Abstract A dynamic model for the scale-up of semibatch loop reactors was developed. The mathematical model comprises the essential parts of the loop reactor: the reaction vessel, the ejector and the circulation loop. Tanks-in-series and axial dispersion concepts were applied on the description of the non-ideal flow pattern of the reactor. The dynamic axial dispersion model was discretized with finite differences with respect to the spatial coordinate, and the created ordinary differential equations were solved with the backward difference method suitable for stiff differential equations. The loop reactor model was tested with a case study, a homogeneously-heterogeneously catalyzed reaction system, reductive alkylation of aromatic amines. Simulations showed that rate equations obtained from laboratory-scale experiments can be successfully combined to the flow model of the loop reactor: the behaviour of a large-scale loop reactor was predicted with satisfactory accuracy.