Supercritical fluid favorable properties

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... 

Enantioselective separation by supercritical fluid chromatography (SFC) has been a field of great progress since the first demonstration of a chiral separation by SFC in the 1980s. The unique properties of supercritical fluids make packed column SFC the most favorable choice for fast enantiomeric separation among all of the separation techniques. In this chapter, the effect of chiral stationary phases, modifiers, and additives on enantioseparation are discussed in terms of speed and resolution in SFC. Fundamental considerations and thermodynamic aspects are also presented. 

Supercritical fluids possess favorable physical properties that result in good behavior for mass transfer of solutes in a column. Some important physical properties of liquids, gases, and supercritical fluids are compared in Table 4.1 [49]. It can be seen that solute diffusion coefficients are greater in a supercritical fluid than in a liquid phase. When compared to HPLC, higher analyte diffusivity leads to lower mass transfer resistance, which results in sharper peaks. Higher diffusivity also results in higher optimum linear velocities, since the optimum linear velocity for a packed column is proportional to the diffusion coefficient of the mobile phase for liquid-like fluids [50, 51]. 

A special area of HP NMR in catalysis involves supercritical fluids, which have drawn substantial attention in both industrial applications and basic research [249, 254, 255]. Reactions in supercritical fluids involve only one phase, thereby circumventing the usual liquid/gas mixing problems that can occur in conventional solvents. Further advantages of these media concern their higher diffusivities and lower viscosities [219]. The most commonly used supercritical phase for metal-catalyzed processes is supercritical CO2 (SCCO2), due to its favorable properties [256-260], i. e., nontoxicity, availability, cost, environmental benefits, low critical temperature and moderate critical pressure, as well as facile separation of reactants, catalysts and products after the reaction. 

Because of their unique characteristics, supercritical fluids have received a great deal of attention in a number of important scientific fields (1-14). Several reasons are given for choosing a supercritical fluid over another solvating system, but choice is governed generally by 1) the unique solvation and favorable mass transport properties (5) and 2) the ease with which the chemical potential can be varied simply by adjustment of the system pressure and/or temperature (13). 

The interfacial tension is a key property for describing the formation of emulsions and microemulsions (Aveyard et al., 1990), including those in supercritical fluids (da Rocha et al., 1999), as shown in Figure 8.3, where the v-axis represents a variety of formulation variables. A minimum in y is observed at the phase inversion point where the system is balanced with respect to the partitioning of the surfactant between the phases. Here, a middle-phase emulsion is present in equilibrium with excess C02-rich (top) and aqueous-rich (bottom) phases. Upon changing any of the formulation variables away from this point—for example, the hydrophilie/C02-philic balance (HCB) in the surfactant structure—the surfactant will migrate toward one of the phases. This phase usually becomes the external phase, according to the Bancroft rule. For example, a surfactant with a low HCB, such as PFPE COO NH4+ (2500 g/mol), favors the upper C02 phase and forms w/c microemulsions with an excess water phase. Likewise, a shift in formulation variable to the left would drive the surfactant toward water to form a c/w emulsion. Studies of y versus HCB for block copolymers of propylene oxide, and ethylene oxide, and polydimethylsiloxane (PDMS) and ethylene oxide, have been used to understand microemulsion and emulsion formation, curvature, and stability (da Rocha et al., 1999). 

It is well known that the properties of supercritical fluids are sensitive to pressure, and thus pressure may drastically influence the catalytic activity or the product selectivity when a reaction takes place in supercritical conditions. The favorable pressure for the Wacker oxidation of styrene is around 16 MPa of total pressure including 3 MPa 02, at which the selectivity toward acetophenone reaches 92 %, while under a total pressure of 9 MPa, the selectivity for acetophenone is lower (86 %). However, C02 with a higher pressure of over 20 MPa might retard the interaction between the substrate and the catalyst, and might cause a low concentration of substrate in the vicinity of the catalyst, thus resulting in a relatively low yield [38]. 

Supercritical fluid extraction (SFE) utilizes the properties of supercritical fluids for extraction of analytes from solid samples. A supercritical fluid (SCF) is a substance above its critical temperature and pressure, when it is between the typical gas and liquid state. Low viscosity and near-zero surface tension and heat of vaporization allow SCFs to penetrate into solids more rapidly than liquid solvents, which leads to more favorable mass transfer. The density of an SCF is close to the liquid density. 

Pure CO2 has been the preferred solvent due to its favorable properties. The CO2 is used in siphoned cylinders to assist the transference of the solvent to the pump. CO2 passes through a cooling system to increase its density before being inserted in the heating system. When required, a vessel containing a modifier can be added to the system in a way similar to that already well known in supercritical fluid extraction (SFE). 

There are several reasons for carrying out the reactions at supercritical conditions. Naturally, some of the reasons are coupled. Nevertheless, they, in general, relate to favorable transport properties, unique solvency characteristics, favorable kinetic considerations, and their sensitivity to operating conditions (manipulated variables). These unique properties lead to opportunities in process synthesis, process intensification, and controllability. These advantages, coupled with the environmentally friendly nature of these processes, make reactions in supercritical fluids attractive. The effect of these properties on opportunities for these favorable reaction environments is summarized in Table 1. 

The supercritical fluids exhibit gas-like viscosities, diffusivities, and liquid-like densities. These favorable transport properties lead to enhanced mass transfer, permeation, and wetting characteristics. The mass transfer limited multiphase reactions will benefit from reduction of a number of phases, as in the case of most oxidation, hydrogenation, or replacement of the more viscous liquid phase with a supercritical or a less viscous expanded liquid phase. The mobility combined with tunability results in effective maintenance of catalyst activity in heterogeneous catalysis. 

If the polarity is considered equivalent to hexane and polar modifiers are added to the supercritical fluid, then the separation may be considered similar to normal-phase HPLC. However, the viscosity and mass transfer properties of supercritical fluids are more favorable and can lead to increased separation efficiencies and decreased analysis times. Berger and Wilson,for example, have demonstrated that separations with up to 260,000 theoretical plates can be achieved by serially coupling 10 HPLC columns without the deleterious pressure effects that would be encountered in separations using a liquid mobile phase. For applications that are not limited by polar matrices, SFC is, therefore, a viable option. 

One of the benefits of using supercritical fluids as the solvent is the strong dependence of the solubility of the solute on the solvent density. This is a property that could be exploited for facilitating the separation of the solute from the solvent as it leaves the column, by dropping its pressure or raising its temperature, thereby lowering its density and the solubility of the solute. As a result, the extract separates into a liquid solute and a vapor solvent. Another favorable property of supercritical fluids as solvents is the high diffusivity of the solute in these fluids compared to that in liquids. Supercritical fluids also have a substantially lower viscosity than liquids. Because of these properties the mass transfer rate of the solute... 

Supercritical fluids exhibit physicochemical properties between those of liquids and gases. These properties favor their introduction into different matrices and also analyte solubility. In addition, SFs exhibit transport properties of gases (high diffusivity). Mass transfer is rapid with SFs. 

Supercritical fluids have a number of distinct advantages over conventional liquid solvents. The adjustable solvent strength and favorable transport properties have already been mentioned and it is these features which really differentiate SCFs tom liquid solvents. Most SCFs are low-molecular-weight gases which have relatively low critical temperatures. Operations may therefore be carried out at moderate temperatures which is desirable in the recovery of thermally labile materials. Perhaps the most important advantage offered by SCFs is that after the release of pressure, components are left virtually free of residual supercritical solvent. 

The high difflisivities and the low viscosities of supercritical and near-critical fluids are well established. Studies have shown that the favorable mass transport properties of dense gases are retained in microemulsions formed in these systems, and multiple benefits are expected for chemical reactions. The diffusion rates of microemulsion droplets in near-critical and supercritical fluids are up to 10 times greater than in liquids. Conventional liquid microemulsion systems often contain nanometer-sized droplets whose diffusion rates are 10-100 times lower than those of molecularly dispersed species. The higher diffusion rates of these droplets in near-critical and supercritical fluids offset the transport limiting effect due to large droplet size. 

Supercritical fluids are effective at much lower temperatures than distillation, and their application in separation avoids degradation and decomposition of heat-labile compounds. Attractiveness of supercritical extraction processes are due to the sensitivity of responses to process variables, promise of complete and versatile regeneration of solvents, energy savings, enhanced solute volatilities, solvent selectivities, favorable transport properties for solvents, and state governed effectiveness of solvents which enables the use of low cost, non-toxic, environmentally acceptable solvents. The impact of inherent characteristics of supercritical fluids on separations is summarized in Table 21.1.5. 

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