Transport properties of supercritical solvents

Both the capillary viscometer (providing about 0.7% accuracy), the theory of which is based on the Hagen-Poiseuille equation and the oscillating disc viscometer (providing about 0.2% accuracy) are applicable to experimental determination of viscosity at high pressures and temperatures. 

Most of the theoretically based estimation methods for viscosity of dense gases rely on modified Enskog theory. Corresponding states based methods are also popular but should be used with care due to their empiric nature. 

The experimental teclmiques used to measure the diffusion are based upon chromatographic methods, NMR (for self-diffusion), and photon correlation spectroscopy 

The so called chromatographic method for the measurement of diffusion coefficients, strictly speaking, is not a chromatographic mefliod since no adsorption/desorption or retention due to partition in two phases are involved in flie method. The experimental system used is however a chromatograph. Diffusion occurs in an empty, inert column on which the fluid phase is not supposed to be adsorbed. The fluid, in which the solute diffuses flows, continuously through the empty column and flie solute, which is introduced into the column at one end, is detected at the other end as effluent concentration. 

The theory of diffusion in flowing fluids is first given by Taylor and Aris. According to Aris, a sharp band of solute, which is allowed to dissolve in a solvent flowing laminarly in an empty tube, can be described in the limit of a long column as a Gaussian distribution, the variance of which, o, in lengtii units is  

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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. 

The mass transport properties of supercritical CO2 are much higher than those of a conventional solvent extraction system, resulting in faster extraction. 

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. 

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. 

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. 

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. 

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