Precipitation of ultrafine organic crystals from the rapid expansion of supercritical solutions over a capillary and a frit nozzle

Benzoic acid, salicylic acid, aspirin or phenanthrene was dissolved in supercritical CO2 and precipitated as a fine powder by the rapid expansion of the supercritical solution over a nozzle (RESS process). A new kind of expansion device is presented (the frit nozzle) which can be manufactured more easily than the capillary nozzle, especially if small holes are required. The performances of the two expansion devices are compared. The process operating conditions (extraction pressure and temperature, pre-and post-expansion temperature, mass flow rate and nozzle design) were varied systematically to analyze their impact on the crystal habit and size of the precipitated material. Except for phenanthrene, the crystals obtained with the frit nozzle were smaller than those precipitated with the capillary nozzle, as can be explained from expansion paths and solubility diagrams. (C) 1997 Elsevier Science B.V.