Explosion in an urea manufactory

In this plant, after conversion of the major part of the carbon dioxide into urea, the off-gasses from the reactor consisted of ammonia, carbon dioxide, hydrogen, oxygen and nitrogen. Prior to venting, this gas mixture was washed with a carbamate solution. This absorption was carried out in a high pressure scrubber, where an explosion occurred.
The internal components were damaged without affecting the vessel itself as it was designed to resit to an explosion of this type. A second explosion occurred later in the same year (April 1974), very imilar to the first one in term of magnitude and damage.

The INITIATING CAUSE was the ignition of a flammable atmosphere in the gas treatment unit of the plant.

Presence of hydrogen in the off-gas mixture was unavoidable, because contained already in the CO2 flow used as raw feedstock. Oxygen was injected in the system to protect the structural materials, made of steel from the very aggressive reaction with carbamide.
The formation of a hydrogen-oxygen flammable and explosive mixture was caused by a very effective adsorption of the other off-gas components. After the stripping, the off-gases contained only negligible amount of NH3 and CO2, leaving oxygen and hydrogen in a ratio falling inside the lower and upper flammability limits.
The IGNITION SOURCE was probably oil traces in the absorption part of the scrubber caused explosion.

The possibility of an explosion had been considered during the design phase, and the scrubber was built able to resist the effect of an internal explosion. Indeed, both explosions caused only damage to its internal components, and did not affect significantly the operation of the plant. Nevertheless, following these events, modifications were brought to the process.

gas scrubber

INDUSTRIAL UREA PRODUCTION PROCESS
Urea is an organic compound with chemical formula CO(NH2)2.
Because of the two amino groups (−NH2), it is the simplest amide (diamide) of carbamic acid and is also called “carbamide”.

The Bosch–Meiser process is at the base of the industrial production of urea, and consists of the reactions:
2 NH3 + CO2 ⇌ NH4CO2NH2 (exothermic)
NH4CO2NH2 ⇌ CO(NH2)2 + H2O (endothermic)

The process conditions are approximately a temperature of 190 °and a pressure of 140 to175 bar.

Because the urea conversion is incomplete, the urea must be separated from the unconverted reactants, including the ammonium carbamate, which is the product of the first reaction. This recycling process occurs in a stripper which decompose the carbamate and separate the gasses from the urea product solution. Oxygen injection is required because ammonium carbamate solutions are highly corrosive to metals, even stainless steel, especially in the hottest parts of the synthesis plant such as the stripper. Continuous injection of a small amount of oxygen (as air) into the plant helps in maintaining a passive oxide layer on exposed stainless steel surfaces.

[From Wikipedia (accesed Feb 2026) https://en.wikipedia.org/wiki/Urea#Industrial_production]

The internals were damaged without affecting the vessel itself as it was designed to take explosions.
The plant was shut down, and the top cover of the scrubber was lifted, the internals were preplaced. Within 4 days after plant shut down, the plant was back on stream again.

According to the Doyeweerd (1975, see reference), 3 methods for explosion prevention were available:
1 . A better removal of hydrogen from the carbon dioxide feed stream. Disadvantage: expensive, because requiring investment for the catalytic hydrogen removal system and for rising the content of inert gas in the scrubber.
2. Elimination of ignition sources. This would require avoiding any possible presence of oil in the high pressure scrubber. Unrealistic because a reciprocating carbon dioxide compressor was used, th source of the oil.
3. Avoiding explosive gas mixtures in presence of high partial oxygen pressures.
The company went for this 3rd option, and changed the operating conditions, resulting in an off-gas containing a higher fraction of ammonia and carbon dioxide, reducing considerably the oxygen partial pressure. To eliminate further ammonia and carbon dioxide the off-gas was then passed through an additional absorption system at a pressure of only some atmospheres.