Energy & Fuels 2004, 18, 1451-1456
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Continuous Formation of CO2 Hydrate via a Kenics-Type Static Mixer Hideo Tajima,* Akihiro Yamasaki, and Fumio Kiyono National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan Received November 18, 2003. Revised Manuscript Received May 6, 2004
Formation of CO2 hydrate using a Kenics-type static mixer was studied experimentally. The flows of liquid CO2 and water were mixed in the static mixer, and CO2 hydrate was formed continuously from the two-phase flow. The patterns of hydrate formation were found to be dependent on the flow velocities of liquid CO2 and water. The flow of agglomerated hydrate chunks in water occurred under relatively CO2-rich conditions, while dispersed flow of tiny particles of CO2 hydrate with small liquid CO2 drops was observed under relatively water-rich conditions. These effects could be explained by two mechanisms occurring in the static mixer, namely, continuous shedding of hydrate films from the interface between liquid CO2 and water induced by the shearing force and breakup of the CO2 drops. The energy consumption by the static mixer for the hydrate formation process was estimated, and it was significantly less than that for a stirring vessel type reactor. A continuous hydrate formation process could be achieved using the static mixer.
Introduction Clathrate hydrates are inclusion compounds with cagelike structures composed of hydrogen-bonded water molecules. A variety of guest molecules can be included in the cages. In general, lower temperature and higher pressure conditions are necessary for stabilizing hydrates, and thermodynamic conditions for hydrate formation depend on the properties of the guest molecules such as size, shape, and interaction with water molecules. Intensive studies have been conducted on the crystalline structures and thermodynamic equilibria of clathrate hydrates, from both experimental and theoretical points of view. Clathrate hydrates have received much attention in environmental and energy application fields in recent years. For example, properties of CO2 hydrate have been studied with respect to the ocean disposal scenario of anthropogenic CO2 as a mitigation measure for global warming.1-4 These scenarios have attracted attention because the ocean is a huge storehouse of CO2. However, because CO2 hydrate is thermodynamically stable under deep ocean conditions (deeper than 450 m), the effect of CO2 hydrate should be considered in regards to the design of the injection of liquid CO2 and the prediction of the fate of the CO2 disposed in the seawater. A new ocean disposal scenario using CO2 hydrate has been proposed by Yamasaki et al.5 In their scenario, a hydrate crystallizer is submerged at intermediate ocean * E-mail:
[email protected]. Fax: +81-29-861-8727. (1) Austvik, T.; Løken, K. P. Energy Convers. Manage. 1992, 33, 659-666. (2) Austvik, T.; Løken, K. P. Energy Convers. Manage. 1993, 34, 1081-1087. (3) Saji, A.; Toshihara, H.; Sakai, H.; Tani, T.; Kamata, T. Energy Convers. Manage. 1992, 33, 643-649. (4) Saji, A.; Noda, H.; Takamura, Y.; Tani, T.; Tanaka, T.; Kitamura, H.; Kamata, T. Energy Convers. Manage. 1996, 36, 493-496.
depths. Liquid CO2 is completely converted into hydrate particles via a hydrate crystallizer. The temperature and pressure of seawater at this depth supply the conditions required for hydrate formation, and therefore no energy is consumed in pressurizing and cooling seawater. The hydrate particles released from the crystallizer fall to the seabed because the density of CO2 hydrate is larger than that of seawater. There are several other scenarios for applications of hydrates in environmental and energy fields. Natural gas storage and transportation in the form of gas hydrates, which was originally proposed by Gudmundsson and Børrehaug,6 has stimulated intensive research in this field. In relation to the research on natural gas storage and transportation, research projects on the exploration of methane hydrate in marine sediments as a potential huge-scale energy source have been started worldwide.7,8 Separation processes of gaseous mixtures using differences in hydrate formation tendencies among the gaseous components have also been proposed,9-11 mainly for environmental applications. To implement the above-mentioned applications, it is essential to develop large-scale and continuous formation processes for gas hydrates. In the laboratory-scale experimental studies, hydrate formation has been con(5) Yamasaki, A.; Wakatsuki, M.; Teng, H.; Yanagisawa, Y.; Yamada, K. Energy 2000, 25, 85-96. (6) Gudmundsson, J. S.; Børrehaug, A. Frozen Hydrate for Transport of Natural Gas. Proceedings of the 2nd International Conference on Natural Gas Hydrates, Toulouse, France, June 2-6, 1996; pp 439446. (7) Ohgaki, K.; Takano, K.; Moritoki, M. Kagaku Kogaku Ronbunshu 1994, 20, 121-123. (8) Khokhar, A. A.; Gudmundsson, J. S.; Sloan, E. D. Fluid Phase Equilib. 1998, 150-151, 383-392. (9) Yoon, J.-H.; Lee, H. AIChE J. 1997, 43, 1884-1893. (10) Kang, S.-P.; Lee, H. Environ. Sci. Technol. 2000, 34, 43974400. (11) Seo, Y.; Lee, H. Environ. Sci. Technol. 2001, 35, 3386-3390.
10.1021/ef034087w CCC: $27.50 © 2004 American Chemical Society Published on Web 07/14/2004
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ducted mainly with stirred-tank-type reactors because turbulent conditions enhance the hydrate formation rates through increasing mass-transfer rates. However, application of stirred-tank-type reactors to large-scale hydrate formation presents several problems. The stirring process for the hydrate formation consumes a large amount of energy, especially in larger scale reactors, because the stirring power consumption is proportional to the fifth power of the diameter of the agitation paddle and the third power of the agitation speed. In addition, it is sometimes difficult to build a continuous hydrate formation process with stirring-tank-type reactors because hydrates formed in the reactor should be separated and removed from the slurry mixtures. Ambiguities in the scale-up of stirred-tank-type reactors would also be disadvantageous for practical applications. As one solution to the above challenges, we propose a new hydrate formation process via static mixers in this paper. Static mixers are motionless mixing devices composed of mixing elements equipped in a straight, empty tube. Fluids are introduced into the static mixer and mix through flowing in the mixer by three kinds of mixing mechanisms, namely, flow reversal, division of flow, and radial mixing.12,13 When applied to hydrate formation, these mixing mechanisms could enhance the mixing of the water and the guest gases, and consequently an effective hydrate formation process could be realized. The contribution of each mechanism will depend on the properties of the target fluids such as viscosity and mutual miscibility. The advantages of utilizing static mixers for the hydrate formation process are as follows. (i) An energy-saving process can be realized because static mixers will consume only the pumping power required for the fluid for the mixing and the pressure drop of the Kenics-type static mixer is relatively low. (ii) A safer process can be achieved because explosion of flammable gases can be avoided, which is crucial for hydrate formation from natural gases, because no mobile parts are used for the mixing operation by static mixers. (iii) Because of the tube-type geometry of the reactor, it would be easier to design a continuous process for hydrate formation. More uniform retention time of the reactants in the mixer would result in more uniform properties of the product hydrates, which is essential for controlling the environmental impact of the ocean disposal process, conversion in the natural gas storage and transportation process, and improvement of the performance in the separation process. In addition, scale-up of the reactor to a large-scale hydrate formation process would be easier because of the simple form of the static mixers. (iv) Despite the above advantages, no study on the hydrate formation process has been conducted with static mixers, and consequently no information is available in the literature on the hydrate formation process using static mixers. (12) Harnby, N.; Edwards, M. F.; Nienow, A. W. Mixing in the Process Industries, 2nd ed.; Butterworth-Heinemann: Oxford, U.K., 1992; pp 225-249. (13) Noritake Co. Ltd. The Basic Technology of Static Mixers (in Japanese); Technical Report No. 20; Noritake Co. Ltd.: Nagoya, Japan, 2000.
Tajima et al.
Figure 1. Mixing elements of the Kenics-type static mixer.
Figure 2. Schematic drawing of the experimental system for hydrate formation. (a) Observations at the premixing unit. Water flow rates: 472, 962, 1573, 2063, and 2981 mL/min. (b) Observations at the observation unit. Water flow rates: 472, 962, 1573, 2063, and 2981 mL/min.
In this paper, therefore, the formation processes of a hydrate in a static mixer were investigated experimentally with a laboratory-scale experimental apparatus, focusing on the effect of the flow velocities of the feed gas and water on the hydrate formation patterns in the static mixer. A Kenics-type static mixer, which is the most widely used,14 was used in the present study. The mixing elements of this mixer are shown in Figure 1. CO2 was used as the guest molecule for hydrates with consideration to the application of the present hydrate formation process to an ocean disposal scenario in the form of CO2 hydrates. Experimental Section Figure 2 shows a schematic flow diagram of the experimental apparatus for the hydrate formation. The premixing unit and the observation unit are placed at the front and back, respectively, of the static mixer. Two streams of prepressurized and cooled flows of liquid CO2 and water are merged in the premixing part, and the two-phase flow is introduced to the static mixer part. As shown in Figure 2, the premixing part is a pressure vessel of stainless steel with a cruciform inner structure; both the longitudinal and transverse flow channels have the same diameter of 10.9 mm and the same length of 120 mm. Two observation windows (25 mm in diameter) made of Pyrex glass plates were attached on the cross of the flow channels. In the premixing part, two flows of water and liquid CO2 were vertically mixed. The static mixer is the type 3/8(14) Pahl, M. H.; Muschelknautz, E. Int. Chem. Eng. 1982, 22, 197205.
Continuous Formation of CO2 Hydrate
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Figure 3. Formation of a liquid CO2 drop covered with a thin hydrate film in the absence of the static mixer. The liquid CO2 flow rate is constant at 46.6 mL/min. (a) observations at the pre-mixing unit for water flow rates of 472, 962, 1573, 2063, 2981 mL/ min. (b) observations at the observation unit for water flow rates of 472, 962, 1573, 2063, 2981 mL/min. N10-522N manufactured by Noritake Co. Ltd., composed of 12 identical mixing elements of which the effective diameter is 3/8 in., the ratio of length to diameter is 1.5, and the thickness is 1.1 mm. The mixing elements are interconnected in series with adjacent elements twisted at 90° each other. This element is called a Kenics-type mixer. The housing pipe was an empty tube (SUS 316) of inner diameter 10.9 mm and length 210 mm. The housing of the mixer was specially designed for highpressure operations with a maximum bearing pressure of 10 MPa. Two static mixers were used in series in all of the experiments except for the comparative experiment with the empty pipe. An observation unit made of a Pyrex glass tube with 10.9 mm inner diameter and 300 mm length was connected to the static mixer at the downstream side. The hydrate formation process was observed directly and recorded with a high-speed digital video camera (VFC-1000 manufactured by FOR.A Co. Ltd.) through the observation unit as well as in the premixing part. Liquid CO2 flow was supplied from a cylinder of high-purity CO2 (>99.9%) by a high-pressure pump (Nippon Seimitsu Co., Ltd., NP-AX-70). The injection flow rate of the high-pressure CO2 could be adjusted over a range from 11.2 to 93.4 mL/min. Water flow was supplied from a tank of deionized water by a high-pressure pump (Fuji Pump Co., Ltd., 2JN224-10V). The flow rate of water could be adjusted over a range from 472 to 2981 mL/min. The experimental pressure was controlled at 7.0 MPa with an accuracy of (0.01 MPa by a pressureregulating valve fitted at the downstream side of the flow system. The temperature of the system was controlled at 277 K with an accuracy of (0.1 K by a cooling jacket over the flow lines where a mixture of water and ethylene glycol at a constant temperature is circulated. The typical isobaric (7 MPa) and isothermal (277 K) conditions used correspond to those for seawater at a depth of about 700 m below the ocean surface.
Results and Discussion CO2 Hydrate Formation without the Static Mixer. Figure 3 shows typical observation results obtained at the premixing unit and the observation unit without the static mixer. In the premixing unit, the liquid CO2 phase is broken up into liquid CO2 drops over the range of experimental conditions used because of the instability of the two-phase flow. Each drop was covered with a thin hydrate film as shown in Figure 3. Without the static mixer, i.e., an empty pipe of the same length as the static mixer is placed in the system instead of the static mixer, the flow patterns at the observation unit (Figure 3b) are very similar to those at the premixing unit (Figure 3a). This result indicates that the empty pipe hardly influences the pattern of the flow over the range of experimental conditions tested. Hy-
Figure 4. Typical patterns of liquid CO2-water flow after passage through the static mixer. Pattern 1: Flow of the agglomerated liquid CO2 drops in the continuous water phase flow. Liquid CO2 flow rate ) 23.2 mL/min. Water flow rate ) 472 mL/min. Pattern 2: Flow of the dispersed liquid CO2 drops in the water flow as a continuous phase. Liquid CO2 flow rate ) 46.6 mL/min. Water flow rate ) 2063 mL/min. Pattern 3: Flow of the mixture of liquid CO2 drops and small hydrate particles dispersed in the continuous water phase flow. Liquid CO2 flow rate ) 93.4 mL/min. Water flow rate ) 2981 mL/ min. Pattern 4: Flow of the chunks of agglomerated hydrate particles in the continuous water phase flow. Liquid CO2 flow rate ) 93.4 mL/min. Water flow rate ) 472 mL/min.
drate formation is limited to the interface between liquid CO2 and water, and thus no hydrate particles were observed. Experimental Observations of CO2 Hydrate Formation with the Static Mixer. Although the flow patterns at the premixing unit with the static mixer were similar to those without the static mixer, a difference appeared in the flow patterns at the observation unit. Various types of hydrate formation processes were observed with the static mixer, depending on the flow velocities of water and liquid CO2. These are roughly divided into the following four patterns, as shown in Figure 4: flows of hydrate chunks, hydrate dispersion, drop agglomeration, and drop dispersion. The liquid CO2 drops were covered with hydrate film in all patterns. Note that all of the experimental conditions studied are within the region where the CO2 hydrate phase is thermodynamically stable.15 The occurrences of the above patterns depended on the flow velocities of liquid CO2 and water. The flow velocity is (15) Sloan, E. D. Clathrate Hydrates of Natural Gases, 2nd ed.; Dekker: New York, 1998.
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Figure 5. Phase diagram of the CO2-water mixed flow. Hydrate line: stoichiometric ratio of CO2 and water in CO2 hydrate, CO2:water ) 1:5.75. Solubility line: solubility of liquid CO2 in water at 7 MPa and 277 K. Symbols: open squares, agglomeration of liquid CO2 drops covered with hydrate film; closed squares, chunks of CO2 hydrate particles; open triangles, dispersion of liquid CO2 drops covered with hydrate film; closed triangles, dispersion of CO2 hydrate particles; open circles, partway between agglomeration and dispersion; closed circles, partway between chunks and dispersion.
defined as the volume flow rate divided by the crosssectional area of the injection pipe of 10.9 mm inner diameter. The dependence of the pattern formations on the flow velocities of liquid CO2 and water is shown graphically in Figure 5. The hydrate line (solid line) in Figure 5 represents the flow ratio of CO2 to water that is equal to the stoichiometric ratio of CO2 and water in the CO2 hydrate (CO2:water ) 1:5.75). The solubility line (dotted line) in Figure 5 represents the flow ratio of liquid CO2 to water that corresponds to the solubility of CO2 in water under the experimental conditions, where “solubility” is defined as the concentration of CO2 dissolved in liquid water in equilibrium with CO2 hydrate. The solubility of CO2 for the present experimental condition was estimated based on the empirical equation in the literature.16 The space can be divided into three regions by these straight lines, namely, the CO2-rich region, the water-rich region, and the transition region. The four patterns of hydrate formation will be discussed in relation to the above regions. Such a variation of the flow pattern may depend on the pressure-temperature conditions. All of the experiments were carried out under isobaric and isothermal conditions, so this paper focuses on the effects of the variation of liquid CO2 and water flow velocities on the fluid patterns. For the CO2-rich region, patterns 1 (agglomerated liquid CO2 drops in water flow) and 4 (CO2 hydrate chunks in water flow) were observed, depending on the CO2 flow velocity. Pattern 1 was mainly observed at relatively lower CO2 flow velocity conditions (
