1CHAPTER 5Toxic Release

andDispersion Models

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Chapter Outline

Introduction Neutrally Buoyant Dispersion Models Pasquill-Gifford Model Toxic Effect Criteria Release Mitigation

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Instructional Learning Objectives

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

Identify release incident Develop source model to describe how materials are

released and rate of release Estimate downwind concentrations of toxic material

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

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Introduction

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

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

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

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

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Introduction

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

Emergency response plan Engineering modification of the process plant Adding appropriate monitoring and preventing system

to eliminate risk of the release materials

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Introduction Two ways the release of toxic materials can be carried away by

the wind characteristic plume or a puff Parameters affecting atmospheric dispersion of toxic materials:

wind speed As the wind speed increases, the plume becomes longer and

narrower atmospheric stability

During the day the air temperature decreases rapidly with the height

At night the air temperature decrease is less Classified to three stability classes: unstable, neutral, stable

Unstable the sun heats the ground faster than the heat can be removed so that the air temperature near the ground is higher than the temperature at higher elevation

Neutral the air above the ground warms and the wind speed increases

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

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ground conditions (buildings, water, trees) Affect the mechanical mixing at the surface and the

wind profile with height Trees and buildings increase mixing

height of release above ground level As the release height increases, the ground level

concentrations are reduced momentum and buoyancy of initial material released

Change the effective height of the release. The momentum of a high-velocity jet will carry the gas

higher than the point of release, resulting much higher effective release height.

Atmospheric Stability

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5Atmospheric Stability

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Release Height Effect

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Neutrally Buoyant Dispersion Models

Can be used to estimate the concentrations downwind of a release in which the gas is mixed with fresh air to the point that the resulting mixture is neutrally buoyant

The models apply to gases at low concentrations, typically in ppm range.

Two types models; plume and puff

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Plume Model Continuous Release

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Puff Model Instantaneous Release

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X =downwind,

Y =crosswind,

Z =vertical)

Coordinate System

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

Equation 5-15 is transformed to rectangular coordinates to yield

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

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Neutrally Buoyant Dispersion ModelsCase 2: Puff with no wind

spherical coordinates

and in rectangular coordinates

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Neutrally Buoyant Dispersion ModelsCase 3: Non-steady-state continuous point release with no

wind

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

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

of direction

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

and eddy diffusivity is a function of direction

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Neutrally Buoyant Dispersion ModelsCase 7: Puff with wind

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

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

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

Hr above the ground

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

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

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Pasquill-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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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 of sunlight. During the day, increased wind speed results in greater atmospheric stability,

whereas at night the reverse is true. This is due to a change in vertical temperature profiles from day to night.

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Pasquill-Gifford Models

Limitations to Pasquill-Gifford Model or Gaussian dispersion Applies only to neutrally buoyant dispersion of gases in

which the turbulent mixing is the dominant feature of the dispersion.

Typically valid for a distance of 0.1-10 km from the release point.

The predicted concentrations are time average. The models presented here assumed 10-minute time

average

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

level, coordinates fixed at release point, constant wind only in 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:

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

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

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

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

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Neutrally Buoyant Dispersion ModelsCase 13: Plume with Continuous steady-state source at height Hr above

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

Example II

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Example II: Apply Eq 5-51

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Example II: Where is max concentration?

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Example I: What is max discharge to result in 10ppm?

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Example I

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Example I

AP Dr Azmi Mohd Shariff CAB2093 Process Safety & Loss Prevention

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

with the puff

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

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Neutrally Buoyant Dispersion ModelsCase 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 Criteria

What concentration is considered dangerous? 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 of exposure to a specific substance:

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Toxic Effect Criteria

ERPG-1 is the maximum airborne concentration below which it is believed nearly all individuals could be exposed 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 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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Realistic and Worst-Case Releases

The realistic releases represent the incident outcomes with a high probability of occurring

The worst-case releases are those that assume almost catastrophic failure of the process, resulting in near instantaneous release of the entire process inventory or release over a short period of time

The worst-case releases must be used to determine the consequences study required by EPA Risk Management Plan

Table 4-5 lists a number of realistic and worst-case releases.

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Realistic and Worst-Case Releases

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Example

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Continuous release of gas (molecular weight of 30) is resulting in a concentration of 0.5 ppm at 300 m directly downwind on the ground. Estimate y and z. 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 at rural area

Solution

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

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Example

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A 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 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.

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.

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

Solution

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Example

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Due to a road accident, there is a leak of chlorine from a tank. Although the leak is quickly stopped, 4 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 stopped.Meteorological conditions: u = 2 m/s, T = 20 0C overcast conditions. stability class D. assume x=y

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Solution

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