Quell a Quality Control Confrontation

Readers recommend improvements for a pH control process

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Steven Vidakovic, associate research fellow
Pfizer Inc., New London, Conn.

TRY VOTING LOGIC
If you are certain that the pH is 1.5 or less, maybe it would be best to keep the probe out of the low pH environment until partial neutralization has occurred. Often it is possible to control the pH by knowledge of the recipe, weight or operating profile. Add base with mass meters and a high performance density or conductivity measurement to verify the strength of base. This will allow tracking the amount of base, without exposing the probes to the less acidic environment.

Another quality assurance problem is precision of pH probes. With two probes it’s hard to know which is correct. A better practice is to have three probes and always take the middle value to be correct. A failed probe should alarm but probe failure, high or low, will allow the process to respond to a probe near the correct value.

A description of the mixture titration curve would have helped. If the pH
target is in a zone with a steep rate of change, dilute the concentration of base by one to three orders of magnitude to complete the neutralization. It will increase water volume but, if the material balance is known, using two different strength acids will improve the repeatability and avoid overshooting the endpoint.

If it is difficult to control the sequence of chemical addition, then automation seems warranted. It may be that your operators need training. Establishing the operating intent and methods of achieving product goals could improve quality.
Gary Holleran, senior engineer
BASF Corp., Beaumont, Texas

IMPROVE THE DESIGN
I believe the process design needs a review. Addition of solid caustic in three different tanks suggests too many variables and process will be out of control as is being experienced.
My solution assumes that chemicals are miscible liquids and will be added only after the desired pH has been achieved.

Using the flow diagram and not having much information of the liquid concentration of the slurry, I would make the following changes to have better process control:
1. Use liquid 50% caustic instead of solid caustic. If solid caustic is not completely dissolved due to uneven mixing, variable pH readings will result; this is due to incomplete and delayed dissolution of solid caustic.
2. Use only one tank with a pump-around system and sufficient residence time for proper mixing; this should give a stable reading. pH probe would be in-line and controlling flow of liquid caustic into the tank. pH probe will have to be strategically located.
3. Check the agitator for proper mixing and design.
4. If addition of caustic as liquid overloads the dryer, re-evaluate the dryer design. Also check the liquid mass balance of the slurry against the dryer design. Necessary changes might have to be incorporated.
Girish Malhotra, president
EPCOT International, Pepper Pike, Ohio

SPLIT THE FLOW
I suggest reconfiguring the tanks to operate in parallel instead of in series. Coordinate your operators so that when one tank is charging material, which has met the pH requirement, the other two feed tanks can be prepared for their batches. I see several advantages in this approach: 1) the material volume in each tank is certain, e.g., the quantity of solid NaOH charged to the tank can be calculated to reach the pH value quickly; 2) there is enough residence time for the probe to attain a reliable pH value; and 3) dividing the dryer demand between three tanks offers greater flexibility for probe maintenance.
Bin Yu, senior process engineer
Rohm and Haas (China) Holding Company Ltd., Shanghai, China


RELY ON CONDUCTIVITY PROBES
I had similar problems of slow pH-probe response with one of our chemical scrubber systems. We monitored pH to identify when the scrubber media was “spent” and required replacement with new media. The pH was erratic, requiring continual recalibration of the transmitters and cleaning and replacement of the probes.

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