ANC CONTROL
EXPERIMENT 3: Addition of sloping glass column above the lime feeder and Tube-length Calculations
INTRODUCTION
In order to overcome the difficulties faced at the end of the second experiment, a new design was considered, which consists of a diagonal column attached at the top of the vertical column. The design would retain small lime particles while allowing the saturated lime water to 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.
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Figure 1: Sloping Column Lime feeder
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Figure 2: the relationship between tube length, capture velocity, and the smallest particle diameter the tube can capture.
We use a 1.3m vertical tube connect with 1.5m slanting tube in our experiment. Under our assumption of flow rate, a length of 1.5m has a capture velocity of 0.12 mm/s, and the smallest particle it can capture is 1.35μm. We could cut the slanting tube for saving space but the capture velocity the tube could provide and the smallest particle size it could capture would both decrease, this relationship could also be found on figure 2.
The lime particles used in our experiment have a nonuniform particle density, it cause some particles with less density fall out at the beginning of our experiment, and the ones which have a larger density than the flocs could be kept in suspension. (Do larger particles necessarily have a larger density?) Many of the larger particles have settling velocities higher than the assumed 10m/day(0.12 mm/s), the particle size could be captured with this velocity is 1.35μm as discussed above. (Remind us here how large a particle has to be.) so we think a 1.5m long slanting tube could help to make a good suspension after the initial period which some of the smallest particals washed out with the effluent water. (Read this sentence again. Please proofread your work. Are you talking about length here?)
To measure if the particle has the rollup risk, the relationship between critical velocity and terminal velocity was also calculated. We assume the particle diameter is small compared to the diameter of the tube, and the flow is fully developed, so we can obtain the linearized equation for critical velocity(floc roll up velocity):
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$$
u\left( {d_
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} \over S}{{V_
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$$
As the particle's size increases, terminal velocity becomes much larger than critical velocity, due to the fact that critical velocity is linear with respect to particle diameter but terminal velocity is proportional to the square of the diameter. However, if the slanting tube's diameter decreases, the critical velocity will increase, theres is more risk the floc would roll up, but with our appartus, the 2.4cm inner diameter could prove the roll up would not happen(see figure 3). (Why? Refer to the figure and explain why? Also, you introduce the concept of critical velocity without any background and very little explanation about what it is.)
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Figure 3: Critical velocity vs Capture velocity
With the new apparatus, as shown in figure-4 below, a fourth trial will be carried out and evaluated. The modifications will be tested to see whether or not it will be successful in maintaining the pH at 12 and if so, for how long.
For this trial, distilled water will be used instead of tap water. It has been observed that after a few hours into any experimental run using lime, the lime instead of remaining in suspension as soluble particles, forms a single mass and becomes insoluble. It is hypothesized (by the previous research team) that this happens because some or all of the lime gets converted into calcium carbonate(which is insoluble)if tap water is used since the water received at Cornell is alkaline in nature. This should not be a problem in Honduras because the raw water to be treated will not be as alkaline. However, under laboratory conditions, in order to get a true estimate of the lime feeder's efficiency (in dissolving lime for a longer period and thereby lasting for a longer time) distilled water will prove to be more accurate because it has no alkalinity and has pH that is that of a solution saturated with respect to atmospheric carbon dioxide. In the pictures below, 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: Apparatus for experiment 3
With the design described on Experiment 3, three trials were done:
Trial 1: Using tap water