Plastic and nonmetallic article shaping or treating: processes – Formation of solid particulate material directly from molten... – Coated particles
Reexamination Certificate
2000-06-16
2002-08-27
Theisen, Mary Lynn (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Formation of solid particulate material directly from molten...
Coated particles
C264S013000
Reexamination Certificate
active
06440336
ABSTRACT:
The invention relates to a process for producing a pulverulent product from a liquid substance or mixture of substances at ambient temperature. A process of this type is disclosed by WO 95/21688.
Pulverulent products are frequently preferred because of their simpler handling in comparison with liquids. In the usual case, for example, the transport and storage of a pulverulent product is less critical than that of a liquid. To produce powders, mechanical processes, such as grinding and agglomeration, and thermal processes, such as crystallization and spray-drying, are known. Substances which are pulverized by such classical processes generally have a melting point which is significantly above the ambient temperature (room temperature). This means that the physical state of these substances is not changed by the pulverization.
Substances whose melting point is beneath the usual ambient temperature cannot be pulverized until they have been solidified by cooling. Even after the pulverization, the solid state of such substances can only be retained with the use of complex cold chains. Another possibility for stabilizing substances which are liquid at usual ambient temperature is applying the substance to be stabilized to finely divided support particles. In this case the support particles are fluidized using a gas stream and the liquid substance to be stabilized is sprayed onto the fluidized support particles. By means of this process, which has become known as fluidized-bed coating, the support particles are coated with a thin film of the liquid substance to be stabilized. The mass ratio between the support and the substance stabilized in this way is determined by the dimensions and the shape of the support particles as well as by the coating thickness. The achievable active ingredient concentrations (active ingredient is taken to mean here the substance which is liquid at room temperature and is to be stabilized) are between 1% by weight and at most about 10% by weight, based on the finished coated support particles. Furthermore, a coating technique of this type can only be employed with certain material combinations in which the mutual wetting and adhesion forces permit the production of a coated support particle. A further restriction is given by the liquid to be stabilized: if its viscosity is too high, it cannot be sprayed, or can only be sprayed with great effort. In the case of some liquids, the sprayability can be improved by dilution with suitable solvents, but this means additional expense and, furthermore, reduces the active ingredient content in the finished stabilized product, quite apart from the fact that the use of many solvents is now undesirable from physiological and environmental aspects.
The previously mentioned process disclosed by WO 95/21688 makes it possible to produce pulverulent solids from liquids. The principle of the process is to dissolve a gas in the liquid to be pulverized under elevated pressure, preferably until a gas-saturated solution is obtained. In comparison with the pure liquid, a solution of this type has a number of favourable properties: thus, usually, the viscosity of this solution in comparison with the pure liquid at the same temperature is decreased by several orders of magnitude and the surface tension is also markedly reduced. The pressurized liquid/gas solution is then passed to an expansion element and there rapidly expanded. In the course of this, the gas present in the liquid/gas solution cools significantly, with great increase in volume, and separates from the liquid. This process leads to the formation of small solid particles which consist entirely of the substance which was previously liquid. The solid particles formed can be separated off by conventional processes and, if desired, can be fractionated.
Although this known process is an elegant and simple method of converting liquids into a pulverulent product, the resulting pulverulent product must still be cooled if the melting point of the processed substance is below the usual ambient temperature.
The object therefore underlying the invention is to provide a process by which substances or mixtures of substances which are liquid at room temperature or at ambient temperature can be stabilized in powder form, with the resulting powder form needing to be stable at room temperature or usual ambient temperature.
This object is achieved according to the invention, starting from the process mentioned at the outset, by means of the fact that a solid, pulverulent auxiliary is admixed to the liquid substance or mixture of substances to be pulverized or to the liquid/gas solution upstream of the expansion element, in the expansion element or downstream, in particular just downstream of the expansion element. In the process according to the invention surprisingly, even relatively small additions of auxiliary are sufficient to stabilize the pulverulent product formed on rapid expansion of the liquid/gas solution. In this process, less solid auxiliary is required the higher the melting temperature of the starting substance to be pulverized. Thus, by the process according to the invention, pulverulent products can be produced which have a high active ingredient content. This means that in the case of many substances or mixtures of substances a comparatively small amount of auxiliary, for example 1 to 90% by weight, preferably 10 to 80% by weight, and particularly preferably only 20 to 50% by weight, is sufficient for stabilizing the resulting powder form. Such high active ingredient concentrations could not be achieved by the processes known hitherto.
In the expansion of the liquid/gas solution, the temperature may fall below the solidification temperature of the substance or mixture of substances, but this is not absolutely necessary in order to obtain the desired pulverulent product. However, it has proved to be expedient, with a number of applications, during the expansion of the liquid/gas solution, to attain a temperature which is at least in the vicinity of the solidification temperature of the substance or mixture of substances.
As gas, in principle, use may be made of any gas which dissolves sufficiently in the liquid substance or mixture of substances to be pulverized. For example, as gas, use can be made of carbon dioxide, a hydrocarbon, in particular methane, ethane, propane, butane, ethene, propene, or a halogenated hydrocarbon, an ether, an inert gas, in particular nitrogen, helium or argon, a gaseous oxide, in particular dinitrogen oxide or sulphur dioxide and ammonia. A mixture of two or more of the abovementioned gases can also be used.
The elevated pressure under which the gas is dissolved in the liquid substance or mixture of substances can be in the range from 5 bar to 800 bar, but preferably the pressure is in the range from 10 bar to 350 bar, and particularly preferably in the range from 20 bar to 250 bar.
Preferably, the dissolution of the gas in the liquid substance or mixture of substances is accelerated by mixing the gas with the liquid substance or mixture of substances. This mixing can be achieved, for example, by shaking or rolling the pressure vessel into which the liquid to be pulverized has been introduced. Alternatively, the solution formed in the pressure vessel can be stirred by means of an agitator. Yet another possibility for achieving good mixture of the liquid to be pulverized with the gas is to recirculate the liquid phase present in the pressure vessel and/or the gas phase, i.e. to pump it out of the pressure vessel and to feed it back to the pressure vessel in the area of the other respective phase. Yet another possibility is the use of a static mixer. Obviously, the abovementioned procedures can also be combined.
The process according to the invention functions in principle with any solid pulverulent auxiliary. However, those which are particularly suitable are auxiliaries having as small a particle size as possible, for which reason, according to a preferred embodiment of the process according to the invention, the particle size is less than
Dirscherl Johann
Steiner Rudolf
Weidner Eckhard
Weinreich Bernd
Adalbert-Raps-Stiftung
Theisen Mary Lynn
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
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