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Ind. Eng. Chem. Res. 2005, 44, 9894-9903
SEPARATIONS Liquid-Phase Composition Determination in CO2-H2O-Alkanolamine Systems: An NMR Study Jana Poplsteinova Jakobsen, Jostein Krane, and Hallvard F. Svendsen* Department of Chemical Engineering, NTNU, Sem Sælands vei 4, NO-7491 Trondheim, Norway
NMR studies were performed investigating the liquid-phase composition in samples where various amounts of CO2 were dissolved in aqueous alkanolamines (butyl-ethanolamine (BEA), methyl-di-ethanolamine (MDEA), and monoethanolamine (MEA)) at various temperatures. Chemical shifts of functional groups present in the systems were determined, and the liquidphase compositions were calculated for 20 and 40 °C with an estimated error of ∼5-10%, largely dependent on the temperature. The obtained speciation was based on the NMR spectra only and represents, thereby, additional and independent information on the systems that could be used for VLE model refinement. The experimental speciation was compared with the speciation predicted by a thermodynamic model with the activity coefficients calculated by the extended UNIFAC model (group contribution method UNIFAC combined with the Debye-Hu¨ckel electrolyte theory). The comparisons showed qualitative agreement and also quantitative agreement for the main species at low and medium loadings. However, for the minor components, and in particular for CO2, the agreement was not satisfactory. Spectra acquired at temperatures above 40 °C (70 and 90 °C), were broadened by fast chemical exchange between the species. The dynamic nature of the system complicated the quantitative evaluation of the spectra; because of the broadening of the peaks, the integration was inaccurate. These spectra could also provide quantitative information and information on the kinetics, but they were not evaluated within the frame of this work. More advanced software for dynamic NMR simulation and regression of kinetic parameters is needed. Introduction Reliable estimates of liquid-phase composition play a key role in the modeling of vapor-liquid equilibria in CO2 absorption processes. A number of thermodynamic models for the CO2-H2O-alkanolamine systems have been proposed in the literature during the past decades.1-9 The models differ mainly in the applied activity coefficient model. The activity coefficient model is crucial for the model accuracy and reliability because it is supposed to provide both accurate representation of the species concentrations and the activity coefficients. The development, and particularly the parameter regression of the activity coefficient models, has relied mainly on phase equilibrium measurements, where the known quantities are the total concentrations of amine and CO2 in the liquid phase and the gas-phase CO2 partial pressure or fugacity. However, the overall composition data alone are not sufficient to uniquely determine the parameters in the formulated activity coefficient models. Additional information on the particular species concentrations or activity coefficients is desired for a proper parameter evaluation. NMR experiments can provide detailed information about the liquidphase composition that can serve this purpose and be used for the evaluation and further refinement of the VLE models. * To whom correspondence should be addressed. Tel.: +47 73594100. E-mail:
[email protected].
NMR studies on CO2 in H2O were performed by Patterson and Ettinger10 and Abbott et al.11 The distribution of CO2 species in mixed H2O-amine systems was investigated by use of NMR by Suda et al.12 and Holmes et al.13 NMR data were also used to determine the carbamate stability constant by Barth et al.,14 Bishnoi and Rochelle,15 and Wang.16 Nevertheless, to our knowledge the individual ionic concentrations have not been evaluated independently solely from NMR data but always in combination with a thermodynamic model and/or equilibrium relationships. It is believed that thermodynamic models with parameters based on both VLE data and independent NMR data will be more reliable and applicable to a wider range of operating conditions. Method and Materials In this work, samples of CO2 dissolved in aqueous alkanolamines were analyzed spectroscopically at various temperatures. The amines investigated were 15 and 30 wt % monoethanolamine (MEA), 9 and 30 wt % butylethanolamine (BEA), and 23 wt % methyl-di-ethanolamine (MDEA). 13C spectra were acquired at temperatures of 20, 40, 70, and 90 °C for amine samples with CO2 loadings within the range of 0.1-1.0 ((mol CO2)/ (mol amine)). To obtain accurate integral areas, the spectra were fitted and integrated using the IGORPro software by WaveMetrics. In contrast to the work by
10.1021/ie048813+ CCC: $30.25 © 2005 American Chemical Society Published on Web 11/10/2005
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Suda et al.,12 the concentrations of all identified species were calculated based on peak positions and peak areas exclusively. Electroneutrality, material balances, and equilibrium constants served only as independent checks of the obtained compositions. The model for the liquidphase composition calculation was excluded from the data evaluation because the goal of the study was to obtain independent data sets that could be used for estimation of the parameters of the activity coefficient models. A 600 MHz Bruker spectrometer was used. Data were acquired by the software XWINNMR developed by Bruker. The temperature of the probe was controlled and the thermostat was calibrated using a sample with 80% ethylene glycol and 20% DMSO. The spectrum of this sample includes two peaks, and the difference in the chemical shifts of the two peaks is temperaturedependent. From this difference (∆δ) in ppm, the temperature in Kelvin can be calculated according to the following equation.
T (K) ) (-108.33∆δ + 460.41)
(1)
The alkanolamines used in this study were MEA with purity 99% obtained from Acros Organics, BEA with purity 98% from Atofina Chemicals, and MDEA with purity 98.5% from Riedel-de-Hae¨n. Dioxane p.a. by Ferak was used as the internal standard. The chemicals used for the calibration were K2CO3 from Merck and KHCO3 from Riedel-de-Hae¨n. 13C is the only naturally occurring carbon isotope with spin, and its natural abundance is only 1.1%. To detect the free CO2 peak, 13C-labeled CO was used, manufactured by Isotec and 2 containing 99 atom % 13C and 6 atom % 18O. Chemical System. The investigated CO2-amineH2O systems are reactive systems. Several chemical reactions are involved that give rise to a large number of formed species. The following species are considered to exist in the liquid phase: amine, amineH+, amineCOO- (if formed), CO2, HCO3-, CO32-, H2O, H3O+, and OH-. The following equilibrium reactions are considered to take place in the liquid phase:
Dissociation of water: 2H2O h H3O+ + OH-
(2)
(3)
Dissociation of bicarbonate ion: HCO3- + H2O h CO32- + H3O+
(4)
Dissociation of protonated amine: amineH+ + H2O h amine + H3O+
(5)
Carbamate reversion to bicarbonate: amineCOO- + H2O h amine + HCO3-
φiyiP ) γixiH∞i exp{vj ∞i (P - Pow)/RT}
(7)
where H∞i and vj ∞i are the Henry’s law constant and partial molar volume for molecular solute i at infinite dilution in the solvent, respectively. The Henry’s law constant for CO2 at infinite dilution in water was taken from Chen et al.,18 and the partial molar volume of CO2 was substituted by molar volume and taken from Brelvi and O’Connell.19 For the solvent (water), the vaporliquid equilibrium is given by
φwywP ) γwxwPow φow exp{vw(P - Pow)/RT}
(8)
The chemical equilibrium constants for the reactions in eqs 2-6 were defined in terms of activity coefficients γi and mole fractions xi:
Kj )
∏i avi
ij
)
∏i (γixi)v
ij
j ) 1, 2, ..., R
(9)
The phase and chemical equilibria were further complemented with mole balances and the electroneutrality equation. The fugacity coefficients were calculated from the Peng-Robinson equation of state (EOS). The activity coefficients were calculated by the extended UNIFAC model that combines an extension of the Debye-Hu¨ckel formula proposed by Guggenheim and Stokes20 with the group contribution method UNIFAC by Wu and Sandler.21 To ensure thermodynamic consistency, the Gibbs-Duhem equation was applied. Thermodynamic Parameters. The thermodynamic parameters needed for the model are the equilibrium constants for all chemical reactions, the parameters in the activity coefficient model, and the Henry’s law constant for CO2 in pure water. The temperature dependency of the equilibrium constants and the Henry’s law constant can be expressed in the form
ln K or ln H ) A + B/T + C ln T + DT
(10)
Coefficients A, B, C, and D for all reactions and for Henry’s law constant are summarized in Table 1. Experimental Section
Dissociation of carbon dioxide: CO2 (l) + 2H2O h HCO3- + H3O+
is based on the infinite dilution reference state and defined by
(6)
Thermodynamic Model. In this work, the thermodynamic model for VLE calculation presented by Poplsteinova17 was used. It is build around phase equilibrium conditions for neutral species and chemical equilibria for all elementary chemical reactions in the system. The equilibrium for volatile solutes (i.e., CO2)
Calibration Experiments. It is not possible to distinguish between HCO3- and CO32- and between the amines and protonated amines (am/amH+) in the NMR spectra, because of the fast-exchanging proton. The proton-exchanging species are represented by a common peak. However, the chemical shift of the common peak depends on the relative amount of the two species. With the help of extensive calibration measurements, functional relationships were obtained for calculation of the particular concentrations of the proton-exchanging species based on the chemical shift. The error in the species ratio calculated from these relationships was estimated to be ∼2.5%. The variations in chemical shift of the common peaks for HCO3-/CO32- and amine/amineH+ were investigated as a function of the species ratios, i.e., the sample pH and temperature. The samples for calibration were prepared as follows. Batch solutions (1 L) of a given concentration of amine, HCl, KHCO3, and K2CO3 were prepared by weighing. A small amount of dioxane was
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Table 1. Coefficients A, B, C, and D in Equation 10 reaction no.
parameter
A
B
C
D
T (°C)
source
2 3 4 5 6 5 -
KH2O KCO2 KHCO3 KMEA KMEACOO KMDEA HCO2
132.899 231.465 216.049 -4.907 37 0.030 669 -83.491 4 170.712 6
-13 445.9 -12 092.1 -12 431.7 -6 166.116 -2 275.19 -819.7 -8 477.711
-22.477 3 -36.781 6 -35.481 9 0 0 10.975 6 -21.957 43
0 0 0 -0.000 985 0 0 0.005 781
0-225 0-225 0-225 0-50 25-120 -95 0-100
Edwards et al.22 Edwards et al.22 Edwards et al.22 Bates and Pinching23 Austgen2 Kamps and Maurer24 Chen et al. 18
added to each batch. Samples with known amine/ amineH+ or HCO3-/CO32- ratio were prepared by mixing the batch solutions in appropriate weight ratios. The pH of each sample was measured using a 702SM Titrino produced by Metrohm and calibrated using buffers of pH ) 12, 8, and 4 by Merck. The particular solution (0.5 mL) was filled into the NMR tubes using a micropipet. For the calibration, Norell 508-Ultra precision NMR sample tubes were used. In addition, one droplet of D2O was injected into each tube to achieve better signal lock. NMR spectra were acquired for each sample at 20 °C, and the chemical shifts of the peaks were recorded. From the set of samples prepared for pH calibration, only five representative samples were chosen for the temperature shift investigation. Figure 1 shows the “titration curve” for MEA/MEAH+ in terms of the change in the chemical shift of the peaks of the 13C spectra. The change in the chemical shift of a peak was evaluated with respect to the shift of the same peak on spectra obtained for samples filled with the pure alkanolamine solutions. From the figure, it can be seen that the effect of pH on the peak shift was most evident for peak S1. This was the peak representing the carbon in the β-position with respect to the nitrogen group. The pH shift was largest for the β-carbon in all three alkanolamines. Further, it can be seen that the change in chemical shift with pH is not linear over the whole pH range. Linear interpolation between the two limiting values will lead to errors, especially in the end parts of the curves. It was, therefore necessary, to perform extensive calibrations over the whole pH range. Figure 2 shows the pH-dependency of the chemical shift of the peak for HCO3-/CO32-. The limiting shifts in samples containing solely CO32- or HCO3- were 168.9 and 161.4 ppm, respectively. This corresponds well with the values 169.5 and 161.3 reported by Abbott et al.11 Again, as in the case of amine/amineH+, linear interpolation between the two limiting peak positions will lead to inaccuracies in the estimated ratios of the particular species. Therefore, calibration was also per-
Figure 1. pH shift of the peaks of MEA in
13C
spectra at 20 °C.
Table 2. Batch Solutions amine solution 15 wt % MEA 30 wt % MEA 9 wt % BEA 30 wt % BEA 23 wt % MDEA
weighed amount (g) mass fraction amine dioxane water amine dioxane water 122.72 256.34 90.89 293.75 238.91
45.05 44.46 23.94 22.50 21.57
836.34 598.22 908.38 685.56 755.49
0.122 0.285 0.089 0.293 0.235
0.045 0.049 0.023 0.022 0.021
0.833 0.665 0.888 0.684 0.744
formed for the HCO3-/CO32- peak shift. The sudden drop between the second-to-last and last points is probably the start of the second step in the titration curve for the protonation of HCO3-. Sample Preparation and Equilibration. Special pressure/vacuum sample tubes 528-PV-7 by Wilmad were used for samples that were loaded with CO2. Batch solutions (1 L) of alkanolamines were prepared by weighing the amine, dioxane, and distilled water to the required weight percent composition. The estimated error of sample preparation is a maximum of 0.5% in concentration of the solvent. Details on the batch solution compositions are given in Table 2. The tubes were filled with 0.4 mL of the batch (unloaded) solution and weighed. One droplet of D2O was added to each sample to improve the lock signal. The addition of D2O diluted the sample by ∼10%. This has, however, no effect on the relative species concentrations because the calculation is based on the total number of moles of the internal standard, dioxane. It only affects the total amine and dioxane mass fractions under which the equilibrium is obtained, which will be ∼10% lower than the values given in Table 2. The sample tube was then connected to the apparatus for CO2 filling and immerged in liquid nitrogen; see Figure 3. Then the whole apparatus and the line to the gas cylinder were evacuated. After evacuation, the valves to the sample tube (2) and vacuum pump (3) were closed and the calibrated gas reservoir of 10.5 mL was filled with 13CO2 to a given pressure. Then the valve (2) was opened to let the 13CO into the sample tube. The pressure drop in the 2
Figure 2. pH shift of the peak of HCO3-/CO32- in 13C spectra at 20 °C.
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Figure 3. Schema of the apparatus for filling the sample tube with CO2.
gas reservoir was observed. After all the CO2 had transferred to the sample tube (a few seconds), the tube was closed (4) and removed from the apparatus. The amount of gas in the sample tube could then be calculated from the volume of the reservoir and the pressure change. Before acquisition of spectra, the samples were conditioned by heating for at least 1 h at the requested temperature in a water bath. A sample holder that was shaking with a high frequency was specially designed for that purpose. Shaking enhanced mixing of the sample and enabled faster equilibration. It was confirmed by a simple test that 1 h conditioning was sufficient. The test consisted of comparison of spectra taken before heating started, in 10 min time intervals within the first hour of conditioning, and finally after conditioning over the whole night. Significant changes in the spectra taken within the first 30-40 min were identified, suggesting that equilibrium was not established within that time interval. However, the spectra taken after 1 h and after a whole night were identical. This led to the conclusion that equilibrium was established within the first hour of conditioning. The test was performed with MDEA solution considering that the equilibrium in the MEA and BEA solutions would be reached even faster. Spectra were acquired after equilibration of the samples at a given temperature. Evaluation of Spectra Spectra of CO2 in Pure H2O. Prior to the experiments with loaded alkanolamine solutions, a few sample spectra of CO2 in pure H2O at 20 °C were acquired. One such spectrum is shown in Figure 4. The peaks for free CO2 and hydrated CO2 in the form of H2CO3, HCO3-, and/or CO32- were identified at 125.3 and 159.5 ppm, respectively. The peak separation of 34.2 ppm is in good agreement with the value of 35.6 ppm reported by Patterson and Ettinger.10 The peak area ratio of hydrated/free CO2 was found to be 2 × 10-3, which also corresponds to the ratio of 1/500 expected by Patterson and Ettinger.10 The CO2 in H2O is present mainly in the form of free CO2 because of the very low degree of hydration of CO2. During the calibration, the chemical shift of a peak entirely due to HCO3- was identified at 161.4 ppm. Hence, the chemical shift of 159.5 ppm suggests that the hydrated CO2 was mainly in the form of HCO3-, except for a small fraction that was in the form of H2CO3 and was responsible for the slight shift toward the lower frequency. Both the ratio of hydrated/
Figure 4. Spectra for CO2 in pure H2O at 20 °C.
Figure 5. Spectra for loaded solution of 15 wt % MEA at 20 °C.
free CO2 and the ratio HCO3-/H2CO3 will be expected to change with CO2 partial pressure and temperature. In one experiment, both the liquid and gas phases were analyzed at the same time, and an additional peak was found at 124.9 ppm; see the detailed picture in Figure 4. This peak was assigned to the CO2 in the gas phase. The chemical shifts for CO2 (aq) at 125.3 ppm and CO2 (g) at 124.9 ppm determined in this work are in agreement with the values of 125.9 and 124.5 ppm reported by Abbott et al.11 Spectra of CO2 in Aqueous Alkanolamine Solutions. A sample spectrum acquired for CO2 in 15 wt % MEA at 20 °C is given in Figure 5. The figure shows the assignment of the peaks to the particular functional groups. Although the approximate ranges of the chemical shifts of the various functional groups were known, the actual position of the peaks varied with the CO2 loading and the temperature. It happened that peaks overlapped each other and even changed position in the spectra. Because of that, every single spectrum had to be treated with special care. Figure 6 illustrates the variations in the spectra with increasing CO2 loading for the solution of 30 wt % MEA. These variations predict the distribution of the species concentrations qualitatively. It can be seen from the spectra that, as the pH decreases with the increasing loading, the peaks for HCO3-/CO32- and MEA/MEAH+ shift toward lower frequencies, confirming that the protonated form of the species gradually prevails. The intensities of the carbamate peaks increase first with increasing loading, reaching their maximum at ∼0.6 (mol CO2)/(mol MEA), and then decrease again, indicating clearly the maximum in the carbamate concentration. The intensity of the peak representing HCO3-/ CO32- increases with increasing loading. To obtain quantitative results, the areas under the spectral peaks were integrated and related to the area of the standard peak. The peaks can be integrated
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Figure 7. Exchange-broadened spectra for 30 wt % MEA at varying temperature. Table 3. Chemical Shifts of CO2-Carbon Types group -/CO2-
HCO3 CO2 -COO
Figure 6. Spectra for 30 wt % MEA solution with varying CO2 loading at 20 °C.
directly in the XWINNMR package that was used for spectrum acquisition. However, the direct integration is not sufficiently accurate for the type of analysis we wanted to perform. This is mainly due to the inexact shape of the peaks and the peak overlaps. Therefore, the experimental spectra were fitted before integration was done. The IGORPro software was used for the fitting and integration of the spectra, since it was especially developed for handling large amounts of experimental data. Quantitative analysis was performed only for spectra acquired at 20 and 40 °C. Broadening of peaks was observed as the temperature increased; see Figure 7. By careful comparison of the spectra at varying temperatures, it was concluded that the broadening is due to an increasing rate of chemical exchange at higher temperatures. The effect of chemical exchange on NMR spectra is known and studied by a branch of NMR spectroscopy called dynamic NMR. Because of the exchange between sites with different chemical shifts, the environment of the magnetic nuclei changes and these changes are observed in the NMR spectra. Kinetic information can be obtained by analysis of these exchange-broadened spectra acquired at different temperatures. First- or pseudo-first-order rate constants in the range 10-1 to 105 can be measured. Evaluation of the high-temperature spectra (70 and 90 °C) based on the line-broadening analysis (simulation of the spectra and regression of kinetic parameters) requires more advanced software for NMR dynamics and was not performed within the frame of the presented work. Results and Discussion Identified Species/Functional Groups and Their Chemical Shifts. The species/groups formed from the
13CO
MEA
BEA
MDEA
161.1-167.8 122.6-126.2 162.6-166.5
160.8-167.5 125.9-126.7 163.6-165.7
161.26-163.9 126
2 that were identified in the samples were the HCO3- and CO32- ions, the COO- group of the carbamate ion, and the free CO2. In addition, peaks for species/ groups resonating at high frequencies were found that were assumed to be products of reactions between CO2 and impurities in the amines. The chemical shifts of the carbons in CO2 species/groups are given in Table 3. In the samples of MEA and BEA, the identified amine species were free amine, protonated amine, and carbamate. In the samples of MDEA, only amine and protonated amine were identified since MDEA does not form carbamate. The chemical shifts of the carbons in the alkanolamines or the chemical shift ranges are given in Table 4. The expected accuracy of the chemical shift is about (0.1 ppm. The ranges signify the change in the chemical shift with pH and temperature. The peaks are not at the same position all the time because the chemical environment around the resonating nuclei changes due to the exchange of protons or CO2 between species. The rate of the chemical exchange is increasing with temperature. Liquid-Phase Compositions. The spectra acquired at higher temperatures were broadened due to fast chemical exchange. So, the peak area integration and, therefore, the speciation calculations were possible only for the spectra acquired at low temperatures (20 and 40 °C), as mentioned above. The particular species concentrations for samples of 15 wt % MEA, 30 wt % MEA, 9 wt % BEA, 30 wt % BEA, and 23 wt % MDEA at 20 and 40 °C are given in the Appendix. Examples of speciation plots for the investigated systems at 20 °C are shown in Figures 8-10. Check of Electroneutrality and Total Alkanolamine Concentration. As a balance check of the obtained ion compositions, the errors in the electroneutrality condition were evaluated in percent of the total number of moles of ions. The average values of the errors in the electroneutrality condition were 10.1% and
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Figure 8. Speciation in 15 and 30 wt % MEA at 20 °C.
Figure 9. Speciation in 9 and 30 wt % BEA at 20 °C.
Figure 11. Electroneutrality check for 15 wt % MEA. Figure 10. Speciation in 23 wt % MDEA at 20 °C.
11.7%, and the maximum errors were 29.2% and 34.4% for data at 20 and 40 °C, respectively. The errors in the electroneutrality condition were all negative, indicating excess of negative charged ions; see the example for the 15 wt % MEA solution in Figure 11. In addition, the errors in the calculated total number of moles of alkanolamine were evaluated. The average error in percent of the total number of moles of alkanolamine as specified by the sample preparation was 3.8% for data
at 20 °C and 8.8% for data at 40 °C. The maximum error in the total amine concentration was 12.3% and 24.2% at 20 and 40 °C, respectively. The error for data at 40 °C was approximately twice as high as the error for data at 20 °C; see Figure 12. This suggests that, already, the spectra at 40 °C might have been affected by chemical exchange and that the integration of the broadened peaks was not accurate enough. The error in electroneutrality is larger than that of the total amine balance. This is because the assignment of the concentrations of amine and protonated amine and the concentrations of
Table 4. Chemical Shifts of Amine-Carbon Types group
MEA/MEAH+
MEACOO-
BEA/BEAH+
BEACOO-
MDEA/MDEAH+
-CH2-OH -CH2-NH2 -CH2-NH
58.9-63.8 42.4-43.3
62.6 44.4
57.8-61.2
61.04
58.7-59.8
48.4-51.2 50.1-51.2
49.8 48.7
-CH2-N< CH3-N< -CH2-CH2-CH3 -CH3
57.3-59.6 41.7-42.7 28.5-31.5 20.1-20.8 13.7-14.3
31.4 20.6 14.4
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Ind. Eng. Chem. Res., Vol. 44, No. 26, 2005 Table 5. Molar-Fraction Based MEA Carbamate Stability Constant 20 °C 40 °C
Figure 12. Amine balance check for 15 wt % MEA.
carbonate and bicarbonate to the individual species is more inaccurate than the evaluation of the total concentration of the sum of the species. Also, as indicated earlier, the concentration of carbonate is most likely overpredicted. An overprediction in this value will lead to an excess of negative ions, as observed in the electroneutrality balance. The error in the obtained species concentrations was estimated based on partial experimental errors and a sensitivity study. The error in the sample preparation was estimated to be 0.5%, the error of integration was estimated to be 1%, and the accuracy of the calibration was estimated to be ∼2.5%. Sensitivity analysis was performed to see what effect inaccuracies within these ranges would have on the resulting species molar fractions. It was found that a 1% error in the peak areas will lead, in average, to a relative error of 0.5% and a maximum error of 1% in the molar fractions. The error introduced by the calibration was shown to have a much more significant effect depending on the species ratio, i.e., loading region. The calibration data were used to establish a relationship between the chemical shift of the common peak and the species ratio. The estimated inaccuracy of 2.5% in the estimation of these species ratios based on calibration may lead to high errors in the species fractions, especially when these fractions are small. Fractions
