1. Neutrally Buoyant Gas Dispersion Instructor: Dr. Simon Waldram
CHEN 459 – Spring ‘09
Neutrally Buoyant Gas Dispersion By
Khaldoon AlObaidi
Instructor: Dr. Simon Waldram

2. Outline Goals of The Project. Introduction. Gas Dispersion Models.
Pasquill-Gifford Model.
Model Inputs/Outputs.
Isopleths.
Demonstration.
A. Part 1 of the model.
B. Cave demonstration.
Recommendations.
References.

3. Goals
1. Investigate the characteristics of neutrally buoyant gas dispersion at different conditions.
2. Determine the isopleth of the dangerous area using the TLV-TWA value.
3. Using the Cave facility to represent the change of concentration over time and distance ( 3-D representation).
* It can be added that comparison between the dispersion of various gases can be studied.
TLV-TWA: The average concentration under which most people can work consistently for eight hours, day in, day out, with no harmful effects. Gas or vapors are expressed in Parts Per Million (ppm),

4. Introduction
Gas is state of matter, consisting of a collection of particles (molecules, atoms, ions, electrons, etc.) without a definite shape or volume that is in more or less random motion.
Weak attractive forces.
Different gases will diffuse and disperse at different rates.
All around us and used in large quantities in industries.
Some are toxic and harmful and their study is required.

5. Gas Dispersion Models
Models have been developed for the study of gas dispersion: all of them consider the conditions that may have an effect on the dispersion of the gas.
Important to provide evacuation scenarios in case of leakage.
Help to determine shape and concentration of the cloud and the area affected.
Help to generate some risk assessments.

6. Pasquill-Gifford Model
Applicable for neutral buoyant gases (or those with densities close to air’s density like carbon monoxide).
Classic Gaussian plume/puff model that was generated originally for smokestack emissions.
The governing equation is
<C> = the average concentration of the chemical in the air.
<uj> = is the average velocity.
Kj = eddy diffusivity with units of area/time

7. Pasquill-Gifford Model
Since Kj changes with position, time, wind velocity, and prevailing weather conditions, new terms were introduced to solve this difficulty by Sutton.
These terms are defined as the standard deviation of horizontal distribution (y) and vertical distribution (z) of pollutant concentration.
Both y and z are themselves functions of x, the distance downwind from the gas release point when the x axis is aligned with the wind direction.

8. Pasquill-Gifford Model
Consider the distance (x, y, z), wind speed, and atmospheric stability as the main parameters to determine the concentration.
All the work of this project is based on this model.

9. Pasquill-Gifford Stability Index

10. The Model Inputs

11. The Model Outputs

12. The Model Outputs
CO release Plume day, 2 kg/s, u = 3.5 m/s, Strong Isolation (B)

13. C&L, p194, Case 15, Puff, instantaneous point release at height h = 20 m
above ground, wind at 2 ms-1, PG dispersion coefficients, stability class E
(slightly stable). Qm = 100 kg
Time t = 5, 100, 200, seconds
7/29

14. Recommendations
It is recommended that further studies should be performed using this model, or another, to investigate the release of gases that are more dense than air.
Also, as an aid to risk assessments, detailed maps can be used to project the release over a specific geographical region.
The model works for specified gases for which the probit equations for specific causative variables have been defined. Other gases and their causative variable probit laws can be added to the Excel Sheets.

15. Thanks To Dr Simon P. Waldram, Senior Professor, as supervisor.
Use of the Cave Facility in the ITS Department was facilitated through the help and work of Mr. Ali Sheharyar.

16. Literature Cited
Crowl/Louvar. Chemical Process Safety Fundamentals with Applications. 2nd. Edition. Prentice Hall PTR: New Jersey; 2002.
Hemond/Fechner-Levy‏. Chemical Fate and Transport in the Environment‏.2nd. Edition. Academic Press; 2000.
Till/Grogan‏. Radiological Risk Assessment and Environmental Analysis‏. Oxford University Press: US; 2008.
Authority Civil Aviation, Qatar Weather Website.

17. THANK YOU! ?
Questions