Trickle-bed reactors are a commonly used type of threephase catalytic reactors in which liquid and gas phases flow concurrently through a bed of catalyst particles. They are usually operated in the trickling regime. A common feature of most trickle-bed reactors is the complexity of the gas-liquid flow and that, in most cases, the overall reaction rate is governed by mass-transfer resistances. Gaseous species must dissolve into and then pass through the liquid phase to reach the catalyst particles. Mass-transfer resistances between the phases affect the conversion and the overall reaction rate. However, the liquid phase is essential to the system and cannot be completely eliminated.According to the recent literature, many attempts have been made to overcome this drawback by means of reducing the mass-transfer resistances. This can be accomplished using other reactor configurations (segregated flow reactors) or by means of cycling of the liquid phase. Segregated flow reactors basically consist of a tubular supported catalytic membrane. The liquid and gas streams are separated as they flow through the membrane (Hatziantoniou and Anderson, 1984;De Vos et al., 1986; Cini and Harold, 1991; Young and Cussler, 1987;Hatziantoniou et al., 1986;Carlsson et al., 1983). In all the cases the solution to overcome the mass-transfer resistance limitations is suggested by means of using another reactor, not a trickle-bed reactor.In the case of cycling, an inlet variable (that is, the liquidflow rate) is periodically changed between two given levels. When the liquid flow is cut off, the bed partially drains. This thins out or eliminates the liquid film surrounding the particles with a decrease in the transport resistance for the gaseous species. This mode of operation could be viewed as a way of segregating, at least partially, liquid and gas phases.Periodic operation of a trickle-bed reactor was studied by Haure et al. (1989). The reaction system under consideration was the sulfur dioxide oxidation over carbon catalyst. In this case, the reactants were present in the gas phase only. The sulfur trioxide formed was removed by the liquid flow (water). The liquid flow was turned on and off and an increase in the oxidation rate of about 30 to 40% was found within a range Correspondence concerning this article should be addressed to P. M. Haure of cycle periods from 2 to 80 min. Under this condition, wetting plays an important role in the behavior of the reactor. The influence of cycling results from the interplay of masstransfer and heat effects.Multiphase reaction-transport interactions at the single catalytic pellet level were studied by Funk et al. (1991) and Harold (1993, 1994). Their approach provides valuable information about local unsteady behavior within the fixed-bed multiphase reactor and gives insight into catalyst performance during periodic operation.The objective of this article is to compare the performance of a laboratory trickle-bed reactor operated in the cycling and conventional modes for a reaction system which is...

When a trickle-bed reactor (TBR) is operated periodically

The catalytic hydrogenation of alpha-methylstyrene to cumene is studied in a laboratoy trickle-bed reactor operated at low liquid flow rates. Under this condition, wetting is not complete, vaporization of the liquid phase may occur, and reaction could proceed via liquid-solid and gas-solid catalysis, which increase the hydrogenation rate sign$-cantly. To determine and quantih the eflect of the gas-solid catalyzed reaction, we propose the use of post-packing sections of increasing lengths. The inert bed acts as an absorber of the gas-phase-produced cumene. The liquid and gas effluent were monitored. Experimental global rates determined fiom liquid cumene concentrations varied with the post-packing length. A simple one-dimensional model developed shows that theoretical and experimental results agree well.