ANC CONTROL

Spring 2010 Mechanisms and 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 ions in the tap water leads to a reaction with the dissolved calcium ions to form a white precipitate which coats the lime particles and inhibits further dissolution. This may also be thought of in terms of a replacement reaction in which hydroxide is exchanged for carbonate on the surface of the solid lime. Then the surface of the particles becomes much less soluble and the lime contained within them is effectively lost. The team is unsure of the specific chemical mechanism at work here.
2. Upflow velocity is the vertical component of the flow velocity. The settling velocity of the particles must be balanced with the upflow velocity in order for particles to remain in suspension. If the fluidized bed is not well-maintained by the upflow velocity in the jet at the entrance to the vertical column, the lime settles into a dense bed at the bottom of the apparatus where preferential flow paths lead to insufficient contact time for full dissolution.
3. Also related to kinetics, the concentration of solid lime particles in the fluidized bed (that is, the volume of solid lime per volume of water) 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 reactor's residence time.

Evaluation and Evolution of Initial Ideas

The test this semester which used distilled water, which should not contain carbonate ions, still failed prematurely, leading the team to believe that the first hypothesis is not the primary reason the feeder does not work. See Experiment 2. Without carbonates, there should be no calcium carbonate precipitation, so another mechanism must have caused the failure. However, the single experiment was not enough to eliminate the carbonates hypothesis as a complicating factor in the lime feeder function.

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Lastly, because the new feeder design with a large-diameter upper arm has a capture velocity sufficiently low to keep the majority of particles out of the effluent, and there is always a large amount of solid lime remaining in the suspension, the third hypothesis of particle thinning does not likely explain the failure of our experiments, although the true failure mechanism may be related, since the smallest particles are carried out under most conditions.

Particle Size

Having demonstrated in experiment 2 that the lime feeders can fail even in the absence of carbonates, the team believes that a significant part of 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 total 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.

Particle Coalescence

In the "Particle Size" section above it was noted that flocculation of the particles in the fluidized bed was observed. This phenomenon of lime particles sticking together was seen again with the particles sliding down the bottom of the tube settler. The team observed a large solid buildup in the smaller reactor which plugged the branched pipe segment except for some small flow paths. It was thought that the initial buildup occurred on the rim in the pipe connection and that additional settling lime added to the formation of the aggregate solid. This behavior has only been seen when the lime is broken up with a blender prior to the experiment, so the tendency of the particles to stick together seems to be related to some extent to particle size. Other surface properties and chemistry may also play a role, but this is not well understood. Clearly, just as the formation of flocs decreases the surface area available for dissolution, the formation of a solid block of lime is detrimental to the performance of the reactor.

Preferential Flow Paths

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.

Chemistry

The carbonate replacement reaction on the surface of the lime particles, mentioned above in the "Beginning of the Semester" section, is the primary chemical mechanism believed to be related to the reactor's failure. Further controlled testing with distilled water should provide further insight into the importance of this phenomenon. In a supplemental test, it was found that even in an effluent sample well below saturation concentration with significant suspended solids the pH decreases over time, which suggests that the leftover solids are largely incapable of dissolution.