Supercritical fluids liquid spray

Supercritical fluid transport involves dissolving the precursor(s) in a supercritical fluid and spraying this liquid into a CVD reactor.43-45 The rapid expansion of the supercritical fluid enhances evaporation of the solute and so aids transport of the precursor to the substrate. This method has the potential advantages of the other methods, but is more limited in the choice of solvent, e.g., C02. It should be noted that this method may not require the evaporation of the precursors and under these conditions would not be classified as a CVD method. 

Colwell, J. D. Senser, D. W. Nielsen, K. A. Influence of Temperature on the Structure of Supercritical Fluid Coating Sprays. In Proceedings of the Sixth Annual Conference on Liquid Atomization and Spray Systems Peters, J., Ed. University of Illinois Urbana, IL, 1993 pp 39-43. 

Suspensions of these spherical particles are used for spray drying to produce large agglomerates which are used as packings for various separation techniques such as High Performance Liquid Chromatography (HPLC) or Supercritical Fluid Chromatography (SFC). They also serve as supports for catalysts. 

In those liquid-fluid systems under consideration, interfacial tension is reduced at elevated pressures which favors creation of new interfacial area by droplet breakage. In addition, the rapid dissolution of the supercritical fluid in the liquid drops leads to a reduction in viscosity and thus to a generally positive mass transfer behaviour in the high-pressure spray extraction. 

Lee, C. Hoy, K.L. Donohue, M.D. Supercritical fluids as diluents in liquid spray application of coatings and apparatus therefor. Union Carbide Corp., USA, EP, 1989. 

The critical temperature of CO2 is 31 °C. Above this temperature, carbon dioxide can be compressed to a very high pressure and relatively high density supercritical fluid. Like a liquid, the supercritical carbon dioxide will mix with or dissolve the blend of coating and low volatility solvent to form a product that is thin enough to be sprayed easily, in very small droplets. 

The limitations of the PGSS process are that the substances to be processed have to be in a liquid state. But most of the products used in the pharmaceutical and food industries will distort before reaching their melting point. Its use is mainly restricted to polymers since the dissolution of supercritical fluid reduces both viscosity and glass transition temperature. The polymer becomes spray able through a conventional nozzle. 

Zeaton, G., Crook, L., Guildenbecher, D., Sojka, P. E. (2005). An experimental study of supercritical fluid jets. Proceedings of the 19th European Confertaice on Liquid Atomization and Spray System, Toronto, Ontario, Canada. 

Nielsen, K. A. Glancy, C. W. Hoy, K. L. Perry, K. M. A New Atomization Mechanism for Airless Spraying The Supercritical Fluid Spray Process. In Proceedings of the Fifth International Conference on Liquid Atomization and Spray Systems Semeijian, H. G., Ed. NIST Pub. 813 Gaithersburg, MD, 1991 pp 361-314. 

After an initial molecular evaluation, a variety of crystal growth strategies are available, but not limited to evaporation and cooling (at varying rates and temperatures), " milling (liquid-assisted or neat), sonication, melting, and reaction crystallization. More non-traditionaf techniques include spray drying, twin-screw extrusion, and supercritical fluids which have all been demonstrated to be scalable processes. 

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