Modeling of cryo-adsorption of hydrogen on MOF-5 pellets: Effect of pellet properties on moderate pressure refueling

Refueling of a single MOF-5 pellet with cryogenic hydrogen at moderate pressures is modeled in this work. A MOF-5 pellet in a discharged state is placed in a hydrogen atmosphere at refueling conditions of 80 K temperature and 30 bar pressure. The absolute adsorption isotherm for hydrogen on MOF-5 is described by the Unilan model, hydrogen is modeled as a real gas, and properties of the compressed MOF-5 pellet are obtained from polynomial fits to low temperature experimental data. Pellet properties change due to the addition of expanded natural graphite (ENG) and various levels of compaction. A parametric study is performed using COMSOL® Multiphysics 4.2 to understand the effect of ENG and pellet compaction on refueling behavior of a single pellet. Specifically, the role of permeability and thermal conductivity, and the effect of anisotropic thermal conductivity are investigated using a two-dimensional axisymmetric model. The variation in pellet refueling time with varying pellet sizes is studied in order to determine the “optimal” pellet size for fast refueling. Results show that both stick-like geometry (h/d ≫ 1) and flat hockey-puck geometry (h/d ≪ 1) provide fast refueling. Conversely, pellets with h/d ≈ 0.5 show the longest refueling times and are therefore not recommended for use in a packed bed design.