Shear rates, mixing

Coprecipitation parameters Critical coprecipitafion parameters that should be under tight control are reaction temperature, shear rate, mixing time, and solvent... 

The overall impact of the various parameters was found to be in the following order shear rate > mixing time > temperature. The combination of slower shear, shorter mixing time, and lower reaction temperature consistently produced quality amorphous MBP. A similar observation was made in the experiment with continuous mode procedure. 

Obviously shear rate in different parts of a mixing tank are different, and therefore there are several definitions of shear rate (/) for average shear rate in the impeller region, oc V, the proportionaUty constant varies between 8 and 14 for all impeller types (2) maximum shear rate, oc tip speed (%NU), occurs near the blade tip (3) average shear rate in the entire tank is an order of magnitude less than case / and (4) minimum shear rate is about 25% of case 3. 

The shear rate between the average velocities is called macroscale shear present in eddies of 500 p.m or larger in size. The shear rate between the fluctuating velocities, present in smaller than 100 p.m eddies, is called microscale shear. A mixing tank, therefore, has several types of shear, ie, macroscale and microscale, maximum and minimum, average in the impeller zone and in the entire tank. 

Drops coalesce because of coUisions and drainage of Hquid trapped between colliding drops. Therefore, coalescence frequency can be defined as the product of coUision frequency and efficiency per coUision. The coUision frequency depends on number of drops and flow parameters such as shear rate and fluid forces. The coUision efficiency is a function of Hquid drainage rate, surface forces, and attractive forces such as van der Waal s. Because dispersed phase drop size depends on physical properties which are sometimes difficult to measure, it becomes necessary to carry out laboratory experiments to define the process mixing requirements. A suitable mixing system can then be designed based on satisfying these requirements. 

Axial-flow turbines are often used in blendiug pseudoplastic materials, and they are often used at relatively large D/T ratios, from 0.5 to 0.7, to adequately provide shear rate in the majority of the batch particularly in pseudoplastic material. These impellers develop a flow pattern which may or may not encompass an entire tank, and these areas of motion are sometimes referred to as caverns. Several papers describe the size of these caverns relative to various types of mixing phenomena. An effec tive procedure for the blending of pseudoplastic fluids is given in Oldshue (op. cit.). 

Solid Dispersion If the process involves the dispersion of sohds in a liquid, then we may either be involved with breaking up agglomerates or possibly physically breaking or shattering particles that have a low cohesive force between their components. Normally, we do not think of breaking up ionic bonds with the shear rates available in mixing machineiy. 

The shear rate available from various types of mixing and dispersion devices is known approximately and also the range of viscosities in which they can operate. This makes the selection of the mixing equipment subject to calculation of the shear stress required for the viscosity to be used. 

For non-New tonian fluids, viscosity data are very important. Every impeller has an average fluid shear rate related to speed. For example, foi a flat blade turbine impeller, the average impeller zone fluid shear rate is 11 times the operating speed. The most exact method to obtain the viscosity is by using a standard mixing tank and impeller as a viscosimeter. By measuring the pow er response on a small scale mixer, the viscosity at shear rates similar to that in the full scale unit is obtained. 

An example of liquid/liquid mixing is emulsion polymerization, where droplet size can be the most important parameter influencing product quality. Particle size is determined by impeller tip speed. If coalescence is prevented and the system stability is satisfactory, this will determine the ultimate particle size. However, if the dispersion being produced in the mixer is used as an intermediate step to carry out a liquid/liquid extraction and the emulsion must be settled out again, a dynamic dispersion is produced. Maximum shear stress by the impeller then determines the average shear rate and the overall average particle size in the mixer. 

At low shear rates, aqueous solutions of polyacrylamide are pseudoplastic. With increasing shear rates and temperature the viscosity of the solutions decrease. At high shear rates during violent mixing and pumping operations the molecular weight of polyacrylamide decreases by destruction of macromolecules. 

Elastomer-plastic blends without vulcanization were prepared either in a two roll mill or Banbury mixer. Depending on the nature of plastic and rubber the mixing temperature was changed. Usually the plastic was fed into the two roll mill or an internal mixer after preheating the mixer to a temperature above the melting temperature of the plastic phase. The plastic phase was then added and the required melt viscosity was attained by applying a mechanical shear. The rubber phase was then added and the mixture was then melt mixed for an additional 1 to 3 min when other rubber additives, such as filler, activator, and lubricants or softeners, were added. Mixing was then carried out with controlled shear rate... 

In this mixing process, contaminants such as solvent and/or diluents as well as their removal problems can be avoided. Degradation of the polymers is avoided by proper maintenance of the viscosity and shearing rates. 

The fluid shear stress actually brings about the mixing process, and is the multiplication of fluid shear rate and viscosity of the fluid [29]. 

Figure 5-27. Shear rate is a function of velocity gradient. By permission, Lightnin Technology Lightnin Technology Seminar, 3rd ed., 1982, Lightnin (formerly Mixing Equipment Co.), a unit of General Signal, p. 1, Section 2A [27].

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