Ask Greg McMillan

Greg's Response:

Because of the pressure to minimize capital costs, a project may sell itself short on the number of stages required. The basic rule from the 1970s was that a well-mixed tank is needed for every two pH units that the influent feed pH is away from the set point. For example, if the set point was 7 pH, one stage for an influent at 5 pH, two stages for an influent at 3 pH, and three stages for an influent at 1 pH would be required. The modern-day version of this old rule would reduce the requirement by one stage if feedforward control or signal linearization could be used effectively or if the set point could be moved to a flatter portion of the titration curve. Today, three stages are rarely used. Even the most difficult systems are tackled by an inline system for the first stage, followed by a well-mixed vessel for the second stage.

One of the most frequently missed opportunities in dramatically reducing the difficulty of control and saving on reagent usage is to shift the set point away from the center to the edge of a control band where the slope is flatter. For example, many environmental systems must keep the effluent between RCRA 2 and 12 pH limits to avoid being classified as hazardous waste. However, a set point at 7 pH is the wrong choice and may lead to oscillations between 2 and 12 pH. A much better choice would be an optimized set point of 4 pH for acidic influent and 10 pH for basic influent.

While a tank with minimal agitation may suffice for blending and smoothing, a vessel with direct reagent addition for pH control must be well mixed with a residence time greater than 5 minutes for maximum process flow and at least 20 times the turnover time.

A dynamic simulation that uses a charge balance with all acids, bases, and salts, verified titration curve, process time constants, transportation, injection, and mixing delays, noise, and measurement and valve 5Rs is critical for finding the best and lowest cost pH system design. 

For much more knowledge, see the ISA book Advanced pH Measurement and Control Fourth Edition (use promo code ISAGM10 for a 10% discount on Greg’s ISA books).

Top 12 Mistakes Made Every Day in pH System Design

1. Incorrect or missing titration curve (incorrect process gain and sensitivity).
2. Improper vessel geometry and agitation patterns (excessive equipment dead time).
3. Backfilled reagent dip tube (excessive reagent injection delay).
4. Incorrect location of reagent injection point (short-circuiting).
5. Gravity flow reagent (excessive reagent injection delay).
6. Incorrect location of reagent control valve (excessive reagent injection delay).
7. Control valve with excessive stick-slip and backlash (limit cycles).
8. Wrong type of electrodes for process conditions (poor 5Rs).
9. Middle signal selection not used (excessive noise and poor 5Rs).
10. Electrodes submersed in a vessel (coating and maintainability problems).
11. Electrodes located in pump suction (bubbles, clumps, and wrenches).
12. Electrodes located too far downstream (excessive measurement delay).