ANC Control

Spring 2010 Hypotheses

Beginning of the Semester

We would like to demonstrate that our lime feeder can run for at least 12 hours with the effluent saturated with dissolved lime (pH 12.6). However, the Fall '09 team was unable to reach to this target with a number of trials. This semester we initially laid out three hypotheses which we believed could account for or contribute to the effluent pH dropping prematurely.

1. The presence of carbonate in the tap water used reacts with the dissolved calcium ions to form a white precipitate which coats the lime particles and inhibits further dissolution.
2. The fluidized bed is not well-maintained by the upflow velocity. When the lime settles into a dense bed at the bottom of the apparatus, preferential flow paths lead to insufficient contact time for full dissolution.
3. Also related to kinetics, the density of lime particles in the fluidized bed may decline as solid lime is lost with the effluent, so that there is not enough available solid lime surface area for the solution to become saturated within the given residence time.

Rejection of Initial Ideas

The tests this semester which used distilled water, which should not contain carbonate ions, still failed prematurely, leading to the rejection of the first hypothesis as the primary reason the feeder does not work. A suitable upflow velocity which maintains a fairly uniform fluidized bed was also found, so that the second hypothesis could be eliminated as the mechanism for failure. Because the new feeder design with a large-diameter upper arm is successful in keeping the particles out of the effluent, the third hypothesis of particle thinning does not likely explain the failure of our experiments, although the true failure mechanism may be related.

Current Thinking

Having eliminated the possibility of chemical interference, we now believe that the problem is related to kinetics. Perhaps the most important variable in determining whether the solution can reach saturation within the reactor's residence time is the surface area over which the water is contacting the solid lime. This is very closely tied to particle size distribution. To maximize available surface area, we would like to maximize the area-to-volume ratio, which is inversely proportional to the particle diameter. In other words, for a given volume of lime, smaller particle sizes yield more available surface area for dissolution.

We now believe that the premature drop in effluent pH we observe may be due to a shift in the particle size distribution in the fluidized bed towards much larger particles, which decreases available surface area enough that reaction kinetics are no longer fast enough to provide a saturated effluent solution. We have come up with three hypotheses for why this could happen:

1. Lime particle flocculation has been observed within the fluidized bed. This has a dramatic effect on contact surface area. We know that it happens within several hours of the addition of lime in our tests, but we have not done enough yet to characterize this process.

2. When the lime slurry is added to the apparatus, there is a broad distribution of particle sizes. The reactor's success at producing saturated effluent initially may be due to the smallest particles in the distribution, which dissolve rapidly. The observed drop in effluent pH may come at the time when the last particles small enough to provide sufficient surface area for saturation have dissolved. The larger, more visible particles which remain may be much more limited in the extent to which they can be dissolved within the residence time of the reactor, so that alone they can only provide an effluent solution orders of magnitude less concentrated than saturation. This would be consistent with the observation that there is always a significant amount of solid lime left over in the reactor, regardless of how long the experiment ran. These large particles would take a very long time to dissolve.

3. The smallest particles, which have the lowest settling velocity, are the most prone to being lost with the effluent. Thus, when solid lime is lost, it contributes to the shift in particle size distribution towards larger particles.

A shift in the particle size distribution leading to insufficient surface area for dissolution is likely some combination of these three phenomena.

Preferential flow paths through the "clearest" water in the reactor may also be a mechanism which contributes to the reactor's failure. For example, when the large apparatus is loaded with 200 grams of lime with a flow rate of 120 ml/min, a significant amount of lime remains suspended in the upper slanted segment, but it settles to the lower side of the tube while a clean stream of water flows up the upper side, avoiding further lime dissolution.