Mixers laminar flow

M 39] [P 37] Using an azo-type competitive reaction, the mixing efficiency could be determined via the selectivity [41]. Using a P-type micro mixer, laminar flow mixing could be investigated (see Figure 1.104). The selectivities measured are far from the ideal behavior of a tubular reactor. 

Mixer Laminar flow width (pm) PI yield (%) P2 yield (%) Selectivity P1 P2... 

When it is deleterious, laminar flow can be avoided by mixing over the cross section. For this purpose static mixers in line can be provided. For very viscous materi s and pastes, screws of the type used for pumping and extrusion are used as reactors. 

Li et al. [36] performed an extensive study on AP in a Sulzer SMX statie mixer with both Newtonian and non-Newtonian fluids. They showed that AP inereased by a faetor of 23 in a SMX statie mixer in the laminar flow regime. Figure 7-24 shows their eorrelation between the Fanning frietion faetor and the Reynolds number for experimental points under various operating eonditions. 

Statie mixers, as reviewed in Chapter 7, eontain mixing elements enelosed in a tubular housing through whieh radial mixing is aehieved. They redistribute fluid aeross the flow ehannel and eonsequently rearrange temperature and eomposition distributions. They are often used to promote mixing in laminar flow systems thus having a pro-nouneed effeet on the RTD. 

The energy to drive the fluid through a static mixer comes from the fluid pressure itself, creating a loss in pressure (usually small) as the fluid flows through the unit. For laminar flow... 

Figure 7.27. Twisted-blade type of static mixer operating in the laminar flow regime (a) Distributive mixing mechanism showing, in principle, the reduction in striation thickness produced (f>) Radial mixing contribution...

Figure 7.29 shows a Sulzer type SMX static mixer where the mixing element consists of a lattice of intermeshing and interconnecting bars contained in a pipe 80 mm diameter. It is recommended for viscous materials in laminar flow. The mixer shown is used in food processing, for example mixing fresh cheese with whipped cream. 

Figure 7.28. Static mixer in laminar flow reduction in relative standard deviation of samples indicating improvement in mixture quality with increasing number n of elements traversed...

The models of Chapter 9 contain at least one empirical parameter. This parameter is used to account for complex flow fields that are not deterministic, time-invariant, and calculable. We are specifically concerned with packed-bed reactors, turbulent-flow reactors, and static mixers (also known as motionless mixers). We begin with packed-bed reactors because they are ubiquitous within the petrochemical industry and because their mathematical treatment closely parallels that of the laminar flow reactors in Chapter 8. 

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. 

Another comparison is due to Van Wageningen et al. (2004) who performed a similar study (in terms of the numerical scheme used) on unsteady laminar flow in a Kenics static mixer. They found that the LB code was 500-600 times faster than FLUENT in terms of simulation time per grid node per time step and that FLUENT used about 5 times more memory than LB. 

The Kenics mixer, Figure 10.14(a), for example, consists of a succession of helical elements twisted alternately in opposite directions. In laminar flow for instance, the flow is split in two at each element so that after n elements the number of striations becomes 2". The effect of this geometrical progression is illustrated in Figure 10.14(b) and points out how effective the mixing becomes after only a few elements. The Reynolds number in a corresponding empty pipe is the major discriminant for the size of mixer, one manufacturer s recommendations being... 

Laminar-flow blending duties involving high viscosity ratios (greater than 1000 1) are classified as difficult. The SMX mixer appears to have the best track record in achieving satisfactory results. 

P 24] Computer simulations were carried out using the software Fluent 6 [68], A 3-D solid model of the T-channel micro mixer was built and named in Gambit The simulations were made solely for the zone of the T-junction, since for all other zones, including the downstream section of the mixing channel, laminar flow was assumed. Thus, a fine mesh of 173 000 brick elements could be used for the solid model. 

FIA is a chemical analysis method based plug-wise injection into a carrier stream [114—116]. These plug samples can be further manipulated, e.g. by reaction to compounds better detectable by a detector. The sample consumption can be reduced via miniaturization however, then usually laminar-flow conditions are given so that micro mixers are needed which are efficient in that regime. 

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. 

Another Mobius-type mixer is described in [142], Again, a two-fold 90° rotation is said to be the basic principle, doubling the number of layers for laminar flow. After the first rotation, the laminated fluids are split and thereafter recombined, restoring the original geometry. 

Behavior of the fluids in the microfabricated channels are different from those in the millimeter scale channels. Miniaturization of micro flow devices opens a new research field, microfluidics which represents the behavior of the fluid in the micro channel [8]. Since the Reynolds number in the micro channel is usually below 200, the flow is laminar and special design concepts are necessary for the fluidic elements of mixers, reaction coils etc. in the pTAS. Some components of flow switches and fluid filters were developed using laminar flow behavior. 

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