Coke formation in steam crackers for ethylene production

Coke formation and equipment fouling in steam cracking furnaces in the ethylene industry still remain a major operation problem. Ethylene producers have been actively seeking ways to reduce coke formation in order to achieve longer furnace run length. To date, many efforts have been made by academics and industrial personnel to understand the coking problem. Based on laboratory scale studies, various coking mechanisms have been proposed in the literature. However, these mechanisms are often reactor-related and contradictory to each other since laboratory scale reactors do not simulate the practical cracking operation in ethylene plants. Therefore, analyzing coke deposits produced in such reactors may not necessarily give results similar to those of coke produced in industrial plants. Due to this consideration, we analyzed two sets of coke sample deposits from typical locations (radiant coil, coil outlet, transfer line exchanger inlet and outlet) in steam cracking furnaces. One set of the samples was taken from an ethane cracking furnace, while the other set was from a naphtha cracking furnace. Analyses were carried out on both the process side and the metal side of the sample deposits. Scanning electron microscopy (SEM) was used for microstructure, proton induced X-ray emission spectrometry (PIXE) for elemental composition, energy dispersive X-ray spectrometry (EDX) for surface elemental composition, and micro-elemental analyzer for C and H contents. Based on our results, this paper aims to provide a closer insight into the coking problem in industrial steam crackers. In addition, the coking mechanisms proposed in the literature (pyrolytic coke, catalytic coke, droplet condensation and mechanically carried over coke) are also briefly reviewed. (C) 2002 Elsevier Science B.V. All rights reserved.