Dense gas antisolvent precipitation:: A comparative investigation of the GAS and PCA techniques

Experimental data are reported on particle size distributions of paracetamol precipitated from an acetone solution using compressed CO2 as an antisolvent. When the solution is sprayed into dense CO2 using the "precipitation with compressed antisolvents" (PCA) process (in two different PCA units), the average particle size is approximately 2mum in the 83-120 bar, 33-62degreesC, 70-138 g/min Of CO2 range. When the operating pressure and temperature are below or close to the critical locus for the CO2 + acetone binary, the particles tend to be spherical and agglomerated, presumably because the surface roughening temperature was exceeded. Well above the critical locus, the particles are less aggregated with distinguishable crystal faces. In contrast, bubbling compressed CO2 through the paracetamol solution (the so-called GAS process) yielded 90-250 mum particles at 25degreesC in the 5-50 g/min of CO2 range. Through the definition of characteristic mass-transfer times (tau(mt)) for the PCA and GAS processes based on published mathematical models, it is shown that the 2 orders of magnitude disparity in the average particle size is mirrored by a similar disparity in the tau(mt) values for the two processes. These results suggest that the PCA and GAS processes, with common underlying mass-transfer mechanisms, may be essentially viewed in a continuum of characteristic mass-transfer time scales, with the higher tau(mt) values yielding progressively larger particles. This result may be useful to rationally interpret and manipulate particle sizes in these processes.