Publikationsserver der RWTH Aachen University
Zur Erzeugung von Mikropartikeln durch Kristallisation mittels CO2-Direktkühlung
Abstract
dc:descriptionThis thesis comprises a theoretical and experimental investigation of the generation of microparticles by rapid cooling crystallization. Extremely high cooling rates are achieved by direct contact cooling with liquid carbone dioxide, which is throttled down to environmental pressure. The starting point of the investigation was a theoretical consideration of the relationship between the resulting particle size distribution, the cooling rate and the kinetic of nucleation and crystal growth. For this, the unsteady crystallization process during a batch cooling was calculated for an idealized crystallization model. It was possible to identify those ranges of the nucleation and crystal growth, which enable the generation of microparticles in principle. The minimum required cooling rates for each kinetic could be estimated on this basis. Finally, this theoretical investigation provided evidence for the existence of those materials, which require very high cooling rates for the production of micron sized particles. It could also be shown, that the influence of the heat of crystallization can be neglected for typical values of crystallization enthalpy, heat capacity of the solution and gradient of the solubility curve. A test bench has been developed for the experimental investigation of this crystallization technique, where the mean values of the cooling rates are in the range of 300000 K/min to 720000 K/min. In the framework of the following experimental investigation 14 solute-solvent systems have been tested systematically with different cooling techniques in order to check their qualification. The experimental strategy allowed the determination of the attainable particle size and the evaluation, if the coolant itself has an additional influence on the crystallization process. Three solute-solvent systems have been found in total, where the generation of micron sized particles bases exclusively on rapid cooling crystallization. The crystallization of the other solute-solvent systems are influenced by the coolant, whereas in all cases the coolant effects the crystallization of smaller particles compared to the pure cooling crystallization. Some solute-solvent systems are not suitable for rapid cooling as the crystallization is too slowly in relation to the cooling rate. It has also been investigated, if the unsteady heat release during crystallization could be a used for an initial estimation of the suitability of a system for the production of micron sized particles by rapid cooling. The differential scanning calorimetry proved to be a helpful testing method for the verification, if a solute-solvent system is in priciple suitable or not for the new technique and if a mechanismen change of the solid state transformation occurs with increasing cooling rate. The solute azelaic acid and the solvent acetone have been chosen as a model system for more a detailed investigation of the crystallization method. It could be shown for this system, that beside the cooling rate the most important parameter for the particle size is the end temperatur and respectivley the equilibrium concentration. A decrease of nucleation at very low temperatures could not be detected here and the smallest particle size of 1 µm is produced at -78 °C. The initial solute concentration has no significant influence on the particle size. Obviously, the attained increase of the supersaturation is compensated by the simultaneous increase of the produced solid mass.
Degree
thesis:*- Grantor dc:publisher
- Publikationsserver der RWTH Aachen University
- Year dc:date
- 2008
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Schleiden, Thomas
- Contributors dc:contributor
-
- Melin, Thomas
Subjects
dc:subject × 8Rights
dc:rights- Statement dc:rights
-
- info:eu-repo/semantics/openAccess
- Language dc:language
- ger