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Energy & Fuels 2005, 19, 1008-1015
Kinetics of Structure H Gas Hydrate Ju Dong Lee, Robin Susilo, and Peter Englezos* Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4 Received October 26, 2004. Revised Manuscript Received February 14, 2005
Experimental data on the kinetics of formation of structure H gas hydrate obtained in a semibatch stirred vessel at pressures of 0.63-1.5 MPa above equilibrium are reported. Methane was used as a guest substance and neohexane, tert-butyl methyl ether, and methylcyclohexane were used as the large molecule guest substance (LMGS). The results indicate that the rates of hydrate formation and the induction times are dependent on the magnitude of the driving force and the type of LMGS. When tert-butyl methyl ether is used as the LMGS, rapid hydrate formation and a much smaller induction time can be achieved. Furthermore, the methane consumption rate for hydrate formation in the presence of tert-butyl methyl ether is 3 times greater than that for a pure methane-water system. It was also observed that, although the induction period was greatly shortened by the memory effect, the rate of gas consumption rate was not affected. Hydrate decompositions were also conducted at a pressure 20% below equilibrium. The system with tertbutyl methyl ether as LMGS exhibited the fastest decomposition rate.
Introduction Structure H (hexagonal) gas hydrate crystals have been known to exist since 1987, when researchers at the National Research Council of Canada reported their formation from water and two guest substances, e.g., methane and neohexane (2,2-dimethyl butane).1-3 The other two well-known hydrate structures are cubic I and cubic II.4 Phase equilibrium data were first presented by Sloan’s group.5,6 Hydrate crystals were observed to form at pressures lower than those required to form methane hydrate at the same temperature. Subsequently, several groups have reported phase equilibrium data for several systems involving methane and a large molecule guest substance (LMGS). The term LMGS was proposed by Mori’s group.7 The fact that structure H (sH) hydrate forms at a much lower pressure than the corresponding structure I (sI) pressure at the same temperature prompted interest in these hydrates as potential media for the storage and transport of natural gas.8,9 Thomas and Dawe reviewed the technical and * Author to whom correspondence should be addressed. Telephone: 1-604-822-6184. Fax: 1-604-822-6003. E-mail address: englezos@ interchange.ubc.ca. (1) Ripmeester, J. A.; Tse, J. S.; Ratcliffe, C. I.; Powell, B. M. Nature 1987, 325, 135-136. (2) Ripmeester, J. A.; Ratcliffe, C. I. J. Phys. Chem. 1990, 94, 87738776. (3) Tse, J. S. J. Inclusion Phenom. Mol. Recognit. Chem. 1990, 8, 25-32. (4) Sloan, E. D. Clathrate Hydrates of Natural Gases, 2nd Edition; Marcel Dekker: New York, 1998. (5) Lederhos, J. P.; Metha, A. P.; Nyberg, G. B.; Warn, K. J.; Sloan, E. D. AIChE J. 1992, 38, 1045-1048. (6) Metha, A. P.; Sloan, E. D., Jr. J. Chem. Eng. Data 1993, 38, 580582. (7) Tsuji, H.; Ohmura, R.; Mori, Y. H. Energy Fuels 2004, 19, 418424. (8) Khokhar, A. A.; Gudmundson, J. S.; Sloan, E. D. Fluid Phase Equilib. 1998, 150-151, 383-392. (9) Ohmura, R.; Kashiwazaki, S.; Shiota, S.; Tsuji, H.; Mori, Y. H. Energy Fuels 2002, 16, 1141-1147.
economic aspects of the various options for transport of natural gas.10 There is a paucity of information on sH hydrate kinetics. Indirect observations were recorded by Hu¨tz and Englezos.11 It was found that, although sH methane hydrates form at much lower pressures than the corresponding sI methane hydrate, they also exhibited interesting kinetic behavior. In particular, hydrate crystal growth was fastest when tert-butyl methyl ether (TBME) was used, slower with 2.2-dimethylbutane (neohexane), and slowest with 2-methylbutane. Tohidi et al. conducted some experiments with the methanemethylcyclohexane-water system.12 They recorded the pressure decrease versus time at a temperature of 276 K with 4.8-6.8 K subcooling. Ohmura et al. also studied sH hydrate formation from methane and methylcyclohexane by spraying water into a high-pressure chamber that contained methane gas.9 It was found that sH hydrate forms at a lower pressure than the corresponding sI hydrate, yet both have the same methane storage rate. Tsuji et al. also used the aforementioned spray system and found that TBME is the most promising LMGS, in terms of rate of hydrate formation.7 Servio and Englezos studied the morphology of sH hydrate from neohexane-methane and water.13 The amount of methane in the neohexane phase was determined to affect crystal growth. The objective of the work undertaken in the present study was to study the kinetics of sH hydrate formation and decomposition. Kinetic studies involve the mea(10) Thomas, S.; Dawe, R. A. Energy 2003, 28, 1461-1477. (11) Hu¨tz, U.; Englezos, P. Fluid Phase Equilib. 1996, 117, 178185. (12) Tohidi, B.; Danesh, A.; Ostergaard, K.; Todd, A. In Proceedings of the 2nd International Conference on Natural Gas Hydrates, Toulouse, France, June 2-6, 1996; pp 229-236. (13) Servio, P.; Englezos, P. Cryst. Growth Des. 2003, 3, 61-66.
10.1021/ef049729+ CCC: $30.25 © 2005 American Chemical Society Published on Web 03/16/2005
Kinetics of Structure H Gas Hydrate
Energy & Fuels, Vol. 19, No. 3, 2005 1009
Figure 1. Schematic of the apparatus used in this research. Table 1. Chemicals Used in This Work chemical
certified purity
supplier
neohexane (NH) tert-butyl methyl ether (TBME) methylcyclohexane (MCH) methane water
99.0% 99.8% g99.0% UHP grade distilled and deionized
Aldrich Chemical Co., Inc. Aldrich Chemical Co., Inc. Aldrich Chemical Co., Inc. Praxair Technology, Inc.
surement of the induction time for crystallization, the determination of the rate of hydrate crystal growth, and the rate of decomposition.14,15 Experimental Section Materials. Methane was used as a guest substance for both sI and sH hydrate formation. Neohexane (NH), tert-butyl methyl ether (TBME), and methylcyclohexane (MCH) were selected as the second guest substance for sH hydrate kinetic experiments. The purities and suppliers of the fluid samples used in the experiments are listed in Table 1. The hydrateforming substances were used without any further purification. Apparatus. A schematic of the apparatus is shown in Figure 1. It consists of a crystallizer (CR), which is a highpressure cell or vessel, with temperature and pressure control systems. The high-pressure cell is an SS316 stainless-steel vessel that is immersed in a temperature-controlled bath. The vessel (SV) supplies gas to the high-pressure cell, and another vessel (RV) is used as a reference vessel, to have accurate pressure measurements for the vessel CR. Both vessels are immersed in an insulated bath. Rosemount smart pressure transducers (model 3051, Norpac Controls, Vancouver, BC, (14) Englezos, P. Ind. Eng. Chem. Res. 1993, 32, 1251-1274. (15) Englezos, P. Rev. Inst. Fr. Pet. 1996, 51, 789-795.
Canada), with a range of 0-13 790 kPa and accuracy of 0.075% of the span, are used. The crystallizer has two circular viewing windows on the front and back. A copper-constantan thermocouple (Omega, accuracy of ( 0.10 K) is inserted into the cell to measure the liquid temperatures during the kinetic experiments. Mixing of the cell contents is accomplished using a magnetic stir bar that is coupled to a set of rotating magnets placed directly underneath the cell. The set of magnets is driven by an electric motor. A typical mixing arrangement creates a vortex that is undesirable when kinetic experiments are performed. This vortex creates a ring that consists of hydrates and occluded water that leaves a small hole at the center of the gas/liquid interface that gradually may disappear. To prevent this vortex and to enhance mixing of the cystallizer contents, a baffle arrangement is used. The baffle has three legs and a horizontal bar, which is placed at the gas/liquid interface. A schematic of the crystallizer and the baffle are shown in Figure 2. Hydrate Formation Procedure. In sI hydrate formation experiments, the crystallizer is filled with 140 cm3 of water. During sH hydrate formation experiments, 100 or 120 cm3 of water and 40 or 20 cm3 of LMGS, corresponding to 200% or 100% of the stoichiometrically required amount of LMGS (NH, TBME, MCH), are charged. The total volume (LMGS + water) is 140 cm3 and results in a gas/liquid water interface that
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Figure 2. Schematic of the crystallizer and baffle used in this research. coincides with the top of the horizontal bar of the baffle. Air in the cell is flushed out by repeating three pressurizations with methane up to 3.0 MPa and depressurizations to atmospheric pressure. Subsequently, the vessels SV, RV, and CR are filled with the methane gas until the desired pressure is obtained. The temperature is then allowed to stabilize in all vessels. After the temperature has stabilized, the stirring in the crystallizer is started; this is time zero in the gas uptake measurement. A stirring rate of 450 rpm was determined to be suitable for this crystallizer and baffle arrangement and is applied to every experimental run. A suitable stirring rate is considered to be one that gives a stable gas consumption rate and a stable temperature following nucleation. The mixing speed of 450 rpm was chosen after performing a series of experiments at different stirring rates, in the range of 400500 rpm, at the same pressure and temperature. During these experiments, the number of moles of the gas consumed, relative to the time and temperature, were recorded. As methane in the vessel CR is consumed for hydrate formation, additional methane gas is automatically supplied from vessel SV and the pressure in the vessel CR is maintained constant with the help of a proportional integral derivative (PID) controller. During an experiment, the data acquisition system scans the pressure and temperature every second and then records the average values every 20 s. At any given time, the number of moles of the gas that have been consumed is the difference between the number of moles of the gas at time t ) 0 (start of the stirring) present in the vessel SV and the number of moles of the gas at time t and is given by the equation
∆n ) VSV
P (zRT )
0
P (zRT )
- VSV
t
(1)
where VSV is the volume of the supply vessel SV, including the tubing, and z is the compressibility factor, which is calculated by Pitzer’s correlations.16 Hydrate Decomposition Procedure. The hydrate decomposition is initiated by reducing the pressure 20% below the corresponding hydrate equilibrium pressure (set point). The pressure is reduced at a rate of ∼100 kPa/min until the desired experimental pressure (set point) is obtained. When the pressure reaches the desired point, mild stirring (280 rpm) is begun and then the number of moles of the decomposed gas versus time is recorded. This is the time zero of the decomposi-
tion experiment. It was noticed that the temperature decreased by ∼2 °C when the pressure was reduced to the set point. Subsequently, the temperature increased back to the previous level within 15 min after mixing was started. To determine the amount of gas decomposed from hydrates, vessel SV is evacuated in advance. As the hydrates are decomposed, the gas released from the hydrate crystals is transferred from vessel CR to vessel SV. A PID controller is used to maintain a pressure in the vessel CR that is constant and equal to the set point. The number of moles of decomposed gas can be calculated by the pressure change in the vessel SV and using Pitzer’s correlation.16 Gas-Phase Analysis. A Varian CX-3400 gas chromatography (GC) system that was equipped for thermal conductivity detection (TCD) and flame ionization detection (FID) is used to determine the gas-phase composition in the crystallizer. The CP-PoraPLOT U capillary column (Chrompack, 25 m × 0.32 m, df ) 10 µm) and the TCD and FID equipment are connected in series. Ultrahigh-purity helium is used as the carrier gas. The gas taken for analysis is transferred from the crystallizer to a sampling tube. The volume of the 1/8 in. diameter stainlesssteel tube is as small as possible, to minimize the sampling effect on the equilibrium. It has on-off valves installed on each end that can sample 0.5 mL. The sampling tube is flushed out 3 times before a sample was collected for analysis. When a sample is taken, the pressure in the crystallizer is reduced by
