TOXIC RELEASE & DISPERSION MODELS

1. TOXIC RELEASE & DISPERSION MODELS Prepared by Associate Prof. Dr. Mohamad Wijayanuddin Ali Chemical Engineering Department Universiti Teknologi Malaysia

2. This case is identical to Case 10. The solution has a form similar to Equation 36. (49) Case 3 : Plume. Continuous, Steady-state Source at Height H, above Ground Level, Wind Moving in x Direction at Constant Velocity u

3. The ground level concentration is found by setting z = 0. (50) The ground centreline concentrations are found by setting y = z= 0. (51)

4. The maximum ground level concentration along the x-axis, <C>max, is found using. (52) The distance downwind at which the maximum ground level concentration occurs is found from (53) The procedure for finding the maximum concentration and the downwind distance is to use Equation 53 to determine the distance followed by Equation 52 to determine the maximum concentration.

5. For this case the centre of the puff is found at x = ut. The average concentration is given by (54) Case 4 : Puff. Instantaneous Point Source at Height H, above Ground Level. Coordinate System on Ground Moves with Puff

6. The time dependence is achieved through the dispersion coefficients, since their values change as the puff moves downwind from the release point. If wind is absent (u = 0), Equation 54 will not predict the correct result. At ground level, z = 0, and the concentration is computed using (55)

7. The concentration along the ground at the centreline is given at any y = z = 0, (56) The total integrated dose at ground level is found by application of Equation 42 to Equation 55. The result is (57)

8. For this case, the result is obtained using a transformation of coordinates similar to the transformation used for Case 7. The result is (58) where t is the time since the release of the puff. Case 5 : Puff. Instantaneous Point Source at Height H, above Ground Level. Coordinate System Fixed on Ground at Release Point

9. The plume model describes the steady state behaviour of material ejected from a continuous source. The puff model is not steady-state and follows the cloud of material as it moves with the wind. As a result, only the puff model is capable of providing a time dependence for the release. The puff model is also used for continuous releases by representing the release as a succession of puffs. For leaks from pipes and vessels, if tp is the time to form one puff, then the number of puffs formed, n, is given by (59) Comparison of the Plume and Puff Models

10. where t is the duration of the spill. The time to form one puff, tp, is determined by defining an effective leak height, Heff. Then, (60) where u is the wind speed. Empirical results show that the best Heff to use is (61) For a continuous leak, (62)

11. and for instantaneous release divided into a number of smaller puffs, (63) where (Qm*)total is the release amount. This approach works for liquid spills, but not for vapor releases. For vapor releases a single puff is suggested. The puff model is also used to represent changes in wind speed and direction.

12. Example 2 On an overcast day, a stack with an effective height of 60 meters is releasing sulfur dioxide at the rate of 80 grams per second. The wind speed is 6 meters per second. Determine a. The mean concentration of SO2 on the ground 500 meters downwind. b. The mean concentration on the ground 500 meters downwind and 50 meters crosswind. c. The location and value of the maximum mean concentration on ground level directly downwind.

13. a. This is a continuous release. The ground concentration directly downwind is given by Equation 51. (51) From Table 2, the stability class is D. the dispersion coefficients are obtained from Figures 10 and 11. The resulting values are sy = 36 meters and sz = 18.5 meters. Substituting into Equation 51 Solution

14. b. The mean concentration 50 meters crosswind is found using Equation 50 and setting y = 50. The results from part a are applied directly,

15. c. The location of the maximum concentration is found from Equation 53, From Figure 11, the dispersion coefficient has this value at x = 1500 m. At x = 1500 m, from Figure 10, sy = 100 m. The maximum concentration is determined using Equation 52,

16. Example 3 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. d. Determine the size of the cloud, based on the TLV, at a point 5 km directly downwind on the ground. Assume the conditions of Part b.

17. 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. Solution

18. 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 2 - 3 m/s wind.

19. 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. 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.

20. 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.

21. 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.

22. d. The downwind centreline concentration is given by Equation 40. (40) The time required for the centre of the plume to arrive is At a downwind distance of 5 km, from Figures 12 and 13, Substituting the numbers provided,

23. where x has units of meters. Rearranging and combining leads to a quadratic equation, The cloud is 164 meters wide at this point, based on the TLV concentration. At 2 m/s, it will take approximately, to pass. An appropriate emergency procedure would be to alert residents to stay indoors with the windows closed and ventilation off until the cloud passes. An effort by the plant to reduce the quantity of chlorine released is also indicated.

24. Effect of Release Momentum and Buoyancy Figure 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 14. 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.

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

26. 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)

27. 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.

28. 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.

29. 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.

30. The release mitigation design procedure is shown in Figure 15. 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.

31. 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.

32. Figure 15 The release mitigation procedure.

33. Table 4 Release mitigation approaches