A Unified Approach for the Optimization of Energy and Water in Multipurpose Batch Plants

Presented in this paper, a unified framework to optimize both water and energy usage simultaneously, together with the scheduling problem, is developed. In this manner, the timing of tasks remains a variable throughout optimization. Due to the fact that a flexible process schedule is employed, an improved result in the form of a better overall production schedule compared to the schedules obtained from optimizing water and energy separately is achievable. Opportunities for direct water reuse and indirect water reuse, using wastewater storage, were explored. Both direct and indirect heat integration were explored for reducing external utilities. The objective was to improve the profitability of the plant by minimizing wastewater generation and utility usage. The results from a case study show that the profit using only external utilities and freshwater was 18 537 cu with requirements of 390 kWh for cooling water, 240 kWh for steam and 816 kg for freshwater. The profit improved to 22 235 cu when opportunities for both direct and indirect water reuse as well as direct and indirect heat integration were explored. In this case, the requirements for cooling water, steam and freshwater were 190 kWh, 10 kWh and 896 kg, respectively.