Hydrodynamics of supercritical antisolvent precipitation: Characterization and influence on particle morphology

The hydrodynamics of a supercritical antisolvent (SAS) precipitation vessel has been investigated so as to dissociate its influence on the morphology of the powder from the influence of physicochemistry. Hydrodynamics characterization experiments were carried out in parallel with SAS precipitations of L-poly(lactic acid) (L-PLA) from methylene chloride solutions under similar conditions. The precipitation vessel behaves as a well-mixed reactor auto-agitated by the introduction of the different phases for all conditions studied. Results of precipitation experiments showed that two different morphologies of L-PLA are obtained, namely, fibers and/or microspheres, depending only on the liquid solution flow rate. Direct visualization of the dispersion of the liquid phase into the continuous supercritical phase in the autoclave showed the formation of an asymmetric laminar jet. The determination Of the jet breakup time, correlated with the morphologies of the solid, allowed for an estimation of the L-PLA precipitation time of between 1 and 4.9 ms at 10 MPa and 308 K.