Virtual Plant Provides Real Insights

Simulation points to a better strategy for controling pH

By Gregory K. McMillan, Emerson Process Management, and Roger D. Reedy and John P. Moulis, Monsanto

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The virtual plant explored the use of a fast inline pH control system to catch up with disturbances, followed by a tank to smooth them out. By putting the pH loop in the recirculation line of a tank, the system could automatically switch between continuous and batch operation. The pH loop deadtime could be kept to less than 2 sec. because of a minimal transportation delay through the mixer, if the delays from reagent injection, control valve stiction and sensor response were minimized.

new pH design
Figure 3. New pH system design -- When pH is out of
range, tank operates in recirculation mode.
Click on illustration for a larger image.

Control logic. When the pH was within range, the effluent would be sent downstream and the level maintained at a minimum to maximize the available space. When the pH was out of range, the tank would operate in the recirculation mode and adjust the pH until the level got above a maximum desired set point at which time it would send the effluent to the other tank if that tank had space. For tank 1 this meant sending the effluent to tank 2. In the unlikely event that tank 2 pH was out of range, it could send the effluent back to tank 1. We verified that linearization of the pH loop was beneficial and minimization of the acid and caustic valve stick-slip was essential. Figure 3 shows the new pH system.

Design details
Test runs showed the pH could be controlled with a 10,000-gal. tank, reducing the installed project cost by more than 50%, if the design had the following details:

 1. A turnover time of less than 2 minutes in the tank at normal level (40%) by the use of a ring of eductors;

 2. A static mixer on a high recirculation stream (1,000 gpm) at the point of re-entry to vessel, with reagent injection via close-coupled isolation and control valves at the mixer inlet and retractable inline pH electrodes at the mixer outlet;

 3. Direct manipulation of close-coupled reagent control valves to achieve fast inline pH control (flow loops necessitate slowing down the pH loop to meet the cascade rule that the primary pH loop be five times slower than the secondary flow loop);

 4. Acid and caustic control valves’ resolution of 0.1% or better and water control valves’ resolution of 0.4% or better;

 5. Reagent demand control where the manipulated variable for the inline system is converted from pH (Y-axis of titration curve) to reagent demand (X-axis of titration curve) by the use of standard signal characterizer blocks in the DCS. The scale is 0 to 100% reagent demand for a 2 to 12 pH range;

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 6. Control logic to keep the caustic valve from plugging by setting the caustic valve to 10% and pulse-width proportionally modulating the caustic isolation valve per the controller output when the caustic valve signal is between 0 and 10%;

 7. Control logic to keep level at the minimum for effective eductor operation whenever the tank pH is within 6 to 9 pH by sending effluent downstream;

 8. Control logic to keep level below a maximum whenever the tank pH is outside 6 to 9 pH by sending flow between tanks (tank 1 to tank 2 and vice versa);

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