Ind. Eng. Chem. Res. 2003, 42, 3179-3184
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APPLIED CHEMISTRY Determination of the Structural Features of Distinct Amines Important for the Absorption of CO2 and Regeneration in Aqueous Solution Danielle Bonenfant,† Murielle Mimeault,‡ and Robert Hausler*,† La STEPPE-Universite´ du Que´ bec a` Montre´ al, C.P. 8888 Succursale Centre-ville, Montre´ al, Canada H3C 3P8, and Faculte´ de Pharmacie, Laboratoire de Toxicologie, Institut de Chimie Pharmaceutique Albert Lespagnol, 3 Rue du Pr. Laguesse, Lille, France
A study of carbon dioxide (CO2) absorption/desorption has been carried out to estimate the influence of the structural features of distinct amines on their CO2 absorption and regeneration. The absorption has been made at two different CO2 flow rates with a series of aqueous 5 wt % ammonia, monoethanolamine (MEA), triethanolamine (TEA), triethylamine, pyridine, pyrrolidine, 2-(2-aminoethylamino)ethanol (AEE), and N-(2-aminoethyl)-1,3-propanediamine (AEPDNH2) solutions, while the CO2 desorption has been performed by heating these solutions. The presence of two or three amino groups in AEE and AEPDNH2, the structure of tertiary amine and alkanolamine, and a nonaromatic ring of pyrrolidine might favor the CO2 absorption, while the structural features of ammonia and pyridine seem to be unfavorable. The tertiary alkanolamine is the most easy to regenerate and looses less of its CO2 loading after regeneration. It appears that AEE and AEPDNH2 would represent interesting compounds which could be used as CO2 absorbents in industrial technologies to prevent CO2 release into the atmosphere. Introduction Some industrial processes use the combustion of coal or gas and annually release a large amount of CO2 into the atmosphere. In numerous cases, the methods used by industries to prevent emanations of gas consist of CO2 removal by chemical absorption/desorption processes with alkanolamine solutions concomitant with a regeneration of amines for reuse. Among the alkanolamines that are the most commonly used in these industrial acid-gas treating methods are monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), diisopropanolamine (DIPA), and methyldiethanolamine (MDEA).1 In addition, certain sterically hindered amines and polyamines also present interesting properties as absorbents due to their high CO2 loading capacities.2-4 In general, the CO2 absorption capacities for the amines depend on several factors including pH of the solution, basicity, concentration, and structural characteristics of amine. In particular, the reaction of aqueous primary amine solutions such as ammonia with CO2 leads to the formation of ammonium bicarbonate as follows:
NH3 + CO2 + H2O f NH4HCO3
(1)
* To whom correspondence should be addressed. Tel.: 514987-3000 (6143). Fax: 514-987-8484. E-mail: Hausler.Robert@ uqam.ca. † La STEPPE-Universite ´ du Que´bec a` Montre´al. ‡ Institut de Chimie Pharmaceutique Albert Lespagnol.
This reversible reaction is constituted by a series of intermediary endothermal-exothermal reactions which are limited to a maximal loading of 0.5 mol of CO2/mol of ammonia.5,6 Moreover, another disadvantage associated with the use of ammonia is the difficultly in separating CO2 and ammonia after the thermal decomposition of ammonium bicarbonate.7 In addition, the sterically unhindered primary and secondary alkanolamines in aqueous solution react directly, instantaneously, and reversibly with CO2 by forming an intermediate zwitterion that is deprotonated by amine giving carbamate through the reactions (2)-(4).8-12
CO2 + R1R2NH a R1R2NH+CO2-
(2)
R1R2NH+CO2- + R1R2NH a R1R2NH2+ + R1R2NCO2- (3) R1R2NH+CO2- + H2O a H3O+ + R1R2NCO2-
(4)
This mechanism involves a second-order reaction where reaction (3) represents the determining step. The contribution of each base present in solution to the deprotonation of zwitterion appears to be dependent on their concentration and basicity that influence principally the catalytic activity of amine. Hence, when an alkanolamine is present at a high concentration, the reaction of deprotonation of zwitterion is a second-order reaction while at a low concentration of alkanolamine, this is a first-order reaction.10 Moreover, an increase of the
10.1021/ie020738k CCC: $25.00 © 2003 American Chemical Society Published on Web 06/17/2003
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basicity of primary and secondary alkanolamines seems also to cause a rise in their reaction rate with CO2. The formation of a stable carbamate limits generally the primary and secondary alkanolamines to a maximum loading of 0.5 mol of CO2/mol of amine. Interestingly, the hydrolyze of carbamate might however increase this limit of CO2 absorption.13,14 In this matter, the degree of hydrolysis of carbamate depends on several factors such as its chemical stability, which is markedly influenced by the temperature. Indeed, the reaction between alkanolamine and CO2 favors the formation of carbamate and bicarbonate at low temperatures whereas the release of CO2 and amine appears rather to be prevalent at high temperatures.15 Among the exceptions for this, the primary alkanolamine MEA reacts with CO2 through a reaction process which is typically exothermal and mediated via a first-order mechanism where the formation of zwitterion involves only a proton transfer that constitutes the determining step.10,14,16-19 This reaction is very rapid and independent of the concentration of OH- ions present in solution.10,20,21 In the case of tertiary alkanolamines such as TEA, they did not react directly with CO2 to form carbamate because these amines lack the free protons.10 In fact, according to the reaction mechanism proposed by Donaldson and Nguyen,20 the tertiary alkanolamines act as bases which catalyze the hydration of CO2 that leads to the formation of bicarbonate.22 The complete mechanism of the reaction of CO2 with a tertiary alkanolamine might be summarized by the formation of a hydrogen bond between free alkanolamine and water which involves the lone-pair electrons of nitrogen that enhances the reactivity between water and CO2.10,22,23 This reaction mechanism includes the following reactions:
CO2 + H2O a H2CO3
(5)
CO2 + OH- a HCO3-
(6)
CO2 + R3N + H2O a R3NH+ + HCO3-
(7)
This reaction, which is considered as a pseudo-firstorder reaction, is less exothermal than that between CO2 and primary and secondary alkanolamines which implicates the formation of carbamate.14,24-27 The tertiary alkanolamines might react at equimolar concentration with CO2 and this confers to them a loading capacity of 1 mol of CO2/mol of amine.13,21 Moreover, the pH of the solution is also a factor that might influence the CO2 absorption rate of tertiary alkanolamines. Notably, the weak basicity of TEA (pKa ) 7.76 at 25 °C) seems to decrease its absorption rate.13,28 Of particular interest, a prior study has also revealed that tertiary alkanolamines are more easy to regenerate and loose less of their absorption capacity after regeneration than the primary alkanolamines.29 The reactions of diverse diamines and polyamines with CO2 have also been investigated.4,30,31 In particular, a reaction mechanism has recently been described about the reaction between CO2 and an aqueous polyamine, bis(3-dimethylaminopropyl)amine (TMBPA) solution, which indicates that 1 mol of TMBPA might react with 3 mol of CO2.4 The presence of a ring in amines may also enhance their reaction rate with CO2. Particularly, it has been observed that the cyclic and rigid structure of quinu-
Figure 1. Chemical structures of amines studied.
clidine might improve the accessibility of lone-pair electrons of the nitrogen for its binding to CO2.32 However, it has been reported that the lone-pair electrons of the nitrogen, which are located in a sp2-hybrid orbital, are usually less available for binding.33 On the basis of aforementioned observations that indicate that the CO2 absorption capacities of amines are related to their structural properties, the present study has been undertaken to compare the effects induced by the presence of one or several amino groups and a ring within the amines upon their CO2 absorption and desorption. Therefore, an estimation of CO2 absorption/desorption has been performed with a series of aqueous 5 wt % amine solutions which are characterized by different structures containing from one to three amino groups including ammonia (primary amine), MEA (primary alkanolamine), TEA, and triethylamine (tertiary alkanolamine and amine), AEE (diamine), AEPDNH2 (polyamine), pyridine (amine with an aromatic ring), and pyrrolidine (amine with a nonaromatic ring) (Figure 1). Moreover, since the CO2 absorption rate is a critical factor, the absorption with these amines has been made at two CO2 flow rates. 2. Experimental Section 2.1. Materials. Ammonia (28-30%) and pyridine (99.9%) have been purchased from Fisher Scientific Co. (Nepean, Ontario, and Fair Lawn, NJ), MEA (99.8%) from J. T. Baker Chemical Co. (Philipsburg, NJ), triethanolamine (98%), and triethylamine (99%) from Anachemia Canada Inc. (Montreal, QC), 2-(2-aminoethylamino)ethanol (99%) from ACROS ORGANICS (New Jersey) and Aldrich Chemical Co. (Milwaukee, WI), N-(2-aminoethyl)-1,3-propanediamine (97%) and pyrrolidine (99%) from Aldrich Chemical Inc. (Milwaukee, WI), and CO2 gas from Praxair Production Inc. (Montreal, QC). All these products have been used without additional purification. 2.2. CO2 Absorption and Desorption Experiments. The aqueous absorbent solutions have been prepared by dilution of an appropriate amount of amines, diamine, and polyamine in demineralized water at a concentration of 5 wt %. Moreover, the concentration of each solution has been verified by the measures of the amount of total organic carbon. A first CO2 absorption has been performed at a partial pressure of
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CO2 of 1 atm with CO2 flows of 1.337 and 3.067 L/min at 23 ( 1 °C on an interval from 0 to 120 s in 150 mL of aqueous absorbent solution within Pyrex cylinders open to the atmosphere. The CO2 absorption has been effectuated within a Pyrex cylinder with a diameter of 4.65 cm and a height of 20 cm, with the exception of CO2 absorption in the pyridine solution, which has been made in a Pyrex cylinder with a diameter of 5.7 cm and a height of 26 cm. The CO2 has been injected at the ambient temperature (23 ( 1 °C) from a CO2 gas reservoir. The CO2 flows have been controlled by using a flowmeter with a stainless steel float from ColeParmer Instrument Co. (Vernon Hills, Illinois, USA). The uniform repartition of CO2 in aqueous absorbent solutions has been assured by using a diffuser localized within these solutions. For all aqueous absorbent solutions, a CO2 re-absorption has been performed after a first desorption at 23 ( 1 °C and at a flow of 3.067 L/min, on an interval of injection of CO2 of 120 s and in the experimental conditions identical to those of the first absorption. A first and second CO2 desorption have been effectuated after absorption performed at a CO2 flow of 3.067 L/min, on an interval of 120 s. The loaded aqueous absorbent solutions have been desorbed in a Pyrex boiling flask with a capacity of 500 mL provided a Pyrex condenser opening at 1 atm. The desorptions have been made at maximum boiling temperatures of each solution at 89 °C for ammonia, 98 °C for triethylamine, 98.599 °C for pyridine, and 100-101 °C for MEA, TEA, AEE, AEPDNH2, and pyrrolidine. The loaded aqueous absorbent solutions have been previously heated on a hot plate stirrer for 5-9 min. The first desorption has been effectuated on an interval from 0 to 210 s. The second desorption has been made after the CO2 re-absorption, during a heating period of 210 s. 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 and desorption 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 has been calibrated with a solution of potassium biphthalate at an organic carbon concentration of 2000 ppm. The absorbent samples have been diluted at the final organic carbon concentrations below 2000 ppm. The organic carbon concentrations of diluted absorbent samples have been measured after injection of 40 µL. 3. Results and Discussion 3.1. Estimation of CO2 Absorption and Reabsorption by Aqueous Amines, Diamine, and Polyamine Solutions. The CO2 absorption patterns obtained at the CO2 flows of 1.337 and 3.067 L/min, in aqueous 5 wt % ammonia, MEA, TEA, triethylamine, AEE, AEPDNH2, pyridine, and pyrrolidine solutions are presented in Figure 2. The results indicate that the amines studied absorb the CO2 more rapidly at a CO2 flow of 3.067 L/min than at 1.337 L/min. In fact, this leads to a higher CO2 loading after 2 min of absorption at 3.067 L/min for all the amines. This suggests that an increase of CO2 flow from 1.337 to 3.067 L/min leads to a rise of the CO2 absorption rate in these amines solutions whose effect might be associated with an increase of mass-transfer rate effects in aqueous phase.34-37
Figure 2. CO2 loading of aqueous 5 wt % MEA ((), TEA (9), AEE (2), triethylamine ()), ammonia (0), pyridine (b), pyrrolidine (O), and AEDPNH2 (4) solutions, measured at a CO2 flow of 1.337 (A) and 3.067 L/min (B) at 23 ( 1 °C and at a partial pressure of CO2 of 1 atm. Table 1. CO2 Loading of Aqueous 5 wt % Amines, Diamine, and Polyamine Solutions as Measured by Absorption, Re-absorption, and First and Second Desorption bCO
aqueousa absorbent ammonia MEA TEA triethylamine AEE AEPDNH2 pyridine pyrrolidine
CO2 loading (mol of CO2/ mol of amine) absorption re-absorption 0.594 0.813 1.220 1.159 1.348 2.451 0.229 1.033
0.595 0.814 1.212 1.026 1.388 2.380 0.229 1.032
2 loading (mol of CO2/ mol of amine)
desorption first second 0.316 0.243 0.104 0.869 0.257 0.597 0.038 0.561
0.324 0.240 0.107 0.846 0.254 0.611 0.038 0.564
a The measures of CO absorption and re-absorption loading 2 have been estimated for the distinct aqueous amine solutions studied at a partial pressure of CO2 of 1 atm, CO2 flow of 3.067 L/min, and ambient temperature and for a period of 2 min. b The estimation of CO2 loading of first and second desorption for the different aqueous amine solutions has been estimated by heating the amine solutions at maximal boiling temperature for 210 s. The CO2 loading values of aqueous amines, diamine, and polyamine solutions measured during the second CO2 desorption were not significantly different from those measured during the first desorption.
As shown in Figure 2B and Table 1, after 2 min of absorption at a CO2 flow of 3.067 L/min, the CO2 loading values expressed as mol of CO2/mol of amine observed
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for the 5 wt % amine solutions are by increasing order as follows: pyridine < ammonia < MEA < pyrrolidine < triethylamine < TEA < AEE < AEPDNH2. In particular, the weaker CO2 loading of pyridine as compared to other tested amines might be due to its aromatic ring because the lone-pair electrons of nitrogen are located in a sp2-hybrid orbital and that might decrease their accessibility for its binding to CO2. In contrast, the higher CO2 loading of pyrrolidine than that obtained for pyridine might be attributed to its nonaromatic ring that can favor the accessibility of lone-pair electrons of the nitrogen to CO2. Furthermore, since the ring of the pyrrolidine is little, this molecule can adopt a spatial organization in solution that favors the loading of CO2. Moreover, the low CO2 loading noticed for primary amine ammonia might be associated with the formation of ammonium bicarbonate that limits the reaction at 0.5 mol of CO2/mol of ammonia.5,6 Accordingly, Yeh and Bai5 have also observed a weak loading value of 1.20 kg of CO2/kg of NH3 (0.464 mol of CO2/ mol of NH3) for CO2 absorption in an aqueous 7 wt % ammonia solution, at a 16 vol % CO2 flow of 2 L/min at 25 °C. In the case of MEA, the low loading value observed for this primary alkanolamine might be due to the formation of a stable carbamate that limits its CO2 absorption. Indeed, it has been reported that the reaction between the primary and secondary alkanolamines such as MEA and DEA with CO2 give generally the loading values of about 0.5 mol of CO2/mol of amine by the formation of a stable carbamate.13,14,38 Thus, since we have obtained the loading values of 0.746 and 0.814 mol of CO2/mol of MEA at the CO2 flow of 1.337 and 3.067 L/min, respectively, ambient temperature, and a pressure of 1 atm, it is likely that a certain amount of carbamate ions generated through the reaction of aqueous 5 wt % MEA solution with CO2 might be hydrolyzed under our experimental conditions. This supports the prior works which indicated that the hydrolysis of carbamate ions might occur with the alkanolamines including MEA at high pressures and this leads to the CO2 loading values greater than 0.5 but less than unity.14,15,38 In particular, Hook15 has reported a loading value of 0.76 mol of CO2/mol of MEA for an aqueous MEA solution at 2.5 M in these experimental conditions. In addition, the high CO2 loading values obtained for triethylamine and tertiary alkanolamine TEA are in agreement with prior studies, which indicated that the tertiary amines might react in equimolar concentration with CO2.13,21 Thus, since it has been reported that the CO2 loading of amines might vary with the pH of solution,13,28 it appears that triethylamine (pKa ) 10.75) and TEA (pKa ) 7.76) might adopt an unprotonated and catalycally active form in our experimental conditions used to measure the CO2 absorption (5 wt % triethylamine: pH ) 11.60; 5 wt % TEA: pH ) 9.46). In support with this, it has also been proposed by Blauwhoff et al.10 that TEA in solution at pH > 9.5 exists principally under an unprotonated form which is catalytically active. However, despite the high CO2 loading observed for these aqueous amine solutions, it is noteworthy that their use as CO2 absorbents could involve certain inconveniences. Indeed, the triethylamine is volatile and aqueous 5 wt % TEA solution foams to a significant degree during absorption and this might constitute a disadvantage in an absorber. Interestingly, the maximal CO2 loading value reached with aqueous 5 wt % AEE solution (1.348 mol of CO2/
mol of AEE) suggests that this diamine might react with more than 1 mol of CO2. In fact, it is likely that the high CO2 loading of AEE might be due to the presence of two amino groups that permit reaction with 2 mol of CO2. In this context, a reaction mechanism has been proposed to describe the reaction between CO2 and the diamines in the membranes, which also indicates that the reaction CO2-diamine produces carbamate ions.31 Hence, the formation of carbamate ions during the reaction CO2-AEE might have contributed to restriction of the CO2 loading. Moreover, the fact that the two nitrogen atoms of AEE are separated by a short organic backbone can create a steric hindrance which may also limit their CO2 binding. Importantly, among all amines studied, the aqueous 5 wt % polyamine (AEPDNH2) solution is the absorbent that shows the high CO2 loading (Table 1). In this context, Dallos et al.4 recently reported a high value of CO2 loading for TMBPA of 2.9 mol of CO2/mol of TMBPA, and these authors have proposed that TMBPA is characterized by a theoretical and maximal loading of 3 mol of CO2/mol of TMBPA. TMBPA is a polyamine which possesses a structure that resembles that of AEPDNH2. Therefore, it is possible that the high CO2 loading of AEPDNH2 might also be due to its capacity to react with 3 mol of CO2. Finally, the CO2 loading values measured for all aqueous 5 wt % amines, diamine, and polyamine solutions studied during the CO2 re-absorption were not significantly different from those measured during the first absorption, except for the re-absorption value of 5 wt % triethylamine solution which was inferior to that of first absorption (Table 1). This significant decrease of 11.5% of CO2 loading for triethylamine might be attributed at its evaporation. Altogether, these results suggest that all of the 5 wt % amine, diamine, and polyamine solutions analyzed show very little loss of CO2 absorption capacity during their regeneration. This observation is important since it suggests that all amines aforementioned could be used as CO2 absorbents in the cyclic system without significant loss of their CO2 absorption capacities during the regeneration. 3.2. Estimation of CO2 Desorption of Aqueous Amine, Diamine, and Polyamine Solutions. The CO2 desorption patterns of aqueous 5 wt % ammonia, MEA, TEA, triethylamine, AEE, AEPDNH2, pyridine, and pyrrolidine solutions obtained after CO2 absorption effectuated at a CO2 flow of 3.067 L/min for 2 min and after heating at temperatures from 89 to 101 °C as a function of amine solution are shown in Figure 3. These desorption curves indicate that 5 wt % ammonia, TEA, AEE, and AEPDNH2 solutions desorb more rapidly than the other amines. Thus, these amines are also those that have the highest CO2 loading values at a CO2 flow of 3.067 L/min, with the exception of ammonia (Figure 2B). On the basis of the percentages of desorption obtained after 210 s of heating at maximal temperatures, the aqueous 5 wt % amine solutions analyzed in our study might be classified as the following: TEA (91.5%) > pyridine (83.7%) > AEE (81.0%) > AEPDNH2 (75.6%) > MEA (66.7%) > ammonia (46.9%) > pyrrolidine (45.7%) > triethylamine (13.1%). Moreover, the CO2 loading values of aqueous amine, diamine, and polyamine solutions measured during the second CO2 desorption were also similar to those measured during the first desorption (Table 1). These results indicate that 5 wt % TEA solution might be regenerated in a greater proportion than the other amines and it looses only a
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CO2 absorption in an absorber could be difficult since this aqueous amine solution foams during absorption. For the AEE, its low cost could however be advantageous industrially. Furthermore, the efficiency of CO2 recuperation of aqueous 5 wt % MEA and pyrrolidine solutions is smaller than those of aqueous AEPDNH2, TEA, and AEE solutions but a use of MEA might present certain advantages due to its acceptable regeneration capacity. In contrast, the aqueous 5 wt % triethylamine, ammonia, and pyridine solutions appear to be less efficient for the CO2 recuperation due to their low absorption and/or regeneration capacity. Thus, it appears that at a partial pressure of CO2 of 1 atm and 23 ( 1 °C, the aqueous 5 wt % polyamine, tertiary alkanolamine, and diamine solutions possess the highest CO2 recuperation capacities. 4. Conclusions
Figure 3. CO2 loading of aqueous 5 wt % MEA ((), TEA (9), AEE (2), triethylamine ()), ammonia (0), pyridine (b), pyrrolidine (O), and AEDPNH2 (4) solutions as a function of time of desorption. The results have been measured after CO2 absorption at a flow of 3.067 L/min and CO2 desorption at the temperatures between 89 and 101 °C and at a partial pressure of CO2 of 1 atm.
small percentage of its absorption capacity (8.5%). This is in agreement with the tests effectuated by Lin and Shyu29 which revealed that the tertiary alkanolamines are easy to regenerate and loose a smaller part of their CO2 absorption capacity after regeneration as compared to primary alkanolamines. The pyridine, AEE, and AEPDNH2 solutions also loose a small part of their CO2 absorption capacity after regeneration, suggesting that those are the amines which are easy to regenerate while 5 wt % MEA solution seems to be the less easy to regenerate. Indeed, this alkanolamine solution losses 33.3% of its absorption capacity after regeneration while ammonia, pyrrolidine, and triethylamine solutions loose a proportion superior to 50% of their regeneration capacity after desorption. In general, the CO2 recuperation capacity of amines is dependent on their absorption and regeneration capacities. In this context, on the basis of our results, the CO2 recuperation capacities expressed in mol of CO2/ mol of amine which have been obtained for the aqueous 5 wt % amine solutions analyzed in this study after a CO2 absorption effectuated at a CO2 flow of 3.067 L/min, at ambient temperature, and for a period of 2 min followed by desorption at maximal boiling temperature for 210 s permit classification of them as follows: AEPDNH2 (1.854) > TEA (1.116) > AEE (1.091) > MEA (0.542) > pyrrolidine (0.471) > triethylamine (0.291) > ammonia (0.279) > pyridine (0.190). These data indicate that the efficacy of CO2 recuperation of aqueous 5 wt % AEPDNH2 solution is markedly superior to those of other amine solutions. This might be due to its great CO2 loading and good regeneration capacity. Additionally, TEA and AEE solutions also have the high CO2 recuperation capacities. However, use of aqueous 5 wt % TEA solution as CO2 absorbent in industrial technologies of CO2 recuperation which implicate a cyclic
The results of the present study revealed that, at 5 wt % in aqueous solution, AEPDNH2, (polyamine), AEE (diamine), and TEA (teritary alkanolamine) are amines studied that possess the greater CO2 loading and recuperation capacity and are among the compounds which show better regeneration capacity. In fact, the presence of three amino groups in AEPDNH2 and their spatial arrangement confer a loading superior to 2 mol of CO2/mol of amine, and therefore it represents a compound which has better structural properties as a CO2 absorbent among the amines studied. In addition, 5 wt % TEA solution seems also to be an amine that is the easiest to regenerate without significant loss of its CO2 loading among all amine solutions studied. Additionally, the structure and pKa of triethylamine, which prevents the formation of carbamate ions during the reaction with CO2, seem to be favorable to its loading, which is only slightly inferior to that of TEA solution in our experimental conditions. Moreover, the nonaromatic ring of pyrrolidine seems also to promote the CO2 absorption but less than the structures of polyamine, diamine, and tertiary amines. However, triethylamine and pyrrolidine appear to be difficult to regenerate and loose about 87 and 54% of their loading during their regeneration, and this contributes to a decrease in their CO2 recuperation capacity. In the case of the pyridine, it seems that its cyclic aromatic structure also defavors markedly its CO2 loading but had little effect on its regeneration, and thereby this compound represents the CO2 absorbent which has the least efficacy among all the amines studied. Thus, it appears that, in our experimental conditions, the structural properties of AEPDNH2, AEE, and TEA confer the greatest CO2 loading compared to those of MEA, pyrrolidine, pyridine, and ammonia and these amines are also among the compounds that are the easiest to regenerate. Therefore, the design of compounds with structural features related to these three types of amines could be advantageous for development of new technological approaches as industry proceeds to the prevention of CO2 emanation in the atmosphere. Acknowledgment 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).
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Resubmitted for review January 29, 2003 Revised manuscript received April 30, 2003 Accepted May 1, 2003 IE020738K
