Heat integration to minimise energy usage in multipurpose batch plants has been in published literature for more than two decades. In most present methods, time is fixed a priori through a known schedule, which leads to suboptimal results. The method presented in this paper treats time as a variable, thereby leading to improved results. Both direct and indirect heat integration are considered together with optimisation of heat storage size and initial temperature of heat storage medium. The resulting model exhibits MINLP structure, which implies that global optimality cannot generally be guaranteed. However, a procedure is presented that seeks to find a globally optimal solution, even for nonlinear problems. Heat losses from the heat storage vessel due to idling are also considered. This work is an extension of MILP model of Majozi [1], which was more suited to multiproduct rather than multipurpose batch facilities. Optimising the size of the heat storage vessel as well as the initial temperature of the heat storage fluid decreased the requirement for external hot utility for an industrial case study by 33% compared to using known parameters.

A unified framework providing for the simultaneous reduction of energy and wastewater in multipurpose batch plants is presented in this paper. Unlike many current methods, the schedule is not predefined. Time is treated as a variable leading to the optimization of the schedule together with water and energy usage simultaneously. Since a flexible process schedule is employed, an improved result in the form of a better overall production schedule compared to schedules obtained from optimizing water and energy separately is achievable. The ability of the current method to handle industrial scale problems is also highlighted using a complex case study. Opportunities for direct water reuse and indirect water reuse, using wastewater storage, are explored as well as direct and indirect heat integration for reducing external utilities. The objective is to improve the profitability of the plant by minimizing wastewater generation and utility usage. The results from three examples are presented.

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