This work concerns the dynamics and control of process networks in which impurities are present and are removed by a small purge stream. Using a singular perturbation analysis, it is shown that the presence of the small purge stream leads to a time scale separation in the dynamics of the network. A method is proposed for the derivation of nonstiff, reduced-order models in each time scale that are suitable for analysis and model-based control. Finally, two case studies are introduced and illustrative numerical simulation results are provided. (c) 2006 American Institute of Chemical Engineers