Chemical Reaction Hysteresis Induced By Capillary Condensation Within The Catalyst Pellets

The fixed-bed catalytic reactor is a widely used multiphase reactor in the chemical industry, but the high parametric sensitivity usually makes the reactor subject to hot spot formation and even runaway.Thus restricting the reaction rate to a reasonable degree is necessary, the present study attempts to accomplish this by monitoring internal wetting fraction through incorporation of capillary condensation into catalyst pellets.Benzene hydrogenation to cyclohexane is applied as the working reaction. At first, the intrinsic kinetics of vapor-phase benzene hydrogenation is studied and kinetic equation based on the Langmuir-Hinshelwood type mechanism is derived. Subsequently, adsorption measurement of benzene and cyclohexane at elevated temperatures and pressures as well as reaction measurement over partially internal wetted catalysts are conducted, two types of operational processes of first adsorption and then desorption or first desorption and then adsorption are investigated. Reaction rate hysteresis is indeed discovered as theoretically expected to exist in mesoporous catalysts. It is revealed reaction rate presents hysteresis loops analogous to the well-known adsorption/desorption isotherms, and the adsorption branch is always above the desorption branch. It shows that reaction rate is not only dominated by the liquid wetting fraction in the catalyst but also affected by the operational history. Multiple hysteresis loops are also obtained by returning at different liquid flow rates. New correlations fitted from the data of adsorption/reaction branches on the relationship between the overall reaction rate and catalyst wetting fraction are proposed, which give a better representation on the reaction rate of a catalyst under partial wetting condition over the conventional linear consideration. Through intrinsic reaction, internal effectiveness factor at different conditions are acquired, which also display hysteresis versus wetting fraction. An increase of temperature leads to a decrease of the internal effectiveness factor. Using support with wide-range pores to prepare catalyst is preferred for pore blocking could occur in a more extensive region. Besides, operation under a suitable reactant flow rate could decrease parametric sensitivity.