For example, take a look at the lag time in the response of acid/base pumps or valves, and the real-time titrant delivery rates... If pH adjustment in an agitated vessel is the goal, ensure that agitation and turnover rates are sufficient in comparison to the rate of titrant addition. It may be necessary to add the acid/base to a separate closed circulation loop to ensure better distribution in very large tanks. It may simply be that the adjustment has to be undertaken slowly to allow sufficient mix time. pH control of slurries is particularly difficult.
The relative location of the pH probe and titrant addition lines will also have a major effect on the operation of the control loops. Poor placement can cause long response delays or, worse, huge pH fluctuations. Try to determine the actual time required for a complete feedback cycle to be completed and compare this to the flowrates of all the streams involved. And finally, pay attention to the titration curve of the system you are trying to control. pH titration curves are not linear, and if you are trying to control pH on the steepest part of the curve, where small adjustments can mean major pH swings, it will only magnify your control problems.I have had some small experience observing the adjustment of pH in large stirred vessels (50 gallon) and above. Patience, I think, is a real virtue in these situations -- back-titrating after you've rocketed past the pH you were trying to hit is really annoying. Slurries, as Mr. McConville notes, are even more problematic and slow to respond.
I think it points to the general undesirability of pH adjustments on scale, especially ones that rely on judgment and metering in small amounts of titrant. It's probably just better to have a process where you add a known amount of titrant and let the agitator do its work.
I'm sure I'm being wrong-headed here somehow -- thoughts?