ABSTRACT A variable axial dispersion model (VADM), accounting for local variations in the axial dispersion coefficient, was compared with the classical constant axial dispersion model (CADM). The two models reproduced the same concentration profile at the reactor outlet in a step‐response experiment, enabling a fair comparison under typical kinetic scenarios. The simulations showed that CADM should be limited to case studies in which convection is the prevailing transport mechanism ( Pe > 50). For lab‐scale PBR with important backmixing effects, the CADM is a good flow model when the apparent reaction order does not change sign during the chemical transformation. If the reaction rate exhibits a maximum along the reactor length (e.g., surface mechanism), averaging the axial dispersion coefficient results in an inappropriate approximation. Nonlinear regression analysis illustrated that using a wrong flow model can lead to large errors in the estimation of the kinetic parameters. Highlights A variable axial dispersion model (VADM) was developed and implemented showing high prediction power. A Péclet number semiempirical function was implemented to simulate different fluid‐dynamic conditions. VADM shown substantial differences with classical axial dispersion models when surface rate expressions are used. Rate expressions passing through a maximum value led to the highest divergency between VADM and CADM. It was demonstrated that high errors can be obtained in parameter estimation when choosing the wrong flow model.