Environ. Sci. Technol. 2010, 44, 2138–2143
Kinetics of Removal of Carbon Dioxide by Aqueous Solutions of N,N-Diethylethanolamine and Piperazine PRASHANTI B. KONDURU,† PRAKASH D. VAIDYA,† AND E U G E N Y Y . K E N I G * ,‡ Department of Chemical Engineering, Institute of Chemical Technology, Matunga, Mumbai - 400 019, India, and Faculty of Mechanical Engineering, University of Paderborn, D-33098, Paderborn, Germany
Received September 20, 2009. Revised manuscript received December 19, 2009. Accepted January 26, 2010.
N,N-Diethylethanolamine (DEEA) is a very promising absorbent for CO2 removal from gaseous streams, as it can be prepared from renewable resources. Aqueous mixtures of DEEA and piperazine (PZ) are attractive for the enhancement of CO2 capture, due to the high CO2 loading capacity of DEEA and high reactivity of PZ. In the present work, for the first time, the equilibrium and kinetic characteristics of the CO2 reaction with such mixtures were considered. Kinetic data were obtained experimentally, by using a stirred cell reactor. These data were interpreted using a homogeneous activation mechanism, by which the investigated reaction was considered as a reaction between CO2 and DEEA in parallel with the reaction of CO2 with PZ. It is found that, in the studied range of temperatures, 298-308 K, and overall amine concentrations, 2.1-2.5 kmol/ m3, this reaction system belongs to the fast pseudo-first-order reaction regime systems. The second-order rate constant for the CO2 reaction with PZ was determined from the absorption rate measurements in the activated DEEA solutions, and its value at 303 K was found to be 24,450 m3/(kmol s).
associated with the carbamate formation is high. Consequently, this results in high solvent regeneration costs. Further, the CO2 loading capacity of such alkanolamines is limited to 0.5 mol of CO2/mol of amine. Tertiary alkanolamines, which have a low reactivity with respect to CO2, contain no hydrogen atom attached to the nitrogen atom, as in case of MEA and DEA, and thus, the carbamation reaction cannot take place. Instead, tertiary amines promote the CO2 hydrolysis reaction forming bicarbonates. The reaction heat released in bicarbonate formation is lower than that of carbamate formation, thus resulting in lower solvent regeneration costs. Moreover, tertiary amines have a high CO2 loading capacity of 1 mol of CO2/mol of amine. N,N-Diethylethanolamine (DEEA) represents a candidate tertiary alkanolamine having good potential for the bulk removal of CO2 from gaseous streams. It comprises two ethyl groups replacing the hydrogen atoms of the amino group in MEA. Diethylamine (obtained from ethanol) and ethylene oxide (obtained from the oxidation of ethylene which, in turn, can be prepared by dehydration of ethanol) react to form DEEA. Ethanol, which is the major raw material for the manufacture of this amine, can be prepared from agricultural products and/or residues. Thus, DEEA is especially promising absorbent, as it can be prepared from renewable resources. In a recent paper, Vaidya and Kenig (4) studied the kinetics of the CO2 reaction with DEEA in aqueous solutions. They found that the second order reaction rate constant attained a value of 173 m3/(kmol s) at 303 K. Besides, they studied the acceleration of the CO2 reaction with DEEA by piperazine (PZ), a cyclic diamine, which is applied as an efficient activator within the activated MDEA technology used by BASF SE. Vaidya and Kenig (4) found that PZ also enhanced CO2 absorption rates in aqueous DEEA solutions. Due to the high CO2 loading capacity of DEEA and high reactivity of PZ, the blend comprising DEEA, PZ, and H2O is attractive for the enhancement of CO2 capture. For the first time, in the present work, the equilibrium and kinetic characteristics of the CO2-PZ-DEEA system in aqueous solutions are considered. Besides, the reaction rate constant for the reaction between CO2 and PZ is determined from the absorption rate measurements of CO2 in the activated DEEA solutions.
Introduction
Theory
CO2 capture from process gas streams represents a crucial step in several chemical processes, e.g., in methane reforming, coal gasification, chemical fertilizer plants, natural gas processing, flue gas treatment, and in ethylene oxide plants. By now, a number of different CO2 separation technologies are available, and these are comprehensively described by Rao and Rubin (1). As a rule, absorption performed with chemical solvents (reactive absorption) is the most feasible option, according to Meisen and Shuai (2). Aqueous solutions of alkanolamines are the highly popular absorbents used in the gas processing industry. Industrially important alkanolamines for CO2 removal, highlighted by Kohl and Nielsen (3), are the primary amine, monoethanolamine (MEA), the secondary amine, diethanolamine (DEA), and the tertiary amine, methyldiethanolamine (MDEA). Primary and secondary alkanolamines react rapidly with CO2 to form carbamates. However, the heat of absorption
There are a few comprehensive reviews on the reactions between CO2 and alkanolamines and the kinetic behavior of such reaction systems available (5–7). Donaldson and Nguyen (8) were the first to propose that tertiary alkanolamines cannot react directly with CO2. Such amines have a base catalytic effect on the hydration of CO2. Some subsequent works on the reaction of CO2 with triethanolamine (TEA) and MDEA are in agreement with this mechanism (9–13). When CO2 is absorbed into an aqueous DEEA solution, base catalysis could be represented as follows (here DEEA is denoted as R2R′N, where R ) -C2H5 and R′ ) -CH2CH2OH):
* Corresponding author phone: +49 5251 60 2408; fax: +49 5251 60 3522; e-mail:
[email protected]. † Institute of Chemical Technology, Mumbai. ‡ University of Paderborn.
R2R′N + CO2 a R2R′NCOO-
(2)
R2R′NCOO- + H2O f R2R′N+H + HCO3
(3)
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k′
R2R′N + H2O + CO2 98 R2R′N+H + HCO3
(1)
The base catalysis reaction could also be explained by a zwitterion mechanism earlier proposed by Yu et al. (12):
10.1021/es902805p
2010 American Chemical Society
Published on Web 02/12/2010
Equation 2 represents a reaction of DEEA with CO2 to form an unstable complex. Equation 3 describes the homogeneous hydrolysis reaction. Water reacts with the zwitterion-type complex, which results in a bicarbonate formation. The following reactions may also take place simultaneously in an aqueous DEEA solution: R2R′N + H2O a R2R′N+H + OH-
(4)
H2O a H+ + OH-
(5)
kOH-
CO2 + OH- {\} HCO3 kS
+ CO2 + H2O {\} HCO3 + H
(6)
(7)
Besides, contrary to the proposal of Donaldson and Nguyen (8), a direct reaction between CO2 and DEEA may occur at extremely high pH. Jorgensen and Faurholt (14) reported such a reaction between CO2 and TEA, another tertiary alkanolamine, thereby resulting in monoalkylcarbonate formation. However, at pH values lower than 12, the contribution of this reaction to the overall reaction can be neglected (15). The amine dissociation reaction (eq 4) is almost instantaneous. The reaction between CO2 and OH(eq 6) is also fast. The total rate of all CO2 reactions in an aqueous solution containing DEEA is represented by the sum of the reaction rates given by eqs 1, 6, and 7: rDEEA ) [kS + kOH-(OH-) + k′(R2R′N)](CO2)
(8)
) kobs(CO2)
(9)
where kobs denotes the observed reaction rate constant which can be measured and is expressed by: kobs ) [kS + kOH-(OH-) + k′(R2R′N)]
(10)
The solvent is in large excess and, therefore, the reaction order with respect to the solvent is assumed to be zero. Hence, eq 8 does not account for the water concentration. The apparent reaction rate constant (kapp), which is used for the analysis of experimental data, is given by kapp ) [k′(R2R′N)]
(11)
kapp can be obtained from kobs as follows: kapp ) kobs - [kS + kOH-(OH-)]
(12)
The reaction given by eq 7 is very slow compared to the other reactions. When the contribution of the reaction given by eq 6 to the overall rate is negligible (as suggested by Benitez-Garcia et al. (15) and Littel et al. (16)), kapp becomes equal to kobs. PZ, which is more effective than other absorption activators, such as MEA and DEA, reacts with CO2 rapidly to form carbamate (17). Besides, the dissociation reaction for PZ may occur (18, 19). The reactions that may take place in presence of PZ (here denoted by R′′(NH)2) can be explained by the homogeneous activation mechanism (20): R′′(NH)2 + 2CO2 a R′′(NHCOO)2
(13)
R′′(NHCOO)2 + 2H2O f R′′(NH2)+ 2 + 2HCO3
(14)
The reaction described by eq 13 occurs simultaneously with that described by eq 2, and CO2 could be transferred by R′′(NHCOO)2 to DEEA. This is reflected by the following reaction:
R′′(NHCOO)2 + 2R2R′N a R′′(NH)2 + 2R2R′NCOO(15) Thus, PZ contributes to absorption of CO2 and enhances the overall rate of absorption. The overall reaction can be regarded as a reaction between CO2 and PZ in parallel with the reaction of CO2 with DEEA. Experimental Section. Materials. N,N-Diethylethanolamine (purity 99%) and anhydrous piperazine (purity 98%), used in all experiments, were purchased from Sisco Research Laboratories Pvt. Ltd., Mumbai. Carbon dioxide, nitrous oxide, and nitrogen cylinders, with a given purity of 99.95%, were purchased from Inox Air Products Ltd., Mumbai. All solutions were prepared using deionized water (conductivity 0.054 µs), which was supplied by a deionized water unit (Millipore India Pvt. Ltd., Mumbai). Experimental Setup. A glass stirred cell reactor with a plane, horizontal gas-liquid interface was used for the absorption studies (see Vaidya and Mahajani (21)). The main advantage of the stirred cell is that the rates of absorption can be measured using a liquid with a single, known composition. This easy-to-use experimental device (inner diameter 97 mm, height 187 mm) was operated batchwise. The total volume of the reactor was 1.45 dm3 and the interfacial surface area was 7.5 × 10-3 m2. The reactor was equipped with a flange made of stainless steel (Sharad Autoclave Engineers, Mumbai). A pressure transducer (Trans Instruments, UK, 0-1 bar) mounted on this flange, coupled with a data acquisition system, enabled measurement of the total pressure inside the reactor. The uncertainty in measuring pressure was (1 mbar. The reactor was also equipped with inlet and outlet ports for gas and liquid. The entire assembly was proven to have no leak. The setup was supplied by a variable speed magnetic drive (Premex Instruments, Switzerland). The gas and liquid were stirred by two impellers, which were mounted on the same shaft. The speed of stirring could be adjusted to the desired value with an accuracy of (1 rpm. The impeller speed during kinetic measurements was limited to 120 rpm to ensure that the gas-liquid interface was undisturbed. The reactor was immersed in a water bath to ensure isothermal conditions. The temperature was adjusted to the desired value with an accuracy of (0.1 °C. The solute gas was passed through a coil, which was also kept in the water bath, before being charged inside the reactor. The stirred cell reactor can also be used to estimate the solubility of the solute in the liquid phase. We used this contactor to measure the solubility of N2O in the activated DEEA solutions, too. Experimental Procedure. A series of experiments on reaction kinetics was conducted at different temperatures (298-308 K). The initial DEEA concentration in solution was kept constant (2 kmol/m3), whereas the PZ concentration was varied (0.1-0.5 kmol/m3). In each experiment, the reactor was charged with 0.4 dm3 of the fresh alkanolamine solution. The gas inside the reactor was then purged with nitrogen to ensure an inert atmosphere. Thereafter, nitrogen was released through the gas outlet port. All the lines were closed and the reactor content attained the desired temperature. CO2 from the gas cylinder was then charged inside the reactor, with this being considered as the starting point for the reaction. The reactor content was stirred at the desired speed of agitation. The decrease in system pressure due to reaction was monitored by the pressure transducer and the “PCO2 vs. t” data were recorded during 30 s using the data acquisition system. These data were plotted for the time interval between t ) 5 s and t ) 25 s and fitted to a third degree polynomial using the least-squares regression. The absorption rates were calculated from the values of the slope - dPCO2/dt. This measurement method based on the fall-in-pressure techVOL. 44, NO. 6, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. CO2 Absorption Rates in Aqueous Mixtures of PZ and DEEA temperature (K)
(DEEA)0 + (PZ)0 (kmol/m3)
PCO2 (kPa)
RCO2 × 106 (kmol/(m2 s))
HCO2(DCO2 kOV) × 107(kmol1/2/(m1/2 s kPa))
2.0 + 0.1 2.0 + 0.2 2.0 + 0.3 2.0 + 0.4 2.0 + 0.5 2.0 + 0.1 2.0 + 0.2 2.0 + 0.3 2.0 + 0.4 2.0 + 0.5 2.0 + 0.1 2.0 + 0.2 2.0 + 0.3 2.0 + 0.4 2.0 + 0.5
5.8 5.6 6.7 5.6 6.1 5.0 5.0 4.3 5.0 6.0 5.0 4.8 5.0 5.7 5.3
1.51 1.82 4.52 6.46 6.84 1.55 2.64 5.09 6.36 7.95 1.73 3.13 5.55 7.36 8.66
1.84 2.29 4.77 8.15 7.92 2.19 3.73 8.37 8.99 9.37 2.44 4.61 7.84 9.13 11.55
298
303
308
nique enabled a simple and straightforward estimation of the absorption rates. Further, no analysis of the liquid phase was required and the pressure decrease was the only factor necessary for the evaluation of the kinetic parameters. The reproducibility of experiments was checked and the error in all experimental measurements was found to be less than 3%.
Results and Discussion Estimation of Physical Properties. The density and viscosity of the blend comprising DEEA, PZ, and H2O were measured at 303 K using a commercial densitometer and a capillary viscometer, respectively, and these values are represented in the Supporting Information. The diffusion coefficients of N2O and CO2 in water, viz. 2.03 × 10-9 and 2.15 × 10-9 m2/s at 303 K, were earlier reported in ref 22. From our viscosity measurements, we estimated the values of the N2O diffusivity in the activated DEEA solutions by using the modified Stokes-Einstein correlation: (DN2Oµ0.80)amine ) const ) (DN2Oµ0.80)water
[
] [
]
P(t) - Pfinal (m′VL) + VG )kLA′t Pinitial - Pfinal VLVG
(17)
To determine the liquid-side mass transfer coefficient kL, the physical absorption of CO2 in water at 303 K was studied and the fall-in-pressure due to absorption was recorded. Afterward, ln [(P(t) - Pfinal)/(Pinitial - Pfinal)] was plotted vs time to estimate the value of kL. The slope of this graph equals -[((m′ VL) + VG)/(VL VG)] kL A′ and its value was found to be 6 × 10-4. Taking the values of other relevant parameters into account (m′ ) 0.8 mol/mol, VG ) 1.05 dm3, VL ) 0.4 dm3, A′ ) 7.5 × 10-3 m2), the value of kL was estimated as 1.6 × 10-5 m/s. It was corrected for the aqueous amine solution (0.001 cm/s) using the surface renewal theory correlation, viz. kL ∝ D. The solubility of CO2 in water at 303 K was found to be 2.73 × 10-4 kmol/(m3 kPa), which agrees well with the published value (22). Chemical Kinetics. The overall reaction of CO2 with aqueous mixtures of DEEA and PZ can be regarded as a 2140
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10 < √M < (Ei - 1)
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(18)
where M is given by
√M )
√DCO kobs 2
(19)
kL
and Ei is the enhancement factor for an instantaneous reaction:
(16)
We measured the solubility of N2O in aqueous DEEA solutions, too (see Supporting Information). The values of DCO2 and HCO2 in solutions were found using the N2O analogy (22). Estimation of kL. For the case of physical absorption in a stirred cell reactor, a mass balance for the solute for both the gas and the liquid phase yields the following expression (23): ln
reaction between CO2 and DEEA in parallel with the reaction of CO2 with free PZ. This suggestion is based on the fact that free PZ transfers CO2 to DEEA thereby regenerating itself (see eq 15). If PZ could not transfer CO2 to DEEA, the free PZ concentration during reaction would quickly decrease and the absorption rate enhancement would not be observed. To study the reaction kinetics, it is essential that the system belongs to the fast reaction regime, without depletion of the amine at the gas-liquid interface (24, 25). The necessary conditions for the fast pseudo-first-order reaction regime are:
Ei ) 1 +
[
(amine)0 Damine z(CO2) DCO2
]
(20)
It is worthy of note that eq 20 is valid only if the film theory is used. In the fast pseudo-first-order reaction regime (E ) M), the rate of absorption is independent of the liquidside mass transfer coefficient kL and hence it should not depend on the agitation speed. We studied this effect experimentally and found practically no change in the absorption rate, while varying the stirring speed in the range 60-120 rpm at 308 K. Hence, it can be concluded that the CO2-DEEA-PZ system belongs to the fast pseudo-first-order reaction regime systems. All further experiments were conducted at a speed of 100 rpm. The amount of inert gas inside the reactor was negligible, and hence we assumed that the gas phase could be treated as almost pure CO2 and the gas-side resistance is negligible. Therefore, the resistance to mass transfer was entirely in the liquid phase. When CO2 concentration in the bulk liquid is negligible, it can be shown, based on the two-film theory of mass transfer (24), that the following relation holds: PCO2 RCO2a
)
1 kLaHCO2E
(21)
For the present case, eq 21 is rearranged to PCO2 RCO2
)
1 HCO2√{DCO2[k2(PZ)0 + k′(DEEA)0]}
and further transformed to
(22)
TABLE 2. Equilibrium and Kinetic Characteristics of CO2-DEEA-PZ System at 303 K (PZ)0 (kmol/ m3)
HCO2(DCO2 kOV) × 107 (kmol1/2/(m1/2 s kPa))
HCO2× 104 (kmol/(m3kPa))
DCO2× 1010 (m2/s)
kOV (m3/(kmol s))
M
Ei
0.1 0.2 0.3 0.4 0.5
2.19 3.73 8.37 8.99 9.37
2.49 2.24 1.31 1.20 1.18
10.8 10.8 9.9 9.9 9.9
718 2570 41200 56700 63700
106 202 807 947 1000
1687 1965 4083 4000 3531
FIGURE 1. Plot of RCO2 vs (PZ)0 at 303 K (initial DEEA concentration ) 2 kmol/m3).
FIGURE 2. Enhancement factors for CO2 absorption at 303 K (initial DEEA concentration ) 2 kmol/m3).
TABLE 3. Comparison of the Enhancement Factors at 303 K (DEEA)0 + (PZ)0 (kmol/m3) 2.0 2.0 2.0 2.0 2.0 2.0
+ + + + + +
0.0 0.1 0.2 0.3 0.4 0.5
PCO2 RCO2
)
PCO2 (kPa)
E
5.5 5.0 5.0 4.3 5.0 6.0
53 106 202 807 947 1000
1 HCO2√{DCO2kOV(DEEA)0}
(23)
with kOV (m3/(kmol s)) given by kOV ) k′ +
k2(PZ)0 (DEEA)0
(24)
The rate of absorption in the formulated amine solution can be expressed as RCO2 ) PCO2HCO2√{DCO2kOV(DEEA)0}
(25)
The CO2 absorption rates in aqueous solutions containing PZ and DEEA at different temperatures and values of the parameter HCO2(DCO2kOV) are presented in Table 1. Using the values of DCO2 and HCO2 at 303 K (see Table 2), the value of kOV was estimated. Thereafter, the conditions given by eq 18 were checked and found to be satisfied. The values of M and Ei, represented in Table 2, provide a further check of our preliminary assumption of a fast pseudo-first-order reaction system. A plot of RCO2 vs (PZ)0 at 303 K is shown in Figure 1. To quantify the acceleration of the CO2 reaction with DEEA by PZ, the enhancement factor due to chemical reaction E
FIGURE 3. Plot of kobs vs (PZ)0 at 308 K (initial DEEA concentration ) 2 kmol/m3). was chosen. We measured the CO2 absorption rate in aqueous DEEA solution. The enhancement factor (E ) M) for the CO2-DEEA system at 303 K and (DEEA)0 ) 2 kmol/m3 was determined and compared with those reported in Table 2. The comparison is shown in Table 3 and it is obvious that the values of E in presence of PZ are higher. Furthermore, the value of E increases with an increase in PZ concentration (Figure 2). The observed reaction rate constants for this system were measured. A plot of kobs vs (PZ)0 at 308 K is shown in Figure 3. The value of kobs () kOV (DEEA)0) rises with PZ concentration. A typical value at (PZ)0 ) 0.1 kmol/ m3 was found to be 2245 L/s. From eq 24 and from the knowledge on k′ at 303 K (viz., 173 m3/(kmol s), see ref 4), the value of the rate constant for VOL. 44, NO. 6, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 4. Kinetic Studies on the CO2-Aqueous PZ Reaction System temp. (K)
(PZ)0 (kmol/m3)
rate constant (m3/(kmol s))
reaction system
303 303 298 303 303 303 298 298
0.10-0.50 0.01-0.21 0.20-0.60 0.06-0.23 0.20-0.60 0.23-0.92 0.60-1.50 0.20-0.80
24450 185 53700 4096 60766 29185 70000 58000
PZ-DEEA-H2O PZ-MDEA-H2O PZ-H2O PZ-AMP-H2O PZ-MDEA-H2O PZ-H2O PZ-H2O PZ-H2O
the CO2-PZ system, k2, was evaluated and it was found to be 24,450 m3/(kmol s) at (PZ)0 ) 0.2 kmol/m3. Earlier kinetic studies of the CO2-PZ system are listed in Table 4 (19, 20, 26–30). The agreement between the reaction rate constants estimated in these works is poor. In our opinion, the deviation in the estimated kinetic parameters, even for identical conditions, is possibly due to the different experimental techniques used for studying reaction kinetics, uncertainties in values of the physical properties used, as well as due to the existence of interfacial turbulence in some types of absorbers, lack of knowledge of the exact gas-liquid interfacial area, and the assumption of a pseudo-first-order reaction. It is worthy of note that the k2 values for all the earlier studies are given for CO2 absorption into the PZ + H2O, PZ + MDEA + H2O, and PZ + AMP + H2O systems, whereas k2 for the present study is for CO2 absorption into the system DEEA + PZ + H2O. We therefore conclude that a detailed knowledge on reaction kinetics with PZ is essential. In a following investigation, we are planning to study CO2 absorption kinetics in aqueous PZ solutions.
Acknowledgments P.B.K. is grateful to the University Grants Commission, New Delhi, for the financial support. P.D.V. gratefully acknowledges his teacher, Dr. V. V. Mahajani, ICT-DAE Professor of Chemical Engineering (Emeritus), Institute of Chemical Technology, Mumbai.
Appendix A Nomenclature a ) gas-liquid interfacial area, m2/m3 A′ ) interfacial surface area of stirred cell, m2 (amine)0 ) initial amine concentration, kmol/m3 (CO2) ) concentration of CO2, kmol/m3 (DEEA)0 ) initial DEEA concentration, kmol/m3 DCO2 ) diffusivity of CO2 in liquid phase, m2/s Damine ) diffusivity of amine in liquid phase, m2/s E ) enhancement factor due to chemical reaction Ei ) enhancement factor for an instantaneous reaction HCO2 ) Henry’s law constant, kmol/(m3 kPa) ks ) forward reaction rate constant in eq 7 k′ ) forward reaction rate constant in eq 1 kapp ) apparent reaction rate constant defined by eq 11 kobs ) observed reaction rate constant defined by eq 10 kOH- ) forward reaction rate constant in eq 6 kL ) liquid-side mass transfer coefficient, m/s k2 ) second-order rate constant for the CO2 reaction with PZ, m3/(kmol s) kOV ) overall reaction rate constant defined by eq 24, m3/ (kmol s) M ) dimensionless number defined by eq 19 m′ ) dimensionless solubility, mol/mol (OH-) ) hydroxyl ion concentration, kmol/m3 PCO2 ) partial pressure of CO2 in bulk gas phase, kPa P(t) ) partial pressure of solute gas at time t, kPa Pinitial ) initial partial pressure of solute gas, kPa 2142
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experimental technique
reference
stirred cell disk column wetted-wall wetted-sphere disk column wetted-wall stirred cell wetted-wall
this work (20) (26) (27) (19) (28) (29) (30)
Pfinal ) partial pressure of solute gas at equilibrium, kPa (PZ)0 ) initial PZ concentration, kmol/m3 R′′(NH)2 ) piperazine R2R′N ) N,N-diethylethanolamine (R2R′N) ) concentration of N,N-diethylethanolamine, kmol/ m3 RCO2 ) specific rate of absorption of CO2, kmol/(m2s) rDEEA ) reaction rate in aqueous DEEA solutions, kmol/(m3 s) t ) time, s T ) temperature, K VG ) volume of gas phase, m3 VL ) volume of liquid phase, m3 z ) stoichiometric coefficient
Appendix B Abbreviations AMP ) 2-amino-2-methyl-1-propanol DEA ) diethanolamine DEEA ) N,N-diethylethanolamine MDEA ) methyldiethanolamine MEA ) monoethanolamine PZ ) piperazine TEA ) triethanolamine
Supporting Information Available The physical properties of the aqueous mixtures of DEEA and PZ at 303 K are represented in this section. This material is available free of charge via the Internet at http:// pubs.acs.org.
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