Prediction Model of Heat and Mass Transfer in Metal Hydride Packed Beds

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Experiments and numerical analyses were made on heat and mass transfer in thin packed beds of LaNi_5 and Lm-Ni metal hydride to develop the prediction model.1. The effective thermal conductivity of LaNi5 and Lm-Ni beds was measured under vacuum condition.2. For each metal hydride bed, the equilibrium absorption isotherms (Pressure-Composition isotherms) were measured and the value of the heat of reaction was determined based on measured isotherms.3. The changes of bed temperature, system pressure and amount of adsorbed hydrogen with time after the beginning of reaction were systematically measured during both of the adsorption and desorption processes in beds of different heat exchanging surface temperatures, final equilibrium pressures and bed thicknesses.4. The adsorption/desorption reaction proceeds fast, as the difference between the system pressure and the plateau equilibrium pressure corresponding to the heat exchanging surface temperature, and the reaction rate reduces as the bed thickness increases.5. Numerical analyses were carried out on heat and mass transfer during adsorption and desorption in LaNi_5 and LmNi metal hydride packed beds, based on a physical model taking account of all elemental heat and mass transfer processes which take place in the bed. Comparing the experimental data, it was clarified that the reaction rate is dominated mainly by the resistance to heat transfer (heat conduction) within the bed and more or less by the resistance to mass transfer (diffusion of hydrogen) within the metal particles, and that the effects of the resistance to mass transfer between metal particles (hydrogen gas permeation) and to heat and mass transfer at the meat particle surface are negligibly small.6. Based on the model considering both of the bed heat conduction resistance and the particle diffusion resistance, calculations of the adsorption/desorption rate were in good agreement with measurements.