NEWS OF THE WEEK
METHANE-C2 CONVERSION New process more than doubles yield ntense industrial research has been focused during the past decade on finding a new source for ethylene and other lower hydrocarbons via catalytic oxidative coupling oi methane. The best conversions achieved thus far have been less than 25L"(, using fixed and fluid ized beds. \ow, researchers at the University oi Minnesota, Minneapolis, have developed a process that produces more than 60r; C2 yields from oxidative coupling of methane. Anna Lee Tonkovich, Robert W. Carr, and Rutherford Aris oi Minnesota's department oi chemical engineering and materials science used a simulated countercurrent moving-bed chromatographic catalytic reactor to convert methane to C2 species. Working on a bench scale with milliliters-per-minute quantities, thev employed a samarium oxide (Sm:0-.) catalyst at temperatures near 1000 K. The research team reports that more than 80('< oi the methane carbon is converted to C : species and it expects optimization will further improve these results. The work is described in last week's Science [262, 221 (1993)1. It was supported by the Department oi Energy. The reactor's success results from rapid separation oi oxygen, methane, and C2 products, this permits shifting the chemical equilibrium in an equilibrium-limited situation. Also, Aris and his coworkers point out, separations that are carried out in chromatographic columns can be used to increase conversions bevond the equilibrium limit oi well-mixed reactors. Work is now under wav in Aris' lab to increase yields by additional manipulations of the apparatus, such as operating in pulsed-flow and lowduty cycles. Although much attention has been given to equilibrium considerations,
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CXTOBER 11, 1W C&E\
Process simulates countercurrent reactor Solid phase (adsorbent and/or catalyst)
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Gas phase out
Feed inlet
Solid phase out
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Inert carrier gas in
The chemical reactor used by Aris and coworkers simulates the process that occurs in a countercurrent moving-bed reactor, as shown here. In this simplified schematic, granular adsorbent and/or catalyst flow slowly downward against a countercurrent stream of inert carrier gas. Feed (for example, methane) is injected partway along the column. Solid and gas flow rates are adjusted so that species that are adsorbed relatively weakly (such as ethylene) move upward with the carrier, whereas those that are adsorbed relatively strongly (such as carbon dioxide) move downward with the solid. methane coupling is not equilibrium limited. It is, simply, a low conversion process because oi the necessity to prevent further oxidation of the desired products. The scientists circumvent this problem by using reaction chromatography—which separates oxygen, methane, and C2 products as they are formed.
The reactor that is being simulated is a countercurrent moving bed in which the solids are appropriate adsorbents and/or catalysts. In it, the solids move down a cylindrical reactor countercurrent to an inert carrier gas, and the reactants (feed) are introduced at some midpoint in the bed. The flow rates of the gas and solids are adjusted to take advantage of the relative adsorptivities of the reactants and products. Products and unreacted feed are removed at the ends of the reactor for appropriate disposal. In the simulation, countercurrent flow is represented by sequentially shifting the entering feed through inlets along a stationary bed. Feed is admitted at each inlet for a prescribed time and then moved to the next inlet, starting at the bottom inlet and going toward the top. This sequence is then repeated indefinitely. The simulation represents the movement of the feed and products, as well as discrete motion of the solids in the opposite direction. Several different arrangements of the discrete stages can be envisioned. The group built and operated a fourstage reactor at atmospheric pressure and under computer-aided control. Aris and his coworkers point out that obtaining the highest yield they observed, 65c\ methane-C: conversion, would require building conventional reactors that are more than 30 times larger. It would also require a much bigger catalyst inventory. And thev tell C&EN that numerous performance enhancements are possible for their process—for example, employment oi better adsorbents, more selective catalysts, and programmed temperature regimes. Industrial sources contacted by C&EN, who request anonymity, generally react favorably to the report, but with some reservations. They express great interest in the data, but considerable doubt as to
whether the reactor's complexities can be adapted to industrial use. Many say they will withhold judgment until extensive economic analysis of a better defined reactor is made. The chief concern is the scaling-up of the reactor by four or five orders of magnitude to a size of world-class interest. There also is concern over the additional pre- and posttreatment required for the various streams. At first glance, a lot of solids-handling equipment would be needed. The industry sources add that the work will probably cause a lot of options to be reexamined. In particular, the catalysis community, which has more or less settled on a stable of potentially useful catalysts, will need to look at other catalysts—especially lanthanide oxides— that have shown high selectivity. The research also should spur reconsideration of adsorbents used in chromatographic reactors. Joseph Haggin
Novel idea developed to destroy toxic chemicals A novel concept for destroying toxic chemicals such as polychlorinated biphenyls (PCBs) and DDT—using mechanical force to break them down chemically—is currently under development by researchers in Australia. The Aussie group is the first to make the surprising suggestion that chemical detoxification might be accomplished by ball milling, a technique normally used in the mining industry for grinding rocks and ore. But the chemical version of this technique is still in an early stage of development, and data to back up its validity are scanty. The mechanochemical technique was conceived by physicist Robert Street of the University of Western Australia (UWA), Perth, and Peter Donecker of the mining company CRA Ltd., Perth. This led to a joint R&D project by CRA staff, professor Paul McCormick and coworkers in UWA's department of mechanical and materials engineering, and environmental chemist Doug Ingraham and coworkers at the Chemistry Centre of Western Australia, also in Perth. Basic studies are continuing at UWA and the Chemistry Centre, with CRA focusing on scaleup and development of a commercial process unit in per-
haps one to two years. The technology is protected by an international patent application. In the process, chemical waste and a reactant such as calcium oxide (quicklime) are placed in a ball mill containing steel balls. As the ball mill operates, colliding balls mechanically activate chemical reactions between the toxic materials and the reactant. 'This results in virtually complete breakdown of the toxic molecules into harmless, environmentally safe by-products such as carbon, calcium hydroxide, and calcium chloride," say the researchers. The method is particularly effective for destroying halogenated organic compounds such as PCBs, DDT, and chlorobenzene, they say. For example, destruction levels of greater than 99.996% were obtained in reactions of PCBs with calcium oxide. In one experiment, six and a half hours were required to completely break down a sample of DDT. However, the group is not communicating very much of its data. "Exact details of the process cannot be revealed for commercial reasons," McCormick tells C&EN. The researchers point out that the technique has a number of advantages over current methods of waste disposal, such as incineration—which can generate toxic emissions. Ball milling occurs in a sealed container at low temperature and does not emit potentially harmful gases. The process can be scaled up to any reasonable size. And ball mills can be mounted on road or rail vehicles for use at toxic waste sites, eliminating transport of toxic chemicals. Asked by C&EN to comment on the technology, chemist Ruth Ann Hathaway of Environmental Analysis South, Cape Girardeau, Mo., who specializes in PCB analysis, cleanup, and incineration, says: "It looks like a pretty good method—actually more feasible than most things I've seen. I don't know that anybody's ever tried [detoxification] at room temperature. There's no reason why it wouldn't work—it's just something we never thought of. Not having to add heat would hold down formation of some of the nasty volatiles you sometimes get with incineration." However, Jurgen H. Exner of JHE Technology Systems, Alamo, Calif., a consultant specializing in hazardous waste treatment and technology commercialization, points out that the technique is still in an early development stage, and there
is "many a step between the laboratory and commercial application." Exner says the initial data provided by the researchers is insufficient to judge the technique's validity. For example, another technology that claimed destruction of PCBs by treatment with quicklime was recently shown to be invalid, because it was found the PCBs were being volatilized, not destroyed. But Exner adds that there is a need for new treatment techniques for chemicals like PCBs and DDT. Existing waste detoxification processes—incineration, thermal desorption, and caustic treatment—all involve high temperatures and produce potentially problematic emissions, he says. And biological treatments using microorganisms are under development but "are still not totally demonstrated." Exner also believes the economics of the Australian process would likely be competitive with costs of existing thermal treatments. Microbiologist John W. Davis of Dow Chemical in Midland, Mich., a specialist in soil and groundwater remediation, also cites the insufficient data. "I am skeptical of techniques that claim destruction of contaminants, yet fail to account for a mass balance of the breakdown products," he says. "If you say something disappears, what does it go to and can you account for everything it breaks down to?" However, Davis adds, "We're always open to examining and evaluating new technologies that might be cost effective for treating contaminated soil." Stu Borman
Russian role in space station questioned Undeterred by the dramatic armed confrontation in Moscow last week, U.S.Russian cooperation in science and technology is forging ahead. However, plans for possible collaboration in building a space station are drawing critical questions in Congress. The Administration is attempting to resolve these doubts through testimony at hearings last week and this week before the Space Subcommittee of the House Science, Space & Technology Committee. National Aeronautics & Space Administration head Daniel S. Goldin was in Moscow last week discussing with top Russian officials implementation of OCTOBER 11, 1993 C&EN
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