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In this paper, we numerically study particle formation in the rapid expansion of supercritical solution (RESS) process in a two dimensional, axisymmetric geometry, for a benzoic acid + CO 2 system. The fluid is described by the classical Navier–Stokes equation, with the thermodynamic pressure being replaced by a generalized pressure tensor. Homogenous particle nucleation, transport, condensation and coagulation are described by a general dynamic equation, which is solved using the method of moments. The results show that the maximal nucleation rate and number density occurs near the nozzle exit, and particle precipitation inside the nozzle might not be ignored. Particles grow mainly across the shocks. Fluid in the shear layer of the jet shows a relatively low temperature, high nucleation rate, and carries particles with small sizes. On the plate, particles within the jet have smaller average size and higher geometric mean, while particles outside the jet shows a larger average size and a lower geometric mean. Increasing the preexpansion temperature will increase both the average particle size and standard deviation. The preexpansion pressure does not show a monotonic dependency with the average particle size. Increasing the distance between the plate and the nozzle exit might decrease the particle size. For all the cases in this paper, the average particle size on the plate is on the order of tens of nanometers. [ABSTRACT FROM AUTHOR]

Abstract: Naproxen has been processed with supercritical fluids in order to improve the dissolution rate and bioavailability. Microparticles of naproxen have been obtained by a Rapid Expansion of Supercritical Solutions (RESS) process in which carbon dioxide has been used as a solvent and methanol as a cosolvent. The influence of extraction pressure (200–300bar) and extraction temperature (60°C and 100°C) on the naproxen precipitation has also been investigated. In general, the morphology of the precipitated particles improved and particle size (PS) decreased in comparison to the raw material. Lower extraction pressure and higher extraction temperature led to a smaller particle size. On the other hand, a supercritical antisolvent (SAS) process has been applied due to the relative medium solubility values of naproxen in supercritical carbon dioxide, with precipitation obtained successfully in all cases. The initial concentration of the solution and the solvent effect has both been analysed. Morphologies and mean diameter ranges have been analysed by scanning electron microscopy (SEM) and the influence on crystallinity of both supercritical processes has been evaluated by X-ray diffraction (XRD) measurements. [Copyright &y& Elsevier]

Abstract: Rapid expansion of subcritical solutions into liquid solvents (RESOLV) was able to consistently produce catechin (CTC)-loaded poly(l-lactide) (PLLA) nanoparticles. The effects of CTC concentration and processing conditions – pre-expansion temperature and pressure, and degree of saturation – on the size and morphology of CTC-loaded PLLA nanoparticles were investigated, as well as the loading capacity (LC), entrapment efficiency (EE), and release of CTC. RESOLV experiments were carried out with 0.1wt% CTC+0.2wt% PLLA and 0.2wt% CTC+0.2wt% PLLA solutions in mixtures of EtOH and CO2 (3:2, wt/wt) with pre-expansion temperature and pressure of 60–100°C and 265–325bar, respectively. The obtained CTC-loaded PLLA nanoparticles were spherical, with average sizes, LC, and EE ranges of ∼30–40nm, 2.4–7.3%, and 4.7–22.0%, respectively. CTC concentration, pre-expansion temperature, and degree of saturation had significant effects on LC and EE of CTC, without affecting the size of composite nanoparticles. The LC and EE of CTC increased with increasing pre-expansion temperature and degree of saturation, and with decreasing CTC concentration. The release profiles of CTC from the nanoparticles exhibited an initial stage of burst release (first 8h) followed by a sustained release (∼1–9 days). Furthermore, the fraction of CTC being released from the nanoparticles decreased with increasing the degree of saturation of subcritical solutions prior to rapid expansion and with increasing the LC of CTC. [Copyright &y& Elsevier]

Abstract: A set of analytical equations is derived for the estimation of the average diameter attained by particles that are generated by the rapid expansion of a supercritical solution (RESS). Applying the simplified model to one specific, but nevertheless representative set of experimental conditions for RESS processing, theoretical findings from earlier work could be generally confirmed and new insight was gained. Particles generated by RESS through an expansion device having a cylindrical section with a small aspect ratio will generally not attain average diameters of more than 20–50nm before passing through the critical cross section since the time available for growth by condensation is limited to a few microseconds. Comparing three different driving mechanisms for growth due to coagulation, it was found that the effects of slip flow differences can be seen as most promising to significantly improve existing RESS models. [Copyright &y& Elsevier]

Abstract: A method is reported for the preparation of highly loaded TiO2–polymer composite particles by rapid expansion of carbon dioxide (CO2) saturated polymer suspensions with high shear mixing. Suspensions of CO2 saturated polymer solution and TiO2 nanoparticles were agitated in high-pressure vessel equipped with a column agitator and mechanical seals. The rotation speed and peripheral velocity were set at 5000rpm and 40.8m/s, respectively. The polymers were poly(lactic acid) (PLA; M.W.=10,000) and poly(styrene)-b-(poly(methyl methacrylate)-co-poly(glycidyl methacrylate)) copolymer (PS-b-PMMA-co-PGMA, M.W.=5000). After agitation and rapid expansion of the CO2 saturated polymer suspensions to an aqueous phase, polymer microspheres were obtained and very small on the order of only micrometers. The structure and morphology of the microspheres were investigated by SEM, TEM, XRD and turbidity measurement. The polymer microspheres obtained were highly loaded TiO2–polymer composites. According to TEM photograph of the cross section of polymer particle, loading ratio of TiO2 in polymer particles is 52%. The TiO2 content in the polymer particles was increased by an increase in the rotational speed of the agitator, and consequently the shear stress in the high-pressure vessel. The shear stress between the column agitator and the inside wall accelerates the dispersion of TiO2 nanoparticles in CO2 saturated polymer suspensions. On the other hand, agglomerated TiO2 nanoparticles were observed on the surface of polymer microparticles produced by rapid expansion of mixtures with conventional mixing, and the loading ratio of TiO2 in the polymer matrix was very low. The loading ratio of TiO2 in the polymer particle was very low and less than 1%. [Copyright &y& Elsevier]

Abstract: l-Poly(lactic acid) (PLA) microspheres were produced by rapid expansion of carbon dioxide (CO2) saturated polymer suspensions. The suspensions of CO2 saturated polymer solution containing ethanol were sprayed through a nozzle to the atmosphere or into water. In this work, to control the particle morphology, the gas-saturated polymer suspensions were expanded through the nozzle to water. After spraying solutions through a nozzle to the atmosphere, polymeric fibers and/or coalesced particles were obtained. On the other hand, polymeric microspheres were obtained after spraying through a nozzle to water. The obtained particles were smaller than those produced by expansion to atmosphere. The expansion of polymer suspensions to water impedes particle''s growth and agglomeration. The particles do not tend to agglomerate after expansion, since the ethanol used as co-solvent on the surface of particles diffuses through the water phase. Furthermore, the interfacial tension between the polymer droplets and water phase contributes to the microspheres formation. Changing the pre-expansion pressure and injection distance between the nozzle and water interface, controls the particle size distribution and morphology of microparticles. [Copyright &y& Elsevier]

By the use of a tiny pressure-transferring probe and capacitor-type sensor, the dynamic pressure signals in the jet flow of the rapid expansion of supercritical carbon dioxide are measured, which distinctively shows the characteristics of quasi-periodical coherent structures. After Fast Fourier Transform (FFT) conversion of the time series signals of the dynamic pressure, there exist three dominant frequency bands in the power spectrum, which correlate with the scale of the dominant eddies in the turbulent field. The dominant frequencies change little with the distance from the nozzle exit or the pre-expansion pressure, while the power density of the dominant frequencies, which correlates with the energy of the dominant eddies in the turbulence, attenuates along the axial direction and with the decrease of the pre-expansion pressure. Through analysis, it is inferred that the nozzle structure and initial conditions remarkably affect the coherent structures in the expansion flow which should be the important factors in the particle nucleation and its growth process in ultra-fine particle preparation by rapid expansion of supercritical fluid solution (RESS). [Copyright &y& Elsevier]

The size distribution of fine powders formed during the rapid expansion of supercritical solutions (RESS) depends on the operating conditions, as well as on the geometry of the expansion device. In order to meet product specifications and improve process control, a fundamental understanding of the interplay between nucleation, condensation, and coagulation during this type of expansion is needed. In this work, we model the particle dynamics resulting from homogeneous nucleation, condensation and coagulation during the subsonic expansion of a non-volatile solute in a supercritical fluid inside a cylindrical capillary. The calculations show that subsonic RESS is a very effective technique for producing particles in the 10–50 nm diameter range. The particle formation process is characterized by delayed nucleation, low particle number concentrations, precipitation of a comparatively small fraction of the total solute mass, and by a narrow size distribution. In a few cases where the expansion trajectory enters the fluid''s vapor–liquid coexistence region, the particle formation exhibits early nucleation, strong coagulation, and higher particle number concentrations. In order to explain and describe quantitatively the much larger particle diameters found in actual RESS experiments, additional condensation and coagulation processes that occur in the transonic flow field outside the expansion device, and their interaction with this complex flow field, would also need to be incorporated. [Copyright &y& Elsevier]