ARTICLE pubs.acs.org/IECR
Solubility and Diffusivity of N2O in Aqueous 4-(Diethylamino)2-butanol Solutions for Use in Postcombustion CO2 Capture Teerawat Sema,*,† Mohamed Edali,† Abdulaziz Naami,† Raphael Idem,*,†,‡ and Paitoon Tontiwachwuthikul*,†,‡ †
International Test Centre for CO2 Capture (ITC), Faculty of Engineering and Applied Science, University of Regina, Regina, Saskatchewan, S4S 0A2, Canada ‡ Joint International Center for CO2 Capture and Storage (iCCS), Department of Chemical Engineering, Hunan University, Changsha, 410081, P. R. China ABSTRACT: In this work, the solubility and diffusivity of nitrous oxide (N2O) in aqueous 4-(diethylamino)-2-butanol (DEAB) solutions were measured. Solubility was measured in a stirred cell reactor over the temperature range of 298343 K and concentration range of 0.683.77 M. On the other hand, diffusivity was measured in a laminar jet absorber over the temperature range of 298318 K and concentration range of 1.02.5 M. An attempt was made to correlate the solubility data with well-known models (semiempirical model, RedlichKister equation, and polynomial model). It was observed that only the polynomial model correlated the solubility of N2O in aqueous DEAB solution satisfactorily with an AAD of 0.1%. Similarly, an attempt was made to correlate the diffusivity data with well-known models (semiempirical model and modified StokesEinstein model). The semiempirical model provided better predicted N2O diffusivity data compared with the experimental data with an AAD of 3.4%. These data can then be used to determine the physical solubility and physical diffusivity of carbon dioxide (CO2) in aqueous DEAB solutions using the “N2O analogy”.
1. INTRODUCTION Greenhouse gas emissions and their potential effects in terms of global warming and climate change problems have become an important issue in recent years.1 Among the greenhouse gases, carbon dioxide (CO2) is considered to be the major target because of its abundance. It is generally agreed that removal of CO2 from large point exhaust gas streams of industrial processes such as electric power plants, refineries, and natural gas processing plants becomes essential in order to make any impact in CO2 emissions mitigation. Several technologies have been used for CO2 removal, such as absorption, adsorption, cryogenics methods, membranes systems, and microbial systems.2 Amine-based CO2 capture technology has been widely used for postcombustion CO2 removal and to provide the most appropriate and costeffective options compared with other technologies,3,4 especially in coal-fired power plants which usually have a low CO2 concentration and a low pressure of the exhaust stream. A newly developed amino alcohol solvent, 4-(diethylamino)2-butanol (DEAB), is considered as a promising solvent to capture CO2 due to its energy efficiency for regeneration, high absorption capacity, and cyclic capacity for CO2 removal.5,6 However, a good solvent not only should provide energy efficiency, high absorption capacity, and cyclic capacity but also should have a fast absorption rate, low degradation rate, and low corrosion rate. In order to develop the reaction kinetics model, several physical, chemical, and thermodynamics properties of the amine solution are required, such as physical solubility of CO2 or CO2 Henry’s constant, physical diffusivity of CO2, density, viscosity, equilibrium constants for the CO2amine system, and CO2 absorption rate data.7 Even though physical solubility and r 2011 American Chemical Society
physical diffusivity of CO2 at various temperatures and concentrations are essential for developing the reaction kinetics model, they cannot be measured directly because CO2 reacts chemically with aqueous amine solutions. Thus, measurements of nitrous oxide (N2O) solubility and diffusivity through the N2O analogy can be used to determine the solubility and diffusivity of CO2 in aqueous amine solutions.810 This is due to the similarity of the molecular weight, chemical configuration, volume, and electronic structure between CO2 and N2O.11 The N2O analogy for determining solubility and diffusivity are shown in the following equations. HeCO2 amine ¼ HeN2 O amine DCO2 amine ¼ DN2 O amine
HeCO2 H2 O HeN2 O H2 O
DCO2 H2 O DN 2 O H 2 O
ð1Þ
ð2Þ
In the present work, new experimental results on N2O solubility and N2O diffusivity of aqueous solutions of DEAB are presented over ranges of temperatures and concentrations. Then, these results are used to establish correlations for the N2O solubility and N2 O diffusivity of aqueous solutions of DEAB. Received: April 18, 2011 Accepted: December 20, 2011 Revised: December 14, 2011 Published: December 20, 2011 925
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Table 1. Solubility of N2O in Pure DEAB Solvent HeN2ODEAB temp. (K)
(kPa m3/kmol)
298
3179
303
3553
313
4423
323
5484
333
7894
343
9043
Figure 1. N2O solubility of pure DEAB, pure MEA,17 pure MDEA,17 and water;18 solid lines are predicted results.
2. EXPERIMENTAL SECTION 2.1. Chemicals. DEAB is a newly developed potential solvent which was synthesized according to the procedure described by Tontiwachwuthikul et al.6 in our solvent synthesis laboratory at the International Test Centre for CO2 Capture (ITC), University of Regina. Aqueous solutions of DEAB of desired concentrations were prepared by adding a known amount of deionized water to predetermined amounts of DEAB. N2O, CO2, and nitrogen (N2) with a purity of 99.9% were supplied by Praxair Inc., Canada. 2.2. N2O Solubility. The apparatus and experimental technique used for determining N2O solubility were modified from the works of Munder et al.12 and Park et al.13 The solubility experiment was performed in a rotary-type 600 mL stainless steel autoclave reactor (model Parr 5500, Parr Instrument Co., Moline, IL) connected to a controller (model Parr 4843, Parr Instrument Co., Moline, IL). The reactor consists of a variablespeed impeller, a heating mantle, cooling coils, a gas feed port, a thermocouple, and a pressure transducer. Initially, the amine solution was degassed using an ultrasonic bath (VWR model 75D, VWR international, ON, Canada), and then 300 mL of the degassed amine solution was loaded into the reactor. The desired temperature and agitation speed were set and controlled by the controller, and then the vacuum was turned on. After shutting down the vacuum pump, the liquid was under a certain pressure due to liquid vaporization. The pressure was measured as equilibrium, PV. Then a certain amount of N2O (nN2O) was fed to the reactor. The amount of nN2O can be determined by measuring the pressure in the reactor before N2O injection (P1) and after N2O injection (P2)
nN2 O ¼ ðP2 P1 Þ
Vg ZN2 O RT
The concentration of N2O at equilibrium is CN 2 O ¼
HeN2 O ¼
g
RA ¼ 4Ce ðDLhÞ1=2
ð3Þ
Ce ¼
P N2 O HeN2 O
ð10Þ
3.1. N2O Solubility in Pure DEAB. The solubility data of pure DEAB solvent at the temperature range from 298 to 343 K are presented in Table 1 and also plotted in Figure 1. As mentioned by Wang et al.,17 the solubility of N2O in pure amine solvent is an exponential function with temperature in Kelvin as shown in eq 11. b2 HeN2 O amine ¼ b1 exp ð11Þ T
The number of moles of N2O dissolved in the liquid phase at equilibrium can be determined by g
ð9Þ
3. RESULTS AND DISCUSSION
ð5Þ
nlN2 O ¼ nN2 O nN2 O
ð8Þ
where Ce* is the equilibrium concentration of the gas at the interface, D is the diffusivity of the gas in the liquid, L is the liquid flow rate, and h is the jet height. A plot of RA and (Lh)1/2 at various flow rates and jet heights should provide a straight line through the origin with a slope of 4Ce*D1/2. The Ce* can be determined by
ð4Þ
PN2 O Vg ZN2 O RT
PN2 O CN 2 O
2.3. N2O Diffusivity. The N2O diffusivities were measured using the laminar jet absorber. A detailed description of the laminar jet absorber and its operation can be seen in Al-Ghawas et al.15 and Aboudheir et al.16 Briefly, the amine solution was degassed by spraying it into a vacuum, and then the degassed amine solution was passed through the temperature-controlled water jacket in order to reach the desired temperature. The degassed amine solution was then passed through the jet nozzle in order to generate a smooth-surfaced rod-like jet in the absorption chamber, continuously. The soapfilm meter was used to measure the rate of absorption (RA). A two-dimensional microscope was used to measure the jet height (h). Finally, the discharged liquid was collected and the liquid flow rate (L) measured. According to the penetration theory for physical absorption, RA can be defined as
The number of moles of N2O in the gas phase at equilibrium (ngN2O) can be determined by n N2 O ¼
ð7Þ
Then the N2O solubility (HeN2O) is defined as
where Vg is the volume of the gas container, R is the gas constant, and ZN2O is the compressibility factor of N2O which can be calculated by the PengRobinson equation of state.14 After the system reached equilibrium, the equilibrium pressure (PT) was measured. The partial pressure of N2O (PN2O) at equilibrium is P N2 O ¼ P T P V
nlN2 O Vl
ð6Þ 926
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Table 2. Solubility of N2O in aqueous DEAB solutions
Table 3. Temperature-Dependence Coefficient (Ai) for the Polynomial Model in eq 17
HeN2Osolution (kPa m3/kmol)
temp. (K)
temp. (K)
0.68 M
1.34 M
1.97 M
3.19 M
3.77 M
298
6676
5285
3965
2699
2416
313
7103
5706
4228
2844
323
7600
5918
4457
333
8102
6354
4857
343
8623
6808
5274
A0
A1
A2
A3
A4
298
7192.7
587
2574.6
932.7
100.9
2665
313 323
7690.2 8767.6
475.1 768.6
2500 1869.2
877.3 741.3
90.5 80
3077
2868
333
9423.7
1099.5
1682
686.5
74.1
3410
3184
343
10063.3
1322.4
1598.5
667.8
71.8
3824
3640
To correlate the N2O solubility results at various concentrations and temperatures, there are several models which have been applied in the literature such as the semiempirical model,17 RedlichKister equation,19 and polynomial model.18 In this study, these three different models were used to develop the predictive models for N2O solubility of aqueous DEAB solution. 3.2.1. Semiempirical Model. It was reported by Wang et al.17 that the semiempirical model gives a good estimation of N2O solubility of various aqueous amine solutions of MEA, diethanolamine (DEA), MDEA, 2-amino-2-methyl-1-propanol (AMP), and diisopropanolamine (DIPA). The model can be written as ln HeN2 O Solution ¼ ϕH2 O ϕDEAB ðk1 þ k2 t þ k3 t 2 þ k4 ϕH2 O Þ þ ϕH2 O ln HeN2 O H2 O
ð14Þ
þ ϕDEAB ln HeN2 O DEAB
where HeN2OH2O can be estimated from the equation proposed by Versteeg and van Swaaij18 as 2284 6 HeN2 O H2 O ¼ ð8:55 10 Þexp ð15Þ T
Figure 2. Solubility of N2O in aqueous DEAB solutions; solid lines are calculated from the polynomial model in eq 17.
Equation 11 can also be written as b2 ln HeN2 O amine ¼ ln b1 þ T
HeN2ODEAB is presented in eq 13. The N2O solubility results of aqueous DEAB solution, which are shown in Table 2, were used to correlate with eq 14 using a nonlinear regression package (NLREG program). It was found that the predicted results from the semiempirical model do not correlate well with the experimental results with an AAD of 6.9%. 3.2.2. RedlichKister Equation. The RedlichKister equation20 is widely used to correlate properties of binary solutions such as density, viscosity, refractive index, and N2O solubility of amine solutions.19,21 In this study, the RedlichKister equation was used as the predictive correlation for N 2 O solubility of aqueous DEAB solutions. The RedlichKister equation can be written as
ð12Þ
A semilog plot of HeN2Oamine and 1000/T should be a straight line as shown in Figure 1. As expected, HeN2ODEAB increases with increasing temperature.17,18 Conversely, the solubility of N 2 O into pure DEAB solution is found to decrease as the temperature is increased. Moreover, Figure 1 also compares He N2O in pure DEAB with that in water, pure monoethanolamine (MEA), and pure methyldiethanolamine (MDEA). It was found that He N2O in pure DEAB is lower than that in water but higher than that in MEA and MDEA. The predictive correlation for HeN2ODEAB can be established as 2460:3 7 HeN2 O DEAB ¼ ð1:1876 10 Þexp ð13Þ T
ln HeN2 O Solution ¼ xH2 O ln HeN2 O H2 O þ xDEAB ln HeN2 O DEAB þ xH2 O xDEAB ½A0 þ A1 ð2xDEAB 1Þ þ A2 ð2xDEAB 1Þ2 þ A3 ð2xDEAB 1Þ3
The predicted values were found to fit well with the experimental values with an absolute average deviation (AAD) of 3.6%. 3.2. N2O Solubility in Aqueous DEAB Solutions. The solubility measurements of aqueous DEAB solutions were done over the temperature range of 298343 K and the concentration range of 0.683.77 M. The results are shown in Table 2 and plotted in Figure 2. It can be observed that HeN2O increases with increasing temperature and decreases with increasing DEAB concentration. Conversely, the N2O solubility into aqueous DEAB solution is found to decrease with increasing temperature and increases with increasing DEAB concentration.
þ A4 ð2xDEAB 1Þ4
ð16Þ
A i is the temperature-dependent coefficient which can be determined for a specific temperature by regression analysis using the NLREG program. It was found that the predicted results from the RedlichKister equation correlate reasonably well with the experimental results with an AAD of 1.7%. 3.2.3. Polynomial Model. It was mentioned by Versteeg and van Swaaij18 that the polynomial model can be used to predict the N2O solubility of aqueous solutions of dimethylmonoethanolamine (DMMEA), DEA, triethanolamine (TEA), monoisopropanolamine 927
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Figure 4. Diffusivity of N2O in aqueous DEAB solutions; solid lines are calculated from the semiempirical model in eq 21.
Figure 3. Parity chart comparing experimental and predicted results of N2O solubility of aqueous DEAB solutions.
Table 5. Viscosity of Aqueous DEAB Solution at Various DEAB Concentrations and Temperatures
Table 4. Diffusivity of N2O in Aqueous DEAB Solutions concentration (M)
temp. (K)
diffusivity (109 m2/s)
1.0
298
1.89
1.0 1.0
308 318
2.16 2.49
concentration of DEAB (M)
298 K
308 K
318 K
1.0
2.39
1.68
1.20
2.0
298
1.04
2.0
4.53
3.05
2.06
2.0
308
1.31
2.5
5.99
3.98
2.64
2.0
318
1.59
2.5
298
0.68
2.5
308
0.91
2.5
318
1.22
viscosity (mPa s)
polynomial model does not deal with estimation of the solubility of N2O in water and pure DEAB. Also, the experimental N2O solubility in aqueous DEAB solutions appears to be a polynomial function with respect to the DEAB concentration as shown in Figure 2. Therefore, the polynomial model was found to provide a near perfect fit between experimental and predicted values. 3.3. N2O Diffusivity in Aqueous DEAB Solutions. The diffusivity of N2O in aqueous DEAB solutions was measured by a laminar jet absorber over the temperature range of 298318 K and concentration range of 1.02.5 M. The experimental values obtained from this study are shown in Table 4 and plotted in Figure 4. The results show that the diffusivity increases with increasing temperature and decreases with increasing DEAB concentration, which corresponds well with the results of Tamimi et al.22 and Ko et al.,23 who used water and aqueous amines solutions of MEA, DEA, DIPA, MDEA, TEA, and AMP. In order to come up with the predictive model for the diffusivity of N2O at various concentrations and temperatures, two models that have been used in the literature were applied. The first model is the modified StokesEinstein model as used by Versteeg and van Swaaij.18 They found that the modified StokesEinstein model was applicable to estimate the N2O diffusivity into aqueous amine solutions of MEA, DEA, TEA, MIPA, and DIPA. The second model is the semiempirical model proposed by Ko et al.23 It has been mentioned that this model provides a good estimation of the N2O diffusivity into aqueous amine solutions of MEA, DEA, DIPA, MDEA, TEA, and AMP. In this study, these two models were used to determine which would provide better predicted results for the N2O diffusivity in aqueous DEAB solutions compared with experimental results. 3.3.1. Modified StokesEinstein Model. The N2O diffusivity in aqueous DEAB solutions depends on both the solvent concentration and the temperature. For the modified StokesEinstein
(MIPA), and DIPA. In this study, the polynomial model is defined as follows HeN2 O Solution ¼ A0 þ A1 C þ A2 C2 þ A3 C3 þ A4 C4 ð17Þ As mentioned earlier, Ai is the temperature-dependence coefficient which can be determined for a specific temperature by regression analysis using the NLREG program. The regression results for Ai coefficients are shown in Table 3. The predicted results using the polynomial model are plotted in Figure 2, where it can be seen that they fit perfectly well with the experimental results. A parity chart that compares the experimental and predicted results is shown in Figure 3 with an AAD of 0.1%. By comparing the three predicting models (the semiempirical model, RedlichKister equation, and polynomial model) for N2O solubility in aqueous DEAB solutions, it can be concluded that the polynomial model provides the best fit among the three as shown in Figure 3. It can be seen that all three predicted models (semiempirical model, RedlichKister equation, and polynomial model) take both concentration and temperature into account as shown in eqs 14, 16, and 17, respectively. In addition, both the semiempirical model and the RedlichKister equation consider the N2O solubility in water and pure DEAB in the models as presented in eqs 14 and 16, respectively. However, both models do not work very well for aqueous DEAB system. This is due to the deviation caused by estimation of the solubility of N2O in water and pure DEAB. On the other hand, the 928
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It is generally accepted that the N2O diffusivity in aqueous DEAB solutions depends on both the solvent concentration and the temperature. For the semiempirical model, the N2O diffusivity correlates directly with both solvent concentration and temperature, resulting in a good prediction as presented in Figures 4 and 5. However, the modified StokesEinstein model correlates the N2O diffusivity with the concentration and temperature via the viscosity, resulting in the large deviation observed in Figure 5. This could be because the aqueous solution of DEAB behaves differently from the StokesEinstein relation.
4. CONCLUSIONS 1 The solubility of N2O in pure DEAB solvent decreases as the temperature is increased. HeN2ODEAB can be calculated using a predictive model (eq 13) with an AAD of 3.6%. 2 N2O solubility into aqueous DEAB solution decreases with increasing temperature and increases with increasing DEAB concentration. HeN2OSolution can be estimated using a polynomial model (eq 17) with an AAD of 0.1%. 3 Diffusivity into aqueous DEAB solution increases with increasing temperature and decreases with increasing DEAB concentration. Diffusivity can be calculated using a semiempirical model (eq 21) with an AAD of 3.4%.
Figure 5. Parity chart comparing experimental and predicted results of N2O diffusivity in aqueous DEAB solutions.
model, the dependence of N2O diffusivity on the concentration and temperature are described in terms of the viscosity (η; Pa s). For various aqueous amine solutions18 the modified StokesEinstein model can be written as DN2 O 3 η0:8 ¼ Constant
ð18Þ
The viscosity of aqueous DEAB solution was calculated based on the results of Maneeintr et al.21 The viscosities of aqueous DEAB solution are presented in Table 5. By determining the constant value in eq 18 using the NLREG program, the modified StokesEinstein model was found to be DN2 O 3 η0:8 ¼ 1:219 1011
ð19Þ
It was found that the predicted results from modified StokesEinstein model do not fit well with the experimental results with an AAD of 10.2%. 3.3.2. Semiempirical Model. The semiempircal model for N2O diffusivity of aqueous amine solution was proposed by Ko et al.23 The model also takes both concentration and temperature into account as shown in eq 20 b3 þ b4 C ð20Þ DN2 O ¼ ðb0 þ b1 C þ b2 C2 Þexp T After regression analysis using the NLREG program, the predictive correlation for the diffusivity of N2O in aqueous DEAB solution can be written as DN2 O ¼ ½ð4:64 108 Þ þ ð8:74 108 ÞC ð 9:5Þ þ ð 3:81CÞ 8 2 þ ð2:86 10 ÞC exp T ð21Þ The predicted results are shown in Figure 4 as solid lines. It can be seen that the predicted values fit well with the experimental values. This can also be confirmed by a parity chart as presented in Figure 5 with an AAD of 3.4%. After determining the predictive models for N2O diffusivity of aqueous DEAB solutions, the modified StokesEinstein model and semiempirical model, it is clear that the semiempirical model provides better estimated results. This can also be confirmed by the parity chart of Figure 5.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: 306-585-4470. Fax: 306-585-4855. E-mail: Raphael.idem@ uregina.ca.
’ ACKNOWLEDGMENT Financial support from the International Test Centre for CO2 Capture (ITC) at the University of Regina and the Natural Sciences and Engineering Research Council of Canada (NSERC) is gratefully acknowledged. ’ NOMENCLATURE Ai = temperature-dependence coefficient AMP = 2-amino-2-methyl-1-propanol C = concentration, mol/L Ce* = equilibrium concentration of gas at interface, mol/L CN2O = concentration of N2O at equilibrium, mol/cm3 D = diffusivity of gas in liquid, cm2/s DCO2amine = diffusivity of CO2 in aqueous amine solution, cm2/s DN2Oamine = diffusivity of N2O in aqueous amine solution, cm2/s DCO2H2O = diffusivity of CO2 in water, cm2/s DN2OH2O = diffusivity of N2O in water, cm2/s DEA = diethanolamine DEAB = 4-(diethylamino)-2-butanol DIPA = diisopropanolamine DMMEA = dimethylmonoethanolamine h = jet height, cm HeN2O = solubility of N2O, kPa m3/kmol HeCO2amine = solubility of CO2 in aqueous amine solution, kPa m3/kmol HeN2Oamine = solubility of N2O in aqueous amine solution, kPa m3/kmol HeN2ODEAB = solubility of N2O in pure DEAB, kPa m3/kmol 929
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HeCO2H2O = solubility of CO2 in water, kPa m3/kmol HeN2OH2O = solubility of N2O in water, kPa m3/kmol L = liquid flow rate, cm3/s MDEA = methyldiethanolamine MEA = monethanolamine MIPA = monoisopropanolamine nN2O = mole of N2O was fed to the stirred reactor, mol ngN2O = mole of N2O in gas phase at equilibrium, mol nlN2O = mole of N2O in liquid phase at equilibrium, mol P = pressure, kPa PN2O = partial pressure of N2O, kPa PV = vapor pressure, kPa PT = equilibrium pressure, kPa R = gas constant, kPa L/mol K RA = rate of CO2 absorption, mol/s t = temperature, °C T = temperature, K TEA = triethanolamine Vg = volume of gas container, L Vl = volume of liquid in container, L ZN2O = compressibility factors
(14) Peng, D.; Robinson, D. B. A new two-constant equation of state. Ind. Eng. Chem. Fundam. 1976, 15, 59. (15) Al-Ghawas, H. A.; Ruiz-Ibanez, G.; Sandall, O. C. Absorption of carbonyl sulphide in aqueous methyldiethanolamine. Chem. Eng. Sci. 1989, 44, 631. (16) Aboudheir, A.; Tontiwachwuthikul, P.; Chakma, A.; Idem, R. A novel design for the nozzle of laminar jet absorber. Ind. Eng. Chem. Res. 2004, 43, 2568. (17) Wang, Y. W.; Xu, S.; Otto, F. D.; Mather, A. E. Solubility of N2O in alkanolamines and in mixed solvents. Chem. Eng. J. 1992, 48, 31. (18) Versteeg, G. F.; van Swaaij, W. P. M. Solubility and diffusivity of acid gases (CO2, N2O) in aqueous alkanolamine solutions. J. Chem. Eng. Data 1988, 33, 29. (19) Hartono, A.; Juliussen, O.; Svendsen, F. H. Solubility of N2O in aqueous solution of diethylenetriamine. J. Chem. Eng. Data 2008, 53, 2696. (20) Redlich, O.; Kister, A. T. Algebraic representation of thermodynamic properties and the classification of solutions. Ind. Eng. Chem. 1948, 40, 345. (21) Maneeintr, K.; Amr, H.; Idem, R. O.; Tontiwachwuthikul, P.; Wee, A. G. H. Physical and transport properties of aqueous amino alcohol solutions for CO2 capture from flue gas streams. Process Saf. Environ. Prot. 2008, 86, 291. (22) Tamimi, A.; Rinker, E. W.; Sandall, O. C. Diffusivity coefficient for hydrogen sulfide, carbon dioxide, and nitrous oxide in water over temperature range 293368 K. J. Chem. Eng. Data 1994, 39, 330. (23) Ko, J.; Tsai, T.; Lin, C.; Wang, H.; Li, M. Diffusivity of nitrous oxide in aqueous alkanolamine solutions. J. Chem. Eng. Data 2001, 46, 160.
Greek Letters
ϕDEAB = volume fraction of DEAB ϕH2O = volume fraction of water η = viscosity, Pa s
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