Ask Greg McMillan

Greg's Response:

Static mixer have noisy pH measurements. Signal filtering can help but a downstream volume is needed to provide smoothing of pH oscillations that can be larger than 6 pH for steep titration curves.

Static mixers have motionless internal elements that subdivide and recombine the flow stream repeatedly and cause rotational circulation to provide radial mixing of the stream but very little axial or back mixing. Consequently, fluctuations in pH over the cross section are smoothed, but fluctuations in pH with time, which are axial, show up unattenuated in the discharge. The equipment dead time is about 80% of the residence time for a static mixer, and most manufacturers are working toward making their static mixers exhibit plug flow to reduce the residence time distribution. Although this is beneficial for many chemical reactions, it makes the discharge pH more likely to oscillate, spike, and violate constraints.

Flow pulses from a positive displacement reagent pump and drops associated with a high-viscosity reagent or low reagent velocity will not be back mixed and will cause a noisy pH signal. Bubbles from a gaseous reagent will also cause a noisy pH signal because the residence time is not sufficient for complete reagent dissolution. Although a static mixer has a poor dead-time-to-time-constant ratio that tends to make a pH loop oscillate, it offers the significant advantage of a small magnitude of dead time and a small volume of off-spec material from a load upset. Also, the reagent injection delay for close coupled reagent valves is much less than for vessels.

The fast correction and small dead time mean that a static mixer used in conjunction with a volume can eliminate the need for a well-mixed vessel. The static mixer can be on the feed or recirculation line of a volume. pH waste treatment systems use static mixers in series separated by volumes. The volumes may simply use an eductor to provide some mixing for smoothing of the oscillations.

Dynamic simulations with mixing nonuniformity and dead time can show the best use of static mixers in conjunction with vessels.

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).

Equipment and the associated piping are the most frequent and largest sources of loop dead time. The lack of understanding of how mixing profiles and especially the fundamental concept of back mixing and the transportation delay of reagent into the mixture, and the transportation delay of changes in process pH appearing at the electrode are the biggest detriments to pH control system performance, assuming a healthy electrode and precise final control element.   To minimize these problems, the distances to and from the mixture are minimized, the axial agitator pumping rate in vessels for back-mixing is maximized, and dead zones are minimized. Strategically employing vessels and static mixers in various pH control system designs and the importance of minimizing dead time will be detailed in future posts on pH control system design.

1. In equipment where a pH control system injects reagent, minimize the dead time contribution to the loop dead time from mixing and transportation delays to less than 6 s.
2. Use large volumes susceptible to large dead times upstream or downstream of the pH control system to average out pH changes for smoother pH control and less reagent consumption, particularly by upstream volumes where influent pH swings above and below the pH set point.
3. If large volumes exist downstream for smoothing pH fluctuations, a static mixer with a close-coupled reagent injection to the mixer inlet and electrodes in the pipeline about 20 pipe diameters downstream of the mixer outlet is an option for fast pH control to minimize new equipment cost.
4. Use static mixers in the feed or recirculation line of the vessel to premix influent and reagent added to the static mixer inlet to minimize mixing and injection delays.
5. Use impellers designed for axial agitation in vessels where a pH control system injects reagent. Keep the vessel diameter-to-height ratio between 0.5 and 1.5 and the agitator pumping rate to give a turnover time (mixing dead time) significantly less than 6 s.
6. In vessels where a pH control system injects reagent, use baffles in the well-mixed vessels to prevent dead zones and promote an axial agitation flow pattern.