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Trial 1 - Fluidizing the Lime Bed
Trial 2 and 3:
In these trials, Process Controller was utilized to automate the experiment. In addition, the apparatus was equipped with a pH probe whose values could be logged into the computer via the Process Controller.
With those additions, the team measured the pH of the effluent, looking for the optimal flow rate that could maintain 12 pH units for 24 hours.
In trial 2, with a flow rate of 40mL/min, the pH dropped from 12 to 11 after about 3.5 hours. In trial 3, with an increased flow rate of 60mL/min and keeping every other parameter constant, no significant changes were noted.
The results of the experiment are on an attached excel file.
<LINK> Trial 2 and 3 on Column experiment
Figure - 1
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1) The arrangement of the pH probe was not stable so the pH data was not accurate.
2) The simple column design was not adequate in giving any range in terms of flow rates that could fulfill the objectives of maintaining a good suspension and maintaining a stable pH for 24 hours on the effluent.
3) The vertical column was not enough to store the required amount of lime to run the experiment for 24 hours.
These problems prompted for a modification in the design of the column leading to the next stage of experiments.
By the end of stage two, changes were made (following Professor Monroe's suggestions) in the design of the water outlet in order to ensure an adequate pH measure. It included putting the pH probe in a vertical tube and making a curved outlet for the effluent. With these changes, the apparatus could get a more stable pH measure. A figure explaining this description is shown below.
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On teh second stage, the team also worked on the relationship between the ratio of flow rate between the lime feeder and the plant with the alkalinity provided by the effluent lime. That information was useful to edit MathCAD calculations and to determine the changes in the Ph at the plant with respect to changes in this flow rate ratio (fraction of water flow rate entering the lime feeder from the entrance tank).
STAGE THREE - Redesign
In order to overcome the difficulties faced at the end of stage two, the team considered a new design, which consists of a diagonal column attached at the top of the vertical column. The design would help the saturated lime-water solution stay inside the apparatus, while still having the needed concentration at the exit. Since the velocity in the slanted tube is affected by the angle, its vertical component is lower than the upflow velocity of the primary column. This decreased velocity allows more lime to settle back into the column and thus prevent unnecessary lime loss. Thus the primary column would be used as a storage vessel for the suspended lime bed while the slanted tube above it would allow for a more uniform saturated lime mixture.
The dimensions of apparatus are determined according to MathCad calculations; the relevant criteria being the length of both columns. Below is the link for the mathCAD file:
The team worked on calculations using the following assumptions for simplification:
- Dfrac = 3: When elementary lime particles coagulate, the density of the larger mass stays the same as that of the original particles. This is unlike what happens in flocs, that have a Dfrac of 2.3.
- Density of lime is 2.211 g/m^3: Particles are uniform.
- Shape Factor of lime particles = 1: The lime particles are perfectly spherical.
- Settling velocity = 10 m/day: Given a flow rate of 80 mL/min (as determined by Trial 1). This velocity corresponds to the finer lime particles that have a diameter of about 1 micrometer.
Results of MathCAD calculations: Theoretically, the maximum length of the tube should not exceed 1.5m. Lime particles will have a larger density than the flocs, which means their settling velocities will be higher than the assumed 10m/day. Also, it is not necesary that ALL lime particles settle down - some amount (not determined yet) will have to fall out of the lime feeder to solve the acidity problem. Consequently, the length of the tube needed will be lower than 1.5m.
The length needed for the pipe in order to obtain a developed laminar flow 'Le', was also calculated and determined in 10cm with the given (above) conditions. This is required to verify whether or not there is a parabolic profile at the end of the pipe. In conclussion, the length of the tube must be greater than Le.
With the new apparatus, as shown in figure-3 below, the team will carry out a fourth trial, and will make the required arrangements for more experiments, checking to see if the modification will be successful in maintaining the pH at 12 and if so, for how long.
Figure - 3 : the picture of new apparatus will be taken this week and put up (since we will get the constructed apparatus only by 10/27/09).
For this trial, distilled water will be used instead of tap water. In the picture below (figure 4), the ANC Control team can be seen carrying the distilled water tank on to the platform where the experiment is to be set up.
Figure - 4 
Experiment 3
FUTURE TASKS
- Is is required to develop several trials with the new enmseble. The team will determine the amount of lime required to maintain a pH of 12 for 24 hours. In addition the team will evaluate the performance of the new emsemble, if it is necessary, the length will be calculated again.
- Simulate Honuran water conditions. Once the team will have the design working well with distilled water, a new experiment will be required to analyze the performance with a lower pH concentration.
- The team is also working on the calculation of required changes according with the natural cariation of flow rate at the inlet tank in the plant.
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