Supercritical fluids physical parameters

Table 12-1. Physical parameters of selected supercritical fluids.

Temperature and pressure are the two most important physical parameters in SFE because together, they define the density and hence, the solvating power of a supercritical fluid. As such, there is an imperative, need to research on the effects of these two parameters so that SFE of the amine surfactant can always be carried out at the optimum conditions. It has been discovered that at 85°C and 100 bar, as high as 96% of the surfactant can be removed within an hour. At constant pressure, high extraction efficiencies can generally be obtained at 50°C and 85°C while at constant temperature, satisfactory efficiencies occur at 100 and 250 bar. Like liquid extraction [3], SFE produces HMS of more enhanced mesoporosity as compared to that of calcination. 

The evaluation of the sublimation pressure is a problem since most of the compounds to be extracted with the supercritical fluids exhibit sublimation pressures of the order of 10 14 bar, and as a consequence these data cannot be determined experimentally. The sublimation pressure is thus usually estimated by empirical correlations, which are often developed only for hydrocarbon compounds. In the correlation of solubility data this problem can be solved empirically by considering the pure component parameters as fitting-parameters. Better results are obviously obtained [61], but the physical significance of the numerical values of the parameters obtained is doubtful. For example, different pure component properties can be obtained for the same solute using solubility data for different binary mixtures. 

Environmental applications of SFE appear to be the most widespread in the literature. A typical example is the comparison of extraction efficiency for 2,3,7,8 -tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) from sediment samples using supercritical fluid extraction and five individual mobile phases with Soxhlet extraction was made (101). The mobile phases, carbon dioxide, nitrous oxide, pure and modified with 2% methanol as well as sulfur hexafluoride were examined. Pure nitrous oxide, modified carbon dioxide and modified nitrous oxide systems gave the recoveries in the acceptable range of 80 to 100%. Carbon dioxide and sulfur hexafluoride showed recoveries of less than 50% under identical conditions. Classical Soxhlet recoveries by comparison illustrated the poorest precision with average extraction efficiencies of less than 65%. Mobile phase choice, still as yet a major question in the science of supercritical fluid extraction, seems to be dependent upon several factors polarity of the solute of interest, stearic interactions, as well as those between the matrix and the mobile phase. Physical parameters of the solute of interest, as suggested by King, must also be considered. Presently, the science behind the extraction of analytes of interest from complex matrices is not completely understood. 

Table 1 groups together differing orders of magnitude of physical parameters for the three states of the same fluid. It should be noted that in spite of high densities (similar to liquids), supercritical fluids are only slightly viscous and, from this point of view, have similar properties to gases. 

What is unique to supercritical fluids compared to typical hquids at ambient conditions is that the variable density allows the same fluid to have a variable solvent power - i.e., the degree of solubdity of a particular component depends upon the density of the supercritical fluid solvent. Now the chemist has three parameters to exploit density (pressure), temperature, and composition. With supercritical fluids it is common to exploit the variable solvent power afforded by adjusting the density first. This allows the same fluid to be used to dissolve famihes of components selectively just by changing a physical parameter (pressure at a given temperature). At the highest densities, the solvent power of the supercritical fluid is very nearly that of the same fluid as a liquid. At that point, it is common to exploit mixtures of typical solvents in the bulk supercritical fluid to achieve more selectivity for an additional range of solute s/analytes. 

Let us pause here for a moment and consider what we have gained in the supercritical region. The density of the supercritical fluid varies in a continuous manner. The variable density results in a variable solvent power, thereby supporting selective solvation with a single fluid the solvent power can be selected and controlled by controlling physical parameters (temperature. 

In this chapter we describe the methods used to calculate solubility isotherms as well as the entire phase diagram for binary and ternary solute-SCF mixtures. The objective of the first part of the chapter is to discuss the relevant physical properties of the solute and solvent pair that are needed to describe the intermolecular forces in operation between molecules in a mixture that ultimately fix solubility levels. A brief description is provided on the application of solubility parameters to supercritical fluids. 

Supercritical fluid extraction (SFE) is a separation technique that uses sc-fluids as separating solvents. Supercritical fluids can replace other solvents in many purification procedures, even in countercurrent extraction. In synthetic chemistry, SFE can be an alternative to conventional methods for purification/isolation of complex products, for example pharmaceuticals, nutraceuticals and vitamins [12, 18j. Since SFE is still quite a young discipline, physical properties and basic parameters for many interesting compounds and mixtures are not yet known (in contrast to classical methods like distillations). Therefore, it must be pointed out that for all applications of sc-fluids the phase equilibria have to be determined properly. Unfortunately, for many technical or industrial applications of procedures based on supercritical fluids, the basic parameters are often not yet known. For industrial implementation, scale-up, miniplant, or pilot plant activities, it is absolutely necessary to have information about phase behaviour, solubility, energy balances and... 

A supercritical fluid is defined as any substance that is above its critical temperature and pressure. Supercritical fluids have physical properties intermediate between liquid and gas phases the solvating power (density) of a SE is similar to that of a liquid, and its diffusivity and viscosity are similar to that of a gas. Carbon dioxide (CO2) is the most widely used SE because of its inertness, low cost, high purity, low toxicity, and low critical parameters (CO2 Tc = 31.3°C, Pc = 72.9 atm). If extraction cannot be achieved using CO2, a more polar SE (e.g., N2O or CHE3) can be used. Alternatively, a polar modifier (MeOH, EtOH, or H2O) may be added to the SE in order to increase the solvating power. Several SEE applications have been reported in peer-reviewed literature for selective isolation of residues from food. " The number of published applications has decreased in recent years, which may be... 

Of these featores, the pressure-dependence of SCF properties dominates or influences virtually every process conducted on polymers. Pressure governs such properties as density, solubility parameter, and dielectric constant changes of more than an order of magnitude are common when pressure is sufficiently increased to transform a gas into a supercritical fluid. This chapter primarily compiles experimental data on the pressure dependence of physical properties of fluid phase polymer-SCF mixtures. Phase equilibria are addressed, including the solubility of polymers in SCFs, the solubility of SCFs in liquid polymers, and the three-phase solid-fluid-fluid equilibria of crystalline polymers saturated with SCFs. Additional thermodynamic properties include glass transition temperature depressions of polymers, and interfacial tension between SCF-swollen polymers and the SCF. The viscosity of fluid phase polymer-SCF mixtures is also treated. 

The physical properties of the solutes also play a crucial role in critical fluid extraction processes, particularly with regard to their molecular structure and temperature-dependent properties. The author has offered some insight into predicting solute solubility in supercritical fluids based upon a correlation between a molecular group structure contribution-solubility parameter correlation [11]. Suffice to say, the introduction of polar functional groups into a compound usually results in the need for a higher extraction pressure and/or temperature [12]. The vapor pressures of the solutes to be extracted or... 

Supercritical fluid extraction (SEE) is a selective technique of sample preparation that enables the preparation of matrices by varying several physical parameters. Nowadays, it is considered to be the best replacement for many extraction technologies, such as accelerated solvent, Soxhlet solvent, microwave assisted extraction and so on. It was originally marketed as a universal extraction tool in 1988 by Isco Inc. (Lincoln, Nebraska, USA), Lee Scientific (Salt Lake City, Utah, USA) and Suprex Corp. (Pittsburgh, Pennsylvania, USA). The basic components of the SFE instmment are a carbon dioxide reservoir, a pump, an extraction vessel, an oven, a restrictor... 

Reacting lipophilic substrates with hydrophilic compounds, as in the case of most transesteriflcation reactions, is one of the major difficulties in lipase-catalyzed reactions. Several parameters need to be considered to overcome this immiscibility problem. One commonly proposed strategy is the use of a nonaqueous medium. In this chapter, the advantages of using nonaqueous media in biochemical synthesis reactions, over aqueous and solvent-free systems, are discussed. The use of hydrophobic solvents is also discussed, followed by a presentation of the alternatives that can overcome the limitations of solvents. The focus of this chapter is mainly on the use of supercritical fluids (SCFs) as a green alternative reaction medium. The chapter also discusses ionic liquids (ILs) as another alternative. These solvents and the factors affecting their physical properties and their effect on the activity and stability of lipase are also discussed. 

TABLE 15.6 Physical Parameters of Selected Supercritical Fluids... 

In this chapter an environmental friendly extraction process of com germ oil based on the use of supercritical CO2 (SC-CO2) is presented. The effect of important operating parameters in supercritical fluid extraction (SEE) processes such as pressure, temperature and flow rate on the extraction kinetics and the quality of the extracted oil is discussed. As for many SC-CO2 extractions of vegetable oils, extraction curves of com germ oil present an initially linear part with a slope close to the oil solubility value in CO2. Then, a second section of the extraction curve is determined by the diffusional resistance in the solid matrix. Characterization of supercritical cmde com oil is presented by showing some properties reported in the literature such as physical parameters, fatty acid composition, neutral lipids, content of tocopherols, acid index, peroxide value, antioxidant capacity and the oxidative stability. 

Although supercritical extraction (SFE) has been known for some time, it is still a relatively new technique to the analytical chemist. Before developing an SFE method, the chemist must understand the composition of the matrix and the analyte properties. The key instrumental parameters affecting the extraction of analytes from the matrix include fluid density, temperature, and fluid composition. Both the make-up of the matrix and the analytes must be considered when selecting the extraction conditions. Consideration of the extraction parameters must be given with respect to their affect on the analytes of interest and on the compounds present in the matrix that may either coextract with the analytes or inhibit their extraction by physical or chemical means. 

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