Low-temperature supercritical drying

In order to avoid the use of flammable and explosive solvents, a much safer process, LTSCD, uses nonflammable solvents and more moderate processing conditions. It requires, however, the replacement of the solvent used during the sol-gel synthesis by one with a considerably lower critical temperature. The solvent most commonly used today for LTSCD is carbon dioxide due to its rather mild critical point conditions (for CO2 Tc = 31.1°C, Pc = 7A MPa) [176,177]. Also, liquid CO2 is well miscible with most of the organic solvents used in the synthesis and generally well suited for the extraction of organic compounds. 

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Stengl, V., Bakardjieva, S., Subrt, J. et al. (2006) Titania aerogel prepared by low temperature supercritical drying. 

Wang, P., Emmerling, A., Tapperfi W. et al. (1991) High-temperature and low-temperature supercritical drying of... 

Three different MgO samples were used in the experiments. AP-MgO with a surface area of 385 m2/g was prepared by a sol-gel technique involving high-temperature supercritical drying described in detail earlier.4,13 CP-MgO (281 m2/g) was obtained by decomposition of Mg(OH)2 prepared by hydration of commercial MgO. In both cases the final preparation step was overnight evacuation at 500°C followed by storage under ambient conditions. Their performance was compared to that of a commercial low surface area sample CM-MgO (10 m2/g). 

Sievers RE, Milewski PD, Sellers SP, Miles BA, Korte BJ, Kusek MD, Clark GS, Mioskowski B, Villa JA. Supercritical and near-critical carbon-dioxide assisted low-temperature bubble drying. Ind Eng Chem Res 2000 39 4831-4836. 

The textural properties (surface area and porosity) of obtained samples depend considerably on the preparation conditions. However, in all cases the high-temperature supercritically dried zirconia aerogels showed larger porosity and higher surface areas compared to the aerogels dried by low-temperamre extraction with supercritical CO2. 

Furthermore, zirconia aerogels dried by low-temperature extraction with supercritical C02(l) were amorphous. However, the high-temperature supercritically dried samples contained predominantly tetragonal zirconia. 

Supercritical Fluid Extraction. Supercritical fluid (SCF) extraction is a process in which elevated pressure and temperature conditions are used to make a substance exceed a critical point. Once above this critical point, the gas (CO2 is commonly used) exhibits unique solvating properties. The advantages of SCF extraction in foods are that there is no solvent residue in the extracted products, the process can be performed at low temperature, oxygen is excluded, and there is minimal protein degradation (49). One area in which SCF extraction of Hpids from meats maybe appHed is in the production of low fat dried meat ingredients for further processed items. Its apphcation in fresh meat is less successful because the fresh meat contains relatively high levels of moisture (50). 

Fig. 1. Crack-free monolithic titania-silica aerogel photos, (a) aerogel prepared by hi -temperature ethanol supercritical drying, (b) aerogel prepared by low-temperature CO2 supercritical drying.

Carbon dioxide, as can most other substances, can exist in any one of three phases—solid, liquid, or gas—depending on temperature and pressure. At low temperatures, carbon dioxide exists as a solid ("dry ice") at almost any pressure. At temperatures greater than about -76°F (-60°C), however, carbon dioxide may exist as a gas or as a liquid, depending on the pressure. At some combination of temperature and pressure, however, carbon dioxide (and other substances) enters a fourth phase, known as the supercritical phase, whose properties are a combination of gas and liquid properties. For example, supercritical carbon dioxide (often represented as SCC02, SC-C02, SC-CO2, or a similar acronym) diffuses readily and has a low viscosity, properties associated with gases, but is also a good solvent, a property one often associates with liquids. The critical temperature and pressure at which carbon dioxide becomes a supercritical fluid are 31.1°C (88.0°F) and 73.8 atm (1,070 pounds per square inch). 

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