(300c) Optimal Production and Maintenance Scheduling in Olefin Plants

Process unit maintenance is defined as all actions appropriate for retaining or restoring the functionality of the unit to such an operating condition that the maintained unit can generally be reused. Maintenance is very important to ensure chemical plant reliability and profitability, as well as to help the plant safety and environmental sustainability. As a main type of maintenance, the preventive maintenance involves preplanned maintenance activities, which will be started, completed, and reused at the scheduled time. It is widely practiced in all chemical plants, especially for the olefin plant. Olefin plants employ multiple cracking furnaces in parallel to convert various hydrocarbon feed stocks to smaller hydrocarbon molecules, mostly ethylene and propylene. The continuous operational performance of cracking furnaces gradually decays because of coke formation in the reaction coils, which requires each furnace to be periodically shut down for decoking. Thus, decoking of cracking furnaces to restore them to original performance is an importance preventive maintenance for olefin plants.

In this paper, the simultaneous scheduling of olefin production and unit maintenance has been proposed. Two new maintenance initiatives: recycled ethane relay cracking and decoking grouping, are introduced to help improve the smart manufacturing in olefin plants. First, as ethane is a byproduct of cracking furnaces, it is separated and recycled as a feedstock of a furnace specially designated by the plant. Since some plants do not have equipment to store recycled ethane, another furnace must be picked up to crack recycled ethane temporary when the designated recycling furnace is in decoking. This kind of maintenance operation is called recycled ethane relay cracking. Second, when a furnace is in decoking maintenance, the throughput of this furnace will be zero and the total throughput of the whole furnace system flowing to the downstream separation process will suffer a sudden decrease. When the furnace is back on production after about two days, the throughput will become normal and the total throughput will suffer another sudden increase. These two kinds of throughput upsets due to furnace maintenance should be minimized for the stable operation of the downstream process. Thus, grouping the decoking operation means the decoking operation of different furnaces shall be lined up as many as possible to reduce the upset to downstream units. To maximize the olefin plant profit as elaborated by previous studies as well as to accomplish the two maintenance initiatives, a novel mixed-integer linear programming (MILP) model has been developed. It can help olefin plants obtain optimal schedules on simultaneous production and unit maintenance. The efficacy of the development has been demonstrated by various case studies.