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

This paper discusses the modelling of xylose hydrogenation kinetics over Ra ney nickel in aqueous solutions, the determination of the hydrogen solubili ty in the reaction mixture as well as evaluation of mass transfer effects i n the reaction system. The hydrogenation experiments were carried out batch wise 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 deg rees C. The catalyst-to-xylose ratio was approximately 5wt-% of the xylose weight normally. The reactor contents were analysed off-line with a high pe rformance 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 xylit ol. The overall hydrogen solubility in the reaction mixture was significant ly lower than in pure water, as expected. The main hydrogenation product wa s xylitol, but small amounts of xylulose and arabinitol were detected as by -products. A;semi-competitive kinetic model, based on hydrogen and xylose a dsorption, was developed. The model accounts for the very different areas c overed by a hydrogen atom and an organic species on the catalyst surface. T he parameters of the kinetic model were determined with non-linear regressi on analysis. It turned out that the kinetic model is able to describe the f ormation of both xylitol and the by-products. The mass transfer effects in the batch hydrogenation were evaluated by using measured viscosities and es timated diffusion and mass transfer coefficients. A process simulator, util izing the kinetic and mass transfer effects, was developed to predict the b ehaviour of industrial reactors. (C) 1999 Society of Chemical Industry.