TOXIC RELEASE

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TOXIC RELEASE DISPERSION MODELS
Prepared by Associate Prof. Dr. Mohamad
Wijayanuddin Ali Chemical Engineering
Department Universiti Teknologi Malaysia
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During and accident, process equipment can
release toxic materials very quickly and in
significant enough quantities to spread in
dangerous clouds throughout a plant site and the
local community. A few examples are - -
Explosive rupture of a process vessel due to
excessive pressure caused by a runaway
reaction. - Rupture of a pipeline containing
toxic materials at high
pressure. - Rupture of a tank containing toxic
material stored above its atmospheric
boiling point. - Rupture of a train or truck
transportation tank following an
accident. Serious accidents (such as Bhopal)
emphasize the importance of emergency planning
and for designing plants to minimize the
occurrence and consequences of a toxic release.
Toxic release models are routinely used to
estimate the effects of a release on the plant
and community environments.
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An excellent safety program strives to identify
problems before they occur. Chemical engineers
must understand all aspects of toxic release to
prevent the existence of release situations and
to reduce the impact of a release if one occurs.
This requires a toxic release model. There are 3
steps in utilizing a toxic release
model. 1. Identify the design basis. What process
situations can lead to a release, and which
situation is the worst? 2. Develop a source model
to describe how materials are released and the
rate of release. 3. Use a dispersion model to
describe how materials spread throughout the
adjacent rates. The main emphasis of the toxic
release model is to provide a tool useful for
release mitigation. The source and dispersion
models predict the area affected and the
concentration of vapor throughout. The design
basis is valuable for eliminating situations that
could result in a release.
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Various options are available based on the
predictions of the toxic release model. To name a
few, these are 1. develop an emergency response
plan with the surrounding community 2. develop
engineering modifications to eliminate the
source of the release 3. enclose the
potential release and add appropriate vent
scrubbers or other vapor removal
equipment 4. reduce inventories of hazardous
materials to reduce the quantity released
and 5. add area monitors to detect incipient
leaks and provide block valves and engineering
controls to eliminate hazardous levels of
spills and leak. These options are discussed
in more detail on release mitigation.
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Design Basis
The design basis describes the various scenarios
leading to toxic release it looks for what can
go wrong. For any reasonably complex chemical
facility, thousands of release scenarios are
possible it is not practicable to elucidate
every scenario. Most toxic release studies strive
to determine the largest practicable release and
the largest potential release. The largest
practicable release considers releases having a
reasonable chance for occurrence. This includes
pipe ruptures, holes in storage tanks and process
vessels, ground spills, and so forth. The largest
potential release is a catastrophic situation
resulting in release of the largest quantity of
material. This includes compete spillage of tank
contents, rupture of large bore piping, explosive
rupture of reactors, and so forth. Table 1
contains examples of largest practicable and
largest potential releases.
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Development of a proper design basis requires
skill, experience, and considerable knowledge of
the process. Hazards identification procedures
are very helpful. The completed design basis
describes - 1. what went wrong, 2. the state of
the toxic material released (solid, liquid, or
vapor), and 3. the mechanism of release
(ruptured pipe, hole in storage vessel, and so
on).
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Table 1 Examples of Largest Practicable and
Largest Potential Releases
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Example 1
Water is treated at a swimming pool using a
100-lb bottle of chlorine. The chlorine is fed
from the bottle through a 1/4-in line to the
water treatment facility. A relief valve on the
tank prevents excessive pressure from rupturing
the tank. Chlorine is stored in the bottle as a
liquid under pressure and will boil when the
pressure is reduced. Identify the release
scenarios.
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Solution
Scenario 1 The bottle of chlorine ruptures,
possibly from dropping the tank while unloading
from a truck. The entire contents is spilled,
with a fraction flashing immediately into vapor
and the remaining liquid forming a boiling pool
on the ground. Scenario 2 A hole forms in the
tank either because of mechanical rupture or
corrosion. A jet of flashing chlorine and a
boiling pool of liquid chlorine forms. Scenario 3
The relief valve fails open, forming a jet and
pool of boiling chlorine. Scenario 4 The feed
line to the treatment plant fails with a jet and
pool of boiling chlorine.
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Scenario 5 A fire develops around the chlorine
tank, heating the tank until the relief valve
opens. Scenario 6 A fire develops around the
chlorine tank, but the relief valve fails closed.
The tank pressure builds until it ruptures,
spilling the entire tank contents explosively.
The largest practicable release could be either
scenarios 2, 3, or 4, depending on the rate of
material release computed using an appropriate
source model. The largest potential release is
scenario 6, releasing the entire tank contents
almost immediately.
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Source Models
 The purpose of the source model is to - 1. The
form of material released, solid, liquid or
vapor 2. The total quantity of material
released and 3. The rate at which it is
released. This information is required for any
quantitative dispersion model study.
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Dispersion Models
Dispersion models describe the airborne transport
of toxic materials away from the accident site
and into the plant and community. After a
release, the airborne toxic is carried away by
the wind in a characteristic plume as shown in
Figure 1 or a puff, shown in Figure 2. The
maximum concentration of toxic material occurs at
the release point (which may not be at ground
level). Concentrations downwind are less, due to
turbulent mixing and dispersion of the toxic
substance with air.
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Figure 1 Characteristic plume formed by a
continuous release of material.
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Figure 2 Puff formed by near instantaneous
release of material.
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A wide variety of parameters affect atmospheric
dispersion of toxic materials - - Wind speed -
Atmospheric stability - Ground conditions,
buildings, water, trees - Height of the release
above ground level - Momentum and buoyancy of
the initial material released As the wind speed
increases, the plume in Figure 1 becomes longer
and narrower the substance is carried downwind
faster but is diluted faster by a larger quantity
of air. Atmospheric stability relates to vertical
mixing of the air. During the day the air
temperature decreases rapidly with height,
encouraging vertical motions. At night the
temperature decrease is less, resulting in less
vertical motion. Temperature profiles for day and
night situations are shown in Figure 3. Sometimes
an inversion will occur.
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Figure 3 Air temperature as a function of
altitude for day and night conditions. The
temperature gradient affects the vertical air
motion.
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During and inversion, the temperature increases
with height, resulting in minimal vertical
motion. This most often occurs at night as the
ground cools rapidly due to thermal
radiation. Ground conditions affect the
mechanical mixing at the surface and the wind
profile with height. Trees and buildings increase
mixing while lakes and open areas decrease it.
Figure 4 shows the change in wind speed versus
height for a variety of surface conditions. The
release height significantly affects ground level
concentrations. As the release height increases,
ground level concentrations are reduced since the
plume must disperse a greater distance
vertically. This is shown in Figure 5.
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Figure 4 Effect of ground conditions on vertical
wind gradient.
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Figure 5 Increased release height decreases the
ground concentration.
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The buoyancy and momentum of the material
released changes the effective height of the
release. Figure 6 demonstrates these effects.
After the initial momentum and buoyancy has
dissipated, ambient turbulent mixing becomes the
dominant effect. Two types of vapor cloud
dispersion models are commonly used the plume
and puff models. The plume model describes the
steady-state concentration of material released
from a continuous source. The puff model
describes the temporal concentration of material
from a single release of a fixed amount of
material. The distinction between the two models
is shown graphically in Figures 1 and 2. For the
plume model, a typical example is the continuous
release of gases from a smokestack. A
steady-state plume is formed downwind from the
smokestack. For the puff model, a typical example
is the sudden release of a fixed amount of
material due to the rupture of a storage vessel.
A large vapor cloud is formed that moves away
from the rupture point.
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Figure 6 The initial acceleration and buoyancy
of the released material affects the plume
character. The dispersion models discussed in
this chapter represent only ambient turbulence.
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Figure 7 Steady-state, continuous point source
release with wind. Note coordinate system x is
downwind direction, y is off-wind direction , and
z is vertical direction.
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Figure 8 Puff with wind. After the initial,
instantaneous release, the puff moves with the
wind.