Transport properties liquids

Transport Properties Although the densities of SCFs can approach those of conventional liquids, transport properties are more favorable because viscosities remain lower and diffusion coefficients remain higher. Furthermore, CO2 diffuses through condensed-liquid phases (e.g., adsorbents and polymers) faster than do typical solvents which have larger molecular sizes. For example, at 35°C the estimated pyrene diffusion coefficient in polymethylmethacrylate increases by 4 orders of magnitude when the CO2 content is increased from 8 to 17 wt % with pressure [Cao, Johnston, and Webber, Macromolecules, 38(4), 1335-1340 (2005)]. 

In this section, we will briefly discuss different testing techniques that are widely used to measure most of the important mass transport properties of fhe diffusion layers. It is important to note that these techniques can also be used with MPLs. The first subsection will explain methods that deal with properties that affect both gas and liquid mass transport, and the other two subsections will discuss only techniques that measure gas and liquid transport properties, respectively. 

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... 

Substances in the sc state have a unique set of physical properties that make them attractive alternatives as reaction solvents. They have high miscibility with gases, liquid-like solvating power, and better-than-liquid transport properties, which invariably provide improved reaction rates. By far, the most commonly used fluid is GO2 because it is inexpensive, nontoxic, nonflammable, environmentally benign, and has low critical constants = 304.2 K = 72.8 bar). Accordingly, it has been lauded as a replacement for volatile organic solvents. The sc fluids also offer the potential to tune the solvent properties and affect yield, rate, and selectivity with pressure. In addition, the morphology of the product can be controlled by rapid expansion of sc solutions, and selective extraction of products from complex mixtures can be achieved by careful choice of solution density. 

Watanabe reported on the concept of ionicity, which is a ratio between the ionic conductivity measured and that deduced from NMR diffteion coefficient data (that accounts for total mobility of ions, even in the form of associated pairs or clusters that do not contribute to charge transport). Ionicity is less than unity if potential charge carriers are not available for transport, and thus reflects the degree of ionic association in the liquid. Transport properties such as viscosity, diffusion coefficient and ionicity do not vary monotonically as the alltyl side chain increases, which is consistent with the appearance of nano-segregated structures at intermediate chain lengths [54]. 

This direct proportionality between the rough hard-sphere transport properties and the Enskog coefficients has formed the basis for many correlations of liquid transport properties (Easteal Woolf 1984 Li etal. 1986 Walker etal. 1988 Greiner-Schmid et al 1991 Harris etal 1993). For a successful data fit, with unique values for Vq and the proportionality factors, it is necessary to fit a minimum of two prt rties simultaneously, with the same Vq values. This is exemplified in the case of methane in Chapter 10. It is further shown in Chapter 10 that successful correlation of transport property data for nonspherical molecular liquids can be made, based on the assumption that transport properties for these fluids can also be directly related to the smooth hard-sphere values. 

Developments in liquid state theory and empirical studies of transport phenomena in liquids can be exploited by restricting the correlation to the region of saturated or compressed liquids away from the critical point (Brush 1962 Schwen Puhl 1988). The basis of one such class of correlations is the recognition that liquid transport properties, and in particular viscosity, can be correlated well in terms of the difference between the molar volume and a compact packing volume which is fluid-specific and a weak function of temperature. The simplest form of this type of correlation is the fluidity ( ) versus molar volume V = p ) approach described by Batschinski (1913) and Hildebrand (1977) for pure fluids based on experimental observations these variables are linearly related to a very good approximation. Hard-sphere theories such as that proposed by Enskog and several modifications of this approach have also been used for liquids. Current work in this area has evolved significantly and is described in detail in Chapters 5 and 10. 

For pure fluids, it is most common to represent the saturated vapor and saturated liquid transport properties as simple polynomial functions in temperature, although polynomials in density or pressure could also be used. Exponential expansions may be preferable in the case of viscosity (Bmsh 1962 Schwen Puhl 1988). For mixtures, the analogous correlation of transport properties along dew curves or bubble curves can be similarly regressed. In the case of thermal conductivity, it is necessary to add a divergent term to account for the steep curvature due to critical enhancement as the critical point is approached. Thus, a reasonable form for a transport property. 

Table 3. Thermodynamic and Transport Properties of Liquid Ethylene ...

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

It follows from this discussion that all of the transport properties can be derived in principle from the simple kinetic dreoty of gases, and their interrelationship tlu ough k and c leads one to expect that they are all characterized by a relatively small temperature coefficient. The simple theory suggests tlrat this should be a dependence on 7 /, but because of intermolecular forces, the experimental results usually indicate a larger temperature dependence even up to for the case of molecular inter-diffusion. The Anhenius equation which would involve an enthalpy of activation does not apply because no activated state is involved in the transport processes. If, however, the temperature dependence of these processes is fitted to such an expression as an algebraic approximation, tlren an activation enthalpy of a few kilojoules is observed. It will thus be found that when tire kinetics of a gas-solid or liquid reaction depends upon the transport properties of the gas phase, the apparent activation entlralpy will be a few kilojoules only (less than 50 kJ). 

Ionic liquids possess a variety of properties that make them desirable as solvents for investigation of electrochemical processes. They often have wide electrochemical potential windows, they have reasonably good electrical conductivity and solvent transport properties, they have wide liquid ranges, and they are able to solvate a wide variety of inorganic, organic, and organometallic species. The liquid ranges of ionic liquids have been discussed in Section 3.1 and their solubility and solvation in... 

Section 3.3. In this section we deal specifically with the electrochemical properties of ionic liquids (electrochemical windows, conductivity, and transport properties) we will discuss the techniques involved in measuring these properties, summarize the relevant literature data, and discuss the effects of ionic liquid components and purity on their electrochemical properties. 

The behavior of ionic liquids as electrolytes is strongly influenced by the transport properties of their ionic constituents. These transport properties relate to the rate of ion movement and to the manner in which the ions move (as individual ions, ion-pairs, or ion aggregates). Conductivity, for example, depends on the number and mobility of charge carriers. If an ionic liquid is dominated by highly mobile but neutral ion-pairs it will have a small number of available charge carriers and thus a low conductivity. The two quantities often used to evaluate the transport properties of electrolytes are the ion-diffusion coefficients and the ion-transport numbers. The diffusion coefficient is a measure of the rate of movement of an ion in a solution, and the transport number is a measure of the fraction of charge carried by that ion in the presence of an electric field. 

Because of the inherent technical difficulties, investigations of transport properties in molten salts are much less common than those of aqueous solutions. However, interpretation of the phenomena seems to be even simpler in molten salts where water is not involved. Molten salt systems are considered to be the simplest liquid electrolytes. Data have been compiled largely due to the great efforts of the Janz group." "... 

L. Copeland, Transport Properties of Ionic Liquids, Gordon and Breach, New York, 1974. 

Gas-expanded liquids (GXLs) are emerging solvents for environmentally benign reactive separation (Eckert et al., op. cit.). GXLs, obtained by mixing supercritical CO2 with normal liquids, show intermediate properties between normal liquids and SCFs both in solvation power and in transport properties and these properties are highly tunable by simple pressure variations. Applications include chemical reactions with improved transport, catalyst recycling, and product separation. 

Lemmon, A. W Jr., H. W. Deem, E. H. Hall and J. F. Walling, 1964, The Thermodynamic and Transport Properties of Potassium, Proc. of High Temperature Liquid Metal Technology Meeting, Vol. 1, 88-114, USAEC Rep. ORNL-3605. (2)... 

Viscoelastic and transport properties of polymers in the liquid (solution, melt) or liquid-like (rubber) state determine their processing and application to a large extent and are of basic physical interest [1-3]. An understanding of these dynamic properties at a molecular level, therefore, is of great importance. However, this understanding is complicated by the facts that different motional processes may occur on different length scales and that the dynamics are governed by universal chain properties as well as by the special chemical structure of the monomer units [4, 5],... 

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