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Can the Residence Time Distribution be too long using Top Entering Mixers:
FLASH OR RAPID MIXER RETENSION TIMES (RESIDENCE TIME DISTRIBUTIONS) THAT ARE TOO LONG.
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 want to begin to grow (coagulation), and if the growing floc is then reintroduced into the shear zone, it disrupts the flocs overall effectiveness. 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 is not the case. Static mixers suffer from the opposite extreme due to too little a mixed residence time, 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. 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 the result. Most plants are then forced to deal with the hand they have been dealt. What confounds the plug-flow concept is that the plug flow device must be designed to handle maximum or peak flow conditions. In most cases, 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, where as the flow is reduced, the mixing efficiencies of plug flow device is also reduced. In other words, at normal flow rates, the mixing efficiency suffers, as you cannot change the laws of physics. To add insult to injury, most plants cannot maintain or guarantee an average or normal flow rate, where additional flow turn-down is then required, which can then becomes down right problematic. To compensate for these flow turndown conditions, you can either add more internal elements or sometimes the flow is split into two (2) separate plug flow streams. The addition of elements requires a greater pressure drop at peak flow and two static mixers, along with the appurtenances, directly add to the bottom line. 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. You might then ask, "How did this 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, for example, where they dumped as much metal and wood within the flash 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:
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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Mixer/Mixing Information |
