Scaled-Up Ocean Injection of CO2âHydrate Composite Particles

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Energy & Fuels 2007, 21, 3300–3309

Scaled-Up Ocean Injection of CO2–Hydrate Composite Particles† C. Tsouris,* P. Szymcek, P. Taboada-Serrano, and S. D. McCallum Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennesee 37831

P. Brewer,* E. Peltzer, and P. Walz Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, California 95039

E. Adams* and A. Chow Massachusetts Institute of Technology, 77 Massachusetts AVenue, Cambridge, Massachusetts 02139

W. K. Johnson Institute of Ocean Sciences of Fisheries and Oceans, Canada, 9860 West Saanich Road, P.O. Box 6000, Sidney, British Columbia V8L 4B2, Canada

J. Summers U.S. Department of Energy, 1000 Independence AVenue, SW, Washington, District of Columbia 20585 ReceiVed April 20, 2007. ReVised Manuscript ReceiVed July 10, 2007

A pilot-scale, three-phase continuous-jet hydrate reactor, developed to produce CO2 hydrate for ocean sequestration, was tested both in the laboratory and at sea. A 72-L pressure vessel was used for laboratory tests; field experiments were performed with a remotely operated vehicle at depths between 1200 and 2000 m off the coast of Monterey, CA. Rapid production of a consolidated sinking CO2–hydrate composite paste was achieved in both settings. The vertical and lateral movement of the extruded hydrate was monitored by the high-definition television camera mounted on the vehicle and with a 675-kHz scanning sonar, along with dissolution rates and associated temperature and pH changes during the injection operations. It was observed that globules of unconverted liquid CO2 occluded in the structure of the hydrate composite largely determine the hydrate composite behavior in the ocean by providing sites for accelerated dissolution, thereby affecting the CO2–hydrate particle orientation, shape, lifetime, and sinking rate. Model calculations predict that largescale releases of these particles (at a CO2 injection rate of ∼100 kg/s) should show a descent depth of nearly 1000 m below their release point, as a result of plume dynamics and the increase in density caused by the CO2 dissolution into the surrounding ocean water.

Introduction Global atmospheric emissions of CO2 are predicted to increase from 7.4 Gt carbon (Gt C or 27 Gt CO2) per year in 1997 to 26 Gt C (95 Gt CO2) per year by 2100;1 approximately 30% of this emission is quickly transferred to the surface ocean by gas exchange; over a period of centuries, some 85% will be transferred to the ocean waters. Direct ocean sequestration of * Corresponding authors: Costas Tsouris, e-mail [email protected], phone 865-241-3246(reactor scale-up, laboratory experiments); Peter Brewer, e-mail [email protected], phone 831-775-1706(field experiments); Eric Adams, e-mail [email protected], phone 617-253-6595(plume simulations). † Disclaimer: This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a nonexclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. (1) IntergoVernmental Panel on Climate Change Special Report on Emissions Scenarios; Cambridge University Press: Cambridge, England, 2000.

CO2 is a possible means to counteract the increase in atmospheric CO2 via accelerating this process. This technique was first suggested over 25 years ago,2 and carbon capture and storage science and technology have recently been reviewed in a Special Report of the Intergovernmental Panel on Climate Change.3 Estimates show that over 300 Gt C could theoretically be stored in the deep ocean without increasing the pH by more than 0.18 units, which is comparable to levels of observed natural variability.3 While indirect ocean CO2 sequestration proceeds apace on a vast scale, questions of acceptability of direct injection under the provisions of the London Convention remain.4 Proposed methods for direct sequestration include liquid CO2 injection at intermediate depths to produce a rising (2) Marchetti, C. On engineering the CO2 problem. Climate Change 1977, 1, 59–68. (3) IntergoVernmental Panel on Climate Change Special Report on Carbon Dioxide Capture and Storage; Cambridge University Press: Cambridge, England, 2005, p 431. (4) Kildow, J. Testing the waters: an analytical framework for testing the political feasibility of scenario-based proposal for disposing of CO2 in the oceans. Energy ConVers. Manage. 1997, 38, S273–S277.

10.1021/ef070197h CCC: $37.00  2007 American Chemical Society Published on Web 10/02/2007

Scaled-Up Ocean Injection of CO2–Hydrate

Energy & Fuels, Vol. 21, No. 6, 2007 3301

Figure 1. (a) Schematic of the pilot-scale CJHR reactor used in preliminary laboratory experiments and field experiments, (b) micrograph of hydrate particles exiting the mixing zone of the CJHR reactor during laboratory experiments, and (c) photograph of the experimental setup for the field experiments shown as mounted on the ROV Tiburon.

and gradually dissolving plume of CO2 droplets;5 sequestration of solid CO2 hydrate to produce a plume of sinking, gradually dissolving particles;6 injection of sinking liquid CO2 at depths greater than 3000 m to form a liquid–CO2 lake at the bottom of the ocean;5 and injection of solid CO2 in the form of dry ice.5 Several aspects have to be taken into consideration when examining an ocean sequestration alternative. The first concerns the environmental impacts of the selected injection method. Solid CO2–hydrate particles sink to greater depths after formation7,8 or to zones of pressure and temperature of increased hydrate-phase thermodynamic stability.9,10 Therefore, sinking hydrate particles present the advantage of slow dissolution rates and less impact to shallow-depth marine environments that are rich in life-forms. Furthermore, the fact that CO2 hydrate can form sinking plumes may ensure longer residence times for CO2 in the ocean. The second aspect to be taken into consideration is the operational costs involved in the injection process. Infrastructure and implementation costs of most injection methods increase with injection depth, while residence times for sequestered CO2 increase with increasing depth.11 (5) Fujioka, Y.; Ozaki, M.; Takeuchi, K.; Shindo, Y.; Herzog, H. J. Cost comparison of various CO2 ocean disposal options. Energy ConVers. Manage. 1997, 38, S273–S277. (6) Steinberg, M.; Cheng, H. C.; Horn, F. A system study for the remoVal, recoVery and disposal of CO2 from fossil fuel power plants; Brookhaven National Laboratory Report OE/CH/0016, 1984. (7) Lee, S. Y.; Liang, L.; Riestenberg, D.; West, O. R.; Tsouris, C.; Adams, E. CO2 hydrate composite for ocean carbon sequestration. EnViron. Sci. Technol. 2003, 37, 3701–3708. (8) Tsouris, C; Brewer, P. G.; Peltzer, E. T.; Waltz, P.; Riestenberg, D.; Liang, L. Hydrate composite particles for ocean sequestration: field verification. EnViron. Sci. Technol. 2004, 38, 1499–1509. (9) Van der Waals, J. H.; Platteeuw, J. C. Clathrate Solutions. AdV. Phys. Chem. 1959, 2, 1–57. (10) Sloan, E. D., Jr. Clathrate Hydrates of Natural Gases, 2nd ed., Marcel Dekker: New York, 1998. (11) Brewer, P. G.; Friederich, G.; Peltzer, E. T.; Orr, F. M., Jr. Direct experiments on the ocean disposal of fossil fuel CO2. Science 1999, 284, 943–945.

A laboratory-scale continuous-jet hydrate reactor (CJHR) was developed at Oak Ridge National Laboratory (ORNL) to produce hydrate.7,12 This reactor was initially designed to produce CO2 hydrate for marine carbon dioxide sequestration12 and was also tested in the field to verify its effectiveness.8,13 The laboratory-scale CJHR was able to achieve conversions of up to 30% in a laboratory setting7,12 and conversions of 45% during field-testing operations.8,13 Because conversions larger than 20% result in effective injections (i.e., sinking, cohesive CO2–hydrate particles) at injection depths greater than 1200 m, the laboratory version of the CJHR was quite successful. However, if the goal to capture approximately 175 Gt C over a period of 50 years is to be achieved,14 intensive and largecapacity CO2–hydrate injectors will be required. This work describes the field testing of a pilot-scale CJHR. The original laboratory-scale reactor design7,8,12,13 was modified to allow for at least a 10-fold increase in volumetric flow while maintaining high conversions. Different injector designs were initially tested in the laboratory for the reactor presented in Figure 1a, and the more promising ones were chosen for the field experiments detailed in this work. Information from field experiments was introduced into a plume-modeling scheme to generate possible outcomes of largescale CO2–hydrate injection operations, where the scale is such that the local dissolved CO2 field is substantially modified. The modeling results are an important contribution to the assessment of the behavior of the plume, the effective injection depth, the (12) West, O. R.; Tsouris, C.; Lee, S.; McCallum, S. Negatively buoyant CO2-hydrate composite for ocean carbon sequestration. AIChE J. 2003, 29, 276–285. (13) Riestenberg, D.; Tsouris, C.; Brewer, P.; Peltzer, E.; Walz, P.; Chow, A.; Adams, E. E. Field studies on the formation of sinking CO2 particles for ocean carbon sequestration: effects of injector geometry on particle density and dissolution rate and model simulation of plume behavior. EnViron. Sci. Technol. 2005, 39, 7287–7293. (14) Pacala, S.; Socolow, R. Stabilization wedges: solving the climate problem for the next 50 years with current technologies. Science 2004, 305, 968–972.

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residence time of CO2 in the ocean, and, ultimately, the environmental impacts. Continuous-Jet Hydrate Reactor The CJHR used in this study constitutes a scaled-up version of the original reactor developed at ORNL.7,8,12,13 The laboratory-scale reactor was designed to produce sinking and cohesive CO2–hydrate particles to ensure successful ocean carbon sequestration. The particles produced by the CJHR were heterogeneous, cohesive aggregates composed of CO2–hydrate particles and unconverted liquid CO2. The characteristics of the hydrate particles produced had to be maintained during the scale-up process. The density of the hydrate particles relies heavily upon the conversion of CO2 into hydrate. The particles will sink when approximately 25% of the injected CO2 is converted into hydrate at depths greater than 1000 m. The formation of CO2 hydrate constitutes a product-limited reaction.15,16 Hydrate formation occurs nearly instantaneously when H2O and liquid CO2 interact at favorable temperatures and pressures in natural seawater.7,12,15,16 This reaction forms a solid layer of CO2 hydrate at the interface of the two species. Essentially, the solid hydrate limits further formation of hydrate, and hence, this process is controlled by mass transfer through the surface area along which the two species interact. Therefore, the potential for hydrate formation is limited by a surface barrier to mass transfer that prevents the interaction between the two reactants. In addition to mass transfer effects, thermal effects are important during hydrate formation. Hydrate formation constitutes an exothermic reaction,10 with rejection of solutes in a manner analogous to freezing. If heat is not effectively dissipated by the system, a considerable localized temperature increase may occur upon reaction at the interface between reactants. A significant increase in temperature may hinder further hydrate formation because the system could move away from the thermodynamically favorable conditions for hydrate formation (i.e., conditions of pressure and temperature inside the hydrate-phase stability zone). The combination of mass transfer barriers with slow dissipation of reaction heat decreases the final conversion achieved during hydrate production. Mass transfer barriers can be reduced via increasing the surface area of interaction between reactants. Larger interfacial areas also increase heat transfer from the interface into the bulk of the reactants, where the excess heat generated by the reaction can be absorbed. Therefore, the pilot-scale CJHR had to be designed to increase surface area (i.e., increase the amount of H2O and CO2 converted into hydrate) while working with larger flow rates of reactants, approximately a 10-fold increase in hydrate production over rates reported earlier.7,8,12,13 Maximizing surface area can be achieved by dispersing one of the hydrate-forming species into the continuous flow of the other hydrate-forming species (i.e., dispersing CO2 in H2O or vice versa). Ideally, the dispersed phase should be sprayed into the continuous phase with the smallest droplet size possible. It is desirable for the dispersion of one reactant into the other to be performed under spray-mode conditions.17 At these hydro(15) Shindo, Y.; Sakaki, K.; Fujioka, Y.; Komiyama, H. Kinetics of the formation of CO2 hydrate on the surface of liquid CO2 droplet in H2O. Energy ConVers. Manage. 1996, 37, 485–489. (16) Shindo, Y.; Lund, P. C.; Fujioka, Y.; Komiyama, H. Kinetics of formation of CO2 hydrate. Energy ConVers. Manage. 1993, 34, 1073–1079. (17) 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, 1240–1246.

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dynamic conditions, the dispersed phase will be present in very fine droplets—increasing interfacial area, reducing mass transfer barriers for the reaction, and maintaining large conversions, even at high flow rates. Furthermore, vigorous mixing will help disperse local regions of high salt concentration caused by brine rejection during hydrate formation, thus preventing the inhibiting effects of high salt concentrations on hydrate formation. The pilot-scale CJHR is a tubular reactor that consists of a 13.3-cm headpiece with interchangeable inputs for liquid CO2 and H2O and a 45.7-cm-long Teflon tube that acts as the mixing zone. The dispersed-phase species flows through a single inlet on the top of the headpiece into an exchangeable, distributing, disc-shaped injector of 1.27-cm length and varying designs (Figure 1a). The reactant that constitutes the continuous phase flows into the reactor through an inlet directly below the disc injector. The two phases mix below the injector to form hydrate, which becomes consolidated in the mixing zone due to hydrate forming at the boundaries of the droplets (Figure 1b). The reactor design allows for the testing of different injection modes and different choices of dispersed phase. Capillaries embedded in the disc injector were chosen to distribute the flow of the dispersed reactant into the continuous one. Capillary sizes and configurations were tested in terms of their ability to produce a spray jet break-up regime at high flow rates and low backpressures. The chosen injector design includes an array of capillaries, six of them in the periphery and one in the center of the distributing disc. The smaller diameters of multiple capillaries with total cross-sectional areas similar to that of a single-capillary injector (i.e., the original design of the laboratory-scale injector) allow for a spray-mode jet break-up regime while sustaining a high flow rate. Experimental Methods During the preliminary laboratory tests, the CJHR was mounted inside the Seafloor Process Simulator (SPS), which is used to simulate CO2–hydrate injection in marine environments.18 The SPS is a cylindrical Hastelloy C-22 vessel of 31.75-cm diameter, 91.44-cm length, and 72-L volume.18 The vessel is equipped with several sapphire windows and sampling ports and can be maintained at a pressure of up to 20 MPa. The SPS allows operation pressures equivalent to those encountered at different ocean depths to be maintained during laboratory experiments. The entire reactor was submerged and at equilibrium with the vessel. A Seabird SBE ST centrifugal pump (Seabird Electronics, Bellevue, WA) was used to circulate water in the SPS through the CJHR at a controlled flow rate into where it was mixed with liquid CO2 injected from outside the SPS by a Haskel ALG15/30 pulsed-flow pump (Haskel, Burbank, CA). The laboratory tests reported in this work were performed with flow rates between 2.00 and 3.00 L/min of H2O and 0.40 and 0.66 L/min of CO2. The recirculation of H2O and the external introduction of CO2 simulated injection operations in the field. Experiments were conducted with both distilled and simulated ocean water. The salinity of simulated ocean water was kept at 35 ppt using Instant Ocean (Aquarium Systems, Mentor, OH). The SPS was configured with a pressure transducer and thermocouples to measure pressure and H2O and headspace temperatures within the SPS. Lab View software was used to monitor and record measured internal pressure and temperature conditions. The SPS was filled with approximately 60 L of distilled or saline water, and nitrogen was used to pressurize the vessel. Gas was periodically vented from the SPS during experiments to maintain (18) Phelps, T. J.; Peters, J.; Marshall, S.; West, O. R.; 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, 1514–1521.



Table 1. Jet Break-up Regimes in Terms of We Numbers for the Preliminary Laboratory Experiments and the Field Experiments Discussed in this Work experiment Lab Lab Lab Lab Lab Lab Lab Lab

1 2 3 4 5 6 7 8

T0967-2 T0967-3 T0967-4 T0969-1 T0969-2 T0970-3 T0970-4 T0970-6 a

capillary diameter (mm)

disp. phase (L/min)

cont. phase (L/min)

H2O/CO2

We

CO2 CO2 CO2 CO2 H2O H2 O H2 O H2 O

1.191 1.191 0.794 0.794 0.794 0.794 1.191 1.191

5.50a 5.50a 5.50a 5.50a 3.00 2.00 3.00 2.00

2.00 2.00 3.00 3.00 0.66 0.66 0.66 0.66

3.0 3.0 4.5 4.5 4.5 3.0 4.5 3.0

7500 7500 25000 25000 8000 3400 2300 1000

CO2 CO2 CO2 seawater seawater seawater seawater seawater

1.191 1.191 1.191 0.794 0.794 0.794 0.794 0.794

3.00 2.50 2.50 8.00 8.00 3.50 3.75 3.50

7.50 7.50 7.50 1.70 1.70 1.50 1.50 1.50

2.5 3.0 3.0 4.7 4.7 2.3 2.5 2.3

2200 1500 1500 55000 55000 11000 12000 11000

disp. phase

Flow rate corrected to account for the pulsed flow obtained from the CO2 pump used in laboratory experiments.

the pressure at a preset value, an approach that caused the pressure to vary within a range of 0.50 MPa during experiments. The experiments were visually recorded with a Sony Firewire (XCD-X710CR) camera connected to a personal computer. Field experiments were conducted more than 50 miles off the shore of Monterey, CA, using a battery of four CJHRs mounted in parallel on the remotely operated vehicle (ROV) Tiburon, as shown in Figure 1c. However, due to pumping restrictions, only one CJHR was in operation at a given time. Experiments were monitored aboard the research vessel (RV) Western Flyer. A centrifugal pump (model 11.400, KC Denmark A/S, Silkeborg, Denmark) was used to circulate ocean water into the operational CJHR. The flow of CO2 was regulated with two hydraulic pumps connected to a 56-L piston accumulator, which was filled with liquid CO2 prior to the ROV dive.19,20 Two injector designs with multiple capillaries of diameters equal to 0.794 and 1.191 mm were selected for the field experiments, based on their ability to produce consolidated sinking hydrate particles at pressures as low as those encountered at depths of 1000 m. The injectors could also work with high flow rates while maintaining low backpressures and not producing clogging problems. Experiments were conducted with either H2O or liquid CO2 as the dispersed phase. Flow rates for H2O ranged from 3.50 to 8.00 L/min, while those for liquid CO2 ranged from 1.50 to 3.00 L/min. Experiments were conducted at depths ranging from 1200 to 2000 m below the surface. Salinity and temperature changed depending upon ambient ocean conditions and were monitored using sensors installed on the ROV Tiburon. Field experiments focused on monitoring the fate of individual hydrate particles, which appeared as elongated extruded rods with a stiff paste-like composition. After the formation of hydrate composites, a single particle was arbitrarily selected, and its vertical movement was monitored by the primary high-definition television camera system installed on the ROV Tiburon21. The selected particle was often followed until it became visually indistinguishable from the surrounding water. The lateral movement of all the particles produced (i.e., the development of a plume) was monitored by a 675-kHz scanning sonar system installed on (19) Peltzer, E. T.; Brewer, P. G.; Nakayama, N.; Walz, P.; Aya, I.; Kojima, R.; Yamane, K.; Nakajima, Y.; Haugan, P.; Hove, J.; Johannessen, T. Initial results from a 4 km CO2 release experiment. Preprints ACS DiV. Fuel Chem. 2004, 49, 429–430. (20) Brewer, P. G.; Peltzer, E. T.; Walz, P.; Aya, I.; Yamane, K.; Kojima, R.; Nakajima, Y.; Nakayama, N.; Haugan, P.; Johannessen, T. Deep ocean experiments with fossil fuel carbon dioxide: creation and sensing of a controlled plume at 4 km depth. J. Mar. Res. 2005, 63, 9–33. (21) Brewer, P. G.; Peltzer, E. T.; Friedrich, G.; Rehder, E. Experimental determination of the fate of rising CO2 droplets in seawater. EnViron. Sci. Technol. 2002, 36, 5441–5446.

Tiburon. The pH of the water in the immediate vicinity of the CJHR was monitored by two pH probes, model SBE 18 (Seabird Electronics, Bellevue, WA), installed on the ROV Tiburon.

Results and Discussion Jet Break-up Regimes for Laboratory and Field Experiments. Laboratory tests of the pilot-scale CJHR were conducted in order to choose injector designs suitable for the field experiments. These tests were also performed to determine optimum operating conditions for the CJHR. Temperature and pressure conditions were selected to reflect those of ambient temperatures encountered in the field work. Dispersed phases (CO2 or H2O), flow rates, and capillary sizes and configurations were selected to yield spray-mode flow regimes. Spray-mode conditions prevail for Weber numbers (We) greater than 324 during dispersion of liquid CO2 in H2O and vice versa.17 However, during hydrate formation, better results are obtained for We numbers greater than 1000. The first part of Table 1 summarizes the results obtained in terms of jet break-up regimes for the two multiple-capillary injectors chosen for the field experiments. Jet break-up regimes are characterized by the We number, which was calculated using the nozzle velocity of the dispersed phase. The laboratory experiments listed in Table 1, which correspond to field conditions, resulted in jet break-up regimes in spray mode (We > 324) in all cases. Furthermore, all but one laboratory experiment achieved We numbers greater than 1000. The utilization of CO2 as the dispersed phase resulted in jet breakup regimes in spray mode with high associated We numbers (We ≈ 7500 and We ≈ 25 000 for the two H2O/CO2 ratios examined). On the other hand, the utilization of H2O as the dispersed phase resulted in lower We numbers (between 1000 and 8000) for the two injectors selected and the different H2O/ CO2 ratios. Figure 2 presents temperature profiles (i.e., temperature in different parts of the CJHR vs injection time) for laboratory experiments at the conditions of the experiments Lab 1 to Lab 4 listed in Table 1. Although the bulk temperature and the temperature at the injector itself (exchangeable distributing disc) remain close to the baseline ambient temperature, the temperature in the mixing zone and at the outlet of the CJHR (outlet of Teflon tube) experience an increase of 4–6 °C in some cases. The more-marked temperature increase occurs in the mixing zone, where the reaction is led to near completion. The increase


Figure 2. Temperature profiles (i.e., temperature vs time of injection) for laboratory experiments (a) at P ) 13.8 MPa and (b) at P ) 11.8 MPa.

in temperature at higher pressures (e.g., P ) 13.8 MPa) does not drive the system to thermodynamically unfavorable conditions for hydrate formation. However, as the pressure decreases, that is, at shallower injection depths, a similar temperature increase may drive the system to conditions of pressure and temperature closer to the equilibrium line of hydrate stability and thus have detrimental effects on hydrate formation. One should note that the magnitude of temperature increase during the reaction is not affected by pressure and that this increase can be controlled by the degree of dispersion or We number achieved during operation of the reactor. The reduction of mass transfer barriers and the thermal effects achieved with better dispersion of one reactant into the other (i.e., higher We numbers) could be directly observed in the degree of consolidation and buoyancy of the hydrate product. In general, during laboratory experiments, the utilization of CO2 as the dispersed phase yielded sinking CO2–hydrate particles at all the conditions reported above, with the degree of consolidation increasing with increasing We number. Experiments with H2O dispersed in CO2 also produced sinking hydrate particles, but the degree of consolidation was less than that of the particles obtained with CO2 dispersed in H2O, a find that corresponds to the lower We numbers achieved at these conditions. Additionally, smaller capillary sizes resulted in moreconsolidated hydrates. Based on the analysis of laboratory results, the 1.191-mm multiple-capillary injector was selected for experiments in the field using CO2 as the dispersed phase,

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and the 0.794-mm multiple-capillary injector was selected for experiments in the field with H2O as the dispersed phase. Additionally, a minimum value of 2.5 for the H2O/CO2 ratio was recommended for the field experiments at P ) 11.8 MPa (equivalent to an approximate depth of 1200 m); the ratio for complete hydrate formation corresponds to 2.36 volumes of H2O per volume of CO2. The selected limiting ratio aims to prevent the system from moving into a regime of water limitation. The second part of Table 1 displays the jet break-up regimes observed during field experiments in terms of values of the We number. During field experiments, higher We numbers were obtained when H2O was used as the dispersed phase than when CO2 was used. When H2O was used as the dispersed phase, We numbers between 10 000 and 55 000 were obtained in the field, a behavior that was expected because higher H2O flow rates were used in the field than in the laboratory for comparable capillary diameters. On the other hand, when CO2 constituted the dispersed phase, We numbers between 1500 to 2200 were obtained, despite the fact that higher flow rates of CO2 were used in the field than during laboratory experiments for comparable capillary diameters. The reason behind the lower We numbers in the field with CO2 as the dispersed phase stems from the fact that the actual CO2 flow during laboratory experiments is higher at the nozzles due to the pulsed-flow characteristics of the pump used. The calculated CO2 flow rate during individual pulses in laboratory experiments was equivalent to 5.5 L/min, which leads to We numbers of 7500 and 25 000 for the 1.191- and 0.794-mm multiple-capillary injectors, respectively. The continuous flow of CO2 from the piston accumulator used in the field experiments was equal to either 2.5 or 3.0 L/min, which leads to We numbers of 1500 and 2200, respectively. Since the jet break-up regime achieved during injection operations is directly related to the characteristics of the hydrate product obtained, it was expected that more-cohesive sinking CO2 hydrates would be obtained with H2O as the dispersed phase during field experiments. Higher conversions were anticipated in this case as well, because higher We numbers or more-pronounced spray-mode regimes would result in smaller barriers to mass and heat transfer for the hydrate formation reaction. In fact, hydrate composite particles with water as the dispersed phase prevailed for longer periods of time while settling: that is, they appeared to dissolve more slowly during field experiments. It may seem contradictory that laboratory and field experiments yield higher-quality hydrate composites for different reactants acting as the dispersed phase. However, it is the quality of the mixing and dispersion achieved during injection (i.e., the size of the droplets of the dispersed phase) that ultimately dictates the quality of the hydrate product. Due to the equipment available for the experiments, finer dispersions were obtained with CO2 as the dispersed phase in the laboratory and water as the dispersed phase in the field. Vertical Movement of CO2–Hydrate Composite Particles. The extruded hydrate emerged as a tubular paste, which broke up into curved rod-like segments, each of which was several tens of centimeters long. For the sake of simplicity, we will refer to these segments as particles. The vertical movement of selected particles from each batch of experiments was visually recorded and is presented in this work as the vertical position of the particle vs time after injection (i.e., depth-vs-time profiles or vertical movement profiles). The particles dissolved as they moved downwards or upwards, and they were followed for up to about 40 min. Experiments



Figure 4. Depth-vs-time profiles (vertical movement) of CO2–hydrate particles produced during experiments T0969, with H2O as the dispersed phase.

Figure 3. Depth-vs-time profiles (vertical movement) of CO2–hydrate particles produced during experiments T0967, with CO2 as the dispersed phase.

performed with different injections and using different species as the dispersed phase are listed with a code (e.g., T0967), which is related to the RV logging system. An additional number is used to identify different conditions of flow rates and injection depths within experiments performed with the same injector and the same fluid as the dispersed phase. Figure 3 presents the vertical movement profiles of CO2–hydrate particles obtained using CO2 as the dispersed phase and the 1.191-mm multiple-capillary injector. The selected CO2–hydrate particle produced at an injection depth of 1750 m was initially neutrally buoyant, sinking to greater depths after 4–5 min. The CO2–hydrate particles produced at injection depths of 2000 m sank almost immediately, though a small period of neutral buoyancy (less than 1 min) was observed in both cases. The “lag” period, or the time interval at which the recently produced particles were neutrally buoyant, can be associated with the amount of unconverted liquid CO2 trapped inside the composite particle between individual hydrate particles. Although CO2 hydrate is denser than ocean water by approximately 10%, liquid CO2 becomes denser than seawater only at depths greater than 3000 m. Therefore, the amount of unconverted liquid CO2 trapped inside the CO2–hydrate particles makes them either buoyant or neutrally buoyant when the hydrate aggregates are initially synthesized. However, the dissolution of liquid CO2 is faster than that of solid hydrate. Therefore, the remaining solid-hydrate-particle backbone sinks as it slowly dissolves, once sufficient liquid CO2 trapped in the hydrate particles has been dissolved. The amount of unconverted liquid CO2 affects not only the relative density of the hydrate composite particles but also their shape. More-malleable composite particles result from higher amounts of occluded liquid CO2 in the particle structure. These particles acquire long helicoidal shapes during injection operations. The shape of the particles and their orientation affect their settling velocity, resulting in the noncontinuous, sometimes nonlinear vertical movement profiles observed in Figure 3.

Figure 5. Depth-vs-time profiles of CO2–hydrate particles produced during experiments T0970, with H2O as the dispersed phase.

Figure 4 presents the vertical movement profiles of particles produced in experiments T0969, in which seawater was used as the dispersed phase at high H2O/CO2 ratios. Cohesive sinking CO2–hydrate particles were produced at lower injection depths than in the case of CO2 as dispersed phase, which indicates a higher conversion. This conclusion is further sustained by two observations. First, the particles traveled longer vertical distances in comparable periods of time than did those produced with CO2 as the dispersed phase. Second, the particles were brittle and broke into short cylinders upon formation (i.e., they contained less unconverted liquid CO2 occluded in their structures). Although particles T0969-1 and T0969-2 were produced at identical conditions and at the same injection depth, one presents larger average sinking velocities than the other. In fact, particle T0969-2 seems to present three different zones in terms of sinking velocities (i.e., different slopes of the depthvs-time profiles). In this case, the shape that individual particles acquired as they dissolved (the aggregates did not dissolve uniformly) determined their orientation in the liquid column and, ultimately, the behavior of the vertical movement profiles.


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Table 2. Calculated Sinking Velocities, Densities, and Hydrate Conversions for Particles from the Field Injections particle from injection

injection depth (m)

Fc (kg/m3)

Fw (kg/m3)

Fh (kg/m3)

average sinking velocity (cm/s)

density (kg/m3)

hydrate conversion, Xh (%)

T0967-4 T0967-3 T0967-2 T0969-1 T0969-2 T0970-6 T0970-4 T0970-3

1750 2000 2000 1500 1500 1200 1500 1600

1002 1014 1014 988 988 972 988 994

1036 1037 1037 1035 1035 1033 1035 1035

1143 1143 1143 1143 1143 1143 1143 1143

5.3 2.9 5.9 2.2 5.5 -0.3 3.5 5.2

1042 1039 1045 1036 1042 1033 1038 1041

16 9 16 15 24 16 16 17

Figure 5 presents the depth profiles for the CO2–hydrate particles produced in experiments T0970, in which H2O was again used as the dispersed phase but with lower H2O/CO2 ratios. In this case, the hydrate particles were also brittle and cylindrical. The vertical movement profiles of particles T0970-4 and T0970-6 present an initial lag time due to occluded liquid CO2 dissolution, as discussed earlier. On the other hand, particles of T0970-3 sank immediately, suggesting that the fraction of occluded unconverted CO2 was insufficient to make the particles buoyant. As expected, the amount of unconverted liquid CO2 increased as injection depth decreased, due to the increasingly thermodynamically unfavorable conditions for hydrate formation found at shallower depths. As in previous cases, particles T0970-4 and T0970-6 sank readily after a critical amount of occluded CO2 was dissolved. In the cases presented in Figure 4, a slight nonlinearity of the vertical movement profiles is noted. The changes in settling velocity could result, in part, from tumbling and reorientation of the particles within the vertical column of water, which affects the drag coefficient, as well as from dissolution of the particles. The preferential orientations of hydrate particles result from local differences in density within the particles, which originate from the initial nonuniform distribution of occluded CO2 and from nonuniform dissolution rates of the solid hydrate particles. Some of the particles shown in Figure 5 have a sawtooth pattern on top, which might reflect the loss of occluded liquid CO2 from the top of these particles. It could also reflect mild eddying in the lee of the cylinder. However, the vertical movement profiles of particles produced with H2O as the dispersed phase present a more linear behavior than comparable particles produced with CO2 as the dispersed phase. This phenomenon occurs because better mixing conditions in the CJHR reactor lead to higher conversion of liquid CO2 to hydrate. In this case, the mixing conditions are, in general, better (e.g., higher We numbers) because of higher flow velocities of the jets through the capillaries of the distributor. On the other hand, for the laboratory experiments in which a pulsed-flow pump was used for liquid CO2 with CO2 as the dispersed phase, results were better because of the lower viscosity of liquid CO2 and because the jet velocity was kept high during each pulse. The average sinking velocities of particles shown in Figures 3–5 are presented in Table 2 and range from approximately zero (neutrally buoyant) to about 6 cm/s, which is consistent with average sinking velocities observed in previous field experiments.13 Using the previously employed drag formulation appropriate for freely falling cylinders,13,22 we calculated the composite particle density from the densities of the three particle constituents (hydrate, unreacted liquid CO2, and seawater). Then, using this composite value, the reactor efficiencies were computed (Table 2). These efficiencies (from 9% to 24%), which are lower than those observed in previous field experiments,13

are attributed to the larger diameter of the current CJHR and the associated difficulties of effecting complete mixing. Although there is scatter in the data, the results for experimental series T0969 and T0970, in which H2O is the dispersed phase, appear to show higher conversion efficiencies than those for series T0967, in which CO2 is the dispersed phase. In our previous field experiments,13 sinking particles were confined within the transparent “bubble box” attached to the ROV. This box contained a scale from which it was possible to estimate shrinkage rates for particle diameters and, thus, dissolution rates. Because the present particles were generally larger and more numerous, it was not practical to confine them to the bubble box; thus, no direct measurement of shrinkage rate was possible. The shrinkage rate for several of the particles involving H2O as the dispersed phase was estimated by observing the time variation of the particle aspect ratio (diameter to length). Within the range observed previously, the inferred diameter shrinkage rates fell between about 2 × 10-6 and10-5 m/s. In conclusion, the use of H2O as the dispersed phase allows for the formation of plumes of solid hydrate composite particles of more uniform shape, higher relative density, almost constant settling velocities, and slower dissolution rates. This corresponds to the fact that higher We numbers can be achieved via the use of H2O as the dispersed phase, thus reducing detrimental mass transfer and thermal effects. Dispersion of CO2–Hydrate Particles. The dispersion of the hydrate particles formed during each injection was monitored via sonar. As an illustration of the kind of images obtained, a series of three sonar images from injection T0970-4 are presented in Figure 6. The center of each image coincides with the location of the ROV at a depth of 1499.3 m. These images were obtained before the ROV was moved to follow a selected particle as it sank in the water column. The hydrate particles drift away from the injection point and from one another as they sink. In this particular case, the particles spread out in the direction of gravity. As hydrate particles slowly dissolve, they become smaller and eventually undetectable by the sonar. Variations of pH during Injections. As shown in Figure 1c, the battery of CJHRs, along with pH probes and thermocouples, was confined in an acrylic box. The change in pH of the water surrounding the CJHRs was monitored during the injection operations. The pH electrodes used are calibrated at the surface, and an algorithm is used to correct the observed output to true pH. The changes in temperature and pressure, as well as effects on the liquid junction potential of the reference electrode can cause offsets at depth, which must be corrected for. Fortunately, via global survey data,23 the pH field of the deep ocean is well known at the level of precision required here, and the electrode values for open ocean water observed were corrected to a background of pH 7.76 by applying a simple offset value.

(22) Isaacs, J. L.; Thodos, G. The free-settling of solid cylindrical particles in the turbulent regime. Can. J. Chem. Eng. 1967, 45, 150–155.

(23) National Oceanographic Data Center, NODC Database OSD, http:// www.nodc.noaa.gov/.



Figure 6. Sonar images obtained during field injection T0970-4 at a depth of 1499.3 m. Scale ) 8 m per division.

Figure 8. A sinking particle plume in an ambient current of velocity ua. The dense particles induce negative plume buoyancy, but eventually separate from the plume at depth hs, as the plume becomes deflected by the ambient current.

Figure 7. Variation in pH vs injection time recorded during field injection T0970-4.

Figure 7 presents the evolution of pH as injection T0970-4 progresses. The initial “lag” time in pH changes coincides with the start-up period of the reactor. As soon as the reaction takes place, a drastic decrease of pH from 7.76 to 6.57 occurs at the injection point. Dissolution of unreacted liquid CO2 is primarily responsible for the sharp decrease in pH. As the reaction progresses and a considerable amount of hydrate particles populate the vicinity of the reactors within the confines of the box, an additional decrease in pH may occur due to the slow dissolution of the occluded liquid CO2 from the hydrate particles as they slowly drift away from the reactor. The spikes and variations of pH may respond to random movement of newly formed particles around the pH probes. When injection stops, the pH returns to the original value as the last particles formed drift away from the reactor. Finally, it should be noted that variations of pH no larger than 1.5 units occur at all times next to the outlet of the CJHR, where concentrations of unreacted liquid CO2 and newly formed particles are the highest. Lesser effects on the pH of the surrounding water are expected with increasing distance from the reactors and as the particles are dispersed and slowly dissolved. Plume Modeling. The laboratory and small-scale field experiments described above have all dealt with small, shortterm releases of CO2. Meanwhile a dense two-phase plume

model24,25 has been applied to the larger continuous release of similar particles discharged at similar depth into a quiescent water environment. Previous predictions for plumes composed of pure hydrate particles showed much greater sinking than did predictions for individual pure hydrate particles,24 suggesting that a similar effect might be achieved with lighter (but still negatively buoyant) composite particles. We examine this possible effect here. The reasons for the greater sinking in a continuous release are (1) the “plume effect”, whereby particles are transported downward at much higher speeds (plume velocity plus settling velocity) due to plume entrainment and (2) the “solute density effect”, whereby the density of entrained seawater in the plume is increased by the dissolution of CO2 from the hydrate. Laboratory experiments with buoyant bubble plumes show that an ambient current may ultimately cause the entrained ambient fluid within the plume to separate from the bubbles, leaving the bubbles to rise at approximately their terminal velocity.26 A sinking particle plume should behave similarly, only upside down (Figure 8). A criterion for the separation depth hs, is given by ref 26 as hs )

5.1B (uaus24)0.88

(1)

where ua is the ambient current speed, us is the particle slip velocity, and B is the kinematic buoyancy flux of the particles. (24) Wannamaker, E. J.; Adams, E. E. Modeling descending carbon dioxide injections in the ocean. Proceedings of the 6th International Conference on Greenhouse Gas Control Technologies, Kyoto, Japan; Pergamon: United Kingdom, 2002; pp 753–758. (25) Wannamaker, E. J.; Adams, E. E. Modeling descending carbon dioxide injections in the ocean. J. Hydraulic Res. 2006, 44, 324–337. (26) Socolofsky, S. A.; Adams, E. E. Multi-phase plumes in uniform and stratified crossflow. J. Hydraulic Res. 2002, 40, 661–672.


Tsouris et al.

Figure 9. Numerical prediction of particle descent as a function of mass loading and ambient current speed ua for plumes consisting of (a) particle T0970-4, (b) particles with the same composition as T0970-4 but with increased diameter of 5 cm, (c) particles with the same composition as T0970-4 but with diameter of 10 cm, and (d) particles with 50% hydrate conversion and 10-cm diameter. The following experimental conditions are valid for all four particles: length ) 30 cm; shrinkage rate ) 6 × 10-6 m/s; release depth ) 1500 m; seafloor at descent depth ) 3000 m.

The kinematic buoyancy flux B is given by N˘ Vg∆F/F, where N˘ is the number of particles released per unit time, V is the particle volume, g is gravity, and ∆F/F is the normalized excess density of the particles. In the presence of ambient stratification, the entrained seawater within the plume may peel and become trapped by the stratification (processes simulated by the quiescent plume model) before the current results in separation. Following Socolofsky and Adams,26 we assume that if the depth of the first peel, as predicted by the quiescent model, is less than the separation depth hs, then the quiescent plume calculations are valid. On the other hand, if the separation depth is less, the particles are assumed to sink individually following separation. Preliminary calculations presented in Adams et al.27 have been updated, assuming a particle slip velocity us of 3.5 cm/s (corresponding to a hydrate conversion efficiency of 16% for particle T0970-4), and a diameter shrinkage rate of 6 × 10-6 m/s. Figure 9 shows predicted depths of sinking for four sizes of composite particle plumes having these characteristics. The smallest particle in part a has a diameter of 2.2 cm and a length of 30 cm, which is typical of particles produced via injection T0970-4. To reflect the potential for scale-up to larger particles, parts b and c show the sinking of particles that have the same estimated hydrate conversion efficiency (and thus density) as part a but with larger particle diameters of 5 and 10 cm, respectively, representing a modest scale-up. Finally, part d of Figure 9 shows the sinking of a particle of 10-cm diameter with (27) Adams, E. E; Chow, A. C.; Brewer, P. G; Peltzer, E. T; Walz, P.; Tsouris, C.; McCallum, S. D.; Szymcek, P.; Summers, J. S.; Bergman, P.; Johnson, K. Direct injection of CO2 hydrate composite particles for ocean carbon storage: field experiments and plume modeling. Presented at the Greenhouse Gas Control Technologies Conference, Trondheim, Norway, June 2006.

an additional increase in hydrate conversion efficiency to 50% (to reflect future improvements in CJHR mixing). Calculations are made for CO2 injection rates of 0.01 to 1000 kg/s and ambient current speeds of ua ) 0, 0.01, 0.05, 0.2, and 0.4 m/s. Greatest sinking is clearly predicted for the largest particles at higher hydrate conversion rates, delivered at the largest CO2 mass loadings. For small mass flow rates, plume descent depths approach those of single particles (shown as dotted lines in Figure 9), approximately 100, 300, 640, and 2800 m for the four particles. (For these calculations, the seafloor is reached at a depth of 3000 m below the release point, which is 1500 m below the water surface.) To illustrate the effect of a plume, Figure 10 plots the predicted “plume effect”, the sinking depth for the plume divided by that for the individual particle. The greatest plume effect occurs with the smallest particles delivered at the highest CO2 injection rates. For example, with a release of 100 kg/s (e.g., from a 400-MW power plant28), the smallest particles are predicted to sink ∼9 times further as part of a plume, than as individual particles, whereas the largest particles will sink to the bottom of the seafloor regardless of whether a plume forms. As reactor size (and hence particle diameter) increases, the plume effect diminishes, as do the consequences of potential plume separation caused by an ambient current. For a coal-fired central power station with a discharge of CO2 of ∼100 kg/s (i.e., a 400-MW power plant28), model results suggest that significant sinking (>2000 m) can occur with the largest of the simulated particles (and even greater sinking is likely with still-larger particles). To achieve this mass delivery (28) Miller, P. J.; Van Atten, C. M. J. North American Power Plant Air Emissions; Commission of Environmental Cooperation of North America Press; Montreal, Canada, 2004.



Figure 10. “Plume effect” for the four particles described in Figure 9.

rate (3.5 orders of magnitude greater than that simulated in the field), one can continue to increase the size of the reactor and the number of inlets. Although there seems to be no practical limit on the reactor diameter or the number of inlet ports, a need exists to increase the conversion efficiency via better mixing. The largest scale-up factor can probably be achieved by using a bundle of reactors, either at the same depth or at different depths. Dividing the mass flow rate among multiple injection points is also a possibility. Conclusions A pilot-scale continuous-jet hydrate reactor (CJHR) was developed and tested for its application in carbon sequestration, both in the laboratory and in the field. Cohesive, sinking CO2–hydrate composite particles were produced in both settings. It was found that the degree of cohesiveness and buoyancy of hydrate particles is determined by the conversion achieved during formation, which can be increased by maximizing the interfacial area between reactants. Maximization of interfacial area in the CJHR was achieved via a fine dispersion of one reactant into the other in the spray-mode jet-break up regime as quantified by the We number. The higher the We number, the finer was the dispersion achieved inside the reactor. In the field experiments, variations of pH during hydrate formation and the fate of hydrate particles after injection of CO2 were investigated. Individual hydrate particles formed during injection operations sank to greater depths as they slowly dissolved. Although no plume formation was observed because of the relatively low flow rates, information on vertical and horizontal movement of individual hydrate particles was utilized

in a plume modeling scheme. Modeling predictions indicate that large-scale production of CO2–hydrate composite particles with the characteristics of the ones obtained during field experiments would show a descent of nearly 1000 m below the injection point due to plume dynamics. Biological impacts were not observed in this study. The present work involved teams conducting laboratory experiments, field experiments, and mathematical modeling; it is vital that such teamwork be continued. Although this topic has been debated for more than a quarter of a century, only recently have these combined skills been applied to small-scale diagnostic studies. Expansion of this research to include issues related to environmental impacts would prove extremely beneficial. Acknowledgment. Gratefully acknowledged is support by the Ocean Carbon Sequestration Program, Office of Biological and Environmental Research, U.S. Department of Energy, Grant KP1202030, under contract DE-AC05-00OR22725 with UTBattelle, LLC, and contract DE-FG02-01ER63078 with MIT. Support for MBARI was provided by the David and Lucille Packard Foundation and the U.S. Department of Energy under contracts DEFC26-00NT40929 and DE-FG03-01-ER63065. We thank the captain and crew of the RV Western Flyer and the pilots of the ROV Tiburon for their excellent work in making the field experiments possible. We also thank Dr. C. S. Wong of the Institute of Ocean Sciences of Canada and Dr. P. Bergman of the U.S. Department of Energy for scientific discussions and Dr. Marsha Savage for editing the manuscript. EF070197H

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