Dispersion phenomenon

Understanding the dispersion phenomenon in industrial accidents

Industrial accidents often involve the release of hazardous materials during storage, transport, or processing, posing significant threats to health and the environment. Understanding the behaviour of these toxic or flammable releases is crucial for predicting their impact on people, equipment, and the environment, and implementing effective safety measures and emergency plans. This chapter provides essential insights into the dispersion phenomenon of accidental releases.

What is dispersion?

Dispersion refers to the spreading of particles or substances within a fluid, which can be either a gas or liquid. This chapter primarily focuses on the dispersion of gases and vapours in the atmosphere. Dispersion relies on three key physical concepts:

• Initial phase

• Atmospheric conditions and terrain features

• Dispersion regimes

Initial phase

Dispersion originates from a source, typically involving the release of a hazardous chemical substance. This release might occur from a high-pressure source, such as compressed gas from a vessel, resulting in an initial turbulent free jet. The velocity of this jet significantly exceeds that of the surrounding air, resulting in friction and turbulence, which characterise the initial phase of dispersion.

Atmospheric Conditions and Terrain Features

Atmospheric conditions like wind velocity and weather stability play a pivotal role in determining the dispersion of chemicals. These conditions dictate how far and wide a chemical cloud spreads. Natural features such as mountain ranges, valleys, and urban canyons can act as barriers or channels, altering the movement of chemicals and leading to complex dispersion patterns.

Dispersion regimes

The behaviour of dispersion is affected by the physical characteristics of the vapour cloud, including its density, molecular weight, and temperature relative to the surrounding air. This leads to three primary dispersion regimes:

• Lighter than air: Chemicals with a molecular weight lower than that of air, such as hydrogen, ammonia, or methane, fall into this category. Buoyant forces play a significant role in concentration profiles, causing chemicals to ascend.

• Neutral gas: Gases with a density similar to that of air, like ethylene, belong to this regime. The cloud disperses horizontally at the release height. This is common in dispersions resulting from phenomena like pool evaporation (such as gasoline spills), as the vapour cloud has ample time to mix thoroughly with the air.

• Dense gas: This regime encompasses gas or vapour clouds with a higher density than air. Gravity heavily influences the dispersion, causing the cloud to drift with the wind. Examples include chlorine and clouds containing liquid droplets that evaporate over time, forming a larger vapour cloud.

A vapour cloud might transition between regimes during dispersion. For instance, a cloud with cold liquid droplets might become lighter as it warms and the droplets evaporate. Once the cloud reaches ambient temperature, it behaves as a neutral gas.

Dispersion typically occurs in a part of the atmosphere called the mixed layer, ranging from 200 to 2000 meters above the ground. Turbulent eddies within this layer distribute materials 1000 times more efficiently than molecular diffusion. Therefore, wind speed and direction, along with atmospheric turbulence, are critical in determining how gas clouds spread. While humidity and temperature are less influential, thermal inversion plays a decisive role (Bosch, 2005).

Conclusion

Industrial accidents involving hazardous materials present significant risks to both human health and the environment. Understanding the behaviour of toxic or flammable releases is crucial for assessing their impact and implementing effective safety measures and emergency plans. This chapter delves into the dispersion of gas or vapour clouds resulting from such accidents, highlighting the key physical concepts behind desperation: the initial phase of dispersion, atmospheric conditions together with terrain features, and the physical properties of substances. These factors influence the spread of chemicals in the atmosphere, leading to various dispersion regimes such as lighter than air, neutral gas, and dense gas. Additionally, the dispersion process occurs within a mixed layer of the atmosphere, where turbulent winds play a critical role in distributing materials. By grasping the dynamics of dispersion, we can better anticipate and mitigate the consequences of industrial accidents, safeguarding both lives and ecosystems.

References

Bosch, C. v. (2005). Methods for the calculation of physical effects 'Yellow book' CPR 14E. The Hague: Ministerie van Verkeer en Waterstaat.