pH Control Loop

I'm after a bit of advice and hope someone can help..?
I have been asked to design a pH control system. Basically, the effluent
stream coming from a plant at <pH2 is pumped into a static mixer where caustic soda is added via a control valve to do the first course adjustment. The stream then runs through another static mixer, again with caustic added via a control valve where the fine adjustment is carried out. The final effluent stream must be between pH 7 and 8
I was wondering if the best way to control would be to have a ph meter feeding a controller on the outlet of each mixer to control the caustic addition via a normal 3 term feedback loop. Or, would some form of feed forward or cascade control loop be more suitable and if so, how would this be acheived.
Any advice would be much appreciated.
Steve
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^Jak^ wrote:

If you have an extra pH meter available, it would do the first loop quite a bit of good. A formula based on empirical experience can predict the valve position for the first reactant addition, with the first PID loop just correcting the error in this prediction. The controller adds the prediction to the PID reaction to create the actual control output. Likewise, the pH meter that runs this first loop can also be an input into a second formula that predicts the second valve position. Of course, those formulas don't mean much if the stream does not flow at a constant rate. If rate varies, a measurement of that needs to be included in the feedforward formulas. If the caustic reactant supply suffers concentration variations or pressure changes, it is helpful to include those pieces of information in the formulas, also, or replace the control valves with metered injection.
A warning. Static mixers are not at all good for this application, because they just mix across the area of the pipe, and do not add any historical averaging (lag) so an abrupt change in the stream pH at the mixer input (that includes poor uniformity across the pipe cross section) still shows up as an abrupt change at the mixer output (but with much better uniformity across the pipe area).
I have designed a fat spot replacement for a static mixer loop that had this problem. It improved the ability of the loop to smooth out step changes by at least a factor of 10. It consists of a section of pipe about 4 times the process pipe diameter and about 5 times as long as it is wide, closed at one end. The input stream enters this fat section near the exit end with a tangential connection, so that the flow causes maximum rotation within the fat pipe. At the same end, the flow exits through concentric dip tube that has two rows of holes, smaller near the entry point, and larger toward the far end. It ends about one of its diameters from the closed end of the large pipe with an open end. This mechanism approximates a mix tank, by storing a historical record of recent process variations and combines samples from this history, so that a step change in the input composition exits as an lag filtered step. It also has quite a bit higher residency time than a static mixer of the same length, allowing reactions to come closer to completion before being measured.
Having this kind of mixer between reactant injection and process measurement allows for quite a bit more gain in the PID controller, and for less error before the controller reacts.
--
John Popelish

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John Popelish has given good advice. Personally I think the best return on investment is to put in flow measurement and scale all controller output proportional to flow. That gets rid of one variable very easily.
Walter.
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