Catalytic wall microchannels

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. 

The theoretical foundation for this kind of analysis was, as mentioned, originally laid by Taylor and Aris with their dispersion theory in circular tubes. Recent contributions in this area have transferred their approach to micro-reaction technology. Gobby et al. [94] studied, in 1999, a reaction in a catalytic wall micro-reactor, applying the eigenvalue method for a vertically averaged one-dimensional solution under isothermal and non-isothermal conditions. Dispersion in etched microchannels has been examined [95], and a comparison of electro-osmotic flow to pressure-driven flow in micro-channels given by Locascio et al. in 2001 [96]. 

To compare the catalytic wall reactor with a packed bed, correct criteria must be chosen [18]. For both the reactors, the outer catalyst surface per void (V oy) volume and the space-time must be identical. Under these conditions, the following relationship between the diameter of the microchannel and the particle diameter holds... 

From a design point of view, it is important to understand how to introduce two separate flows into one microchannel. In addition, the relative velocities of the flows have a significant influence on the resulting pattern of the multiphase flow. Another important aspect is how to introduce the catalysts active phase for a heterogeneous reaction where the solid catalyst is coated on the wall and/or placed as a packed bed inside a reactor. Even though the packed bed reactors are easier to fabricate than catalytic wall microreactors (CWM), CWMs are still favoured in most cases due to lower pressure drop and as they exhibit higher heat transfer rates (Kin et al, 2006). 

There are two main ways to incorporate the catalyst in a microreactor as a packed bed [57] or as a coating [58]. The advantage of the second method is that industrial catalysts direct in the desired sieve group can be used. Also, the accessible catalyst contents are much higher than those in catalytic wall reactors. Because of the high-pressure loss in the microchannel and a less efficient heat removal as in coated reactors, the packed-bed microreactors can play only a minor role [19]. 

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. 

In addition to packed and wall-coated systems, numerous researchers have investigated the fabrication of membranes, within microchannels, in which catalytic material can be incorporated. Employing a protocol developed by Kenis et al. (1999), Uozumi et al. (2006) deposited a poly(acryla-mide)-triarylphosphane palladium membrane (PA-TAP-Pd) (1.3 pm (wide), 0.37 mmol g-1 Pd) within a glass microchannel [100 pm (wide) x40pm (deep) x 1.4 cm (long)]. Once formed, the membrane was used to catalyze a series of Suzuki-Miyaura C-C bond-forming reactions, the results of which are summarized in Table 21. 

SAMs on microchannel walls have been studied for surface properties in microreactors,59 to control surface wetting,60 to create zones for specific immobilization of proteins and biomolecules,61 and to conduct catalytic reactions.62 And a pH sensing monolayer confined to a glass microchannel has been reported by our group.32... 

To avoid high-pressure drop and clogging problems in randomly packed micro-structured reactors, multichannel reactors with catalytically active walls were proposed. The main problem is how to deposit a uniform catalyst layer in the microchannels. The thickness and porosity of the catalyst layer should also be enough to guarantee an adequate surface area. It is also possible to use methods of in situ growth of an oxide layer (e.g., by anodic oxidation of a metal substrate [169]) to form a washcoat of sufficient thickness to deposit an active component (metal particles). Suzuki et al. [170] have used this method to prepare Pt supported on nanoporous alumina obtained by anodic oxidation and integrate it into a microcatalytic combustor. Zeolite-coated microchannel reactors could be also prepared and they demonstrate higher productivity per mass of catalyst than conventional packed beds [171]. Also, a MSR where the microchannels are coated by a carbon layer, could be prepared [172]. 

Another interesting example was provided by Teplyakov et al. [6] at the last EuroMembrane Congress in Taormina, in September 2006. The authors deal with processes using porous ceramics with catalytic coating in microchannel walls. This... 

In general, the geometric surface area of the microchannels in a typical microreactor is insufficient to carry out catalytic reactions at high performance. Consequently, the specific surface area must be increased, either by chemical treatment of the channel walls or by coating them with a porous layer. The porous layer may serve directly as a catalyst or as a support for the catalytically active components. Various techniques to introduce the catalyst have been developed and are summarized in the following sections [147,148]. 

The sol-gel method is widely used to obtain oxide layers on the walls of microchannels. This method is advantageous because a large variety of compositions can be produced, and porosity and surface texture can be tailored. The sol-gel method is also used for the preparation of particulate porous catalytic supports [155,201,202], The colloidal metal oxide sols can be prepared by various methods such as reactions of metal salts with water or by hydrolysis and polycondensation of metal alkoxides. The latter is the most versatile procedure and has been investigated extensively. Often the sol contains varying concentrations of solid particles, and the procedure is no longer a sol-gel but rather a hybrid method, with the coating medium being a mixture between a sol and a suspension (Table 3). 

Now we can estimate the pressure drop in all devices with the presented relations Equations 6.5 and 6.7 for the foam Equations 6.9 and 6.10 for the microchannel reactor and Equation 6.4 for the packed bed with spherical particles. For the microchannel reactor we suppose that 60% of the cross section of the reactor is occupied by the channel walls and catalytic layer (see Figure 6.7). Therefore, the channel volume available for the fluid corresponds to the void volume in the packed bed i.e. =0.4 = e. For a given superficial fluid velocity u, the velocity in the void volume is given by = u/e. From Figure 6.10 it becomes evident that the pressure drop in packed bed reactors are several times higher than in foam reactors. The difference can be explained by the high porosity in the foam (efoam = .9) compared to the packed bed = 0.4). The lowest pressure and, therefore, the lowest energy dissipation is found for the multichannel microreactor. 

Even if the flow conditions of liquids on the microscale are almost laminar and therefore numerical simulations with high accuracy are applicable, there are several reasons for the basic necessity for experimental flow visualization. In most cases, for instance, the exact data of geometries and wall conditions of microchannels and data on chemical media such as diffusion coefficients and reaction rates are unknown. Furthermore, in cases of chemical reactions, the interaction between mass transport and conversion are not calculable to date, especially if simultaneous catalytic processes take place. Therefore, the visualization of microscale flow is a helpful tool for understanding and optimizing microchannels. 

Figure 10.10a shows propane conversion contours obtained from 2D CFD calculations for catalytic propane combustion in a non-adiabatic microchannel for the conditions mentioned in the caption [23]. Unlike the homogeneous combustion case, the preheating and combustion zones in catalytic microburners overlap since catalytic reactions can occur on the hot catalyst surface close to the reactor entrance. Figure 10.10b shows a discontinuity in the Nu profile, similar to the homogeneous combustion problem. In this case, it happens at the boundary between the preheat-ing/combustion zone and the post-combustion zone. At this point, the bulk gas temperature (cup-mixing average) and wall temperatures cross over and the direction... 

So far, we have considered catalytic materials that conform to the side walls of a microreactor. A downside to a functionalized coating at a channel wall is the limited catalytic surface area that can be provided. As an alternative, thin-film technology can be used for depositing catalytic materials on more complex three-dimensional surfaces inside microchannels [49]. Impregnation methods can also be used on porous silicon surfaces [50]. The mass transfer rates described for such structures are sufficient for all but the fastest heterogeneous reactions. 

On ceramic monoliths, foams or metaOic microchannels, catalytically active species can be deposited directly on the structured wall, when the microstructure aheady... 

In the transverse direction, the symmetry boundary condition is appropriate at r = 0. Additionally, a condition at the wall is required. In the case of a microchannel reactor where a first-order reaction is occurring in the catalytic layer, this is given by... 

A microchannel reactor configuration, in which catalytic endothermic (hydrocarbon SR) and exothermic (hydrocarbon combustion) reactions can be coupled, is shown in Figure 11.8 [ 24]. The reactor is composed of parallel groups of endothermic and exothermic channels which are separated by thin solid walls. The reactive flows are considered to be co-current. Each channel is square shaped, and the inner walls of the channels are wash-coated with a porous supported metal catalyst specific for the reaction type. Washcoat thickness is assumed to be uniform... 

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