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Direct measurement of formation and dissociation rate and storage capacity of dry water methane hydrates
Fuel Processing Technology 92 (2011) 1617–1622
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Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
Direct measurement of formation and dissociation rate and storage capacity of dry water methane hydrates Gaowei Hu, Yuguang Ye ⁎, Changling Liu, Qingguo Meng, Jian Zhang, Shaobo Diao Key Laboratory of Marine Hydrocarbon Resources and Environmental Geology, Ministry of Land and Resources, Qingdao 266071, China Qingdao Institute of Marine Geology, Qingdao 266071, China
a r t i c l e
i n f o
Article history: Received 21 January 2011 Received in revised form 3 April 2011 Accepted 6 April 2011 Available online 6 May 2011 Keywords: Dry water Methane gas hydrates Methane storage Structural characteristics
a b s t r a c t Dry water (DW) has been recently demonstrated to be an effective medium for methane storage in a hydrated form. Here, a series of experiments have been carried out on dry water methane hydrates (DW–MH) to investigate their formation and dissociation rates, storage capacity and structural characteristics. The result shows that the storage capacity of MH increases at least 10% by using DW relative to using surfactants like sodium dodecyl sulfate (SDS) solution. Also, it is found that controls on pressure–temperature (P–T) condition have influences on the induction and reaction time of DW–MH formation, i. e. the induction and reaction time are much shorter when the reaction cell is cooled to ~ 3 °C first. On the basis of Raman spectra, the hydration number is calculated as 5.934 ± 0.06 at different positions of the DW–MH, which suggests that the sample is very homogeneous. The dissociation process of the DW–MH sample exhibits a rapid release of methane gas at the first stage of dissociation. Although hydrate dissociation is prevented by the effect of self preservation, most methane gas has released from the hydrate, however, before the self preservation occur. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Gas hydrates, also known as clathrates, are ice-like, crystalline inclusion compounds composed of water molecules surrounding gas molecules (usually methane) under certain pressure and temperature [1]. As one volume of methane hydrate (MH) can yield as much as 180 v/v STP methane [2], it is suggested that it may be economically feasible to transport natural gas in a hydrated form. In particular, the transportation cost is expected to be 18–24% lower than with liquefied natural gas (LNG) transportation [3,4], and the transportation is more secure than LNG or pipeline transportation [5]. The methane hydrate storage chain consists of three main parts, the hydrate formation, transportation and regasification [6]. Several works have been done to promote the application industry [7]. For example, it is found that the hydrate formation conditions may change to be much milder with various additives [8–10], while the formation rate and storage capacity can be largely enhanced by stirring, bubbling, or adding surfactants [11–15]. Recently, a more attractive method, using dry water as a medium for MH formation, has been used to promote the MH formation rate and storage capacity significantly [16,17]. DW is a free-flowing powder prepared by mixing water and hydrophobic silica particles (H18) at high speeds [18]. The highly distributed gas-liquid interface in “dry water” powder can be used to greatly increase the rate of heterogeneous gas-liquid reactions [19].
Wang et al. [16] is probably the first one who report methane storage in DW–MH. Using DW in storing system, they have dramatically enhanced the formation rate and methane uptake of MH. In their work, the methane uptake kinetics in DW–MH at 0 °C were investigated and shown that the CH4 capacity is related to the mixing speed of making DW. Particularly, the CH4 capacity (~175 v/v at STP) is close to the maximum capacity when DW is prepared at a mixing speed of 19,000 rpm. Carter et al. [17] develop this work and demonstrate that DW can be used to increase the kinetics of formation of other gas clathrates, such as CO2 and Kr. Besides, they found that the addition of gellan gum allows effective reuse of DW over several hydrate formation-dissociation cycles without the need for reblending. However, some fundamental issues are still very vague and need to be clarified, such as the reaction time of DW–MH formation, the hydration number, cage occupancies and storage capacity of DW–MH, and the characteristics of DW–MH dissociation. The purpose of this work was to study the formation and dissociation rate and storage capacity of DW–MH. The gas storage capacity was determined by a dedicated apparatus. The hydration numbers of DW–MH were directly measured with a Micro-Laser Raman Spectroscopy, with which the homogeneity of methane distribution in DW–MH was also studied. 2. Experimental section 2.1. Dry water preparation
⁎ Corresponding author at: Qingdao Institute of Marine Geology, Qingdao 266071, China. Tel.: + 86 532 85755850; fax: + 86 532 85771905. E-mail address: [email protected] (Y. Ye). 0378-3820/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2011.04.008
The hydrophobic silica nanoparticles (H18) were kindly supplied by Wacker Chemie. In a high-speed mixing blender (Blendtec, Glass
1618
G. Hu et al. / Fuel Processing Technology 92 (2011) 1617–1622
was taken out, numbered and submerged into liquid nitrogen for following measurements. 2.3. Hydration numbers and homogeneity measurement
Fig. 1. Dry water prepared with 5 g hydrophobic silica nanoparticles (H18) and 95 g water mixing at 22,000 rpm for 90 s.
Jug Blender, Part #40-501, 1000 milliliter), 5 g H18 was added to 95 g de-ionized water. Mixing was carried out at three different speeds (speed 1:12,000 rpm; speed 2:17,000 rpm; speed 3:22,000 rpm) for 90 s. The DW was produced as a free-flowing white powder which could be poured from one vessel into another (Fig. 1). 2.2. Hydrate formation and take out The apparatus for hydrate formation is presented in Fig. 2. It is composed of four functioning units: (1) a high-pressure reaction cell (max volume 130 cm3) for simulating appropriate pressure and temperature of hydrate formation, in which there is a platinum (Pt100) resistance thermometer with precision of ±0.1 °C used for measuring the temperature of the sample, (2) a gas compressor and a pressure transducer (Shanghai Tianmu company, precision ±0.1 MPa) responsible for gas pressure control, (3) a cooling system and jacketed coolant around the reaction cell for temperature control, and (4) a computer system for measuring and interval-logging data of temperature, pressure and time. An easy-unlock switch was mounted on the reaction cell so that the hydrate samples can be rapidly taken out after it is formed. For more details of the apparatus, see reference [20]. In each experiment, 20 g of DW was loaded into the reaction cell for MH formation. Two types of controls on P–T condition were conducted to form hydrate. In type I, high pressure methane gas (8–9 MPa) was introduced into the vessel before temperature decreasing. In type II, Gas was introduced after the temperature is at a certain temperature (~3 °C) which is directly conducive for hydrate formation. In this way, the induction and reaction time of the hydrate formation under different control conditions were investigated (reaction time means the interval time between time point of hydrate begins to form to the time point of hydrate formation finishes). After the DW–MH was formed, high pressure gas was released and the easy-unlock switch was rotated a few circles to open the reaction cell within 1–2 min. And then the DW–MH
A portion of DW–MH sample was placed in a steel mortar in contact with liquid nitrogen and powdered with a steel rod. The powdered samples were put into a microscope hot and cold stage (−196 °C) and analyzed with a Raman Spectroscopy (type: Renishaw, in Via), to specify their structural characteristics such as hydration numbers and cage occupancies. The Raman analytical procedures have been described by Liu et al. [21]. Raman spectra were taken with a high-resolution (spatial resolution, horizontal 1 μm × vertical 2 μm) Leica microscope and with a Ar+ laser wavelength of 514.5 nm at ~100 mW. Because the hydrated gas is methane, we only scanned the Raman frequency shift interval in the C–H stretch region from 2860 to 2960 cm− 1. In order to investigate the homogeneity of DW–MH hydration numbers, Raman spectra were measured at different positions of a sample (Fig. 3). In each measurement, 20× microscopic lens was used to give a panoramic picture and five different positions were selected for further Raman spectra measurements (upper part of Fig. 3). After that, the microscopic lens was changed to 50×, and then the photo and Raman spectra of position one was measured (lower part of Fig. 3). In the same way, the photos and Raman spectra of positions 2–5 were obtained with the 50× microscopic lens. 2.4. Storage capacity and dissociation rate measurement A portion of DW–MH sample (~ 1 g) was measured by a dedicated apparatus (Fig. 4) to determine the storage capacity (vgas/vwater) directly [22]. The experimental processes were as follows: (1) the volume (v0) of standard test tube part was calibrated. (2) at atmosphere, valve K1 and K3 were closed while K2 was opened, then the system was vacuumed and the pressure P1 was recorded. After that K2 was closed while K3 was opened (at this time pressure changed to P2), so that the air pressure of the standard test tube part is: P0 = P2 − P1. (3) a portion of DW–MH sample was put into the sample test tube which is partly submerged in liquid nitrogen. All the valves (K1, K2 and K3) were opened when the whole system was vacuumed. After that K2 was closed and liquid nitrogen cup was moved away from the sample test tube so that hydrate can dissociate under room temperature (the dissociation rate can be measured at this stage). As hydrate dissociation finished, the liquid nitrogen cup was moved to submerge the standard test tube, to accumulate all the methane gas dissociated from hydrate (pressure P3 was recorded after
Fig. 2. Schematic diagram of the hydrate-formation apparatus.
G. Hu et al. / Fuel Processing Technology 92 (2011) 1617–1622
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Fig. 3. Upper: Photo of sample DW–MH-1 under 20× microscopic lens; p1 to p5 are five different positions for measuring Raman spectra. Lower: Photo and Raman spectra of point p1 measured by 50× microscopic lens.
accumulation finished). Lastly, valve K1 was closed and the liquid nitrogen cup was taken away from the standard test tube, then the methane gas released under room temperature and the pressure was recorded to P4. So the total volume of methane gas can be calculated with vgas = v0 × (P4 − P3) / (P2 − P1). (4) the volume of the hydrated water was measured with gravimetric method after DW–MH was dissociated, i.e. vwater = Wwater / ρwater = (W1 − W2) / ρwater, where W1 is the weight of tube and water, W2 is the weight of the dry tube, and ρwater is density of water (1 g/cm3). 3. Results and discussions 3.1. Induction and reaction time of DW–MH formation The formation conditions of DW and DW–MH samples are shown in Table 1, and the hydrate formation processes by two different types of P-T condition-controls are shown in Fig. 5. As shown in the formation process of DW–MH-1 (Fig. 5, left), the induction time, which means the interval time between time point of P-T conditions become conducive for methane hydrate formation to the time point of hydrate begins to
form, was about 243 min. And the reaction time of hydrate formation of DW–MH-1 was about 166 min. The induction and reaction time were much shorter with the second type of P-T condition-control, however. As shown in Fig. 5, right, the induction time and reaction time of sample DW–MH-2 were about 4 min and 70 min, respectively, when hydrate was formed under a certain temperature (~3 °C) which is directly conducive for hydrate formation. Our results are similar to Wang et al. [16], who have investigated the first type of P-T condition for DW–MH formation (although their DW is prepared at a different mixing speed of 19,000 rpm). In their studies, the reaction time of DW–MH was about 160 min (when gas capacity achieve about 158 v/v STP), which is very close to the first type result of 166 min in our experiments. Notably, the reaction time under the second type of P–T condition is much shorter than that under the first type. So we formed two other samples, DW–MH-2 and DW–MH-3, under the second type of P–T condition to investigate the effect of mixing speed of making DW on the induction time and reaction time (Table 1 and Fig. 6). The results indicate that when the mixing speed is lower, the induction time will be a little longer while the reaction time is remained as 70–77 min. Thus, from the above
Fig. 4. Schematic diagram of apparatus for measuring storage capacity and dissociation rate of gas hydrates.
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Table 1 Parameters of DW–MH samples. Sample No.
Mixing speed of DW
P-T conditions
Induction time Reaction time (min) (min)
DW–MH-1 DW–MH-2 DW–MH-3 DW–MH-4 DW–MH-5
Speed Speed Speed Speed Speed
Type Type Type Type Type
243 32 8 4 6
3 2 1 3 3
I II II II II
0
μ w ðhÞ−μ w ðh0 Þ = Δμ w
166 75 70 70 77
Note: type I: high pressure gas (8–9 MPa) was introduced into the reaction cell under room temperature, then the temperature was decreased to form hydrates; type II: after the reaction cell was cooled to ~ 3 °C, high pressure gas (8–9 MPa) was introduced to form hydrates.
experiments, it is suggested that DW–MH may be formed under the second type of P–T condition to enhance the formation rate. 3.2. Hydration numbers, homogeneity and storage capacity 3.2.1. Hydration numbers and homogeneity According to the peak intensities or peak area of Raman spectra, the structural characteristics such as hydration numbers and cage occupancies of MH can be obtained [21,23]. The cage occupancy ratio of methane in large and small cages is: θL = θS = IL = 3IS
ð1Þ
where θL, θS are the absolute occupancies of large and small cages and IL, IS are the integrated intensities of large and small cages from Raman spectra. For structure I hydrate, the chemical potential of hydrate can be calculated with: μ w ðhÞ−μ w ðh0 Þ = −RT½3lnð1−θL Þ + lnð1−θS Þ = 23
where μ w(h) is the chemical potential of water molecules in the lattice of empty hydrate, μ w(h0) is the chemical potential of water molecules in the reference state. At equilibrium with ice,
ð2Þ
ð3Þ
where Δμ 0w is the chemical potential difference between the water in the empty hydrate lattice and ice, and the value was found to be 1297 J mol− 1 [24]. Combining Eqs. (1), (2), and (3), the occupancy ratios can be used to calculate the absolute occupancies of θL and θS, which in turn can be used to obtain hydration numbers from n = 23 = ð3θL + θS Þ
ð4Þ
With the methods described in Section 2.3, the structural characteristics of three samples were measured. Here the result of DW–MH-1 was selected as an example to show the hydration numbers and homogeneity of DW–MH (Table 2). The results show that the hydration numbers measured at different positions of DW–MH are hardly changed, and the value can be given as 5.934 ± 0.06. It indicates that the DW–MH sample is very homogeneous. The hydration number of DW–MH is smaller than that of hydrate formed by CH4 + 300 ppm sodium dodecyl sulfate (SDS) solution, which is 6.12 ± 0.05 [21]. It demonstrates that the storage capacity of DW–MH is larger than that of SDS–MH. Also, it is found that the occupancy of large cage of DW–MH is more than 99%, while the average occupancy of small cage of DW–MH is about 89.50%. Since the large cage has been nearly absolutely filled with methane, it suggests that efforts should be focused on enhancing the methane uptake of small cages for further elevation of storage capacity of DW–MH. 3.2.2. Storage capacity The measured storage capacity (vgas/vwater at STP) of DW–MH samples are shown in Table 3. The results exhibit that gas storage of DW–MH has a small increase when the mixing speed of preparing DW
Fig. 5. P-T changes during hydrate formation in a sealed system (left, DW–MH-1; right, DW–MH-4).
Fig. 6. Induction time and reaction time of DW–MH-2 (left, DW prepared at mixing speed 3) and DW–MH-3 (right, DW prepared at mixing speed 1).
G. Hu et al. / Fuel Processing Technology 92 (2011) 1617–1622
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Table 2 Structural characteristics of DW–MH-1. Sample position
IL
IS
θS
θL
n
DW–MH-1 (p1) DW–MH-1 (p2) DW–MH-1 (p3) DW–MH-1 (p4) DW–MH-1 (p5) Average
33847 46482 37216 48182 30454
9723 14067 11042 14750 9371
0.8558 0.9024 0.8844 0.9130 0.9178 0.8950
0.9931 0.9939 0.9936 0.9942 0.9943 0.9940
5.997 5.921 5.951 5.904 5.896 5.934
is increased. The DW prepared at the highest mixing speed (~22,000 rpm) shows a methane uptake of 203.93 vgas/vwater STP, which can be converted to about 163.14 vgas/vhydrate STP owing to one volume methane hydrate usually contains 0.8 volume of water [1]. In Table 3, the storage capacity of DW–MH samples was compared with that of SDS-MH. It shows that at least 10% of storage capacity is enhanced when methane storages in DW, comparing with it storages in SDS solutions. 3.3. Dissociation rate and self-preservation of DW–MH A typical dissociation process of DW–MH-1 is shown in Fig. 7. DW–MH sample was placed in sample test tube of the storage capacity measurement apparatus (Fig. 4), and the test tube was partly submerged into liquid nitrogen so that hydrate can be stable. After the whole system was vacuumed, the liquid nitrogen cup was moved away and the DW–MH sample was warmed by room temperature. About 3 min later, the pressure began to increase, which indicates that DW–MH start to dissociate. In Fig. 7, it shows that the whole dissociation process can be described by four stages. At the first stage, the dissociation rate is very rapid. Almost 55% of total methane gas is released in about 2 min. The dissociation rate at the second stage is much lower (~15 times lower) than that at the first stage, however, probably it is caused by the self preservation behavior of MH [25]. And then the dissociation rate rebounds at the third stage since the methane gas burst after the ice trap is thawed. However, the rate is lower than that at the first stage because the total mass of gas is smaller. In the last stage, the gas pressure has a small increase by the warm temperature. About the self preservation behavior of DW–MH during hydrate dissociation, it occurred in our experiments after that more than half of the methane gas was released. Thus, more efforts need to be made to improve usage of this behavior during the MH storage chain. 4. Conclusions In this paper, the results of direct measurements on formation rate, structural characteristics, storage capacity and dissociation rate of five DW–MH samples were reported. The conclusions are made as follows: (1) the formation rate can be improved by controlling the P-T condition of hydrate formation, i. e. the induction and reaction time of DW–MH formation are 243 min and 166 min respectively when the reaction cell is pressurized before cooled, while they are 4 min and 70 min respectively when the reaction cell is cooled first; although the formation time is reduced compared with Wang et al.
Fig. 7. Dissociation process of sample DW–MH-1 under free space and room temperature (~ 20 °C).
[16], further reduction of DW–MH formation time may possibly be made by a more appropriate P–T condition; (2) the hydration number of DW–MH sample is 5.934 ± 0.06, and the sample shows to be very homogeneous from Raman spectroscopy measurements; (3) DW can be used to increase at least 10% of the storage capacity of MH, compared with surfactants such as SDS solutions, further elevation of storage capacity of DW–MH should be focused on enhancing the methane uptake of small cages; and (4) although hydrate dissociation can be prevented by the effect of self preservation, more than half of total methane gas has released before the self preservation occur. Acknowledgements This work was supported financially by “National Natural Science Foundation of China (41072037)”, “Ministry of Land and Resources' Public Benefit Research Foundation (201111026)” and the “Natural Gas Hydrate in China Sea Exploration and Evaluation Project (GZH2002002)”. Comments by two anonymous reviewers have greatly helped in clarifying the presentation. References [1] E.D. Sloan Jr., C.A. Koh, Clathrate Hydrate of Natural Gases, 3rd Ed. Taylor & Francis-CRC Press, London, 2007. [2] E.D. Sloan Jr., Fundamental principles and applications of natural gas hydrates, Nature 426 (2003) 353–359. [3] J.S. Gudmundsson, M. Mork, O.F. Graff, Hydrate non-pipeline technology, 4th International Conference on Gas Hydrate, Yokohama, Japan, 2002, pp. 997–1002. [4] H. Kanda, Economic study on natural gas transportation with natural gas hydrate (NGH) pellets, 23rd World Gas Conference, Amsterdam, Netherlands, 2006, p. 11. [5] W. Hao, J. Wang, S. Fan, W. Hao, Evaluation and analysis method for natural gas hydrate storage and transportation process, Energy Conversion and Management 49 (2008) 2546–2553. [6] N.J. Kim, J.H. Lee, Y.S. Cho, W. Chun, Formation enhancement of methane hydrate for natural gas transport and storage, Energy 35 (2010) 2717–2722. [7] X. Lang, S. Fan, Y. Wang, Intensification of methane and hydrogen storage in clathrate hydrate and future prospect, Journal of Natural Gas Chemistry 19 (2010) 203–209. [8] B. Tohidi, A. Danesh, A.C. Todd, R.W. Burgass, K.K. Østergaard, Equilibrium data and thermodynamic modelling of cyclopentane and neopentane hydrates, Fluid Phase Equilibria 138 (1997) 241–250. [9] Y. Guo, S. Fan, K. Guo, Y. Chen, Storage capacity of methane in hydrate using calcium hypochlorite as additive, 4th International Conference on Gas Hydrate, Yokohama, Japan, 2002, pp. 1040–1043. [10] T. Uchida, R. Ohmura, I.Y. Ikada, J. Nagao, S. Takeya, A. Hori, Phase equilibrium measurements and crystallographic analyses on structure-H type gas hydrate
Table 3 Storage capacity (vgas/vwater, STP) measured in this study. Sample No.
v0 (cm3)
P1 (kPa)
P2 (kPa)
P3 (kPa)
P4 (kPa)
vgas (cm3)
W1 (g)
W2 (g)
vwater (cm3)
vgas/vwater
DW–MH-1 DW–MH-5 DW–MH-2 DW–MH-3 SDS-MH
80.85 80.85 80.85 80.85 80.85
1.3 1.3 1.3 1.3 1.3
55.4 55.4 55.4 56.2 56.6
2.0 17.4 1.3 11.3 4.1
42.9 134.9 26.5 29.5 38.7
61.18 175.67 37.57 26.76 50.63
35.83 35.98 35.79 35.58 36.01
35.53 35.10 35.60 35.44 35.73
0.30 0.88 0.19 0.14 0.28
203.93 199.62 197.75 197.61 180.30
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G. Hu et al. / Fuel Processing Technology 92 (2011) 1617–1622 formed from the CH4–CO2–neohexane–water system, The Journal of Physical Chemistry. B 110 (2006) 4583–4588. B. Zhang, Q. Wu, D. Sun, Effect of surfactant tween on induction time of gas hydrate formation, Journal of China University of Mining and Technology 18 (2008) 0018–0021. U. Karaaslan, E. Uluneye, M. Parlaktuna, Effect of an anionic surfactant on different type of hydrate structures, Journal of Petroleum Science and Engineering 35 (2002) 49–57. H. Ganji, M. Manteghian, K. Sadaghiani zadeh, M.R. Omidkhah, H.R. Mofrad, Effect of different surfactants on methane hydrate formation rate, stability and storage capacity, Fuel 86 (2007) 434–441. H. Ganji, M. Manteghian, H.R. Mofrad, Effect of mixed compounds on methane hydrate formation and dissociation rates and storage capacity, Fuel Processing Technology 88 (2007) 891–895. Z. Sun, R. Wang, R. Ma, K. Guo, S. Fan, Natural gas storage in hydrates with the presence of promoters, Energy Conversion and Management 44 (2003) 2733–2742. W. Wang, C.L. Bray, D.J. Adams, A.I. Cooper, Methane storage in dry water gas hydrates, Journal of the American Chemical Society 130 (2008) 11608–11609. B.O. Carter, W. Wang, D.J. Adams, A.I. Cooper, Gas storage in “dry water” and “dry gel” clathrates, Langmuir 26 (2010) 3186–3193. B.P. Binks, R. Murakami, Phase inversion of particle-stabilized materials from foams to dry water, Nature Materials 5 (2006) 865–869.
[19] B.O. Carter, D.J. Adams, A.I. Cooper, Pausing a stir: heterogeneous catalysis in “dry water”, Green Chemsitry 12 (2010) 783–785. [20] Q. Chen, C. Liu, Y. Ye, Experimental study on geochemical characteristics of methane hydrate formed in porous media, Journal of Natural Gas Chemistry 18 (2009) 217–221. [21] C. Liu, Y. Ye, Q. Meng, Determination of hydration number of methane hydrates using Micro-Laser Raman Spectroscopy, Spectroscopy and Spectral Analysis 30 (2010) 963–966. [22] C. Liu, Y. Ye, H. Ren, S. Diao, J. Zhang, Experimental techniques of directly measuring the gas storage capacity in methane hydrate, Natural Gas Industry 25 (2005) 44–47. [23] H. Lu, I. Moudrakovski, M. Riedel, G. Spence, R. Dutrisac, J. Ripmeester, F. Wright, S. Dallimore, Journal of Geophysical Research 110 (2005) B10204. [24] D.W. Davidson, Y.P. Handa, J.A. Ripmeester, Xenon-129 NMR and the thermodynamic parameters of xenon hydrate, The Journal of Physical Chemistry 90 (1986) 6549–6552. [25] S. Takeya, J.A. Ripmeester, Anomalous preservation of CH4 hydrate and its dependence on the morphology of hexagonal ice, ChemPhysChem: A European Journal of Chemical Physics and Physical Chemistry 11 (2010) 70–73.
Contents lists available at ScienceDirect
Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
Direct measurement of formation and dissociation rate and storage capacity of dry water methane hydrates Gaowei Hu, Yuguang Ye ⁎, Changling Liu, Qingguo Meng, Jian Zhang, Shaobo Diao Key Laboratory of Marine Hydrocarbon Resources and Environmental Geology, Ministry of Land and Resources, Qingdao 266071, China Qingdao Institute of Marine Geology, Qingdao 266071, China
a r t i c l e
i n f o
Article history: Received 21 January 2011 Received in revised form 3 April 2011 Accepted 6 April 2011 Available online 6 May 2011 Keywords: Dry water Methane gas hydrates Methane storage Structural characteristics
a b s t r a c t Dry water (DW) has been recently demonstrated to be an effective medium for methane storage in a hydrated form. Here, a series of experiments have been carried out on dry water methane hydrates (DW–MH) to investigate their formation and dissociation rates, storage capacity and structural characteristics. The result shows that the storage capacity of MH increases at least 10% by using DW relative to using surfactants like sodium dodecyl sulfate (SDS) solution. Also, it is found that controls on pressure–temperature (P–T) condition have influences on the induction and reaction time of DW–MH formation, i. e. the induction and reaction time are much shorter when the reaction cell is cooled to ~ 3 °C first. On the basis of Raman spectra, the hydration number is calculated as 5.934 ± 0.06 at different positions of the DW–MH, which suggests that the sample is very homogeneous. The dissociation process of the DW–MH sample exhibits a rapid release of methane gas at the first stage of dissociation. Although hydrate dissociation is prevented by the effect of self preservation, most methane gas has released from the hydrate, however, before the self preservation occur. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Gas hydrates, also known as clathrates, are ice-like, crystalline inclusion compounds composed of water molecules surrounding gas molecules (usually methane) under certain pressure and temperature [1]. As one volume of methane hydrate (MH) can yield as much as 180 v/v STP methane [2], it is suggested that it may be economically feasible to transport natural gas in a hydrated form. In particular, the transportation cost is expected to be 18–24% lower than with liquefied natural gas (LNG) transportation [3,4], and the transportation is more secure than LNG or pipeline transportation [5]. The methane hydrate storage chain consists of three main parts, the hydrate formation, transportation and regasification [6]. Several works have been done to promote the application industry [7]. For example, it is found that the hydrate formation conditions may change to be much milder with various additives [8–10], while the formation rate and storage capacity can be largely enhanced by stirring, bubbling, or adding surfactants [11–15]. Recently, a more attractive method, using dry water as a medium for MH formation, has been used to promote the MH formation rate and storage capacity significantly [16,17]. DW is a free-flowing powder prepared by mixing water and hydrophobic silica particles (H18) at high speeds [18]. The highly distributed gas-liquid interface in “dry water” powder can be used to greatly increase the rate of heterogeneous gas-liquid reactions [19].
Wang et al. [16] is probably the first one who report methane storage in DW–MH. Using DW in storing system, they have dramatically enhanced the formation rate and methane uptake of MH. In their work, the methane uptake kinetics in DW–MH at 0 °C were investigated and shown that the CH4 capacity is related to the mixing speed of making DW. Particularly, the CH4 capacity (~175 v/v at STP) is close to the maximum capacity when DW is prepared at a mixing speed of 19,000 rpm. Carter et al. [17] develop this work and demonstrate that DW can be used to increase the kinetics of formation of other gas clathrates, such as CO2 and Kr. Besides, they found that the addition of gellan gum allows effective reuse of DW over several hydrate formation-dissociation cycles without the need for reblending. However, some fundamental issues are still very vague and need to be clarified, such as the reaction time of DW–MH formation, the hydration number, cage occupancies and storage capacity of DW–MH, and the characteristics of DW–MH dissociation. The purpose of this work was to study the formation and dissociation rate and storage capacity of DW–MH. The gas storage capacity was determined by a dedicated apparatus. The hydration numbers of DW–MH were directly measured with a Micro-Laser Raman Spectroscopy, with which the homogeneity of methane distribution in DW–MH was also studied. 2. Experimental section 2.1. Dry water preparation
⁎ Corresponding author at: Qingdao Institute of Marine Geology, Qingdao 266071, China. Tel.: + 86 532 85755850; fax: + 86 532 85771905. E-mail address: [email protected] (Y. Ye). 0378-3820/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2011.04.008
The hydrophobic silica nanoparticles (H18) were kindly supplied by Wacker Chemie. In a high-speed mixing blender (Blendtec, Glass
1618
G. Hu et al. / Fuel Processing Technology 92 (2011) 1617–1622
was taken out, numbered and submerged into liquid nitrogen for following measurements. 2.3. Hydration numbers and homogeneity measurement
Fig. 1. Dry water prepared with 5 g hydrophobic silica nanoparticles (H18) and 95 g water mixing at 22,000 rpm for 90 s.
Jug Blender, Part #40-501, 1000 milliliter), 5 g H18 was added to 95 g de-ionized water. Mixing was carried out at three different speeds (speed 1:12,000 rpm; speed 2:17,000 rpm; speed 3:22,000 rpm) for 90 s. The DW was produced as a free-flowing white powder which could be poured from one vessel into another (Fig. 1). 2.2. Hydrate formation and take out The apparatus for hydrate formation is presented in Fig. 2. It is composed of four functioning units: (1) a high-pressure reaction cell (max volume 130 cm3) for simulating appropriate pressure and temperature of hydrate formation, in which there is a platinum (Pt100) resistance thermometer with precision of ±0.1 °C used for measuring the temperature of the sample, (2) a gas compressor and a pressure transducer (Shanghai Tianmu company, precision ±0.1 MPa) responsible for gas pressure control, (3) a cooling system and jacketed coolant around the reaction cell for temperature control, and (4) a computer system for measuring and interval-logging data of temperature, pressure and time. An easy-unlock switch was mounted on the reaction cell so that the hydrate samples can be rapidly taken out after it is formed. For more details of the apparatus, see reference [20]. In each experiment, 20 g of DW was loaded into the reaction cell for MH formation. Two types of controls on P–T condition were conducted to form hydrate. In type I, high pressure methane gas (8–9 MPa) was introduced into the vessel before temperature decreasing. In type II, Gas was introduced after the temperature is at a certain temperature (~3 °C) which is directly conducive for hydrate formation. In this way, the induction and reaction time of the hydrate formation under different control conditions were investigated (reaction time means the interval time between time point of hydrate begins to form to the time point of hydrate formation finishes). After the DW–MH was formed, high pressure gas was released and the easy-unlock switch was rotated a few circles to open the reaction cell within 1–2 min. And then the DW–MH
A portion of DW–MH sample was placed in a steel mortar in contact with liquid nitrogen and powdered with a steel rod. The powdered samples were put into a microscope hot and cold stage (−196 °C) and analyzed with a Raman Spectroscopy (type: Renishaw, in Via), to specify their structural characteristics such as hydration numbers and cage occupancies. The Raman analytical procedures have been described by Liu et al. [21]. Raman spectra were taken with a high-resolution (spatial resolution, horizontal 1 μm × vertical 2 μm) Leica microscope and with a Ar+ laser wavelength of 514.5 nm at ~100 mW. Because the hydrated gas is methane, we only scanned the Raman frequency shift interval in the C–H stretch region from 2860 to 2960 cm− 1. In order to investigate the homogeneity of DW–MH hydration numbers, Raman spectra were measured at different positions of a sample (Fig. 3). In each measurement, 20× microscopic lens was used to give a panoramic picture and five different positions were selected for further Raman spectra measurements (upper part of Fig. 3). After that, the microscopic lens was changed to 50×, and then the photo and Raman spectra of position one was measured (lower part of Fig. 3). In the same way, the photos and Raman spectra of positions 2–5 were obtained with the 50× microscopic lens. 2.4. Storage capacity and dissociation rate measurement A portion of DW–MH sample (~ 1 g) was measured by a dedicated apparatus (Fig. 4) to determine the storage capacity (vgas/vwater) directly [22]. The experimental processes were as follows: (1) the volume (v0) of standard test tube part was calibrated. (2) at atmosphere, valve K1 and K3 were closed while K2 was opened, then the system was vacuumed and the pressure P1 was recorded. After that K2 was closed while K3 was opened (at this time pressure changed to P2), so that the air pressure of the standard test tube part is: P0 = P2 − P1. (3) a portion of DW–MH sample was put into the sample test tube which is partly submerged in liquid nitrogen. All the valves (K1, K2 and K3) were opened when the whole system was vacuumed. After that K2 was closed and liquid nitrogen cup was moved away from the sample test tube so that hydrate can dissociate under room temperature (the dissociation rate can be measured at this stage). As hydrate dissociation finished, the liquid nitrogen cup was moved to submerge the standard test tube, to accumulate all the methane gas dissociated from hydrate (pressure P3 was recorded after
Fig. 2. Schematic diagram of the hydrate-formation apparatus.
G. Hu et al. / Fuel Processing Technology 92 (2011) 1617–1622
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Fig. 3. Upper: Photo of sample DW–MH-1 under 20× microscopic lens; p1 to p5 are five different positions for measuring Raman spectra. Lower: Photo and Raman spectra of point p1 measured by 50× microscopic lens.
accumulation finished). Lastly, valve K1 was closed and the liquid nitrogen cup was taken away from the standard test tube, then the methane gas released under room temperature and the pressure was recorded to P4. So the total volume of methane gas can be calculated with vgas = v0 × (P4 − P3) / (P2 − P1). (4) the volume of the hydrated water was measured with gravimetric method after DW–MH was dissociated, i.e. vwater = Wwater / ρwater = (W1 − W2) / ρwater, where W1 is the weight of tube and water, W2 is the weight of the dry tube, and ρwater is density of water (1 g/cm3). 3. Results and discussions 3.1. Induction and reaction time of DW–MH formation The formation conditions of DW and DW–MH samples are shown in Table 1, and the hydrate formation processes by two different types of P-T condition-controls are shown in Fig. 5. As shown in the formation process of DW–MH-1 (Fig. 5, left), the induction time, which means the interval time between time point of P-T conditions become conducive for methane hydrate formation to the time point of hydrate begins to
form, was about 243 min. And the reaction time of hydrate formation of DW–MH-1 was about 166 min. The induction and reaction time were much shorter with the second type of P-T condition-control, however. As shown in Fig. 5, right, the induction time and reaction time of sample DW–MH-2 were about 4 min and 70 min, respectively, when hydrate was formed under a certain temperature (~3 °C) which is directly conducive for hydrate formation. Our results are similar to Wang et al. [16], who have investigated the first type of P-T condition for DW–MH formation (although their DW is prepared at a different mixing speed of 19,000 rpm). In their studies, the reaction time of DW–MH was about 160 min (when gas capacity achieve about 158 v/v STP), which is very close to the first type result of 166 min in our experiments. Notably, the reaction time under the second type of P–T condition is much shorter than that under the first type. So we formed two other samples, DW–MH-2 and DW–MH-3, under the second type of P–T condition to investigate the effect of mixing speed of making DW on the induction time and reaction time (Table 1 and Fig. 6). The results indicate that when the mixing speed is lower, the induction time will be a little longer while the reaction time is remained as 70–77 min. Thus, from the above
Fig. 4. Schematic diagram of apparatus for measuring storage capacity and dissociation rate of gas hydrates.
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Table 1 Parameters of DW–MH samples. Sample No.
Mixing speed of DW
P-T conditions
Induction time Reaction time (min) (min)
DW–MH-1 DW–MH-2 DW–MH-3 DW–MH-4 DW–MH-5
Speed Speed Speed Speed Speed
Type Type Type Type Type
243 32 8 4 6
3 2 1 3 3
I II II II II
0
μ w ðhÞ−μ w ðh0 Þ = Δμ w
166 75 70 70 77
Note: type I: high pressure gas (8–9 MPa) was introduced into the reaction cell under room temperature, then the temperature was decreased to form hydrates; type II: after the reaction cell was cooled to ~ 3 °C, high pressure gas (8–9 MPa) was introduced to form hydrates.
experiments, it is suggested that DW–MH may be formed under the second type of P–T condition to enhance the formation rate. 3.2. Hydration numbers, homogeneity and storage capacity 3.2.1. Hydration numbers and homogeneity According to the peak intensities or peak area of Raman spectra, the structural characteristics such as hydration numbers and cage occupancies of MH can be obtained [21,23]. The cage occupancy ratio of methane in large and small cages is: θL = θS = IL = 3IS
ð1Þ
where θL, θS are the absolute occupancies of large and small cages and IL, IS are the integrated intensities of large and small cages from Raman spectra. For structure I hydrate, the chemical potential of hydrate can be calculated with: μ w ðhÞ−μ w ðh0 Þ = −RT½3lnð1−θL Þ + lnð1−θS Þ = 23
where μ w(h) is the chemical potential of water molecules in the lattice of empty hydrate, μ w(h0) is the chemical potential of water molecules in the reference state. At equilibrium with ice,
ð2Þ
ð3Þ
where Δμ 0w is the chemical potential difference between the water in the empty hydrate lattice and ice, and the value was found to be 1297 J mol− 1 [24]. Combining Eqs. (1), (2), and (3), the occupancy ratios can be used to calculate the absolute occupancies of θL and θS, which in turn can be used to obtain hydration numbers from n = 23 = ð3θL + θS Þ
ð4Þ
With the methods described in Section 2.3, the structural characteristics of three samples were measured. Here the result of DW–MH-1 was selected as an example to show the hydration numbers and homogeneity of DW–MH (Table 2). The results show that the hydration numbers measured at different positions of DW–MH are hardly changed, and the value can be given as 5.934 ± 0.06. It indicates that the DW–MH sample is very homogeneous. The hydration number of DW–MH is smaller than that of hydrate formed by CH4 + 300 ppm sodium dodecyl sulfate (SDS) solution, which is 6.12 ± 0.05 [21]. It demonstrates that the storage capacity of DW–MH is larger than that of SDS–MH. Also, it is found that the occupancy of large cage of DW–MH is more than 99%, while the average occupancy of small cage of DW–MH is about 89.50%. Since the large cage has been nearly absolutely filled with methane, it suggests that efforts should be focused on enhancing the methane uptake of small cages for further elevation of storage capacity of DW–MH. 3.2.2. Storage capacity The measured storage capacity (vgas/vwater at STP) of DW–MH samples are shown in Table 3. The results exhibit that gas storage of DW–MH has a small increase when the mixing speed of preparing DW
Fig. 5. P-T changes during hydrate formation in a sealed system (left, DW–MH-1; right, DW–MH-4).
Fig. 6. Induction time and reaction time of DW–MH-2 (left, DW prepared at mixing speed 3) and DW–MH-3 (right, DW prepared at mixing speed 1).
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Table 2 Structural characteristics of DW–MH-1. Sample position
IL
IS
θS
θL
n
DW–MH-1 (p1) DW–MH-1 (p2) DW–MH-1 (p3) DW–MH-1 (p4) DW–MH-1 (p5) Average
33847 46482 37216 48182 30454
9723 14067 11042 14750 9371
0.8558 0.9024 0.8844 0.9130 0.9178 0.8950
0.9931 0.9939 0.9936 0.9942 0.9943 0.9940
5.997 5.921 5.951 5.904 5.896 5.934
is increased. The DW prepared at the highest mixing speed (~22,000 rpm) shows a methane uptake of 203.93 vgas/vwater STP, which can be converted to about 163.14 vgas/vhydrate STP owing to one volume methane hydrate usually contains 0.8 volume of water [1]. In Table 3, the storage capacity of DW–MH samples was compared with that of SDS-MH. It shows that at least 10% of storage capacity is enhanced when methane storages in DW, comparing with it storages in SDS solutions. 3.3. Dissociation rate and self-preservation of DW–MH A typical dissociation process of DW–MH-1 is shown in Fig. 7. DW–MH sample was placed in sample test tube of the storage capacity measurement apparatus (Fig. 4), and the test tube was partly submerged into liquid nitrogen so that hydrate can be stable. After the whole system was vacuumed, the liquid nitrogen cup was moved away and the DW–MH sample was warmed by room temperature. About 3 min later, the pressure began to increase, which indicates that DW–MH start to dissociate. In Fig. 7, it shows that the whole dissociation process can be described by four stages. At the first stage, the dissociation rate is very rapid. Almost 55% of total methane gas is released in about 2 min. The dissociation rate at the second stage is much lower (~15 times lower) than that at the first stage, however, probably it is caused by the self preservation behavior of MH [25]. And then the dissociation rate rebounds at the third stage since the methane gas burst after the ice trap is thawed. However, the rate is lower than that at the first stage because the total mass of gas is smaller. In the last stage, the gas pressure has a small increase by the warm temperature. About the self preservation behavior of DW–MH during hydrate dissociation, it occurred in our experiments after that more than half of the methane gas was released. Thus, more efforts need to be made to improve usage of this behavior during the MH storage chain. 4. Conclusions In this paper, the results of direct measurements on formation rate, structural characteristics, storage capacity and dissociation rate of five DW–MH samples were reported. The conclusions are made as follows: (1) the formation rate can be improved by controlling the P-T condition of hydrate formation, i. e. the induction and reaction time of DW–MH formation are 243 min and 166 min respectively when the reaction cell is pressurized before cooled, while they are 4 min and 70 min respectively when the reaction cell is cooled first; although the formation time is reduced compared with Wang et al.
Fig. 7. Dissociation process of sample DW–MH-1 under free space and room temperature (~ 20 °C).
[16], further reduction of DW–MH formation time may possibly be made by a more appropriate P–T condition; (2) the hydration number of DW–MH sample is 5.934 ± 0.06, and the sample shows to be very homogeneous from Raman spectroscopy measurements; (3) DW can be used to increase at least 10% of the storage capacity of MH, compared with surfactants such as SDS solutions, further elevation of storage capacity of DW–MH should be focused on enhancing the methane uptake of small cages; and (4) although hydrate dissociation can be prevented by the effect of self preservation, more than half of total methane gas has released before the self preservation occur. Acknowledgements This work was supported financially by “National Natural Science Foundation of China (41072037)”, “Ministry of Land and Resources' Public Benefit Research Foundation (201111026)” and the “Natural Gas Hydrate in China Sea Exploration and Evaluation Project (GZH2002002)”. Comments by two anonymous reviewers have greatly helped in clarifying the presentation. References [1] E.D. Sloan Jr., C.A. Koh, Clathrate Hydrate of Natural Gases, 3rd Ed. Taylor & Francis-CRC Press, London, 2007. [2] E.D. Sloan Jr., Fundamental principles and applications of natural gas hydrates, Nature 426 (2003) 353–359. [3] J.S. Gudmundsson, M. Mork, O.F. Graff, Hydrate non-pipeline technology, 4th International Conference on Gas Hydrate, Yokohama, Japan, 2002, pp. 997–1002. [4] H. Kanda, Economic study on natural gas transportation with natural gas hydrate (NGH) pellets, 23rd World Gas Conference, Amsterdam, Netherlands, 2006, p. 11. [5] W. Hao, J. Wang, S. Fan, W. Hao, Evaluation and analysis method for natural gas hydrate storage and transportation process, Energy Conversion and Management 49 (2008) 2546–2553. [6] N.J. Kim, J.H. Lee, Y.S. Cho, W. Chun, Formation enhancement of methane hydrate for natural gas transport and storage, Energy 35 (2010) 2717–2722. [7] X. Lang, S. Fan, Y. Wang, Intensification of methane and hydrogen storage in clathrate hydrate and future prospect, Journal of Natural Gas Chemistry 19 (2010) 203–209. [8] B. Tohidi, A. Danesh, A.C. Todd, R.W. Burgass, K.K. Østergaard, Equilibrium data and thermodynamic modelling of cyclopentane and neopentane hydrates, Fluid Phase Equilibria 138 (1997) 241–250. [9] Y. Guo, S. Fan, K. Guo, Y. Chen, Storage capacity of methane in hydrate using calcium hypochlorite as additive, 4th International Conference on Gas Hydrate, Yokohama, Japan, 2002, pp. 1040–1043. [10] T. Uchida, R. Ohmura, I.Y. Ikada, J. Nagao, S. Takeya, A. Hori, Phase equilibrium measurements and crystallographic analyses on structure-H type gas hydrate
Table 3 Storage capacity (vgas/vwater, STP) measured in this study. Sample No.
v0 (cm3)
P1 (kPa)
P2 (kPa)
P3 (kPa)
P4 (kPa)
vgas (cm3)
W1 (g)
W2 (g)
vwater (cm3)
vgas/vwater
DW–MH-1 DW–MH-5 DW–MH-2 DW–MH-3 SDS-MH
80.85 80.85 80.85 80.85 80.85
1.3 1.3 1.3 1.3 1.3
55.4 55.4 55.4 56.2 56.6
2.0 17.4 1.3 11.3 4.1
42.9 134.9 26.5 29.5 38.7
61.18 175.67 37.57 26.76 50.63
35.83 35.98 35.79 35.58 36.01
35.53 35.10 35.60 35.44 35.73
0.30 0.88 0.19 0.14 0.28
203.93 199.62 197.75 197.61 180.30
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