Modeling and simulation of absorption-desorption cyclic processes for hydrogen storage-compression using metal hydrides

This work is aimed to develop and analyze reduced and simplified lumped models of cyclic processes for hydrogen storage and thermal compression using metal hydrides. Rigorous models involve several thousands of variables whereas reduced models we are interested in involve only several tens of variables. The models here presented reproduce the main dynamic behavior of rigorous models and experimental data found in the literature. Furthermore, the main tradeoffs arisen in process design are well described with these models, which is always an objective of optimal process design. In the first part of the work, a simplified lumped model is developed and validated by comparing the simulations outcome with numerical results and experimental measurements obtained from the literature for absorption and desorption individual processes. Our model is then used to simulate the process behavior using real parameters and constraints required by continuous recovery and compression systems such as those found in the metal treatment industry. The simulation results are used to improve the process performance by adjusting some key parameters of the system. These results are also used to perform a sensitivity analysis, i.e. evaluate the storage/compression system behavior when introducing variations to parameters such as operating conditions, reactor design, and material properties. Finally, we further reduce the model by considering that the inlet and outlet hydrogen flow is approximately constant. This particular specification is usually required by continuous processes in the metal treatment industry where hydrogen flow must remain constant. This requirement allows considering reaction rate as a constant. The constant reaction rate constraint allows integrating the ordinary differential equations; hence the system no longer has differential and algebraic equations but just algebraic equations. As a consequence of the simplification, the number of equations to be solved is reduced from over 15,000 to less than 50, maintaining an excellent match in the results. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.