Modelling of kinetics and mass transfer in the hydrogenation of xylose over Raney nickel catalyst

Abstract

This paper discusses the modelling of xylose hydrogenation kinetics over Raney nickel in aqueous solutions, the determination of the hydrogen solubility in the reaction mixture as well as evaluation of mass transfer effects in the reaction system. The hydrogenation experiments were carried out batchwise in an automatic laboratory-scale reactor. The reactor system operated at a pressure range of 40–70 bar and at temperatures between 80 and 140 °C. The catalyst-to-xylose ratio was approximately 5 wt-% of the xylose weight normally. The reactor contents were analysed off-line with a high performance liquid chromatograph. Hydrogen solubility in the reaction medium was determined with a gas-chromatographic system. The solubility was found to remain fairly constant during the hydrogenation. Only a slight increase in the hydrogen solubility was detected as xylose was hydrogenated to xylitol. The overall hydrogen solubility in the reaction mixture was significantly lower than in pure water, as expected. The main hydrogenation product was xylitol, but small amounts of xylulose and arabinitol were detected as by-products. A semi-competitive kinetic model, based on hydrogen and xylose adsorption, was developed. The model accounts for the very different areas covered by a hydrogen atom and an organic species on the catalyst surface. The parameters of the kinetic model were determined with non-linear regression analysis. It turned out that the kinetic model is able to describe the formation of both xylitol and the by-products. The mass transfer effects in the batch hydrogenation were evaluated by using measured viscosities and estimated diffusion and mass transfer coefficients. A process simulator, utilizing the kinetic and mass transfer effects, was developed to predict the behaviour of industrial reactors. © 1999 Society of Chemical Industry