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Neutralization and pH Adjustment: Short Circuiting, pH Variations, and Process Control Problems:

SHORT CIRCUITING - PROCESS PROBLEMS:
Complex reactions may require the staging of multiple tanks instead of using a single tank however if you apply the 80/20 rule, 95% of all neutralization of pH adjustment applications are straightforward requiring only a single tank design.  The most common process related problems involve short circuiting and/or wild dosing variations and loss of process control.  The obvious solution may be related to discharge location in relation to where the reactants are added.  This can generally be correct by using a common dip-tube which can discharge a few inches above the suction side of the impeller (above a downward pumping hydrofoil for example) which is the preferred feed location as compared to dumping the neutralization reactants on a quiescent liquid surface of the tank.  

A more common problem, that may not be intuitive, as it is often overlooked, is related to mixing flow patterns that are dominated by the angular component of mixing.  In many cases, due to the initial cost of using a plastic polypropylene and polyethylene tank,  anti-swirl baffles cannot be easily applied to these tanks where for one reason or another the angular component of mixing dominates.  In this case, reactants A & B, like two horses on a merry-go-round never meet.  In many cases the angular offset mounting arrangement can be used however it cannot be used effectively for all tank bottom configurations (tall cones). When swirl results, the vertical component of mixing suffers and more often than not is non existent.  Concentration variations can then stratify vertically in the tank, where the discharge pH probe indicates wild variations, causing a lack of control over the reaction.  If this is a problem, in most cases, it can be readily solved with the addition of a single anti-swirl baffle placed in a strategic location.  

It is also possible to establish staging within a single tank.  By using dual (2) impellers, the upper impeller being upward pumping and the lower being a downward pumping design, three zonal regions are established within the tank.  The top up-pumping region, the center re-circulation zone and the lower downward pumping region.  These zonal regions act as physical barriers like internal staged zones which reduce the effects of short circuiting.    

10.03.11 

Industrial Portable Mixers 
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