Fluidization Velocity Experiment-Single Media

Purpose

The purpose of this experiment is to compare the experimental results to the empirical fluidization velocity model covered in the literature research section (Walter J. Weber).

Concept of Experiment

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, (b)). 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, (a)). 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 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 measured fluidization velocity vs the empirically predicted 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.


Figure 2: Predicted Fluidization Velocity vs Measured Fluidization Velocity
We believe there are two main sources for the error (more of which are discussed on the main on the Clear Well Filtration Page).
#1)Wall Friction: We can attribute the increase in error as flow rate increased due to the increase in wall friction on the test vial. We can minimize this by increasing the size of our bench scale experiments.
#2) Sand Properties Parameters: We might have used an incorrect D60 and porosity for the filter bed in our equations. We tested this in our Mathcad code and found that if we increased the porosity from 0.4 to 0.5, the graph changed to Figure 3, thereby decreasing the error at %30 expansion to only 30% error:


Figure 3: Fluidization Velocity Experiment-porosity change
Then we also found that if we instead increased the d60 from 0.55 to 0.9, the graph changed to Figure 4, thereby decreasing the error at 30% expansion to only 16.7% error:


Figure 4: Fluidization Velocity Experiment-d60 change
This shows that small errors in our d60 or porosity term could easily account for the major errors we have. Therefore, future experiments should carefully measure these properties before conducting experiments.

The above sources of error will be very difficult to control for the actual filtration design. Consequently, we surmise that we need to apply a safety factor of around 10-30% when applying the empirical fluidization velocity equation. The follow up experiments for multi-media experimentation with larger bench scale model will further specify the safety factor required and we expect the larger scale model to reduce the overall error.