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Can the Residence Time Distribution be too long using Top Entering Mixers:


Optimal Top Entering Flash/Rapid Mixer Tank: 40 Seconds

Believe it or not, having a retention time that is too long is almost never considered, but it is more common that you might think.  Again, the objective of a Flash or Rapid Mixer is to intensely mix the coagulant to form a pin floc, and then move the pin floc on to the primary floc chamber.  If the pin-floc remains within the flash mixing chamber for too long a time, it will begin to grow, and if the growing floc is then reintroduced into the shear zone, due to the excessive retention time, it will disrupt the flocs overall effectiveness, as growing floc does not want to be exposed to elevated levels of fluid shear.

With this stated, you might then consider that a plug-flow static mixer would then be considered superior to continuous-stir-flow-reactors (CSFR), but this couldn't be further from the truth.  Static mixers suffer from the opposite extreme due to too little a mixed residence time (a few seconds only), which definitely influences the overall chemicals efficiency dramatically.  In short, plug-flow is reliant upon instantaneous mixing, where in fact, down-stream mixing, or uncontrolled channel mixing, is vital and dependent for the completion of the reaction as the reaction is not instantaneous.  If you are unable to enforce timely mixing, inadequate mixing will result in either chemical overdosing or the loss of water quality or both.  With channel mixing, since you cannot adequately influence and enforce timely mixing, you have to live with whatever result that you get.  In other words, most plants are then forced to deal with the hand they have been dealt, which might explain why it is common to see static mixers replaced with CSFR's.  

What confounds the plug-flow concept is that the plug flow device must be designed to handle maximum or peak flow conditions, which typically are twice the nominal flow rates.  Maximum or peak flow is rarely realized.  What is generally not understood is that there is a direct relationship between mixing performance and flow rate.  Common sense tells us that when flow is reduced, the mixing efficiencies of plug flow is also reduced.  You cannot change the laws of physics.  In other words, at normal flow rates, the mixing efficiency suffers.  To add insult to injury, most plants cannot maintain or guarantee an average or normal flow rate, where a significant additional flow turn-down is then required.  You might consider this to occur only occasionally however, during the summer months a 4:1 turndown from peak flow can occur for weeks at a time, which can then becomes down right problematic.  To compensate for these flow turndown conditions, most design the system into two (2) separate plug flow streams, as designing additional internal elements in one static mixer increases additional pressure at both nominal and peak flow.  Again, there is no good solution.  The cost of adding elements or the division of streams into two plug flow devices, along with the appurtenances, makes the plug flow option even less attractive.      

Water Treatment Plants, for example, may be in use for decades, where an original flash mixer needs replacement or an upgrade.  At peak or maximum flow, it is not uncommon to see retention times of 2 minutes and greater using alternative and/or outdated designs such as horizontal or vertical rake-type impellers.  The problematic retention time then exacerbates itself under nominal flow conditions (say a 2:1 flow turn-down) at 4 minutes or more.  To compensate, some have divided the flash mixing chamber into two (2) separate chambers, adding a second top entering mixer, where they rotate operation of  mixer-1 and mixer-2 on a year-on-year-off basis.   You might then ask, "How did a 2/4 minute design retention time occur"?  The larger Flash or Rapid Mixer chamber volume was originally required to accommodate an alternative or out-dated design such as a vertical or horizontal paddle mixer design.  The technology of the time was to dump as much metal and wood within the flash mixing chamber as they could to achieve a specified G-factor.  It sounds ludicrous, but selling 1950's technology in the 1980's or 1990's, or even today, is still not all that uncommon.

The following solutions can be applied to enhance the process result using hydrofoil technology in a chamber that is too large, with a residence time that is too long:

Split the flash mixing chamber, with a nominal flow rate retention time of 80 seconds or larger into two separate chambers, installing mixer-1 and mixer-2 into their respective chambers, alternating the service in each chamber on a yearly basis.   

Lower the mixing intensity, by applying a variable speed device to your existing AC motor.  Lowering the intensity may actually increase your yield by lowering the probability of reintroducing growing floc within the impeller shear zone.  

Use zonal mixing.  In the case of having an upward plug flow scheme within the mix chamber, or using an overflow weir, for example, using an impeller only in the lower region of the tank rather than using dual impellers (mixing throughout the tank) is preferred, again due to reintroducing the growing floc into the impeller shear zone.

Add a false top or bottom - A physical barrier will limit the mix volume to the preferred residence time distribution and prevent the effluent from reintroducing itself back into the shear zone.   

Constrain the tank volume by adding or moving the location of one of the  tank walls.

Install a new separate flash mixing chamber prior to the primary and secondary chamber.

In the newly created downstream chamber, some have added a low speed floc mixer, prior to introduction into the primary floc chamber.     

Most or all of these design considerations may or may not be feasible for your specific application.  The alternative to doing nothing with a reduction in process efficiency (floc that will not perform at its optimum).  The result is generally poor water quality with elevated dosing or chemical usage.  


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