Drying and gas or vapor contact with solids – Process – With contacting of material treated with solid or liquid agent
Reexamination Certificate
2000-09-11
2003-02-11
Lovering, Richard D. (Department: 1712)
Drying and gas or vapor contact with solids
Process
With contacting of material treated with solid or liquid agent
C034S351000, C034S505000, C252S062000, C423S338000
Reexamination Certificate
active
06516537
ABSTRACT:
The present invention relates to a process for drying microporous, fluid-containing particles and a process for the preparation of microporous, three-dimensionally networked particles in which this drying process is used.
It is known that hydrogels, e.g. silica hydrogels which can be prepared by precipitating gels from waterglass, can be dried under supercritical conditions to give microporous, three-dimensionally networked silica particles. In the supercritical drying, the interfacial tension of the fluid contained in the microporous particles is completely or substantially eliminated with the object of substantially avoiding shrinkage of the microporous particles during drying, since characteristic properties of the microporous particles are wholly or partly lost on shrinkage. In the case of gels, such a product obtained by supercritical drying is referred to as an aerogel. In contrast to conventional drying without special precautions, in which the gels suffer a large volume contraction and xerogels form, only a small volume contraction (<15%) thus takes place during drying close to the critical point.
The prior art for the preparation of aerogels by means of supercritical drying is described in detail in, for example, Reviews in Chemical Engineering, Volume 5, No. 1-4, pages 157-198 (1988) in which the pioneering work by Kistler is also described.
The literature also discloses the continuous preparation of aerogel powders, cf. for example U.S. Pat. No. 2,572,321, DE-C-1 030 312, U.S. Pat. No. 2,868,280 and U.S. Pat. No. 5,032,555. A suspension of a finely divided, fluid-containing gel is brought to supercritical pressure with respect to the fluid phase of the gel by means of a pump. To reach the supercritical temperature of the fluid phase of the gel, the suspension flows through a heat exchanger and is then released via a pressure control valve. The aerogel is deposited in cyclones and on filters. The disadvantage of the powder method is that the total amount of liquid required for suspending has to be brought to a supercritical temperature, which results in a corresponding energy demand. Further disadvantages are the abrasion in the expansion valve and the dust production by the product. Apart from this, special filter apparatuses, such as expensive, encapsulated belt filters, etc., have to be used for the preparation of the finely divided, fluid-containing gel in any wash or salt-removal step and, if a hydrogel is present, in an exchange of the water for, for example, flammable fluid.
In addition to the continuous preparation of aerogel powders, the continuous preparation of silica gel is also known, cf. in this context U.S. Pat. No. 2,436,403, U.S. Pat. No. 2,485,249, U.S. Pat. No. 2,466,842 and U.S. Pat. No. 2,956,957 and Ullmanns Encyklopädie der technischen Chemie, 4th revised edition, Volume 21, pages 460-461. The gels are obtained, inter alia, also in bead form.
WO-A-95 06 617 relates to hydrophobic silica aerogels which are obtained by reaction of a waterglass solution with an acid at a pH of 7.5 to 11, substantial removal of ionic components by washing with water or dilute aqueous solutions of inorganic bases while maintaining the pH of the hydrogel in the range from 7.5 to 11, displacement of the aqueous phase contained in the hydrogel by an alcohol and subsequent supercritical drying of the alcogel obtained.
A process for the preparation of silica aerogel on the pilot scale has been 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, which are all carried out batchwise: preparation and aging of silica hydrogel, comminution of the hydrogel to give granules, separation of salt from the gel formed, exchange of the water in the gel for alcohol, introduction of the gel, dried by allowing to drip off, into a pressure container, heating of the pressure container, reduction of the pressure to atmospheric pressure, evacuation of the pressure container and subsequent removal of the aerogel. The disadvantage of this process is that all steps are carried out batchwise and are therefore very time-consuming, labor-intensive and expensive. White mentions no continuous process for the preparation of granules and for salt removal. In the water/alcohol exchange, White prefers, for the liquid phase, a procedure to be described as covering with the layer/impregnating/drainage and which constitutes intermittent treatment of the solid bed with liquid. White believes constant flowthrough to be less economical.
According to U.S. Pat. No. 3,672,833, the known processes for removing salt from gels and for exchanging water for other solvents are extremely tedious and hence expensive processes. In order to overcome this, gel preparation from lower alkyl orthosilicates is proposed there. However, these require a great deal of energy for their preparation.
EP-B-0 331 852 discloses a continuous process for extracting caffeine from green coffee beans, in which, the green coffee beans are passed as a moving bed countercurrently to supercritical carbon dioxide. DE-A- 35 32 957 describes an apparatus in which a particulate feedstock in the moving bed is subjected to countercurrent extraction with a solvent in the liquid or supercritical state, for example with supercritical carbon dioxide. The only specifically mentioned extraction is that of rapeseed oil from rape.
It is an object of the present invention to provide an improved, more economical process for drying microporous, fluid-containing particles and an improved, more economical process for the preparation of microporous, three-dimensionally networked particles with the use of the drying process, the abovementioned disadvantages of the prior art being avoided.
We have found that this object is achieved, surprisingly, if the fluid-containing particles to be dried are fed as a moving bed countercurrently to a drying fluid which is present under at least near-critical conditions. We have also found that an essentially continuous preparation of microporous, three-dimensionally networked particles is possible if, in addition to the drying process, any washing and/or salt removal or fluid exchange in the pores of the microporous particles and removal of sorbed gases or substances in the moving bed are also carried out by the countercurrent method. The present invention therefore relates to a process for drying microporous, fluid-containing particles, in which the fluid-containing particles to be dried are fed as a moving bed countercurrently to a drying fluid, the interfacial tension of the fluid being reduced in comparison with the interfacial tension of the fluid at room temperature, at near-critical to supercritical pressure of the fluid, preferably to a value in the range from 0 to {fraction (1/10)}, in particular from 0 to {fraction (1/20)}, of the interfacial tension at room temperature.
The range in which the present invention is preferably applied 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 significantly increase, that the thermal conductivity of the product does not significantly increase and that preferably no shrinkage above 15%, in particular no shrinkage above 10%, occurs. This circumstance can also be described in terms of 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, Sherwood, McGraw Hill, 1977, page 601 et seq., the interfacial tension at the temperature (and pressure) to be tested be ing measured and being compared with that at room temperature and atmospheric pressure under otherwise identical conditions.
In a further embodiment, the present invention relates to a process for the preparation of microporous, three-dimensionally networked particles by
(a) preparation of microporous particl
Gall Martin
Köster Herbert
Kratzer Horst
Reichert Wolfgang
Schelling Heiner
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