Solvents, supercritical drying

Using inorganic salts as precursors, alcohol as the supercritical drying agent, and a batch process a solvent-exchange step was necessary to remove water from the gel. 

Another supercritical C02 extraction method employed vegetable oil as co-solvent. Sun-dried tomatoes (50 per cent moisture content) were further dried at 70°C and 20mbar for 70-80h. The final product, containing about 6 per cent moisture, was ground to a particle size of 1 mm. 

Cracking of PS because of capillary forces can be circumvented if one avoids crossing the liquid-vapor boundary in the phase diagram of the solvent. This is the case for supercritical drying [Ca4] or freeze drying [Ami], as shown in the inset of Fig. 6.12. 

Distortions of patterns due to capillary forces can be avoided altogether by applying supercritical drying (SCD) [83,84]. In a supercritical liquid, the surface tension becomes neghgible (y 0) and the capillary force vanishes. However, depending on the solvent, high pressure (1-10 MPa) and elevated temperatures (40-400 °C), may be required [85]. This factor renders this method not imiversally applicable for wet processing. For example, if SCD was to be used for isopropanol removal from SU-8, temperatures over 235.2 °C and pressures over 4.8 MPa would be needed [85]. 

The final approach to the reduction of eluent consumption is the optimal recycling of solvents. Indeed, preparative and industrial chromatography can be designed as a unit operation that includes solvent recycling dry feed mixture is injected while dry separated compounds are recovered. Many techniques can be applied depending on the situation in isocratic (that is with a constant mobile phase composition) or gradient conditions, and with organic and/or supercritical eluents. 

Different researchers have utilized solvent exchange prior to supercritical drying. In this method, the solvent (alcohol) was substituted with liquid Carbon Dioxide. This method was applied to the production of silica aerogel by Tewari et al. and Rangarajan et al. (7, 8). 

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