Some chemical reactions in the liquid phase can generate drastic changes of the rheological behavior of the liquid. This often happens in agitated tanks entirely baffled equipped with impellers which generate circulations which depend on the hydrodynamic regime - laminar or turbulent. Results issued from Computational Fluid Dynamics (CFD) are presented here concerning fields of velocity components and local energy dissipation, corresponding to a liquid initially with a low viscosity becoming highly viscous Newtonian and then pseudo-plastic. It clearly appears that the flow patterns generated by the two propellers settled in the vessel are quite different, especially with large recirculations and dead zones in the laminar flow range. Depending on the rheological properties of the fluid, energy dissipation occurs in the vicinity of the propellers and/or near the walls.