Preventive maintenance is essential for every chemical production site to prevent failure and accidents, however it upsets material and utility flows inside the site and also causes production loss. In order to minimize the loss, maintenance of each plant unit has to be carefully scheduled together with considerations on site-wide material and utility balances.This will involve both production and utility systems, and indeed is a very complicated problem. A scheduling strategy is then employed to handle the problem efficiently. It divides the scheduling into two steps, long-term and short-term. Long-term maintenance scheduling determines the combination of plant shutdown in a period of two to five years.Based upon the long-term schedule, a short-term maintenance scheduling optimizes the exact timing of plant shutdown, overhaul, inspection and startup within a maintenance period of four to ten weeks. Short-term maintenance scheduling involves pre-set utility and material demand profiles during a plant shutdown, overhaul and startup making it a very challenging task. In this paper, a multi-period mixed integer linear programming (MILP) model, a Site-Model, is proposed as an aid to optimize short-term site-wide maintenance schedule. A special formulation is also developed to deal with the pre-set utility and material demand profiles in the Site-Model.

Mitsubishi Kasei and Mitsubishi Petrochemical merged to become Mitsubishi Chemical in 1994 making it the largest chemical company in Japan. This allows two original companies' production sites, especially for those are physically built next to each other, to improve themselves by multi-site integration. Mitsubishi Yokkaichi production plant site is one of the typical examples, which consists of three individual plant sites in the old company structure. Each plant site contains a utility plant to generate steam and electricity for chemical production. After the company merge, connecting steam and electricity among plant sites have been carried out for better flexibility, efficiency as well as capability of the utility systems. Besides these, there are still many other improvement alternatives. A Site-Model, which includes all three utility plants and production units, was then developed to explore further opportunities. In this paper, applications at Mitsubishi Yokkaichi production site are presented to illustrate the features of the site-model.

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