(32a) Cold Box Fouling in a Cracker

Serious fouling of a coldbox led to high pressure drop increase over the first cracked-gas pass heat-exchanger. Pressure drop eventually limited the plant capacity and production rate had to be reduced. Root cause investigations were initiated including extensive analysis of the fouling material. The main contributing root cause for the cold box plugging was identified as the co-cracking of a C4-recycle stream from the Butadiene plants containing NO containing inhibitor residuals leading to NOx formation in the furnaces. These NOx species are assumed to react in gas phase with dienes to form a non-explosive, non-shock sensitive NOx-polymer material. The non-explosive, non-shock sensitive NOx-polymer material found in the heat exchanger is a completely new phenomenon in Dow and in the ethylene industry, different from the known NOx-gums formed in the low temperature zone of the cold box reported in Dow and ethylene industry. The intent of this paper is to share the current knowledge on the composition and mechanism of formation of such polymer and recommendations to understand its reactivity properties which is essential for the development of efficient and safe cleaning procedures, like chemical cleaning with warm methanol.