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