2. Defining, Crystallization !!!
Crystallization refers to the formation of solid
crystals from a homogeneous solution.
Technique used to purify solid compounds.
Based on the principles of solubility.
Impurities are excluded from the growing crystals
and the pure solid crystals can be separated from the
dissolved impurities by filtration.
A few parts per million may alter the product beyond
recognition.
6. Kinetics and Mechanisms of Crystallization
Crystallization process consists of two stages
Formation of small particles or nuclei;
Growth of the nuclei
If the number of nuclei can be controlled then the
size of the crystal can also be controlled.
7. 1. Crystal Nucleation
The creation of crystalline bodies within a supersaturated
fluid, is a complex, often ill-defined event, and nuclei may
be generated by many different mechanisms
Primary Nucleation—in the absence of crystals
primary nucleation are based on sequences of
bimolecular collisions and interactions in a
supersaturated fluid, which result in the buildup of
lattice-structured bodies that may or may not achieve
thermodynamic stability.
8. Secondary Nucleation—in the presence of crystals
can take place only if crystals of the species under consideration
are already present.
This is mainly the case in working crystallizers
where
B is the rate of secondary nucleation (birthrate),
Kb is the birthrate constant,
MT is the slurry concentration (magma density),
N is a term that gives some measure of the intensity of agitation
in the system (e.g., the rotational speed of an impeller), and
ΔC is the supersaturation.
The exponents j, l, and b vary according to the operating
conditions
9. 2. Crystal Growth
1. mass transport, either by diffusion or convection from
the bulk solution to the crystal face
2. Surface reaction in which the growth units are
integrated into the crystal lattice
where
D is the diffusion coefficient,
v the molar volume,
ΔC the supersaturation, and
L the crystal size; i.e., the diffusion-controlled growth
rate is directly proportional to the degree of
supersaturation
10. 3. Growth–Nucleation Interactions
Crystal nucleation and growth in an
industrial crystallizer cannot be considered in
isolation because they interact with one
another and with other system parameters in
a complex manner.
The rate of growth of a crystal in a solution is
dependent on the temperature and
concentration of the liquid at the crystal face.
13. Designing a Crystallizer
Five main types of information are generally
required to design a crystallization process:
1) Solubility and phase relationships
2) Metastability limits
3) Nucleation characteristics
4) Crystal growth characteristics
5) Hydrodynamics of crystal suspensions
14. Solubility & Phase Relationships
Solubility and phase relationships influence
the choice of crystallizer and method of
operation.
These data must be obtained by using the
materials to be encountered in the plant
because traces of impurity often have a
considerable effect on phase relationships.
16. Nucleation characteristics
The processes of nucleation and growth are
both exceedingly complex
They are influenced greatly by temperature,
supersaturation and impurities.
A knowledge of these system characteristics
is essential in design.
17. Crystal growth characteristics
Specify the type of crystal growth like veiled
growth, Spike wise growth
The crystals are present in large quantities,
settling is hindered;
Further complications can arise if their shapes
are irregular.
18. Hydrodynamics of crystal suspensions
Crystal suspension velocities must also be
known so that liquor circulation rates in
fluidized-bed crystallizers
Agitation rates in stirred vessels can be
specified
22. Desalination of Seawater by Freezing
a) Washer–melter;
b) Scraper;
c) Wash column
d) Screens
e) Decanter
f) Heat exchanger
g) Compressor
h) Crystallizer
