(Redirected from Crystallization processes)
'Crystallization' is the (natural or artificial) process of formation of solid
crystals from a uniform
solution. Crystallization is also a chemical solid-liquid separation technique, in which mass transfer of a
solute from the liquid solution to a pure solid crystalline phase occurs.
Process
The crystallization process consists of two major events, '
nucleation' and '
crystal growth'.
''Nucleation'' is the step where the
solute molecules dispersed in the
solvent start to gather into clusters, on the
nanometer scale (elevating solute concentration in a small region), that becomes stable under the current operating conditions. These stable clusters constitute the nuclei. However when the clusters are not stable, they redissolve. Therefore, the clusters need to reach a critical size in order to become stable nuclei. Such critical size is dictated by the operating conditions (
temperature,
supersaturation, etc.). It is at the stage of nucleation that the atoms arrange in a defined and
periodic manner that defines the
crystal structure — note that "crystal structure" is a special term that refers to the internal arrangement of the atoms, not the macroscopic properties of the crystal: size and shape.
The ''crystal growth'' is the subsequent growth of the nuclei that succeed in achieving the critical cluster size. Nucleation and growth continue to occur simultaneously while the supersaturation exists. Supersaturation is the driving force of the crystallization, hence the rate of nucleation and growth is driven by the existing supersaturation in the solution. Depending upon the conditions, either nucleation or growth may be predominant over the other, and as a result, crystals with different sizes and shapes are obtained (control of crystal size and shape constitutes one of the main challenges in industrial manufacturing, such as for pharmaceuticals). Once the supersaturation is exhausted, the solid-liquid system reaches equilibrium and the crystallization is complete, unless the operating conditions are modified from equilibrium so as to supersaturate the solution again.
Many compounds have the ability to crystallize with different crystal structures, a phenomenon called
polymorphism. Each polymorph is in fact a different thermodynamic solid state and crystal polymorphs of the same compound exhibit different physical properties, such as dissolution rate, shape (angles between facets and facet growth rates), melting point, etc. For this reason, polymorphism is of major importance in industrial manufacture of crystalline products.
== Crystallization in
nature ==
Snow flakes are a very well known example, where subtle differences in ''crystal growth'' conditions result in different
geometries.
There are many examples of natural process that involve crystallization.
Geological time scale process examples include:
★ Natural (mineral)
crystal formation (see also
gemstone);
★
Stalactite/
stalagmite, rings formation.
Usual time scale process examples include:
★
Snow flakes formation (see also
Koch snowflake);
★
Honey crystallization (nearly all types of honey crystallize).
Artificial methods
For crystallization to occur the solution must be
supersaturated. This means that the solution has to contain more
solute entities (molecules or
ions) dissolved than it would contain under the equilibrium (saturated solution). This can be achieved by various methods, with 1) solution cooling, 2) addition of a second solvent to reduce the solubility of the solute (technique known as anti-solvent or drown-out), 3) chemical reaction and 4) change in pH being the most common methods used in industrial practice. Other methods, such as solvent evaporation, can also be used.
'Applications:'
There are two major groups of applications for the ''artificial crystallization'' process: ''crystal production'' and
purification.
Crystal production
From a
material industry perspective:
★ Macroscopic crystal production, for supply the demand of natural-like crystals with methods that "accelerate time-scale" for massive production and/or
perfection:
★
★
ionic crystal production;
★
★
covalent crystal production.
★ ''Tiny size crystal''s:
★
★
Powder,
sand and smaller sizes: using methods for
powder and
controlled (nanotechnology fruits) forms.
★
★
★ ''Mass-production'': on
chemical industry, like salt-powder production.
★
★
★ ''Sample production'': small production of tiny crystals for
material characterization.
Controlled recrystallization is an important method to supply unusual crystals, that are needed to reveal the molecular structure and nuclear forces inside a typical molecule of a crystal. Many techniques, like
X-ray crystallography and
NMR spectroscopy, are widely used in chemistry and biochemistry to determine the structures of an immense variety of molecules, including inorganic compounds and
bio-macromolecules.
★
★
Thin film production.
Massive production examples:
★
"Powder salt for food" industry;
★
Silicon crystal wafer production.
★ Production of sucrose from sugar
beet, where the sucrose is crystallized out from an aqueous solution.
Purification
Well formed crystals are expected to be pure because each molecule or ion must fit perfectly into the lattice as it leaves the solution. Impurities would normally not fit as well in the lattice, and thus remain in solution preferentially. Hence, molecular recognition is the principle of purification in crystallization. However, there are instances when impurities incorporate into the lattice, hence, decreasing the level of purity of the final crystal product. Also, in some cases, the solvent may incorporate into the lattice forming a ''solvate''. In addition, the solvent may be 'trapped' (in liquid state) within the crystal formed, and this phenomenon is known as ''inclusion''.
Thermodynamic view
The nature of a crystallization process is governed by both thermodynamic and kinetic factors, which can make it highly variable and difficult to control. Factors such as impurity level, mixing regime, vessel design, and cooling profile can have a major impact on the size, number, and shape of crystals produced.
Now put yourself in the place of a molecule within a pure and ''perfect crystal'', being heated by an external source. At some sharply defined
temperature, a bell rings, you must leave your neighbours, and the complicated architecture of the crystal collapses to that of a liquid. Textbook thermodynamics says that melting occurs because the
entropy, S, gain in your system by spatial randomization of the molecules has overcome the
enthalpy, H, loss due to breaking the crystal packing forces:
This rule suffers no exceptions when the temperature is rising. By the same token, on cooling the melt, at the very same temperature the bell should ring again, and molecules should click back into the very same crystalline form. The entropy decrease due to the ordering of molecules within the system is overcompensated by the thermal randomization of the surroundings, due to the release of the heat of fusion; the entropy of the universe increases.
But liquids that behave in this way on cooling are the exception rather than the rule; in spite of the second principle of thermodynamics, crystallization usually occurs at lower temperatures (supercooling). This can only mean that a crystal is more easily destroyed than it is formed. Similarly, it is usually much easier to dissolve a perfect crystal in a solvent than to grow again a good crystal from the resulting solution. The nucleation and growth of a crystal are under kinetic, rather than thermodynamic, control.
Equipment for Crystallization
1. ''Tank crystallizers''. Tank crystallization is an old method still used in some specialized cases. Saturated solutions, in tank crystallization, are allowed to cool in open tanks. After a period of time the mother liquid is drained and the crystals removed. Nucleation and size of crystals are difficult to control. Typically, labor costs are very high.
2. ''Scraped surface crystallizers''. One type of scraped surface crystallizer is the Swenson-Walker crystallizer, which consists of an open
trough 0.6m wide with a semicircular bottom having a cooling jacket outside. A slow-speed spiral
agitator rotates and suspends the growing crystals on turning. The blades pass close to the wall and break off any deposits of crystals on the cooled wall. The product generally has a somewhat wide crystal-size distribution.
3. ''Double-pipe scraped surface crystallizer''. Also called a ''votator'', this type of crystallizer is used in crystallizing
ice cream and plasticizing
margarine. Cooling water passes in the
annular space. An internal
agitator is fitted with spring-loaded scrapers that wipe the wall and provide good heat-transfer coefficients.
4. ''Circulating-liquid evaporator-crystallizer''. Also called ''Oslo crystallizer''. Here
supersaturation is reached by evaporation. The circulating liquid is drawn by the screw pump down inside the tube side of the condensing stream heater. The heated liquid then flows into the vapor space, where flash evaporation occurs, giving some
supersaturation.The vapor leaving is condensed. The supersaturated liquid flows down the downflow tube and then up through the bed of fluidized and agitated crystals, which are growing in size. The leaving saturated liquid then goes back as a recycle stream to the
heater, where it is joined by the entering fluid. The larger crystals settle out and
slurry of crystals and mother liquid is withdrawn as a product.
5. ''Circulating-magma vacuum crystallizer''. The magma or suspension of crystals is circulated out of the main body through a circulating pipe by a
screw pump. The magma flows though a heater, where its temperature is raised 2-6 K. The heated liquor then mixes with body slurry and
boiling occurs at the liquid surface. This causes
supersaturation in the swirling liquid near the surface, which deposits in the swirling suspended crystals until they leave again via the circulating pipe. The vapors leave through the top. A steam-jet
ejector provides
vacuum.
References
★ Glynn P.D. and Reardon E.J. (1990) "Solid-solution aqueous-solution equilibria: thermodynamic theory and representation". Amer. J. Sci. 290, 164-201.
★
★ Geankoplis, C.J. (2003) "Transport Processes and Separation Process Principles". 4th Ed. Prentice-Hall Inc.
External links
★
Industrial Crystallization
See also
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