OPTIMAL SCHEDULING OF HEAT-INTEGRATED MULTIPURPOSE PLANTS

A systematic mathematical framework for scheduling the operation of multipurpose batch/semicontinuous plants involving heat-integrated unit operations is presented. The approach advocated takes direct account of 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 takes 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 formulations. Indirect heat integration utilizes a heat transfer medium (HTM) which acts as a mechanism both for transferring heat from one operation to another and for storing energy over time. This provides a degree of decoupling with respect to the timing of the operations involved. The mathematical formulation presented in this paper is based on a detailed characterization of the variation of the mass and energy holdups of HTM over time. In particular, it takes account of the limitations on energy storage due to heat losses to the environment. A modified branch-and-bound procedure is proposed for the solution of the resulting nonconvex mixed integer nonlinear programming problem.