Due to the development of computers one may have the impression that the differences between methods of fluid mechanics and of chemical reaction engineering are diminished. But important differences survive, and their disregard can lead to misunderstandings. The main difference is the domination of the mass related view in reaction engineering and of the position related view in hydrodynamics. The analysis of the fundamentals result in the incorporation of the reaction kinetics and of the residence time distribution. Taking this into account, the dispersion model and the convection diffusion model are compared and the character of the axial dispersion coefficient and of the boundary conditions are discussed. It can be seen, that a direct transformation of hydrodynamics results is not possible. The simulation of the life history of molecules and particles offer an alternative for this.