Influence of Cyclopentane and SDS on Methane Separation from Coal

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Influence of Cyclopentane and SDS on Methane Separation from Coal Mine Gas by Hydrate Crystallization Dong-Liang Zhong,*,†,‡ Kun Ding,† Jin Yan,‡ Chen Yang,† and Dong-Jun Sun† †

College of Power Engineering, Chongqing University, Chongqing 400044, China State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400044, China

‡

ABSTRACT: The influence of cyclopentane (CP) on methane separation from low-concentration coal mine gas (30% CH4, 60% N2, and 10% O2) by gas hydrate formation was investigated in the present work. Overpressure was used as the driving force. Experiments were carried out at 286.6 K and (2.0−5.0) MPa, and the cyclopentane concentration was fixed at 7.0 wt %. The experimental results indicated that the gas uptake increased with the increase of driving force, but the methane recovery decreased with the increase of driving force. A ∼ 6% higher methane recovery was obtained under the low driving force ΔP = 1.5 MPa as compared to that obtained in the presence of tetra-n-butyl ammonium bromide (∼ 27%). Then, the anionic surfactant sodium dodecyl sulfate (SDS) was employed to enhance hydrate formation from low-concentration coal mine gas in the presence of cyclopentane. Surface tensions of the solution (H2O/CP/SDS) were measured under different SDS concentrations and reported. It was found that gas uptake and the rate of hydrate formation were dependent on SDS concentration, but the presence of SDS did not show clear influence on methane recovery. The methane recovery obtained in the presence of SDS was ∼33.3%, while that obtained without SDS was 33.1%.

1. INTRODUCTION The recovery of methane from coal mine methane (CMM) gas is a desire for energy exploration, safety guarantee of coal mine operations, and reduction of greenhouse gas emissions.1−3 Gas hydrate crystallization has been commonly used for CO2 capture4−7 and recently has been considered as a potential approach to separate CH4 from low-concentration CMM gas (CH4/N2/O2 or CH4/N2).8−10 However, the phase equilibrium pressures for gas hydrates formed from CMM gas mixtures at given temperatures are significantly higher comparing to those for gas hydrates synthesized from pure methane. For instance, the equilibrium hydrate formation pressure at 275.15 K for the low-concentration CMM gas with a mole composition of 30% CH4, 60% N2, and 10% O2 is 7.78 MPa, but for methane is only 3.11 MPa.11 Therefore, thermodynamic promoters are needed for hydrate formation from the low-concentration CMM gas, which will shift the phase equilibrium conditions to low pressures and high temperatures. Additives such as tetrahydrofuran (THF), tetra-n-butyl ammonium bromide (TBAB), and cyclopentane (CP) have been found to be the promising thermodynamic promoters for hydrate formation from lowconcentration CMM gas, and the incipient phase equilibrium data measured in the presence of these additives have been reported in recent studies.10−13 On the other hand, a better understanding of the kinetics of hydrate formation will highlight how to efficiently recover methane from the CMM gas and promote the development of the hydrate-based technology for CH4 separation from lowconcentration CMM gas. Some researchers have noticed the importance and studied the kinetics of hydrate formation from the CMM gas. In our previous work,10 we carried out a kinetic investigation on methane separation from the simulated CMM gas in the presence of TBAB and reported the methane recovery and separation factor of CH4 from the CMM gas. It © 2013 American Chemical Society

was concluded that the phase equilibrium pressures for gas hydrate formed from the CMM gas were largely decreased in the presence of TBAB, but the methane recovery obtained was only 27% which should be increased to a higher value if this hydrate-based method is practically used on the industry scale. Increasing the rate of hydrate formation is another concern in the development of the hydrate-based method for methane separation from the CMM gas mixture. The use of surfactants will be an option to shorten the nucleation time and enhance the rate of hydrate formation. The anionic surfactant sodium dodecyl sulfate (SDS) is a well-known promoter for the enhancement of gas hydrate formation. Link et al.14 formed methane hydrate in a high-pressure view cell and concluded that SDS was one of the best surfactants to maximize the methane uptake. Okutani et al.15 reported an experimental study on the effects of three surfactants (SDS, STS, and SHS) on the formation of gas hydrates in a quiescent methane/liquid system. They found that SDS has the highest solubility in water among the three homologues and is very effective for increasing both the rate of hydrate formation and the final water-tohydrate conversion ratio. Okutani et al.15 also reported the observations of macroscopic hydrate growth on the reactor wall and concluded that hydrate formation was enhanced due to the capillary mechanism in which liquid water was driven to flow upward through the porous hydrate layer by capillary suction in the presence of surfactant. Yoslim et al.16 investigated the effect of anionic surfactants (SDS, STS, and SHS) on hydrate growth from a gas mixture of 90.5% CH4 and 9.5% C3H8 and concluded that the addition of SDS significantly increased the gas consumption for hydrate formation as compared to pure Received: July 31, 2013 Revised: November 11, 2013 Published: November 18, 2013 7252

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water. Yoslim et al.16 also observed the morphology of hydrate crystals at the gas/water interface in the presence of SDS and concluded that the porous fiber-like hydrate crystals formed in the presence of SDS is responsible for the increase of gas consumption. Therefore, in the present work, SDS was employed as a candidate to promote hydrate formation in the CMM gas system. Based on the phase equilibrium data measured in the presence of cyclopentane,13 the purpose of this work is to study the influence of cyclopentane and the anionic surfactant sodium dodecyl sulfate (SDS) on the kinetics of hydrate formation and methane separation from the low-concentration CMM gas mixture with a mole composition of 30% CH4, 60% N2, and 10% O2 under moderate pressure and temperature conditions.

were used instead of 140 cm3 deionized water. Similar to the experiments performed without SDS, the volume ratio of SDS solutions to cyclopentane was maintained at 10:1, and therefore 14 cm3 cyclopentane was added into the SDS solutions. When SDS solutions were prepared and filled into the crystallizer, the experiments were carried out following the above procedure. Two experimental runs were performed under each experimental condition. 2.3. Calculation of the Amount of Gas Mixture Consumed. Based on the compositions of the gas mixture at the beginning and at the end of the experiments, the number of moles of the gas mixture incorporated into the hydrate crystals was calculated by eq 1.

ΔnH = ng ,0 − ng ,t =

⎛ PV ⎞ ⎛ PV ⎞ ⎜ ⎟ − ⎜ ⎟ ⎝ ZRT ⎠0 ⎝ ZRT ⎠t

(1)

where ng is the number of moles of the gas mixture in the crystallizer at time 0 and time t, P is the pressure in the crystallizer, T is the temperature of the gas phase, V is the volume of the gas phase, and Z is the compressibility factor calculated by the Pitzer correlation for the second Virial coefficient (eq 2).17

2. EXPERIMENTAL SECTION 2.1. Materials. The gas mixture with a mole composition of 30% CH4, 60% N2, and 10% O2 was purchased from Chongqing Rising Gas with a reported uncertainty in the composition of ±0.05%. The composition of the gas mixture was selected to simulate the typical low-concentration CMM gas recovered from underground coal mines. Cyclopentane and SDS were purchased from Chongqing Oriental Chemical Co., Ltd. with a certified mass purity of 95%. Deionized water was used in all experimental runs. 2.2. Apparatus and Procedure for Hydrate Formation. A detailed description of the apparatus was given elsewhere in the literature.11 Briefly, the stainless-steel reactor was a 600 cm3 vessel. A speed-adjustable electromagnetic stirrer was inserted into the reactor for solution agitation. Two platinum resistance thermometers with an uncertainty of 0.1 K were used to measure the temperatures of the gas and the liquid solution, respectively. A pressure transducer (Banna Electronics, Inc., Cranberrry, NJ) with an uncertainty of 0.01 MPa was used to measure the pressure of the gas phase. A gas chromatograph (SC-2000, Chongqing Chuanyi Analyzer,Chongqing, China) with an uncertainty of 0.1% was used to determine the compositions of the gas mixtures remaining in the vessel and releasing from hydrate. The experimental procedure for hydrate formation in the presence of cyclopentane was given as follows. Prior to the experiments, the crystallizer was cleaned with deionized water and dried. Then, it was filled with 140 cm3 deionized water and 14 cm3 cyclopentane. The volume ratio of water to cyclopentane was fixed at 10:1, and the cyclopentane mass concentration was 7.0 wt %. The crystallizer and the tubing were purged with the low-concentration CMM gas mixture three times, and hence the air remaining in the reactor was flushed out. Once the temperatures and the pressure of the reactor contents reached desired values, the crystallizer was separated from the gas cylinder by closing the inlet and outlet valves. Then the electromagnetic stirrer was started at a constant speed of 150 rpm. This was considered to be time zero for the experiments of hydrate formation. The temperatures and the pressure were recorded by a data acquisition unit (Agilent 34970A) and logged into a computer every 10 s. Hydrate formation was confirmed by visual observations through the viewing windows. The experiments were terminated when no significant pressure drop was observed in the crystallizer. The composition of the gas mixture remaining in the vessel was measured using the gas chromatograph (GC) at the end of the hydrate formation experiments. Subsequently, the crystallizer was quickly vented to atmospheric pressure and isolated again by closing the vent valve. Then the crystallizer was heated to 288.15 K at a rate of 0.1 K/s until hydrate in the vessel was fully dissociated. The gas mixture decomposed from hydrate was sampled when the temperatures and the pressure in the reactor were stabilized, and the composition was determined by GC. It should be noted that four parallel experimental runs were carried out under each experimental condition, and fresh solutions were used for all experiments. For the experiments investigating the influence of SDS on methane recovery from the low-concentration CMM gas, a total of 140 cm3 SDS solution with three SDS concentrations (300, 500, and 700 ppm)

Z = 1 + B0

Pr P + ωB1 r Tr Tr

(2) 0

1

where the equations of Abbott were used for B and B . The rate of hydrate formation is customarily considered to be the rate of gas consumption and is calculated using the forward difference method as follows

ΔnH,t +Δt − ΔnH,t ⎛ dΔnH ⎞ ⎟ = ⎜ , Δt = 10s ⎝ dt ⎠t Δt

(3)

The average of these rates is calculated every 30 min and reported. average rate of hydrate formation(R av ) dΔnH dΔnH ⎡ dΔnH ⎢ dt 1 + dt 2 + ··· + dt =⎢ m ⎢ ⎣

( ) ( )

( )

m

⎤ ⎥ ⎥, m ⎥ ⎦ (4)

= 180

2.4. CH4 Recovery and Efficiency. Split fractions and separation factors were two metrics used by Linga et al.18,19 to assess the separation efficiency of a gas mixture by gas hydrate crystallization. In this work, the CH4 recovery or split fraction (R) of CH4 was calculated as follows:20

R=

H nCH 4 feed nCH 4

× 100% (5)

nfeed CH4

is the moles of CH4 supplied into the crystallizer, and nHCH4 where is the moles of CH4 captured in hydrate at the end of the experiments. There are three components (CH4, N2, and O2) included in the lowconcentration CMM gas mixture, and the hydrate may not distinguish N2 and O2 because the hydrate characteristics of O2 such as phase equilibrium pressure is nearly the same as that of N2,8 so the separation factor (S) in this work was defined using the following equation:20 S=

H nCH (n Ngas2 + nOgas2 ) 4 gas nCH (n NH2 + nOH2) 4

ngas CH4,

(6)

ngas O2

are the moles of CH4, N2, and O2 in the gas phase at where the end of the experiments, respectively. The symbols nHN2 and nHO2 are the moles of N2 and O2 incorporated into the hydrate crystals at the end of the experiments, respectively. The split fraction (R) indicates the efficiency of CH4 recovered from the low-concentration CMM gas by hydrate formation, and the separation factor (S) demonstrates the extent of separating CH4 from the low-concentration CMM gas. A higher separation factor (S) 7253

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Table 1. Experimental Conditions and Results for Gas Hydrate Formation from the Gas Mixture with a Mole Composition of 30% CH4, 60% N2, and 10% O2 in the Presence of Cyclopentanea

a

exp. no.

Texp (K)

Pexp (MPa)

ΔPb (MPa)

tind (min)

xgas CH4 (%)

xHCH4(%)

ΔnH (mol)

u(ΔnH) (mol)

R (%)

S

1 2 3 4 5 6 7 8 9 10 11 12

286.6

2.0

1.5

286.6

3.5

3.0

286.6

5.0

4.5

140 135 145 142 120 113 116 125 25 16.3 23 25

27.5 28.2 27.2 27.5 24.6 24.2 24.6 24.0 25.5 25.7 26.0 25.5

53.2 53.9 50.5 51.0 53.3 56.9 53.6 55.8 54.3 54.1 53.2 57.0

0.0681 0.0694 0.0673 0.0685 0.0835 0.0824 0.0829 0.0831 0.1526 0.1522 0.1517 0.1531

0.0045 0.0045 0.0045 0.0045 0.0058 0.0057 0.0057 0.0058 0.0104 0.0103 0.0102 0.0104

35.5 35.9 29.3 31.7 30.9 29.2 30.0 30.2 28.8 28.7 26.8 29.7

2.9 3.2 2.3 2.6 7.9 6.4 7.6 7.7 3.8 4.1 2.8 4.3

Standard uncertainties u are u(T) = 0.1 K and u(P) = 10 kPa. bΔP = Pexp − Peq.

average values of the final gas consumption were 0.0683, 0.083, and 0.1524 mol corresponding to ΔP = 1.5, 3.0, and 4.5 MPa, respectively. This can be explained by that the increase of driving force caused a larger amount of gas hydrate formed in the crystallizer, and therefore more amount of the gas mixture was captured into the hydrate crystals. However, the methane recovery (R) was observed to decrease with the increase of driving force. As seen in Table 1, the average values of methane recovery (R) obtained at ΔP = 1.5, 3.0, and 4.5 MPa were 33.1%, 30.1%, and 28.5%, respectively. This is probably because the hydrate cavities that used to accommodate CH4 molecules under low overpressures were occupied by N2 or/and O2 molecules with the increase of the overpressure. As a result, more N2 or/and O2 gas were incorporated into the hydrate phase, and the methane recovery (R) was reduced under the higher driving force. This can be preliminarily confirmed by the results of GC measurements of the initial and recovered gases. For example, GC measurements showed that the initial gas supplied to the vessel was 30% CH4, 60% N2, and 10% O2 in mole fraction, which agrees with the composition reported by the gas supplier. The recovered gas at ΔP = 1.5 MPa contained 35.3% N2 (experiment 1) and contained 36.3% N2 at ΔP = 4.5 MPa (experiment 11), indicating that more N2 might be incorporated into the hydrate phase under the higher driving force. However, further work on spectroscopic measurements such as Raman is needed to quantitatively verify the nature and the composition of gas molecules in the gas hydrates formed with the CMM gas mixture. It was also found that the methane recovery (R) was significantly increased in the presence of cyclopentane (∼33.1%) as compared to that obtained in the presence of TBAB (∼27%).20 Figure 1 shows the temperature profile and gas uptake curve for the experiment performed at 286.6 K and 2.0 MPa, and at the cyclopentane mass fraction of 7.0 wt % (experiment 2 in Table 1). As seen in the figure, because hydrate crystallization is an exothermic process, thus the induction time was identified by a sudden temperature rise in the liquid and gas phase at 135 min. Temperature spike normally occurred in liquid phase because of the initiation of hydrate crystallization. Note that temperature increase in gas phase at the nucleation point was also observed in the present system. One proper explanation is that cyclopentane was dispersed at the gas/liquid interface due to the agitation and its immiscibility with water (its density is smaller than water), thus hydrate nucleation would first occur at the gas/liquid interface at the nucleation point, and then

indicates the ability of the hydrate to separate CH4 from the CMM gas mixture is stronger.

3. RESULTS AND DISCUSSION The incipient equilibrium conditions for gas hydrate formed from the CMM gas mixture in the presence of cyclopentane (7.0 wt %) were measured using an isochoric step-heating method and reported in our previous work.13 It was found that cyclopentane was a better thermodynamic promoter than TBAB because the incipient equilibrium hydrate formation conditions obtained in the presence of cyclopentane were greatly reduced as compared to those obtained in the presence of TBAB and in pure water. The measured equilibrium point (Teq = 286.6 K, Peq = 0.5 MPa) in the presence of cyclopentane was selected for kinetic experiments in the present work. The experimental conditions were summarized in Table 1. The temperature was fixed at 286.6 K, and overpressure was used as the driving force (ΔP = Pexp − Peq) for hydrate formation. The effect of driving force was examined by varying the overpressure from 1.5 to 4.5 MPa. As seen in Table 1, experiments 1−4 were carried out at 286.6 K and 2.0 MPa (ΔP = 1.5 MPa), experiments 5−8 were carried out at 286.6 K and 3.5 MPa (ΔP = 3.0 MPa), and experiments 9−12 were carried out at 286.6 K and 5.0 MPa (ΔP = 4.5 MPa). The results of hydrate formation from low-concentration CMM gas mixture were presented in Table 1 as well. The CH4 concentration in the gas phase (xgas CH4) was measured at the end of hydrate formation, and the CH4 concentration in the hydrate H phase (x CH ) was measured when hydrate was fully 4 decomposed. The final gas consumption (ΔnH), CH4 recovery (R), and separation factor (S) were calculated using eqs 1, 5, and 6, respectively. The combined standard uncertainties in ΔnH were also calculated from the uncertainties in P, T, V, and Z. Induction time indicates the onset of hydrate nucleation, which is usually determined by visual observations21or pressure measurements22 or a sudden increase in the temperatures of gas and liquid phases along with visual observations.20,23 Generally, the induction time decreases with the elevation of driving force. This is also observed in the present work. As seen in Table 1, the average induction times obtained under the driving forces of ΔP = 1.5, 3.0, and 4.5 MPa were 140.5 min (experiments 1− 4), 118.5 min (experiments 5−8), and 22.3 min (experiments 9−12), respectively. Meanwhile, the final gas consumption was observed to increase with the increase of driving force. The 7254

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Figure 1. Temperature profile and gas uptake curve for the experiment carried out at 286.6 K and 2.0 MPa in the presence of cyclopentane (7.0 wt %).

Figure 2. Temperature profile and gas uptake curve for the experiment carried out at 286.6 K and 5.0 MPa in the presence of cyclopentane (7.0 wt %).

quick growth of hydrate downward to the liquid phase and upward to the gas phase were followed. This growth mechanism is similar to that reported by Lim et al.24 who observed the morphology of carbon dioxide-hydrogen-cyclopentane hydrates in an unstirred system. Therefore, the release of hydration heat at the gas/liquid interface and at the nucleation point caused the temperature spike occurring both in liquid and gas phase. This phenomenon was also observed in other experiments in this work. Gas uptake is the number of moles of the gas mixture incorporated into the hydrate crystals, which was calculated by eq 1. As shown in Figure 1, the gas uptake curve exhibited a two-stage ascending pattern in which the gas mixture was consumed rapidly (∼ 0.062 mol, 90% of total gas uptake) during the first stage (from time zero to ∼300 min) and then consumed slowly during the second stage (from ∼300 min to the end of the experiment). Obviously, the gas uptake curve at the first stage indicated that gas hydrate grows quickly once hydrate nucleation occurred at the induction time. Progressively, gas hydrate might accumulate at the gas/liquid interface and prevent the transport of gas molecules to the reaction interface. As a result, the gas uptake curve at the second stage gradually reaches a plateau with a decreasing tendency in the growth rate. Figure 2 shows the temperature and gas uptake profiles for the experiment carried out under a higher driving force (experiment 10 in Table 1). The pressure was increased to 5.0 MPa, but other experimental conditions such as the temperature and the cyclopentane concentration were the same as experiment 2 and maintained at 286.6 K and 7.0 wt %, respectively. As seen in the figure, the induction time was identified at 16.3 min by a sudden increase in the gas and liquid temperature. Even though the solution used in this experiment was the same as those used in other experiments, it was found that the induction time was shortened as compared to other experiments (experiments 1−8). Also, the gas uptake was greatly increased at the end of the experiment (∼0.152 mol) as compared to the experiments performed under the low driving forces (experiments 1−8). This indicates that the increase of driving force caused a larger amount of gas hydrate formed in the crystallizer, and therefore more amount of the gas mixture was enclathrated into the hydrate crystals. However, the developing trend of the gas uptake can also be divided into two stages as that observed in Figure 1. Gas uptake quickly increased to 0.131 mol (90% of the total gas consumption)

from time zero to about 175 min, and then the growth rate of the gas uptake curve decreased, and the gas uptake curve was approaching a plateau at the end of the experiment. The experimental results reported in Table 1 indicated that the methane recovery (R) obtained at ΔP = 1.5 MPa was higher than those obtained at ΔP = 3.0 and 4.5 MPa, but the nucleation process was prolonged under the low driving force (ΔP = 1.5 MPa). Therefore, it is necessary to promote hydrate nucleation under the low driving force. The anionic surfactant sodium dodecyl sulfate (SDS) is a well-known promoter for the reduction of nucleation time and the enhancement of gas hydrate formation.14−16 We introduced SDS into the present system and investigated the influence of SDS on hydrate formation and methane recovery from the low-concentration CMM gas mixture in the presence of cyclopentane. Table 2 shows the experimental conditions for hydrate formation from the CMM gas when SDS was added into the solutions containing water and cyclopentane. The temperature and pressure were fixed at 286.6 K and 2.0 MPa (ΔP = 1.5 MPa), and three SDS concentrations (300, 500, and 700 ppm) were used. The combined standard uncertainties in ΔnH were calculated from the uncertainties in P, T, V, and Z. As seen in Table 2, the induction times measured in the presence of SDS were significantly decreased as compared to those obtained in the absence of SDS (experiments 1−4 in Table 1). This indicated that the surfactant SDS promoted hydrate nucleation in the system including CMM gas and cyclopentane. In addition, the final gas consumption obtained in the presence of SDS was increased as compared to that obtained without SDS. For example, the average value of gas consumption obtained at the SDS concentration of 300 ppm was 0.0739 mol (experiments 1−2 in Table 2), while that obtained in the absence of SDS was 0.0683 mol (experiments 1−4 in Table 1). This indicated that the amount of gas hydrate formed in the presence of SDS was increased, which is in good agreement with what reported in the literature.25 This result is evidence that SDS can be used as an effective promoter to enhance hydrate formation from the low-concentration CMM gas in the presence of cyclopentane. Interestingly, the methane recovery (R) was nearly maintained constant as compared to that obtained in the absence of SDS (∼33%). Figure 3 shows the temperature and gas uptake profiles for the experiment performed at 286.6 K, 2.0 MPa, and at the cyclopentane mass fraction of 7.0 wt % and the SDS 7255

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Table 2. Experimental Results for Gas Hydrate Formation from the Gas Mixture with a Mole Composition of 30% CH4, 60% N2, and 10% O2 in the Presence of Cyclopentane and SDSa

a

exp. no.

Texp (K)

Pexp (MPa)

ΔPb (MPa)

SDS (ppm)

tind (min)

xgas CH4(%)

xHCH4(%)

ΔnH (mol)

u(ΔnH) (mol)

R (%)

S

1 2 3 4 5 6

286.6

2.0

1.5

300

286.6

2.0

1.5

500

286.6

2.0

1.5

700

70 68 50 67 75 69

25.7 25.9 26.8 26.8 24.9 25.6

54.2 55.1 53.3 52.9 55.9 54.8

0.0732 0.0745 0.0754 0.0735 0.0728 0.0731

0.0050 0.0051 0.0050 0.0049 0.0050 0.0050

33.8 33.3 32.7 31.8 34.4 33.5

3.2 2.9 3.9 2.7 3.3 2.7

Standard uncertainties u are u(T) = 0.1 K and u(P) = 10 kPa. bΔP = Pexp − Peq.

Figure 3. Temperature profile and gas uptake curve for the experiment carried out at 286.8 K and 2.0 MPa in the presence of cyclopentane (7.0 wt %) and SDS (500 ppm).

Figure 4. Effect of SDS on the rate of hydrate formation from CMM gas in the presence of cyclopentane (Time zero is the nucleation point).

concentration of 500 ppm (experiment 3 in Table 2). As seen in the figure, the induction time was determined at 50 min by a sudden increase in the temperatures of the liquid and gas phase. It should be noted that the temperature spike which occurred at the nucleation point was observed to be much higher in the liquid phase as compared to that in the gas phase. This can be explained by that hydrate nucleated easily and widely at the nucleation point when SDS was added and mixed with the liquid solution, and thus large hydration heat was simultaneously released to the liquid phase. As a result, temperature in the liquid phase was increased rapidly before hydration heat was removed by the coolant circulating around the reactor. Similar to the gas uptake curves obtained in the absence of SDS, the gas uptake curve obtained in the presence of SDS also shows a two-stage ascending pattern. In the first stage (from time zero to ∼100 min), gas consumption increased quickly to 0.06 mol (90% of the total gas consumption) due to the significant hydrate growth following hydrate nucleation at the induction point. In the second stage (from ∼100 min to the end of the experiment), the rate of gas consumption was reduced. This is because the agglomeration of gas hydrate at the hydrate/gas interface may prevent gas molecules transporting through the hydrate layer to the reaction interface. Accordingly, the rate of hydrate formation was decreased, and the process of hydrate formation was interrupted. Figure 4 compares the rates of hydrate formation from CMM gas in the presence of SDS and in the absence of SDS. The rate of hydrate formation was calculated by eq 4. As seen in the figure, time zero was the nucleation point. The rates of hydrate formation obtained in the presence of SDS were higher than those obtained in the absence of SDS, indicating that the addition of SDS into the liquid solution enhanced the growth of

hydrate formed from the CMM gas. The comparison of hydrate formation rates also shows that the SDS concentration of 500 ppm was an optimum concentration among the SDS concentrations (300, 500, and 700 ppm) tested for the acceleration of hydrate formation in the presence of cyclopentane. This is probably due to the reduction of surface tensions when SDS was added into the solutions.26,27 Thus, we measured the surface tensions of the solutions (H2O/CP/SDS) used in the present work. The surface tensiometer of MiniLab ILMS (GBX, France) with an uncertainty of 0.01 mN/m was used. The testing temperature was fixed at 293 K, and the solutions were in contact with air during the testing process. The results of surface tensions were presented in Figure 5. It can be seen that surface tensions of the solutions (H2O/CP/ SDS) used in the present work were greatly reduced as compared to the surface tensions reported by Watanabe et al.27 This contributed to the fast hydrate formation observed in this work (experiments 1−6 in Table 2). It can be also seen that surface tensions of the solutions decreased with the increase of SDS concentration. The surface tensions γ at 300, 500, and 700 ppm SDS were 38.47, 32.68, and 31.43 mN/m, respectively. It should be noted that the kinetic experiments showed that the rate of hydrate formation at 500 ppm SDS was higher than 300 and 700 ppm SDS (Figure 4). The reason might be that the surface tension at 500 ppm SDS was smaller than 300 ppm, and hence hydrate nucleated faster at 500 ppm SDS than 300 ppm. Also, at the nucleation point, hydrate nucleated rapidly at 700 ppm SDS as compared to 500 ppm because the surface tension obtained at 700 ppm was lower than 500 ppm SDS. Shortly, a larger amount of hydrate may agglomerate at the gas/liquid 7256

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4. CONCLUSIONS In the present work, cyclopentane was used to promote methane recovery from low-concentration coal mine gas (30% CH4, 60% N2, and 10% O2). The methane recovery obtained under the low driving force ΔP = 1.5 MPa was approximately 33.1%, which was significantly increased as compared to that obtained in the presence of TBAB. Also, the methane concentration of the gas mixture released from hydrate (∼52%) was higher than that obtained in the presence of TBAB (∼41%). The anionic surfactant SDS was employed to enhance hydrate formation from low-concentration coal mine gas in the presence of cyclopentane. It was found that the surface tensions of the solution (H2O/CP/SDS) decreased with the increase of SDS concentration, and the gas uptake and the rate of hydrate formation were dependent on SDS concentration. However, the methane recovery was not significantly influenced by the presence of SDS, which was nearly the same as those obtained in the absence of SDS.

Figure 5. Variation of surface tension γ under different SDS concentrations. SDS solutions were in contact with air, and the γ data were obtained at 0.1 MPa and 293 K.

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AUTHOR INFORMATION

Corresponding Author

interface and thus block gas molecules to transport to the reaction interface. As a result, the rates of hydrate formation at 700 ppm SDS were quickly dropped below the values at 500 ppm SDS. Figure 6 shows the comparison of methane concentrations of gas hydrates formed from the same low-concentration CMM

*(D.L. Zhong) Phone/Fax: +86-23-65102473. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the National Natural Science Foundation of China (No. 51006129), Chongqing Science and Technology Commission (cstc2013jcyjA90012), and Visiting Scholar Foundation of the State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University (No.2011DA105287-FW201211) is greatly appreciated.

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REFERENCES

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Figure 6. Concentration of methane gas recovered from lowconcentration CMM gas by hydrate crystallization in the presence of TBAB and in the presence of cyclopentane.

gas mixture in the presence of cyclopentane and in the presence of TBAB. As seen in the figure, compared to the original CH4 content in the low-concentration CMM gas (30%), the CMM gas mixture was greatly purified via gas hydrate crystallization, indicating that hydrate crystallization is an effective approach to separate CH4 from low-concentration CMM gas. In addition, CH4 content that was released from the hydrate formed in the presence of cyclopentane (52%) was much higher than that obtained in the presence of TBAB (∼41%).20 This indicates that cyclopentane was more efficient than TBAB to concentrate the low-concentration CMM gas using hydrate formation. Meantime, the CH4 recovery (R) obtained in the presence of cyclopentane was increased and was much higher than that obtained in the presence of TBAB. 7257

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