Nucleation
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Learn about the process of nucleation. - primary/secondary nucleation - homogeneous/heterogeneous nucleation - nucleation rate
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Nucleation
- 1. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 869993. Nucleation
- 2. Crystallization process • Crystallization process includes two steps: • nucleation • crystal growth. • In supersaturated solution, constituent particles gather together in clusters. When they reach the critical cluster size, they become stable nuclei. • This process is called nucleation. Nuclei are like little baby crystals. Picture: Kati Jordan CC0
- 3. Formation of nuclei • The formation of nuclei is a difficult process to envisage. • Adequate amount of constituent particles (atoms, ions or molecules) have to be attached into a fixed lattice to form a stable nucleus. For example, a stable ice nucleus contains about 100 molecules. • An unstable nuclei may either grow or redissolve. The minimum size of a stable nucleus is called its critical size. • Nucleation requires a supersaturated solution. It is not at equilibrium. The solution crystallizes to achieve equilibrium.
- 4. Nucleation • The region, where the solution is only slightly supersaturated, is called metastable zone. In metastable zone nucleation is not spontaneous. Some seeds are needed. • The rate of nucleation increases with the level of supersaturation. Above the metastable zone, nucleation becomes spontaneous. • The width of the metastable zone is dependent on a number of conditions, e.g. the rate of cooling. The crystal growth occur in the metastable zone. Picture: Kati Jordan CC0
- 5. Nucleation • Nucleation can be divided into primary and secondary nucleation. • In secondary nucleation, the formation of nuclei is induced by the existing crystals. It is the main method in industrial crystallizers. • In primary nucleation, no crystals are involved. If the nucleation is caused by some foreign particles (e.g. dust), process is called heterogeneous nucleation. • If nucleation occurs spontaneously, it is called homogeneous nucleation. It is not possible in industrial crystallizers because of the presence of impurities and low levels of supersaturation. Picture: Kati Jordan CC0
- 6. Nucleation rate • Nucleation rate effects on crystal size distribution and crystal habit. • It depends on the level of supersaturation and the seed surface area. • The rate increases with increasing supersaturation. • The nucleation rate is usually given in units of the number of nuclei formed / (m³ · s). • It can also be determined as an induction time. • Induction time is a time interval between the creation of supersaturation and the appearance of crystals. It can range from micro seconds to days.
- 7. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 869993. References • Beckmann & Beckmann, W. 2013. Crystallization: Basic Concepts and Industrial Applications. Weinheim: Wiley‐VCH Verlag GmbH & Co. KGaA, pp. 17-25. • McCabe, W. L., Smith, J. C. & Harriott, P. 2005. Unit operations of chemical engineering. 7th ed. Boston: McGraw-Hill, pp. 936-945. • Mullin, J. W. & W Mullin, J. 2001. Crystallization. Butterworth-Heinemann, pp. 181-215. • Myerson, Myerson, A. S. & Myerson, A. 2002. Handbook of Industrial Crystallization. Butterworth-Heinemann, pp. 43-53. Videos: • Dive deeper into the process of nucleation: https://youtu.be/Jbb6QDiE9dI • Learn about the classical nucleation theory: https://youtu.be/cpPjN9x9NNE