causitc and ammonium pH control

[YouthfulEngineer]

I am investigating a control system for pH control for an ammonia stripping column.
The ammoniacal liquor contains both ammonia (NH3) and ammonium ions (NH4+). Caustic (NaOH) is added to liberate ammonia as product which is distilled off.
The pH of the remaining effluent however needs to be steady between 9.5 and 10.5 pH. The current system which uses a centrifugal pump and control valve system gives quite a highly fluctuating pH value. The control valve is controled via the pH sensor readings.

My question is whether the control of effluent pH would be improved if the ammonium ion concentration of the feed was monitored to provide the set point for the amount of caustic sent to the column. In effect providing a feed forward control rather than the current pH monitoring (feed back control)

sorry for the long winded background.

 

[BenThayer]

look at your pH curve. if you are trying to control on the vertical section of the curve, it will be extremely difficult to do this.

you may need larger tank and more retention time. are you just doing it via an in-line mixer on the way into the stripper right now or do you have some capacity in it with some mixing via recycle, etc.?

and do you have a flow meter on the feed as well? you might be able to have a ratio control relating the feed flow (or control valve position) and add the caustic with a very small trim valve to do the final pH adjust...

 

[mbeychok]

Probably others will disagree with me, but it has been my experience that pH controllers rarely (if ever) work well in large scale process units.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[25362]

It has been said that pH control with weaker bases such as soda ash, will require more reagent for neutralization, but control is easier. See Shinskey's Process Control Systems Chapter 10, McGraw-Hill. Carbonates may buffer in the range you wish to regulate.

 

[IronMike]

Another significant effect on pH is the temperature. If your temperature control range is wide, especially at the point of pH measurement, you will see a lot of variability in pH in addition to that caused by the caustic variability. Long time ago I ran a stripper with temperature and pH as input controls. Not trusting the online pH instrument, I took samples and had them analyzed in the lab. The pH 11.5 controlled on line went up to 12.5-13.0 in lab. Difference was temperature - samples had cooled to room temp overnight. Later did test on a sample, heating and measuring pH at the same time. The pH decreased about 1+ units as temperature increased from ambient to 150F and reversed same amount as sample cooled. And YES - the meter was temperature compensated. Lot of folks don't realize that pH is really hydrogen ion activity which is f(Temp).