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Integration of Cyclic Hoist Scheduling and Water-Reuse Network Design for Environmentally Benign Manufacturing

Qiang Xu, Department of Chemical Engineering, Lamar University, Beaumont, TX 77710

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 hoist(s). Each hoist is programmed to carry jobs from one unit to the other, based on a preset operational schedule. An optimal hoist schedule can improve production efficiency and reduce manufacturing cost significantly. Hoist schedule development, especially cyclic hoist scheduling (CHS) has been commonly understood a key factor to maximize the manufacturing productivity.

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 minimum. It is well known for its environmental significance in process pollution prevention. In this paper, a novel methodology, which integrates CHS and WRND technologies for simultaneous consideration of productivity and environmental concerns in manufacturing, is developed. It demonstrates that economic and environmental thrusts can be efficiently incorporated to create a win-win situation for material-handling industries.

In this methodology, there are two basic objectives that should be considered in a two-layer optimization scienario. The upper layer objective is to minimize the cycle time that responds to maximizing the production rate. A new MINLP-based CHS model is developed for this purpose. Processing time information of each process unit operation is passed to the lower layer. Since time issues are involved in lower layer, dynamic simulation has to be employed. The objective in the lower lever is then designed to minimize the total freshwater usage for the process operation. It can be accomplished by a proposed dynamic WRND model. It is very possible that multiple scheduling alternatives may exists for a same CHS problem. The iterative computation between these two layers will eventually lead to the most desirable solution. The proposed methodology excels previous works with the important feature that it identifies optimial CHS schedules and water-reuse network designs simultaneously. The efficacy of the methodology is demonstrated by solving an industrial electroplating example.