Temperature Fronts and Patterns in Catalytic Systems

Abstract

Current understanding and open questions concerning formation and motion of temperature fronts and patterns in packed-bed reactors and on single catalytic pellets are reviewed. For the single-reaction case, it is possible to readily predict the maximal temperature of fronts formed in packed-bed reactors following sudden changes in the feed conditions (wrong-way behavior) or during reverse-flow operation. There is a need to extend these relations for systems involving multireactions and reversible changes in the pellet state. Complex temperature patterns may be generated on catalytic surfaces by global interaction and surface nonuniformities. Simulations predict that even more intricate patterns may evolve in packed-bed reactors. It is still unresolved which of these complex patterns exist in commercial reactors, what is their impact, and which control policies can stabilize beneficial temperature patterns. The predictions of several important dynamic features, such as hot-spot formation, await further analysis of models which account also for the momentum transport and changes in the physical properties of the fluid in the reactor.

Theoretical investigation of steady state multiplicities in solid oxide fuel cells*

“…From the analysis of classical chemical fixed bed reactors, a number of mechanisms are known to be responsible for the formation of hot spots, e.g. the decrease in feed temperature or interaction between exothermic and endothermic reactions [23,24]. These mechanisms are also relevant for high temperature fuel cells.…”

“…width of cell (m) c P molar heat capacity (J mol )1 K )1 ) C SE coefficient in Equation(24) (W )1 m )1 ) c t total gas concentration (mol m )3 ) d thickness of electrodes (m) D eff diffusion coefficient (m 2 s )1 ) E activation energy (J mol )1 ) E SE coefficient in Equation (24) (J mol )1 ) F Faraday constant (96485 C mol )1 ) D R G free enthalpy of reaction at T ref (J mol )1 ) H height of gas channels (m) D R H heat of reaction (J mol )1 ) i current density (A m )2 ) I total cell current (A) _ n molar flow (mol s )1 ) p pressure (Pa) R ohmic resistance (W) R gas constant (8.314 J mol )1 K )1 ) D R S entropy of reaction at T ref (J mol )1 K )1 ) t time (s) U Cell cell voltage (V) y j molar fraction z space coordinate (m)Greek symbols a heat transfer coefficient (W m )2 K )1 ) b 1/2 coefficients in Equation (23) c pre-exponential kinetic factor (A m )2 ) g overpotential (V) h charge transfer coefficient k heat conductivity of the solid (W m )1 K )1 ) m stoichiometric coefficient q resistivity (W m) q S density of the solid (kg m )3 )…”

Transversal thermal patterns in packed‐bed reactors with simple kinetics: Bifurcation criterion and simulations

“…Recent reviews of the Correspondence concerning this article should be addressed to O. Nekhamkina at aermwon@tx.technion.ac.il. dynamics, and patterns of single catalytic wires, pellets, and reactors have been presented by Luss 9 and Luss and Sheintuch 10 and of electrochemical systems by Kiss and Hudson 11 and Krischer. 12 The current knowledge and understanding about the formation of transversal temperature patterns has been reviewed by Viswanathan et al 13 The obtained results are kinetics-dependent 14 and in the present study we focus on kinetics that admits multiple steady state solutions, with one or two stable states (i.e., a non-oscillatory case).…”

Dynamic responses of packed bed reactors

“…Consider, for example, a sudden and persistent decrease of feed temperature. The resulting wrong-way response (Boreskov and Slinko, 1965;Pinjala et al, 1988;Luss, 1997) illustrated in Fig. 2a, consists of a transient temperature rise.…”