Rapid Expansion of Supercritical Solution (RESS) is used generally to produce fine particles for the food, cosmetics, and pharmaceutical industries.
Organic material is dissolved in supercritical carbon dioxide and is used to rapidly expand the supercritical solution through an expansion nozzle. Due to an abrupt decrease of pressure to atmospheric, very high supersaturation values are achieved which lead to small particle sizes.
Furthermore, as the pressure change traveled at the speed of sound, a rapidly propagating mechanical perturbation is obtained and led to a uniform condition within the nucleating medium — hence in principle to narrow particle size distributions.
Previous RESS reports indicate that the RESS process is an extremely attractive technology to form small particles with a narrow particle size distribution.
RESS process consists of two steps:
- Dissolving the solid substance in a Super Critical Fluid (SCF)
- Formation of particles due to supersaturation
In the RESS process, SCF-CO2 is first pumped at desired pressure and temperature to the extraction chamber containing solid substance(s) through a heat exchanger. The supercritical solution expands rapidly in the precipitator causing a collapse of pressure and temperature, which favors the formation of drops and particles.
The SCF percolates and dissolves the solid substance(s) in the extractor and then the resulted solution is depressurized through a heated nozzle or capillary at supersonic speed into a low-pressure chamber. The supercritical solution is expanded adiabatically in the chamber, which leads to a rapid drop in temperature and pressure and spontaneous formation of droplets/particles. During the rapid expansion of the supercritical solution, the density and solvent power decrease significantly, resulting in supersaturation of the solution and consequently precipitation of desire particles free of a residual solvent.
The RESS process advantages are its simplicity, effectiveness and the absence of organic solvents.
RESS – SC: There is also a variation of the traditional RESS that uses a solid cosolvent (RESS-SC). This concept is widely used in the field of extraction with SCF and is called co-extraction. Generally, the RESS process is a batch-type, but there is also a continuous RESS process, which allows obtaining micro-particles in a constant and continuous way.