Proceedings (senza ISBN) del "XIV Congresso dell Divisione di Chimica Industriale della Società Chimica Italiana: Nuovi Orientamenti nell’Industria Chimica”, Milano (I), 3-4 ottobre 2001. -
In many cases the dimensions of the particles and the size distributions in solids obtained from chemical reactions, extractions processes, or crystallization operations do not correspond to the desired standards for the practical applications. A recrystallization process is needed in order to satisfy the standards. The milling process, the thermal recrystallization, and the liquid-antisolvent recrystallization are the most commonly techniques used. Applying these methods may cause a problem with decomposition of the solid due to the high temperatures involved and furthermore it is difficult to obtain a completely solvent-free product. These problems are avoided by use of processes based on supercritical fluids (SCF).
The two main techniques related to supercritical fluid recrystallization processes are the Rapid Expansion of a Supercritical Solution (RESS) and Gas Anti-Solvent recrystallization (GAS). In the RESS process a supercritical solution of the solvent and the solute(s) is expanded through a nozzle and the expansion decreases the solvent power of the supercritical solvent causing the precipitation of the solute. The RESS technique can only be applied to solutes soluble in a SCF, excluding a lot of heavy substances and almost all polymers, and thus making the technique of little interest for practical applications.
The GAS technique takes the advantage of the miscibility of the SCF with the organic solvent. When increasing the pressure by adding the anti-solvent (usually CO2), the solubility of the anti-solvent in the solvent is increased, and at a certain point the reduced solvent power of the mixed solvent causes solute precipitation. Finally, the solvent can be extracted by exploiting its solubility in the anti-solvent, and a dry and pure solute can be obtained.
