Principle of separation
The crystallization process starts with the partial freezing of the initialAdvantages
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 operatingProcess 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 gradeStatic
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,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 *References