Spring 2010 Hypotheses
Beginning 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.
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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.