Crystallization
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Transcript of Crystallization
Crystallization
What is Crystallization
Formation of solid particles within a homogenous phase
• e.g. formation of solid NaCl from salt solution
• Formation of ice, snow from water vapor
• Natural minerals, e.g. gemstones, diamonds
Significance of study of crystallization:
• Crystals are very pure form of materials
• Crystals formed from impure solution are still pure
• Very practical method of obtaining very high purity
material in satisfactory condition for storage
Snow flakes
Rock salt crystalsNatural diamond crystal
• Magma: two phase mixture of mother liquor and crystals of all sizes
• Mother liquor: the part of a solution left over after crystallization
• Supersaturated solution: solution with concentration greater than equilibrium concentration
• Invariant crystal: the crystal maintains geometric similarity
• Characteristic length: definition of size of crystal
• Overlapping principle: different faces of crystal grow at a different rate and later the shape and appearance of the crystal
– Faces with low velocity survive crystal growth
Terms
sp is surface area of the crystal vp is the volume of the crystal
p
p
s
vL
6
Equilibrium Curve
• Solubility is usually a strong function of temperature, e.g. KNO3, MnSO4
• Most inorganic substances crystallize with water of crystallization– In some systems several hydrates might be formed
• Fundamental driving force for crystallization is change in chemical potential but mostly expressed as concentration driving force
KNO3
Solu
bili
ty
Temperature
NaCl
MnSO4
Solubility-supersolubility diagram
Supersaturation
From solutions
• solution with concentration greater than equilibrium concentration
• Generated using – Temperature change (cooling or heating)
– Evaporating the solvent
– Addition of third component (salting out, precipitation)
• Degree of supersaturation is given by
• Saturation ratio is given by
• Relative supersaturation:
*ccycm
*c
cS
1
Sc
c*
Example
• At 293 K, a supersaturated solution of sucrose contains 2.45 kg sucrose/kg water. If the equilibrium saturation value is 2.04 kg/kg water, what is the supersaturation ratio in terms of kg/kg water and kg/kg solution?
Formation of Crystals
• Rupture of existing crystals
• Nucleation
– Primary nucleation (homogenous nucleation)• Solute molecules clustering to form aggregates or embryos that grow into
nuclei
– Secondary nucleation (heterogeneous nucleation)• Contact nucleation
– Breakage of dendritic growths on the crystals
– Catalytic effect of solid particles in the supersaturated solution
• Fluid shear nucleation
– Sonication, shear due to impellers, etc
Formation of Crystals
• Caking
– Frequently cake or cement together on storage
– Depend on crystal size, shape, moisture content, storage conditions
– Minimized by airtight packing, deposition of inert dust on crystals
• Washing
– To remove mother liquor content and for removal of impurities
– Crystals are separated using filtration, centrifugation
– Cake is washed with another insoluble liquid
– Multiple washing stages might be required
Crystal Growth
Design of crystallizers
• Set supersaturation limit in the crystallizer
• Calculate solution recirculation rate from material balance
• Estimate maximum growth rate assuming cubic crystals for given supersaturation
• Calculate crystal growth for different desupersaturations
• Mass of crystals in suspension and the suspension volume are calculated assuming a value for the voidage which is often about 0.85
• Solution up-flow velocity is calculated for very small crystals– Usually measured exerimentally
• Crystallizer area and diameter are calculated, and height is estimated using (volume of suspension/cross-sectional area)
Equipment
• Cooling Crystallizers
– Agitated vessels
– Scraped surface crystallizers
• Evaporating crystallizers
– Vacuum cooling crystallizers
Equipment
• Cooling Crystallizers
– Agitated vessels
– Scraped surface crystallizers
• Evaporating crystallizers
– Vacuum cooling
– Forced circulation
Forced circulation Swenson crystallizer
Equipment
• Cooling Crystallizers
– Agitated vessels
– Scraped surface crystallizers
• Evaporating crystallizers
– Vacuum cooling
– Forced circulation
– Fluidized bed
Oslo fluidized bed crystallizer