Supercritical drying with carbon dioxide

Carbon aerogels can be prepared from the organic gels mentioned above by supercritical drying with carbon dioxide and a subsequent heat-treating step in an inert atmosphere. 

Figure 9.5. Density of cellulose aerogels prepared by Innerlohinger et al. [19] using NMMO as a solvent and supercritical drying with carbon dioxide (redrawn on the basis of their data using different units).

It is less well known, but certainly no less important, that even with carbon dioxide as a drying agent, the supercritical drying conditions can also affect the properties of a product. Eor example, in the preparation of titania aerogels, temperature, pressure, the use of either Hquid or supercritical CO2, and the drying duration have all been shown to affect the surface area, pore volume, and pore size distributions of both the as-dried and calcined materials (34,35). The specific effect of using either Hquid or supercritical CO2 is shown in Eigure 3 as an iHustration (36). 

However, even with carbon dioxide as a drying agent, the supercritical drying conditions can affect the properties of a product. Other important drying variables include the path to the critical point, composition of the drying medium, and depressurization. 

Using solvents at the non-polar end of the spectrum offers a means of selectively extracting specific actives. An example from the nutraceutical sector is the selective extraction of ginkgo biloba with a high level of acetone in an ace-tone/water solvent mixture, which produces a 50 1 extract (50 kg of dried herb produces 1 kg of dried extract) standardised on 24% flavone glycosides and 6% terpenes. Sub- and supercritical forms of carbon dioxide produce very-fine quality extracts, and here again there is the possibility of using the difference in polarity between the sub- and supercritical states to assist in selective extraction. 

Miscellaneous Patents. The commonality in this section is the diversity of the patents. For example, grouped together are patents describing a coal slurry combustion process, the dry cleaning of clothes with carbon dioxide, the separation of inorganic salts from water, and the growth of crystals from supercritical solution. 

Christianson, D. D., J. P. Friedrich, G. R. List, K. Warner, E. B. Bagley, A. C. Stringfellow, and G. E. Inglett. 1984. Supercritical fluid extraction of dry-milled corn germ with carbon dioxide. J. Food Sci. 49 229. 

Christianson, D.D. et al. Supercritical fluid extraction of dry-milled com germ with carbon dioxide, J. Food Sci., 49, 229,1984. 

The rM5-modified alcogel is then subjected to supercritical drying. The supercritical drying process is based on the carbon dioxide extraction method [4]. The standard process parameters pressure and temperature versus time are shown in Figure 28.3. The substitution of carbon dioxide for alcohol was performed at a supercritical condition of carbon dioxide. The temperature of substitution is 80°C and the pressure is 16 MPa. At this condition, alcohol is soluble in supercritical state carbon dioxide with every concentration. The whole process of supercritical drying takes ordinarily about 2 days. The maximum size of TMSA obtained by the above process is approximately 200 x 300 mm. ... 

---