(31d) Chemical Plant Startup Simulations for Flare Emission Reduction

HRVOC (Highly Reactive Volatile Organic Compounds) emissions from industries are highly responsible for ground-level ozone pollution.  Especially, HRVOC emissions from flares of MSS (maintenance, startup and shutdown) & events are the biggest.  Thus, reduction of flare emissions could significantly reduce the total HRVOC emissions, and could reduce ozone pollution.  On the other hand, reduction of flare emissions could save raw materials and intermediate materials, decrease startup/shutdown time, and thus reduce the total cost of startup and shutdown operations.

In this paper, startup procedures of an ethylene plant were simulated with dynamic simulation.  The ethylene plant is a typical front-end demethanizer process.  Simulation models have all the major process facilities, including cracking furnaces, quench towers, charge gas compressor (CGC) system, chilling train, demethanizer, deethanizer, depropanizer, debutanizer, C2 converter, C3 converter, C2 splitter and C3 splitter.  Dynamic simulation covers all startup steps which involve flare emissions.  The simulation results have been validated with real data from ethylene plants.

Three startup scenarios were simulated with dynamic models.  The first scenario is a “baseline” scenario, which means no flare reduction methods are used in the scenario.  The overall emissions of the baseline scenario are compared with emission data from actual startup events reported to local air quality authorities, and they are within normal range. 

The second scenario simulates startup procedure with certain flare reduction methods, including commissioning distillation columns before CGC starting, using gas from columns to help CGC start, and precooling chilling train.  An optimized startup schedule is built based on dynamic simulation.  Comparing the baseline scenario, the second scenario reduces total startup time by 20%, and reduces total vent gas by 47%.

The third scenario simulates nitrogen startup procedure, which means use nitrogen to start CGC and precooling chilling train.  The startup schedule is also optimized based on simulation results.  Comparing the baseline scenario, the third scenario reduces total startup time by 36%, and reduces total vent gas by 58%.