Separation-layer micro mixer

Figure 1.5 Photograph of the liquid-flow splitting unit for liquid/liquid processing with three tanks and six separation-layer micro mixers [8].

Reactor 22 [R 22] [Separation-layer Micro Mixer Tube] - Reaction System... 

Reactor type Concentric separation-layer micro mixer-tube reactor Tube outer diameters 2.0 mm 3.0 mm 4.0 mm... 

Separation-layer micro mixers are specially tools for mixing solutions which react fast or tend to foul otherwise [39, 53, 135-138], The most prominent example of such processes is probably the generation of particles by immediate precipitation, as e.g. for calcium carbonate formation. Separation-layer mixers thus overcome the limits of normal micro mixers, which tend to clog under such conditions. 

The concentric separation-layer micro mixer is constructed as an assembly of stacked plates for feed supply with three tubes, performing lamination for mixing, set into one another (see Figure 1.116) [39, 53, 136-138] (see also [135]). The tubes are inserted into a frit. The three feed lines are each connected to a tube. In this way, a tri-layered concentric fluidic system is achieved. Besides mixing three solutions, a major application for the device is to separate the two fluids to be mixed by a separation layer, usually being the solvent of the two solutions. This is to delay the mixing process in order to avoid unwanted fouling problems at the mixer outlet This is particularly valuable for spontaneous precipitation reactions which are the main field of application of the mixer. 

Figure 1.116 Schematic of the construction of the concentric separation-layer micro mixer and magnified concentric fluid layer arrangement when leaving the mixer [53] (source IMM).

P 49] CFD simulations were made for monitoring the flow patterns within a droplet which is generated at a concentric separation-layer micro mixer [39], Diffusion-convection equations of two user scalars have to be solved in addition to the corresponding equation for the volume fraction of the fluids within a multiphase CFD simulation. 

The separation-layer micro mixer was mounted about 2 cm above a funnel-shaped glass element which was connected to a glass tubular reactor, not being cooled [53], The end of the tube was set about 2 cm above a glass beaker collecting the solutions. All experiments were made using the micro mixer and the mixer-tubular reactor set-up only. 

M 54] [P 48] Separation-layer micro mixers with concentric multi-layered outlets can be operated in a droplet-forming mode [53] If fast precipitating solutions are contacted in this way with a solvent layer for initial separation, the part of the droplet close to the tube outlets remains transparent, which demonstrates that a tri-layered system still exists with the two reacting solutions not being intermixed, as evidenced by calcium carbonate formation in aqueous solutions as described in [39,136], At the droplet end cap the layers collide and circulation flow sets in. As a result, mixing is achieved and precipitation occurs. The circulation patterns are visualized by the particle trajectories. 

M 54] [P 48] CFD simulations for the flow in the separation-layer micro mixer predict a stable, almost irrotational flow pattern in the inlet region, which is in line with the experimental findings of a transparent region mentioned above [39], This pattern is maintained until the droplet end cap. Changes only occur when the droplet breaks up and falls, inducing rotational flow. 

Figure 4.91 Demonstration of flow guiding in a liquid flow-splitting unit which consists of three tanks, six interdigital separation-layer micro mixers, a multiple channel segmenter and six tubular sections [140].

The functioning of the liquid-flow guiding was demonstrated with the formation of iron rhodanide as a test reaction in a device built of three liquid-flow splitting units and six interdigital separation layer micro mixers (see Figure 4.91) [141]. 

Separation layer mixers use either a miscible or non-miscible layer between the reacting solutions, in the first case most often identical with the solvent used [48]. By this measure, mixing is postponed to a further stage of process equipment. Accordingly, reactants are only fed to the reaction device, but in a defined, e.g. multi-lamination-pattem like, fluid-compartment architecture. A separation layer technique inevitably demands micro mixers, as it is only feasible in a laminar flow regime, otherwise turbulent convective flow will result in plugging close to the entrance of the mixer chamber. 

Both concentric and stacked fluid layer arrangements, corresponding to difler-ent versions of separation mixers, were developed, allowing either a drop- or streamlike injection of liquids in a reaction tube attached to the micro mixer [48],... 

This electrokinetically driven micro mixer uses localized capacitance effects to induce zeta potential variations along the surface of silica-based micro channels [92], The zeta potential variations are given near the electrical double layer region of the electroosmotic flow utilized for species transport. Shielded ( buried ) electrodes are placed underneath the channel structures for the fluid flow in separate channels, i.e. they are not exposed to the liquid. The potential variations induce flow velocity changes in the fluid and thus promote mixing [92],... 

The separation-layer technique benefits from the unique feature of micro mixers, such as to operate in a laminar flow regime [135], By the absence of convective recirculation patterns, at least close to the inlet, the separation layer remains as a barrier between the solution to be mixed, as long as it is not passed by molecules owing to diffusive transport. 

Mixer 54 [M 54] Concentric Separation-layer Interdigital Micro Mixer... 

Concentric separation-layer interdigital micro mixer... 

Mixer type Planar separation-layer interdigital micro mixer Feed channel width, depth 60 pm, 500 pm... 

Channel structures are etched on two plates which are later positioned face-to-face to give the overall fluid structure [140], In the region where the channels overlap, they are separated by the separation plate defined by an etch stop layer. The channel covered by this structured plate was generated by underetching in the <100> direction through slits in the plate. The micro mixer is assembled from a silicon and a glass wafer connected by anodic bonding. 

One solution to the problem mentioned above is not to rely solely on flow symmetry, but to achieve equidistribution by pressure-loss adjustment by means of flow resistors (see Figure 4.98) [140, 141, 148). In the flow sequence consisting of a main stream tube, fluid inlet, damping tank, distribution tubes and micro device such as a micro mixer, the main pressure drop is nearly always on the last side. The separation layer mixer acts here as a pressure restrictor similar to the sparger mentioned above. This requires accurate control over structural precision of micro fabrication. 

The combustion catalyst was impregnated onto the a-alumina layer on the Fecralloy surface (see Section 10.2.1). The catalysts were reduced prior to testing, which does not seem very viable for a later practical application. The anode off-gas was mixed upstream of the reactor by laboratory equipment and then further mixed with air in a separate micro-mixer. The steam reforming side of the reactor was operated at an... 

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