Supercritical liquid diffusion

Supercritical fluid chromatography (SFC) is a GC method of analysis of compounds in systems where normal GC presents resolution difficulties (Lee and Markides, 1987). A supercritical fluid has properties at a critical temperature intermediate between a liquid and a gas. At and above this critical temperature, a gas cannot be compressed into a liquid, irrespective of the pressure, but it solvates solid matter as if it were a liquid. A supercritical fluid diffuses freely into and out of adsorbent pores with a minimum of resistance. A major advantage of SFC chromatography is its ability to effect separation of oligomers without derivatization. 

Saim and Subramaniam [38] and Ginosar and Subramaniam [39] also found that the in situ extraction of the coke compounds by near-critical or supercritical reaction mixtures prevents pore plugging that otherwise occurs at subcritical (gas-like) conditions. Although the coke laydown decreased at supercritical (liquid-like) conditions, the isomerization rates were lower and deactivation rates were higher due to pore diffusion limitations in the liquid-like reaction mixtures. It was therefore concluded that near-critical reaction mixtures provide an optimum combination of solvent and transport properties that is better than either subcritical (gas-like) or dense supercritical (liquid-like) mixtures for maximizing the isomerization rates and for minimizing catalyst deactivation rates. These findings indicate that catalytic reactions which require liquid-like reaction media for coke extraction and heat removal, yet gas-like diffusivities for enhanced reaction rates, can benefit from the use of near-critical reaction media. 

In order to overcome some of these restrictions in the development of the impregnation processes Perman [56] suggests using a liquid solvent (preferentially water) to solubilize the drug. This solvent is insoluble in the supo critical fluid. The system is then pressurized with die supercritical fluid in order to swell the polymer and to allow rapid difi isional transport of the solute into the polymeric substrate. At the end, pressure is released and the liquid diffuses out of the matrix. The process proposed seems in some extent similar to the recrystallization processes since the impregnation of the solutes is partially enhanced by the decrease of the drug solubility in the liquid solvent. The most important parameters are ... 

Supercritical extraction involves the use of a gaseous solvent medium at a temperature just above its critical temperature while increasing the pressure to the critical pressure for that solvent. Under these conditions, the solvent has the properties of both gas and liquid, diffusing like a gas while having the density of a liquid (Anon., 1989). 

Reaction schemes exploiting supercritical fluid diffusivities. The dif-fusivity of a dilute solute in a supercritical fluid, somewhat removed from the critical point, is typically an order of magnitude greater than in liquid solvents at comparable temperatures. Thus, radical initiators under supercritical fluid conditions are able to escape more readily from solvent cages, and the rate coefficient for the initiation process is markedly increased. Processes propagated by free radicals, such as polymerisation, are rate enhanced for this reason, as are enzymatic reactions. 

Heterogeneous catalytic reactions in supercritical solvents Obviously, a solid catalyzed reaction takes place only on the active sites of the porous catalyst with the implication of some mass and heat transport steps prior to and after the reaction. The first step is the diffusion of the reactants through the film surrounding the catalyst particle to the external surface of the catalyst, followed by diffusion of the reactants into the catalyst pore to the active site in the pores. These steps are limited by the dif-fusivity and viscosity of the reactants. In the case of a supercritical fluid phase reaction, the diffusivity is higher than the liquid diffusivity, viscosity is less than the liquid viscosity and therefore, the rate of transfer to the active site will be higher. After the adsorption, reaction and desorption steps, the products have to diffuse out of the pore, and again... 

These values are as much as one hundred times larger than those typically observed in conventional liquids. The improved transport rates in SCFs versus liquid solvents are important in practical appheations including supercritical extraction. Furthermore, carbon dioxide diffuses through condensed-hquid phases (e.g., adsorbents and polymers) faster than do typical solvents which have larger molecular sizes. 

The use of both sub- and supercritical fluids as eluents yields mobile phases with increased diffusivity and decreased viscosity relative to liquid eluents [23]. These properties enhance chromatographic efficiency and improve resolution. Higher efficiency in SFC shifts the optimum flowrate to higher values so that analysis time can be reduced without compromising resolution [12]. The low viscosity of the eluent also reduces the pressure-drop across the chromatographic column and facilitates the... 

The development of CO2 fixation reactions in supercritical CO2 attracts increasing attention due to its gas-like low viscosities and high diffusivities and its liquid-like solubilizing power. Matsuda et al. attempted to carry out the con-... 

A supercritical fluid exhibits physical-chemical properties intermediate between those of liquids and gases. Mass transfer is rapid with supercritical fluids. Their dynamic viscosities are nearer to those in normal gaseous states. In the vicinity of the critical point the diffusion coefficient is more than 10 times that of a liquid. Carbon dioxide can be compressed readily to form a liquid. Under typical borehole conditions, carbon dioxide is a supercritical fluid. 

Table 3.12 Density, viscosity, and diffusion coefficient of gas, liquid and supercritical fluids...

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