(22d) Evolution of Coking Mechanisms in Aging Ethane Steam Cracking Coils: From Radical to Catalytic Coking

Coking in ethane steam cracking coils poses a significant operational challenge, reducing efficiency and increasing maintenance demands over time. As coils age, changes in coking mechanisms result in more frequent decoking, negatively impacting plant performance and economics [1]. This study investigates the evolution of coking mechanisms throughout the coil's operational life and examines the effects on coil performance.

Experiments were conducted using an inductively heated electrobalance (IHEB) setup at the Laboratory for Chemical Technology (LCT) of Ghent University to measure coking rates under controlled conditions. Samples included artificially aged materials, conditioned in the IHEB, and ex-service coils retrieved from reactors at the end of their operational life. The IHEB setup simulates industrial steam cracking conditions, enabling realistic assessments of material-gas interactions at different aging stages. An inductive heater allows independent control of metal temperature, while a magnetic suspension balance (MSB) captures real-time sample weight changes, offering high measurement accuracy. Additionally, experiments were performed on a well-established test rig at Schmidt + Clemens under realistic operating conditions, as documented in recent studies [2], to serve as a reference and verify the findings.

To gain deeper insights into material aging, a detailed metallurgical investigation was performed using scanning electron microscopy (SEM) to assess the samples' microstructural changes. This allowed the identification of base material conditions, oxide layer depletion, and exposure of catalytic sites such as Ni and Fe as the material aged.

In addition to quantifying coking rates, the morphology of coke deposits formed on coils at various aging stages was analyzed. Two types of heterogeneous coking mechanisms are believed to exist: catalytic and non-catalytic [3]. Catalytic coking occurs in the initial cracking phase when the catalytic sites of the material are exposed to the gas phase. Non-catalytic (radical) coking, mainly dependent on the feedstock characteristics and operational temperatures, predominates once these catalytic sites are covered with coke. The results indicate a pronounced shift in coking mechanisms as coils age. Coking is predominantly radical in the early stages, with an intact oxide layer. As the coils near the end of their operational life, chromia depletion and increased surface roughness expose more Ni and Fe surfaces, enhancing catalytic activity [4]. This leads to a transition from radical to catalytic coking, characterized by faster coke formation rates and filamentous coke structures.

The study demonstrates a clear progression from non-catalytic to catalytic coking mechanisms as coils age, driven by chromia depletion and surface composition changes. Addressing these changes through material and operational strategies can improve the efficiency and sustainability of ethane steam cracking processes.

References:

1. Jakobi, D., et al., Tailor - Made Materials For High Temperature Applications: New Strategies For Radiant Coil Material Development, in CORROSION 2009. 2009. p. NACE-09155.
2. Jakobi, D. and P. Karduck, Behavior of High-Temperature Tube Materials in Sulfur-Containing Steam-Cracking Conditions, in Corrosion 2018. 2018, Corrosion: Phoenix, Arizona, USA.
3. Mohamadzadeh Shirazi, H., et al., Effect of Reactor Alloy Composition on Coke Formation during Butane and Ethane Steam Cracking. Industrial & Engineering Chemistry Research, 2024. 63(5): p. 2100-2112.
4. Baker, R.T.K., D.J.C. Yates, and J.A. Dumesic, Filamentous Carbon Formation over Iron Surfaces, in ACS Symposium Series. 1982, American Chemical Society: Washington DC.