OPTICAL-CELL EVIDENCE FOR SUPERHEATED ICE UNDER GAS-HYDRATE-FORMING CONDITIONS

We previously reported indirect but compelling evidence that fine-grai ned H2O ice under elevated CH4 gas pressure can persist to temperature s well above its ordinary melting point while slowly reacting to form methane clathrate hydrate. This phenomenon has now been visually verif ied by duplicating these experiment in an optical cell while observing the very slow hydrate-forming process as the reactants were warmed fr om 250 to 290 K at methane pressures of 23 to 30 MPa. Limited hydrate growth occurred rapidly after initial exposure of the methane gas to t he ice grains at temperatures well within the ice subsolidus region. N o evidence for continued growth of the hydrate phase was observed unti l samples were warmed above the equilibrium H2O melting curve. With co ntinued heating, no bulk melting of the ice grains or free liquid wate r was detected anywhere within the optical cell until hydrate dissocia tion conditions were reached (292 K at 30 MPa). even though full conve rsion of the ice grains to hydrate requires 6-8 h at temperatures appr oaching 290 K. In a separate experimental sequence, unreacted portions of H2O ice grains that had persisted to temperatures above their ordi nary melting point were successfully induced to melt, without dissocia ting the coexisting hydrate in the sample tube, by reducing the pressu re overstep of the equilibrium phase boundary and thereby reducing the rate of hydrate growth at the ice-hydrate interface. Results from sim ilar tests using CO2 as the hydrate-forming species demonstrated that this superheating effect is not unique to the CH4-H2O system.