Chemical Engineering Journal 176–177 (2011) 3–13 Contents lists available at ScienceDirect Chemical Engineering Journal j ourna l ho mepage: www.elsevier.com/locate/cej Criteria for kinetic and mass transfer control in a microchannel reactor with an isothermal first-order wall reaction João P. Lopes a , Silvana S.S. Cardoso b , Alírio E. Rodrigues a,∗ a Laboratory of Separation and Reaction Engineering, Associate Laboratory LSRE/LCM, Department of Chemical Engineering, Faculty of Engineering, University of Porto, Portugal b Department of Chemical Engineering and Biotechnology, University of Cambridge, UK a r t i c l e i n f o Article history: Received 26 November 2010 Received in revised form 25 March 2011 Accepted 20 May 2011 Keywords: Microreactors Monoliths Kinetic control Diffusional control Regimes of operation a b s t r a c t Analytical expressions for distinguishing between different reaction and mass transport regimes in an isothermal microchannel reactor with first-order wall reaction are presented. These expressions are explicit functions of the Damköhler number (Da) and of the Graetz parameter (˛ Pe m /z), as well as of the degree of mass transport control () which is usually set arbitrarily. The power law addition of contribu- tions from fully developed and developing concentration profile conditions allows a correct description for all values of ˛ Pe m /z. The scaling analysis suggests that the relative importance of mass transfer effects compared to reaction should be assessed by the rescaled Damköhler number Da*, defined with the correct scales for external and internal diffusion. It is also shown that a Da - ˛ Pe m /z parametric map is appropriate for identifying the boundaries between regimes, which can be directly calculated from the dimensionless parameters in a very convenient manner. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Coated-wall microchannel reactors have been put forward as a promising design concept [1–3] for technologies related with energy generation [4–7], biocatalysis [8], systems with strict safety requirements [9] or environmental applications [10]. On one hand, miniaturization leads to an obvious enhancement of transfer rates, yielding higher selectivity and more efficient heat sup- ply/removal [11,12]. On the other, attaching the catalytic coating to the walls of the channel provides significant higher accessi- ble surface area without increasing pressure drop [13]. For these reasons, microreactors can operate under isothermal conditions with precise temperature control and increased process safety [13]. However, the introduction of the catalyst may lead to the appear- ance of external (as well as internal) mass transfer limitations. Working on the microscale (channels with diameters in the order of a ∼ 100 m) privileges surface effects (such as an hetero- geneous reaction) over homogenous processes dependent on the channel volume. In the analysis of the competition between surface reaction and transport towards the catalytic surface, two limiting regimes are usually identified by the chemical reaction engineering community [14–18]: the kinetically controlled regime (when reac- tion is slow and radial concentration gradients are negligible) and ∗ Corresponding author. Tel.: +351 22 508 1671; fax: +351 22 508 1674. E-mail address: arodrig@fe.up.pt (A.E. Rodrigues). the mass transfer controlled regime (when a fast reaction leads to nearly instantaneous annulment of the concentration at the inter- face). The definition of operating regimes is of extreme importance for almost all studies involving catalytic monoliths or microchannel reactors. For example, (1) Measurement of intrinsic kinetic parameters. The presence of significant radial concentration gradients makes it impossi- ble to measure directly the intrinsic activity and selectivity of the catalytic layer [19]. To be certified that mass transfer and kinetics are independently evaluated, a number of methods are available [20–25], involving a selective enhancement of mass transfer or reaction rates; (2) Measurement of mass transfer parameters. At high temperatures, when the different mass transfer resistances must be accounted for, the evaluation of transport parameters may lead to incon- sistent results if wall concentration annulment is incorrectly assumed [15,26–30]. This has led to controversy in literature, where observed Sherwood numbers were lower than the the- oretical minimum due to failing in accounting for finite wall reaction rates; (3) Design of the microreactor and choice of operating conditions. The geometric and operating parameters are included in the dimen- sionless numbers of the model and must be chosen so that the required performance is achieved. The change in operating con- ditions may lead to a change in the regime, which might take conversion to follow a different asymptote; 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.05.088