Mixer separation-layer

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... 

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. 

This separation layer mixer is constructed as an assembly of stacked stainless-steel plates having three tubes [48], These tubes are placed into each other and are inserted into a fit. The plates contain three feeding lines for reactants 1 and 2 as well... 

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

P 5] Layers of 4,4 -bipyridyl (0.3 mol 1 in dichloromethane), ethyl bromoacetate (0.3 mol in dichloromethane) and a separation layer of dichloromethane were fitted into each other by means of a concentric separation mixer (three-fluid nozzle with three tubes having diameters of 1.5, 3 and 4 mm, slotted into each other) [78]. Thereby, two circular liquid layers of a thickness of 200 pm and a center stream of 1.5 mm diameter were generated. The reaction temperature was 22 °C. The reaction solution was inserted as droplets or a continuous stream either directly or via... 

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 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... 

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. 

Concentric separation-layer interdigital micro 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).

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

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. 

Layers of calcium nitrate (40 mmol T1 in water CaN03 4 H20), potassium carbonate (40 mmol 1 1 in water) and a separation layer of water are fitted into each other by means of the concentric separation mixer [53], The reaction temperature is 22 °C. The reaction solution forms droplets in a dodecane reservoir and inserted as such a segmented flow in the tubular reactor [137, 138],... 

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. 

Figure 1.121 Distributions of one reacting solution, the separation-layer solution and another reacting solution in black, gray and white, respectively, at time steps (from left to right) t = 0.03, 0.20, 0.27 and 0.30 s (top). All inlet velocities were 100 mm s. The dimensions of the separation-layer mixer are specified in [135]. The velocity distributions are given as well (bottom). Left t = 0.07 s right t = 0.76 s. Further specifications are given in [135].

Robustness of the planar separation-layer mixer concept... 

M 54] [P 50] For the concentric separation-layer mixer detailed investigations on the robustness of the reactor concept were undertaken [53], The operation time until plugging was monitored for three well-known fast-precipitating reactions from inorganic and organic chemistry. 

Figure 1.122 Photographs documenting the extent and location of calcium carbonate precipitation in droplets generated by the concentric separation layer mixer. The photos were rotated by 90° to allow a better comparison ([53] source IMM).

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 separation layer mixer used a barrier liquid to delay the mixing until the exit of the mixer is reached by the fluid. Here the actual mixing is processed outside the reactor inside the developing bubble (for more detailed information, see Section 1.3.13, Droplet Separation-Layer Mixing). 

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 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]. 

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