2013 1(12)

THE EFFECT OF NONISOTHERMAL CONDITIONS ON DISSOCIATION OF METHANE HYDRATE AT NEGATIVE TEMPERATURES

Nakoryakov V.E., Misyura S.Ya.

Institute of Thermophysics SB RAS, Novosibirsk, Russia

Introduction

Natural gas hydrates from underwater deposits are the alternative and promising energy sources. Now the technologies of production, storage and transportation of methane hydrate powders are being developed actively. Among the fundamental problems of hydrate growth and dissociation there are the related disciplines: the problems and plasticity, physical chemistry microsurface and heat and mass transfer. The rates of gas hydrate production and dissociation regulate the intensity of heat and mass transfer. At hydrate dissociation not only the degree of temperature and pressure deviation from the equilibrium values should be taken into account, but also the rates of heat and mass transfer as well as the typical rates of physical processes such as crystallization, diffusion, and material plasticity.

The dissociation of gas hydrates

The available experimental data relate mainly to the quasi-isothermal processes of generation and dissociation. In the current study different high rate of heat and mass transfer were implemented via a change in heat fluxes. Dissociation of methane hydrate in the air atmosphere at the pressure of 1 bar was studied experimentally. The maximal rate of hydrate dissociation was observed at methane combustion above the interface (hydrate-gas), and it was an order higher than the rate of dissociation without combustion. Methane concentration was measured by the gravimetric, volumetric and X-ray diffraction method. The measurement of instantaneous rates of dissociation allowed determination of several characteristic time ranges with different reaction rates and distinguishing the short-time regime of abnormally low dissociation rate: the phenomenon of self-preservation for the non-isothermal case. The powder of methane hydrate was produced in the reactor-crystallizer with application of forced liquid circulation. The average diameter of particles of methane hydrate powder was 1.9 mm. The diameter loading container was 45 mm, thickness of walls made of thermally-insulated material was 2 mm, and container height was 6 mm. The temperature of powder surface was measured by the thermal imager. The thermocouples were located inside the powder: at the axis and near the left and right lateral walls. The thermocouples allowed determination of the average volumetric temperature of the sample.

According to thermal imaging measurements, the motion of the thermal front is not uniform: the temperature distribution is non-uniform over the surface, and there are high local and average temperature gradients along the container diameter.  After combustion the temperature field becomes uniform. The high heat flux at combustion eliminated high temperature non-uniformities. During combustion adhesion of some particles of methane hydrate was observed, and large aggregations with the size of about 10 mm were formed.

Kinetics of gas hydrate

Kinetics of hydrate formation and dissociation relates to the difference of Gibbs free energy ∆G, which depends on component volatility and thermodynamic parameters Ð, Ò, V. The rate of methane hydrate growth at its formation and the rate of ice film growth at dissociation are connected with ∆G in the form of exponential dependence. However, the rate of gas hydrate dissociation will depend not only on characteristic driving forces, but also on the structures formed at dissociation: the size and shape of ice micrograins, dislocation density, density and size distribution of pores. The shape of grains and their sizes depend on the conditions of hydrate dissociation, e.g., on ∆Ð and degree of gas hydrate subcooling relative to the point of ice melting as well as on the heat flux (heating rate). Under the conditions of annealing the transition from methane hydrate to the structure of hexagonal ice occurs through the intermediate metastable liquid state. In this metastable state the process of dissociation will be determined by characteristics times: τcri, τdif, τpla (crystallization, diffusion and plasticity). Then, for the temperature of powder surface Ò < 0°C, criteria K₁=t _dif /t_cri  and  K₁=t _pla /t_cri will determine kinetics of dissociation.

Conclusion

When gas hydrate dissociates, it is necessary to take into account not only the degree of temperature and pressure deviation from the equilibrium values, but also the rate of heat and mass transfer as well as the characteristic rates of physical processes such as crystallization, diffusion, and material plasticity. The maximal rate of hydrate dissociation was observed at methane combustion above the interface (hydrate – gas), and it was an order higher than the rate of dissociation without combustion. The measurement of instantaneous rates of dissociation allowed the distinguishing of several characteristic time periods with significantly different reaction rates and the short-term regime of abnormally low dissociation rate: the “self-preservation” phenomenon.   

© SPIC "Kappa", LLC 2009-2013