Materials processing with supercritical solvents

What makes supercritical fluids so attractive in this domain is their sensitivity to a large number of processing variables in a region where transition from a single or multiphase system into another is rather simple through a variety of paths. 

One can start with a homogeneous phase and use pressure, temperature, mass separating agents, other external fields such as electromagnetic or irradiation, to nucleate and grow, or react or fractionate, to form new material with unique performance characteristics. In the homogenization step, supercritical fluids are used to solubilize. If solubilization in the supercritical fluid is not possible, the supercritical fluid can be used to induce phase separation as an anti-solvent in a subsequent step. 

The supercritical fluids are effective in heterogeneous environments as well. They penetrate into porous environment loaded with additives or, used as a pure supercritical fluid to clean, dry (extract), coat, impregnate and process (e.g. extrude) a low viscosity solution. 

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The supercritical fluid extraction (SFE) is a process in which a highly compressed gas (fluid) is brought into contact with a relatively non-volatile solid or liquid at temperatures at or slightly above the critical temperature of the solvent. Under such conditions, the condensed phase will begin to volatize, which is interpreted as the supercritical fluid phase (Vayisoglu et al., 1996). The SFE is one of the best methods to obtain hqnid fuels from coals. The SFE extraction is carried out in an autoclave at above the critical temperature and the pressure of the solvent. The yield of soluble material increases with increasing pressure (Paul and Wise, 1971). 

The PCA process uses supercritical fluid drying to help preserve fine microstructures in the material. Supercritical fluid drying is a technique that has been used for many years to dry biological materials and, more recently, aerogels (qv). The original solvent is replaced by exchange with a supercritical fluid, such as C02, and the system is depressurized above the critical temperature of the SCF. SCFs have no vapor—liquid interface. Thus fine microstructures are... 

Decontamination of soils using supercritical fluids is an attractive process compared to extraction with liquid solvents because no toxic residue is left in the remediated soil and, in contrast to thermal desorption, the soils are not burned. In particular, typical industrial wastes such as PAHs, PCBs, and fuels can be removed easily [7 to 21]. The main applications are in preparation for analytical purposes, where supercritical fluid extraction acts as a concentration step which is much faster and cheaper than solvent-extraction. The main parameters for successful extraction are the water content of the soil, the type of soil, and the contaminating substances, the available particle-size distribution, and the content of plant material, which can act as adsorbent material and therefore prolong the extraction time. For industrial regeneration, further the amount of soil to be treated has to taken into account, because there exists, so far, no possibility of continuous input and output of solid material for high pressure extraction plants, so that the process has to be run discontinuously. 

In addition to the process benefits, there are cost and environmental benefits associated with the supercritical process. The supercritical fluid process has low operating energy costs when compared to other alternative solvent processes and the cost of the carbon dioxide used to supply the system is orders of magnitude less than the purchase costs of chlorofluorocarbons, especially with the added taxes imposed by the federal government. In addition, carbon dioxide is a more environmentally friendly material and does not have the disposal costs associated with other alternatives. 

If large quantities are used for technical processes, e.g. for cleaning, the recovery and reuse of the microemulsion or at least of a considerable amount of the most expensive components is desired. Therefore, strategies are needed to separate contaminants from the organic microemulsion components. Separation is usually more complicated than from ordinary solvents and often requires several steps [39, 40]. In particular, the separation of waste materials from the surfactants is usually very difficult or often even impossible. The temperature-dependent phase behaviour of bicontinuous microemulsions, however, can sometimes be beneficially used for separation [41]. Easy separation, at least from the unpolar solvent, can be achieved from microemulsions with supercritical liquids [42]. 

Fractionation of a polymer into its separate oligomers, as reported by Jentoft and Gouw, is an impressive achievement for supercritical fluid solvents. However, in general it is achieved only in conjunction with chromatography with materials processed at microgram to milligram levels. Supercritical fluid... 

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