Impact of Initial Surface Roughness and Aging on Coke Formation during Ethane Steam Cracking

Alloy composition and morphology of the inner wall of steam cracking reactors are well-known key factors that affect their coking tendency. The effect of surface roughness on the coking tendency remains uncharted to date and has been studied here for a 35/25 Ni/Cr wt % alloy in a quartz jet stirred reactor equipped with an electro-balance under coil outlet industrially relevant ethane steam cracking conditions: <i>T</i><sub>gas phase</sub> = 1173 K, <i>P</i><sub>tot</sub> = 0.1 MPa, and <i>X</i><sub>C<sub>2</sub>H<sub>6</sub></sub> = 70%. Up to 6 times higher initial coking rates have been observed during cyclic aging in an <i>R</i><sub>α</sub> surface roughness range of 0.15–7 μm, and cyclic aging proved to have an effect mainly on the catalytic coking behavior. No effect was observed on the asymptotic coking rates. Scanning electron microscopy, energy diffractive X-ray surface analysis, and cross section elemental mappings suggest that the effect of surface roughness and aging on the catalytic coking rate derives mainly from changes in the metal surface composition. The amounts of metallic Ni and Fe show an increasing tendency with increasing surface roughness, explaining the pronounced coke deposition. Using Ekvicalc, thermodynamic calculations were performed proposing that the amount of Cr<sub>2</sub>O<sub>3</sub> gradually decreases followed by an increase of manganese chromite, MnCr<sub>2</sub>O<sub>4</sub>.