Polymer inhibitors are used within an ethylene plant in locations which are susceptible to aggressive polymer formation i.e. Primary Fractionator, Compressors, Pyrolysis Gasoline Strippers and within the Gas Separation units. Over the years, the use of such additives has been extended to newer process units in the plant to improve overall unit reliability and run-length. Most of these plants also undergo significant turn-around changes as part of reliability improvement or profitability improvement. Such changes often result in some recycle of streams from one section of the plant to another. Conventional polymer inhibitors make use of nitrogen based compounds in the form of amines, nitroxyl compounds and/or aminophenols to provide polymer inhibition.
Most polymers within ethylene complexes are formed through the reaction of free radical alkyl groups and are catalyzed by a number of factors including temperature, oxygen and active metal sites. Often these polymers are initiated due to Diels Alder reactions and at times, corrosion products have also been known to contribute to the polymerization rate. Most commercial inhibitors were formulated to provide quick termination of radicals such that propagation of the polymer chain is hindered.
Since the early 80s when the Berre facility in Europe had an incident that could be related to NOx decomposition in the cold box, olefins plants have been cautious about monitoring NOx compounds and many are known to monitor NOx precursors in their system. More recently in 2008, an incident pertaining to NOx compounds was reported at a Port Arthur facility. NOx compounds or NOx gums are formed when a conjugated di-olefin, most probably butadiene, reacts with NOx gases that condense and accumulate. The heat of reaction is enough to initiate a decomposition which then leads to detonation.
With the general use of polymer inhibitors on the rise, clubbed with the increased recycle or re-processing of process streams, there exists a strong likelihood of NOx formation in such plants on account of usage of the polymer inhibitors. This paper will compare the NOx formation rate of some of the commonly used polymer inhibitors and also present interesting results in polymerization tests of a Non-Nitrogen based inhibitor. It is believed that the reduction in addition of Nitrogen compounds that have a higher likelihood of generating NOx in the plant will be a huge step towards reducing NOx accumulation in the cold box section of the ethylene plant.
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