1.0Introduction
The spread or release of toxic chemicals with the intent to
result in harm is known as the chemical attack. During an accident,
process equipment can release toxic materials quickly and
in-significant quantities to spread in dangerous clouds throughout
a plant site and the local community. In the chemical process
industry, raw materials are converted into commercial products.
Exothermic chemical reactions can lead to a thermal runaway if the
heat generation rate exceeds the heat removal rate. A runaway
reaction is a reaction that is out of control because the heat
generation rate from the reaction exceeds the rate at which the
heat is removed from the system by the cooling media and the
surroundings (CSB, 2002). A runaway reaction is also known as one
of the common cause resulting in overpressure. During runaway
reactions, which tend to accelerate with rising temperature,
extremely high volumes of non-condensable with high energy can
cause the internal pressure of a vessel or pipeline to rise
rapidly. Runaway reactions are continuing to be a problem in the
chemical industry. A recent study showed that 26% of our major
chemical plant accidents are due to runaways. Pressure build-up
during the runaway is caused by an increase in vapour pressure of
liquid components and by the production of non-condensable gases.
As a runaway reaction proceeds, the increased generation rate of
vapour increases the vapour velocity, the mass flow rate, and the
inlet temperature in the overhead condenser. Basically, pressure
relief valves may not provide adequate protection because of their
relatively slow response time. In such a situation, vapour
depressuring systems, rupture discs and emergency vents are
preferred. In this paper, a detailed survey of the incidents caused
by runaway reactions was performed. The results of this study can
be used to identify the source of risk, to improve safety, to
reduce loss, and to design safer operation procedures. In the end
of the report, the inherent safety (IS) index of the process are
analysed using the Chemical and Exposure Index followed by design
modification that is proven safe through:I. Re-calculation of the
IS index, andII. Analyse the consequence contour before and after
the implementation of the IS principle using PHAST software.
U.S. Chemical Safety and Hazard Investigation BoardThe U.S.
Chemical Safety and Hazard Investigation Board (CSB) is an
independent federal agency charged with investigating industrial
chemical accidents at fixed facilities. It is designed to conduct
scientific investigations as to the root cause of chemical
accidents and is not an enforcement or regulatory body. A thorough
CSB investigation of an industrial accident can take several
months, even sometimes over a year due to the complexity of the
situation.
2.0 Literature Review
The U.S. Chemical Safety and Hazard Investigation Board (CSB)
conducted a comprehensive investigation of a runaway chemical
reaction at MFG Chemical (MFG) in Dalton, Georgia on April 12, 2004
that resulted in the uncontrolled release of a large quantity
highly toxic and flammable allyl alcohol and allyl chloride into
the community. The U.S. Chemical Safety and Hazard Investigation
Board (CSB) finds out that the runaway chemical reaction rapidly
pressurized the reactor causing the manway seal to fail, and then
activated the overpressure safety device. Unable to contain the
toxic vapour or stop the runaway reaction, the release continued
until the chemical reaction ceased. CSB concluded that this
incident was avoidable. An attempt to manufacture a new product
resulted in a runaway reaction that over-pressurized the reactor,
activated the emergency vent, and released toxic vapour into the
atmosphere, exposing and injuring facility employees, nearby
residents, and emergency responders. The release forced more than
200 families from their homes. One MFG employee sustained minor
chemical burns and 154 people received decontamination and
treatment at the local hospital for chemical exposure, including 15
police and ambulance personnel assisting with the evacuation. The
reactor continued venting toxic vapour for nearly eight hours and
the evacuation order lasted more than nine hours.
2.1Background
On the night of April 12, 2004, during an attempt to make the
first production batch of triallyl cyanurate (TAC) at MFG Chemical,
Inc. (MFG), at their Callahan Road facility in Dalton, Georgia. a
runaway chemical reaction released highly toxic and flammable allyl
alcohol and toxic allyl chloride into the nearby community. At
approximately 9:30 PM, the reaction went out of control and
over-pressurized a 4000-gallon reactor. The runaway reaction caused
the release of highly toxic and flammable allyl alcohol vapour and
toxic allyl chloride vapour into the community. The dense vapour
continued to escape from the reactor for more than eight hours.
Neither the Dalton Fire Department emergency responders nor MFG
personnel had the personnel protective equipment required to enter
the process area safely to attempt to stop the vapour release. The
Dalton Fire Department promptly ordered an evacuation of all
residents and businesses within a one-half mile radius of the
facility. The Dalton Police Department then dispatched officers to
the neighbourhoods to alert the residents to evacuate. In this
business workforce, there are 3 companies involved. The
relationship of these companies is illustrated below:Manufacturer
of allyl alcoholLyondell Chemical CompanyMFG Chemical CompanyGP
Chemical
Client for TAC
Issue a PO
Figure 1: Relationship between companies involved
2.2Investigation of Incident
The incident likely involved hazardous chemical reactions.
Therefore, the CSB launched an investigation to determine the root
and contributing causes of the incident. The CSB team began the
investigation with the MFG management and senior engineering
personnel responsible for chemical process development and followed
by a detailed examination on the process equipment, the chemical
transport and storage tanker (isotanker) and the reactor cooling
system. The investigation team also contracted modelling of the
vapour cloud release. The key findings based on their investigation
are presented below: There was a runaway reaction at the MFG
facility during the TAC synthesis. The runaway reaction resulted
when operators added the entire quantity of each reactant, as well
as the catalyst, to the reactor at once, and was then unable to
control the reaction rate. MFG did not conduct an adequate
evaluation of the reactive chemistry hazards involved in
manufacturing triallyl cyanurate before attempting the first
production batch. Readily available technical literature, including
specific TAC synthesis accident histories would have alerted them
to the reactive chemistry hazards involved. Lyondell Chemical (the
allyl alcohol manufacturer) did not clearly communicate to MFG
management or GPC (the allyl alcohol buyer) that MFG would be
required to implement the EPA Risk Management Program regulation,
including conducting appropriate design reviews and preparing
comprehensive emergency plans, before receiving the allyl alcohol
shipment at the MFG facility. MFG did not provide a hazardous
vapour/liquid containment system on the reactor emergency vent. The
runaway reaction released allyl alcohol and allyl chloride into the
atmosphere and into a nearby creek. MFG did not develop the
comprehensive process hazards analysis, pre-startup review, and
emergency response elements required by the OSHA PSM standard and
the EPA Risk Management Program regulation. MFG and GPC did not
apply industry best practices for toll manufacturing where MFG did
not share certain critical process safety information with GPC, and
GPC did not ensure that MFG had addressed all hazards associated
with the process before attempting to produce the first production
batch. This industry best practices are highlighted in Guidelines
for Process Safety in Outsourced Manufacturing Operations (CCPS,
2000).
2.3MFG Studies in TAC Synthesis
MFG personnel only conduct a research to confirm there are no
any restrictions that could adversely affect their TAC production.
However, they did not conduct detailed literature research
addressing the reactive chemistry hazards involved in the process.
The following chemical equation shows the synthesis of triallyl
cyanurate by reacting cyanuric chloride with allyl alcohol in the
presence of a catalyst: Allyl Alcohol + Cyanuric Chloride +
Catalyst TAC + HCl + Catalyst (1)HCl is the by-product formed from
this reaction. The complete conversion of the cyanuric chloride is
achieved by using an excess amount of allyl alcohol. The
neutralization reaction between caustic soda and HCl is exothermic.
Therefore the synthesize of the fixed-volume batch of TAC is
carried out using a 4000-gallon reactor with an external cooling
jacket. However MFG disregard the reaction between allyl alcohol
and cyanuric chloride which is also highly exothermic which could
lead to significant heat generation. The synthesis of TAC is
illustrated through the flowchart attached in the appendix of this
report.Figure 2: Basic TAC Process Diagram
2.4Process Upset
A short time after loading the allyl alcohol, the operators
noticed that the reactor temperature had increased from 32F to
about 72F, presumably due to the addition of the warm allyl
alcohol. Ten minutes later, the operators noted that the
temperature had already climbed to 103F. The temperature continued
to increase rapidly to 118 F, well above the peak temperature of
about 100F that they expected. Unknown to the engineers and
operators, it was almost at the temperature at which the exothermic
decomposition reaction occurs. Rapidly increasing pressure in the
reactor caused the manway gasket to blow out. Dense, white vapour
immediately began to spray out of the manway. The rupture disc blew
open about 30 seconds later, sending additional white vapour out of
the end of the 4-inch vent pipe near the base of the reactor. The
last observed reactor temperature was 124F (51C). The runaway
chemical reaction incident in the TAC process involved two
reactions: (1) the desired synthesis reaction to form the products;
and (2) an undesired decomposition reaction. The heat produced by
the undesired decomposition reactions raised the temperature and
pressure of the reactor as follows: PropertyHigh Thermal
Inertia
Maximum exoterm temperature (0C)424
Maximum exoterm pressure (bar)103
The detailed timeline of the incident occurrence were attached
in the Appendix of this report.
3.0Analyze Inherent Safety (IS) Index
For this project, the IS index that will be used is the Chemical
Exposure Index (CEI), because of the toxic release incident.
Furthermore, the toxic had been release is in the form of gas. The
Chemical Exposure Index (CEI) provides a simple method of rating
the relative acute health hazard potential to people in neighboring
plants or communities from possible chemical release incidents.
Therefore, to calculate the Airborne Quantity, the following
equation will be applied which is suitable for the gas release
incident. The equation can be seen as follow:
Where:Pa = Absolute pressure (Pg + 101.35)Pg = Pressure gauge
(kPa)D = Diameter of the hole (mm)MW = Molecular Weight of Allyl
alcohol (MW = 58.1)T = Temperature (C)
From the scenario, the value of pressure gauge is assume bigger
than the set pressure of rupture disc to blew down which is, 75
psig, because of poor monitoring system for pressure. So, the
assumption value for pressure gauge is Pg = 80 psig. From the
equation, the parameter needed is absolute pressure. So, the
calculation of Pa as follows:1. Change the unit from psi to kPa
2. Calculate Pa
Furthermore, since the reactant inside the reactor which is
allyl alcohol is vaporize from liquid to gas and escape to the
environment, therefore, the temperature of the reaction is also
assume bigger than the boiling point of the allyl alcohol which is
97C. Therefore, as per the calculated maximum exoterm temperature
from the literature review, T =424 C From the scenario, there are 2
source of leak, first from the 18-inch manway gasket and 4-inch
vent pipe. So, the value of Airborne Quantity (AQ) is equal to the
total for both leaks. So, the value of AQ must be calculated for
leak at diameter of 4-inch and 18-inch, respectively.1. Calculation
for 4-inch diameter.Since the diameter is 4-inch, the size of the
pipe that needs to be account is 2- inch (50.8 mm)Substitute the
value into the equation:
2. Calculation for 18-inch diameter.Since the diameter is
18-inch, the size of the pipe that needs to be account is 20% of
cross section area.CSA = 254.47 inch20.20 (254.47) = 50.89 inch2
Substitute the value into the equation:
Then, calculate the Chemical Exposure Index (CEI) value:
Allyl alcohol use AEGL instead of ERPG. Replace ERPG with AEGL
in the equation. Assume the duration for the exposure of the toxic
is 15 minutes. AEGL for Allyl alcohol is:ppmmg/m3
AEGL 1 2.15.37
AEGL 2 4.210.7
AEGL 3 130333
Therefore:1. 4-inch diameter
2. 18-inch diameter
Since, CEI value more than 1000, therefore, CEI = 1000. Then,
calculate the Hazard Distance for both situations.
1. 4-inch diameterFor AEGL 1;
For AEGL 2;
For AEGL 3;
2. 18-inch diameterFor AEGL 1;
Since, HD value more than 10 000, therefore, HD = 10 000.For
AEGL 2;
Since, HD value more than 10 000, therefore, HD = 10 000.For
AEGL 3;
Therefore, further reviews as well as design modification need
to be done on the process to reduce the impact if the accident
happens. 4.0Sequence of Vapour Dispersion Modelling in PHAST
The flowchart below illustrates the procedure that leads to the
Chemical Exposure Index calculation starting from the scenario
selection to PHAST toxic vapour release modelling. This procedure
is prior to the implementation of the design modification.
ScenarScenario: Reactant in warm condition & poor cooling
system Figure 3: Procedure for CEI calculation before Design
Modificationio: Reactant in warm condition & poor cooling
system
Wind direction and speed, ambient temperature, mixing
(discharge) height, and atmospheric stability are typical inputs
used to simulate vapour plume transport and dispersion. Since the
released occurred after sunset at approximately 9:30PM and with
calm wind condition, a slight stable atmosphere (Pasquill-Gifford
class E) was chosen. There were two discharge point involved in the
release: A leaking gasket on an 18-inch diameter reactor manway on
top of the reactor. Followed by discharge through a 4-inch diameter
vent line after the reactor rupture disc blew.
Based on the analysis shown below, the rupture disc vent pipe
discharge was directed downward, close to the ground. The hazard
distance of the release is shown in map below:Figure 4: Hazard Zone
for Rupture Disk Release
ERPG/AEGL-1ERPG/AEGL-2ERPG/AEGL-3Figure 5: Hazard Zone for
Manway Gasket Release
5.0Proposed Design Modification
Figure 6: Procedure for CEI Calculation after Design
Modification
Moderate the size of the reactorMFG should recalculate the
ability of the reactor to remove access heat since the heat removal
capacity of a reactor equipped with an external jacket is directly
proportional to the ratio of the jacketed surface area to reactor
volume. Thus, the engineers need to improvise the calculations as
the surface-to-volume decreases as the reactor volume increases.
Reduce the controlling temperatureApart from moderating the reactor
size, reducing the control temperature will reduce the exothermic
rate of reaction and also easy to control especially in large scale
production reactor. By reducing the temperature inside the reactor,
the pressure will also reduce to a safer level. Controlling an
exothermic reaction depends on the interaction among the kinetics
and reaction chemistry; the plant equipment design; and the
operating environment. Rearrange the reaction processThe reaction
intended is to synthesize triallyl cyanurate (TAC) by reacting
cyanuric chloride with allyl alcohol in the presence of a catalyst.
However, the reaction produces hydrogen chloride (HCl) as a
by-product. In order to ensure complete conversion of the cyanuric
chloride, the procedure specified an excess amount of allyl alcohol
and also adding sodium hydroxide to neutralize HCl. The
rearrangement reaction of TAC is carried out by heat-treating TAC
in the presence of a catalyst. In the preferred embodiment of the
present invention, the rearrangement reaction is carried out in a
reaction solvent (for example, xylene) in the presence of a copper
catalyst.Applying the American Institute of Chemical Engineers
Center for Chemical Process Safety, Guidelines for Process Safety
in Outsourced Manufacturing Operations (CCPS, 2000)It is
recognizable as an industry recognized best practice that provides
comprehensive guidance for safe tolling operations.
Recommends that the client become familiar with the tollers
planned CCPS endorses the client, which is the GP Chemical (GPC) to
become familiar with MFGs planned operation and audit the health,
safety and environmental practices as part of the clients product
stewardship responsibilities. GPC should ensure MFG specifically
addressed the hazards of production-scale manufacturing of TAC.
Ensuring training programCCPS best practice guidelines recommend
that the GPC ensure that the training program at the MFGs facility
meets process safety, and environmental risk management training
recommendations and requirements. GPC should request and review the
MFG employee-training program to make sure that adequate training
addressing the hazardous chemicals involved in TAC production.
Make audit during operationsThe guidelines also mention
regarding the client (GPC) should make audits during ongoing
operations in order to assure that operations are going as planned
and obligations are being met. GPC should visit the MFG facility
more often, or actively participate in the verification runs or the
attempt to make the first full-scale production batch.
Evaluating good process safetyThe guidelines recommend
evaluating good process safety practices even, when a candidate
toller is not currently regulated by a governmental process safety
requirement and the proposed toll project will not trigger
regulation. Actions need to be taken although the raw materials are
not regulated by the Risk Management Program (RMP) or Process
Safety Management (PSM).
Discuss and implement any changes madeCCPS best practice
guidelines recommend that MFG share any techniques, information or
experience learned as part of the contractual agreement with the
client. MFG should discuss and agree on any changes made to the
equipment, chemicals, technology or procedure of the tolling
arrangement with the client (GPC). MFG should share all process
information with GPC and also adhere to the procedures that have
been made. The operators also need to consider any actions that
might increase the probability of a runaway reaction if the full
production quantity is added of each raw material to the
reactor.
Applying and implementing Management of Change (MOC)CCPS
guidelines recommend MFG to discuss and agree on any changes made
to the equipment, chemicals, technology or procedure of the tolling
arrangement with GPC. If covered under PSM or RMP, any deviation
from the original design specifications is considered a change. Any
change requires MFG and GPC to address the hazards and risks
associated with the production process. Ensure the hazard
evaluations address critical process controls, overpressure
protection, alarms, and other equipment such as vent
collection/containment devices to minimize the possibility and
consequences of a toxic or flammable release.
Conduct process hazard analysesThe CCPS guidelines also
mentioning for every new situation or changes, a process hazard
analysis should be conducted. All aspects (including human factors)
should be considered while performing PHA to identify potential
problems caused by the scale-up.
Conducting intensify observation Guidelines of CCPS recommend
augmenting observation during scale up of the critical process
characteristics that were designed in pilot testing to take into
account the order-of-magnitude changes in vessel size and quantity
of materials that may have been engineered into the new process.
MFG should adequately evaluate the hazards associated with the
scale-up of the process, such as evaluation of the heat removal
capability of the production reactor compared to the bench-scale
testing.
Provide comprehensive emergency response Create a comprehensive
emergency response plan and provide equipment and training that is
appropriate to the duties assigned to employees in the event of an
emergency.A summarized version of the step-by-step layers are shown
in the flowchart below:
7.0Reference
Health and Safety Executive (HSE) [Online], 2007. Chemical
Reaction Hazards and the Risk of Thermal Runaway,
www.hse.gov.uk/pubns/indg254.htm, London, UK: HSE.
CCPS. Inherently Safer Chemical Processes, A Life Cycle
Approach, New York: AIChE, 2004.
Investigation Report on Toxic Chemical Vapor Cloud Release.
April 12, 2014. U.S. Chemical Safety and Hazard Investigation
Board.Booth, A.D. et. al. Design of emergency venting system for
reactors - Part 1, Trans IChemE, vol. 58 (1980) 75-79.Health and
Safety Executive (HSE), 2000. Designing and Operating Safe Chemical
Reaction Process, Norwich, U.K., HSE Books, 2000.
8.0Appendix
Figure 7: Overpressure rupture disc & vent pipe on top of
reactor
Figure 8: 18-inch Reactor Manway
Figure 9: Toxic Vapour Release Timeline