Phase Equilibrium Data of Gas Hydrates Formed from a CO2 + CH4

Article pubs.acs.org/jced

Phase Equilibrium Data of Gas Hydrates Formed from a CO2 + CH4 Gas Mixture in the Presence of Tetrahydrofuran Dong-Liang Zhong,†,‡ Zheng Li,‡ Yi-Yu Lu,*,† and Dong-Jun Sun‡ †

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

‡

ABSTRACT: In this work, the phase equilibrium conditions for gas hydrates formed from a CO2/CH4 gas mixture (0.4 CO2 and 0.6 CH4 in mole fraction) in the presence tetrahydrofuran (THF) were measured using the isothermal pressure search method and reported. The THF mole fractions used were 0.005, 0.01, 0.03, and 0.05 respectively. It was found that the equilibrium hydrate formation conditions obtained in the presence of THF shifted to high temperatures and low pressures as compared with those obtained using the same gas mixture in pure water. For the hydrates formed at a given temperature, the phase equilibrium pressure was observed to decrease as the THF mole fraction increased from 0.005 to 0.05. Therefore, it was confirmed that THF can be used as an effective thermodynamic promoter for CO2 separation from the CO2/CH4 gas mixture by hydrate formation. The heat of hydrate dissociation was also determined based on the measured phase equilibrium data of the gas hydrates formed from the CO2/CH4 gas mixture in the presence of THF. It was found that structure II hydrate was formed from the CO2/CH4 gas mixture in the presence of THF. Then the impact of driving force on CO2 separation from the CO2/CH4 gas mixture was investigated. The results indicated that the competition between CO2 and CH4 molecules for hydrate cage occupancy became stronger with the increase of driving force. Thus, a lower pressure at the given temperature is preferred for CO2 separation from the CO2/CH4 gas mixture by hydrate formation in the presence of THF.

1. INTRODUCTION Clathrate hydrates or gas hydrates are icelike crystalline inclusion compounds formed by water and small-sized gas molecules (CH4, C2H6, CO2, etc.) under low-temperature and high-pressure conditions.1 The naturally occurring gas hydrates in permafrost regions or in the ocean sediments have been considered as a potential energy source for the 21st century because of the huge methane content.2 In addition, gas hydrate crystallization can be used for various industrial purposes such as gas storage and transportation,3 desalination,4−6 separation of close-boiling point compounds,7,8 refrigeration and air conditioning,9−11 and gas separation processes,12,13 particularly CO2 capture from flue, fuel, and industrial gases.14−17 The separation of CO2 by gas hydrate formation has been extensively studied for different types of gas mixtures.18−20 Because of the high separation efficiency, this technology has been considered as a potential economical approach for CO2 capture as compared to the conventional CO2 separation processes.21,22 CO2-containing gas mixtures are generally split into two species. One species is the gas mixture, such as flue gas (CO2/N2) and fuel gas (CO2/H2), in which the phase boundary of gas hydrates formed from the individual gas component is far from each other.23,24 As a result, CO2 can be separated from these gas mixtures economically and effectively if the hydrate formation conditions are properly selected. The other species is the gas mixture, such as the contaminated natural gas (CO2/CH4), in which the phase boundaries of gas hydrates formed from each gas component are very close.25,26 © XXXX American Chemical Society

Consequently, the selectivity of CO2 by hydrate formation will be reduced due to the competitive enclathration of CO2 and the other gas component (CH4 in the contaminated natural gas) into the hydrate crystals. The recovery of shale gas (one type of the unconventional natural gases) has attracted the attention of researchers worldwide because of its huge reserves on the globe.27 A novel technique recently proposed for the recovery of shale gas is to inject supercritical CO2 into the shale gas environment, and therefore, CH4 stored in the shale is displaced by the injected CO2.28,29 This approach not only provides a long-term sequestration for the greenhouse gas CO2 but also enhances the production of shale gas. However, as the injection process is maintained for a certain period, CO2 will partially flow out of the shale gas environment and the product gas might be contaminated by CO2 when the shale rocks are saturated with CO2 gas. Therefore, the recovered shale gas is a CO2/CH4 gas mixture and should be processed for CO2 removal before it is utilized or transferred to consumers. The recovered CO2 could be circulated and reused for the production of shale gas. The problem with the separation of CO2 from the CO2/CH4 gas mixture by hydrate formation is that both CO2 and CH4 molecules might be incorporated into the hydrate crystals under the hydrate formation conditions and, thus, affect the Received: August 11, 2014 Accepted: November 13, 2014

A

dx.doi.org/10.1021/je5007482 | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Table 1. Purities and Suppliers of Materialsa

CO2 separation efficiency.30 On the other hand, the rate of hydrate formation should be promoted if this technique is practically used. Recent studies have shown that the reduction of hydrate formation conditions or the use of chemical additives could improve the hydrate formation kinetics and increase the CO2 separation efficiency.24,31,32 It should be noted that better understanding of the thermodynamic behaviors of gas hydrates formed from the CO2/CH4 mixture is essential for the improvement of the formation kinetics of gas hydrates formed from such gas mixture. Lee et al.33 reported the phase equilibrium behaviors for gas hydrates formed from a CO2/ CH4 gas mixture (50:50) in the presence of tetrahydrofuran (THF) and studied the hydrate cage occupancy of CH4 and CO2 molecules using Raman spectroscopy. Ricaurte et al.25 studied the kinetics of CO2 separation from a CO2/CH4 gas mixture (0.75 CO2 and 0.25 CH4 in mole fraction) by hydrate formation in the presence of THF. Unfortunately the driving force was not quantitatively determined because they did not find the equilibrium data for the CO2−CH4−THF mixed hydrates, and the phase equilibrium data reported by Lee et al.33 cannot be used because the composition of the CO2/CH4 gas mixture was different. It should be noted that equilibrium data of the CO2−CH4−THF mixed hydrates will vary with the change of gas composition and THF concentration, and the structure of the CO2−CH4−THF mixed hydrates might change as well with the variation of the CO2/CH4 gas composition. In this work, the hydrate-based separation process was proposed to separate CO2 from the shale gas that was recovered by CO2 injection. A CO2/CH4 gas mixture (0.4 CO2 and 0.6 CH4 in mole fraction) was used to simulate the shale gas according to the preliminary field data, and THF was employed as a thermodynamic promoter to reduce the hydrate phase equilibrium conditions. The phase equilibrium data are of great importance to the development of the hydrate-based separation process for CO2 separation from the shale gas recovered by CO2 injection. However, to the best of our knowledge, phase equilibrium data of the CO2−CH4−THF mixed hydrates formed from the CO2/CH4 gas mixture (0.4 CO2 and 0.6 CH4 in mole fraction) using THF as a promoter have not been reported in the literature. Therefore, the purpose of this work is to measure and report the phase equilibrium data of gas hydrates formed from the model shale gas (CO2/ CH4 gas mixture) at different THF concentrations. Then the structure of the CO2−CH4−THF mixed hydrates was determined in the presence of THF. The driving force (overpressure) for hydrate formation was quantitatively determined based on the measured phase equilibrium data, and the impact of driving force on CO2 recovery from the CO2/CH4 gas mixture (0.4 CO2 and 0.6 CH4 in mole fraction) was also studied in the presence of THF.

material recovered shale gasb tetrahydrofuran

supplier Chongqing Rising Gas Chongqing Oriental Chemical

purity 0.9995 (mole fraction) 0.99 (mass fraction)

analysis method GC GC

a Deionized water was used in all experiments. bMole fraction of the gas mixture was 0.4 CO2 and 0.6 CH4.

2.2. Apparatus and Procedure. The schematic diagram of the experimental apparatus is shown in Figure 1. It consists of a high-pressure vessel constructed of 316 stainless steel. The volume of this vessel is 375 cm3. As seen in Figure 1, it was immersed in a water bath to maintain the temperature of the reactor contents at a desired value. The temperature of the water bath was controlled by an external refrigerator. Two quartz windows were equipped in the front and rear sides of the vessel, allowing a visual access to hydrate formation/ decomposition inside the vessel. A magnetic stir bar was used to agitate the vessel contents, which was coupled with an electromagnetic plate beneath the vessel. Two copper− constantan thermocouples (Omega Engineering Corporation, U.S.A.) with the uncertainty of ±0.1 K were inserted into the vessel to measure the liquid and gas temperatures. A pressure transducer (EJX430A, Yokogawa Electric Corporation, Tokyo, Japan) with the uncertainty of 0.04% in the range of 0 MPa to 16 MPa was used to measure the pressure in the vessel and connected to a data acquisition unit (GE Automation Corporation, U.S.A.). The data acquisition unit communicated with a computer, and thus, the real time measurements of temperature and pressure were recorded in the computer. A vent valve installed at the outlet of the vessel was used for venting and sampling the gas mixture remaining in the reactor at the end of the experiment. A gas chromatograph (GC-2014, Shimadzu Corporation, Kyoto, Japan) with the uncertainty of 0.001 mole fraction was used to analyze the composition of the gas mixture sampled at the end of the experiment. The incipient equilibrium hydrate formation pressures at given temperatures were determined using the isothermal pressure search method.34 Prior to the experiments, the reactor was cleaned with deionized water and dried. Subsequently, it was filled with 140 cm3 of liquid water or THF solution. Once the temperature of the liquid phase (inside the reactor) reached the desired value, the gas mixture (CO2/CH4) from the gas cylinder was injected into the reactor and discharged three times to remove any air present in the system (reactor and the tubing). Then the reactor was pressurized with the gas mixture from the gas cylinder to a pressure well above the expected equilibrium hydrate formation pressure (∼2 MPa in excess) corresponding to the given temperature. When the temperature and pressure were stabilized, the magnetic stir bar was started to agitate the reactor contents and promote hydrate formation. Once a small amount of hydrate crystals were formed and could be seen through the viewing windows, the pressure was reduced to a value that ensures the hydrate crystals completely decomposed. This process was repeated three times to eliminate any hysteresis that is associated with hydrate formation. Then, the pressure of the system was increased to a value approximately 0.5 MPa above the estimated equilibrium pressure and thus a small amount of hydrate was formed. Subsequently, the pressure was gradually decreased by a step of 0.05 MPa, and the system was kept at least 4 h under each

2. EXPERIMENTAL SECTION 2.1. Materials. Table 1 reports the specifications and supplier names of the materials used in this work. The gas mixture with mole fractions of 0.4 CO2 + 0.6 CH4 was supplied by Chongqing Rising Gas with a composition uncertainty of ±0.0005. This gas composition was selected to simulate the shale gas recovered by CO2 injection. Tetrahydrofuran (THF) was purchased from Chongqing Oriental Chemical Co., Ltd., with a certified mass purity of 0.99 (gas−liquid chromatography method). The uncertainty in the mass fraction of tetrahydrofuran is 0.01. Deionized water was used in all experimental runs. B



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Figure 1. Schematic diagram of the experimental apparatus.

pressure condition. If there was still very small amount of hydrate crystals remaining in the system overnight and the pressure was constant, the pressure at the given temperature was considered to be the incipient equilibrium pressure. This procedure was repeated for the measurements of incipient hydrate formation pressures at different temperatures in liquid water and in the presence of THF (0.005, 0.01, 0.03, and 0.05 mole fractions). When the incipient hydrate formation pressure was determined, the gas mixture remaining in the vessel was sampled and the gas composition was analyzed by the gas chromatograph.

3. RESULTS AND DISCUSSION 3.1. Validation of Apparatus. This was the first occasion that the experimental apparatus was used to measure the incipient equilibrium hydrate formation conditions. Therefore, an initial study was performed to validate the accuracy of the experimental apparatus. The incipient equilibrium conditions for gas hydrates formed from the 0.4 CO2 + 0.6 CH4 gas mixture in liquid water were measured using the experimental procedure described above. The results are shown in Figure 2 and compared with the data reported by Adisasmito et al.35 and Seo et al.36 As can be seen in Figure 2, the results obtained from our experimental setup are in good agreement with the data in the literature14,35,36 and compare well with the predictions by CSMHyd.37 This result indicates that the experimental apparatus and procedure adopted in this work are reliable for the measurements of hydrate phase equilibrium data. It also can be seen in Figure 2 that the hydrate formation pressures obtained from the CO2/CH4 mixture are lower than those of pure CH4 but higher than those of pure CO2 at given temperatures. This indicates that the CO2/CH4 mixed hydrates are more stable than CH4 hydrate but less stable than CO2 hydrate. It should be noted that the phase equilibrium conditions for the CO2/CH4 mixed hydrates will approach to those of CO2 hydrate as CO2 composition increases in the CO2/CH4 gas mixture. 3.2. Hydrate Phase Equilibrium Data for the CO2/CH4/ THF System. The phase equilibrium data of gas hydrates formed from the CO2/CH4 mixture in the presence of THF are reported in Table 2 and are plotted in Figure 3. The mole

Figure 2. Phase equilibrium data of gas hydrates formed from different hydrate-forming gases in liquid water: ■, CH4, Adisasmito et al.;35 △, CH4, Mohammadi et al.;14 ●, CO2, Adisasmito et al.;35 ○, 0.4 CO2 + 0.6 CH4 (in mole fraction), Seo et al.;36▲, 0.4 CO2 + 0.6 CH4, this work; □, 0.4 CO2 + 0.6 CH4, Adisasmito et al.;35 solid line, 0.4 CO2 + 0.6 CH4, predicted by CSMHyd.37

fractions of THF used in this study are 0.005, 0.01, 0.03, and 0.05, respectively. As seen in Figure 3, the phase equilibrium data obtained in the presence of THF are significantly reduced as compared to those obtained in liquid water. For example, the phase equilibrium pressure for gas hydrates formed in the presence of 0.01 mol faction of THF is 0.258 MPa at 276.95 K, whereas that for gas hydrates formed in pure water is 2.637 MPa at the same temperature. This result indicates that the presence of THF has shifted the CO2/CH4 mixed hydrates formation conditions to lower pressures and higher temperatures, and is consistent with the promotion effect of THF reported in the literature.38−41 From the view of industrial application, the low-pressure and high-temperature conditions are needed because of the operation cost. Therefore, THF can be used as a potential thermodynamic promoter for the reduction of phase equilibrium conditions of the CO2/CH4 mixed hydrates. C



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to 1.319 MPa while the temperature increases to 284.85 K. The equilibrium pressure was also observed to increase with the increase of temperature at other THF mole fractions (0.005, 0.03, and 0.05). This means that the formation of the CO2/ CH4 mixed hydrates at a given THF concentration will become difficult with the increase of the system temperature. This result agrees with the pressure−temperature relationship in the hydrate formation equilibrium conditions obtained in the absence of THF. In addition, lower phase equilibrium conditions for the CO2/ CH4 mixed hydrates were obtained as the mole fraction of THF increases from 0.005 to 0.05. For example, the phase equilibrium pressures obtained at 0.005 mole fraction of THF were in the range of (0.621 to 2.247) MPa at (277.15 to 284.95) K, and that obtained at 0.03 mole fraction of THF were in the range of (0.2 to 1.235) MPa at (280.25 to 288.15) K. When the mole fraction of THF increases to 0.05, the phase equilibrium pressures were in the range of (0.427 to 1.795) MPa at (283.25 to 291.15) K. Interestingly, for the hydrates formed at a given temperature, the equilibrium pressure obtained at 0.03 THF mole fraction is observed to be very close to that obtained at 0.05 THF mole fraction. This result indicates that increasing THF concentration will not give a significant thermodynamic promotion for the formation of the CO2/CH4 mixed hydrates when the mole fraction of THF exceeds 0.03. The original composition of the CO2/CH4 gas mixture in the gas cylinder is 0.4 CO2 and 0.6 CH4 in mole fractions. As seen in Table 2, the vapor composition at the equilibrium state is different from the original gas composition at the beginning of the experiments. Note that the composition of CO2 in the vapor phase is lower than that in the original gas mixture. This is due to that the solubility of CO2 in liquid water is much higher than that of CH4 under given pressure and temperature conditions. Based on the incipient equilibrium pressure, temperature, and the vapor composition measured at the equilibrium state, we calculated the heat of hydrate dissociation (ΔHd) using the Clausius−Clapeyron equation, which is given as follows:

Table 2. Phase Equilibrium Data of Gas Hydrates Formed with the CO2 + CH4 Gas Mixture in the Presence of THF along with the Mole Fractions of CO2 and CH4 in the Vapor Phase at the Equilibriaa xTHF

T K

MPa

xCO2

xCH4

0.005

277.15 279.15 280.95 283.05 284.95 276.95 277.15 278.95 280.85 282.95 284.85 287.15 289.15 291.35 280.25 282.05 283.75 286.15 288.15 283.25 285.05 287.15 289.25 291.15

0.621 0.866 1.211 1.603 2.247 0.258 0.302 0.477 0.727 1.011 1.319 1.741 2.563 3.423 0.200 0.355 0.574 0.860 1.235 0.427 0.645 0.930 1.343 1.795

0.338 0.345 0.367 0.334 0.331 0.396 0.383 0.361 0.381 0.349 0.347 0.340 0.340 0.357 0.302 0.392 0.350 0.352 0.343 0.330 0.371 0.345 0.317 0.334

0.662 0.655 0.633 0.666 0.669 0.604 0.617 0.639 0.619 0.651 0.653 0.660 0.660 0.643 0.698 0.608 0.650 0.648 0.657 0.670 0.629 0.655 0.683 0.666

0.01

0.03

0.05

P

Vapor composition

a

Mole fraction of the CO2 + CH4 gas mixture was 0.4 CO2 and 0.6 CH4. Standard uncertainties u are u(T) = 0.1 K, u(P) = 6.4 kPa, and u(x) = 0.001.

ΔHd d(ln P) = d(1/T ) zR

(1)

where P and T are the pressure and temperature in the reactor at the hydrate equilibrium state, R is the universal constant, and z is the gas compressibility determined by the Pitzer correlation for the second virial coefficient42 z = 1 + B0

Pr P + ωB1 r Tr Tr

(2) 0

1

where the equations of Abbott were used for B and B . Figure 4 shows the Clausius−Clapeyron plot from the experimental data of the CO2/CH4 gas mixture in equilibrium with hydrate formed in the presence of THF. It can be clearly seen that the data points obtained at different THF mole fractions (0.005, 0.01, 0.03, and 0.05) fall on lines respectively, and the four lines are almost in the same constant slope, indicating that a constant heat of hydrate dissociation was obtained for the CO2/CH4 mixed hydrates formed in the presence of THF. According to the slope of the logarithm of the hydrate dissociation pressure against the reciprocal temperature, the heat of hydrate dissociation for gas hydrates formed from the CO2/CH4 gas mixture in the presence of THF

Figure 3. Phase equilibrium conditions for gas hydrates formed with the 0.4 CO2 + 0.6 CH4 gas mixture (in mole fraction) in the presence of tetrahydrofuran (THF): ●, pure water (xTHF = 0), predicted by CSMHyd;37 □, xTHF = 0.005; ■, xTHF = 0.01; ○, xTHF = 0.03; ▲, xTHF = 0.05.

It also can be seen in Figure 3 that the equilibrium pressure obtained at a given mole fraction of THF increases with the increase of temperature. For example, the equilibrium pressure for the CO2/CH4 mixed hydrates formed in the presence of 0.1 mole fraction of THF is 0.258 MPa at 276.95 K and increases D



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conditions and results were reported in Table 3. As seen in Table 3, the experiments were carried out at two pressures of 2.8 and 4.3 MPa, and at a fixed temperature of 277.15 K in the presence of xTHF = 0.01. The results include induction time (tind), final gas uptake (ΔnH), CO2 mole fraction in the gas phase (xgas CO2) remaining in the reactor at the end of the experiments, CO2 mole fraction in the hydrate phase (xHCO2), CO2 recovery (R), and separation factor (S). Final gas uptake is the amount of gas mixture consumed during the hydrate formation process. It was calculated using the method reported in the previous work13 and is given as follows: ΔnH = ng,0 − ng,t =

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

(3)

where ng is the number of moles of the gas mixture in the reactor at time 0 and time t, P is the pressure in the reactor, T is the temperature of the gas phase, V is the volume of the gas phase, and z is the gas compressibility calculated by eq 2. CO2 recovery or split fraction (R) indicates the recovery efficiency of CO2 from the total CO2 supplied to the reactor and was calculated by the following equation.

Figure 4. A Clausius−Clapeyron plot based on the hydrate equilibrium data obtained from the CO2/CH4 gas mixture (0.4 CO2 and 0.6 CH4 in mole fraction) in the presence of THF: □, xTHF = 0.005; ■, xTHF = 0.01; ○, xTHF = 0.03; ▲, xTHF = 0.05.

was calculated and was found to be 57.2 ± 5.72 kJ·mol−1. This value is in the range of the structure II hydrate dissociation heat reported by Sloan et al.,37 indicating that the CO2/CH4 gas mixture forms structure II hydrate in the presence of THF. This result agrees with what was reported in the literature that structure II gas hydrate is formed when THF is introduced into the hydrate-forming system.33,43 For the CO2/CH4 gas mixture, THF molecules and CO2 molecules might occupy the large cavities of the structure II hydrate, and CH4 molecules will favorably enter the small cavities or compete with CO2 to enter the large cavities. The cage occupancy of gas hydrates formed in the CO2/CH4/THF system can be determined by the hydrate characterization such as Raman or XRD spectroscopy. Interestingly, the CO2/CH4 gas mixture forms structure I hydrate in the absence of THF,44 in which CH4 molecules is able to occupy both the small and large cavities but CO2 molecules would only occupy the large cavities. 3.3. Impact of Driving Force on CO2 Separation in the CO2/CH4/THF System. Based on the measured incipient phase equilibrium data (Table 2), the effect of driving force (overpressure) on CO2 separation in the CO2/CH4/THF system was investigated using the same apparatus. Overpressure (ΔP) was defined as the difference between the experimental pressure (Pexp) and the phase equilibrium pressure (Peq) at a given temperature (ΔP = Pexp − Peq).37 The experimental

R=

H nCO 2 feed nCO 2

× 100% (4)

H where nfeed CO2 is the moles of CO2 supplied to the reactor and nCO2 is the moles of CO2 incorporated into the hydrate phase at the end of the experiments. Separation factor (S) represents hydrate selectivity of CO2 from the gas mixture (CO2/CH4) and was calculated by the following equation.

S=

gas H × nCH4 nCO 2 gas H × nCH nCO 2 4

ngas CO2

(5)

ngas CH4

where and are the numbers of moles of CO2 and CH4, respectively, in the gas phase at the end of the experiments. nHCH4 is the number of moles of CH4 incorporated into the hydrate crystals at the end of the experiments. Induction time was defined as the period from the beginning of the experiment to the occurrence of hydrate nucleation. The appearance of hydrate nuclei was determined by visual observations through the viewing windows. It can be seen in Table 3 that induction times obtained at the two driving forces were both short (less than 10 min). This indicates that the

Table 3. Experimental Conditions and Results for CO2 Separation from the CO2/CH4 Gas Mixture in the Presence THFa exp. no. 1 2 3 4 5 6 7 8

exp. state fresh memory fresh memory fresh memory fresh memory

Pexp

ΔPb

tind

final gas uptake

MPa

MPa

min

mol

2.8

2.5

4.3

4.0

8.5 7.0 5.5 4.8 5.3 4.8 5.3 5.1

0.1099 0.1083 0.1029 0.1018 0.1083 0.1092 0.1194 0.1129

xgas CO2

xHCO2

R

S

% 0.293 0.316 0.291 0.329 0.377 0.373 0.382 0.363

0.543 0.512 0.538 0.556 0.418 0.407 0.426 0.443

54.8 50.4 53.5 46.8 24.8 23.8 28.2 29.4

3.6 2.8 4.0 2.6 1.8 1.6 1.8 2.3

a

Gas composition: 0.4 CO2 and 0.6 CH4 in mole fraction; Constant conditions: T = 277.15 K, xTHF = 0.01, liquid volume =140 cm3. bDriving force ΔP = Pexp − Peq, Peq = 0.3 MPa at 277.15 K. E



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nucleation of gas hydrates formed from the CO2/CH4 gas mixture was promoted by the presence of THF. In addition, it was found that the induction times obtained in memory solutions were shorter than those obtained in fresh solutions under the same pressure and temperature conditions. This agrees with the fact that hydrate nucleation in the solutions that has experienced hydrate formation (memory solutions) usually occurs faster than that in fresh solutions. Figure 5 shows the effect of driving force on CO2 mole fraction in the gas phase and in the hydrate phase at the end of

Figure 6. Effect of driving force on CO2 recovery (R) and separation factor (S) from the gas mixture of 0.4 CO2 + 0.6 CH4 in mole fraction. The experiments were carried out at 277.15 K and xTHF = 0.01.

mixture were greatly reduced in the presence of THF, but CO2 recovery (R) and separation factor (S) were decreased significantly at higher driving force due to the strong competition between CH4 and CO2 for the occupation of hydrate cavities. In the case of CO2 separation in the CO2/ CH4/THF system, a lower pressure at the given temperature is preferred for CO2 recovery from the CO2/CH4 gas mixture and the hydrate selectivity of CO2 will be increased. This is favorable for future industry applications because the gas compression cost will decrease at the lower pressure. Meantime, a higher CO2 separation efficiency will be acquired.

Figure 5. Effect of driving force on CO2 mole fraction in the gas phase (remaining in the reactor) and in the hydrate phase at the end of the experiments. The experiments were carried out at 277.15 K and xTHF = 0.01.

4. CONCLUSIONS The phase equilibrium conditions for gas hydrates formed from the 0.4 CO2 + 0.6 CH4 gas mixture in the presence THF were measured using the isothermal pressure search method. The results indicated that the presence of THF has shifted to the hydrate equilibrium formation conditions to high temperatures and low pressures as compared with those obtained using the same gas mixture in pure water. Therefore, THF can be used as a potential thermodynamic promoter for CO2 separation from the CO2/CH4 gas mixture by gas hydrate formation. For the hydrates formed at a given temperature, the equilibrium pressure was decreased with the THF mole fraction increasing from 0.005 to 0.05. The heat of hydrate dissociation was determined based on the phase equilibrium data of gas hydrates formed the CO2/CH4 gas mixture in the presence of THF. It was found that the CO2/CH4 gas mixture forms structure II gas hydrate in the presence of THF. The impact of driving force on CO2 separation from the CO2/CH4 gas mixture indicated that the competition between CO2 and CH4 for hydrate cage occupancy became stronger with the increase of driving force. As a result, a lower pressure at the given temperature is preferred for CO2 separation from the CO2/CH4 gas mixture by hydrate formation in the presence of THF.

the experiments. It can be clearly seen that CO2 mole fraction in the hydrate phase (xHCO2) was higher than 0.4 in the original gas mixture that was supplied to the reactor at the beginning of the experiments. It was also higher than that in the gas phase remaining in the reactor (xgas CO2) at the end of the experiments. This is evidence showing the preference of CO2 enclathration in the hydrate phase as compared to CH4 in the presence of THF. However, when the driving force (ΔP) was increased from 2.5 to 4.0 MPa CO2 mole fraction in the hydrate phase was observed to decrease sharply from 0.537 to 0.424. Meantime, the CO2 mole fraction in the gas phase remaining in the reactor was increased from 0.307 to 0.374. This indicates that strong competition between CH4 and CO2 molecules for the occupation of hydrate cavities occurred with the increase of driving force, and as a result, the amount of CO2 captured in the hydrate phase was reduced at higher driving force. Figure 6 shows the effect of driving force on CO2 recovery (R) and separation factor (S) in the presence of THF. As seen, CO2 recovery (R) decreased significantly with the increase of driving force. For example, CO2 recovery (R) obtained at ΔP = 2.5 MPa was 51.4%, but it decreased to 26.6% at ΔP = 4.0 MPa. The separation factor (S) was 3.3 at ΔP = 2.5 MPa and was reduced by 42% at ΔP = 4.0 MPa. This result indicates that more CH4 were incorporated into the hydrate phase at higher driving force, and thus, the amount of hydrate cavities that were able to trap CO2 molecules became smaller, causing the decrease of CO2 recovery (R) and separation factor (S). This is in good agreement with what was reported in Figure 5. Therefore, it was found that the incipient phase equilibrium conditions for gas hydrates formed from the CO2/CH4 gas

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The financial support from the Ministry of Education Innovation Research Team (IRT13043), the National Key F



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Basic Research Program of China (No.2014CB239206), and the National Natural Science Foundation of China (No.51006129) is greatly appreciated. Notes

The authors declare no competing financial interest.

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Phase Equilibrium Data of Gas Hydrates Formed from a CO2 + CH4

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