(521ej) Dynamic Rate Analysis for Low Frequency Operation of Chemical Reactors

Periodic operation of catalytic reactors has been shown to be a viable method of improving reactant conversions and product selectivities. However, mathematical complexity of transient design equations complicates the ability to identify regions of process enhancement through dynamic relative to conventional steady state (SS) operation. In this study, we describe methods to determine limits of process enhancement during low frequency quasi-steady state (QSS) operation of chemical reactors. During QSS operation, the net concavity of rate expressions directly correlates to differences between local QSS and SS rates. In general, QSS rates are enlarged by convex and reduced by concave relationships. Rate expression convexity stems from apparent reaction orders outside the range 0 to 1 whereas concavity arises from orders within this range. These effects can be applied to determine limits of dynamic enhancement for complex Langmuir Hinshelwood rate expressions. For instance, fig. 1a shows that conversion versus the Damkohler number (Da) in the QSS reactor surpasses that achieved by the SS reactor for a binary reaction involving competitive adsorption over a catalyst surface. Fig. 1a also shows that the local QSS rate enhancement (Δ) as a function of Da reaches a maximum at Da=0.22 before rapidly dropping below zero. The sudden decrease in Δ is a result of changing convexity as the dominant inhibiting reactant B is consumed at higher Da, resulting in a shift from a negative to positive apparent reaction order at its concentration wave minima. Fig. 1b shows that the apparent reaction order of the SS solution at the concentration wave minima for species B changes from negative to positive at the Da of maximum enhancement. Here, we have demonstrated that analysis through SS material balances can offer great insight into determining enhancement limitations during low frequency dynamic operation of chemical reactors.