(566b) Emission Source Characterization During An Ethylene Plant Shutdown

Ethylene plant shutdowns not only generate huge amounts of emissions (VOC), but also waste tremendous raw material that could be utilized to generate more products. A well-planned shutdown would reduce the risks on safety, reliability, business and environment. However, fundamental and quantitative studies on shutdown flaring emission are still lacking.  During an ethylene plant shutdown, plant usually cut off feed and flared off all off-spec products until the plant was de-inventoried. It would be executed following three steps: (i) Preparation of shutdown. Feed rates are reduced to minimize the liquid inventories in facilities ahead of the shutting down. (ii)Shutting down. The plant is shut down, beginning with shutting down Cracked Gas Compressor (CGC), followed by gradually cutting off the furnace feed. (iii) Decommissioning including liquid discharge, vapor discharge and N₂ purge. Almost all of the emissions come from shutting down and decommissioning.

In this paper, plant-wide dynamic simulations are employed to model the preparation of shutdown to get initial status for shutting down and decommissioning. The emission during shutting down has been counted. The decommissioning which generates most of flare source is then investigated to quantify and characterize flaring emission source during an ethylene plant shutdown. Liquid discharge is calculated by governing equation. Dynamic simulations for vapor discharge and N₂ purge are built based on isolated sections such as CGC. Deep insights of the emission source distribution and dynamic emission profiles are provided. The durations of decommissioning events calculated in this paper will help decision makings on planning plant shutdowns. The study enriches emission inventories with details for industry point sources, which have no accurate resolution on shutdowns. It also provides detailed technical support for both the industry and environmental agencies on evaluating and developing cost-effective flare minimization control strategies in the future.