The simplest model of a bubbling fluidized bed, with uniform bubbles exchanging matter with a dense phase of catalytic particles which promote 2 continuum of parallel first order reactions is considered. It is shown that the system behaves like a stiffed tank with two feeds; the one, direct at the inlet; the other, distributed from the bubble train. The basic results can be extended to cases of catalyst replacement for a single reactant and to Astarita’s uniform kinetics for the continuous mixture.