Research Article Influence of Bubble Evolution on the Effective Kinetics of Heterogeneously Catalyzed Gas/Liquid Reactions. Part I: Reactions with Gaseous Products For heterogeneously catalyzed multiphase reactions the formation of bubbles may have an influence on mass and heat transfer as well as on the effective reac- tion rate. This first of two contributions deals with the Ni-catalyzed decomposi- tion of H 2 O 2 , which was used as a model system for an (almost) isothermal reac- tion with a gaseous product. (In part II the strongly exothermic hydrogenation of hexene will be analyzed, where gas/vapor bubbles may be generated by overheat- ing of the catalyst.) The discharge of O 2 bubbles formed by decomposition of H 2 O 2 enhances the external mass and heat transfer up to one order of magnitude. This is in analogy to the well-known phenomena during nucleate boiling. The ex- periments and theoretical considerations also show that the internal mass transfer depends on the intensity of the reaction and thus on the H 2 O 2 concentration, which is in contradiction to the classical Thiele approach. This discrepancy could be explained by a modified model that takes the formation of bubbles into account. Keywords: Bubble evolution, H 2 O 2 decomposition, Mass and heat transfer, Oscillation model Received: December 23, 2009; revised: March 4, 2010; accepted: March 8, 2010 DOI: 10.1002/ceat.200900624 1 Introduction Since the fundamental studies of Thiele [1] and Zeldovich [2] on the interplay of a chemical reaction and the internal mass transfer in catalytic particles it is well-known that the diffusion of reactants may limit the achievable effective reaction rate. This effect, i.e., the pore effectiveness factor, is also calculable by the Thiele modulus. The same is true for the influence of external heat and mass transfer, which is considered by correla- tions for the external heat and mass transfer coefficients and the corresponding Sherwood and Nusselt numbers. All these theories are straightforward, if the reactants are present either as liquids or gases. For multiphase reactions, where a gaseous reactant is formed from a liquid reactant or for highly exother- mic gas/liquid reactions in trickle-bed or bubble column reac- tors where bubble formation may also occur, these theories are not valid or at least only to a limited extent. The effect of bubble formation and its influence on mass and heat transfer were already described and discussed for electrolysis and dissolution processes [3–7]. A common feature of these processes is the generation of a gaseous reactant, which leads at a certain point to the formation of gas bubbles. A summary about the electrolytic gas evolution is given in [3], where nucleation, growth, and detaching of bubbles is de- scribed. Correlations for mass transfer at the electrodes are giv- en and the analogy to heat transfer during nucleate boiling is also mentioned. The superposition of bubble-induced and convective mass transfer is described in [4]. Bubble evolution during dissolution of magnesium in hydrochloric acid was studied in [5]. The improved mass transfer is explained by the convective flow caused by the evolution of bubbles. The influ- ence of external stirring on the dissolution rate was also deter- mined. At low concentrations, the effective rate is distinctly in- fluenced by external stirring while for high concentrations (and with it high dissolution rates) this effect becomes less, because the stirring effect of the H 2 bubbles becomes domi- nant. In the case of decomposition of calcite by nitric acid, the impact of stirring was related to a decrease of the de- parture diameter and thus to an improvement of the mass transfer [6]. Chem. Eng. Technol. 2010, 33, No. 6, 911–920 © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cet-journal.com Thomas Oehmichen 1 Leonid Datsevich 1 Andreas Jess 1 1 Department of Chemical Engineering, University of Bayreuth, Bayreuth, Germany. – Correspondence: Prof. Dr. A. Jess (jess@uni-bayreuth.de), Department of Chemical Engineering, University of Bayreuth, D-95440 Bayreuth, Germany. Bubble evolution 911