Brine-Saturated

Article pubs.acs.org/IECR

CO2 Hydrate Formation Characteristics in a Water/Brine-Saturated Silica Gel Mingjun Yang, Yongchen Song,* Lanlan Jiang, Yu Liu, and Yanghui Li Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, Dalian University of Technology, Dalian, Liaoning 116024, China ABSTRACT: Hydrate-based technology is a promising method for gas separation and seawater desalination. There is little information about the combination of the two applications. The CO2 hydrate formation and dissociation in saline water (3 wt % sodium chloride) and deionized water in a silica gel fitted vessel are experimentally investigated by using a magnetic resonance imaging (MRI)-based pool measurement system. Three experimental cases were conducted with different procedures. MRI images and mean intensity (MI) were obtained using a spin echo multislice pulse sequence. From this study, it is found that the hydrate formation in saline solution is rapid compared to that in deionized water. It is caused by the “structure making” of ions. Hydrate is formed more rapidly in the flowing process than in the cooling process due to the additional mechanical effect. The so-called “memory effect” was identified for the hydrate dissociated solution, for which the nondissociated hydrate crystals exist. It shows that the twice displacement is superior for experimental stability. Additionally, MR images show that the rapidly formed hydrate can cause blockages of the experimental loop. The sensitivity of the MRI system is high when the temperature is above 284.15 K. This causes a rapid decrease in the MI as the temperature increases. hydrate induction time.32 Though the presence of porous media increased the hydrate phase equilibrium pressure slightly,13,33 it is also a potential choice for hydrate-based technology improvement. A hydrate phase equilibrium shift is reported with the presence of silica gel, but the microscale investigation showed that porous media could provide a favorable carrier for hydrate formation.33,34 The microimaging study showed a fast conversion of water to hydrate in the silica gel pores, which indicated that the formation of hydrate in silica gel pores offers feasibility for recovering CO2 from flue gases.31 A nearly 4-fold increase in the number of moles of the gas incorporated in the hydrate per mole of water is observed.35 When silica gel is used for CO2 capture, the gas obtained from the hydrate dissociation contained more than 95 mol % CO2 (70 mol % in the bulk water hydrate).30 Furthermore, silica gel with a larger surface area leads to higher gas consumption and reduces the induction time,9 and silica sand can be an effective porous media for the separation of CO2 from a fuel gas mixture in a fixed bed setup.10 Significant progress has been made in mitigating the pressure and improving the hydrate formation rate and gas capacity.11,12,36−40 However, there is limited information about the kinetic data of hydrate formation, especially in porous media.41,42 Magnetic resonance imaging (MRI) is an effective tool for hydrate investigations in porous media because it noninvasively maps water protons with high space resolution in three dimensions.43−47 The rates and mechanisms of hydrate formation in coarse-grain porous media can be

1. INTRODUCTION Hydrate-based technology is a promising method for gas separation, gas transportation and storage, seawater desalination, and cold storage.1−6 The basic principle of these applications is that guest molecules are enclathrated into hydrate cages formed by water molecules at “high pressure and low temperature”.7,8 Thus, the combination of gas separation and seawater desalination is feasible by hydrate formation, although few investigations have been conducted. Substantial progress has been made for hydrate-based gas separation.9−14 Most of the current investigations focus on mitigating the hydrate formation pressure and on improving the hydrate formation rate and gas capacity. Additives are used to decrease the hydrate phase equilibrium pressure or to promote the hydrate formation rate. Tetrahydrofuran (THF),15−17 tetrabutyl ammonium bromide (TBAB), 18,19 cyclopentane (CP),20−22 and cyclobutanone23 are potential thermodynamic promoters that occupy the large cavity of the structure II hydrate (s II). The hydrate structure change causes a dramatic decrease of the hydrate phase equilibrium pressure. Sodium dodecyl sulfate (SDS) is a popular kinetic additive used to increase the hydrate formation rate and gas capacity by improving the gas/liquid contact.24−27 Stirring can improve the contact of the gas and liquid, and it is widely used when the guest is insoluble in water.3,14,28 Because the energy consumption for mechanical agitation is tremendous,29 the industrial use of hydrate-based technology requires that hydrate crystallization be performed without mechanical agitation.4 Porous media is used as a carrier for hydrate formation. The presence of porous media decreases the hydrate induction time and improves the ratio of water to hydrate conversion.9,30,31 The mechanism involves the pore space providing interfaces for the interaction of liquid and gas. In our previous investigations, the presence of glass beads promoted methane hydrate formation by decreasing the © 2014 American Chemical Society

Received: Revised: Accepted: Published: 10753

March 26, 2014 May 6, 2014 May 9, 2014 May 9, 2014 dx.doi.org/10.1021/ie5012728 | Ind. Eng. Chem. Res. 2014, 53, 10753−10761

Industrial & Engineering Chemistry Research

Article

residual solution saturation by gas flooding at high flow rate and high pressure difference, where the outlet valve was open. The displacement was performed twice for each cycle to obtain a stable initial solution saturation. Then, the vessel was pressurized to a designed value with the gas mixture, and MRI began to monitor the solution distribution. The vessel was cooled to allow hydrate formation with constant pressure. For Case 3, the thermostat bath temperature was set to 279.65 K (Cycle 3-1) and 273.35 K (Cycle 3-2). After the vessel pressure was constant (6.00 MPa), the MRI began to obtain images. The gas mixture was then injected stably into the solution saturated silica gel to allow hydrate formation, and the back pressure was kept at a designed value. Hydrate was present for all cycles. The depressurization and/or heating methods were used to allow hydrate dissociation, which was shown individually in the following section.

obtained.48 Hydrate formation was found to be faster in a bed with a lower water content and smaller particle size.49 Hydrate-based seawater desalination is also a potential application,5 although there are only limited data, especially in combination with CO2 capture. To obtain kinetic mechanism data for hydrate formation and dissociation in silica gel, the characteristics of hydrate formation were investigated experimentally using MRI for some operation processes. The knowledge obtained from this work is sufficiently general and is expected to be used in other hydrate-based applications.

2. EXPERIMENTAL INVESTIGATION 2.1. Experimental Apparatus and Materials. The core apparatus was a high-pressure vessel, which was made of nonmagnetic material. As shown in Figure 1, the auxiliary

3. RESULTS AND DISCUSSION Three experimental cases were performed with different procedures in this study. Hydrate formation and dissociation were visually investigated first in partly water saturated (Case 1) and partly brine saturated (Case 2, 3% sodium chloride) silica gel. The depressurization/heating method was used to make hydrate dissociation. CO2 gas was injected vertically upward into the cooled silica gel with a flow rate of 1.0 mL·min−1 at 6.00 MPa (Case 3, 3% sodium chloride). The sagittal planes were obtained to display the solution distribution in silica gel. The field of view (FOV) was 40 mm × 40 mm with a 2.0 mm thickness for all images. The mean intensity (MI) data was for FOV in this investigation. 3.1. CO2 Hydrate Formation in Partly Water Saturated Silica Gel by Cooling (Case 1). After pure CO2 was injected into partly water saturated silica gel, the pressure and temperature were kept at 0.10 MPa and 279.65 K. The temperature (T), pressure (p), and mean intensity (MI) changes for Cycle 1-1 are shown in Figure 2a. Figure 3a−f shows the MRI images. For this cycle, the MRI began to obtain images when the initial T and p were stable. The T and p were maintained for 50 min to verify the stability of the MI. The significant p increase caused an MI fluctuant from 58 to 80 min, shown in Figure 3a,b. The cooling process (to 271.45 K) was performed at 64 min to make hydrate formation. There is also a slight MI fluctuation during the cooling process, which may be caused by the temperate effects on the tuning of the MRI. MI decreased suddenly at 148 min, which was caused by CO2 hydrate and/or ice formation. The hydrate formation process is shown in Figure 3b−d. The bright signal (liquid solution) first disappeared at the lower section of the vessel wall. After MI decreased to 0.01, T increased to 273.35 K at 330 min. MI increased during the heating process and then remained constant. That is to say, ice was present in this cycle. The backpressure was set to atmospheric pressure at 420 min to make CO2 hydrate dissociation. MI was fairly constant from 370 to 438 min and then increased rapidly from 0.016 to 0.026 due to hydrate dissociation. The MI increase indicated CO2

Figure 1. Schematic diagram of the apparatus.

apparatuses were an MRI, a data acquisition system, syringe pumps, a thermostat bath, a backpressure regulator, and a vacuum pump. A pressure transducer (Nagano Co., Ltd., Japan) and a thermocouple (Yamari Industries, Japan) were connected to the vessel. A high precision syringe pump (260D, Teledyne Isco Inc., America) was used to increase the pore pressure and maintain a constant flow. A back pressure regulator (BP-2080M) was made by JASCO Corporation, Japan. A thermostat bath (F-25 me, JULABO Labortechnik GmbH) was used to drive the FC-40 (3 M Company, USA) flowing through the surround jacket of the vessel. MRI (400 MHz, Varian Inc., USA) was used to visualize and quantify liquid solution (contains 1H) distributions in silica gel, but it did not detect 1H contained in solids, such as hydrate crystals or ice. MRI images were conducted using a spin echo multislice pulse sequence (SEMS). The experimental materials are shown in Table 1. Deionized water was used in all experiments. 2.2. Experimental Procedures. After silica gel was packed into the vessel tightly, the vessel was connected to the experimental system and vacuumed. The configured solution was injected and pressurized to 8.00 MPa to saturate the silica gel and verify the tightness. Then, the experiments were conducted with different methods. For Cases 1 and 2, the pore solution in silica gel was partly displaced to obtain an initial Table 1. Properties and Suppliers of Materials material

purity, %

CO2 NaCl silica gel

99.9 99.5 100

particle size, nm

supplier

0.42−0.84

Dalian Guangming special gas co., Ltd., China Shenyang Xinxing reagent factory, China Anhui Liangchen Silicon Material Co., Ltd., China (pore size 8.0−10 nm) 10754

dx.doi.org/10.1021/ie5012728 | Ind. Eng. Chem. Res. 2014, 53, 10753−10761


Article

Figure 2. MRI mean intensity, temperature, and vessel pressure changes during the cooling process in water/CO2 saturated silica gel, Cycle 1-1 (a); Cycle 1-2 (b).

initial experimental T was the same as that of Cycle 1-1. The T, p, and MI changes are shown in Figure 2b. Figure 3g−l shows the MRI images for Cycle 1-2. For this cycle, the MRI images were obtained when the initial T and p were stable. The T and p were kept for 160 min, and the constant MI indicated that there was no hydrate formation. The vessel was cooled to 273.35 K at 164 min to make hydrate formation. Similar with that of Cycle 1-1, there was a slight MI fluctuation during the cooling process. MI remained constant from 200 to 520 min, which indicated that there was no hydrate formation during the first cooling process. T was decreased to 271.45 K at 400 min to promote hydrate formation further. MI decreased suddenly at 528 min, which may be caused by CO2 hydrate and/or ice formation. The images for hydrate formation are shown in Figure 3h,i. After MI decreased to 0.008, T increased to 273.35 K. As shown in Figure 3j, the ice melting caused an MI increase. MI increased to 0.024 and stayed nearly constant. That is to say, ice formation caused the MI decrease at 528 min. The backpressure was set to atmospheric pressure at 646 min to make CO2 hydrate dissociation. The vessel was heated at 660 min, and MI increased sharply to 0.036 due to the hydrate

Figure 3. Distribution of the solution saturation during the cooling process in water/CO2 saturated silica gel, Cycle 1-1 (a−f); Cycle 1-2 (g−l).

hydrate dissociation, which also demonstrated the CO2 hydrate formation from 150 to 215 min. Once there was no CO2 hydrate formation, there was no MI increase during the depressurization process. The step-function pressure decrease indicated the hydrate blockage present in this cycle. After hydrate dissociated completely at atmospheric pressure, T was increased to 279.65 K. Then, the silica gel was saturated again, and the initial residual solution saturation was obtained. After p increased to 6.00 MPa, Cycle 1-2 was conducted. The

Figure 4. Sagittal planes, MI histogram, and the results after subtracting the highest MI value for Cycle 1-2. 10755



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Figure 5. MRI mean intensity, temperature, and vessel pressure changes during the cooling process in water/CO2 saturated silica gel, Cycle 2-1 (a); Cycle 2-2 (b); Cycle 2-3 (c).

Figure 6. Distribution of the solution saturation during the cooling process in brine/CO2 saturated silica gel, Cycle 2-1 (a−f); Cycle 2-2 (g−l); and Cycle 2-3 (m−r).

or with 1.5 K superheating for 20 h. It also suggests that the memory effect is in the bulk water phase and is probably due to residual hydrate structure.50 Since the T is increased only to 279.65 K in this case, and the time periods is short. The memory effect is obvious for Cycle 1-2. The memory effects of hydrate dissociation water is a promising parameter for hydrate technology application. The step-function pressure change indicated the presence of hydrate blockage in Cycle 1-2. It disappeared at 705 min, and meanwhile MI reached the highest value K. Then, the p remained at 0.1 MPa, whereas MI decreased sharply with the T increase because of the effects of a relatively high temperature on MRI tuning, Figure 3k,l. The sagittal planes, MI histogram,

dissociation. The MI increase demonstrated CO2 hydrate formation during the pressurization process, which was not imaged in this study. Once there was no CO2 hydrate formation, MI stopped increasing. Compared with Cycle 1-1, the hydrate formation is obviously rapid and easy for Cycle 1-2. This indicated that the hydrate dissociated water had a “memory effect”. Recently, Sefidroodi et al. investigated the memory effect for CP hydrate formation with various degrees of superheating and time periods. It was found that hydrate could be consistently obtained when the superheating above the equilibrium temperature for CP hydrates (7.7 K) was no more than 2−3 K. They said that the memory effect did not always disappear with as much as 8.4 K superheating for 20 min 10756



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Figure 7. MRI mean intensity, temperature, and vessel pressure changes during the cooling process in water/CO2 saturated silica gel, Cycle 3-1 (a); Cycle 3-2 (b).

MRI images for Cycle 2-2. After the p and T reached the desired initial values, MRI began to monitor the solution distribution in the silica gel. The vessel was cooled to 273.35 K at 10 min to make hydrate formation. There was a slight MI fluctuation during the cooling process. MI decreased suddenly at 22 min, which was caused by the CO2 hydrate. The hydrate formation process in the FOV is shown in Figure 6h−j. After decreasing to 0.001, MI remained nearly constant from 51 to 320 min. The p decreased to atmospheric pressure to make CO2 hydrate decomposition at 300 min. There was no obvious hydrate blockage. MI increased rapidly to 0.017 due to the hydrate dissociation and then remained constant. The MI was similar for the beginning and the end of this cycle. Then, MI stayed nearly constant until the end of Cycle 2-2, shown in Figure 3l. The initial p and T of Cycle 2-3 were the same with that of Cycle 2-2. The T, p, and MI changes are shown in Figure 5c. T began to decrease at 7 min to promote hydrate formation. The sharp MI decrease present at 25 min indicates the hydrates formation. MI decreased dramatically (0.031 to 0.025) at this time (25 to 51 min). To ensure the water conversion to hydrate, the vessel was cooled again at 400 min to promote ice formation. After the T reached 271.45 K, the MI decreased again at approximately 627 min. Then, MI remained constant, and the backpressure was set to atmospheric pressure to dissociate hydrates at 710 min. MI increased gradually to 0.021 from 719 to 734 min due to hydrate dissociation. Then, the MI remained nearly constant. The vessel was heated from 744 min to melt the ice. MI increased again at 750 min and finally reached 0.030. Then, the MI decreased again due to the effects of high temperature on the MRI system, which was similar to that of Cycle 1-2. Figure 6m−r shows the sagittal plane images obtained using MRI for Cycle 2-3. The residual solution in silica gel changed with hydrates formation (n) and (q) because hydrate formation squeezed the pores solution. The bright signal indicates that the water was not totally consumed (o). The hydrates formation in the pores partitioned the solution and gas in the silica gel. From the experiments, it is found that hydrate formation rate in saline solution is higher than that in deionized water. It may be caused by the effect of ions on the hydrogen-bond network. It is widely believed that the presence of salts in water engenders structural changes in the hydrogenbond network.51 It is found that ions can be classified as “structure makers” and “structure breakers”. Since ions tightening the hydrogen bonding having relative viscosity of

and the results after subtracting the highest MI value (present at 704 min) for Cycle 1-2 were shown from left to right in Figure 4. To make a comparison, an MRI image obtained before hydrate dissociation was introduced in Figure 4a. After the hydrate dissociated totally, we obtained the water distribution, Figure 4b. The images during the heating process are shown in Figure 4c−f. The sagittal planes during the heating process show similar profiles. When subtracted from the highest MI value, we obtained a nearly uniform image. It indicated that the effects of high temperature were on the whole vessel. That is to say, the MI decrease was not caused by the change of the water distribution, but it was caused by the sensitivity of the MRI system to high temperature. 3.2. CO2 Hydrate Formation in Partly Brine Saturated Silica Gel by Cooling (Case 2). Salt solution (sodium chloride, NaCl) was introduced in Case 2. The experimental procedure was similar to that of Case 1, where hydrate formation was driven by cooling. Three cycles were conducted for this case. The p and T were kept at 4.00 MPa and 279.65 K for Cycle 2-1. The T, p, and MI changes are shown in Figure 5a. Figure 6a−f showed the MRI images of Cycle 2-1. The T and p were kept constant for 20 min, and then, the vessel was cooled to 273.35 K. There was a slight MI fluctuation during the cooling process. After the T decreased to 273.35 K, the MI, p, and T remained constant until 475 min. This indicated that there was no hydrate formation in this duration. The p was increased to 5.00 MPa by gas injection to provide a further “drive force” for hydrate formation. Hydrate did not form in 1 h. Then, the p was increased to 6.00 MPa at 540 min. CO2 hydrate formation caused an MI decrease suddenly at 570 min. Because the T was higher than the ice point, there was no ice present. The hydrate formation process in FOV is shown in Figure 6b,c. After MI decreased to 0.028, the p decreased to atmospheric pressure to initiate CO2 hydrate decomposition. The solution distribution changed during the depressurization process, Figure 6d. There was no obvious hydrate blockage to prevent the p decrease. The MI increased rapidly to 0.035 due to hydrate dissociation, demonstrating the formation of CO2 hydrate at approximately 570 min. Then, MI remains nearly constant until the end of Cycle 2-1. There was lots of residual water remaining in the vessel after hydrate formation, Figure 6c. The initial p and T were 6.00 MPa and 279.65 K for Cycle 22, which was conducted to measure the hydrate formation during a cooling process (from 279.65 to 273.35 K). The T, p, and MI changes are shown in Figure 5b. Figure 6g−l shows the 10757



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Figure 8. Distribution of solution saturation during the gas injection process in saline saturated silica gel, Cycle 3-1 (a-f); Cycle 3-2 (g-l).

aqueous solutions were designated as “structure making”,52 Na+ is a “structure maker” in this study. It enhances the hydrogenbond structure in the NaCl + water + CO2 system. When the memory effects of water phase (dissociated from hydrate and ice) are compared, the enhancement of hydrogen-bond structure promotes hydrate formation in this study. 3.3. Hydrate Formation in Cooled Brine Saturated Silica Gel by CO2 Flowing (Case 3). There were two cycles in this case. The flowing process at 279.65 K was measured (Cycle 3-1). The initial p and T were kept at 6.00 MPa and 279.65 K for Cycle 3-1. The flow rate was 1.0 mL·min−1, and the flow direction was vertically upward. The T, p, and MI changes are shown in Figure 7a. After the initial vessel T and p were stable, MRI began to monitor the solution distribution in the silica gel. Then, CO2 was injected into the vessel to create a flow. MI decreased suddenly when the CO2 flood front reached the FOV 25 min later, which was caused by CO2 flooding displacement. There was no water migration before the CO2 flood front reached the FOV. The absence of hydrate in this process is discussed later in this study. After the residual water saturation was obtained, MI remained nearly constant until 300 min. In this duration, T was decreased to 273.35 K at 125 min. MI decreased again at 300 min due to hydrate formation. The rate of MI decrease was smaller than in the other cycles, which may be caused by the lower water saturation after CO2 flooding. Then, the MI remained constant again from 400 min after hydrate formation. The depressurization process was conducted at 410 min, where the backpressure was set to atmospheric pressure. There was a sharp MI increase from 412 to 445 min due to hydrate dissociation. The p increase from 411 to 420 min indicated the presence of hydrate blockage, and the heating process was performed at 418 min. When the hydrate dissociated totally, MI was nearly constant again. After T was higher than 284.15 K, MI decreased again due to the effect of high temperature on the MRI system. It has been discussed in the last section. The final MI value was similar to that after CO2 displacement (approximately 45 min later), so we proposed that the first MI decrease was not caused by hydrate formation. The displacement process in the FOV is shown in Figure 8b, whereas the hydrate formation process is shown in Figure 8b−d. There was little residual water remaining in the vessel after hydrate formation. A concentrated solution zone appeared in the FOV after hydrate dissociation, shown in Figure 8e. The last image of this cycle is shown in Figure 8f, which is similar to that after hydrate formation (Figure 8d), indicating that the solution distribution scarcely changed during the hydrate dissociation process.

For Cycle 3-2, the initial experimental p and T were set to 6.00 MPa and 273.35 K, and the flow rate was 1.0 mL·min−1. The flow direction was vertically upward. The T, p, and MI changes are shown in Figure 7b. MRI began to monitor the solution distribution in silica gel at the start of CO2 flowing. The MI decreased sharply at 23 min and then remained constant until 154 min. The sharp decrease of the image brightness is shown in Figure 8g,h. This was caused by CO2 displacement and hydrate formation. The p increase from 46 to 75 min indicated the presence of hydrate blockage. The other evidence for the presence of hydrate blockage was that the p curve was a broken line during the depressurization process. The CO2 flow was stopped, and the backpressure was set to atmospheric pressure at 154 min to initiate hydrate dissociation. The p decreased to atmospheric pressure at 165 min. Figure 8i,j shows an obvious change during hydrate dissociation. MI increased to 0.033 and then stayed constant after the hydrate dissociation, indicating the presence of hydrate in this cycle. Once there was no hydrate formation, MI did not increase during the depressurization process. When T was higher than 284.15 K, MI also decreased. The heating process was conducted at 168 min. The image brightness decreased gradually with the T increase, as shown in Figure 8i−l. From the experiments, the hydrate formed in the flowing process (Case 3) more rapidly than in the cooling process (Case 1 and 2). This can be explained by the promotion of perturbation, such as stir and spraying. The stir can promote the contact of liquid water and hydrate former gas, so it is strong in hydrate formation.53 The gas flow causes the extra pressure and creates a good mix of hydrate-forming gas and water, which is the crucial condition that needs to be satisfied in order to produce a high rate of hydrate formation.54 Jerbi et al. used two experimental protocols to measure the CO2 hydrate formation, by gas injection in a precooled solution and by water/CO2 mixture cooling with a constant gas quantity in the loop. They found that after 1200 s the hydrate fraction reached is slightly higher with gas injection than with cooling (6.5% vs 5.6%). They also proposed that, when the system is forced by the CO2 injection, there is an additional mechanical effect compared to cooling.55 3.4. Comparison of Hydrate Saturations for Different Cycles. Hydrate was quantified using MI data in this study. Because hydrate concentrates former gas (164) and water (0.8), one volume of water converts into 1.25 volumes of hydrate.56 That is to say, the hydrate saturation can be calculated using the following equation: 10758


Industrial & Engineering Chemistry Research S h = 1.25 ×

(I0 − Ii) × S W0 × 100% I0

Article

9%. Although the hydrate saturation was high for Cycle 3-1, the residual solution saturation was also high, indicating that the solution movement must take place for the next pressurization. This may cause the hydrate saturations decrease for the next hydrate formation process. In other words, Case 1 may be a potential choice when considering hydrate saturation for hydrate-based technology.

(1)

where SW0 is the initial water saturation and I0 and Ii are the MI at the initial time and i minutes. The hydrate saturations for all cycles are shown in Figure 9. The effects of ice on the MI are

4. CONCLUSIONS The characteristics of CO2 hydrate formation and dissociation in silica gel were experimentally investigated using MRI. Seven experimental cycles were conducted in this study. From the experiments, it is found that hydrate formation in saline solution is rapider than that in deionized water, but the hydrate saturation was lower. The hydrate formed in the flowing process (Case 3) was rapider than that in the cooling process (Case 1 and 2). The twice displacement makes MI rarely decrease during the depressurization process (only in Cycle 21). The hydrate-dissociated water had a “memory effect” because of the existence of nondissociated hydrate crystals. The hydrate restricted the contact of the hydrate former gas and solution and/or caused blockages in some cycles. When the temperature was above 284.15 K, the MI decreased significantly as the temperature increased. The different experimental procedures affect hydrate saturations. The highest hydrate saturation was 45% (Cycle 3-1), and the lowest one was 9% (Cycle 2-1) in this investigation.

Figure 9. Quantified hydrate saturations for all cycles.

removed when calculating the hydrate saturation. Because ice forms in different stages, the methods are different with each cycle. For Case 1, the initial solution saturation was 33%. The hydrate saturations were 18.8% and 17.8% for the two cycles. For Cycle 1-1, ice and hydrate formed simultaneously. The hydrate saturation was calculated using the MI data after ice melting. For Cycle 1-2, hydrate formed first, and ice formed second. The difference of the two hydrate saturations may be caused by the different “driven force” or mechanism for the two cycles. It is a perspective for hydrate-based technology. For Case 2, the initial solution saturation was 33%, which was also obtained by twice gas displacement at high differential pressure. The lowest hydrates saturation appeared in the first cycle, and the difference was approximately 10% and 5% when compared with Cycles 2-2 and 2-3. Considering that high T also caused the MI changes, we did not calculate the hydrates saturation when the vessel T was higher than 284.00 K. Because the initial solution saturation was obtained with twice gas displacement at a high pressure difference, there were minimal MI changes during pressurization and depressurization. We neglected the solution displacement as the gas mixture flushed into the vessel. For Case 3, the initial solution saturation was 100%. The highest hydrate saturation appeared in Cycle 3-1 (approximately 45%), which was higher than that of Cases 1 and 2. The difference of hydrate saturations for Cycles 3-1 and 3-2 was caused by the experimental procedure. For Cycle 3-1, hydrate formed after the CO2 flooding. That is to say, hydrate formed in the partly saturated silica gel, whereas hydrate formed when the gas flood front interacted with the solution for Cycle 3-2. The solution saturations after hydrates dissociation of the last two cycles were also different, 33% and 70% for Cycles 3-1 and 3-2. As a whole, hydrate saturation of Cycles 1-1, 1-2, and 2-2 was similar and was slightly higher than that of Cycle 2-3. This means that Case 1 is better than Case 2, mainly due to the presence of salt, which decreased the water activity. The second cycle hydrates saturation of Case 2 was higher than that of the other two cycles. The lowest hydrate saturation for Case 2 was

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is financially supported by the National Natural Science Foundation of China (51106018, 50736001, and 51227005), the Major National Science and Technology Programs of China (2011ZX05026-004), the High-tech Research and Development Program of China (2006AA09A209-5, 2013AA09250302), the Major State Basic Research Development Program of China (2011CB707304), and the Fundamental Research Funds for the Central Universities of China (DUT13LAB19).

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REFERENCES

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