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

SHORT CIRCUITING - PROCESS PROBLEMS:
Complex reactions may require staging instead of using a single tank however, 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 reactant discharge location, which can generally be correct by using a common dip-tube.  In other words, having a dip tube discharge a few inches above the suction side of the impeller (above a downward pumping hydrofoil for example), is the preferred feed location as compared to dumping the neutralization reactants on a quiescent liquid surface.  

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 plastic polypropylene and polyethylene tanks, an angular offset mounting arrangement is used, 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.  More importantly, when swirl results,, the vertical component of mixing also suffers mightily, where concentration variations stratify vertically in the tank, where the discharge pH probe indicates wild variations, causing a lack of control over the feed reactant chemical.  This problem, in most cases, can be readily solved with the addition of a single anti-swirl baffle placed in a strategic location.     

08.27.09 

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