D
Dispersion Models
This section presents the basics of two dispersion models relevant for gas propagation from a stationary point source. It also summarizes timescales for global mass
transport in the atmosphere and an exchange model for the penetration of gas into a
building.
In addition, emissions can also be released from spontaneous point sources and
sources elevated far above the ground (e.g., a chimney, thermal chimney effect, etc.).
Large obstacles can also stand in the way of an emission and affect its propagation.
In these cases, many other and more complex propagation models exist and are
introduced in other literature (CPD, 1996; Elvers, 2000).
D.1
Heavy Gas Dispersion
Examples of heavy gas spills include Flixborough in 1974 and Bhopal in 1984 (see
Appendix A). In heavy gas dispersion, the spreading of the substance after release
is affected by the wind velocity and displacement due to gravity. The validity of
this type of model is limited to being within close range of the release site (less
than 100 m). The schematic in Fig. D.1 illustrates the model used for heavy gas
dispersion.
Conditions for propagation by heavy gas dispersion:
• The density of the gas (ρ Gas ) must be greater than the density of the air (ρ Air ),
meaning that the density difference ( = ρ Gas − ρ Air ) is the driving force of
propagation
• The stationary point source of gas (mass flow ˙
M) is constant and located on a
grid at x, y, z = 0 (as seen in Fig. D.1)
• x-direction: wind direction (u = wind speed)
• y-direction: propagation occurs due to ρ at a speed of v g
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
K. Hungerbühler et al., Chemical Products and Processes,
https://doi.org/10.1007/978-3-030-62422-4
279
Dispersion Models
This section presents the basics of two dispersion models relevant for gas propagation from a stationary point source. It also summarizes timescales for global mass
transport in the atmosphere and an exchange model for the penetration of gas into a
building.
In addition, emissions can also be released from spontaneous point sources and
sources elevated far above the ground (e.g., a chimney, thermal chimney effect, etc.).
Large obstacles can also stand in the way of an emission and affect its propagation.
In these cases, many other and more complex propagation models exist and are
introduced in other literature (CPD, 1996; Elvers, 2000).
D.1
Heavy Gas Dispersion
Examples of heavy gas spills include Flixborough in 1974 and Bhopal in 1984 (see
Appendix A). In heavy gas dispersion, the spreading of the substance after release
is affected by the wind velocity and displacement due to gravity. The validity of
this type of model is limited to being within close range of the release site (less
than 100 m). The schematic in Fig. D.1 illustrates the model used for heavy gas
dispersion.
Conditions for propagation by heavy gas dispersion:
• The density of the gas (ρ Gas ) must be greater than the density of the air (ρ Air ),
meaning that the density difference ( = ρ Gas − ρ Air ) is the driving force of
propagation
• The stationary point source of gas (mass flow ˙
M) is constant and located on a
grid at x, y, z = 0 (as seen in Fig. D.1)
• x-direction: wind direction (u = wind speed)
• y-direction: propagation occurs due to ρ at a speed of v g
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
K. Hungerbühler et al., Chemical Products and Processes,
https://doi.org/10.1007/978-3-030-62422-4
279