In-line Mechanical Mixers

For operation in a laminar flow regime, we get the same time effects when we keep the residence time the same  

If we keep the shear rate the same in both scales, we also keep the shear stresses the same  

This leads immediately to keeping the length/diameter ratio the same, and 

For the turbulent flow regime, the residence time is again held the same, and now the energy dissipation is held constant for any reactive or two-phase effects. This leads to keeping 

In some cases there may be a flow regime change so that the Reynolds number, which always increases on scale-up, must always be compared on both scales. Scale-up is most reliable when both large and small scale systems are operating in the same flow regime. As mentioned in Section 7-10, care should be taken when operating in the transition regime. 

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When the flow is laminar, either single or multiphase, there is only one design class option static or motionless mixers. Other pipeline mixing devices described for turbulent flow are not usable for even the simplest mixing applications in the laminar regime. All rely on turbulence and cannot function at low Reynolds numbers. The only alternative technology is in-line dynamic mixers, which include extruders, rotor-stator mixers, and a variety of rotating screw devices. None of these has the benefits of simplicity and the little or no maintenance characteristic of static mixers. In-line mechanical mixers are discussed briefly later in the chapter. 

In areas such as miscible liquid blending, the formation of emulsions, solid dispersions such as paints, and dry powder mixing, it is understandable, therefore, that several criteria have been developed to assess mixture quality . It is unfortunate that of the many definitions presently available for mixture quality, in solid or liquid mixtures, none is universally applicable. In the case of powder mixing, further details may be found in Chapter 2, while for liquid mixing more information is presented in Chapters 8 and 10 for mechanically agitated vessels. Chapter 9 for Jet mixers and Chapter 12 for in-line static mixers. 

Figure 7.9 Laminar blending in a Chemineer-Kenics in-line static mixer simplified mixing mechanism...

Finally, mixing can be induced by physically splicing the fluid into smaller units and re-distributing them. In-line mixers rely primarily on this mechanism, which is shown in Figure 7.4. 

For CSTRs, was assumed to be 1, while for in-line mixers mix was assumed to be between 0.5 and 0.9. A surface reaction rate of 1.5 /Js was assumed to allow for a possible change in reaction mechanism at high acid concentrations not ruled out by the experimental data. The particle size distribution was assumed to be given by Eq. (57) ... 

Sfatic mixers offer certain advantages over dynamic in-line mixers and continuous stirred tanks. Table 9.18 is a summary of the characteristics of a static mixer compared with a conventional mechanically stirred vessel. 

Figure 1 Block diagram of the key components of the continuous reactor for hydrogenation of organic compounds at Nottingham [31]. SCCO2, H2 and the organic substrate were mixed in a heated mixer. The mixture was then passed through a reactor containing a fixed bed catalyst (usually a supported noble metal). There was optional on-line FTIR monitoring before the product and CO2 were separated by expansion. More recent reactors have used static rather than mechanical premixers.

Figure 4.16 Curves of the sedimentation of BaS04 particles. Rotation rate of mechanical mixer in volume device 300 (1) 1500 (2) 2500 (3) rpm and tubular turbulent device (4). Points - experiment lines - approximation function...

Crystallinity Impurity Solvate Incomplete dissolution pH Temp Amount Miscibility w/solvent Mixer type(high-shear in-line versus suspended mixer homogenizer) Mixer design Rotor/stator design and gap Tip speed S/AS flow rates and ratio Batch size Processing time Temperature Too slow Incomplete removal of solvent Temp. Time Air flow Humidity Shear/temp Milling mechanism... 

S.3.2 Laminar Flow. In laminar flow there is no radial mixing without a motionless mixer, mechanical in-line mixer, or a stirred tank. The choice is... 

There are few published data for power draw and pumping capacity in either batch or in-line rotor-stator mixers. Even less is known about the velocity fields in these devices, so there is little hard evidence to support proposed mechanisms for dispersion and emulsification. As a result it is often necessary to rely on equipment vendors for scale-up rules. Although many vendors have facilities for customer trials, few have well-equipped laboratories for acquisition of basic data for performance characterization. In reality, it is difficult to know how many vendor data are available, since many consider the information to be proprietary. Until recently, there has been little academic interest in high-shear mixers. This work is only starting to appear in the open literature, and it is important for the practitioner to stay informed as a body of knowledge evolves. 

Waste rubber crumb from a number of diene rubbers such as SBR, butadiene rubber (BR) and polyisoprene has been compounded with cheap mineral fillers and additives, such as monoethanolamine, hy Ushmarin and co-workers [32] at Chuvash State University and used to manufacture general rubber goods such as car mats and sleeping policemen for railway lines. They found that the use of a two stage mixing process in an internal mixer produced materials with the best physical and mechanical properties. 

Preparation is accompHshed by simple blending of the diluent into the hot base asphalt. This is generally accompHshed in tanks equipped with coils for air agitation or with a mechanical stirrer or a vortex mixer. Line blending in a batch circulation system or in a continuous fashion (40) is used where the volume produced justifies the extra faciUties. A continuous, line-blending system is appHcable to the manufacture of cutback asphalts and asphalt cements (Fig. 8). 

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