Identification of a reaction boundary layer in membrane reactors for hydrogen production

Integrated membrane reactors for the production of pure hydrogen at low temperatures are currently attracting much interest. In this work, we developed a model of a membrane reactor for low temperature methane steam reforming, which was solved numerically using COMSOL Multiphysics. The system studied consists of an annular reactor, packed with a catalyst. A hydrogen permeable membrane is supported on the inner wall of the reactor, allowing the selective removal of hydrogen as it is being produced through the steam reforming reaction. The behavior of the reactor may be described as the result of a competition between the steam reforming reaction, which tends to bring the system towards equilibrium, and hydrogen permeation, which moves the system away from equilibrium conditions. The presence of a region of the reactor, close to the wall on which the membrane is supported, in correspondence of which the system is not capable of reaching equilibrium, was noticed. The extent of this region, which we will refer to as a reaction boundary layer, depends on the operating conditions and on reactor geometry. The latter quantity may be defined as a function of the ratios between the length of the reactor and its inner radius, L/R1, and the ratio between the outer and inner radii, R2/R1. The effects of the main operating parameters on the thickness of the reaction boundary layer has been studied, along with the influence of the ratio R2/R1. This procedure has allowed an optimization of reactor design when considering a fixed membrane area and feed flow rate.