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 | 32 Volume 2, Issue 1
Societal Risk Assessment of Styrene Release Scenario A case study
Diptendu Das* and Ramdas Bhattacharya**
*Atomic Energy Regulatory Board (AERB), Mumbai
**Former Director and Vice Chairman, AERB
Email: diptendudas@aerb.gov.in, dir.ipsd@gmail.com
Abstract
A societal risk assessment study has been carried out to validate the consequence effect distance and to estimate worst case
scenario effect of styrene gas release recently from a polymer chemical plant near Visakhapatnam, India. Significant
quantities (about 820 MT out of 1842 MT) of styrene stored in a tank (tank capacity 3090.96 KL/ 2250 MT) leaked into
the environment. Considering the toxic nature of styrene vapour, a study has been carried out on different leak scenarios
including worst case scenario and its subsequent dispersion (heavy gas dispersion) to find out the effect distances.
The styrene vapour release scenario is simulated using Areal Location of Hazardous Atmospheres (ALOHA) Software.
Various Scenarios of releases are assumed, and different wind speeds and stability classes are considered to simulate the
scenarios. Based on the site map of the incident spot (point source of release) threat zones are generated. The effect
distance and postulated exposures on population and inhabitants around the vicinity of the site of release is studied. The
emergency preparedness and disaster mitigation measures to handle such emergency are assessed from the above study.
Study reveals that as the temperatures of the styrene gas increases from 20
o
C to 149
o
C due to polymerisation runaway
reaction the effect distance increases. It is estimated that at the worst-case scenario, a radius of 7km distance can get
affected at this temperature of release and population and other living beings and environment can get affected, if proper
mitigation measures were not in place.
It is worth mentioning that maintaining styrene below 20
o
C and preventing polymerization reaction is essential failing
which such accident can take place. In this case, cooling was not efficient, resulting in polymerization of styrene which is
exothermic in nature and runaway reaction took place causing the accident. This accident resulted in 11 fatalities and
thousands other got affected.
The worst case scenario considered in this study would help the plant authorities as well as the regulators for putting
adequate emergency planning and disaster management in place depending on this consequence effect distance and risk
assessment study.
Keywords: Societal Risk, Chemical Disaster, Styrene Vapor Leak, Wind Stability
1. Introduction
Styrene vapour release occurred in the early morning of
7th May 2020, from a polymer chemical plant near
Visakhapatnam, India. This Styrene vapour uncontrolled
release occurred from one of the Styrene storage-tanks
(M6 Tank). 11 fatalities were reported subsequent to the
release and inhalation of the toxic gas and around 600
people got affected due to styrene exposure. As per the
Manufacture, Storage and Import of Hazardous
Chemical Rules (MSIHC) Rules, 1989 this accident falls
under the category of major accident. This accident is
analyzed and various scenario are simulated to find out
effect distances for same accident as well as for other
probable conditions.
Release of any stored chemical can be of two types:
a) A common type of leakage due to faulty valves
or due to small hole in the cylinder.
b) A major release due to bursting or rupture of a
cylinder (tank), which is commonly termed as
catastrophic failure and the most unlikely event.
It is therefore necessary to estimate the distances where
the effect of toxic chemicals pose detrimental effect to
the health of the exposed individuals due to toxicity.
Continuous leakages (plume) may be treated by using
conventional atmospheric dispersion model with
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, ‘Drepresents -
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.
Daytime insolation
Night time
conditions
Anyti
me
Surf
ace
wind
spee
d
(m/s)
Stro
ng
Moder
ate
Slig
ht
<=3/8
cloudi
ness
Heav
y
overc
ast
<2
2-3
3-4
4-6
>6
A
A-B
B
C
C
A-B
B
B-C
C-D
D
B
C
C
D
D
F
F
E
D
D
D
D
D
D
D
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 | 34 Volume 2, Issue 1
The concentration of styrene is analysed based on Acute
Exposure Guideline Levels (AEGL). AEGLs are
calculated for five relatively short exposure periods 10
minutes, 30 minutes, 1 hour, 4 hours, and 8 hours as
differentiated from air standards based on longer or
repeated exposures. AEGL “levels” are dictated by the
severity of the toxic effects caused by the exposure, with
Level 1 being the least and Level 3 being the most
severe.
All levels are expressed as parts per million or
milligrams per cubic meter (ppm or mg/m3) of a
substance above which it is predicted that the general
population could experience, including susceptible
individuals: Level 1 denotes notable discomfort,
irritation, or certain asymptomatic non-sensory effects.
However, the effects are not disabling and are transient
and reversible upon cessation of exposure. Level 2
denotes Irreversible or other serious, long-lasting
adverse health effects or an impaired ability to escape.
Level 3 denotes Life-threatening health effects or death.
Styrene is heavy gas and dispersion pattern is shown
below:
Figure 1: Heavy Gas dispersion mechanism [CPQRA,
2000]
2. Dispersion Modeling
Atmospheric dispersion of vapors is key element for
consequence analysis. Typically, the dispersion
calculations provide an estimate of the geographical area
affected and the average vapor concentrations expected.
The preliminary calculations require an estimate of the
released rate of the gas based on release temperature,
pressure and release diameter and the atmospheric
conditions (such as wind speed, stability class), surface
roughness etc.
For a continuous release of gas as per Pasquill-Gifford
plume model [Pasquill et al., 1974]
󰇛

󰇜



󰇡

󰇢

󰇡

󰇢
 (1)
Where,
<C> (x, y, z) = average concentration (mass/
volume)
G = continuous release rate (mass/time)
σ
x
, σ
y
&
σ
z
= dispersion coefficient in the x, y and
z directions (length)
u = wind speed in x direction
(length/time)
y = cross-wind direction (length)
z = distance above the ground (length)
H = height of the source above the
ground level plus plume rise (length)
Assumptions:
i. Styrene Tank were placed in open yard i.e. on
ground. So, it is assumed that it is a ground
level release (H = 0).
ii. We are interested in ground level (z = 3m)
concentration at different downwind distances
(x) and also for a particular downwind distance
what is the concentration in various cross-wind
locations (y) (as at lower wind speed, wind
movement is very erratic).
So, as per the assumptions equation (1) reduces to
󰇛

󰇜

󰇩
󰇪
(2)
Figure 2: Atmospheric dispersion showing plume
release
Various release scenarios are considered for the styrene
dispersion study as discussed below
Simulation of Styrene release:
Scenarios were considered to study the effect of
parameter such as temperature of liquid and the leak size
as mentioned below.
Styrene Tank (filled with 820 MT of liquid Styrene) was
stored in vertical cylindrical fixed roof tank [Ref. 10]
inside the plant. Assuming a hole size of 2 mm circular
diameter and the release is from vertical cylindrical fixed
roof tank top vent and dip hatch vent. The leak was
developed over a period time due to corrosion. ALOHA
models are used to analyse Styrene dispersion
The schematic diagram of a Styrene Tank is shown in
Fig. 3.
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 | 35 Volume 2, Issue 1
Figure 3: Schematic of Styrene Tank
The input process conditions and styrene properties used
for the analysis is shown in the Table 2.
Table 2: Input Data used for Styrene release
Pressure inside Tank
(abs) (P
a
)
1 atm
Storage temperature
(T)
30ºC (During runaway, the
temperature reaches 149
0
C)
Molecular weight of
Styrene (MW)
104.15
Diameter of hole (D)
2 mm, 2inches, 1m, 2m , 5m
for different scenarios
Normal boiling point
(T
b
)
145.1° C [at atmospheric
pressure]
Vapor pressure
0.011 atm
AEGL-1 (60 min):
20 ppm
AEGL-2 (60 min):
130 ppm
AEGL-3 (60 min):
1100 ppm
IDLH:
700 ppm
LEL:
11000 ppm
UEL:
61000 ppm
Simulation of Styrene release has been done using
ALOHA 5.4.1.2. It allows the user a choice of several
accident scenarios, then uses an appropriate source
algorithm to inject the material into the air over a limited
time. A flat homogeneous earth surface is assumed. It
provides several source points like Direct, Puddle, Tank
etc.
Stability class D is considered for release analysis, Wind
direction taken based on wind rose diagram Wind speed
= 13.0487 miles/hour from WNW at 10 meters
Ground Roughness: open country
Air Temperature: 30° C, Stability Class: D
No Inversion Height, Relative Humidity: 61%
Release Duration: ALOHA limited the duration to 1
hour
2.1 Scenario 1: Release of Styrene gas from tank top
vent /hole size is 2mm; Tank- room temperature
Concentration profile (using ALOHA simulation) of
Styrene as a function of distance from the source is
represented in Fig.4
Figure 4: ALOHA study result showing an effect-
distance for different levels of concern (2mm hole leak,
room temperature)
It is observed from the analysis that the leak size of 2mm
(pin hole leak) at room temperature does not contribute
affected area more than 100m. Therefore, the leak size is
increased to 2 inch diameter and results are shown in
Scenario 2.
2.2 Scenario 2: Release of Styrene gas from tank top
vent /hole size is 2inch; Tank, room temperature
Leak size Concentration profile (using ALOHA
simulation) of Styrene as a function of distance from the
source for 2 inch diameter leak size at room temperature
is represented in Fig.5.
It is seen from the study that 2mm leak and 2inch leak at
room temperature, the effect distance are within 100 m
and impact of it cannot reach outside plant boundary.
Now to study the effect of temperature rise due to
polymerization reaction again the styrene release was
studied at 69 deg C. The effect distance from the source
for 2 inch diameter leak size at room temperature is
represented below.
2.3 Scenario 3: Release of Styrene gas from tank top
vent /hole size is 2mm , 69 degree Celsius
temperature
Study revealed that due to exothermic runway reaction,
temperature of styrene increased from room temperature
30 degree Celsius to 149 degree Celsius in the Vizag
release case. So, in this study, styrene release rate was
estimated at room temperature and also at intermediate
temperature of 69 degree Celsius along with the
maximum temperature of 149 degree Celsius (boiling
point) as described below.
Float
18m
Fixed Tank
12m
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Figure 5: ALOHA study result showing an effect-
distance for different levels of concern (2inch hole leak,
room temperature)
It is seen from the study that for 2mm leak size at 69 deg
C temperature the effect distance is around 1.5 km.
Hence it is understood temperature of liquid is critical
parameter, which needs to be controlled. The effect
distance and dispersion profile of styrene release at
69deg C for 2mm leak is shown in Fig. 6
Figure 6: ALOHA study result showing an effect-
distance for different levels of concern (2mm hole leak,
69
0
C temperature)
2.4 Scenario 4: Release of Styrene gas from tank
top vent /hole size is 2mm, 145 degree Celsius
temperature
Further when leak size was increased from 2mm to 2
inch and liquid temperature was increased from room
temperature to 149 deg C the effect distance was found
around 7 km, which is resemblance to the real accident
scenario. Figure 7 depicted the 2inch leak of styrene at
149 deg C.
Figure 7: ALOHA study result showing an effect-
distance for different levels of concern (2inch hole leak,
149
0
C temperature)
2.5 Scenario 5: Release of Styrene gas from tank top
vent /hole size is 2m dia leak (catastrophic failure),
145 degree Celsius temperature
Finally, to understand the worst-case scenario, bigger
size leak was assumed. Leak sizes were assumed from
1m diameter to 2m diameter. it was observed that room
temperature leak was initially confined to smaller area.
But at higher temperature (149 deg C) styrene leak
would cause worst case scenario and more than 10 km
distance would be affected. The styrene release profile is
shown in Figure 8.
Figure 8: ALOHA study result showing effect-
distances for different levels of concern (2inch hole leak,
149
0
C temperature)
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The Effect distances obtained at different scenario are
shown in Table 3.
Table 3: Comparison of different simulation method
results
Scenario
Concentration
of Styrene
(ppm)
Styrene effect
distance
2mm Leak,
room
temperature
20
60 m
2inch Leak,
room
temperature
20
120 m
2mm leak, 69
degree Celsius
20
1.5 Km
2inch leak, 149
degree Celsius
20
8 Km
Catastrophic
failure (2m dia
release)
20
>10 Km
2.6 Styrene Release rate estimation
Based on the styrene dispersion modeling the various
release rate were estimated for different scenarios. For 2
inch diameter leak at room temperature it is estimated
that around 1kg/minute of styrene may leak, and within
1 hour, maximum 60 kg styrene leak is possible.
However, in real case the leak was much more. Hence
room temperature leak could not cause such a havoc.
The styrene release profile for 2m leak at room
temperature is shown in Fig. 9
Figure 9: ALOHA study result showing styrene leak at
room temperature (2inch hole leak, 149
0
C temperature)
However, when styrene release was studied at 149 deg C
from 2nch hole, it was found the leak rate is very high,
around 1 ton per minute (fig. 10) and within 1 h, 60 ton
styrene may leak.
Fig 11. shows room temperature release from 2inch leak
size. It is seen that since there is no temperature rise in
the styrene inventory hence the leak rate is gradual and
step wise increase noted. Thus, action time available to
prevent more leak is more in room temperature release
scenario.
Figure 10: ALOHA study result showing styrene leak at
room temperature (2inch hole leak, 149
0
C temperature)
In the worst-case scenario i.e., catastrophic release case
it is found the leak rate is very high (around 1000
ton/minute). Hence, such releases would be
instantaneous and within no time the leak would spread
to public domain. Hence disaster preventive action time
in such case is very minimal.
Figure 11: ALOHA study result showing styrene leak at
room temperature (2inch hole leak, at room temperature)
In actual case, however, around 800 Ton styrene release
took place within one hour. Hence, it is not a
catastrophic failure accident. Continuous styrene release
took place during the accident. Therefore, plume model
is sufficient to analyze dispersion of styrene in this
accident. Dispersion analysis results were found
matching with the actual accident scenario.
Figure 12: ALOHA study result showing styrene leak at
room temperature (catastrophic failure, at room
temperature
Figure 13 shows ALOHA study result showing Threat
Zone (in yellow color of radius 1 km, red color shows
the source of leakage) for styrene leak at 149 degree
Celsius.
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 | 38 Volume 2, Issue 1
Figure 13: ALOHA study result showing Threat Zone
of 1 km radius for styrene leak at 149 degree Celsius (2
inch hole leak, Google Maps)
The above figure depicts RR Venkatapuram village
which was most affected during the leakage scenario.
Figure 14 shows ALOHA study result showing Threat
Zone (in yellow color of radius 10 km, red color shows
the source of leakage) for styrene catastrophic failure
case. However, concentration of styrene cloud is
estimated around 20 ppm, which is less than 50ppm
(TLV of styrene).
Figure 14: ALOHA study result showing Threat Zone
of 10 km radius for styrene leak at 149 degree Celsius
(Catastrophic failire, Google Maps)
The above figure depicts Meghadrigedda reservoir,
Marriott hotel, Visakhapatnam airport, Simhachalam
area which were under the exposure zone.
3. Disaster Management:
Eemergency Response made in the incident was the
immediate evacuation of the affected inhabitants from
the threat zone. Police and ambulance (were informed at
first) made the rescue and evacuation possible. National
Disaster Response Force (NDRF), other emergency
responders assisted. Emergency response was adequate.
There is no antidote available as per literature, however
PPE must be used to avoid exposure. Using copious
amount of water may not be beneficial due to low
solubility of styrene in water (0.03% at room
temperature). Hence immediate medical treatment is
advisable on styrene exposure.
4. Conclusion
In this paper, Styrene dispersion study has been carried
out using ALOHA. From the study, it has been observed
that for ALOHA the room temperature release does not
disperse very fast the styrene vapor even in the
catastrophic failure scenario. However, when the
temperature of release of Styrene from tank increased
from room temperature to higher temperatures (69 and
149 degree Celsius), the release distance found to be
much higher. The worst case release would be
catastrophic failure at higher temperature (149 degree
Celsius). The study reveals that cause of accident is due
to runaway reaction of the exothermicity of
polymerization of styrene. Hence, the cooling of styrene
storage is very essential, and addition of polymer
inhibitor also prevents polymerization, thereby
preventing temperature rise. However, appropriate
emergency planning and disaster management must be
in place and practiced even if such preventive actions are
taken.
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(AICHE), 1994
[2] Guidelines for Consequence Analysis of Chemical
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[3] Guidelines for Chemical Process Quantitative Risk
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[4] Giby Joseph, Styrene transfer hose failure, Journal of
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125(2004)
[5] Biswajit Ruj, Pradip Kumar Chatterjee, Toxic release
of Styrene and off-site emergency scenario A case
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[6] Aurélia Dandrieux, Gilles Dusserre, James Ollivier,
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[7] Pasquill, F., Atmospheric Diffusion: The Dispersion
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nd
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[8] OREDA, Offshore Reliability Data Handbook, 4
th
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[9] Yashoda Tammineni, Teja Dakuri, Vizag Gas Leak-
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International Journal of Research and Development
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[10] The report of High Power Committee on Styrene
Vapor Release Accident at M/s LG Polymer India
Ltd., 4
th
Edition, 2002 Under the Chairmanship of
Sri Neerabh Kumar Prasad, IAS Special Chief
Secretary to Government, EFS&T Department GoAP
(2020)