CFD model for charge and discharge cycle of adsorptive hydrogen storage on activated carbon

摘要

High surface area activated carbons and other microporous adsorbents have generated a significant amount of interest over the past decade as storage media for hydrogen and natural gas, due to their high storage capacity at low temperatures and their use in gas purification processes. This paper uses computational fluid dynamics (CFD) to simulate the charging and discharging of a sorption-based hydrogen storage system. The CFD model is based on the mass, momentum and energy conservation equations of a system formed of gaseous and adsorbed hydrogen, an activated carbon bed and steel tank walls. The adsorption process is modeled using the Dubinin-Astakov adsorption isotherms extended to the supercritical regime. The model is implemented using Fluent. In our study, we can obtain accuracy peak temperature of simulation due to a non-constant isosteric heat of adsorption is used, derived from the model isotherms. We adopt piecewise heat capacity to consider the heat capacity of the adsorbed phase of hydrogen. We can make a conclusion that the simulated temperatures without consideration of heat capacity for hydrogen in adsorbed phase (c_pa), rise faster and reach higher peaks than the simulated temperatures with consideration of c_pa, and diverge more from experimental results. Also, we study the changes of temperature, pressure and adsorption during the charging and discharging processes as well as when the system is idle (which we define as dormancy) in the case of room temperature water cooling. The results are compared with experimental data from a storage unit cooled with room temperature water. The simulated pressure is in a good agreement with the experimental values. The simulated temperature profiles are also generally in good agreement with the experimental values, except close to the inlet and the wall. In addition, we have studied the effect of quality of the mesh on the accuracy and stability of the numerical computation and the influence of the mass flow rates on temperature and adsorption capacity.