Steady-state behavior of the nonisothermal partially wetted and filled catalyst

Abstract

The behavior of the single catalyst pellet plays a crucial role in hot-spot formation within the trickle-bed reactor. In this study the steady-state behavior of the partially wetted and filled catalytic pellet within which an exothermic gas phase catalytic reaction occurs is examined. An idealized picture that captures the essential physics is modelled. The slab-type pellet is wetted on one face by a flowing liquid, a fraction of which enters the pores by capillary action. The volatile pure liquid component can vaporize within the pores and subsequently decompose into a volatile product. The impact of several kinetic and physical parameters on the steady-state behavior is determined. The steady-state position of the intraparticle gas—liquid interface, the degree of catalyst overheating, and the catalyst effectiveness are sensitive functions of the interaction between the liquid imbibition and vaporization, capillary condensation, diffusion, and exothermic reaction processes. The dependence of catalyst effectiveness on the Thiele modulus is shown to assume one of several different forms corresponding to the existence of different rate-controlling regimes. As the catalytic activity is increased, the transition between a mostly liquid-filled, low-rate state to a mostly gas-filled, high-rate state may be single valued or in the form of a counterclockwise hysteresis. Multiplicity is predicted to exist over a wide range of conditions, including the case of a reaction with a zero activation energy. For a sufficiently large activity and under thermally mild conditions intraparticle diffusion limits the overall rate. Under thermally intensive conditions the gas—liquid interface cannot be sustained within the sufficiently active pellet. Modifications of the model to handle these cases are discussed.