Simultaneous optimization of production scheduling and water-using network for batch processes

Compared to hierarchical methods, simultaneous optimization of batch production scheduling and water-using network can ensure the integrity of solution space and achieve better trade-offs between production and water-using costs. However, the high time-space coupling between two modules inevitably leads to a combinational explosion of alternative configurations, thereby the choice of proper time representation is critical to control model complexity. In order to overcome drawbacks of high complexity of the global continuous time model and solution space reduction of the unit-specific event point model, a hybrid time model was proposed which consisted of unit-specific event point for schedule model, non-uniform discrete time for water-using network model and auxiliary time correlation constraints. A mixed-integer nonlinear programming (MINLP) model was then formulated based on state task network (STN) and water-using network superstructure. Moreover, a solution strategy was also developed, which integrated hierarchical methods (MILP-NLP) and stochastic disturbance to provide efficient initial values. The results of two illustrative examples demonstrate that synchronous optimization significantly decreases water consumption compared to hierarchical methods, and the proposed methodology can avoid loss of solution space as well as reduce computational complexity. © 2016, Editorial Board of “Journal of Chemical Engineering of Chinese Universities”. All right reserved.