Experiment on the Separation of Air-Mixed Coal Bed Methane in THF

May 30, 2012 - CBM is the main cause of underground gas explosion accidents in the coal .... temperature of the reactor is measured by platinum resist...
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Experiment on the Separation of Air-Mixed Coal Bed Methane in THF Solution by Hydrate Formation Qiang Sun,† Xuqiang Guo,*,† Aixian Liu,† Jin Dong,‡ Bei Liu,† Jingwen Zhang,§ and Guangjin Chen† †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China CNOOC Research Institute, Beijing 100027, China ‡ Department of Chemical Engineering and Material Science, University of Minnesota, Minneapolis 55455, United States §

ABSTRACT: Because of the technology limitation and safety requirement, a large amount of air-mixed coal bed methane is released directly to the atmosphere. It leads to severe waste of resources and exacerbates the greenhouse effect. This work tries to alleviate this problem by separating CH4 from a mixture of CH4 and N2 via hydrate separation method. Several experiments were systematically carried out in 6 mol % tetrahydrofuran (THF) solution to separate CH4/N2 gas mixtures containing 4.90 to 71.23 mol % CH4. The results that THF can significantly reduce the formation pressure of CH4−N2 hydrate materialize the possibility of separating CH4 and N2 using hydrate separation technology in industrial scale. CH4 can be separated effectively from CH4/N2 mixture and concentrated in hydrate phase with the presence of THF. The recovery of CH4 ranges from 34.06% to 58.16% and the separation factor is between 2.29 and 5.17. A two-stage separation process of 46.28 mol % CH4/53.72 mol % N2 with recycle is designed to increase the recovery of CH4. The calculated results indicate that CH4 concentration in hydrate gas could reach 82.61 mol % with a recovery yield of 47.28%. Hydrate separation technology is an effective way to process and utilize the airmixed coal bed methane. Multistage separation process does have wide application prospects.

1. INTRODUCTION Coal bed methane (CBM) is an unconventional natural gas resource associated with coal.1 It is a kind of clean energy with a total amount of 240 × 1012 m3 all over the world.2 However, CBM is the main cause of underground gas explosion accidents in the coal mining, as well.3 It is very important for us to utilize this natural resource adequately and properly. Currently, most extracted CBM is mixed with air because of the exploitation technology limitations and reservoir-forming conditions. This CBM has a relatively low content of CH4, usually ranging from 10 to 70 mol %. The potential explosion after mixing CH4 with air results in tremendous difficulty and risk to process and separate the CBM−air mixture.4−7 Consequently, this kind of air-mixed CBM is generally vented to the air directly.8 Being one type of the greenhouse gases, CH4 is accounted for up to 18% of the global greenhouse effect.9 The global annual discharge of CH4 from CBM alone is as high as 554 × 108 m3.10 It leads to severe environmental pollution and serious waste of resources. Therefore, research on the technology to process and utilize these air-mixed CBM is significant and demanding. It could efficiently mitigate the problem of global warming and utilize natural resources more effectively. So far, hydrate separation method11 has been reported to be one of the most appropriate approaches to separate CH4 from air-mixed CBM.12 Hydrates are nonstoichiometric compounds formed by small molecular gas and water under favorable conditions of temperatures and pressures.13,14 Hydrate separation technology is based on the differences of compositions in residual gas phase and solid (hydrate) phase after the mixed gas hydrates formation.15,16 With advantages such as mild experimental conditions, concise technological process, and low energy consumption,17,18 hydrate separation technology has attracted great attention over the past few © 2012 American Chemical Society

decades and has been widely applied to separate various gas mixtures. For example, Kang16 and Seo19 separated CO2 from fuel gas via hydrates formation. Linga and Englezos20,21 also successfully captured CO2 from gas mixture using the same method. Since the hydration reactions and subsequent separation processes are conducted with the presence of water, the humid condition can effectively prevent CH4 from explosion.22 Thus, hydrate separation technology is very suitable to separate CBM−air mixtures.23 Though the actual air-mixed CBM contains a mixture of CH4, N2, and O2, the O2 content is far less than CH4 and N2 content. Furthermore, the O2 hydrate formation condition is very close to that of N2. Thus, air-mixed CBM could be treated as a mixture containing only CH4 and N2.4 Roberts24 and Jhaveri25 found that, at 273.15 K, the formation pressures of CH4−H2O and N2−H2O hydrates are 2.63 and 14.26 MPa, respectively. These relatively high formation pressures demand a high level of safety and cost in the experimental setup and the separation process, restricting the extensive application of hydrate separation technology. Besides, high pressure also increases the explosion range of CH4. Therefore, it is necessary to use appropriate hydrate thermodynamics promoter to decrease the formation pressure of CH4 and N2 hydrate. Tetrahydrofuran (THF) is an answer among numerous alternatives. It can significantly reduce the formation pressure of gas hydrates.26 For example, the formation pressure of CH4 hydrate in 6 mol % THF solution is reduced to 0.13 MPa at 277.7 K27 from 2.63 MPa at 273.15 K and that of N2 hydrate in 5 mol % THF solution is 2.46 MPa at 284.75 K,28 as compared Received: November 30, 2011 Revised: April 22, 2012 Published: May 30, 2012 4507

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parts. The lower part has two rectangle viewing windows so we can see the inside of the reactor. The upper part contains a piston. The piston can move up and down to control the gas pressure by turning the hand pump, which transfers the pressurized liquid (28 wt % ethylene glycol solution) between the hand pump and piston. Gas pressure in the reactor is determined by a pressure sensor with accuracy of ±0.01 MPa. The air bath (CW-YF-1, manufactured by Shanghai experimental apparatus general factory) keeps the temperature inside the reactor constant, with the accessible window from 243.15 to 323.15 K. The temperature of the reactor is measured by platinum resistance thermometer with accuracy of ±0.1 K. A magnetic stirrer is fixed at the bottom of the reactor to constantly stir the liquid. Stirring speed is controlled by adjusting the voltage regulator. The vacuum pump is used to remove air from apparatus and suck liquid into the reactor. 2.2. Methods. CH4−N2−THF hydrate formation conditions were determined with constant temperature method.32 The clean hydration reactor was filled with an appropriate amount of 6 mol % THF solution and CH4−N2 gas mixture. The air bath and magnetic stirrer were then set up to keep the reaction mixture at uniform concentration and desired temperature. Pressures were controlled by the hand pump. First, high pressure was applied for the gas hydrate to form for the first time, which quickly decomposed completely at low pressure. In the next step, pressure was increased slowly until a trace of hydrate (i.e., several crystal particles) was observed again, and the pressure and temperature were kept constant. If the trace of hydrate could exist for a long time, between 4 and 6 h,32 the combination of pressure and temperature was considered to be one set of hydrate formation data. If the trace of hydrate completely disappeared within 4 h, the gas pressure should be slowly increased again until the trace of hydrate regenerated and coexisted with the gas mixture for at least 4 h. The new combination of pressure and temperature was then obtained. A serious of hydrate formation conditions of CH4−N2−THF were obtained after varying temperatures of the reaction mixtures. The experiment on the single-stage separation of CH4/N2 mixture was carried out in 6 mol % THF solution under constant pressure and temperature. The reactor was cleaned by deionized water, rinsed with THF solution, and left under vacuum for 10 min to remove any air and impurities from the pipeline and reactor. Then, an appropriate amount of THF solution, determined by gas to liquid ratio, was filled into the reactor. The air bath was launched to keep reactor temperature reached at desired experimental value. CH4 and N2 gas mixture was then introduced into the reactor until the gas pressure reached the desired value. The magnetic stirrer was turned on and used to homogenize the reaction mixture to start the hydration reaction immediately. Reaction time was recorded synchronously from this point. As the reaction progressed, the gas pressure decreased. The hand pump should be rotated at once to maintain constant pressure in the reactor. The pump reading was also recorded with time to monitor the pressure loss over time. The hydration reaction is considered to reach or be close to equilibrium after the gas pressure in the reactor remains unchanged for about 1 h. The composition of residual gas phase was analyzed by gas chromatography. Then, the reactor temperature was reduced to 268.15 K, and the residual gas in reactor was released in a short time. It has been reported that, when the temperature is 268.15 K at atmospheric pressure, CH4 hydrate dissociates very slowly.33 This means that the hydrate phase composition will not change after the momentary release of the residual gas in our experiments. Subsequently, the reactor temperature was increased to 293.15 K. After the solid hydrate dissociated totally, the reaction system reached a gas−liquid equilibrium, and the gas composition was analyzed again using gas chromatography. The gas− liquid equilibrium data were calculated with Hysys to investigate the influence of gas solubility on hydrate gas composition. A series of separation experiments on CH4 and N2 mixtures were conducted under different temperatures and pressures. The multistage separation process of CH4 and N2 was designed and simulated in this work to improve the efficiency of separation. It is actually a series of interrelated single-stage separation processes. After the first single-stage separation, the hydrate gas composition was measured, and it was used as the feed gas in the second stage

to 14.26 MPa at 273.15 K. The presence of THF enables the separation of CH4 and N2 to be completed at much lower pressure. The price of THF (industrial grade, ≥99.5%) is only about 25 yuan/kg.29 Compared to the high cost of experimental equipments and energy consumption at high pressure, THF is definitely a great alternative to make the separation of CH4 and N2 more feasible and reduce the cost of separation process. This is also the reason that THF was included in this work. It has been reported that THF, gas molecules, and water molecules form structure II (SII) hydrate with a ratio of 1:2:17 (8THF·16Gas·136H2O),30 so the optimal concentration of THF solution is 5.56 mol %. Our previous experimental results demonstrated that the formation pressure of gas hydrate in 6 mol % THF solution is lower than that in 5 mol %.31 Consequently, 6 mol % THF solution is used in this work, taking into consideration of factors such as safety and economics.

2. EXPERIMENTAL SECTION 2.1. Materials and Apparatus. Table 1 lists the specifications of the experimental materials. THF and deionized water were weighed

Table 1. Specifications of Experimental Gas and Reagents materials

purity

suppliers

CH4 N2 THF

99.99 mol % 99.999 mol % analytical reagent

deionized water

15 × 106 Ω·cm

Beijing Bei temperature gas factory Beijing Bei temperature gas factory Beijing Modern Eastern Fine Chemicals Co., Ltd. water distillation unit (SZ-93, Shanghai Yarong Biochemistry Instrument Factory, China)

using an electronic balance (SL502N, made in Shanghai Minqiao Precise Science Instrument Co., Ltd.) with accuracy of ±0.01 g. The gas compositions were determined by gas chromatograph (Agilent 7890). A schematic sketch of the experimental setup is illustrated in Figure 1. It is mainly composed of the following sections: a high-pressure hydration reactor, a hand-pump, an air bath, a stirring system, and a vacuum pump. The maximum volume of the hydration reactor is 420 mL, and the designed working pressure is 20 MPa. It consists of two

Figure 1. Schematic diagram of the experimental apparatus: 1, gas cylinder; 2, vacuum pump; 3, air bath; 4, hydration reactor; 5, piston; 6, glass window; 7, magnetic stirrer; 8, temperature sensor; 9, pressure sensor; 10, experimental data acquisition system; 11, hand pump; 12, gas outlet; 13, liquid inlet/outlet. 4508

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separation. Then, the single-stage separation process was repeated again, and the composition of hydrate gas after several stages separation was obtained and analyzed subsequently.

will come to gas−liquid equilibrium state, and the gas pressure generally ranges from 0.3 to 0.7 MPa. To investigate the influence of gas solubility on the hydrate gas composition, we applied Hysys to calculate the gas−liquid equilibrium results, as shown in Figure 3. It shows equilibrium of 46.28% CH4/ 53.72% N2 in 6 mol % THF solution under 293.15 K and 0.7 MPa. The normalized composition and yield of equilibrium gas phase are provided in Table 3. It also shows the results of the same gas−liquid system under 293.15 K and 0.3 MPa. We can see that the gas solubility of CH4/N2 mixture in THF solution is very limited, which could be ignored in theory, and the composition of equilibrium gas is very close to that of feed gas. This close match verifies the reliability of experimental method used in this work. 3.3. Separation of CH4−N2 via Hydrate Formation. The experimental results are shown in Table 4. The recovery rate or split fraction (S.Fr.)20 of CH4 in the hydrate phase and the separation factor (S.F.)37 are calculated as follows:

3. RESULTS AND DISCUSSION 3.1. Formation Conditions of CH4−N2−THF Hydrates. In this work, the formation conditions of CH4−N2−THF hydrate were measured first, as shown in Table 2 and Figure2. Table 2. Hydrate Formation Pressures of CH4 and N2 Gas Mixture at Different Temperatures in 6 mol % THF Solution P, MPa

a

T, K

4.90%a CH4 95.10% N2

15.99% CH4 84.01% N2

29.21% CH4 70.79% N2

46.28% CH4 53.72% N2

279.15 281.15 283.15 285.15 287.15

0.75 1.09 1.60 2.28 3.10

0.48 0.71 1.03 1.49 2.10

0.34 0.50 0.75 1.08 1.54

0.26 0.40 0.60 0.88 1.20

H nCH 4

S. Fr. (%) =

Gas mixtures compositions are given as mole fractions.

S. F. =

F nCH 4

× 100 (1)

H yCH yNG 4

2

G yNH yCH

(2)

2

4

nHCH4

nFCH4

where and stand for the numbers of moles of CH4 in hydrate gas and in feed gas, respectively. yHCH4 and yGCH4 are the mole fractions of CH4 in hydrate gas and in residual gas, respectively. yGN2 and yHN2 are the mole fractions of N2 in residual gas and in hydrate gas, respectively. nHCH4 and nFCH4 are calculated using the following equation: H H nCH = n HyCH 4 F F nCH = n FyCH 4

Figure 2. Hydrate formation conditions of CH4−N2 in 6 mol % THF solution.

(3)

4

(4)

4

where nH and nF refer to the total number of moles of hydrate gas and feed gas, respectively. yFCH4 is the mole fraction of CH4 in feed gas. nH and nF are given by

Figure 2 also shows the hydrate formation condition of 4.90% CH4/95.10% N2 gas mixture predicted by Chen−Guo hydrate model.34 This model has been widely applied to predict the hydrates formation conditions of various systems for its high accuracy and concise calculation process.35,36 In Figure 2, it demonstrates that the formation pressure of CH4−N2 hydrate is significantly reduced with the presence of THF in water. It greatly reduces the cost of experimental setup and materializes the widespread application of the hydrate formation separation technology. Data in Figure 2 also indicate that the higher CH4 content in CH4/N2 gas mixtures, the lower formation pressure of CH4−N2−THF hydrate. This occurs because CH4 forms hydrate more easily than N2 under the same condition. In the same CH4−N2−THF system, the gas hydrate formation pressure increases with temperature. It means that high temperature discourages the formation of CH4−N2−THF hydrate. Consequently, the temperature in our experiments is set to be lower than 281.15 K. 3.2. Gas Solubility of CH4−N2 in THF Solution. The mole fraction of CH4 in feed gas ranges from 4.90% to 71.23% by molar mass. Several single-stage separation experiments were conducted. During the separation process in this work, after CH4−N2−THF hydrate totally dissociated, the reaction system

nH =

P HV H z HRT H

nF =

P FV F z FRT F H

(5)

(6) H

where P and T refer to the pressure and temperature of hydrate gas, respectively. PF and TF are experimental pressure and temperature, respectively. R is gas constant, with a value of 8.3145 J/(mol·K). zH and zF are gas compressibility factors of hydrate gas and feed gas respectively, which are calculated by PT equation of state38 in this work. VH and VF stand for the volume of hydrate gas and feed gas respectively. VF is given by VF = VR − VL R

(7) L

where V is defined as the volume of reactor. V is the volume of liquid medium added in the reactor before the hydration reaction. The volume of hydrate gas VH is calculated using the following equation VH = VE − VL 4509

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Figure 3. Gas−liquid equilibrium results calculated by Hysys after CH4−N2−THF hydrate dissociated.

where VE stands for the total volume of both hydrate gas and liquid medium after hydrate completely dissociated, and it is given by the following expression

Table 3. Composition and Yield of Equilibrium Gas Phase at Gas−Liquid Equilibrium State under 293.15 K normalized mole fraction, %

gas yield, %

component

0.3 MPa

0.7 MPa

0.3 MPa

0.7 MPa

CH4 N2

46.30 53.70

46.32 53.68

99.81 99.74

99.53 99.37

⎛ ΔL ⎞⎟ V E = V R ⎜1 − ⎝ h ⎠

where h refers to the height of the reactor. ΔL stands for the traveling distance of the piston in the reactor, and it is obtained by measuring the lengths of piston rod before and after the reaction. The experimental data in Table 4 show that, with the pressure window of 0.6−1.3 MPa and temperature window of 278.15−281.15 K, CH4 and N2 mixtures can be separated to some extent via hydrate formation with the presence of THF. For example, after the single-stage separation of 29.21% CH4/ 70.79% N2, CH4 could be concentrated in hydrate phase with an average mole fraction (yHCH4) of 53.95%. Similarly, for 46.28% CH4/53.72% N2, yHCH4 can be achieved on average at 69%. The average increases of CH4 concentrations from various feed gases (yFCH4) to hydrate gases in Table 4 are plotted respectively in Figure 4, and they are arranged in the ascending order of yFCH4. For a 7.23% CH4/ 92.77% N2 mixture, shown as the second experiment in Figure 4, yCH4H is 16.83% after singlestage separation. If a 16.83% CH4/83.17% N2 mixture is separated sequentially, yHCH4 is more than 25.77% according to the third experiment in Figure 4. By such analogy, yHCH4 could reach more than 90% after a continuous five-stage separation of 7.23% CH4/ 92.77% N2. Figure 5 shows the average decreases of CH4 concentrations from different feed gases to residual gases (yGCH4) in Table 4 in the descending order of yHCH4. We can assume that CH4 concentration could decrease from 46.28% in feed gas to about 10% in residual gas after five-stage separation. It demonstrates that CH4 could be effectively separated from

Table 4. Experimental Results of the Separation of CH4 and N2 via Hydrate Formation in 6 mol % THF Solution gas a

4.90% CH4 95.10% N2 7.23% CH4 92.77% N2 15.99% CH4 84.01% N2

29.21% CH4 70.79% N2

46.28% CH4 53.72% N2

71.23% CH4 28.77% N2 a

P, Mpa

T, K

GCH4, mol %

HCH4, mol %

S.Fr., 100%

S.F.

1.3 1.3 0.8

278.15 280.15 278.15

3.59 3.25 5.48

12.45 12.30 16.83

36.08 43.91 34.57

3.82 4.18 3.49

0.6 0.7 1.2 1.3 0.9 1.1 1.1 1.3 0.7 0.7 0.9 0.9 1.1 1.1 0.7

279.15 279.15 281.15 281.15 279.15 279.15 281.15 281.15 279.15 281.15 279.15 281.15 279.15 281.15 279.15

13.00 12.37 12.70 10.34 22.53 19.07 21.16 19.07 39.41 35.42 37.61 34.85 38.38 31.91 61.53

27.40 24.43 26.38 24.85 55.09 54.30 51.46 54.93 70.48 69.64 69.93 69.38 65.74 68.84 87.46

34.06 43.01 38.23 58.16 36.98 50.57 46.15 51.01 32.28 44.83 39.05 47.81 40.75 55.49 44.36

2.53 2.29 2.46 2.87 4.22 5.04 3.95 5.17 3.67 4.18 3.86 4.24 3.08 4.71 4.36

(9)

Gas mixtures compositions are given as mole fractions.

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separation process is definitely necessary to obtain high CH4 concentration in hydrate, especially for those CH4/N2 mixtures with a low concentration of CH4. The S.Fr. of CH4 in our experiments ranges from 34.06% to 58.16%. Generally speaking, higher S.Fr. could be obtained at higher reaction pressure. This is because high pressure produces more CH4 and N2 hydrates. However, yHCH4 decreases slightly with the increase of pressure, as shown in Table 4. We believe more N2 hydrate forms instead at relatively higher pressure. In terms of temperature, we found that higher temperature leads to higher yHCH4. This is because N2 hydrate formation pressure is much higher than that of CH4. As temperature increases, more N2 dissociates from hydrate phase than CH4 does. This means that the temperature effect on N2 hydrate is greater than that on CH4 hydrate in both the formation and dissociation processes. Besides S.Fr., we calculated S.F. in this work. The value is between 2.29 and 5.17, also supporting the fact that CH4 and N2 could be separated effectively via hydrate formation. 3.4. Preliminary Design of Two-Stage Hydration Separation Process. The average S.Fr. of single-stage separation in Table 4 is 43.18%. A S.Fr. of only about (43.18%)n will be achieved after a n-stage hydration process. Therefore, recycles of residual gas are necessarily required to increase the recovery yield of CH4 in multistage separation, which is plotted in Figure 7. Because the common yFCH4 in airmixed CBM ranges from 30% to 50%,6 we choose 46.28 CH4/ 53.72% N2 mixture as the feed gas of a two-stage separation process. In Figure 7, the feed gas forms hydrate in reactor 3 with liquid medium for the first time. The residual gas escapes from the top of the reactor and the hydrate is transferred by the unreacted liquid into the decomposer 4, where the hydrate dissociates and the CH4-rich gas after one-stage separation (CH4-rich gas I) is obtained. In the next stage, CH4-rich gas I is pressurized by booster 5, and forms hydrate in reactor 6 for the second time. After that, the residual gas is discharged from the reactor, and the hydrate moves to the decomposer 7. Then, the CH4-rich gas after two-stage separation (CH4-rich gas II) is collected at the outlet of decomposer 7 after hydrate dissociates. The liquid medium flow out of decomposer 4 and 7 can be pressurized and cooled, and then form hydrate again with gas mixture. Consequently, there is no loss of liquid, in theory. The residual gases out of hydration reactors 3 and 6 are recycled to form hydrate after being pressurized by booster

Figure 4. Increases of CH4 concentration from different feed gases to hydrate gases in individual single-stage separation.

Figure 5. Decreases of CH4 concentration from different feed gases to residual gases in individual single-stage separation.

CH4/N2 mixture and concentrated in hydrate phase after multistage hydrate formation in THF solution. The principle of multistage hydration process of CH4−N2 is illustrated in Figure 6. According to Chen−Guo hydrate model, THF molecules occupy 51264 cavities first in SII hydrate formation process of CH4−N2−THF. CH4 and N2 molecules then compete to enter into 51262 cavities, but CH4 does so faster than N2 because CH4 forms hydrate more easily than N2. Therefore, the CH4 concentration in residual gas phase decreases continuously, and the content of CH4 in hydrate phase continues to increase. It should also be noted that, in a single-stage hydration reaction, yHCH4 increases with the yFCH4 after separation. Thus, multistage

Figure 6. Principle of multistage hydration process of CH4−N2−THF. 4511

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solution. It shows that THF can significantly reduce the formation pressure of CH4−N2 hydrate. It enables the separation of CH4 and N2 via hydrate formation at a low pressure of 0.7−1.3 MPa, which effectively reduces the risk of CH4 explosion and materializes the industrial application of this separation technology. Experiments to separate different CH4− N2 mixtures using hydrate formation were systematically conducted in 6 mol % THF solution. Several single-stage hydration separations were conducted under different conditions. The experimental results indicate that CH4 can be effectively separated from CH4−N2 mixture and remained in hydrate phase in 6 mol % THF solution by hydrate separation technology under optimal combination of operating pressure and temperature. While multistage separation process is applied, higher concentration of CH4 in hydrate gas can be achieved. For example, yHCH4 could be more than 90 mol % after five-stage separation of 7.23%CH4/92.77%N2. As a typical composition of air-mixed CBM, the 46.28%CH4/53.72%N2 mixture is designed to be separated with a two-stage hydration separation. The calculation results indicate that yHCH4 could be anticipated to reach about 82.61% with S.Fr. of 47.28% under appropriate conditions. Multistage hydrate separation technology is an effective way to process and utilize CBM−air mixture. The technology has a broad prospect in industrial application.

Figure 7. Schematic of two-stage separation of CH4/N2 via hydrate formation: 1, gas cylinder; 2, 5, 12, gas boosters; 3, 6, hydration reactors; 4, 7, decomposers; 8, CH4-rich gas outlet; 9, liquid medium tank; 10, pump; 11, cooler.

12. The separation results are calculated and presented in Table 5. In Table 5, residual gas of both reactor 3 and 6 is recycled twice. At first, 1 mol of 46.28%CH4/53.72%N2 mixture is fed. After two-stage separation, 0.12 mol of hydrate gas with yHCH4 of about 85% and 0.88 mol of residual gas with yGCH4 of 41.11% are obtained, and then, the residual gas is recycled to form hydrate for the first time. After another two-stage separation, 0.08 mol hydrate gas with yHCH4 of about 82% is added in CH4-rich gas product, accompanied with 0.80 mol of residual gas with yGCH4 of 36.84%, which is pushed back to hydration reactor for the second time. Similarly, 0.06 mol hydrate gas with yHCH4 of about 79% turns up after the last two-stage separation, leaving 0.74 mol of residual gas with yGCH4 of 33.19%. Finally, 1 mol of 46.28%CH4/53.72%N2 yields 0.26 mol of 82.61% CH4/17.39% N2 in total. According to eq 1, the theoretical value of S.Fr. is 47.28%. The results in Table 5 are only simulation values, and we believe they could be obtained by appropriately adjusting and controlling the reaction conditions and time.

■

AUTHOR INFORMATION

Corresponding Author

*Telephone: +86 10-89731003. Fax: +86 10-89731003. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by China National Science and Technology Plan (2009CB219504, 2012CB215004) and National Natural Science Foundation of China (20925623, U1162205), which are greatly acknowledged.

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REFERENCES

(1) Yang, Z. Q.; Grace, J. R.; Lim, C. J.; Zhang, L. Energy Fuels 2011, 25, 975−980. (2) Yang, M. Energy Policy 2009, 37, 2858−2869. (3) Kedzior, S. Int. J. Coal Geol. 2009, 80, 20−34. (4) Zhao, J. Z.; Zhao, Y. S.; Shi, D. X. J. China Coal Soc. 2008, 33, 1419−1424. (5) Nie, L. H.; Xu, S. P.; Su, Y. M.; Liu, S. P. Chem. Ind. Eng. Prog. 2008, 27, 1505−1511.

4. CONCLUSIONS We measured the hydrate formation conditions of CH4−N2 gas mixtures with different CH4 compositions in 6 mol % THF

Table 5. Simulated Results of Two-Stage Separation of 46.28%CH4/53.72%N2 via Hydrate Formation incoming material calc. counts

a

items

feed

1

yCH4, %a

46.28

2

n, mol yCH4, %

1.00 0

3

n, mol yCH4, %

final

1st stage

recycle

2nd stage

gas

hydrate

gas

hydrate

36.26

69.00

59.00

85.00

0 41.11

0.69 33.00

0.31 66.00

0.19 56.00

0.12 82.00

0 0

0.88 36.84

0.67 30.00

0.22 63.00

0.13 53.00

0.08 79.00

n, mol yCH4, %

0 0

0.80 33.19

0.63 0

0.17 0

0.10 0

0.06 82.61

n, mol

0

0.74

0

0

0

0

0.26

yCH4 refers to CH4 concentration in different gas mixtures, and the values are given as mole fraction. 4512

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dx.doi.org/10.1021/ef300330b | Energy Fuels 2012, 26, 4507−4513