dispersion models

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Toxic Release and Dispersion Models Chapter 5 An Equal Opportunity University

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Page 1: Dispersion Models

Toxic Release and Dispersion Models

Chapter 5

An Equal Opportunity University

Page 2: Dispersion Models

2

Toxic releases• In an accident, significant quantities of

toxicants can be released from process equipment– Disperses throughout the plant and

neighboring community– Bhopal MIC incident is an example of the

consequences of unmitigated dispersion of toxicants

• Our jobs as engineers to prevent the existence of release situations

and reduce impact should one occur

Page 3: Dispersion Models

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What affects dispersion?• Dispersion models describe airborne

transportation of toxic materials from the source

• Wind carries away material in a plume– Maximum concentration at release point

• Parameters affecting dispersion– wind speed– Atmospheric stability– Ground conditions (buildings, water,

trees)– Height of release above ground level– Momentum and buoyancy

Page 4: Dispersion Models

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How these parameters contribute

• Wind speed– Increasing wind speed results in narrower and longer

plumes• Atmospheric stability

– Air/ground temperature affects vertical mixing of the air– Three classes

unstable– Sun heats the ground faster than heat can be removed (early morning)– Enhanced turbulence

neutral– Air above ground warms and wind speed increases to reduce solar

warming effects– Air temperature difference does not affect turbulence

stable– Sun cannot heat ground as fast as it is cooling (temperature near the

ground is lower than upper air temperatures)– Influence of buoyance suppresses turbulence

Page 5: Dispersion Models

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How these parameters contribute

• Ground conditions– Affects mechanical mixing at the surface

and the wind profile with height – Vertical obstacles increase mixing

Page 6: Dispersion Models

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How these parameters contribute

• Release height significantly affects ground level concentrations

Page 7: Dispersion Models

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How these parameters contribute

• Buoyant and momentum of the material released change the effective height of the release

Sufficiently mixed with air to be neutrally

buoyant

Page 8: Dispersion Models

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Neutrally Buoyant Dispersion Models

• Estimate the concentration downwind of a release in which the gas is mixed with fresh air to the point the resulting mixture is neutrally buoyant

• plume model– Steady-state concentration of material

released from a continuous source

Page 9: Dispersion Models

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• puff model– Temporal concentration of a material from

a single release of a fixed amount

Neutrally Buoyant Dispersion Models

Page 10: Dispersion Models

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Other things to consider…• Often want to

consider multiple conditions to determine worst cases– sitting– Design mitigation– Emergency

response plans

http://www.geoassociates.net/

Page 11: Dispersion Models

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Worst Case Scenario• Worst case scenarios to consider

– Highest concentration in a plume is at the release point

– Highest concentration in a puff is at the center of the puff

– In design, assume weather conditions resulting in the highest concentration estimates

• Keep in mind the applicability of the model you choose to the release concentration (i.e., only a few ppm or do you need a more robust model)

Page 12: Dispersion Models

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Toxic Effect Criteria• You’ve chosen/developed a model of

dispersion, how do we determine how dangerous it is?

• TLV-TWA is too conservative for emergency conditions

• Alternatives?

Page 13: Dispersion Models

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Toxic Effect Criteria• Emergency response planning guidelines

(ERPG) for air contaminants – American Industrial Hygiene Association (AIHA)– ERPG-1

Max airborne concentration below which it is believed nearly all individuals could be exposed for up to 1 hour with only mild, transient effects

– ERPG-2 Max airborne concentration below which it is believed nearly

all individuals could be exposed for up to 1 hour without experiencing/developing irreversible health effects impairing ability to take protective action

– ERPG-3 Max airborne concentration below which it is believed nearly

all individuals could be exposed for up to 1 hour without experiencing/developing life-threatening health effects

Page 14: Dispersion Models

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Toxic Effect Criteria• Others…

– NIOSH Immediately Dangerous to Life or Health (IDLH)

– National Academy of Sciences/National Research Council’s emergency exposure guidance levels (EEGLs) and short-term public emergency guidance levels (SPEGLs)

– ACGIH’s TLV-STEL and TLV-C– OSHA’s PEL– New Jersey Dept. of Environmental Protection

Toxicity Dispersion (TXDS)– EPA’s toxic endpoints as part of RMP

Page 15: Dispersion Models

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Release Mitigation• Part of the consequence modeling procedure• Inherent safety

– Inventory reduction, chemical substitution, process attenuation

• Engineering design– Spill containment, emergency relief devices

• Management– Policies, procedures, maintenance programs

• Warning systems• countermeasures

– Curtains (air, water, steam), deliberate ignition of explosive cloud

• Emergency response