pH titration/control

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Thread Starter

Dai Ly

Control Engineers,

I am looking to automate the control pH. Currently, manufacturing operators test pH of a container. If the pH is low, caustic is added and if the pH is high, acid is added until the desired setpoint is reached. If overshoot occurs, then the appropriate caustic/acid is added to achieve the setpoint.
I am looking at 2 possibilities:
1) a pH probe in the container (could be 10 liters capacity or a 1000 liters capacity) reads the pH and the H+ ion concentration. A caustic inlet to the container, and a acid inlet to the container to add the necessary amount to satisfy the set pH.
2) a sample of the process fluid is taken from the container via a robot. pH control is then done on the sample. The appropriate proportioned amount would then be added to the container.

Have any of you experienced/know of someone who has done this sort of control? I would appreciate any feedback.

Regards,
Dai Ly
[email protected]
 
A

Allen Nelson

Before I was a controls engineer, I was a chemist in a QC lab (don't ask).

We used to do your method 2 (except that the robot was a guy with a dixie cup on a stick). Titrating the sample, then scaling that amount to the batch size worked pretty well. A second adjustment was sometimes needed, but we rarely overshot (and thus didn't waste caustic/acid.)

There are some problems with your method 1 approach. You cannot pre-calculate the amount of acid / base to meter into your batch, because of buffering. And if you try to use the pH meter in real-time control, you have to make sure that you have complete mixing and reacting during your adjustment. On the lab bench, we would sometimes "overshoot" the pH, stop the titration, let it mix, watch the pH settle back, add some more, mix and wait some more, add some more, etc., to get the correct titer. DEPENDING ON YOUR PRODUCT, you may have the same problem. If you are not careful, you will think you have overshot when you haven't, and take inappropriate action.

Automated pH control can be tricky. Be careful and good luck!

Allen Nelson
 
I used to work at a plant that ran alot of cooling towers that needed constant ph control. It was a completely automated system that incorported a ph controller and 2 dosing pumps. if the ph measured low, the caustic dosing pump will start dosing very quickly and slow down as it got near setpoint. The same happened if the ph was to high, the acid dosing pump would kick in and operate the same. Sort of PID control.
 
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Johan Vereecken

> 1) a pH probe in the container (could be 10 liters capacity or a 1000 liters capacity) reads the pH and the H+ ion concentration. A caustic inlet to the container, and a acid inlet to the container to add the necessary amount to satisfy the set pH.


I'd recommend this method with a big (at least 1000L; the bigger, the better) container. If the inlet flow is not too high and its pH varies much, the liquid will buffer itself mostly. So less caustic/acid will be needed. Don't forget to stir. Use a pH controller connected to 2 dosing pumps with some hysteresis (eg. 1 Sorensen above and below the desired pH). It might need some testing to find an acceptable hysteresis and dosing flow.
During operation, only the causitic/acid will have to be added and the pH-sensor will have to be calibrated every 2 weeks. If the liquid is very dirty/sticky, then a self-cleaning electrode would be a good choice.

Regards.

Johan Vereecken
 
> I used to work at a plant that ran alot of cooling towers that needed constant ph control. It was a completely automated system that incorported a ph controller and 2 dosing pumps. if the ph measured low, the caustic dosing pump will start dosing very quickly and slow down as it got near setpoint. The same happened if the ph was to high, the acid dosing pump would kick in and operate the same. Sort of PID control.
===============================================

Control Engineers,

Thank you for all who replied to the origin of this post. I think this approach of the PID control with 2 pumps for the respective dosing of acid/caustic would be a likely approach for our application. Do you remember the controller in use (ie. PLC/DCS) Did you just have a single loop for the control? Any inputs is always greatly appreciated!

Regards,
Dai Ly
[email protected]
 
W
Generally speaking, pH controllers are proportional controllers, not PID controllers. You can, of course, do PID control with any device that has the math functions built in. Of the 20 or so manufacturers of pH control
systems, I can't think of one that provides PID as standard. Mostly this is because chemical metering pumps are operated in discrete pulses, not via an analog positioner, so it is much more difficult to use PID control than simple proportional control.

Walt Boyes
 
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Patrick Viskens

Control Engineers

To control pH with a PID controller use a non linear PID algorithm. You set deviation limits around your setpoint. When the measurement has a value in between the limits the proportional band of your controller will be set to a higher value ( 3 .. 5 times more ) so you work slowly to your desired setpoint. The output of the PID controller has a split range which has two time proportional output contacts. The non linear PID algorithm with good tuned time proportional output contacts does the job. Depending on the kind of application ( installation ) you can use Otto Schmitt dead time compensation, feed forward compensation, ...

Kind Regards,
Patrick Viskens
[email protected]
 
Dear Walt, PID controls can be used on chemical dosing pump provided that the pump speed can be controlled using a variable speed drive. We use this method for controlling the lime and alum dosing of our previous water treatment projects. Regards, C.P.Ku
 
I have a couple questions regarding titration. I am set to do a titrating next week on a raw groundwater sample and wanted to know how much and what kind of NaOH (caustic) solution I should take with me. 2% solution? 8% solution? How many mL should I take with me? Also, is there a standard method or EPA approved method for performing titration curves? Where can I find it? Thanks
 
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