(245o) Olefin Furnace System Scheduling with the Consideration of Relay Cracking of Recycled Ethane/Propane

Ethylene is the most important product with the largest bulk productivity in the petrochemical industry. An ethylene plant employs multiple cracking furnaces in parallel to convert various hydrocarbon feed stocks to smaller hydrocarbon molecules, mostly ethylene and propylene. Among the furnaces, one furnace is normally specifically designated to crack recycled ethane and propane which are the byproducts of the cracking reaction. 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. Since some plants do not have buffers to store recycled ethane and especially recycled propane, another furnace must be picked up to relay cracking recycled ethane/propane temporary when the designated recycling furnace is in decoking. Since ethane and propane gas are easier to form coke than liquid feeds, plants normally prefer to cracking recycled ethane/propane at the beginning of the batch rather than at the ending of the batch on the temporary relay furnace to have more flexibility in adjusting production schedule. Thus it is better to let the batch starting time of the relay furnace equal to the batch ending time of the recycling furnace. When the recycling furnace finishes the previous batch and is in decoking, a relay furnace has just finished decoking and can crack recycled ethane/propane temporary. When the recycling furnace finishes decoking and starts a new batch, the recycled feed will be switched back to the designated recycling furnace and the temporary relay furnace will crack its own feed. To realize above object, a MINLP model has been developed and demonstrated by case studies.