Static mixers laminar flow

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. 

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. 

Static mixers are typically less effective in turbulent flow than an open tube when the comparison is made on the basis of constant pressure drop or capital cost. Whether laminar or turbulent, design correlations are generally lacking or else are vendor-proprietary and are rarely been subject to peer review. 

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. 

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

Static mixers, as reviewed in Chapter 7, contain mixing elements enclosed in a tubular housing through which radial mixing is achieved. They redistribute fluid across the flow channel and consequently rearrange temperature and composition distributions. They are often used to promote mixing in laminar flow systems thus having a pronounced effect on the RTD. 

Mixing in static mixers considered as chemical reactors was essentially studied by Nauman (165, 166). This author proposed a model which consists of a tubular reactor comprising N zones in laminar flow (parabolic velocity profile). Mixing between each zone is achieved accross a plane by a permutation of the radial position of fluid particles (r — , in this way the flowrate... 

Commercially available static mixer reactors (SMR) as large as 1.8 m in diameter are operating in the production of nylon, silicones, polystyrene, polypropylene, and other polymers. Removing heat from a highly exothermic reaction in a static mixer equipped with a simple cooling jacket is limited by heat transfer to small diameters, typically less than 6 in. A new type of static mixer has been developed to overcome laminar heat-flow transfer problems. It is called the SMR mixer-heater exchanger-reactor (Mutsakis et al, 1986). 

Figure 2 shows an example of a static mixer and a schematic representation of how such structures operate—see, for example, [1] and [2]. The open intersecting channels divide the main fluid stream into a number of substreams. In addition to the lateral displacement caused by the obliquity of the channels, a fraction of each substream shears off into the adjacent channel at every intersection. This continuous division and recombination of the substreams causes transition from laminar to turbulent flow at Reynolds numbers (based on channel hydraulic diameter) as low as 20Q-300 and results in... 

Emulsions are usually prepared by the application of mechanical energy produced by a wide range of agitation techniques. These disrupt droplets by the application of either shear forces in laminar flow or inertial forces in turbulent flow. Emulsifying devices ranging from simple hand mixers and stirrers to the use of propeller or turbine mixers, static mixers, colloid mills, homogenizers, and ultrasonic devices have been used. 

Solvent Mixing. During gradient operation, the HPLC solvents that comprise the mobile phase must be mixed adequately, most commonly with either a static or dynamic mixer. Static mixers rely on laminar-flow dynamics, whereas dynamic mixers use magnetic stirrers. Solvent viscosity affects mixing characteristics inadequate mixing is detected by many UV detectors and may be expressed as an unstable baseline. 

The constant has the numeric value of 64 for empty pipe, as is well known. The following numerical values apply for the static mixers whose characteristics are given in Fig. 8.5 Kenics 450 SMX (LP) 640 SMX (standard) 4000 and SMV 5600 Ross-ISG mixer 1.2 x IO. The pressure drop of these mixers is, in the laminar flow range, thus about a factor of 7 10 60 90 or 200 higher than in an empty pipe [416]. 

---