AbstractA new mathematical model is presented model is presented for use in the design and optimization of fluidized‐bed fermenters. Unlike previous models, the biomass particle size is not a required input parameter, but is predicted as a consequence of the process by which the fermenter reaches a steady state. Both tower fermenters and supported‐film bioreactors are included in the analysis. The differences between them are explained as a consequence of the different effects of added biomass on the particle settling velocity and the tendency of a fluidized bed to stratify. A detailed qualitative treatment of solids mixing allows the model to predict the varying biomass concentration through a tower fermenter and the more constant concentration in the supported film reactor. Other features of this analysis are the inclusion of an axial dispersion term to allow for different liquid mixing conditions, and the introduction of a variable transformation that eliminates the need for a computer solution. A sample design problem is included.