Catalysis of Organic..

Salmi et al. 18722. Modeling and Optimization of ComplexThree-Phase Hydrogenations and Isomerizationsunder Mass-Transfer Limitation andCatalyst DeactivationTapio O. Salmi, Dmitry Yu. Murzin, Johan P. Wärnå, Jyri-Pekka Mikkola,Jeanette E. B. Aumo and Jyrki I. KuusistoAbstractÅbo Akademi, Process Chemistry Centre, FI-20500 Turku/Åbo, Finlande-mail: Tapio.Salmi@abo.fiInternal and external mass transfer limitations in porous catalyst layers play a centralrole in three-phase processes. The governing phenomena are well-known since thedays of Thiele (1) and Frank-Kamenetskii (2). Transport phenomena coupled tochemical reactions is not frequently used for complex organic systems. A systematicapproach to the problem is presented.Industrially relevant consecutive-competitive reaction schemes on metal catalystswere considered: hydrogenation of citral, xylose and lactose. The first case study isrelevant for perfumery industry, while the latter ones are used for the production ofsweeteners. The catalysts deactivate during the process. The yields of the desiredproducts are steered by mass transfer conditions and the concentration fronts moveinside the particles due to catalyst deactivation. The reaction-deactivation-diffusionmodel was solved and the model was used to predict the behaviours of semi-batchreactors. Depending on the hydrogen concentration level on the catalyst surface, theproduct distribution can be steered towards isomerization or hydrogenation products.The tool developed in this work can be used for simulation and optimization ofstirred tanks in laboratory and industrial scale.IntroductionMost parts of heterogeneously catalyzed reactions are carried out in porous catalystlayers, such as catalyst particles in fixed beds, fluidized beds, slurry reactors or incatalyst layers in structured reactors, such as catalytic monoliths, fibers or foams (4 -6). In the manufacture of organic chemicals, three-phase processes are very frequent,catalytic three-phase hydrogenation being the most typical example. Suchmanufacturing processes very often involve a strong interaction of kinetics, masstransfer limitations and catalyst deactivation. Mass transfer might play a central rolein various stages of a catalytic three-phase process: at the gas-liquid interface as wellas inside the porous catalyst layer. In the presence of organic components, catalystdeactivation due to poisoning and fouling retard the activity and suppress theselectivity with time. At elevated reaction temperatures, sintering of the catalyst cantake place. Production of organic fine and specialty chemicals is often carried out in