The coking kinetics of heat resistant austenitic steels in hydrogen-propylene atmospheres

The kinetics of coke formation on several commercial Fe-Ni-Cr alloys used in the construction of ethylene steam crackers, nickel and copper were investigated over the temperature range 450 to 1000‡ C in hydrogen-propylene atmospheres using a tubular microbalance reactor. Between 900 and 1000‡ C steady state coking was controlled by gas-phase pyrolysis and similar coking rates were observed for all materials. At 900‡ C alloy carburization led to lengthy periods of parabolic kinetics before the onset of steady state. Below 900‡ C alloy coking rates were between those of copper and nickel. Major differences in coking rates of alloys were only observed below 800‡ C and arise from variations in the effectiveness of chromium carbide scales in excluding the hydrocarbon atmosphere from contact with the underlying nickel-rich and iron-rich alloy.