Polymerization Reactions Inhibitor Modeling - Styrene and Butyl Acrylate Incidents Case Studies

Inhibitors are chemical substances that are used in small amounts to suppress the polymerization reaction of a monomer. An inhibitor has to be completely consumed before a polymerization reaction can proceed at normal rates. The time required to completely consume the inhibitor is often referred to as an ”induction” time. Inhibitors react with polymerization initiation radicals to produce products that cannot induce further reaction. Inhibitors are different from reaction ”retarders”. A retarder does not suppress the reaction but merely slows it down, i.e. the reaction continues to increase at a slower rate until the retarder is consumed. Some impurities in monomers can act as retarders.

Small amounts of inhibitors can substantially prolong the shelf life of a reactive monomer. Common polymerization inhibitors, typically antioxidants, include MEHQ (monomethyl ether hydroquinone), TBC (4-t-butylcatechol), HQ (hydroquinone), PTZ (phenothiazine), etc. The effectiveness of most commonly used inhibitors depends on the presence of dissolved oxygen to convert free radicals to peroxy radicals that in turn react with the inhibitor to stabilize the monomer. Both inhibitors and oxygen deplete over time. Understanding inhibitor requirements is essential for polymerization reactions safety.

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Two Case Studies

The first case study is on a styrene storage runaway incident. The incident occurred on the 7th of May 2020 in India and resulted in loss of life and numerous injuries. News and local reports about the incident indicate that the likely cause was a combination of loss of cooling and loss of inhibitor associated with a prolonged COVID-19 shutdown. In this case study we model suspected runaway reaction scenarios using Process Safety Office® SuperChems™ and the inhibitor models developed in this paper to determine if the scenarios are plausible.

M/s LG Polymers Pvt Ltd is located in R. R. Venkatapurm, Vizakhapatnam district, India. The plant is surrounded by residential areas as shown in Figure 2. On the 25th of March 2020, the unit was closed, due to the nationwide lockdown to curb the spread of COVID-19 [15, 16]. The unit was proposed to resume operations on the 7th of May 2020. In the early hours on the 7th of May 2020, a vapor release from a styrene monomer storage tank occurred. The tank had an inventory of 1830 tons of styrene monomer at the time of the incident. The vapor leak drifted westbound into residential areas. On the same day, 12 people died from exposure to styrene vapors and 585 were hospitalized.

The second case study is on a butyl acrylate runaway incident. The butyl acrylate runaway reaction and subsequent vapor cloud explosion occurred at the Synthron, LLC facility located in Morganton, NC on January 31, 2006. The runaway reaction incident resulted in one death and multiple injuries, and later led to the bankruptcy of the company. The incident was caused in part by the erroneous scale-up of the original reaction process recipe. In this case study we used Process Safety Office® SuperChems™ to model the runaway reaction scenario and to examine the impact of inhibitor on the thermal stability of butyl acrylate.

The normal size recipe was scaled up to produce a batch of 6,080 lbs of acrylic polymer in a 1,500 gallon reactor, which is 12 percent more than the normal amount of polymer produced in a single batch. On January 30, the operator(s) added solvents and a portion of the monomer to the reactor. On the morning of January 31, the operator(s) heated the reactor by applying steam to the reactor jacket. Finally, the senior operator started the reaction by pumping the additional remaining monomer into the reactor all at once. Several minutes later, a vapor release from the reactor occurred. The energy release and irritating vapor pushed the operator and other employees out of the building. An operator re-entered the building with a respirator and started emergency cooling water flow to the reactor jacket. However, the rate of energy release exceeded the cooling capacity of the reactor condenser. The building exploded less than 30 seconds after he exited.

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