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164
Note-The present status of the equipment is the result of a rather long period of experimentation and growth. The reliability and accuracy of the method are shown by the fact that during September, 1928, it has been necessary to make only six reactivations in order to keep the relative humidity below 4 per cent at all times.
Humidity Readings
A pair of wet and dry bulb thermometers are placed so that small amounts of the dry air can be by-passed out and humidity readings taken from time to time. Wet and dry
Vol. 21, No. 2
bulbs have sometimes been considered as rather inaccurate, especially when being used in air which is under 10 per cent relative humidity. However, the Weather Bureau of the U. S. Department of Agriculture, in a bulletin on “Psychrometry” by C. F. Marvin, asserts that this method is accurate if the velocity of the air passing the wet bulb is 4.57 meters (15 feet) per second, a condition which has been complied with in these determinations. In addition to this, gravimetric determinations using phosphorus pentoxide have been found to check the wet and dry bulbs to within 1 per cent.
Low-Temperature Carbonization of Coal’ S. W. Parr UNIVERSITY
OF
ILLINOIS, URBANA, ILL.
TTEMPTS at the low-temperature carbonization of coal, which have been continuous and indeed cumulative as to intensity for the past twenty-five years, seem to have attained to a number of proposed processes a t the present time of about two hundred and fifty. The normal procedure for introducing a discussion under fi heading such as is used as a label for this paper would be to propose a definition. Any attempt a t a definition for the low-temperature carbonization of coal brings with it the feeling that such an effort would have a characteristic closely related to charityin that it must of necessity cover a multitude of sins. Let me for the moment pass by the matter of a definition. I will revert to it a little later after I have set forth a few topics bearing upon certain fundamental factors involved in formulating a definition.
A
relating to the temperature a t which the process is to be carried out. This line of cleavage is a temperature line and is drawn very closely along the zone of 450-500’ C. (900’ F.). Carbonizations a t temperatures below this line are distinctive in behavior and products, and the attending conditions are fixed and definite, and indicate in what manner and with what material the procedure may be undertaken. Similarly, carbonizations a t temperatures above this line are distinctive in behavior and products, and the attending conditions yield results that are fixed and definite from both physical and chemical standpoints. These two statements regarding two distinct zones of carbonization a t once suggest a reason for not attempting a t the outset a definition of the term “low-temperature carbonization.” Let us briefly review the behavior, especially of a chemical sort, in these two zones. Temperature Curves in Lower Coking Zone
Figure 1
A historical review may also be set aside even though it is fin exceedingly interesting account-especially interesting to me because my own first experiments were carried out in the early months of 1902, and it is generally conceded that with added years one grows reminiscent, that is to say, historically inclined. A most admirable historical r6sum6 has been set forth in the recent book by F. M. Gentry and any elaboration of those facts a t this time would be superfluous. I would like to address myself rather to some of the more salient factors that many investigational workers have developed. Two Carbonization Zones A review of the procedure and the results as they exist today shows a very marked cleavage into two fundamental ideas 1 Presented before the Second International Conference on Bituminous Eoal, Pittsburgh, Pa., November 19 to 24, 1928.
I
I n the first or lower zone, as we would expect, the first 100 degrees of temperature are required to drive off the free moisture, and until this work is completed the temperature d o e s n o t r i s e above 100’ C. What occurs in the next 200 degrees is of fundamental importance, though it has, as a rule, been neglected by students of carbonization phenomena. We find that the temp e r a t u r e curves from any standard processas, for example, tests 6, 11, and 12 in Figure 1 -show an i n d i s p o s i tion to rise until after 200’ C . h a s been passed. If no influence other than simple heat conductivitv were oDerating after i00’ C. had / been r e a c h e d , t h e n these from and Figure 2-Cross ~~
\-I./
\ ~
Section of Oven Used for Low-Temperature Carbonization
February, 1929
INDUSTRIAL A N D ENGINEERING CHEMISTRY
a t 100"up to, say, 500" C. would be straight lines. Some influence is o p e r a t i v e which tends to depress these lines and keep them below 200" C., and in tests 6 and 11 below 300" C., for a considerable length of time. I n explaining t h i s p h e n o m e n o n we are greatly assisted by the following information which h a s been obtained in recent years from various sources. Below 300" C. the coal is still in the granular or non-pasty stage. Below t h e p a s t y stage there has been no decomposition of a sort to deliver hydrocarbon vapors. There is, however, a rearrangement or condensation of organic constituents carrying hydroxyl and carboxyl groups of such Figure 3-New Type Low-Tem erature a natureas to split off Carbonization Retort Capable o f Erpansion t o Facilitate Discharge c a r b o n dioxide and water. The resultant heat effect of these reactions over this range of temperature is endothermi? and is substantially the same as the absorption of heat that occurs below 100" C. in the vaporization of the free water content of the coal. This latent heat of what we will call "oxygen decomposition"-and I believe this is a proper designation for it-is affected in a decided way by the manner in which the heat is applied to the coal. I n test 12 the thermocouple is inserted in the center of the back door, or discharge end, of the oven. The application of heat here is slow and in the long-drawn-out time of 11 hours the endothermic reactions have spread themselves in a f a i r l y even m a n n e r from the seventh to the eleventh hour. From our own studies on other phases of the problem it is very evident that a long, slow application of heat has a marked influence uDon the attending reactions, and also upon the subsequent reactions a t a further a d v a n c e i n temperature. This lower curve, for example, a t the eleventh hour of heat application has exhausted the endothermic type of reaction, and from 200" C. a t 11 hours to 500" C. a t 12l/2 hours the rise is in a straight line, showing substantially only the accession of heat coming by way of conductivity from the heat of the oven. The curve for test 6 shows a more rapid application of heat, reaching 200" C. in 6 instead of 11hours, and 2 Hollings and D~~~~~n~~~pp,a,rBItgUnBltifoOnT
Temperature of Coals
107, llO6T (1915).
Cobb, J . Chem. Soc.,
165
300" C. in 8 hours, where the line flattens out for ll/z hours, or until the endothermic process is complete, when the normal accession of heat from the oven is alone operative for 1hour, or until a temperature of 400" C. is reached. We are not just now concerned with the extension of this line beyond 400" C. Test 11, as indicated by the upper curve, shows a still more positive addition of heat, reaching 300" C. in 7 hours instead of 8, with a much shorter period of endothermic reactions. This again has a marked effect on the subsequent behavior, for after 30 minutes we proceed very quickly into the zone where decomposition is exothermic and the products are hydrocarbons instead of the oxides of carbon. This type of reaction is not only a function of the time required for attaining these critical temperatures, but it is decidedly of more pronounced and positive character when it follows a quick accession of heat in the lower ranges, and is far less pronounced-in fact, almost annulled-where the accession of heat over the endothermic range is unduly prolonged. May I stress this point a little even a t the risk of repetition? I n our own experiments a very extended series of data emphasize the fact that those reactions which occur, say below 300-350" C., have a profound and, indeed, governing effect upon the reactions above 350" C.-that is, after the pasty stage has been reached-and furthermore, that the time factor is all important. The coal in test 12 after 12 hours has a
5 LAMP BANK
PYROMETER
Figure 5-Apparatus for Determination of Ignition Temperature of Coke
behavior beyond 350" C. which bears no resemblance whatever to the curve for test 11, which has reached the same temperature after only 7l/z hours. Indeed, the zone below 300" C. would be very properly designated as the conditioning stage for establishing a certain chemical status from which very profound and fundamental differences in the behavior occur above the critical temperature, which for this particular coal, from the Pennsylvania seam, is about 350" C. This time-temperature factor for bringing about certain very important chemical conditions as a basis for proceeding into the next zone is made evident by marking the points a t which the resultant temperatures in each of the three tests become exothermic-at 350" C. in test 11 a t 7 l / ~hours; a t 400" C. in test 6 after hours; and a t 450" C. in test 12 after 121/z hours. This basic reactional condition seems indeed to be a straight-line function of the time. Attention should also be called to the pronounced character of the curves immediately following these points. The curve following the shortest time-that is, 11-is sharp,
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I1VD USTRIAL A N D ENGINEERIXG CHEMISTRY
positive, and altogether different from either of the other two, where the conditioning period was more prolonged. One further fact should be noted from this chart. The ultimate temperature attained after 14 hours, when the carbonization process has been completed, differs in each test and is evidently a resultant of those temperature effects of an exothermic character which have been produced a t and immediately following the critical stage, as a t 350", 400", and 450" C. Products of Low-Temperature Zone
We will revert to this chart again, but for the moment we are concerned only with results below 450" or 500" C. (900" F.), which I think we may properly designate as the true low-temperature zone. Below this range we have the usual products of tar, gas, and coke, but each has its special characteristics as follows: Tms-The yield of tars is from two and one-half to three times as great as the yield from the high-temperature process using the same coal. This is because there is very little secondary decomposition following the primary decomposition of the original substance undergoing carbonization. It is possible, of course, that some of the waxes or fossil resins are volatilized with little or no decomposition, but for the most part the condensable material is the primary decomposition product resulting from the breaking down of the lignose type of organic substance which has descended by geological
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processes from the original plant material. In these tars the aliphatic series of hydrocarbons predominates, so that the product is more like petroleum oil than tar. This material lends itself to cracking processes under specific types of control for the production of motor spirit, lubricating, fuel, or Diesel oil, or hydrogen and carbon black. It would seem to have indifferent or undetermined values for application in the wood-preserving industry. GAS-The yield of gas is small owing to the small amount of secondary decomposition taking place in the tar. The range in volume is from 3000 to 5000 cubic feet per ton of coal carbonized. The gas is of high heating value and may average as high as 3l/2 or 4 million B. t. u. per ton as compared with 5l/2 or 6 million in the case of the gas from standard processes. SOLIDRESmuE-This is a char or semi-coke. It is high in volatile matter, averaging from 10 to 15 per cent. The limiting temperature of 500" C. is indicated for the most part because of the metal containers within which the process is to be carried out, such heat being maintained as shall not burn out the iron. On this account, therefore, the application of the usual methods for discharging the mass a t the end of the carbonization period are inoperative owing to the swelling and sticking of the charge. This may be illustrated by a cross section of one of the ovens emdoved in our own work in Figure 2.3 The charge, having bee; slbjected to the prescribed- heating, would not drop from the retort. The mounting of a screw pressure device, and later of a pneumatic plunger, was ineffective. Later the walls of the r e t o r t were made capable of expansion by lever action, as in Figure 3. A similar device is seen in the modified Figure 9-Variation i n Ignition Temperc o a l i t e r e t o r t of ature of Coke a s a Function of Coking Davidson in England. Temperature This general behavior of sticking in the retort has been met by numerous devices for agitating the charge while being heated, such as is found in the Carbo-coal process in this country and the fusion process in England. The solid material thus produced lends itself directly to combustion for power purposes, but must be briquetted if 3
Parr and O h , University of Illinois Eng. Expt. Sta., Bull. 60 (1912).
February, 1929
INDUSTRIAL A N D ENQINEERINQ CHEMISTRY
intended for domestic consumption. Its low ignition temperature suggests a storage hazard on account of the tendency toward spontaneous combustion unless this is obviated by briquetting or otherwise. Definition of Low-Temperature Carbonization Only such brief references have been made to the factors involTred in low-temperature carbonization as involve the
Figure IO-Prrheafer from the Discharge End
fundamental principles governing the condit;ol,s prescribed, As tims considered the definition of Gentry applies--namely:
167
Figure 4 shows ail apparatus for measuring the temperature of ignition of coal, and Figure 5 a similar apparatus for coke. Two thermometers are employed in eacli case, mercurial for coal and thermocouples for coke. One thermometer measures the temperature of the oven-that is! the space surrounding the coal sample; the other measures the temperature within tlie sample mass. A stream oi oxygen is enused to pass through the sample of coal in the apparatus shown in Figure 4. In the otber device two exactly parallel samples are used, one being a blank mithout. the stream of oxygen. Figures 6, 7, and 8 sliow the temperature within tlie coal to have a lag compared with the surrounding or oven temperature until a certain temperature effect is produced due to the coinhation of the oxygen with the coal. A sharp rise in temperature occurs, and the point where the eosl-temperature line crosses the oven-temperature line is ta,ken as the ignition point. A fresh coal sample irom Montgomery County. Ill., has an ignition t e m p r a t w e of 140' C. (Figure 6); Saline County, Ill., coal (Figure 7) has ail ignition temperature of 150" C.; Pocnliontas coal (Figure 8) ha,s about 215' C. Using the second type of apparatus (Figure 5 ) and with coke made from the snme coals, we obtain the results shown in Figure 9. The ignition point of coke carboniaed at 500" C. is only 148" C., substantially as low as the coal from which i t was made. Coke made at, 1OOO" C. (1900" F.) bas an ignition point somewhere abont or beyond 600' C., this tempirature the limit Of the appar'Ltus.
Low-temperature carbonization* * * is taken to mean thc destructive distillation of coal at or below the cracking temperature of the hydrocarbons in pzimary tar.
Midtemperature Coking
I will now discuss briefly the conditions and the resulting products which are inlierent above the line of cleavage and in the range between 500" and 750' C. (900" and 1400" F.). And first, why a limiting temperature of 750" C.? The most important reason will become evident upon considering the phenomenon 01 ignition and the temperature a t vvliich it occurs under various circumstances.
Figure 12--plrr
Process Coke from T e s W oil Vartovs Goats
But especially interesting is tlie fact that a t about 750" C. (1400" F.) there is an intersection of the lines representing the higher and the lower ranges of carhonization as measured in terms of their ignition temperatures. It might be appropriate to designate this as the "midtemperature coking zone'' as distinct from the true low-temperature zone, below 500" C. (900" F.). The question arisesis carbonization a t this temperature entitled to a specific recognition of this sort? My answer is decidedly in the affirmative and a few salient reasons are given herewith: (1) Coal carbonized at this temperature (750' C.) has less than 5 per cent of volatile matter, showing that the decomposition processes within the coal substance are substantially comnla," l,.c.".
Fiaure Ll-Parr
Procees Experimenfal Plant, Showin$ Coke Side of Oven
( 2 ) The low-temperature ignition point is believed to be due to the physical state of the carbon residue and not to hydrogen or methane evolved at the various points where ignition berins. I t should be recalled that the ignition noiiit for bvdroren is >00" C., and for methane, above- 600" C. Supar-carbon prepared in the cold by dehydrating with concentrated sulfuric acid shows this same eeneral characteristic of ienition at ternperatures in direct proportion to a given heat io which it has bcen subjected.
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Table I-Commercial COAL
No.
East Kentucky, Harlan Co.
691
Pennsylvania, Pittsburgh Seam
750
Alabama, Mary Lee Seam
975
Southern Illinois, Franklin Co.
1122
Central Illinois, Vermilion Co.
883
Indiana Fourth Vein, Vermilion Co.
634
Per ton of coal of 2000 Ibs.
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Yields, Midtemperature Coking Processo
FIXED
ASH MOISTUREVOLATILE CARBON Per cent Per cent Per cent Per cent 55.95 4.84 2.30 36.90 57.27 4.95 Dry 37.77 55.37 8.99 1.40 34.23 56.16 9.12 Dry 34.72 56.40 12.99 2.70 27.91 57.97 13.35 Dry 28.68 50.00 8.40 7.50 34.10 54.05 9.10 Dry 36.85 40.80 12.00 10.20 37.00 45.44 13.36 Dry 41.20 44.77 6.81 13.42 35.00 51.71 7.86 Dry 40.42
(3). The gas yield at this temperature is substantially all that IS available a t any temperature, measured in terms of heat units-that is, for coals of the Illinois type, 5 '/z t o 6 million B. t. u. per ton. At higher temperatures the volume of gas may be greater, but the heat value per cubic foot will be less. (4) The tars, not having been subjected t o such violent secondary decomposition, are larger in amount, more uniform in composition, and of greater value, owing to the higher percentages of the active principles, the creosote oils, required in wood preservation. Their specific gravity is greater than 1.1; hence they readily separate by gravity and are drawn off as dry tar-that is, with less than 3 per cent of water present. ( 5 ) By strict observance of the fundamental principles already demonstrated by the temperature logs of carbonization at different rates as t o time and reproducing the zonal reactions separately, the theoretical time for the actual carbonization process should be within 3l/2 t o 5 l / 2 hours. This can best be illustrated by referring again t o the timetemperature charts already shown. I n Figure 1 the active process of decomposition in test 11 obviously begins a t about 350' C. Assuming, now, t h a t the most desirable results are secured when a temperature of 750' or possibly 800' C. has been attained, we have 4 t o 41/2 hours for the actual time of carbonization.
Midtemperature Coking Experimental Plant These conditions have been strictly observed in the experimental plant operating as shown by the accompanying photographs and figures. This plant has been operating con-
COKE
Lbs. 1319 1350 1350 1371 1459 1500 1249 1350 1153 1284 1125 1300
TAR Gallons 17.0 17.4 16.0 16.2 9.73 10.00 13.0 14.0 14.2 15.8 10.4 12.0
GAS Feet 8050 8242 8700 8840 8136 8360 8215 8881 7833 8723 7305 8438
GAS PRODUCTION Av. per Total cu. f t . perton
B. 1.
u.
700 700 700 700 650 650 650 650 650 650 650 650
B. t .
Y.
5,636,000 5,769,000 6,095,000 6,180,000 5,288,250 5,435,000 5,340,000 5,773,000 5,091,660 5,670,000 4,748,910 5,486,000
tinuously 24 hours a day for 365 days. A preliminary or conditioning temperature below the softening point of the coal was regularly employed. The carbonization reactions were complete a t from 750" to 800" C. and within a time limit of from 4 to 5 hours. Secondary ,decomposition effects on the hydrocarbon vapors were definite, resulting in a gas'of slightly lower volume but of thermal value substantially equivalent to that produced by the standard high-temperature process. There was a corresponding increase in the condensable products, the tars proving to be remarkably uniform and of exceptionally high grade for creosoting purposes. Naphthalene formation was very slight. The coke was dense, firm, and of exceptionally high quality, especially conforming in combustion properties to the ignition temperatures shown in Figure 9. The oven employed was of standard type as manufactured by the Russell Engineering Company, of St. Louis, now the Improved Equipment-Russell Engineering Corporation, New York City. Experiments were conducted on carlots of coal from as far west as Iowa, as far south as Birmingham, Ala., and from the Pittsburgh and Cambria County seams in the East, with substantially all types from eastern and western Kentucky, Indiana, and Illinois. Without exception a highgrade coke was produced. Typical examples showing the amount and character of the yields are given in Table I.
Identification of Rayon' Wm. D. Grier* 150 WILLIAMST., NEW YO=, N. Y.
H E phenomenal growth of the so-called artificial silk, or rayon, industry in the last few years has developed a need for simple and accurate methods for the identification of the various types now on the market. Methods detailed in much of the recent literature seem to have been developed from work upon paper-making materials and, while of undoubted value in the identification of the various fibers used for that purpose, especially in the differentiation of chemical and ground wood, rag stock, lignified or unlignified fibers, and the like, give uncertain results in the study of rayon. A careful trial of many of the methods suggested indicates that tests of a purely chemical nature, dependent on color reactions with such reagents as Heraberg's reagent, solutions of iodine in potassium iodide, either with or followed by sulfuric acid, together with those depending on afinity for certain types of dyes, are, with one or two exceptions, inconclusive.
T
1 2
Received September 7, 1928. President, The New York Microscopical Society.
Comparatively few of the writers on the subject have laid much stress upon the use of the microscope, although Chamot, Behrens, and others have demonstrated that methods of combined microscopical and chemical analysis are of the highest value in the examination of technical materials. With this thought in mind the writer, after examination of authentic samples of practically all the leading types of rayon now on the market has arrived a t the conclusion that their characteristics, as exhibited under the microscope (subject in one or two cases to a simple chemical test of a confirmatory nature) are sufficiently constant to afford a reasonably certain means of identification. For. the sake of brevity it will be understood that whenever the appearance or structure of a rayon fiber is mentioned in this article, it means its appearance as exhibited under a moderate magnification of 200 diameters more or less, by transmitted light, mounted in a drop of distilled water, under a cover glass of medium thickness, and in either side view or transverse section.
