Plastic and nonmetallic article shaping or treating: processes – Formation of solid particulate material directly from molten...
Reexamination Certificate
2000-07-10
2003-04-22
Theisen, Mary Lynn (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Formation of solid particulate material directly from molten...
C264S012000, C264S013000, C425S006000, C425S007000
Reexamination Certificate
active
06551532
ABSTRACT:
This invention relates to a method for forming particles of a substance. It also relates to a mixing chamber for use in forming particles of a substance.
BACKGROUND ART
The use of supercritical fluids in particle forming processes has been described in several documents. A supercritical fluid can be defined as a fluid at or above its critical pressure and critical temperature simultaneously. Such fluids are interesting in particle formation since their solving power of different substances undergoes large changes as a result of changes in the physical characteristics of the surroundings, which characteristics can be relatively easily controlled, such as pressure. This property make supercritical fluid a medium highly appreciated for having a solving power being controllable by pressure and temperature changes, which is particularly useful in extraction and atomization of different substances, such as substances used in pharmacy. Further, supercritical fluids are normally gases under ambient condition, which eliminates the evaporation step needed in conventional liquid extraction.
There are several techniques related to this phenomenon used today, one of which is known as rapid expansion of supercritical solutions (RESS) and another that is known as gas anti-solvent precipitation (GAS). In the GAS technique a substance of interest is dissolved in a conventional solvent, a supercritical fluid such as carbon dioxide is introduced into the solution, leading to a rapid expansion of the volume of the solution. As a result, the solvent power decreases dramatically over a short period of time, triggering the precipitation of particles. Documents referring to this are for example J. W Tom and P. G. Debenedetti in J. Aerosol SCI., 22 (1991), 555-584; P. G. Debenedetti et al in J. Controlled Release, 24(1993), 27-44 and J. W. Tom et al in ACS Symp Ser 514 (1993) 238-257; EP 437 451 and EP 322 687.
A modification of the GAS system has recently been developed, which is called the SEDS (solution enhanced dispersion by supercritical fluid) process, which utilises supercritical fluid technologies for particle formation.
This technique is described in WO95/01221, which reveals a method for the formation of a particulate product, which comprises the co-introduction of a supercritical fluid and a vehicle system comprising at least one substance in solution or suspension into a particle formation vessel. The temperature and pressure inside the particle formation vessel are controlled, such that dispersion and extraction of the vehicle occur substantially simultaneously by the action of the supercritical fluid.
The method described in the aforementioned document is particularly developed for use in Gas Anti Solvent (GAS) techniques. These techniques are useful in situations where the solid of interest does not dissolve in, or has very low solubility in a supercritical fluid. The solute is therefore in a first step dissolved in a conventional solvent. The solution of the solvent and the substance is commonly known under the term “vehicle system”. The term “vehicle” is herein a fluid, which dissolves a solid or solids to form a solution, or which forms a suspension of a solid or solids, which do not dissolve or have a low solubility in the fluid. The vehicle can be composed of one or more fluids.
In a second step of the procedure, the vehicle is extracted by the supercritical fluid, which has a sufficient solubility for the vehicle in concern when held in a supercritical condition. As a result, extraction and droplet formation of the vehicle occurs substantially simultaneously by the action of the supercritical fluid. The particles thus formed by the substance previously carried in the vehicle system are collected in a particle vessel and the remaining supercritical fluid and vehicle products can optionally be brought through a cleaning system for possible reuse. The term “particle” as used herein can include products in a single-component or multi-component, as mixtures of one component in a matrix of another form.
In the description of the method described above, the importance of maintaining control over the working conditions, especially the pressure is set out. It is thus necessary to eliminate any uncontrolled pressure fluctuation across the particle formation vessel and ensure a uniform dispersion of the vehicle. Through a high degree of control of parameters such as temperature, pressure and flow rate of both vehicle system and supercritical fluid and the simultaneous co-introduction of the vehicle system and the supercritical fluid into the particle formation vessel, droplet formation occurs when the fluids come into contact with one another.
In the document WO95/01221 is further an apparatus for performing the method described. The apparatus is provided with means for co-introduction of the vehicle system and the supercritical fluid into the particle formation vessel. This means consists of a nozzle, having coaxial passages serving to carry the flow of the vehicle system and of the supercritical flow, respectively. The outlet end of the particle formation chamber is conical, with an angle of taper typically in the range of 10 to 50 degrees. The document teaches further that an increase in the angle may be used for increasing the velocity of the super-critical fluid introduced to the nozzle and hence the amount of physical contact between the supercritical fluid and the vehicle system. It is further imposed that control of parameters such as size and shape in the resulting particulate product will be dependent upon variables including the flow rates of the supercritical fluid and/or the vehicle system comprising the substance, the concentration of the substance in the vehicle system, and the temperature and pressure inside the particle formation vessel.
In another patent document, WO96/00610, the method is improved by introducing a second vehicle, which is both substantially miscible with the first vehicle and substantially soluble in the supercritical fluid. The corresponding apparatus is consequently provided with at least three coaxial passages. These passages terminate adjacent or substantially adjacent to one another at the outlet end of the nozzle, which end is communicating with a particle formation vessel. In one embodiment of the nozzle the outlet of at least one of the inner nozzle passages is located a small distance upstream (in use) of the outlet of one of its surrounding passages. This allows a degree of mixing to occur within the nozzle between the solution or suspension, that is the first vehicle system, and the second vehicle. This pre-mixing of the solution and the second vehicle does not involve the supercritical fluid. It is in fact believed that the high velocity supercritical fluid emerging from the outer passage of the nozzle causes the fluids from the inner passages to be broken up into fluid elements. From these fluid elements the vehicles are extracted by the supercritical fluid, which results in the formation of particles of the solid previously solved in the first vehicle. The useful maximal taper of the conical end is in this document also augmented up to 60 degrees.
Another technique for particle precipitation using near-critical and supercritical antisolvents has later been described in WO97/31691. This document mentions the use of specialized nozzles for creating extremely fine droplet sprays of the fluid dispersions. The method involves passing the fluid dispersion through a first passageway and a first passageway outlet into a precipitation zone, which contains an antisolvent in a near- or supercritical condition. Simultaneously an energizing gas stream is passed along and through a second passageway outlet proximal to the first fluid dispersion outlet. The passage of the energizing gas stream generates high frequency waves of the energizing gas adjacent to the first passageway outlet in order to break up the fluid dispersion into small droplets.
The disclosed prior art of producing small particles by use of supercritical fluid as an antisolvent to release
Boissier Catherine
Glad Håkan
AstraZeneca AB
Theisen Mary Lynn
White & Case LLP
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