Modeling of On-line Catalyst Addition Effects in a Short Contact Time Reactor

This paper describes a computational study of the partial oxidation of ethane to ethylene in a shortcontact-time reactor (SCTR), using a two-dimensional computational fluid dynamics model with full heat and mass transport. Detailed heterogeneous and homogeneous chemical kinetic mechanisms are employed to describe the chemistry. Emphasis is placed on simulating recent experiments in which the platinum catalyst is added to the front face of the reactor while it is operating (“on-line” catalyst addition). Our simulations indicate that the fundamental behavior of the ethane SCTR prepared with catalyst added online is the result of coupled heterogeneous and homogeneous chemical processes. It seems clear that low CH4 selectivity results from the lack of heterogeneous CH4 production downstream in SCTRs prepared with the catalyst added on-line. Total ethane consumption and ethylene production rates are less strongly affected because the homogeneous route for these processes compensates for the loss of heterogeneous activity, whereas the production of methaneis much more effective as a heterogeneous process. These results indicate that the improved performance observed as a result of on-line catalyst addition is due to a shift from heterogeneous ethane decomposition to homogeneous decomposition. This limits the total production of methane while increasing the selectivity to ethylene. In addition to predictions of ethane conversion and ethylene selectivity, the model also predicts the production of all other major products: H2O, H2, CH4, CO, and CO2.