Science & Technology: CHEMISTRY
WEIRD FLUIDS, BIG BUSINESS
NutraSweet Co. puts eggs through the wringer to make Eggcellent, its new egg-yolk powder with 74% of the fat and 90% of the cholesterol removed. It dehydrates them, turning the golden spheres into a powdery glob. Then, it shoots in carbon dioxide that has been transformed under high pressure into a so-called supercritical fluid. This liquid-like gas becomes a dieter's best friend, extracting fatty triglycerides and cholesterol but leaving phospholipids--the fats that help foods stick together. Introduced late last year, Eggcellent is showing up in grocery items ranging from waffles to mayonnaise.
As NutraSweet's success shows, supercritical fluids are finally delivering on some of the high hopes of the 1980s, when supercritical decaffeination stirred excitement. These mysterious fluids--actually gases or liquids that turn into a new state of matter above a "critical" temperature and pressure--are being used or tested for such varied applications as distilling petroleum and destroying U.S. Army tear-gas stockpiles.
Why are supercritical fluids finally reaching critical mass? For starters, tougher standards for air and water quality and food and drugs have pushed industries toward the ultraclean technology. Engineers have also learned from years of high-pressure experimentation how to coax desired properties from supercritical fluids by tweaking their temperature and pressure. Getting Eggcellent out of egg yolks "took more hours than I care to remember," says Kenn Vest, the product's marketing development director.
Scientists have known since the late 19th century that certain materials took on strange properties when sufficiently heated and squeezed. Carbon dioxide, for instance, goes supercritical at 88F and 73 times normal atmospheric pressure. Above that point, it has the density of a liquid but behaves like a gas in the way it diffuses into cracks and crevices. It's a gentle but effective solvent. Supercritical water is something else entirely: Mix it with oxygen, and the combination oxidizes nearly anything it touches--the same as fire.
The petrochemical industry was the first to exploit supercritical fluids. Beginning in the 1950s, it used a supercritical version of the hydrocarbon gas pentane to distill heavy crude oils and tar. By the late 1970s, German coffee company HAG mastered the use of supercritical carbon dioxide to decaffeinate coffee. General Foods Corp. bought HAG in 1979, and in 1988, its Maxwell House Coffee Co. unit refitted a plant in Houston for supercritical extraction (diagram). Decaffeination used to depend on methylene chloride, which is now a suspected carcinogen. The other big decaf makers simply switched to another chemical; Maxwell House says it picked supercritical extraction for Sanka and other brands because it better preserved the coffee's flavor--and adds that its customers are pleased.
TEFLON AND SPRAYS. The 1990 amendments to the Clean Air Act are giving supercritical fluids a boost by requiring the phaseout of chlorofluorocarbons. Supercritical carbon dioxide could substitute for CFCs in the making of fluoro-polymer plastics such as DuPont Co.'s Teflon. In production of emulsion polymers such as polyvinyl chloride, the building blocks called monomers don't always mix well in supercritical fluids. So Joseph M. DeSimone, a University of North Carolina chemist, is leading research on a kind of chemical "soap" to improve the mixing. Aside from polymer production, supercritical fluids can be used to create evenly sized, microscopic particles from substances dissolved in them. Phasex Corp. of Lawrence, Mass., and Dura Pharmaceuticals Inc. of San Diego are exploiting that property to create better inhalable drug sprays.
In environmental cleanup, water is the most promising of the supercritical fluids. Last year, Eco Waste Technologies of Austin, Tex., completed the world's first commercial system to treat wastes by supercritical water oxidation. Running at 1,000F and 270 times atmospheric pressure, it treats about five gallons of liquid waste per minute at a laboratory and pilot plant in Austin owned by Huntsman Corp. Such harmful organic materials as benzene, hexane, and aniline are turned into carbon dioxide, oxygen, nitrogen, and pure water, plus small traces of carbon monoxide. Huntsman says the process costs about half the $1.25 per gallon that it pays for incinerating the liquid waste. So far, the system won't work on any chlorinated or fluorinated compounds, such as CFCs.
MULTIPURPOSE? For the U.S. Defense and Energy departments, an environmentally friendly way to wipe out harmful wastes could be a big boon. One U.S. Navy officer estimates that the Defense Dept. alone could use up to 25 supercritical-water-oxidation plants, each one handling 6,000 gallons a day. Indeed, Defense and Energy are spending $15 million to $20 million every year just to develop the technology. They are cooperating with such companies as Foster Wheeler Development, Martin Marietta Energy Systems, and Modell Development based in Framingham, Mass.
Enthusiasm for supercritical fluids may overreach at times. For instance, Los Alamos National Laboratory and Hughes Environmental Systems Inc. are spending $3 million to study the use of supercritical carbon dioxide in dry cleaning as a substitute for harmful perchloroethylene. But corner dry cleaners might be better off simply picking a less-harmful chemical solvent. Phasex founder and President Val Krukonis argues that supercritical fluids aren't suited for most kinds of cleaning. Even Charles A. Eckert, a chemical engineering professor at Georgia Institute of Technology and vice-president of the International Society for the Advancement of Supercritical Fluids, says supercritical fluids should be "a tool, not a goal." Still, in several fields, this high-pressure state of matter could take a lot of pressure off the environment.By Lars Hansen in New York