Article pubs.acs.org/IECR
CO2 Solubility in a Mixture Absorption System of 2‑Amino-2-methyl1-propanol with Ethylene Glycol C. Zheng, J. Tan, Y. J. Wang, and G. S. Luo* The State Key Lab of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China S Supporting Information *
ABSTRACT: A mixture system of sterically hindered amine 2-amino-2-methyl-1-propanol (AMP) with ethylene glycol (EG) was developed. The solubility of CO2 in mixture solutions of AMP−EG was measured at temperatures from 303.2 to 353.2 K and partial pressures of CO2 from 5 to 120 kPa. The concentration of AMP ranged from 0.4 to 1.0 mol/L. A model based on a zwitterion mechanism concerned with parameters independent of temperature and ion strength was derived, which could fit the experimental data very well. The calculated heat of absorption of CO2 in AMP−EG solution showed a decline compared to that in AMP−H2O solution. Comparing the solubilities of CO2 in different solvents with the same chemical absorber AMP, we found that the solvent has a significant influence on the solubility of CO2. Two possible aspects make contributions to the influence: deprotonation process and ion strength. The study of the influence of the solvent provides guidance for a reasonable selection and good design of the absorption process for CO2. Compared to AMP−DEG and AMP−TEG solutions, the loading of CO2 in AMP−EG solution is higher especially at lower partial pressure of CO2; the viscosity of AMP−EG solution is also lower. Compared to AMP−H2O solution, AMP−EG solution has higher circular loading of CO2 while the temperature changes especially at lower desorption temperature.
1. INTRODUCTION Carbon capture and storage (CCS) is one of the key topics in the field of environmental science and energy. Carbon dioxide (CO2), mainly from combustion of fossil fuels, accounts for the largest amount of greenhouse gases emitted to the atmosphere.1 Postcombustion capture (PCC) is proposed to be more suitable for CO2 capture for the existing power-generation facilities which produce most of the CO2.2 Solvent absorption, solid sorbent adsorption, membrane separation, and cryogenic fractionation are four developed PCC technologies.3 Because of the low concentration of CO2 (∼14% by volume) in the flue gas from coal-fired power plants,4 amine-based solution absorption, mainly represented by monoethanolamine (MEA)-based aqueous solution, has been well studied.5 CO2 is also a component in natural gas, biogas, and landfill gas.6 For the reduction of the calorific value brought about by the presence of CO2, the concentration of CO2 in natural gas as fuel is required to be less than 2%.7 Powerful chemical absorbents are also suited for the separation of CO2 in natural gas; the key problem is energy consumption. “Next-generation” CCS technologies such as ionic liquids, metal−organic frameworks, and chemical looping have received more and more attention in recent years aiming at lower costs of energy and payment.2 The traditional amine-based absorption process (especially for aqueous MEA solution) has some disadvantages, such as the evaporation of water and the high heat of absorption, which lead to an increasing energy consumption, and the high temperature of stripping, which leads to the degradation of products.5,8 Despite these disadvantages, the method of advanced amine solvent is also a forthcoming technology.9 In our previous work,8,10 mixture absorption systems of amines and glycols have been studied. For the mixture system, a © 2013 American Chemical Society
solvent of diethylene glycol (DEG) or triethylene glycol (TEG) was used to avoid evaporation and the sterically hindered amine 2-amino-2-methyl-1-propanol (AMP) suggested by Sartori and Savage11 was used to reduce the chemical reaction heat between CO2 and amine. The absorption system is promised to have lower energy consumption. For a reasonable selection of the solvent and good design of the absorption process, more glycols as solvents must be studied and their performances should be compared. Therefore, the aim of this work is to determine the absorption performance of the AMP−EG system for CO2. A model was developed to fit the experimental data. Meanwhile, an overview and a comparison among AMP−water, AMP−EG, AMP−DEG, and AMP−TEG absorption systems were carried out.
2. EXPERIMENTAL SECTION 2.1. Materials. Analytically pure ethylene glycol (EG) was purchased from Beijing Modern Oriental Fine Chemistry Co., Ltd. Analytically pure 2-amino-2-methyl-1-propanol (AMP) was purchased from Shanghai Aladin. Carbon dioxide (CO2) and nitrogen (N2; 99.995 mol %) were purchased from Beijing Huayuan Gas Chemical Industry Co., Ltd. The mixture solutions of AMP and EG were compounded at 25 °C. 2.2. Experimental Setup. Figure 1 shows the experimental setup, which consists of four parts: transportation (1−4), reaction (5−8), separation (9−10), and measurement (8, 11). The transportation part includes an advection pump with a measurement accuracy of ±1% used to pump the liquid Received: Revised: Accepted: Published: 12247
June 7, 2013 July 21, 2013 July 30, 2013 July 30, 2013 dx.doi.org/10.1021/ie401805n | Ind. Eng. Chem. Res. 2013, 52, 12247−12252
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Figure 1. Experimental setup.
Figure 2. Solubility of CO2 in AMP−EG solution at different concentrations of AMP (points, experimental data; lines, model correlation results). (a, top left) CAMP = 0.400 mol/L; (b, top right) CAMP = 0.600 mol/L; (c, bottom) CAMP = 1.00 mol/L.
solution and two mass flow meters with a measurement accuracy of ±1.5% used to deliver a mixture gas of N2 and CO2. The reaction part includes a water bath with a resolution of ±0.1 K used to maintain the temperature, a back-pressure valve used to maintain the pressure, a membrane dispersion microcontactor,13 and a capillary tube long enough to provide enough residence time to ensure the reaction reaches equilibrium. The separation part includes a phase separator and a reservoir. The measurement part includes a pressure sensor with an accuracy of ±0.5% set in the middle of the capillary tube and before the back-pressure valve to determine the total pressure and a gas chromatograph (GC) (Tianmei, 7890II) using the method of a thermal conductivity detector (TCD) applied to measure the composition of gas samples online. The accuracy of the measurements by GC was ±0.5%. The method and reliability of measurement were explained in detail in our previous work.8,10,13 The maximum relative standard deviation of CO2 solubility is 4.33%.
3. RESULTS AND DISCUSSION 3.1. Solubility of CO2 in AMP−EG Solution. The solubility of CO 2 in AMP−EG solution at different concentrations of AMP is shown in Figure 2. The loading of CO2 increases with the decrease of temperature and the increase of partial pressure of CO2 in the gas phase at different concentrations of AMP. The solubility of CO2 increases significantly with the increase of AMP concentration. The solubility of CO2 in AMP−EG solution is sensitive to both temperature and partial pressure of CO2. 3.2. Modeling of CO 2 Absorption in AMP−EG Solution. Three hypotheses are made for the reactive absorption of CO2 in AMP−EG solution: (1) N2 is insoluble in AMP−EG solution; (2) EG is nonvolatile at experimental temperature; (3) the absorption reaction occurs only in the liquid phase. A Henry’s law relation is used to describe the physical absorption of CO2. The equilibrium solubility is proportional to the partial pressure of CO2 in the vapor phase. CO2 (g) ↔ CO2 (l) 12248
(1)
dx.doi.org/10.1021/ie401805n | Ind. Eng. Chem. Res. 2013, 52, 12247−12252
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Table 1. Results of Parameters with Confidence Intervals
H=
no. of points
A
B
C × 10−2
D
114
−34.83 ± 0.827
9543 ± 321
−4096 ± 270
1.711 ± 0.135
pCO
A combined Henry’s law and chemical equilibrium constant is expressed by
2
[CO2 (l)]
(2)
B C + 2 θ + DcAMP,0θ (9) T T cAMP,0 represents the total concentration of AMP, and θ represents the loading of CO2 (mol of CO2/mol of initial amine). The three adjustable parameters A, B, and C were suggested by Gabrielsen.16 A and B represent the standard temperature dependence of the chemical equilibrium constant; C represents the dependence of temperature and the loading of CO2, which itself depends on temperature, pressure, and absorbent composition. The adjustable parameter D suggested by Astarita17 represents the ionic strength dependence in a nonideal system. In the nonaqueous solution of AMP−EG, the ionic strength mainly depends on [RNHCOO−] and [BH+]. 3.3. Parameter Regression. The equilibrium constant, Kexp, could be calculated with the experimental data from this work within the range of loading. For AMP−EG solution at concentrations of 0.40−1.0 mol/L, the value of CO2 absorbed in EG physically cannot be ignored. The calculation consists of the following expressions:
14
ln K = A +
The zwitterion mechanism suggested by Caplow and reintroduced by Danckwerts15 is used to describe the reaction between CO2 and AMP: CO2 (l) + RNH 2 ↔ RNH 2+COO−
K1 =
(3)
[RNH 2+COO−] [CO2 (l)][RNH 2]
(4)
In which RNH2 represents AMP here. The zwitterion is then deprotonated by bases existing in the solution to produce a carbamate ion and a protonated base (here “B” represents EG): RNH 2+COO− + B ↔ RNHCOO− + BH+ [RNHCOO−][BH+] [RNH 2+COO−][B]
K2 =
(5)
(6)
In this study, experimental data show that it has a loading of CO2 (mol of CO2/mol of AMP) from 0.30 to 0.85, so that the molar ratio of the chemical reaction between AMP and CO2 could be 1:1. From eqs 1, 3, and 5, the absorption of CO2 in AMP−EG solution could be expressed by a single equilibrium: CO2 (g) + RNH 2 + B ↔ RNHCOO− + BH+ K=
[CO2 (l)] =
Kexp =
⎛ ⎜C − ⎝ CO2
[RNH 2] = cAMP,0 − [RNHCOO ]
pCO
2,exp
H
(
⎛ pCO ,exp ⎜cAMP,0 − CCO2 + 2 ⎝
CCO2 represents the total amount of CO2 absorbed by solution (mol/L). [B]0 represents the initial concentration of EG (mol/ L). [B] represents the concentration of EG in solution (mol/ L). We use the method of a modified Marquardt routine to estimate the parameters A−D. ⎛ ⎛K ⎞⎞ calc, i ⎟ ⎟⎟ ⎟ ⎝ ⎝ Kexp , i ⎠⎠
∑ ⎜⎜ln⎜⎜
(11) −
(8)
2
(10)
[RNHCOO−] = [BH+] = CCO2 − [CO2 (l)]
(7)
= [B]0 − [B]
[RNHCOO−][BH+] pCO [RNH 2][B]
Obj =
pCO ⎛ M ⎞ 2 ⎜⎜1 − cAMP,0 AMP ⎟⎟ H ⎝ ρAMP ⎠
2
M
1 − cAMP,0 ρAMP AMP
pCO
2,exp
H
(1 − c
(12)
⎞2 ⎟ ⎠
)
MAMP AMP,0 ρ AMP
)⎞⎠[B] ⎟
(13)
⎡ ∂ ⎛ ΔG ⎞⎤ ΔH ⎟⎥ = − ⎢ ⎝⎜ ⎣ ∂T T ⎠⎦ p T2
(15)
⎛ θ⎞ ΔHCO2 = R ⎜ −9543 + 819200 ⎟ ⎝ T⎠
(16)
The heat of absorption of CO2 in AMP−EG solution could be calculated by eq 16. A comparison of the calculated heat of CO2−AMP−EG, the calculated heat of CO2−AMP−H2O derived by Park et al.,18 and the experimental heat of CO2− AMP−H2O measured by Arcis et al.19 is shown in Figure 3. The comparison shows that the heat of absorption decreases as the loading of CO2 increases. It also indicates that the absorption heat of the CO2−AMP−EG system is lower than that of the CO2−AMP−H2O system. 3.4. Comparison of Different Solvents. Using the model derived in this work, we could predict the solubility of CO2 at higher concentrations of AMP−EG solution. We could also use the model of the CO2−AMP−H2O system derived by
(14)
The results of the parameters with confidence intervals are shown in Table 1. Using these parameters, we could calculate the solubility of CO2 in AMP−EG solution at different temperatures, partial pressures of CO2, and AMP concentrations. The comparison between experimental data (points) and model correlation results (lines) is shown in Figure 2. They are in good agreement. Using the Gibbs−Helmholtz equation, we could predict the heat of absorption of CO2 in AMP−EG solution: 12249
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Figure 5. Physical absorption of CO2 in different solvents.8,10,20
Figure 3. Comparison of absorption heats for different systems.
Gabrielsen et al.16 and the model of the CO2−AMP−DEG (TEG) system derived in our previous work10 to predict the solubility of CO2 at the same conditions with the CO2−AMP− EG system. Figure 4 shows the comparison of different absorption systems. At a concentration of 2.0 mol of AMP/L of solution, the solubility of CO2 at different temperatures and different partial pressures of CO2 is calculated. The solubility of CO2 in different solutions decreases following the order AMP− water > AMP−EG > AMP−DEG > AMP−TEG. The comparison shows that the solvent makes a contribution to the loading of CO2. Figure 5 shows that the solubility of CO2 in pure solvent decreases following the order TEG ≈ DEG > EG > H2O; for the nonpolar CO2 molecule, this order indicates that the polarity of the solvent decreases in the order H2O > EG > DEG ≈ TEG. For the same absorbent AMP, chemical reaction between CO2 and AMP makes no difference in the loading of CO2. The solvent mainly has an influence on the deprotonation process and ionic strength, which leads to different reactions except for the reaction between CO2 and AMP. For AMP−H2O system, the deprotonation process consists of the following equations: RNH 2+COO− + H 2O → RNHCOO− + H3O+
RNHCOO− + H 2O ↔ RNH 2 + HCO3−
(18)
RNH 2 + H3O+ ↔ RNH3+ + H 2O
(19)
In a polar solvent as H2O, there are more kinds of ions, which could promote the process of deprotonation. As a result, the solubility of CO2 in aqueous solution is higher than that in nonaqueous solution. For the nonaqueous system AMP−EG, the EG molecule reacts with the zwitterion to produce a carbamate ion and a protonated base as in eq 5. All the zwitterions turn to carbamate. For nonaqueous systems AMP− DEG and AMP−TEG, because of a lower concentration of hydroxyl and lower polarity, the production of ions would be restrained, so that a partial deprotonation model was derived to describe the experimental data. The molecular weight of DEG is lower than that of TEG, so the molecular concentration of DEG is higher than that of TEG, which promotes the deprotonation process from eq 5. Physical property data of solvents are listed in Table 2. The solubility of CO2 in AMP−DEG or AMP−TEG solution is lower compared with that in AMP−H2O or AMP−EG solution, especially at low partial pressures of CO2. The low solubility of CO2 leads to a low CO2 recovery at the same concentration and flow rate in the process of absorption. The
(17)
Figure 4. Comparison of different absorption systems (2 mol/L). 12250
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Table 2. Physical Property Data of Solvent mol weight sp gravity (20/20 °C) boiling point at 101.3 kPa (°C) vapor press. at 101.3 kPa, 20 °C (Pa)
H2O
EG
DEG
TEG
18.02 1.00 100.0 2338
62.07 1.12 197.3 7.40
106.1 1.12 244.8 1.33
150.2 1.13 289.4 AMP−EG > AMP−DEG > AMP−TEG. Two possible aspects make a contribution to the influence: the deprotonation process and the ion strength. Compared to AMP−DEG and AMP−TEG solutions, AMP−EG solution has a higher loading of CO2 especially at lower partial pressures of CO2; its viscosity is also lower because the viscosity of EG is lower as shown in Figure 6.12 The viscosities of these solvents
Figure 6. Viscosities of pure glycols.
decrease significantly as the temperature rises. Compared to AMP−H2O solution, AMP−EG solution has a higher circular loading of CO2 while the temperature changes. The study of the influence of solvent provides guidance for a reasonable selection and good design of the absorption process for CO2 qualitatively and quantitatively. We will study the kinetic characteristics of AMP in glycols in our further work.
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Article
ASSOCIATED CONTENT
S Supporting Information *
Tables including loadings of CO2 in AMP−EG solution and solubility of CO2 in different solutions at 313.2, 333.2, and 353.2 K. This material is available free of charge via the Internet at http://pubs.acs.org. 12251
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(18) Park, S. H.; Lee, K. B.; Hyun, J. C.; Kim, S. H. Correlation and prediction of the solubility of carbon dioxide in aqueous alkanolamine and mixed alkanolamine solutions. Ind. Eng. Chem. Res. 2002, 41, 1658. (19) Arcis, H.; Rodier, L.; Coxam, J. Y. Enthalpy of solution of CO2 in aqueous solutions of 2-amino-2-methyl-1-propanol. J. Chem. Thermodyn. 2007, 39, 878. (20) Li, M. H.; Chang, B. C. Solubilities of carbon dioxide in water + monoethanolamine + 2-amino-2-methyl-1-propanol. J. Chem. Eng. Data 1994, 39, 448.
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