Environmental Engineering Reference
In-Depth Information
7.3.6 Pressured Fluid Extraction
The pressured fluid extraction (PFE), or more commonly called accelerated solvent
extraction (ASE), uses elevated temperature (100-180 C) and pressure (1500-2000
psi) to achieve analyte recoveries equivalent to those from Soxhlet extraction, using
less solvent and taking significantly less time than the Soxhlet procedure. The fully
automated device has been commercially available since 1995 by Dionex Corp.,
USA. The EPA has validated a group of water insoluble or slightly water soluble
SVOCs using a recommended mixture of solvents (acetone, CH 2 Cl 2 , hexane). A
total of 10-30 g dry (air drying or by mixing with anhydrous Na 2 SO 4 ) and ground
solid samples of small particle size in the range of 100-200 mesh (150-75
m) are
loaded into the sealed extraction cell (11-33 mL). The cell is heated to the extraction
temperature, pressurized with the appropriate solvent system with a pump, and
extracted for five minutes, and the extract is then collected.
The mechanisms for the enhanced extraction efficiency at high temperature and
pressure in the PFE system compared to extractions at or near room temperature and
atmospheric pressure are suggested to be the result of the increased analyte solubility
and mass transfer effects, and the disruption of surface equilibria. At high temperature,
the solubility and diffusion rate of analytes are increased whereas solvent viscosity is
decreased. High pressure keeps solvent liquefied above its boiling point and allows
solvent to penetrate matrix readily. Elevated temperature and pressure also disrupt the
analyte-matrix bonding such as van der Waals' forces, hydrogen bonding, and dipole
attractions of the solute molecules and active sites on the matrix (Dean, 1998).
m
7.3.7 Supercritical Fluid Extraction
Unlike all other extractions using solvents, the extracting solvent in supercritical
fluid extraction (SFE) is CO 2 in its supercritical fluid (SCF) state. A SCF is defined
as a substance above its critical temperature (T c ) and critical pressure (P c ). The
critical point represents the highest temperature and pressure at which the substance
can exist as a vapor and liquid in equilibrium. This formation of SCF can be
illustrated by the phase diagram of pure CO 2 (Fig. 7.12). As can be seen, CO 2 in its
solid, liquid, and gas phases can coexist at the triple point. The gas-liquid
coexistence curve is known as the boiling curve. If we move upward along the
boiling curve, increasing both T and P, then the liquid becomes less dense as a result
of thermal expansion and the gas becomes more dense as the pressure rises.
Eventually, the densities of the two phases converge and become identical, the
distinction between gas and liquid disappears, and the boiling curve comes to an end
at the critical point (T c ¼31.1 C; P c ¼74.8 atm for CO 2 ).
The CO 2 in this SCF state is characterized by physical and thermal properties
that are between the pure liquid and gas form of CO 2 . SCF has a gas-like high mass
transfer coefficients and a liquid-like high solvent property. Besides, SCF has low
viscosity and almost zero surface tension. The high diffusivity of SCF makes it
possible to readily penetrate porous and fibrous solids. Consequently, SCF can offer
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