Permeability of a deformable metal hydride bed during hydrogen absorption

Heat transfer in porous metal hydride (MH) beds is the major bottleneck in scaling up of the metal hydride technology, while a role of mass transfer limitations in operation is generally overlooked in lab-scale experiments with relatively small beds. We present relations for gas permeability within the Kozeny-Carman approach and a dimensionless parameter to compare heat and mass transfer limitations in metal hydride devices. From experiments on hydrogen absorption and desorption in a flow-through reactor containing 1 kg of La0.9Ce0.1Ni5 alloy we experimentally determined an apparent permeability and showed, that the "breathing' of the bed can lead to appearance of dual porosity due to fracturing of the bed, which leads to increased permeability at absorption of hydrogen (similar to 20 mu m(2)) comparing with desorption (similar to 0.3 mu m(2)). Particle swelling leads to decrease of porosity both due to decrease of pores and due to closure of fractures, thus deformation of the metal hydride bed leads to decrease of permeability during absorption. The effect can be important for flow-through reactors with thick layers of metal hydride.