New techniques brighten outlook for plasma Induction plasma produces ultrapure titanium dioxide; high-yield arc process converts coal to acetylene Many industrial chemists and chem ical engineers dismiss plasma chemistry as an interesting laboratory exercise with little future in the chemical proc ess industries. Sure, they admit, Huels has been operating plasma acetylene plants in West Germany since 1940, but that's a special case; usually, plasma processing is just too inefficient in its use of electricity to be rewarding. There is a determined minority, however, that refuses to relegate plasma processing to the limbo of the laboratory. Spokesmen for this mi nority group stated their case at a symposium held during the American Institute of Chemical Engineers/Instituto de Ingenieros Quimicos de Puerto Rico joint meeting in San Juan. No, plasma can't, compete with ex isting processes simply as a heat source, the pro-plasma forces con ceded. But there are cases where it offers something unique that makes it worthwhile despite the high energy costs. One potential use is in making ultrapure pigments. Another is in converting coal to acetylene. One speaker—who turned out to be a devil's advocate—suggested using a variation of plasma processing to make low-cost ammonia and hydrogen from organic wastes. Induction. Dr. Peter H. Dundas, director of process development for TAFA/Ionarc, Bow, N.H., noted that, until recently, induction heating of gases was not commercially feasibletechnology had not advanced to the point that megawatt systems could successfully run continuously. Now, he said, TAFA's 1-Mw. induction heater can do just that; now the ques tion is: "How can the chemical proc ess industry use oxygen, hydrogen, or chlorine heated without contamination to 2000° K.?" One use would be to make pigmentgrade titanium dioxide, Dr. Dundas suggests. Typically, the pigment is made by oxidizing very pure titanium tetrachloride at about 1900° K. Ti tanium tetrachloride is highly reactive at this temperature, as is the nascent chlorine released by the reaction. Consequently, expensive refractories must be used to construct the oxida tion reactors. Reactants are heated in one of two ways: with outside heat transferred through refractory heat exchangers, or by controlled combustion of carbon 56
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monoxide added to the reactant stream. Either method entails high capital investments, Dr. Dundas says, and each has certain operating dis advantages, so pigment makers have investigated other ways to heat the reactants, including d.c. and a.c. arc plasmas and induction plasmas. The trouble with arcs, Dr. Dundas says, is that electrode material is eroded, producing colored oxides that degrade the white titanium dioxide. With induction heating there are no electrodes, hence no electrode erosion problem. Also, he notes, at room temperature the metallic parts of the TAFA induction heater resist attack by reactive gases. Since these parts are kept at room temperature by water cooling, they are not a source of product contamination. Torch. Induction plasmas work best at 8000° to 10,000° K., accord ing to Dr. Dundas, but the best tem perature for the oxidation of titanium tetrachloride is about 1900° K. The plasma torch in the TAFA heater op erates in a sheath mode, which makes it possible to heat a part of the re actants to 8000° K. within a sheath cooler gas. The average downstream enthalpy can be adjusted to optimum by controlling the relative flow rates of core gas and sheath gas. TAFA studies have shown that a 1Mw. plant operating at 30% efficiency
could produce some 27 tons per day of titanium dioxide. Operating and capital costs, amortized over 15 years, would be about $15 per hour. Is such a plant economically attrac tive? Dr. Dundas doesn't know—the costs of competing systems are wellguarded secrets, he says, so it will be up to the pigment makers to decide. Confident. Richard E. Gannon and Val J. Krukonis of Avco Corp., Lowell, Mass., are confident that their plasma process for converting coal to acetylene will be economically attractive when it goes into commercial use, perhaps within a decade. "We didn't choose acetylene, it chose us," Mr. Gannon said, noting that when hydrocarbons are heated above 1200° C. and then rapidly quenched, acetylene is the primary pyrolysis product. In the Avco system, coal is injected into a hydrogen plasma produced in a 120-kw. rotating-arc reactor. Down stream, more coal is injected into the still-hot gases, to increase overall ef ficiency. Hydrogen, argon, or water may be used to quench the hot gases. Best yields are obtained with a hydro gen quench, Mr. Gannon notes; studies with deuterium-labeled hydrogen indi cate that the "quench" also takes part in the conversion reaction. The acetyl ene is stripped from the cooled arc gases by conventional means.
At present, the process is achieving yields of 30 to 35%, with a gross energy consumption of 4 to 5 kwh. per pound of acetylene. Projecting these results, Mr. Gannon says that the technique could deliver acetylene for about 5.25 cents per pound, com pared to the 8 to 11 cents per pound current selling price. With process improvements, he adds, acetylene will become cheap enough to challenge ethylene as feedstock for vinyl mono mers. Expedient. Plasma processes may sometimes be economically expedient, said physical chemist David R. Safrany of Bechtel Corp. Laboratory, Belmont, Calif., but "philosophically" they're unsatisfying. The only good process is an exothermic process, Dr. Safrany believes. If an endothermic step is absolutely necessary—for instance, to produce hydrogen for the Haber am monia synthesis—then the energy should come from the sun, for free. We can get hydrogen from partial oxi dation of hydrocarbons, but natural conversion of photosynthesized carbo hydrates to hydrocarbons (coal, petro leum, or natural gas) takes much too long. Dr. Safrany proposes a shortcut —perhaps the ultimate in plasma proc esses: "Large masses of organic com pounds could be vaporized and de composed with a thermonuclear de vice. Upon cooling, the . . . fragments would recombine not to the original compounds but to the thermodynamically favored . . . hydrogen and car bon monoxide." Since no chemistry is involved—only thermodynamic equil ibration—any organic substance, such as sewage, garbage, or lignin, could be used as raw material. Cyclic process. Decontamination would be accomplished by an indirect cyclic process. "The hydrogen, which contains most of the radioactivity, would be used to produce ammonia," Dr. Safrany explains. "The ammonia would be oxidized to nitric oxide, which does not contain hydrogen and which therefore would not be radio active." The carbon monoxide—not radioactive—would be reacted with water to produce "clean" hydrogen, which could then be used to synthe size uncontaminated ammonia. Dr. Safrany calculates that by us ing a 10-megaton nuclear device to vaporize about 3 Χ 10 6 tons of or ganic wastes, it is theoretically pos sible to make hydrogen for 0.5 cent per pound and ammonia for 0.1 cent per p o u n d - " a n order of magnitude less than by conventional methods." There would be some logistical prob lems, he admits, but the process ap pears to be scientifically feasible. Now all the chemical engineers have to do is work out the details.
SCIENCE/TECHNOLOGY By James H. Krieger Technology Editor
Coal gasification: from concept to reality There is a great feeling of satisfaction in watching a process or development move from concept to reality. There is even greater satisfaction when a whole technology makes the transition. Perhaps it is premature, but that is the feeling that accompanied the recent announcement by Panhandle Eastern Pipe Line Co. and Peabody Coal Co. that they had agreed to undertake a program which could lead to a combined venture for commercially producing high-quality pipeline gas from coal (C&EN, May 11, page 35). Not that pipeline gas from coal is going to emerge from the venture overnight. Panhandle Eastern and Peabody talk about the facility contemplated being in operation by the end of the decade. While saying that results of industry and government research to date are encouraging enough to start the project, they also point out that substantial technological problems must be solved. The significant point is that a specific commercial operation is now to be investigated by industry. Not a research project. Not a pilot plant. Not a demonstration plant. The talk is not vague con jecture about what might be possible "someday." Panhandle Eastern and Peabody envision a facility that would have a capacity of producing the equivalent of up to 300 million cu. ft. of natural gas a day, con suming about 6 million tons of coal annually. The ultimate objective, they say, is to construct and operate a gasification plant at a Peabody coal mine and to transport the gas produced as a supplemental supply for Panhandle Eastern's market area in the central midwest and Great Lakes region. This is the way it was supposed to happen. When the Depart ment of Interior's Office of Coal Research was formed in 1960 and set about to speed development of coal gasification technology, it envi sioned development of that technology to the point where utilities com panies would find it attractive enough to commercialize. In a sense, development has consisted of continuous refinement of the technology. Coal gasification goes far back in industrial history. Several score processes attest to its technical feasibility. But the goal of current gasification development is to produce a pipeline gas that will be interchangeable with natural gas and to do so at a price that will make it competitive with natural gas. This means that the product gas must be pumpable at 1000 p.s.i.g. and must have a heating value of about 1000 B.t.u. per standard cu. ft. Specific gravity must be within a certain relatively narrow range, and content of carbon monoxide, sulfur, inerts, and water must be below certain levels. Product gas from gasifiers varies widely, depending on the process, but none approaches pipeline quality. A selling price for the pipeline gas of about 50 cents or less per million B.t.u. was the economic goal. The extent of the challenge can be seen from the first cost projections, which put gas price at about $1.00 per million B.t.u. Prior to the early sixties, some individual coal companies had been carrying on development of gasification technology. The American Gas Association had also been supporting a fair amount of work at the Institute of Gas Technology. An infusion of funds from OCR speeded up this work and got additional projects under way. Gasification is only one of a number of utilization projects spon sored by OCR. Another is a liquids program. And tied in with these technologies is the production of electrical power by various means. Commercialization of these would not only aid in pollution control, but, with relatively vast coal reserves in the U.S., it would provide needed flexibility for any national program aimed at conserving natural resources. We hope more announcements of pending commercialization are forthcoming.
JUNE 1, 1970 C&EN 57
