Flare Minimization during Ethylene Plant Turnaround Operations via Dynamic Simulation and Optimization

Flaring emissions during CPI (chemical process industry) plant startups, shutdowns, and upsets causes negative environmental and social impacts and also results in tremendous material and energy losses to plants. For instance, an ethylene plant with 1.2 billion pounds of ethylene production per year may flare five million pounds of ethylene during one typical startup, which generates 15.4 million pounds of CO2, 7.5 K lbs of NOX, 40 K lbs of CO, 100 K lbs of highly reactive VOCs. Obviously, minimizing flaring and CO2 emissions bring significant benefits to environmental, societal, and CPI industrial sustainability.

Unfortunately, current practices in flare minimization almost exclusively depend on industrial experience and “end-of-the-pipe” control strategies. For instance, the installation of flare gas recovery units (FGRU) can capture flare gas for recycle and reuse. However, it is less desirable since the “waste gas” is already generated.  Besides, the capital expenditure and operating cost of the FGRU is considerable. Thus, current practices in flare minimization are not enough for CPI today because of increasingly strict environmental regulations and economic competitions.

This presentation introduces systematic methodologies that have been developed at Lamar University for industrial flare minimization via plant-wide dynamic simulation and optimization. Since off-specification streams are inevitable during plant turnaround operations, to significantly reduce flaring emission, they must be either recycled to the upstream process for online reuse, or stored somewhere temporarily for future reprocessing, when the plant manufacturing returns to stable operation.  Thus, the off-spec products will be able to be reused instead of being flared.  This can be achieved through the identification of viable operation strategies through plant-wide dynamic simulation and optimization. The dynamic simulation/optimization provide an insight into process dynamic behaviors, which is crucial for a plant to minimize the flaring; meanwhile, maintain the operating safety. The effectiveness of the developed methodologies will be demonstrated by multiple case studies involving plant start-up, shutdown, and process upset from different ethylene processes.