Wednesday, 4 June 2008

Design Errors: Static Mixers

A quiet sort of a week in the main. Cyril continues his research into treatment of PCBs in groundwater. I have been engaged with the pharmaceutical client mentioned previously on here.

They have what must be one of the worst effluent treatment plants I have ever seen in terms of fundamental design flaws. One of the biggest of them was the use of static mixers to mix acid and coagulant with effluent, which was pumped by diaphragm pumps. This showed a fundamental misunderstanding of the nature of static mixers in the system designer. Static mixers blend fluids across the body of the mixer. If you feed them with a pulsating flow from a diaphragm pump, and an out-of synch. pulsating flow from a dosing pump, you end up with constantly varying degrees of mix in the outlet flow. The normal rule of sampling at least 10 pipe diameters downstream to get 95% degree of mix no longer applies. Your pH probe for example will see extreme variation in measures pH, and instead of gaining control to +/- 0.1pH units as you should from a system like this, you will be lucky to see +/- 0.5 pH units.

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