A comprehensive framework for synthesis and design of heat-integrated batch plants: Consideration of intermittently-available streams

Significant opportunities for energy savings in batch plants exist through Heat Integration of intermittently-available material transfer streams between processing and storage units. Such opportunities may, however, be limited by the structure of the process flowsheet and the scheduling in place. This work presents results obtained when the design, scheduling and Heat Integration of multipurpose batch plants are carried out simultaneously. A rigorous stream-task-oriented scheduling formulation is presented, which enables explicit description of the movements of the intermittently-available streams. This thorough treatment of streams opens up more degrees of freedom than realised in previous stream Heat Integration formulations. On such a basis, design and Heat Integration constraints are added, to result in a mixed integer nonlinear programming formulation. The optimisation objective is to maximise annual net profit, which is a trade-off between production throughput, capital costs, and operational costs arising from hot and cold utility consumption. When applied to a literature example, results show higher annual net profit and reduction in utility costs by as much as 15%, compared to when each of the design, scheduling and Heat Integration steps are carried out in sequence.