Anti-coking Materials for Steam Crackers

Steam cracking is one of the most important chemical processes in the world, which provides olefin monomers ethylene and propylene for the chemical industry. The boom of shale gas in the past decade has spurred the investment in steam cracking, with construction of new crackers and expansions of existing crackers planned. There has been growing interest in improving the operational profits of steam crackers by applying a catalytic coating on the internal wall of cracker tubes to reduce coke formation and increase the on-line production time. Ceria-based oxides have been widely used in the three-way catalysts for gasoline car emission control and have shown excellent activity in carbon oxidation due to their redox property. The addition of cerium oxide has also been shown to reduce the coke formation over catalysts used for steam reforming of hydrocarbons. Therefore, ceria-based oxides are promising anti-coking coating materials for steam crackers. Moreover, a methodology for reaction of carbon needs to be developed to obtain reproducible kinetics.
In this study, a methodology has been developed by using coke oxidation with dioxygen over cerium-zirconium mixed oxides as a model reaction and was validated with classic gas phase CO oxidation. The surface-area-based reaction rates were obtained by evaluating the active catalyst surface area in the mixture of catalyst and coke, which correlated with the reaction rates of CO oxidation. The composition Ce0.8Zr0.2O2 was found to be the most active catalyst for coke oxidation. Detailed kinetic studies of both coke and CO oxidation revealed the important role of lattice oxygen.
The developed method was then used to study coke oxidation/gasification with dioxygen and steam over alkaline-earth-metal-doped Ce0.8Zr0.2O2. The alkaline earth metal compounds were highly dispersed on the surface of Ce0.8Zr0.2O2. The presence of alkaline earth metals promoted the catalytic activity of coke gasification with steam but decreased the rate of coke oxidation with dioxygen.