A Numerical Simulation For Dispersion Of Low-Level Leakage Of Characterized Pollutants Over The Residential Areas

Low-level leakage is inevitable in chemical industrial process and it is difficult for people to find and repair immediately because of the small quantity. The leakage and dispersion of pollutants is affected by many factors, such as environmental conditions, leakage quantity and building layout. This paper defined the leaking of pump, valve and flange, volatilization of pollutants caused by maintenance and unloading, organized and unorganized emissions as the low-level leakage; identified the hazard with the safety system analysis method; set up the source model according to the hazard consequence and calculated the quantity of characterized pollutants; built the physical and mathematical model with computed fluid dynamical software( Fluent software); simulated the dispersion regularity of the characterized pollutants over the residential area in different environmental conditions, different leakage and different building layout and analyzed the influence of bad environmental conditions on the downstream residential area.The results show that, the impact of wind velocity on pollutant concentration is related to leaking rate. There is a critical leaking rate, Qc, when the rate of leakage from the source is greater than the Qc, pollutant concentrations increase with increase in wind speed. On the contrast, when the rate of leakage from the source is less than the Qc, pollutant concentrations decrease with decrease in wind speed. If the rate of leakage was large, the trend that benzene pollutant concentrations of residential areas varied with the changes of leaking rate is significant. However, if the rate of leakage is small, reducing the rate again has little effect on reducing the pollutant concentrations in residential areas. If the wind speed is very small, the pollutant concentrations of sensitive points decrease as the distance between the point and the source increases. If the wind speed is substantially large, the pollutant concentrations of sensitive points are not only associated with the distance between the point and the source, but also with the rate of leakage. The pollutant dispersion has the same direction with wind and its concentration increase with increase in the level of atmospheric stability but increase first and then decreases as air humility increase; as the surface roughness increased, benzene concentration decreased. The results also showed that, different H/W of building resulted in different concentration in residential area. The concentration in street canyon increased with increase in H/W. If H/W is lower (such as 0.1 and 0.5), the high concentration area is mainly located at windward of street. However, If H/W is higher, such as 1 and 2, the high concentration area is mainly located at leeward of street. The concentration of street in"set-down"pattern is more than in"set-up"pattern. The concentration of benzene and xylene exceed the limit when wind velocity is 7m/s, wind direction is northwest, the level of atmospheric stability is D, air humidity is 75% and surface roughness is 0.001.