For the design? scale-up, simulation, and optimization of biological processes in three phase fluidized bed reactors, a mathematical model that is able to predict flow behavior and mass transfer considering kinetics of biological reaction in a wide range of operating conditions is needed. In this paper a dynamic model is developed based on mass balances of a disc-shaped reactor volume element taking into account residence time distribution of fluid phases by the axial dispersion model (ADM). Finally a simulation of the dihydroxyacetone production using gluconobacter oxydans (DSM 50049) was performed showing that the specific yield of a three phase bed reactor is strongly influenced by operating conditions.