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he first supercritical water oxidation (SWO) reactor is being built in Austin, TX, by the Texaco Chemical Company to treat organic wastes. It is an indication that, after decades of research, supercritical fluid-based technologies finally may have arrived. SWO and supercritical fluid extraction (SFE) are slowly emerging as environmentally friendlier approaches toward dealing with the myriad toxic-waste problems. In pilot-plant studies at several companies and universities, the two techniques have been used to treat agents for chemical weapons, municipal solid waste, contaminated soils, and waste from the chemical industry. First discovered in 1821 by a French scientist, Baron Charles Cagniard de la Tour, supercritical fluids (SCFsl are fluids at high temperature and pressure that exhibit characteristicsbf both a liquid and a gas. They possess unique physical properties, including a high dissolving power and an ability to sustain combustion and oxida-
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tion. These unique properties form the basis of SWO and SFE. SWO and SFE are approaching maturity at a time when communities nationwide are increasingly reluctant to have new incinerators and landfills in their backyards. Unlike incinerators, SWO reactors produce much more controlled emissions, harmless byproducts, and relatively nontoxic, easy-to-discharge effluents. The supercritical state The “supercritical state” is shown schematically in Figure 1. Above its critical temperature, a substance exists as a supercritical fluid; at low pressures, the fluid has gas-like properties. Nitrogen “gas” in laboratory cylinders is an example of a supercritical fluid. Carbon dioxide and water are the two most widely used SCFs. The preferred fluid for SWO methods is water (supercritical at 374 “C and 2 2 1 bars), and the preferred fluid for SFE operations is carbon dioxide Isuuercritical at 31 “C and 74 bars). What senarates SCFs from fluids under ordinary conditions is their extraordinary power to dissolve organic compounds. For example, many materials now identified as hazardous waste-such as polychlorinated biphenyls (PCBs) and obsolete explosives-are organic compounds that are insoluble in water at ordinary temperatures and pressures and are difficult to treat; however, they readily dissolve in an SCF such as water. Both SWO and SFE are successful because of the extraordinary solvating powers of SCFs. SCFs, unlike ordinary fluids, also can sustain combustion and oxidation-processes that are often necessary for destroying toxic waste and a necessary component of SWO. SCFs sustain these reactions because they mix well with nonpolar organic compounds like oxygen, carbon dioxide, methane, and other alkanes.
Supercritical water oxidation During SWO, waste such as sludge is pressurized and heated until its aqueous component enters the supercritical stage (Figure 2). It is then burned in an insulated reactor where the dissolved components further break and readily combine with oxygen. The end products of the oxidation process most often are water and carbon dioxide. Very little CO and no detectable NO, or SO, is produced in the oxidation process. In
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Phase diagram of a pure substance showing the supercritical state (above its critical temperature. a substance exists as a supercriticalfluid)
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contrast, incineration oxidation reactions produce much more of these unwanted byproducts. SWO has successfully degraded dioxin, PCBs, benzene, DDT, urea, cyanide, ammonia, and dozens of other toxic substances (I). SWO reactors offer greater than 99.99% destruction e#iciency for many toxins and, because, unlike incinerators, they are closed systems, they can control emissions better. They also require less fuel because they operate at lower temperatures-500-600 “C compared with 2000-3000 “C needed for an incinerator. Several companies, such as Modar Incorporated (Natick, MA), Modec Incorporated (Framingham, MA), and Eco-Waste Technologies (Austin,TX), have conducted successful pilot-plant studies with SWO to treat sludge and other waste. Eco-Waste Technologies (contractor for the Texaco SWO reactor) also has been carrying out a military program, sponsored by the Defense Advanced Research Projects Agency, to destroy chemical-warfare agents and solidrocket propellants. Richard Lyon, president of EcoWaste, says that his company landed the Texaco SWO contract after facing stiff competition from alternative proposals for an incinerator as well as pretreatment methods. According to Lyon, the plant, lo-
cated at Austin Research Laboratories, would process 5 gallmin of organic wastes 24 hours per day. Supercritical fluid extraction Once confined to the petroleum industry (for separation and purification) and the coffee industry (for caffeine removal), SFE could provide a superior method for treating soils contaminated with organic pollutants such as diesel oil and PCBs. Scientists at the Westinghouse Hanford Company (Richmond, WA) and the Phasex Corporation (Lawrence, MA) have used SFE in successful pilot-scale experiments to remove various chemicals from large amounts (up to 5 kg) of contaminated soil. During the extraction, as the SCF (carbon dioxide) passes through the soil at high temperatures and pressures, it dissolves the soil’s organic pollutants. After the contaminants are dissolved, the temperature and pressure are reduced. The fluid changes to a gas, and the contaminants precipitate and are removed. The offgas is then recycled with no additional waste created. SFE could be used on site with the help of mobile units. In addition, soil structure is not permanently damaged during the SCF-based treatment, as in incinerator remediation; the soil can be returned to the site Envimn. Sci. Technol.. VOl. 27, No. 5. 1993 807
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from where it was removed and replanted. Also, no secondary waste streams are produced that could cause atmospheric, soil, or groundwater pollution. Limitations of the methods Two significant problems mentioned by experts in using SWO reactors are corrosion of reactor surfaces because of high temperatures and pressures, and lack of fundamental understanding of many aspects of the supercritical state. Because of corrosion, special alloy reactors must be used, thus increasing cost. Soil treatment using SFE suffers from several scale-up constraints, including decreasing efficiency. SFE is described by experts as an emerging technology, not yet ready for large-scale operations. It must also be noted that SCF-based methods are not a panacea for all toxic waste. In many cases bioremediation and other methods can be equally or more efficient. Prospects Because of the concerns people have about landfills and incinerators, SCF-based methods present an attractive alternative. But cost is a concern.
According to Lyon and other SWO experts who have done cost estimates, SWO is cost effectivewhen its total costs are compared with the total costs required to set up an incinerator. Costs vary depending on the nature of the toxic waste. Although it is too early to speculate whether SWO and SFE can ever replace incinerators, at the very least they provide a promising alternative technology for treating hazardous waste. SFE methods for soil treatment are also promising but would require further scale-up operations before widespread use. Reference (1) Glanz, J. R@D 1992, Feb.,98-100
Vinod K. Join is a stoff writer at the
American Chemical Society News Service who covers science and science policy. He has a Ph.D. in physical chemistry from the University of New Orleans and an M.A. degree in science, technology, and public policy from the George Washington University.
