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Ind. Eng. Chem. Res. 2004, 43, 1460-1465
Crystalline Microparticles of Controlled Size Produced by Supercritical-Assisted Atomization E. Reverchon* and A. Spada Department of Chemical and Food Engineering, University of Salerno,Via Ponte Don Melillo, 84084 Fisciano (SA), Italy
Supercritical-assisted atomization (SAA) is a promising technique that can be used to produce micron- and submicron-sized particles of several kinds of compounds using water or organic solutions. It is based on the solubilization of supercritical CO2 in the liquid solution formed by the solvent and the (solid) solute and on the subsequent atomization of this solution using a thin-walled nozzle. In this work, supercritical-assisted atomization operated at pressures below 12 MPa and at temperatures between 65 and 95 °C has been applied to sodium chloride, potassium iodide, and ammonium chloride to test the possibility of producing microcrystals of controlled dimension. The influence of the concentration of the liquid solution was investigated. Cubic crystals were obtained for two of the tested materials; crystalline particles were also produced in the case of ammonium chloride. A mechanism for crystal formation during SAA at different concentrations is proposed. 1. Introduction The production of micron- or submicron-sized particles with controlled particle sizes (PSs) and particle size distributions (PSDs) is of significant interest in several industrial fields. Coloring materials, catalysts, superconductors, polymers, and pharmaceutical compounds are only some categories of products that can be used as micron-sized particles.1,2 Traditional micronization techniques, such as jet milling and liquid solvent precipitation, do not provide efficient control of the size and size distribution in the micrometer range. Generally, irregular particles are produced that range between 0.1 and 10 and even 20 µm. Moreover, solvent residues and/or thermal degradation can decrease the quality of the products. Therefore, some supercritical-fluid-based micronization processes have been proposed to overcome these limitations. Such processes can take advantage of some specific properties of gases at supercritical conditions, such as continuous modulation of the solvent power and large diffusivities, as well as opening up the possibility of achieving solventless or organic-solvent-reduced operation. Thus, it is, in principle, possible to tune the particle size and distribution and to obtain a reduced environmental impact. Of the various supercritical-fluid-based techniques, the most successful until now is supercritical antisolvent (SAS) precipitation.3-9 One of the prerequisites for a successful SAS precipitation is the complete miscibility of the liquid, and the simultaneous insolubility of the solute, in supercritical carbon dioxide. For these reasons, SAS is not applicable to the precipitation of watersoluble compounds because of the very low solubility of water in carbon dioxide at suitable process conditions. Moreover, high-pressure phase equilibria among the three components, namely, the liquid, the solute, and the supercritical antisolvent, can largely condition the final results.10 * To whom correspondence should be addressed. Tel.: +39 089964116. Fax +39 089964057. E-mail:
[email protected].
Very recently, a new category of supercritical-fluidbased micronization techniques has been introduced that can be even more effective in producing particles with a good control of PS and PSD. These processes are an evolution of the traditional atomization methods, in which supercritical CO2 is used as one of the process fluids. A first supercritical-based atomization method was proposed by Sievers and co-workers11,12 and called CAN-BD (carbon dioxide assisted nebulization with a bubble dryer). In this process, prior to atomization, supercritical CO2 and the liquid solution are introduced into a tee connection with a near-zero internal volume (
