OPTIMAL SCHEDULING OF HEAT-INTEGRATED MULTIPURPOSE PLANTS

A systematic mathematical framework for scheduling the operation of mu ltipurpose batch/semicontinuous plants involving heat-integrated unit operations is presented. The approach advocated takes direct account o f the trade-offs between maximal exploitation of heat integration and other scheduling objectives and constraints. Both direct and indirect heat integration are considered. In the former case, heat transfer tak es place directly between the fluids undergoing processing in the heat -integrated unit operations, and therefore a degree of time overlap of these operations must be ensured. It is shown that this involves only relatively minor modifications to existing detailed scheduling formul ations. Indirect heat integration utilizes a heat transfer medium (HTM ) which acts as a mechanism both for transferring heat from one operat ion to another and for storing energy over time. This provides a degre e of decoupling with respect to the timing of the operations involved. The mathematical formulation presented in this paper is based on a de tailed characterization of the variation of the mass and energy holdup s of HTM over time. In particular, it takes account of the limitations on energy storage due to heat losses to the environment. A modified b ranch-and-bound procedure is proposed for the solution of the resultin g nonconvex mixed integer nonlinear programming problem.