Article pubs.acs.org/EF
Effect of Graphite Nanoparticles on Promoting CO2 Hydrate Formation Shi-dong Zhou,*,†,‡ Yi-song Yu,† Miao-miao Zhao,† Shu-li Wang,† and Guo-Zhong Zhang‡ †
Jiangsu Key Laboratory of Oil and Gas Storage and Transportation Technology, Changzhou University, Changzhou, Jiangsu 213016, People’s Republic of China ‡ College of Pipeline and Civil Engineering, China University of Petroleum, Qingdao, Shandong 266580, People’s Republic of China ABSTRACT: The effects of the graphite nanoparticles on the CO2 hydrate formation process were experimentally studied by measuring the induction time and amount of gas consumed. A suspension of 0.4% graphite nanoparticles was injected into the hydrate formation reactor, while pressure and temperature were maintained at 3.5 MPa and 277.15 K, and the magnetic stirrer speed was set at 300 rpm. The reaction lasted 800 min. The CO2 hydrate formation process and the amount of gas consumed were studied in both pure water and water containing a 0.4% graphite nanoparticle suspension. The induction time of hydrate formation was measured under various pressures. The results show that the nanographite particles had a positive effect on hydrate formation. In comparison to pure water, the induction time of CO2 hydrate formed in the presence of the graphite nanoparticles decreased by 80.8%, while the maximum CO2 consumption increased by 12.8%. In addition, the hydrate reaction was 98.8% complete within 400 min in the presence of nanographite particles. Graphite nanoparticles played a vital role in promoting CO2 hydrate formation.
1. INTRODUCTION Gas hydrate is a non-stoichiometric complex, in which gas molecules are trapped in a host lattice formed by water molecules.1 Nowadays, gas hydrate formation had been studied for natural gas storage and transportation,2 gas separation,3,4 separation of close boiling point compounds,5,6 seawater desalination,7,8 and thermal storage.9 These technologies have a bright future in their respective fields. However, to industrialize them, the rate of gas hydrate formation and storage capacity should be increased. Generally, four methods are used to promote hydrate formation, including stirring, bubbling, spraying, and adding surfactants. Linga et al.10 designed a new apparatus with a stirrer to enhance the rate of gas hydrate formation. Luo et al.11 proved that bubbling of gas in a continuous liquid phase was an efficient way to promote hydrate formation. Spraying was applied to promote hydrate formation of hydrophobic gas by Gnanendran and Amin.12 Lirio et al.,13 Fazlali et al.,14 and Karimi et al.15 had reported the effect of sodium dodecyl sulfate (SDS) on enhancing hydrate formation. In addition, ultrasonic and magnetic fields16 were also applied. Although favorable effects have been proven in the research of experiments, it is hard to industrialize them because of large initial investment. Thus, a new way should be proposed to improve the rate of hydrate formation. Actually, the process of gas hydrate formation is controlled by heat and mass transfer. To promote hydrate formation, a special additive to improve the properties of heat and mass transfer is needed. Nanofluids, in which there is high heat and mass transfer, were proposed at the end of the 20th century and considered to be a perfect additive for hydrate formation.17 Copper nanoparticles, which were used to promote HFC134a hydrate formation, were reported by Li et al.18 for the first time. Then, Park et al.19,20 employed carbon nanotubes as the additive to promote methane hydrate formation. The results © 2014 American Chemical Society
showed that the hydrate storage capacity increased and the induction time decreased. Arjang et al.21 received the same results by studying the effect of synthesized silver nanoparticles on promoting methane hydrate formation. Mohammadi et al.22 proved that an appropriate amount of SDS mixed with silver nanoparticles can be used to increase the rate of hydrate formation. In addition, polymer nanocomposites and nanosilica were used in methane hydrate formation by Ganji et al.23 and Chari et al.24 Almost all of them obtained the same results that nanofluids can accelerate the rate of gas hydrate formation and increase the hydrate storage capacity. However, most of the nanoparticles that were used to promote hydrate formation were metals and oxides, which will deteriorate under acidic conditions (pH < 7). Conversely, metalloid materials are acidresistant. However, it is certain that the industrial application of the hydrate technologies will always occur under an acidic environment. Hence, metalloids are a good choice in promoting hydrate formation. Thermal properties of nanoparticles are key factors to determine the heat-transfer coefficient of a hydrate formation system.25 Therefore, a material with high thermal conductivity must be found to promote hydrate formation. In this survey, the graphite nanoparticle was selected because it has a high heat-transfer coefficient and a low price among metalloid and oxide nanoparticles. Heat-transfer coefficients of several metalloids are exemplified in Table 1. Furthermore, lubrication is an important property of graphite nanoparticles, and the mechanical devices may benefit from it. Finally, graphite nanoparticles can be recycled easily and without producing secondary pollution. Received: January 10, 2014 Revised: June 1, 2014 Published: June 8, 2014 4694
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been completed, the magnetic stirrer was turned on with a speed of 300 rpm. Next, CO2 was captured to form the hydrate, and the pressure in the reactor decreased. The pressure stability was seen as a sign of the system reaching the steady state. Finally, the temperature and pressure were recorded by a data acquisition system.
Table 1. Heat-Transfer Coefficients of Some Metalloid Materials material
water
heat-transfer coefficient (W m−1 K−1)
0.61
graphite Al2O3 129
40
molecular sieve
glass
SiO2
0.1
0.7
7.6
3. RESULTS AND DISCUSSION 3.1. Effect of Additives on the Hydrate Formation Process. The results of the whole process of hydrate formation were found by two experiments. The 200 mL distilled water or nanofluid were injected into the hydrate formation reactor, while pressure and temperature were controlled at 3.5 MPa and 277.15 K, respectively, and the magnetic stirrer speed was set at 300 rpm. The reaction of hydrate formation lasted 800 min. Pressure−time (P−t) profiles of the two experiments within 800 min were plotted in Figure 2. A very interesting
2. EXPERIMENTAL SECTION 2.1. Materials. Distilled water was prepared in our laboratory, and all properties were consistent with industry standards. Graphite nanoparticles with a purity of 99.9% and particle average fineness of 50 nm were purchased from Xuzhou Jiechuang New Material Technology Co., Ltd. (China). Carbon dioxide with a purity of 99.8% was obtained from Changzhou Jinghua Industrial Gases Co., Ltd. (China). Aqueous solutions were prepared by an accurate analytical balance with an accuracy of ±0.0002 g. 2.2. Apparatus. The schematic diagram of CO2 hydrate formation is illustrated in Figure 1. In summary, the experimental apparatuses consist of an intake system, a fluid system, a data acquisition system, a cooling system, and a hydrate reaction system. The reactor is a SS-316 round vessel with a volume of 500 mL and which can withstand a pressure of 30 MPa. The operating temperature is from 273.15 to 288.15 K. In addition, the reactor needs to prevent any heat loss. A magnetic stirrer installed in the reactor is used to mix carbon dioxide and aqueous solution with a speed range of 0−1000 rpm. The core unit of the cooling system is a thermostat (258.15−268.15 K). The water flowing through the wall jacket is used to cool the system. A mechanical system with an electromotor is applied to increase the decline rate of the temperature. Two platinum resistance thermometers (Pt 100) with an accuracy of ±0.1 K (253.15−293.15 K) were inserted into the reactor and wall jacket to measure the temperature of the reaction system and water bath. A BD pressure transducer (0−10 MPa) is used to measure pressure and was produced by BD Sensors Co., Ltd. (Germany). Both the temperature and pressure are tracked and recorded at 1 s intervals through a data acquisition interface. 2.3. Procedure. At first, the interior of the hydrate formation reactor was carefully washed 3 times with distilled water, and then inner parts were evacuated with the suction pump. Second, a suspension of 0.4% graphite nanoparticles (200 mL) was injected into the reactor, and the air hidden inside it was removed quickly by a suction pump again. Third, the temperature of the reactor was controlled at 277.15 K by the cooling system, and then carbon dioxide was introduced into the reactor until the pressure reached the set value (2.5, 3.5, 4.5, 5.5, and 6.5 MPa). When all steps mentioned above had
Figure 2. P−t and T−t profiles of CO2 hydrate formation within 800 min under the conditions of 3.5 MPa and 277.15 K.
phenomenon had been observed in pure water and graphite nanoparticle suspension. The pressure decreased acutely in the initial stages of hydrate formation, and then the rate of pressure change decreased. However, the pressure that dropped in nanofluid was faster than that in pure water, especially in the first 300 min. Besides, Figure 3 shows that the rate of pressure decreased in pure water quickly within 30−65 min. However, it has not been seen obviously in a graphite nanoparticle
Figure 1. Schematic illustration of the experimental apparatus. 4695
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3.2. Effect of Additives on the Induction Time. The induction time is an important parameter to characterize the gas hydrate formation. Currently, there are two definitions of induction time. One is the time period from the moment of initial equilibrium state to the time of nucleation being finished. The other is the time period from the moment of initial equilibrium state to the time when crystals become visible. Both of the two definitions were proposed by Kashchiev and Firoozabad.26 The latter definition was used in our research. From this point, a series of experiments were performed to illustrate the effect of graphite nanoparticles on the induction time under the conditions of 277.15 K and different pressures, and the magnetic stirrer speed was 300 rpm. The final result is determined by comparing the induction time data to the temperature index obtained from P−t and T−t profiles. The data were acquired by an average value in three experiments. Figure 4 provides the process of the hydrate formation at
Figure 3. Partial enlarged drawing of Figure 2 within 100 min.
suspension, except at the beginning of hydrate formation. There are some reasons to explain these phenomena. When the system reached saturation, a large number of hydrate crystal nucleii appeared and hydrate formed quickly. With the further reaction of hydrate formation, the rate of hydrate formation slowed. In addition, Figure 2 shows temperature−time (T−t) profiles of CO2 hydrate formation corresponding to the P−t profiles. In comparison to the pure water, the temperature of graphite nanoparticle suspensions dropped sharply at the beginning of hydrate formation. Then, the temperature stabilized at 277.15 K. However, Figure 3 shows there was an increasing tendency appearing in the temperature curves of pure water and graphite nanoparticle suspensions, but the time of temperature rise was different. The temperature rise during hydrate formation in the graphite nanoparticle suspensions began in the initial stage of hydrate formation, but the phenomenon appearing in pure water was from 30 to 60 min. Furthermore, the time from the beginning to stabilization in pure water was much longer than that in graphite nanoparticle suspension. It means that graphite nanoparticles have a good effect on heat transfer. The main reasons for these phenomena are explained as follows. Gas was introduced into the reactor and causes the temperature to increase, and then the temperature dropped under the action of the cooling system. Besides, hydrate formation is an exothermic process. Therefore, with the large amount of hydrate formation, the temperature will rise and cannot be controlled at 277.15 K accurately in the rest of the time. When our results obtained from Figure 3 are compared to those by Arjang et al.,21 the phenomenon of partial elevation cannot be seen obviously in the T−t curve of nanoparticle suspensions. Moreover, the pressure curve of graphite nanoparticle suspensions decreased sharply in the initial stage of hydrate formation. These may be due to the short induction time under the action of stirring. In addition, the fact that hydration reaction heat can be exported timely is another reason for these phenomena.
Figure 4. Process of the hydrate formation at 277.15 K and 3.5 MPa.
277.15 K and 3.5 MPa. The time from beginning to ti is the induction time. Figure 5 presents the induction time of CO2
Figure 5. Induction time of CO2 hydrate formation at 277.15 K and different pressures.
hydrate formation under different pressures. The result shows that the presence of the graphite nanoparticles can greatly reduce the induction time of hydrate formation. For example, at 3.5 MPa, the induction time of hydrate formation in graphite nanoparticle suspensions was 8.6 min, but the time increased to 47.3 min without graphite nanoparticles. A similar behavior also appeared at 3.5, 4.5, 5.5, and 6.5 MPa. According to statistical 4696
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analysis, the induction time of CO2 hydrate formation decreased by 80.8% on average in graphite nanoparticle suspensions relative to the formation in pure water. Besides, Figure 5 shows that the induction time of gas hydrate formation decreased with increasing pressure, while it decreased gently in graphite nanoparticle suspensions (Figure 5). The results provide strong evidence that the main factors controlling the gas hydrate formation are the process of mass and heat transfer in the gas−liquid interface. Graphite nanoparticles had a positive effect on gas hydrate formation and can greatly decrease its induction time. Three reasons may contribute to the decrease in the induction time. To begin with, the nucleation of crystals includes two pathways, heterogeneous nucleation and homogeneous nucleation, but the former more easily forms the hydrate crystal nucleus. Graphite nanoparticles increased the inhomogeneity of the hydrate formation system, thereby providing an environment of heterogeneous nucleation. Thus, the time from the beginning to the appearance of hydrate crystals is reduced. In addition, gas hydrate formation is an exothermic process, and effectively removing the heat of reaction is a good way to accelerate the rate of hydrate formation. However, the heat-transfer coefficient of the system in the presence of graphite nanoparticles will be increased. Finally, graphite nanoparticles provide a high specific surface/volume ratio for the gas−liquid reaction interface of hydrate formation. 3.3. Amount and Rate of Gas Consumed. The amount and rate of gas consumed are important parameters in the gas hydrate formation, considering that the volume of the hydrate formation apparatus is fixed. The amount of CO2 gas consumed is calculated as follows:27 ⎛ PV PV ⎞ 1 ΔnCO2 = ⎜ 0 0 − t t ⎟ ZtTt ⎠ R ⎝ Z0T0
Figure 6. Amount of CO2 consumed within 800 min under the conditions of 3.5 MPa and 277.15 K.
consumed in the presence of graphite nanoparticles was faster than that in pure water. The following reasons can be used to explain this phenomenon. When the system reached saturation, hydrate appeared in the gas−liquid interface and increased the gas−liquid mass-transfer resistance. Therefore, the speed of gas consumption slowed. Besides, the observation that the rate of gas consumed in nanofluid was faster than that in pure water may be related to the properties of graphite nanoparticles. Hydrate formation terminated after 300 min. When the data obtained from a data acquisition system were analyzed, the number of moles of CO2 consumption in pure water and graphite nanoparticle suspensions were 0.218 and 0.246 mol, respectively, which is shown in Table 2. Thus, the maximum
(1)
Table 2. Amount of CO2 Consumed within 800 min under the Conditions of 3.5 MPa and 277.15 K
where R is the universal gas constant, P and T are the pressure and temperature in the reactor, which are obtained from the data acquisition system, Z is the compressibility factor, which is calculated by the Peng−Robinson28 equation of state, subscripts 0 and t stand for the conditions of the reactor at time t = 0 and time t, V0 is the volume occupied by the carbon dioxide in the initial stage of hydrate formation, and the volume of the solution will not change with the increasing pressure. Therefore, constant V0 is 300 mL. In contrast, it is more difficult to obtain the value of Vt because the formation of the gas hydrate changes the volume of the gas inside the reactor. Mohammadi et al.22 adopts a feasible way (eq 2) to solve this problem, and our study also takes this approach Vt = Vcell − VS + VRW − VH
mole of gas consumed (mol) time (min)
in pure water
in nanofluid (0.4% graphite nanoparticles)
60 120 180 240 300 400 500 600 700 800
0.137 0.171 0.188 0.199 0.207 0.211 0.215 0.218 0.218 0.218
0.164 0.196 0.217 0.229 0.238 0.243 0.246 0.246 0.246 0.246
(2)
gas consumption increased by 12.8% because of the presence of nanographite particles. In addition, Table 2 shows hydrate formation in the presence of graphite nanoparticles almost continues to 400 min with a rapid speed, and hydrate reaction had achieved 98.8% completation in this stage.
where Vcell is the volume of the vessel (500 mL), VS is the volume of the reaction aqueous solution (200 mL), VRW is the volume of water reacted, and VH is the volume of hydrate produced. Both VRW and VH were calculated by the formula provided by Mohammadi et al.22 These formulas only contain two parameters (P and T) Figure 6.shows the amount of CO2 gas consumed in the experiments with graphite nanoparticles and pure water within 800 min, which were conducted at P = 3.5 MPa and T = 277.15 K, and the magnetic stirrer speed was 300 rpm. CO2 molecules were trapped with the fast rate in the initial stage of hydrate formation, and then the rate of gas consumption slowed until the system reached a steady state. However, the rate of gas
4. CONCLUSION Because of the high heat-transfer coefficient and specific surface area, graphite nanoparticles were selected to promote CO2 hydrate formation. A series of experiments was performed under certain conditions, and hydrate reaction time lasted 800 min. The results showed that nanographite particles had a positive effect on hydrate formation. In comparison to pure 4697
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(12) Gnanendran, N.; Amin, R. The effect of hydrotropes on gas hydrate formation. J. Pet. Sci. Eng. 2003, 40 (1−2), 37−46. (13) Lirio, C. F. d. S.; Pessoa, F. L. P.; Uller, A. M. C. Storage capacity of carbon dioxide hydrates in the presence of sodium dodecyl sulfate (SDS) and tetrahydrofuran (THF). Chem. Eng. Sci. 2013, 96, 118−123. (14) Fazlali, A.; Kazemi, S. A.; Keshavarz-Moraveji, M.; Mohammadi, A. H. Impact of different surfactants and their mixtures on methane hydrate formation. Energy Technol. 2013, 1, 471−477. (15) Karimi, R.; Varaminian, F.; Izadpanah, A. A.; Mohammadi, A. H. Effects of different surfactants on ethane hydrate formation kinetics: Experimental and modeling studies. Energy Technol. 2013, 1/9, 530− 536. (16) Park, S. S.; Kim, N. J. Study on methane hydrate formation using ultrasonic waves. J. Energy Chem. 2013, 19 (5), 1668−1672. (17) Khaleduzzaman, S. S.; Mahbubul, I. M.; Shahrul, I. M.; Saidur, R. Effect of particle concentration, temperature and surfactant on surface tension of nanofluids. Int. Commun. Heat Mass Transfer 2013, 49, 110−114. (18) Li, J. P.; Liang, D. Q.; Guo, K. H. Formation and dissociation of HFC134a gas hydrate in nano-copper suspension. Energy Convers. Manage. 2006, 47 (2), 201−210. (19) Park, S. S.; Lee, S. B.; Kim, N. J. Effect of multi-walled carbon nanotubes on methane hydrate formation. J. Energy Chem. 2010, 16 (4), 551−555. (20) Park, S. S.; An, E. J.; Lee, S. B.; Chun, W. G.; Kim, N. J. Characteristics of methane hydrate formation in carbon nanofluids. J. Energy Chem. 2012, 18 (1), 443−448. (21) Arjang, S.; Manteghian, M.; Mohammadi, A. Effect of synthesized silver nanoparticles in promoting methane hydrate formation at 4.7 and 5.7 MPa. Chem. Eng. Res. Des. 2013, 91 (6), 1050−1045. (22) Mohammadi, A.; Manteghian, M.; Haghtalab, A.; Mohammadi, A. H.; Rahmati-Abkenar, M. Kinetic study of carbon dioxide hydrate formation in presence of silver nanoparticles and SDS. Chem. Eng. J. 2014, 237, 387−395. (23) Ganji, H.; Aalaie, J.; Boroojerdi, S. H.; Rezaei Rod, A. Effect of polymer nanocomposites on methane hydrate stability and storage capacity. J. Pet. Sci. Eng. 2013, 112, 32−35. (24) Chari, V. D.; Sharma Deepala, V. S. G. K.; Prasad Pinnelli, S. R.; Murthy, S. R. Methane hydrates formation and dissociation in nano silica suspension. J. Nat. Gas Sci. Eng. 2013, 11, 7−11. (25) Sajadi, A. R.; Kazemi, M. H. Investigation of turbulent convective heat transfer and pressure drop of TiO2/water nanofluid in circular tube. Int. Commun. Heat Mass Transfer 2011, 38 (10), 1474−1478. (26) Kashchiev, D.; Firoozabad, A. Induction time in crystallization of gas hydrates. J. Cryst. Growth 2003, 250 (3−4), 499−515. (27) Introduction to Chemical Engineering Thermodynamics; Smith, J. M., Ness, H. V., Abbott, M., Eds.; McGraw-Hill: New York, 2001. (28) Peng, D. Y.; Robinson, D. B. A new two constant equation of state. Ind. Eng. Chem. Res. 1976, 15, 59−64.
water, the induction time of CO2 hydrate formed in the presence of graphite nanoparticles decreased by 80.8%, while the maximum CO2 consumption increased by 12.8%. In addition, the hydrate reaction was 98.8% complete within 400 min in the presence of nanographite particles. There are enough reasons to believe that these results obtained from CO2 may inspire us to study the effect of graphite nanoparticles on promoting hydrate formation of natural gas.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-0519-83290280. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the funding from the National Natural Science Foundation of China (Grant 51176015) and the Science and Technology Innovation Foundation of the China National Petroleum Corporation (CNPC) (Grant 2011D50060606). In addition, the Jiangsu Province, China Graduate Education Innovation Project (Grant CXZZ13_0735) also provided financial support.
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REFERENCES
(1) Sun, Q.; Guo, X. Q.; Liu, A. X.; Dong, J.; Liu, B.; Zhang, J. W.; Chen, G. J. Experiment on the separation of air-mixed coal bed methane in THF solution by hydrate formation. Energy Fuels 2012, 26 (7), 4507−4513. (2) Javanmardi, J.; Nasrifar, K. H.; Najibi, S. H.; Moshfeghian, M. Economic evaluation of natural gas hydrate as an alternative for natural gas transportation. Appl. Therm. Eng. 2005, 25 (11−12), 1708−1723. (3) Li, X. S.; Zhan, H.; Xu, C. G.; Zeng, Z. Y.; Lv, Q. N.; Yan, K. F. Effects of tetrabutyl-(ammonium/phosphonium) salts on clathrate hydrate capture of CO2 from simulated flue gas. Energy Fuels 2012, 26 (4), 2518−2527. (4) Eslamimanesh, A.; Mohammadi, A. H.; Richon, D.; Naidoo, P.; Ramjugernath, D. Application of gas hydrate formation in separation processes: A review of experimental studies. J. Chem. Thermodyn. 2012, 46, 62−71. (5) Tumba, K.; Naidoo, P.; Mohammadi, A. H.; Richon, D.; Ramjugernath, D. Phase equilibria of clathrate hydrates of ethane + ethene. J. Chem. Eng. Data 2013, 58 (4), 896−901. (6) Tumba, K.; Hashemi, H.; Naidoo, P.; Mohammadi, A. H.; Ramjugernath, D. Dissociation data and thermodynamic modeling of clathrate hydrates of ethene, ethyne and propene. J. Chem. Eng. Data 2013, 58 (11), 3259−3264. (7) Khawaji, A. D.; Kutubkhanah, I. K.; Wie, J. M. Advances in seawater desalination technologies. Desalination 2008, 221 (1−3), 47− 69. (8) Peterson, T. N.; Petticrew, C.; Naidoo, P.; Mohammadi, A. H.; Ramjugernath, D. Experimental measurements and thermodynamic modeling of the dissociation conditions of clathrate hydrates for (refrigerant + NaCl + water) systems. J. Chem. Eng. Data 2014, 59 (2), 466−475. (9) Artem, A. S.; Andrey, Y. M.; Tatyana, V. R. Methane capacity of double tetrabutylammonium bromide + methane ionic clathrate hydrates. Energy Fuels 2012, 26 (6), 3711−3716. (10) Linga, P.; Kumar, R.; Lee, J. D.; John, R.; Peter, E. A new apparatus to enhance the rate of gas hydrate formation: Application to capture of carbon dioxide. Int. J. Greenhouse Gas Control 2010, 4 (4), 630−637. (11) Luo, Y. T.; Zhu, J. H.; Fan, S. S.; Chen, G. J. Study on the kinetics of hydrate formation in a bubble column. Chem. Eng. Sci. 2007, 62 (4), 1000−1009. 4698
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