(587a) Spatial and Temporal Phenomena in An Autothermal Oxidative Ethane Dehydrogenation Reactor Observed by Capillary Sampling

The oxidative dehydrogenation (ODH) of ethane is attractive as an alternative route for ethylene production because it is capable of producing yields and selectivities that are comparable to the well established steam cracking technology. In contrast to steam cracking, ODH operates with net heat generation; hence autothermal operation can be realized in adiabatic short contact time reactors. Platinum based monolith supported catalysts have been used in such reactors since the early 1990's, and when doped with Sn, produce very high (~80%) selectivity to ethylene when reactors are co-fed with H₂. Experimental and modeling work has begun to converge to a description of the process as a combination of heterogeneous exothermic steps to provide heat for the primarily homogeneous production of ethylene. The catalyst is hypothesized to play a critical role in the selective oxidation of the feed components where the desired scenario is a fast oxidation of co-fed H₂, which preserves the feed ethane for ethylene production. Although experiments have been conducted for measuring species downstream of the catalyst region, measurements of species within the catalytic region are scarce in the literature, but are critical in validating existing models for the ODH process. In addition, the role of the catalyst is important in transient processes, as its heat storage and transport influence startup and shutdown behavior.

In the present work, the ODH of ethane was performed in a short contact time reactor (~ 8 ms) with Pt and Rh based catalysts, which were supported on 45 pores per linear inch alumina foam monoliths. Spatially resolved measurements of gaseous species, surface temperature, and gas temperature were measured via a capillary sampling technique. A small hole was drilled through the monolith support prior to catalyst impregnation and allowed for the insertion of a quartz capillary used to withdraw a small gas sample from inside the catalyst region, or sheath a temperature probe. The gaseous species profiles for C₂H₆/O₂ ratios ranging from 1-2 (with simulated air) show distinct trends. In the first half of the catalyst, O₂ and C₂H₆ react exothermically to partial oxidation products with little or no C₂H₄ production. At the end of this zone, the temperature reaches a maximum (900 °C-1100 °C), yielding primarily H₂O and CO for Pt and H₂, CO, and H₂O for Rh. Following the exothermic zone, C₂H₄ and trace CH₄ production is observed over Pt, whereas reforming of C₂H₆ with intermediate water yields additional CO and H₂ on Rh.

In addition to spatially resolved steady state measurements, the dynamic response to feed changes (including initial startup) was evaluated. For the short contact time reactor employed in this study, the transient responses to step changes in C₂H₆/O₂ exist on distinct time scales for the reactor temperature and chemical species. For pre-aged catalysts, gaseous products respond to feed changes in much less time than temperature. In addition, measurements during the startup of fresh Pt catalysts show that the aging process occurs on the order of tens of minutes on the basis of effluent composition and temperature changes.