Mixing quality

Agitation and mixing may be performed with several objectives  

Enhancement of heat exchange between the fluid and the boundary of a container. 

Enhancement of mass transfer between dispersed phases. 

When the ultimate objective of these operations is the carrying out of a chemical reaction, the achieved specific rate is a suitable measure of the quality of the mixing. Similarly the achieved heat transfer or mass transfer coefficients are measures of their respective operations. These aspects of the subject are covered in other appropriate sections of this book. Here other criteria will be considered. 

The uniformity of a multiphase mixture can be measured by sampling of several regions in the agitated mixture. The time to bring composition or some property within a specified range (say within 95 or 99% of uniformity) or spread in values—which is the blend time—may be taken as a measure of mixing performance. Various kinds of tracer techniques may be employed, for example  

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Table 3. Mixing Quality Descriptions for Chemscale Numbers,...

The table data show that the stress/strain properties of compositions are improved by additional dispersion (mixing). Ultrasonic analysis is sufficiently reliable and informative as a means of mixing quality assessment. The very small change of the characteristics for filled compositions (chalk + kaolin) can be due to the fact that these fillers are readily distributed in the matrix as they are. 

A breakthrough of application of continuous mixers in rubber mixing cannot be expected until the performance of the final product desires a substantially more consistent mixing quality. 

To improve the mixing quality in the tubular reactor, Kenics type in-line static mixer reactor was employed. The in-line static mixers were designed to mix two or more fluids efficiently since an improved treinsport process such as flow division, radial eddying, flow constriction, and shear reversal eliminated the gradients in concentration, velocity and temperature. However, only 70 % conversion was achieved with one Kenics mixer unit. As shown in Table 2, five mixer units were required to achieve the maximum conversion. 

The most important modification of these materials was the discovery of the effect of adding stannous fluoride (Foster Dovey, 1974, 1976). Originally added to provide fluoride release, it was found to improve the mixing qualities of the cement and to increase strength by about 50 %. This is reflected also in improved adhesion to enamel and dentine (Section 5.7.4). 

The final quality and reproducibility of rubber compounds mixed from scratch on a two-roll mill, or finalisation of compound batches mixed in an internal mixer, depend to a large degree on the skill of the mill operative. In order to overcome to some extent the reliance on operative skill and to ensure reproducible mix quality many mills are now fitted with stock blender units. These consist of a driven twin roller system, of similar overall length to the mill rolls, fitted above the mill frame. 

The rectangular frame in Fig. 10a enables a similar mixing quality in the cell chamber as in the cell of Fig. 9 (for reduced mixing requirements less tubes are sufficient). Ring-shaped frames as shown in Fig. 10b can be particularly easy machined by turning, or simply can be cut from thick-walled tubes, including glass. 

The extrudate samples and the solidification experiment clearly show the importance of the melting process on the mixing quality of the extruder discharge. The cases shown here are for color masterbatches in natural or white resins, but the concept applies to any solid mixture or blend added to the hopper of an extruder. Color masterbatches were used here because it is much easier to visualize than other compositional variations or thermal gradients. 

Figure 8.32 Kenics mixer and mixing quality as a function of the helix angle [58] (courtesy of Han Meijer of Technische Universiteit Eindhoven)...

Atlanta, Georgia measurement of mixing quality in chlorine dioxide pulp bleaching ... 

The enthalpy of formation data appear in Table 1. Where there are numerous data for structurally related compounds, plotting the enthalpy of formation versus the number of carbons can reveal at a glance the quality of the data. For the alkyl hydroperoxides especially, the data are seen to be of mixed quality as evidenced by the lack of linearity in the plot (Figure 1). The error bars for the ethyl and n-propyl hydroperoxides are so large as to render the data meaningless. Also, the enthalpy of formation of n-propyl hydroperoxide is more negative than that of 1-hexyl hydroperoxide, another indication of its gross inaccuracy. The C2 and C3 hydroperoxide data will not be considered any further. 

In other words, the mixing time (at Fr = const) required to attain a certain mixing quality increases with the square of the drum length L. In order to reduce the mixing time, the component to be mixed would have to be added in the middle of the drum or simultaneously at several positions. 

The expression for the mixing work, necessary for a given mixing quality, can be obtained by multiplying both process characteristics (48) and (49) ... 

P 41] Semi-analytical calculations were performed for determining mixing quality [39]. Assuming uniform lamella formation and the absence of lamellae tilting, and... 

Figure 1.111 Flow rate dependence of the Dushman mixing quality of the interdigital concentric consecutive microstructured mixers of the StarLam series and other micro mixers taken for benchmarking [130].

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