Scientists perfect chlorine-recycling process


A cheap and environmentally friendly chlorine-recycling process developed by USC researchers may dramatically cut the volume of hazardous chlorine gas being manufactured, the amount of toxic chlorine waste to be disposed of and the environmental dangers of transporting both products on the world's roads, rails and waterways.

"The new recycling process should result in a substantial economic gain for industry as well as improvement for the environment," said Ronald G. Minet, an adjunct professor of chemical engineering who is spearheading industrial applications of the catalytic carrier process.

The process has been patented by an interdisciplinary team of scientists: chemical engineers Minet and Theodore T. Tsotsis, holder of the Robert E. Vivian Chair in Energy Resources; and Sidney W. Benson, professor emeritus of chemistry in the College of Letters, Arts and Sciences.

Chlorine is a gaseous chemical element widely used to kill germs in water supplies and to keep swimming pools algae-free. Millions of tons of chlorine are consumed annually in the reactions that produce such everyday goods as plastics, paints and packaging materials.

In the course of these reactions, most of the chlorine is incorporated into the products being manufactured. But some chlorine frequently combines with hydrogen, becoming hydrogen chloride (HCl) - a corrosive and hazardous waste material that dissolves in water to form hydrochloric acid.

The amounts of chlorine and toxic HCl waste involved are huge. Of all chemicals used in the world today, chlorine ranks eighth in volume.

Industrial uses exist for HCl, but chlorine processes produce much more of the by-product than the market can absorb - resulting in a toxic-waste disposal problem. Chlorine gas is highly toxic if released into the atmosphere, so shipment via public carrier also poses environmental risks.

Chemists have always known that it's theoretically possible to convert the HCl waste back to its components - hydrogen and chlorine - and thus recycle the chlorine. But existing processes to do this have been inefficient and expensive.

As a result, companies making products like titanium dioxide (a paint pigment) or polyurethane (a plastic) have found it more economical to bring in a continuing supply of new chlorine and to dispose of their plants' accumulating stocks of hydrochloric acid as waste.

According to Minet, American manufacturers bought and consumed 12 million tons of chlorine in 1994. The recycling process, he said, has the potential to reduce chlorine consumption by approximately 3 million to 4 million tons per year. Recycling would also save approximately 6 million megawatts of electric power - the amount of energy required to extract this much chlorine from raw materials - as well as the fuel used to transport waste and chlorine gas.

In Europe and Japan, where environmental regulations regarding chlorine and HCL are at least as stringent as those in the United States, similar volumes of material are involved.

USC's catalytic carrier process is a refinement of the Deacon process, originally patented in 1868, which uses a copper compound as a catalyst to oxidize HCl with air, producing water and chlorine. Prior attempts to make the Deacon process work in continuous industrial applications have been economically unsuccessful, because of difficulties associated with handling the very hot and extremely corrosive mixture created by the reaction, the problem of incomplete reactions and the rapid loss of catalyst activity.

Benson's analysis of the basic chemistry, done in collaboration with postdoctoral researcher Mohammed Hisham, suggested a solution: to use an apparatus in which different phases of the reaction would occur in separate vessels. Benson's analysis suggested that the two-stage apparatus would function far more efficiently, avoiding the corrosion and catalyst deterioration problems.

Minet - who has been conducting investigations in the Zona Franca plant of the Carburos Metalicos S.A. company, in Barcelona - estimates that a full-scale installation using the process will recover chlorine from waste HCl at a cost of less than $80 per ton. Chlorine on the open market costs as much as $250 per ton, making the catalytic carrier process economically attractive. Tsotsis and USC research associates Hai-yen Pan and Max Keith Mortensen worked with Minet on these studies.

"We believe we are well on the way to solving the technology problems involved in applying the process to industrial production," Minet said, after achieving a short test run of a four-kilogram-per-hour pilot plant.

Three patents for the process have been issued to USC, and a fourth is pending.

"Environmental groups have been concerned in recent years about problems associated with the shipping and storing of chlorine," Benson said. "This process has the potential to alleviate much of that concern."

Minet will report his findings at the ninth International Symposium on Large Industrial Plants, to be held in October in Antwerp, Belgium.



Chemist Sidney W. Benson and chemical engineer Ronald G. Minet with a prototype of the chlorine-recycling apparatus they believe can have a major impact on the worldwide environmental problems associated with the hazardous element.

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Updated: 2/9/98