Numerical Simulation And Experimental Study Of La-Ni Hydriding Kinetics Based On The Varying-size Model

Particle size often varies during gas–solid heterogeneous reactions.For example,metal hydrides?MH?can reversibly absorb and desorb H₂,accompanied by the expansion and shrinkage of the alloy particles,respectively,especially in the La-Ni system.Although the traditional Shrinking-Core Model?SCM?can be used to describe the H₂absorption/desorption process,the calculation results from SCM show relatively large deviations from the experimental data without considering the factor of particle expansion/shrinkage.Therefore,based on the actual expansion of particles in the hydrogenation of La-Ni alloys,we defined the lattice deformation factor???,modified the particle radius and density terms and proposed a Varying-Size Model?VSM?for MH particles.Three types of control mechanism including H₂ dissociative chemisorption,H internal diffusion,and surface reaction were obtained.Hydriding experiments on the systems of six typical La-Ni alloys(LaNi₅,LaNi_4.5Al_0.5,LaNi_4.5Co_0.5,LaNi_4.5Fe_0.5,LaNi_4.5Fe_0.25Co_0.25.25 and LaNi_4.0Fe_0.5Co_0.5)were performed under quasi-isothermal and variable-pressure conditions of 30?,50?,70?and 1.2 MPa,0.8MPa,0.4 MPa,respectively.We explored the effects of temperature and supply hydrogen pressure on hydrogen absorption rate and amount,and obtained that MH has better hydrogen storage performance under low temperature and high pressure conditions.Also the effect of replacing Ni elements with different kinds of elements was compared,indicating that Al,Fe,Co elements can increase the hydrogen storage capacity at a lower temperature.Based on the experimental results,we have determined that the rate-controlling step for both VSM and SCM of these alloys is internal diffusion of hydrogen atoms.The simplified forms of the VSM diffusion equations were also developed to facilitate their practicability:LaNi₅:1.426-1.419?1-X?^2/3-0.046?1-X?^5/3-0.95X+0.038?1-X?²=^t/?'_diffLaNi_5-x-yM_xN_y:1.362-1.35?1-X?^2/3-0.069?1-X?^5/3-0.90X+0.057?1-X?²=^t/?'_diffThe four kinetics models?JMA first-order models,JDM,SCM,and VSM?were used to simulate and calculate the rate constant k of each alloy,and the kinetic parameters E_a?A?E_D and D₀ of each alloy were summarized.Then the fitting effect between several models and experimental data was compared,verifying that the VSM has the highest accuracy.Finally,the sensitivity analysis of multiple parameters in VSM shows that the particle size and temperature can significantly affect hydriding reaction rate,while the pressure is the least sensitive factor.