At about 6 a.m. on December 13, 1994, two explosions rocked the ammonium nitrate (AN) facility at the Port Neal, Iowa nitrogen products manufacturing complex operated by Terra Industries. Four people were killed and 18 injured. Approximately 5,700 tons of anhydrous ammonia were released, and releases of ammonia continued for nearly six days following the explosion. Groundwater under the facility was contaminated by chemicals released as a result of the blast.
Figure 1. Terra's Port Neal, Iowa complex before the explosion. The ammonium nitrate plant is indicated.
Figure 2. Post-explosion aerial photograph of the Port Neal plant
The Port Neal plant produced an 83 percent AN solution by reacting ammonia and nitric acid in a vessel called a neutralizer. The original neutralizer was replaced in 1980 and a major modification and upgrade of the plant occurred in 1992. A scrubber and new control system were also added in September 1994. In the two days prior to the explosion, the nitric acid plant was shut down for maintenance. With the nitric acid plant not operating, the AN facility was also shut down.
Figure 3. Inadvertent flow of nitric acid and chlorides from the absorption column into the neutralizer
Figure 4. Superheated steam injection into the neutralizer
Exponent was retained to investigate the cause of the explosion. Exponent determined that the accident occurred due to unsafe plant operations including poor maintenance and inadequate employee training. Specifically, during the shutdown period, the pH of the neutralizer vessel contents dropped to an unusually low level and leaks in other equipment led to the introduction of chloride ions that catalyzed the final reaction. Unaware that the 18,000 gallon-capacity neutralizer vessel was in a highly acidic and contaminated condition, Terra employees injected superheated steam to try to keep the vessel contents from freezing due to the winter cold. The energy from injected superheated steam led to the runaway chemical reaction of the sensitized ammonium nitrate solution and resulted in the subsequent explosive detonations.
Figure 5. and 6. Exponent tests
McGoran B, Reza A, et al. Evaluation of a chemical plant explosion and lessons learned. The 1994 detonations at the Terra Industries ammonium nitrate plant. Proceedings, 2006 Annual Meeting of the Chinese Mechanical Engineering Society and 1st Annual Meeting of the Chinese Academy of Engineering, Mechanics and Transportation Engineering Division, pp. 252–257, 2006.