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In this experiment, a bench-scale granular filter was backwashed. Our bench scale model consists of a 5 cm deep sand filter with a diameter of 2.5 cm (see Figure 1, left image). The sand (classified as D60) has a diameter of 0.5mm and porosity of 0.4. The diameter of the flow control orifice is 0.2 cm. We essentially introduced a backwash flowrate of water of known velocity from the bottom and measured the bed expansion. An attenuator, or small tank filled with water, was installed between the pump and the filter to eliminate the pulsing action of the pump (see Figure 1, right image). The bed expansion flow rate was increased from 20 mL/min to 380 mL/min (0.4-9 mm/s) (380 mL/min being the max rated flowrate for our bench scale pump configuration). The fluidization velocity is based on the ratio of original filter depth to expanded filter depth, so the velocity doesn't change with a larger depth.
Figure 1: (a) Fluidization Velocity Experiment
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Set-Up (b) 5 cm Filter Bed of Sand in Tube Unexpanded & Expanded
Results and Discussion
Our target expansion was 30% expansion, and we found to achieve this, the flow rate had to be 340 ml/min, or 8 mm/s. However, at this flow rate, the error between calculated and experimental was 110%.
We plotted the experimentally found fluidization velocity vs the calculated fluidization velocity as as target bed expansion was increased (see Figure 2). As expected, higher bed expansion required high fluidization velocity. However, the difference between calculated and actual velocity increased as the flow rate increased. therefore, the experimental and the calculated data had a roughly direct relationship with calculated data having a steeper slope.
Experiment results data.
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