Bhopal Revisited

The release of a chemical intermediate, methyl isocyanate (MIC), at Bhopal, India, in December 1984 is considered the worst industrial chemistry accident on record. Understanding the root cause of the tragedy is important to any chemist or chemical engineer involved in process safety.

The release of a chemical intermediate, methyl isocyanate (MIC), at Bhopal, India, in December 1984 is considered the worst industrial chemistry accident on record. Understanding the root cause of the tragedy is important to any chemist or chemical engineer involved in process safety.

In "The Black Box of Bhopal: A Closer Look at the World's Deadliest Industrial Disaster," author Themistocles D'Silva offers excellent technical insight into the tragedy at Bhopal, which began with contamination in the infamous storage Tank E-610—an MIC holding tank in which a runaway reaction occurred—followed by the release of a cloud of poison gas. This insightful book stretches from an explanation of how economic and political development in India contributed to the accident through the detailed chemistry involved.

D'Silva's viewpoint is heavily influenced by the fact that he is a Ph.D. chemist formerly employed by Union Carbide Corp., which owned a majority stake in the facility. His chemical background centered on the synthesis of carbamate insecticides including the commercial product 1-naphthyl-N-methylcarbamate (trade name Sevin) formed from a reaction using the MIC intermediate. He was a member of the Union Carbide scientific investigation subteam charged with replicating the composition of residue left behind in Tank E-610.

The strength of D'Silva's book is the integration of original documentation (located in the appendix) that demonstrates just how complex the relationships were between Union Carbide; Union Carbide India Ltd. (UCIL), the subsidiary that operated the Bhopal plant; the Indian federal government; and the local government of the Indian state of Madhya Pradesh. For example, D'Silva uses India's 1947 Foreign Exchange Regulation (control of currency and economic development) to explain how Union Carbide's equity in UCIL dropped from 60% to 50.9% by sale of UCIL stock to Indian banks and trusts. These details highlight how the political/economic climate for UCIL ultimately contributed to the degradation of safety controls at the plant.

D'Silva offers the most details yet published on the chemistry of the events that led to the accident, which I will discuss in more detail. He condenses volumes of information into the key issues surrounding the accident and its aftermath: history of the plant's construction and operation, investigation of the gas leak, and subsequent litigation and outcome.

The weakness of the book is the lack of presentation from the workers' and affected peoples' point of view. For example, he places little emphasis on the substandard training received by workers at the pesticide plant and he does not critique the lack of emergency preparedness training for the surrounding community.

Here's a brief recap of the details of the accident. Early morning on Dec. 3, 1984, a release of toxic MIC gas occurred from a UCIL pesticide plant located on the northern side of Bhopal. Several thousand kilograms of the gas escaped into a calm environment in which there was a mild northwest wind. Being denser than air, the gaseous MIC sank toward the ground and infiltrated shantytowns located next to the plant and then moved southward into the main city. It is estimated that more than 200,000 people were exposed to the gas and that more than 2,500 people died as a direct result. Panic surrounded the city from that morning until weeks later when the remaining MIC on the plant site could be neutralized.

This tragedy was initiated by a runaway reaction in a 15,000-gal storage tank for MIC—Tank E-610. All parties involved agree that MIC reacted exothermically with water to form methylamine and carbon dioxide. The methylamine then reacted exothermically with MIC to form dimethylurea (DMU). DMU reacted exothermically to form trimethylbiuret (TMB). The temperature inside the tank rose exponentially, creating a pressure cooker that eventually released its contents at approximately 250 °C and 50 lb-per-sq-in-gauge.

Methyl isocyanate can also exothermically polymerize into a trimer (MIC trimer) at higher temperatures, and indeed 40 to 50% of the residue remaining in the tank at Bhopal was this trimer. The intended use of the storage tank was to hold MIC only, but a significant amount of water entered the tank. The author explains the chemistry involved in much greater detail. For example, gas and liquid chromatograms of the core residue are presented that demonstrate the many products (15 peaks shown) that were formed during the runaway reaction.

From a safety point of view, isocyanates can always react exothermically, resulting in reaction runaways. The chemistries are complicated because the isocyanates, -N=C=O, have two adjacent double bonds. These structures desire to react, exothermically, to form more stable molecules. Of course, reactivity is the main advantage of the isocyanates as intermediates.

Proper controls and design are imperatives when working with isocyanate chemistry. These controls were in place at the Bhopal plant; however, as the author details, at the time of the accident, three major controls were off-line due to plant management decisions. New information for me was an explanation of the role that chloroform played in the runaway reaction. In the Union Carbide studies that followed the accident, water and MIC alone did not duplicate the residue found in the storage tank. Above 100 °C, DMU and TMB in the presence of chloroform formed the main cyclic residues found in the tank (MIC trimer, dimethyl isocyanurate, and trimethyltriazenedione). Chloroform was used as a solvent in the MIC preparation process at Bhopal.

Normally, chloroform was separated from MIC before the MIC went into the storage tank. The simulation studies found that the tank residue could be matched with a mixture consisting of 85% MIC, 12% chloroform, and 3% water heated up to 225 °C. Thus, Tank E-610 was contaminated with water and chloroform. The neighboring storage tank held essentially MIC only. The presence of chloroform in Tank E-610 was acknowledged in a press release issued by Union Carbide in 1985; however, I hadn't appreciated its role in the underlying reaction mechanisms until reading this book.

The author describes why the pesticide plant was unprofitable and how that, in turn, led to poor maintenance decisions and contributed to the subsequent accident. An attempt to manufacture α-naphthol on the site failed. α-Naphthol was the other intermediate combined with MIC to form the pesticide. During the plant construction phase, UCIL elected to use α-naphthol formed from the on-site combination of the naphthalene and chlorosulfonic acid.

The downfall of this pathway was the production of a toxic side-product, β-naphthol. The author explains that UCIL had overcome this side-product formation in pilot-plant tests via more controlled reaction conditions. However, after the process unit was built on-site, it did not perform to expectations. UCIL invested $2.5 million to resolve the problem before giving up and importing α-naphthol from Union Carbide. This failure alone led to an imbalance in payments and doomed the plant's profitability.

Another factor that damaged the profitability of the plant was the influx of competitive products into India, specifically synthetic pyrethroids. The forecasted demand for MIC upon which the plant was built was never realized. It was operating at about one-third of its capacity and became a losing proposition for UCIL. D'Silva acknowledges that UCIL was looking at plans to dismantle the plant and sell portions to potential buyers in Brazil and Singapore.

The plant was nearing a milestone on closure, and the last batch of MIC had been produced in October 1984. Under pressure from the adverse business conditions, plant management cut operating costs through layoffs and the removal or neglect of maintenance on safety controls. Included were decisions related to three key units devoted to mitigating or preventing MIC releases to the surroundings: shutting down the storage tank refrigeration unit, which when operating, kept the water/MIC reaction rate low enough to prevent runaway should the two be in combination; neglecting replacement of corroded pipe on a flare tower; and placing on standby a caustic solution scrubber tower.

D'Silva notes that if the refrigeration unit had been available, more time would have been available for the plant crews to make a plan of action, because the reaction rates would have been slowed. A neighboring third tank used for off-spec material—Tank E-619, with the same capacity as Tank E-610—was available for storage and was nearly empty. With time to analyze, the crews could have split the contaminated MIC and lessened the overall runaway reaction. Splitting the contents of Tank E-610 also would have lessened the pressure buildup and could have contained the gases generated. Readers, the message is clear: Be leery of tampering with safety units when looking for methods to cut costs.

The author goes into great detail explaining complexities of the interrelationships of parent company, local company, federal government, and local government. His evidence is from court documentation filed in U.S. lawsuits that followed the accident. He provides a detailed description of the litigation pathway that followed the accident.

Eventually, all court cases in the U.S. were combined into one lawsuit placed under Judge John F. Keenan in the U.S. District Court for the Southern District of New York. He dismissed the lawsuit based on grounds that the proper forum for the case was India and not the U.S. In India, the lawsuits were eventually settled out of court. Union Carbide paid $420 million to the Indian government; UCIL paid $45 million, plus $5 million paid earlier to the International Red Cross.

D'Silva's book is an excellent Bhopal resource. It does not give the whole story of Bhopal, skimping on the details about the suffering of the people affected. Nevertheless, it details many of the Union Carbide facts behind the event with copies of original documentation. The author's firsthand knowledge of agricultural pesticide chemistry is another strength. I recommend the book to any chemist or engineer working in isocyanate chemistry as well as to anyone working in process safety. Transfer of technology to a developing country is an elaborate challenge from both a safety and environmental point of view.