Proceedings of CUChE Alumni Symposium 2022
On “Circular Economy on Sustainable Basis: The Role of Chemical Engineers”
CUChEAA ISBN: 978-81-954649-1-3
December 2022 P a g e | 33 Volume 2, Issue 1
continuous exposure to a person present nearby whereas
instantaneous release (puff) may cause short time
Exposure with higher concentration. Again, depending
upon the nature of gas released (dense/lighter than air)
dispersion can be classified as Gaussian dispersion (for
lighter gases) and Heavy gas dispersion (for heavy
gases).
The conventional Pasquill-Gifford model holds good
results for gas dispersions for both Plume and Puff
release.
Table1. Meteorological Conditions Defining Pasquill-
Gifford Stability Classes [CPQRA, 2000]
Where, ‘A’ represents - extremely unstable conditions,
‘B’ represents - moderately unstable conditions, ‘C’
represents - slightly unstable conditions, ‘D’ represents -
neutral conditions, ‘E’ represents - slightly stable
conditions and ‘F’ represents - moderately stable
conditions [CPQRA, 2000]
The dense (heavy) gas dispersion or Gaussian (light)
dispersion depends upon the following factors:
a) Molecular weight of the gas, b) Release
temperature of the gas, c) Presence of spray
(minute droplets in gas) and d) Temperature
and humidity of ambient air.
Styrene (M.W. 104.15) gives heavier than air cloud both
at ambient temperature and at its boiling point (145 ºC).
The droplets of liquid suspended in the gas vaporize by
taking latent heat of vaporization from the gas, thus cool
it (making it heavier) while the effect of water droplets
condensing out from humid air by adding heat of
condensation to the gas, making it lighter. Thus, low air
humidity (dry air) and a large flash-off of liquid droplets
make it denser than air cloud.
Other factors influencing gas dispersion are:
a. Atmospheric Stability
The atmospheric conditions have been divided into six
classes of stability by Pasquill. Class A represents
unstable conditions of strong sunlight, clear sky and high
level of atmospheric turbulence conditions which
promote rapid mixing and quick dispersion of any
released gases. Class F represents stable conditions
occurring at night and consisting of light winds, low
level turbulence and inversion conditions. Class D is in
between and known as the neutral condition.
b. Effect of wind Meandering on Evacuation or
Protection action zones
The direction of wind is rarely steady over any
significant period of time, and it tends to shift back and
forth between various directions. This shifting over time
is referred to as meandering. The practical significance
of wind meandering is that an area larger than that
predicted by the application of dispersion models may
require evacuation or other means like sheltering
populations in place during an actual emergency.
Other influencing factors in gas dispersion are source
geometry, elevated discharge, local terrain, discharge
velocity and Threshold Limit Value (TLV) or other
effect concentrations for toxic gases and presence of
mists, fumes, aerosols, fine dusts etc.
The Britter and McQuaid (1988) [CPQRA, 2000] model
has been used for heavy gas dispersion study. The model
is best suited for instantaneous or continuous ground
level release of dense gas. Following a typical puff
release, a cloud having similar vertical and horizontal
dimensions (near the source) may form. The dense cloud
slumps under the influence of gravity increasing its
diameter and reducing its height. Considerable initial
dilution occurs due to the gravity-driven intrusion of the
cloud into the ambient air. Subsequently the cloud height
increases due to further entrainment of air across both
the vertical and horizontal interface. After sufficient
dilution occurs, normal atmospheric turbulence
predominates over gravitational forces and typical
Gaussian dispersion characteristics are exhibited.
Joseph (2004) analyzed Styrene transfer hose
catastrophic failure. In this study mainly failure
mechanism (corrosion) has been studied during loading
and unloading time. However, the atmospheric Styrene
concentration has not been studied after release. Ruj et
al. (2012) studied offsite emergency planning aspects
using Complex Hazards Air Release Model (CHARM)
software package. However, using fundamental principle
of the heavy gas dispersion, analysis has not been carried
out in this case. Dandrieux et al. (2002) carried out small
scale experiment on Styrene release and dispersion.
However, the effect of release and dependency of
weather parameter is not studied. So there is a scope for
studying Styrene accidental release scenario using
fundamental principle of heavy gas dispersion. In this
paper, we have investigated concentration of Styrene at
various distances by using heavy gas dispersion model
(Britter and Mcquaid, 1988) and DOW’s Chemical
Exposure Index (CEI) [Dow’s CEI, AICHE, 1994]
method. Subsequently, the effect distance results have
been verified using ALOHA software. Also emergency
mitigating measures adopted during the release has been
outlined in this paper. Yashoda et al .(2020) studied the
Vizag styrene release incident, however, effect of
temperature on styrene release and societal risk aspects
were not covered . Hence this study is carried out to
understand the impact of temperature rise on styrene
release.