Fractional crystallization is a stage-wise separation technique that relies upon the liquid-solid phase change and enables the purification of multi-component mixtures, as long as none of the constituents can act as a solvent to the others. Thanks to the level of selectivity that can be achieved in solid-liquid equilibrium, it is ultimately possible to reach high purities for the selected components. This one-page reference provides basic information on fractional solvent-free melt crystallization.
Principle of separation
The crystallization process relies on the partial freezing of the initial liquid mixture by gradually decreasing its temperature. The frozen solid phase exhibits a different chemical composition to the remaining residual liquid. This phenomenon is the basic physical principle behind the fractional melt crystallization process.
Crystals are allowed to grow on a cooling surface, which separates the feedstock from the coolant medium, as polycrystalline layers. The heat released by the solidification process is withdrawn through the crystalline layer or from the bulk. The driving force results from the net effect of temperature and concentration gradients across both solid and liquid phases.
In a hypothetical case where crystallization continued indefinitely, purified products could theoretically be recovered in 100% pure forms. In practice, trade-offs required at industrial scale make it impossible to operate under these ideal conditions. Nevertheless, a number of strategies, such as partial melting of the solid fraction (sweating), address these challenges and help reach extremely high purity levels.
In a fractional crystallization process (Figure 1), the main steps are crystallization, draining, sweating and total melting. During the crystallization step, high-purity crystals are formed on the cooling surface, while the impurities are mostly concentrated in the remaining liquid (draining). The sweating phase then enhances the purification process and end-product quality through partial melting of the impurities entrapped within or between crystals. Finally, the remaining crystallized material (final purified product), can be melted to remove it from the column and continue with downstream activities.
Three different technologies and their respective equipment can be used for fractional melt crystallization:
Falling-film crystallizer. In falling-film crystallization, the crystals grow from an agitated melt (falling-film product) inside tubes that are cooled by a co-current cooling medium flow, which runs on the other side of the tubes. In this case, crystals grow on the inside of the tube from the falling film of melt. Reproducible (and high) transfer rates are achieved on both side of the tube. The resulting shear at the crystal-liquid interface transports impurities rapidly into the bulk of the melt.
This technology is robust and easy to operate. In addition, it can handle high throughputs while achieving extremely high purities. More precisely, a typical feed has concentrations between 90 and 99%, and end products can reach 99.99% purity or greater. Glacial acrylic acid, optical-grade bisphenol A and battery-grade ethylene carbonate are purified to their highest grade using this technology.
Static crystallizer. In static crystallization, crystals are grown from a stagnant melt. Versatility and robustness are intrinsic to this technology, which can purify the most challenging products, including those characterized by high viscosities and high or low melting points. Isopulegol, phosphoric acid, wax and paraffins, anthracene/carbazole and satellite-grade hydrazine are chemicals that benefit from static crystallization processes.
Suspension crystallizer. In suspension crystallization, crystals are generated on a cooling surface wall and then scraped off, so they continue to grow in size within a stirred vessel, in suspension (slurry). The solid-liquid separation of the slurry can be performed either by using a wash column or a centrifuge. More complex to operate than the technologies mentioned previously, suspension crystallization has a key advantage of high separation efficiency. In effect, products with purities of 99.5% and above can be achieved in a single crystallization stage, from feedstocks with concentrations of 75–90%. This can drive considerable energy savings. Para-xylene and halogenated aromatics are purified using this technology, which can also concentrate aqueous feedstocks, such as food and drink concentrates, or wastewater.
Combining these technologies can enhance their respective features. For example, combining falling-film with suspension and static crystallization is common.
Editor’s note: The content for this column was authored by Sulzer Chemtech (www.sulzer.com).