Estimation of the characteristic time scales in the supercritical antisolvent process

Simple engineering correlations and a linear jet breakup model are used to estimate the orders of magnitude of characteristic times of the various processes involved in the supercritical antisolvent method (SAS) of particle formation. The characteristic times of jet breakup, mass transfer, and nucleation are studied under typical conditions of pressure and temperature in the two-phase regime for a mixture of carbon dioxide and ethanol. The results of the calculations suggest that the jet breakup phenomenon is much faster than the mass transfer process. This difference in the characteristic times indicates that only a small amount of the supercritical fluid has had time to diffuse into the liquid jet stream by the time the breakup takes place. The characteristic nucleation times can vary appreciably with process conditions (e.g., supersaturation) and the specific properties of the mixture where precipitation occurs (e.g., liquid-solid interfacial tension). On the basis of the present calculations, two different regimes can be anticipated: a diffusion-limited regime leading to a precipitation front and a nucleation-limited, well-mixed regime.