(659g) Hydrodynamics Modeling and Analysis of Rapid Expansion Systems of Supercritical Solutions (RESS)

Particle formation by methods utilizing supercritical fluids (e.g. CO,₂) is a subject of great interests in many industrial applications such as pharmaceuticals, foods, natural products and specialty chemicals ^[1,2,3,4]. Strict regulations on the use of organic solvents and their residual level in the end products form a major limitation to the traditional processing techniques. The particle formation using supercritical fluids (SCF) involves minimal or no use of organic solvents while the processing condition is relatively mild. Additionally, the SCF technology involves a growing of the particles in a controlled fashion to attain the desired morphology. This feature offers advantages as compared to some conventional techniques of particle size reduction (e.g. milling and grinding) so that the SCF-based particle formation is especially desirable for thermally sensitive materials. The SCF technology for particle formation is rapidly evolving, as reflected by the number of modified processes reported since its inception ^[5,6,7]. These include static supercritical fluid process (SSF), rapid expansion of supercritical solutions (RESS), particle from gas-saturated solutions (PGSS), gas anti-solvent process (GAS), precipitation from compressed anti-solvent (PCA), aerosol solvent extraction system (ASES), supercritical anti-solvent process (SAS), and supercritical anti-solvent process with enhanced mass transfer (SAS-EM).

Previous studies have shown that the RESS process be one of the most promising techniques. The technique offers several advantages over conventional processes, which are either mechanical (crushing, grinding and milling), or equilibrium controlled (crystallization from solution). The RESS process can lead to very high super-saturation ratios, which leads to uniform conditions within the nucleation medium, and hence in principle to narrow particle size distributions and to form small particles. RESS can be used to precipitate mainly organic solutes upon expansion of a dilute solution from a supercritical or near critical solvent. This RESS concept can be implemented in relatively simple equipment, although particle collection from the gaseous stream is not easy.

The expansion step in RESS occurs through such a throttling device as an orifice, capillary tube or needle valve, which can support a large pressure drop without freezing. The RESS process can produce a variety of precipitate morphologies, including thin films, fine diameter fibers, and narrow size distribution particles. Changes in pressure, throttling device dimensions, concentrations of solute in the supercritical fluid and temperature cause wide variations in precipitate morphology (e.g. fibers to spheres). An increase in pre-expansion temperature under certain experimental conditions can lead to a transition of powder to fibrous morphology. Similarly, a decrease in pressure can also result in a powder to fiber morphology transition. This suggests that solutions with lower density are more likely to produce particles of a fibrous morphology in the RESS process. In addition to the dynamic variables of pressure, temperature and concentration, a fluid dynamic process variable ? the geometry of the throttling device - has also been found to influence the morphology of the precipitate ^[8,9].

In this paper, we study the hydrodynamic behavior through a capillary nozzle in the RESS process using Computational Fluid Dynamics (CFD). The CFD simulation is focused on the pre-expansion chamber of the RESS process consisting of three successive steps. They are at the stagnation chamber (i.e. the reservoir), at the nozzle inlet and along the nozzle itself. The solute considered in this study is benzoic acid and the supercritical solvents are CO2 and CHF3 respectively. The study is aimed at identifying the effect and sensitivity of the design parameters (i.e. operating conditions such as pressure and temperature) in the pre-expansion path of the RESS process on the particulate formation. From the CFD simulation, it is found that high pre-expansion pressures and low pre-expansion temperature favor for the formation of small particles. Furthermore, for mixture of benzoic acid/carbon dioxide, the nucleation process inside the capillary nozzle could not be avoided. According to the classical nucleation theory, our analysis reveals that the lower nucleation rates should results in a delayed precipitation. Hence, it would be less time available for growth, resulting in smaller particles. The CFD modeling and analysis of RESS process provides information for understanding the basic trends that relate the particle formation with the expansion process conditions. It is therefore of important for optimization, control and design studies ^[10].

Keywords: RESS, Hydrodynamics; CFD Modeling; Particle formation

References

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