pH sensor problem


Thread Starter

Albert Avidor

Hii List,

I have an assignment for controlling ph level in an acid scrubber.
The system work as follows:
Acid vapors are coming from an industrial acid cleaning system to the scrubber
a caustic pump is pumping caustic to the scrubber to neutralize the acid. Right now the pH sensor is in the bottom of the scrubber.I would like to move the pH sensor to the stack in order to control the outgoing gazes to the atmosphere.I have been told that the pH sensor will not work in the stack. Can any body help to solve this
Albert Avidor
You have been told correctly. The pH sensor will not work in the outgas stream.

The reason wet scrubbers are designed and controlled the way you are doing it now, is that if you control the pH of the scrubbing liquid, you do not have to control the acidity of the stack gas. The stack gas will be properly scrubbed, and all you have to do is take the required grab samples for analysis.

You need to ensure proper mixing in the scrubber basin, so that the reaction of the caustic is complete, and you need to make sure that the residence time between the injection point and the sample intake to the pH control system (you do have an analyzer/controller, yes?) is short enough so that you will see realtime changes in pH...

There are several excellent books on pH, including one by Greg McMillan which is available at the ISA Bookstore, Dr. Bela Liptak describes such a system in his IEH, as well. Both are excellent references.

You can also get knowledgeable advice from the vendor. pH control is not an exact science...and in order to be a successful vendor, especially of an application like wet-scrubber control, which is a very common application, you _have_ to know what you are doing. I suggest you discuss the
application with either the vendor who sold the system to begin with, or one of that vendor's competitors, or both and compare notes on what they told you. Betcha they both tell you much the same thing.


Walt Boyes (see I _can_ talk about something other than standards <grin>)

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