Fractional crystallization (chemistry)
Separation technique using liquid–solid phase change for high purity.
Fractional crystallization is a stage-wise separation technique in chemistry that relies on the liquid–solid phase change. It fractionates mixtures via differences in crystallization temperature and enables the purification of multi-component mixtures, provided none of the constituents 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.
- field
- Chemistry
- known_for
- Stage-wise separation technique using liquid–solid phase change
- principle
- Partial freezing of initial liquid mixture; solid phase has different composition than remaining liquid
- advantages
- Purification of close boilers, low thermal stress, inherently safe, solvent-free, emission-free, lower energy consumption than distillation
- process_steps
- Crystallization, draining, sweating, total melting
- technologies
- Falling-film, static, suspension crystallizers
Lore & Background
The crystallization process begins with the partial freezing of the initial liquid mixture by slowly decreasing its temperature. The frozen solid phase subsequently has a different composition than the remaining liquid, a principle comparable to distillation but operating between liquid and solid phases. Crystals grow on a cooled surface or as a suspension in the liquid, with heat released during solidification withdrawn through a cooling surface or via the liquid. In theory, 100% of the product could be solidified and recovered, but in practice strategies such as partial melting of the solid fraction (sweating) are applied to reach high purity levels.
Reader's Guide
Fractional crystallization offers several advantages over other separation technologies. It makes the purification of close boilers possible, allowing very high purities even for challenging components. Because of lower operating temperatures, thermal stress on the product is very low, which is particularly relevant for products that would otherwise oligomerize or degrade. The technology is inherently safe, operating at low pressures and low temperatures, and uses no solvents while being emission-free. Since the latent heat of solidification is 3–6 times lower than the heat of evaporation, energy consumption is much lower compared to distillation. The process involves key steps: crystallization, draining, sweating, and total melting. Three technologies exist: falling-film crystallizers (high separation efficiency, very high purities up to 99.99 wt.-% or greater), static crystallizers (versatile, robust, suitable for challenging products), and suspension crystallization (high separation efficiency, considerable energy savings). Applications include purification of glacial acrylic acid, optical grade bisphenol-A, battery grade ethylene carbonate, isopulegol, phosphoric acid, wax, paraffins, anthracene/carbazole, satellite-grade hydrazine, paraxylene, halogenated aromatics, and aqueous feeds.
Did You Know?
- Fractional crystallization can achieve very high purities, such as 99.99 wt.-% or greater using a falling-film crystallizer.
- The latent heat of solidification is 3–6 times lower than the heat of evaporation, making the process more energy-efficient than distillation.
- The sweating step is a controlled partial melting process that releases impurities trapped within or between crystal structures.
- Fractional crystallization operates at low pressures and low temperatures, making it an inherently safe technology.
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