A catalytic process that could help paper mills save millions of dollars a year by converting a polluting by-product into formaldehyde, a useful product, has been discovered and patented by an engineering professor at Lehigh University in Bethlehem, Pa.
Israel E. Wachs, professor of chemical engineering, says his method, which was tested by Georgia-Pacific Corp. (G-P) in mobile pilot plants for two years, succeeded in converting a methanol-water waste stream contaminated by sulfur compounds and small amounts of hydrocarbons known as terpenes into formaldehyde. The process also significantly minimizes most of the emissions of carbon dioxide (CO2) and sulfur dioxide (SO2), two potentially harmful by-products of traditional pollution-control methods, Wachs says.
Andrew G. Gibson, an industrial consultant and former process- improvement manager at G-P, says the company found that a mill producing 2,000 tons a day of pulp would save between $500,000 and $1 million a year using the new method to get rid of the contaminated methanol waste stream. There are approximately 150 such mills in the U.S. and Canada, says Gibson, and 300 more worldwide.
Methanol and sulfur compounds, called mercaptans, are produced at paper mills when logs are digested under intense heat and pressure with a caustic solution in sulfite. The process separates lignin, a polymeric resin that holds together the cell walls of plants, from cellulose, the material used to make paper. Methanol and mercaptans, foul-smelling compounds, were once released by paper mills into streams. The conventional method now of disposing of methanol and mercaptans is incineration at 1,500 degrees Fahrenheit, an expensive process often requiring additional fuel--usually natural gas--that emits CO2, a greenhouse gas, and SO2, an ingredient in acid rain.
"Using conventional pollution-control methods," Wachs says, "paper mills at best can only convert an extremely bad pollutant, contaminated methanol waste streams, into moderately bad pollutants, CO2 and SO2 - and at a major cost. Consequently, the paper mills are not completely solving the pollution emissions, but just taking a band-aid approach."
Wachs's process also produces formaldehyde, a reactive compound important to the paper mill industry used to make the resins in particle board, and also to make molding compounds, brake linings and other products.
Gibson says G-P asked Wachs in the mid-1990s to develop a new catalytic process capable of converting the contaminated methanol waste stream into CO2 and SO2 at mild temperatures in contrast to the energy-intensive incineration method at 1,500 degrees F. Existing methanol-oxidation catalysts made of bulk metal oxides or metallic silver, Gibson says, were useless in paper mills because they were deactivated by the sulfur compounds and by the high steam concentration present in the stripper overhead gas containing the methanol.
Wachs conducted experiments in the microreactors at Lehigh's Zettlemoyer Center for Surface Studies using tiny amounts - about 100 mg - of a catalyst of vanadium pentoxide on a titania support. Not only did his catalyst incinerate the waste streams to CO2 and SO2 at about 600 degrees F., much lower than the 1,500 degrees F. using conventional non-catalytic methods, but the catalyst was not poisoned by the sulfur compounds and high concentrations of water (~50%).
But two things - the rapid catalytic reaction rate and the total conversion to CO2 and SO2 - made Wachs suspect that he was failing to notice intermediate reaction steps and products. "It seemed to me that the reaction was being pushed so quickly from step A to step C that it was not possible to detect the intermediate reaction step B. So the reaction was slowed down and it was discovered that in that step formaldehyde was being made in very high concentrations. This was the discovery of a new catalytic reaction that was previously unknown and was a new way of making formaldehyde that was economically and ecologically favorable."
To further improve the efficiency of the catalyst, Wachs deposited the active vanadia-titania catalytic material on an inert ceramic, which minimized over-oxidation of the desired formaldehyde and minimized the negative influences of excessive exothermic heat of this reaction.
Wachs, who holds 23 patents, believes the new catalytic reaction, selective oxidation of mercaptans to formaldehyde, can also be applied to the chemical, natural gas and petroleum industries. He has filed a dozen more patents describing those possible applications. "The approach used in this project may be beneficial in treating other emission mixtures containing reactive mercaptans," he wrote in a paper titled "Converting Waste Gases to Value-Added Chemicals." The paper, which was co-authored with Gibson, and also with Tom Burgess and Saul Furstein of G-P, was delivered by Gibson at the 2001 Spring Meeting of the American Institute of Chemical Engineers (AIChE) in April in Houston.
Different versions of the paper will be published by the Technical Association of the Pulp and Paper Industry (TAPPI) Journal, AIChE Environmental Progress and Applied Catalysis: Environmental in the coming year.
Materials provided by Lehigh University. Note: Content may be edited for style and length.
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