While restarting his chemical plant at 6:45 a.m., the operator saw an orange plume of nitrogen oxides (NOx) coming out from the stack of the nitric acid production unit. He shut the unit down, but the plume continued exiting the stack for several hours. Residents saw the cloud and voiced their strong concerns, which were relayed by local elected officials. Despite 800 kg of NOx being released to the atmosphere, nearby air-quality monitoring stations did not register any exceedances of air quality limits.

The production process involves oxidizing ammonia to NOx, which react with water inside an absorption tower to form nitric acid. The gas phase now only contains traces of NOx, which are treated using selective catalytic reduction (SCR).

At the time of the accident, a priming failure occurred on a pump used to feed weak acid to the absorption tower. The water flow rate inside the tower was no longer sufficient to produce nitric acid. As a result, a high concentration of NOx formed at the tower’s outlet and was discharged to the atmosphere via the stack in the form of an orange cloud.

The source of the problem was a failure of the condenser level, which prevented correct adjustment of the water level. The condenser emptied itself and the pump failed to prime.

In addition, the technicians were working in an old control room to be as close as possible to the facility. As a result, they did not know how much concentrated NOx had been released and thus could not respond to the deviation in NOx emissions.

For safety reasons, the acid concentration was measured at the outlet of the bleaching column. As the condenser’s pump had failed to prime, the weak acid produced in the condenser was no longer discharged and flowed to the bleaching column, skewing the measured concentrations down. The result was that the technicians did not increase the flow of water in the condenser to reduce NOx emissions.

The inspection authorities for classified facilities asked the operator to:

• Conduct a technical and economic feasibility study into how to prevent discharges during shutdowns. The operator considered that the cost of the proposed solution outweighed the environmental benefits and instead suggested improving the reliability of the processes to reduce the number of shutdowns.
• Model the dispersion of the pollution plume using worst-case atmospheric conditions of stability and temperature inversion (pictures of the accident showed high stability due to the morning hour and the presence of clouds).
• Ensure that at least three air-quality monitoring stations are working during the unit’s shutdown and restart phases.
• Provide them with the concentrations measured the next time the unit is restarted.