Supercritical drying process types and their applications
    Currently, supercritical drying technique is mainly used gel (water gel, alcohol gel, etc.) desolventising drying process, reported in the literature supercritical drying operation There are many ways, this is mainly discussed several typical supercritical drying process is characterized by its application.
 A high temperature supercritical drying of organic solvents
    The critical temperature of the water, the critical pressure, and water in a supercritical state prone to dissolve the gel problem, so hydrogels are not suitable for direct supercritical drying. Using inorganic (water glass) hydrogels prepared, need alcohols (such as the alcohol) replacing water in the water is methanol gel gel gel supercritical methanol, and then dried (critical point of methanol 240 ℃, 7.93MPa), which is used in high-temperature 1931 Kistlcr supercritical methanol were prepared by drying the first airgel - silica aerogels. Subsequently, the high temperature supercritical ethanol (or acetone) were prepared by drying the alumina airgel, Ti02 airgel, titanium oxide aerogels, organic aerogels, carbon aerogels and so on.
Alcohols such as methanol as a solvent with a high temperature supercritical drying, a surface of the gel network esterification, the resulting airgel surface hydrophobicity, small easily absorb moisture in the air, the nano-porous structure is very stable, long-term storage. However, the critical temperature of the alcohol is relatively high, and because flammable. Methanol is also toxic.
(2) low-temperature drying of supercritical carbon dioxide
    1985 Tewari such as the use of supercritical carbon dioxide as the drying medium, the supercritical drying process operating temperature is substantially reduced. Temperature close to the critical temperature of carbon dioxide, and non-toxic, non flammable, the operating conditions, the solid carbon dioxide basic skeleton of the gel is chemically inert, is a purely physical process. Only while the supercritical carbon dioxide prior to drying temperature has a relatively time-consuming solvent substitution process, that is, first the liquid solvent within the gel pores (such as methanol, ethanol, acetone, etc.) after replacement with liquid carbon dioxide, supercritical carbon dioxide and then dried. Low temperature drying process supercritical carbon dioxide pressure versus time curve, the drying cycle takes about 40h. Supercritical carbon dioxide with low-temperature drying of airgel, which has strong hydrophilic surface, put a long time will absorb moisture from the air, usually by heating (100-250 ℃) to remove without affecting its nanoporous structure. Recently, Aerogels are mostly using this supercritical drying method.
3 drying temperature supercritical carbon dioxide extraction
    If the low-temperature drying of supercritical carbon dioxide solvent replacement process used liquid carbon dioxide into a supercritical carbon dioxide fluid, when the operation is to supercritical carbon dioxide extraction temperature drying process. Supercritical carbon dioxide with low-temperature drying operation method compared to low-temperature drying of supercritical carbon dioxide extraction method so that the entire drying time is further reduced, significantly reducing operating costs. However, during operation, to ensure the operating temperature and pressure should be above the curve of binary mixtures critical, because above the critical curve, methanol and carbon dioxide is completely miscible single phase (supercritical fluid) region, there is no surface tension; below the curve in the critical steam to carbon dioxide and liquid coexisting a yeast two-phase zone, the resulting table
Hang tension will lead to the structure encountered gas condensate dirty destruction.
4 supercritical reaction temperature rapid drying
   Operation, the first synthetic silica airgel precursor liquid used was added to the reaction pressure in a desiccator, and the position of the snake tube heater temperature to 300 ℃ (about 15min), pressure was increased to about 20MPa, then, condensate Glue has been basically formed. In the relief valve under the control of the reaction temperature in ethanol miss small dryer, the dryer when the reaction pressure is reduced to 0.2MPa (about 15min), heating was stopped, the outer pressure vessel to pass into the cooling fluid drier reaction cooled and the reaction with dry compressed air dryer purge silica airgel was cooled to room temperature and then the reaction pressure vessel dryer removed from the outer and open, you can get a dry silica airgel. The whole process (including mixed, gelation, aging and drying in a high-temperature rapid supercritical reaction once the dryer) - as required 3h (but not less than 1h). This supercritical drying method than the conventional silica airgel obtained significantly higher elastic modulus three times, its production cost is lower 8 times. During the rapid heating, the outer pressure vessel to eliminate fluid expansion stress generated.
5 pre-pressurized inert gas supercritical drying
In order to prevent the gel pores to reach the liquid solvent evaporation occurs before the supercritical state, generally using an excess of solvent liquid (previous several belong in this case) before using quiet or heating gas (plus a nitrogen, argon, etc. ) pre-pressurized measures. It is reported that, during the drying operation before the supercritical nitrogen pre 8MPa avoid gel contraction can also shorten the drying time. It was also found that, in the preparation of aerogels Zr02, the pre nitrogen plus a gel can be prevented or reduced shrinkage during the drying and destruction, maintaining the pore structure Zr02 airgel.

 

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