Crystallization is the process by which a homogeneous solution yield solid particle. This is done by concentrating the solution to saturation state. It is technique used in obtaining pure chemical substances for production, packaging and storage. Industrially, crystallization can serve as a purification process. It is worthy of note that almost all chemical processes adopts this technique in production.
The solid substance is called crystal in its pure state. Crystals are organized array of atoms, molecules and ions in 3D lattice. These arrangements are detected with the aid of X-ray diffraction. In accumulation of crystal from one stage to another, a fixed polyhedron is maintained and this is called invariant crystal. This accumulation must be without hindrance.
Crystal growth is diffusion process. It is done on the solid surface through a liquid medium. There is an exchange of ions between the solid surface and crystal in order to build a lattice. Crystal growth can be in DIFFUSION or interfacial stage but must occur at supersaturated condition. The equation for mass transfer can be given as
NA = molar flux, m = rate of mass transfer (mol/h), Sp = crystal surface area, Ky =mass transfer coefficient and (y-y’) = driving force for mass transfer( y in concentration).
Growth rate (G)
Where ρm =the molar density.
Factors affecting crystallization
- Degree of saturation
- Population density and size of crystal
- Agitation intensity
Crystallization equipment and processes
Crystallization equipment is called crystallizers. They are classified according to the precipitation processes:
- By evaporation of a solution
- By adiabatic evaporation and cooling
- By cooling hot concentrated solution
Crystallizers can be operated continuous or batch process. It is advice to operate on continuous process. For commercial operations, crystallizers must be operated on super-saturated condition since it is at this condition, crystallization can occur. Super saturation can be obtained depending on the solubility curve of the solution. It can be of the following ways: cooling, evaporation and intermediate conditions.
Crystallization process is guided by both thermodynamic and kinetic properties. These properties make the process difficult to handle or control. Example is the entropy of the system. As the temperature increases the entropy increases, thus, causing melting of the liquid. This state continues until a state of randomness. When the temperature is reduced, the molecules tend to come to rest.
Forms of crystallizers
- Vacuum crystallizer: this helps achieve adiabatic evaporative cooling used to create super saturation. The saturated solution is injected at the system pressure and temperature above boiling point. The magma volume is checked by controlling the liquid and crystallizing solid. The solution formed is is allowed to cool to the EQUILIBRIUM temperature. Then the magma as product is withdrawn at the bottom. The theoretical yield of crystal is directly proportional to the difference between the feed concentration and solubility of solute at equilibrium temperature.
- Draft tube crystallizer
- Baffle crystallizer
This is the clustering of the molecules of magma to form a defined and stable solid crystal in a unit time. This crystal formation takes place in two steps: nucleation and growth of macroscopic size. Nucleation gives a defined crystal structure. It can be primary or secondary.
Primary nucleation: this is the stage where there is no crystal formed. Example is the formation of fog. It could be homogeneous or heterogeneous.
- Homogeneous type: in the absence of precipitation reaction, homogeneous nucleation occurs. There is no influence of any wall or impurity in the system. It follows the sequence below:
Cluster=embryo=nucleus= and crystal
- Heterogeneous type: this is when a solid such catalyst influence the rate of nucleation; the formation of nucleus is reduced.
This is the formation of nuclei attributable to the influence of an existing macroscopic crystal in the magma. It can still be defined as when a nucleation process is initiated by the presence or contact with other crystal in the magma. It is grouped into fluid shear and contact nucleation.
- Fluid shear: this is when a supersaturated solution is transferred over a growing crystal surface at a substantial velocity, due to the boundary shear force, sweeping away nuclei that would be incorporated into the crystal, thereby making the swept away crystal appear as new crystal.
- Contact nucleation: mostly, it is employed in the industrial crystallizers. It occurs at low super saturation condition. this enhances good growth rate of crystal. It has a less striking energy at the surface of a crystal.
From chemical kinetics, the rate of nucleation is given as
Where; Na is the Avogadro’s number
R &C are gas constant and frequency factor
C is proportional to the concentration of individual particles an collision rate with embryo at the critical size in required nucleus.
W.L. Mc Cabe, J.C smith (2005); unit operation of chemical engineering, seven editions. Mc graw-Hill. New your.
Alan foust et.al. Principle of unit operation second edition (1980); John Willey & sons