Forming a Structure-H Hydrate Using Water and Methylcyclohexane

Feb 12, 2009 - Methylcyclohexane Jets Impinging on Each Other in a Methane ... Using methane and methylcyclohexane as the hydrate-forming gas and the ...
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Energy & Fuels 2009, 23, 1619–1625

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Forming a Structure-H Hydrate Using Water and Methylcyclohexane Jets Impinging on Each Other in a Methane Atmosphere Tetsuya Murakami,† Hisashi Kuritsuka, Hideyuki Fujii, and Yasuhiko H. Mori* Department of Mechanical Engineering, Keio UniVersity, Yokohama 223-8522, Japan ReceiVed October 12, 2008. ReVised Manuscript ReceiVed January 12, 2009

This paper reports the experimental examination of a novel device for the continuous, high-rate formation of a structure-H hydrate. The device is designed such that two precooled liquids, water and a hydrophobic large-molecule guest substance (LMGS), such as methylcyclohexane, are injected in the form of co-planar cylindrical jets into a hydrate-forming gas phase confined in a high-pressure chamber, where LMGS provides guest molecules to fit into the 51268 cages of a structure-H hydrate. The liquid jets collide, thereby forming a radially expanding sheet, which, in turn, sprinkles from its rim tiny water/LMGS compound droplets into the gas phase. Using methane and methylcyclohexane as the hydrate-forming gas and the LMGS, respectively, we have performed a series of hydrate-forming experiments to examine the operational function of the twinjet device. The observed hydrate-formation behavior, including the rate of hydrate formation depending upon the degree of the LMGS cooling, is presented and discussed.

Introduction This paper first reports the experimental examination of a novel hydrate-forming operation that we recently devised with the intention of providing a new attractive option for designing industrial-scale operations of forming clathrate hydrates from, for example, natural gas for its storage and transport or biogases for separating undesirable (toxic or incombustible) species, such as hydrogen sulfide and carbon dioxide, thereby making them sufficiently methane-rich. The operation is characterized by use of twin liquid jets impinging on each other in a hydrate-forming gas confined in a high-pressure chamber. One of the jets is liquid water, while the other is a hydrophobic liquid having a substantially low freezing point. If precooled much below the water freezing point, the latter liquid possibly functions as a coolant for removing the heat released by the hydrate formation. Moreover, the liquid may also serve, with the help of the surrounding hydrate-forming gas, as a companion hydrate guest, if it is one of the large-molecule guest substances (LMGSs), whose molecules may fit into the 51268 cages of the structure-H hydrates.1 The conceptual design of a twin-jet hydrate-forming system for realizing the above idea is illustrated in Figure 1. The system is equipped with two liquid-circulation loops, one for water and the other for the LMGS, and a high-pressure chamber charged with a hydrate-forming gas, such as methane. * To whom correspondence should be addressed. Telephone: +81-45566-1522. Fax: +81-45-566-1495. E-mail: [email protected]. † Current address: Flight Crew Center, All Nippon Airways Co., Ltd., Haneda Airport, Tokyo 144-8522, Japan. (1) The use of a LMGS for the purpose of forming a structure-H hydrate from natural gas (or methane) was first proposed by Khokhar et al.2 with the expectation of enabling a rather high gas-storage capacity at a pressure substantially lower than that required for forming the structure-I methane hydrate. Subsequently, the hydrate formation from methane or a naturalgas-simulating (methane + ethane + propane) mixture in the presence of a LMGS was experimentally3-6 and theoretically7 studied in the laboratory of the present authors. As for the possible advantage of using a LMGS in the industrial operations of forming hydrates from natural gas (or similar gas mixtures), consult the paper by Kobayashi et al.6

Figure 1. Conceptual illustration of a twin-jet hydrate-forming system.

The two loops merge in the chamber such that the two liquids injected into the chamber in the form of co-planar cylindrical jets collide, forming a radially expanding sheet, which, in turn, sprinkles from its rim tiny water/LMGS compound droplets into the gas phase.8 Having passed through the gas phase, these droplets, possibly containing hydrate crystals, coalesce into a liquid pool occupying the lower portion of the chamber, in which the LMGS, the hydrate, and water are gravitationally separated into three layers. Water and the LMGS may be separately drained out of the chamber, pumped back into the circulation loops to flow through in-line heat exchangers, thereby being cooled to prescribed levels, and then injected again into the chamber. The twin-jet hydrate-forming scheme outlined above has some similarity to the water-spray scheme, which has already been tested by several research groups using laboratory-scale hydrateforming equipment,3-6,9-12 in that water is dispersed in the (2) Khokhar, A. A.; Gudmundsson, J. S.; Sloan, E. D. Gas storage in structure H hydrates. Fluid Phase Equilib. 1998, 150/151, 383–392. (3) Ohmura, R.; Kashiwazaki, S.; Shiota, S.; Tsuji, H.; Mori, Y. H. Structure-I and structure-H hydrate formation using water spraying. Energy Fuels 2002, 16, 1141–1147.

10.1021/ef800880f CCC: $40.75  2009 American Chemical Society Published on Web 02/12/2009

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continuous phase of a hydrate-forming gas inside a chambertype reactor and that, for continuing the hydrate formation inside the reactor, water needs to be externally circulated. As we recently claimed elsewhere,13 the twin-jet hydrate-forming scheme is possibly free from the major disadvantages of the conventional water-spray scheme, such as (a) a relatively large power consumption for pressurizing water to be sprayed and (b) an insufficient function of discharging heat released by the hydrate formation.14 Because the twin-jet scheme uses simple tubular nozzles or orifices to release water and a hydrophobic liquid (possibly a LMGS) in the form of cylindrical jets, high pressurization of the liquids in excess of the pressure inside the reactor is not necessary.15 Disadvantage a is thus eliminated in the twin-jet scheme. Disadvantage b may also be avoided or at least reduced by intensive cooling of the hydrophobic liquid, while it is being pumped through a circulation loop equipped with an external heat exchanger (Figure 1).16 From a fluid-mechanical viewpoint, the twin-jet hydrateforming scheme has a similarity to a particular type of propellant injectors for bipropellant rocket engines, in which liquid-fuel

and liquid-oxygen jets impinge on each other,20 and to impinging-jet reactors that have been developed for various chemical engineering processes including the liquid-liquid extraction, absorption, and desorption of gases, drying of solids, crystallization, precipitation, etc. (see, for example, refs 21-23). Besides such an engineering development, a number of fine fluid-mechanical studies have been performed to explore the nature of the formation and disintegration of geometrically beautiful liquid sheets resulting from the oblique collision of identical cylindrical jets (see, for example, refs 24-26). However, none of these fluid-mechanical studies has been extended to deal with the impingement of two jets of different liquids immiscible with each other or with the impingement in a high-pressure gaseous atmosphere corresponding to a guestgas phase inside a hydrate-forming reactor. Although the mechanistic nature of the impingement of a water jet and a hydrophobic-liquid jet still needs to be investigated, we do not go into details about this issue in this paper. Instead, we focus on the availability of the jet-impingement operations for industrial hydrate production from, for example, natural gas. This paper reports the first laboratory

(4) Tsuji, H.; Ohmura, R.; Mori, Y. H. Forming structure-H hydrates using water spraying in methane gas: Effects of chemical species of largemolecule guest substances. Energy Fuels 2004, 18, 418–424. (5) Tsuji, H.; Kobayashi, T.; Ohmura, R.; Mori, Y. H. Hydrate formation by water spraying in a methane + ethane + propane gas mixture: An attempt at promoting hydrate formation utilizing large-molecule guest substances for structure-H hydrates. Energy Fuels 2005, 19, 869–876. (6) Kobayashi, T.; Imura, N.; Ohmura, R.; Mori, Y. H. Clathrate hydrate formation by water spraying in a methane + ethane + propane gas mixture: Search for the rate-controlling mechanism of hydrate formation in the presence of methylcyclohexane. Energy Fuels 2007, 21, 545–553. (7) Tsuji, H.; Kobayashi, T.; Okano, Y.; Ohmura, R.; Yasuoka, K.; Mori, Y. H. Thermodynamic simulations of isobaric hydrate-forming operations: Formulation of computational scheme and its application to hydrate formation from a methane + ethane + propane mixture. Energy Fuels 2005, 19, 1587–1597. (8) The formation of water/LMGS compound droplets assumed here is supported by the principle of surface-free-energy minimization in a hypothetical system containing a water droplet and a LMGS droplet initially separated by a gas phase. Once the droplets are brought into mutual contact, the droplet of the LMGS, which has a lower surface tension than water, should immediately, although not completely, engulf the water droplet, thereby reducing the total surface free energy of the system. The compound droplet thus formed should not spontaneously disintegrate into the water and LMGS droplets again. A comprehensive discussion on the surfacefree-energy issue concerning the compound-droplet formation is given in the following paper: Mori, Y. H.; Nagai, K.; Funaba, H.; Komotori, K. Cooling of freely falling liquid drops with a shell of an immiscible volatile liquid. J. Heat Transfer 1981, 103, 508–513. (9) Rogers, R.; Yevi, G. Y.; Swalm, M. Hydrates for storage of natural gas. Proceedings of the 2nd International Conference on Natural Gas Hydrates, Toulouse, France, June 2-6, 1996; pp 423-429. (10) Yoshikawa, K.; Kondo, Y.; Kimura, T.; Fujimoto, T. Method and apparatus for producing hydrates (in Japanese). Patent Abstracts of Japan, Publication 2000-256226, Sept 19, 2000. Released online in Patent and Utility Model Gazette DB of the Japan Patent Office (accessible at http:// www.ipdl.inpit.go.jp/homepg_e.ipdl). (11) Nagamori, S.; Ono, J.; Nagata, K. Device for continuously producing gas hydrates (in Japanese). Patent Abstracts of Japan, Publication 2000-264852, Sept 26, 2000. Released online in Patent and Utility Model Gazette DB of the Japan Patent Office (accessible at http://www.ipdl.inpit. go.jp/homepg_e.ipdl). (12) Miyata, K.; Okui, T.; Hirayama, H.; Ihara, M.; Yoshikawa, K.; Nagayasu, H.; Iwasaki, S.; Kimura, T.; Kawasaki, T.; Kikuchi, K.; Terasaki, D. A challenge to high-rate industrial production of methane hydrate. Proceedings of the 4th International Conference on Gas Hydrates, Yokohama, Japan, May 19-23, 2002; pp 1031-1035. (13) Mori, Y. H. Method and apparatus for producing structure-H hydrates (in Japanese). Patent Abstracts of Japan, Publication 2006-176467, July 6, 2006. Released online in Patent and Utility Model Gazette DB of the Japan Patent Office (accessible at http://www.ipdl.inpit.go.jp/homepg_e. ipdl). (14) Mori, Y. H. Recent advances in hydrate-based technologies for natural gas storagesA review. J. Chem. Ind. Eng. (China) 2003, 54 (supplement), 1–17.

(15) The pressure excess of the liquid (water or the hydrophobic liquid coolant) required to be jetted into the reactor through an orifice (or nozzle) can readily be estimated on the basis of classical hydraulics, when the type and the hole size of the orifice as well as the liquid flow rate are specified. For the water flow rate of 100 cm3/min through a simple sharp-edged orifice with a 1 mm hole diameter (the conditions relevant to our experiments in this study), the pressure excess is estimated to be no more than 6 kPa. Nearly the same pressure excess is estimated for jetting the hydrophobic coolant. In contrast, the pressure excess required for spraying water at the same flow rate using the full-cone-type spray nozzle used in our previous studies3-6 (H. Ikeuchi and Co., model 1/4MJ006BW) is as high as ∼0.7 MPa. Thus, we can expect that, despite the necessity of the twin circulation loops, one for water and the other for the hydrophobic coolant, in the twinjet system, the total liquid-pumping power required there should be significantly lower than that required for the corresponding water-spraying system. (16) Fukumoto et al.17 and, more recently, Matsuda et al.18 examined an alternative way to overcome difficulty (b): spray water against the surface of a cooled metal block exposed to the gas phase inside a hydrate-forming reactor such that the heat released by the hydrate formation on the surface may immediately be removed out of the reactor by the heat conduction through the block. Although its effectiveness for hydrate formation was confirmed, this cooling technique is apt to be beset with a problem of hydrate accumulation on the surface, which provides an additional thermal resistance to the heat removal. Most recently, Fujita et al.19 extended the above cooling technique by replacing the solid metal block for conductive cooling by a porous plate through which an intensively cooled hydrophobic liquid coolant seeps out to form a continuous film flowing down the plate surface. The problem of hydrate accumulation was successfully eliminated by this device at the cost of additional power consumption for pumping the hydrophobic coolant across the porous plate. (17) Fukumoto, K.; Tobe, J.; Ohmura, R.; Mori, Y. H. Hydrate formation using water spraying in a hydrophobic gas: A preliminary study. AIChE J. 2001, 47, 1899–1904. (18) Matsuda, S.; Tsuda, H.; Mori, Y. H. Hydrate formation using water spraying onto a cooled solid surface in a guest gas. AIChE J. 2006, 52, 2978–2987. (19) Fujita, S.; Watanabe, K.; Mori, Y. H. Clathrate-hydrate formation by water spraying onto a porous metal plate exuding a hydrophobic coolant. Accepted for publication. Manuscript AIChE-08-11391.R1. (20) Rupe, J. H. The liquid-phase mixing of a pair of impinging streams. Progress Report 20-195; Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, 1953. (21) Berman, Y.; Tamir, A. Extraction in thin liquid films generated by impinging streams. AIChE J. 2000, 46, 769–778. (22) Kudra, T.; Mujumdar, A. S. Impinging streams dryers for particles and pastes. Drying Technol. 1989, 7, 219–266. (23) Saien, J.; Zonouzian, S. A. E.; Dehkordi, A. M. Investigation of a two impinging jets contacting device for liquid-liquid extraction processes. Chem. Eng. Sci. 2006, 61, 3942–3950. (24) Taylor, G. I. Formation of thin flat sheets of water. Proc. R. Soc. London, Ser. A 1960, 259, 1–17. (25) Bush, J. W. M.; Hasha, A. E. On the collision of laminar jets: Fluid chains and fishbones. J. Fluid Mech. 2004, 511, 285–310. (26) Bremond, N.; Villermaux, E. Atomization by jet impact. J. Fluid Mech. 2006, 549, 273–306.

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Figure 2. (a) Functional design and (b) structural details of the experimental hydrate-forming chamber. The chamber is equipped with two orifices each drilled on the side wall of a liquid-feeding tube horizontally inserted into the chamber. The tubes can rotate about their axes, hence the collision angle of the two liquid jets issuing from the orifices can be changed.

experiments of hydrate formation using a jet-impingement device. The experiments employed methane and methylcyclohexane (MCH) as the guest gas and the LMGS, respectively, from which structure-H hydrates were expected to be formed. Independently precooled water and MCH were injected into a pressurized methane atmosphere in the form of twin jets, resulting in their oblique collision accompanied by intensive atomization. Experimental Section Apparatus. Figure 2 illustrates the geometrical design of the hydrate-forming test chamber, which is the central portion of the experimental apparatus that we constructed in this study. Because we did not intend to be heavily involved in the fluid-mechanical aspect of jet impingement throughout this study, we decided not to use a pair of sufficiently long cylindrical nozzles for the purpose of establishing fully developed Poiseulle flows of the test liquids, water and MCH, before their injection. Instead, we used a pair of orifices, 1.0 mm in diameter, drilled on the side walls of two horizontally oriented, stainless-steel liquid-supply tubes with a 6 mm outer diameter and 4 mm inner diameter, which were so inserted into the chamber as to be aligned on the same horizontal plane and parallel to each other, with a distance of 30 mm between their axes. The end of each tube was plugged such that the liquid pumped into the tube could be issued only through the orifice into the chamber, taking the form of a cylindrical jet. The tube could be rotated about its axis to change θ, the inclination angle of the jet axis measured from the vertical. By adjusting both the axial locations and rotational orientations of the two orifices, we could locate the axes of the two liquid jets on a common vertical plane such that they were symmetrically oriented about a vertical axis lying on the plane, thereby intersecting at an angle of 2θ. By employing the simple twin-orifice arrangement described above, we could minimize the horizontal area of the internal space of the test chamber. The test chamber that we constructed was a cylindrical stainless-steel column with a 100 mm inner diameter and 410 mm internal height (see Figure 2b). The chamber was equipped with three vertically aligned circular Pyrex-glass windows, each having a 50 mm diameter viewing area, on either of its front and back sides. The upper windows allowed us to observe the behavior of the jet impingement, while the lower windows were used for observing the hydrate sedimentation in the MCH and water phases layered in the lower portion of the chamber.

Figure 3. Schematic of the entire experimental setup. The MCH circulation loop illustrated here is termed type II in this paper. Neither the cooler nor the silica-gel container was incorporated in the type I loop.

The layout of the entire experimental system including some measurement instruments is illustrated in Figure 3. Two sheathed resistance thermometers were inserted into the gas and liquid phases occupying the upper and lower portions, respectively, of the inside space of the chamber. For measuring the pressure inside the chamber, a digital pressure gauge [Valcom model VPRH(IS)-A410.00MPa-2S(HH)-IU] was attached to the chamber. To control its temperature at a prescribed level, the chamber was immersed in a water bath equipped with an immersion-type cooler and a PIDcontrolled heater integrated with a stirrer. The bath itself was made of transparent poly(methyl methacrylate) plates such that we could observe the inside of the chamber across the bath wall. A high-pressure methane cylinder was connected to the test chamber to continuously supply research-grade methane (99.9 mass % certified purity, supplied by Toyoko Kagaku Co., Tokyo, Japan) to the chamber, thereby compensating for the loss of gaseous methane because of its dissolution into the liquids and also hydrate formation inside the chamber. The line connecting the methane cylinder and the test chamber was equipped with a pressure regulator (Yamato model YR-5062) and a mass-flow meter (Oval model F-111S-A-12-11N). This allowed us to measure the instantaneous rate of methane supply to the test chamber within an uncertainty of (5 cm3/min NTP (converted to a volumetric flow rate at a temperature of 273.15 K and pressure of 101.3 kPa). The data obtained by the mass-flow meter were recorded, simultaneously

1622 Energy & Fuels, Vol. 23, 2009 with the pressure and temperature data, by a data logger (Graphtech model GL450) every 1/5 s and stored in a memory card. Also connected to the chamber were two liquid-circulation loops, one for water and the other for the MCH. The loop for the water was assembled such that water sucked from the bottom of the chamber could be cooled to the temperature of the water bath and supplied back into the chamber through the orifice located at an upper portion of the chamber. The loop consisted of a double plunger pump (Nihon Seimitsu Kagaku Co., model NP-EX-200T), a pulse damper (Uniflows, model RD20033S), a Swagelok teetype particulate filter with 230 µm pore size, a helical-tube-type heat exchanger immersed in the water bath, and stainless-steel connecting tubes. The pump was controlled by a frequency inverter (Mitsubishi Electric model FR-B750) to adjust the circulatory flow rate of water. The heat exchanger was axially long enough to allow the temperature of the internal water flow to approach that of the water bath within 0.1 K. To circulate the MCH, two different loop assemblies, types I and II, were constructed for alternative use. Type I was identical to the assembly of the water-circulation loop, except for the tube for sucking the MCH being inserted into the chamber from its top to a height of 120 mm above the bottom wall. Type II was modified such that (i) the heat exchanger was immersed not in the water bath for temperature-controlling the test chamber but in an external cooling bath (a Neslab RTE-740 bath circulator) to enable the MCH cooling to various temperature levels independent of the temperature setting of the test chamber and the circulating water and (ii) a stainless-steel tank (2250 cm3 inside volume) holding a fixed silica-gel bed was installed to remove water dissolved or dispersed in the MCH-rich liquid pumped from the test chamber. It should be noted that the above device for dehydrating the MCH-rich liquid was found to be crucial for preventing the loop from plugging because of hydrate and/or ice formation inside the loop as far as the liquid was to be cooled to a temperature below 0 °C while flowing through the heat exchanger. A resistance thermometer was used to monitor the temperature in the bath of the aqueous ethylene glycol solution for cooling the MCH. However, the actual temperature of the MCH issuing into the test chamber, TMCH, was higher than the temperature in the bath because of the heat transfer from the surroundings to the MCH supply line between the bath and the orifice inside the test chamber. Thus, a calibration was made to correlate the bath temperature over the range from 0 to -27 °C to the TMCH directly measured by a resistance thermometer in advance of hydrate-forming experiments. Procedure of Hydrate-Forming Experiments. Each experimental run was commenced by evacuating the test chamber, the water and MCH circulation loops, and the methane-gas supply line between the pressure regulator and the test chamber. The chamber was then charged with deionized and distilled water in the amount of 850 cm3 and reagent-grade MCH (99.0 mass % certified purity, supplied by Junsei Chemical Co., Tokyo) in the amount of 400 cm3 (when using the type I loop) or 2000 cm3 (when using the type II loop). These amounts of water and MCH were determined to position the interface between the two liquid phases near the elevation of the center of the lowest viewing window of the chamber and to allow the MCH layer to extend to about 50 mm in thickness above the interface such that the tip of the MCH-suction tube safely remained in the bulk of the MCH layer throughout the hydrateforming operation. Subsequently, the chamber was charged with methane gas until the pressure inside the chamber, p, was increased to within (0.1 MPa of the target pressure of 3.0 or 3.7 MPa. After the temperature inside the chamber, T, as well as that in the surrounding water bath were confirmed to be held constant at 2.0 ( 0.1 °C, the plunger pumps were started to supply the water and MCH into the gas phase inside the test chamber in the form of jets. The volume flow rate of the water, V˙water, was fixed at 100 cm3/min, but that of MCH, V˙MCH, was adjusted at 114 cm3/min; this was to make the axial momenta of the two jets balance each other. In addition to the acquisition of temperature, pressure, and gas flow rate data, a videographic recording of the inside of the test chamber was occasionally performed with the aid of backlighting across the viewing windows on the rear and front sides of

Murakami et al. the chamber. The above operation was continued until either of the two jets (the MCH jet in most cases) was significantly disturbed by hydrate formation on the periphery of the orifice producing the jet. Experimental Conditions and Parameters. Although the twinjet assembly installed in the test chamber allowed us to vary the angle between the jet axis, 2θ (Figure 2), we fixed it at 120° throughout the experiments in which we obtained the results shown below. The selection of this angle was based on our as-yet unpublished observations of the liquid atomization caused by the collision of water and MCH jets injected into air at normal pressure from twin tubular nozzles with which 2θ was varied between 60° and 120°. Angles larger than 120° was avoided to prevent the top surface of the chamber from being heavily wetted by the liquids radially sprayed from the point of the jet collision. The only operational parameter that we intentionally varied in this study was the MCH-jet temperature, TMCH. In the experiments using the type II MCH-circulation loop, we controlled TMCH at 2.0, -2.0, -5.6, or -9.3 °C independent of the water-jet temperature, Twater, which was practically equal to the water-bath temperature (2.0 ( 0.1 °C). On the basis of TMCH and Twater, we can determine the mixed-mean jet temperature Tjet (the temperature that would be available if the two liquids forming the jets were thermally equilibrated with each other) as follows:

Tjet )

V˙waterFwatercp,waterTwater + V˙MCHFMCHcp,MCHTMCH V˙waterFwatercp,water + V˙MCHFMCHcp,MCH

(1)

where F and cp denote the mass density and specific heat, respectively, of the species indicated by the attached subscripts. Although we expected to observe hydrate formation at an isobaric condition, the pressure inside the test chamber, p, underwent some irregular variation during each experimental run because of occasional imbalance between the methane gas consumption by hydrate formation and the methane gas supply passively controlled by the pressure-regulating valve (Figure 3). The variation in p was within (0.10 MPa about a certain mean pressure throughout the hydrate-forming period in each experimental run. Such mean pressures fell in either of the two following ranges: 3.00-3.11 and 3.72-3.78 MPa. For each experimental run, we calculated Teq, the four-phase (methane/water/MCH/hydrate) equilibrium temperature corresponding to the mean pressure, using CSMHYD, a phase-equilibrium calculation program.27 Once we determined the two representative temperatures, Tjet and Teq, for each run, we can evaluate the effective jet subcooling ∆Tsub defined as

∆Tsub ) Teq - Tjet

(2)

We assume ∆Tsub to be a key parameter related to the temperature driving force for hydrate formation in the water/MCH/methane disperse system radially extending around the point of jet collision. This parameter will be used later in presenting the experimental data on the hydrate formation rates.

Results and Discussion Qualitative Observations. Figure 4 shows snapshot pictures of the impingement of the water and MCH jets in the methane atmosphere at three different pressures. In every picture, we find an elliptic liquid sheet radially extending, although being shifted downward, from the point of the jet collision. The sheet tended to shrink with an increase in the pressure. We also noted, irrespective of the pressure level, tiny droplets sprinkled from the periphery of the liquid sheet. Presumably, the sheet is a (27) CSMHYD, a phase-equilibrium calculation program package accompanying the following book: Sloan, E. D., Jr. Clathrate Hydrates of Natural Gases, 2nd ed.; Marcel Decker, Inc.: New York, 1998. The pressure versus Teq curve based on this program is given in Figure 1 in the paper by Ohmura et al.3

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Figure 4. Snapshot pictures of the impingement of water and MCH jets in methane atmosphere at three different pressures. The type I MCHcirculation loop was used. Twater ) TMCH ) 2.0 °C. The formation of an elliptic liquid sheet and the sprinkling of tiny droplets from the periphery of the sheet are observed irrespective of the pressure level.

Figure 5. Sequence of the accumulation of hydrate crystals near the MCH/water interface. The type II MCH-circulation loop was used. p ≈ 3.0 MPa, Twater ) 2.0 °C, and TMCH ) -2.0 °C.

dual film composed of a water layer and an MCH layer, and the droplets ejected from the sheet are compound globules each composed of water and MCH phases. The sprinkling of such droplets into the methane gas should strongly increase the twophase (water/MCH, water/methane, and MCH/methane) interfacial areas and the three-phase (water/MCH/methane) contact line per unit volume, thereby providing a favorable condition for hydrate formation. It is technically difficult, however, to visually discern whether or not hydrate crystals are formed on the droplets while falling through the methane gas phase. In our experiments, we could visually detect hydrate formation only in the liquid phases occupying the lower portion of the test chamber. Figure 5 exemplifies a sequence of the accumulation of hydrate crystals near the MCH/water phase boundary. These hydrate crystals were possibly released from the droplets when they fell onto the free surface of the MCH-rich liquid layer and then sedimented to the phase boundary. Alternatively, they may have formed inside the MCH-rich liquid phase containing methane and water not only in the form of dissolved molecules but, as demonstrated in Figure 6, in the form of macroscopic bubbles and droplets. In Figure 6, we find many water droplets, mostly encapsulating methane bubbles, piled on the MCH/water phase boundary. This situation is essentially the same as the one that Ohmura et al.3 observed in their experiments of hydrate formation using water spraying into a methane gas phase, below

Figure 6. Sedimentation of water droplets above the MCH/water interface. The type I MCH-circulation loop was used. p ≈ 3.0 MPa, and Twater ) TMCH ) 2.0 °C. Note that many of the water droplets encapsulated methane bubbles.

which the MCH and water phases were superimposed. As discussed in detail by Ohmura et al.,3 such compound water/ methane droplets may effectively help the formation of the structure-H methane/MCH hydrates near the phase boundary. Quantitative Representation of Experimental Results. Figure 7 shows the time evolution of Vg, the cumulative volume (NTP) of methane supplied to the test chamber after the instant t ) 0 when we first recognized the hydrate crystals at the MCH/ water phase boundary in the test chamber, during each experimental run. Graph a is a plot of the Vg versus t data obtained with the type I MCH-circulation loop, while graph b is a plot of the Vg versus t data obtained with the type II MCH-

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Figure 7. Time evolution of Vg, the cumulative volume (NTP) of methane supplied to the test chamber after the first appearance of hydrate crystals. (a) Compiled Vg(t) data obtained using the type I MCHcirculation loop. Twater ) TMCH ) 2.0 °C. (b) Compiled Vg(t) data obtained using the type II MCH-circulation loop. p ) 3.06 ( 0.10 MPa, and Twater ) 2.0 °C.

circulation loop. Before discussing what the Vg versus t data in Figure 7 indicate about the hydrate-formation characteristics, we note the relation between Vg and Vg,uptake, the cumulative volume (NTP) of methane uptake into the hydrate, which we could not directly measure. In principle, Vg may be equated to Vg,uptake on the conditions that the following quantities are held practically unchanged throughout the hydrate-forming period (t g 0) in each run: (i) the pressure p inside the test chamber, (ii) the total quantity of methane physically dissolved in the liquids held in the experimental system (the test chamber plus the liquid-circulation loops), and (iii) the volume of the gas phase inside the test chamber. We assume that condition i was satisfied with reasonable accuracy in every experimental run, because the magnitude of the variation in pressure was (3% at most. As for conditions ii and iii, however, it is more difficult to estimate how the actual experimental conditions were in disagreement with them and how such a disagreement affected the relation between Vg and Vg,uptake. This issue is qualitatively discussed below. Because methane is quite soluble in MCH, even a relatively slight change in the amount of methane dissolved in the MCHrich liquid phase may significantly affect Vg. In general, the concentration of methane in MCH in equilibrium with a methane gas phase, cs, must be higher than that in equilibrium with a hydrate, csh, under a given p-T condition. Thus, if methane was dissolved in the MCH-rich liquid phase close to cs before inception of the hydrate formation (t e 0), hydrate crystals could subsequently form, consuming the methane molecules dissolved in the MCH-rich liquid phase, besides those in the gas phase, until the concentration of the dissolved methane decreased to csh. Such an availability of the dissolved methane for hydrate formation could function to keep Vg from increasing with time in proportion to Vg,uptake. Obviously, this effect could be much more significant when the experimental system incorporating the type II loop and holding a larger amount

Murakami et al.

of MCH was used. The experiments employing the type II loop also suffered another problem that possibly confused the relation between Vg and Vg,uptake. Because of the higher temperature in the silica-gel container than that in the test chamber, some fraction of methane that had been dissolved in the MCH-rich liquid was released inside the silica-gel container, forming a gas phase. The growth of this gas phase inside the silica-gel container suppressed the suction of the MCH-rich liquid from the test chamber into the circulation loop, while allowing the pumping of the liquid from the silica-gel container to the test chamber to be injected into it at the prescribed flow rate. The imbalance between the flow rate of the MCH-rich liquid from the test chamber to the silica-gel container and that from the silica-gel container to the test chamber resulted in an increase in the entire liquid-phase volume and a decrease in the residual gas-phase volume inside the test chamber and thereby a reduction in the gas flow into the chamber. Summarizing the above discussion, we can conclude that the Vg(t) values recorded for each experimental run using the type II MCH-circulation loop were possibly much lower than the corresponding Vg,uptake(t) values in the same run. The Vg versus t curves obtained with the type I MCHcirculation loop (Figure 7a) exhibit a common feature; i.e., each curve consists of two characteristic regimes, the former gentleslope regime and the latter steep-slope regime, bordered at some intermediate stage of the hydrate-forming process. Even in a group of experimental runs operated under the same pressuretemperature condition, the duration of the former regime was considerably different from run to run. The visual observations revealed that the transition from the former to the latter regimes in each run synchronized with a sudden change in appearance of the water jet from a smooth, transparent cylinder to a turbid cylinder with a rough surface. Such a change in the water jet appearance was presumably ascribable to the onset of incorporating tiny hydrate particles from the test chamber into the water-circulation loop. Despite the particulate filter installed in the loop, hydrate particles of several tens of micrometers or even smaller in size were possibly circulated through the loop, being suspended in the liquid water. Such particles could work, when injected with water into the test chamber, as hydrate nuclei on which hydrate crystals could continuously form and grow, leading to a rapid increase in Vg with time t. Such regime-regime transitions in Vg versus t relations and in water-jet appearance also occurred in the experimental runs employing the type II MCH-circulation loop when TMCH was adjusted to -5.6 or -9.3 °C (Figure 7b). When TMCH was higher, i.e., at 2.0 or -2.0 °C, the former gentle-slope regime continued throughout each run presumably because of the dissociation of the hydrate particles while flowing through the silica-gel container. Because of the significant changes in the rate of methane gas supply, V′g ()dVg/dt), during many of the experimental runs, we define V˙g, the representative gas supply rate in each run, as ∆Vg divided by τ, where ∆Vg denotes the increase in Vg during the period from the instant when V′g had increased to the 50% of its maximum value in the run to the end of the run, and τ denotes the length of the period. The V˙g values thus evaluated are plotted versus the effective jet subcooling ∆Tsub (eq 2) in Figure 8. We note that, as expected, V˙g tends to increase with an increase in ∆Tsub and that the V˙g values obtained with the type II loop are generally much lower than those obtained with the type I loop at equivalent levels of ∆Tsub. As already discussed, the V˙g values obtained with the type II loop may have been substantially lower than the corresponding Vg,uptake values. Presumably, the V˙g values obtained with the type I loop are closer to the corresponding Vg,uptake values.

Hydrate Formation by Water Spraying

Figure 8. Average rate of methane supply to the test chamber versus effective jet subcooling, i.e., the deficiency of the mixed-mean jet temperature (eq 1) from the four-phase (methane/water/MCH/hydrate) equilibrium temperature corresponding to the system pressure p in each experimental run.

Finally, we compare the V˙g values obtained in this study to those deduced from the data that Ohmura et al.3 obtained in their experiments in which water was sprayed at the flow rate of 90 cm3/min into a methane gas phase underlayered by superimposed MCH and water layers at a pressure of 2.8 MPa. It should be noted that the operational conditions in the two studies were not necessarily equivalent to each other; i.e., the above water flow rate and the system pressure in the study by Ohmura et al.3 were slightly lower than those in this study (100 cm3/min and 3.0 or 3.7 MPa), and the subcooling of the water spray in the former study was about 4.8 K, which was slightly lower than the lowest ∆Tsub value in this study (5.3 K). Despite such differences in the operational conditions between the two studies, the comparison may still be instrumental for us to have, although even roughly, an idea about the potential of the jetimpingement technique for hydrate production. The V˙g values evaluated from the Vg versus t data given by Ohmura et al. (Figure 7 in ref 3) are no more than ∼65 cm3/min (NTP), which may be compared to the V˙g values of ∼90 cm3/min (NTP) that we obtained in this study using the type I loop when ∆Tsub ≈ 5.3 K (see Figure 8). On the basis of this comparison, we can expect that the jet-impingement technique has a hydrate productivity almost equivalent or even superior to that of the water spraying technique under equivalent operational conditions. Concluding Remarks This study has revealed the potential utility of an unconventional jet-impingement technique for hydrate formation;

Energy & Fuels, Vol. 23, 2009 1625

water and a LMGS (a sH-hydrate-forming hydrophobic liquid) are injected into a hydrate-forming gas phase in the form of cylindrical jets, which collide with each other, causing simultaneous atomization of the two liquids. A series of experiments using methylcyclohexane and methane as the LMGS and the hydrate-forming gas, respectively, has demonstrated that the jet-impingement technique yields a hydrate formation rate comparable to that available using the water spraying technique, if the water and LMGS jets in the former are cooled to the same temperature level as that of the water sprays in the latter. Higher hydrate formation rates are available by precooling the LMGS to temperatures considerably below the water freezing point. Despite its high potential for high-rate hydrate production, the jet-impingement technique is accompanied by a risk of accidental breaks in the continuous hydrate-forming operations because of the blocking of orifices for the water/LMGS jetting by hydrate formation around their mouths or because of the plugging of the loop through which the LMGS is drained out of the hydrate-forming reactor and pumped back, via an external cooler, to the reactor to be jetted there again. The incorporation of tiny hydrate particles into the loop, being suspended in the flowing LMGS, is favorable for maintaining the hydrate formation rate inside the reactor at a high level. However, excessive growth of these particles, while flowing in the loop, with the help of methane and water dissolved in the surrounding LMGS should be avoided to prevent the loop from being plugged. Technical refinement to overcome the above-mentioned problems is required to make the jetimpingement technique acceptable for industrial hydrateforming operations. Acknowledgment. This study was supported by the Industrial Technology Research Grant Program (Grant 05A45004) from the New Energy and Industrial Technology Development Organization (NEDO) of Japan in the fiscal years of 2006 and 2007 and by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (Grant 20246040) in the 2008 fiscal year. We thank Dr. Ryo Ohmura, Department of Mechanical Engineering, Keio University, for his useful advice about our experimental work. We are also indebted to Hidehiko Tsuda, former student in the Department of Mechanical Engineering, Keio University, for his help with the preliminary twin-jet hydrate-forming experiments prior to this study. EF800880F