(31e) Dynamic Simulation for Flare Minimization Strategy in an Ethylene Plant Shutdown

Ethylene plant shutdowns generate a great amount of flare emissions that inevitably damage air quality of the surrounding environment.  During an ethylene plant shutdown, plant generally cut off feed and flared off all off-spec products until the plant was de-inventoried.  Here, an optimal shutdown strategy is designed for reducing flare emission and preventing losses of hydrocarbon materials.  In this paper, dynamic simulations are employed to model the operations of shutdown for both the general strategy and the optimal strategy.

First, the general shutdown strategy is simulated and dynamic emission profile is presented. The shutdown of an ethylene plant 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.

Second, the optimal shutdown strategy is simulated to recover more liquid and vapor hydrocarbon materials in the facilities.  Compared with the basic shutdown strategy, the optimal shutdown strategy includes certain flare reduction methods.  For example, after shutting down the furnaces, gas from other facilities is used to keep CGC running to help recover leftover hydrocarbon materials in the flash drums and columns, rather than send hydrocarbon materials to flare.  In the dynamic model, recycle lines are built and the operations are simulated to monitor the key process variables and study the feasibility. So, through the dynamic simulations, the shutdown strategy can be investigated to help the shutdown reduce the risks on safety, reliability, business and environment.