This article investigates the modeling of mixing phenomena occur at unsteady stirring conditions in that agitated vessels. In particular, a new model of a torus reactor including a well-mixed zone and a transport zone is proposed. The originality of the arrangement of ideal reactors developed here is due to the time-dependent location of the boundaries between the two zones. This concept is applied to a model of the positive influence of unsteady stirring condition on a homogenization process; the model avoids mass balance discontinuity during the transitions from steady to unsteady stirring conditions. To ascertain the reliability of the model proposed, experimental runs with highly viscous fluids have been carried out in an agitated tank. The impeller used is a nonstandard helical ribbon impeller, fitted with an anchor at the bottom. The degree of homogeneity in the tank is recorded using a conductivity method after a tracer injection. It is shown that, for a given agitated fluid and mixing system, the parameters of the model are easy to estimate and that modeling results are in close agreement with experimental ones. Moreover, it appears that this model allows the easy derivation of the control law, which is a great advantage when optimizing the dynamics of a mixing process.