Supercritical fluid-adsorbate-adsorbent

KING ET AL. Supercritical Fluid-Adsorbate-Adsorbent Systems 65... 

Adsorption and Desorption Adsorbents may be used to recover solutes from supercritical fluid extracts for example, activated carbon and polymeric sorbents may be used to recover caffeine from CO9. This approach may be used to improve the selectivity of a supercritical fluid extraction process. SCF extraction may be used to regenerate adsorbents such as activated carbon and to remove contaminants from soil. In many cases the chemisorption is sufficiently strong that regeneration with CO9 is limited, even if the pure solute is quite soluble in CO9. In some cases a cosolvent can be added to the SCF to displace the sorbate from the sorbent. Another approach is to use water at elevated or even supercritical temperatures to facilitate desorption. Many of the principles for desorption are also relevant to extraction of substances from other substrates such as natural products and polymers. 

Wong, D.C.L. van Compemolle, R. Nowlin, J.G. O Neal, D.L. Johnson, G.M. Use of supercritical fluid extraction and fast ion bombardment mass spectrometry to identify toxic chemicals from a refinery effluent adsorbed onto granular activated carbon. Chemosphere 1996, 32, 621. 

Wong, J. M., N. Y. Kado, P. A. Kuzmicky, H.-S. Ning, J. E. Woodrow, D. P. H. Hsieh, and J. N. Seiber, Determination of Volatile and Semi-volatile Mutagens in Air Using Solid Adsorbents and Supercritical Fluid Extraction, Anal. Chem., 63, 1644-1650 (1991). 

Cholesterol Extraction from Beef Tallow fay SC-CO2 in the Presence of Adsorbents. The utilization of SC-CO2 extraction in the presence of adsorbers enhances the fractionation of solutes dissolved in the supercritical fluid. This procedure has been reviewed by King (29). 

Decontamination of soils using supercritical fluids is an attractive process compared to extraction with liquid solvents because no toxic residue is left in the remediated soil and, in contrast to thermal desorption, the soils are not burned. In particular, typical industrial wastes such as PAHs, PCBs, and fuels can be removed easily [7 to 21]. The main applications are in preparation for analytical purposes, where supercritical fluid extraction acts as a concentration step which is much faster and cheaper than solvent-extraction. The main parameters for successful extraction are the water content of the soil, the type of soil, and the contaminating substances, the available particle-size distribution, and the content of plant material, which can act as adsorbent material and therefore prolong the extraction time. For industrial regeneration, further the amount of soil to be treated has to taken into account, because there exists, so far, no possibility of continuous input and output of solid material for high pressure extraction plants, so that the process has to be run discontinuously. 

If a liquid is used as die mobile phase, the technique used is liquid chromatography (LC). The solid adsorbent is constrained in a tube or column through which the liquid mobile phase flows. Any number of solvents, buffer solutions, or supercritical fluids can be used as liquid mobile phases. High-pressure liquid chromatography (HPLC) is used if pressure is needed to force die liquid phase through the tube. If the liquid phase moves over a thin adsorbent surface propelled by capillary action, die technique used is thin-layer chromatography (TLC). In general, two types of surfaces are used as the solid phase. 

It appears that temperatures in the range of 500.to 670 K for the adsorption of n-paraffins in the 560-740 K boiling range on type 5A molecular sieves should be considered for investigating the combination supercritical fluid volatility amplification-vapor phase molecular sieve process. The pressure of operation will be governed by the critical pressures of potential solvents with critical temperatures in this range. The supercritical solvent of choice should not be adsorbable on type 5A molecular sieves and should not be expensive. 

The utilization of supercritical fluids in conjunction with adsorbents and active solids is well documented in the technical literature. The most frequently cited applications involve the use of dense gases for the regeneration of adsorbents (1) and as mobile phases in supercritical fluid chromatography (2). Numerous... 

Fundamental studies on the adsorption of supercritical fluids at the gas-solid interface are rarely cited in the supercritical fluid extraction literature. This is most unfortunate since equilibrium shifts induced by gas phase non-ideality in multiphase systems can rarely be totally attributed to solute solubility in the supercritical fluid phase. The partitioning of an adsorbed specie between the interface and gaseous phase can be governed by a complex array of molecular interactions which depend on the relative intensity of the adsorbate-adsorbent interactions, adsorbate-adsorbate association, the sorption of the supercritical fluid at the solid interface, and the solubility of the sorbate in the critical fluid. As we shall demonstrate, competitive adsorption between the sorbate and the supercritical fluid at the gas-solid interface is a significant mechanism which should be considered in the proper design of adsorption/desorption methods which incorporate dense gases as one of the active phases. 

Figure 1. Reported conditions for the supercritical fluid desorption of selected adsorbates from activated carbon.

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