Chemistry: physical processes – Physical processes – Crystallization
Patent
1997-02-19
2000-05-16
Straub, Gary P.
Chemistry: physical processes
Physical processes
Crystallization
23300, 4222451, B01J 204, B01D 902
Patent
active
060631384
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the controlled formation of particulate products using supercritical fluids. It provides a method and apparatus for the formation of a substance in particulate form, and the particulate product of such a method.
2. Description of Prior Art
The use of supercritical fluids (SCFs) and the properties thereof have been extensively documented; see for instance J. W. Tom and P. G. Debenedetti, "Particle Formation with Supercritical Fluids--A Review", J. Aerosol. Sci., 22 (5), pp555-584 (1991). Briefly, a supercritical fluid can be defined as a fluid at or above its critical pressure (Pc) and critical temperature (Tc) simultaneously. Such fluids have been of considerable interest, because of their unique properties. These characteristics include: liquids. fluid density with only slight changes in pressure, which in turn results in highly controllable solvation power. Supercritical fluid densities typically range from 0.1-0.9 g/ml under normal working conditions. Thus, selective extraction with one supercritical fluid is possible. which eliminates the evaporation/concentration step needed in conventional liquid extractions. non-degrading atmospheres for sensitive and thermolabile compounds, due to their inertness and the moderate temperatures used in routine working conditions. Carbon dioxide is the most extensively used SCF due to its cheapness, non-toxicity, non-flammability and low critical temperature.
These characteristics have led to the development of several techniques of extraction and particle formation utilising supercritical fluids. In particular, two particle formation methods have been identified.
"Rapid expansion of supercritical solution" (RESS) (see, for instance, J. W. Tom and P. G. Debenedetti, supra) involves the dissolution of the solute of interest in a supercritical fluid, followed by rapid expansion of the resulting supercritical solution to atmospheric pressure, resulting in the precipitation of solute particles.
"Gas anti solvent" (GAS) recrystallisation (P. M. Gallagher et al, "Supercritical Fluid Science and Technology", ACS Symp. Ser., 406, p334 (1989)) is particularly useful in situations when the solute of interest does not dissolve in, or has a very low solubility in, a supercritical fluid or a modified supercritical fluid. In this technique, the solute is dissolved in a conventional solvent. A supercritical fluid such as carbon dioxide is introduced into the solution, leading to a rapid expansion of its volume. As a result, the solvent power decreases dramatically over a short period of time, triggering the precipitation of particles.
The concept of spraying liquid mixtures into supercritical fluids such as carbon dioxide, or vice versa, has also been employed in solvent extraction procedures for a decade (see for instance R. J. Lahiere & J. R. Fair in Ind. Eng. Chem. Res., 26, pp2086-2092 (1987)).
More recently, U.S. Pat. No. 5,043,280 describes a method for manufacturing a preparation comprising a substance, such as a medically useful substance, and a carrier, such as a pharmaceutically acceptable carrier, which avoids or lacks a solvent residue, or at least reduces the solvent residue to a toxicologically harmless amount. The method essentially involves the use of a fluid, at a supercritical state when introduced into a spray tower, to extract a solvent from sprayed solution(s) of a substance and a carrier, to form a sterile product containing the substance embedded in the carrier. It should be noted, however, that the method has no means for controlling the physical properties of the particulate product formed.
In many fields, and especially in the fields of pharmaceuticals, photographic materials, ceramics, explosives and dyes, there is a need for techniques whereby a particulate product may be obtained with consistent and controlled physical criteria, including particle size and shape, quality of the crystalline phase, chemical purity and enhanced handling and fluidizing properties.
In addition, i
REFERENCES:
patent: 5043280 (1991-08-01), Fischer et al.
Hanna Mazen
York Peter
Bradford Particle Design Limited
Straub Gary P.
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