A reactor model for a downer-regenerator circulating fluidized-bed (CFB) during the partial oxidation of n-butane to maleic anhydride is presented. Upflow reactors (risers) suffer from severe solids back mixing and gas-solids-separation, in comparison down flow reactors exhibit a more uniform gas-solids flow and reduced backmixing, resulting in narrower residence time distributions. Due to the sensitivity of the VPO catalyst to over-reduction, downer reactors present an interesting alternative to riser reactors. The reactor models for the downer and the regenerator fluidized-bed are coupled with reduction and oxidation kinetics for the catalyst, respectively. The influence of the solids residence time distributions for the combined system of both reactors on the oxidation state of the catalyst is explored by a novel newly developed oxygen loading distribution. Simulation results suggest the limited solids-flux in downers restrict the maximum butane concentrations, while the scale-up is predicted to be uncritical.