Staging Distribution of Oxygen in Circulating Fast Fluidized-Bed Membrane Reactors for the Production of Hydrogen

A mathematical model is used to simulate coupled steam and oxidative reforming reactions of methane for heat integration and to improve the yield of hydrogen in a circulating fast fluidized-bed membrane reactor (CFFBMR). A composite membrane of a very thin layer of palladium silver alloy is used for the removal of hydrogen, shifting the thermodynamic equilibrium and achieving the production of pure hydrogen that is suitable for fuel cells from one integrated unit. A distributed oxygen feed through discrete injection points along the length of the reformer to control the heat in the reaction medium is compared with oxygen feeding through dense perovskite membranes and the conventional direct feeding of oxygen (co-feed). Equispaced and orthogonal injection points for oxygen distribution, as well as uniform and nonuniform oxygen flow distribution, are investigated. The results indicate that the staging distribution of oxygen enables one to control the hot-spot temperatures and to operate the reactor under safer operability conditions in contradistinction to the possibility of thermal runaway for the conventional co-feed mode of operation. The results also show that the nonuniform oxygen flow distribution is superior, with regard to hydrogen yield over uniform oxygen flow distribution, for both equispaced and orthogonal injection systems. This is due to the good control and high oxygen fluxes at the injection points. The equispaced injection system gives better hydrogen yield than the orthogonal injection system in a certain region of parameter space and vice versa in other regions. Among the advantages of distributed oxygen feed is that the reformer can be operated at low feed temperatures.