Supercritical fluids, properties solubility parameters

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... 

With traditional solvents, the solvent power of a fluid phase is often related to its polarity. Compressed C02 has a fairly low dielectric constant under all conditions (e = 1.2-1.6), but this measure has increasingly been shown to be insufficiently accurate to define solvent effects in many cases [13], Based on this value however, there is a widespread (yet incorrect ) belief that scC02 behaves just like hexane . The Hildebrand solubility parameter (5) of C02 has been determined as a function of pressure, as demonstrated in Figure 8.3. It has been found that the solvent properties of a supercritical fluid depend most importantly on its bulk density, which depends in turn on the pressure and temperature. In general higher density of the SCF corresponds to stronger solvation power, whereas lower density results in a weaker solvent. 

Important solvent properties of SC-CO2 (e.g., dielectric constant, solubility parameter, viscosity, density) can be altered via manipulation of temperature and pressure. This unique property of a supercritical fluid could be exploited to control the behavior (e.g., kinetics and selectivity) of some chemical processes. 

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. 

Thermodynamic Properties The variation in solvent strength of a supercritical fluid from gaslike to liquidlike values may be described qualitatively in terms of the density, p, or the solubility parameter, 5 (square root of the cohesive enei density). It is shown for gaseous, liquid, and SCF CO2 as a function of pressure in Fig. 22-17 according to the rigorous thermodynamic definition ... 

The Hildebrand solubility parameter, 6, is a semi-quantitative entity related to the thermodynamic properties of dense gases (supercritical fluids) and solutions.t The solubility parameter in calories per cubic centimeter is calculated from the equation ... 

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 is another method used in PHA recovery. Supercritical carbon dioxide is widely used, due to its high solubility with other compounds. It is safe (low toxicity and reactivity) and convenient to use. Furthermore, supercritical fluids possess the unique properties of high densities and low viscosities, making them a suitable choice of solvent for extraction. The efficiency of this extraction method is highly dependent on the optimisation of the operating parameters, such as temperature, pressure, modifier as well as the exposure times [8]. 

Adidharma and Radosz provides an engineering form for such a copolymer SAFT approach. SAFT has successfully applied to correlate thermodynamic properties and phase behavior of pure liquid polymers and polymer solutions, including gas solubility and supercritical solutions by Radosz and coworkers Sadowski et al. applied SAFT to calculate solvent activities of polycarbonate solutions in various solvents and found that it may be necessary to refit the pure-component characteristic data of the polymer to some VLE-data of one binary polymer solution to calculate correct solvent activities, because otherwise demixing was calculated. GroB and Sadowski developed a Perturbed-Chain SAFT equation of state to improve for the chain behavior within the reference term to get better calculation results for the PVT - and VLE-behavior of polymer systems. McHugh and coworkers applied SAFT extensively to calculate the phase behavior of polymers in supercritical fluids, a comprehensive summary is given in the review by Kirby and McHugh. They also state that characteristic SAFT parameters for polymers from PVT-data lead to... 

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... 

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. 

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