Model To Describe Mass-Transfer Enhancement by Catalyst Particles Adhering to a Gas−Liquid Interface

Abstract

A model is presented to describe mass-transfer enhancement in slurry reactors by catalyst particles adhering to the gas−liquid interface. This model is a combination of the particle−interface adhesion−dehesion (PIAD) model and the gas-to-liquid-to-solid (GLS)−gas-to-solid (GS) model. The PIAD model is a dynamic description of the equilibrium between the catalyst particle adhesion and dehesion rates at the gas−liquid interface. These rates determine the average residence time of the particles at the gas−liquid interface. The GLS−GS model is a combination of the classical, resistances-in-series, GLS mass-transfer model and a direct GS mass-transfer model. The average particle residence time at the gas−liquid interface, the solid−liquid partition coefficient, and the reaction rate determine the mass-transfer rate by shuttling of the particles between the gas−liquid interface and the bulk liquid. The model parameters are determined from mass-transfer and reactivity experiments, performed with two different slurry systems and two Pd-catalyzed reactions, i.e., oxidation of glucose (aqueous liquid) and hydrogenation of α-methylstyrene (organic liquid), with carbon and silica catalysts in a laboratory-scale surface-aeration stirred-slurry reactor with a known flat gas−liquid interfacial area. The mass-transfer coefficient under reactive absorption conditions is higher than that under nonreactive, physical absorption conditions. Experimental and theoretical mass-transfer enhancement factors under physical and reactive absorption conditions agree well. The GS mass-transfer coefficient increases with the mixing intensity, but the GLS mass-transfer coefficient increases more, finally leading to a decrease of the mass-transfer enhancement factor with the mixing intensity. The mass-transfer model is able to predict physical and reactive mass-transfer rates as a function of the mixing intensity and catalyst concentration.