Deactivation-induced dynamics of the reaction front in a fixed-bed catalytic membrane reactor: Methane cracking as a case study

Abstract

Thermo-Catalytic decomposition (TCD) of methane can be regarded as a cornerstone towards the development of greenhouse gas-free processes for pure hydrogen production. Most studies of TCD focused on process schemes where the extraction of hydrogen from the gaseous CH4−H2 mixture is accomplished in a unit separated from the reaction environment. In this article, we investigate numerically a different setup that involves the use of a semi-batch annular fixed-bed membrane reactor. The permeselective membrane allows to lower the reaction temperature, overcoming equilibrium limitations. The intrinsic time-dependency of the process (induced by catalyst deactivation due to massive deposition of the solid carbon product), together with spatial concentration gradients triggered by hydrogen permeation through the membrane give rise to a non-trivial dynamical behavior of the reactor. Specifically, we observe that a localized reaction front develops near the membrane at the early stage of the process. At later times, the front moves away from the membrane zone throughout the bed as larger and larger portions of the catalyst become inactive. The front thickness and dynamics are found to have a strong influence upon the overall timescales of the reaction. A dimensionless analysis of the dependence of the reactor efficiency on the pressure and on the catalyst activity (here quantified by the Damköhler number) is carried out by assuming 550 °C as a working temperature. An optimal working pressure is found at relatively high Damköhler value. Qualitatively different operating modes of the membrane reactor in different regions of the pressure-Damköhler parameter space are identified and interpreted.