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Waste plastic yields high-quality fuel oil.

Ironically, after all the trouble of reclaiming plastic waste from gooey trash, recycled products often cost more and look worse than virgin plastics -- a situation that displeases consumers.

But fuel chemists M. Mehdi Taghiei and his colleagues at the University of Kentucky in Lexington report a new, efficient way of converting plastic waste into high-quality, saturated fuel oil.

"It's good oil, too--much like imported crude oil," Taghiei said this week in Chicago at a meeting of the American Chemical Society. "This oil is even lighter and easier to refine into high-octane fuel than imported oil. It has no sulfur and fewer impurities." Similarly, the chemists found they could liquefy plastic with coal, also producing high-quality fuel.

The researchers mixed various types of plastic with zeolite catalysts, including HZSM-5 and tetralin, in a sand bath, then placed the slurry in a "tubing-bomb" reactor. Pressurizing the mixture with hydrogen and heating it to 420 degrees C for an hour caused high-molecular-weight plastics to break down into smaller compounds, similar to those in crude oil. Furthermore, oil yields proved high: Milk jugs generated 86 percent oil, soda bottles, 93 percent. Polyethylene, another common soft plastic, eked out 88 percent. When liquefied with coal in a roughly half-and-half mixture, the plastics turned into even better oil.

"In terms of the economics of this process, we have done some estimates," says Kentucky chemist Gerald P. Huffman, a coauthor of the report. "To convert coal and plastic simultaneously into oil right now costs about $27 or $28 per barrel, compared with $18 to $20 per barrel for imported oil. But we're quite confident that we can drive the cost of converted oil down to roughly the cost of imported oil. This process may be commercially viable within five to 10 years."

Plastics today account for roughly 40 percent of landfill trash, says Taghiei. However, of the total volume of plastic entering the waste stream, only 3.7 percent gets recycled, he adds. Why so little? The reason lies not only in high cost, he says, but also in contamination and impurities. Thus recycled polyethylene, for example, costs 10 percent more than virgin polyethylene.

Other methods exist for converting plastic to oil, Taghiei says, but usually they produce "unsaturated and unstable oils of low yield and low value." On the basis of the current rate of plastic disposal, he estimates that the United States could produce some 80 million barrels of oil a year.

This work stems from the Consortium for Fossil Fuel Liquefication Science, a five-university project sponsored by the Department of Energy. Overall, the project aims to make oil by liquefying many types of waste hydrocarbons with coal, using such garbage as paper, agricultural waste, sewage sludge, and rubber tires, as well as plastics.

Taghiei and Huffman argue that plastic-to-oil conversion plants could eventually supply the United States with substantial amounts of oil on an ongoing basis. Indeed, says Huffman, Germany has already started building a promising 200-ton-per-day plastic-to-oil reactor in the city of Bottrop.
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Author:Lipkin, Richard
Publication:Science News
Date:Aug 28, 1993
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