Supercritical Thinking

By Andrea Dace

Jagdish Dhawan says his interest in recycling began from his days as a student in India, where "we do not have the luxury of wasting anything." Dhawan, a chemical engineering professor at the University of South Alabama (USA), has since developed a process for recycling scrap tires from automobiles, trucks, and aircraft. The method is novel in that it recovers compounds that have commercial value, such as carbon black and a light synthetic crude oil that can be refined into a fuel like gasoline.

Dhawan’s invention will help manage the scrap tire solid waste that is replenished at a rate of almost 300 million tires a year in the U.S. Stockpiles of discarded scrap tires have been an environmental and health problem almost since the invention of the automobile. In the 1970s, during the Arab oil embargo, manufacturers began to look into burning scrap tires as an alternative energy source. It was discovered, however, that the sulfur added to tire rubber during vulcanization is carried into exhaust gases as sulfur dioxide, a compound linked to acid rain. Consequently, the U.S. EPA began requiring the removal of sulfur from exhaust vented into the environment. Wary of added cost, industries balked at developing scrap tire incineration as an energy source. Today, the majority of scrap tires are reused or recycled in other ways. One prevalent mechanical recycling solution is to grind scrap tires into crumb rubber that can be added to products like building materials and highway surfaces.

What is unique about Dhawan and co-inventor Richard Legendre’s patented chemical process is that the reaction occurs under supercritical conditions -- certain pressures and temperatures where gases can behave as liquids, or liquids as gases. Supercritical behavior of gases and liquids has been observed since World War I. Decaffeinating coffee with supercritical carbon dioxide is probably one of the most familiar applications of the technology, but the unusual chemical reactions still offer a range of tantalizing possibilities to industry.

Dhawan says he remembers reading surprising statistics on the numbers of discarded tires, bottles, and aluminum cans not being recycled in the U.S. when he moved to Mississippi to study. Dhawan was offered scholarships from several American universities after finishing his master’s at the University of Kanpur in India with the highest GPA. He chose the best offer and entered the Ph.D. program at the University of Mississippi.

After graduating with a doctorate in Chemical Engineering, Dhawan was able to study supercritical processes in depth when he went to work for one of the largest Canadian engineering companies. His job was to look at technology that could be used on Canadian coal to produce gasoline. Eight years later, Dhawan returned to the U.S. to teach at USA. He says he decided to return to teaching for professional freedom. "In academia, you think. You express ideas. You get feedback;" he says, "Then you go back to the lab to work." In industry, he felt limited studying subjects only for their profit potential to the company. Also, he didn’t care for the confidentiality agreements. "You cannot share your findings, or even talk about them."

Back in the academic environment, Dhawan came up with the ideal application for the idea that was to become his patent. He remembers going to a seminar presented by two professors from MIT on PCBs, the transformer cooling oils that had been linked to cancer in humans. Even though a global effort was underway to remove PCBs from the nation’s power plants, nothing yet was known as effective for disposal. The researchers from MIT described a method that could break the compounds down safely using only water, but under supercritical conditions. He started thinking about solvents at supercritical conditions to dissolve hydrocarbon rubber, the substance of scrap tires.

Dhawan was teaching fulltime at USA when he started to work out his own supercritical process. As soon as he finished his classes, he says he would go down to the room he called his lab. He started to get material results, then he called in Legendre, a fellow chemistry professor at USA, to help with the chemical analysis. The process calculations were very complex; the final solution contains about 300 different molecules. Today, Dhawan notes with satisfaction that the original equations have since been verified by several other institutions.

When Dhawan sent in his initial application to patent the process, he was surprised when the claim was denied. Reviewers informed him there were already many patents describing ways to render tires for petroleum. Dhawan investigated the earlier patents, and realized that the majority of claims were various pyrolysis methods where the tire is heated at high temperatures in the absence of air. Fumes from the volatile compounds in the melted rubber can be condensed into a liquid known as tire oil. The petroleum reclaimed under pyrolysis is useful as a fuel, but it needs further processing to remove the sulfur. While pyrolysis is environmentally friendly because there is no exhaust gas to clean, high processing temperatures can degrade other recoverable compounds. By comparison, Dhawan’s supercritical method has a better energy budget because the reaction occurs at a lower temperature. Also, it produces not only a higher quality oil, but other valuable compounds while the sulfur in the tire rubber is transformed into carbon. Dhawan proved to the U.S. Patent Office that the methods described in the earlier patents were not as efficient as his, nor did they recover materials as economically viable. He got the patent in 1995.

In a standard agreement as a USA employee, Dhawan’s work had become the property of the school’s College of Engineering. Then in 2000, the patent was licensed for commercialization by a company that eventually became Advanced Recycling Sciences (ARS), based in Tustin, California. The university was paid a lump sum for a license period of seven years. After the university deducted its overhead, the remaining amount was distributed to Dhawan and Legendre. Had ARS wanted to sell the synthetic crude end product, the process could have complemented its existing capabilities. ARS already produced a granulated product from scrap tires, which is the reactor feed. Instead, ARS decided to develop a larger, scalable processing unit, so that it could market the process itself to businesses that would buy a skid-mounted plant. To date, ARS’s commercialization plan has progressed, but at a slowed pace. Dhawan thinks that it could take another two years or longer, depending on the economy. Large-scale commercialization requires not only technical expertise and manpower, but a great deal of financing. Dhawan does believe that the process will be widely accepted eventually because of its environmental and economic advantages compared to other methods of recycling scrap tires.

Now an American citizen, Dhawan has taught at USA for 22 years. Currently, he is working with his students on a prototype of a "chemi-car," a car fueled by chemicals that has won several awards in annual competitions. The car they have developed is fueled by sugar produced from agricultural biomass or even grain alcohol. "This is exciting," says Dhawan of his latest adventure.

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Andrea Dace is a freelance writer in Williamsburg, Virginia