An old proverb says that people who live in glass houses shouldn’t throw stones. But if you were trapped inside a greenhouse with the temperature building to stifling levels—as some scientists say is happening on Earth—you’d certainly want to break open a window, if you could.
One way to shatter the industrial practices that generate atmospheric greenhouse gases may be to implement the precepts of so-called green chemistry. Its supporters describe this philosophy as pollution prevention at the molecular level. It focuses on developing chemical products and processes that reduce or eliminate the production and use of substances that are hazardous to people or the environment.
The principles of green chemistry often seem less like revolutionary concepts than simple common sense. An overarching theme is that it’s better to prevent waste than to treat it or clean it up after it’s been created. Other goals include developing chemical reactions that use or generate substances that have little or no toxicity to human health, as well as ones that start with renewable raw materials rather than nonrenewable resources such as petroleum products.
Many of the concepts of green chemistry were formulated in the late 1980s when scientists began to think of industries in terms of their effect on the environment. Green chemistry began to take hold in earnest after Congress passed the Pollution Prevention Act of 1990. Today, several practical applications of the research are poised to break out of the lab and become part of the industrial recipe.
The significant growth in the green-chemistry movement in the past decade has been driven by several factors, including new knowledge about which chemicals are harmful, the ever-increasing ability of chemists to control the substances that reactions generate, and the increased costs of using and disposing of hazardous chemicals.
Although the guidelines of green chemistry are applicable to all types of pollution, greenhouse-gas emissions have been a particular target. Researchers have used green chemistry to develop alternatives to the use of gases, such as chlorofluorocarbons, that can strongly contribute to Earth’s warming. They have also begun to develop processes that can reduce or eliminate industrial emissions of nitrous oxide, one of the most significant greenhouse gases. Manufacturers are looking with interest at and, in some cases, enthusiastically adopting these new practices.
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In many instances, manufacturing processes use or generate synthetic chemicals that, pound for pound, absorb much more of the sun’s infrared radiation than natural greenhouse gases, such as carbon dioxide, do. These humanmade chemicals include chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and other perfluorinated compounds (PFCs).
For example, CFC-12, one of the gases scheduled to be phased out of production because of its role in atmospheric ozone depletion, absorbs solar infrared wavelengths almost 16,000 times as effectively as carbon dioxide does. Sulfur hexafluoride, a PFC used in the production of semiconductors, electrical insulation, and magnesium, has a global warming potential almost 24,000 times that of an equal weight of carbon dioxide.
Not only are these gases strong absorbers of greenhouse radiation, their chlorine and fluorine bonds make them exceptionally long-lived in the environment. Data show that sulfur hexafluoride may persist in the atmosphere for up to 3,200 years.
Ironically, in the quest to replace some of these gases in today’s industrial processes, researchers have turned repeatedly to carbon dioxide, the first greenhouse gas to be recognized. A liquid only above 31ºC and 73.8 atmospheres of pressure(*see correction, below), carbon dioxide is now widely used as a solvent to replace volatile organic chemicals in processes ranging from decaffeinating coffee (SN: 2/3/96, p. 71) to dry cleaning and industrial degreasing.
Dow Chemical Co. in Midland, Mich., has developed a technique that uses pure carbon dioxide, instead of CFCs and HCFCs, as the propellant, or blowing agent, in the manufacture of polystyrene foam. Dow’s Gary Welsh says that the company’s worldwide licensing of this technology eliminates the use of about 3.5 million pounds of CFC-12 and HCFC-22 each year. Because the carbon dioxide used in the process is obtained either from natural sources or as a byproduct of other industrial processes, there is no net increase in the global quantity of the greenhouse gas.
“It doesn’t make economic sense to burn hydrocarbons to generate carbon dioxide,” Welsh says. “There’s enough of it out there already.”
Another company is phasing out CFCs as blowing agents in its production of many of its rigid polyurethane foams, which are used as insulation in products such as water heaters, refrigerators, and roofing materials. Stepan Co. of Northfield, Ill., has replaced the greenhouse gases with carbon dioxide, which is produced when water reacts with one of polyurethane’s components, says Brad Beauchamp, business manager for polyurethane systems at Stepan. The product is known as water-blown polyurethane.
Stepan’s original process used CFC-11 as a blowing agent. That gas was phased out in 1992 in favor of HCFC-141b. This greenhouse gas, however, will no longer be produced in the United States after Jan.1, 2003, in accordance with the Montreal Protocol on Substances that Deplete the Ozone Layer. From 85 to 90 percent of a CFC blowing agent ends up in the bubbles, or cells, in the foam, but the remainder is released to the atmosphere.
There’s little difference between the performance or the production costs of the water-blown foams and those of the CFC-blown products, Beauchamp says. Although carbon dioxide has a slightly lower insulating value than the greenhouse gases, company researchers were able to increase the insulating efficiency of the product by decreasing the size of the cells in the foam.
The water-blown polyurethane foam now makes up about 28 percent of the company’s sales in rigid urethane foams, a figure that Beauchamp says should rise steadily until the phase-out of HCFC-141b.
One of the more potent natural greenhouse gases is nitrous oxide, which absorbs the sun’s infrared radiation more than 200 times as effectively as carbon dioxide does. Although the vast majority of human-generated nitrous oxide results from automobile exhaust, about 10 percent—400,000 metric tons each year—comes from the production of adipic acid, which is used to make artificial resins and plastics such as nylon. Manufacturers typically produce adipic acid by oxidizing benzene at high temperatures and pressures in a multistage process. The final step uses nitric acid and generates nitrous oxide as a byproduct.
The sheer quantity of these nitrous oxide emissions, as well as the other noxious by-products of nitric acid use, have made the traditional way of producing adipic acid a tempting target for green chemistry. Scientists at Nagoya University in Japan reported in 1998 that they had developed a way to streamline the oxidation reaction and replace the nitric acid with highly concentrated hydrogen peroxide. The researchers say that the resulting process is cleaner, safer, and less corrosive, produces no nitrous oxide, and can probably be conducted at large scale with no operational problems.
A venture-capital group recently developed a one-step oxidation process for adipic acid production. The new method eliminates nitric acid and instead uses concentrated acetic acid—in essence, industrial-strength vinegar—as a solvent. This technique results in higher yields of adipic acid and lower equipment costs, and it eliminates emissions of nitrous oxide and nitric acid by-products, says Mag Fouad. He’s vice president of technology at Fluor Daniel in Sugar Land, Texas, a firm that fine-tuned the process and is demonstrating the technology to potential customers.
Data obtained during 3 years of operation at a pilot plant in Poulsbo, Wash., showed that manufacturers could reduce their equipment costs more than 30 percent. They could also save more than 20 percent in operating costs because of decreased energy requirements and the reduced need to collect and treat hazardous waste products, Fouad says.
Although the processes developed by the Japanese scientists and the Fluor Daniel researchers are more environmentally friendly than the traditional way of producing adipic acid, they still use nonrenewable petroleum products as their raw materials. Karen M. Draths and John W. Frost, both chemistry professors at Michigan State University in East Lansing, have harnessed several techniques of green chemistry to develop an approach that overcomes this disadvantage.
Draths and Frost use a genetically modified Escherichia coli bacterium to ferment glucose, a simple sugar, to produce a chemical called cis, cis-muconic acid. This chemical is then made to react with hydrogen gas to generate adipic acid under conditions of a moderate pressure, about 3 atmospheres, and about room temperature.
“The best thing about this process is that you don’t start with benzene, which is a nonrenewable resource,” Frost says. “There’s only one way for [benzene’s] price to go, and that’s up.”
Frost says the modified E. coli uses genes inserted from two other bacteria to produce enzymes that together generate a synthesis reaction not found in nature. Draths and Frost first described this reaction in 1994. Since then, they’ve tailored the fermentation reaction so that chemicals toxic to the bacteria are produced more slowly and can be removed from the solution before they kill the E. coli host bacteria. After 48 hours in the fermenter, the solution contains about 35 grams of adipic acid per liter of fluid.
Unpublished work conducted during the past 18 months shows that the process can easily be scaled up for use in 100,000-gallon fermenters, Frost says.
For anyone fearing that genetically engineered E. coli may escape and turn midwestern cornfields into large pools of adipic acid, Frost reports that the bacteria can’t live in the environment outside the fermenter. “These bacteria compare to a normal E. coli like a domestic poodle compares to a wolf,” Frost says. “There’s no way they can live ‘in the wild.'”
Supporters of the green-chemistry movement argue that its pollution prevention makes economic sense for companies as well as environmental sense for the community. “There is an incredible interest in green chemistry overseas,” says Dennis L. Hjeresen, acting director of the Green Chemistry Institute, which is based at Los Alamos (N.M.) National Laboratory. International chapters of the institute have formed in about a dozen countries, including Great Britain, Italy, Japan, and China.
Green chemistry is important for the developing world, Hjeresen says. For example, China is already the world’s number-two emitter of carbon dioxide, and the government there is concerned about its environmental impact, he reports. Chinese scientists are turning to green chemistry as a potential way to reduce pollution as the country’s industrialization and population grow steadily. In May, Hjeresen traveled to Guangzhou to attend the third international symposium on green chemistry in China.
In the United States, the Environmental Protection Agency has developed software that enables scientists to design environmentally friendly reactions for synthesizing chemicals and to see what others have done in this field. Also, the American Chemical Society has put together an educational program.
Although many of the movement’s principles originally appeared to be a radical departure from the traditional methods of chemical engineering, green chemistry now seems both logical and obvious to a new generation of chemists, Hjeresen says.
“Kids who are going through college now have grown up in a world that’s always celebrated an Earth Day,” he explains. “You tell them about the principles of green chemistry, and they immediately ‘get it.'”
In the meantime…recycle
In cases where chemists have yet to design a process that reduces the emission of greenhouse gases, companies must either abate the emissions as they leave the smokestack or capture and recycle them. Researchers at Air Liquide, located in Countryside, Ill., have developed a system that can collect emissions of sulfur hexafluoride and other perfluorinated gases from the semiconductor-manufacture process and recycle them back into production.
David Li, former manager of process research for Air Liquide, described the system in March at the annual meeting of the American Chemical Society. The equipment uses a polymer membrane to capture more than 98 percent of the sulfur hexafluoride, perfluoromethane, and perfluoroethane from the exhaust and then concentrate the gases almost 5,000-fold, to more than 99 percent purity. Li says that this still isn’t pure enough for semiconductor manufacturers, who are hesitant to use the recycled gases unless they are more than 99.999 percent pure.
Although attaining that desired degree of purity in these recycled gases would be costly now, Li says the company is conducting research that aims to bring down that expense. In the meantime, he and his colleagues are marketing the current equipment to producers of magnesium castings, who also use and emit sulfur hexafluoride but don’t demand extremely high purity of the recycled gas.
Air Liquide’s marketing targets the companies that supply these castings to automobile manufacturers. Li says that while the auto industry has recently been using the lighter magnesium components to increase gas mileage and reduce emissions of carbon dioxide, ironically, this change exacerbates the greenhouse effect by emitting significant amounts of sulfur hexafluoride during manufacture.
Li says that his company’s system can, in effect, decrease the cost of the sulfur hexafluoride by as much as 40 percent—not bad for a chemical that typically costs $10 to $15 per pound.
Air Liquide is now working with a major magnesium producer to implement such a system, Li says. He adds that the Environmental Protection Agency has asked the company to showcase the technology to demonstrate its utility to the industry.
“This type of system cuts down on emissions and helps save the [castings] company’s bottom line,” Li says. “Interest in this technology will skyrocket if there’s ever an emissions tax on these gases.”
Correction: The article incorrectly states that carbon dioxide is a liquid only above 31 degrees C and 73.8 atmospheres. In fact, that’s the point above which it is a supercritical fluid, in which there’s no difference between the liquid and gas states. Carbon dioxide can be liquefied at temperatures between -56.6 degrees C and 31 degrees C at pressures as low as 5.2 atmospheres.