SPECIAL REPORT
Gael D. Ulrich, University of New Hampshire
Flame-generated particles have found practical use ever since prehistoric cave dwellers collected soot from their fires to paint scenes on cave walls. Today, a wide variety of particulate commodities are made by flame technology. The most important of these are carbon black, fumed silica, titanium dioxide (titania), and fiber optic fume. Now, too, as understanding of flame processes improves, flame-generated particles promise to become even more significant, not only as commodity products but as specialty chemicals as well. In the late 1960s, while I was working for the research division of a company that produced flame-synthesized chemicals, such as carbon black and fumed silica, one of our customers found that high concentrations of fumed silica produced unusual elasticity in a particular polymer. The commercial manifestation of this discovery was the superball, a toy rubber ball with an extraordinarily high bounce. The temporary bounce that simultaneously showed up, as a result, in the company's sales volume also became known within the company as a "superball," one that both the research and marketing staffs periodically were urged to duplicate. In the realm of flame-generated particles, fiber optic fume, used for producing optical waveguides, probably is the most recent similar sales superball. But with the flexibility, product quality, and cleanliness inherent in flame technology, I believe that still other superballs are likely to be discovered. One of the distinguishing characteristics of flamesynthesized particles is the variation in size possible with the elemental or primary particle—that is, the particle that, cemented to others, forms the clustered frameworks that are known as aggregates. In commercial products, these primary particles can range from as small 22
Auguste, 1984 C&EN
as 6 nm to as large as 600 nm. Experimental materials have been made in which the particles are even smaller. The size of the aggregates, moreover, also can vary because they may be formed from one to as many as 1000 primary particles, depending on the base substance and manufacturing conditions. Such wide variations in two properties suggest great product flexibility. Versatility is limited, unfortunately, because the .two properties are somewhat interdependent. Conditions that produce small primary particles, for instance, also yield aggregates with large populations (and vice versa). Thus, there are no small primary particles that are not aggregated or any aggregates having a large population of big primary particles. The crystalline nature of flame-synthesized products also varies, depending on chemical composition and the tempering time provided by the flame. Carbon black and silica, for example, tend to be amorphous by nature, but titania and alumina are prone to form crystals. Titania and alumina also are amorphous, however, if they are not kept at high temperatures long enough to permit the particles to crystallize. Additional valuable characteristics—such as purity, uniformity, and flexibility—are consequences of flame synthesis technology. This unique combination of properties contributes to the strength, beauty, stability, and other desirable properties of many consumer goods. Flame technology has many advantages for manufacturing that have either resulted in its replacing older processes or provided a means for manufacturing entirely new products that could not be made otherwise. Carbon black production provides an example of the former. The channel process, the first modern process for making carbon black, was patterned after the ancient ones. Now it has been almost totally superseded by
Flame
Synthesis Of Fine Particles furnace processes. In the channel process, natural gas, burning in an atmosphere deficient in oxygen, produces smoke from which carbon black is collected on cold metal channels located above the flame. (The same phenomenon is demonstrated when one places a table knife in the luminous zone of a candle flame. The tenacity and density of the black smudge made when the knife is placed on a friend's cheek testifies to the pigmentary strength of carbon black.) The carbon black is mechanically scraped from the channels into hoppers, from which it is conveyed and prepared for shipment. Thermal black, requiring longer flame residence times, is produced in large brick-filled furnaces by a cyclic process involving alternate heating and "make" cycles. Natural gas is fed downward over a hot (above 1600 °F) brick checkerwork, where it pyrolizes to produce smoke containing carbon particles. About half the carbon black remains on the brickwork; the rest is carried off in the gas stream to be collected in a bag filter. When the brickwork cools to a predetermined point, the production cycle is interrupted. The reactor then is regenerated by admitting hydrogen, air, and natural gas, which heats the brick and ignites the carbon that has deposited on it. By the time the brick becomes clean via combustion, it is hot again and ready for another cycle. The channel and thermal processes are both very dirty. A desire to reduce environmental emissions, coupled with other factors, has led to the replacement of these older processes by modern furnace production methods using flame technology. In furnace processes, preheated residual oil feedstock is injected into a flame produced by burning natural gas or liquid fuel with air. Particles of carbon are generated in the flame, quenched while still suspended in the gas stream, and then removed by a collector such as a fabric filter. Furnace processes not only are cleaner but give higher yields.
Product quality and flexibility and production throughput also are usually higher in furnace processes. Fumed silica, like channel black, originally was produced by being collected on a cooled surface—in this case, a rotating aluminum drum. Production shifted rather quickly, however, to a furnace process quite similar to that used in modern carbon black plants. Flexibility and purity are vital in producing silica fibers for optical waveguides. The core of a waveguide is rich in a dopant, such as germania. The concentration of the dopant and its radial distribution provide the graded refractive index that is the key to waveguide performance. For long-distance, distortion-free communication, purity is crucial because contamination, even in the parts-per-billion range, can cause appreciable attenuation losses. By controlling the processing atmosphere and avoiding contact with metal surfaces, the purity of the final product is almost equal to that of the raw materials fed to the flame. No competitive techniques can match the purity achieved by flame processing. Another asset of flame technology is the ease with which effluent streams can be processed. A case in point is titanium dioxide, an important pigment in paints and plastics. The pigment originally was manufactured by precipitation from a liquid—the sulfate process. Emergence of the flame or chloride process, based on burning purified titanium tetrachloride, owes much to the ease with which the gas stream can be cleaned. The particles are collected in fabric filters economically and with high efficiency. By-product chlorine is scrubbed from the effluent gases and is recycled to the reactors in which rutile ore is chlorinated. The cleaned gas stream can then be discharged to the atmosphere; there are no water emissions to speak of. Auguste, 1984 C&EN
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Special Report
A handful of major commodity chemicals are produced by flame processes Probably the most important flamegenerated aerosol is carbon black. It is also the oldest manufactured pigment. Made in small amounts by primitive peoples, lampblack was produced in significant quantities by the ancient Chinese, who burned vegetable oils in an oxygen-starved flame, and then scraped sooty deposits from the surrounding cold surfaces. These deposits, when dispersed in suitable liquids, made excellent inks and paints. Since then, carbon black's role as a pigment in printing inks and black paints has not been surpassed. Carbon black matured dramatically to become a major commodity chemical when researchers discovered its remarkable ability to strengthen and reinforce rubber. With 3 to 4 kg of carbon black in every tire, automotive use alone accounts for more than 4 billion kg per year of production worldwide. Although all the various grades of carbon black are comprised of submicron particles, there are large variations in characteristics from grade to grade. These range from materials having small primary particles, such as the channel blacks, to the larger-sized thermal
Lampblack first was produced In quantity by the ancient Chinese blacks. Fine-particle grades are efficient pigments; coarser blacks, on the other hand, can be incorporated more readily into polymers to yield strong, tough, rigid finished objects. Other blacks, including tire-reinforcing grades, fill the spectrum between these extremes.
Various metal oxides are a second important group of flame-generated aerosols. These include fumed silica, titanium dioxide, and uranium oxide. Fumed silica was developed as a substitute for carbon black in hydrocarbon-limited Germany during World War II. It is formed by the combustion of silicon tetrachloride or some other silane with a hydrogen-containing fuel and oxygen. Minuscule liquid droplets, precipitated by the reaction, form a "smoke" that resembles carbon black in particle size and structure. In context with its more ancient relative, it has, rather carelessly, been called "white soot." After the war, silica fume was found to have useful properties in its own right. Today, nearly 100 million kg is produced in the world per year. More expensive than carbon black, fumed silica is compounded into silicone rubbers and glass-fiber-reinforced polyester resins. It also is used in numerous other specialty products, ranging from cake mixes to toothpaste—applications where superior performance and lack of color justify its premium price. Fumed silica sells for about 100 times the price of
Commercially available carbon blacks are produced In a wide variety of particle sizes, ranging (left to right) from a high-color pigment grade with a surface area of 210 sq meters per g through rubber-reinforcing grades, 90 sq meters per g and 30 sq meters per g, to a medium thermal grade, 7 sq meters per g
In uranium dioxide manufacture, the same is true and is of even more significance. Flame-generated powder used to form fuel pellets for nuclear power plants is produced by combustion of uranium hexafluoride in an oxidizing flame. Gaseous emissions from this dry process are cleaned more thoroughly and with less expense than is possible for liquid effluents from aqueous precipitation processes. 24
Auguste, 1984 C&EN
Low capital cost is another advantage common to flame technology. Burners, reactors, coolers, collectors, and effluent cleaning devices used to collect dry, flame-generated particles are less expensive than comparable equipment used in liquid-phase plants. Flame technology, to be sure, has some disadvantages, usually stemming from raw material costs. Except for carbon black, the products mentioned above are based
During the Initia/ step In making optical waveguides, silica and germanla, produced In high purity from silicon and germanium tetrachlorides In the two flames at left, are deposited on the boule, which will then be heated and collapsed to form a glassy rod from which filaments can be drawn natural sand of the same purity and chemical composition. It is about 10 times more expensive than liquid-pre cipitated silica (silica gel). But its cost is justified by the value of its enormous external surface area and its aggregated structure. Flame-produced (chloride process) titanium dioxide has emerged as a re-
Fumed silica, more expensive than carbon black, has many specialty uses, Including as a reinforcing agent or flow-control additive
placement for its longer-established liquid-precipitated counterpart made by the sulfate process. About 2 billion kg of titanium dioxide are consumed annually worldwide. Because of its large refrac tive index, it is the premier pigment in white or tinted paints and plastics. This asset is enhanced through sophisticated combustion processing, which yields a narrow size distribution, precisely con trolled particle diameter, and crystalline form that can be manipulated to provide optimum performance in paints, paper, plastics, and other products. These qualities, combined with the large ca pacity and minimal environmental im pact of flame-based titanium dioxide plants, account for commercial domi nance of the chloride process. Uranium oxide, perhaps the world's most expensive flame aerosol, is made when uranium hexafluoride is oxidized in a flame. The powder is pressed and sintered to form fuel pellets for nuclear reactors. Currently, most uranium dioxide fuel is produced by wet precip itation processes. In this industry, how ever, environmental considerations are even more important than they are in titanium dioxide manufacture. Because it is so simple and clean, the flame process promises to supplant the wet process, especially if uniformity and sinterability can be matched or im proved. Light can be transmitted over a tor tuous path through a transparent rod or filament. In the telecommunications and computer industries, such optical waveguides are used to convey coded laser signals over long distances and with minimal distortion. Modern optical waveguides are filaments of silica, usually less than 100 μιτι in diameter. In commercial waveguide manufac ture, silicon and germanium tetrachlo rides are used as raw materials. They react in an oxidizing flame to form par ticles that precipitate and deposit on a growing silica mandrel or boule. Three
on expensive metal chlorides or fluorides. For carbon black, quality is accompanied by a price that is difficult to beat. Made in high yields from relatively cheap raw materials (fuel oil or natural gas), it is a tenth as expen sive as fumed silica. Silica, however, frequently is used in place of carbon black when, because of color, electrical conductivity, or other reasons, carbon black is not suit able. Fumed uranium dioxide is certainly expensive, but
Flame-synthesized titanium dioxide, the most Important white pigment, Is produced with a narrow size distribution and precise particle diameter different geometric arrangements of flame and substrate are employed commercially. In each case, the parti cle-covered preform is heated and col lapsed to form a glassy rod from which hair-thin filaments are drawn. As with uranium dioxide, a commercial role for this product is already established, and it promises to grow rapidly with the ex pansion of the telecommunications, computer, and electronics industries.
Flame-generated particles of uranium dioxide can be sintered Into pellets for fueling nuclear power plants
it suffers no cost penalty relative to material made by the wet process because both processes use the same raw material. As a general rule, flame-generated particulate mate rials require expensive raw materials. But their unique properties and the simplicity of the flame process often compensate for this disadvantage. Fiber optic fume is a good illustration of this rule. For this product, raw maAugust 6, 1984 C&EN
25
Special Report Collisions of flame-formed particles form larger aggregates and agglomerates • Agglomerates
Aggregates
Molten spherical primary particles Flame (reaction zone)
t t t Reagents: SiCI 4 , H2, air
^r SiCI
4
2100 Κ
+ 2H2 + 0 2 + 3.76N2
•-Burner tube
t t t Fumed silica is formed when silicon tetrachloride reacts in a hydrogen flame to form single spherical droplets of silicon dioxide. These grow through collision and coalescence to form larger droplets. As the droplets cool and begin to freeze, but continue to collide, they stick but do not coalesce, forming solid aggregates, which in turn continue to collide to form clusters known as agglomerates
terial cost is less important than purity and unifor mity. It is tempting to anticipate other conceptual or generic products that might be manufactured profitably in a flame. Perhaps a farfetched but very-large-tonnage ex ample is portland cement. It becomes quickly evident to even a casual visitor to a cement plant that inordinate amounts of energy are spent to fire and grind the clinker. It is conceivable that quality improvements plus energy and capital cost savings might be possible if an aerosoltype flame process were employed in making cement. Unfortunately, reducing the raw material to a particle size where rapid flame reactions would be feasible is a major challenge. I understand, nevertheless, that aerosol flame techniques have been explored by some cement companies. Mixed oxides comprise a relatively unexplored and undeveloped avenue of flame technology. The precision with which composition can be controlled undoubtedly contributes to the success of flame technology in making fiber optic fume. Because a great number of different compounds can be precipitated in a flame and a wide range of composition (from zero to 100% of one or many components) is feasible, the number of potential coformed aerosols is almost infinite. Since precipitation occurs from the gas phase within microseconds, mixtures are likely to be homogeneous—even down to the mo lecular lattice in some cases. Catalysis, one would think, is a field in which such 26 Auguste, 1984 C&EN
precisely made aerosols might be useful. Surface areas of combustion-precipitated materials are, by nature, large and totally accessible, an important asset in catal ysis. Even though a variety of flame-generated particles has been available for a number of years, I know of no case where they are used as catalysts in commercial quantities. (Because of their well-defined surface char acteristics, they are used extensively in laboratory test ing, however.) Cost may be one deterring factor. The difficulty of supporting light, fluffy powders in conventional ways is another problem, although unconventional tech niques deserve attention if they can take advantage of the unique properties of these materials. It seems, for example, that transport reactors, which carry the catalyst along with the process stream, would offer significant heat transfer and kinetic advantages in some situations. Fluid beds are also possible with these fluffy powders if gas flow is modest. Since particles already can be re moved from process streams with high efficiency, sep aration of particles from reactor exit gases should be relatively easy. Until recent years, the science of catalysis probably had not evolved to the point where it could take ad vantage of the options available. About 16 years ago, I recommended the use of flame-synthesized mixed ox ides to a researcher in a prominent catalyst laboratory. He suggested that I send along samples as I made them, saying he would "put them on the shelf along with other materials and try them as occasion permitted." It appears that catalyst research then was about as sophisticated as flame aerosol technology was at that time. Science has advanced in both realms since then. If catalyst technol ogy has progressed to the point where a specific opti mum type of particle can be identified, combustion aerosol science can, I believe, fill the order. Poor communication between catalyst research and aerosol science may be one reason why there has not been more collaboration. A prominent textbook on chemical engineering reactor design, for example, states that particles having a diameter as small as 30 nm "can not, at present, be produced economically." This repre sents a limitation of the wet-chemical precipitation techniques that have been used traditionally to make catalysts. But flame aerosols having primary particle diameters a fifth this size are made routinely in com mercial tonnages. And their diameter can be reduced even further in many cases. The precision ceramics industry also appears to be a natural market for flame-synthesized particles. I have participated in programs which produced sample quantities of mullite, alumina, and zirconia for ceramic and refractory testing. (Fumed alumina for nonceramic specialty chemical applications has been available commercially for many years; it is almost as old as fumed silica but has not become so prominent.) One can envision the manufacture of other oxides, borides, carbides, or nitrides by flame techniques. Arcs, plasmas, and electrically augmented flames now are being used to make these materials. In many respects, particle formation in these systems is similar to that in flames. Where electrical augmentation permits the use
of cheaper raw materials, how ever, arc, plasma, and electric furnace processes may be more economical. Historically, the ceramics in dustry has depended on less-ex pensive materials, but there must be some critical applications that would justify the cost of premium flame-synthesized ceramics. Op tical waveguides and uranium oxide, in fact, might well be cate gorized as ceramic materials in several respects. Recent progress in under standing the processes by which fine particles are formed in flames probably will broaden the appli cations of flame technology. For years, plant engineers have known that they could vary par ticle and aggregate sizes by vary ing raw materials concentrations and high-temperature residence times. Early researchers felt that nucleation and surface deposition were the underlying mechanisms behind particle growth. With sil ica, the simple fact that particle growth continues long after all of the silica has precipitated is evi dence against this belief. Also, for silica- and many other oxideforming flames, thermodynamics indicates that nucleation is an es Growth in size of fumed silica particles as they are carried further from the flame is shown by these four electron micrographs. All samples were taken sential element of the chemical from the same flame but at different distances from the flame front: upper left, reaction. Therefore, it occurs very 8 milliseconds residence time, specific surface area 360 sq meters perg; upper rapidly within the flame and must right, 13 milliseconds, 350 sq meters perg; lower left, 86 milliseconds, 200 sq cease before particle growth itself meters per g; lower right, 137 milliseconds, 150 sq meters per g ceases. In such flames, where the chemical steps are extremely rapid and complete, stable aerosol for oxide aerosols have been derived (see selected droplets are formed almost instantaneously. These un readings). In the early stages of particle formation, before doubtedly collide because of Brownian motion, which aggregates begin to form, kinetic theory can be em is the cause of the coagulation that, in turn, is the pri ployed to predict the growth of primary particles. This mary factor in particle growth. yields an expression for either primary particle con As these droplets cool and begin to freeze but continue centration or size in terms of solids loading and time: to collide, they stick but do not coalesce, forming ag gregates. Collisions among aggregates continue even 1 after they have become solids. But the aggregates are Ν =· 1 / 5 Co M0 6 / 5 held together by physical rather than fusion bonds, where Ν = particle concentration (number per cubic forming clusters known as agglomerates. Agglomerates meter), Co = concentration of condensed species (mol can be decomposed readily by high-energy agitation ecules per cubic meter), c = sticking coefficient (diwhen the product is incorporated into a liquid. Aggre mensionless), A = collision constant, t = time(s). gates, on the other hand, because they are glued by fu This equation for particle concentration is valid for a sion bonds, are much stabler and cannot be fragmented cloud of spherical particles that are uniform in size. appreciably, even with intense agitation. The primary Strictly, although particles may be spherical, they are not particle, the smallest discernible building block of the uniform, but this approximation agrees within 20% with aggregate, is almost invariably nonporous. Thus, the a more rigorous analysis. particle diameter measured by gas adsorption is that of the primary particle. The equation indicates that coagulation is extremely Mathematical equations based on this growth model fast, especially in the early phases of growth when parAugust 6, 1984 C&EN
27
Special Report Laser technique measures growth of silica particles as they move from flame Particles formed in a silica-producing flame scatter a laser beam as it ap proaches and leaves the flame. The burner shown here is adjusted to pro duce a cone-shaped flame, rather than the flat flame shown in the diagram on page 26. The laser beam entering from the right initially is scattered slightly by dust in the room. It suddenly becomes brighter when it encounters larger flame-generated particles that have grown from the time they were first produced near the burner rim. As the beam approaches the core of the flame, it is scattered by younger and younger particles and becomes dimmer because these particles are smaller. Within the core, the gases have not yet reacted, so there are no particles to scatter the beam. As the beam emerges through the flame toward the left, this scattering behavior is reversed. Data from this laser light-scattering technique are plotted as the points in the lefthand graph. The points represent measurements at varying flow rates and temperatures but at the same indicated silica concentration. The solid curves are theoretically predicted growth rates calculated using a comprehensive growth model. As expected, coagulation rate is a strong function of concentration and aggregate population. It decreases dramatically as the population becomes smaller (or the aggregate mass be comes larger). The rather marked break in each curve coincides with a shift from free-molecule to continuum Brownian behavior [at aggregate masses of about 3000 Χ 10 6 atomic weight units (5 X 10" 1 5 g) and aggregate diameters of about 400 nm in these flames]. Although aggregate mass is depen dent on residence time and concentra tion, primary particle size is dictated by residence time and temperature. The lower right graph, for example, shows specific surface areas formed at two
temperatures for some of the silica ag gregates represented in the graph on the left. Classical fusion theory can be used to derive a relationship between particle diameter and residence time. To inte grate the relationship, a boundary con dition and the cooling rate must be known. (Surface tension, one parameter in the theory, is rather insensitive to temperature, but viscosity, the other parameter, increases exponentially as Atomic weight units 4
10
Γ 5.0 mol· % « O
August 6, 1984 CAEN
Specific surface area, sq meter/g t
_ ^ ^ ^ ^ ^ ^ |
^^^^^^· ^^^ ·
·
/ ••••j^—1
/ 103
*^^^Ôimote%SK>2
#
b VT LJr
·
c# · M ·
102 0 •ι ι ι ι 1 ι ι ι ι Ι ι ι ι ι Ι ι ι 0
50 100 Growth time (milliseconds)
tides are small and populous. Solids concentration and residence time are the most significant variables af fecting particle population. Temperature is relatively unimportant until growing particles cool to the point where they no longer coalesce. Then, according to this idealized picture, growth ceases. To test the model, one can study the growth of pri mary particles. Experiments involving titanium dioxide 28
the particles cool, so that the cooling rate is vital.) This introduces some ar bitrariness into the data fit. Neverthe less, using the same values for initial particle diameter and cooling rate, the two curves in the righthand graph were plotted using the theoretical fusion ex pression. The results, both theoretical and experimental, obviously are highly sensitive to temperature, as these curves are for flames that differ by only 40 Κ in temperature.
150 Growth time (milliseconds)
particles in a flame, lead aerosols in a shock tube, and carbon black particles under dilute conditions in a lab oratory furnace all showed good agreement with the equation when a sticking coefficient of unity was used. Even though carbon black is not precipitated cleanly by the chemical reaction, the chemistry evidently occurs rapidly enough so that coagulation (of viscous tarry droplets or semisolid particles) controls growth in its
Number of particles decreases rapidly as flame-formed primary particles coagulate Number of particles per cc
1.5 χ 1013ΓΤ
!
I
1.0 χ 101
0.5 χ 1013
are described in my work with N. S. Subramanian done several years ago at the University of New Hampshire. More recently, John W. Riehl and I have employed a laser light-scattering technique to measure the in-situ aggregate mass of growing silica particles. In practical terms, theory tells us that a spectrum of particles can be produced, each varying in aggregate mass and structure, depending on how concentration, residence time, and temperature are manipulated. We then can predict the kinds of particles to expect in par ticular flames, and we can design burners and reactors to produce custom particles. This is not a revolutionary concept. Manufacturers have discovered many of the same principles through experience. What is exciting is that combustion science has advanced to a stage where, if a need arises for parti cles having specific characteristics, they probably can be produced in a flame quickly and efficiently. D
Selected Readings
oL_j 0
ι
ι
ι
ι
iiT^^Ba—-
0.5 Growth time (milliseconds)
1.0
Note: Change in particle population predicted from equation in text and based on silica at a flame concentration of 7 mole %.
latter stages. In my lab, we have attempted similar measurements for silica. Using surface area (as measured by nitrogen adsorption) to infer the primary particle size, we were chagrined to find growth rates two orders of magnitude slower than the equation predicted. Why does a Brownian collision mechanism describe growth of metals, other oxides, and carbon black (at low concentrations) but not silica? Evidently, silica particles do not fuse rapidly when they collide. Instantaneous coalescence is a tacit assumption in the idealized theory. For some materials, and in dilute systems, this is a good assumption. Silica, on the other hand, has an anomalous viscosity that is 100,000 times greater than that of other refractory oxides at the same temperature. For this compound, fusion seems to be so slow that single spherical particles do not exist at all. Rather, aggregates or floes emerge as the true colliding partners from the earliest stages of nucleation and chemical reaction. In this case, the simplified model must be amended to include aggregates even in the early stages. These aggregates, however, also must collide by Brownian motion, just as single particles do. Meanwhile, as long as the primary particles are fluid, they will grow by fu sion within the aggregate. If the number of primary particles per aggregate is reasonably large, collision and coalescence can be treated independently. Collision rate controls aggregate mass, whereas fusion rate controls primary particle size. Electron micrographs of silica samples collected at increasing distances from the flame front seem to substantiate this picture. Theoretical equations pertaining to this refined model
Dannenberg, E. M., "Carbon Black," in Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 4, Wiley-lnterscience, New York, 1978, p. 631. Medalia, A. I., Heckman, F. Α., "Morphology of Aggregates—II. Size and Shape Factors of Carbon Black Aggregates from Electron Microscopy," Carbon, 7, 567 (1969). Ulrich, G. D., "Theory of Particle Formation and Growth in Oxide Synthesis Flames," Combus. Sci. TechnoL, 4, 47 (1971). Ulrich, G. D., Subramanian, Ν. S., "Particle Growth in Flames—III. Coalescence as a Rate-Controlling Process," Combus. Sci. TechnoL, 17, 119(1977). Ulrich, G. D., Riehl, J. W., "Aggregation and Growth of Submicron Oxide Particles in Flames," J. Colloid Interface Sci., 87, 257
(1982). Gael D. Ulrich is professor of chemical engineering at the University of New Hampshire, Durham. After earning bach elor's and master's degrees in chemical engineering at the University of Utah, he re ceived his doctorate from Massachusetts Institute of Technology in 1964. Before joining the University of New Hampshire faculty in 1970, Ulrich conducted research on particle-synthesis flames in industry. At the university, he has consulted and continued his research on the formation and growth of oxide particles in flames. He recently completed a textbook, "A Guide to Chemical Engineering Process Design and Economics," pub lished last January by John Wiley & Sons. Reprints of this C&EN special report will be available at $3.00 per copy. For 10 or more copies, $1.75 per copy. Send requests to: Distribution, Room 210, American Chemical Society, 1155—16th St., N.W., Washington, D.C. 20036. On orders of $20 or less, please send check or money order with request. Auguste, 1984 C&EN
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