Drying and gas or vapor contact with solids – Process – Gas or vapor contact with treated material
Reexamination Certificate
2000-09-11
2002-08-27
Doerrler, William (Department: 3749)
Drying and gas or vapor contact with solids
Process
Gas or vapor contact with treated material
C034S329000, C034S340000, C034S351000, C034S338000, C034S349000, C034S389000, C264S042000, C264S049000, C264S050000, C252S062000, C423S338000
Reexamination Certificate
active
06438867
ABSTRACT:
The present invention relates to a process for drying microporous, fluid-containing particles, a process for the preparation of microporous, three-dimensionally networked particles in which this drying process is used, and an apparatus for carrying out the drying process.
It is known that hydrogels, e.g. silica hydrogels, which can be prepared by precipitating a gel from waterglass, can be dried under supercritical conditions to give microporous, three-dimensionally networked silica particles. During this supercritical drying, the interfacial tension of the fluid contained in the microporous, three-dimensionally networked particles is completely or substantially eliminated, with the object of substantially avoiding shrinkage of the microporous, three-dimensionally crosslinked particles during the drying, since characteristic properties of the microporous, three-dimensionally crosslinked particles are completely or partly lost in the case of shrinkage. In the case of gels, such a product obtained by supercritical drying is referred to as an aerogel. In contrast to the conventional drying without particular measures, during which the gels suffer a large volume contraction and xerogels form, only a small (<15%) volume contraction thus occurs on drying close to the critical point.
The prior art for the preparation of aerogels by means of supercritical drying is described in detail, for example, in Reviews in Chemical Engineering, Volume 5, No. 1-4, pages 157-198 (1988), in which the pioneering work of Kistler is also described.
In the known processes for the preparation of an aerogel, the required heat for circumventing the two-phase region of the fluid contained in the pores of the particles to be dried is supplied by conduction through the container wall (cf. Reviews in Chemical Engineering, Volume 5, No. 1 to 4 (1988); Ind. Eng. Chem. Res. 30 (1991), 126-129; and Journal of Materials Science 29 (1994), 943-948). It is known that the wall/volume ratio becomes more unfavorable with increasing container volume, correspondingly increasing the batch times on scale-up. Furthermore, the thickness of the pressure-resistant container wall increases with the container diameter. In the case of heat supply externally into a thick-walled container under pressure, thermal stresses in the container wall limit the temperature difference between the inner surface and outer surface of the pressure-resistant container, so that the specific heat supplied by unit volume (watt/m
3
) into the pressure-resistant container is additionally reduced.
WO-A-95 06 617 relates to hydrophobic silica aerogels, which are obtained by reacting a waterglass solution with an acid at a pH of from 7.5 to 11, substantially removing ionic components from the resulting hydrogel by washing with water or dilute aqueous solutions of inorganic bases while keeping the pH of the hydrogel at from 7.5 to 11, displacing the aqueous phase contained in the hydrogel by an alcohol and then subjecting the alcogel obtained to supercritical drying.
A process for the preparation of a silica aerogel on the pilot scale is described by White in Industrial and Engineering Chemistry, Volume 31 (1939), No. 7, pages 827-831, and in Trans. A. J. Chem. E. (1942), 435-447. The process comprises the following steps: preparation and aging of a silica hydrogel, comminution of the hydrogel to give granules, separation of the salt from the gel formed, replacement of the water in the gel by alcohol, introduction of the gel, dried so that it no longer drips, into a pressure-resistant container, heating of the pressure-resistant container, reduction of the pressure to atmospheric pressure, evacuation of the pressure-resistant container and subsequent removal of the aerogel. The disadvantage of this process is that all steps are carried out batchwise and are thus very time-consuming, labor-intensive and expensive. White does not mention any continuous processes for the preparation of granules or for the removal of salt. In the water/alcohol exchange, White prefers, for the liquid phase, a procedure to be described as “covering with a layer/impregnation/drainage”, which procedure constitutes intermittent treatment of the solids bed with liquid. White believes that flow-through uniformly as a function of time is less economical.
According to U.S. Pat. No. 3,672,833, the known processes for the removal of salt from gels and for the replacement of water by other solvents are extremely tedious and hence expensive processes. To circumvent this, this U.S. patent proposes preparing the gel from lower alkyl orthosilicates. However, these require considerable energy in their preparation.
It is an object of the present invention to provide an improved, more economical process for drying microporous, fluid-containing particles, an apparatus suitable for carrying out this process and an improved, more economical process for preparing microporous, three-dimensionally networked particles with the use of the drying process, the abovementioned disadvantages of the prior art being avoided.
We have found, surprisingly, that this object is achieved if the heat required for heating to temperatures which are at least close to the critical temperature of the fluid is supplied by convection. We have furthermore found that this measure can be carried out particularly advantageously in an apparatus in which a pressure container has an inner container and a pressure-withstanding outer container, a gap being provided between the inner and the outer container, and the apparatus has suitable measuring and control apparatuses and pumps and heat exchangers. We have furthermore found that microporous, three-dimensionally networked particles can be prepared in a particularly advantageous manner if, in addition to the use of the abovementioned drying process, any required washing and/or removal of salt or a fluid exchange in the pores of the microporous particles and any required removal of sorbed gases or substances are carried out in each case in a moving bed by the countercurrent method.
The present invention therefore relates to a process for drying microporous, fluid-containing particles by reducing the interfacial tension of the fluid, preferably to 0 to 1/10, in particular to 0 to 1/20, of the interfacial tension of the fluid at room temperature, by appropriately increasing the temperature at from close to the critical pressure to supercritical pressure of the fluid. The novel process comprises supplying the heat required for the temperature increase by convection.
The present invention also relates to an apparatus for carrying out this drying process, which comprises a pressure container having an inner container and a pressure-withstanding outer container and suitable measuring and control apparatuses and pump apparatuses and heat exchangers, the inner container being provided for holding the particles to be dried and a gap or space being provided between the inner container and outer container.
The region in which the procedure is preferably carried out according to the invention can be defined by the fact that the microporous particles do not lose their properties during the drying; this means that, for example, the apparent density of the product does not increase significantly, that the thermal conductivity of the product does not increase significantly, and that preferably no shrinkage above 15%, in particular above 10%, occurs. This situation can also be described by the fact that the aerogel may not become a xerogel (gel dried at atmospheric pressure).
The abovementioned interfacial tension is determined as described in The Properties of Gases and Liquids by Reid, Brausnitz and Sherwood, McGraw Hill, 1977, page 601 et seq., the interfacial tension at the temperature (and pressure) to be tested and that at room temperature and atmospheric pressure being measured under otherwise identical conditions and compared.
In a further embodiment, the invention relates to a process for the preparation of microporous, three-dimensionally networked particles by
(a) preparing microporo
Gall Martin
Köster Herbert
Kratzer Horst
Reichert Wolfgang
Schelling Heiner
BASF - Aktiengesellschaft
Doerrler William
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Shulman Mark
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