Supercritical Fluids Supercritical fluids (SCF) are materials at elevated temperature and pressure that have properties between those of a gas and a liquid. Supercritical Fluids Extraction Organics in soils, sediments, or water are dissolved in the fluid at elevated temperature and pressure conditions. The dissolved organics are released from the SCF at lower temperatures and pressures. Supercritical Water Oxidation (SCWO) Air and contaminated water are brought together above the critical point of water. Complete oxidation of organic components occurs rapidly. 116
Critical Point (CP) For temperatures and pressures above this point, the fluid is single-phase. It is neither a liquid nor a gas. In a pressure-temperature phase diagram, the critical point (TC, PC) represents the highest values of T and P where vapor/liquid equilibrium can exist. Critical constants Solvent Carbon dioxide Water Ammonia Benzene Toluene Cyclohexane Temperature ( o C) 31.1 374.15 132.4 288.5 320.6 281.0 Pressure (atm) 73.0 218.4 111.5 47.7 41.6 40.4 Density (g/cm 3 ) 0.460 0.323 0.235 0.304 0.292 0.270 117
Fig. 9-38. Pressure/temperature phase diagram for water. 118
Theorem of Corresponding States All gases when compared at the same reduced temperature (T/TC) and reduced pressure (P/PC), have approximately the same compressibility factor and all deviate from ideal gas behavior to about the same degree. Near the critical point, a small change in pressure can create a large change in density at constant temperature. At the higher density the SCF acts as a more effective solvent. 119
In most cases, the viscosity changes rapidly near the critical point. The lower solvent viscosity and higher diffusivity permits much higher mass transfer rates of organic contaminants into SCFs. 120
Properties of supercritical water Density and dielectric constant drop. Inorganic solubility decreases Organic solubility increases 121
Process Description SCF extraction: The contaminated stream is fed into the extraction vessel. The extraction fluid is pressurized and heated to the critical point in a compressor, before it is continuously loaded into the extraction column. The organic contaminant in the contaminated stream dissolves in the SCF. The SCF is then expanded by passing it through a pressure reduction valve. The organic contaminant is separated from the extracting fluid after expansion. The SCF is recompressed and recycled to the extraction vessel. 122
Fig. 9-36. Schematic diagram of an SCF process for extracting organic compounds from water. 123
Supercritical Water Oxidation: Pressurized and heated water (at supercritical conditions) containing organic contaminants is mixed with compressed air in a reactor. The organics are oxidized in a short time Inorganic compounds are separated because of their low solubility in water at supercritical conditions. Gases (mainly CO2 and N2) are removed from the effluent water in liquid-vapor separators. 124
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Design Considerations Selection of solvents Distribution (partition) coefficient, K K Ce / Cr where Ce = concentration of organic in the extract at equilibrium Cr = concentration of organic in the raffinate (effluent) at equilibrium The distribution coefficient is used to determine the quantity of extraction solvent needed to extract a compound from a feed. Recoverability For SCF extraction, reactions between the solvent and solute are undesirable. Density Critical temperature and pressure Toxicity Chemical reactivity (Essential for SCWO) Interfacial tension Cost Hazard In SCWO, organic compounds are destroyed rather than removal. 126
SCF processes are emerging technologies and few full-scale applications exist. Bench scale studies in conjunction with SCF equipment manufacturers are necessary to ensure that the process is both feasible and cost-effective for a specific application. 127