Costas Tsouris, Peter Brewer, Edward Peltzer, Peter Walz,
David Riestenberg, Liyuan Liang and Olivia R. West
: Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6181
: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, California 95039-9644
Environmental Science & Technology (2004) 38: 2470-2475. doi: 10.1021/es034990a
Received: 2003 September 8.
Revision received: 2004 January 27.
Accepted: 2004 February 3.
This paper reports on the formation and dissolution of CO2/seawater/CO2-hydrate composite particles produced during field experiments in Monterey Bay, CA using a CO2 injector system previously developed in the laboratory. The injector consisted of a coflow reactor wherein water was introduced as a jet into liquid CO2, causing vigorous mixing of the two immiscible fluids to promote the formation of CO2 hydrate that is stable at ambient pressures and temperatures typical of ocean depths greater than ~500 m. Using flow rate ratios of water and CO2 of 1:1 and 5:1, particulate composites of CO2-hydrate/liquid CO2/seawater phases were produced in seawater at depths between 1100 and 1300 m. The resultant composite particles were tracked by a remotely operated vehicle system as they freely traveled in an imaging box that had no bottom or top walls. Results from the field experiments were consistent with laboratory experiments, which were conducted in a 70 L high-pressure vessel to simulate the conditions in the ocean at intermediate depths. The particle velocity and volume histories were monitored and used to calculate the conversion of CO2 into hydrate and its subsequent dissolution rate after release into the ocean. The dissolution rate of the composite particles was found to be higher than that reported for pure CO2 droplets. However, when the rate was corrected to correspond to pure CO2, the difference was very small. Results indicate that a higher conversion of liquid CO2 to CO2 hydrate is needed to form negatively buoyant particles in seawater when compared to freshwater, due primarily to the increased density of the liquid phase but also due to processes involving brine rejection during hydrate formation.
© 2004 American Chemical Society.
Gratefully acknowledged is support by the Ocean Carbon Sequestration Program, Office of Biological and Environmental Research, U.S. Department of Energy, Grant KP120203, under Contract DE-AC05-00OR22725 with UT-Battelle, LLC. Support for MBARI was provided by the David and Lucile Packard Foundation and by the U.S. Department of Energy under Contracts DE-FC26-00NT40929 and DE-FG03-01-ER63065. We thank the captain and crew of the RV Point Lobos and the pilots of the ROV Ventana for making the field experiments possible and Dr. Marsha Savage for editing the manuscript.