Article pubs.acs.org/IECR
CO2 Solubility in a Mixture Absorption System of 2‑Amino-2-methyl-1-propanol with 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, People’s Republic of China ABSTRACT: A mixture system of sterically hindered amine 2-amino-2-methyl-1-propanol (AMP) with glycol was developed as the working system to reduce energy consumption. The solubility of CO2 in the mixture solutions of AMP−diethylene glycol (DEG) and AMP−triethylene glycol (TEG) was determined by a fast measurement method. The results show that the physical absorption can be enhanced using TEG or DEG as a solvent. The heat of absorption of the CO2−AMP−glycol system ranges from 50 kJ/(mol CO2) to 62 kJ/(mol CO2) as the loading of CO2 (mol CO2/mol AMP) ranges from 0.2 to 0.8, which is much lower than that between monoethanolamine (MEA) and CO2. The absorption performance of the mixture system is sensitive both to temperature and pressure, which is different from the process with MEA as an absorbent. A reaction assumption of AMP with CO2 in DEG or TEG was suggested and a model for predicting the solubility of CO2 is developed accordingly. The prediction results show that the model can predict the absorption performance well. Comparing the MEA−TEG and AMP− TEG systems, we found that the AMP−TEG system would have much lower energy consumption in the CO2 absorption− regeneration process than the MEA−TEG absorption system.
1. INTRODUCTION CO2 capture and storage (CCS) is becoming a global engineering project, because CO2 makes the largest contribution to greenhouse gases.1−3 Chemical absorption is generally considered as a reliable technology for CO2 capture. Aqueous solutions of alkanolamines have been used for CO2 capture since the 1930s.8 Monoethanolamine (MEA), as a primary amine, is widely used in many industrial processes such as postcombustion capture in the CO2 capture process from a power plant4−7 for its rapid reaction rate with CO2. However, the heat of absorption is ∼88 kJ/mol CO2, which means that the regeneration of MEA requires a lot of energy and high temperature. Some studies also show that the degradation rate of MEA increases as temperature rises.9−12 Therefore, it is highly required to develop new chemical absorption systems. One alternative is to use new solvents with low volatility to prevent the excessive energy consumption caused by solvent evaporation. Tan et al.13 developed a mixture system of MEA and triethylene glycol (TEG) for CO2 capture. It has been proved that TEG as solvent results in low energy consumption with nearly no solvent evaporation and avoidance of thermal degradation. Recently, the potential use of ionic liquids (ILs) for CO2 capture has received more and more attention.14−20 Imidazolium-based ILs,15,16 functional ILs based on chemical absorption,17−19 and hydroxylammonium-based ILs20 have been suggested. Another way to reduce the energy consumption is to use new absorbents by decreasing the chemical reaction heat. A group of amines called sterically hindered amines was suggested by Sartori and Savage.21 2-Amino-2-methyl-1-propanol (AMP) is a typical form of sterically hindered primary alkanolamine. Its amino group is attached to a tertiary carbon atom. Two additional methyls compared with MEA occupy the space around the amino group; therefore, sterically hindered alkanolamines do not tend to form stable carbamate with CO2.22 Yamada et al.23 © 2012 American Chemical Society
studied CO2 absorption in the aqueous solution of AMP using density functional theory. Arcis et al.24 determined the absorption heat of CO2 with AMP in the aqueous solution. Their results show that the reaction heat is much lower than that of MEA with CO2. On the basis of previous work,13 this work develops a mixture chemical absorption system for CO2 absorption. The objective of this study is to reduce the energy consumption further in the CO2 capture process. The mixture system mainly contains AMP and glycol. AMP is used to reduce the reaction heat. Diethylene glycol (DEG) or TEG is used to reduce evaporation of solvent. The solubility of CO2 in different absorption systems such as AMP−TEG and AMP−DEG is determined. A comparison among different absorption systems is presented. The interaction between physical and chemical absorption processes and the influence of solvent on the solubility of CO2 are discussed. A model to predict the absorption capability of the mixture system is developed.
2. EXPERIMENTAL SECTION 2.1. Materials. Triethylene glycol (TEG), ethylene glycol (EG), and diethylene glycol (DEG), supplied by Tianjin Guangfu Fine Chemical Research Institute, were reagent-grade with a mass fraction purity of ≥0.99. 2-Amino-2-methyl-1propanol (AMP), supplied by Shanghai Aladin, was reagentgrade with a mass fraction purity of ≥0.97. Carbon dioxide (CO2) and nitrogen (N2) (at 99.995 mol %) was supplied by Beijing Huayuan Gas Chemical Industry Co., Ltd. 2.2. Determination of Solubility of CO2. A rapid measurement method using a membrane dispersion microcontactor Received: Revised: Accepted: Published: 11236
March 17, 2012 July 9, 2012 August 2, 2012 August 2, 2012 dx.doi.org/10.1021/ie3007165 | Ind. Eng. Chem. Res. 2012, 51, 11236−11244
Industrial & Engineering Chemistry Research
Article
Figure 1. The experimental setup.13
Figure 2. Solubility of CO2 in (a) ethylene glycol (EG) and (b) diethylene glycol (DEG).
CO2 in pure DEG and EG was determined first and the results were compared with the literature results,25,26,29 which are shown as lines in Figures 2a and 2b. The two figures also show the solubility of CO2 in pure DEG and EG, respectively, which indicate that the solubility is proportional to the partial pressure of CO2 in the vapor phase, consistent with Henry’s Law very well. The measured Henry coefficients in this work are in good agreement with published data with a relative error of AMP−TEG
• Figure 5a shows the linear fit of ln H vs 1/T, ΔH = −11.82 kJ/(mol CO2) • Figure 5b shows the linear fit of ln K1 vs 1/T, ΔH1 = −26.74 kJ/(mol CO2) • Figure 5c shows the linear fit of ln K2 vs 1/T, ΔH2 = −39.83 kJ/(mol CO2) Equations 1 and 3 could occur for every mol of CO2, and eq 5 could occur partially. The overall heat of absorption can be expressed by [NHCOO−] ΔH2 ΔCCO2
T (K)
Using K1 and K2 determined by the experimental data, we could calculate the solubility of CO2 in the AMP−TEG solution at different concentrations and partial pressures of CO2. The predicted results are in good agreement with the experimental data, as shown in Figure 7a−d. 3.4. Comparison of the Absorption Abilities of Different Working Systems. Using this model, we can predict the solubility of CO2 in the working systems of AMP−TEG and AMP−DEG, while the solubility of CO2 in the AMP−H2O solution can be calculated using Park’s model.27 All predicted results for different systems are shown in Figures 8a and 8b. By comparing the solubility of CO2 in these systems, we noticed considerable differences among the three systems. The order of the solubility of CO2 in these systems is
Assuming that K is independent of pressure, the equation is given below:
ΔHoverall = ΔH + ΔH1 +
PCO2 (kPa)
It clearly shows that solvent has an obvious influence on the CO2 loading. The most possible reason is the effect of solvent on the deprotonation process, which leads to different reaction mechanisms. We also calculated the solubility of CO2 at different concentrations of AMP, and the results are shown in Figures 9a and 9b. The results show that the absorption process is controlled by chemical absorption at high AMP concentration and pressure. However, at low AMP concentration and pressure, it is more likely controlled by physical absorption. It will be very helpful for the regeneration process. Table 5 shows Table 5. Comparison of CO2 Solubility in AMP−TEG and MEA−TEGa
(17)
We measured the absorption heat of CO2 in the AMP−DEG solution directly, which gives that ΔH value of −60.98 ± 1.40 kJ/(mol CO2) at a CO2 loading (mol CO2/mol AMP) of 0.3, in comparison with the absorption heat of CO2 in the AMP− H2O solution, which gives that ΔH = −70.32 ± 2.51 kJ/(mol CO2) under the same condition. Arcis has reported the heat of absorption of the CO2−AMP−H2O system; the heat of absorption vs the loading of CO2 (mol CO2/mol AMP) derived from our mechanism model is shown in Figure 6. More details and experimental data of absorption heat will be shown in our next work. 3.3. Absorption of CO2 in the AMP−TEG Solution. The solubility of CO2 in the AMP−TEG solution was determined using the same method, and the results are shown in Figure 7a−d. The absorption characteristics of the AMP−TEG solution are similar to those of the AMP-DEG system. The experimental data are shown in detail in Table 3. Using the same method, we could get the value of K1 and K2 in the CO2−AMP−TEG system. The result is shown in Table 4.
loading of CO2 (mol CO2/L) AMP−TEG MEA−TEG
T = 313.2 K
T = 333.2 K
T = 353.2 K
0.611 0.927
0.288 0.774
0.129 0.491
a
Data taken from ref 13. The partial pressure of CO2 is 20 kPa; the concentration of amine is 2 mol/L.
the comparison of CO2 solubility in the solutions of AMP− TEG and MEA−TEG at the same partial pressure of CO2. At a constant partial pressure of 20 kPa and AMP concentration of 2 mol/L, the circular CO2 loading between 313.2 K and 353.2 K is ∼0.482 mol CO2/L AMP−TEG, which is slightly higher than 0.436 mol CO2/L MEA−TEG under the same operating conditions.13 Using the solubility data of CO2 in the MEA−TEG solution,13 we could make a comparison between MEA and AMP at different partial pressures of CO2; this is shown in Figure 10. We notice that the solubility of CO2 in the AMP−TEG solution 11242
dx.doi.org/10.1021/ie3007165 | Ind. Eng. Chem. Res. 2012, 51, 11236−11244
Industrial & Engineering Chemistry Research
Article
Figure 8. Comparison of solubility in different systems: (a) T = 313.2 K and (b) T = 333.2 K.
Figure 9. prediction of CO2 solubility in the AMP−TEG solution at higher concentrations of AMP.
the MEA−TEG solution, the value is only 0.224 mol CO2/L MEA−TEG solution. As a result, absorbing CO2 at a higher pressure and desorbing CO2 at a lower pressure, using AMP as an absorbent, will have better performance than using MEA as an absorbent.
4. CONCLUSIONS The solubility of CO2 in pure diethylene glycol (DEG) and ethylene glycol (EG) was determined, and the solubility of CO2 in pure triethylene glycol (TEG) or DEG is significantly larger than that in water (H2O). These observations indicate that using TEG or DEG as the solvent for CO2 absorption can enhance the physical absorption. The solubility of CO2 in an AMP−glycol solution (where AMP is 2-amino-2-methyl-1propanol) was determined, and the results show that the mixture system is sensitive to both temperature and pressure. An assumed reaction between CO2 and AMP with a molar ratio of 1:1 is suggested. A prediction model based on this reaction assumption has been developed, and the model could predict the solubility very well. The heat of absorption of the CO2− AMP−DEG system ranges from 50 kJ/(mol CO2) to 62 kJ/ (mol CO2) as the loading of CO2 (mol CO2/mol AMP) ranges from 0.2 to 0.8, which is much lower than that between monoethanolamine (MEA) and CO2. It indicates that the chemical bond between CO2 and AMP is weaker than that between CO2 and MEA. Comparing the AMP−TEG system with the MEA−TEG system, using AMP−TEG for CO2 absorption could have much lower energy consumption in the CO2 capture process. For the mixture system in this work, using TEG as a solvent to avoid the evaporation of the solvent, the absorption system could significantly decrease the energy cost; using AMP as an absorbent to weaken the chemical bond between CO2 and alkanolamine, the system could decrease the energy cost further in the regeneration process.
Figure 10. Comparison of CO2 solubility in the AMP−TEG and MEA−TEG solutions.
Table 6. Comparison of CO2 Solubilities in AMP−TEG, AMP−H2O, and MEA−TEGa loading of CO2 (mol CO2/L)
AMP− H2O AMP− TEG MEA− TEG a
PCO2 = 10 kPa
PCO2 = 20 kPa
PCO2 = 60 kPa
PCO2 = 100 kPa
0.998
1.201
1.533
1.698
0.168
0.288
0.636
0.871
1.114
0.692
0.774
0.876
0.916
0.954
PCO2 = 160 kPa
The temperature is 333.2 K; the concentration of amine is 2 mol/L.
is more sensitive to the partial pressure of CO2 than that in the MEA−TEG solution. Table 6 shows a comparison of CO2 solubilities in the solutions among AMP−TEG, AMP−H2O, and MEA−TEG. While the partial pressure of CO2 changes from 100 kPa to 10 kPa, the circular loading of CO2 in the AMP−TEG solution is 0.703 mol CO2/L AMP−TEG solution; in the AMP−H2O solution, it is 0.700 mol CO2/L AMP−H2O solution; and in 11243
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We found that the AMP−TEG system has lower CO2 loading at low CO2 partial pressure, compared to the MEA− TEG system. That may cause some problems in absorption processes with low partial pressures of CO2, such as postcombustion capture; the partial pressure of CO2 is ∼15 kPa in the post-combustion effluent. If we increase the pressure by compressing the gas, an extra energy consumption will be needed. This absorption system also has a disadvantage of its higher viscosity, which would bring a higher pressure drop. Of course, further research must be done to allow the new system to be used in industrial processes. In our future work, we will measure the enthalpy of solution of CO2 in AMP− glycol solutions to verify the model. We will also study on the dynamic characteristics of the CO2−AMP−Gly system.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86-10-62783870. Fax: +86-10-62783870. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We would like to acknowledge the support of the National Natural Science Foundation of China (No. 21036002), Chinese National Programs for High Technology Research and Development (No. 2008AA062301), and SRFDP (No. 20090002110070) for this work.
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REFERENCES
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