.2 uptake. The inverted esters do show a slight enhancement of CO.sub.2 uptake over the normal diesters. However, costs are higher for these solvents and they tend to have higher melting points.

[0080] Following the initial experimental CO.sub.2 solubility screening test, additional characterizations of the solvents by water solubility, H.sub.2 solubility (computational models), and physical properties (melting point, boiling point, viscosity) were undertaken. Water absorption tests done using the Karl Fisher titration technique combined with results from the scientific literature for the CASSH series are compared with the most common least hydrophilic commercialized solvents, PC and TBP, with the data shown in FIGS. 11A-C. As shown in FIGS. 11A-C, selected diester solvents in the CASSH series have significantly lower water solubilities than current commercial solvents. Water vapor absorption tests by a gravimetric microbalance using humidified nitrogen show similar results, as shown in FIGS. 12A and 12B. The water solubility in the DEPG is only shown to moderate humidity levels because water vapor absorption at higher humidity is very high with long equilibration times.

[0081] A sample of DEPG was stored in an open vial within a sealed cabinet purged with humidified N.sub.2 over two weeks and the weight of the sample was recorded at intervals with the results shown in FIG. 13. The sample weight increased by 30% in 300 hours and the rate of weight increase had not yet leveled off at that point. FIG. 13 illustrates a salient features of the invented method, very low water adsorption compared to current commercial solvents.

[0082] For applications at lower temperatures and higher pressures, CO.sub.2 uptake deviates much more from Henry's Law than does H.sub.2 to give CO.sub.2 isotherms like those in FIG. 14. The pronounced upturn in the CO.sub.2 uptake at high pressure will nearly double the uptake predicted from Henry's Law which in turn will nearly double the working capacity and CO.sub.2/H.sub.2 selectivity of the solvent compared to that from a solvent with a simple linear isotherm.

[0083] A summary of properties for the CASSH solvents are compared against the commercial solvents PC, which has the most similar properties in terms of CO.sub.2 and water solubility and physical properties, and DPEG. The results are summarized in FIG. 10. The six CASSH solvents listed in FIG. 10 are shown to have similar properties to those of PC but with lower water solubilities. For a low temperature CO.sub.2 capture system working at an absorber temperature of 10° C., all CASSH solvents are highly superior to DEPG due to the large difference in water uptake. For PC, which operates at lower temperatures than DEPG, CASSH 2-6 would be particularly suitable in the range of between approximately −15° C. and approximately 0° C. due to their lower melting points. Mixtures of CASSH solvents could also be applied for absorption processed which operate below 0° C. These mixtures could include other diester derivatives other than those listed as CASSH solvents, for example di-isobutyl succinate or isopropyl glutarate. Importantly, as shown in FIG. 10, the CASSH solvents are superior to current commercial solvents for CO.sub.2 solvation given their unique combination of reduced water absorption, low volatility, and high CO.sub.2 capacities.

[0084] While numerous diester derivatives are feasible for capture in the range of between approximately −15° C. and approximately 25° C., derivatives derived from diacids consisting of 4-10 carbons in length with terminal alkyl groups ranging from C1 to C4 are most favored due to factors such as melting points, boiling points, viscosities, and predicted CO.sub.2 uptake based on the correlation shown in FIG. 9.

[0085] In an embodiment, it can be determined whether diesters other than those shown and described herein and in the accompanying figures based on where those solvents would land on the correlation plot shown in FIG. 9. Any diester, tri-ester, or tetra-ester with a computed “moles of ester functional group per liter” (computed by EQUATION 1 below) greater than 8 would be predicted to have a competitive CO.sub.2 solubility according to the correlation shown in FIG. 9. The effectiveness of this solvent would then simply be evaluated based on physical properties with optimal values of viscosity (<6 cP), melting point (<5° C.), boiling point (>215° C.), and water solubility (<1.6 mol/L) and material cost to manufacture.

moles ester functional group per liter=(number of ester functional groups in the molecule)(density of multiester)/(molecular weight of multiester)   EQUATION 1

[0086] FIG. 10 provides a table summarizing physical properties and CO.sub.2 solubility parameters for CASSH solvents as compared to commercial solvents PC and DEPG, wherein a * indicates calculated values according to method described in Shi et al., “Computational Screening of Physical Solvents for CO.sub.2 Pre-combustion Capture.” J Phys Chem B. 2021 Dec. 16; 125(49):13467-13481, the entirety of which is incorporated by reference herein.

[0087] The CO.sub.2/H.sub.2 selectivities listed for CASSH solvents were computed using the relevant Henry's Constants for CO.sub.2 and H.sub.2. These values are expected to be significantly higher under low temperature conditions due to the deviation from Henry's Law for CO.sub.2 uptake. PC results are taken from Burr et al., “Which physical solvent is best for acid gas removal?” Hydrocarbon Processing, 2009, vol. 88, is. 1, pp 43-50 the entirety of which is incorporated by reference herein.

[0088] While the above discussion mainly focused on solvents designed for typical process operating conditions in the range of absorber temperatures between approximately −15° C. and approximately 10° C. with CO.sub.2/H.sub.2 mixtures containing CO.sub.2 partial pressures in the range of between approximately 15 and approximately 30 bar and regeneration conditions between approximately 25° C. and approximately 35° C., between approximately 1 bar to approximately 2 bar, initial IGCC-CO.sub.2 capture modeling studies indicate the benefits for operating the absorber at warmer temperatures near approximately 40° C. with higher regeneration temperatures of between approximately 70 and approximately 100° C. using waste heat from a chemical/power plant. The higher operating temperatures in this process would require thermally stable solvents with high boiling points (low vapor pressures) in excess of 275° C. while maintaining low water uptake and low viscosities. Of the widely used commercial solvents, only DEPG or TBP would meet the boiling point and stability requirements. However, as noted above, DEPG is very hydrophilic and would suffer greatly under the higher water partial pressures at 40° C. compared to those at 10° C. For these proposed warm gas operations, CASSH-1 and CASSH-2 would be suitable diester solvents due to their high CO.sub.2 uptake and extremely low water solubility, with CASSH-3 also an option.

Tri-Ester and Tetra Ester

Characterization and Performance Detail

[0089] As described above and shown in FIG. 3, an embodiment of the invention uses tri- and tetra esters as CO.sub.2 solvents in the invented method. These multi-ester molecules are particularly suitable for warm gas operations. Exemplary multi-esters (more than two) are shown in FIGS. 15A-C. which are derived by transesterification of either trimethylolethane or trimethylolpropane, respectively. These higher molecular weight solvents provide low vapor pressure options with “moles of ester functional group per liter” values in the favorable region of the correlation plot shown in FIG. 16.

[0090] Tri-ester and citrate solvents can be tailored in a similar fashion to the diester CASSH solvents to optimize their CO.sub.2 capacity and other physical properties. The plots in FIGS. 17A-17B and FIG. 18 show the water mitigation and competitive CO.sub.2 capture of tri-esters TME-OAc3 and TME-OiPr3 as well as CASSH-1 and CASSH-2. The water uptake in these solvents is exceptionally low compared to the DEPG and significantly lower than that of the most hydrophobic commercial solvent TBP. Tri-ester TME-OAc3 is the lowest molecular weight tri-ester. It is the least hydrophobic tri-ester, but shows the highest CO.sub.2 uptake and significantly outperforms DEPG while maintaining a water uptake lower than TBP. The hydrophobicity of the tri-ester can be enhanced by adding carbons to the ester groups. Replacing the methyl groups of tri-esters TME-OAc3 with isopropyl groups gives TME-OiPr3 which shows very low water vapor absorption while maintaining a CO.sub.2 uptake at 40° C. which is nearly identical to that of DEPG and TBP. As observed for the diester solvents, the tri-ester solvents also possess high CO.sub.2/H.sub.2 absorption selectivity. In fact, the CO.sub.2/H.sub.2 selectivity of 83 for TME-OAc3 (calculated using the method in Shi et al., “Computational Screening of Physical Solvents for CO.sub.2 Pre-combustion Capture.” J Phys Chem B. 2021 Dec. 16; 125(49):13467-13481) is among the highest of the solvents shown in TABLE 3. The high molecular weights of tri-ester solvents along with CASSH-1 and CASSH-2 diesters provide the low vapor pressures required for efficient operation at elevated temperatures relative to tradition chilled gas capture.

[0091] FIG. 16 is plot for some diester and tri-ester solvents which shows the correlation of 40° C. CO.sub.2 Henry's constants to the molar concentration of ester functional groups within a liter of the pure solvents. As shown in FIG. 16, While tri-esters are slightly below the correlation of diesters, they have an advantage of good capacity at 40° C. with very low vapor pressure.

Pilot Testing Detail

[0092] Pilot scale testing was performed by the Energy & Environmental Research Center at the University of North Dakota (UND EERC) by shipping solvents to their precombustion capture facility and testing at the pilot scale. Flow and gas make-up parameters for the pilot scale testing is shown in TABLE 2 below. TABLE 3 provides pilot testing performance data for various solvents.

TABLE-US-00002 TABLE 2 Average shifted syngas conditions and composition entering absorber column for each trial Value Parameter Syngas total pressure, MPa 4.88 Syngas temperature, ° C. 37.5 Syngas flow rate, std. m.sup.3 .Math. h.sup.−1 3.8 Syngas composition, dry, avg (mol %) CO.sub.2 52.0 H.sub.2 13.1 N.sub.2 32.7 CH.sub.4 1.6 CO 0.2 H.sub.2S 0.5

TABLE-US-00003 TABLE 3 Pilot plant campaign at UND EERC solvent absorption performance results for DEPG, PEG-PDMS-3, CASSH-1 and TBP at high solvent flow rates (approx. 37-45 L/hr). Solvent Performance PEG- Temperature* parameter PEGDME PDMS-3 CASSH-1 TBP 10° C. Solvent temperature 10.1 − 13.8  9.5 − 20.5 10.4 − 28.5 10.7 − 13.9 inlet-outlet, ° C. CO.sub.2 gas uptake, 1.64 ± 0.10 1.72 ± 0.10 2.27 ± 0.03 1.50 ± 0.06 mol × L − 1 H.sub.2S gas uptake, 0.014 ± 0.001 0.014 ± 0.001 0.021 ± 0.001 0.015 ± 0.001 mol × L − 1 25° C. Solvent temperature 25.0 − 27.0 25.3 − 33.6 25.6 − 32.6 25.1 − 26.8 inlet-outlet, ° C. CO.sub.2 gas uptake, 1.46 ± 0.14 1.63 ± 0.02 1.66 ± 0.04 1.65 ± 0.04 mol × L − 1 H.sub.2S gas uptake, 0.011 ± 0.001 0.012 ± 0.001 0.015 ± 0.001 0.016 ± 0.001 mol × L − 1 40° C. Solvent temperature ** 40.0 − 46.6 40.6 − 48.0 39.8 − 41.4 inlet-outlet, ° C. CO.sub.2 gas uptake, ** 1.64 ± 0.07 1.91 ± 0.05 1.90 ± 0.01 mol × L − 1 H.sub.2S gas uptake, ** 0.014 ± 0.001 0.018 ± 0.001 0.018 ± 0.001 mol × L − 1 55° C. Solvent temperature ** 54.3 − 63.7 55.4 − 57.4 55.5 − 58.5 inlet-outlet, ° C. CO.sub.2 gas uptake, ** 1.55 ± 0.10 1.67 ± 0.05 1.92 ± 0.01 mol × L − 1 H.sub.2S gas uptake, ** 0.014 ± 0.001 0.016 ± 0.001 0.018 ± 0.001 mol × L − 1 Water content of solvent at 4000 550 1550 1670 end of trial, ppm

[0093] For the pilot testing whose data is shown in TABLE 3, CO.sub.2 partial pressure in syngas was ˜2.6 MPa and H.sub.2S partial pressure in syngas was ˜0.02 MPa. For 10° C.-25° C. inlet cases, the solvent was regenerated at 43° C. For the 40-55° C. inlet cases, the solvent was regenerated at approximately the temperature that it leaves the absorption column, no additional heat was supplied if the solvent was already greater than 43° C.

[0094] In TABLE 3, a* indicates that An absorber temperature range is provided for each nominal solvent temperature. The lower temperature is the absorber inlet solvent temperature (solvent enters at the top of the absorption column) and the higher temperature is the solvent temperature at the bottom of the absorption column. As the syngas enters at approximately 38° C. and the absorption process is exothermic, there is a temperature gradient from the top to bottom of the column. PEGDME solvent testing was not conducted at the highest solvent inlet temperature of 55° C. and data was not included if mass balance errors were greater than 20% (which was the case for data collected with PEGDME at 40° C.).

Corrosion Detail

[0095] A final, but significant advantage of the instant invention applications is its anticipated effect on reducing corrosion rates. Corrosion rate models were calculated using OLI software package for carbon steel are shown in TABLE 4. There is a dramatic decrease in corrosion rate predicted as the water content in the solvent is decreased. Even a relatively hydrophilic solvent such as DEPG shows a marked decrease in corrosion rates over aqueous solutions of K.sub.2CO.sub.3 and methyldiethanolamine (MDEA) due to lower water concentrations. When the solvent becomes even more hydrophobic as with the CASSH series of solvents, the corrosion rate drops many orders of magnitude below DEPG, and well below the ideal rate of 0.01 mm/yr. With a predicted corrosion rate of practically zero, hydrophobic solvents will provide exceptional benefits to preserving process infrastructure against degradation and produce substantial savings in initial construction and yearly maintenance costs.

TABLE-US-00004 TABLE 4 H.sub.2O K.sub.2CO.sub.3 in H.sub.2O wet MDEA With 1% H.sub.2S 0.85 0.21 0.852 Without 1% H.sub.2S 25.6 0.08 0.70

[0096] TABLE 4 depicts the calculated corrosion rate of carbon steel at 40° C. in equimolar CO.sub.2/H.sub.2 at 50 bar (in mm/yr). Models were performed using OLI software package on generic carbon steel. Ideal corrosion rate is <10 μm/yr, though <50 μm/yr can be acceptable for many applications.

[0097] Corrosion data has been collected for CASSH-1, the data provided in FIG. 19. As shown in FIG. 19, corrosion was not detected in testing with CASSH-1 even though the tests with CASSH-1 were allowed to run longer than every other solvent.

[0098] FIG. 19 provides the corrosion rates measured for carbon and stainless steels using various pre-combustion solvents, as calculated by OLI software. It is found that the hydrophobic CASSH solvent shows greatly reduced corrosion rates compared solvents which contain or admit water.

[0099] The measured corrosion rates in FIG. 19 reflect the average (and standard deviation) of two methods (gravimetric and spectroscopic) for stainless steel SS304 and general carbon steel C1020. OLI Corrosion Analyzer simulations of the corrosion rate for those cases which could be simulated used SS304 and carbon steel C10100. The solvent disub-4PEG was disclosed previously by the inventors in U.S. Pat. No. 10,589,228, the entirety of which is incorporated by reference herein.

[0100] The relative corrosion rates of C1020 carbon steel in the presence of CASSH-11, dry DEPG, and DEPG containing only 2 wt. % water was studied under the following conditions: 21° C. and 100 psig CO.sub.2, 21° C. and 400 psig CO.sub.2, 40° C. and 100 psig CO.sub.2, and 40° C. and 400 psig CO.sub.2. The corrosion rates were determined gravimetrically according to ASTM G1-03, Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens. The rates obtained are illustrated in FIG. 20, where dry CASSH-1 and dry disub-4PEG show superior corrosion rates to dry TBP, dry Poly(ethylene glycol) dimethyl ether (DME PEG), and DME PEG with 2% H.sub.2O and 10% H.sub.2O added. The moisture content experienced by carbon capture solvents may easily exceed these nominal levels under actual capture conditions. The ability to use carbon steel in place of stainless steel for capture system equipment would result in substantial capital cost reductions.

[0101] The solvents used in the invented method are superior to solvents used in the prior art. The presently used solvents 18 are low in viscosity, hydrophobic, low in vapor pressure, and high in CO.sub.2/H.sub.2 absorption selectivity, and low in water solubility, for example. The solvents are well suitable to remove CO.sub.2 from a stream of syngas that has gone through the water gas shift, i.e. remove CO.sub.2 from a combined stream of CO.sub.2, H.sub.2 and H.sub.2O. As physical solvents, the solvents described herein are suitable for a combined temperature swing/pressure swing operation in the exemplary temperature range between approximately −15° C. to approximately 100° C. with exemplary CO.sub.2 partial pressures in the range of approximately 15 to approximately 30 bar during absorption and between approximately 10° C. and approximately 180° C. and between approximately 1 to approximately 25 bar for regeneration.

[0102] The solvents described herein have superior properties in certain aspects over prior art solvents and also share beneficial or necessary properties of prior art solvents to aid in dissolving CO.sub.2 in the invented method as described above. For example, the solvents described herein have similar boiling points, viscosities, and CO.sub.2 and H.sub.2 solubilities as common commercial solvents used in similar applications, but with significantly lower water solubilities. The disclosed solvents are also stable under the intended operating conditions and have environmental advantages including low toxicity, high biodegradability, and do not contain any chemical elements other than H, C, or O that can cause turbine damage if any solvent should make its way into the combustion process.

[0103] Considering the aforementioned advantages and disadvantages of PC, NMP, and DEPG, it is apparent that a hydrophobic solvent with similar physical properties which can operate at low temperatures with comparable CO.sub.2 solubility could offer significant benefits as a CO.sub.2 mitigation solvent. The current invention addresses this need through the development of hydrophobic CO.sub.2 physical solvents.

[0104] Having described the basic concept of the embodiments, it will be apparent to those skilled in the art that the foregoing detailed disclosure is intended to be presented by way of example. Accordingly, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or alterations and various improvements of the subject matter described and claimed are considered to be within the scope of the spirited embodiments as recited in the appended claims. Additionally, the recited order of the elements or sequences, or the use of numbers, letters or other designations therefor, is not intended to limit the claimed processes to any order except as may be specified. All ranges disclosed herein also encompass any and all possible sub-ranges and combinations of sub-ranges thereof. Any listed range is easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, tenths, etc. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, etc. As will also be understood by one skilled in the art all language such as up to, at least, greater than, less than, and the like refer to ranges which are subsequently broken down into sub-ranges as discussed above. As utilized herein, the terms “about,” “substantially,” and other similar terms are intended to have a broad meaning in conjunction with the common and accepted usage by those having ordinary skill in the art to which the subject matter of this disclosure pertains. As utilized herein, the term “approximately equal to” shall carry the meaning of being within 15, 10, 5, 4, 3, 2, or 1 percent of the subject measurement, item, unit, or concentration, with preference given to the percent variance. It should be understood by those of skill in the art who review this disclosure that these terms are intended to allow a description of certain features described and claimed without restricting the scope of these features to the exact numerical ranges provided. Accordingly, the embodiments are limited only by the following claims and equivalents thereto. All publications and patent documents cited in this application are incorporated by reference in their entirety for all purposes to the same extent as if each individual publication or patent document were so individually denoted.

[0105] All numeric values are herein assumed to be modified by the terms “about” or “approximately,” whether or not explicitly indicated. The terms “about” or “approximately” generally refer to a range of numbers that one of skill in the art would consider equivalent to the recited value (e.g., having the same function or result). In many instances, the terms “about” and “about” include numbers that are rounded to the nearest significant figure.

[0106] The recitation of numerical ranges by endpoints includes all numbers within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5).

[0107] One skilled in the art will also readily recognize that where members are grouped together in a common manner, such as in a Markush group, the present invention encompasses not only the entire group listed as a whole, but each member of the group individually and all possible subgroups of the main group. Accordingly, for all purposes, the present invention encompasses not only the main group, but also the main group absent one or more of the group members. The present invention also envisages the explicit exclusion of one or more of any of the group members in the claimed invention.