Transport properties of supercritical fluids

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... 

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 review here proposed shows clearly that the use of a membrane in the presence of a supercritical fluid makes it possible the design of very attractive and powerful processes to improve transfer or reaction, to set in contact different phases, to fluidify highly viscous liquids, etc. or for the preparation of new generations of membranes. This is to be connected to the specific thermodynamic and transport properties of supercritical fluids and the particular environment that is created for all these operations. 

Vesovic, V. and Wakeham, W. A., Transport Properties of Supercritical Fluids and Fluid Mixtures, Supercritical Fluid Technology Reviews in Modem Theory and Applications, CRC Press, Boston (1991)... 

Sengers, J.V. (1994) Effects of critical fluctuations on the thermodynamic and transport properties of supercritical fluids, in E. Kiran and J. M. H. Levelt Sengers (eds.). Supercritical Fluids, Kluwer, Dordrecht, 231-271. 

Fig. 1. Pressure tunability of density and transport properties of supercritical fluids in the near-critical region.

Vesovic, V. Wakeham, W.A. (1991). Transport properties of supercritical fluids and fluid mixtures, in Critical Fluid Technology, s. TJ. Bruno, J.F. Ely. Boca Raton CRC Press, pp. 245-289. 

The transport properties of the supercritical fluids fall somewhat in between the gas and the liquid and also depend on how removed one is from the critical point. Dense gasses have the solubilizing power of liquids and the mobility of gasses as depicted in Table 20.1.3. There are quite a few empirical correlations and theoretical models, which are primarily extensions of corresponding low-pressure liquid and gas counter parts. Similarly, some of the classical experimental methods can be used for measurement of transport properties of supercritical fluids. A rather brief overview of the methods applicable for supercritical fluids will be presented since specialized reviews in the area give a good account of the state of the art. " " For engineering purposes, one can use applicable property estimation methods available in flowsheet simulators such as ASPEN PLUS, PROll, G-PROMS, and CHEMCAD. These methods are discussed in a text classical in the field." ... 

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. 

In the first edition we discussed several examples of employing certain properties of supercritical fluids in ways other than merely extractive in this edition we add several new examples that exploit transport, surface tension, and other properties of supercritical fluids and their solutions. 

The special properties of supercritical fluid (SCF) solvents [7,8] for PTC reactions bring substantial environmental and econonric advantages. First, they permit the use of totally benign solvents, especially CO2, and the solvent separation from product becomes quite facile. Moreover, since PTC processes always involve mass transfer, the lower viscosity and higher diffusivity of SCFs significantly enhance transport. 

NMR techniques can be used to study supercritical fluids as the employed pressures range from 0.1 to 80 MPa. The reason for renewed interest in the properties of supercritical fluids can be traced to the great promise of supercritical fluid extraction techniques. High pressure NMR spectroscopy can not only be used to investigate transport and intermolecular interactions in compressed supercritical... 

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... 

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

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. 

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. 

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