Reaction rate hysteresis in a single partially internally wetted catalyst pellet: Experiment and modelling

The hydrogenation of cyclohexene over gamma-alumina supported palladium is studied here with a view to examining the multiplicity and hysteresis effects for a single pellet. As many as three steady states with different degrees of internal wetting are found, and depending on the initial condition different branches can be attained. Percolation effects appear to be important on the branches with substantial liquid filling, where large supercritical pores connected through subcritical ones are not filled during condensation, but can remain filled during evaporation. Condensation transitions are noticed when the catalyst is cooled and found to be consistent with the Cohan condition, while on each steady state branch the Kelvin equilibrium is followed. States in-between these branches are also achievable, since on cooling condensation does not occur until the Cohan condition is satisfied. Thermal history therefore plays a crucial role in determining the catalyst state. Evaporative transitions are sharper for pellets with distinct micropore and macropore size distributions, and require detailed analysis of steady state stability for their prediction.