The dynamic behavior of a stand alone flash with a constant heating rate and a corresponding reactor-separator system with recycle is analyzed by means of different model formulations which are frequently applied. It is shown that the predictions of the different model formulations are rather close for the stand alone flash. In particular, in all cases, a stable steady state is predicted. In contrast to this, it is shown that the dynamics of the corresponding reactor-separator system can depend on the specific model formulation. For a rigorous model with a dynamic energy balance, a new type of bifurcation is found, where the eigenvalues have a pole with a change of sign. Similar effects are reported for systems with negligible and finite transportation delay of the recycle. For the latter, multiple periodic solutions are found for a model with a quasistatic energy balance, which will disappear because of the new type of bifurcation if a rigorous dynamic energy balance is applied.