(241b) Three-Phase Mass Transfer in Pillared Micro Channels

Continuous processing of fine chemicals in micro fabricated reactors offers numerous advantages compared with batchwise production in larger sized reactors. For example, gas-to-liquid and liquid-to-solid mass transfer rates are considerably increased in micro channels because of the high surface area to volume ratios. Furthermore, the plug flow of reactants results in much better control of the residence time distribution which leads to higher selectivities and yields. Finally, the excellent heat management prevents hot spot formation and provides novel windows of operation. This means that reactions can be performed safer and more sustainable. Much research has been done on two-phase flow patterns in micro structured reactors [1, 2]. In these narrow channels, the channel orientation is not relevant as the gravitational force is almost negligible and the flow is mainly determined by capillary and pressure forces. This results in the disappearance of stratified flow and different flow regimes are observed [2]. A recent trend in the design of three-phase microreactors for catalyzed reactions focuses on the use of structured catalyst supports, i.e. a structured geometry with a high surface area with a metal catalyst, like Pd, deposited on that structure and/or on the micro channel walls. The catalytic surface area of these structured microreactors is sufficiently large to accommodate fast reactions. However, for slow reactions at low temperatures, still larger surface areas are required. Therefore, surface enhancing structures have to be etched on the micro structured support to further enlarge the surface area to volume ratio (Figure 1). A proper design of a micro structured multiphase reactor requires a comprehensive understanding of the three-phase flow hydrodynamics. The pressure drop, the liquid hold-up and the flow regimes and their transitions need to be known as a function of the micro channel and structure geometries. Moreover, the relation of the hydrodynamics to the gas-liquid and liquid-solid mass transfer needs to be established. In Figure 2 an overall picture is given of the relevant mass transfer steps for the structured microreactor in Figure 1. In this work, micro channels having a length of 6.6 cm, a width of 1000 µm and a depth of 50 µm etched in silicon wafers were used. To increase the catalytic support area from 4.2x10⁴ m_sup² m_reac^-3 to 9.5x10⁴m_sup² m_reac^-3, micro pillars of 3 µm in diameter and 50 µm in length were fabricated in the second axial length part of these channels (Figure 1). The flow channels and pillars were etched in silicon in one step using the Bosch process and powder blasting of the wafer was done to create inlet and outlet holes to the flow channels. Finally, anodic bonding with a Pyrex wafer was used to complete the chips. Making pillars in micro channels requires therefore no extra step in the synthesis. To introduce the gas and liquid to the micro channel, a cross shaped mixer was used. Nitrogen/water systems were investigated for superficial gas velocities of 0.3 to 15 m/s (Re_G = 7.5 - 375) and superficial liquid velocities of 0.01 to 1.67 m/s (Re_L = 0.95 - 158). Video analysis of the two-phase flow in these water/nitrogen systems was performed using a Zeiss Axiovert Microscope coupled to a high-speed camera (maximum 10,000 frames s^-1). Figure 3 shows a number of observed flow patterns at different gas and liquid velocities. The pillar structure hardly influences the two-phase flow and the observed flow patterns and regimes are similar to their counterparts in non-pillared micro channels [2-4]. Furthermore, specifically for the bubbly and Taylor flow regime (Figure 3(c,d)), the rates of overall mass transfer were estimated using the mass transfer model configuration of Figure 2. A very thin liquid film is present around the pillars. The capillary force rapidly squeezes the liquid in the film to the top and the bottom of the channel. The film is refreshed by liquid when the bubble has passed. This will result in very high gas-liquid and liquid-solid mass transfer rates due to the continuously alternating gas-liquid environment exposed to this film and the short diffusion lengths. The oxidation of sodium formate over a Pd catalyst was chosen as a model reaction. This reaction is kinetically (pseudo) half order in oxygen. The intrinsic kinetics is very fast and the overall reaction rate is only limited by mass transfer resistances. This overall reaction rate can therefore be further increased by increasing the gas-liquid and liquid-solid mass transfer interfacial areas. Palladium was deposited on the silicon pillars by means of an ion-exchange procedure with silanol groups present on the surface. In normal slurry bubble reactors, overall mass transfer coefficients of 0.2 s^-1 were obtained using 1% Pd on carbon particles and a liquid volume of 5.0?10^-4 m³. For the pillared micro reactor in this study, overall mass transfer coefficients of the order of at least 1.0 s^-1 are expected [5]. References: [1] Bretherton, F.P., J. of Fluid Mech. 10 (1960), 166-188 [2] Triplett, K.A., Ghiaasiaan, S.M., Abdel-Khalik, S.I., Sadowski, D.L., Int. J. of Multiphase Flow 25 (1999), 377-394 [3] Warnier, M.J.F., Rebrov, E.V., de Croon, M.H.J.M., Hessel, V., Schouten, J.C., Chem. Eng. Journal (2007), in Press [4] Hetsroni, G., Mosyak, A., Segal, Z., Pogrebnyak, E., Int. J. of Multiphase Flow 29 (2003), 341-360 [5] Kreuzer, M.T., Peng Du, Heiszwolf, J.J., Kapteijn, F., Moulijn, J.A., Chem. Eng. Sc. 56 (2001) 6015-6023