Info about Liquor

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Blending of Miscible Substances into Liquids

The degree of mixing is a very important indicator of the extent of homogeneity achieved during blending operations. The performance of an agitator is often characterized by the time and energy required to attain a given degree of mixing. In practice, a relatively low, but known, volume of a miscible tracer is added to the contents of the mixer and the time required for the tracer concentration in the mixer to attain 95% of its final estimated concentration is measured; this is known as mixing time. Allowable limits are often placed on the composition variances of blended products. Other statistical methods are also available to describe product uniformity, which can be used for controlling the operation, as well as for testing mixing equipment. Blending of solute with nonNewtonian liquids (or to result in the formation of nonNewtonian solution) could pose special problems. Bingham plastic fluids (fluids exhibiting shear only when the applied stress exceeds a threshold value known as yield stress), when agitated by small impellers, tend to form a cavern around the impeller, where the shear stress is high; there is little or no agitation in the regions remote from the impeller. A different kind of problem can be encountered with certain viscoelastic substances, such as those having a consistency similar to a thick sauce or jelly: these substances develop ‘normal forces’ as a result of elastic properties, in addition to the usual shear forces caused by viscous effects. The normal forces oppose the formation of vortices, and, instead of the liquid level falling around the rotating shaft as described earlier, it rises and climbs up the shaft. The phenomenon is known as the Weissenberg effect.