5 toxic release dispersion models

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CHAPTER 5CHAPTER 5CHAPTER 5CHAPTER 5

Toxic ReleaseToxic Releaseandand

Dispersion ModelsDispersion Models

Chapter OutlineChapter Outline

IntroductionIntroductionNeutrally Buoyant Dispersion ModelsNeutrally Buoyant Dispersion ModelsNeutrally Buoyant Dispersion ModelsNeutrally Buoyant Dispersion ModelsPasquillPasquill--Gifford ModelGifford ModelToxic Effect CriteriaToxic Effect CriteriaRelease MitigationRelease Mitigation

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Instructional Learning ObjectivesInstructional Learning Objectives

After completing this chapter, students should be able to After completing this chapter, students should be able to do the following:do the following:do the following:do the following:

Identify release incidentIdentify release incidentDevelop source model to describe how materials are Develop source model to describe how materials are released and rate of releasereleased and rate of releaseEstimate downwind concentrations of toxic material Estimate downwind concentrations of toxic material

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using dispersion model using dispersion model Predict impact/effect due to the released of materialsPredict impact/effect due to the released of materials

IntroductionIntroduction

Toxic release model represents first 3 steps in consequence Toxic release model represents first 3 steps in consequence modeling procedure:modeling procedure:modeling procedure:modeling procedure:

1.1. Identifying release incident (what process situations can Identifying release incident (what process situations can lead to a release?)lead to a release?)

2.2. developing source model to describe how materials are developing source model to describe how materials are released and rate of releasereleased and rate of release

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3.3. estimating downwind concentrations of toxic material estimating downwind concentrations of toxic material using dispersion model (once downwind concentrations using dispersion model (once downwind concentrations known, several criteria available to estimate impact @ known, several criteria available to estimate impact @ effect)effect)

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Based on the predictions of toxic release, the following Based on the predictions of toxic release, the following options can be done for performing release mitigation:options can be done for performing release mitigation:options can be done for performing release mitigation:options can be done for performing release mitigation:

Emergency response planEmergency response planEngineering modification of the process plantEngineering modification of the process plantAdding appropriate monitoring and preventing system Adding appropriate monitoring and preventing system to eliminate risk of the release materialsto eliminate risk of the release materials

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Two ways the release of toxic materials can be carried away by Two ways the release of toxic materials can be carried away by the wind the wind –– characteristic plume or a puffcharacteristic plume or a puffParameters affecting atmospheric dispersion of toxic materials:Parameters affecting atmospheric dispersion of toxic materials:

•• wind speedwind speedAs the wind speed increases, the plume becomes longer and As the wind speed increases, the plume becomes longer and narrowernarrower

•• atmospheric stabilityatmospheric stabilityDuring the day the air temperature decreases rapidly with the During the day the air temperature decreases rapidly with the heightheightAt night the air temperature decrease is lessAt night the air temperature decrease is lessCl ifi d t th t bilit l t bl t l t blCl ifi d t th t bilit l t bl t l t bl

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Classified to three stability classes: unstable, neutral, stableClassified to three stability classes: unstable, neutral, stable•• Unstable Unstable –– the sun heats the ground faster than the heat can the sun heats the ground faster than the heat can

be removed so that the air temperature near the ground is be removed so that the air temperature near the ground is higher than the temperature at higher elevationhigher than the temperature at higher elevation

•• Neutral Neutral –– the air above the ground warms and the wind speed the air above the ground warms and the wind speed increasesincreases

•• Stable Stable –– the sun cannot heat the ground as fast as the ground the sun cannot heat the ground as fast as the ground cools; the air of higher density is below air of lower densitycools; the air of higher density is below air of lower density

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•• ground conditions ground conditions (buildings, water, trees)(buildings, water, trees)Affect the mechanical mixing at the surface and the Affect the mechanical mixing at the surface and the wind profile with heightwind profile with heightTrees and buildings increase mixingTrees and buildings increase mixing

h i ht f l b d l lh i ht f l b d l l•• height of release above ground levelheight of release above ground levelAs the release height increases, the ground level As the release height increases, the ground level concentrations are reducedconcentrations are reduced

•• momentum and buoyancy of initial material releasedmomentum and buoyancy of initial material releasedChange the effective height of the release. Change the effective height of the release. The momentum of a highThe momentum of a high--velocity jet will carry the gas velocity jet will carry the gas higher than the point of release, resulting much higher than the point of release, resulting much

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g p , gg p , ghigher effective release height. higher effective release height.

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Neutrally Buoyant Dispersion ModelsNeutrally Buoyant Dispersion Models

C b d t ti t th t ti d i d fC b d t ti t th t ti d i d fCan be used to estimate the concentrations downwind of a Can be used to estimate the concentrations downwind of a release in which the gas is mixed with fresh air to the point release in which the gas is mixed with fresh air to the point that the resulting mixture is neutrally buoyantthat the resulting mixture is neutrally buoyantThe models apply to gases at low concentrations, typically The models apply to gases at low concentrations, typically in in ppmppm range.range.Two types models; plume and puffTwo types models; plume and puff

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X =downwind,

Y =crosswind,

Z =vertical)

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Neutrally Buoyant Dispersion ModelsNeutrally Buoyant Dispersion ModelsCase 1: SteadyCase 1: Steady--state continuous point release with no windstate continuous point release with no wind

Eddy diffusion or eddy dispersion or turbulent diffusion is any diffusion process by which substances are mixed in the atmosphere

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Equation 5-15 is transformed to rectangular coordinates to yield

Neutrally Buoyant Dispersion ModelsNeutrally Buoyant Dispersion ModelsCase 2: Puff with no windCase 2: Puff with no wind

spherical coordinates

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and in rectangular coordinates

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Neutrally Buoyant Dispersion ModelsNeutrally Buoyant Dispersion ModelsCase 3: NonCase 3: Non--steadysteady--state continuous point release with no state continuous point release with no

windwind

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Neutrally Buoyant Dispersion ModelsNeutrally Buoyant Dispersion ModelsCase 4: SteadyCase 4: Steady--state continuous point source release with windstate continuous point source release with wind

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Neutrally Buoyant Dispersion ModelsNeutrally Buoyant Dispersion ModelsCase 5: Puff with no wind and Eddy Diffusivity is a function Case 5: Puff with no wind and Eddy Diffusivity is a function

of directionof direction

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Neutrally Buoyant Dispersion ModelsNeutrally Buoyant Dispersion ModelsCase 6: SteadyCase 6: Steady--state continuous point source release with wind state continuous point source release with wind

and eddy diffusivity is a function of directionand eddy diffusivity is a function of direction

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Neutrally Buoyant Dispersion ModelsNeutrally Buoyant Dispersion ModelsCase 7: Puff with windCase 7: Puff with wind

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Neutrally Buoyant Dispersion ModelsNeutrally Buoyant Dispersion ModelsCase 8: Puff with no wind and with source on groundCase 8: Puff with no wind and with source on ground

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Neutrally Buoyant Dispersion ModelsNeutrally Buoyant Dispersion ModelsCase 9: SteadyCase 9: Steady--state plume with source on the groundstate plume with source on the ground

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Neutrally Buoyant Dispersion ModelsNeutrally Buoyant Dispersion ModelsCase 10: Continuous steadyCase 10: Continuous steady--state source with source at height state source with source at height

HHrr above the groundabove the ground

For this case the ground acts as an impervious boundary at a distance H from the source.

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If Hr = 0, Equation 5-36 reduces to Equation 5-35 for a source on the ground.

PasquillPasquill--Gifford Models Gifford Models

Cases 1 through 10 all depend on the specification of a value for the eddy diffusivity Kj.

In general, Kj changes with position, time, wind velocity, and prevailing weather conditions.

Although the eddy diffusivity approach is useful theoretically, it is not convenient experimentally and does not provide a useful framework for correlation.

Sutton solved this difficulty by proposing the following definition for a dispersion coefficient:

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definition for a dispersion coefficient:

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The dispersion coefficients are a function of atmospheric conditions and the distance downwind from the release.

The atmospheric conditions are classified according to six different stability classes, shown in Table 5-1.

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The stability classes depend on wind speed and quantity The stability classes depend on wind speed and quantity of sunlight. During the day, increased wind speed results of sunlight. During the day, increased wind speed results in greater atmospheric stability,in greater atmospheric stability,whereas at night the reverse is true. This is due to a whereas at night the reverse is true. This is due to a h i ti l t t fil f d t i hth i ti l t t fil f d t i htchange in vertical temperature profiles from day to night.change in vertical temperature profiles from day to night.

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PasquillPasquill--Gifford ModelsGifford Models

Limitations to Limitations to PasquillPasquill--Gifford Model or Gaussian dispersionGifford Model or Gaussian dispersionApplies only to neutrally buoyant dispersion of gases inApplies only to neutrally buoyant dispersion of gases inApplies only to neutrally buoyant dispersion of gases in Applies only to neutrally buoyant dispersion of gases in which the turbulent mixing is the dominant feature of the which the turbulent mixing is the dominant feature of the dispersion.dispersion.Typically valid for a distance of 0.1Typically valid for a distance of 0.1--10 km from the release 10 km from the release pointpoint..The predicted concentrations are time average. The predicted concentrations are time average. The models presented here assumed 10The models presented here assumed 10--minute timeminute time

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The models presented here assumed 10The models presented here assumed 10 minute time minute time averageaverage

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Neutrally Buoyant Dispersion ModelsNeutrally Buoyant Dispersion ModelsCase 11: Puff with instantaneous point source at ground Case 11: Puff with instantaneous point source at ground

level, coordinates fixed at release point, constant wind only level, coordinates fixed at release point, constant wind only in x direction with constant velocity uin x direction with constant velocity u

The ground-level concentration is given at z = 0:

The ground-level concentration along the x axis is given at y = z = 0:

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The center of the cloud is found at coordinates (ut, 0,0). The concentration at the center of this moving cloud is given by

Neutrally Buoyant Dispersion ModelsNeutrally Buoyant Dispersion ModelsCase 12: Plume with continuous steadyCase 12: Plume with continuous steady--state source state source

at ground level and wind moving in x direction at at ground level and wind moving in x direction at constant velocity uconstant velocity u

This case is identical to case 9. The solution has a form similar to Equation 5-35:35:

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ExampleExample

Continuous release of gas (molecular weight of 30) is resulting in a concentration of 0.5 ppm at 300 m resulting in a concentration of 0.5 ppm at 300 m directly downwind on the ground. Estimate σy and σz. Assume that the release occurs at ground level and that the atmospheric conditions are worst case.

Assume u=2 m/s and stability is class F

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SolutionSolution

At 300 m = 0.3 km, sy = 11.8 and sz = 4.4.

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ExampleExampleA gas with a molecular weight of 30 is used in a particular process. A source model study indicates that for a particular accident outcome 1.0 kg of gas will be released i t t l Th l ill t d l l Th instantaneously. The release will occur at ground level. The plant fence line is 500 m away from the release.

a. Determine the time required after the release for the center of the puff to reach the plant fence line. Assume a wind speed of 2 m/s.

Assume u=2 m/s and stability is class F

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b. Determine the maximum concentration of the gas reached outside the fence line.

c. Determine the distance the cloud must travel downwind to disperse the cloud to a maximum concentration of 0.5 ppm. Use the stability conditions of part b.

SolutionSolution

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ExampleExample

Due to a road accident, there is a leak of chlorine from a tank. Although the leak is quickly stopped, 4 g q y pp ,kg of chlorine are released; the release can be considered instantaneous. Downwind, on the road, several cars have stopped at a distance of 200 m.Calculate the time required for the centre of the cloud to reach the cars. Then calculate the maximum concentration at the location where the cars are

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stopped.Meteorological conditions: u = 2 m/s, T = 20 0C overcast conditions. stability class D. assume σx=σy

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SolutionSolution

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Neutrally Buoyant Dispersion ModelsNeutrally Buoyant Dispersion ModelsCase 13: Plume with Continuous steadyCase 13: Plume with Continuous steady--state source at height Hstate source at height Hrr above above

ground level and wind moving in x direction at constant velocity uground level and wind moving in x direction at constant velocity u

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Neutrally Buoyant Dispersion ModelsNeutrally Buoyant Dispersion ModelsCase 14: Puff with instantaneous point source at height HCase 14: Puff with instantaneous point source at height Hrr above above ground level and a coordinate system on the ground that moves ground level and a coordinate system on the ground that moves

with the puffwith the puff

For this case the center of the puff is found at x = ut. The average concentration is given byconcentration is given by

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Neutrally Buoyant Dispersion ModelsNeutrally Buoyant Dispersion ModelsCase 14Case 14

Puff with Instantaneous Point Source at Height Hr above Ground Level and a Coordinate System Fixed on the Ground at the Release

Point

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Toxic Effect CriteriaToxic Effect Criteria

• What concentration is considered dangerous?• TLV-TWA is for worker exposures, and not design for short-TLV TWA is for worker exposures, and not design for short

term exposures under emergency conditions.• One of the recommended method by Environmental

Protection Agency (EPA) is by using emergency response planning guidelines (ERPGs) for air contaminants issued by the American Industrial Hygiene Association (AIHA)

• Three concentration ranges are provided as a consequence

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of exposure to a specific substance:


• ERPG-1 is the maximum airborne concentration below which it is believed nearly all individuals could be

d f t 1 h ith t i i ff t thexposed for up to 1 hr without experiencing effects other than mild transient adverse health effects or perceiving a clearly defined objectionable odor.

• ERPG-2 is the maximum airborne concentration below which it is believed nearly all individuals could be exposed for up to 1 hr without experiencing or developing irreversible or other serious health effects or

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symptoms that could impair their abilities to take protective action.

• ERPG-3 is the maximum airborne concentration below which it is believed nearly all individuals could be exposed for up to 1 hr without experiencing or developing life-threatening health effects.

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•• Examples of ERPGs in unit ppmExamples of ERPGs in unit ppm

ERPGERPG--11 ERPGERPG--22 ERPGERPG--11•• AcetaldehydeAcetaldehyde 1010 200200 10001000•• AcroleinAcrolein 0.10.1 0.50.5 33•• Vinyl AcetateVinyl Acetate 55 7575 500500

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Realistic and WorstRealistic and Worst--Case ReleasesCase Releases

The realistic releases represent the incident The realistic releases represent the incident outcomes with a high probability of occurringoutcomes with a high probability of occurringg p y gg p y gThe worstThe worst--case releases are those that assume case releases are those that assume almost catastrophic failure of the process, almost catastrophic failure of the process, resulting in near instantaneous release of the resulting in near instantaneous release of the entire process inventory or release over a short entire process inventory or release over a short period of timeperiod of timeThe worstThe worst--case releases must be used to case releases must be used to determine the consequences study required by determine the consequences study required by

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determine the consequences study required by determine the consequences study required by EPA Risk Management PlanEPA Risk Management PlanTable 4Table 4--5 lists a number of realistic and worst5 lists a number of realistic and worst--case releases.case releases.

Realistic and WorstRealistic and Worst--Case ReleasesCase Releases

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5 toxic release dispersion models

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