ANC CONTROL

EXPERIMENT 3: Addition of sloping glass column above the lime feeder

OVERVIEW

In order to overcome the difficulties faced at the end of the second experiment, a new design was considered, which consists consisted of a diagonal column attached at the top of the vertical column. The design would help the retain small lime particles while allowing saturated lime -water solution stay inside the apparatus, while still having the needed concentration at the 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. This decreased velocity allows more lime

The angle of inclination and laminar flow regime allows lime particles 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 were determined in MathCad. The two constraints are the tube's length and the terminal velocity of the particle. This terminal velocity should be larger than both the capture velocity and the critical velocity of settling. The length should be large enough to let the flow in the slanting tube to become a fully developed flow; the relevant criteria can be found in our Mathcad file. MathCad file

Calculations were made using the following assumptions for simplification:

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, which 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.

CALCULATIONS ANALYSIS

It was assumed that the smallest particle the tube could capture has the same terminal velocity as the capture velocity, and a longer tube can capture smaller sized particles (The relation is shown in figure 1). The tube length at 1.5m has a capture velocity of 10m/day,and the smallest particle it can capture is 0.00135mm. Lime particles will have a larger density than the flocs, which means that their settling velocities will be higher than the assumed 10m/day. Also, it is not neccesary 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 less than 1.5m.

The relationship between critical velocity and terminal velocity was also calculated, 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, there will be a certain amount of small particles that roll up the tube, which would not happen in this case.

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Figure 1, the relationship between tube length, capture velocity, and the smallest particle diameter the tube can capture.

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lime particles to settle back to the column below, making the process more economical.

The detailed calculations and assumptions regarding the design of the sloped column of the lime feeder can be found here

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Figure 1: Sloping Column Lime feeder

With the new apparatus, as seen in figure-1 above, four sets of trials were carried out and evaluated.
The apparatus was first tested with tap water in trial 1 and trial 2 and then with distilled water in trial 3 and 4.
It had been observed that after a few hours into any experimental run, the lime, instead of remaining in suspension as soluble particles, formed a single mass and became insoluble. It was hypothesized (by the previous research team) that this happened because some or all of the lime got converted into calcium carbonate(which is insoluble)if tap water was used since the water at Cornell is alkaline in nature. This should not be a problem in Honduras because the raw water to be treated is not 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 was used because it has no alkalinity. In the pictures below

The length needed for the pipe in order to obtain a developed laminar flow 'Le', was also calculated and determined to be 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 conclusion, the length of the tube must be greater than Le.

With the new apparatus, as shown in figure-3 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.

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.
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