Coke formation in steam crackers for ethylene production

Coke formation and equipment fouling in steam cracking furnaces in the ethy lene industry still remain a major operation problem. Ethylene producers ha ve been actively seeking ways to reduce coke formation in order to achieve longer furnace run length. To date, many efforts have been made by academic s and industrial personnel to understand the coking problem. Based on labor atory scale studies, various coking mechanisms have been proposed in the li terature. However, these mechanisms are often reactor-related and contradic tory to each other since laboratory scale reactors do not simulate the prac tical cracking operation in ethylene plants. Therefore, analyzing coke depo sits 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 c oil, coil outlet, transfer line exchanger inlet and outlet) in steam cracki ng furnaces. One set of the samples was taken from an ethane cracking furna ce, 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 depos its. Scanning electron microscopy (SEM) was used for microstructure, proton induced X-ray emission spectrometry (PIXE) for elemental composition, ener gy dispersive X-ray spectrometry (EDX) for surface elemental composition, a nd micro-elemental analyzer for C and H contents. Based on our results, thi s paper aims to provide a closer insight into the coking problem in industr ial steam crackers. In addition, the coking mechanisms proposed in the lite rature (pyrolytic coke, catalytic coke, droplet condensation and mechanical ly carried over coke) are also briefly reviewed. (C) 2002 Elsevier Science B.V. All rights reserved.