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
