Dispersion of Heavy Petroleum Gases in the Atmosphere

Abstract

This research addresses hazards of liquefied petroleum gases in its applications. The hazards are caused by the leakage of gases and valued by risk analysis and assessment in safety management. However, to accurately calculate the risks, detailed knowledge on processes and development of leaked gases are critical. Hence, the aim of this PhD is to investigate the processes and development of the gas leakage dispersion.

The leakage of a liquefied natural gas or liquefied petroleum gas experiences evaporating and mixing with ambient air to form a vapour cloud, heavier than air. When concentration of the vapour cloud lies within the limit of flammability and if the cloud make contact with the source of ignition and then the cloud could ignite that in case, a large and strong explosion will firstly occur and then fire follows, which could cause damage to the people and properties. This research study is therefore developing a novel approach for analysing vapour cloud dispersion at petroleum gas processing facilities. This is achieved based on existing computational fluid dynamics technology and tools such as ANSYS-CFX, FDS, and in-house solvers. Using these tools, the process for liquefied gas leakages and dispersions are simulated and the characteristics of petroleum gas vapour clouds are analysed. Three main case studies and flashing jets of liquefied petroleum gases are simulated and explored. Effects of the obstacles which are placed behind the leakage source with different height on heavy gas dispersion in the atmosphere are investigated under various conditions of the stratified flows. Sub-scale structures, which are of the order of obstacle lengths, heights and dominated by obstacle configurations were created in the gas clouds. Such sub-scale structures significantly impact the formation of gas clouds. Finally, recommendations for computational modelling of petroleum gas clouds are developed. The achieved results from this research evidently show consistency when compared intensively with published data by other researchers.

The study discusses thermodynamic properties of atmosphere along with the properties of petroleum gas. Properties of heavy gas clouds and the process of their formation has been discussed. Also, combustion reactions are reviewed alongside the happened accidents and the simulation of fire explosions is discussed.

In this work, the formation of flashing jet has been analysed with an example of liquefied petroleum gas. Two-phase flow has been brought into consideration through the Eulerian
Lagrangian model and gas phase calculations are done through Navier-Stokes’s equation. Thermodynamic properties of the liquefied gas along with their storage conditions inside the liquefied tanks are discussed. Furthermore, Liquefied mist has been tracked through Lagrangian particles. Procedure for the mathematical modelling of liquefied mist along with the dynamics of the particle has been discussed. Distribution of the mist droplets size have been discussed. Change in the droplet’s temperature has been linked with heat and mass transfer through different equations.

The findings from this work will provide a framework to analyse the behaviour of liquefied natural gas or liquefied petroleum gas leakages that will be benefitted to investigate accidental fires and explosions. The findings also provide a basis for risk assessment and management.