Determining the Mixing Quality

For small values of A, the return flow fraction flowing across the flight tip increases, thus increasing the potential for dispersion. This is in agreement with experimental results, e. g., Fig. 9.11. The return flow fraction also increases with a larger gap width s. However, the absolute shear and extension rates are then lower, which can have an adverse effect on dispersion. 

A twin screw extruder with a diameter of 32 mm has a clearance of s=0.15 mm between the screw and barrel. The screw is double-flighted. At a throughput of 50 kg/h (density p = 1000 kg/m3), a speed of 300rpm, and a melt-containing screw length of 1S/D=10, the return flow fraction results in  

This means that, in the screw zone under consideration, every fluid element passes over the flight tip an average of 3.5 times. The shear rate in the clearance at the flight tip is 3350s 1. 

The above equations represent a very simplified approach for simple conveying elements. For special mixing and dispersing elements such as kneading discs, eccentric discs or grooved elements, the approach must be expanded. A simple estimate is, however, possible. 

The mixing quality describes the distribution of a desired attribute in the volume under consideration. This attribute can be the concentration of an additive or filler, a color value or, in the case of a blend, the fraction of a certain polymer. Chemical reactions can also be 

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Figure 9.18 Example for determining the mixing quality in a random sample and the relationship between the mixing quality and the size of the individual samples...

One method of determining the mixing quality is to use optical evaluation. A suitable set-up is shown in Fig. 9.19. Here, the main stream being fed to the extruder is a clear oil. A side stream containing a tracer is introduced at some arbitrary position along the screw. This approach allows the length available for mixing to be varied. The extruder can be equipped... 

Figure 9.19 Experimental set-up for determining the mixing quality downstream of an extruder optical evaluation of gray-scale values to determine concentration...

Figure 9.21 Determining the mixing quality from a model system consisting of silicone oil and iron oxide static image of a cuvette and concentration curve after turning the tracer stream on and off...

Figure 1.199 Determination of the mixing quality of the different impinging jet micro mixers using a competing reactions approach based on measuring UV-Vis absorption ([53] source IMM).

The mixed quality of proteins in the diet The values for protein, noted earlier, refer to high-quality proteins such as those foimd in eggs, fish, and meat. In the real world, the diet contains a mixture of proteins from plant and animal sources. The mixed quality of the proteins, as determined by their component amino acids as well as their variable digestibility, suggests a correction factor in the calculation of the RDA for protein. With this correction, a protein requirement of about 0.64 g protein/kg body weight is calculated. 

With the help of the ID simulation performed in MOREX, some process parameters cannot be determined. In particular, this refers to the mixing quality which is achieved when glass fibers are added. Therefore, it is decided to examine the flow conditions by means of a 3D simulation in BEMFlow. 

Y. Men, B. Werner, Determination of the segregation index to sense the mixing quality of scale-up for pilot- and production-scale microstructured mixers, in Proceedings of the 12th European Conference on Mixing, Bologna, 27-30June 2006, 2006. 

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. 

The official permission to use a synthetic colorant in food is determined by its quality and safety. Detailed and accurate analysis became compulsory in order to verify purity and quantify the labeled concentrations of colorants in food. For the analysis of synthetic colorants added to food products, (1) simple and rapid methods are used to determine their presence, (2) accurate and precise methods evaluate then-concentrations, or (3) certain methods evaluate their degradations to unstable and unsafe forms. This chapter is dedicated to these three methods used to identify and quantify synthetic colorants as pure or mixed pigments in foodstuffs. 

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