Pressure drop microreactor

In addition to absolute pressure measurements, pressure sensors can be used to determine flow rates when combined with a well-defined pressure drop over a microfluidic channel. Integration of optical waveguide structures provides opportunities for monitoring of segmented gas-liquid or liquid-liquid flows in multichannel microreactors for multiphase reactions, including channels inside the device not accessible by conventional microscopy imaging (Fig. 2c) (de Mas et al. 2005). Temperature sensors are readily incorporated in the form of thin film resistors or simply by attaching thin thermocouples (Losey et al. 2001). 

The increased interfacial area in the microreactor led to an increased pressure drop. The energy dissipation factor, the power unit per reactor volume, of the microreactor process was thus higher (sv = 2-5 kW/m3) than that of the laboratory trickle-bed reactors (sv = 0.01-0.2 kW/m3) [277]. This is, however, outperformed by the still larger gain in mass transfer so that the net performance of the microreactor is better. 

Selection of the laboratory reactor requires considerable attention. There is no such thing as a universal laboratory reactor. Nor should the laboratory reactor necessarily be a reduced replica of the envisioned industrial reactor. Figure 1 illustrates this point for ammonia synthesis. The industrial reactor (5) makes effective use of the heat of reaction, considering the non-isothermal behavior of the reaction. The reactor internals allow heat to exchange between reactants and products. The radial flow of reactants and products through the various catalyst beds minimizes the pressure drop. In the laboratory, intrinsic catalyst characterization is done with an isothermally operated plug flow microreactor (6). 

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. 

The main drawback of randomly packed microreactors is the high pressure drop. In multitubular fixed-bed microreactors, all channels must be packed identically, or supplementary flow resistances must be... 

In addition to flow maldistribution, small deviations in the channel diameter (which typically originate from imperfect manufacturing) cause a broadening of the RTD. The deviations may also be the result of nonuniform coating of the channel walls with catalyst material (Section 5). If the number of parallel channels is large (i.e., N > 30), a normal distribution of the channel diameters with a standard deviation a can be assumed. The effect of the diameter variation on the pressure drop in the microreactor can be estimated on the basis of the relative standard deviation, G = ffd/ t [81 ] ... 

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. 

Another solution was proposed by McGovern et al. [128]. Their microreactor is designed such that solid catalyst is suspended in the reaction channel by an arrangement of catalyst traps. The construction allows the use of commercial catalyst, and the pressure drop across the bed can be controlled by engineering the packing density. The reactor behavior was characterized by using the hydrogenation of liquid o-nitroanisole to o-anisidine as a model reaction. 

For the flow conditions given in Example 4-4 in a l(K)0-f( length of Ij-in. schedule 40 pipe (oip = 0.0118). the pressure drop is less than 10%, However, for high volumetric flow rates through microreactors, the pressure drop may be significant. 

Example 4-6 Calculating X in a Reactor with Pressure Drop Example 4 7 Gas-Phase Reaction in Microreactor—Molar Flow Rate Example 4-8 Membrane Reaeior Example CDR4.1 Spherical Reactor Example 4.3.1 Aerosol Reactor Example 4-9 Isothermal Semibatch Reactor Profe.ssional Reference Shelf R4.1. Spherical Packed-Bed Reactor. ... 

An elegant means of gathering intrinsic data is the use of microtechnology, where heat and mass transfer as well as pressure drop can be characterized with great accuracy. Combined with proper resources (like catalyst particle size), microreactors provide direct access to the imderlying mechanisms and the kinetics of the various steps. 

Kashid, M. N., Agar, D. W. (2007). Hydrodynamics of liquid-liquid slug flow capillary microreactor flow regimes, slug size and pressure drop. Chemical Engineering Journal, 131, 1-13. 

The drawback of randomly packed microreactors is the high pressure drop. In multitubular micro fixed beds, each channel must be packed identically or supplementary flow resistances must be introduced to avoid flow maldistribution between the channels, which leads to a broad residence time distribution in the reactor system. Initial developments led to structured catalytic micro-beds based on fibrous materials [8-10]. This concept is based on a structured catalytic bed arranged with parallel filaments giving identical flow characteristics to multichannel microreactors. The channels formed by filaments have an equivalent hydraulic diameter in the range of a few microns ensuring laminar flow and short diffusion times in the radial direction [10]. 

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. 

In the previous sections, the mass transfer and the pressure drop properties of three different microstructured devices for fast catalytic reactions have been assessed. In the present chapter, we compare their mass transfer performance while considering the energy demand in order to choose an appropriate design of microreactor for an eventual catalytic reaction. 

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). 

As the channel width shrinks, transverse rates of transport increase at the expense of enhanced pressure drop. Consequently, one should make microreactors small, but not too small. An operation diagram, similar to that one shown in Figure 10.4a, can provide the necessary size for operating in the kinetically limiting regime. These simple analysis results can be verified via detailed CFD simulations [2]. This dimensional group concept can also be used for complex geometries [2], such as the post-microreactor [9,10], to estimate the distance of structural elements in order to ensure kinetic control. 

Figure 11.7 shows another important aspect of mass transfer to the wall in microreactor or monolith applications the external mass transfer improves with decreasing velocity. This implies that the mass transfer increases with a decrease in pressure drop. This behavior is related to the fact that 8 decreases when U decreases and is very different from intuitively expected behavior. The notion that enhancement... 

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