Catalytic wall microreactors pressure drop

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). 

To combine the advantages of packed-bed and catalytic wall microreactors, catalytic bed microreactors were proposed recently. In this novel reactor design, the catalyst is applied on metallic filaments or wires which are incorporated in a microreactor, leading to a low pressure drop and a nanow residence time distribution [87-89]. By insertion of metallic wires a uniform gas distribution and a reduced risk of temperature gradients is obtained. However, similarly to catalytic wall microreactors, an increase in the specific surface area of the grid or wire is required. In addition to metallic wires and grids, modified ceramic tapes can also be used as a catalyst support [90]. 

This kind of reactor is easy to fabricate, commonly operates with laminar flow, and is used for catalyst screening and for the production of chemicals. However, micropacked bed reactors (MPBRs) usually have a high pressure drop dimng the passage of gases. Therefore, catalytic wall microreactors are more suitable. 

Besides maldistribution, small deviations in the channel diameter introduced during the manufacturing process cause an enlargement of the RTD. The deviations may also be because of a nonuniform coating of the channel walls with catalytic layers. If the number of parallel channels is large (N > 30), a normal distribution of the channel diameters with a standard deviation a can be assumed. The relative standard deviation, = cr /d influences the pressure drop over the microreactor [27] ... 

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. 

The highest pressure drops usually occur in packed-bed microreactors with axial flow design. A reduced pressure drop while maintaining catalytic area can be achieved by a cross-flow design packed-bed microreactor [82]. To keep pressure drops down, the use of catalytic wall and catalytic bed microreactors is recommended. 

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