(153c) Dynamic Simulation Targeting Emission and GHG Reduction Under Abnormal Operations: Start-up of an Ethylene Plant By Total Recycle and Intermediate Storage
Flaring is the safety way for any chemical processing facilities to protect personnel, equipments and their surrounding environment especially during process upsets. Flared gases contain natural gas, highly reactive volatile organic compounds (HRVOCs) and in some cases, carbon monoxide (CO), react with atmospheric oxygen in combustion reactions, and theoretically create carbon dioxide (CO2) and water. CO2 generated from flaring is considered a major industrial source in greenhouse gases (GHG). Besides, flare emissions release volatile organic compounds (VOCs) and pollutants into the atmosphere, which react with nitrous oxide (NOx) in sunlight to generate ground-level ozone and smog. This phenomenon often raises concern within the chemical process industry (CPI) such as chemical plants or oil and gas processes. Chemical processes, especially refineries and petrochemical plants, are usually designed to utilize their on-site produced gas in a balance. Hence, more gas recovery from flaring also brings better thermal efficiency and more environmentally friendly operations. Flaring is very essential during turnaround operations as a large quantity of inert and off-specification gases needs to be ejected until the whole system reaches normal conditions. Flare minimization (FM) has been practiced not only during regular operations but also when processes are going through start-ups, shutdowns and malfunctions (SSM).
A typical startup of an ethylene plant sends approximately 400,000 - 1,000,000 lbs of materials to flare and generates 200,000 lbs of CO and VOCs. Hence, managing flare emission during startup, shutdown and malfunction of an ethylene plant are very important. This research continues the previous work on introducing a generic approach of using dynamic simulation for emission and GHG reduction under abnormal operations, demonstrated by a case study of an front-end de-ethanizer ethylene plant start-up. The base case employs recycling off-specification streams from distillation columns to charge gas compressor (CGC) inlet as substitute partially for furnace feed in order to shorten the total start-up time and reduce the amount of hydrocarbons sent to flare. However, there is significant flaring occurring in this scenario during the transient period when the sub-sections are connected and brought from after-commissioning status to normal operation. Therefore, the optimal case presented in this work investigates different approaches of using temporary storage units to accumulate and recycle this flaring quantity to either furnace feed or CGC inlet, ultimately targeting flareless start-up for the whole ethylene plant. Large-scale dynamic simulation, emission characterization, planning/scheduling, and result evaluation are carried out simultaneously to achieve the desired outcome. It is proven that the outcome contributes a great deal in evaluating new strategies and predicting process dynamic behaviors. It also helps achieving more precise timing to control the recycle loop; hence, flaring is potentially minimized while the start-up time is shortened.