Recrystallization of organic compounds from supercritical fluids and high-pressure gas-expanded liquid solvents

Abstract

The precipitation of organic particles from supercritical fluids (SF) by expansion (SFX) has become an interesting alternative to milling without thermal decomposition. Through the rapid-expansion process, a dramatic change of the solute supersaturation ratio is created causing subsequent precipitation with a narrow particle size distribution. It was found that β-carotene precipitates from SF ethylene and ethane have the feed material crystallinity. However, SF CO₂ reacted with β -carotene and did not give characteristic β -carotene X-ray spectra. The mean particle sizes of these precipitates were in the submicron range (ca. 0.3 μm). Increased solubility was obtained by addition of toluene as co-solvent in SF ethylene. It was found that the mean size of β -carotene particles, generally remained unchanged if the toluene concentration was less than one and one-half mole percent. The GAS (Gas Anti-Solvent) process, wherein a high pressure gas is dissolved in a liquid solvent, thus precipitating a solid, has successfully been applied to recrystallize some difficult-to-comminute materials. It is essential to obtain the experimental solubility of the high pressure gas in the liquid solvent in order to model the GAS recrystallization process. A new method to measure compressed gas solubility and solvent expansion behavior was developed using a high precision densitometer. Theoretical predictions of phase equilibria and solubility of the compressed gas dissolved in the liquid solvent for three binary asymmetric systems was done in this study. Solids precipitation from liquid solvents, using dissolution of high pressure CO₂ as an anti-solvent to create supersaturation, is a potentially attractive crystallization process. Separation and purification of β -carotene mixtures was done in this study by this anti-solvent salting out technique. It was found that total carotene was successfully separated from epoxy oxides, as well as trans β -carotene from cis isomers. The separation can be carried out in batch or continuous modes. It is suggested that the purification (or fractionation) of an isomer mixture might be done using a mixed solvent if the desired compound dissolves in a highly expanded solvent and the undesired compound dissolves in a less expanded solvent.