Wall Microchannels

To avoid internal mass transfer resistances in the porous catalytic layer, its thickness, must be limited. To ensure an effectiveness factor of 0.95 in an isothermal catalyst layer, the following criterion must be fulfilled [14]  

In the case of strongly exothermic and endothermic reactions, the reactions may give rise to a temperature profile within the catalytic layer, which is dependent on reaction enthalpy AH ), activation energy ( ), and the thermal conductivity of the porous catalytic material X ). For quasi-isothermal behavior, the observed rate, j, should not differ from the rate that would be observed at constant temperature by more than 5%, and thus the resulting criterion for quasi-isothermal catalytic wall behavior is given by 

In general, the thickness of the catalytic layer is kept sufficiently small and internal temperature and concentration profiles can be neglected. Therefore, only the transfer from the bulk of the fluid to the catalyst wall must be considered. 

The importance of the external mass transfer on the transformation rate is characterized by the ratio between the mass transfer time and the characteristic reaction time f (Eq. (11.1) and Eq. (11.3)). The influence of external mass transfer on the observed transformation rate can be neglected, if is at least 10 times smaller than f (Doll 0.1). 

Owing to the small channel diameters in MSR, laminar flow can be considered in which the fluid flows in parallel layers without lateral mixing. This situation occurs when the ratio between inertial forces to viscous forces is relatively low. The ratio is characterized by the Reynolds number, which is deflned as follows for circular tubes Re = u-d -pg)ft, where u is the mean fluid velocity. Laminar flow is stable for Re 2000. At higher Reynolds number, inertia forces become dominant, producing flow instabilities such as eddies and vortices, and the flow becomes turbulent. 

---

Combined with the use of precious metal catalyzed washcoats deposited on the walls, microchannel reactors can realize nearly 10 times reduction in reactor size compared with that of a process that utilizes catalyst particles. The washcoat thickness is usually less than lOOjit and provides greater structural stability. This stability arises from smaller thermal expansion ratios and lower temperature gradients. 

Natrajan VK, Christensen KT (2010) Non-intrusive measurements of convective heat transfer in smooth-and rough-wall microchannels laminar flow. Exp Fluids 49 1021-103710... 

Baviere R, Gamrat G, Favre-Marinet M, Le Person S (2006) Modeling of laminar flows in rough-wall microchannel. JFluids Eng Trans ASME 128(4) 734—741... 

Tjerkstra RW et al (2000) Multi-walled microchannels free standing porous sihcon membranes for use in microTAS. J Microelectromech Syst 9(4) 495-501 Vadjikar RM et al (1994) Morphology of self-supporting porous sihcon layers. J Mater Sci Lett 13 222-224... 

J., and Gandia, L.M. (2011) Computational fluid dynamics simulation of ethanol steam reforming in catalytic wall microchannels. Chem. Eng. /., 167 (2-3), 603-609. 

Karakaya et al. [186] compared the productivity and selectivity of SR catalysts in a catalytic wall microchannel and PB configuration at identical weight hourly space velocity (WHSVs). In general, the microchannel configuration outperforms the PB one in both productivity and selectivity. Enhanced heat transfer in the microchannel configuration, resulting in uniform temperature distribution within the catalyst layer, explains the obtained results. Moreover, the high temperamre used, 750°C, increases the rate of the reverse water gas shift (R-WGS) and hence the CO selectivity. 

Fig. 2.33a-c Boiling in the central part of microchannels. Tls = 0.14 m/s, q = 220 kW/m. 1 Clusters of liquid droplets at the bottom of the channel. 2 Clusters of the liquid droplets on the side-wall. 5 Steam. Reprinted from Hetsroni et al. (2003b) with permission... 

Davies J, Maynes D, Webb BW, Woolford B (2006) Laminar flow in a microchannel with super hydrophobic walls exhibiting transverse ribs. Phys Fluids 18 087110... 

Mala GM, Li D, Werner C (1997b) Flow characteristics of water through a micro-channel between two parallel plates with electro kinetic effects. Int J Heat Fluid Flow 18 491 96 Male van P, Croon de MHJM, Tiggelaar RM, Derg van den A, Schouten JC (2004) Heat and mass transfer in a square micro-channel with asymmetric heating. Int J Heat Mass Transfer 47 87-99 Maranzana G, Perry I, Maillet D (2004) Mini- and micro-channels influence of axial conduction in the walls. Int J Heat Mass Transfer 47 3993 004 Maynes D, Webb BW (2003) Full developed electro-osmotic heat transfer in microchannels. Int J Heat Mass Transfer 46 1359-1369... 

The advantages of microreactors, for example, well-defined control of the gas-liquid distributions, also hold for photocatalytic conversions. Furthermore, the distance between the light source and the catalyst is small, with the catalyst immobilized on the walls of the microchannels. It was demonstrated for the photodegradation of 4-chlorophenol in a microreactor that the reaction was truly kinetically controlled, and performed with high efficiency [32]. The latter was explained by the illuminated area, which exceeds conventional reactor types by a factor of 4-400, depending on the reactor type. Even further reduction of the distance between the light source and the catalytically active site might be possible by the use of electroluminescent materials [19]. The benefits of this concept have still to be proven. 

Xu, Y, Platzer B., Concepts for the simulation of wall-catalyzed reactions in microchannel reactors with mesopores in the wall region, Chem. Eng. Technol. 24, 8 (2001) 773-783. 

Method //was designed for a bonded microreactor. After the channel was sealed with a Pyrex top plate using the anodic bonding technique, the liquid precursor was infiltrated into the microreactor through the outlet of the reactor under slight pressure and withdrawn. A thin film of solution remained on the walls of the microchannel. 

To process the methanol and water mix, a CuZnAl catalyst was wash-coated onto the microchannel walls. The alumina was deposited by dipping the plates into a 20% alumina suspension, which also included a stabilizer and a binder. After any excess was wiped off, the plates were calcined at 600 °C for 1 h in air. Air was removed from the pores by placing the calcined plate in a vacuum. The alumina wash-... 

In order to detect the analyte specifically, a complex has to be formed first. To this end, the revelation moiety (e.g. an enzyme-labelled antigen or antibody) is for instance incubated in the chip so as to bind to the analyte that has previously been captured within the microchannel. In another scheme, the analyte solution is first mixed with the revelation moiety, and the formed complex is then incubated in the chip in order to be captured on the bed of antibodies coating the walls of the micro-channel. After a washing step (to remove the excess affinity partner), the microchannel is filled with the substrate which shall thus react... 

Figure 3. Schematic drawing of a microchannel plate detector for neutron radiography. Channel diameters range from 6 to 12 p.m, and typical wall thickness are about 2 p.m. The high voltage depends on the particular MCP used and can range from 2 to 6 kV.

---