Influence of the Reactor Material Composition on Coke Formation during Ethane Steam Cracking

An experimental study of the coking tendency of nine different materials was carried out in a quartz electrobalance setup with a jet stirred reactor (JSR) under industrially relevant ethane steam cracking conditions: T-material = 1159 K, P-tot = 0.1 MPa, chi(ethane) = 73%, dilution delta = 0.33 kg(H2O)/kg(HC). A strong influence of the composition of the materials on the coking rate as a function of time on-stream was observed. The initial coking rate varied from 5 X 10(-4) g.m(-2).s(-1) to 27 X 10(-4) g.m(-2).s(-1), while the asymptotic coking rate changed in the range of 2 X 10(-4) g.m(-2).s(-1) to 6 X 10(-4) g.m(-2).s(-1). SEM and EDX analyses of coked and uncoked coupons revealed that the composition of the oxide layer in contact with the cracked gas, formed after the initial preoxidation or decoking, has an important influence on the amount of coke deposited. Materials that formed a thin Al2O3 layer on the coupon surface showed a higher coking resistance. A uniform surface composition and a high resistance to spalling and fractures are other important characteristics of good materials.