ARTICLE pubs.acs.org/IECR
Experimental Study on the Separation of CH4 and N2 via Hydrate Formation in TBAB Solution Qiang Sun,† Xuqiang Guo,†,* Aixian Liu,† Bei Liu,† Yusheng Huo,‡ and Guangyin Chen† † ‡
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China, Henan Zhong Yuan Green Energy High-Tech Co., Ltd., Puyang 457000, Henan, China ABSTRACT: In order to reduce the emission of coal bed methane mixed with air (can be regarded as a CH4 and N2 mixture), which will be helpful to adequately utilize the natural resources and protect environment, the separation of CH4 and N2 via hydrate formation in tetra-n-butylammonium bromide (TBAB) solution was systematically studied in this work. The CH4-N2 hydrate formation conditions were determined in TBAB solution first, and then the separation experiments were carried out in TBAB and TBAB-sodium dodecyl sulfate (SDS) solution, respectively. The experimental results show that CH4 and N2 form a hydrate much easier after adding TBAB to water. The composition of CH4 in the hydrate after single-stage equilibrium separation in TBAB solution can be increased from 46.25 mol % to 67.86 mol %. At the same conditions, the composition of CH4 after separation in TBAB-SDS solution is 68.66 mol % and the reaction time shortens greatly. Besides, the recovery of CH4 is more than 47%, and the gas storage capacity of hydrate is 19-21 m3/m3. Higher composition and recovery of CH4 are expected to be obtained if multistage separation is applied. It indicates that CH4 can be concentrated effectively from CH4 and N2 via hydrate formation in TBAB solution. Since the hydrate separation technology can substantially avoid the explosion problem caused by CH4 and on the basis of the results obtained in this work, we may say that this technology is quite suitable for the separation of coal bed methane mixed with air and has broad prospects for industrial applications.
1. INTRODUCTION Coal bed gas, also known as coal bed methane (CBM), is widely generated and stored in coal seams. The main component of CBM is methane (CH4), so CBM is commonly considered as a clean and efficient source of energy. China possesses abundant CBM resources with a total amount of 31 1012 m3 within 2000 m underneath the ground.1 There are two ways to exploit CBM,2 and the most widely applied method is the collection of CBM through the extraction system of coal mines. Thus, most exploited CBM has a low concentration of CH4 mixed with air, which presents difficulties and risks to the technology of CH4 processing and transport. Currently, CBM is usually released into the atmosphere. Not only are the natural resources of CBM enormously wasted, but air environment is polluted greatly.3 CH4 is a kind of greenhouse gas, its greenhouse effect is 21 times stronger than that of CO2, and it contributes to 18% of the global greenhouse effect.4 The volume of CBM discharged from China is up to 194 108 m3 per year, which ranks first in the world.5 Therefore, the government has already issued related rules to resolve the problem.6 Therefore, it is very necessary to study the separation and recycling of CH4 from CBM mixed with air. At normal temperature and pressure, the explosion range of CH4 in air is 5 mol % to 15 mol %, and the explosion range can expand at high temperature and pressure.7 The potential risk restricts the application of conventional separation methods, such as pressure swing adsorption (PSA) and membrane separation, to separate the CBM and air mixture. During the separation process by these conventional methods, the concentration of CH4 in feed gas or product gas is certain within the explosion range. Once a spark or hotspot appears, a dangerous situation exists. Therefore, we need an alterative safe and efficient way to separate the CBM r 2010 American Chemical Society
Table 1. Different Gas Hydrate Formation Pressures at 273.15 K
formation pressures (MPa)
CH4
N2
O2
2.56
14.30
11.10
and air mixture, and we think that the separation method via hydrate formation8 can meet this purpose effectively. Hydrate is a cage-like crystalline compound formed at low temperature and high pressure by water and small molecular weight gas.9 Different conditions are required for different gases to form hydrates. So long as favorable temperatures and pressures are maintained, some kinds of gas will form hydrates first, and then the gas mixture will be separated by solid-gas two-phase separation. Table 1 shows the hydrate formation conditions of the main components of the CBM and air mixture at 273.15 K.10 From Table 1 we can see that CH4 forms hydrate much easier than N2 and O2 at the same temperature. Hence, the CBM and air mixture can be separated readily via hydrate formation in theory. Because the separation process is conducted in contact with water and the gas humidity after separation is very high, security will be definitely ensured.11 A low boiling point gas mixture could be separated via hydrate formation within a range 273.15-293.15 K.12 Theoretically, the hydrate separation technology consumes less energy than cryogenic separation, loses less gas pressure than PSA, and costs less than membrane separation. Considering all the advantages of the hydrate separation techReceived: August 15, 2010 Accepted: December 10, 2010 Revised: November 3, 2010 Published: December 31, 2010 2284
dx.doi.org/10.1021/ie101726f | Ind. Eng. Chem. Res. 2011, 50, 2284–2288
Industrial & Engineering Chemistry Research
ARTICLE
Table 2. Purity and Suppliers of Experimental Materials materials
purity
suppliers
CH4
99.99 mol %
Beijing AP Beifen Gases Industry Co., Ltd., China
N2
99.999 mol %
Beijing AP Beifen Gases Industry Co., Ltd., China
TBAB
industrial grade
Shenglong Fine Chemical Co., Ltd., Dangshan, China
deionized water
15 106 Ω 3 cm
water distillation unit (SZ-93, Shanghai Yarong Biochemistry Instrument Factory, China)
Table 3. CH4-N2 Hydrate Formation Conditions in Pure Water Predicted by the Chen-Guo Model
formation
274.15 K
276.15 K
278.15 K
280.15 K
282.15 K
5.79
7.08
8.62
10.55
12.85
pressures (MPa)
nology, we think it is quite suitable to use it for the separation of CBM mixed with air, and this technology does have wide application prospects.
2. EXPERIMENTAL WORK CBM mixed with air can be regarded as a mixture of CH4, N2, and O2. The concentration of O2 is relatively low. Our previous experiments indicate the separation property of the O2 hydrate is close to that of N2, so the CBM and air mixture can be considered as a CH4 and N2 system.13 The representative concentration of CH4 in CBM mixed with air is 30 mol % to 50 mol %.14 The gas composition in this work is 46.25 mol % CH4 and 53.75 mol % N2. The compositions of the gas phase were measured by a gas chromatograph (HP6890). The purity and suppliers of experimental materials are given in Table 2. The hydrate formation conditions of the experimental gas in pure water were predicted by the Chen-Guo hydrate model.15 The model has been widely accepted for its high precision. The calculated results are shown in Table 3. From Table 3 we can see that the formation pressures of the gas hydrate are relatively high. Therefore, to realize the separation of CH4 and N2 in pure water, more expensive reaction equipment with more strict safety standards will be required, which would increase the cost of production and impede industrial application of the method. Thus, an appropriate hydrate thermodynamics accelerator should be used to reduce the formation pressure of CH4-N2 hydrate. It is well-known that tetra-n-butylammonium bromide (TBAB) can reduce the gas hydrate formation pressures greatly16 by forming a semicage hydrate with water. Especially, it is suitable for separating and recovering gases such as H2S and CH4 via hydrate formation. Therefore, TBAB was chosen in our experiments as the hydrate thermodynamics accelerator. As used by Li17 et al., 0.29 mol % TBAB was adopted in this work. Although Li17 et al. chose the concentration of TBAB for a CO2 system, we think that this concentration is also suitable for a CH4 system because both CO2 and CH4 form structure I hydrates in pure water and structure ΙΙ hydrates in tetrahydrofuran (THF) solution. Therefore, they both should also form hydrates with the same kind of structure in TBAB solution. Such low concentrations of TBAB enable a relatively high gas storage capacity of hydrate,18 low cost of production, and good environmental protection,19 which are beneficial to the industrial application. 2.1. Experimental Apparatus. Figure 1 shows the schematic sketch of the experimental apparatus. It is mainly composed of four sections: a reactor, a hand-pump, an air bath, and a data
Figure 1. Schematic sketch of the experimental apparatus: (1) gas cylinder; (2) pressure sensor; (3) air bath; (4) hand-pump; (5) piston; (6) reactor; (7) glass window; (8) temperature sensor; (9) magnetic stirrer; (10) gas inlet/outlet; (11) liquid inlet/outlet.
acquisition system. The reactor is made of stainless steel. The maximum working volume of it is 420 mL, and the designed maximum working pressure is 20 MPa. It includes two sections. The lower section can be seen through two lathy glass viewing windows. The upper section equips with a piston, which is connected to the hand-pump. The piston can move up and down to control the pressure in the reactor by turning round the pump. The air bath (CW-YF-1) is manufactured by Shanghai Experimental Apparatus General Factory. It besieges the whole reactor to maintain a constant temperature ranging from 243.15 to 323.15 K. The temperature inside the reactor is measured by a platinum resistance thermometer. The accuracy of temperature is within (0.1 K. The pressure of the gas in the reactor is measured by a pressure sensor with a deviation less than 0.01 MPa. All these numerical values are displayed in the data acquisition system. Besides, a magnetic stirrer is fixed at the bottom of the reactor to constantly stir the solution mixture. 2.2. Formation Conditions of the CH4 and N2 Gas Mixture Hydrate. First, the formation conditions of the CH4 hydrate in pure water were measured and compared with literature data to inspect and verify the accuracy of the experimental apparatus and method adopted in this work. We found the relative error is less than 0.2%, and it indicates that the results in this work are reliable. The method called “pressure search”20 was used to measure the formation conditions of the CH4 and N2 mixed gas hydrate in TBAB solution. After the reactor was purged thoroughly, an appropriate amount of TBAB solution and CH4þN2 gas mixture were introduced into it. The air bath and stirrer were turned on to adjust the reactor temperature to the desired experimental value. By turning the hand-pump, hydrate formed quickly at high pressure and decomposed totally at low pressure. After a trace of 2285
dx.doi.org/10.1021/ie101726f |Ind. Eng. Chem. Res. 2011, 50, 2284–2288
Industrial & Engineering Chemistry Research
ARTICLE
hydrate (only several crystal particles could be seen) formed again by slowly increasing the gas pressure, the pressure and temperature of the system at that moment should remain unchanged. If the trace of hydrate crystals could exist for 4 h, the current pressure and temperature could be regarded as one set of hydrate formation data. If the trace of hydrate crystals totally disappeared in 4 h, then the gas pressure should be increased again until the trace of hydrate crystals regenerated and coexisted with the gas mixture for at least 4 h. The hydrate formation pressure at current temperature was then finally obtained. In the same way, different formation pressures of the CH4 and N2 mixed gas hydrate in TBAB solution were obtained at different temperatures. 2.3. Separation of the CH4 and N2 Gas Mixture via Hydrate Formation. Sodium dodecyl sulfate (SDS) is a sort of anionic surfactant and a hydrate kinetic accelerator with good performance.21 SDS can accelerate the reaction speed and shorten the reaction time. Rogers22 proposed that when the concentration of SDS solution is more than 242 mg/L, gas hydrates will form quickly. SDS solution with a concentration of 500 mg/L was tested in this work to investigate the promotion effect of SDS on the hydrate separation process. Separation of CH4 and N2 via hydrate formation was studied in TBAB and TBAB-SDS solution, respectively. An appropriate amount of TBAB solution was filled into the reactor after it was purged thoroughly. When the reactor temperature reached the desired experimental value by adjusting the air bath temperature, CH4 and N2 mixed gas was introduced Table 4. Formation Conditions of CH4-N2-TBAB Hydrate
formation pressure (MPa)
276.15 K
278.15 K
280.15 K
282.15 K
0.51
0.88
1.32
1.83
into reactor until the gas pressure reached the experimental value. Then the stirrer was turned on and the reaction began. The reaction time was recorded synchronously. While the reaction came to equilibrium over about 8-10 h, the gas composition was detected by gas chromatography. Then the reactor temperature was reduced to 268.15 K to keep the hydrate phase stable, and the equilibrium gas was released to atmosphere pressure. It is believed that when the temperature is less than or equal to 268.15 K at atmospheric pressure, the CH4 hydrate dissociates very slowly, at least for a while.23 Arguably, the transitory evacuation process of the gas phase would not make the composition of hydrate phase change. By increasing the reactor temperature, the hydrate thawed completely, and the gas composition after separation was detected by gas chromatography. Finally, the TBAB solution was replaced by TBAB-SDS solution and the separation of CH4 and N2 via hydrate formation was investigated in the same
3. RESULTS AND DISCUSSION 3.1. Formation Conditions of the CH4-N2-TBAB Hydrate. The formation conditions of the CH4-N2-TBAB hydrate were measured and are shown in Table 4. To investigate the effect of TBAB solution on hydrate formation, we compared the data in Table 4 with the data in Table 3 (the formation conditions of the CH4-N2 hydrate in pure water calculated with the Chen-Guo hydrate model15), and the results are given in Figure 2. Figure 2 shows that the formation pressure of the CH4-N2-TBAB hydrate is much lower than that of the CH4-N2 hydrate at the same temperature. This suggests that the separation of the CH4 and N2 mixed gas via hydrate formation can be realized at relatively low pressures in TBAB solution, which is beneficial for the industrial application of the technology. 3.2. Separation of the CH4 and N2 Gas Mixture via Hydrate Formation. In this work, the separation experiments were carried out at 274.15 K and 0.8 MPa, and the gas to liquid ratio is 60:1. We chose 0.8 MPa (two times of that of hydrate forhyphen-qj;mation at the same temperature) to supply an impetus for the experiments and realize separation of CH4 and N2 more effectively because the reaction rate will be increased at high pressures. However, the reaction pressure should not be too high for the consideration of safety and the potential industrial applications. The results of separating CH4 and N2 via hydrate formation are shown in Tables 5 and 6. The recovery or split fraction (S.Fr.)24,25 of CH4 in the hydrate phase and the separation factor (S.F.)24,25 are calculated as follows.
S:Fr: ¼
S:F: ¼
Figure 2. Hydrate formation conditions of the CH4 and N2 mixed gas in TBAB and water solutions.
nH CH4 nfeed CH4
G nH CH4 nN2 G H nN2 nCH4
ð1Þ
ð2Þ
Table 5. Experimental Results of Separating CH4 and N2 via Hydrate Formation in TBAB Solution composition (mol %) components
feed gas
residual gas
CH4
46.25
30.99
N2
53.75
69.01
hydrate
gas storage capacity of hydrate (m3/m3)
recovery or S.Fr. of CH4 (%)
S.F.
67.86
20.74
50.72
4.70
32.14 2286
dx.doi.org/10.1021/ie101726f |Ind. Eng. Chem. Res. 2011, 50, 2284–2288
Industrial & Engineering Chemistry Research
ARTICLE
Table 6. Experimental Results of Separating CH4 and N2 via Hydrate Formation in TBAB-SDS Solution composition (mol %) components
feed gas
residual gas
CH4
46.25
32.24
N2
53.75
67.76
hydrate
gas storage capacity of hydrate (m3/m3)
recovery or S.Fr. of CH4 (%)
S.F.
68.66
19.04
47.11
4.60
31.34
Figure 3. Reaction processes of separating CH4 and N2 via hydrate formation in different solutions.
where nCH4feed is defined as the numbers of moles of CH4 in the feed gas, nCH4H and nN2H are the numbers of moles of CH4 and N2 in the hydrate phase at the end of the experiments, and nCH4G and nN2G are the numbers of moles of CH4 and N2 in the residual gas phase at the end of the experiments. The experimental data show that after single-stage equilibrium separation in TBAB solution, the composition of CH4 in the hydrate phase increases from 46.25 mol % to 67.86 mol %. At the same reaction conditions, the composition of CH4 in the hydrate phase increases to 68.66 mol % in TBAB-SDS solution. In theory, the compositions of CH4 in the hydrate phase with TBAB and TBAB-SDS should be same; however, we find a 0.8 mol % difference between them because in an 8-10 h separation process, the presence of SDS on the gas-liquid contact can adsorb relatively more CH4 into the hydrate phase. If the reaction time is infinite, the compositions of CH4 in the solution with and without SDS should be strictly same. The gas storage capacity of hydrate in this work is 19-21 m3/m3, less than the results obtained by N. H. Duc26 because CH4 requires a higher pressure than that of CO2 to form hydrate under the same conditions. The recovery or split fraction of CH4 we obtained is more than 47.11%, and the separation factor is 4.60-4.70. These results demonstrate that CH4 can be effectively concentrated from the CH4 and N2 mixed gas via hydrate formation. It is certain that if a multistage separation is applied, the composition of CH4 in the hydrate phase, the recovery rate of CH4, and the gas storage capacity of the hydrate will be much higher. In separation experiments, except for the points mentioned above, the reaction speed/reaction time is another area that definitely needs to be considered. We found that the main reaction time is within 4 h in TBAB-SDS solution in this work, about 3 h less than that in TBAB solution, as shown in Figure 3. This illustrates that SDS can boost the reaction speed and shorten the reaction time greatly, which can improve the production efficiency and benefits industrial applications.
4. CONCLUSIONS The formation conditions of the CH4-N2-TBAB hydrate were measured in this work. The results show that TBAB can reduce the formation pressure of gas hydrate, which enables the separation of CH4 and N2 mixed gas via hydrate formation at relatively low pressures and makes industrial application of the technology possible. The separation experiment results show that CH4 can be concentrated from CH4 and N2 mixed gas via hydrate formation. After adding SDS to the TBAB solution, we can speed up the reaction greatly and the reaction time is shortened. In addition, the present work shows that the composition of CH4 after single-stage equilibrium separation is 68.66 mol % with a relatively good recovery of CH4. As more separation stages are applied, higher composition and recovery of CH4 will be obtained. The separation method proposed in this work, i.e., via hydrate formation in TBAB solution with SDS, provides an effective and safe way for separation of CH4 from CBM mixed with air. The data and knowledge obtained are expected to apply to real applications for the separation of CBM mixed with air via hydrate formation on an industrial scale. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors thank the China National Natural Science Foundation Council for Grants 20676146, China National Science and Technology Plan 2006AA09A208, 2009CB219504, and Research Fund for the Doctoral Program of Higher Education 20070425001. ’ REFERENCES (1) Boyer, C. M.; Bai, Q. Z. Methodology of coal-bed methane resource assessment. Int. J. Coal Geol. 1998, 35, 349–368. (2) Zhu, Z. M.; Shen, B.; Jiang, G. Present situation and development direction of exploitation and utilization of coalbed methane. Multipurp. Util. Miner. Resour. 2006, 6, 40–42. (3) Zhang, B. Y.; Wu, Q.; Zhu, Y. M. Effect of THF on the thermodynamics of low-concentration gas hydrate formation. J. China Univ. Min. Technol. 2009, 38, 203–208. (4) Badr, O.; Probert, S. D.; O’Callaghan, P. W. Methane: A greenhouse gas in the Earth’s atmosphere. Appl. Energy 1992, 41, 95–113. (5) Pilcher, R. C.; Cote, M. M.; Collings, R. C.; et al. Recent trends in recovery and use of coal mine methane. In The 3rd international conference of pollution-reducing technology of methane and nitrous oxide. China National Coal Association: Bejing, 2003, pp 5-12. (6) Emission standard of coalbed methane/coal mine gas (on trial). General Administration of Quality Supervision, Inspection and Quarantine (AQSIQ). GB 21552-2008, 2008. (7) Zou, R. X.; Bao, Z. R.; Lin, D. J. The explosion experiment of methane and air. J. Chem. Educ. 1986, 1, 10–12. 2287
dx.doi.org/10.1021/ie101726f |Ind. Eng. Chem. Res. 2011, 50, 2284–2288
Industrial & Engineering Chemistry Research
ARTICLE
(8) Fan, S. S.; Cheng, H. Y.; Chen, G. J.; et al. Separation technique based on gas hydrate formation. Modern Chem. Ind. 1999, 19, 11–14. (9) Englezos, P. Clathrate hydrates. Ind. Eng. Chem. Res. 1993, 32, 1251–1253. (10) Qiu, J. H.; Chen, Z. H. Present situation and prospect of clathrate hydrates and hydrate technology. Jiangsu Chem. Ind. 2005, 33, 1–4. (11) Ballard, A. L.; Sloan, E. D. Hydrate separation process for closeboiling compounds. Proceedings of the 4th International Conference on Gas Hydrate, 2002, pp 19-23. (12) Li, C. B.; Wu, G. D.; Jing, F. Q. Experimental investigation and numerical computation of methane combustion and explosion suppressed by vapor. China Saf. Sci. J. 2009, 19, 118–124. (13) Zhao, J. Z.; Zhao, Y. S.; Shi, D. X. Experiment on methane concentration from oxygen-containing coal bed gas by THF solution hydrate formation. J. China Coal Soc. 2008, 33, 1419–1424. (14) Zheng, Z.; Wang, S. L. Separation technology for coalbed methane-air mixture based on hydrate. J. Filtr. Sep. 2008, 18, 5–9. (15) Chen, G. J.; Guo, T. M. Thermodynamic modeling of hydrate formation based on new concepts. Fluid Phase Equilib. 1996, 14, 43–65. (16) Kamata, Y.; Yamakoshi, Y.; Ebinuma, T.; et al. Hydrogen sulfide separation using tetra-n-butyl ammonium bromide semi-clathrate (TBAB) hydrate. Energy Fuels 2005, 19, 1717–1722. (17) Lu, T.; Zhang, Y.; Li, X. S.; et al. Equilibrium conditions of hydrate formation in the systems of CO2-N2-TBAB and CO2-N2-THF. Chin. J. Process Eng. 2009, 9, 541–544. (18) Li, X. S.; Xu, C. G.; Chen, Z. Y.; et al. Tetra-n-butyl ammonium bromide semi-clathrate hydrate process for post-combustion capture of carbon dioxide in the presence of dodecyl trimethyl ammonium chloride. Energy 2010, 35, 3902–3908. (19) Lin, W.; Delahaye, A.; Fournaison, L. Phase equilibrium and dissociation enthalpy for semi-clathrate hydrate of CO2þTBAB. Fluid Phase Equilib. 2008, 264, 220–227. (20) Sun, Z. G.; Shi, L.; Fan, S. S.; et al. Study of the measuring methods of gas hydrate phase equilibrium. Chem. Eng. Oil Gas 2001, 30, 164–166. (21) Link, D. D.; Edward, P. L.; Heather, A. E. Formation and dissociation studies for optimizing the uptake of methane by methane hydrates. Fluid Phase Equilib. 2003, 211, 1–10. (22) Zhong, Y.; Rogers, R. E. Surfactant effect on gas hydrate formation. Chem. Eng. Sci. 2000, 55, 4175–4187. (23) Tang, J. F.; Li, X. G.; Li, Y. X.; et al. Experimental study on natural gas hydrate stability. Nat. Gas Ind. 2008, 25, 125–128. (24) Linga, P.; Kumar, R.; Englezos, P. The clathrate hydrate process for post and pre-combustion capture of carbon dioxide. J. Hazard. Mater. 2007, 149, 625–629. (25) Linga, P.; Adeyemo, A.; Englezos, P. Medium-pressure clathrate hydrate/membrane hybrid process for postcombustion capture of carbon dioxide. Environ. Sci. Technol. 2007, 42, 315–320. (26) Duc, N. H.; Chauvy, F.; Herri, J. M. CO2 capture by hydrate crystallization - A potential solution for gas emission of steelmaking industry. Energy Convers. Manage. 2007, 48, 1313–1322.
2288
dx.doi.org/10.1021/ie101726f |Ind. Eng. Chem. Res. 2011, 50, 2284–2288
