Trickle Bed Reactors: Effect of liquid flow modulation on catalytic activity.

AYUDE M; CASSANELLO M; MARTINEZ O; HAURE P

Osaka, Japón

Congreso; Fifth International Conference on Unsteady-State Processes in Catalysis (USPC-5); 2006

Catalytic oxidation of organic solutions is frequently carried out in Trickle BedReactors (TBR) operated at steady state regime. Platinum group metals (particularly Pt andPd) and metal oxide catalysts are often active for the oxidation under close to ambientconditions. However, the attainment of viable reaction rates is determined by the lowsolubility of oxygen in water at atmospheric pressures and the wetting of the catalyst thatlimits the access of oxygen to the particle. So, reactor performance may be enhanced bypulsing directly the liquid loading, i.e. switching the liquid flow rate between a minimum(usually zero) and a maximum value (or ON-OFF cycling) while the gas flow is kept constant.In this way, the external surface coverage of the catalyst particles varies periodically, so thesupply of oxygen is improved. Not only mass transfer rates to the surface of the catalyst areenhanced but internal concentration profiles change accordingly Ayude et al., 2005).Performance enhancements achieved with liquid flow modulation have been reported byseveral authors (Silveston and Hanika, 2002).Additionally, there is some evidence that liquid modulation may also affect catalystactivity. For instance, semiconductor catalysts (such as the CuO/Al2O3 catalyst used byMassa et al. (2005) for the periodic oxidation of phenol solutions) are active participants andits surfaces have a dynamic character that depends on the properties of the reactants.Therefore, catalyst activity is enhanced as the catalyst is exposed to richer oxygenconcentrations. So, for metal oxide catalysts, liquid flow modulation may improve masstransport of reactants and also catalyst activity.The aim of this work is to study the effect of slow ON-OFF liquid flow modulation oncatalyst activity for the case of noble metal catalysts. The oxidation of aqueous solutions ofethanol (0.3M) over 0.5% Pd/Al2O3 spherical egg-shell catalyst was considered. The reactionwas carried out in a batch-recycled differential TBR operated at 1 atm and 70°C.