Introduction – Problem statement + Aim and Objectives
Background
The purpose of the Chemical Process Safety and Risk Management module is to equip
students with skills to identify, analyse, and manage hazards at laboratory and industrial
scales.
This study focuses on two hazardous chemicals commonly used in water treatment and
industry: chlorine gas and nitric acid. Both pose serious risks if accidentally released, stolen,
or involved in equipment failure.
The scenario simulated involves a stolen chlorine cylinder, creating a potential hazardous
release near populated areas, including a school located 22 km away. The study evaluates
dispersion, risk, and emergency response, with a similar scenario performed for nitric acid for
comparative assessment.
Problem Statement
On 4 April 2018, a pressurized liquefied chlorine cylinder (3 ft tall, silver, labeled
“CHLORINE UN 1017 INHALATION HAZARD”) was stolen from an industrial facility.
Concerns arose about large-scale toxic gas release downwind, potentially affecting the public.
Emergency services required hazard prediction and dispersion modelling to plan mitigation.
The scenario is repeated with nitric acid to compare hazard potential and consequences.
Aim of the Study
To determine the potential consequences of a large-scale releasing involving chlorine gas and
nitric acid by performing atmospheric dispersion modelling using CAMEO tools (ALOHA
and MARPLOT), and assess risks to nearby populations.
Objectives
To identify and describe the hazardous properties, behaviour, and risks associated with
chlorine gas and nitric acid.
To examine past incidents involving chlorine and nitric acid releases to understand real-world
consequences and lessons learned.
To evaluate possible causes, release scenarios, and immediate consequences for both
chemicals.
To use CAMEO, ALOHA, and MARPLOT for modelling dispersion of chlorine and nitric
acid.
To compare the different the hazard zones with respect to the exposure impacts and potential
risk to human populations (including the school located 22 km away).
To propose preventive and emergency response measures based on simulation results.
Chemicals involved + Past events
CHLORINE GAS (Cl2)
Key Properties

Yellow-green, pungent odour

Toxic inhalation hazard (TIH)

Liquefied under pressure; vaporizes rapidly when released

2.5 times denser than air → hugs the ground & spreads far

Reacts with organics, ammonia → may form explosive compounds

Causes lung damage, pulmonary edema (excessive fluid accumulation), irritation at
low ppm
Industrial Uses



Water disinfection
Bleaching agent
Chemical manufacturing (PVC-polyvinyl chloride, solvents-methylene chlorine in
pharmaceutical manufacturing for synthesis, extraction, and purification processes of
antibiotics and vaccines)
NITRIC ACID (HNO3)
Key Properties

Highly corrosive, strong oxidizer

Gives off nitrogen dioxide (NO₂) fumes (brown)

Can form toxic red/brown cloud when released

Reacts violently with metals & organics

Causes burns, respiratory damage
Industrial Uses

Fertilizer manufacturing

Explosives (TNT, ammonium nitrate)

Metal cleaning & refining

Water treatment in small concentrations
The two chemicals considered in our study, chlorine gas and nitric acid, are commonly used
in industry but pose significant hazards if released. Chlorine is a dense, toxic gas that
vaporizes rapidly and can form explosive compounds with other chemicals. Nitric acid is
highly corrosive and a strong oxidizer, releasing toxic nitrogen dioxide fumes when spilled.
Understanding their properties is critical, as it informs both the potential consequences of
accidental releases and the emergency response measures. Next, we will look at real-world
incidents to see how these hazards manifest in practice before moving on to a detailed hazard
analysis.
PAST INDUSTRIAL INCIDENTS
1. Graniteville Chlorine (train) Disaster (2005 — USA)
On Wednesday, 5 January 2005, in Graniteville, a growing community on the outskirts of
Aiken and Augusta in South Carolina, a railroad worker failed to reset a switch that diverted
trains onto a siding. The following day, a northbound Norfolk Southern freight train
consisting of three locomotives and 42 cars veered onto the siding and collided with a parked
train.
Among the cars involved, three tanker cars were carrying chlorine, and the impact ruptured
the tanks, releasing approximately 90 tons of chlorine gas into the atmosphere. Workers near
the site reported smelling chlorine immediately and experienced burning of the eyes and
lungs, along with nausea and vomiting. Within the first hour after the collision, eight people
died as a dense yellow-green chlorine cloud settled close to the ground around the accident
site.
This incident clearly demonstrates how chlorine gas forms a heavy toxic plume that can
travel significant distances, making it highly relevant to the chlorine dispersion scenario
modelled in this study.
2. Arizona Nitric Acid tanker spill
An accident involving nitric acid that closely matches our study occurred in Arizona in
February 2023. A tanker truck transporting liquid nitric acid overturned on Interstate 10 near
Tucson, leading to a loss of containment. Although nitric acid is transported as a liquid, the
spill rapidly produced toxic fumes due to evaporation and partial decomposition into nitrogen
dioxide. Authorities closed the highway and issued evacuation and shelter-in-place orders
while hazmat teams managed the release. The primary hazard was inhalation of corrosive
nitric acid and nitrogen dioxide vapours, making this event directly comparable to the
atmospheric dispersion scenario modelled using ALOHA and MARPLOT.
Methodology
Slide 1: Methodological Framework
This study adopts a case-study–based consequence assessment approach consistent with
emergency planning and chemical process safety principles. A worst-credible release scenario
is analysed to evaluate and compare the potential impacts of chlorine gas and nitric acid.
Slide 2: Scenario Direction and Study Flow
The study begins with interpretation of the problem statement, followed by definition of the
release scenario. The chemical type and storage conditions are then identified, after which an
appropriate modelling approach is selected to evaluate potential consequences.
Slide 3: Location and Time Definition
The scenario is geographically defined at Boston, Massachusetts, as specified in the problem
statement. The release is assumed to occur from an unsheltered single-storey structure, with
the time set to the early afternoon to reflect realistic atmospheric conditions.
Slide 4: Source and Tank Data Consideration
The release source is defined based on the cylinder dimensions provided in the problem
statement, namely a pressurised container approximately three feet in height and eight inches
in diameter. These dimensions are used to characterise the source geometry and ensure that
the dispersion modelling is based on realistic storage conditions.
Slide 5: Assessment Scope and Assumptions
To maintain a conservative safety perspective, a worst-credible release is assumed.
Emergency response actions and mitigation measures are not considered during the
dispersion modelling stage , allowing the analysis to focus solely on potential public
exposure.
Slide 6: Link to Modelling and Results (Handover Slide)
Based on this methodological framework, atmospheric dispersion and spatial impact
assessment were performed using ALOHA and MARPLOT. The simulation results are
presented in the next section.