CFD Study for Dispersion of Hydrocarbons Near Flare

Flare stacks are key system in the petrochemical process industries, where off gases containing hydrocarbons are combusted before sending it to atmosphere.  This study discuses a molecular seal gas leak upstream of a flare stack in a commercial scale petrochemical plant.  This leak is difficult to fix during the continuous plant operation, so it had to wait until next turndown time.  To continue operation, risk analysis requires the calculations of the distribution of the leaked gas in the proximity of the flare to rule out potential fire/explosion.  Flammability limits (LEL: Lower explosive limit / UEL: Upper explosive limit for each individual species are identified.  The diffusivity coefficient of each species in air is determined from available empirical correlations.  The combustible hydrocarbon dispersion in the atmosphere from the flare line is predicted using CFD modeling.  Computational domain for CFD model consist of the atmosphere surrounding the flare stack near leak.  Prevailing wind velocity and direction data is used for the atmospheric domain boundary conditions.  A 2D model is used to identify the extent of computational domain of interest for the 3D simulations.  The preliminary 2D model results are shown in Figure 1.  The extent of the flammable zone for the Methane gas is shown in Figure 1b.  3D simulations will also be presented to capture the effects of the flare stack body on flow turbulence and leak gas distribution including parametric study to assess wind velocity/direction and flare gas composition.