In previous work in our laboratories and in the ocean, we have investigated the formation and dissociation of composite CO2 particles made of CO2 hydrate, liquid CO2, and water. The composite is formed by partially converting liquid CO2 into hydrate at mid-ocean depth (1000-1200 m) conditions. Partial conversion of CO2 into hydrate enables injection of CO2 in seawater in the form of a dense composite material that will drive CO2 to ocean depths well below the injection point. Thermodynamic conditions allowing the formation of negatively buoyant composite particles have been established for a laboratory-scale, continuous-jet hydrate reactor (CJHR). An investigation has been performed to explore the issues related to the scale-up of the CJHR. Hydrate was formed using two CJHR geometries; the first sprayed water in CO2, and the second sprayed CO2 in water. Using a plot of Ohnesorge vs. Reynolds numbers allowed flow rates to be selected that would yield a spray regime and maximize hydrate formation. The effect of varying pressure and liquid flow rates on hydrate behavior was observed. Depending on these parameters, hydrate particles were observed to sink, float, or be neutrally buoyant. A two-order-of-magnitude scale-up in the flow rates of the two fluids has been achieved with the larger CJHR geometries without losing the important characteristics of the hydrate particles (i.e., density and cohesiveness). Temperature changes as a result of hydrate formation were also monitored. A mathematical model has been developed to predict the fate of sinking CO2 hydrate particles after release in the seawater. These results can guide further field and laboratory investigations related to the scale-up of ocean CO2 sequestration. (c) 2007 American Institute of Chemical Engineers.