In situ optical monitoring of the solution concentration influence on supercritical particle precipitation

We report a novel approach for the measurement of the location of particle formation in the supercritical antisolvent process (SAS). The measurement strategy is based on in situ Raman and elastic light scattering. In the SAS process, paracetamol was used as the solute, ethanol as the solvent and carbon dioxide as the antisolvent. Experiments were performed under miscible conditions for the binary system ethanol and carbon dioxide at 313 K and pressures between 10 MPa and 17.5 MPa. For high paracetamol concentrations in the injected ethanol solution, particles were found to start precipitating after jet breakup in a multi-phase flow. For low paracetamol concentrations, precipitation starts later in a one-phase flow, when the transient interface (phase boundary) between the injected solution and the supercritical carbon dioxide has diminished.