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KINETICS, CATALYSIS, AND REACTION ENGINEERING Estimation of the CO2 Absorption Capacities in Aqueous 2-(2-Aminoethylamino)ethanol and Its Blends with MDEA and TEA in the Presence of SO2 Danielle Bonenfant,† Murielle Mimeault,‡ and Robert Hausler*,† De´ partement de Ge´ nie de la Construction, STEPPE-EÄ cole de Technologie Supe´ rieure, 1100, Notre-Dame Ouest, Montre´ al (Que´ bec), Canada H3C 1K3, and Department of Biochemistry and Molecular Biology, College of Medicine, Eppley Cancer Institute, 7052 DRC, UniVersity of Nebraska Medical Center, 985870 Nebraska Medical Center, Omaha, Nebraska 68198-5870
A study of carbon dioxide (CO2) and sulfur dioxide (SO2)/CO2 mixtures absorption has been carried out in aqueous 2-(2-aminoethylamino)ethanol (AEE) solution and its blends with N-methyldiethanolamine (MDEA) and triethanolamine (TEA) to estimate the influence of SO2, MDEA, and TEA on the CO2 absorption capacity of the AEE. The CO2 absorption loading has been estimated in 15 wt % AEE alone and in the presence of either 5 and 10 wt % MDEA or 5 and 10 wt % TEA solutions with 100 vol % CO2 and 5.03 and 15.02 vol % SO2/CO2 mixtures at a starting temperature of 296 K and flow rates of 3.067, 3.229, and 3.605 L/min, respectively. The results revealed that the presence of SO2 in the gas decreases the CO2 absorption rate and loading in the AEE solution as a function of the concentration of SO2. The additions of 5 and 10 wt % of MDEA and TEA do not seem to influence the CO2 absorption rate in the AEE solution. Moreover, the addition of MDEA increases slightly the CO2 absorption capacity of AEE, while TEA decreases the absorption capacity of AEE in the absence and presence of SO2. These effects were enhanced with increases of MDEA and TEA. Altogether, the results indicated that the blend of 15 wt % AEE + 10 wt % MDEA represents an interesting solvent which could be used as absorbent for the removal of CO2 from emission into the atmosphere by industries. 1. Introduction Fossil fuel combustion from refineries and the energy industry is one of the principal source of CO2 emission into the atmosphere.1 Several technologies are available to reduce the emission of CO2 from industrial gas streams, but the chemical absorption with alkanolamines as solvent, such as monoethanolamine (MEA), diethanolamine (DEA), di-2-propanolamine (DIPA), triethanolamine (TEA), and methyldiethanolamine (MDEA), is the most used for low to moderate CO2 partial pressures.2-5 This method is efficient and usually permits removal from 75 to 90% of the CO2 emitted into the atmosphere.2 The primary and secondary alkanolamines react directly and reversibly with the CO2 through the formation of a zwitterion intermediate which is deprotonated by the bases present in solution to form a stable carbamate,6-10 although the formation of carbamate increases the reaction rate but limits generally the loading to 0.5 mol of CO2/(mol of amine).11,12 However, the hydrolysis of an amount of carbamate, especially at high pressure, might increase the CO2 loading values near 1 mol of CO2/(mol of amine).13,14 In contrast, tertiary amines do not react directly with CO2 to form carbamate. In aqueous solutions, tertiary amines rather catalyze the hydrolysis of CO2 to form bicarbonate ions and the protonated amine.15,16 Generally, tertiary amines react more slowly with CO2 than primary * To whom correspondence should be addressed. Tel.: (514) 396 8499. Fax: (514) 396 8584. E-mail:
[email protected]. † STEPPE-E Ä cole de Technologie Supe´rieure. ‡ University of Nebraska Medical Center.
and secondary amines. Moreover, tertiary amines such as TEA and MDEA are weaker bases than primary and secondary alkanolamines, and this low basicity contributes to decrease their CO2 absorption rate.12,17 However, the tertiary alkanolamines present the advantage of reacting at equimolar concentration with the CO2.11,12,18 The structure of diamines contains two amino groups which may confer to them a higher CO2 absorption capacity than 1 mol of CO2/(mol of amine). It has been suggested that unhindered diamine reacts rapidly and reversibly with the CO2 and that this reaction leads to the formation of a carbamate ion.19,20 The results from our prior studies indicated that an unhindered diamine, 2-(2-aminoethylamino)ethanol (AEE), possesses a higher CO2 absorption capacity than those of primary, tertiary, and cyclic amines, including MEA, TEA, triethylamine, pyridine, and pyrrolidine. In particular, AEE showed a CO2 loading varying from 1.35 to 1.12 mol of CO2/(mol of amine) in aqueous 5-25 wt % AEE solutions at 296 K and atmospheric pressure.21,22 Several studies have shown that the blends of primary and secondary amines with tertiary amines present the high rate of absorption of primary and secondary amines combined with the CO2 absorption capacity of the tertiary amines. Notably, it has been shown that the addition of a small amount of MEA to an aqueous solution of MDEA and TEA permits significant enhancement of its CO2 absorption rate and CO2 loading.4,5,23,24 Horng and Li4 have also shown that an increase of the concentration of TEA in an aqueous blend of MEA + TEA results in a rise of the solubility of CO2. Moreover, we have
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observed that the addition of 10 wt % MDEA in a solution of 15 wt % AEE leads to a small increase of the CO2 absorption loading of AEE and to a marked rise of its desorption capacity.22 In spite of many advantages, the amines are considered as nonselective solvents to remove CO2 because they can also interact with other pollutants, especially with SO2, and that might impair the removal of CO2.2,25,26 The presence of SO2 might represent a major problem due to its concentration which is typically 700-2500 ppm in industrial gas streams from coalfire plants.2 Because the SO2 is a more acidic gas than CO2, its regenerative removal requires use of weaker bases than primary amines. The strong basicity of primary amines results in an irreversible reaction with SO2 which produces corrosive and heat-stable salts, reducing thereby the CO2 absorption rate and capacity of the absorbent.2,27,28 In this context, a mathematical model developed by Rao and Rubin2 has notably indicated that for the CO2 capture system using 15-50 wt % MEA solutions as solvent, the absorption of the industrial gas stream which contains a typical concentration of SO2 might lead to a loss of 2 mol of MEA/(mol of SO2). A recently developed system process for SO2 capture showed that SO2 may also react with diamine.29 It has been shown that SO2 might react with the monoprotonated diamine to form the doubly protonated diamine and a bisulfite ion (HSO3-). The present study has been undertaken to determine whether the CO2 absorption properties of diamine AEE, alone or in combination with MDEA or TEA, are influenced by the presence of SO2 in solution. Therefore, the CO2 absorption capacities of aqueous AEE, AEE + MDEA, and AEE + TEA solutions have been estimated in the presence of SO2 at several concentrations and compared to results obtained in the absence of SO2. 2. Experimental Section 2.1. Materials. Monoethanolamine (99.8%) was purchased from J. T. Baker Chemical Co. (Philipsburg, NJ), 2-(2-aminoethylamino)ethanol (99%) from ACROS ORGANICS (NJ), triethanolamine (98%) from Anachemia Canada, Inc. (Montreal, QC), N-methyldiethanolamine (99+%) from Aldrich Chemical Co. (Milwaukee, WI), CO2 gas from Praxair Production Inc. (Montreal, QC), and 5.03 vol % SO2/CO2 and 15.02 vol % SO2/CO2 gas mixtures from Air Liquide (Montreal, QC). All these products have been used without additional purification. 2.2. CO2 Absorption Experiments. The absorbent solution was prepared by dilution of appropriate amounts of AEE in demineralized water at the concentration of 15 wt %. The blends were prepared by the mixture of 15 wt % AEE with MDEA or TEA at 5 and 10 wt %. The concentration of each solution was verified by measurement of the amount of total organic carbon. A first CO2 absorption was performed with 100 vol % CO2, 5.03 vol % SO2/CO2, and 15.02 vol % SO2/CO2 mixtures at flow rates of 3.067 L/min (100 vol % CO2), 3.229 L/min (5.03 vol % SO2/CO2 mixture), and 3.605 L/min (15.02 vol % SO2/ CO2 mixture) in aqueous 15 wt % AEE, 15 wt % AEE + 10 wt % TEA, and 15 wt % AEE + 10 wt % MDEA solutions at a starting temperature of 296 K on an interval from 0 to 10 min. These flow rates were adjusted to maintain a constant CO2 partial pressure in the 100 vol % CO2 and SO2/CO2 mixtures. The CO2 absorption was also measured with 100 vol % CO2 and 5.03 vol % SO2/CO2 in 15 wt % AEE + 5 wt % TEA and 15 wt % AEE + 5 wt % MDEA solutions in the same experimental conditions for a period of 10 min. The CO2 absorption was performed in a 150 mL absorbent solution within a Pyrex cylinder with a diameter of 4.65 cm and a height of 20 cm, open to the atmosphere. The CO2 was injected from CO2 and gas mixture reservoirs, and the gas flow
Table 1. CO2 Loading Obtained for 100 vol % CO2 and 5.03 and 15.02 vol % SO2/CO2 Mixtures in Aqueous 15 wt % AEE Solution and Its Blends with 5 and 10 wt % MDAE and TEA CO2 loadinga (mol of CO2/(mol of amine)) amine solution
100 vol % CO2
5.03 vol % SO2/CO2
15.02 vol % SO2/CO2
15 wt % AEE 15 wt % AEE + 5 wt % MDEA 15 wt % AEE + 10 wt % MDEA 15 wt % AEE + 5 wt % TEA 15 wt % AEE + 10 wt % TEA
1.189 1.231 1.267 0.990 0.976
1.048 1.103 1.137 0.926 0.893
0.804 0.817 0.773
a
The estimations of CO2 loading values were made at flow rates of 3.067 L/min (100 vol % CO2), 3.229 L/min (5.03 vol % SO2/CO2 mixture), and 3.605 L/min (15.02 vol % SO2/CO2 mixture) at a starting temperature of 296 K for a period of 10 min. The CO2 partial pressure was maintained constant in pure and SO2/CO2 mixtures.
Figure 1. Loadings of 100 vol % CO2 by aqueous 15 wt % AEE ([), 15 wt % AEE + 10 wt % MDEA (9), and 15 wt % AEE + 10 wt % TEA (2) solutions measured at a CO2 flow rate of 3.067 L/min and at a starting temperature of 296 K.
was controlled by using a flowmeter with a stainless steel float from Cole-Parmer Instrument Co. (Vernon Hills, IL). The uniform repartition of gas in the aqueous amine solutions was assumed by using a diffuser located within the solutions. The CO2 concentration of absorbent solutions has been determined by subtraction of amounts of total organic carbon measured in the samples before and after the CO2 absorption by using an analyzer DC-85A TOC from Dohrmann Division Co. (Santa Clara, CA), provided there was a furnace for combustion with a continuous flux of oxygen at 800 °C and a tube for catalysis containing cobalt oxide on alumina. The analyzer was calibrated with a solution of potassium biphthalate at an organic carbon concentration of 2000 ppm. The absorbent samples were diluted at the final organic carbon concentrations below 2000 ppm. The organic carbon concentrations of the diluted absorbent samples were measured after injection of 40 µL. 3. Results and Discussion 3.1. Estimation of CO2 Absorption in AEE, AEE + MDEA, and AEE + TEA Solutions in the Absence of SO2. The CO2 absorption loading values and patterns obtained in aqueous AEE, AEE + MDEA, and AEE + TEA solutions are presented in Table 1 and Figure 1. The results indicated that the addition of 10 wt % MDEA and TEA does not affect the
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% SO2/CO2, and 15.02 vol % SO2/CO2 decrease from 1.189 to 1.048 and 0.804 mol of CO2/(mol of amine), which correspond to the losses of CO2 absorption capacity of 11.9 and 32.4%, respectively. The addition of 5 and 10 wt % MDEA increases slightly the CO2 absorption capacity of AEE solution loaded with SO2/CO2 mixtures, but these values remain inferior to those of the AEE solution loaded with pure CO2. In contrast, the addition of 5 and 10 wt % TEA leads to a drop of CO2 loading of AEE solution in the presence of 5.03 and 15.02 vol % SO2. The losses of CO2 loading in AEE solution loaded with SO2/ CO2 mixtures that occur by addition of TEA are estimated at 11.6% for 15 wt % AEE + 5 wt % TEA solution loaded with 5.03 vol % SO2/CO2, and at 14.7 and 3.7% for 15 wt % AEE + 10 wt % TEA solution loaded with 5.03 and 15.02 vol % SO2/CO2 mixtures. In addition, the loading with SO2/CO2 mixtures seems also to decrease the CO2 absorption rate in 15 wt % AEE, 15 wt % AEE + 10 wt % MDEA, and 15 wt % AEE + 10 wt % TEA solutions, as a function of the concentration of SO2. 4. Conclusion Taken together, the results revealed that the CO2 absorption capacity of 15 wt % AEE solution might be slightly enhanced by the addition of MDEA in both the absence and the presence of SO2 in absorbed gas. In contrast, the addition of TEA decreases the CO2 absorption capacity of AEE solution and results in a greater loss of CO2 loading when the absorbed gas contains SO2. Therefore, the mixtures containing AEE + MDEA might represent the more interesting solvents as those constituted with AEE + TEA, which could be used for the capture of the CO2 from industrial gas streams containing SO2. Acknowledgment
Figure 2. Loadings of CO2 by aqueous 15 wt % AEE ([), 15 wt % AEE + 10 wt % MDEA (9), and 15 wt % AEE + 10 wt % TEA (2) solutions loaded with 5.03 vol % SO2/CO2 (A) and 15.02 vol % SO2/CO2 mixtures (B), measured at CO2 flow rates of 3.229 (A) and 3.605 L/min (B) and at a starting temperature of 296 K.
absorption rate of CO2 in 15 wt % AEE solution. Nevertheless, the addition of 5 and 10 wt % MDEA increases the CO2 loading in 15 wt % AEE solution from 3.5 to 6.6%. This increase of the CO2 loading might be due to an increase of the amount of bicarbonbate ions formed at high loading when MDEA is present in the AEE solution, as previously suggested.22 In contrast, the addition of 5 and 10 wt % TEA decreases the CO2 loading in AEE of 16.7 and 18.0%. This decrease of the CO2 loading might be associated with the lower catalytic activity and basicity of TEA than those of MDEA. In addition, several studies revealed that the CO2 loading values obtained for 5 and 24.73 wt % TEA solutions are inferior to those observed for 5 wt % AEE and 20.1 wt % MDEA solutions.21,23 3.2. Estimation of CO2 Absorption in AEE, AEE + MDEA, and AEE + TEA Solutions in the Presence of SO2. The CO2 absorption loading values and patterns obtained in AEE, AEE + MDEA, and AEE + TEA solutions loaded with SO2/CO2 gas mixtures are presented in Table 1 and Figure 2. These results indicate that the presence of 5.03 and 15.02 vol % SO2 in the gas injected decreases the CO2 loading in 15 wt % AEE solution and this decrease is more marked when the percentage of SO2 increases. The CO2 loading values for AEE solution after 10 min absorption with 100 vol % CO2, 5.03 vol
We thank Dr. Fre´de´ric Monette for his technical assistance. This work has been supported by grants from the Natural Sciences and Engineering Research Council of Canada (NSERC). Literature Cited (1) He, B.; Zheng, X.; Wen, Y.; Tong, H.; Chen, M.; Chen, C. Temperature Impact on SO2 Removal Efficiency by Ammonia Gas Scrubbing. Energy ConVers. Manage. 2003, 44 (13), 2175-2188. (2) Rao, A. B.; Rubin, E. S. A Technical, Economic, and Environmental Assessment of Amine-Based CO2 Capture Technology for Power Plant Greenhouse Gas Control. EnViron. Sci. Technol. 2002, 36, 4467-4475. (3) Yoon, S. J.; Lee, H. Kinetics of Absorption of Carbon Dioxide into Aqueous 2-Amino-2-ethyl-1,3-propanediol Solutions. Ind. Eng. Chem. Res. 2002, 41, 3651-3656. (4) Horng, S.-Y.; Li, M. Kinetics of Absorption of Carbon Dioxide into Aqueous Solutions of Monoethanolamine + Triethanolamine. Ind. Eng. Chem. Res. 2002, 41, 257-266. (5) Liao, C.-H.; Li, M. Kinetics of Absorption of Carbon Dioxide into Aqueous Solutions of Monoethanolamine + N-Methyldiethanolamine. Chem. Eng. Sci. 2002, 57 (21), 4569-4582. (6) Danckwerts, P. V. The Reaction of CO2 with Ethanolamines. Chem. Eng. Sci. 1979, 34 (4), 443-446. (7) Blauwhoff, P. M. M.; Versteeg, G. F.; van Swaaij, W. P. M. A Study on the Reaction between CO2 and Alkanolamines in Aqueous Solutions. Chem. Eng. Sci. 1984, 39 (2), 207-225. (8) Caplow, M. Kinetics of Carbamate Formation and Breakdown. J. Am. Chem. Soc. 1986, 90, 6795-6803. (9) Versteeg, G. F.; van Swaaij, W. P. M. On the Kinetics between CO2 and Alkanolamines Both in Aqueous and Non-aqueous SolutionssI. Primary and Secondary Amines. Chem. Eng. Sci. 1988, 43 (3), 573-585. (10) Glasscock, D. A.; Critchfield, J. A.; Rochelle, G. T. CO2 Absorption/ Desorption in Mixtures of Methyldiethanolamine with Monoethanolamine or Diethanolamine. Chem. Eng. Sci. 1991, 46 (11), 2829-2845.
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ReceiVed for reView June 5, 2007 ReVised manuscript receiVed September 12, 2007 Accepted September 14, 2007 IE070778U
