Measuring Nitrous Oxide Mass Transfer into Non-Aqueous CO2BOL

Article pubs.acs.org/IECR

Measuring Nitrous Oxide Mass Transfer into Non-Aqueous CO2BOL CO2 Capture Solvents Greg A. Whyatt, Charles J. Freeman, Andy Zwoster, and David J. Heldebrant* Pacific Northwest National Laboratory, Richland, Washington 99325, United States ABSTRACT: This paper investigates CO2 absorption behavior in CO2BOL solvents by decoupling the physical and chemical effects using N2O as a nonreactive mimic. Absorption measurements were performed using a wetted-wall contactor. Testing was performed using a “first generation” CO2-binding organic liquid (CO2BOL), composed of an independent base and alcohol. Measurements were made with N2O at a lean (0.06 mol CO2/mol BOL) and rich (0.26 mol CO2/mol BOL) loading, each at three temperatures (35, 45, and 55 °C). Liquid-film mass transfer coefficients (kg′) were calculated by subtracting the gas film resistancedetermined from a correlation from literature−from the overall mass transfer measurement. The resulting kg′ values for N2O in CO2BOLs were found to be higher than that of 5 M aqueous MEA under comparable conditions, which is supported by published measurements of Henry’s coefficients for N2O in various solvents. These results suggest that the physical solubility contribution for CO2 absorption in CO2BOLs is greater than that of aqueous amines, an effect that may pertain to other nonaqueous solvents.

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INTRODUCTION

Global climate change is largely being driven by emissions of carbon dioxide (CO2) emitted from power generation and transportation sources. One emissions-reduction option being considered is the removal of CO2 from stationary power generation sites, such as coal-fired power plants, via flue gas “scrubbing” technologies. Having first been patented in the 1930s by Bottoms, gas scrubbing technologies have been used extensively in the oil and gas industries for over 80 years.1,2 Only recently has CO2 scrubbing been evaluated and developed for flue gas from coal-fired power plants. The most mature CO2 scrubbing technologies are proprietary formulations of CO2reactive amines dissolved in water.3 Several of these aqueous amine solvents are close to commercial-scale deployment for coal-fired-power-plant flue gas, but have projected energy requirements as large as 30% of the total power plant output.4,5 The greatest opportunity for reducing the energy penalty of gas scrubbing technologies is to reduce the energy penalty associated with thermal regeneration of the solvent, more specifically the energy lost to boiling and condensing large amounts of water. As such, new classes of solvent technologies are being designed to operate with little-to-no water as a solvent, providing similar chemical fixation as amine-based chemistries using organic solvents where lower specific heats project substantial energy savings.6−12 CO2-binding organic liquids (CO2BOLs)13 are one class of water-lean solvents, predicated on the switchable ionic liquid chemistry pioneered by Jessop (Figure 1).14 However, despite promising energetic predictions for CO2BOLs significant viscosity increases with loading have limited their applicability. Nevertheless, the mass transfer behavior of this solvent at lower CO2 loadings is considered indicative of a number of anhydrous solvent options if proved to be viable with significant viscosity decreases. © 2016 American Chemical Society

Figure 1. CO2 binding chemistries of a CO2BOL alkylcarbonate. R = C6H12−.

For applications targeting CO2 scrubbing from flue gas a wetted-wall contactor apparatus is commonly used to quantify absorption behavior.15−18 Earlier work reported wetted-wall contactor data for CO2 absorption in a CO2BOL solvent.19 Here, the liquid film mass transfer coefficients (kg′) were compared to available data from aqueous MEA and piperazine solvents. Despite having much higher viscosities CO 2 absorption in CO2BOLs was determined to be similar to that of the aqueous MEA, under similar driving forces. A unique, inverse temperature dependence of kg′ was also observed with the CO2BOLs solvent, which indicated higher physical solubility of CO2 in the organic solvent compared to water.19 To further our understanding of dissolution and diffusion of CO2 into CO2BOLs and aqueous amine solvents a nonreactive surrogate for CO2 was used in the wetted-wall absorption experiments. N2O was chosen as the CO2 surrogate based on its similar kinetic diameter, as verified in similar measurements.20 This paper presents results of N2O absorption in a CO2BOL solvent and compares that data to past CO2 absorption measurements to decouple the physical and chemical components of absorption. Received: Revised: Accepted: Published: 4720

January 28, 2016 March 25, 2016 March 28, 2016 March 28, 2016 DOI: 10.1021/acs.iecr.6b00390 Ind. Eng. Chem. Res. 2016, 55, 4720−4725

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Industrial & Engineering Chemistry Research

Figure 2. Representative illustration of the WWC setup and configuration for CO2BOL testing.

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EXPERIMENTAL SECTION All measurements were performed on a custom-built wettedwall column (WWC) apparatus (see Figure 2).16 Here, solvent flows up through a stainless tube and back down on the outside of the same tube, creating a consistent film that is then exposed to upward flowing gas of varying CO2 or N2O concentration. The changes in gas concentration are used to assess gas species absorption.19 The tube outer diameter and height are 1.25 and 9.09 cm, respectively whereas the inside diameter of the chamber in which the tube is centered is 2.30 cm. Column surface area was 36.93 cm2 and the hydraulic diameter of annulus was 1.05 cm prior to any adjustment for film thickness. The solvent flow rate was 350 ccm for all measurements. The CO2BOL solvent was blended (1:1 by mole) from diazabicyclo[5.4.0]-undec-7-ene (DBU) and 1-hexanol, purchased from Aldrich, and dried over 4 Å molecular sieves prior to use. The solvent was handled under a nitrogen atmosphere until loaded with CO2. N2O (99.6%) CO2 and N2 gases (both 99.998%) were purchased from Oxarc and used without further purification. Dry ice (99% purity by mass) was also purchased from Oxarc. To minimize water contamination, dry ice and solvents were mixed with little or no contact with ambient air. CO2 loadings were selected to target a desired equilibrium pressure (P*) in the solvent. Unloaded solvent was then added, under nitrogen, to a Chemglass 5 L jacketed tank reactor, equipped with an integrated stirrer with a bladed shaft and a N2 bubbler. Outside layers of dry ice were broken off to provide a “dry” surface prior to massing. Once the dry ice sublimed to reach the desired mass, it was removed and added to the reactor. After the dry ice was added, the solution was stirred vigorously until all the CO2 had been absorbed. Loaded solvents (up to 1.2 L) were then delivered to the WWC via an integrated gear pump. All sweep gases were blended from the bottled N2O and nitrogen using Brooks Instrument SLA5800 series mass flow controllers (MFC), which are accurate within ±0.9% of set point. All MFCs were recalibrated for mixture gases prior to use using a primary calibrator (Mesa Laboratories, Model Definer 220) to ±1% accuracy. Gas flow was reduced from 6 slpm to 100 sccm in order to achieve a reliable detectable change in gas composition during experiments.

Gas composition of the stream exiting the WWC was analyzed by a quadrupole mass spectrometer (MKS, Cirrus 200 amu) sampling at atmospheric pressure and double-checked by gas chromatography (Agilent Model 3000A Micro GC). The mass spectrometer signals for N2O (30 amu) and N2 were calibrated before each WWC experiment; with gas mixtures blended to the match the sweep gas, enabling full coverage of the test concentration range. Gas sampling was performed first in bypass to confirm the gas concentration followed by the column. Mass spectrometer sampling was performed at 4 s intervals and GC sampling was performed at 4 min intervals. Wetted-Wall Column Analysis Sweeps and Conditions. Figure 2 shows a process flow diagram for the WWC system used for both the CO2BOL and MEA experiments. Each solvent was loaded to a targeted CO2 loading, added to the WWC and recirculated at 350 mL/min, then heated to the desired temperature (e.g., 35 °C). Next, nitrogen gas was flowed at a constant rate in the annulus outside of the falling liquid film at 100 sccm (referenced to 21.1 °C). CO2 concentrations were set to minimize flux and N2O concentrations were added to the nitrogen stream to create positive flux conditions. Here, zero flux corresponds to the equilibrium partial pressure (P*) for the gas in question. N2O dissolved in solution was purged after each experiment prior to measuring the next condition. N2O was removed from solvents by sweeping a blended gas of CO2 and N2 (at a given P* of CO2) through the solvent to release any dissolved N2O without changing the loading of CO2 in solution. Blended gas was passed through the solvent until there was no GC signal of N2O remaining on rich solvent, though lean solvent loadings there was some residual N2O (∼959 Pa) for which the driving force was accounted for in the analysis. Temperature conditions of 35, 45, and 55 °C were measured for both the lean and rich CO2 loadings in the solvent. Both the inlet gas and solvent temperatures were set to these targets by controlling the temperature of an oil jacket surrounding the apparatus. Only bulk gas and liquid temperatures were measured. Local temperature variations due to moisture loss, heat of adsorption, etc. were assumed to be minimal. The total gas pressure was measured and reported for each test. The extent of N2O adsorption was determined by comparing the ratio between N2O and N2 on a gas chromatograph (GC) and 4721

DOI: 10.1021/acs.iecr.6b00390 Ind. Eng. Chem. Res. 2016, 55, 4720−4725

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Industrial & Engineering Chemistry Research Table 1. Mass Transfer Coefficients for N2O in DBU 1-Hexanola run #

loading (mol CO2/mol BOL)

N2O conc. in gas (vol %)

T (°C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.26 0.26 0.26 0.26 0.26 0.26 0.26 0.26 0.26

3 9 15 3 9 15 3 9 15 3 9 15 3 9 15 3 9 15

35 35 35 45 45 45 55 55 55 35 35 35 45 45 45 55 55 55

kg, gas - calculated, Bishnoi 3.33 3.33 3.33 3.34 3.34 3.34 3.32 3.32 3.32 3.28 3.33 3.33 3.35 3.35 3.35 3.37 3.37 3.37

× × × × × × × × × × × × × × × × × ×

10−07 10−07 10−07 10−07 10−07 10−07 10−07 10−07 10−07 10−07 10−07 10−07 10−07 10−07 10−07 10−07 10−07 10−07

log mean driving force (Pa) 1893 7269 12813 7339 12836 1828 7289 12865 2798 8282 13960 2741 8280 13944 2809 8339 13942

flux (mol/s/m2)

KG, overall

5.46 × 10−05 2.89 3.03 × 10−04 4.17 5.12 × 10−04 4.00 not measured 2.77 × 10−04 3.78 5.03 × 10−04 3.92 8.92 × 10−05 4.88 3.27 × 10−04 4.49 5.47 × 10−04 4.25 9.00 × 10−05 3.22 2.84 × 10−04 3.43 4.39 × 10−04 3.14 9.63 × 10−05 3.51 2.83 × 10−04 3.41 4.41 × 10−04 3.17 7.01 × 10−05 2.50 2.56 × 10−04 3.07 4.34 × 10−04 3.12

× 10−08 × 10−08 × 10−08 × × × × × × × × × × × × × ×

10−08 10−08 10−08 10−08 10−08 10−08 10−08 10−08 10−08 10−08 10−08 10−08 10−08 10−08

kg′, liquid 3.16 × 10−08 4.77 × 10−08 4.54 × 10−08 4.26 4.44 5.72 5.19 4.88 3.57 3.82 3.47 3.92 3.80 3.50 2.70 3.38 3.43

× × × × × × × × × × × × × ×

10−08 10−08 10−08 10−08 10−08 10−08 10−08 10−08 10−08 10−08 10−08 10−08 10−08 10−08

a

KG, kg, and kg′ values all in units of mol/(s m2 Pa). Apparatus dimensions OD = 1.25 cm, height = 9.09 cm, ID = 2.30 cm. Column surface area was 36.93 cm2 and the hydraulic diameter of annulus was 1.05 cm prior to adjustments for film thickness. The solvent flow = 350ccm for all measurements.

mass spectrometer (MS) when passing through the column relative to when the gas was bypassed around the column. Prior to the GC, the gas was passed through a drierite column to remove water. For N2O measurements, the gas was also passed through an ascarite column to eliminate CO2. The ascarite was necessary due to a potential for the CO2 peak and N2O peak to overlap at higher N2O/CO2 concentrations that would have impacted the accuracy of the N2:N2O ratio determination. All data reported in the table and figures below are from the GC due to the higher accuracy compared to the MS.

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RESULTS AND DISCUSSION

Testing was performed on a 1:1 molar mixture of DBU and 1hexanol by first loading the solutions to a desired lean (0.06 mol CO2/mol BOL) and rich (0.26 mol CO2/mol BOL) loading. N2O absorption tests were performed at three different N2O gas concentrations (3, 9, 15% by volume). These concentrations were measured at the lean and rich loadings, each at 35, 45, and 55 °C. All tests were performed at nearatmospheric pressure. The data collected from the WWC are tabulated in Table 1 and plotted in Figure 3. The intent was to have very low CO2 flux, such that loading was steady and there was no additional phase drift to interfere with N2O transport. At less than 1% CO2 P* the CO2 content was set to 1%. For P* > 1%, the % CO2 was experimentally determined based on where the bypass concentration matched the column concentration. As shown in Table 1, N2O flux increases with increasing N2O concentration in the feed gas at each temperature for both the lean and rich loadings. The slope of each linear fit in Figure 3 corresponds to the overall mass transfer coefficient, KG. Note that all three temperatures were fit for each of the rich and lean solvent data sets. The slope, or KG, for the lean solvent set was equal to 4.08 × 10−08 mol/(s m2 Pa), whereas the slope of the rich solvent was 3.18 × 10−08 mol/(s m2 Pa), indicating a higher KG for the lean solvent compared to the rich.

Figure 3. N2O Flux vs log mean driving force for rich and lean solutions of DBU 1-hexanol. The linear slope of each data set corresponds to the overall mass transfer coefficient, KG. Data is also shown in Table 1.

Next, the liquid phase mass transfer coefficients (kg′) were calculated for the CO2BOLs solvent by accounting for the gas film mass transfer coefficients (kg) in the overall mass transfer (KG) measurement. Gas film mass transfer was determined by a

(

relationship from Bishnoi:21 Sh = 1.075 Re × Sc ×

d 0.85 h

)

where d is defined as the hydraulic diameter of the annulus (1.05 cm) and h is the length of the column (9.09 cm). It should be noted that the kg values were based on the geometric area of the tube, and did not include a film thickness, as that value was not measured. A sensitivity check of the absence of film thickness showed minimal changes in calculated kg′. All of the mass transfer data are presented in Table 1. Here, the kg′ values for N2O into CO2BOLs range from 2.72 × 10−08 to 5.82 × 10−08 mol/(s m2 Pa) for all of the conditions tested. Measurements of N2O mass transfer into 5 M MEA made on the same test apparatus have yielded kg′ values in the range of 4.6 × 10−9 to 8.7 × 10−9 mol/(m2 Pa s).22 For comparison, Jiru 4722

DOI: 10.1021/acs.iecr.6b00390 Ind. Eng. Chem. Res. 2016, 55, 4720−4725

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Figure 4. Comparison of kg′ for both N2O (15% in gas) and CO219 into DBU-1-hexanol.

measured kg′ values for N2O into 5 M MEA of 3.38 × 10−09 and 4.17 × 10−09 mol/(s m2 Pa) at 298 and 323 K, respectively, using a different experimental apparatus.23 Therefore, it is concluded that the kg′ for N2O in CO2BOL appears to be an order of magnitude higher than for 5 M MEA under comparable loadings. We hypothesize that the higher kg′ values for N2O into DBU 1-hexanol compared to 5 M MEA are due to higher solubility of N2O into organic solutions compared to water. CO2 is known to be more soluble in DBU24 than water.23,25 Jiru et al. measured the solubility of N2O in water and 5 M MEA measuring Henry’s constants using NaCl as a bicarbonate salt surrogate. These measurements indicated low solubility of both N2O and CO2 at all loadings due to the expected “salting out” effect.23 Physical CO2 solubility in 5 M MEA was derived from the N2O analogy method.23 Higher solubility of CO2 compared to N2O has been verified by multiple groups for water. Henry’s constants (Pa m3/mol), expressed as a ratio of gas phase pressure divided by liquid phase concentration for these gases into water had a N2O/CO2 ratio corresponding to 1.4, indicating lower solubility of N2O compared to CO2.26−28 Monteiro and Svendson have recently raised questions regarding the accuracy of the N2O analogy method in aqueous solvents due to limited solubility data for CO2 and N2O in concentrated solutions.29 Greater uncertainty exists for nonaqueous solvents, where even less data is available to validate the accuracy of the N2O analogy. As expected, there are no available data for N2O solubility in DBU or 1-hexanol, though there are published reports of N2O in solvents of comparable polarity. It has been shown by Sada et al. that increasing mole fractions of alcohols in aqueous solutions results in increased N2O solubility.30 For the lean form of the solvent, we can estimate N2O solubility from measured solubility in alcohols or glymes of comparable polarity.31 The ratio of Henry’s constant for N2O/CO2 in tetraethylene glycol dimethyl ether (TEG-DME) at 313 K was 1.08.32 Shiflett et al. measured the N2O and CO2 solubility in a range of butylmethylimmidazolium (bmim) ionic liquids, with Henry’s constant ratios for N2O/CO2 of 1.24 [bmim(SCN)] to 2.14 bmim N(CN)2, indicating higher solubility of CO2 than N2O.33

Figure 4 shows that for N2O over the 35 to 55 °C temperature range, the liquid film mass transfer coefficient does not show a strong dependence on temperature. The temperature trend between 45 and 55 °C appears to show a slight increasing trend for the lean solvent and a slight decreasing trend for a rich solvent. It is speculated that the increases in diffusion related to the increase in temperature and reduction in viscosity as temperature increases are being approximately offset by a reduction in driving force related to a reduction in solubility with increasing temperature. Comparing the N2O mass transfer coefficient at the two solvent loadings shows a modest reduction (e.g., 24% at 35 °C) as the loading is increased from 0.06 to 0.26 mol CO2/mol alkalinity, which is postulated to be primarily due to increasing viscosity with higher loading. Comparing to the behavior of CO 2 mass transfer coefficient,19 the lean solvent at 35 °C shows a mass transfer coefficient that is a factor of 75 higher than measured for N2O. This is attributed to synergistic effects of high solubility19 with rapid reaction.34,35 In the lean solvent, the mass transfer coefficient decreases by a factor of 5 as temperature increases from 35 to 55 °C. Because reaction and diffusion rates should both increase with increasing temperature, it is concluded that the decline is related to a reduction in solubility of CO2 with temperature. The mass transfer coefficient for CO2 into rich solvent shows only a factor of ∼19 increase relative to N2O, which we interpret to be due to an increase in diffusional resistance occurring near the surface of the rich solvent. It is hypothesized that the polar ionic species formed from reaction with CO2 are concentrated at the gas−liquid interface, creating a barrier to diffusion that limits the rate of CO2 uptake. Consistent with this hypothesis, the mass transfer coefficient in the rich solvent is relatively insensitive to temperature as reductions in solubility are offset by diffusion rates. Another possible explanation is that at higher temperature, the increased reaction kinetics result in an elevated concentration of polar CO2 reaction products (alkylcarbonates) at the liquid surface that have a low mobility relative to CO2 and reduce mass transfer of CO2 into solution, either through reduced solubility as a thin layer on the surface, or by forming a physical barrier to diffusion. This is likely, as a heterogeneous molecular structure has been experimentally observed by 4723

DOI: 10.1021/acs.iecr.6b00390 Ind. Eng. Chem. Res. 2016, 55, 4720−4725

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Industrial & Engineering Chemistry Research Liauw36,37 and modeled in our group.38 If the reduction in mass transfer requires a critical concentration of alkylcarbonate, the critical concentration could be reached with slower kinetics in a rich solvent compared to a lean solvent. This may be consistent with the observation that in the rich solvent the mass transfer was significantly lower at 35 °C compared to lean solvent and kg′ was relatively insensitive to temperature for both N2O and CO2. Either way, it is apparent that CO2BOL and other waterlean solvent systems may have unique or heterogeneous molecular structures that would require new film theories to model.39

for the United States Department of Energy. This article was submitted in response to the best-of presentations in “Carbon Management: Recent Advances in Carbon Capture, Conversion, Utilization & Storage” symposium at the American Chemical Society’s 250th meeting in Boston, MA.

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(1) Bottoms, R. R. Process for Separating Acidic Gases. U.S. Patent 1,783,901, December 3, 1930. (2) Kohl, L.; Nielsen, R. B. Gas Purification, 5th ed.; Gulf Publishing Company: Houston, TX, 1997. (3) Rochelle, G. T. Amine scrubbing for CO2 capture. Science 2009, 325, 1652−1654. (4) Adams, D.; Davison, J. Capturing CO2; IEA Greenhouse Gas R&D Programme Report; IEA: Cheltenham, U. K., 2007. (5) Cousins, A.; Wardhaugh, L. T.; Feron, P. M. H. A survey of process flow sheet modifications for energy efficient CO2 capture from flue gases using chemical absorption. Int. J. Greenhouse Gas Control 2011, 5, 605−619. (6) Bates, E. D.; Mayton, R. D.; Ntai, I.; Davis, J. CO2 Capture by as Task-Specific Ionic Liquid. J. Am. Chem. Soc. 2002, 124, 926−927. (7) Gurkan, B.; Goodrich, B. F.; Mindrup, E. M.; Ficke, E. L. E.; Massel, E.; Seo, S.; Senftle, T. P.; Wu, H.; Glaser, M. F.; Shah, J. K.; Maginn, E. J.; Brennecke, J. F.; Schneider, W. F. J. Molecular Design of High Capacity, Low Viscosity, Chemically Tunable Ionic Liquids for CO2 Capture. J. Phys. Chem. Lett. 2010, 1, 3494−3499. (8) Qi, G.; Wang, Y.; Estevez, L.; Duan, X.; Anako, N.; Park, A. H.; Li, W.; Jones, C. W.; Giannelis, E. P. High efficiency nanocomposite sorbents for CO2 capture based on amine-functionalized mesoporous capsules. Energy Environ. Sci. 2011, 4, 444−452. (9) Wang, C.; Luo, H.; Jiang, D.; Li, H.; Dai, S. Carbon dioxide capture by superbase-derived protic ionic liquids. Angew. Chem., Int. Ed. 2010, 49, 5978−5981. (10) Blasucci, V. M.; Hart, R.; Pollet, P.; Liotta, C. L.; Eckert, C. A. Reversible ionic liquids designed for facile separations. Fluid Phase Equilib. 2010, 294, 1−6. (11) Perry, R. J.; Davis, J. L. Amino Disiloxanes for CO2 Capture. Energy Fuels 2012, 26, 2512−2517. (12) Im, J.; Hong, S. Y.; Cheon, Y.; Lee, J.; Lee, J. S.; Kim, H. S.; Cheong, M.; Park, H. Steric hindrance-induced zwitterionic carbonates from alkanolamines and CO2: highly efficient CO2 absorbents. Energy Environ. Sci. 2011, 4, 4284−4289. (13) Heldebrant, D. J.; Yonker, C. R.; Jessop, P. G.; Phan, L. Organic liquid CO2 capture agents with high gravimetric CO2 capacity. Energy Environ. Sci. 2008, 1, 487−493. (14) Jessop, P. G.; Heldebrant, D. J.; Li, X. W.; Eckert, C. A.; Liotta, C. L. Green chemistry: reversible nonpolar-to-polar solvent. Nature 2005, 436 (7054), 1102. (15) Dugas, R. E. Carbon Dioxide Absorption, Desorption, and Diffusion in Aqueous PZ. and MEA. Ph.D. Dissertation, University of Texas, Austin, TX, December 2009. (16) Dugas, R. E.; Rochelle, G. T. CO2 Absorption Rate into Concentrated Aqueous Monoethanolamine and Piperazine. J. Chem. Eng. Data 2011, 56 (5), 2187−2195. (17) Chen, X.; Rochelle, G. T. Aqueous Piperazine Derivatives for CO2 Capture: Accurate Screening by a Wetted Wall Column. Chem. Eng. Res. Des. 2011, 89 (9), 1693−1710. (18) Pacheco, M. A.; Kaganoi, S.; Rochelle, G. T. CO2 absorption into aqueous mixtures of diglycolamine® and methyldiethanolamine. Chem. Eng. Sci. 2000, 55 (21), 5125−5140. (19) Mathias, P. M.; Zheng, F.; Heldebrant, D. J.; Zwoster, A.; Whyatt, G.; Freeman, C. J.; Bearden, M. D.; Koech, P. K. Measuring the Absorption Rate of CO2 in Nonaqueous CO2-Binding Organic Liquid Solvents with a Wetted-Wall Apparatus. ChemSusChem 2015, 8 (21), 3617−3625. (20) Hirschfelder, J. O.; Curtiss, C. F.; Bird, R. B. Molecular Theory of Gases and Liquids; Wiley: New York, NY, 1964.

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CONCLUSIONS In conclusion, the rate of N2O absorption was measured in a DBU-1-hexanol (CO2BOL) solvent to decouple the chemical reactivity and physical dissolution behavior of CO2. N2O flux was measured on a WWC at both lean (0.06 mol CO2/mol BOL) and rich (0.26 mol CO2/mol BOL) loading at 35, 45, and 55 °C at three concentrations of N2O in the feed gas. Liquid-film mass transfer coefficients (kg′) for N2O were calculated by removing gas film mass transfer contributions via a relationship from Bishnoi et al.21 The measured liquid-film mass transfer coefficient for N2O in a CO2BOL solvent was 1 order of magnitude higher than that reported for 5 M MEA, which is attributed to higher physical solubility of gas in 1hexanol versus water. The kg′ for N2O was not a strong function of temperature between 35 and 55 °C, indicating that the significant decrease in kg′ seen for CO2 in the lean solvent is not likely due to a diffusional resistance. More likely, this is the result of the coupled effect of the reaction kinetics acting on a reduced dissolved CO2 concentration as temperature increases. In the rich solvent, the mass transfer was significantly lower at 35 °C compared to lean solvent and kg′ was relatively insensitive to temperature for both N2O and CO2. This may indicate that in the rich solvent a diffusional barrier limits adsorption, possibly due to polar species formed during CO2 reaction, concentrating at the gas liquid interface.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*D. J. Heldebrant. E-mail: [email protected]. Tel.: (509)-372-6359. Author Contributions

G. A. Whyatt designed the test matrix and performed analysis of data, A. Zwoster measured the data, D. J. Heldebrant and Charles J. Freeman analyzed data and drafted the paper. Funding

Funding for this work was provided by the Department of Energy’s Office of Fossil Energy’s Carbon Capture Simulation Initiative (CCSI). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This contribution was identified by Dr. Camille Petit (Imperial College London, UK) as the Best Presentation in the session “ENFL: Carbon Management: Recent Advances in Carbon Capture, Conversion, Utilization & Storage” of the 2015 ACS Fall Meeting in Boston, MA. The authors acknowledge the Department of Energy’s Office of Fossil Energy, Carbon Capture Simulation Initiative (CCSI) for funding. Pacific Northwest National Laboratory is proudly operated by Battelle 4724

DOI: 10.1021/acs.iecr.6b00390 Ind. Eng. Chem. Res. 2016, 55, 4720−4725

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DOI: 10.1021/acs.iecr.6b00390 Ind. Eng. Chem. Res. 2016, 55, 4720−4725