Critical Fluid Extraction. Organic compounds, primarily PCBs and PAHs, appear to be the most amenable to extraction from soils and sludges with a technique utilizing liquefied gas, typically carbon dioxide, propane, butane, and sometimes alcohol. High pressure and moderate temperatures are used to compress the gas to fluid state. At the critical temperature and pressure, where the fluid state occurs, the mass transfer capabilities of the "critical fluid" are at their best. The critical fluid extraction process begins with the addition of hazardous waste to a vessel containing a critical fluid. The organics move to the top of the vessel with the critical fluid and are pumped to a second vessel. There, the temperature and pressure are decreased causing the contaminants to volatilize from the critical fluid. The concentrated organics are then recovered and the critical fluid is recycled. Extraction efficiencies between 90 and 98 percent have been demonstrated using PCB-laden
sediments. Volatile and semivolatile organics in liquid and semi-solid wastes have been removed with 99.9 percent efficiencies in the laboratory [6]. An estimate of performance and cost for the implementation of such a system can be developed with a bench- or pilot-scale test, and a full-scale design can be formulated from a successful pilot test. However, the cost of implementing this technology is generally high --$100,000 to $1,000,000 in 1990 dollars -- due to the complexity of the process and the need to maintain high pressures.
This complexity also makes it difficult to predict the efficiency of the process. Finally, critical fluid extraction, together with treatment and disposal methods, can constitute a full and permanent
solution to a contaminated site [6].