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pH control in water treatment plant

This case study concerns the control of pH of residual waste water in a refinery. The control of pH was achieved by adding more or less lime to a decanter tank which was being fed by two separate water flows of different flow rates and time-varying pH. The combined flow rate of the two streams often changed greatly and very rapidly, making the precise control of pH very difficult for the existing control system which had been refined over several months of adjustment and tuning. The changes were between 100 and 170 m3 per hour in a tank of 300 m3 capacity. The pH was measured after the lime had been added using an on line analyser. Dead time in the system was very high, typically 50 minutes. The diagrams below show the improvement of ADEX control over the existing method with a maximum variation of 0.2 compared with 0.89 over a 12 hour period.

The ADEX strategy

Value of pH was not used directly for calculating the control signal but rather a linearised value produced by a pH signal treatment block. Due to the long dead time and the changes in flow rates, the control periods were also adjusted automatically to compensate.

The control signal applied was based on the ADEX controller output multiplied by the ratio between the actual flow rate and a nominal flow rate, taken to be 150 m3/ hour. The controller worked on a delay of 4 control periods.

Existing Control Scheme Performance

pH Control is achieved using the previous method over 12 hours pH value varies between 9.90 and 10.79.

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