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Control pH on pipeline

March 25, 2014

It is and will be extremely difficult to control the pH in a flowing pipeline if it is a strong acid strong base titration. There just is not enough residence time in a pipeline of any size that is flowing at a good rate for pH control.

What you will observe will be large cycles of high pH followed by low pH, and most of the time pH will be way above or below your control point.

The best way to control pH will be to allow the stream to flow into a titration tank of appropriate size ( 10 to 30 minute flow volume), and then to flow into an attenuation tank of about 5 to 10 times the volume of the titration tank. Bothe the titration tank and the attenuation tank should have an agitator and mixing baffles with moderate mixing.

The level needs to be adjustable in the titration tank to find the “sweet spot” for residence time. And then you need to use a Foxboro 762C Programmable Controller with the EXACT self-tuning algorithm. Half of the pH curve needs to be programmed into the Output Characterization ONLY, such that response at or near set point is at or near zero, with the one half pH curve sloping up and away from zero to the 100 % response point.

Once the process is running, observe the pH chart. If it is spiking up and down (indicating too low a residence time) raise the level in the titration tank. When it settles down but will still be spiking with less amplification, turn on the EXACT tuning and watch the chart. If and when the spiking becomes much less frequent with some time the pH running for brief periods at set point, TURN OFF the EXACT Tuning.

The level in the attenuation tank should be kept high at about 7 or 8 times the total volume of the titration tank. The pH coming out of the attenuation tank should be approximately at set point and only wandering very slightly.

An in-line static mixer fitted with an injection lance, and a pH meter a suitable distance downstream controlling a piston diaphragm dosing pump can add acid and safely and effectively control pH with an accuracy of 0.1 pH units.

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