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1. The presence of carbonate ions in the tap water used reacts 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.
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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.