causitc and ammonium pH control
causitc and ammonium pH control
(OP)
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.
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.





RE: causitc and ammonium pH control
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...
RE: causitc and ammonium pH control
Milton Beychok
(Visit me at www.air-dispersion.com)
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RE: causitc and ammonium pH control
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.
RE: causitc and ammonium pH control