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《Geochimica》 2003-02
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Methane hydrate decomposition at constant temperature and pressure

P>SUN Chang yu, CHEN Guang jin, GUO Tian min<P>(Research Laboratory of High Pressure Fluid Phase Behavior and Property, University of Petroleum, Beijing 102200, China)<P>  
The kinetics of methane hydrate decomposition is studied using a Sapphire cell apparatus. The kinetic data of methane hydrate decomposition at temperatures from 0.6 to 5.8 ℃ and pressures from 1.0 to 3.0 MPa are measured using constant decomposition pressure method, and at temperature of lower than 0 ℃ . The influence of temperature and pressure on decomposition rate is examined. It is revealed that the decomposition rate of methane hydrate is proportional to difference between gas fugacity at the equilibrium state and that at the actual decomposing state. When temperature is higher than 0 ℃ , the influence of mass transfer could be ignored and hydrate decomposition could be described by intrinsic reaction theory. The average deviation between the experimental gas amount and the calculated value by intrinsic reaction theory is less than 4.8% . When temperature is lower than 0 ℃ , water generated from the hydrate decomposition would transform into ice rapidly at the surface of hydrate crystal. The released gas diffuses from the hydrate and ice mixture layer to the bulk of gas phase. The thickness of hydrate and ice mixture layer would increase with the hydrate decomposition. In this case, the intrinsic reaction rate is very quickly compared with that of diffusion rate, so the hydrate decomposition process is controlled by gas diffusion. The thickness of hydrate layer is reduced correspondingly with the hydrate continuous decomposition and the boundary of ice hydrate moved toward water ice phase. The hydrate decomposition when temperature is lower than 0 ℃ could then be described as a moving boundary phenomenon. When the decomposition time is long, the Fick diffusion coefficient should be revised according to hole size among the ice layer because the ice layer became compact with the increasing of its thickness which resulting in gas diffusion not abide by Fick diffusion law. The results by the model match the experimental data perfectly showing that diffusion theory could be successfully applied to hydrate decomposition kinetics when temperature is lower than 0 ℃ .P
【Fund】: 国家自然科学基金(29806009)
【CateGory Index】: P736.4
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