NOx in the Cold Box
Nitrogen oxide (NOx) contamination in the cryogenic train is a sneaky safety issue for ethylene units. Parts per billion levels of NOx are enough to cause buildup of enough explosive material in a cold box to cause a loss of containment event once triggered by a warm-up event. As with many safety problems, this phenomenon was demonstrated in a dramatic fashion when Shell’s plant in Berre, France exploded in 1990. The root cause was traced back to brown gums formed from traces of NOx in gas from a catalytic cracker that the olefins plant processed, and launched a new field of chemistry for the ethylene industry.
The hydrocarbon feeds to most ethylene plants contain little to no nitrogen in them, so NOx buildup can often be a surprise when first discovered in a cold box (cryogenic train). Thus the nitrogen ingress is usually associated with some sort of acute or chronic contaminantion. Common sources include gases from catalytic crackers, antifouling additives such as hydroxylamine or nitroso-based additives, or even something like the nitrate-contaminated water carryunder issue that led to loss of containment at BASF Port Arthur in 2008.
Compounds with N-O bonds can crack in pyrolysis furnaces to form NO (nitric oxide) and NO2 (nitrogen dioxide). NO2 is an acid gas, and reacts with water to form nitrous and nitric acids which are removed by the caustic tower:
The NO, however, passes right through the caustic tower, the driers, the contaminant removal beds and the acetylene converter on its way to the cryogenic train. There, temperatures below -75 °C (-100 °F) allow NO to dimerize and form N2O4:
Remember, these processes take place over years: Shell Berre started feeding cat cracker gas in 1982, and the cold box exploded during a warmup in 1990. The cryogenic temperatures act as a lens, concentrating up reactants as they deposit as solids on the metal walls inside the process equipment. Since units typically operate for years at a time, this means that very slow reactions have an opportunity to progress. One of the rate limiting steps towards the formation of explosive material is the oxidation of NO in the presence of traces of water:
The nitrous acid formed is unstable, and dissociates to form NO2 and nitric acid:
When ammonia is present, as it sometimes is when nitrogen contamination is an issue, both the nitrous acid and nitric acid can react to form explosive NOx salts:
The salts alone are stable enough to survive ambient temperature, but not elevated temperatures or severe mechanical shock. So why were there loss of containment events associated with NOx? The reason is since NO2 has an unpaired electron on the nitrogen atom, it is a free radical and readily attacks diolefins such as butadiene, forming NOx gums.
Butadiene is not normally present in the “back” of the cold box where temperatures become very cold. However, during shutdowns or plant trips, BD can diffuse to the back of the box if there are no protections in place. This is what is thought to have happened at Shell Berre in 1990. Once formed, the gums are thermally unstable, and when temperatures rise to ambient they can detonate. Once the gums explode, they act as a primary explosive for the larger mass of salt deposits. Very little material is needed for loss of containment - as little as ten grams of TNT-equivalent material can cause loss of containment.
NOx management is a game of washes. During an outage, after clearing the hydrocarbon but before allowing the equipment to warm up, alcohol-based solvents are injected into the cold box and allowed to soak to dissolve the dangerous deposits. Some companies elect to analyze the washes to monitor NOx buildup in their facilities. Shiloh Scientific Consulting has deep expertise in NOx wash analysis and can help you interpret the results to ensure safety in your plant.
