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Energy & Fuels 1999, 13, 624-628
Hydrate Formation on Surfaces of Buoyant Liquid CO2 Drops in a Counterflow Water Tunnel Ho Teng and A. Yamasaki* National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba 305, Japan Received October 15, 1998. Revised Manuscript Received February 16, 1999
An experimental investigation of hydrate formation on surfaces of buoyant liquid CO2 drops was conducted in a counterflow water tunnel simulating conditions in the deep ocean. The major component of the experimental system was a tapered polycarbonate test section that could stand pressures up to 300 bar. In the experiments, through proper control of the pressure, temperature, and flow rate of the downwardly flowing water, CO2 drops released were suspended stably in the test section. Because the test section was transparent, the behavior of the CO2 drops in suspension could be examined visually and the process of hydrate formation on surfaces of the drops could be recorded by a video camera. It was found that the hydrodynamic stability of the buoyant CO2 drops influenced hydrate formation considerably. Small drops had a spherical shape, and in general, they were hydrodynamically stable; hydrate formed rapidly on such drops. Large drops were often nonspherical in shape, and because they were hydrodynamically unstable, their shapes varied with time; for such drops, it took a longer period of time for hydrate to cover their surfaces. For drops whose linear dimensions were on the order of a few centimeters, a continuous process of hydrate formation on and hydrate shedding from the surfaces of the drops was observed. The CO2 concentration in water was found to affect hydrate formation significantly. For both small and large drops, an increase in the CO2 concentration accelerated the process of hydrate formation. The phenomena observed in the present investigation were not reported previously.
Introduction Ocean disposal of anthropogenic CO2 has been proposed as a means of mitigating the global warming.1,2 The proposed depths for the ocean disposal are below the mixed layer, typically, at depths greater than 500 m. At these depths (where pressures of seawater are greater than 50 bar), in many oceans temperatures of seawater are below 283 K and, therefore, CO2 is in the liquid state. Since these pressures and temperatures fall within the region for hydrate formation, CO2 hydrate may be encountered.3 Natural formation of CO2 hydrate in the deep ocean has been found in the Okinawa trough.4 CO2 hydrate is a binary clathrate compound, and its chemical formula is 8CO2‚46H2O or CO2‚ 5.75H2O under the stoichiometric condition.5 Due to the low solubility, CO2 effluent in the ocean will break up into drops. Since the mole fraction of CO2 in the hydrate is much larger than the CO2 solubility in seawater, the most possible location for hydrate formation is the CO2seawater interface. Being a solid crystal, the hydrate interphase formed between the CO2 drops and seawater may induce significant changes in the physical and chemical behavior of the CO2 drops in the ocean. Several investigators6-11 have reported laboratory experiments of hydrate formation on CO2 drops in water * To whom correspondence should be addressed. E-mail: akihiroy@ nimc.go.jp. (1) Steinberg, N. M. DOE Report; US-DOE/CH/00016-2, 1984. (2) Golomb, D. S. Energy Convers. Manage. 1993, 34, 967. (3) Song, K. Y.; Kobayashi, R. SPE Form. Eval. 1987, 500. (4) Sakai, H.; Gamo, T.; Kim, E.-S.; Tsutsumi, M.; Tanaka, T.; Ishibashi, J.; Wakita, H.; Yamano M.; Omori, T. Science 1990, 248, 1093. (5) Sloan, E. Clathrate Hydrate of Natural Gases; Marcel Dekker: New York, 1990.
or artificial seawater simulating deep ocean conditions. In their experiments, however, the CO2 drops were either at rest or held by a net, and thus, the drops were basically in a spherical shape. The density of liquid CO2 varies with the ocean depth, and liquid CO2 is with neutral buoyancy only at a depth about 3000 m in the ocean; thus, in general, CO2 drops released in the ocean are in a buoyant (either positive or negative) motion. If the buoyant velocities are not very low, then the shapes of the drops are hardly spherical due to the hydrodynamic instability of the drops. This hydrodynamic instability is induced by the resistance to the buoyant motion of the drops and internal flows within the drops. Because the hydrodynamic effect due to the buoyant motion on the shape of drops and flow at surfaces of the drops were not included, the previous experiments may not simulate the actual physical and chemical behavior of the CO2 drops in the ocean. For the purpose of investigating the hydrodynamic effect on hydrate formation on surfaces of the CO2 drops in the ocean, we developed a counterflow water tunnel that could simulate the buoyant motion of the CO2 drops at depths down to 3000 m in the ocean. In this work, we will report our experimental findings. (6) Aya, I.; Yamane, K.; Yamada, N. HTD (ASME) 1992, 215, 17. (7) Austvik, T.; Løken, K. P. Energy Convers. Manage. 1992, 33, 659. (8) Austvik, T.; K. P. Løken Energy Convers. Manage. 1993, 34, 1081. (9) Saji, A.; Yoshida, H.; Sakai, H.; Tanii, T.; Kamata, T. Energy Convers. Manage. 1992, 33, 643. (10) Saji, A.; Noda, H.; Takamura, Y.; Tanii, T.; Tanaka, T.; Kitamura H.; Kamata, T. Energy Convers. Manage. 1995, 36, 493. (11) Shindo, Y.; Lund, P. C.; Fujioka, Y.; Komiyama, H. Energy Convers. Manage. 1993, 34, 1073.
10.1021/ef980225j CCC: $18.00 © 1999 American Chemical Society Published on Web 04/17/1999
Buoyant Liquid CO2 Drops
Figure 1. Schematic diagram of the experimental apparatus. (1) Test Section (Polycarbonate tube); (2) CO2 injection nozzle; (3) Flow stabilizer; (4) Pressure gauge; (5) Needle valve; (6) CO2 injection pump; (7) High-pressure circulation pump; (8) Thermal bath; (9) Heat exchanger; (10) CO2 cylinder; (11) Flow rate transducer and flow controller; (12) Pressure sensor; (13) Water injection pump; (14) Water tank; (15) Piston-type pump; (16) Pressure controller; (17) Electromagnetic valve; (18) Nitrogen cylinder; (19) Thermocouple and thermometer.
Experimental Section Figure 1 shows the counterflow water channel for simulating the buoyant motion of and hydrate formation on the CO2 drops in the ocean. The test section of the system (1) was a tapered, transparent polycarbonate tube 250 mm in length and 40 (at the top) and 60 mm (at the bottom) in inner diameters. The pressure of the system was controlled with an accuracy of 0.5 bar by a piston pump with a pressure control unit (1518). The temperature of the system was controlled by a heat
Energy & Fuels, Vol. 13, No. 3, 1999 625 exchanger (9) where the water in the system exchanged heat with the coolant from a thermal bath (8): via controlling the rate of heat exchange, the system temperature could be controlled at an accuracy of 0.5 K. Because pressures of the system were in the range between 50 and 300 bar, the CO2 drops tested were with positive buoyancy. For the purpose of making the buoyant CO2 drops suspended in the test section, the water in the system was circulated in a way such that the direction of water flow in the test section was opposite that of the buoyant motion of the drops. The water circulation was driven by a circulation pump (7) whose speed could be controlled with changing the frequency of the drive motor via a frequency controller. The rate of the water circulation was measured by a flow-rate transducer (11). To make the drops suspend stably in the test section, velocity distributions both upstream and downstream of the test section were regulated by a section of the flow channel filled with drinking straws of different lengths (3). Liquid CO2 entered the system through an orifice at the bottom of the test section. The system pressure was controlled at a value between 50 and 60 bar when liquid CO2 was introduced into the system. After the CO2 entered the system, the system pressure was increased rapidly to the set value. To make the CO2 temperature close to that of the system, the CO2 from a supply cylinder (10) also passed through the heat exchanger before it entered the system. The phenomena of hydrate formation on the CO2 drops in suspension in the test section were recorded by a CCD camera.
Experimental Observations and Discussion CO2 drops of various sizes were tested. The sizes of the CO2 drops tested were controlled via the orifice size, CO2 injection rate, water temperature, velocity of the
Figure 2. Formation of hydrate on small liquid CO2 drops. Water conditions: temperature 276 K, pressure 50 bar. Period of time after liquid CO2 was injected into the system: (a) 0.17 min, (b) 1.0 min, (c) 6.0 min, (d) 19.5 min.
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Figure 3. Hydrate formation on a liquid CO2 plug. Water conditions: temperature 276 K, pressure 50 bar. Period of time after liquid CO2 was injected into the system: (a) 6.0 min, (b) 13.0 min, (c) 19.82 min, (d) 19.87 min, (e) 19.92 min, (f) 20.1 min, (g) 20.3 min, (h) 26.0 min.
downwardly flowing water, and the CO2 concentration in water. Through proper control of the pressure, temperature, and water velocity, the drops released into
the test section were in a reasonably stable suspension. The system pressures covered a range from 50 to 100 bar, and the system temperatures were between 276
Buoyant Liquid CO2 Drops
Energy & Fuels, Vol. 13, No. 3, 1999 627
Figure 4. Hydrate shedding from a CO2 drop with linear dimension of the inner diameter of the test section. Water conditions: temperature 276 K, pressure 50 bar. Time sequence: (a) 0 min, (b) 0.83 min, (c) 8.0 min, (d) 10.0 min.
and 283 K. Under experimental pressures and temperatures, the shapes of the suspended CO2 drops were nonspherical when sizes of the drops became larger than 15 mm. Commonly, the nonspherical drops were hydrodynamically unstable and their shapes varied with time. Figure 2 presents the process of hydrate formation on the drops whose diameters were between 5 and 15 mm. These drops were basically in a spherical shape. A thin hydrate film formed rapidly on their surfaces after the drops entered water (Figure 2a) and the higher the CO2 concentration in water, the faster the process of hydrate formation. For these drops, no obvious hydrodynamic effect on hydrate formation was noticed. When interacting with each other, the drops did not combine to form a larger one but, instead, they agglomerated to form a grape-like cluster (Figure 2b-d). The speed of the agglomeration process was noticed to depend strongly on the chemical conditions on the drop surfaces. This suggests that surfaces of the drops be highly chemically reactive. The mechanism of the agglomeration may be due to hydrogen bonding, because the incomplete hydrogen bonds on hydrate surfaces are always arranged toward the water phase, and when the drops covered with hydrate are in contact, the hydrogen bonds tend to complete, which holds the drops together. The drop cluster formed was apparently stable both physically and chemically over a period of a few hours. This is probably due to the fact that the experimental system was small, and thus, the content of CO2 in the ambient water easily reached a relatively high level.
Otherwise, as reported in the literature,8 the hydrate formed would dissolve in water given enough time if the CO2 concentration in water were negligibly low. Figure 3 shows the phenomenon of hydrate formation on a very large CO2 drop from a liquid plug. Due largely to the hydrodynamic effect (note that velocities at surfaces of the buoyant drops and the velocity of the ambient water were on same order of magnitude), formation of hydrate on the CO2 drops released was delayed, which allowed the physical process to take place before the chemical reaction for hydrate formation occurs: the drops released coagulated to form a larger liquid body (Figure 3a). Via proper control of the injection rate, water temperature, and velocity of the downwardly flowing water, a liquid plug was produced (Figure 3b). The phenomenon of hydrate formation on the liquid plug was significantly different from that on a small drop. It took up to about 20 min for hydrate formation to start. Hydrate was observed to form first on the rear of the plug (Figure 3c) and then spread very rapidly to the entire surface of the plug, and simultaneously, hydrate was shed from the rear of the plug downstream continuously (Figure 3d-g). During this process, the size of the liquid plug reduced very rapidly and soon became a large drop (Figure 3h). Although a similar phenomenon was observed on the large drop, the speed of hydrate shedding decreased with a decrease in the drop size and hydrate shedding changed from a continuous process to an intermittent process whose period was inversely proportional to the drop size
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(Figure 4 a-d). When the drop size reached a certain critical value, hydrate shedding stopped completely. After the shedding stopped, the hydrate film on the drop surface looked apparently stable. The mechanism for the phenomenon shown in Figures 3 and 4 may be explained as follows. At a given pressure and temperature, a liquid CO2 plug (or a large CO2 drop) has a high buoyant velocity and thus a high counterflow velocity is required for the plug to suspend in the test section. This also results in a high velocity on the surface of the plug and, consequently, leads to a fast internal flow in the plug. Because the upstream water contacting with the front of the plug has a low CO2 content and because the high surface velocity induces a strong convective mass transfer of CO2 from the front to rear of the plug, in a thin boundary layer on the water side of the plug, the concentration of CO2 in the rear becomes much higher than that in the front. Due to this characteristic, hydrate forms easier in the rear of the plug. However, the internal flow in the rear of the plug is usually hydrodynamically unstable, and vortices may also form in the wake of the liquid plug. Since the plug cannot change its shape easily (which is similar to a Taylor bubble in a tube), instead of counteracting to the hydrodynamic instability via changing its shape as the drops do, the liquid plug counteracts the hydrodynamic instability with tail flapping. The hydrodynamic disturbance on the surface at the rear of the plug significantly retards the growth of the hydrate along the surface, although hydrate formation continues in the rear of the plug. Formation of the hydrate in the rear tends to stabilize the shape of the plug. When the hydrate film becomes thick enough, the surface instability can be considerably stabilized. This allows hydrate film to grow along the surface. Once growth of the hydrate film starts, it spreads very rapidly along the surface. Because of the hydrodynamic disturbance both
Teng and Yamasaki
in and out of the plug (i.e., internal flow and vortices in the wake flow) and the shear forces at the surface of the plug, hydrate was shed from the rear of the plug. The speed of the shedding depends on the mechanism of the shedding. For the shedding from a plug, the internal disturbance and surface shearing mechanisms dominate the process. For a large drop, the vortex mechanism is the main contributor. For a given drop size, the shedding has a particular frequency which is related to the vortex street in the wake flow. The above explanation agrees with our experimental observation. Summary Hydrate formation on surfaces of buoyant liquid CO2 drops in the ocean was simulated in a counterflow water tunnel. Through proper control of the pressure, temperature, and flow rate of the downwardly flowing water, CO2 drops released were stably suspended in the test section. The investigation covered a range of drop sizes from a few millimeters to the linear dimension of the test section. It was found that the drop hydrodynamics influenced the phenomena of hydrate formation significantly. Small drops were usually hydrodynamically stable and spherical in shape, and hydrate formed rapidly on such drops. While large drops are, in general, hydrodynamically unstable and, thus, their shapes vary with hydrodynamic disturbances, it took longer for hydrate to cover the surfaces of such drops. Hydrate shedding from surfaces of large drops was observed. When the linear dimension of the drops reached that of the test section (e.g., a liquid plug), hydrate formation on and shedding from the surfaces of the drops became a continuous process. An increase in the CO2 concentration accelerated the process of hydrate formation. EF980225J
