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.

Hh

Ll

p}

Optional Technology >99%

*

Ll

Ff

Jkljui

,hki

Mhj