Scrubber pH control

R

Thread Starter

Ron Davis

I am working with controlling an exhaust scrubber which is handling HCl fumes from an etching process. I would like to know how others are controlling their scrubbers to achieve neutral pH in their exhaust stream. More specifically, where is the best place to locate the pH probe? Is more than one sensor needed?

The pH probe is currently located in the scrubber itself. The measured pH levels drop rapidly (from ~8.0 to ~3.0 in the space of 4 or 5 seconds).
These drops appear even as we are pumping caustic solution into the wash stream. Eventually the system responds and the pH rises up to ~12.0 and
stabilizes for a period of time before dropping back down again. I have been told that the exhaust fumes come in a steady stream with no surges.

Thanks in advance,
Ron Davis
[email protected]
 
Are you using a Beckman style probe? We discovered that the CL ions have a tendency to migrate into the probes on our scrubbers and show an indicated pH that is markedly different than actual. You can change the dosing of NaOh to an intermittent "slug" and increase the dilution water added with it to alleviate this type of problem. You could relocate the probes to a recirc
line that has a high rate of flow to give more accurate sampling. NOTE: we have also observed that the Caustic soda sticks to the probes and cause drift of as much as one pH unit.
 
A

Andre Leeuwis

Best to put the pH probe in the circulation line. This way you will measure the pH after it is mixed with the added caustic.

The pH of a liquid can change realy fast when you add caustic to it. This stops when you are in the buffer zone. In this zone (I'm not sure but I
believe something like 6-8 pH) the pH will be stable for an amount of time while you are still adding caustic. After you add a certain amount of
caustic you will leave this buffer zone again and the pH will go up very fast again.

For this reason it is better to add caustic for a certain time. Wait for some time and check if the pH is ok. If not, add another amount of caustic,
etc. This will make sure that you will not overshoot your pH.

Regard,
Andr=E9
 
M

Michael Johnson

Maybe a flow meter inline with the exhaust fumes ( like an orifice plate and DP transmitter ) before the scrubber as a volumetric correction factor the pH controller. Meaning, for every volume increase in exhaust fume into the scrubber add a proportional ( or fixed ) amount of caustic solution.
 
A

Andre Leeuwis

Best to put the pH probe in the circulation line. This way you will measure the pH after it is mixed with the added caustic.

The pH of a liquid can change realy fast when you add caustic to it. This stops when you are in the buffer zone. In this zone (I'm not sure but I
believe something like 6-8 pH) the pH will be stable for an amount of time while you are still adding caustic. After you add a certain amount of
caustic you will leave this buffer zone again and the pH will go up very fast again.

For this reason it is better to add caustic for a certain time. Wait for some time and check if the pH is ok. If not, add another amount of caustic,
etc. This will make sure that you will not overshoot your pH.

Regard,
Andr=E9
 
I agree. I have set up a few of these systems (8 or more). Probe in recirc line and add caustic in timed slugs. Have your control program ignore pH alarms for an equilibrium time after each caustic addition. Also helps to only add chemicals after a blowdown.
 
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