Theory

The technical feasibility and economic viability of separation and purification of an organic mixture by melt crystallization entirely depend on its solid-liquid phase equilibrium (SLE). A survey of SLE data in 1986 demonstrated that more than 70% of organic substances have melting points between 0°C and 200°C, indicating that no special thermal media are needed to achieve separation (Figure 1).

The classification of about 1500 binary systems in the International Critical Tables (Figure 2) has revealed that more than 50% form simple eutectic mixtures and 25% form an intermolecular solid compound which forms simple eutectics with the two constituent components. This type also includes the formation of more than two solid compounds. The formation of a pure solid phase in one crystallization step is feasible for roughly 85% of the systems. The formation of solid-solutions is limited to only 10%, for which complete separation in a single crystallization step is fundamentally impossible.

Even for eutectic systems, however, the product from industrial crystallizers is not 100% pure. This is caused by the adherence of mother liquor to the crystal surface and by the inclusion/entrapment of mother liquor inside the crystals. Further purification is required and this can be achieved by washing and by sweating.

• Washing is done by contacting the crystals with a fresh solvent or, preferably, with pure melted product. This operation removes adhering impurities and is widely used in commercial plants.

• Sweating can be induced by exposing the impure solid phase to slightly elevated temperatures. This operation removes locally dispersed included impurities. Upon a temperature increase, the fraction of liquid inside the crystals increases according to the lever-rule whereby the more impure sections

173 273 373 473 573 Melting temperature (K)

Figure 1 Distribution of atmospheric melting points of organic components (after Matsuoka and Fukushima, 1986).

173 273 373 473 573 Melting temperature (K)

Figure 1 Distribution of atmospheric melting points of organic components (after Matsuoka and Fukushima, 1986).

Figure 2 Distribution of binary phase diagrams of organic mixtures (after Matsuoka and Fukushima, 1986).

melt preferentially due to a lower melting point. The impure liquor comes out of the crystalline body through cracks and pores. Sweating can also be applied to solid solution systems and plays an important role as a purification mechanism in layer and suspension growth processes.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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