During the early morning of May 7, 2020, a toxic vapor cloud engulfed an area over a radius of around 3 kilometers whose source was determined to be the LG Polymers chemical plant, located in the R. R. Venkatapuram village of the Visakhapatnam district, Andhra Pradesh, India.
The National Disaster Response Force (NDRF) reported the death toll to be 11, with thousands sick due to exposure to the gas.
A report published by the National Green Tribunal Committee claimed poisonous styrene monomer vapor was responsible for the severe loss of life and long term health impairment of the residents. Did the plant fail to adhere to the safety protocols? What can we learn to avoid this type of accident from happening in the future? We will try to answer some of these questions in the paragraphs to follow.
How fatal can styrene be for us?
Styrene(ethylbenzene, C6H5-C2H3) is an inflammable colorless liquid derived from benzene and is used as a raw material in polystyrene synthesis. Depending on the intensity of inhalation, a person can experience various health issues. Short term exposure results in mucous membrane and eye irritation, nausea, jolt, loss of consciousness, and gastrointestinal effects. A person could find it challenging to inhale. Long term exposure may lead to permanent damage to the central nervous system.
Synthesis of polystyrene
The most common method to synthesize polystyrene on a commercial scale is the free-radical polymerization using benzoyl peroxide as initiator. The reaction is highly exothermic with
ΔH = 71 KJ/mol. However, the reaction can be self-initiated, as postulated by Mayo. According to the reaction mechanism suggested by him, two styrene monomers combine to form a Diels-Alder styrene dimer (AH), which transfers an H atom to a third monomer (molecular assisted homolysis) and generate mono radicals to start self-polymerization.
What triggered the self-polymerization of styrene in Vizag?
Since the reaction is exothermic and monomer molecules are thermally unstable, content inside the reactor should be circulated regularly to avoid temperature runaway. The plant being unoperated for an extended period due to the nationwide lockdown, the stock of 1800 tons of styrene in a storage tank of capacity 2400 mt remained stagnant. This could have led to the Diels-Alder reaction resulting in self-polymerization. With the concentration of monomers being high, the polymerization reaction auto-accelerated(R_P = K[M]2). The lack of automation to monitor the temperature change in the upper half of the tank caused styrene to reach its onset temperature(the temperature at which heat released by the reaction can no longer be removed from the reactor and subsequent temperature rise is detected). Once it reached the onset temperature(66°C), it took only 15-20 minutes for styrene to reach its boiling point, which is 146°C. And according to the NGT(National Green Tribunal) committee, this is how styrene vapor formed and leaked out the plant.
Should storage of styrene be of paramount importance?
The ideal temperature for the storage of styrene is 15-18°C, and at no point shouldn’t be allowed above 25°C. Tertiary Butyl Catechol(TBC) is used as an inhibitor to avoid self-polymerization and lower the tank’s temperature. The plant didn’t have a stock of TBC. So the temperature kept on rising inside the reactor, and this aided the formation of vapor.
The refrigeration unit should be monitored round the clock to keep the temperature below 20°C. But the investigation of the NGT committee pointed out that the unit had not been operated 24 hours before the incident.
Was gross negligence on the company’s part the reason for this incident?
Industrial safety regulations are flagrantly violated in chemical plants leading to cataclysmic disasters. Authorities should take a more proactive approach and clamp down on violations with hefty fines.
Emphasis should be put on urban planning in areas surrounding high-risk chemical plants to avoid major loss of life.