[labnetwork] Fume Scrubber pH Drift

[Kuhn, Jeffrey G]

Dr. Khbeis,

We have a Harrington fume scrubber at Birck. Your experience is the opposite of ours. In our case, the sump pH typically climbs to around 9.2 and stays there. We are fine with that since our POTW requirements call for a discharge pH of 5.0 - 10.0. The scrubber system was built with a caustic feed pump assuming that the pH would typically drop during normal operations, but in my five years at Birck the caustic pump has never been required.

Without knowing your facility's chemicals, gases, or processes, I assume that your scrubber's incoming airstream is similar to ours since we are connected to the same types of equipment. If that is true, then my first thought is that perhaps there is a gas leak somewhere in one of your cabinets or process tools since your scrubber pH drops even when tools are idle. Do you have a toxic gas monitoring system? If so, it should have identified the leak and be in alarm if it is functioning correctly. I do not think that the beaker quantities of acids typically used in fume hoods would produce enough fumes to cause the pH to drop so quickly.

This is a bit of a long shot, but do you use ultrapure water for the scrubber sump make-up? If so, the moving airstream would quickly dissolve CO2 into the water and form a dilute carbonic acid. In that event, I could see the pH dropping to as low as 3.5. The cost of producing UPW typically precludes its use in such applications but the formation of carbonic acid would indeed cause the pH to drop.

Please feel free to contact me directly should you have any additional questions and I will do my best to assist you.

Regards,

Jeff Kuhn
Facility Engineer
Birck Nanotechnology Center
Purdue University
1205 W. State St.
West Lafayette, IN 47907
Ph:  (765) 496-8329
Fax: (765) 496-2018



From: labnetwork-bounces at mtl.mit.edu [mailto:labnetwork-bounces at mtl.mit.edu] On Behalf Of Michael Khbeis
Sent: Tuesday, July 24, 2012 1:29 PM
To: Fab Network
Cc: Leonard Hixson
Subject: [labnetwork] Fume Scrubber pH Drift

Dear Colleagues,

I was wondering if any of you had experience in pH swings on fume scrubbers.  We inherited the facility and associated scrubber that is attached to all our gas cabinets, GRC, and fume hoods.  We have little documentation on the PM schedule and no Operating and Maintenance manuals. Basically, we are neutralizing or raising pH to 10, but then over the course of a few minutes up to several hours the system will drop to pH of <5.  Acceptable range in our AOP is 5.5 - 11. We see this swing even when tools are idle - there does not appear to be an operational indicator that tracks the system response.

System description at a high level: Age of system is 15 years.  Last media change - unknown, but plastic media looks mostly normal. Manual fill/drain, system recirculates with addition of caustic soda to balance out acidification - also manual dispense when pH is out of range.  We started a bacterial kill that was recommended in the weekly PM schedule about 3 weeks ago and starting seeing this instability in pH a week later.  We halted the PM schedule to see what impact there is, if any.

Remedies attempted:
            1) Added sodium bicarbonate to increase buffering of system.  Minimal impact.
            2) Multiple drain/flush cycles - pH was normal (7) for few minutes then dropped to 4 shortly after.
            3) pH probes were recalibrated and verified to be operational independently.

Any recommendations or anecdotal experience would be appreciated. I thank you for participating in this community.

Gratefully,

Dr. Michael Khbeis
Associate Director
Microfabrication Facility (MFF)
University of Washington
Fluke Hall, Box 352143
(O) 206.543.5101
(C) 443.254.5192
khbeis at uw.edu<mailto:khbeis at uw.edu>


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