Supercritical solvent technology

The two fluids most often studied in supercritical fluid technology, carbon dioxide and water, are the two least expensive of all solvents. Carbon dioxide is nontoxic, nonflammable, and has a near-ambient critical temperature of 31.1°C. CO9 is an environmentally friendly substitute for organic solvents including chlorocarbons and chloroflu-orocarbons. Supercritical water (T = 374°C) is of interest as a substitute for organic solvents to minimize waste in extraction and reaction processes. Additionally, it is used for hydrothermal oxidation of hazardous organic wastes (also called supercritical water oxidation) and hydrothermal synthesis. 

The addition of a dispersed droplet phase (forming a microemulsion) provides a convenient means of solubilizing highly polar or ionic species into the low polarity environment of the SCF phase. Hence, the combination of supercritical solvents with microemulsion stractures provides a new type of solvent with some unusual and important properties of potential interest to a range of technologies. These droplets have high diffusion rates in SCF and the properties of the continuous phase can be readily controlled by manipulation of system pressure (Beckman et al., 1995). 

The process involves the use of supercritical fluids rather than liquids as solvents. A fluid is in the supercritical state when its pressure and temperature exceed the pl ical properties which defines its critical point. Carbon dioxide is by far the most widely used supercritical solvent. Many other selected fluids have potential use for SFE technologies. 

Over the past decade, much progress in supercritical fluid technology has occurred. For example, supercritical fluids have found widespread use in extractions (2-5), chromatography (6-9), chemical reaction processes (10,11), and oil recovery (12). Most recently, they have even been used as a solvent for carrying out enzyme-based reactions (14). Unfortunately, although supercritical fluids are used effectively in a myriad of areas, there is still a lack of a detailed understanding of fundamental processes that govern these peculiar solvents. 

A fluid is supercritical when it is compressed beyond its critical pressure (Pc) and heated beyond its critical temperature (r, ). Supercritical fluid technology has emerged as an important technique for supercritical fluid extraction (SFE). In many of the industrial applications, it has replaced conventional solvent-based or steam extraction processes, mainly due to the quality and the purity of the final product and environmental benefits. 

Supercritical Fluid Technology Reactions Table 3 Heterogeneous catalytic reactions in supercritical solvents 2921... 

Rapid expansion of supercritical solutions (RESS) processing is used to prepare microspheres. Microencapsulation takes place when a pressurized supercritical solvent containing the shell material and the active ingredient is released through a small nozzle the abrupt pressure drop causes the desolvation of the shell material and the formation of a coating layer around the active ingredient (74). A prerequisite for this technology is that the compounds effectively dissolve in the SCF, which limits its application. 

Mukhopadhyay M, Dalvi SV. Solid solubility prediction from partial molar volume fraction of solvent in antisolvent-solvent mixture. Proceedings of Super Green-2002, 1st International S5mposium on Supercritical Fluid Technology for Energy and Environmental Applications, Suwon, Korea, Nov. 3-6, 2002. 

A chemical destruction method that has been used for the treatment of PCBs in contaminated dielectric liquids or soil is based on the reaction of a polyethylene glycol/potassium hydroxide mixture with PCBs (De Filippis et al. 1997). This method can be used successfully for the destruction of higher chlorinated PCBs with an efficiency of >99%, but was found to be unsuitable for the treatment of di- and trichlorobiphenyls due to low destruction efficiencies (Sabata et al. 1993). Irradiation of PCBs in isooctane and transformer oil by y-radiation resulted in degradation of PCBs to less chlorinated PCBs and PCB-solvent adducts (Arbon et al. 1996). Supercritical fluid technology has shown promise as a method for extraction of PCBs from soils, coupled with supercritical water oxidation of the extracted PCBs (Tavlarides 1993,1998a). Hofelt and Shea (1997) demonstrated the use of semipermeable membrane devices to accumulate PCBs from New Bedford Harbor, Massachusetts water. Another method showing... 

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