Multiphase, Microdispersion Reactor for the Continuous Production of

Jun 2, 2009 - During experiments, the reactor was submerged in water inside the SPS and received water from the surrounding through a submersible pump...
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Ind. Eng. Chem. Res. 2009, 48, 6448–6452

RESEARCH NOTES Multiphase, Microdispersion Reactor for the Continuous Production of Methane Gas Hydrate Patricia Taboada-Serrano,†,‡ Shannon Ulrich,‡ Phillip Szymcek,‡ Scott D. McCallum,‡ Tommy J. Phelps,‡ Anthony Palumbo,‡ and Costas Tsouris*,†,‡ Georgia Institute of Technology, Atlanta, Georgia 30332, and Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830

A continuous-jet hydrate reactor originally developed to generate a CO2 hydrate stream has been modified to continuously produce CH4 hydrate. The reactor has been tested in the Seafloor Process Simulator (SPS), a 72-L pressure vessel available at Oak Ridge National Laboratory. During experiments, the reactor was submerged in water inside the SPS and received water from the surrounding through a submersible pump and CH4 externally through a gas booster pump. Thermodynamic conditions in the hydrate stability regime were employed in the experiments. The reactor produced a continuous stream of CH4 hydrate, and based on pressure values and amount of gas injected, the conversion of gas to hydrate was estimated. A conversion of up to 70% was achieved using this reactor. 1. Introduction Greenhouse gas emissions could be substantially reduced if automobiles were powered by direct injection of compressed natural gas instead of conventional gasoline.1 However, the technology currently available for transport and storage of natural gas hinders developments in this area. Current natural gas transport and storage methods with high energy densities include: (1) liquefied natural gas (LNG),1 (2) compressed natural gas (CNG),1 (3) chemically liquefied natural gas (GTL),2,3 and (4) adsorbed natural gas (ANG).4 All these methods require large investments in terms of materials for high-pressure systems and/ or have large associated costs in terms of energy required for compression or liquefaction.1-4 Additionally, methods like ANG or GTL are highly dependent on the physicochemical characteristics of the processes (e.g., surface area and sorption capacity of adsorbents or phase behavior of mixtures of methane and different solvents), and the development of technology for their industrial application has been rather slow.3,4 Gas hydrates are a viable alternative to current gas transport and storage systems,5-7 and compared to other technologies, a safer medium because gas hydrate dissociation can be a slow process, releasing gas at a controllable rate. Gas hydrates are crystalline structures composed of repetitive cages formed by hydrogen-bonded water molecules that are stabilized by the presence of a guest gas molecule in each cage.8 Gas hydrates have a large capacity for gas storage in a solid, stable form at moderate pressures and temperatures near the melting point of ice.8 One unit volume of methane hydrate may contain an amount of gas equivalent up to 187 units of volume of methane gas at standard conditions of pressure and temperature, and methane hydrate is stable at 2.55 MPa at 273.15 K. These conditions compare favorably against the higher pressures (around 20 MPa) and the lower temperatures (113 K) required for CNG and LNG, respectively.1,6 Furthermore, recent studies * To whom correspondence should be addressed. Tel.: 865-241-3246. E-mail: [email protected]. † Georgia Institute of Technology. ‡ Oak Ridge National Laboratory.

found that while undergoing rapid dissociation, gas hydrates can be preserved in a metastable thermodynamic-equilibrium state at pressures as low as 0.1 MPa and temperatures as high as 268 K. Normally, the hydrate equilibrium temperature for a pressure of 0.1 MPa corresponds to 193 K.5,6 This unique behavior further enhances the suitability of gas hydrates for transport and storage of natural gas because it would enable the utilization of low pressure and low heat-load transport technologies.5-7 The main challenge in the application of gas hydrates for methane gas transport and storage lies in the current lack of a method for the continuous production of methane hydrate suitable for scale-up to industrial applications.6,7 Most of the current methods used for hydrate production are batch or semibatch and are, therefore, suitable only for laboratory or small-scale operations.9-13 Furthermore, the very few successful methods for continuous gas-hydrate production have only used liquid carbon dioxide as a guest species.14-21 In this work, we present the preliminary tests of a variation in design of our continuous-jet hydrate reactor (CJHR)sinitially designed for the production of CO2 hydrate from liquid CO2 and watersthat allows its application for the continuous production of methane hydrate, using methane gas as a guest species. The reactor discussed here constitutes the first compact unit for continuous gas-hydrate production from a guest gas species, with exception for a closed-loop, ejector-type reactor of semibatch operation described elsewhere.22 2. Experimental Section 2.1. Multiphase, Microdispersion Reactor. Gas hydrate formation is expected when the guest species and water are put in contact at conditions of pressure and temperature where the hydrate phase is thermodynamically stable. However, formation of gas hydrate may not happen spontaneously because it constitutes a crystallization process ultimately determined by the energy available to promote the formation of a new solid interface (i.e., the energy necessary for crystal nucleation and growth).8 Additionally, gas-hydrate formation is an exothermic

10.1021/ie8019517 CCC: $40.75  2009 American Chemical Society Published on Web 06/02/2009

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process which can be self-inhibiting due to inadequate dissipation of heat: local increases in temperature may drive the conditions outside of the zone of hydrate thermodynamic stability.8 Most of the hydrate formation methods rely on vigorous mixing between the guest species and water, which are usually immiscible, in order to create and maximize interfacial area that serves as nucleation site. This mixing also promotes efficient dissipation of the heat of reaction. The hydrate reactors commonly encountered in the literature for this purpose are either continuously stirred tanks and in-line mixers,9,10,14,15 or compressed gas vessels in which water is sprayed.11,12 The common disadvantage of these reactors is that they are not suitable for scale-up to continuous industrial operations. Creating very fine dispersions of a hydrate-forming species into water in a continuous manner was the method of choice for the design of our CJHR. The original reactor design was tubular and was comprised of one capillary tube inserted in another tube of larger diameter.16-19 A hydrate-forming species, originally liquid water, was pumped at high velocity through the capillary tube, while liquid carbon dioxide was introduced through the outer tube, creating a fine dispersion of water in liquid CO2. It was found that creating dispersions of water into liquid CO2 within the spray regime (Weber numbers higher than 324) would maximize hydrate conversion.23 Instant conversion into hydrate at the interface of the fine water droplets took place, leading to the formation of a continuous stream of complex liquid CO2/hydrate/water particles, which would be extruded from the reactor mixing zonesi.e., the tubular section immediately following the end of the capillary tube. Recently, a pilot scale reactor based on the original design was built and tested in the laboratory and in the field.20,21 More than 1 order of magnitude increase in flow of both reactants was achieved, while keeping high hydrate conversions, via the design of a multiple-capillary distributor disk inside the body of the hydrate reactor. Additionally, the flow pattern was switched from coflow to perpendicular flow as a means to create even finer dispersions, i.e., the species to become the continuous phase is introduced laterally in the pilot-scale CJHR. The basic design of the pilot-scale CJHR was adopted for the production of gas hydrate using methane gas and water. In contrast with the original CJHR, where either liquid CO2 or water could be used as dispersed phase with similar outcomes in terms of conversion, methane gas must constitute the dispersed phase in the multiphase, microdispersion reactor. Although the basic dimensions and layout of the CJHR were maintained,20,21 the multiple capillaries in the distribution disk were replaced by multiple nozzles of conical shape for two reasons. The first reason for this is to further increase the gas velocity at the injection point, while reducing the diameter of the gas bubbles formed. Finer dispersions are needed for successful hydrate formation when dealing with gaseous species.13 The second reason was to prevent clogging in the distributing disk due to water invasion of the gas injection distributor brought about by capillary effects. Finally, the length of the reactor mixing zone was shortened to reduce backpressure. A picture of the multiphase, microdispersion reactor is shown in Figure 1a. A schematic of the original distribution disk used with liquid carbon dioxide and the updated design used with methane gas are shown in Figure 1b. 2.2. Experimental Methods. The multiphase, microdispersion reactor was mounted inside the Seafloor Process Simulator (SPS),24 a 72-L cylindrical Hastelloy C-22 vessel depicted in Figure 1c. The SPS has 40 sampling ports including several sapphire windows for observation and several sampling ports.

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Figure 1. (a) Photograph of the multiphase, microemulsion reactor. (b) Schematic of the multicapillary and multinozzle distribution disks. (c) Photograph of the Seafloor Process Simulator (SPS).

The SPS can withstand pressures of up to 20 MPa. The low temperatures required for hydrate formation are achieved by placing the whole vessel inside of a walk-in cold room. Chemically pure methane gas (Air Liquide, Houston, TX) was introduced into the SPS through one of the sampling ports using a Haskel gas booster (Haskel Inc., Burbank, CA) located outside the cold room. This particular model of gas booster is a positive displacement gas pump that delivers gas in strokes. However, the mean value of methane gas flow injected into the reactor was set to a constant value by regulating the inlet pressure of methane to the gas booster and the drive air pressure, as explained in the manual provided by the manufacturer. Water, stored inside the SPS, was recycled and injected into the reactor using a Seabird SBE ST 3 (Seabird Electronics, Bellevue, WA) pump at selected flow rates via regulation of the power supplied to the pump by an Epsco D-612T DC power supply (Epsco Inc., Itasca, IL). The pressure inside the vessel, the temperature of the head space and the temperature of the water were recorded by a pressure transducer and two thermocouples connected to a logging system developed in Labview, following the same protocol used in previous work.20,21 Hydrate production was recorded with a Sony Firewire (XCD-X710CR) video camera installed in one of the sapphire observation windows of the SPS. One advantage of having the water used for hydrate production stored inside the SPS is that the large volume of water served as a thermal sink, efficiently maintaining the temperature of the reactor at a desired value and minimizing the impacts of temperature increase in the headspace of the vessel due to accumulation of unconverted methane gas. At the beginning of each experiment, the SPS was pressurized with nitrogen until the conditions of pressure and temperature were inside the methane-hydrate thermodynamic stability zone. Nitrogen was used to prevent premature methane-hydrate formation during pressurization. The conditions of pressure and temperature selected for the experiments are well below the nitrogen-hydrate stability field and the stability field of nitrogen-methane gas mixture hydrate.8 Because of this, the methane-hydrate production strictly due to the operation of the reactor could be unambiguously quantified. Nitrogen gas was also used to regulate the pressure when needed and to flush the hydrate reactor and the SPS after experiments.

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2.3. Determination of Methane Conversion into Hydrate. No direct measurement of the amount of hydrate formed during each injection could be performed; therefore the pressure, the temperature of the gas in the head space, the temperature of the liquid inside the SPS, and amounts of gas introduced into the SPS were monitored and recorded every second during the experiments with the objective to obtain an indirect estimate of the amount of methane incorporated into the hydrate formed. One should note that the mass of water (60 kg) inside the SPS is constant during the experiments: no water is either introduced or removed from the SPS. This information was used for solving mass balances on water, nitrogen gas, and methane (the only species present at any given time during experiments), with the following assumptions: • Three phases are present: gas, liquid, and hydrate. • Evaporation of water is negligible due to the low temperatures and high pressures; therefore, there is no water in the gas phase. • Gas-liquid thermodynamic equilibrium prevails at all conditions of pressure and temperature, determining the concentrations of nitrogen and methane in the liquid phase. • The liquid phase and hydrate phase are incompressible. • Two stages were considered: (1) a pressurization stage via the introduction of nitrogen to the SPS until the operating conditions were achieved and (2) an injection stage, when methane was injected through the reactor to form methane hydrate. The mass balance in the pressurization stage included only nitrogen, since water was neither added nor removed from the SPS. The Peng-Robinson equation of state and Henry’s law were used to describe the nitrogen in the gas phase and the nitrogen dissolved in the liquid phase. The mass balance equation was as follows: L G in + mN2 ) ) δmN2 d(mN2

(1)

where the terms on the left-hand side (lhs) of the equation correspond to changes of the mass of nitrogen in the gas phase plus the mass of nitrogen in the liquid phase at equilibrium and the term on the right-hand side (rhs) of the equation corresponds to the amount of nitrogen injected into the SPS within a given period of time. The mass balance in the methane injection stage included nitrogen and methane, but only methane was injected through the reactor into the SPS in this stage. The amount of nitrogen remained constant, and methane was distributed in three phases: the (1) gas phase, (2) liquid phase as dissolved gas, and (3) hydrate phase. The Peng-Robinson equation of state was used to describe the methane in the gas phase, and Henry’s Law was used to describe the dissolution of methane in the liquid phase. The two mass balances solved in this stage were the following: L G + mN2 ))0 d(mN2 G L H in d(mCH4 + mCH4 + mCH4 ) ) δmCH4

(2)

where the terms on the lhs of the second equation correspond to changes of the mass of methane in the gas, liquid, and hydrate phases at equilibrium and the term on the rhs of the equation corresponds to the amount of methane injected into the SPS through the hydrate reactor within a given period of time. Methane conversion into hydrate achieved by the reactor is calculated as the amount of methane present in the hydrate phase at the end of the injection divided over the total amount of

Figure 2. Range of experimental thermodynamic conditions of temperature (273.65-275.65 K) and pressure (6.8-10.0 MPa) used for continuous hydrate formation by the reactor shown in Figure 1. These conditions are within the methane hydrate stability zone. I corresponds to ice, Lw corresponds to liquid water, V corresponds to water vapor, and H corresponds to hydrate. The triangular symbols correspond to predicted points of hydrate equilibrium.

methane introduced by the gas booster during injection through the hydrate reactor. 3. Results and Discussion The reactor was tested via injection of methane gas and water for a fixed period of time (between 90 and 150 s, depending on the effectiveness of methane conversion). Since a continuous process of hydrate production was tested inside an enclosed vessel, restricting the amount of time for the reactor to function was critical for safety reasons and in order to keep the discharge pressure within a range of values that would allow for the methane-gas flow rate to be maintained at a constant value by the booster pump. The pressure inside the SPS increased steadily during experiments due to accumulation of unconverted methane gas, and the gas flow provided by the gas booster was highly dependent on the discharge pressure, i.e., the pressure inside the SPS. An average methane-gas flow rate of 4.74 L/s measured at standard conditions of pressure and temperature was maintained during all the experiments. The flow rates of water injected into the reactor varied between 0.012 and 0.016 L/s. These flows were determined by the capacity of the pump used. During all the experiments, the temperature and pressure in the SPS were maintained within the ranges of 273.65-275.65 K and 6.8-10.0 MPa, respectively, values that are well within the hydrate stability zone as shown in Figure 2. Two different distributors were tested during the experiments. The first one was a multicapillary distributor with a capillary diameter of 184.6 µm that proved optimal for the continuous production of CO2 hydrate.20,21 The second one varied in design to inject methane gas through multiple nozzles. The smallest diameter of the nozzles at the point of injection was 61.5 µm. Methane hydrate composite particles of 25.4-mm diameter and about 30 to 40 mm long were continuously ejected from the reactor mixing zone during operation with both distributors, as shown in Figure 3. The original design for the CJHR and the variation of design implemented for work with methane gas successfully produced methane hydrate in a continuous manner. In contrast with the large liquid CO2/hydrate/water composite particles obtained with the CJHR,20,21 the particles obtained in this case are basically methane hydrate. Unconverted methane coalesced into large bubbles and was ejected as a side-product

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Figure 3. Close-up of methane hydrate particle being extruded from the multiphase, microemulsion reactor mixing zone.

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instructed in the booster pump’s manual, it was assumed that the highest uncertainties in the experiment come from this operation. The methane conversion vs Re curves show an apparent plateau in the conversion of methane gas into hydrate for both distributors, indicating that the maximum conversion can only be marginally increased with increasing water velocity. In fact, the conversion achieved at the highest Re is only marginally higher than the one at the preceding Re value. This feature is a possible indication that the overall hydrate formation rate is no longer determined by the interfacial area. Either the limitations of the distributor design, in terms of producing fine methanebubble dispersions, have been met or the controlling step or resistance to hydrate formation has changed from an external mass transfer resistance to an internal mass transfer resistance. Finally, it should be noted that the methane conversion into hydrate as calculated in this work assumes that the methane that cannot be accounted for in the liquid and gaseous phases is located in the hydrate structure. Hydrate formation takes place instantaneously at the interface between the fine methane bubbles and the continuous aqueous phase, forming a thin layer of hydrate surrounding the bubbles, which thickens at decreasing velocity with increasing mass transfer resistance.8,9,11,13,20 Therefore, the conversion reported in this work is an apparent conversion, which includes methane stored within the hydrate structure itself (i.e., one molecule of methane inside each hydrate cage) and methane trapped in between hydrate crystals as gas bubbles. These methane gas bubbles are not part of the methanehydrate structure. 4. Conclusion

Figure 4. Methane gas conversion into hydrate (Xmethane) as a function of Re number for water flow.

from the reactor mixing zone. The size of unconverted-methane bubbles decreased significantly with increasing water velocity; and, in general, less unconverted methane was observed with the multinozzle distributor (the new design). This is an indication that finer dispersions were achieved with increasing water velocity and when using the new distributor, leading to higher conversions of methane into hydrate. Figure 4 presents the behavior of methane gas conversion into hydrate as a function of Reynolds (Re) number for water injected into the reactor as calculated at the injection point. As expected from the visual observations, methane conversion into hydrate increases with increasing water flow rate and with the utilization of the multinozzle distributor. Additionally, the replacement of multiple capillaries by nozzles in the distribution disks has a greater impact on methane conversion into hydrate at larger water flow rates. One should note that the improvements in methane conversion with the utilization of the multinozzle distributor fall within the range of experimental uncertainty predicted for the measurements. The experimental uncertainties were predicted by propagation of a 5% uncertainty in the value of the gas flow delivered by the booster pump. Since the gas flow delivered by the booster pump was kept constant by continuously regulating the pressure of the gas entering the pump, and the air-drive pressure of the booster, as

A modification in design of the distributor for the dispersed phase in the CJHR resulted in an improved multiphase, microemulsion reactor for the continuous production of methane hydrate. Preliminary results discussed in this work indicate that conversions as high as 70% of methane stored in the hydrate phase can be achieved with the current restrictions of operation imposed by our experimental facilities. Higher conversions may be achieved after proper optimization of operating conditions for the reactor is pursued. The capability to continuously produce hydrate from water and a guest gas mixture in gaseous phase is not only important for methane gas transport and storage. Utilization of gas hydrates for gas separations, including CO2 capture from flue gas and CH4/CO2 and CO2/H2 separations, is in principle possible based on thermodynamic equilibrium grounds.8 Gas hydrate formation is already being explored as a possible separation method for HCF-134a from gas mixtures.25 Acknowledgment Funding for this work was provided by the Department of Energy, National Energy and Technology Laboratory, to Oak Ridge National Laboratory under Contract No. DE-AC0500OR22725 with UT-Battelle, LLC, and to Georgia Institute of Technology under Contract No. DE-FC26-06NT42963. Literature Cited (1) Maclean, H. L.; Lave, L. B. Environmental implications of alternative-fueled automobiles: air quality and greenhouse gas tradeoffs. EnViron. Sci. Technol. 2000, 34, 225. (2) Xia, S.; Ma, P.; Guo, Y.; Hua, C. Determination and study of the solubility of methane in mixtures of methanol plus various hydrocarbons at high pressures. J. Chem. Eng. Data 2006, 51, 1035.

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(16) West, O. R.; Tsouris, C.; Liang, L. Method and apparatus for efficient injection of CO2 in oceans. United States Patent No. 6,598,407, 2001. (17) West, O. R.; Tsouris, C.; Lee, S.; McCallum, S. D.; Liang, L. Negatively buoyant CO2-hydrate composite for ocean carbon sequestration. AIChE J. 2003, 49, 283. (18) Lee, S.; Liang, L.; Riestenberg, D.; West, O. R.; Tsouris, C.; Adams, E. CO2-hydrate composite for ocean carbon sequestration. EnViron. Sci. Technol. 2003, 37, 3701. (19) Tsouris, C.; Brewer, P.; Peltzer, E.; Walz, P.; Riestenberg, D.; Liang, L.; West, O. R. Hydrate composite particles for ocean carbon sequestration: field verification. EnViron. Sci. Technol. 2004, 38, 2470. (20) Szymcek, P.; McCallum, S. D.; Taboada-Serrano, P.; Tsouris, C. A pilot-scale continuous jet hydrate reactor. Chem. Eng. J. 2008, 135, 75. (21) Tsouris, C.; Szymcek, P.; Taboada-Serrano, P.; McCallum, S. D.; Adams, E.; Chow, A.; Brewer, P.; Peltzer, E.; Walz, P.; Johnson, W. K.; Summers, J. Scaled-up Ocean Injection of CO2-hydrate Composite Particles. Energy Fuels 2007, 21, 3300. (22) Tang, L.-G.; Li, X.-S.; Feng, Z.-P.; Lin, Y.-L.; Fan, S.-S. Natural gas hydrate formation in an ejector loop reactor: preliminary study. Ind. Eng. Chem. Res. 2006, 45, 7934. (23) Riestenberg, D.; Chiu, E.; Gborigi, M.; Liang, L.; West, O. R.; Tsouris, C. Investigation of jet breakup and droplet size distribution of liquid CO2 and H2O systems—implications for CO2 hydrate formation for ocean carbon sequestration. Am. Mineral. 2004, 89, 1240. (24) Phelps, T. J.; Peters, J.; Marshall, O. R.; West, L.; Blencoe, J. G.; Alexiades, V.; Jacobs, G. K. A new experimental facility for investigating the formation and properties of gas hydrates under simulated seafloor conditions. ReV. Sci. Instrum. 2001, 72, 1514. (25) Seo, Y.; Tahima, H.; Yamasaki, A.; Takeya, S.; Ebinuma, T.; Kiyono, F. A new method for separating HFC-134a from gas mixtures using clathrate hydrate formation. EnViron. Sci. Technol. 2004, 38, 4635.

ReceiVed for reView December 17, 2008 ReVised manuscript receiVed May 19, 2009 Accepted May 22, 2009 IE8019517