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Natural Gas Hydrate Formation in an Ejector Loop Reactor: Preliminary Study Liang-Guang Tang, Xiao-Sen Li,* Zi-Ping Feng, Yu-La Lin, and Shuan-Shi Fan Guangzhou Research Center for Gas Hydrate, Guangzhou Institute of Energy ConVersion, The Chinese Academy of Sciences, No 2. Nengyuan Road, Wushan, Tianhe District, Guangzhou, People’s Republic of China, 510640
A natural gas hydrate formation system based on the ejector-type loop reactor (ELR) has been built. Three types of flow patterns in the reactor are observed with variation of the gas entrainment rate, i.e., singlebubble regime, intermediate regime, and jet regime. The flow pattern under the free suction state of the ejector is in the jet regime. The microbubble generated from ELR with a static mixer can shorten the induction time for hydrate formation significantly. However, it cannot improve the hydrate formation rate as the gas entrainment rate decreased with the static mixer on. The hydrate formation rate of the ELR system is dependent on the subcooling, gas entrainment rate, and pressure and is fairly comparable with the formation rate of both water spraying and stirring reactor in the literature. Introduction Gas hydrates are crystalline clathrate compounds and formed by inclusion of light gases in a water lattice.1 Gudmundsson2 proposed that the natural gas hydrate, once formed at a high pressure, can be preserved at atmospheric pressure and at a temperature below the water-freezing point, typically at -15 °C, due to the “self-preservation” effect. Because of the favorable storage ability and pressure-temperature conditions compared to compressed natural gas (CNG) or liquefied natural gas (LNG), interest in the possibility of storing and transporting natural gas in the form of hydrates has been increasing in recent years 3. The first step for development of hydrate-based technology for natural gas transportation involves high-speed hydrate formation. The major difficulties to be overcome for high-speed hydrate formation include the following: (1) the natural gas supplied to the hydrate formation reactor must be well mixed with water and (2) an effective cooling device for removing the heat of hydrate formation from the inside of the reactor must be present. There are two main methods to produce gas hydrates in the literature, i.e., gas dispersion, that is, to introduce gas bubbles in water continuous phase, and liquid dispersion, that is, to introduce water droplets in gas continuous phases.4 Gudmundsson et al.5 developed a stirred tank reactor where gases were introduced through the liquid phase and dispersed by an impeller. The gas-dispersion-type reactor has the advantages of effective heat removal through the wall of the reactor or any cooling device and the possibility to completely convert the water into hydrate. However, it involves two main disadvantages. One is that a compressor is needed to make the excess gas that has not been hydrated out of the reactor. The other is the power required for the stirring increases with the 5th power of the impeller size, which hinders scale-up of this kind of reactor.4 Mitsubishi Heavy Industries6 proposed the liquid-dispersiontype of reactor design. From the top of the reactor water is sprayed into the gas phases and the formed hydrate slurry is withdrawn from the reactor and filtrated. Tsuji et al.7 successfully applied the water spaying technique to produce structure H hydrate. This kind of system has the advantages of increased * To whom correspondence should be addressed. Tel: (8620)87057037. Fax: (8620)87057037. E-mail:
[email protected].
gas/water interfacial area and simple reactor design due to no mechanical stirrer being required. However, the disadvantage is obvious. First, hydrate formation heat cannot be effectively removed due to a substantial proportion of the reactor volume being occupied by gas. Second, the hydrate-to-water volume ratio in the reactor cannot be very high.4 To overcome the first disadvantage, Fukumoto et al.8 sprayed water against a copper block exposed to a hydrate-forming gas while the block was steadily chilled at its backside, and this technique was extended by Matsuda et al. recently.9 To effectively produce gas hydrate for commercial use, great effort has been made. Zhong et al.10 found that the formation rates of gas hydrates can be increased multiple orders of magnitude if the host water is a micellar solution containing sodium dodecyl sulfate or related surfactant. Tajima et al.11 suggested a Kenics-type static mixer in forming the CO2 hydrate. Takahashi et al.12 patented a microbubble technology in forming gas hydrate. In this patent, water was introduced into the apparatus by a pump spiraled up along the wall and went down to the outlet along the center of the apparatus. Gas was automatically introduced from the gas inlet, and a twirl of gas formed along the center axis was forced out from the outlet with circulating water. The mixture of gas and water was dispersed by the force of circulation, and the shearing force generated at the outlet separated the mixture into fine bubbles. A summary of current methods is given in Table 1. All the developed hydrate formation techniques have their own disadvantages. Development of new hydrate formation technology is necessary. The patented microbubble technology works based on a similar principle of the ejector-type loop reactor (ELR) widely used in chemical reaction engineering. The principle of the ELR is utilization of the kinetic energy of a high-velocity liquid jet to entrain the gas phase and create a fine dispersion of the two phases.13 As the resulting gas-liquid stream leaves the ejector and enters the reactor vessel, a secondary gas dispersion of bubbles is obtained in the bulk fluid. The ELR has the following advantages compared to the stirred tank reactor: (1) favorable mass transfer and mixing characteristics between gas and liquid phases, (2) an external heat exchanger can be suitably inserted into the liquid circulation loop, eliminating the disadvantages of internal coils installation, (3) gas recirculation ensures complete gas utilization without extra compressor, and (4) the
10.1021/ie0609259 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/14/2006
Ind. Eng. Chem. Res., Vol. 45, No. 23, 2006 7935 Table 1: Summary of the Current Hydrate Formation Techniques hydrate formation technique gas dispersion liquid dispersion advanced technique static system with surfactant static mixer microbubble
comments large energy consumption when scaling up hard for heat transfer hard for heat transfer high flow resistance working at low pressure
absence of moving parts, eliminating sealing problems and allowing easier operation at elevated pressure.14 Due to these advantages, Havelka et al.15 suggested that the ELR will be a standard part of the process of reactor selection for reactions in gas-liquid and gas-liquid-solid systems. However, the technology has not been applied into the gas hydrate formation. A natural gas hydrate formation system based on ELR has been built. Preliminary experiments have been carried out to prove the feasibility of this reactor. Experimental Apparatus Figure 1 shows the schematic flow diagram of the ELR-based experimental apparatus for hydrate formation. The flow loop mainly consists of four parts, a reactor, an ejector, a heat exchanger, and a circulation pump, and is situated in a temperature-controlled room varied from 273.15 to 293.15 K. The reactor was made of borosilicate glass and is transparent in appearance for visualization. It has an internal diameter of 14 cm with an effective volume of 4.62 L and can safely operate up to 5.0 MPa. It was sealed by two pieces of stainless steel flanges on both sides connected by four screws. Two platinum resistance thermocouples (Pt100) were used to measure the solution and gas temperature in the reactor, with an accuracy of (0.1 K. A KELLER PA-21S/80400 pressure transducer within 0.01 MPa accuracy was used to measure the pressure inside the reactor. A pressure regulator can maintain constant pressure in the reactor within (0.01 MPa. A D07-11AM/ZM gas flow meter was used to measure the gas consumption rate during hydrate formation. The flow meter has a capacity of 0-10 SLM (standard liter per minute) at an accuracy of 1% of full scale and a repeatability of within 0.2% of the flow rate. The gas was precooled before entering into the reactor. An external heat exchanger was used to remove the heat of hydrate formation. An electromagnetic valve controls the flow rate of the coolant through a PID controller (model XSC5CHRC1V0), to keep the circulation water at a constant temperature, with an accuracy of (0.1 K. A schematic plot of the ejector used in the experiment is plotted in Figure 2. It was revised from commercial usage and was not optimized for the experimental conditions at this stage. Liquid is supplied to the ejector after the heat exchanger by a circulation pump via nozzle, and the fast liquid jet produced by the nozzle entrains and disperses the gas phase through the diffuser into the reactor again. The circulation pump used is a three-cylinder cyclic pump (model W3020-0.57/7) with a maximum flow rate of 0.57 m3/h, and the flow rate can be adjusted by a transducer. The flow rate of the liquid was measured by a floating liquid flow meter with an accuracy of 1.5% of full scale (LZD-15). The gas entrainment rate after dehydration is measured by a similar gas flow meter (D0711AM/ZM) used for measuring the gas consumption rate. To reduce the bubble diameter formed due to gas dispersion in the reactor, a SV static mixer (model SV-2.3/20) was also tested. Static mixers are motionless mixing devices composed
Figure 1. Schematic plot of the NGH formation system with ELR.
of mixing elements equipped in a straight, empty tube. Fluidgas jet from the ejector was introduced into the static mixer and further mixed through flowing in the mixer before entering into the reactor by three kinds of mixing mechanisms, namely, flow reversal, division of flow, and radial mixing. The SV static mixer is bought from a commercial usage with a maximum dispersion ability of 1-2 µm. Experimental Section The natural gas used had a composition of 92.02% CH4, 3.01% C2H6, and 4.97% C3H8. The water used was distilled water. Before each experiment, the whole flow loop system was first cleaned with distilled water. The water was injected into the reactor and pressurized with natural gas to about 2 bar to check for leakages of the system. The flow loop was then evacuated to about 1 mbar with the vacuum pump. Gas was re-injected into the reactor to a pressure about 5 bar lower than the hydrate equilibrium pressure at experimental temperature. Afterward, the heat exchanger starts to work and the liquid and gas circulation began to start at a low liquid circulation rate. The system stood for several hours to reach gas dissolution and constant temperature. During this stage a small amount of gas was injected due to the low solubility of the gas in water. Afterward, circulation stopped and gases were injected to the experimental pressure in a quick way. The data acquisition system was started. The experiment was initiated by switching the liquid circulation pump on. The hydrate begins to form. The experiment was terminated at a specified time or no gases flow into the reactor. Results and Discussion 1. Jet Patterns in Non-Hydrate Formation Conditions. When one gas phase (dispersed phase) is injected into the ambient liquid (continuous phase), the formed jet is hydrodynamically unstable and breaks up into bubbles. In the experimental conditions (P ) 3.0 MPa, T ) 298.15 K), by fixing the liquid injection rate at 7.0 L/min and adjusting the gas entrainment rate, different bubble regimes can be observed in the case without a static mixer, as shown in Figure 3a-c.
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Figure 2. Scheme of the ejector (adapted with permission from ref 15).
Figure 3. Jet patters of the ELR.
At low gas entrainment rate, a jet of gas phase is formed. Bubbles formed at the tip of the jet and broke away from the
jet as individual drops (Figure 3a). This is the single-bubble regime and happens with a gas flow Reyonld’s number less
Ind. Eng. Chem. Res., Vol. 45, No. 23, 2006 7937 Table 2: Experimental Condition for All Runs no.
pres., MPa
temp., K
nominal subcooling, ∆Tsub, K
liquid injection rate, L/min
gas entrainment rate (SLM)
static mixer
1 2 3 4 5 6 7 8 9 10 11 12 13 14
3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.5 3.5 3.5 3.5 3.0 3.0 3.5
276.25 278.35 280.05 280.15 280.25 280.15 282.65 281.55 281.55 281.55 281.45 278.15 282.65 281.55
7.9 5.8 4.1 4.0 3.9 4.0 1.5 3.9 3.9 3.9 4.0 6.0 1.5 3.9
8.5 8.5 9.0 8.5 8.0 7.5 8.5 8.5 8.0 7.5 7.0 8.6 8.5 8.5
19.0 19.0 21.7 18.3 16.7 13.0 18.7 30.5 27.5 24.7 18.3 11.7 11.3 20.2
no no no no no no no no no no no yes yes yes
than 200.16 No accurate measurement of the bubble size distribution is made in the experiment, but the observed bubble diameter during this regime is approximately 2-3 mm. As the gas entrainment rate increased, the gas flow Reyonld’s number increased and the uniformity of the bubbles decreased. Large bubbles of irregular shapes were observed together with smaller spherical ones (Figure 3b). This is the intermediate regime and happens with a gas flow Reyonld’s number of approximately 200-2100.16 At even higher gas velocities the gas stream approaches the appearance of a continuous jet that breaks up to 7.6-10.2 cm above the ejector. The stream consists of large, closely spaced, irregular bubbles with a rapid swirling motion. These bubbles disintegrate into a cloud of smaller ones of random size distribution (Figure 3c), and this is called the jet regime. The flow at the condition of free gas suction of the ejector occurs in this regime. In the case of a static mixer the bubble size is greatly reduced and a smoke-like material is dispersing from the static mixer, suddenly resulting in a milky state of aqueous solution in the vessel, as shown in Figure 3d. The flow properties cannot be visualized in this case and are independent of the gas entrainment rate. A total of 14 hydrate formation experiments have been conducted, and Table 2 lists all the experimental conditions. The nominal subcooling for hydrate formation was calculated by obtaining the hydrate formation pressure-temperature condition with the feed gas composition using the program CSMHYD.1 The actual subcooling should decrease from the nominal one due to preferential consumption of C2H6 and C3H8, suggested by Mori.4 At the free suction state the gas entrainment rate is strongly dependent on the liquid injection rate and the operating pressure. In general, the gas entrainment rate increases with increasing liquid injection rate and pressure, as given in Table 2. With the static mixer, the gas entrainment rate decreases obviously due to flow resistance. For example, at 3.0 MPa and a liquid injection rate of 8.5 L/min, the gas entrainment rate decreases from 19.0 SLM in the case without a mixer (run 2) to 11.3 SLM (run 13) with a mixer. 2. Induction Time during Hydrate Formation. A typical gas consumption rate with time during hydrate formation is plotted in Figure 4. When the experiment starts, the gas consumption rate increases sharply (
