REAGENT DOSING DIFFICULTIES
The pH scale is logarithmic, so when a solution approaches neutrality (pH = 7), a small addition of reagent can cause a large change in pH (Figure 2). The further the solution is from 7.0, the more reagent is needed to adjust pH.
If the reaction tank is large and the pH sensor is located far from the reagent dosing point, it can take a long time for the sensor to detect the change. The sensor will continue to call for reagent long after enough has been added to achieve neutralization. This leads to overshoot and oscillation, a pH value higher or lower than desired, and wasted reagent.
In a single-tank batch control application, you can use a timer to prevent overshoot by delaying the addition of reagent — thus allowing enough holdup time, mixing and chemical reaction so the sensor achieves a representative pH value. While avoiding dead time is desirable in most control loops, dead time often is a necessity in pH control.
A control valve isn't a pipette and has great difficulty dispensing wide ranges of volume. Therefore, a single-tank system may require two different size valves for adding reagent, with the valves configured as a single split-range controller. When the pH is far from set point, relatively large amounts of reagent are needed, so the larger control valve is used. However, when the pH approaches set point, only a few milliliters of reagent may be necessary to reach it, so the smaller valve dispenses the final amount.
Another common mistake is trying to adjust pH in a pipe. This usually is a bad idea because the reagent often doesn't mix properly with the process fluid. Reaction tanks with agitators are best for ensuring more complete mixing.
Reagents may be more viscous than water, and require time to mix and react. You should locate tank exits to give reagent the maximum time to react. For example, when using heavy reagents, the tank should discharge at the top instead of at the bottom.
Batch control may not be practical when there's a high volume of waste, and may not work well when the holdup time in the reaction tank is less than five minutes or when wide swings occur in the pH value of the waste. Instead, consider a continuous neutralization process (Figure 3).
One reaction tank will suffice if the change required is less than 2 pH units and the amount of inflow will allow a 3–5-minute holdup time for reactions to take place. Two tanks usually are required if the change is 3–5 pH units; three tanks generally are necessary if the change is larger than 5 pH units.
A submersible sensor measures the pH value and transmits it to the automation system, which opens a control valve as needed to add reagent. Each tank should have an agitator, and generally needs 1–3 minutes of holdup time.
DOWN AND DIRTY
The two biggest mechanical challenges facing pH sensors are temperature extremes, which can crack the glass sensor, and process fluids that coat or poison the sensor elements (Figure 4). Specific process-fluid-induced problems include:
1. High concentrations of hydroxyl ions (very high pH) can shorten the life of the electrode.
2. Hydrofluoric acid can dissolve pH glass.
3. Sulfides can poison a silver-based reference electrode.
4. Undissolved solids and liquids can coat a pH sensor.
5. Fluids can contaminate the reference solution, causing drift.
6. Certain fluids can degrade electrical connections within the sensor.
Manufacturers of pH sensors usually have solutions for all these problems. For example, to combat coating, a jet spray cleaner can be attached to the sensor to keep it clean. To avoid reference solution poisoning, its chamber can be refilled with a highly viscous reference gel using a syringe.
The sterilization required in many biopharmaceutical processes to avoid cross-batch contamination or unwanted growths can harm pH sensors. The sterilization system typically uses steam; exposure to high temperature steam and rapid thermal shock can shorten a sensor's life.