(343c) Coupling Electroplating Process Design and Operation for Simultaneous Productivity Maximization, Energy Saving, and Wastewater Minimization

Over 20,000 electroplating workshops throughout the US annually produce numerous plated workpieces for many pillar US industrials, such as the automotive, aerospace, electronic, refinery, and petrochemical industries. However, electroplating plants also generate tremendous amounts of hazardous or toxic waste in the forms of wastewater, sludge, and spent solution, which includes over 100 toxic chemicals and metals that are potentially hazardous to human health and to the environment [1]. It has been estimated that a normal electroplating plant annually produces about 55,000 tons of waste and wastewater and 50 tons of hazardous sludge containing many hazardous heavy metals and chemicals, such as chromic acid, nickel sulfate, and zinc cyanide [2]. Because of the large number of electroplating shops throughout the U.S., the pollution generation costs the industry hundreds of millions of dollars per year for waste treatment and disposal [3]. The environmental problems have resulted in the electroplating industry becoming the second most regulated one in the nation. Therefore, waste source reduction for the electroplating industry is an urgent need. In practice, when energy and productive efficiencies are also taken into account, the design and operation of an electroplating process becomes very complicated and thus need in-depth studies.

Facing this challenge, this work simultaneously addresses three important issues in the design and operation of an electroplating process, i.e., productivity improvement, energy saving, and wastewater minimization. In the operation side, the cyclic hoist scheduling (CHS) is employed to minimize the production cycle time so as to improve the productivity. A plating model measuring the electricity usage during the electroplating operation addresses the optimal energy consumption under product quality specifications. In the process design side, the rinsing dynamics based dynamic water-reuse network design (DWRND) targets the minimum wastewater generation through the optimal water allocation among all rinse units. The three aspects of hoist scheduling, energy consumption, and wastewater generation are integrated into a multi-objective mixed-integer dynamic optimization (MIDO) model. This developed MIDO model presents unique features such as the multiple scheduling alternatives for the CHS problem, the discontinuity inside the DWRND model, and the trade-off among the three objectives. To efficiently solve the developed MIDO model, the orthogonal collocation on finite elements method is used to transform the MIDO model into a mixed-integer nonlinear programming (MINLP) model. Due to the discontinuity inside the DWRND model, special treatments are required during the MIDO model discretization process. Because of the multiple scheduling alternatives for the CHS problem, a solution cut strategy is conducted to identify the potential best solutions in an iterative way. The MINLP model is efficiently solved to identify the optimal results, such as the Pareto frontier of the multi-objective optimization problem, which will provide important technical supports for the electroplating process design and operation. The proposed methodology and its efficacy have been demonstrated with an alkali zinc electroplating process.

References

1. Duke, L. D. (1994), Hazardous Waste Minimization: Is It Taking Root in U.S. Industry? Waste Minimization in Metal Finishing Facilities of the San Francisco Bay Area, California. Waste Management, 14, 49-59.

2. Kushner, J. B; Kushner, A. S. (1994). Water and Waste Control for the Plating Shop, 3rd ed.; Hanser Gardner Pub.: Cincinnati, OH.

3. Load, J. R.; Pouech, P.; Gallerani, P. (1996). Process Analysis for Optimization & Pollution Prevention. Plating Surf. Finishing, 83, 28-35.