(638a) Analysis of (side) Product Inhibition in the Measurement of Reaction Rates in Packed-Bed Flow Reactors: The Case of Ethane Partial Oxidation

While it is common practice to measure reaction rates below 10% conversion with the goal of minimizing packed bed rate gradients, beds can be integral in nature at conversions even as low as 0.1% when rates exhibit high enough sensitivities to product concentrations. Despite the ubiquitous nature of catalytic product inhibition, mathematical analyses of bed scale gradients for non-power law models do not currently exist in the open literature. We present herein not only a mathematical and experimental analysis of product inhibition, but evidence for a unique case in which the side product of a partial oxidation reaction (CO₂) exhibits a greater inhibitory effect on reaction rates compared to the by product (H₂O) of the desired partial oxidation reaction.

Using ethane oxidation over bulk nickel oxide as an example, we show how product co-feeds can be used to gain insights into reaction kinetics that are then used to analyze integral bed data obtained in the absence of product co-feeds. Rates are a very strong function of conversion, with partial oxidation rates decreasing by up to 50% between 0.2-2% ethane conversion. Although lowering the ethane conversion narrows the gap between measured and true rates, it leads to an amplification of normalized rate gradients that make rate estimation harder at lower rather than higher conversions. Partial oxidation rate data over pure nickel oxide, unlike those over niobium-doped nickel oxide, can only be explained when the effect of both CO₂ and water on measured rates are considered- observations that are rationalized within a framework of carbonate mediated product inhibition that decreases in relative preponderance with increasing niobium loadings. Our study provides a mathematical foundation for analyzing product inhibition for non-power law rate expressions and extends this analysis to cases where side products also inhibit reaction rates.