Microchannel overlapping

Continuous Contactor with Partly Overlapping Channels Solute transfer can occur between immiscible phases each flowing in separate adjacent but displaced microchannels, having only a small conduit in which the fluid interface is stable (partial overlap) [267,268]. 

A difference in the bulk conductivity of the micro- and nanochannels results from the doublelayer overlap, even if they are filled with the same buffer. At low buffer concentrations, the interface conductivity (at the nanochannel) starts to dominate over the conductivity of the buffer ions. This results in a current being carried by different ratios of cations to anions in a nanochannel compared to a microchannel. Assuming that A+ and B have equal concentrations in the microchannel, the relationship between the ion fluxes in the microchannel (J ) and nanochannel (J ) for the cations can be described as ... 

Figure 50.37 has four cross-sectional planes, xi, yi, y2, and X2, respectively. The two x-cross-sectional planes represent the current passing through the microchannels and the two y-planes represent the current through the nanochannel. xi and yi are at the cathode end and X2 and y2 are at the anode end. At both x-planes, where no double-layer overlap exists, the current is carried by two cations and two anions. At both y-planes, where the interface conductivity starts to dominate the bulk conductivity, the current is carried by three cations and one anion. This results in an ion-enrichment at the cathode side and an ion-depletion at the anode side for both cations and anions. 

The nanochannels usually connect two bigger chaiuiels (microchannels) to each other (see Fig. 2). The EDL covers most or the entire domain of the nanochannels. The EDL may also overlap in the smaller nanochannels or for the lower values of bulk ionic concentration (decrease of the bulk ionic crmcentration increases the EDL thickness). Unlike the microchannels, huge volume of the nanochannels are covered with the EDL which are not electrically neutral (the EDL thickness is usually negligible in the microchannels). The electrically charged domain of the nanochannels notably influences the i(Miic mass transfer of the co-ions and the counterions. As an example, consider that... 

The electric double layers formed at the microchannel walls do not overlap, and the Debye-Huckel linearization principle remains as valid. 

Similarly to partially overlapping channels, microchannels with mesh contactors (Figure 7.2h) are used to create the partial contact of fluids. The advantage of these contactors is that both modes of operation, cocurrent and countercurrent, can be apphed. Besides, the flow is stabilized because of the solid support between two fluids. The solid contactors are porous membrane [9, 10] and metal sheets with sieve-like structure [11]. Similarly to parallel flow, the mass transfer in both cases is only by diffusion and the flow is under laminar flow regime dominated by capillary forces. The membrane contactor has the advantage of being flexible with respect to the ratio of two fluids. In addition to flow velocities, the mass transfer is a function of membrane porosity and thickness. In another type of microextractor, two microchaimels are separated by a sieve-like wall architecture to achieve the separation of two continuous phases. However, the hydrodynamics in both types of contactors is more complex because of interfadal support and bursting of fluid... 

Figure8.il SEM image of cross-section of silicon lass bonded structure with overlapping microchannels to form contactor.

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... 

EDL field strength is of the order of 10 V/m, which is significantly higher than the typical electric field strengths ( kV/m) that are commonly employed to actuate fluid dynamic transport through microchannels. The far-stream boundary condition is applicable. This, however, loses its validity when the EDLs formed in vicinity of two narrowly-separated solid surfaces effectively protrude into the central plane that is located midway between the two surfaces and overlap with each other. 

Two classes of gas-liquid microchannel reactors were developed in the past years -continuous-phase contacting falling film, overlapping charmel, mesh, and annular flow approaches, and dispersed-phase contacting by Taylor flow reactors, micromixers for bubble and foam formation, and miniaturized packed bed microreactors, which follow classical trickle-bed operation at smaller scale. Recently integration of operations inside a microdevice has been studied and led to the development of membrane microreactors. 

Figure 2.11 Schematic representation of fluid-fluid microstructured reactors (a micromixer settler b cyclone mixer c interdigital mixer d microchannel with partial overlap e microchannel with membrane or metal contactor f microchannels...

The microchannels can be divided into various types such as microchannels with partial overlapping (Figure 2.1 Id), microchannels with mesh contactors (Figure 2.lie) (porous membrane, sieve-like structure, etc.), microchannels with inlet Y- or T-shaped contactor (Figure 2.1 If), microchannels with static mixer (Figure 2.11g), and multichannel contactors with intermediate redispersion units (Figure 2.11h). 

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