pH control

T

Thread Starter

Tang Chin Teng

I'm doing my thesis for B Eng. I have read books by Shinskey but still cannot get the idea.. hope that anyone out there can show me some light...
 
W
We'd be happy to shed some light, but you'll have to help.

What specifically do you want to know about pH Control?

If you've read some books, either Shinskey or McMillan, you must have some specific questions.
Let's hear them.

What do you want to know?

Do you want to start from "this is pH and this is what the measurement means?"
Or do you want to start from "I have this specific application and it is not working and I don't know why?"

Where do you want to start?

Walt Boyes

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Walt Boyes -- MarketingPractice Consultants
[email protected]
21118 SE 278th Place - Maple Valley, WA 98038
253-709-5046 cell 425-432-8262 home office
fax:801-749-7142
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B
Read my article in the 2000, October issue (starting on page 18) of Control Magazine titled: "The Nasty Variable to Control" and you will "get it".
Best regards, Bela Liptak
 
D
PH control is difficult because you have to study the constitution of the chemical from basic theorem.
Industry work often assign the control engineers to design control strategy or commission.
The problem is that the control engineers are not chemical specialist. He has to pay attention to many project.
A fast strategy is to use timer in PLC to take place of PID control. Of course basic study is necessary but time to job completion is also
important.
You could also visit below discussion board
http://www.fansi.net/Discussion/tech1.asp?Class1=Facility-FMCS
to get your idea.

Best Regards,

Daniel Lee 10/25/2000
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T

Tang Chin Teng

right now I'm doing a basic simulation by using matlab. I'm using the model in the journal written by Yeong-Koo Yeo and Tae-In Kwon which is printed in Ind. Eng. Chem. Res., Vol 38, No. 3, 1999. Well the model is proposed by Mcavoy. I'm have no trouble in transforming the mixing dynamics to tranfer function but I faced difficulty in transforming the process equations which are when the base and the acid reacted into transer function....

Thanks to Mr. Boyles.. I know about pH measuments already... as you see above, i'm facing the problem in stimulating the application...

Thanks to Mr.Liptak.. so can I find the article on the net?? i'm the pioneer batch of chemical engineering in the uni which i'm studying.. so it's hard for me to find the magazine in the library..

and thanks to Mr.Lee and Mr.Heavner too...
 
A
> I'm have no trouble in transforming the mixing dynamics to transfer
> function but I faced difficulty in transforming the process equations
> which are when the base and the acid reacted into transfer function....
>
Hi

The pH modelling I have seen done before, develops a model of chemical reaction by considering the mass balance of ions in the particular solution being used. Combining this with the standard definition of pH (pH = -log10 [H+]) and equilibrium conditions, gives the expected non-linear dynamic equation. I guess you have this done already.

This non-linear dynamic equation can be represented directly in a non-linear modelling package (e.g. Simulink for Matlab), using
appropriate blocks etc. If you must represent it as a *linear* transfer function (i.e. if you are using Matlab without Simulink), you must
linearise the non-linear equation at an appropriate operating point. Clearly this linearised model will only be approximate and will become invalid as the operating point moves away from that used initially. To get around this you could use multiple linear models.

I hope this helps.


Andy [email protected]___

Advanced Control Technology Club, Industrial Systems and Control Ltd.,
50 George Street, Glasgow, G1 1QE Tel: (+44) 0141 553 1111
http://www.isc-ltd.com/actclub.html Fax: (+44) 0141 553 1232
______________________________________________________________________
 
A
> From: Tang Chin Teng <[email protected]>
> i just couldn't find a way to transform it into a linear equation....

You could:
1) use a software package to do this - most have linearisation functions (e.g. linmod in Matlab)

2) re-formulate the non-linear differential equation using small incremental changes in the variables (and time) to give a linear
approximation. This may require a bit of mathematical manipulation - most graduate level maths books should explain ways to do this.

Hope this is helpful
Andy Clegg
[email protected].com
Advanced Control Technology Club, Industrial Systems and Control Ltd.,
50 George Street, Glasgow, G1 1QE
Tel: (+44) 0141 553 1111
http://www.isc-ltd.com/actclub.html
Fax: (+44) 0141 553 1232
 
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