Energy Fuels 2010, 24, 2530–2535 Published on Web 03/26/2010
: DOI:10.1021/ef901446n
Thermodynamic Promotion of Tetrahydrofuran on Methane Separation from Low-Concentration Coal Mine Methane Based on Hydrate Baoyong Zhang* and Qiang Wu Department of Safety Engineering and Technology, Heilongjiang Institute of Science and Technology, Harbin 150027, China Received December 1, 2009. Revised Manuscript Received March 18, 2010
This paper proposed a new hydrate-based gas separation (HBGS) method especially for recovering methane from low-concentration coal mine methane (CMM). The main purpose of this study was to evaluate the thermodynamic promotion of tetrahydrofuran (THF) on methane recovery from lowconcentration CMM. Temperature and pressure conditions for incipient hydrate formation (vapor-aqueous liquid-hydrate) were closely examined at various different THF concentrations (1.00, 0.10, and 0.01 mol L-1) in gas hydrate systems (CH4-N2-O2-THF-H2O) and at various initial operating pressures (8, 10, and 14 MPa). Subsequently, the phase equilibrium parameters of hydrate formation for the CH4-N2-O2-H2O system were calculated using the Chen-Guo model and the Patel-Teja equation of state. The difference between experimental and calculated values indicates that the addition of THF significantly reduced the equilibrium formation conditions. The higher the THF concentration and the initial operation pressure, the more significant the promotion effect of THF. THF with a concentration of 1 mol L-1 was found to promote CH4 capture the best under an initial operation pressure of 14 MPa in the range of those examined. The present study illustrates the concept and provides thermodynamic data that will aid in the design and production of an effective hydrate-based methane recovery process.
low-concentration CMM to the atmosphere. Some gas separation techniques [i.e., membrane separation,3 pressureswing adsorption (PSA),4 and low-temperature liquefaction5] have been developed in last few decades. Although these methods have proven successful for the recovery of CH4 from CMM, they still have some problems associated with lowconcentration CMM: membrane separation, product gas loss and high cost; PSA, low recovery rate and safety problems; low-temperature liquefaction, strict operational requirements and high energy consumption. Accordingly, a new, more effective, and practical separation technique completely different from the existing conventional methods needs to be explored. A low-concentration CMM separation technique based on hydrate, as a new separation concept, was proposed by Wu and Zhang in 2005.6 They confirmed the thermodynamic and kinetic validities of the CH4 recovery from the low-concentration CMM, by performing the gas hydrate formation and dissociation experiments. Gas hydrates, the ice-like non-stochiometric crystalline solids, have some valuable characteristics, including mild pressure-temperature conditions for the formation of hydrate, high storage capacity, and safe storage. It was suggested to use these properties of gas hydrates in separating and concentrating noble gases and components of mixed gas
1. Introduction Coal mine methane (CMM), consisting mainly of methane (CH4), is not only one of the major disasters for coal mines but also a significant source of clean energy. Coal-related methane resources in China buried to a depth of 2000 m are over 34 trillion m3, 12.5% of the world’s total, ranking third in the world (China University of Petroleum).1 These factors forced the development of drainage technologies applied in Chinese coal mines. The technologies include in-mine boreholes in the same coal seams, adjacent seam CMM drainage, and in-mine gob boreholes. China has rapidly expanded its use of CMM drainage technology in the past decade. Drained and recovered CMM increased from less than 2 billion m3 in 1994 to 4.7 billion m3 in 2007.2 However, the concentration of methane in recovered CMM in many coal mines is low; CMM normally has a concentration of 10-35 mol % and a maximum concentration of 60 mol %. Over 70% of the recovered CMM in China has a concentration of less than 30 mol %.2 Methane is an explosive gas in the range of 5-15 mol %. Because of safety concerns, there are limited options to use low-concentration CMM (less than 30 mol % methane) globally, resulting in the direct emission of the majority of *To whom correspondence should be addressed. Telephone: 86-45188036392. E-mail:
[email protected]. (1) China University of Petroleum. Feasibility study of coal bed methane production in China. A technology report for the EU-China Energy and Environment Programme funded by the European Union and the National Development and Reform Commission of China (EuropeAid/120723/D/SV/CN), March 2008. (2) International Energy Agency. Coal mine methane in China: A budding asset with the potential to bloom. An assessment of technology, policy, and financial issues relating to CMM in China, based on interviews conducted at coal mines in Guizhou and Sichuan Provinces, February 2009. r 2010 American Chemical Society
(3) Fuertes, A. B. Preparation and characterization of adsorptionselective carbon membranes for gas separation. Adsorption 2001, 7, 117– 129. (4) Olajossy, A. Methane separation from coal mine methane gas by vacuum pressure swing adsorption. Chem. Eng. Res. Des. 2003, 81, 474– 482. (5) Wu, J.; Sun, Z.; Gong, M. Security technology of methane recovery from coalbed gas with oxygen. Nat. Gas Ind. 2009, 29 (2), 113–116. (6) Wu, Q.; Zhang, B. Experimental on mine gas separation based on hydration mechanism. J. China Coal Soc. 2009, 34 (3), 361–365.
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: DOI:10.1021/ef901446n
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Florusse et al.13 have on pure gases and N2/CO2 mixtures. reported the first result that the decomposition temperature of H2 hydrate can be increased substantially if the large cavities in the hydrate of the cubic structure II are filled with THF molecules, whereas H2 singly occupies the small cage of structure II H2/THF hydrate.14 H2 singly occupies the small cage of structure II H2/THF hydrate. Isothermal phase equilibria (pressure-composition in the gas phase) for the quaternary system of H2-CO2-THF-H2O have been measured in the presence of the gas hydrate phase.15 The lowest three-phase equilibrium pressure is obtained under the condition that the mole fraction of THF in water is 0.056. The Raman spectroscopy results show that the H2 and CO2 molecules competitively occupy the S cage of structure II (i.e., the H2 molecule is enclathrated in the hydrate cages with a small amount of THF at considerably low pressure). Vapor-hydrate equilibrium data for the CH4-H2-THFH2O system for the separation of hydrogen from the CH4-H2 feed mixture were obtained to check the single-stage separation efficiency with the presence of THF as a promoter.16 The previous research for simple or mixed hydrates of CH4, N2, H2, and THF have indicated a positive relationship between THF and hydrate-forming conditions. The present study is a part of an ongoing effort to document the importance of THF in application to the hydrate-based technology. The thermodynamic parameters of incipient hydrate formation for the CH4-N2-O2-THF-H2O system were carefully determined. The phase equilibrium parameters of gas hydrate formation for the CH4-N2-O2-H2O system were calculated by the Chen-Guo model. The thermodynamic effect of THF was evaluated by comparing the measured thermodynamic parameters (incipient hydrate formation conditions) for the CH4-N2-O2-THF-H2O system with the calculated phase equilibrium results for the CH4-N2-O2-H2O system. The optimal THF concentration and initial operation pressure were also investigated, and the low-concentration CMM separation technique was developed.
Figure 1. Principle diagram of low-concentration CMM hydration separation.
(i.e., N2-CO2,7 H2-CO2,8,9 and H2-CH410). The main components (CH4, N2, O2, etc.) of low-concentration CMM, which is known as a mixed gas, can form hydrates, although the phase equilibrium conditions are significantly different. For example, at constant temperature (273.15 K), the phase equilibrium pressures of CH4, N2, and O2 are 2.56, 14.3, and 11.0 MPa, respectively. Through pressure control, CH4 can be changed into hydrate and separated from CMM, while the other(s) are kept in gaseous form, as shown in Figure 1. In past few years, Wu and Zhang succeeded in separating the CMM with the CH4 concentration as low as 14.28 mol % by performing 37 group tests. This preliminary result indicated the feasibility of low-concentration CMM separation based on hydrate. The initial and critical step for the development of hydrate-based technology involves highspeed hydrate formation. The major difficulty to be overcome for high-speed hydrate formation is the promotion of thermodynamic equilibrium conditions. Tetrahydrofuran (THF), a hydrate promoter, can reduce the required hydrate formation pressure and enhance the corresponding kinetic rate. This has interested many scientists and engineers. In the past few decades, many hydrate-based experiments have been performed to evaluate the thermodynamic promotion of THF. Kang et al.7 studied the hydrate phase equilibrium of the gas mixture CO2-N2-THF at the temperature range of 272-275 K with different compositions of vapor phase. Their results indicated that the addition of a small amount of THF, which lowered the equilibrium pressure and raised the temperature, showed a significant hydrate promotion effect. Significant hydrate phase boundary shifts upon the addition of 1% THF have also been reported for experiments
2. Experimental Section 2.1. Materials. The AR-grade THF was obtained from Tianjin Kaitong Chemical Reagent Co., Ltd., China. CMM gas (26.00 mol % CH4, 59.10 mol % N2, and 14.90 mol % O2) was produced by Harbin Liming Gas Co., Ltd., China. THF and distilled water were weighed using a balance with a precision of 0.1 mg. 2.2. Apparatus. A schematic diagram of the experimental apparatus used in this work is illustrated in Figure 2. It consists of a high-pressure reactor, which is immersed in a temperaturecontrolled thermotank. The cylindrical 316 stainless-steel reactor has an inner diameter or 9.5 cm, length of 14.7 cm, and total available volume of about 1 L. The reactor can attain a maximum pressure of 30 MPa. The cell has two circular glass viewing windows on the front and back. A cold light illuminator, connected to one of the glass windows, provides ample light
(7) Kang, S.-P.; Lee, H. Recovery of CO2 from flue gas using gas hydrate. Environ. Sci. Technol. 2000, 34, 4397–4400. (8) Sugahara, T.; Murayama, S.; Hashimoto, S.; Ohgaki, K. Phase equilibria for H2 plus CO2 plus H2O system containing gas hydrates. Fluid Phase Equilib. 2005, 233 (2), 190–193. (9) Linga, P.; Kumar, R. N.; Englezos, P. Gas hydrate formation from hydrogen/carbon dioxide and nitrogen/carbon dioxide gas mixtures. Chem. Eng. Sci. 2007, 62 (16), 4268–4276. (10) Luo, Y.; Zhu, J.; Chen, G. Numerical simulation of separating gas mixtures via hydrate formation in bubble column. Chin. J. Chem. Eng. 2007, 15 (3), 345–352. (11) Linga, P.; Adeyemo, A.; Englezos, P. Medium-pressure clathrate hydrate/membrane hybrid process for postcombustion capture of carbon dioxide. Environ. Sci. Technol. 2008, 42 (1), 315–320. (12) Susilo, R.; Alavi, S.; Ripmeester, J.; Englezos, P. Tuning methane content in gas hydrates via thermodynamic modeling and molecular dynamics simulation. Fluid Phase Equilib. 2007, 263 (1), 6–17.
(13) Florusse, L. J.; Peters, C. J.; Schoonman, J.; Hester, K. C.; Koh, C. A.; Dec, S. F.; Marsh, K. N.; Sloan, E. D. Stable low-pressure hydrogen clusters stored in a binary clathrate hydrate. Science 2004, 306, 469–471. (14) Hester, K. C.; Strobel, T. A.; Sloan, E. D.; Koh, C. A.; Huq, A.; Schultz, A. J. Molecular hydrogen occupancy in binary THF-H2 clathrate hydrates by high resolution neutron diffraction. J. Phys. Chem. B 2006, 110 (29), 14024–14027. (15) Hashimoto, S.; Murayama, S.; Sugahara, T.; Ohgaki, K. Phase equilibria for H2 þ CO2 þ tetrahydrofuran þ water mixtures containing gas hydrates. J. Chem. Eng. Data 2006, 51 (5), 1884–1886. (16) Wang, X.; Sun, C.; Yang, L.; Ma, Q.; Tang, X.; Zhao, H.; Chen, G. Vapor-hydrate equilibria for the methane þ hydrogen þ tetrahydrofuran þ water system. J. Chem. Eng. Data 2009, 54, 310–313.
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Figure 2. Schematic diagram of natural gas hydrate (NGH) formation apparatus.
Aqueous solutions containing 1, 0.1, and 0.01 mol L-1 THF are used to form the mixed CH4, N2, and O2 hydrates. It is clear that the gas hydrate formed at 1.36-17.70 °C under pressure ranging from 7.24 to 13.70 MPa. The temperature of methane liquefaction is -161.5 °C at 1 atm pressure for low-temperature liquefaction separation. It is clear that the operating conditions of this new technique are milder. To verify the feasibility of the new hydrate-based separation method, the CH4 mole fraction in the hydrate phase was measured. Table 1 indicates that the CH4 mole fractions increase after separation. With regard to tests 13, 23, and 33, CH4 mole fractions increase by 15.76, 14.21, and 12.06% through the first separation stage, for THF concentrations of 1, 0.1, and 0.01 mol L-1, respectively. It can be concluded that the larger the THF concentrations, the larger the CH4 molar concentration in the hydrate phase. This result indicates that the new hydrate-based separation technique can effectively recover CH4 from low-concentration CMM. 3.2. Phase Equilibrium Parameters of Gas Hydrate Formation for the CH4-N2-O2-H2O System. The thermodynamic effect of THF can be evaluated by comparing the measured thermodynamic parameters (incipient hydrate formation conditions) for the CH4-N2-O2-THF-H2O system to the calculated phase equilibrium results for the CH4-N2O2-H2O system. At present, few published data or other information are available on the phase equilibrium of gas hydrate formation for the CH4-N2-O2-H2O system; however, at least four thermodynamic models can calculate the phase equilibrium conditions, including HWHYD, PVTSIM, Ng-Robinson model, and Chen-Guo model. In this study, we chose the Chen-Guo model17,18 and the Patel-Teja (PT) equation of state (EOS)19 to calculate the phase equilibrium conditions. Most of the existing thermodynamic models for predicting hydrate formation are various modifications of the van der Waals-Platteeuw model, which cannot reflect the instability of chemical composition for hydrate. The Chen-Guo model can explain the instability of chemical composition for
for the charge-coupled device (CCD) imaging system. The imaging system records the macroscopic phenomena of gas hydrate formation. A thermocouple and a pressure sensor are connected to the high-pressure cell to measure its internal temperature and pressure. The measurement range of the thermocouple lies between -10 and 60 °C, with an accuracy of (0.01 °C. The maximum pressure of the pressure sensor equals 40 MPa, with an accuracy of 0.3%. A data-logger and a personal computer are used to record the temperature at every 30 s. 2.3. Procedures. Each experimental run was performed according to the following sequence of steps. (1) Reagent input: the reactor was washed with distilled water, evacuated, and then purged with CMM in turn to ensure the reactor was thoroughly clean and air was absent. The THF solution of 500 mL was then pumped into the reactor using the liquid feed pump. CMM gas was introduced into the system rapidly from a gas feed cylinder to the desired value (8, 10, or 14 MPa). (2) Refrigeration: turn on the refrigeration system and set the thermotank temperature to the desired value with the rate of cooling of 1 °C/min. (3) Data recording: the system was then left waiting to establish thermal equilibrium and the nucleation of hydrate until a trace amount of hydrate crystal was observed in the gas/liquid interface. Afterward, the temperature T and pressure P in the reactor changing with time were recorded. The hydrate crystal growth and morphology changed, and a series of representative pictures were recorded by the CCD imaging system with a resolution of 1628 1236. (4) Determination of CH4 concentrations of the vapor phase: after an equilibrium state of three-phase coexistence was established, the equilibrium composition was measured. The vapor phase was analyzed at least 3 times by gas chromatography (the sample volume was 100 μL using the GC4000A gas chromatograph). The average concentration was then taken as an equilibrium vapor-phase composition. (5) Determination of CH4 concentrations of the hydrate phase: after the corresponding hydrate phase compositions were measured when the vapor-phase analysis was finished, the reactor was evacuated to ensure the gas was absent. The hydrate in the reactor was then dissociated by gradually increasing the temperature until vapor and water formed, and the corresponding vapor composition was analyzed by gas chromatography.
3. Results and Discussion (17) Chen, G. J.; Guo, T. M. Thermodynamic modeling of hydrate formation based on new concepts. Fluid Phase Equilib. 1996, 122 (1), 43– 65. (18) Chen, G. J.; Guo, T. M. A new approach to gas hydrate modeling. Chem. Eng. J. 1998, 71, 145–151. (19) Patel, N. C.; Teja, A. S. A new cubic equation of state for fluids and fluids mixtures. Chem. Eng. Sci. 1982, 37 (3), 463–473.
3.1. P-T Parameters for Incipient Hydrate Formation and CH4 Mole Fraction in the Hydrate Phase. Incipient hydrate formation parameters and CH4 mole fraction in the hydrate phase are determined for the CH4-N2-O2-THF-H2O system, and the overall results are summarized in Table 1. 2532
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: DOI:10.1021/ef901446n
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Table 1. Incipient Hydrate Formation Conditions for the CH4-N2-O2-THF-H2O System incipient gas hydrate formation conditions test ID 11 12 13 21 22 23 31 32 33
molar concentration of THF (mol L-1)
initial operation pressure (MPa)
temperature (°C)
pressure (MPa)
8.00 10.00 14.00 8.00 10.00 14.00 8.00 10.00 14.00
13.74 16.18 17.70 3.42 5.95 9.19 1.36 5.74 8.65
7.29 9.58 13.70 7.52 9.31 13.11 7.24 9.22 13.02
1.00 0.10 0.01
j
P
θj ¼
j
f j Cj P 1þ f j Cj j
ð2Þ
j
X
xi ¼ 1:0
ð3Þ
j
where the notations used in the above formula are listed in the Nomenclature and the PT EOS is used for calculating fi. The PT EOS can be summarized as follows: RT a ð4Þ P ¼ V - b VðV þ bÞ þ cðV - bÞ where a, b, and c are given by a ¼ Ωa RðRTc Þ2 =Pc
ð5Þ
b ¼ Ωb RTc =Pc
ð6Þ
c ¼ Ωc RTc =Pc
ð7Þ
41.76 40.21 38.06
when 1 mol L-1 THF is added in the aqueous solution. This implies that the aqueous solution containing THF can be used to form the mixed gas hydrate of CH4, N2, and O2 under more favorable conditions. As a result, the CH4-N2-O2THF system can possibly be applied for an industrial process without significant compression costs. The thermodynamic promotion of THF can be evaluated by the pressure difference between the phase equilibrium data for the CH4-N2-O2-H2O system and the experimental data of incipient hydrate formation conditions for the CH4-N2-O2-THF-H2O system. A larger pressure difference shows a larger thermodynamic promotion. To seek the optimal THF concentration with different initial operating pressures, the variations of pressure difference (THF effect) with different initial operating pressures were investigated, as shown in Figure 4. In this study, we find that THF with 1 mol L-1 has the optimal thermodynamic promotion effect under an initial operation pressure of 14 MPa among those examined in the range of 8-14 MPa. 3.4. Promotion Mechanism of THF. THF, which is a small hydrocarbon molecule and relatively volatile, can form the structure II hydrate,20 where two kinds of cavities exist: one is the basic cavity, also called the big cavity, which is a tetrakaidecahedron composed of 12 pentagons and 2 hexagons, and another is the linked cavity, called the small cavity, which is a dodecahedron composed of 12 pentagons. THF molecules uniformly dissolve in the water to form molecular clusters with the water molecules around them, and the molecular clusters link with each other to form the basic hydrates.21 Research22 indicates that, when the ratio of the gas molecule diameter to the cavity diameter is in the range of 0.76-1.00, CH4, N2, and O2 can be absorbed into the small cavities of the structure II hydrates and form the mixed hydrates. The higher concentration of THF solution results in more clusters formed by THF molecules with water molecules; therefore, more empty linked cavities will be occupied by small size molecules (CH4, N2, O2, etc.). The THF molecules just fully occupy the large cavities because of their proper sizes, and the small size molecules (CH4, N2, O2, etc.) dissolved in water may move into the empty linked cavities; thus, the stable hydrates are formed. The thermodynamic conditions vary in accordance with the different hydrate structures.22 The addition of THF alters the structures of the hydrates and then decreases the equilibrium
hydrate. In addition, Chen and Guo have proposed an alternative statistical mechanics-based hydrate model, and the mathematic expressions are simplified.17 The proposed hydrate model is adequate for engineering applications, especially for gas mixtures. The accuracy and reliability of the Chen-Guo model were verified in refs 17 and 18. The hydrate formation mechanism described in the Chen-Guo model is a two-step process: (1) a quasi-chemical reaction process to form basic hydrate and (2) an adsorption process of smaller gas molecules in the linked cavities of basic hydrate. The equations used in the model are as follows: X R θj Þ ð1Þ fi ¼ xi fi 0 ð1 -
X
mole fraction of CH4 in hydrate (mol %)
On the basis of the eqs 1-7 and the parameters of CH4, N2, and O2 seen in ref 17, the phase equilibrium parameters of the gas hydrate formation can be calculated for the CH4-N2-O2-H2O system. 3.3. Thermodynamic Promotion of THF. The phase equilibrium data for the CH4-N2-O2-H2O system are shown in Figure 3 along with the experimental data of incipient hydrate formation conditions for the CH4-N2-O2-THFH2O system. It is clear that the incipient hydrate formation pressure is notably less than the calculated phase equilibrium pressure and gradually decreases with an increasing amount of THF addition. The addition of a small amount of THF to water expands the hydrate stability region. For example, the CMM hydrate formation pressure under the gas compositions of 26.0 mol % CH4, 59.1 mol % N2, and 14.9 mol % O2 is abruptly shifted from about 50.8 to 13.7 MPa at 290.9 K
(20) Subramanian, S; Sloan, E. D. Molecular measurements of methane hydrate formation. Fluid Phase Equilib. 1999, 158 (1), 813–820. (21) Luo, Y. T.; Zhu, J. H.; Chen, G. J. Experimental studies and modeling of kinetics for methane hydrate formation with THF promoter in bubble column. J. Chem. Ind. Eng. 2006, 57 (5), 1153–1158. (22) Sloan, E. D.; Koh., C. A. Clathrate Hydrate of Natural Gases; Taylor and Francis Group, LLC: New York, 2008; pp 45-80.
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Figure 3. Comparison of the experimental and calculated values of the gas hydrate formation pressure in three systems: (a) 1 mol L-1 THF, (b) 0.1 mol L-1 THF, and (c) 0.01 mol L-1 THF.
formation conditions were largely shifted to lower pressures by adding a small amount of THF. The higher the THF concentration, the more notable the thermodynamic promotion of THF. Accordingly, THF is confirmed to act as a hydrate promoter and extend the hydrate stability region. This type of hydrate promoter can play an important role in changing high-pressure conditions to milder ones, which provides a great advantage when it is applied to the real hydrate-based process. This study can also be applied in such fields as carbon dioxide separation, hydrogen energy storage, and gas solidification.
pressure. The use of THF is very attractive for the development of an economic CMM separation process.
Acknowledgment. Our heartfelt thanks go to Professor E. Dendy Sloan, Jr., Professor Guang-jin Chen, Professor Qinglan Ma, and the editor for their help in experiments, modeling, and editing. The authors also thank the four reviewers for their critical comments that helped improve the manuscript. Additionally, we gratefully acknowledge the financial support provided by the National Natural Science Foundation (50904026 and 50874040), Provincial Natural Science Foundation of Heilongjiang (B200710), and Innovation Talent of Science and Technology Foundation of Harbin (2008RFQXG111).
4. Conclusions
Nomenclature
Figure 4. Variations of pressure difference (THF effect) with different initial operation pressures.
fi =fugacity of gas component i fi0 = fugacity of the gas phase in equilibrium with the unfilled basic hydrate i
The thermodynamic promotion of THF on low-concentration CMM hydration separation was investigated on the basis of calculated and experimental results. The hydrate 2534
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Cj =Langmuir constant of gas component j xi =mole fraction of basic hydrate i θj=fraction of linked cavities occupied by gas component j P=pressure Pc =critical pressure R=universal gas constant
T=thermodynamic temparature Tc =critical temparature V=molar volume a, b, and c=parameters in PT EOS Ωa, Ωb, and Ωc =constants in EOS R=parameter in the three-parameter γ function
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