11    INFLUENCE OF WATER ON SUPERCRITICAL
FLUID EXTRACTION
Tanya Smyth Pratte, Graduate Student
Selma Guigard. Graduate Student
Richard G. Zytner, Assistant Professor
Warren H. Stiver, Assistant Professor
School of Engineering, University of Guelph
Guelph, Ontario, Canada, NIG 2W1
INTRODUCTION
Supercritical Fluid Extraction (SFE) has many applications including separation of organics
from water, coffee and tea decaffeination, food and seed oil extractions, solvent and monomer
extractions from polymeric materials and regeneration of granular activated carbon.' Some potential advantages of SFE over conventional processes are reduction in energy requirements,
high quality of extracted products that are free of solvents, cheap solvents and gentle treatment
of natural products.2 Major areas of environmental research involving SFE include regeneration
of adsorbents, oxidation of hazardous wastes, extraction of organics from aqueous streams and
leaching of contaminated soil samples.3 In regard to the remediation of contaminated soils, the
focus of much of the research has been on factors that control the rate of extraction and improvements on the extraction process on a small scale.3-4
One of the factors affecting SFE applications in site remediation is the water content of a contaminated soil. In addition to the natural water levels present at the site, the first step following
excavation may be a soil washing operation for a volume reduction benefit. In a laboratory setting, drying of samples prior to extraction is viable; however, drying may be too expensive at
contaminated site, where tons of soil are involved. Therefore, the effects of water on the extraction rate and on the final amount of contaminant extracted can be crucial in determining if SFE is
a feasible option for soil remediation.
Water could influence the extraction by changing the thermodynamics of the process or by
changing the kinetics of the extraction. From a thermodynamic perspective, the water may affect
both the affinity of the contaminant for the soil as well as changing the properties of the supercritical fluid. Water will preferentially sorb to mineral sites in the soil and can cause swelling and
shrinking of the clay constituents in soil. As a result, the sorption of the contaminant is altered.
Supercritical carbon dioxide phase will become more hydrophilic as water is dissolved into it.
thus increasing the overall polarity of the solvent phase. From a kinetic perspective, water may
act as a physical barrier between the supercritical phase and the contaminated soil surface. Extraction of a contaminant from the soil would require passing through this barrier, which could
be limiting for low solubility constituents.
The main objective of the research discussed in this chapter was the investigation of the impact water has on the supercritical fluid extraction of organics from soil.
BACKGROUND
Supercritical fluids are fluids that are above their critical temperature and pressure. At these
conditions the gas and liquid phases of the fluid are indistinguishable. In the supercritical domain
the fluid has a number of properties that are advantageous for its use as an extraction fluid. Supercritical fluids have low viscosities and high solute diffusivities, compared to ordinary liquids.
Mass transfer is enhanced due to its high buoyant forces while the low surface tension allows
51 si Purdue Industrial Waste Conference Proceedings. 1996, Ann Arbor Press. Inc.. Chelsea. Michigan 48118.
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