Transport phenomena versus intrinsic kinetics: Hydrogen sorption limiting sub-process in metal hydride beds

This paper discusses and compares the different sub-processes that occur during the hydrogen sorption of practical systems based on metal hydrides, i.e. intrinsic kinetics, heat transfer and hydrogen transport. Derived from their modeling equations, a resistance analysis is developed on these hydrogen sorption sub-processes for the first time. This analysis allows quantifying how strongly each sub-process affects the overall sorption kinetics in a hydride bed and thereby the sorption-rate limiting sub-process can be identified. It was found that in the case of the hydrogen absorption of sodium alanate material the heat transfer resistance is the dominant and rate limiting sub-process, with the exception of small geometries. Besides, the resistance due to hydrogen transport is negligible in comparison to the overall absorption resistance. As a consequence, simulations and designs of scaled-up systems based on sodium alanate material always require heat transfer optimization as one of the foremost considerations.