ADVANTAGES OF SUPERCRITICAL FLUIDS

Supercritical fluids have long been known for their abilities to dissolve organic contaminants. Their ability to display a wide range of solvent characteristics and the ability to tune solubility with small changes in temperature and pressure were identified early on in our search for alternative cleaning methods. The gas-like diffusivity and low surfece tension combined with liquid-like densities were important 

Perfluoropolyether fluids (ex., Krytox ) are commonly used as lubricants, dielectric fluids or heat transfer fluids in a number of space and military products. These fluids and other perfluorolubricants are almost impossible to dissolve in solvents other than halocarbons. Additionally, other halogenated damping fluids have compatibility problems with some of the common solvents which can dissolve the fluid. Early experiments with supercritical carbon dioxide indicated that it would dissolve both halogenated damping fluids and perfluoropolyether lubricants and thus was a candidate as a replacement solvent cleaning process. 

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. 

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Supercritical Extraction. The use of a supercritical fluid such as carbon dioxide as extractant is growing in industrial importance, particularly in the food-related industries. The advantages of supercritical fluids (qv) as extractants include favorable solubiHty and transport properties, and the abiHty to complete an extraction rapidly at moderate temperature. Whereas most of the supercritical extraction processes are soHd—Hquid extractions, some Hquid—Hquid extractions are of commercial interest also. For example, the removal of ethanol from dilute aqueous solutions using Hquid carbon dioxide... 

The advantages of supercritical fluid chromatography for polymer separations have been illustrated in the literature for many years. A recent example is the separation of long-chain polyprenols using SFC with matrix-assisted laser-desorption ionization TOF mass spectrometry [10]. The generic name for 1,4-polyprenyl alcohols is polyprenol these compounds generally have smaller polymerization chains of less... 

The further optimization and development concerning stability and selectivity of the organometallic catalyst in these kinds of media and the application of isolation methodologies similar to CESS (catalysis and extraction using supercritical solutions [43]) together with the physical and chemical advantages of supercritical fluids can lead to high potential catalyst matrices that fulfil the requirements of industrial processes both for bulk and fine chemicals. 

Figure 15.4 shows a schematic diagram of a supercritical fluid extraction apparatus. The advantages of supercritical fluid extraction (SFE) are as follows ... 

A great advantage of supercritical fluids is the possibility to separate the extracted solute from the solvent by simple pressure reduction to subcritical pressures. The solubility of most solutes in supercritical fluids virtually vanishes and therefore the solutes precipitate at these conditions. For example, pyrene solubil-... 

As fluids in a subcritical state retain most of the advantages of supercritical fluids, some extractions can be undertaken under these conditions. 

Supercritical fluid extraction has been applied to extract phenolic acids from a variety of plant samples. It uses high-pressure to force carbon dioxide to be a mixture of liquid and gas phases, which is called a supercritical fluid. The liquid and gas phase mixture of carbon dioxide can more readily permeate the sample matrix than only the gas phase of carbon dioxide. The compounds solublized in the liquid phase of carbon dioxide are extracted from the sample matrix and collected after they elute from the outlet of the system. The biggest advantage of supercritical fluid extraction is that there is no, or less, organic solvent involved in the extraction, due to the use of carbon dioxide supercritical fluid as the major solvent.. The carbon dioxide readily evaporates as gas phase at the system outlet. Thus, unlike other solvent extraction methods, there is no evaporation step for the extraction, making this an environmentally friendly method. However, the system is much more expensive and delicate than the other novel technology extraction... 

The advantages of supercritical fluid extraction over conventional solvents is that the supercritical fluids have a higher diffusion rate than liquids, lower viscosity than liquids, and higher vapor pressure that allows easy evaporation of solvent. The lower viscosities and higher diffusion rates of supercritical... 

From the viewpoint of chemical processes, the main advantages of supercritical fluids are... 

The application of supercritical fluids, for example SCCO2, as an environmentally acceptable replacement for conventional solvents, is well documented in the industry. Based on the work of Zosel, the decaffeination of coffee and tea using SCCO2 was the first industrial use of this technology [1]. The advantages of supercritical fluids are not only useful in separation techniques, for example supercritical fluid extraction (SFE) or supercritical chromatography (SFC), their application as process solvents is well recognized [2, 3]. 

Although the relatively poor solvent power of carbon dioxide may be used to advantage for selective separations, as explained above, it is a key technical issue that is limiting its widespread use. It can, in fact, spoil the economics of otherwise highly attractive processes, namely integrated reaction-separation. The recent trend is to tackle this problem of solubility by using biphasic mixtures of COj and liquid compounds, where the liquid phase retains some of the advantages of supercritical fluids, but with enhanced solvent power. 

Table I. Possible advantages of supercritical fluids for waste remediation/ treatment, and as an alternative solvent for waste minimization.

A major advantage of supercritical-fluid chromatography (SFC) is that it offers the advantage of liquid-like solubility, with the capability to use a nonselective gas-phase detector such as flame ionization detector. Other advantages of using supercritical fluids for extractions are that they are ine ensive, contaminant free, and less costly to dispose safely than organic solvents. 

Particle Mass Transfer Coefficients. It is well known that the advantages of supercritical fluids in extractions are based on their optimal property values, which lie between those of gases and liquids. Thus, with high diffiisivity and low viscosity values, they are normally higher tlian those of liquids. Wakao and Kaguei s [45] correlation for mass transfer from particles is a good starting point. It has been successfully applied by a... 

In recent years, fluids have been exploited above their critical temperatures and pressures and the term supercritical fluids has been coined to describe these media. The greatest advantages of supercritical fluids are typically not too far (say 10-50 K) above their critical temperatures. Nitrogen gas in a cylinder is a fluid, but is not usually considered as a supercritical fluid, but more often described by an older term as a permanent gas. Sometimes substances are exploited a little below the critical temperature in the liquid region where the behaviour is similar to that of a supercritical fluid. For this reason some people prefer the term near-cri-... 

Despite many potential advantages of supercritical fluid solvent, there are relatively few examples of electrosynthesis in supercritical fluids. Consequently, the technologies are relatively... 

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