CO2 Hydrate Formation and Dissociation in Cooled Porous Media: A

Aug 5, 2013 - CO2 Hydrate Formation and Dissociation in Cooled Porous Media: A Potential Technology for CO2 Capture and Storage ... Citing Articles; R...
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CO2 Hydrate Formation and Dissociation in Cooled Porous Media: A Potential Technology for CO2 Capture and Storage Mingjun Yang,† Yongchen Song,*,† Lanlan Jiang,† Ningjun Zhu,† Yu Liu,† Yuechao Zhao,† Binlin Dou,† and Qingping Li‡ †

Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, Dalian University of Technology, Dalian 116024, People’s Republic of China ‡ China National Offshore Oil Corporation Research Center, Beijing 100027, People’s Republic of China S Supporting Information *

ABSTRACT: The purpose of this study was to investigate the hydrate formation and dissociation with CO2 flowing through cooled porous media at different flow rates, pressures, temperatures, and flow directions. CO2 hydrate saturation was quantified using the mean intensity of water. The experimental results showed that the hydrate block appeared frequently, and it could be avoided by stopping CO2 flooding early. Hydrate formed rapidly as the temperature was set to 274.15 or 275.15 K, but the hydrate formation delayed when it was 276.15 K. The flow rate was an important parameter for hydrate formation; a too high or too low rate was not suitable for CO2 hydration formation. A low operating pressure was also unacceptable. The gravity made hydrate form easily in the vertically upward flow direction. The pore water of the second cycle converted to hydrate more completely than that of the first cycle, which was a proof of the hydrate “memory effect”. When the pressure was equal to atmospheric pressure, hydrate did not dissociate rapidly and abundantly, and a long time or reduplicate depressurization should be used in industrial application.

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recovery for the flue gas mixture in the presence of THF, obtained in their work, were greater than the ones reported in the literature with TBAB.18 One of the key elements for successful development of hydrate technology is the improvement of gas/liquid contact to increase the hydrate growth rate.19 Although hydrate crystallization is able to capture CO2, the power required for mechanical agitation was found to be very significant. If hydrate-based technology is used industrially, hydrate crystallization will be carried out without mechanical agitation.18 Microscale investigations showed that porous media can provide a favorable carrier for hydrate formation. Seo et al. found that the dispersed water in silica gel reacts readily with the gas, thus obviating the need for a stirred reactor and excess water.20 Adeyemo concluded a near 4-fold increase in moles of gas incorporated in the hydrate per mole of water and that the water-to-hydrate conversion was also improved.21 There are also a lot of investigations concerning CO2 hydrate in porous media recently,22−24 which provide some guidance for hydratebased CO2 storage in marine sediment. Lee et al. investigated the formation of a sinking CO2 hydrate for ocean carbon sequestration.25 Kwon et al. explored the dissociation behavior

INTRODUCTION Fossil fuel power plants are producing about one-third of all CO2 emissions worldwide, and the proportion has reached 42% in China. Hence, they are the prime targets for CO2 capture and storage (CCS). Hydrate-based technology is a promising method for CCS, which is a novel concept and uses CO2 hydrate to trap CO2 molecules in a lattice of water molecules. CO2 hydrate can form under high pressure and low temperature with the presence of water. It is a potential technology for CO2 capture from CO2-containing gas streaming (CO2/N2 or CO2/H2) or CO2 storage as a hydrate form in ocean and marine sediments.1 Hydrate-based CO2 capture is the most promising long-term CO2 capture technology because its energy penalty may be as small as 6−8%.2 Because the hydrate-based technology was proposed for gas separation, lots of investigations have been carried out to mitigate hydrate formation conditions and increase the hydrate formation rate, which can improve the separation efficiency and decrease the process cost.3−8 The most popular thermodynamic additives are tetrahydrofuran (THF) 9 − 1 1 a n d t e t r a -n -butylammonium bromide (TBAB);12−16 both of them can decrease the hydrate formation pressure. Linga et al. investigated gas hydrates from CO2/N2 and CO2/H2 gas mixtures in a semi-batch stirred vessel at a constant pressure and temperature of 273.7 K, and they found that the hydrate growth rate from CO2/H2 mixtures was the fastest.17 Then, they reported that the gas uptake and CO2 © 2013 American Chemical Society

Received: Revised: Accepted: Published: 9739

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of the CO2 hydrate in sediments in relation to pore fluid pressure evolution and sediment particle size.26 Lamorena et al. characterized an enhanced CO2 hydrate formation in marine sediment suspensions obtained from a gas hydrate deposit.27 Magnetic resonance imaging (MRI) is an effective apparatus for investigations in physical, chemical, life, and clinical sciences. It non-invasively maps the proton with high-space resolution in three dimensions. Having designed and constructed a high-pressure vessel to safely withstand 40.0 MPa, Hirai et al. observed the CO2 hydrate growth of a water droplet in liquid CO2 at 20.0 MPa.28 They demonstrated the effective performance of the apparatus as well as the perfect performance of MRI for hydrate investigations. Then, they measured hydrate thickness growth with MRI and the phenomenon applied to advanced CO2 ocean dissolution technology.29 The presence of porous media might be a challenge for hydrate investigation using MRI, and the baffle quickly disappears.30,31 Kvamme et al. applied MRI to visualize the conversion of CH4 hydrate to the CO2 hydrate within Bentheim sandstone matrix.32 Then, they investigated experimentally the rates and mechanisms of hydrate formation in coarse-grain porous media.33 Most of these experiments were conducted in a vessel fitted within a MRI instrument, which provides a unique method of monitoring hydrate formation by the loss of signal intensity. Most of the following experiments performed by them were also focused on the exchange of carbon dioxide for methane in the hydrate.34−37 Bagherzadeh et al. studied the methane hydrate formation in an unconsolidated silica sand bed using MRI, and they found that hydrate formed faster in bed with a lower water content and smaller particle size.38 Although lots of studies have been carried out to investigate hydrate-based CCS, they are mostly focused on thermodynamic properties. There is only very limited information on the kinetic data of CO2 hydrate formation, especially with CO2 flowing in porous media. One challenge to implement the hydrate-based technology for CCS, including CO2 capture and marine sediment CO2 sequestration, is to visualize the formation and dissociation of CO2 hydrate. This study focuses on analyzing the hydrate formation and dissociation process when CO2 flows into porous media, which may be helpful for understanding CCS technology based on hydrate formation. The knowledge obtained from this work is sufficiently general and is expected to be useful in other applications.

Figure 1. Schematic diagram of the experimental apparatus.

Experimental Procedures. The glass beads (BZ-01, particle diameter of 0.105−0.125 mm, As-One Co., Ltd., Japan) were used to form a porous medium. The interstitial porosity, permeability, density, and surface area are 36.4%, 7.8 μm2, 2.6 g/mL, and 0.00524 m2 g−1, respectively. CO2 (99.9%) was provided by Dalian Guangming Special Gas Co., Ltd., China. The deionized water was used in all experiments. MRI images were conducted using fast spin-echo multislice pulse sequence (FSEMS), and the experimental parameters are as follows: echo time (TE), 5.62 ms; repetition time (TR), 3.0 s; image data matrix, 128 × 128; field of view (FOV), 40 × 40 mm, with 2.0 mm thickness; number of slices, 10; number of images for averaging, 1; and acquisition time, 36 s. After the glass beads were packed into the vessel tightly with deionized water, the vessel was reconnected to the system and a vacuum pump was used to discharge the residual gas. Then, the bath temperature was decreased to the designed value, and the vessel pressure was also increased to the designed value by water injection to saturate the glass beads (100% water saturated). After the pressure and temperature were kept constant, MRI began to obtain images. CO2 was then injected stably into the vessel at a designed flow rate, and back pressure was kept at a designed value. When there was no water distribution change in the obtained images, the constant flow was ended and changed to a constant pressure process. The temperature was then increased gradually to make the hydrate decompose. Once there was no water distribution change in the obtained images, the vessel was cooled again to make hydrate formation again or the experiment was ended. This was determined by the field data analysis of the lab assistant. Generally, when the hydrate quantity of first hydrate formation was large, the second hydrate formation process would be carried out. It was the end of one case, and then the porous media was saturated again for the next batch experiment.

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EXPERIMENTAL SECTION Experimental Apparatus. The schematic diagram of the experimental apparatus was shown in Figure 1. The highpressure vessel is made of polyimide, and its effective dimension is Φ 15 × 200 mm. 1H MRI produces images of protons contained in liquids but does not image protons contained in solids, such as hydrate crystals or ices, because of their much shorter relaxation times. This makes MRI a potent tool to distinguish between the solid hydrate and liquid phase from which the hydrate forms.28 The pressure transducers (Nagano Co., Ltd., Japan) are connected to the vessel. The high-precision Syringe pump (260D, Teledyne Isco, Inc., Lincoln, NE) is used to increase pore pressure and keep CO2 flowing constantly. A back-pressure regulator (BP-2080-M) is made by JASCO Corporation, Japan. A thermostat bath (F-25 me, JULABO Labortechnik GmbH) is introduced to control the temperature precisely.

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RESULTS AND DISCUSSION With CO2 flowing in a cooled porous medium, the formation and dissociation of CO2 hydrate was visually investigated using MRI. Five flow rates (0.2, 0.5, 0.8, 1.0, and 1.5 mL min−1), three pressures (3.0, 4.0, and 5.0 MPa), three temperatures (274.15, 275.15, and 276.15 K), and two flow directions (vertically up and vertically down) were experimentally investigated. It should be noted that the vessel temperature was increased to 288.00 K to make hydrate dissociate for each heating process. Operating conditions of the experiments were shown in Table 1. It should be emphasized that the discussions on mean intensity (MI) data were all for FOV. By the way, the 9740

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Table 1. Operating Conditions and Results of the Experiments with Parameter Variation maximum hydrate saturation case case case case case case case case case case

1 2 3 4 5 6 7 8 9 10

T (K)

p (MPa)

flow rate (mL min−1)

275.15 275.15 275.15 275.15 275.15 275.15 276.15 274.15 275.15 275.15

4 4 4 4 5 3 4 4 4 4

1.0 1.5 0.5 0.8 1.0 1.0 1.0 1.0 1.0 0.2

flow direction vertically vertically vertically vertically vertically vertically vertically vertically vertically vertically

upward upward upward upward upward upward upward upward downward upward

first cycle

second cycle

0.589 0.371 0.290 0.324 0.391

0.256

0.284 0.541 0.202 0.287

0.398 0.609 0.281

0.589

Figure 2. Change in the MRI mean intensity of water and vessel pressure during the formation and dissociation of CO2 hydrate for cases 1 (a, 1.0 mL min−1), 2 (b, 1.5 mL min−1), 3 (c, 0.5 mL min−1), 4 (d, 0.8 mL min−1), 10 (e, 0.2 mL min−1), and 9 (f): () pressure and (- - -) MI.

(cases 1, 2, 3, 4, and 10). Because CO2 flow rates were different from each other, the CO2 flood front reached the FOV at different times. Considering the continuity demands of CO2 capture, cases 1, 3, and 4 were carried out with 2 times hydrate formation and dissociation, while cases 2 and 10 were carried out once because of the low efficiency of CO2 capture.

reproducibility test experiments are carried out at 273.65 K and 3.00 MPa. Although the largest hydrate saturation difference is 3.4%, the residual water saturations after hydrate dissociation are 45.6, 46.1, and 46.0%, respectively. Effects of Flow Rates on CO2 Hydrate Formation and Dissociation. The pressure and MI were shown in Figure 2 9741

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For case 1, MI decreased suddenly when the CO2 flood front reached the FOV about 18 min later, as shown in Figure 2a, which was caused by CO2 flooding displacement and CO2 hydrate formation. The water migration cannot be detected before the CO2 flood front reached the FOV. As seen from Figure S1a of the Supporting Information, the residual bright signal near the vessel wall indicated that the water was not totally consumed. The water migration caused by hydrate formation was not obvious, and most of them were local migration. The first heating process was performed at 1 K min−1 to make CO2 hydrate decomposition 40 min later. The increase of the pressure and MI indicated that CO2 hydrate began to dissociate at 52 min, which demonstrated that the CO2 hydrate had formed as CO2 flowing in cool watersaturated porous media. The released gas also drove some water away to the outlet of the vessel. The vessel was cooled to 275.15 K again at 120 min to form hydrate. The pressure and MI decrease at 161 min, indicating the hydrate formation in the FOV. The sudden CO2 flow drove some water away from the FOV and caused MI to decrease at 184 min. MI increased to a maximum value at 215 min, which indicated the end of the second hydrate dissociation process. When the flow rate was set to 0.5 and 0.8 mL min−1, CO2 hydrate formed and dissociated twice. The sharp decrease of MI was mainly caused by CO2 flooding displacement at 34 min for case 3. By comparison of the hydrate dissociation section in Figure 2c to that in Figure S1c of the Supporting Information, we concluded that the MI decrease from 34 to 43 min was mainly caused by CO2 flooding displacement for case 3. The first heating process was started at 95 min to make hydrate dissociation, and the hydrate blockage disappeared at 113 min. The second cooling process began 165 min later, and MI suddenly dropped at 189 min. The shorter time for the second hydrate formation might be a proof of “memory effect” of hydrate. The pressure decreased at 302 min because of the opening of the outlet valve. The MI decrease at 275 min was caused by the water migration, and the MI increase at 308 min indicated generous hydrate dissociation in the FOV. MI increased to 0.000 32 and was kept constant after 317 min. There was no pressure increase for the first cycle of case 4, as shown in Figure 2d. The sharp MI decrease was also caused by CO2 flooding displacement and CO2 hydrate formation at 25 min. After the CO2 flow was stopped, a pressure decrease was caused by hydrate formation outside the pale of the FOV, which was similar to that of case 1. The first heating process began at 60 min. The vessel was cooled again at 100 min, while MI increased sharply from 104 min, obtained a maximum at 118 min, and then decreased sharply. This was caused by the dissociation and formation of hydrate in the FOV. When the outlet valve was opened at 150 min, the sharply pressure decrease indicated the disappearance of the hydrate block. MI sharply increased at 212 min because of the CO2 hydrate dissociation and reached a maximum at 226 min. MI decreased sharply at 11 min for case 2 (shown in Figure 2b). In comparison to Figure S1b of the Supporting Information, there was no hydrate formation in the FOV before the hydrate block appearance at 19 min. The vessel was heated at 41 min, and MI decreased smoothly to 0.0002 at 60 min. Considering the lower hydrate saturation, case 2 was ended at 89 min. Then, we considered that a lower flow rate experiment should be carried out to supplement the data necessary for this study. The flow rate of case 10 was 0.2 mL min−1. The MI curve decreased at 92 min and was kept

constant from 108 to 115 min. This was mainly caused by CO2 flooding displacement. The vessel pressure increase from 102 to 143 min was caused by the blockage of hydrate formation outside the pale of the FOV. MI decreased gradually until 162 min later because of hydrate formation. The saturation of CO2 hydrate during the dissociation process for cases 1, 2, 3, 4, and 10 was calculated using the method by Yang et al.31 and shown in Figure 3. The expression

Figure 3. Quantified hydrate saturation for cases 1 (a, 1.0 mL min−1), 2 (b, 1.5 mL min−1), 3 (c, 0.5 mL min−1), 4 (d, 0.8 mL min−1), and 10 (e, 0.2 mL min−1).

of residual water saturation at i minutes (SWi) was shown as follow: S Wi =

IiS W0 × 100% I0

(1)

where SW0 is the initial water saturation (100% for this study) and I0 and Ii were the MRI mean intensity of liquid water at initial time and i minutes. Because 1 volume of fresh water can form 1.25 volumes of gas hydrates at standard temperature and pressure, the saturation of CO2 hydrate (Shi) in the porous medium at i minutes can be calculated using the following equation: S hi = 1.25

(I0 − Ii)S W0 × 100% I0

(2)

The reliability of eq 2 has been verified using a isochoric process, where the hydrate saturation can be calculated using two methods (water- and gas-based methods). The hydrate saturation obtained by the two methods becomes close to each other. The highest CO2 hydrate saturation was 60.9%, appearing in the second process of case 4, and the lowest CO2 hydrate saturation was 28.7%, appearing in the first process of case 10. The descending order of hydrate saturations for the first hydrate formation was cases 1, 2, 4, 3, and 10, and that of the second hydrate formation was cases 4, 3, and 1. For case 1, CO2 released from hydrate was partly effluent from the vessel. When CO2 flowing in the vessel was started again at 180 min, water was flushed from the FOV again because of the high pressure difference. For cases 4 and 3, the lower calculated hydrate saturation for the first process was mainly caused by a shorter heating time, which could not make hydrate dissociation completely. That was to say that the maximum water saturation was not obtained after the first heating process. 9742

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Figure 4. Change in MRI mean intensity of water and vessel pressure during the formation and dissociation of CO2 hydrate for cases 5 (a, 5 MPa) and 6 (b, 3 MPa): () pressure and (- - -) MI.

After the first section of this investigation, the following conclusions could be obtained: (1) Hydrate blockage appeared frequently, and it was a key restricted factor for hydrate-based CO2 capture. There was no hydrate blockage in case 1, where the CO2 flow was stopped early. This provided a guidance for the optimization operation. (2) Considering the similar water distribution of two dissociation processes, the water saturation in the FOV was kept constant after the first hydrate formation process. It was favorable for the reuse of pore water, which could ensure the feasibility of continuous operation. It was also crucial for the industrial application of hydrate-based CO2 capture technology. Because of the invariable water quantity, the hydrate saturation could be calculated during the second hydrate formation process, such as that of cases 3 and 4. (3) For cases 3 and 4, the MI after the second hydrate dissociation process was higher than that in the first process. This proved that the first hydrate dissociation process was not sufficient; therefore, the heating time should be extended properly to make hydrate dissociate completely. When hydrate saturation was calculated, the insufficient dissociation caused the low hydrate saturation after the first hydrate formation process. (4) When the pressure was set to atmospheric pressure, the hydrate was not dissociated rapidly and abundantly. The reduplicate depressurizing was applied in these cases, which was also a guidance for hydrate-based CO2 capture. (5) The lower hydrate saturation for the second hydrate formation process of case 1 was undesired. It was mainly caused by the abrupt CO2 flooding, which drove out some water from the FOV. This phenomenon could be avoided by keeping a constant pressure as the temperature was decreased, such as cases 3 and 4. (6) For case 2, the flow rate was too high to form an abundant hydrate, but for case 10, the flow rate was too low to form an abundant hydrate. Both of them were not suitable for CO2 hydrate formation. (7) There were hollows in the MI curve before the first hydrate dissociation process for cases 2, 3, 4, and 10, which might be caused by the second hydrate formation and dissociation of CO2 hydrate during the heating process. This should be avoided. Effects of Pressures on CO2 Hydrate Formation and Dissociation. Because the CO2 flow rate was the same for cases 1, 5, and 6, MI decreased as the CO2 flood front reached the FOV about 18 min later. As seen from Figures 2a and 4a, CO2 hydrate formed at the site where the CO2 flood front arrived for cases 1 and 5. The sharp MI decrease was caused by the CO2 flooding displacement and CO2 hydrate formation. Through the analysis of the overall MI curve of case 6, as shown in Figure 4b, the MI decrease was mainly caused by CO2

flooding displacement. The heating procedure started at 40 min for case 1, 54 min for case 5, and 37 min for case 6. For case 5, the hydrate blockage disappeared at 64 min. MI decreased because of the second hydrate formation at 120 min. The outlet valve was opened at 194 min and closed soon. MI did not change until 244 min, which demonstrated that hydrate dissociated gradually using the depressurizing method, and the pressure propagation velocity was low in the hydratecontaining porous media. For case 6, because of the disappearance of hydrate blockage, CO2 released from the hydrate had been partly flushed from the vessel. After the vessel was heated at 37 min, MI decreased gradually because of the CO2 flooding. It was kept constant until 133 min, which indicated that there was no hydrate formation in the FOV. Once hydrate existed in the FOV, MI would change during the hydrate dissociation process. The CO2 hydrate saturations for cases 5, 7, 8, and 9 were shown in Figure 5. Considering hydrate was partly dissociated

Figure 5. Quantified hydrate saturation during hydrate dissociation for cases 5, 7, 8, and 9.

in the first heating process, CO2 hydrate saturation for the first cycle of case 5 might be higher than that of case 1. There was a hollow at 75 min for case 5, which might be caused by the reformation and redissociation of CO2 hydrate during the heating process. For case 5, the pressure had a little increase during the second hydrate formation process (4.20−4.40 MPa), which was caused by the gas−liquid coexistence of CO2. For case 6, the operation pressure was too low to form an abundant hydrate, which could not meet our desire. As a whole, the first 9743

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Figure 6. Change in MRI mean intensity of water and vessel pressure during the formation and dissociation of CO2 hydrate for cases 7 (a, 276.15 K) and 8 (b, 274.15 K): () pressure and (- - -) MI.

a proof of the “memory effect” of hydrate, and it also appeared in cases 1, 3, 4, 5, and 6. Effects of Flow Directions on CO2 Hydrate Formation and Dissociation. As seen from Figure 2f, when the flow direction was vertically downward, the sharp MI decrease was caused by the CO2 flooding displacement at 18 min. There was no hydrate formation for case 9 from 18 to 27 min in the FOV (shown in Figure S1f of the Supporting Information). After hydrate formation in the FOV, hydrate blockage appeared at 35 min. The pressure decrease from 48 to 50 min was caused by hydrate formation. After the heating process was started at 50 min, the pressure stopped decreasing and increased from 60 min. MI increased from 60 min, and the sharp increase appeared at 90 min. By comparison of cases 1 and 9, we conclude that hydrate formed easily in the vertically upward flow direction and the hydrate saturation was higher than that in the vertical downward flow direction. This was caused by the effects of gravity. With CO2 flowing vertically downward, porous water was easily driven away by gravity and CO2 flooding and missed the chance to contact with CO 2 sufficiently. Even more hydrate particles might be moved by the resultant force of gravity and CO2 flooding and also affected the contact between water and CO2. Therefore, the vertically upward experiments were more appropriate. The investigation results indicated that the following knowledge should be considered for hydrate-based CO2 capture. For the hydrate formation process, hydrate blockage should be avoided to improve the gas uptake. Hydrate formed rapidly in 274.15 and 275.15 K, but there was a induction time when the temperature was 276.15 K. The hydrate “memory effect” made the pore water convert to hydrate more completely for the second hydrate formation process, which implied that the hydrate-based CO2 capture technology was highly efficient in continuous production. The flow rate was an important parameter on hydrate formation in the flowing system, with 0.5−1.0 mL min−1 being the potential choice for this investigation, while the experimental results indicated that the flow rate did not significantly affect the residual water saturation after hydrate dissociation. The first heating time can be extended properly to make hydrate dissociation completely, which was important to improve the gas uptake for the second hydrate formation, while the long time or reduplicate depressurization was also necessary to make hydrate dissociate completely. The action of gravity promoted the gas−liquid contact in the vertically upward flow, which made hydrate form easily and massively. Overall, the experimental results provide some basic fundamental data for continuous hydrate-based gas

hydrate saturation of case 5 was lower than that of case 1, but it was the opposite for the second process. In other words, CO2 hydrate formation of case 5 excelled over that of case 1. However, for case 5, the presence of gas−liquid coexistence of CO2 made the system more complicated and it was hard to control and evaluate. Effects of Temperatures on CO2 Hydrate Formation and Dissociation. As seen from panels a and b of Figure 6, when the temperature was set to 276.15 and 274.15 K, CO2 hydrate formed at the site where the CO2 flood front arrived. The sharp decrease of MI was caused by the displacement of CO2 flooding and CO2 hydrate formation. There was no hydrate formation for case 7 from 18 to 28 min, as shown in Figure 6a. MI increased from 41 to 79 min for case 1 and from 80 to 89 min for case 8, which declared that the MI decrease was not caused singly by CO2 flooding. After the first sharp decrease, the MI curve of case 7 had a short horizontal section about 20 min later. For case 8, the presence of a short horizontal section from 18 to 21 min showed that the hydrate formation process paused, which might be caused by the hydrate blockage. After the second sharp decrease, MI decreased slowly after the heating process was carried out at 45 min for case 7 and 35 min for case 8 at 1 K min−1. There were also horizontal sections for MI curves of the two cases. Then, the hydrate dissociation process started, and MI increased sharply and was finally kept constant. The vessel was cooled again for case 8 at 139 min, and the second hydrate formation process started at 155 min and finished at 166 min. The second heating process was carried out at 210 min, and the hydrate disappeared at 315 min. As shown in Figures 3 and 5, the highest CO2 hydrate saturation was 58.9% for cases 1, 7, and 8, presenting in the first process of case 1 and the second process of case 8. CO2 hydrate saturation for the first process of case 8 was the real value, because the heating time was sufficient for hydrate dissociation. The hydrate saturation for the second process of case 8 was higher than that for the first process, which was caused by the high residual water saturation before hydrate formation and the low residual water saturation after hydrate formation. By comparison of the effects of temperatures on hydrate formation, we concluded that hydrate formed rapidly when the temperature was set to 274.15 and 275.15 K. When the temperature was set to 276.15 K, the 10 min delay of hydrate formation took place and the hydrate saturation was low. The lowest MI after the second hydrate formation process was smaller than that of the first process, which meant that the pore water was converted to hydrate more completely. This might be 9744

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separation in cooled porous media. The flow rates, pressures, temperatures, and flow directions are all crucial control parameters for hydrate formation in a gas-flowing system. It provides fundamental data for the conceptual design of the hydrate-based flow-through CO2 capture process. The knowledge obtained from this work is sufficiently general and is expected to be useful in other applications.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed description of water distribution in the porous media because of CO2 flowing and hydrate formation for cases 1, 2, 3, 4, 10, and 9 (Figure S1) and cases 5, 6, 7, and 8 (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-411-84709093. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This project is financially supported by the National Natural Science Foundation of China (50736001 and 51106018), the High-Tech Research and Development Program of China (2006AA09A209-5), the Major State Basic Research Development Program of China (2009CB219507), the Scientific Research Foundation for Doctors of Liaoning Province (20111026), and the Fundamental Research Funds for the Central Universities (DUT13LAB19).

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