Microchannel reactors pressure drop

Four elements of microchannel scale-up models will be described pressure-drop design, heat-transfer design, reactor design, and mechanical and manufacturing designs. 

Numbering up microchannels to large-scale capacity reactors is driven by a rigorous understanding of pressure drop in every parallel circuit Passive flow distribution permits sufficient flow to each channel. No serious evaluation of microvalves or actuators has been undertaken for high-capacity systems with thousands to tens... 

As described above, microchannel reactor scale-up requires integrated models, which include the reaction chemistry with heat transfer, pressure drop, flow distribution, and manufacturing tolerances. The culmination of scale-up models is their successful demonstration. 

We prepared microchannel reactor employing stainless steel sheet 400tan thick patterned microchannel by a wet chemical etching. The microchannel shape and dimension were decided by computer simulation of flow distribution and pressure drop of the reactants in the microchaimel sheet. Two different types of patterned plates with mirror image were prepared [5]. The plate has 21 straight microchannels which are 550/an wide, 230/an deep and 34mi long as revealed in Fig. 1(b). 

Some of these will be discussed here with various attributes that they possess. Aside from the conventional substrates, others that are currently under development are short contact time (SCT) reactors that consist of screens, mesh, or expanded metal that are typically fabricated from high-temperature FeCrAl alloys. Reticulated foams that combine the very low pressure drop of monoliths with the improved transport of SCT reactors are often used. There are flat plate and microchannel reactors and recently microelectromechanical system (MEMS) reactor geometries that have been fabricated. In comparison to monolith or pellet beds, SCT substrates seem to allow Prox reactors to operate at significantly lower water concentrations before the onset of the hydrogen oxidation reaction, that is, the high-temperature steady state. 

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]. 

As an alternative to the above-mentioned reactor, Pattekar et al. [39] designed a radial flow packed-bed MSR to reduce the pressure drop compared to randomly packed microchannels. This design provides an order of magnitude reduction in pressure drop while maintaining the compact design of the microreactor, and seems to be easier to construct. 

Many reported microreactors use micropacked beds for gas-liquid-solid reactions. One advantage of micropacked beds is the commercial availability of active and selective catalysts, for example, for hydrogenation reactions. Furthermore, the particle sizes of these catalysts, which are typically used in suspension reactors, are in the micrometer range and well suited for the use in microchannels. However, proper design of the reactor is required to maintain an acceptable pressure drop. 

The drawbacks of randomly packed beds in microchannels are the high pressure drop and effects related to the nonuniform packing of the small catalyst particles, namely, channeling and maldistribution of the fluids. A large RTD results, which diminishes the reactor performance and, in the case of sequential reaction networks, the product selectivity. The reactor or the catalyst may be modified such that a structured bed is obtained. 

Although MSRs have been shown to be suitable for the optimization of many synthetic procedures, they have not yet received enough attention for catalytic chemistry. The main reason for the reluctance to apply them is the difficulty of introducing a solid catalyst into the microchannels of the reactor. Micropacked-bed reactors are easy to fabricate, but they usually produce a high pressure drop during the flow of gases. To overcome this problem, microstructured packings such as foams or fibrous supports may be used instead. 

In liquid-liquid systems, the flow pattern describes the spatial distributions of the two phases in the microchannels and is strongly related with the performance of the micro-reactors, since it influences the pressure drop, and heat and mass transfer... 

Kashid and Agar (2007) investigated the effects of various operating conditions on pressure drop in a PTFE microchannel reactor with a Y-junction as mixing zone. They developed a theoretical prediction for pressure drop based on the capillary pressure and the hydrodynamic pressure drop without the presence of a continuous film and for a constant contact angle between the dispersed plug and the channel wall (Fig. 2.11a). 

Example 6.2 Comparison of pressure drop in microstructured packed bed reactor, microchannel reactor and foam reactor... 

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. 

Commenge, J.-M., Falk, L., Corriou, J.-P, and Matlosz, M. (2002) Optimal design for flow uniformity in microchannel reactors. AIChE /., 48 (2), 345-358. Wirth, K.-E. (2010) Pressure drop in fixed beds. Part L1.6, in VDI-Heat Atlas, Springer, Berlin, New York, Heidelberg. Dietrich, B., W.S., Kind, M., and Martin, H. (2009) Pressure... 

In microreactors, the friction factor is not independent of wall surface roughness. Moreover, molecular interaction with the walls increases relative to intermolecular interactions when compared to macro-scale flows. In macro-scale systems, two boundary conditions will be applied, that is, a no-slip-flow in which the fluid next to the wall exhibits the velocity of the fluid normally being zero in the most common conditions, and a slip flow in which the velocity of the fluid next to the wall is not zero, and is affected by the wall friction effects and shear stress at the wall. In the case of the slip-flow conditions, a significant reduction in the friction pressure drop and thus reducing the power consumption required to feed the fluid into the microchannel reactor. For most cases in microreactors, the = 0.1 continuum flow with slip boundary conditions is applied. In addition, the pressure drop inside the microreactor is minimal in comparison to that of macro-scale systems (Hessel et ai, 2005b). 

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). 

Multiphase packed-bed or trickle-bed microreactor [29, 30] Standard porous catalysts are incorporated in silicon-glass microfabricated reactors consisting of a microfluidic distribution manifold, a single micro-channel reactor or a microchannel array and a 25-pm microfllter. The fluid streams come into contact via a series of interleaved high aspect ratio inlet chaimels. Perpendicular to these chaimels, a 400-pm wide channel is used to deliver catalysts as a slurry to the reaction chaimel and contains two ports to allow cross-flow of the slurry. High maldistribution, pressure drop and large heat losses may occur... 

The specifically designed microchannel reactor consists of microstructured stacked plates (Fig. 1). The geometry of the plates and of the stack itself is optimised to avoid mixing in the entrance and outlet area and to distribute evenly the flow between the different channels [10]. Since the flow in the structure is always laminar, the pressure drop can be easily estimated, and the optimisation of the reactor design is fiicilitated. 

---