Air_Dispersion_Modeling_&_ALOHA.ppt
- 1. Air Dispersion Modeling & ALOHA Prepared by: Environment Section, HSEF Department Haldia Petrochemicals Limited
- 2. Bhopal Tragedy: Environmental Perspective Incident – Leak of Methyl Isocynate Apart from system failure, other important reasons for mass casualties were – Low atmospheric temperature Timing of accident Wind direction Release height Molecular weight higher than air The casualties could have been minimized, if the incident would have occurred in daytime !!!!
- 3. Relevance in HPL Perspective Handling of large quantity of hydrocarbons Heavier than air Highly Flammable in nature Large no. of small sources capable to create havoc at local levels Handling of highly toxic gas like Chlorine Very low TLV Heavier than air
- 4. What is Dispersion? Dispersion is “Diffusion” of air contaminants in air Atmospheric dispersion modeling is the mathematical simulation of how air pollutants disperse in the ambient atmosphere. The dispersion models are used to estimate or to predict the downwind concentration of air pollutants emitted from sources such as industrial plants and vehicular traffic. The models are typically employed to determine whether existing or proposed new industrial facilities are or will be in compliance with the National Ambient Air Quality Standards (NAAQS)
- 5. Influencing Factors to Dispersion Meteorology Wind Speed Wind Direction Atmospheric Stability Ambient Air Temperature Height to the bottom of inversion
- 6. Wind Wind is the natural horizontal motion of the atmosphere. It occurs when warm air rises, and cool air comes in to take its place Wind is caused by differences in pressure in the atmosphere. The pressure is the weight of the atmosphere at a given point. The height and temperature of a column of air determines the atmospheric weight. Because cool air weighs more than warm air, a high pressure mass of air is made up of cool and heavy air. Conversely, a low pressure mass of air is made up of warmer and lighter air. Differences in pressure cause air to move from high pressure areas to low pressure areas, resulting in wind.
- 7. Atmospheric Stability Atmospheric stability refers to the vertical motion of the atmosphere. Unstable atmospheric conditions result in a vertical mixing. Typically, the air near the surface of the earth is warmer in the day time because of the absorption of the sun's energy. The warmer and lighter air from the surface then rises and mixes with the cooler and heavier air in the upper atmosphere causing unstable conditions in the atmosphere. This constant turnover also results in dispersal of polluted air. Stable atmospheric conditions usually occur when warm air is above cool air and the mixing depth is significantly restricted.
- 8. Atmospheric Stability Adiabatic Lapse Rate : Negative of temperature gradient in the atmosphere in adiabatic conditions. It equals to ~ 1o C/100 m. Environmental Lapse Rate: Equals wet adiabatic lapse rate - ~ 0.6o C/100 m A B C D E ELR>ALR Unstable ELR=ALR Neutral ELR<<ALR Strongly Stable Temperature A l t i t u d e
- 9. Stability Class A – Very Unstable B - Unstable C – Slightly Unstable D - Neutral E – Slightly Stable F - Stable
- 10. Stability Class Wind Speed, m/s Daytime Solar Insolation Night time Cloud Cover Strong Moderate Slight <50% >50% <2 A A-B B E F 2-3 A-B B C E F 3-5 B B-C C D E 5-6 C C-D D D D >6 C D D D D
- 11. Inversion Stable atmospheric conditions usually occur when warm air is above cool air and the mixing depth is significantly restricted. This condition is called a temperature inversion. During a temperature inversion, air pollution released into the atmosphere's lowest layer is trapped there and can be removed only by strong horizontal winds. Because high-pressure systems often combine temperature inversion conditions and low wind speeds, their long residency over an industrial area usually results in episodes of severe smog.
- 13. Influencing Factors Emission Parameters Source location Source height Source diameter Exit velocity Exit Temperature Mass flow rate
- 14. Influencing Factors Terrain Elevation - Both source as well as receptor Location, height and width of any obstruction
- 15. Building Effects or Downwash Building effects or downwash: When an air pollution plume flows over nearby buildings or other structures, turbulent eddies are formed in the downwind side of the building. Those eddies cause a plume from a stack source located within about five times the height of a nearby building or structure to be forced down to the ground much sooner than it would if a building or structure were not present. The effect can greatly increase the resulting near-by ground-level pollutant concentrations downstream of the building or structure. If the pollutants in the plume are subject to depletion by contact with the ground (particulates, for example), the concentration increase just downstream of the building or structure will decrease the concentrations further downstream
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