Experimental Study of Conditions for Methane Hydrate Productivity by

Oct 8, 2015 - ABSTRACT: Various saturated methane hydrates (MHs) formed in porous media were swapped with CO2 to investigate the principles of ...
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Experimental Study of Conditions for Methane Hydrate Productivity by the CO2 Swap Method Jiafei Zhao,† Lunxiang Zhang,† Xiaoqing Chen, Zhe Fu, Yu Liu,* and Yongchen Song* Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, School of Energy and Power Engineering, Dalian University of Technology, Dalian, Liaoning 116024, People’s Republic of China ABSTRACT: Various saturated methane hydrates (MHs) formed in porous media were swapped with CO2 to investigate the principles of replacement reactions in different regions surrounded by three curves: (L−V)CO2, (H−V)CH4, and (H−V)CO2. The replacement percentage was analyzed to evaluate the productivity of MHs, with the replacement reactions in zone A [above (H−V)CO2, above (H−V)CH4, and above (L−V)CO2], zone B [above (H−V)CO2, above (H−V)CH4, and below (L− V)CO2], and zone C [above (H−V)CO2, below (H−V)CH4, and below (L−V)CO2]. Temperature conditions under the ice point caused restrictions in the replacement process. The two key factors affecting this phenomenon are the area of MHs for the replacement reactions and the diffusibility of CO2 in the three zones. Pressure signals of replacement procedures in the three zones were discussed to investigate the possible CO2−CH4 exchange kinetics. Two replacement stages were observed: surface replacement and inner layer replacement. Pressures in zone C decreased instead of recovering to the equilibrium line of the MHs. We also analyzed other affecting factors, such as the pressure and temperature. In zone C, replacement percentage decreased as the pressure increased at the same replacement temperature and MH saturation. In zone B, replacement percentage increased as the temperature increased with the same pressure and MH saturation, although this tendency was not obvious when the temperature was below the freezing point.

1. INTRODUCTION Natural gas hydrates (NGHs) are crystalline solids formed by gas molecules trapped in water cavities,1 which exist below the ocean floor and in permafrost.2 Currently, hydrates are considered to be a potential unconventional energy resource.1 It is conservatively estimated that the amount of carbon bound in NGHs is twice the amount of carbon to be found in all known fossil fuels on Earth.3 Because each volume of hydrates can contain as much as 184 volumes of gas [standard temperature and pressure (STP)], hydrates are also considered to be a means of storing mass.1 CO2 is a type of greenhouse gas, the emission of which is harmful to human development.4 Storage of CO2 in hydrate reservoirs by replacing entrapped natural gas is considered an interesting option for the safe longterm storage of CO2.3 In the process of exploiting natural gas from methane hydrates (MHs), researchers have explored four ways to decompose the crystalline inclusion compounds: depressurization, thermal stimulation, inhibitor injection, and CO 2 replacement.5 Among these four methods, carbon dioxide replacement has offered the most stable long-term solution for the storage of a greenhouse gas. The benefits include methane production without requiring heat6 and the stabilization of the ocean floor7 without causing geological deformation. Additionally, carbon dioxide hydrates are thermodynamically more stable than MHs.8 This method has attracted considerable research. The process of MHs swapped by carbon dioxide has been demonstrated to be feasible both kinetically9,10 and thermodynamically.2,11−13 The concept of injecting CO2 into the MH layer under the seafloor and permafrost to replace methane gas was proposed by Ebinuma in 1993.13 As a key indicator of the commercial viability of mining MH by replacement, the replacement © XXXX American Chemical Society

percentage has been studied through experiments and simulations. However, the statistics of replacement percentage reported in the literature exhibit different upper limits. One limit is given in the study of McGrail et al.14 Because of the differences in chemical affinity for CO2 versus CH4 in the structure I (sI) hydrates, the mole fraction of methane would be reduced to approximately 0.48 in the hydrate and rise to a value of 0.7 in the gas phase at equilibrium. Ota et al.2 conducted experiments using in situ Raman spectroscopy with liquid CO2 and determined that 65% of the bulk-scale methane remained in the hydrate form after 307 h of replacement at 273.2 K. In experiments by Ota et al.15, the calculated compositions of the hydrate phase after 280 h at 3.2 MPa (by successive CO2 swap) were 0.69 for CH4 (hCH4) and 0.31 for CO2 (hCO2). These values almost coincided with the corresponding conditions at 277 h (hCH4, 0.73; hCO2, 0.27). In research by Zhou et al.4, 13.2% methane was replaced after 43 h. The reaction conditions were below the MH equilibrium line but above the carbon dioxide hydrate line. Higher limits were presented in the following studies. Experiments by Komai et al.16 have reported that up to 80% MH was replaced in 12 h at 2 MPa and at 274 ± 0.5 K by the bulk phase scale. The data were obtained from the peak strength for Raman shifts. It was found that, in addition to the production of ice crystals, the reformation of carbon dioxide hydrate occurred at the surface and inside the sample of MH. Lee et al.17 examined the limiting composition of mixed hydrates in the bulk phase with nuclear magnetic resonance (NMR) spectra and assumed that at least Received: April 24, 2015 Revised: October 8, 2015

A

DOI: 10.1021/acs.energyfuels.5b00913 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Schematic diagram of the experimental apparatus.

Meanwhile, other researchers have shown interest in the effect of the reaction time, water saturation, and hydrate saturation on the replacement percentage. The experimental data about the effect of time reported by Wang et al.25 and Li et al.26,27 show that the variation tendency of the decomposed CH4 hydrate and formed CO2 hydrate during the replacement process is the same as that of the experiment by Ota et al.15 These results show that the highest ratio of decomposed CH4 hydrate and formed CO2 hydrate usually occurs in the early stages of reaction and the replacement rate becomes extremely slow after the early stages of reaction. Yuan et al.28 performed experiments on methane recovery from NGH using pressurized liquid CO2 and pointed out that the replacement ratio increases with the decrease of hydrate saturation and the increase of water saturation. In addition, although the factors influencing the replacement percentage have been researched for many years, a further study is still required on other important parameters, such as fluid mixing, multiphase fluid flow, porous media, volume ratios, etc. There appear to be discrepancies in the effects of the temperature, pressure, and different displacers in the published literature. Thus, several comparative experiments were conducted in this work, and the replacement percentage has been discussed systematically to reveal the effects of the pressure and temperature. To study the appropriate conditions for the replacement reaction and find the appropriate state of CO2 to swap MHs, basic statistics are needed to be derived and discussed on the basis of our experiments. In this study, we conducted 40 replacement experiments, explored the replacement percentage with operating points located at A [above (H−V)CO2, above (H−V)CH4, and above (L−V)CO2], B [above (H−V)CO2, above (H−V)CH4, and below (L−V)CO2], and C [above (H−V)CO2, below (H− V)CH4, and below (L−V)CO2] zones in the phase equilibrium diagram systematically, and considered the effects of MH saturation in porous media. The effects of the temperature and pressure at the same zone were studied, especially when ice was formed. Pressure trends in the three zones are discussed to investigate the possible CO2−CH4 exchange kinetics.

64% of methane should have been recoverable as a result of considerable enrichment of CO2 in the hydrate. Yuan et al.18 considered different CO2 states and compared the replacement percentage using different displacers in the literature. By comparison, liquid CO2 offers a higher replacement percentage than gaseous CO2. Researchers have used experimental methods and simulations to search for the optimal conditions for using CO2 to replace CH4 from the hydrate. Experiments about the effect of the pressure, temperature, and different displacers on the replacement percentage have been extensively carried out in the past few years. Zhao et al.9 have reported that both higher initial temperatures and pressures are beneficial for improving the replacement rate and efficiency. Deusner et al.19 showed that reservoir conditions, including the temperature, pressure, and permeability, have a major influence on the production process during the flow-through experiments. They found that CH4 production from CO2 injection was most efficient at 8 °C, while gas recovery was apparently independent of the pressure. In contrast, Masaki et al.15 investigated the effects of the pressure on the CH4−CO2 replacement in CH4 hydrate using quantitative analysis with in situ laser Raman spectroscopy and found that the CH4−CO2 replacement at the boundary of the liquid and hydrate phases (273.2 K and above 3.60 MPa) proceeded faster than at the boundary of the gaseous and hydrate phases (273.2 K and 3.26 MPa). For the replacement in the liquid phase, strong pressure dependence was not observed. Meanwhile, Komai et al.16 found that the formation rates of gas hydrate greatly changed with the temperature, especially in the temperature range between 269 and 275 K. Yuan et al.18 simulated the variations of the mole fraction of CH4 in the gas phase and CO2 in the hydrate phase under different pressures (3.5, 3.15, 2.5, and 2.0 MPa) with respect to time. These researchers found that, as a result of higher driving forces, the mole fraction of CH4 in the gas phase and CO2 in the hydrate phase both increased and the system pressure decreased. Moreover, in the enhanced gas hydrate recovery (EGHR) report for the U.S. Department of Energy,20 the authors noted that gaseous CO2 is more efficient than liquid CO2 because, with liquid CO2 injection, thermodynamic conditions can either favor CO2 or CH4 cage occupation. B

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The original data of the experiments to explore the replacement percentage of zones A, B, and C under different MH saturations are shown in Table 2. Experimental conditions and calculations are included. Typical phase diagrams of methane (gas/hydrates) and carbon dioxide (gas/liquid/hydrates) are shown in Figure 2.8 The operating points of this study are located at zone A (above LCO2−VCO2), zone B (between LCO2−VCO2 and water−HCH4−VCH4), and zone C (between water−HCH4−VCH4 amd water−HCO2−VCO2). It can be observed that zone C offers the optimum condition for the replacement reaction where CO2 hydrates exist and CH4 hydrates decompose. 2.3. Calculations of CH4 Replacement. In calculation of the replacement percentage, the mass balance of CH4 before and after hydrate formation and before and after the replacement was determined by means of the following equations: There are two conditions for the calculation, and the sI MH composition can be expressed by the formula 8CH4·46H2O. Note that MHs formed in the lab are probably not ideal, and the value of 6 as the hydrate number allows the possibility of empty cages:1 (1) The ratio of water molecules to gas molecules in hydrate (called the hydrate number) is 6.22 The density of MHs is 0.918 g/cm3. (2) The porosity of BZ08 is 0.399. The conservation of volume of the pore is calculated as follows:

2. EXPERIMENTAL SECTION 2.1. Apparatus. A schematic of the experimental apparatus used in this work is shown in Figure 1. The unit consists of four parts: a reaction cell, signal conversion device to record pressure and temperature, gas-supplying system, and gas chromatography to analyze the gas composition. The reactor is made of stainless steel and has a set of quartz windows on either side for direct observation of the hydrate formation and dissociation process. Thus, the inside volume is 288 mL and consists of two parts: the column made by stainless steel (255.3 mL with a diameter of 50 mm and height of 130 mm) and the quartz windows (35.6 mL with a length of 112 mm, width of 26.5 mm, and height of 6 mm). The temperature of the cell during the experiment is controlled by a glycol bath. One pressure and one temperature sensor are attached to the cell. Four thermocouples are distributed evenly to record the temperature gradient of the cell. The signals are input into the signal conversion device to record the real-time pressure/ temperature (P/T) conditions. Details of the materials and devices used in these experiments are listed in Table 1.

Table 1. Materials and Devices Used in Experiments item

model

CH4 and CO2 gas glycol bath

F38-EH

data collector mass flow meter

ADAM4000 D08

gas chromatogram pressure reducing value

GC7900 YQD-370

precision

provider

99.99% 0.1 K

Dalian Date Gas Co., Ltd. Julabo Technology (Beijing) Co., Ltd. Advantech Beijing Sevenstar Electronics Co., Ltd. Techcomp (China) Co., Ltd. Shanghai Regulator Factory Co., Ltd.

0.3%

0.2 MPa

Vpore = p08 Vcell

(1)

Vpore = Vw,total + VCH4,total = Vw,for + VCH4,for + VH,for

(2)

where Vpore is the volume of the pore and p08 is the porosity of BZ08, which is typically set at 39.9%. The subscript “total” represents the volume of water and gas injected into the cell under a certain pressure. The subscript “for” represents the condition after hydrate formation. The conservation of methane before and after hydrate formation is shown as follows:

2.2. Procedure. First, the reactor was tightly packed with quartz glass beads (Asone, Co., Ltd., Japan), BZ08 (0.710−0.990 mm, with an average of 0.8 mm), and uniformly saturated with 20 mL of deionized water. A certain amount of methane was subsequently injected into the cell through a cooler for hydrate formation. The temperature of the bath was set at 273.65 K. After several hours, the formation of hydrate was considered to be completely finished under this condition after the pressure and temperature of the cell had stabilized for 1 h. The saturation of MHs across runs was controlled by the amount of methane gas injected. After the formation of MHs, the temperature was decreased to 268.15 K to reduce hydrate decomposition in the process of releasing residual CH4 gas by vacuum pump. The temperature at 268 K is located in the range of 242−271 K, where sI hydrates have selfpreservation phenomenon when releasing P to 0.1 MPa.21 Chilled CO2 gas was subsequently injected to maintain a pressure above the MH equilibrium pressure at 273.65 K controlled by a high-pressure pump (260D, Teledyne Isco, Inc., Lincoln, NE) and thermostatic bath (F25-ME, Julabo, Inc., Germany) filled with Fluorinert FC-40 (3M, St. Paul, MN). The process of gas release and CO2 fill was repeated quickly and finally stabilized at the designed pressure. In the later part of this work, the pressure trend of the replacement reaction will be introduced. During the reaction, the pressure varied only slightly. The value of the pressure normalized with the recorded pressure after 2 h of reaction time when the diffusion stage was finished. After CO2 gas was injected, the bath temperature was set according to experimental design. The replacement reaction proceeded under a constant volume to simulate commercial production. After a fixed reaction time, the residual mixed gas was released and the mixed hydrates decomposed. Samples of both the residual gas and the decomposed gas were collected. The total gas volume was measured. The samples were analyzed for composition using gas chromatography. From the volume of the collected gas mixture and the percentage of methane gas gained by gas chromatography, the amount of remaining MHs was calculated.

i i i nCH = nCH + nCH 4,total 4,H 4,G

(3)

i nCH = VCH4,total,s/22.4 4,total

(4)

i nCH = VCH4,forPfor /z forRTfor 4,G

(5)

niCH4,total

is the total number of moles of methane in the cell, where niCH4,H is the number of moles of methane in the hydrate phase in the cell, niCH4,G is the number of moles of methane released before the injection of CO2, and VCH4,total,s is the volume of total methane injected under standard conditions. MH saturations are calculated as follows:

s = VH,for /Vpore

(6)

i VH,for = MHnCH /ρ 4,H

(7)

where MH is the molar mass of MHs when the hydrate number is 6 (CH4·6H2O) and ρ is the density of MHs. The conservation of methane before and after replacement is calculated as follows: i i ηRe = [(nCH − nCH4,remain)/nCH ] × 100% 4,H 4,H

(8)

nCH4,remain = xCH4,ranknM gas,tank

(9)

nM gas,tank = VM gas,tank /22.4

(10)

where VM gas,tank is the volume of the mixed gas from the mixed hydrate decomposition under standard conditions, xCH4,tank is the mole fraction of methane in mixed gas from decomposition, and ηRe is the replacement percentage (which is used to describe the dynamics of the replacement23). 2.4. Models of the CO2−CH4 Exchange Process. One approach for analyzing the physical mechanisms of the exchange process is to C

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Energy & Fuels Table 2. Experimental Conditions and Results (Effects of the Conditions of Different Saturations and Zones) zone A

zone B

above freezing point

below freezing point

zone C

saturation replacement percentage (%) initial water (mL) residual water (mL) before the replacement pressure (MPa) temperature (K) after the replacement pressure (MPa) temperature (K) saturation replacement percentage (%) initial water (mL) residual water (mL) before the replacement pressure (MPa) temperature (K) after the replacement pressure (MPa) temperature (K) saturation replacement percentage (%) initial water (mL) residual water (mL) before the replacement pressure (MPa) temperature (K) after the replacement pressure (MPa) temperature (K) saturation replacement percentage (%) initial water (mL) residual water (mL) before the replacement pressure (MPa) temperature (K) after the replacement pressure (MPa) temperature (K)

0.096 13.35 20.00 11.22 3.67 273.65 3.77 273.45 0.144 17.58 20.00 6.77 3.11 273.65 3.00 273.65 0.110 6.41 20.00 13.10 3.01 273.65 2.96 272.35 0.148 34.32 20.00 6.39 1.95 273.65 1.78 274.85

0.122 27.10 20.00 8.76 3.60 273.65 3.71 273.45 0.170 27.75 20.00 4.34 3.01 273.65 2.94 273.15 0.134 6.07 20.00 7.69 2.89 273.65 3.40 272.15 0.168 39.13 20.00 4.56 1.85 273.65 1.73 273.24

0.164 28.67 20.00 4.93 3.64 273.65 3.76 273.35 0.176 24.98 20.00 3.80 3.17 273.65 3.10 273.65 0.165 10.93 20.00 4.88 3.01 273.65 2.91 267.35 0.171 34.58 20.00 4.25 1.70 273.65 1.38 274.95

0.185 47.32 20.00 3.04 3.57 273.65 3.67 273.85 0.200 21.91 20.00 1.56 3.00 273.65 2.98 273.30 0.168 8.67 20.00 4.56 2.97 273.65 2.82 268.45 0.188 26.38 20.00 2.72 2.01 273.65 1.84 273.65

0.214 43.86 20.00 0.31 3.58 273.65 3.66 273.85 0.245 18.40 25.00 2.50 2.60 273.65 2.68 273.30 0.172 14.54 20.00 4.20 3.02 273.65 2.85 271.15 0.240 25.01 25.00 3.04 1.71 273.65 1.49 273.53

hydrate film. The general CO2−CH4 exchange has similar physical steps, because it must occur at the surface first and then the diffusion is limited as the interior layers of the hydrate particles are further exposed to CO2. The concepts can be expressed as follows: Avrami model

α = 1 − exp(k1 − t n)

(11)

shrinking core model

(1 − α)1/3 =

2k 2(t − t *) /r + (1 − α*)1/3

(12)

where α is the replacement percentage at time t, α* is the replacement percentage when diffusion through the hydrate film starts at time t*, k is the rate constant with the subscript indicating the growth stage, n is the Avrami exponent, and r is the radius of the particle. The replacement percentage as a function of time is regressed with both equations to obtain the rate constants (k1 and k2) and the Avrami exponent (n).

3. RESULTS AND DISCUSSION The replacement experiments were conducted to explore the replacement percentage operating points located at zones A (liquid CO2 zone), B (gaseous CO2 and NGH stable zone), and C (NGH unstable zone) in the phase equilibrium diagram. Hydrate saturation, pressure, and temperature conditions were studied to discuss their effects on the replacement percentage. Temperature and pressure signals during the replacement reaction in zones A, B, and C were analyzed to investigate the possible CO2−CH4 exchange kinetics. MH saturation in porous media was discussed together with the replacement conditions

Figure 2. Replacement conditions of carbon dioxide and MHs. compare the data using physical diffusion/reaction models. The Avrami equation29−31 and shrinking core model32 are well-known physical models for crystallization kinetics. The Avrami model is applicable in the initial stage of the reaction of the hydrate film, while the shrinking core model can be applied to the diffusion through the D

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Energy & Fuels in the three zones. The effects of the temperature and pressure at the same zones were especially studied when ice was formed. 3.1. Pressure and Temperature Trends during Replacement of Different Zones. Panels a−c of Figure 3 show the temperature and pressure trends of the typical reaction period of zones A, B, and C from MH formation to the end of hydrate dissociation. In Figure 3a, the first stage is MH formation from 0 to approximately 1000 min. The following is the procedure for reducing the temperature to the hydrate selfpreservation range before the release of residual methane gas. From 1200 min onward, the procedure of the replacement reaction was performed for approximately 50 h. The details have been introduced in section 2.2. The same process was adapted and shown in panels b and c of Figure 3. The designed pressure and temperature for the replacement reaction were 3.60 MPa and 273.5 K in zone A, 3.00 MPa and 273.5 K in zone B, and 1.70 MPa and 273.5 K in zone C. Figure 3d shows the replacement percentage trend of the exchange process and fitting line using the two kinetic models (eqs 11 and 12). The Avrami model fits the data from the first 5 h of the reaction; while, the shrinking model best fits the data from 20 to 50 h. During the first 5 h, the amount of CH4 production was more than 60% of the total production over 50 h. The gray zone in Figure 3d is defined as the transition region, where the replacement percentage growth rate drops dramatically. Generally, it was assumed that there were two stages in the exchange process:33 (i) fast surface reaction from 0 to 5 h and (ii) gradual slow process as a result of resistance to diffusion through the formation of the mixed hydrate layers from 5 to 50 h. Therefore, the rate-limiting step is the second stage. In this stage, CO2 must penetrate into the inner part of the CH4 hydrate particles; this reaction rate is slow. Because the thermal effect of the CO2−CH4 hydrate exchange reaction is 3.49 kJ/mol, 6.4% of the MH formation is due to the thermal effect. Thus, any temperature changes during the 50 h replacement reaction in panels a−c of Figure 3 is kept constant by the temperature of the glycol bath. When the replacement condition was located in zone A, the pressure of the replacement reaction increased from 3.60 to 3.71 MPa. The pressure curve had an upward trend. Such a tendency accounts for the accumulation of the gas phase, which is attributed to the partially decomposed MHs and the change of the hydrate number caused by the formation of mixed hydrates. The upward pressure trend is inconsistent with previous studies.24 As shown in Figure 3b (in zone B), the replacement reaction started at 3.00 MPa and ended at 2.98 MPa. The pressure trend was relatively more steady than in zone A because gaseous CO2 is not as efficient as liquid CO2 and the reaction rate and scale are smaller than those of zone A. The slight decrease of pressure reflects gas being consuming caused by more CO2 hydrate formation as a result of extra free water than the CO2/ CH4 exchange. In zone C, the replacement pressure and temperature conditions were located at MHs in the unstable zone. At 273.5 K, the MH equilibrium pressure is 2.70 MPa. However, the pressure decreased as the reaction proceeded, declining from 1.70 to 1.38 MPa. In this experiment, 4.0 g of extra water (after MHs) was formed. The first declining stage was caused by the consumption of CO2 to form CO2 hydrates with extra water. This sharp increase may have been due to the decomposition of MHs because the pressure is below the

Figure 3. Temperature and pressure trends of the typical reaction period of zones A, B, and C: (a) P/T trend of zone A, (b) P/T trend of zone B, and (c) P/T trend of zone C. (d) Replacement percentage comparisons for Avrami (orange line) and shrinking core (blue line) models.

phase equilibrium line. The last declining stage was the reformation of mixed hydrates from free gas. E

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conditions were designed on the basis of the phase equilibrium diagram: in zone A, the state of CO2 is liquid; in zone B, CO2 is gaseous and MH is stable; and in zone C, CO2 is gaseous and MH is unstable. This work was intended to study the replacement efficiency of liquid and gaseous CO2 together because the phase change of CO2 is caused by changes in the temperature and pressure. The detailed experimental conditions are in Table 2. As shown in Figure 4, when saturation increased, the replacement percentage in zone A increased but decreased in zone C. In zone B, the replacement percentage increased to a certain point and decreased when the MH saturation was limited for experiments above the freezing point. Under the same replacement condition above the freezing point, zones A and C have a higher replacement percentage, while zone C is relatively more effective at low MH saturation. Conditions in zone B are the least effective. For replacement temperature below the freezing point, the replacement percentage was the lowest, and as saturation increased, the increase of the replacement percentage was notably slight. This finding confirms the results reported in the literature that liquid CO2 is more effective than gaseous CO2 when located in zone B because liquid CO2 has a better ability to diffuse. While gaseous CO2 in zone C is not discussed in the literature, it has a higher replacement percentage and is favorable for CO2 formation and CH4 decomposition. For MH deposits with lower saturation, zone C is more effective as a result of adequate void spaces for gaseous CO2 to diffuse. Therefore, a favorable temperature and pressure enhanced the replacement reaction. For higher saturation, fewer voids restrict the diffusion of gaseous CO2, leading to a higher replacement percentage in zone A. In zone B, as MH saturation increased, the parabolic trend reflected the interactions of two factors. One factor is the increase of the reaction area as a result of more MHs. The other factor is the increase of the barrier for CO2 to penetrate because gaseous CO2 is not as efficient as liquid CO2. Furthermore, the formation of ice restricted the

The replacement reaction undertaken in this zone was the most effective compared to zones A and B because the temperature and pressure conditions support the formation of CO2 hydrates and the dissociation of CH4 hydrates. 3.2. Replacement Percentage on Conditions of Different Saturations and Zones. The replacement percentage in different zones was determined and shown as a function of hydrate saturation in Figure 4. Experimental

Figure 4. Replacement percentage of runs with different MH saturations (◆, operating points located in zone A above the freezing point; ■, operating points located in zone B above the freezing point; ●, operating points located in zone B below the ice point; and ▲, operating points located in zone C above the freezing point).

Table 3. Experimental Conditions and Results (Effects of the Temperature and Pressure) effect of the temperature on the replacement percentage

effect of the pressure on the replacement percentage

saturation replacement percentage (%) initial water (mL) residual water (mL) before the pressure replacement (MPa) temperature (K) after the pressure replacement (MPa) temperature (K) saturation replacement percentage (%) initial water (mL) residual water (mL) before the pressure replacement (MPa) temperature (K) after the pressure replacement (MPa) temperature (K) F

0.165 10.93 20.00 4.88 3.01

0.168 8.67 20.00 4.56 2.97

0.166 10.48 20.00 4.77 2.99

0.165 14.12 20.00 4.88 3.05

0.164 28.69 20.00 4.93 3.21

0.165 34.98 20.00 4.88 3.17

273.65

273.65

273.65

273.65

273.65

273.65

2.91

2.82

2.89

2.93

3.08

3.01

267.35

268.45

270.45

271.45

273.15

273.85

0.160 56.60 20.00 5.31 1.68

0.168 39.13 20.00 4.56 1.85

0.165 25.45 20.00 4.88 2.57

0.172 14.54 20.00 4.20 3.02

0.166 10.48 20.00 4.77 2.99

0.160 19.38 20.00 5.31 3.58

0.164 28.67 20.00 4.93 3.64

273.65

273.65

273.65

273.65

273.65

273.65

273.65

1.60

1.73

2.45

2.85

2.89

3.62

3.76

274.25

273.24

273.15

271.15

270.45

273.45

273.35

DOI: 10.1021/acs.energyfuels.5b00913 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels replacement reaction by blocking the path for carbon dioxide to transform. 3.3. Effects of the Temperature and Pressure on Replacement Percentage. In this part, six experiments were conducted to explore the effects of the temperature on the replacement percentage under the same pressure of 2.90 MPa with the temperature ranging from 267 to 275 K. The operating points of these runs were all located in zone B (below the phase line of CO2 and above the MH line). Seven experiments were conducted to explore the effects of the pressure on the replacement percentage under the same temperature of 273.5 K, while the pressure ranged from 1.50 to 3.80 MPa. At 273.5 K, the equilibrium pressure for MHs was 2.30 MPa and the pressure for CO2 phase change was 3.50 MPa. The operating points of these seven experiments covered the region of zones A, B, and C. The detailed experimental conditions are in Table 3. Figure 5a shows that, in zone B, the replacement percentage increased from 0.2 at 273.5 K to 0.35 at 275 K. Because of lower activation energy, a higher temperature enhanced a higher level of the chemical reaction. Below 273 K, ice makes it more difficult for CO2 molecules to diffuse into MHs, thereby causing the deposit to exhibit a lower replacement percentage. When the temperature is above the freezing point, the barrier of CO2 diffusion by water is eliminated and the temperature becomes the controlling factor. This result confirms the studies in the literature.16 The formation rates of CO2 increased greatly with increasing temperature, especially between 269 and 275 K. In Figure 5b, operating points located in zones A and C with a higher average replacement percentage were more efficient than in zone B. When the pressure was in zone C, the replacement percentage decreased as the pressure increased. This trend was not observed in zones A and B. Yuan et al.18 divided the replacement procedure into four steps: diffusion of CO2 molecules, dissociation of MH, rearrangement of CH4 and CO2 molecules in cavities, and diffusion of CH4 molecules into the gas phase. In zone A, CO2 is in the liquid phase, which enhances the diffusion of CO2 molecules. Note that, as pressure increased, the diffusion became more efficient. In zone C, a lower pressure enhanced the dissociation of MH and affected the rearrangement of the mixed gas molecules into cavities, thereby making it favorable for CO2 hydrate formation.

Figure 5. Replacement percentage of runs with changing temperature or pressure: (a) replacement percentage of different reaction temperatures and (b) replacement percentage of different reaction pressures.

4. CONCLUSION The swap procedure could be divided into two stages: surface exchange and inner layer exchange. Pressure changes in zone C did not increase as expected, thereby indicating a large scale of the replacement reaction. The replacement percentage in zones A and C is higher than that in zone B, and the temperature conditions under the freezing point restricted the replacement process. As saturation increased, the replacement percentage increased in zones A and B and decreased in zone C. The effects of increasing MH saturation in different zones reflect the interactions of two factors: one is the increase in the reaction area as a result of more MHs. The other is the increase in the barrier for CO2 to penetrate. The effect of the temperature and pressure on the replacement percentage is also discussed. The replacement percentage decreased as the pressure increased at the same replacement temperature and same MH saturation in zone C. The replacement percentage increased as the temperature increased with the same pressure, and the MHs were saturated in zone B. However, this trend was not obvious when the temperature was below the freezing point. Taking the

industrial application into consideration, further research to improve the replacement reaction in zones A and C by the combined method should be conducted.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions †

Jiafei Zhao and Lunxiang Zhang contributed equally to this work and should be regarded as co-first authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study has been supported by the Major Program of the National Natural Science Foundation of China (51436003), the National High Technology Research and Development Program of China, “863 Program” (Grant 2013AA09250302), G

DOI: 10.1021/acs.energyfuels.5b00913 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

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and the Natural Science Foundation of China (Grants 51227005, 51376034, and 51276028).

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NOMENCLATURE Vpore = volume of pore in the cell (mL) p08 = porosity of BZ08 glass sands Vcell = total volume of the cell (mL) Vw,total = volume of water saturated in the glass sands (mL) VCH4,total = volume of CH4 injected in the cell under certain pressure (mL) Vw,for = volume of water remaining in the liquid phase after hydrate formation (mL) VCH4,for = volume of CH4 remaining in the gas phase after hydrate formation (mL) VH,for = volume of MH formed in the cell (mL) i nCH = total number of moles of CH4 in the cell (mol) 4,total i nCH = initial number of moles of CH4 in the hydrate phase 4,H (mol) i nCH = number of moles of CH4 in the gas phase (mol) 4,G VCH4,total,s = volume of CH4 injected in the cell in standard conditions (mL) VCH4,release = volume of released CH4 before injection of CO2 in standard conditions (mL) s = saturation of hydrate in the cell MH = molar mass of MH in the ideal form (g/mol) ρ = density of MH (g/cm3)

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DOI: 10.1021/acs.energyfuels.5b00913 Energy Fuels XXXX, XXX, XXX−XXX