Supercritical fluids transport 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... 

A paiticularly attiactive and useful feature of supeicritical fluids is that these materials can have properties somewhere between those of a gas and a hquid (Table 2). A supercritical fluid has more hquid-hke densities, and subsequent solvation strengths, while possessiag transport properties, ie, viscosities and diffusivities, that are more like gases. Thus, an SCF may diffuse iato a matrix more quickly than a Hquid solvent, yet still possess a Hquid-like solvent strength for extracting a component from the matrix. 

Transport Properties Although the densities of supercritical fluids approach those of conventional hquids, their transport properties are closer to those of gases, as shown for a typical SCF such as CO9 in Table 22-12. For example, the viscosity is several orders of magnitude lower than at liquidlike conditions. The self-diffusion coefficient ranges between 10" and 10" em /s, and binaiy-diffusiou coefficients are similar [Liong, Wells, and Foster, J. Supercritical Fluids 4, 91 (1991) Catchpole and King, Ind. Eng. Chem. Research, 33,... 

TABLE 22-12 Density and Transport Properties of a GaS/ Supercritical Fluid/ and a Liquid... 

The same types of catalyst have been employed in 1-octene hydroformylation, but with the substrates and products being transported to and from the reaction zone dissolved in a supercritical fluid (carbon dioxide) [9], The activity of the catalyst is increased compared with liquid phase operation, probably because of the better mass transport properties of scC02 than of the liquid. This type of approach may well reduce heavies formation because of the low concentration of aldehyde in the system, but the heavies that do form are likely to be insoluble in scC02, so may precipitate on and foul the catalyst. The main problem with this process, however, is likely to be the use of high pressure, which is common to all processes where supercritical fluids are used (see Section 9.8). 

A way around this issue may have been found with the use of supercritical fluids. These materials, such as liquid carbon dioxide, have many interesting properties from the point of view of pharmacutical processing since they combine liquid-like solvent properties with gas-like transportation properties. Small changes in the applied pressure or temperature can result in large changes of the fluid density and, correspondingly, the solvent capacity and properties of the resultant particles. 

A gaseous pure component can be defined as supercritical when its state is determined by values of temperature T and pressure P that are above its critical parameters (Tc and Pc). In the proximity of its critical point, a pure supercritical fluid (or a dense gas as it is alternatively known) has a very high isothermal compressibility, and this makes possible to change significantly the density of the fluid with relatively limited modifications of T and P. On the other hand, it has been shown that the thermodynamic and transport properties of supercritical fluids can be tuned simply by changing the density of the medium. This is particularly interesting for... 

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). 

After completing a Ph.D. degree and postdoctoral research in analytical chemistry, Saul Goldman joined the University of Guelph in 1972. As a reformed experimentalist, he developed a research program based on statistical mechanics,277 with special interests in supercritical fluid extraction, spectra of endohedral fullerenes,278 the transport of ions through biological channels, and the molecular basis for the properties of liquids and solutions.279... 

To understand any extraction technique it is first necessary to discuss some underlying principles that govern all extraction procedures. The chemical properties of the analyte are important to an extraction, as are the properties of the liquid medium in which it is dissolved and the gaseous, liquid, supercritical fluid, or solid extractant used to effect a separation. Of all the relevant solute properties, five chemical properties are fundamental to understanding extraction theory vapor pressure, solubility, molecular weight, hydrophobicity, and acid dissociation. These essential properties determine the transport of chemicals in the human body, the transport of chemicals in the air water-soil environmental compartments, and the transport between immiscible phases during analytical extraction. 

Fluids are highly compressible along near-critical isotherms (L01-1.2 Tc) and display properties ranging from gas-like to Liquid-Like with relatively small pressure variations around the critical pressure. The liquid-like densities and better-than-liquid transport properties of supercritical fluids (SCFs) have been exploited for the in situ extraction of coke-forming compounds from porous catalysts [1-6], For i-hexene reaction on a low activity, macroporous a catalyst, Tiltschcr el al. [1] demonstrated that reactor operation at supercritical... 

The deactivation of solid acid catalysts, such as those used in reforming and alkylation practice, by coking occurs because the coke precursors that are formed either in the fluid phase or on the catalyst have relatively low volatilities at the operating pressure and temperature. Supercritical media have been shown to offer a unique combination of solvent and transport properties for the in situ extraction of coke-forming compounds from porous catalysts. Reported investigations of the supercritical decoking concept are summarized elsewhere [1]. 

Employing 1-hexene isomerization on a Pt/y-ALOj reforming catalyst as a model reaction system, we showed that isomerization rates are maximized and deactivation rates are minimized when operating with near-critical reaction mixtures [2]. The isomerization was carried out at 281°C, which is about 1.1 times the critical temperature of 1-hexene. Since hexene isomers are the main reaction products, the critical temperature and pressure of the reaction mixture remain virtually unaffected by conversion. Thus, an optimum combination of gas-like transport properties and liquid-like densities can be achieved with relatively small changes in reactor pressure around the critical pressure (31.7 bars). Such an optimum combination of fluid properties was found to be better than either gas-phase or dense supercritical (i.e., liquid-like) reaction media for the in situ extraction of coke-forming compounds. 

The operating pressure is obtained from the vapor pressure and the partial pressure of the gaseous educts and products. In this process, the temperatures applied are between 150 and 500 °C. In recent times, supercritical fluids have attracted a great deal of attention as potential extraction agents and reaction media in chemical reactions. This has resulted from an unusual combination of thermodynamic properties and transport properties. As a rule supercritical reactions like hydrolysis or oxidation are carried out in water. Above the critical point of water, its properties are very different to those of normal liquid water or atmospheric steam. 

The general properties of supercritical fluids make them an attractive alternative to liquid solvents in column operations where transport effects come into play. If supercritical CO2 is employed as the solvent, this advantage is further supplemented by the non-flammable, non-toxic nature of the fluid, and the relative ease of solvent recovery. Supercritical solvents also offer the potential to greatly enhance thermally driven separations through dramatic changes in component solubility, adsorptive characteristics, and thermal conductivity near the critical region. 

One promising way to proceed seems to be the extraction of the halogenated components by supercritical fluids. This technique is called supercritical fluid extraction (SFE). Because of their special properties supercritical fluids show solubilities like organic solvents and transport properties like gases. Especially carbon dioxide with it s low critical data (Tc = 31,3 °C, pc = 7,28 MPa) was found to be a good candidate for the extraction of organic substances. There are several applications of the SFE-technique with SC-CO2 for instance in the field of ... 

In addition to conventional liquid chromatography, supercritical fluid chromatography (SFC), using a supercritical fluid as mobile phase (mostly scf-C02), has attracted attention in the last decades [58, 164, 168, 169]. Supercritical fluids provide a favourable medium for the transport of solutes through a chromatographic column because they resemble a gas in terms of viscosity, a liquid in terms of density, and are intermediate between these two phases in terms of diffusivity. For some physieal properties of supercritical solvents, see Section 3.2. 

Supercritical fluids possess characteristics that make them interesting for use as polymerization media. A supercritical fluid exists at temperatures and pressures above its critical values. In the supercritical state, the fluid exhibits physical and transport properties intermediate between the gaseous and liquid state. This is illustrated in Table 2. SCFs have liquid-like densities, but gas-like diffusivities. These intermediate properties can provide advantages over liquid-based processes. In particular, the higher diffusivities of SCFs reduce mass transfer limitations in diffusion-controlled processes. Additionally, lower energy is required for processing the supercritical fluid because its viscosity is lower than that of most liquids, and because the need to vaporize large quantities of liquid is avoided. 

A number of other important potential applications of a micellar phase in supercritical fluids may utilize the unique properties of the supercritical fluid phase. For instance, polar catalyst or enzymes could be molecularly dispersed in a nonpolar gas phase via micelles, opening a new class of gas phase reactions. Because diffusivities of reactants or products are high in the supercritical fluid continuous phase, high transport rates to and from active sites in the catalyst-containing micelle may increase reaction rates for those reactions which are diffusion limited. 

A substance is said to be in the gaseous state when heated to temperatures beyond its critical point. However, the physical properties of a substance near the critical point are intermediate between those of normal gases and liquids, and it is appropriate to consider such supercritical fluid as a fourth state of matter. For applications such as cleaning, extraction and chromatographic purposes, supercritical fluid often has more desirable transport properties than a liquid and orders of magnitude better solvent properties than a gas. Typical physical properties of a gas, a liquid, and a supercritical fluid are compared in Table 1. The data show the order of magnitude and one can note that the viscosity of a supercritical fluid is generally comparable to that of a gas while its diffusivity lies between that of a gas and a liquid. 

Microemulsions have the ability to partition polar species into the aqueous core or nonpolar solutes into the continuous phase (See Fig. 1). They can therefore greatly increase the solvation of polar species in essentially a nonpolar medium. The surfactant interfacial region provides a dramatic transition from the highly polar aqueous core to the nonpolar continuous-phase solvent. This region represents a third type of solvent environment where amphiphilic solutes can reside. Such amphiphilic species will be strongly oriented in the interfacial film so that their polar ends are in the core of the microemulsion droplet and the nonpolar end is pointed towards or dissolved in the continuous phase solvent. When the continuous phase is a near-critical liquid (7)j = r/7 > 0.75) or supercritical fluid, additional parameters such as transport properties, and pressure (or density) manipulation become important aids in applying this technology to chemical processes. 

The question arises as to whether the microemulsion phase retains the high transport rates for which supercritical fluids are well know.f lf Various spectroscopic studies that are described in the following section have been used to directly measure the transport properties in supercritical fluid or near-critical liquid microemulsions. 

Why use a supercritical fluid for cleaning, especially since elevated pressures are required One reason is that supercritical fluids offer a combination of liquid-like solubilities and gas-like transport properties which are superior to many liquids. The economic viability of a cleaning process often depends on how much solvent is needed to remove a given amount of material and how much energy must be added to the system to remove the dirt from the part. The low cost and ease of recyclability of SCF CO2 make it an excellent choice for cleaning applications. 

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