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Quell a Quality Control Confrontation

ChemicalProcessing.com

Readers recommend improvements for a pH control process

This Month’s Puzzler
A batch process that consists of a series of feed tanks for a fluidized rotary dryer relies on pH probes to control product quality. There are two steps: pH adjustment, then chemical addition. The pH is adjusted from 1.0 to about 6.5. If the pH is too low, the acid bound to the solid will corrode the dryer and the customer’s equipment. Also, low or high pH will cause a problem with another ingredient added to the customer’s product. Unfortunately, we can’t depend on the probes. They suffer from slow response, especially with negative drops, for example, from 1.0 to 1.1. Also, because of poor residence time in the process, some chemicals added during the pH adjustment have caused problems such as silicon gelling of the probes (in acid solution). The quality assurance department is convinced that we should be able to meet our customer’s needs if we can achieve an accuracy of 0.25 pH, i.e., both probes agreeing that closely. The pH probes last about a week with some product runs and only a few hours with others. Can you suggest improvements in this process or how it’s controlled?

FOCUS ON MIXING
Meter NaOH in by using powder feeders sized accordingly. Add bigger agitators and possibly larger motors to incorporate the powders completely and reduce reaction time.
Errol Williams, production engineer
AVEKA Inc., Woodbury, Minn.

USE ZETA POTENTIAL
I suggest using zeta potential measurements in place of pH for a few reasons. In measuring pH, what is really being measured is a mV signal, which is then converted to a pH value. It is this mV value which is important. This can be better measured via a zeta potential measurement. Zeta potential will vary as pH varies.

Once the shape of this relationship is known for a given material, then the process can be better controlled. (This will require development in a laboratory to define the relationship between zeta potential and pH.) For aqueous-based systems the rule of thumb is that a system will be kinetically stable so long as it is outside the electro-neutral region of ±30mV. If a system is sensitive to pH variation, then it is best controlled using zeta potential and adjusting the solution to a specific mV range.
Michael Sakillaris, technology fellow
Rohm & Haas, Marlborough, Mass.

CHANGE PARAMETERS
I suggest you look at increasing the solution temperature or circulating the product in the pH control vessel to increase the residence time. Also look at the possibility of using a density measuring instrument to control you pH.
Lukas Koster, engineer
Ardeer Engineering, Modderfontein, South Africa

pH EQUALS SLOW RESPONSE
First you have to accept the fact that pH and slow response go hand-in-hand
with each other. With pH control you have to think outside the “normal
process control strategy” box. Here are some ideas: 1) when the process is too tough on the pH probes, don’t use them — measure something else instead; 2) don’t put the probes into the process fluid except when you need to measure the pH; 3) different pH process solutions require different pH probes; and 4) change the way the acidic slurry is neutralized.

An instrument is not necessary for neutralization. Try dead reckoning. For example, measure the amount acidic slurry added to tank one and then calculate the amount of dry NaOH needed for that amount based on a titration curve. Don’t put the pH probe into the acidic slurry tank in the first place. Use the tank level to determine the weight of slurry added to the tank. Titrate the slurry to only 5.5 pH in the first tank. Then the other two tanks only need to go from a pH of 5.5 to 6.5. It is best to use the titration calculation on those tanks as well. Then you have the remaining two tanks to adjust the final pH.

We have found a lot of success with NaOH-addition-based titrations in waste treatment applications but lots of problems with on-line dynamic pH adjustment control. You will need separate titration curves and calculations for each product neutralized.

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.