Detection of oxides of nitrogen

The on-line detection of oxides of nitrogen in light hydrocarbon streams by modified chemiluminescence detection and/or by dry colorimetric detection

by Surinder Thind,
Applications Chemist


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.

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