Catalysis of Organic..

194Modeling Mass Transfer Hydrogenationconcentration in the liquid phase plays a crucially important role for the production:in the case of external mass transfer limitation of hydrogen, the isomerisationreaction, which does not need any hydrogen, is favoured (Fig. 3).Heavy mass-transfer limitationsModerate mass-transfer limitationsconc. / wt-%504540353025201510500D-xylosexylitolD-xyluloseD-arabitolconc. / wt-%605040302010023466992115138161184207230253276299t/min0D-xylosexylitolD-xyluloseD-arabitol23466992115138161184207230253276299t/minHeavy mass-transfer limitations and moderatelydeactivated catalystLight mass-transfer limitations50conc. / wt-%504540353025201510D-xylosexylitolD-xyluloseD-arabitolconc. / wt-%4540353025201510D-xylosexylitolD-xyluloseD-arabitol5500024487296120144168192216240264288t/min023466992115138161184207230253276299t/minFigure 3. Simulation of hydrogenation of xylose.ConclusionsWe have presented a general reaction-diffusion model for porous catalyst particles instirred semibatch reactors applied to three-phase processes. The model was solvednumerically for small and large catalyst particles to elucidate the role of internal andexternal mass transfer limitations. The case studies (citral and sugar hydrogenation)revealed that both internal and external resistances can considerably affect the rateand selectivity of the process. In order to obtain the best possible performance ofindustrial reactors, it is necessary to use this kind of simulation approach, whichhelps to optimize the process parameters, such as temperature, hydrogen pressure,catalyst particle size and the stirring conditions.