Crystallizer Growth and Nucleation Rates
Use this window to define the rate equations for the crystal
growth rate and nucleation rate.
Usage
Crystal Growth Rate
You must supply the Rate Constant for the rate equation in
ft/sec or m/sec. Growth rates are typically in the range 2E-7 to
2E-8 m/sec. By default, the rate is directly proportional to S, the
Supersaturation Ratio. You may change this by overriding the
default Exponential Factor, Gexp. Factors are usually in the range
0.0 to 2.5, ans has a default value of 1.0.
Crystal Nucleation Rate
The Nucleation Rate RG is the number of crystals nucleated per
unit time per unit liquid volume.
RG = KG SGexp
You must supply the Rate Constant KG for nucleation and specify
its dimensional units. By default, the rate is directly
proportional to the Supersaturation Ratio, S,
where S = (X - Xeq)/ Xeq.
X is the mole fraction of solute in the liquid and Xeq is the
mole fraction of solute in the liquid at crystallizer temperature.
Exponent Gexp has a default value of 1.0.
You may change this by overriding the default Exponential
Factors. Typical values for the Supersaturation Ratio Factor are in
the range 0.5 to 2.5 for secondary nucleation and up to 10 for
primary nucleation. If an exponent is specified for the Impeller
Speed, you may need to change the default value of 100 RPM.
CRYSTALLIZATION FROM SOLUTION
1. The feed to a crystallizer should be slightly
unsaturated.
2. Complete recovery of dissolved solids is obtainable by
evaporation,
but only to the eutectic composition by chilling.
Recovery by melt crystallization also is limited by the
eutectic
composition.
3. Growth rates and ultimate sizes of crystals are controlled
by
limiting the extent of supersaturation at any time.
4. Crystal growth rates are higher at higher temperatures.
5. The ratio S C=Csat of prevailing concentration to
saturation
concentration is kept near the range of 1.021.05.
6. In crystallization by chilling, the temperature of the
solution is
kept at most 128F below the saturation temperature at the
prevailing concentration.
7. Growth rates of crystals under satisfactory conditions are
in
the range of 0.10.8 mm/hr. The growth rates are
approximately
the same in all directions.
8. Growth rates are influenced greatly by the presence of
impurities
and of certain specific additives that vary from case to
case.
9. Batch crystallizers tend to have a broader crystal size
distribution
than continuous crystallizers.
10. To narrow the crystal size distribution, cool slowly through
the
initial crystallization temperature or seed at the initial
crystallization
temperature.
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