Quell a Quality Control Confrontation

Readers recommend improvements for a pH control process

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With the titration curves you may be able to get away with only using two tanks. This way you can get longer residence time for the NaOH solids mixing by alternating using tanks one and two for the acidic neutralization. Then tank three is used for the final adjustment from tanks one and two.

Continual exposure to a harsh environment is often unnecessary and expensive. Consider using a retractable housing for the pH probes. This way the probes are only in contact with the process fluid during the time when you need to measure the pH. When retracted then the probe tips should be flushed with water to get the corrosive material off. While the solids are mixing and the pH is changing, don’t measure the pH. Wait until the acid is neutralized and then insert the probe to check the final result.

Don’t use the same probe type for the acidic solution tank as for the neutral. Work with your supplier to determine what is best for each tank application.

Go back to the drawing board! Change the process. Consider only filling the first tank one-third; then add all the NaOH normally needed for a full tank of pH 5.5. This will over-neutralize the slurry to an alkaline mixture. But then top off the rest of the tank with the acidic slurry until the desired pH is achieved. This provides two benefits. First, you get a longer mixing time for the solid NaOH and, second, it can be easier to control the pH with the slurry addition rather than through the solids addition. This approach should be tested in the lab first and make sure the materials of construction are appropriate.

Also, consider using NaOH solution for the second and third tanks to improve the adjustment response time.
James Loar, applications engineer
Ciba, Basel, Switzerland

LOOK AT OTHER VENDORS
There are several pH probes that are designed for working in a harsh environment. The only caveat is that they are only accurate at one pH calibration point. If all you need to do is control at pH 6.5 and not worry too much about the absolute accuracy at lower and higher readings, it should work.
John Pocreva, engineer
Numeric-data-solutions, Lagrangeville, N.Y.

INSTALL A DRAFT TUBE
As neutralization reactions depend mainly on agitation, modify the reactor (see Figure 1: http://www.chemicalprocessing.com/Media/MediaManager/Figure_1_Draft_Tube_Model.pdf) and procedure as follows:
1) prepare an NaOH solution (30%) and add it instead of solid, as liquids are easier to mix;
2) keep the longitudinal baffles as per original design;
3) install a feed well to direct NaOH solution flow to the slurry;
4) construct a draft tube to direct flow to avoid short-circuiting and maximize mixing efficiency — the draft tube eliminates the effect of the longitudinal baffles as flow streams travel along the tank walls instead of towards them;
5) relocate the NaOH solution feeding point to be at the middle of the new draft tube;
6) install a riser to avoid short-circuiting and to keep mixture in the reactor longer;
7) make a new opening and install an overflow line at the top side of the tanks — this would maximize residence time, mixing time and minimize short-circuiting;
8) flip (or change the type of) agitator blades — this is necessary because of the draft tube; and
9) relocate pH meter away from the dead zone (of poor mixing), e.g., install it at the overflow.
Zeid Matouk, senior process engineer
Jordan Abyad Fertilisers and Chemicals Co, Amman, Jordan

MODEL IT
Try modeling the chemical reaction; measure (weigh) accurately quantities of material going in and start with an accurate pH measurement of the slurry at the beginning of the process. This should be followed by occasional measured confirmation of the pH at intervals to compare with predicted pH, as the calculated amounts of -OH is added to the slurry. This way probes will last longer, especially if they are retrieved and rinsed during the non-measuring periods.

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