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{{crystallization In chemistry, fractional crystallization is a stage-wise separation technique that relies on the liquid–solid phase change. This technique fractionates via differences in crystallization temperature and enables the purification of multi-component mixtures, as long as none of the constituents can act as solvents to the others. Due to the high selectivity of the solid–liquid equilibrium, very high purities can be achieved for the selected component.


Principle of separation

The crystallization process starts with the partial freezing of the initial
liquid Liquid is a state of matter with a definite volume but no fixed shape. Liquids adapt to the shape of their container and are nearly incompressible, maintaining their volume even under pressure. The density of a liquid is usually close to th ...
mixture by slowly decreasing its temperature. The frozen solid phase subsequently has a different composition than the remaining liquid. This is the fundamental physical principle behind the melt fractionating process and quite comparable to distillation, which operates between a liquid and the gas phase. The
crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
s will grow on a cooled surface or alternatively as a suspension in the liquid. The heat released by the solidification process is withdrawn through a cooling surface or via the liquid. In theory, 100% of the product could be solidified and recovered. In practice, various strategies such as partial melting of the solid fraction (sweating) need to be applied in order to reach high purity levels.


Advantages

Fractional crystallization has various advantages over other separation technologies. First of all, it makes the purification of close boilers possible. This allows for very high purities even for challenging components. Furthermore, because of the lower operating
temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
, the thermal stress applied to the product is very low. This is in particular relevant for products that would otherwise oligomerize or degrade. Next, fractional crystallization is usually an inherently safe technology, because it operates at low pressures and low temperatures. Also, it does not use any
solvents A solvent (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for p ...
and is emission-free. Finally, since the latent heat of solidification is 3–6x lower than the heat of
evaporation Evaporation is a type of vaporization that occurs on the Interface (chemistry), surface of a liquid as it changes into the gas phase. A high concentration of the evaporating substance in the surrounding gas significantly slows down evapora ...
, the energy consumption is – in comparison to distillation – much lower.


Process steps

Fractional crystallization involves several key steps: # Crystallization: This is the initial phase where the material to be purified is cooled. As it cools, high-purity crystals begin to form on the cooling surface. The purity is achieved because the impurities tend to remain in the liquid phase rather than being incorporated into the crystal structure. # Draining: After the formation of the crystals, the next step is to remove the residual liquid that contains a higher concentration of impurities. This process of draining helps to separate the pure crystals from the impure liquid. # Sweating: This phase is a controlled partial melting process. It further purifies the product by melting only a small portion of the crystal. The melting causes the impurities trapped within or between the crystal structures to be released and separated. # Total Melting: In the final step, the remaining crystallized material, which is now the purified product, is completely melted. This total melting facilitates the removal of the pure substance from the crystallization equipment and prepares it for downstream processing.


Crystallizers

There are three differenct fractional crystallization technologies available:


Falling-film

In the falling-film crystallizer, crystals grow from a melt that forms a thin film along the inside of cooled tubes. A concurrent cooling medium flows on the outside of these tubes. This arrangement allows for reproducible and high transfer rates of heat, facilitating the growth of crystals from the falling film of melt. The solid–liquid separation of the resulting slurry can be accomplished using a wash column or a centrifuge. This technology is more complex than others but offers the advantage of high separation efficiency and very high purities. A typical feed has concentrations between 90–99%, which is purified up to 99.99 wt.-% or greater. For example, glacial acrylic acid, optical grade
bisphenol-A Bisphenol A (BPA) is a chemical compound primarily used in the manufacturing of various plastics. It is a colourless solid which is Solubility, soluble in most common organic solvents, but has very poor solubility in water. BPA is produced on a ...
and battery grade ethylene carbonate can be purified to their highest grade using a falling-film crystallizer.


Static

The static crystallizer allows crystals to grow from a stagnant melt, making it a versatile and robust technology. It can purify highly challenging products, including those with most challenging properties, such as high viscosities and high or low melting points. Examples of applications include isopulegol,
phosphoric acid Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, ...
, wax and paraffins, anthracene / carbazole and even satellite-grade
hydrazine Hydrazine is an inorganic compound with the chemical formula . It is a simple pnictogen hydride, and is a colourless flammable liquid with an ammonia-like odour. Hydrazine is highly hazardous unless handled in solution as, for example, hydraz ...
.


Suspension

In suspension crystallization, crystals are generated on a cooling surface and then scraped off to continue growing in size within a stirred vessel in suspension or slurry. The solid–liquid separation is performed either through a wash-column or a centrifuge. This method is more complex to operate, but offers the advantage of a high separation efficiency, which translates to considerable engery savings. Examples of applications include paraxylene, halogenated aromatics, and also aqueous feeds.


See also

* Cold Water Extraction * Fractional crystallization (geology) * Fractional freezing * Laser-heated pedestal growth * Pumpable ice technology *
Recrystallization (chemistry) Recrystallization is a broad class of List of purification methods in chemistry , chemical purification techniques characterized by the dissolution of an impure sample in a solvent or solvent mixture, followed by some change in conditions that en ...
* Seed crystal *
Single crystal In materials science, a single crystal (or single-crystal solid or monocrystalline solid) is a material in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no Grain boundary, grain bound ...


References


"Small Molecule Crystalization"
( PDF) at
Illinois Institute of Technology The Illinois Institute of Technology, commonly referred to as Illinois Tech and IIT, is a Private university, private research university in Chicago, Illinois, United States. Tracing its history to 1890, the present name was adopted upon the m ...
website *
Fractional Solvent-Free Melt Crystallization
a
Chemical Engineering
website
Sulzer Fractional Crystallization Technologies
C. A. Soch, Fractional Crystallization, The Journal of Physical Chemistry 1898 2 (1), 43-50; DOI: 10.1021/j150001a002 Fractionation Phase transitions Methods of crystal growth