Many chemical processing and manufacturing activities involve material handling in multi-stage process systems. Coating processes, such as electroplating and polymeric coating processes, are typical examples, where operations are performed by hoists. Each hoist is programmed to carry jobs from one unit to the other, based on a preset operational schedule. The cyclic hoist scheduling (CHS) has been well developed for productivity improvement. Water-reuse network design (WRND), on the other hand, targets the optimal design of water allocation network, such that fresh water consumption and wastewater generation could be the minimized. It is well known for its environmental significance in waste water minimization.

A recent work has proposed to integrate these two technologies by a two-layer optimization method, where the upper layer employs a MINLP CHS model to minimize the cycle time that responds to maximizing the production rate, and the lower layer utilizes a dynamic WRND model to minimize the total freshwater usage. Processing time information of each process unit operation is passed between the two layers, and the iterative computation between these two layers will eventually lead to the most desirable solution for both CHS and WRND.

In this paper, a new methodology for simultaneous consideration of CHS and WRND is developed. The CHS and WRND problems are integrated into one Mixed-Integer Dynamic Optimization (MIDO) model. The simultaneous approach, based on orthogonal collocation on finite elements, is used to transform MIDO model into a Mixed-Integer Nonlinear Programming (MINLP) model for solution identification. This developed MIDO model presents unique features as the multiple scheduling alternatives may exist for the same CHS problem, meanwhile the jump discontinuity may exist in the WRND problem.

The proposed methodology can successful tackle these difficulties. It can identify the optimal CHS schedule and water-reuse network design simultaneously. The proposed simultaneous method and its advancement have been demonstrated with an electroplating process. It demonstrates that economic and environmental thrusts can be simultaneously considered to create a win-win situation for manufacturing.