INDUSTRIAL A N D ENGINEERISG CHEMISTRY
September, 1930
Company-or rolled and welded of plate steel from 3 to 6 inches in thickness and total weight up to 200 t o n s - e . g., A. 0. Smith Corporation-are being used with heating jackets, special internal coils, or direct fire. In contradistinction to such monsters of steel whose mass so firmly attaches them to the earth, heat-transfer equipment has taken, not only wings, but ballonets as well. The largest change in lifting power of a lighter-than-air craft on a cruise is the steady burning of the tons of fuel required by the engines. The increased buoyancy may be compensated for by valving, when hydrogen fills the ballonets. Such practice is wasteful, however, and may not be considered for the U. S. Savy airships using helium. One of the methods used for maintaining the total mass of the airship and load constant is by condensing and collecting the water formed in burning the hydrogen in the gasoline molecules instead of exhausting it as vapor. The exhaust gases are passed through a lightmetal cooler which condenses the superheated steam present and collects the water for storage as ballast. It is possible to obtain a greater weight of water than the original weight of any fuel burned in aviation engines and thus maintain the altitude desired without valving the buoyant gas. A heat interchanger which has several advantages over the more ordinary tubular type has been developed in Germany and more recently applied in this country by the Combustion Engineering Company and others for the cooling and heating of large volumes of gas. A pack of cards may be imagined to be supported horizontally with all the cards slightly separated and arranged with alternate surfaces face to face, back to back,
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face to face, etc. If between each two faces cold gas is passed and between each two backs hot gas is passed, there will be an interchange of heat through the cards. Such a n interchange built of unembossed “cards” of sheet metal and with suitable connections for gas flues may be built more cheaply per unit of heat-transfer surface than any other. A recent installation of a battery of these “decks” contains approximately nine acres of gas film transferring heat to or from metal. To obtain this surface in tubes as originally considered would have required more than 200 miles of 1-inch iron pipe. Conclusion
No one would be unfortunate enough to assume that all or any large fraction of the unusual equipment used for transferring heat in recent years has been mentioned above. The incomplete descriptions attempted serve only to show the versatility of technology in adapting apparatus and processes to obtain the advantages of higher or lower temperatures or temperature differences. Most of the individual subjects of discussion have beeF completely described numerous times in the technical press and for some of them complete bibliographies involving hundreds of references have been prepared. To such bibliographies and the numerous treatises on individual phases of heat transfer one must look for records of past accomplishments while only necessity and native ingenuity can hold the answer of future types of heat transferring operations.
Aluminum in the Chemical Industry’ H. V. Churchill ALUMINUM RESEARCH LABORATORIES, NEWKEBSINGTOB, PA.
HE development of modern processes in chemical technology for the manufacture of useful materials has been paralleled by the development of special materials for the construction of apparatus. Metallurgical research in general has kept pace with chemical research, but many of the problems of chemical technology are so specific as to make it necessary for the chemical engineer to take the products of modern metallurgy and discover for himself the chemical utility of newly developed metals and alloys for his particular purposes.
T
yses Due to Corrosion Resistance
It is only within recent years that the metallurgist has concerned himself with the development of specific corrosionresistant materials. Before this application of quantitative corrosion resistance as a measure of the utility of a metal, mechanical and physical properties were the gage whereby the metallurgist evaluated his product. As a result of this, the literature is replete with information as to the physical and mechanical properties of materials and scanty with respect to chemical behavior under specific conditions. This condition will persist until the users of materials of construction freely publish reports of their experience with specifically described materials under particular conditions. These published data should include, not only complete information as to environment and exposure, but also full and complete descriptions of the materials used. This latter requirement is important in the case of many metals, such as steel, brass, bronze, and aluminum. These names of metals 1
Received July 12, 1930
are not specifically descriptive. For example, in chemical engineering literature the term “aluminum” is indiscriminately applied to any metal which has aluminum as a principal constituent. The fallacy of such a procedure is illustrated in Table I, which gives the chemical composition of several aluminum alloys in common use. Table I-Composition TRADE DESIGNATION Si High purity
9s ~-
5lSC 17.50
No. 43 No. 47
AI
Fe
Cu
hln
Mg
Zn
%
%
%
%
%
%
%
a
( e a
.. 1.25
..
..
99.50 99.OOb 97.75b
a
3s 25SC
of S o m e C o m m o n A l u m i n u m Alloys
1.00 0.75
a n
D
L1
6.00
a
I
a a
a
..
0:60
a
4.40 4.00 a
..
0:80 0.50
.. .. .. ..
0:50
*. .. ..
..
..
..
.., . ..
97.50b
93.50h 94.006 94.00h
13.00 n 5 ,. 8.5.OOb h’o. 12 8.00 a o . . 9O.OOb x o . 112 7.50 1 . 2 0 1.6 89.00s 5 Silicon iron and copper are impurities incidental to practically all aluminum. When’they are not definitely specified, their total is fixed by the AI content. b Aluminum contents here expressed are approximate. They are fixed by the actual total of alloying ingredients and incidental impurities. c These alloys may be heat-treated to produce improved mechanical properties.
..
It is not surprising, with such diversity of composition, that variation in chemical behavior should also occur. While it is always hazardous to be didactically specific in predicting the behavior of any metal under new conditions in the absence of test data, a general classification of wrought aluminum alloys would rank the metal in order of general corrosion resistance as follows: High purity (most resistant), 2S, 35, followed by those alloys which may be heat-treated. As a matter of fact, practically all aluminum chemical apparatus
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INDUXTRIAL A N D ENGINEERING CHEMISTRY
is built from high-purity metal, 2s or 3s. The alloys 51S, 25S, and 17s are used for structural or supporting members not actually in contact with chemical reagents. I n the case of casting alloys, silicon alloys such as Nos. 43 and 47 are more resistant to corrosion than the copper alloys such as KO.12 or 112. Constructors of chemical apparatus in which aluminum casting alloys are used generally employ silicon alloys in place of the older and better known copper alloys. Uses Based on Improvement in Quality of Product
Until alloys of aluminum are developed which are resistant to alkaline attack, the use of aluminum-base metals in the chemical industry must be confined to neutral media or to the handling of those acid materials which do not seriously attack the metal. Typical of the use of aluminum with acid materials is its employment in synthetic acetic acid production. The use of aluminum in this industry is advantageous from several standpoints, but principally on the basis of immunity to attack and secondarily on improved quality of acid produced. This later point is of interest elsewhere in the chemical industry. There are many chemical products on the market which, on account of color or contamination, are not so valuable as they would be if the objectionable color or contamination were absent. Practically no aluminum salts impart color to solutions. The presence of minute amounts of aluminum salts in most chemical products is not objectionable on other grounds; for example, the non-toxicity of aluminum salts is an advantage, particularly in relation to edible materials. The acetic acid industry employs three types of aluminum metal. I n wrought form 2s and 3 s are used and in castings No. 43 is employed. Another interesting use of aluminum in acid media where the use has other basis than resistance to corrosion attack or lack of color effect is in the production of organic acids by fermentation or mold activity. Citric and gluconic acids are now made by this process. Aluminum containers are here used because they are non-toxic to bacterial ferments or molds. Metals which are toxic toward such ferments or molds cut the yield to only a fraction of what is obtained in aluminum containers. Resistance to corrosion is also a factor in this application of aluminum, but the primary reason for using aluminum is given above. One interesting exception to the rule that aluminum is unsatisfactory when used in connection with alkaline materials is found in its very successful employment in ammonia condensers in by-product coke plants. Typical of the industries which use aluminum in media which are neutral or only mildly acid is the turpentine and rosin industry. Here the use of aluminum is especially desirable on account of the favorable color of the resultant product. I n this industry, however, the use of aluminum tends to vary directly in accordance with the market price of rosin. When the premium for light-colored rosin is sufficiently large to pay more than the excess cost of aluminum over other containers, aluminum is used. Aluminum is also used, to some extent, in the varnish industry, because it does not discolor the resultant varnish. Aluminum does not stand u p in an altogether satisfactory manner in varnish kettles; nevertheless, it is used because the color of the varnish produced by processing in aluminum is quite satisfactory. Available Forms
Apart from considerations of chemical behavior, it is essential that the metals being considered for use in the chemical industry be available in a variety of forms and that they be amenable to fabrication, to forming, and to joining in a variety of ways. A brief list of the forms in which aluminum and aluminum alloys are available illustrates the general adapta-
T’ol. 22, No. 9
bility of the metal to fabrication processes. Aluminum o r aluminum alloys are available in die castings, sand castings, permanent mold castings, forgings, sheet, extruded sections, structural shapes, foil, powder, wire, rivets, bolts, nuts, flat wire, screen, perforated sheet, bottle closures, drums, barrels, collapsible tubes, tank cars, tanks, pans, shingles, corrugated sheet, and other forms too numerous to mention. Aluminum and aluminum alloys can be welded by arc, butt, spot, or gas processes, or they can be readily joined by rivets or bolts. They are amenable to forming by stamping, by hot or cold pressing, by spinning, or by hammering over forms. Position in Electromotive Series
One important consideration often overlooked in the design of aluminum apparatus is the position of aluminum in the electromotive series. Aluminum is electronegative to practically all of the other metals in industrial use. This makes i t imperative that aluminum in electrolytes be kept free from electrical contact with other metals, lest a galvanic couple be set up which will seriously corrode the aluminum. Generally speaking, aluminum should be used alone in chemical a p paratus. The presence of other metal is hazardous even if precautions are taken to insulate or isolate the aluminum, since, if any attack does occur on the other metal, salts of that metal will go into solution. Metallic salts in solution tend to attack aluminum and aluminum alloys. This phenomenon has often led to failure where the performance of aluminum would have been satisfactory if it had been the sole metal in contact with the solution or other materials being handled. If it is essential that other metals be present in contact with aluminum, it should be borne in mind that the potential between aluminum and zinc is very low, the potential between aluminum and galvanized iron only slightly higher, that of iron and aluminum slightly higher than the latter, and all three of these substantially lower than the potentials developed between aluminum and copper or aluminum and brasses or bronzes. Effects of High Temperatures
Aside from the usual stimulating effect of high temperatures on corrosion, it is also important to realize that high temperatures may have other effects upon the properties of aluminum. It is important in this respect to realize that no serious oxidation or scaling of the aluminum takes place when aluminum IS exposed to air at temperatures up to the melting point. The mechanical properties of pure aluminum in the annealed condition are not appreciably altered by exposure to elevated temperatures, but in cases where pure aluminum has been hardened by cold work it is appreciably softened or annealed by exposure to heat. I n general, the strength, hardness, and elastic modulus of aluminum decrease on raising the temperature and plasticity is increased until a hot short condition is approached. Aluminum alloys are appreciably stronger than pure aluminum a t ordinary temperatures and most of them maintain greater strength a t elevated temperatures. Exceptions to this rule are those alloys of aluminum which contain appreciable quantities of zinc. Comparatively little aluminum in the form of high-strength alloys is used in the construction of chemical apparatus. This is due t o the fact that high-strength aluminum is not so resistant t o chemical attack as the simpler types of the metal and its alloys. Thermal Conductivity
The high thermal conductivity of aluminum is an important factor in many of its applications in the chemical industry. High thermal conductivity leads to evenness of heat distribution, so that local overheating effects are less liable to occur.
INDUSTRIAL AND ENGINEERING CHEMISTRY
September, 1930
in aluminum apparatus than in apparatus built of metal of lower conductivity. The thermal conductivity of tiluminum is affected by the addition of other elements, especially those that enter into solid solution. I n the case of the common alloys of aluminum, however, the conductivity never falls below about half that of pure aluminum. This is interesting when one considers brasses and bronzes, the thermal conductivity of which may be only one-fourth to one-tenth that of pure copper. Aluminum Paint and Aluminum Foil The use of aluminum paint in and about chemical plants is so well known as to make more than brief mention here superfluous. Bluminurn paint, in addition to supplying necessary and essential protection to the surface or material painted with it, also helps greatly in solving factory and mill lighting problems. The use of aluminum paint as a reflector of heat is well known. It is common practice to paint storage tanks and similar containers with aluminum paint in order to cut down evaporation losses of the materials being stored or handled. S o t so well known is the recently projected practice of using aluniinum foil in a somewhat similar manner. Stor-
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age tanks or containers may be coyered on the outside with aluminum foil, which is applied by much the same methods used in applying wall paper. Suitable adhesives are used which are usually of an asphaltic or bituminous nature. The advantage of foil over aluminum paint here is higher initial reflectivity and retention of that superiority over comparatively long periods of time. Conclusion
No one material of construction solves all the construction and operating problems of chemical engineering. Certain materials have specific advantages for particular purposes. Aluminum and aluminum alloys are no exception to this general rule. They are of little interest to workers in alkaline media. They are being used to an increasing extent in acid media where the acidity is organic in nature and in industries where the media are neutral. To the chemical engineer i t is a valuable accessory metal which should help to solve many troublesome problems. Its use is also economical when, despite not altogether satisfactory performance in corrosion resistance, favorable thermal, electrical, mechanical, or physical advantages make its use desirable.
Minimum Voltage to Reduce Aluminum Oxide' Albert Broadus Newmanz and George Granger Brown DEPARTMENT OF CHEMICAL ENGINEERING, UNIVERSITY
OF
MICHIGAN, ANN ARBOR, MICH.
The specific heat of crystalline alumina has been potential (IO)or "free energy" determined by a method involving the diffusion of ( AF = AH - T A S ) of formaprocess for the producheat through a cylinder of the powdered material. tion (12) of aluminum oxide tion of metallic alumiThe results of this determination have been combined from aluminum and gaBeous num involves the electrolysis with the thermodynamic properties of alumina as oxygen a t any temperature of a solution of aluminum given in the literature to compute the minimum within the range c o v e r e d . oxide a t temperatures betheoretical voltage required to reduce alumina to Having a numerical value for tween 900" and 1000" C:. I n molten aluminum and gaseous oxygen. It is probable the AF of formation a t any view of the commercial imthat this value is reduced in the commercial process temperature, the minimum portance of this process, it by oxidation of the carbon anode. Making allowance electromotive force required becomes of interest to calfor this reduction in potential, the theoretical minifor the d e c o m p o s i t i o n of culate the m i n i m u m elecmum voltage at 950" C. is comuuted to be 0.947. aluminum oxide into alumitrical energy which would num a n d o x y g e n a t t h a t be required if the process could be operated under ideal conditions. Commercially. temperature may be easily calculated from the equation mechanical conditions may outweigh energy efficiency, but NFE = - AF. Furthermore, the equilibria between carbon, such a calculation serves as a useful basis for comparison with oxygen, carbon monoxide, and carbon dioxide a t that temperature being known, the reduction of minimum electroactual operation. The method of calculation used depends upon the third motive force due to anode reactions can also be calculated. I n an attempt to assemble the necessary data for these law of thermodynamics (11) and requires the following numerical data: (a) the absolute entropies of solid aluminum, calculations, it appeared that reliable results had been re gaseous oxygen a t atmospheric pressure, and crystalline ported covering every item except the heat capacity of aluminum oxide; ( b ) the heat of formation of aluminum oxide aluminum oxide from room temperature upward. For this from solid aluminum and gaseous oxygen; (c) the heat reason the following experimental work was undertaken to capacities of gaseous oxygen and of crystalline aluminum obtain this necessary information. oxide, both as functions of temperature up to that of the Heat Capacity of Crystalline Aluminum Oxide commercial electrolytic process; (d) the heat capacity of solid aluminum from room temperature to the melting point; GENERAL DESCRIPTIOX OF THE METHoD-The method was ( e ) the melting point of solid aluminum; (f) the latent heat a modification of the one proposed by Brown and Furnas (2) of fusion of aluminum; (9) the heat capacity of liquid aluminum from the melting point up to the temperature of the A common method of determining the heat capacity of a liquid is to apply a known quantity of heat to a known weight commercial electrolytic process. From these data one can calculate the thermodynamic of the liquid by means of a submerged heating element, stirring the liquid before, during, and after heating. In this way, 1 Received February 20, 1930. Adapted from a thesis submitted by subject to suitable corrections, the uniform temperature rise Mr Newman to the Graduate School of the University of Michigan in is easily measured by means of a thermometer. I n the case partial fulfilment of the requirements for the degree of doctor of philosophy. of a powdered solid the possibility of maintaining the powder 2 Present address, Department of Chemical Engineering, Cooper a t a uniform temperature by stirring does not exist. HowUnion, N e w York, N. Y.
HE present commercial
T
I
