KINETICS AND MASS TRANSFER IN HYDROFORMYLATION – BULK OR FILM REACTION? Tapio Salmi, Andreas Bernas, Johan Wärnå, Päivi Mäki-Arvela, Dmitry Yu. Murzin Laboratory of Industrial Chemistry and Reaction Engineering, Process Chemistry Centre, Åbo Akademi University, Biskopsgatan 8, FI-20500 Åbo/Turku, Finland Abstract: The kinetics of a gas-liquid reaction, alkene hydroformylation was studied in the presence of a homogeneous catalyst in a pressurized laboratory-scale semibatch reactor. A reactor model was developed comprising both complex kinetics and liquid-phase mass transfer. The model was based on the theory of reactive films. The model is able to predict under which circumstances the hydroformylation process is affected by liquid-phase diffusion of the reactants. Experimental data and model simulations are presented for the hydroformylation of propene in the presence of a homogeneous rhodium catalyst. Keywords: gas-liquid reaction, kinetics and mass transfer, hydroformylation INTRODUCTION A lot of theories have been developed for the quantitative description of gas-liquid reactions, such as the film theory, the penetration theory of Higbie and the surface renewal theory of Danckwerts (Danckwerts 1970). The crucial issue in all of these theories is the degree of backmixing and the local turbulence in the liquid side of a gas-liquid reactor. In general, it is agreed that the interplay of kinetics and diffusion in the liquid phase affects the overall reaction rate, in case that the intrinsic reaction rate is moderate, rapid, very rapid or instantaneous. Only for slow and very slow reactions, a simplified treatment is possible. The main part of the previous research work in this field has concerned inorganic systems, for which the reactions often are rapid, for instance, neutralisation reactions used in gas absorption. Organic synthesis reactions are in most cases considered to be slow or very slow compared to the diffusion processes. In the ultimate case, this means that the gas-liquid reaction system can be treated as a pseudo-homogeneous system, just the solubilities of the gas-phase components are included in the model. The presence of a homogeneous catalyst can complicate the picture, since the catalyst concentration appears in the rate equation, and the process can be slow or rapid, depending on the catalyst concentration level. In case of organic reactions, the mechanisms are typically very complicated comprising several elementary steps and reaction routes. This implies that the simple analytical expressions for zero and first order kinetics developed for the evaluation of the role of the mass transfer in the gas-liquid process (relations between the Hatta number and the enhancement factor) are not any more valid. In the laboratory scale, the problem is surmounted by introducing enough of stirring effect to suppress the mass transfer limitation. In industrial scale, this is typically not possible, and thus it is anticipated that many organic gas- liquid reactions carried out industrially are strongly influenced by liquid-phase mass transfer limitations. In this study, we present the modelling principles for gas-liquid semibatch reactors, where organic transformations take place. The experimental case study is hydroformylation of alkenes, which is the most important homogeneously catalyzed gas-liquid reaction applied industrially. The aim of the modelling is to reveal, under which circumstances the reaction is kinetically controlled and under which circumstances the gas-liquid mass transfer plays a crucial role. In spite of the industrial importance, quantitative studies on hydroformylation kinetics and mass transfer effects have been very scarce. Research papers on the intrinsic kinetics have appeared only recently (Bernas et al. 2008; Murzin et al. 2009) and mass transfer coupled to chemical reactions is in this connection completely oublié. Hydroformylation, or oxo synthesis, is carried out in the presence of a homogeneous catalyst, nowadays organic rhodium complexes are used. Hydroformylation implies a reaction between an alkene molecule,