Transport-reaction-permeation regimes in catalytic membrane reactors for hydrogen production. The steam reforming of methane as a case study

Catalytic reactors equipped with dense metallic membranes for hydrogen production are attracting increasing interest in view of their potential to overcome yield limitations associated with chemical equilibrium. A transport-reaction-permeation isothermal model, fully coupled with momentum transport through the packed bed, is here considegd for the species participating to the methane steam-reforming reaction in an annular catalytic reactor fed by a pre-reformed (equilibrium) mixture. Model predictions are validated by comparison with experimental data available in the literature. Large variations of the mixture density are found as a consequence of nonuniform composition profiles, which, in turn, trigger sizeable radial convective fluxes of hydrogen towards the membrane. Nonlinearities associated with chemical equilibrium, reaction kinetics, membrane permeation, and density dependence on composition interact with one another, making the overall equipment response markedly complex. A thorough dimensionless analysis of the response of the reactor vs the operating pressure is carried out, which shows the occurrence of non-monotonic behavior characterized by a maximum of efficiency associated with a critical pressure value. A scaling law of critical efficiency vs the main dimensionless parameters is proposed, which can provide a rationale for the design of reactor geometry and operating conditions.