Ask Greg McMillan

Greg's Response:

Lime feeders have a transportation delay that is proportional to the length of the feeder divided by its speed. This transportation delay may be several minutes. To eliminate the need to increase the vessel size and the agitation power, the lime rotary valve speed can be base loaded, and the pH controller can manipulate the conveyor speed or the influent flow. If the pH controller manipulates the waste flow, the dissolution time associated with an increased lime delivery rate is also eliminated. The level controller on the influent tank slowly corrects the lime rotary valve base speed if the waste inventory gets too high or low.

The dissolution time of pulverized dry lime can be greatly reduced by slaking the lime and making it into a slurry. For example, the dissolution time for pulverized dry lime of about 32 min, can be reduced as lime slurry to about 8 min. The dissolution time of lime slurry increases with age due to the agglomeration of small particles into larger particles, even though the lime slurry storage tank is mildly agitated. If the pH controller manipulates the lime feeder or water addition to the lime slurry storage tank, the equipment time constant of the slurry tank has the same effect as a slow valve time constant. To prevent adding this time constant to the loop, the lime feeder speed should be manipulated by a slurry tank level controller and the water addition rate ratioed to the lime feeder speed. The pH controller should manipulate the diluted lime slurry discharge flow that feeds the neutralization vessel. The diluted lime slurry discharge flow must be kept flowing by using a recycle loop to prevent settling and plugging in the reagent lines. A throttled globe valve will plug. The liner of a pinch diaphragm valve will fail due to erosion. Pulse width modulation of an on-off ball valve is the best alternative because it has the fewest maintenance problems and is relatively inexpensive to replace periodically.

Dynamic simulations with automation, equipment, and process dynamics including dissolution time are needed to determine the best lime 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).

The time for solids to dissolve can be horrendous (minutes). Although the time for bubbles to dissolve is faster (seconds), the noise from fluctuations in concentration, besides the delay to complete dissolution, is problematic. For these reasons, using lime and ammonia for pH control is undesirable. Additional problems stem from highly viscous reagents, such as 97% sulfuric acid, that result in laminar flow in control valves and greater difficulty in mixing with process streams. Highly concentrated reagents cause extremely large reagent delivery delays due to low reagent flows (e.g., <1 gal/h). Strong acids and strong bases cause this same problem, and the additional difficulty of a steeper titration curve and the consequential need for more precise final control elements and possibly more neutralization stages. The following best practices are offered to provide more realizable reagents for pH control.

1. Avoid reagents that have solids or bubbles, take more than a second to react, are highly concentrated, or are strong acids or strong bases.
2. Increase the residence time in equipment and piping to provide complete dissolution of reagents and uniformity of reagents in the process stream.
3. If a conveyor is used, manipulate the motor speed of a gravimetric feeder rather than the flow dumped on the conveyor inlet to eliminate the conveyor transportation delay.
4. Use pressurized reagent pipelines with a coordinated isolation valve close coupled to the reagent control valve outlet to prevent process backflow into the reagent pipeline.
5. Minimize the volume between the reagent valve and its injection point into the process.
6. Avoid using dip tubes by injecting reagent into the feed or recirculation stream. 
7. Dilute reagents using tight concentration control in a recirculation stream to a diluted reagent storage tank.
8. Use Coriolis mass flow meters for accurate reagent flow and concentration measurement by using a high-resolution density measurement for reagent dilution and pH flow ratio control.