| Summary: | 碩士 === 立德大學 === 資源環境研究所 === 96 === In this study, fluidized-bed crystallizer (FBC) was applied to crystallize the
wastewater containing KH2PO4 by adding CaCl2. The optimal operation conditions of
FBC were investigated under controlling the concentrations of KH2PO4 and CaCl2
within the range of metastable region. The recirculation flow rates were varied and the
operation time was lengthened to further investigate the effect of hydrodynamics on
growth rate of crystal.
The experimental results showed that the best crystallization conditions were
illustrated as follows: pH = 10, Ca/P = 1, Ca = 10 mg/L, P = 7.8 mg/L, influent flow
rate = 4.8 L/hr, recirculation flow rate = 2.4 L/hr, surface loading rate = 0.916
Kg-P/m2-hr. The results of a continuous-flow 168-hours experiment indicated that the
crystallization rate reached the value 89.08 %. Furthermore, with a greater
recirculation flow rate, the greater the specific energy dissipation rate (ωs) is, but the
smaller the crystallization rates (G) is. Another observation phenomena was found
that the greater specific weight, was the pellet the lower part the pellet would move,
and the pellet should be drawn out of when their crystallization amount reaches a
certain utmost value. By the study of EDS (qualitative and quantitative analysis of
element for crystal) and the SEM observation of crystal surface, the result shows that
the phosphate content in the crystal was about 13.316 %. The appearance of the
crystal phase on the bed top appeared more round and loose, while appearing more
needle-like at the bed bottom. The correlated mathematical model among G, relative
saturation level (σ), and specific energy dissipation rate in the FBC was finally
established: 048 . 1 501 . 0 263 . 2 03 291 . 6 r L E G
s
− −
− = ω .
From the model of the growth rate for crystallization, the conclusion can be
III
made that G will decrease as L or ωs increases, but will increase as r
increases. The correlations can be applied in estimating the growth of crystal
operation, and design of the fluidized-bed crystallizer.
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