Wall-coated monolith channels mass transfer

Two types of catalyst systems are commonly used (1) packed beds and (2) monolith blocks. The first type is a perforated container containing spherical beads coated with catalytic ingredients it allows the reactants to flow through the catalytic beds. It has high external surface area and high mass transfer efficiency between reactants and catalyst. The second type has the walls of monolithic honeycomb wash coaled with the catalyst ingredients and allows the reactants to flow through the channels in the monolith. 

Recently, the hydrogenation a mixture of toluene, styrene and 1-octene, representing a model feed for hydrotreating in the refining industry, was performed in monolith reactors [37]. One is a y-alumina monolith of diameter 1 cm and 15 or 30 cm long and the other is a more conventional cordierite monolith with a wall-coated layer of y-alumina. In both monoliths, the channels size is 1-2 mm and the catalyst is based on Ni. Substantial alkene conversions of more than 50% were observed in the small-channel reactors, which was attributed to the intensified mass-transfer rate generally measured in monolith reactors [16]. 

The mass transfer rate in monolith reactors is mainly controlled by four mechanisms - gas/solid, gas/liquid, liquid/solid, and pore diflhsion. The reaction takes place on the surface and in the pores of the wash coat layer at the channel wall. The reactants are transferred from the bulk liquid and gas phases to the solid surface, and the reactants from the gas phase are also transferred from bulk gas phase to bulk liquid phase and from there to the solid surface. The three mass transfer coefficients involved are gas-liquid, liquid-solid and gas-solid. In addition to these factors, diffusion of the reactants into the pores and products out of the pores also influences the overall mass transfer rate. The correlations for liquid-solid and gas-liquid mass transfer have been reported by Roy et.al. [24], where Sherwood number is expressed... 

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