(235d) A CFD-Based Process Design and Operation Analysis of Multi-Tube Membrane Reactor for Hydrogen Production

Hydrogen is considered an energy carrier of the future primarily owing to its environmental friendliness as it produces only water when combusted. As fuel cells become more widespread as portable energy generators, demand for hydrogen is expected to increase. Hence it is important to develop technologies for producing hydrogen at various scales with higher yields and portability. Currently, hydrogen is mainly produced from fossil fuels through steam reforming (SR). SR provides the highest hydrogen yield per methane feed along with long-term stability. The SR reaction is highly endothermic and an external heat source is needed to retain the temperature. On the other hand, some weak exothermic reactions also take place simultaneously such as the methanation reaction (inverse SR of methane) and the water-gas shift reaction. These reactions lead to impurities like methane and carbon dioxide. In the separation to obtain pure hydrogen, membranes are often used due to their high hydrogen selectivity at high pressure. A membrane reactor (MR) is attractive because it not only does the separation but also induces the forward reaction to produce more hydrogen by way of Le Chatelier’s principle as the amount of hydrogen in the reaction mixture is reduced. This can help overcome the limitation of equilibrium to obtain a higher hydrogen yield. The MR system includes catalytic chemical reactions and the physical permeation through the membrane. The conventional MR has multiple tubes in order to increase the surface area of the membranes while minimizing its volume. The reaction rates depend on both the temperature and the pressure, which strongly affect the hydrogen production. The locations of the membrane tubes affect the mass and heat transfer because they block one another to serve as obstacles for mass and heat transfer.

To observe the internal phenomena and spatial distributions of the key variables, 3D computational fluid dynamics (CFD) simulations are conducted. The kinetics of SR^1,2 and the hydrogen permeation through the membrane³ are modeled based on the experimental data and incorporated into the CFD by using the user-defined functions in the FLUENT^TM software package. In order to reduce the computational cost, the symmetry boundary condition method, which is applicable when the physical geometry of interest has a mirror symmetry, is used in parallel with the direction of gravity. A lab-scale model is firstly validated by comparison with the experimental results of a single-tube MR³. Sensitivity analysis involving the key operating variables of pressure and temperature is conducted. Then, the four-tube MR with a same total surface area as the single-tube MR is designed and simulated. It can be said that the heat and mass transfer is heavily influenced by the location of the membrane tubes relative to the center axis of the reactor, resulting in different velocity and temperature distributions, and hydrogen yields. The simulation result demonstrates that design and operation of multi-tube MRs can be enhanced by performing CFD simulations and conducting sensitivity analysis to identify key variables influencing the hydrogen yield and optimizing them.

1. Xu J, Froment GF. Methane Steam Reforming, Methanation and Water-Gas Shift: 1. Intrinsic Kinetics. AIChE Journal. 1989;35.
2. Praharso, Adesina A, Trimm D, Cant N. Kinetic study of iso-octane steam reforming over a nickel-based catalyst. Chemical Engineering Journal. 2004;99(2):131-136.
3. Kim C-H, Han J-Y, Kim S, et al. Hydrogen production by steam methane reforming in a membrane reactor equipped with a Pd composite membrane deposited on a porous stainless steel. International Journal of Hydrogen Energy. 2018;43(15):7684-7692.