Transcript No Slide Title

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
Largest practicable release : large release with a reasonable chance to
occur.
Rupture of small bore piping, 1-inch maximum.
Partial flange gasket blowout of large diameter piping (for example, 50%
blowout of a 2-inch line resulting in an equivalent hole diameter of 1inch).
Failure of a ¾-inch fusible plug on a 1-ton cylinder.
Generally limited release duration (15 minutes typical based on time
required for operator intervention to stop the leak).
Largest potential release : catastrophic release of maximum amount of
material.
Rupture of a 2 or 3 inch liquid line.
Tank truck rupture on a highway (3 or 4 inch hole size assumed, typical).
Typically, entire source vessel inventory spilled.
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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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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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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 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
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formed that moves away from the rupture point.
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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The puff model can be used to describe a plume; a plume is
simply the release of continuous puffs. If, however, steady-state
plume information is all that is required, the plume model is
recommended since it is easier to use. For studies involving dynamic
plumes (for instance the effect on a plume due to a change in wind
direction), the puff model must be used.
Consider the instantaneous release of a fixed mass of material,
Qm*, into an infinite expanse of air (a ground surface will be added
later). The coordinate system is fixed at the source. Assuming no
reaction or molecular diffusion, the concentration, C, of material due
to this release is given by the advection equation.
C

u j C   0

t
x j
(1)
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where uj is the velocity of the air and the subscript j represents the
summation over all coordinate directions, x, y, and z. if the velocity, uj,
in Equation 1 is set equal to the average wind velocity and the equation
is solved, one would fond that the material disperses much faster than
predicted. This is due to turbulence in the velocity field. If one were
able to specify the wind velocity exactly with time and position,
including the effects due to turbulence, Equation 1 would predict the
correct concentration. Unfortunately, no models are currently available
to adequately describe turbulence. As a result, an approximation is
used. Let the velocity be represented by an average (or mean) and
stochastic quantity;
u j  u j  u 'j
(2)
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where <uj> is the average velocity and uj’ is the stochastic
fluctuation due to turbulence. It follows that the concentration, C, will
also fluctuate as a result of the velocity field, so,
C  C  C'
(3)
where <C> is the mean concentration and C’ is the stochastic
fluctuation. Since the fluctuations in both C and uj are around the
average or mean values, it follows that,
u 'j  0
(4)
C'  0
Substituting Equation 2 and 3 into Equation 1 and averaging the
result over time, yields,
 C
t


x j
u
j

C 

u 'j C '  0
x j
(5)
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The terms <uj>C’ and uj’<C> are zero when averaged (<<uj>C’> =
<uj><C’> = 0), but the turbulent flux term <uj’C’> is not necessarily zero
and remains in the equation.
An additional equation is required to describe the turbulent flux. The
usual approach is to define an eddy diffusivity, Kj (with units of area/time),
such that
 C
'
'
u jC  K j
(6)
x j
substituting Equation 6 into Equation 5 yields,
 C
t


x j
u
j
C



x j

 C
K j

x j





(7)
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If the atmosphere is assumed to be incompressible,
 uj
x j
(8)
0
and Equation 7 becomes
 C
t
 uj
 C
x j


x j

 C
K j

x j





(9)
Equation 9, together with appropriate boundary and initial
conditions, forms the fundamental basis for dispersion modelling. This
equation will be solved for a variety of cases.
The coordinate system used for the dispersion models is shown in
Figures 7 and 8. The x-axis is the centreline directly downwind from the
release point and is rotated for different wind directions. The y-axis is
the distance off of the centreline and the z-axis is the elevation above the
release point. The point (x,y,z) = (0,0,0) is at the release point. The
coordinates (x,y,0) are level with the release point, and the coordinates
(x,0,0) are along the centreline, or x-axis.
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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.
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