Fluidization Velocity Experiment-Single Media

Purpose

Purpose of this experiment is test the validity of the empirical Fluidization Velocity Equation covered in the literature research section.

Concept of Experiment

In this experiment, we simulated the backwashing of a granular filter with a sand test vial as the bench scale model. Our bench scale model consists of a 5 cm deep sand filter with a diameter of 2.5 cm. The sand has D60 of 0.5mm and porosity of 0.4. The diameter of the flow control orifice is 0.2 cm. Please see figure 1 below. We essentially introduced a backwash flowrate of water of known velocity from the bottom and measured the bed expansion. The attentuator between the pump is there to eliminate the pulsing action of the pump. We measured the bed expansion as we increased the flow rate from 20 mL/min to 380 mL/min, which is the maximum flow rate possible with our pump configuration.

Figure 1: Fluidization Velocity Experiment-Single Media Diagram

Results and Discussion

We plotted the actual fluidization velocity vs the calculated fluidization velocity as the target bed expansion is increased. As expected, higher bed expansion required high fluidization velocity. The difference between calculated and actual increased as the velocity increased. Click here for the Experiment 1 results data.

Figure 2: Calculated Fluidization Velocity vs Actual Fluidization Velocity

We believe the following to be sources of error.
>> Human error: Despite our best attempt at being consistent, there will always be human error in observing the bed expansion visually.
>> 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.
>> Sand Properties Parameters: We might have used an incorrect D60 and porosity for the filter bed in our equations.
>> Preferential flow: Despite our best attempt to keep the test tube as level as possible, we might have introduced preferential flow in our experiment causing an unbalanced backwash flow.

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.