Enhancement of the Liquid-Side Mass Transfer in a Falling Film Catalytic Microreactor by In-Channel Mixing Structures Evgeny V. Rebrov,* ,† Thijs Duisters, ‡ Patrick Lö b, § Jan Meuldijk, ‡ and Volker Hessel ‡ † Queen’s University Belfast, Stranmillis Road, BT9 5AG Belfast, United Kingdom ‡ Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands § Institut fü r Mikrotechnik Mainz GmbH, Carl-Zeiss-Str. 18-20, 55129 Mainz, Germany * S Supporting Information ABSTRACT: Catalytic octanal oxidation with oxygen was performed at 100 °C and the total pressure of 5 and 10 bar in a falling ﬁlm microreactor with varying reaction plates bearing diﬀerent in-channel mixing structures. The liquid ﬂow rate was changed in the range of 3.3-17.5 mL/min. The liquid-sided mass transfer over grooved or ﬁnned structured plates was enhanced by factors of 1.12 and 1.20, respectively, compared to that on a standard plate with 16 microchannels with dimensions of 1200 μm × 400 μm. The liquid ﬂow rate over the structured plates could be increased by 60%-80% without any loss of octanal conversion. A two-dimensional convection and diﬀusion model adopted from Al-Rawashdeh et al. [Chem. Eng. Sci. 2008, 63, 5149] was formulated to simulate the reactor behavior, and its predictions describe the experimental results in terms of octanal conversion with an accuracy of 4.3% when the actual temperature distribution in the reactor plate is taken into account. 1. INTRODUCTION The transformation of aldehydes to carboxylic acids is an important reaction in organic synthesis as the carboxylic acids are versatile intermediates in a variety of synthetic trans- formations. The commonly used oxidants for this trans- formation are NaClO 2 , 1 K 2 Cr 2 O 7 , 2 pyridinium chlorochro- mate, 3 quinolinium dichromate, 4,5 KMnO 4 , 6 hydrogen per- oxide, 7 and some others, including stoichiometric 4,8 and catalytic methods using Ru/CeO 2 , 9 Ag 2 O/CuO, 10 and Au/C catalysts. 11,12 Stoichiometric routes employ explosive oxidation reagents or those containing poisonous metals and they have struggled with increasingly stringent environmental laws. Therefore, environmentally benign catalytic oxidation by molecular oxygen is of great importance. Several catalysts for aldehydes oxidation by molecular oxygen have been proposed, including homogeneous catalyst systems, such as Ni(acac) 2 13 and Keggin-type heteropolyanions [PW 9 O 37 (Fe 3-x Ni x (OAc) 3 )] (9+x)- . 14 The falling ﬁlm microreactor (FFMR) utilizes a multitude of thin liquid ﬁlms that move by gravitational force, providing a typical liquid residence time of few seconds up to 1 min. The very large speciﬁc gas/liquid interface area of 20 000 m 2 /m L 3 provides an excellent mass-transfer rate between the phases. The FFMR has been widely applied for diﬀerent chemical reactions, such as photochemical chlorination, 15 sulfonation, 16 and hydrogenation. 17 Chemical reaction was limited by the mass transfer within the liquid phase. Zanﬁr et al. 18 modeled CO 2 absorption in a NaOH solution with a two-dimensional (2D) model, and compared the results of calculation with experimental data. The authors concluded that carbon dioxide (CO 2 ) is consumed within a short distance from the gas-liquid interface and neither liquid ﬁlm thickness nor the thickness of the gas ﬁlm signiﬁcantly inﬂuenced the CO 2 conversion. Commenge et al. 19 investigated the gas-phase residence time distribution (RTD) in a FFMR, and observed that the formation of recirculation loops at the gas inlet and a jet eﬀect considerably increased the mixing within the gas phase. Al- Rawashdeh et al. 20 studied the inﬂuence of liquid ﬂow distribution on conversion, showing that an uneven ﬂow distribution lowers the conversion, compared to an ideal equal distribution only, by 2%. They also investigated the eﬀect of surface wettability of the reaction plate on the actual meniscus shape at the gas/liquid interface during the absorption of CO 2 by an aqueous NaOH solution. The conversion decreased by 20% over a hydrophobic surface, compared to a hydrophilic surface, because of a reduction of the interfacial area. Zhang et al. 21 correlated the contact angle with the ﬂuid proﬁle in rectangular microchannels. They reported liquid mass transport coeﬃcients in the range from 5.8 × 10 -5 m/s to 13.4 × 10 -5 m/s and demonstrated the inﬂuence of surface tension and viscosity. Hecht and Kraut 22 applied thermographic imaging method to study the dependence of the reaction rate along the reactor length. They reported that enhanced mass-transfer rate is a combined result of entrance eﬀects and temperature nonuniformity along the reactor length. Sobieszuk et al. 23 observed enhanced mass-transfer rates in a FFMR with 0.3 mm × 0.6 mm microchannels with an aqueous solution of monoethanolamine (MEA). The CO 2 -MEA-H 2 O system is known to demonstrate the Marangoni eﬀect in the macro- scale, 24 which is thought to be responsible for higher mass- transfer rates. Stroock et al. 25 showed that mixing occurring in liquids can be further enhanced by patterned topographies on surfaces. Slanted grooves at the channel bottom induce convective motion perpendicular to the main ﬂow. Special Issue: CAMURE 8 and ISMR 7 Received: September 13, 2011 Accepted: April 24, 2012 Published: April 24, 2012 Article pubs.acs.org/IECR © 2012 American Chemical Society 8719 dx.doi.org/10.1021/ie301058h | Ind. Eng. Chem. Res. 2012, 51, 8719-8725