Membrane reactors, combining production and separation of pure hydrogen in one vessel, outperform conventional fuel processors for small-scale hydrogen production. Among different membrane reactor concepts, the fluidized bed membrane reactor shows advantages in terms of optimal gas-catalyst contact, heat management and reduced bed-to-membrane mass transfer limitations; the drawbacks are the limits of the fluidization regimes which set minimum and maximum feed flowrates. In this work, a comprehensive model of a fluidized bed membrane reactor is developed. The reactor produces hydrogen (around 3 Nm(3)/h) for the 5 kW(ei)-class PEMFC-based m-CHP systems using natural gas or bio-ethanol as feedstocks. Two different permeate options have been analyzed, namely vacuum and sweep gas configurations, together with detailed fluid-dynamics at the feed side. For the vacuum case the hydrogen output simulated with an ideal approach, i.e. without bubble-emulsion phase distinction, kinetic and mass transfer limitations, is overestimated by 10% with respect to the full detailed model. In the sweep gas configuration the detailed models haves the hydrogen produced by the ideal model. The gas diffusion through the thick porous support of the membranes is the most limiting phenomena, and the reactor size and operation conditions must be selected carefully considering this to reach the desired output.