Classification cutting fluids

In large-scale industrial crystallization processes, the most commonly used classification device is a hydrocyclone. The advantages of the hydrocyclone are its high capacity within a small equipment volume and an easily adjustable cutting size by the control of the feed-flow rate and the ratio of the up-flow to the down-flow. The fluid bed or elutriation leg is another method often used for classification in crystallization processes. With the elutriation leg, the feed or clear solution is fed from the bottom of the elutriation leg. The up-flow velocity is set based on the settling velocity of the cutting size... 

Elutriation and fluid classification methods are also highly relevant to solid-liquid separation problems—they use the same or similar mechanisms for analysis as many separators. Use is made here of the size-dependent nature of dynamic separation processes and most of these methods are based on the analytical cut size defined in chapter 3, Efficiency of Separation . 

In some classification applications, the required cut point is very sharp and the rheology of both separated phases is such that they remain quite fluid. In this type of application the pond used would be relatively deep, and separation would be akin to a liquid/liquid separation, using a hydraulic balance under some form of baffle. 

It can be noted that the production of toxins is not one of the properties on which the classifications are based, as production of the toxins cuts across species, even to include an occasional CNS. Production of the toxins by any of the CNS is considered rare, if indeed actual however, there is one report that coagulase-negative staphylococci from several different species do produce one or more of the identified toxins (Valle et al.,1990). This is a debatable question because one of the sensitive detection methods, ELISA, was used to assay for toxin production. The amount of toxin produced by these strains was less than 10 ng/ml of culture supernatant fluid. The fact that the ELISA methods are sensitive to less than 1 ng/ml of culture supernatant fluid, it is questionable whether a strain producing less than 10 ng/ml can be considered toxin-positive. It is an open question as to how much toxin a strain should produce to be labeled a toxin producer, as this has not been defined. Some of the strains are under study to determine if the toxin production reported can be verified and whether these strains can produce enterotoxin in foods. 

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