Abstract:

Producers of high purity monomers (ethylene) have identified the need to measure the presence of Oxides of Nitrogen (NOₓ) at very low PPB levels, and take steps to avoid potentially hazardous conditions, in the Cold Box. NOₓ (NO and NO₂) in the olefins processing train is a safety concern. There is a possibility of accumulation of unstable liquid or solid nitrogen oxide (N₂O₃) and nitrogen oxide-organic solids in the cold processing equipment. In the Cold Box of ethylene production plant at temperature, 130 C to 170 C, oxides of nitrogen may combine with dienes (gums) and form potentially explosive nitrated resins. During, shut down, at the time the olefins recovery train is allowed to warm up, any cracked gas flow passing through sections where N₂ O₃ has deposited, will lead to the formation of extremely hazardous conditions. This is due to the reaction of NO₂ and N₂ O₄ with heavier olefinic materials. The NO₂ can react with olefinic materials to form “gums”. These ‘gums’ formed are explosive at cryogenic temperatures. NOₓ deposits represent the presence of a powerful oxidizer in a system filled with flammable materials. Even though, in theory, Nitric Oxide (‘NO’) is the only NOₓ species that reaches the cryogenic ethylene recovery unit, it is also required to measure other species that belong to NOₓ group. If NOₓ levels are below five parts per billion in the cracked gas, then accumulations would not be expected to occur. However, if the concentration is over 30 ppb, accumulations in one form or another are assured.