Energy Fuels 2010, 24, 432–438 Published on Web 11/10/2009
: DOI:10.1021/ef900863y
Direct Observation of the Effect of Sodium Dodecyl Sulfate (SDS) on the Gas Hydrate Formation Process in a Static Mixer Hideo Tajima,† Fumio Kiyono,‡ and Akihiro Yamasaki*,§ †
Graduate School of Science and Technology, Niigata University, 2-8050 Ikarashi, Niigata 950-2181, Japan, ‡National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba 305-8569, Japan, and §Department of Materials and Life Science, Seikei University, 3-3-1 Kichijoji-kitamachi, Musashino, Tokyo 180-8566, Japan Received August 8, 2009. Revised Manuscript Received October 16, 2009
Direct observation and the measurement of the formation rate of the gas hydrate of HCFC-22 in a Kenicstype static mixer were conducted in a laboratory-scale experimental apparatus. This study focused on the effect of sodium dodecyl sulfate (SDS) addition on the hydrate formation process under the SDS concentration below the critical micelle concentration (cmc). The hydrate film was formed at the surface of bubbles in any case, but the appearance of the hydrate film and its behavior were significantly changed with increasing the SDS concentration. Without SDS, smooth and homogeneous hydrate film was formed at the surface of HCHC-22 bubbles. The HCFC-22 gas was trapped in the shell of the rigid hydrate film, and the bubbles agglomerated each other to form a grape-like structure. From the addition of SDS, the hydrate film formed with a rougher and heterogeneous surface; the hydrate film seems to have a loose structure and easily collapsed from the buoyant motion of the HCFC-22 gas trapped in the shell. The enhancement of the gas hydrate formation by the SDS addition can be attributed to the mass transfer through the loose hydrate film and the surface renewal by the film collapse.
take place at the interface between the water and gas phases.12 The gas hydrate formed at the interface, in turn, would hinder the mass transfer of the gas into the aqueous phase. As a result, the formation of the gas hydrate may not be complete; only the thin film of the gas hydrate is formed at the interface. For the further formation of the gas hydrate to achieve higher conversions, continuous contact of the two phases of water and gas is necessary. In laboratory-scale experimental systems, vigorous agitation is usually applied to promote the hydrate formation rates, which may continuously break the hydrate films at the interface. The authors have developed a new type of continuous formation method of gas hydrates using a static mixer.13,14 The static mixer is a motionless mixing device with specially designed mixing elements inserted in a housing pipe. When the fluids pass through the static mixer, the mixing of the fluids takes place in the mixer depending upon the shape of the mixing elements. The static mixer can be used to produce various types of gas hydrates.13,14 Another method to promote the hydrate formation is the use of a promoter.15-29 A well-known promoter is a surfactant,
Introduction Gas hydrates are inclusion compounds with cage-like structures composed of hydrogen-bonded water molecules. Several applications have been proposed in environmental and energy fields using the inclusion abilities in the framework of gas hydrates. The applications include natural gas transportation in the form of natural gas hydrates,1 hydrogen storage,2,3 gas separation,4-7 ocean disposal of anthropogenic CO2 as a countermeasure for global warming,2,8 and desalination of seawater.9-11 These applications would require an efficient formation or production process of gas hydrates and the elucidation of the formation mechanism of gas hydrates. When gas hydrate is formed from the two-phase system of water and gas, the gas molecules should be taken up in the aqueous phase; the formation of gas hydrates is considered to *To whom correspondence should be addressed. Telephone: 81422373887. Fax: 81422373871. E-mail:
[email protected]. (1) Gudmundsson, J. S.; Borrehaug, A. Proceedings of the 2nd International Conference on Natural Gas Hydrates, Toulouse, France, 1996; pp 439-446. (2) Ohgaki, K.; Takano, K.; Sangawa, H.; Matsubara, T.; Nakano, S. J. Chem. Eng. Jpn. 1996, 29, 478–483. (3) Khokhar, A. A.; Gudmundsson, J. S.; Sloan, E. D. Fluid Phase Equilib. 1998, 150-151, 383–392. (4) Hnaptow, M. A.; Happel, J. U.S. Patent 5,434,330, 1995. (5) Yoon, J.-H.; Lee, H. AIChE J. 1997, 43, 1884–1893. (6) Kang, S.-P.; Lee, H. Environ. Sci. Technol. 2000, 34, 4397–4400. (7) Seo, Y.; Lee, H. Environ. Sci. Technol. 2001, 35, 3386–3390. (8) Golomb, D. S. Energy Convers. Manage. 1993, 34, 967–976. (9) Yamasaki, A.; Wakatsuki, M.; Teng, H.; Yanagisawa, Y.; Yamada, K. Energy 2000, 25, 85–96. (10) Knox, W. G.; Hess, M.; Jones, G. E.; Smith, H. B. Chem. Eng. Prog. 1961, 57, 66–71. (11) Kubota, H.; Shimizu, K.; Tanaka, Y.; Makita, T. J. Chem. Eng. Jpn. 1984, 17, 423–429. (12) Sloan, E. D. Clathrate Hydrates of Natural Gases, 2nd ed.; Marcel Dekker, Inc.: New York, 1998; pp 108-132. r 2009 American Chemical Society
(13) Tajima, H.; Yamasaki, A.; Kiyono, F. Energy Fuels 2004, 18, 1451–1456. (14) Tajima, H.; Yamasaki, A.; Kiyono, F. Energy Fuels 2005, 19, 2364–2370. (15) Zhong, Y.; Rogers, R. E. Chem. Eng. Sci. 2000, 55, 4175–4187. (16) Karaaslan, U.; Parlaktuna, M. Energy Fuels 2000, 14, 1103–1107. (17) Karaaslan, U.; Parlaktuna, M. Energy Fuels 2001, 15, 241–246. (18) Karaaslan, U.; Parlaktuna, M. Energy Fuels 2002, 16, 1413–1416. (19) Karaaslan, U.; Uluneye, E.; Parlaktuna, M. J. Pet. Sci. Eng. 2002, 35, 49–57. (20) Link, D. D.; Ladner, E. P.; Elsen, H. A.; Tayor, C. E. Fluid Phase Equilib. 2003, 211, 1–10. (21) Sun, Z.; Ma, R.; Wang, R.; Guo, K.; Fan, S. Energy Fuels 2003, 17, 1180–1185. (22) Sun, Z.; Wang, R.; Ma, R.; Guo, K.; Fan, S. Energy Convers. Manage. 2003, 44, 2733–2742.
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: DOI:10.1021/ef900863y
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sodium dodecyl sulfate (SDS), of which the acceleration effect of the hydrate formation was first reported by Zhong and Rogers.15 They attributed the acceleration to the formation of micelles, although their concentration range of SDS was far below the critical micelle concentration (cmc). There is some debate on the effect or even the existence of the micelle in the hydrate-water system.25-29 When the surfactant concentration reaches the cmc, anionic surfactants play an important role in the fast nucleation and growth of the gas hydrate.24 The overall hydrate formation rate is increased with the use of the anionic surfactant in comparison to non-ionic and cationic surfactants.16,21-23 The formation rates of the structure I hydrate are enhanced more than those of the structure II hydrate by anionic surfactants.19 The detailed function and mechanism of surfactants on the gas hydrate formation have not yet being clarified. In most of the previous studies, the effect of the surfactants on the hydrate formation was investigated in an agitation vessel or even in static vessels without agitation. Little information is available on the effect of surfactant addition on the hydrate formation process in the continuous hydrate formation process, such as in a static mixer. In this study, the effect of the surfactants on the hydrate formation was investigated in a static mixer. The use of a glass tube as a housing pipe of the static mixer enabled the direct observation of the hydrate formation. Because the static mixer is a flow-type reactor, the formation of gas hydrates in the mixer can be easily followed visually. In addition, the investigation of the hydrate formation in the static mixer is of practical importance for the implementation of continuous formation processes of gas hydrates by the static mixer. Chlorodifluoromethane (HCFC-22), a ozone layer depleting compound and greenhouse gas was used as a model gas of the hydrate formation. Hydrate formation of HCFC-22 can be applied to recovery and separation processes of HCFC-22 from stocks, such as old refrigerators and air conditioners. HCFC-22 forms the structure I hydrate, and the hydrate formation condition is rather mild, e.g., 0.329 MPa at 283.1 K, compared to the liquedfaction.30
Figure 1. Schematic drawing of the experimental system for the hydrate formation: 1, gas cylinder; 2, water tank; 3, water supply pump; 4, mass flow controller; 5, static mixer unit; 6, digital video camera; 7, cooling unit; 8, pressure regulating valve; 9, mass flow meter; 10, pressure gauge; 11, thermometer.
Experimental Section Gas hydrate formation was carried out with deionized water and HCFC-22 (>99.9%, Takachiho Chemical Industrial Co., Ltd.). SDS (>95%, Wako Pure Chemical Industries, Ltd.) was used as purchased. SDS aqueous solutions (∼1000 ppm) were prepared by dissolving SDS in deionized water. The concentration of SDS was changed in the range of 0-1000 ppm, which is far below the cmc of SDS.31-33
Figure 2. Appearance of the Kenics-type static mixer.
Figure 1 shows a schematic drawing of the experimental system for hydrate formation. The static mixer unit as a hydrate formation reactor was set to allow for a vertical upward flow of gas. The housing tube of the mixer is made of a Pyrex glass tube, with an inner diameter of 11.0 mm and 455 mm in length. The mixer is a Kenics type with 24 elements, as shown in Figure 2, provided by Noritake Co., Ltd. The cross-sectional area of the mixer is divided into two approximately semi-circular passages by means of a number of helical thin partitions. Each of the mixing elements is twisted through 180° and is right- or left-hand rotation. Elements of alternate rotation are joined sequentially, so that perpendicularity is maintained between the rearward and forward edges of adjacent elements. First the mixer was filled with SDS solution dissolved in deionized water (52.8 mL). Then, HCFC-22 gas is injected at the bottom of the static mixer with a constant flow rate at 198 mL/ min from a nozzle (2 mm orifice diameter). This flow rate was selected because the effect of SDS is most noticeable among the flow conditions studied. Under a lower flow rate, the mixer was plugged with bubbles with hydrate, while under a higher flow rate, the effect of SDS is not clearly observed. The flow rate was controlled with a mass flow controller. The upward flow of the
(23) Sun, Z.; Wang, R.; Ma, R.; Guo, K.; Fan, S. Int. J. Energy Res. 2003, 27, 747–756. (24) Li, J.; Liang, D.; Guo, K.; Wang, R. J. Colloid Interface Sci. 2005, 283, 223–230. (25) Gnanendran, N.; Amin, R. J. Pet. Sci. Eng. 2003, 40, 37–46. (26) Gnanendran, N.; Amin, R. Fluid Phase Equilib. 2004, 221, 175–187. (27) Watanabe, K.; Imai, S.; Mori, Y. H. Chem. Eng. Sci. 2005, 60, 4846–4857. (28) Watanabe, K.; Niwa, S.; Mori, Y. H. J. Chem. Eng. Data 2005, 50, 1672–1676. (29) Di Profio, P.; Arca, S.; Germani, R.; Savelli, G. Chem. Eng. Sci. 2005, 60, 4141–4145. (30) Chun, M.-K.; Yoon, J.-H.; Lee, H. J. Chem. Eng. Data 1996, 41, 1114–1116. (31) Flockhart, B. D. J. Colloid Sci. 1961, 16, 484–492. (32) Stephen, H.; Stephen, T. Solubilities of Inorganic and Organic Compounds; Pergamon Press: Oxford, U.K., 1963; p 145. (33) Kaneshina, S.; Tanaka, M.; Tomida, T.; Matuura, R. J. Colloid Interface Sci. 1974, 48, 450–460.
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Figure 3. Time variation of the hydrate formation process of HCFC-22 with deionized water in the static mixer (without SDS). Pressure, 0.4 MPa; temperature, 283 K.
Figure 4. Time variation of the hydrate formation process of HCFC-22 with SDS solution (400 ppm) in the static mixer. Pressure, 0.4 MPa; temperature, 283 K.
HFC-22 were found to be covered with a thin hydrate film. The bubbles covered with the hydrate film gradually accumulated and agglomerated in the mixer with time. Finally, after 80 s, the bubbles were completely covered with the hydrate film and the static mixer was almost filled with the agglomerated bubbles covered and connected with the hydrate film. The boundary of the bubbles, however, can be clearly distinguished by the hydrate film. In addition, once the agglomerated bubbles are formed, no noticeable changes were observed for a long period. These results indicate that the hydrate formed without SDS has a relatively rigid structure. Figure 4 shows the time variation of the hydrate formation in the static mixer for the conditions with the SDS concentration at 400 ppm, the pressure at 0.40 MPa, and the temperature at 283 K. In 10 s after the flow started, the bubbles were covered with hydrate films and the formation of small particles of solid hydrate was observed. These small particles seem to be peeled off from the film of the hydrate formed on the bubble surface. The inflowing bubbles was quickly covered with hydrate film and agglomerated. The mixer was filled with the bubbles after 60 s. The appearance of the hydrate film surface in this case is significantly different from that of the hydrate film formed without SDS (Figure 3). The hydrate film formed without SDS is smooth and homogeneous, but the hydrate film formed with SDS is rough and heterogeneous; the hydrate film formed with SDS seems to contain some small particles in it.
bubbles was observed, which were mixed with the SDS solution in the static mixer to form hydrates. The present system can be regarded as a semi-batch system (flow system for gas and batch system for water). The system temperature was controlled within a range of (0.1 °C by the water/ethanol circulation jacket attached to the mixer. The system pressure was controlled by a pressure-regulating valve equipped at the downstream side of the static mixer unit, with which the pressure could be controlled within the variation range of (0.01 MPa. The gas flow rates were measured by a mass flow meter, after the gas had passed through the static mixer unit. The gas uptake in the reactor is determined by the difference between the inlet and outlet gas flow rates. Hydrate formation was confirmed by both visual observations and variations in the outlet gas flow rates. Pressure and temperature were recorded with a data-logging system. The size of the HCFC-22 bubbles was measured by a graphical analysis of the captured images of the video-recorded mixing process (Image J-1.33u, developed by the U.S. National Institute of Health with a PC). The hydrate formation process was observed and recorded on a digital video camera recorder through the glass housing pipe. The data were analyzed on a PC.
Results and Discussion Effect of the SDS Concentration on the Morphology of the HCFC-22 Hydrate Film on the Bubble Surface. Figure 3 shows the time variation of the mixing and hydrate formation behaviors in the static mixer without SDS. The pressure and temperature are 0.40 MPa and 283 K, respectively. Within 10 s after the gas flow was started, the bubbles of 434
Energy Fuels 2010, 24, 432–438
: DOI:10.1021/ef900863y
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Figure 5. Snapshots of the HCFC-22 bubbles with hydrate under various SDS concentrations at 80 s after the gas injection started. Pressure, 0.4 MPa; temperature, 283 K.
Figure 6. Cumulative gas uptake in the static mixer for the conditions of 0-200 ppm SDS. Pressure, 0.4 MPa; temperature, 283 K.
Figure 7. Cumulative gas uptake in the static mixer for the conditions of 250-1000 ppm SDS. Pressure, 0.4 MPa; temperature, 283 K.
With time, the hydrate film was broken because of the buoyant motion of the bubbles in the hydrate shell. The results indicate that the hydrate film formed with SDS is less rigid compared to the one formed without SDS. The escaped
bubbles were quickly recovered with the hydrate film. The bubbles experienced the hydrate film formation and its collapse and reformation cycles, and finally, the mixer was filled by the clear hydrate crystals. These results indicate that 435
Energy Fuels 2010, 24, 432–438
: DOI:10.1021/ef900863y
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HCFC-22 hydrate was formed inside the bubbles. Finally, the mixer was filled with the hydrate with an unclear boundary of the bubbles. The above difference in the appearance of the hydrate film depends upon the SDS concentration. Figure 5 shows snapshots of the HCFC-22 bubbles covered with hydrate film, of which all are recorded at 60 s after the gas flow was started. For the cases with the SDS concentration up to 100 ppm,
the hydrate films are smooth and homogeneous. When the SDS concentration was increased over 100 ppm, the surface of the hydrate film changed to be rougher and heterogeneous, which seems to contain small particles in the films. The morphological change of the hydrate film is more noticeable for the conditions with the SDS concentration higher than 250 ppm. These direct observation results suggest that SDS molecules produce a significant change in the characteristic of the hydrate film formed on the bubble surface for the concentration of SDS greater than 250 ppm. A significant change in the hydrate character by adding SDS has been reported in previous research for natural gas hydrate formation in a quiescent system,15 where the hydrate formation was developed extremely fast in more than 242 ppm SDS solution. However, SDS molecules cannot make a micellar conformation themselves because the cmc of SDS is about 2500 ppm at 283 K,31 which is far above the present study. Thus, no micelle formation would not take place under these conditions. Effect of the SDS Concentration on the Apparent Rate of the HCFC-22 Hydrate Formation. Time variation of the gas uptake in the static mixer during the hydrate formation is shown in Figure 6 (SDS concentrations are smaller than 200 ppm) and Figure 7 (SDS concentrations are 250 ppm and greater). The gas uptake rate, which is equivalent to the hydrate formation rate, can be expressed by the slope of the time variation curve. The hydrate formation rates are compared on the basis of the slope of gas uptake curve under the steady-state conditions, where the slope is constant after the induction period. When the SDS concentration was greater than 250 ppm, the gas uptake rate was significantly enhanced by the addition of SDS compared to the case without SDS. The gas uptake rate on the SDS concentration was almost constant for the concentration range of SDS higher than 250 ppm. To clarify the effect of SDS on the hydrate formation rate, the steady-state gas uptake rate can be considered. For each case of the SDS concentration, a constant gas uptake rate was observed. In Figure 8, the gas uptake rate (mol s-1) under the steady-state conditions was plotted against the SDS concentration. The steady-state gas uptake rate increased with an increase in the SDS concentration up to 300 ppm, and after that, the rate was almost independent of the SDS concentration. The increase of the hydrate formation rate may be attributed to the interfacial area of the bubble.12 Sometimes the addition of SDS would decrease the bubble in water. According to the literature data,34 the surface tension was approximately in a linear relation to the logarithmic SDS concentration in water below cmc, especially between 200 and 2300 ppm SDS. Figure 9 shows the drop size (Sauter mean diameter, D32) of the HCFC-22 bubble observed for
Figure 8. Effect of the SDS concentration on the hydrate formation rates (maximum). Pressure, 0.4 MPa; temperature, 283 K.
Figure 9. Sauter mean diameter of HCFC-22 bubbles in the static mixer before hydrate formation. Pressure, 0.4 MPa; temperature, 283 K.
Figure 10. Snapshots of the HCFC-22 bubbles covered with the hydrate film under various conditions. SDS concentration = 250 ppm.
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the cases with various SDS concentrations. The Sauter mean diameter is defined as P ni di 3 ð1Þ D32 ¼ Pi 2 i ni di
smooth and homogeneous with a decreasing temperature (Figure 10a; 281 K and 0.4 MPa) or an increasing pressure (Figure 10b; 283 K and 0.6 MPa). These variations correspond to a shift of the conditions to be more favorable for hydrate formation thermodynamically. On the other hand, the apparent rate of hydrate formation increased with a decrease in the temperature or an increase in the pressure. Mechanism of Hydrate Formation with and without SDS in the Static Mixer. On the basis of the above results, the mechanism for hydrate formation with SDS in the static mixer can be speculated as follows. The mechanism is shown schematically in Figure 12. When the inflow bubbles of HCFC-22 are in contact with the water phase in the static mixer, the nucleation of hydrate will occur at the interface of the bubbles and the water phase. This nucleation will occur more frequently at the downstream side of the bubbles because the separation of vortex occurs at the downstream side. The growth of the hydrate will follow the nucleation, and the surface of the bubbles will be covered with hydrate. Once the hydrate is formed, direct contact of the water and gas will be prevented by the hydrate itself. Consequently, the hydrate growth will be retarded: only a thin film of the hydrate will be formed, which covers the bubble surface. Without SDS, the nucleation and growth of the hydrate film will take place from pure gas and water, the hydrate film will be composed of “pure” hydrate crystal, and some defects of the hydrate film will be recovered by further hydrate growth. This is the reason for the smooth and homogeneous appearance of the hydrate film formed without SDS (Figure 12a). The bubble covered with the smooth hydrate film will be agglomerated and blockaded in the mixer. When SDS is added in the system, SDS will be more concentrated at the interface of the bubbles and water because of its amphiphilic property. Under such a situation, the nucleation will occur by the direct contact of HCFC-22 and water at the bubble surface. The nucleation will be somewhat enhanced because the surfactant SDS would reduce the surface tension between the bubbles and water. However, because of SDS molecules at the surface, the growth of the hydrate at the surface will be partially retarded. In this case, it is likely for small particles of the hydrate to be formed in addition to the hydrate film. Thus, the hydrate film formed with SDS addition appeared to have a rough and
where ni is the number distribution of the bubble with the diameter di. From the addition of SDS, D32 decreased about 30% but it is almost constant by increasing the SDS concentration. Therefore, the enhancement effect of the addition of SDS may not be explained by the increase in the interfacial area only. It can be considered that the enhancement effect should be attributed to the morphology change of the hydrate film formed on the bubbles. Before the mechanism of the enhancement is discussed, the effect of the temperature and pressure on the effect of the SDS addition will be reported in the next section. Effects of Thermodynamic Conditions on the Appearance and Apparent Formation Rate of Hydrate. Effects of the temperature and pressure on the hydrate formation are shown in Figure 10 (appearance of the hydrate film) and Figure 11 (gas uptake), with the SDS concentration at 250 ppm. As shown in Figure 10, the rough and heterogeneous hydrate film at 283 K and 0.4 MPa became more
Figure 11. Cumulative gas uptake for various conditions. SDS concentration = 250 ppm.
Figure 12. Speculated mechanism of the hydrate formation on the bubble surface (a) without SDS and (b) with SDS.
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heterogeneous surface. This hydrate film will allow for the direct contact of the gas and water phases. As a result, the hydrate growth point may not be limited at the surface of the bubbles but the hydrate will grow toward the inside of the bubbles. The hydrate film is not rigid because of the SDS adsorbed at the surface and is easily broken, which resulted in the enhancement of the stripping of hydrate film and the escaping of unconverted gas. The addition of SDS will change the morphology and properties (packing density and mechanical strength) of the hydrate film formed at the bubble surface. Under a lower temperature and higher pressure, the appearance of the hydrate formed on the bubble surface is smooth and homogeneous irrespective of the SDS addition or not. We consider that this is because the formation of
hydrate is too quick under such conditions, so that the effect of SDS cannot be clearly observed. Conclusion Direct observation of the hydrate formation process in a Kenics-type static mixer demonstrated that the addition of SDS in the aqueous phase changes the morphology of the hydrate film formed at the surface of bubbles. With SDS, the surface of the hydrate film became rougher and heterogeneous compared to the hydrate film with a homogeneous and smooth surface formed without SDS. The above significant change of the surface appearance of the hydrate film occurred at the SDS concentration higher than 250 ppm, which corresponds to the SDS concentration that shows the saturation point of the increasing formation rate of hydrate against the SDS concentration. The morphological change could be explained in the SDS molecules adsorbed at the interface of bubbles and water.
(34) Matsuura, R.; Kimizuka, H.; Miyamoto, S.; Shimozawa, R. Bull. Chem. Soc. Jpn. 1958, 31, 532–538.
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