Since we learn by example, here is a neutralizer tank that has the major mistakes I have seen in waste treatment system pH design and installation. Each mistake is the result of a fundamental lack of understanding of the heightened sensitivity of pH control systems to dead time, process and measurement noise, insufficient process attenuation (filtering) and valve backlash and stiction.
1. Insufficient number of stages of neutralization (inadequate rangeability and sensitivity)
2. Improper vessel geometry and agitation patterns (excessive equipment dead time)
3. Backfilled reagent dip tube (excessive reagent delivery delay)
4. Incorrect location of reagent injection point (excessive reagent delivery delay)
5. Gravity flow reagent (excessive reagent delivery delay)
6. Incorrect location of reagent control valve (excessive reagent delivery delay)
7. Control valve with excessive stick-slip (poor sensitivity and excessive variability)
8. Electrodes submersed in vessel (coating and maintainability problems)
9. Electrodes located in pump suction (bubbles, clumps, and wrenches)
10. Electrodes located downstream in recirculation line (excessive measurement delay)
The example serves to illustrate many underlying principles and resulting guidance required to meet the extreme challenges and achieve the extraordinary opportunities for concentration control. In pH control systems reagent piping and injection must minimize transportation delays, volumes must have proper geometry and agitation to maximize the process time constant that filters pH oscillations, several volumes may be needed to provide multiple process filters in series, valves must be able to precisely make extremely small changes in reagent flow, and electrodes must be installed to minimize noise and response time and maximize reliability.