DISSOLUTION OF LIQUID CO2 INTO WATER AT HIGH-PRESSURES - A SEARCH FORTHE MECHANISM OF DISSOLUTION BEING RETARDED THROUGH HYDRATE-FILM FORMATION

The rate of dissolution of CO2 from a liquid CO2 phase to an adjoining liquid water phase is known to be retarded significantly by the forma tion of a thin clathrate hydrate film spread over the interphase bound ary. Much account is currently made of this ''barrier'' effect in the investigation of ocean disposal, or sequestration, of CO2, an option t o mitigate the emission of CO2 into the atmosphere. However, the prima ry, dominant mechanism of the ''barrier'' effect provided by hydrate f ilms is still unclear and sometimes delusively explained. This paper a ims to discuss possible mechanisms of the effect and reasonably estima te the dominant one. The mechanism thus estimated is a reduction of th e solubility of CO2 in liquid water in thermodynamically stable hydrat e/liquid-water coexisting conditions at a temperature below T-h, the h ydrate/liquid/liquid equilibrium temperature under a given pressure, f rom that in metastable (< T-h) or Stable (greater than or equal to T-h ) hydrate-free conditions, which causes a smaller driving force for th e diffusive, or convective, CO2 transfer from hydrate-film surfaces to the adjoining liquid water phase compared to that for the transfer fr om hydrate-free CO2/water interfaces. It is also suggested that liquid CO2 drops, freely buoying up in the sea, may also suffer an additiona l mechanism of retarding dissolution: a hydrate film covering each dro p should suppress the tangential mobility of its surface, thereby caus ing a decrease in the surface-to-seawater mass transfer coefficient fo r CO2. (C) 1998 Elsevier Science Ltd.