A concept for an iterative multi-scale modeling and simulation of gas/liquid bubble column reactors with or without recycle has been developed and discussed for a specific example. It is based upon the interplay of the two-phase mass transfer and reaction with the detailed (unsteady state) hydrodynamics. A "zone model" contains one-dimensional mass balances for the gas and the liquid phase and balance equations which are necessary to describe the change of the mean bubble diameter of the bubble size distribution. Energy balances can also be added. Detailed information of the liquid and gas flow pattern is provided to the zone model by the hydrodynamic model, while the hydrodynamic model gets its required information about local mean bubble sizes and the local mass flux between gas and liquid from the zone model.