Experimental Study on the Effect of Pressure on the Replacement

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Experimental Study on the Effect of Pressure on the Replacement Process of CO2-CH4 Hydrate below the Freezing Point Xuemin Zhang, Yang Li, Ze Yao, Jinping Li, Qingbai Wu, and Yingmei Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02655 • Publication Date (Web): 05 Dec 2017 Downloaded from http://pubs.acs.org on December 9, 2017

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Fig. 1. Schematic diagram of the experimental apparatus. 139x67mm (96 x 96 DPI)

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Experimental Study on the Effect of Pressure on the Replacement Process of CO2-CH4 Hydrate below the Freezing Point Xuemin Zhang a,b, Yang Li a,b, Ze Yao a,b, Jinping Li a,b, Qingbai Wu a,c, Yingmei Wang a,b a

Western China Research Center of Energy & Environment, Lanzhou University of Technology, Lanzhou 730050, China b

Key Laboratory of Complementary Energy System of Biomass and Solar Energy，Gansu Province, Lanzhou 730050, China c

State Key Laboratory of Frozen Soil Engineering, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Science, Lanzhou, 730000, China

ABSTRACT The recovery of natural gas from CH4-hydrate deposits in permafrost environments through injection of CO2 is considered to be a suitable strategy for CH4 production and CO2 storage. In order to study the replacement characteristics of CO2-CH4 hydrate below the freezing point, the experiment was conducted in ice powder with particle size of 800 µm at different injection pressure (3.6 MPa, 4.0 MPa and 4.5 MPa) of CO2. The experimental results showed that, the average replacement rate and efficiency increased with the increasing of injected pressure of CO2 gas. And the average replacement rate and efficiency reached up to 0.403 mmol/h and 13.20% when the injected pressure of CO2 was at 4.5 MPa. The results also indicated that, compared with the temperature conditions above the freezing point, the replacement rate of CO2-CH4 hydrate was slow below the freezing point. It provided a theoretical guidance for gas production from methane hydrate using CO2-CH4 replacement method in permafrost region in the future. KEYWORDS methane hydrate; replacement characteristics; permafrost region; replacement rate; efficiency 1. INTRODUCTION Natural gas hydrate (NGH) is a non-stoichiometric crystalline compound composed of water and methane gas under low temperature and high pressure. It widely exists below the ocean floor and in the permafrost regions [1], where the necessary conditions of low temperatures and high pressures exist for hydrate stability [2]. And there is a large amount of NGHs in nature [3, 4], so it is regarded as a potential future energy resource with great commercial exploitation potential. Recently, natural gas hydrates have received considerable attention because of their potential as a future energy resource [5, 6]. There has great energy potential and resource prospect for gas hydrate in Qinghai-Tibet Plateau permafrost. Therefore, it is meaningful to study the replacement process of CO2-CH4 hydrate for both the storage of carbon dioxide and the exploitation of natural gas hydrate resources in situ in permafrost regions. Many researchers have investigated the replacement process of CO2-CH4 hydrate. Uchida et al. [7] firstly demonstrated the replacement of CO2-CH4 hydrate occurring in the interface. Ota et al. * Corresponding author. Tel./fax: +86 931 2976 332. E-mail: [email protected] (X. Zhang).


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[8, 9] studied the replacement process of CO2-CH4 hydrate with saturated liquid CO2 and high pressure CO2 at 273.2 K. The result showed that the replacement proceeded faster at the boundary of liquid and hydrate phase than that at the boundary of gaseous and hydrate phase, and the replacement rates were dependent on pressure and phase conditions with the driving force related to fugacity differences. Li et al. [10] found that temperature had an obvious influence on the replacement process of CO2-CH4 hydrate and higher temperature was beneficial for replacement reaction. Wang et al. [11] obtained the same conclusions through experimental investigation. Yuan et al. [12, 13] experimentally studied the replacement of CO2-CH4 hydrate in porous sediments. The results showed that the replacement rate and amount of CH4 increased with the increase of hydrate saturation in the sediments. Deusner et al. [14] carried out the replacement of CO2-CH4 hydrate using supercritical CO2. The results showed that CH4 production and CO2 retention is improved under conditions of slow CO2-hydrate formation. Espinoza et al. [15] experimentally studied the CO2-CH4 replacement in hydrate-bearing sand. The results indicated that CO2-CH4 replacement occurred without a loss of stiffness in sediments. Moreover, Mc Grail et al. [16] found that CO2 emulsion could enhance the replacement process of CO2-CH4 hydrate and improved the replacement rate. Park et al. [17] studied the replacement process of CO2-CH4 hydrate by use of CO2 and N2 mixture. The result showed that the replacement efficiency rise significantly. Kvamme et al. [18] draw the same conclusion in studying the replacement process by use of CO2 and N2 mixture. Cha et al. [19] studied the kinetics of methane replacement with CO2 and N2 gas in porous silica gel by in situ NMR spectroscopy. The results showed that the amount of methane evolved from the large cages was larger than that of the small cages. Although there have been many researches reporting the replacement process of CO2-CH4 hydrate under different conditions, few studies have focused on the replacement of CO2-CH4 hydrate below the freezing point. Meanwhile, the temperature of permafrost region is usually below zero. So it is necessary to study the replacement process of CO2-CH4 hydrate below the freezing point. In this article, the replacement characteristic of CO2-CH4 hydrate was experimentally investigated below the freezing point. And the influence of pressure, temperature and reaction time on the replacement process of CO2-CH4 hydrate was discussed through analyzing the experimental results. 2. EXPERIMENTAL SECTION 2.1. Apparatus and materials The schematic diagram of the experimental apparatus is shown in Fig. 1. The experimental apparatus consists of a high-pressure vessel whose internal volume is 210 mL and the maximum working pressure is 10 MPa, a gas cylinder, which supplies methane and controls the pressure of the system, a vacuum pump for vacuumizing the vessel before experiment, and a data acquisition system which include temperature sensor (with an accuracy of ±0.1 K) and pressure sensor (with an accuracy of ±0.5%), data taker of Agilent 34970A and computer. Inside the high-pressure vessel, two temperature sensors are installed at different positions and employed to measure the temperature of the system. The high-pressure vessel is the key part of the system and temperature is controlled by a coolant alcohol with a precision of ±0.1 K (made in Ningbo Tianheng Instrument Co., Ltd.), where the working temperature ranged from 253 K to 373 K. The vessel is immersed into the constant temperature alcohol bath controlled by low temperature circulator. Portable gas analyzer GA5000 with an accuracy of ±0.5%, is furnished by Shenzhen Onuee


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Electronics Ltd. and employed to measure the gas composition in gas phase at any point. Methane used in the experiment was furnished by Foshan Walt Gas Co., Ltd. with purity of 99.99%. Carbon dioxide was furnished by Lanzhou Special Gas Co., Ltd. with purity of 99.99%. Nitrogen was furnished by Chongqing Zhaoyang Gas Co., Ltd with purity of 99.9%. The deionized distilled water was made in our laboratory, used to make ice in the experiments.

Fig. 1. Schematic diagram of the experimental apparatus. 1-Gas cylinder, 2-Reducing valve, 3-Rotary screw valve, 4-Vacuum pump, 5-Preessure gauge, 6-Rotary screw valve, 7-Rotary screw valve, 8-Pressure sensor, 9-Stirrer, 10-Reaction cell, 11-Temperature sensor, 12- Temperature sensor, 13-Temperature sensor, 14-Pressure transducer, 15- Constant temperature alcohol bath, 16- Data acquisition, 17- Computer

2.2. Experimental procedures The replacement process of CO2-CH4 hydrate mainly includes two steps with formation process of CO2 hydrate and decomposition process of CH4 hydrate. Therefore, it needs to compound methane hydrate in situ before the replacement process of CO2-CH4 hydrate. 2.2.1 The formation process of CH4 hydrate Methane hydrate was compounded in ice powder with particle size of 800 µm. The decomposition process of CH4 hydrate was very slow under this condition [20~22]. It slowed down the decomposition process of CH4 hydrate during the experiment. Firstly, a certain amount of distilled water was frozen and crushed and sieved under the condition of liquid nitrogen protection. Then we obtained ice powder with particle size of 800 µm. Afterwards, a given amount of ice powder was added the vessel and sealed it quickly. Then the vessel was immersed into the constant temperature alcohol bath and methane was injected into high-pressure vessel to reach 8.0 MPa. And regulated the temperature of constant temperature alcohol bath to 273.2 K and the vessel was cooled to form methane hydrate. Until the pressure change of system was ignorable, it meant that the formation reaction has completed. The variation of temperature and pressure were measured and recorded by Agilent 34970A data acquisition instrument. 2.2.2 The replacement of CO2-CH4 hydrate The replacement process of CO2-CH4 hydrate was investigated below the freezing point. When the pressure of CH4 gas kept steadily in the vessel, it was considered that the formation process of


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CH4 hydrate was finished. At that time the temperature was quickly reduced to 268 K. Methane hydrate has strong self-preservation effect at this temperature under the atmospheric pressure [1, 20]. When the temperature arrived 268 K, CH4 gas was released rapidly and made the vessel maintain atmospheric pressure in the vessel. After that, the vessel was swashed with CO2 gas at least 3 times. And carbon dioxide was injected into the vessel to reach the corresponding initial pressure and the coolant temperature was rapidly adjusted to 272.2 K. Then the replacement experiment of CO2-CH4 was conducted below the freezing point under the condition of different injection pressure of CO2. Took out a handful of gas from the vessel for every 24 h and measured the gas composition content of CH4 and CO2 gas in the mixture using portable gas analyzer GA5000. The replacement process was monitored and the replacement rate and efficiency was calculated according to the experimental data. After the experiment finished, the amount of CH4 in initial state and the residual amount of CH4 gas was calculated according to the Soave Redlich Kwong (SRK) Equation of State [23]. During the formation process of CH4 hydrate, the amount of CH4 hydrate was calculated according to the variation of CH4 amount before and after the experiment. The amount of injected CO2 was measured by gas flowmeter installed on the CO2 cylinder. The replaced amount of CH4 was calculated according to the total amount of gas mixture and the gas composition content of CH4 gas in the gas phase during the replacement process. Then the average replacement rate and efficiency was obtained from the through experiment. 2.3. Data processing of experiment During the replacement process of CO2-CH4 hydrate, the total amount of CH4 and CO2 in initial state is calculated according to the PVT data. And the amount of CH4 hydrate is calculated according to the consumption of CH4 gas in the initial state. The amount of replaced CH4 is calculated according to the mole fraction X CH of CH4 gas in gas phase at some point during the 4 replacement process. The amount of CH4 and CO2 in the initial state is calculated as followed:

nCH 4 ,H = nTotal,CH 4 − nCH 4 ,G

(1)

niCH

(2)

4 ,H

= nCH 4 , H − nTotal ,G ⋅ X CH4

nCO2 ,H = nTotal,CO 2 − nCO2 ,G i nCO = nTotal ,G ⋅ X CO2 ,G 2

(3)

(4)

The amount of CH4 and CO2 in gas phase and hydrate phase is calculated as followed:

nCH 4 ,G = nTotal,G ⋅ X CH 4

(5)

nCO2 ,G = nTotal,G ⋅ X CO2

(6)

nC H 4 ,H = n Ci H

4 ,H

− n C H 4 ,G


(7)

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n CO 2 ,H = n iCO 2 ,G − n CO 2 ,G Where

(8)

nTotal,G is the total amount of CH4 and CO2 in gas phase, nCH4 , H is the amount of CH4 in

hydrate phase,

nCO2 ,H are

the amount of CO2 in hydrate phase,

X CH 4

and

4 ,H

is the total amount of

i nCO is the total amount of CO2 gas in gas phase before ,G

CH4 hydrate before replacement, and the replacement reaction.

niCH

2

X C O2

are the mole fraction of CH4 and CO2 in gas phase,

respectively. During the replacement process, the replacement rate is calculated by the following equation:

v=

nCH 4 ,G

t

(9)

After the replacement process of CO2-CH4 hydrate, the replacement efficiency of CH4 is calculated as follows:

η=

nCH 4 ,G i nCH 4, H

× 100%

(10)

Where v is the average replacement rate for replacement process of CO2-CH4 hydrate,

nCH 4 ,G

is

the replaced amount of CH4 and during the replacement process, t is the reaction time which replacement process was continuing; η is the replacement rate of replacement process,

niCH

4 ,H

is

the total amount of CH4 in hydrate before the replacement reaction. According to the mole fraction changes of CH4 and CO2 in gas phase and the amount of CH4 hydrate in initial state, the amount of replaced CH4 hydrate can be calculated. Then the average replacement rate and efficiency can be obtained during the replacement process under different conditions.

3. Results and discussion 3.1. The influence of pressure on the replacement process of CO2-CH4 hydrate In this study, to evaluate the influence of pressure on the replacement process, the replacement experiments were carried out at different initial pressure such as at 3.6 MPa, 4.0 MPa and 4.5 MPa. Fig. 2 shows the mole fraction variations of CH4 gas in gas phase over time during the replacement process at different initial pressure. Clearly seen from Fig. 2, the mole fraction of CH4 in gas phase increases with the injection pressure of CO2 gas increased at some point. This shows that, the injection pressure of CO2 has an important influence on the replacement process of CO2-CH4 hydrate. During the replacement process of CO2-CH4 hydrate, more CH4 gas was replaced at higher injection pressure of CO2. The experimental results indicated that, the higher the injected pressure of CO2 was, the more the replaced amount of CH4 gas. So the replacement rate of CO2-CH4 hydrate was faster at higher pressure of CO2. This is because the pore size of ice powder is the main channel for CO2 gas diffusion and migration. The higher pressure of CO2 gas


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is conductive to the diffusion process of CO2 gas in ice powder. Meanwhile, higher pressure of CO2 results in higher concentration of CO2 gas in a certain area. It provides larger driving force for the replacement process of CO2-CH4 hydrate. It is beneficial for the replacement of CO2-CH4 hydrate to some extent. Therefore, the replacement rate is larger and more CH4 gas is replaced in per unit time at higher injected pressure of CO2 below the freezing point. And the mole fraction of CH4 becomes larger in gas phase at some point.

Fig. 2. Changes of mole fraction of CH4 in gas phase over time

Fig. 3-4 show the replacement efficiency variations of CH4 hydrate and the amount variations of CO2 hydrate during the replacement process at different injection pressure. From Fig. 3 and Fig. 4, the replacement efficiency of CH4 hydrate and the formed amount of CO2 hydrate increase during the experiment over time. This indicated that, the higher injected pressure of CO2 was conductive to the replacement process of CO2-CH4 hydrate below the freezing point. So the replacement efficiency of CH4 hydrate and the formed amount of CO2 hydrate are larger at higher injection pressure. Simultaneously, clearly seen from Fig. 3 and Fig. 4, the variation of replacement efficiency and the formed amount of CO2 hydrate changed fast at the earlier stage of replacement process. But it changes slowly after 80 h during the experiment. That is probably because that the replacement reaction occurred in the interface of CO2 and CH4 hydrate. With the replacement process continuing, CO2 gas formed hydrate in situ and CH4 gas was released from methane hydrate. And CO2 hydrate formed in the surface of methane hydrate. Furthermore, hydrate film thickened gradually during the replacement process of CO2-CH4 hydrate. It blocked the diffusion process of CO2 gas in ice powder and formation process of hydrate. So the replacement efficiency variation of CH4 hydrate slows down gradually. Therefore, the replacement efficiency and the formed amount of CO2 hydrate change faster at earlier stage and they become slow after 80 h during the experiments.


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Fig. 3. Changes of replacement efficiency of CH4 hydrate over time

Fig. 4. Amount of formed CO2 hydrate in the replacement process over time

3.2. The influence of temperature on the replacement process of CO2-CH4 hydrate The influence of temperature on the replacement process of CO2-CH4 hydrate was investigated in this study. Table 1 shows the comparison of replacement rate and efficiency under the condition of different injection pressure of CO2 below the freezing point. Table 2 shows the comparison of replacement efficiency under different injection pressure of CO2 at 274.15 K. From Table 1, the replacement rate and efficiency increase with the injection pressure of CO2 gas increasing. This is because higher pressure is beneficial for the diffusion process of CO2 gas in porous media of ice powder. It results in higher concentration of CO2 gas in a certain area and provides larger driving force for the replacement process of CO2-CH4 hydrate below the freezing point. The results indicate that, the higher the injection pressure of CO2 is, the larger the replacement rate and efficiency are. However, compared with the condition above the freezing point [24], the replacement process of CO2-CH4 is slow below the freezing point. The amount variation of replaced CH4 is lesser in per unit time below the freezing point. So the replacement rate and efficiency is also smaller than that during the replacement process above the freezing point. Influenced by the temperature below the freezing point, the formation and decomposition process of hydrate is slow. The results agree well with the conclusions from their research by Takeshi et al. [25]. And the replaced amount of CH4 reduces gradually in unit time. Therefore, the replacement process of CO2-CH4 hydrate is slow below the freezing point.


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Table 1. Comparison of replacement rate and efficiency under different CO2 injection pressure Injection pressure (MPa)

Average replacement rate (mmol/h)

Average efficiency (%)

3.6

0.264

8.63

4.0

0.333

10.89

4.5

0.403

13.20

Table 2. Comparison of efficiency under different CO2 injection pressure at 274.15 K [24] Injection pressure (MPa)

Average efficiency (%)

3.97

20.0

4.56

26.4

4.84

44.9

6.26

9.1

3.3. The influence of reaction time on the replacement process of CO2-CH4 hydrate Fig. 5 shows the replaced amount variation of CH4 over time during the replacement process at different pressure of CO2 gas. Clearly seen from Fig. 5, the replaced amount of CH4 increases over time during the replacement process at different injection pressure of CO2. This was because the replacement process of CO2-CH4 hydrate occurred in the interface of CO2 and CH4 hydrate. Longer contact time is more advantageous to the replacement process of CO2-CH4 hydrate. Therefore, the replaced amount of CH4 increases as time went by during the experiment. Furthermore, as seen from Fig. 5, the replaced amount variation of CH4 changes fast at earlier stage. But it becomes slow after a period of time. The reason was that the replacement reaction occurred in the interface of CO2 and CH4 hydrate. Carbon dioxide formed hydrate in situ and CH4 was released from hydrate during the replacement process. And hydrate film thickened gradually and it blocked the diffusion process of CO2 gas in ice powder and the replacement process of CO2-CH4 hydrate. So the replacement rate of CO2-CH4 hydrate slows down gradually during the experiment. Therefore, the replacement rate of CO2-CH4 hydrate is faster at earlier stage of experiment and it slows down gradually after a period of time during the replacement process of CO2-CH4 hydrate.

Fig. 5. Changes of mount of replaced CH4 in the replacement process over time at different pressure

In a word, experimental results show that the replacement rate and efficiency is larger at higher injected pressure of CO2 gas during the experiment. But the replacement of CO2-CH4 hydrate is


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relatively slow below the freezing point. And the replaced amount of CH4 increases with time went by during the replacement process. Longer reaction time is conductive to the replacement process to some extent. And the replaced amount variation of CH4 is faster in earlier stage. But it slows down gradually after a period of time during the replacement process of CO2-CH4 hydrate below the freezing point. This will provide an important theoretical foundation for replacement process of CO2-CH4 hydrate below the freezing point. 4. CONCLUSIONS In this work, the replacement process of CO2-CH4 hydrate was conducted below the freezing point at different injection pressure (3.6 MPa, 4.0 MPa and 4.5 MPa) of CO2. And the influence of pressure, temperature and reaction time on the replacement process of CO2-CH4 hydrate was discussed through experiment. The experimental results showed that, under the condition of different injected pressure of CO2, the average replacement rate and efficiency were larger at higher injected pressure of CO2 below the freezing point. It increased with the increase of injected pressure of CO2 gas. And the average replacement rate and efficiency reached up to 0.403 mmol/h and 13.20% when the injected pressure of CO2 gas was at 4.5 MPa. The results also indicated that, the replacement rate of CO2-CH4 hydrate was slow below the freezing point. And the replaced amount variation of CH4 was faster at earlier stage. But it slowed down gradually after a period of experiment during the replacement process of CO2-CH4 hydrate. This will provide an important theoretical guidance and references for natural gas hydrate exploitation in permafrost regions. ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of China (No. 51266005) and Natural Science Foundation of Gansu Province (No. 1606RJZA082), Open Fund of Natural Gas Hydrate Key Laboratory, Chinese Academy of Sciences (No. Y607kh1001). REFERENCES [1] Sloan, E. D.; Koh, C. A., 2008. Clathrate Hydrates of Natural Gases, third ed. CRC Press Taylor and Francis Group, Boca Raton. [2] Sloan, E. D. Fundamental principles and applications of natural gas hydrates. Nature 2003, 426, 353–363. [3] Luo, S. S., Liu, H. J., Sun, Y. Advancement on natural gas production from hydrate in deep-sea sediments with CO2. China Resour. Compr. Util. 2008, 26, 19–23. [4] Milkov, A. V.; Sassen, R. Economic geology of offshore gas hydrate accumulations and provinces. Marine and Petroleum Geology 2002, 19, 1–11. [5] Kvenvolden, K. A. A review of the geochemistry of methane in natural gas hydrate. Organic Geochemistry 1995, 23, 997–1008. [6] Moridis, G. J.; Sloan, E. D. Gas production potential of disperse low-saturation hydrate accumulations in oceanic sediments. Energy Conversion and Management 2007, 48, 1834–1849. [7] Uchida, T.; Takeya, S.; Ebinuma, T. Replacing methane with CO2 in clathrate hydrate: observation using Raman spectroscopy. In: Proceedings of the Fifth International Conference on Greenhouse Gas Control Technologies. Collingwood, Australia 2001, 523–527. [8] Ota M.; Morohashi K.; Abe Y.; Watanabe, M.; Smith Jr, R. L.; Inomata, H. Replacement of


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Experimental Study on the Effect of Pressure on the Replacement

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