0. TOXIC RELEASE &
DISPERSION MODELS

1. Example 2
Chlorine is used in a particular chemical process. A source model study indicates that for a particular accident scenario 1.0 kg of chlorine will be released instantaneously. The release will occur at ground level. A residential area is 500 m away from the chlorine source. Determine
a. The time required for the centre of the cloud to reach the residential area. Assume a wind speed of 2 m/s.
b. The maximum concentration of chlorine in the residential area. Compare this with a TLV for chlorine of 0.5 ppm. What stability conditions and wind speed procedures the maximum concentration?
c. Determine the distance the cloud must travel to disperse the cloud to a maximum concentration below the TLV. Use the conditions of Part b.

2. Solution
a. For a distance of 500 m and a wind speed of 2 m/s, the time required for the centre of the cloud to reach the residential area is
This leaves very little time for emergency warning.

3. b. The maximum concentration will occur at the centre of the cloud directly downwind from the release. The concentration is given by Equation 41.
(41)
The stability conditions are selected to maximize <C> in Equation 41. This requires dispersion coefficients of minimum value. From Figures 12 and 13, this occurs under stable condition. From Table 2, this will occur at night with a m/s wind.

4. Figure 12 Horizontal and vertical dispersion coefficient for puff model.

5. Table 2 Atmospheric Stability Classes for Use with the Pasquill-Gifford Dispersion Model
Stability class for puff model :
A,B : unstable
C,D : neutral
E,F : stable

6. Assume a slow moving cloud of 2 m/s
Assume a slow moving cloud of 2 m/s. from Figures 12 and 13, at 500 m, sy = 5.2 m and sz = 2.2 m. also assume sx = sy. From equation 41,
Assuming a pressure of 1 atm and a temperature of 298°K, the concentration in ppm is 737 ppm. This is much higher than the TLV of 0.5 ppm (Table 2.8). Any individuals within the immediate residential area, and any personnel within the plant will be excessively exposed if they are outside and downwind from the source.
X ppm   =   (Y mg/m3)(24.45)/(molecular weight)

7. c. From Table 2 - 8, the TLV of 0. 5 ppm is 1. 45 mg/m³ or 1
c. From Table 2 - 8, the TLV of 0.5 ppm is 1.45 mg/m³ or 1.45×10-6 kg/m³. The concentration at the centre of the cloud is given by Equation 41. Substituting the known values,
This equation is satisfied at the correct distance from the release point. A trial and error procedure is required. The procedure is
1. Select a distance, x.
2. Determine sx, sy, and sz using Figures 12 and 13.
3. Check if dispersion coefficients satisfy above equation.

8. The procedure is continued until the equation is satisfied
The procedure is continued until the equation is satisfied. This produces the following results,
The distance is interpolated to about 10.3 km. This is quite a substantial distance considering that only 1.0 kg of chlorine is released.

9. Effect of Release Momentum and Buoyancy
Figure 5.6 indicates that the release characteristics of a puff or plume are dependent on the initial release momentum and buoyancy. The initial momentum and buoyancy will change the effective height of release. A release that occurs at ground level but in an upward spouting jet of vaporizing liquid will have a greater “effective” height than a release without a jet. Similarly, a release of vapor at a temperature higher than the ambient air temperature will rise due to buoyancy effects, increasing the “effective” height of the release.
Both of these effects are demonstrated by the traditional smokestack release shown in Figure The material released from the smokestack contains momentum, based on its upward velocity within the stack pipe, and it is also buoyant, since its temperature is higher than the ambient temperature.

10. Figure 14 Smokestack plume demonstrating initial buoyant rise of hot gases.

11. Thus, the material continues to rise after its release from the stack
Thus, the material continues to rise after its release from the stack. The upward rise is slowed and eventually stopped as the released material cools and the momentum is dissipated.
For smokestack releases, Turner suggests using the empirical Holland formula to compute the additional height due to the buoyancy and momentum of the release,
(64)

12. where
ΔHr is the correlation to the release height, Hr
ūs is the stack gas exit velocity, in m/s
d is the inside diameter, in m
ū is the wind speed, in m/s
P is the atmospheric pressure, in mb
Ts is the stack gas temperature, in °K
Ta is the air temperature, in ° K
For heavier than air vapors, if the material is released above ground level, the material will initially fall towards the ground until it disperses enough to reduce the cloud density.

13. Effect of Buildings and Structures
Building and structures provide barriers to vapor clouds and ground releases. The behaviour of vapor clouds moving around buildings and structures is not well understood.

14. Release Mitigation
The purpose of the toxic release model is to provide a tool for performing release mitigation. Release mitigation is defined as “lessening” the risk of a release incident by acting on the source (at the point of release) either -
1. in a preventive way by reducing the likelihood of an event which could generate a hazardous vapor cloud; or
2. in a protective way by reducing the magnitude of the release and/or the exposure of local persons or property.

15. The release mitigation design procedure is shown in Figure 5. 15
The release mitigation design procedure is shown in Figure Once the toxic release model is completed, it is used to predict the impact of the release. This includes the area and number of people affected and the manner in which they are affected. At this point, a decision is made whether the hazards are acceptable. If the hazards are acceptable, the process is operated. If the hazards are unacceptable, a change is made to reduce the hazard. This includes changing the process, the operation of the process, or invoking an improved emergency procedure. A new toxic release model is developed for the process incorporating the changes and the release impact is again assessed. The procedure is continued until the hazards are reduced to acceptable levels.

16. The best method for preventing a release situation is to prevent the accident leading to the release in the first place. However, engineers must be prepared in the event of an accident. Release mitigation involves -
1. Detecting the release as quickly as possible;
2. Stopping the release as quickly as possible; and
3. Invoking a mitigation procedure to reduce the impact of the release on the surroundings.
Once a release is in vapor form, the resulting cloud is nearly impossible to control. Thus, an emergency procedure must strive to reduce the amount of vapor formed.
Table 4 provides additional methods and detail on release mitigation techniques.

17. Figure 15 The release mitigation procedure.

18. Table 4 Release mitigation approaches