Dynamics and control of heat exchanger networks: A multi-time scale perspective

The generic networks of counter current heat exchangers, designed to minimize the external utility consumption at steady state, are presented. Detailed dynamic models of these networks comprise of systems of hyperbolic partial differential equations (PDE). In the limit of tight energy integration, i.e., when large heat transfer coefficients, leading to small temperature differences, are present, the resulting model exhibits two ? time scale behavior, with the fast dynamics associated with the temperature dynamics of the individual streams and the slow dynamics associated with the total enthalpy content of the streams in a unit. A model reduction framework for the resulting system of stiff hyperbolic PDE and a controller design framework that exploits this time scale separation are proposed. The analysis leads to a classification of the manipulated inputs (internal streams and external utilities) that act in each time scale and can be used to achieve these control objectives. The theoretical results are demonstrated by simulations on prototype heat exchanger network configurations. This is an abstract of a paper presented at the AIChE Annual Meeting (Salt Lake City, UT 11/4-9/2007).