Supercritical fluid-adsorbate-adsorbent systems

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

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. 

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. 

Introducing adsorbents into a supercritical fluid extraction system is an alternative attractive method to improve the selectivity for the citrus oil processing. Several applications using silica gel as adsorbent have been reported in the last decade [1-3]. Yamauchi and Saito [3] fractionated lemon peel oil into 3 fractions with gradual increase in pressure by supercritical fluid chromatography, where terpene rich fraction, ester rich, and alcohol and aldehyde rich fraction were obtained at 10 MPa, 20 MPa, and 20 MPa with ethanol as a cosolvent,... 

In this chapter, we present the results of computational studies on the above mentioned novel inorganic systems namely AlPOs, carbon nanotubes and supercritical fluid extraction from the adsorbed phase over ceramics. Multi-technique computational methods such as Computer Graphics (CG), molecular mechanics (MM), quantum chemistry (QC) and molecular dynamics (MD) were applied. The attempts made to design synthetic sorbents at molecular level are reviewed. 

The dynamic behaviour of the supercritical fluid and adsorbed species on MgO(OOl) surface under the supercritical condition at 400K and 198 atm (Tr — 1.18, Pr = 1.50) is shown in Fig. 15. After the supercritical fluid molecules condensed around the adsorbed species, the adsorbed species was extracted from MgO(OOl) surface by supercritical fluid after nearly 3000 time steps. In the system containing both supercritical fluid and adsorbed species, the extraction of adsorbed species occurred at low temperatures compared to the situation in the absence of fluid system. 

Prior studies utilizing adsorbents in the presence of supercritical fluid media have been reviewed by King (3), who has commented on the lack of fundamental knowledge on adsorbate(sorbate)/adsorbent(sorbent)/supercritical fluid systems. Indeed, with the exception of the sorbent regeneration studies performed at Critical Fluid Systems in the last... 

Both adsorption from a supercritical fluid to an adsorbent and desorption from an adsorbent find applications in supercritical fluid processing.The extrapolation of classical sorption theory to supercritical conditions has merits. The supercritical conditions are believed to necessitate monolayer coverage and density dependent isotherms. Considerable success has been observed by flic authors in working with an equation of state based upon the Tofli isoterm. It is also important to note that the retrograde behavior observed for vapor-hquid phase equilibrium is experimentally observed and predicted for sorptive systems. 

The fipronil standard, more than 99% pure, was obtained from Chem Service (West Chester, Pennsylvania, U.S.A.). The fipronil standard solution was prepared in ethyl acetate. Mallinckrodt (Phillipsburg, New Jersey, U.S. A.) supplied all the nanograde solvents for pesticide-residue analyses. Several adsorbents—such as octadecyl silane (35-75 mesh, Supelco), chromossorb (100-120 mesh, Johns Manville), florisil (100-200 mesh, Riedel), silica gel 60 (70-230 mesh, Merck), and XAD-7 (20-60 mesh, Rohm and Haas)—were tested as sorbents in the SPE cartridges. Different modifiers (hexane, acetone, methanol), various temperature (50, 60°C), and pressure conditions (120, 250, and 300 atm) were tested for supercritical fluid extraction. The water was purified with a Millipore Milli Q Plus System (Bedford, Massachusetts, U.S.A.). 

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