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Solubility of CO2 in aqueous mixtures of diethanolamine with methyldiethanolamine and 2-amino-2-methyl-1-propanol
Fluid Phase Equilibria 150–151 Ž1998. 721–729
Solubility of CO 2 in aqueous mixtures of diethanolamine with methyldiethanolamine and 2-amino-2-methyl-1-propanol Florentino Murrieta-Guevara, Ma. Esther Rebolledo-Libreros, Ascencion ´ Romero-Martınez, ´ Arturo Trejo ) Instituto Mexicano del Petroleo, ´ Subdireccion ´ de Transformacion ´ Industrial, Gerencia de InÕestigacion ´ Aplicada de Procesos, Eje Central Lazaro Cardenas 152, 07730 Mexico City D.F., Mexico ´ ´
Abstract Using the static method with recirculation of the vapor phase, experimental data for the solubility of CO 2 in aqueous mixtures of known composition of diethanolamine ŽDEA. with methyldiethanolamine ŽMDEA. and DEA with 2-amino-2-methyl-1-propanol ŽAMP. have been obtained in the CO 2 partial pressure range 3–3000 kPa. The data for DEA–MDEA solutions were obtained at 313.15 K and are reported at four different compositions: 10 wt.% DEA–15 wt.% MDEA, 10 wt.% DEA–20 wt.% MDEA, 20 wt.% DEA–10 wt.% MDEA and 10 wt.% DEA–35 wt.% MDEA, data for the solution of 10 wt.% DEA–20 wt.% MDEA were also obtained at 393.15 K. The data for DEA–AMP solutions were obtained at 313.15 and 373.15 K and are reported at two different compositions: 25 wt.% DEA–5 wt.% AMP and 20 wt.% DEA–10 wt.% AMP. The results are given as the partial pressure Ž p . of CO 2 against its mole ratio a Žmol CO 2rmol alkanolamine., in the range of temperature studied. The solubility of CO 2 in all the studied systems decreases with an increase in temperature and increases with an increase in the partial pressure of CO 2 , at a given temperature, and it is a strong function of the composition of the blend of alkanolamines in solution. The aqueous mixture with 10 wt.% AMP, at 313.15 K, shows higher capacity to absorb CO 2 than any of the other mixtures studied here. From the experimental solubility results, exothermic values of the enthalpy of solution, D Hs , were derived. q 1998 Elsevier Science B.V. All rights reserved. Keywords: 2-Amino-2-methyl-1-propanol; Aqueous mixtures; Carbon dioxide; Diethanolamine; Experimental data; Gas solubility; Methyldiethanolamine
1. Introduction The removal of carbon dioxide ŽCO 2 . is of great importance in refining, gas-processing, and petrochemical industries. The current industrial process for acid–gas Ž e.g., H 2 S and CO 2 . removal is )
Corresponding author. Tel.: q525-587-3967; Fax: q525-587-3967; E-mail: [email protected]
0378-3812r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 1 2 Ž 9 8 . 0 0 3 5 2 - 5
722
F. Murrieta-GueÕara et al.r Fluid Phase Equilibria 150–151 (1998) 721–729
to absorb the acid gases in an aqueous solution of an alkanolamine, traditionally diethanolamine ŽDEA.. Mixtures of a primary Ž monoethanolamine, MEA. or a secondary alkanolamine Ž DEA. with a tertiary Ž e.g., methyldiethanolamine, MDEA. alkanolamine have been used to improve the energy efficiency of the process w1,2x, in fact, aqueous mixtures of alkanolamines formulated in base of MDEA or sterically hindered alkanolamines have been extensively used to treat gas streams contaminated with acid gases w3x. The selection of the best aqueous mixture of alkanolamines is based on the very well known reaction mechanism with the acid gases, for CO 2 this depends on the type of alkanolamine. Primary and secondary alkanolamines can react quickly with CO 2 through the carbamate reaction and, therefore, exhibit high rates of acid–gas removal. Tertiary alkanolamines cannot form the carbamate and must undergo the much slower acid–base reaction. Thus, they are able to carry out a high total CO 2 removal, but at much lower rates. Sterically hindered alkanolamines, like 2-amino-2-methyl-1propanol Ž AMP., are characterized by the presence of a bulky substituent group near the nitrogen atom. With a hindered alkanolamine, the carbamate can form, but it is unstable. The formation of a stable carbamate causes a stoichiometric absorption or loading limitation of 0.5 mol of CO 2 per mole of alkanolamine, thus, the goal when using an aqueous mixture of alkanolamines has been to maximize the desirable qualities of the individual alkanolamines, that is, to retain much of the high absorption rates of primary or secondary alkanolamines, to offer low regeneration costs, and to decrease both corrosion and circulation rates. Therefore, the experimental investigation of the equilibrium solubility of selected acid gasraqueous mixtures of alkanolamines systems is of fundamental importance for reaching this goal. Reported experimental data on the solubility of CO 2 in mixtures of alkanolamines of industrial importance are not plentiful. Some solubility data in aqueous mixtures of MEA with MDEA, in a very low pressure range, w4,5x, at atmospheric pressure w6x, and also in a large range of pressure, up to 20 MPa, w7x have been reported, for a limited range of compositions of the alkanolamines and at different temperatures. Experimental data are available for CO 2 in aqueous blends of DEA with MDEA at very low pressures w4x, in a low pressure range w5x, and in a medium pressure range w8x, at different compositions of the alkanolamines and at different temperatures. The experimental information on the solubility of CO 2 in aqueous mixtures of MEA or DEA with AMP is even more scarce. There are reported data at three different temperatures and in a low pressure range w9x, and also for the solubility of mixtures of CO 2 q H 2 S in aqueous solutions of DEA with MDEA at different compositions, at low pressure w10x. Hence, in order to increase the availability of experimental data on the solubility of CO 2 in aqueous blends of a secondary with a tertiary and with a hindered alkanolamine we report in this work experimental data for the solubility of CO 2 in aqueous solutions of mixtures of different composition of DEA with MDEA and DEA with AMP in the pressure range 3–3000 kPa. The data for the solutions of DEA–MDEA blends were obtained at 313.15 and 393.15 K, for the former temperature we studied four different compositions: 10 wt.% DEA–15 wt.% MDEA, 10 wt.% DEA–20 wt.% MDEA, 20 wt.% DEA–10 wt.% MDEA and 10 wt.% DEA–35 wt.% MDEA, whereas for the latter temperature we report data for the system with 10 wt.% DEA–20 wt.% MDEA. The data for the solutions with DEA–AMP were obtained at 313.15 and 373.15 K and are reported at two different compositions: 25 wt.% DEA–5 wt.% AMP and 20 wt.% DEA–10 wt.% AMP. The results are given as the partial pressure Ž p . of CO 2 against its mole ratio a Žmol CO 2rmol alkanolamine.. From these results, values of the enthalpy of solution, D Hs , were derived.
F. Murrieta-GueÕara et al.r Fluid Phase Equilibria 150–151 (1998) 721–729
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2. Experimental 2.1. Materials The MDEA and AMP were obtained from Aldrich with a quoted purity of 99 mol% and 95 mol%, respectively. The sample of DEA was obtained from Merck with a reported purity of 98 mol%. Each of the three alkanolamines was further purified by repeated fractionation in a stream of dried nitrogen under controlled reduced atmosphere in an all-glass still and then stored over molecular sieve in order to eliminate any traces of water during handling of the samples. The purified samples were analyzed by gas–liquid chromatography which showed no impurities using a lower limit of detection of 0.05 mol%; the water used to prepare the mixtures was distilled twice. The sample of CO 2 was the same as that used in previous work w11,12x; its minimum purity was 99.7 mol%. 2.2. Apparatus and procedure The experimental apparatus used in the present study is the same as that described in detail in previous work w13–15x. Comprehensive information on the measurement procedure is also given in these references. The temperature at equilibrium was controlled within "0.02 K in the range studied, and the equilibrium total pressure was measured with an accuracy of "1.0 kPa for pressures up to 20 kPa and of "3.5 kPa above 20 kPa w11x. 3. Results and discussion In order to establish the accuracy of the experimental data obtained in this work, the solubility of CO 2 in an aqueous solution of 30 wt.% DEA at 313.15 and 373.15 K was measured. An estimate of the error was obtained by comparing our results with interpolated literature values w16x. Fig. 1
Fig. 1. Comparison of solubility data for CO 2 in aqueous solution of DEA 30 wt.%. ): This work; v and `: literature w16,17x. The full lines are at 313.15 K and the dashed lines are at 373.15 K.
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F. Murrieta-GueÕara et al.r Fluid Phase Equilibria 150–151 (1998) 721–729
presents the comparison, where it may be observed that there exists good agreement, particularly at 313.15 K in the whole pressure range; the absolute average deviation being 8% of the CO 2 loading. Fig. 1 also includes a directly measured literature set of data in the range 0–2500 kPa, at 373.15 K w17x, and the agreement with our data is also better than 10%. Experimental equilibrium solubilities were obtained for CO 2 in aqueous mixtures of 25 wt.% DEA–5 wt.% AMP and 20 wt.% DEA–10 wt.% AMP, both at 313.15 and 373.15 K. Table 1 lists the results as CO 2 partial pressure, p, vs. CO 2 loading, a . Within the temperature and pressure ranges studied here it is observed that the solubility of CO 2 , for a given composition of the alkanolamines, changes regularly, that is, it increases with an increase in pressure, at a given temperature, and decreases as the temperature increases. The effect on the solubility of CO 2 of changing by 5 wt.% the composition of the AMP in the alkanolamine mixture of constant overall composition is very important, since at 313.15 K and at a given partial pressure, for example, 500 kPa, the interpolated value of a is 0.862 for the mixture with 5 wt.% AMP and 0.912 for the mixture with 10 wt.% AMP. The same conclusion is obtained if these results are compared with that corresponding to an aqueous solution of 30 wt.% DEA, at the same pressure and temperature as above. This result shows that the absorption capacity of the aqueous solution of these blended amines increases as the concentration of the AMP increases, as expected when using a hindered alkanolamine. The reported literature data on the solubility of CO 2 in the same aqueous amine blends as above w9x are not directly comparable with the results of this work, since although the total alkanolamine composition is the same, i.e., 30 wt.%, the composition of the individual alkanolamines is not the Table 1 Solubility of CO 2 in aqueous mixtures of DEA–AMP System
Temperature ŽK.
pCO 2 ŽkPa.
a Žmol CO 2 r mol alkanolamine.
DEA 25%–AMP 5%
313.15
162 905 1565 2136 2611 2908 237 703 1443 1980 2538
0.806 1.014 1.095 1.146 1.178 1.200 0.393 0.522 0.572 0.628 0.674
22 46 277 601 872 150 735 1415 2191 2597
0.389 0.689 0.871 0.959 1.00 0.331 0.570 0.675 0.732 0.788
373.15
DEA 20%–AMP 10%
313.15
373.15
F. Murrieta-GueÕara et al.r Fluid Phase Equilibria 150–151 (1998) 721–729
Table 2 Solubility of CO 2 in aqueous mixtures of DEA with MDEA System
Temperature ŽK.
pCO 2 ŽkPa.
a Žmol CO 2 r mol alkanolamine.
DEA 10%–MDEA 15%
313.15
3.5 8.0 29.7 70.0 470.8 917.2 1516.5 2178.4 2612.7
0.234 0.394 0.585 0.717 0.941 1.003 1.058 1.099 1.119
DEA 10%–MDEA 20%
313.15
2.8 13.9 40.7 98.7 209.5 498.9 794.2 1298.0 1621.1 2031.1 2510.5 2833.6
0.248 0.438 0.589 0.718 0.811 0.902 0.958 1.002 1.027 1.048 1.071 1.086
DEA 20%–MDEA 10%
313.15
4.5 12.5 43.1 99.0 418.7 1133.3 1814.3 2377.1
0.185 0.356 0.584 0.712 0.858 0.979 1.036 1.056
DEA 10%–MDEA 35%
313.15
3.8 8.3 15.3 27.4 46.5 74.0 324.5 762.2 1676.0 2245.8 2638.3
0.120 0.202 0.291 0.395 0.510 0.596 0.814 0.901 0.976 1.000 1.010
DEA 10%–MDEA 20%
393.15
46.6 112.8 247.3 432.1
0.038 0.085 0.158 0.223
725
726
F. Murrieta-GueÕara et al.r Fluid Phase Equilibria 150–151 (1998) 721–729
Table 2 Žcontinued. System
Temperature ŽK.
pCO 2 ŽkPa.
a Žmol CO 2 r mol alkanolamine.
DEA 10%–MDEA 20%
393.15
765.0 1396.9 2122.8 2655.4
0.304 0.407 0.475 0.510
same. Nonetheless, we plotted three sets of data at different compositions of the alkanolamines blends from the literature w9x together with our two sets of data, at 313.15 K and in the range 0–350 kPa, and established that the results from both works follow the same general features, e.g., as the concentration of AMP in the solution increases so does the CO 2 solubility. The experimental results for the solubility of CO 2 in aqueous mixtures of DEA–MDEA at different compositions, at 313.15 and 393.15 K, are given in Table 2. Fig. 2 shows the results for the four different compositions studied at 313.15 K. It is observed that, in the composition range studied in this work, 25–45 wt.% of total amine concentration, the solubility of CO 2 is a strong function of the overall composition of the alkanolamines in the aqueous solutions. Also, for a given total amine concentration, e.g., 30 wt.%, the equilibrium solubility of CO 2 increases as the composition of MDEA increases. Within the pressure and temperature ranges investigated the solubility shows the typical behavior of increasing with pressure, at a given temperature, and decreasing as the temperature increases. We have carried out a comparison of some of our results with literature data w5x in the range of pressure 0–500 kPa at 313.15 K. The comparison shows that there exists consistency among the different sets of data since the solubility data change regularly as the total amine concentration changes in the range 25–43.8 wt.% and also the data change regularly as the composition of MDEA
Fig. 2. Solubility of CO 2 in aqueous mixtures of DEA with MDEA at 313.15 K. l: DEA 20 wt.%–MDEA 10 wt.%; I: DEA 10 wt.%–MDEA 15 wt.%; `: DEA 10 wt.%–MDEA 20 wt.%; =: DEA 10 wt.%–MDEA 20 wt.%
F. Murrieta-GueÕara et al.r Fluid Phase Equilibria 150–151 (1998) 721–729
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changes in the range 15–35 wt.% for a constant composition of DEA. A second comparison was performed with literature data w8x, in the range 0–3000 kPa at 393.15 K, and it is observed that all the solubility results show regular behavior in the composition range considered. At this point it is convenient to carry out a comparison between the solubility of CO 2 in the aqueous mixtures of DEA–MDEA, DEA–AMP and the aqueous solution of DEA for a total composition of alkanolamines of 30 wt.%, at 313.15 K. The effect of the sterically hindered alkanolamine AMP on the solubility of CO 2 is in agreement with the reaction mechanism of this gas with secondary and tertiary alkanolamines; it is known that the reactivity for CO 2 is in the order primary) secondary) tertiary alkanolamines. The reactivity of CO 2 with sterically hindered alkanolamines is comparable to that with primary or secondary alkanolamines without formation of a stable carbamate. This fact allows the achievement of the high thermodynamic absorption capacity of 1 mol of CO 2rmol of alkanolamine. The mixture with 5 wt.% AMP has a slightly lower absorption capacity than the mixtures with MDEA; however, the mixture with 10 wt.% AMP has a higher capacity than any of the systems studied in this work. Overall, it may be observed that the aqueous solutions of mixtures of two alkanolamines present higher absorption capacity than the aqueous solution of DEA. Both results are consistent with the well-known reaction mechanism between CO 2 and hindered alkanolamines, and essentially they could be explained as due to the decrease in the formation of a stable carbamate. Knowledge of the enthalpy of solution, D Hs , of acid gases in aqueous mixtures of alkanolamines is of particular interest when considering heat input for the design of energy efficient heat transfer equipment in a gas treating process. Thus, we have obtained approximate values of D Hs from the experimental solubility data reported here using the procedure discussed in detail in previous work w14x. Our results of D Hs for CO 2 in 30 wt.% DEA, 10 wt.% DEA–20 wt.% MDEA, 20 wt.% DEA–10 wt.% AMP, and 25 wt.% DEA–5 wt.% AMP are shown in Table 3, at an average Table 3 Heat of solution D Hs of CO 2 in aqueous mixtures of DEA with MDEA and AMP System
Temperature ŽK.
a Žmol CO 2 r mol alkanolamine.
y D Hs ŽkJrmol.
DEA 10%–MDEA 20%
353.15
0.1 0.2 0.3 0.4 0.5
66.9 67.9 65.6 62.5 61.2
DEA 20%–AMP 10%
343.15
0.5 0.6 0.7
52.5 58.6 57.1
DEA 25%–AMP 5%
343.15
0.5 0.6 0.7
76.4 71.0 64.9
DEA 30%
343.15
0.4 0.5 0.6 0.7
53.4 53.0 49.8 46.0
F. Murrieta-GueÕara et al.r Fluid Phase Equilibria 150–151 (1998) 721–729
728
temperature. The exothermic D Hs values are consistent with the negative temperature coefficients of solubility for CO 2 . It can be seen that within "10% D Hs is a linear function of a for all the systems considered and it changes slightly with the individual concentration of the blended amines in the solution. Our data for the aqueous solution of 30 wt.% DEA were compared in a limited range of values of a with interpolated data from the literature w16x and the agreement found is good.
4. Conclusions Although aqueous mixtures of alkanolamines formulated with tertiary or sterically hindered alkanolamines are extensively used in the purification of gas streams contaminated with acid gases, solubility data of acid gases for these systems are scarce in the open literature. Hence, in this work, data for the solubility of CO 2 in aqueous mixtures of a secondary Ž DEA. with a tertiary Ž MDEA. and with a sterically hindered alkanolamine Ž AMP. have been obtained over ranges of temperature and pressure of industrial interest. Our results show that the amine blends with AMP are superior to those with MDEA.
5. List of symbols
a p D Hs
mole ratio Žmol CO 2rmol alkanolamine. pressure Ž kPa. enthalpy of solution Ž kJrmol.
Acknowledgements We gratefully acknowledge the financial help from Fondo de Apoyo al Desarrollo de Proyectos de Investigacion y Tecnologica con Instituciones de Educacion ´ Basica ´ ´ ´ Superior ŽFIES. del Instituto Mexicano del Petroleo, under Project FIES-95F-141-II. ´
References w1x T. Chakravarty, U.K. Phukan, R.H. Weiland, Chem. Eng. Prog. 81 Ž1985. 32. w2x L. Katti, R.A. Wolcott, Fundamental aspects of gas treating with formulated amine mixtures, 1987, Paper No. 5b, presented at the AIChE National Meeting, Minneapolis, MN. w3x M.D. Anderson, M. J Hegarty, J.E. Johnson, Flexible selective solvent design, Proc. Annu. Conv. Gas Proc. Assoc. 71 Ž1992. 292–309. w4x D.A. Glasscock, J.E. Critchfield, G.T. Rochelle, Chem. Eng. Sci. 46 Ž1991. 2829. w5x D.M. Austgen, G.T. Rochelle, C.-C. Chen, Ind. Eng. Chem. Res. 30 Ž1991. 543. w6x D.P. Hagewiesche, S.S. Ashour, H.A. Al-Ghawas, O.C. Sandall, Chem. Eng. Sci. 50 Ž1995. 1071. w7x F. Jou, F.D. Otto, A.E. Mather, Ind. Eng. Chem. Res. 33 Ž1994. 2002. w8x O.F. Dawodu, A. Meisen, J. Chem. Eng. Data 39 Ž1994. 548. w9x D.J. Seo, W.-H. Hong, J. Chem. Eng. Data 41 Ž1996. 258.
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w10x w11x w12x w13x w14x w15x w16x w17x
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I.-S. Jane, M.-H. Li, J. Chem. Eng. Data 42 Ž1997. 98. F. Murrieta-Guevara, E. Rebolledo-Libreros, A. Trejo, Fluid Phase Equilibria 86 Ž1993. 225. F. Murrieta-Guevara, E. Rebolledo-Libreros, A. Trejo, Fluid Phase Equilibria 95 Ž1994. 163. E. Rebolledo-Libreros, Study on the solubility of carbon dioxide and hydrogen sulfide in mixtures of sulfolane with alkanolamines, MSc Thesis, National Polytechnic Institute of Mexico, 1990, Mexico City Žin Spanish.. ´ F. Murrieta-Guevara, E. Rebolledo-Libreros, A. Trejo, J. Chem. Eng. Data 37 Ž1992. 4. F. Murrieta-Guevara, E. Rebolledo-Libreros, A. Trejo, Fluid Phase Equilibria 73 Ž1992. 167. J.I. Lee, F.D. Otto, A.E. Mather, J. Chem. Eng. Data 17 Ž1972. 465. M.L. Kennard, A. Meisen, J. Chem. Eng. Data 29 Ž1984. 309.
Solubility of CO 2 in aqueous mixtures of diethanolamine with methyldiethanolamine and 2-amino-2-methyl-1-propanol Florentino Murrieta-Guevara, Ma. Esther Rebolledo-Libreros, Ascencion ´ Romero-Martınez, ´ Arturo Trejo ) Instituto Mexicano del Petroleo, ´ Subdireccion ´ de Transformacion ´ Industrial, Gerencia de InÕestigacion ´ Aplicada de Procesos, Eje Central Lazaro Cardenas 152, 07730 Mexico City D.F., Mexico ´ ´
Abstract Using the static method with recirculation of the vapor phase, experimental data for the solubility of CO 2 in aqueous mixtures of known composition of diethanolamine ŽDEA. with methyldiethanolamine ŽMDEA. and DEA with 2-amino-2-methyl-1-propanol ŽAMP. have been obtained in the CO 2 partial pressure range 3–3000 kPa. The data for DEA–MDEA solutions were obtained at 313.15 K and are reported at four different compositions: 10 wt.% DEA–15 wt.% MDEA, 10 wt.% DEA–20 wt.% MDEA, 20 wt.% DEA–10 wt.% MDEA and 10 wt.% DEA–35 wt.% MDEA, data for the solution of 10 wt.% DEA–20 wt.% MDEA were also obtained at 393.15 K. The data for DEA–AMP solutions were obtained at 313.15 and 373.15 K and are reported at two different compositions: 25 wt.% DEA–5 wt.% AMP and 20 wt.% DEA–10 wt.% AMP. The results are given as the partial pressure Ž p . of CO 2 against its mole ratio a Žmol CO 2rmol alkanolamine., in the range of temperature studied. The solubility of CO 2 in all the studied systems decreases with an increase in temperature and increases with an increase in the partial pressure of CO 2 , at a given temperature, and it is a strong function of the composition of the blend of alkanolamines in solution. The aqueous mixture with 10 wt.% AMP, at 313.15 K, shows higher capacity to absorb CO 2 than any of the other mixtures studied here. From the experimental solubility results, exothermic values of the enthalpy of solution, D Hs , were derived. q 1998 Elsevier Science B.V. All rights reserved. Keywords: 2-Amino-2-methyl-1-propanol; Aqueous mixtures; Carbon dioxide; Diethanolamine; Experimental data; Gas solubility; Methyldiethanolamine
1. Introduction The removal of carbon dioxide ŽCO 2 . is of great importance in refining, gas-processing, and petrochemical industries. The current industrial process for acid–gas Ž e.g., H 2 S and CO 2 . removal is )
Corresponding author. Tel.: q525-587-3967; Fax: q525-587-3967; E-mail: [email protected]
0378-3812r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 1 2 Ž 9 8 . 0 0 3 5 2 - 5
722
F. Murrieta-GueÕara et al.r Fluid Phase Equilibria 150–151 (1998) 721–729
to absorb the acid gases in an aqueous solution of an alkanolamine, traditionally diethanolamine ŽDEA.. Mixtures of a primary Ž monoethanolamine, MEA. or a secondary alkanolamine Ž DEA. with a tertiary Ž e.g., methyldiethanolamine, MDEA. alkanolamine have been used to improve the energy efficiency of the process w1,2x, in fact, aqueous mixtures of alkanolamines formulated in base of MDEA or sterically hindered alkanolamines have been extensively used to treat gas streams contaminated with acid gases w3x. The selection of the best aqueous mixture of alkanolamines is based on the very well known reaction mechanism with the acid gases, for CO 2 this depends on the type of alkanolamine. Primary and secondary alkanolamines can react quickly with CO 2 through the carbamate reaction and, therefore, exhibit high rates of acid–gas removal. Tertiary alkanolamines cannot form the carbamate and must undergo the much slower acid–base reaction. Thus, they are able to carry out a high total CO 2 removal, but at much lower rates. Sterically hindered alkanolamines, like 2-amino-2-methyl-1propanol Ž AMP., are characterized by the presence of a bulky substituent group near the nitrogen atom. With a hindered alkanolamine, the carbamate can form, but it is unstable. The formation of a stable carbamate causes a stoichiometric absorption or loading limitation of 0.5 mol of CO 2 per mole of alkanolamine, thus, the goal when using an aqueous mixture of alkanolamines has been to maximize the desirable qualities of the individual alkanolamines, that is, to retain much of the high absorption rates of primary or secondary alkanolamines, to offer low regeneration costs, and to decrease both corrosion and circulation rates. Therefore, the experimental investigation of the equilibrium solubility of selected acid gasraqueous mixtures of alkanolamines systems is of fundamental importance for reaching this goal. Reported experimental data on the solubility of CO 2 in mixtures of alkanolamines of industrial importance are not plentiful. Some solubility data in aqueous mixtures of MEA with MDEA, in a very low pressure range, w4,5x, at atmospheric pressure w6x, and also in a large range of pressure, up to 20 MPa, w7x have been reported, for a limited range of compositions of the alkanolamines and at different temperatures. Experimental data are available for CO 2 in aqueous blends of DEA with MDEA at very low pressures w4x, in a low pressure range w5x, and in a medium pressure range w8x, at different compositions of the alkanolamines and at different temperatures. The experimental information on the solubility of CO 2 in aqueous mixtures of MEA or DEA with AMP is even more scarce. There are reported data at three different temperatures and in a low pressure range w9x, and also for the solubility of mixtures of CO 2 q H 2 S in aqueous solutions of DEA with MDEA at different compositions, at low pressure w10x. Hence, in order to increase the availability of experimental data on the solubility of CO 2 in aqueous blends of a secondary with a tertiary and with a hindered alkanolamine we report in this work experimental data for the solubility of CO 2 in aqueous solutions of mixtures of different composition of DEA with MDEA and DEA with AMP in the pressure range 3–3000 kPa. The data for the solutions of DEA–MDEA blends were obtained at 313.15 and 393.15 K, for the former temperature we studied four different compositions: 10 wt.% DEA–15 wt.% MDEA, 10 wt.% DEA–20 wt.% MDEA, 20 wt.% DEA–10 wt.% MDEA and 10 wt.% DEA–35 wt.% MDEA, whereas for the latter temperature we report data for the system with 10 wt.% DEA–20 wt.% MDEA. The data for the solutions with DEA–AMP were obtained at 313.15 and 373.15 K and are reported at two different compositions: 25 wt.% DEA–5 wt.% AMP and 20 wt.% DEA–10 wt.% AMP. The results are given as the partial pressure Ž p . of CO 2 against its mole ratio a Žmol CO 2rmol alkanolamine.. From these results, values of the enthalpy of solution, D Hs , were derived.
F. Murrieta-GueÕara et al.r Fluid Phase Equilibria 150–151 (1998) 721–729
723
2. Experimental 2.1. Materials The MDEA and AMP were obtained from Aldrich with a quoted purity of 99 mol% and 95 mol%, respectively. The sample of DEA was obtained from Merck with a reported purity of 98 mol%. Each of the three alkanolamines was further purified by repeated fractionation in a stream of dried nitrogen under controlled reduced atmosphere in an all-glass still and then stored over molecular sieve in order to eliminate any traces of water during handling of the samples. The purified samples were analyzed by gas–liquid chromatography which showed no impurities using a lower limit of detection of 0.05 mol%; the water used to prepare the mixtures was distilled twice. The sample of CO 2 was the same as that used in previous work w11,12x; its minimum purity was 99.7 mol%. 2.2. Apparatus and procedure The experimental apparatus used in the present study is the same as that described in detail in previous work w13–15x. Comprehensive information on the measurement procedure is also given in these references. The temperature at equilibrium was controlled within "0.02 K in the range studied, and the equilibrium total pressure was measured with an accuracy of "1.0 kPa for pressures up to 20 kPa and of "3.5 kPa above 20 kPa w11x. 3. Results and discussion In order to establish the accuracy of the experimental data obtained in this work, the solubility of CO 2 in an aqueous solution of 30 wt.% DEA at 313.15 and 373.15 K was measured. An estimate of the error was obtained by comparing our results with interpolated literature values w16x. Fig. 1
Fig. 1. Comparison of solubility data for CO 2 in aqueous solution of DEA 30 wt.%. ): This work; v and `: literature w16,17x. The full lines are at 313.15 K and the dashed lines are at 373.15 K.
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presents the comparison, where it may be observed that there exists good agreement, particularly at 313.15 K in the whole pressure range; the absolute average deviation being 8% of the CO 2 loading. Fig. 1 also includes a directly measured literature set of data in the range 0–2500 kPa, at 373.15 K w17x, and the agreement with our data is also better than 10%. Experimental equilibrium solubilities were obtained for CO 2 in aqueous mixtures of 25 wt.% DEA–5 wt.% AMP and 20 wt.% DEA–10 wt.% AMP, both at 313.15 and 373.15 K. Table 1 lists the results as CO 2 partial pressure, p, vs. CO 2 loading, a . Within the temperature and pressure ranges studied here it is observed that the solubility of CO 2 , for a given composition of the alkanolamines, changes regularly, that is, it increases with an increase in pressure, at a given temperature, and decreases as the temperature increases. The effect on the solubility of CO 2 of changing by 5 wt.% the composition of the AMP in the alkanolamine mixture of constant overall composition is very important, since at 313.15 K and at a given partial pressure, for example, 500 kPa, the interpolated value of a is 0.862 for the mixture with 5 wt.% AMP and 0.912 for the mixture with 10 wt.% AMP. The same conclusion is obtained if these results are compared with that corresponding to an aqueous solution of 30 wt.% DEA, at the same pressure and temperature as above. This result shows that the absorption capacity of the aqueous solution of these blended amines increases as the concentration of the AMP increases, as expected when using a hindered alkanolamine. The reported literature data on the solubility of CO 2 in the same aqueous amine blends as above w9x are not directly comparable with the results of this work, since although the total alkanolamine composition is the same, i.e., 30 wt.%, the composition of the individual alkanolamines is not the Table 1 Solubility of CO 2 in aqueous mixtures of DEA–AMP System
Temperature ŽK.
pCO 2 ŽkPa.
a Žmol CO 2 r mol alkanolamine.
DEA 25%–AMP 5%
313.15
162 905 1565 2136 2611 2908 237 703 1443 1980 2538
0.806 1.014 1.095 1.146 1.178 1.200 0.393 0.522 0.572 0.628 0.674
22 46 277 601 872 150 735 1415 2191 2597
0.389 0.689 0.871 0.959 1.00 0.331 0.570 0.675 0.732 0.788
373.15
DEA 20%–AMP 10%
313.15
373.15
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Table 2 Solubility of CO 2 in aqueous mixtures of DEA with MDEA System
Temperature ŽK.
pCO 2 ŽkPa.
a Žmol CO 2 r mol alkanolamine.
DEA 10%–MDEA 15%
313.15
3.5 8.0 29.7 70.0 470.8 917.2 1516.5 2178.4 2612.7
0.234 0.394 0.585 0.717 0.941 1.003 1.058 1.099 1.119
DEA 10%–MDEA 20%
313.15
2.8 13.9 40.7 98.7 209.5 498.9 794.2 1298.0 1621.1 2031.1 2510.5 2833.6
0.248 0.438 0.589 0.718 0.811 0.902 0.958 1.002 1.027 1.048 1.071 1.086
DEA 20%–MDEA 10%
313.15
4.5 12.5 43.1 99.0 418.7 1133.3 1814.3 2377.1
0.185 0.356 0.584 0.712 0.858 0.979 1.036 1.056
DEA 10%–MDEA 35%
313.15
3.8 8.3 15.3 27.4 46.5 74.0 324.5 762.2 1676.0 2245.8 2638.3
0.120 0.202 0.291 0.395 0.510 0.596 0.814 0.901 0.976 1.000 1.010
DEA 10%–MDEA 20%
393.15
46.6 112.8 247.3 432.1
0.038 0.085 0.158 0.223
725
726
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Table 2 Žcontinued. System
Temperature ŽK.
pCO 2 ŽkPa.
a Žmol CO 2 r mol alkanolamine.
DEA 10%–MDEA 20%
393.15
765.0 1396.9 2122.8 2655.4
0.304 0.407 0.475 0.510
same. Nonetheless, we plotted three sets of data at different compositions of the alkanolamines blends from the literature w9x together with our two sets of data, at 313.15 K and in the range 0–350 kPa, and established that the results from both works follow the same general features, e.g., as the concentration of AMP in the solution increases so does the CO 2 solubility. The experimental results for the solubility of CO 2 in aqueous mixtures of DEA–MDEA at different compositions, at 313.15 and 393.15 K, are given in Table 2. Fig. 2 shows the results for the four different compositions studied at 313.15 K. It is observed that, in the composition range studied in this work, 25–45 wt.% of total amine concentration, the solubility of CO 2 is a strong function of the overall composition of the alkanolamines in the aqueous solutions. Also, for a given total amine concentration, e.g., 30 wt.%, the equilibrium solubility of CO 2 increases as the composition of MDEA increases. Within the pressure and temperature ranges investigated the solubility shows the typical behavior of increasing with pressure, at a given temperature, and decreasing as the temperature increases. We have carried out a comparison of some of our results with literature data w5x in the range of pressure 0–500 kPa at 313.15 K. The comparison shows that there exists consistency among the different sets of data since the solubility data change regularly as the total amine concentration changes in the range 25–43.8 wt.% and also the data change regularly as the composition of MDEA
Fig. 2. Solubility of CO 2 in aqueous mixtures of DEA with MDEA at 313.15 K. l: DEA 20 wt.%–MDEA 10 wt.%; I: DEA 10 wt.%–MDEA 15 wt.%; `: DEA 10 wt.%–MDEA 20 wt.%; =: DEA 10 wt.%–MDEA 20 wt.%
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changes in the range 15–35 wt.% for a constant composition of DEA. A second comparison was performed with literature data w8x, in the range 0–3000 kPa at 393.15 K, and it is observed that all the solubility results show regular behavior in the composition range considered. At this point it is convenient to carry out a comparison between the solubility of CO 2 in the aqueous mixtures of DEA–MDEA, DEA–AMP and the aqueous solution of DEA for a total composition of alkanolamines of 30 wt.%, at 313.15 K. The effect of the sterically hindered alkanolamine AMP on the solubility of CO 2 is in agreement with the reaction mechanism of this gas with secondary and tertiary alkanolamines; it is known that the reactivity for CO 2 is in the order primary) secondary) tertiary alkanolamines. The reactivity of CO 2 with sterically hindered alkanolamines is comparable to that with primary or secondary alkanolamines without formation of a stable carbamate. This fact allows the achievement of the high thermodynamic absorption capacity of 1 mol of CO 2rmol of alkanolamine. The mixture with 5 wt.% AMP has a slightly lower absorption capacity than the mixtures with MDEA; however, the mixture with 10 wt.% AMP has a higher capacity than any of the systems studied in this work. Overall, it may be observed that the aqueous solutions of mixtures of two alkanolamines present higher absorption capacity than the aqueous solution of DEA. Both results are consistent with the well-known reaction mechanism between CO 2 and hindered alkanolamines, and essentially they could be explained as due to the decrease in the formation of a stable carbamate. Knowledge of the enthalpy of solution, D Hs , of acid gases in aqueous mixtures of alkanolamines is of particular interest when considering heat input for the design of energy efficient heat transfer equipment in a gas treating process. Thus, we have obtained approximate values of D Hs from the experimental solubility data reported here using the procedure discussed in detail in previous work w14x. Our results of D Hs for CO 2 in 30 wt.% DEA, 10 wt.% DEA–20 wt.% MDEA, 20 wt.% DEA–10 wt.% AMP, and 25 wt.% DEA–5 wt.% AMP are shown in Table 3, at an average Table 3 Heat of solution D Hs of CO 2 in aqueous mixtures of DEA with MDEA and AMP System
Temperature ŽK.
a Žmol CO 2 r mol alkanolamine.
y D Hs ŽkJrmol.
DEA 10%–MDEA 20%
353.15
0.1 0.2 0.3 0.4 0.5
66.9 67.9 65.6 62.5 61.2
DEA 20%–AMP 10%
343.15
0.5 0.6 0.7
52.5 58.6 57.1
DEA 25%–AMP 5%
343.15
0.5 0.6 0.7
76.4 71.0 64.9
DEA 30%
343.15
0.4 0.5 0.6 0.7
53.4 53.0 49.8 46.0
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temperature. The exothermic D Hs values are consistent with the negative temperature coefficients of solubility for CO 2 . It can be seen that within "10% D Hs is a linear function of a for all the systems considered and it changes slightly with the individual concentration of the blended amines in the solution. Our data for the aqueous solution of 30 wt.% DEA were compared in a limited range of values of a with interpolated data from the literature w16x and the agreement found is good.
4. Conclusions Although aqueous mixtures of alkanolamines formulated with tertiary or sterically hindered alkanolamines are extensively used in the purification of gas streams contaminated with acid gases, solubility data of acid gases for these systems are scarce in the open literature. Hence, in this work, data for the solubility of CO 2 in aqueous mixtures of a secondary Ž DEA. with a tertiary Ž MDEA. and with a sterically hindered alkanolamine Ž AMP. have been obtained over ranges of temperature and pressure of industrial interest. Our results show that the amine blends with AMP are superior to those with MDEA.
5. List of symbols
a p D Hs
mole ratio Žmol CO 2rmol alkanolamine. pressure Ž kPa. enthalpy of solution Ž kJrmol.
Acknowledgements We gratefully acknowledge the financial help from Fondo de Apoyo al Desarrollo de Proyectos de Investigacion y Tecnologica con Instituciones de Educacion ´ Basica ´ ´ ´ Superior ŽFIES. del Instituto Mexicano del Petroleo, under Project FIES-95F-141-II. ´
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