pH Control

[JBando]

Hello there:

F1 ! F1 !
Help ! Help !

I am trying to tune a PID conroller for a pH control system.
the inlet stream comes at pH=1.5-2.5, and caustic is added to get a pH = 6.5 (ideally 7.0, but I will refrain to discuss about that right now). I have a Equal % Open Control valve, and the Tanks has a residence time of 15-20 mins (DEAD TIME). What I see is the following:
SP=6.5
When PV = 3.5, CV tries very hard and opens 100%, and remains like that, as teh Residence Time is 15 mins (pH is measured at the TANK Exit). By the time the new pH information reaches and shuts off, caustic addition is too much- thus resulting in high pH around 8.5. too much offshoot.

What I have done:

I set intially manual mode to get the pH around 6.2, then turn on AUTo. It did nto do a good job. I changed the Gain to 0.01 from 0.5, turn off the Integral action (originally 20), and set K_D=0. I do not see any improvement. Any pointers how to TUNE the loop?

thanks,

Bando

 

[hacksaw]

pH is never simple, especially with large dead time.

Liptak advise integral time (in min/repeat) to your dead time (15-20 min) and the proportional gain for stable response (about 1/3 of that which causes cycling). assuming that your neutralization process is well behaved.

You also need to linearize the valve or change the trim for linear flow with % opening. If you need the reangeability, then use split ranged linear valves. You may be able to characterize the controller output to achieve same.

The eq% characteristic causes you to limit controller gain to keep the loop stable when the valve is wide open. This impacts how the loop closes in on the set point when the valve is more nearly closed.

There are special controllers (with a non-linear gain characteristic) that accomplish the same thing.


There are special reagent feed valves that have a linear characteristic and 100:1 turn-down. Fisher makes one. These are frequently used in tough app's like yours.

We always used two tanks, with the second tank serving as a mixing / polishing tank to tighten the pH control. You may be able to do it with one, but you'll have a lot of work to make it happen.

 

[RGCook]

It sounds like you have way too much gain. Try reducing it 20% at a time. As you do that, time the peak-to-peak overshoot and set your integral to three times that amount. Leave derivative at zero.

The other responses are also very good. I am trying to give you the basics to make what you have work, if possible.

 

[JBando]

Thanks for your input Hacksaw & RGcook.
I will implement your suggestions in the coming week.
I will keep you posted.

Hacksaw, in fct we are implementing the strategy for tighter control go get PV=7.0. For that purpose, we are instaaling the new tank to do the great amount of job, and do the finishing the job in tank 2. As the feed pH varies, we see variation in pH exiting from tank almost 1 (6.5-7.5pH)

I was following the controller setting for the existing tank, but looks like each component has to be similar for implementation. I will keep you posted.

Thanks,

Jay

 

[saxon]

jbando, pH is one the most difficult process variables to control accurately, if not the most difficult. One question, have you guys worked up a titration curve (pH vs. ratio of reagent to influent flow) on the acid/base system in question? If you haven't I doubt you will ever get the system under control. This curve will tell you lots about the acid/base system and what is required to control it.

From the information you have supplied, it sounds as if you're trying to control a strong acid/strong base system. The curve for this type of reaction is a vertical straight line between approx. pH 2 thru 11. The reaction rates and kinetics in this region are almost instananeous. Hence your problems with over shoot or under shoot of the set point due to action/reaction time constraints and limitations of the control system. Another problem you may be facing is a changing pH strength in the influent stream or reagent stream over time. If this is occuring, the PID settings you set for one ratio of reagent to influent flow will be all wrong for the changes that have occured. No matter how much you fiddle with PID settings in the controller you're going to lose control.

My recommendation is get the titration curve first, then make sure the system is capable of providing the type of control you want. BTW, also get a copy of "pH control" by Gregory Mcmillan, published by Instrument Society of America. It's the bible of pH control.

Hope this helps.
saxon

 

[djs]

Hello;
I agree that pH is very difficult to control. See the following web site:

http://www.omega.com/techref/phtour.html

Perhaps a cascade type control: have a ratio controller followed by a PID controller. The idea is that the ratio controller gets the pH close does the coarse pH adjustment, and then the PID does the fine tuning. You may have to change out your controller for this to work.

 

[JBando]

Hello Everybody:

I had to change the valve trim. And i noticed that the dead time of the system was too high. So I am getting a retractable probe, and place it in the tank (earlier it is in in the Exit line. Let see- I will keep you posted the update. Thanks for your help.

Jay

 

[IRstuff]

This site, while pushing their PID tuning software has several articles on tuning PID controllers specifically for PH control:

http://www.expertune.com/articles.html

TTFN

 

[Assumptions]

On the topic of tuning I just completed some catching up on my reading last night and came across a series of good articles on tuning. There were three articles in Chemical Engineering (August 2002, Jan and Feb. 2003)
If you have access to these I would recommend that you have a look.

Hope this may jelp.

 

[JBando]

Hello Everybody:

Well, One setp closer to perfect situation:
I am ordering a retractable pH probe for measurement.
Also, changing the Valve Trim for finner control.
As of now, I am able to control close to 6.55-6.75.
i really appreciate your help, and suggestions.
Assumptions, I will see if I can get those article.
By any chance do you hvae those in electronic format?
If so, can you please forward those to me?

Thanks,

Jay

jbando@ureach.com

 

[Assumptions]

JBando : Don't have an electronic copy. However, the publisher will provide copies of atricles.

Tel: 212-621-4674 ? jschweikart@che.com

Don't know what the cost is?

Sorry, can't he;p further.

Good luck in your search.

 

[JBando]

Thanks.. Assumptions...