Diffusivities in supercritical fluids

Non-globular molecules are not expected to obey this relationship. The self diffusion in supercritical fluids is an order of magnitude faster than in ordinary liquids with molecules of similar sizes (Eckert, Knutson and Debenedetti 1996). 

The development of mass transfer models require knowledge of three properties the diffusion coefficient of the solute, the viscosity of the SCF, and the density of the SCF phase. These properties can be used to correlate mass transfer coefficients. At 35 C and pressures lower than the critical pressure (72.83 atm for CO2) we use the diffusivity interpolated from literature diffusivity data (2,3). However, a linear relationship between log Dv and p at constant temperature has been presented by several researchers U>5) who correlated diffusivities in supercritical fluids. For pressures higher than the critical, we determined an analytical relationship using the diffusivity data obtained for the C02 naphthalene system by lomtev and Tsekhanskaya (6), at 35 C. 

In addition to density, diffusivity of the supercritical fluids is higher than that of liquid solvents, and can be easily varied. For typical conditions, diffusivity in supercritical fluids is of the order of lO cm /sec as compared to 10 for gases and 10 for liquids. Typical viscosity of supercritical fluids is of the order of 10 g/cm/sec, similar to that of gases, and about 100-fold lower than that of liquids. High diffusivity and low viscosity provide rapid equilibration of the fluid to the mixture to be extracted, hence extraction can be achieved close to the thermodynamic limits. However, the main extraction benefit of supercritical fluids is their adjustable density that provides adjustable solvent strength. The compounds of choice can be dissolved/extracted in the supercritical fluid at high pressure and then this fluid mixture is carried to another vessel where simple lowering of the pressure... 

Another important topic in the study of supercritical fluids is viscosity effects. Several research groups have used well-established probes to examine the effect of viscosity on rotational diffusion in supercritical fluid systems. 

Reactions. Supercritical fluids are attractive as media for chemical reactions. Solvent properties such as solvent strength, viscosity, diffusivity, and dielectric constant may be adjusted over the continuum of gas-like to Hquid-like densities by varying pressure and temperature. Subsequently, these changes can be used to affect reaction conditions. A review encompassing the majority of studies and apphcations of reactions in supercritical fluids is available (96). 

The diffusion coefficients of small molecular weight solutes in supercritical fluids and liquids are typically in the range of 1 x 10 " and 10 cm /s [18,19], respectively, while those of EFLs are typically intermediate [4] between these two values (10 cm /s). 

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. 

Bioreactions. The use of supercritical fluids, and in particular C02, as a reaction media for enzymatic catalysis is growing. High diffusivities, low surface tensions, solubility control, low toxicity, and minimal problems with solvent residues all make SCFs attractive. In addition, other advantages for using enzymes in SCFs instead of water include reactions where water is a product, which can be driven to completion increased solubilities of hydrophobic materials increased biomolecular thermostability and the potential to integrate both the reaction and separation bioprocesses into one step (98). There have been a number of biocatalysis reactions in SCFs reported (99—101). The use of lipases shows perhaps the most commercial promise, but there are a number of issues remaining unresolved, such as solvent—enzyme interactions and the influence of the reaction environment. A potential area for increased research is the synthesis of monodisperse biopolymers in supercritical fluids (102). 

This makes it possible to tune solvent properties to optimize chromatographic separations. Because of the lower viscosity and higher diffusivity of supercritical fluids compared to common solvents, a higher mobile phase velocity can be used in the column, leading to a higher process throughput than that of liquid chromatography. 

Supercritical fluid chromatography provides increased speed and resolution, relative to liquid chromatography, because of increased diffusion coefficients of solutes in supercritical fluids. (However, speed and resolution are slower than those of gas chromatography.) Unlike gases, supercritical fluids can dissolve nonvolatile solutes. When the pressure on the supercritical solution is released, the solvent turns to gas. leaving the solute in the gas phase for easy detection. Carbon dioxide is the supercritical fluid of choice for chromatography because it is compatible with flame ionization and ultraviolet detectors, it has a low critical temperature. and it is nontoxic. 

As discussed before, the contribution of the current modes are not important for diffusion in dense neat liquids at low temperature. This is because at large viscosity, the decay of the transverse current mode is rapid. In the gas phase, however, the decay of the current modes can be rather slow, and this can contribute significantly to diffusion. The transverse current mode is expected to play an important role here also because it is precisely the range where the current mode plays important role in hard-sphere fluids. Preliminary work is being done to find an important role of transverse current modes in the diffusion of supercritical fluids [201]. 

Wu, B.C. Klein, M.T. Sandler, S.I. The Influence of Diffusion on Reactions in Supercritical Fluid Solvents, Paper presented at AIChE National Meeting, Orlando, Fla. March 18-21,1990. 

We report on steady-state and time-resolved fluorescence of pyrene excimer emission in sub- and supercritical C02. Our experimental results show that, above a reduced density of 0.8, there is no evidence for ground-state (solute-solute) interactions. Below a reduced density of 0.8 there are pyrene solubility complications. The excimer formation process, analogous to normal liquids, only occurs for the excited-state pyrene. In addition, the excimer formation process is diffusion controlled. Thus, earlier reports on pyrene excimer emission at rather "dilute pyrene levels in supercritical fluids are simply a result of the increased diffusivity in the supercritical fluid media. There is not any anomalous solute-solute interaction beyond the diffusion-controlled limit in C02. 

In supercritical fluids, the possibility of local composition enhancements of cosolvent about a solute suggests that we should see enhancement of anion fluorescence if the water cosolvent clusters effectively about the 2-naphthol solute. Although in liquids the water concentration must be >30% to see anion emission, the higher diffusivity and density fluctuations in SCFs could allow stabilization of the anion at much lower water concentrations provided that the water molecules provide sufficient structure. Therefore the purpose of these experiments was to investigate 2-naphthol fluorescence in supercritical CO 2 with water cosolvent in the highly compressible region of the mixture to probe the local environment about the solute. 

Roberts, C. B. Zhang, J. Brennecke, J. F. Chateauneuf, J. E. Laser Flash Photolysis Investigations of Diffusion-Controlled Reactions in Supercritical Fluids. J. Phys. Chem. 1993a, 97, 5618-5623. 

A number of other methods, not falling within any of the earlier-mentioned categories, may prove useful for process intensification. Some of them, such as supercritical fluids, are already known and have been applied in other industries (104,105). Because of their unique properties, especially the high diffusion coefficient, supercritical fluids are attractive media for mass transfer operations,... 

General overview of several studies of transport and intermolecular interactions in compressed supercritical fluids is presented. The unique aspects of the instrumentation used in these studies are emphasized. First, the results of NMR studies of self-diffusion in supercritical ethylene and toluene are discussed. These experiments used the fixed field gradient NMR spin-echo technique. Second, the novel NMR technique for the determination of solubility of solids in supercritical fluids is described. 

Some extractions are only limited by diffusion and especially the internal diffusion. In certain cases like basil oil extraction or ginger extraction, the solubility of the extract in supercritical fluid is very high but the access in the matrix is more difficult, so that the concentration of... 

The prospect of using enzymes as heterogeneous catalysts in scC02 media has created significant interest. Their low viscosity and high diffusion rates offer the possibility of increasing the rate of mass-transfer controlled reactions. Also, because enzymes are not soluble in supercritical fluids, dispersion of the free enzymes potentially allows simple separations without the need for immobilization. 

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. 

The diffusion coefficients of solutes in supercritical fluids are in between those they possess in liquids and gases. Because diffusion coefficients in SFs are higher than those in liquids, mass transfer is usually more favourable in the former. The diffusion coefficient can also be altered to advantage as diffusivity in a supercritical fluid decreases with increasing pressure and increases with increasing temperature, especially in the vicinity of the critical point. 

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