Study of mass transport regimes in membrane steam reformers

The success of membrane reactors for hydrogen separation depends significantly on the development of membranes having high permeability and selectivity with respect to hydrogen. Pd-based membranes have been proposed and their performance is being continuously improved. In conditions of high permeating flux, external mass transfer resistances, such as those due to the transport of hydrogen from the bulk of the catalytic bed to the membrane surface, may limit the performance of the reactor. In the present work, a two-dimensional model has been developed and the influence of different operating conditions on the performance of a methane steam reforming reactor has been investigated. An isothermal reactor has been considered as a first step for a more complete analysis. The velocity profile has been evaluated through Darcy's law for flow through porous media, in which gas velocity is correlated to pressure drops and bed permeability. Both axial and radial components of velocity have been accounted for. Mass transport equations for all species present have been developed b;, considering consumption/production through the steam reforming reaction, and transport by convection and dispersion in the radial and axial directions. Variations of gas density due to pressure gradients and composition changes have also been accounted for. In these conditions, full coupling between mass and momentum transport has been obtained. The membrane has been considered to have infinite selectivity towards hydrogen and Sieverts' law has been used to describe hydrogen flux through the membrane. Dimensionless parameters which govern the system's behavior have been identified and, according to the value of these parameters, simplified models have been proposed. The equations of the general model have been solved using the finite elements solver COMSOL Multiphysics. Integral quantities, such as hydrogen production, permeation, and recovery, as well as concentration and mass flux profiles within the reactor have been evaluated. The most important results of this study are the definition of an optimal operating pressure, whose value depends on membrane permeability and reaction rate, and the identification of a reaction boundary layer, close to ttre membrane, in which all the hydrogen-consuming reaction occurs. These flndings may serve as a basis for the optimization of reactor design.