Coal Carbonization-The
Plastic Stage
JAMES H. LUM1 AND HARRY A. CURTIS,2 Yale University, New Haven, Conn.
apparatus and used it in measuring the initial temperature of softening of various coals, but did not attempt to interpret the pressure-temperature curves obtained. Recently Bunte and Lohr (9, 3) have reported a study of the plastic state by the Layng-Hathorne apparatus. Great importance was ascribed to the change of slope of the gas pressure-temperature curve. I n view of the conflicting opinions as to the correlation of the gas pressure-temperature curve obtained in the Foxwell procedure with the condition of the coal during carbonization, the present authors undertook, first, a study of the factors influencing the results, and subsequently, a correlation of the pressure-temperature data with data on plasticity obtained by another method. FIGURE 1. GENERALARRANGEMENT OF APPARATUS Apparatus Used A . Nitrogen cylinder E. Driers B. Oxygen removers F . Coal fusion furnace The general arrangement of apparatus used for C . Wash bottles J . Manometer D. Flowmeter studying the influence of various factors in the Foxwell method is shown in Figure 1. Figure 2 shows the details of the furnace, which was essenT IS WELL known that many bituminous coals soften tially the same as the Ball and Curtis apparatus, except when heated, presumably because of a fusion of some that the gas pressure was measured in the space below the components of the coal and a fluxing of the more infuscoal charge instead of in the steel tube bringing the nitrogen ible parts by the fused material. The degree of plasticity to the apparatus. reached varies greatly for different coals, in some cases there The nitrogen was drawn a t constant rate from a high-pressure being only a mild sintering together of the individual parcommercial cylinder of the gas. It was passed through an oxygenticles, while in other cases the fusion is so complete that the individual particles disappear in the plastic mass of coal undergoing carbonization. If the degree of fusion is sufficient to close the interstices between coal particles, the gas being generated by the decomposing coal causes the mass to swell and become spongy. As carbonization proceeds, the plasticity grows less and eventually a cellular coke is formed.
H I
Of the methods for studying the plastic stage in coal carbonization, the gas-flow test devised by Foxwell (5) has been widely used. Foxwell placed a small charge of crushed coal in a horizontal tube electrically heated. An inert gas was passed slowly through the tube as the temperature was raised, and the resistance of the coal charge to the gas flow was measured. As the coal softened, the gas pressure required to maintain constant gas flow increased to a maximum and then decreased to nearly its initial value. A curve with resistance to gas flow as ordinates and temperatures as abscissas was then plotted for each coal. Foxwell evidently believed that the temperature at the beginning of the resistance increase marked the beginning of the plastic condition and that the temperature when the resistance to gas flow had again fallen to its initial value marked the end of the plastic condition of the coal. He regarded the magnitude of the maximum &stance and the area under the resistance-temperature curve as criteria of the degree of plasticity of the coal. It will be shown in the present paper that some of these conclusions are in error. Layng and his associates (6, 7, 8) used a modified Foxwell apparatus. They believed that the plastic temperature range lay between the points of initial pressure increase and maximum pressure, and they attached importance to the slope of the gas pressure-temperature curve between these points. These conclusions are also in error. Lloyd (9) maintained approximately constant gas pressure in his apparatus and measured the decrease in the rate of gas flow as the coal softened. He believed that he waB able to correlate the rate of flow-temperature curve with the coking characteristics of the coal. Ball and Curtis ( 1 ) greatly improved the Foxwell
-1 2
J’
FIGURE 2. COALFUSION FURNACE G. Electric furnace H. Iron container for lead bath
Present address, Thomas & Hochwalt Laboratoriea, Inc., Dayton, Ohio. Preaent address, Tennesste Valley Authority, Knoxville, Tenn.
I. J.
327
Lead Manometer
K. Coal charge L. Copper wool support for coal charge M . Thermocouple
VOL. 7, NO. 5
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absorption apparatus (11) using reduced ammoniacal cuprous chloride, then through sulfuric acid wash bottles, through an orifice flowmeter and a bubbling tube, and finally through a drying tube containing calcium chloride. The purified, dry nitrogen was then passed through the apparatus shown in Figure 2, and the gas ressure-temperature curve determined-i. e., gas pressure reagngs on the manometer, J, versus tem erature of the coal charge, as measured by the thermocouple,
d
Screen size of coal: all pass 10-mesh, not more than 25 per cent 80-mesh Rate of heating: rapidly to within 10' to 20' of the softening temperature, thereafter at 2' C. per minute I n the study of the effects of the factors in the gas-flow test, a single coal of medium volatile matter content and normal plastic characteristics was used in all determinations.
Variables in the Foxwell Procedure With a given apparatus and a given coal, the variables which most influence the results obtained are presumably the following: rate of gas flow through the coal charge, packing of the crushed coal in the fusion tube, length of the coal charge in the fusion tube, screen analysis of the coal used, and rate of heating of the coal as softening point is approached and during the plastic stage. It will be noted on referring to the previous articles on the Foxwell method that the values selected for the above variables differed widely. Rates of gas flow were in the range 10 cc. per minute per square centimeter cross section of fusion tube, up to 177.3 cc., and in some cases were not held constant during the run; little attention was paid to the packing of the coal charge; the length of coal charge was in the range 2.5 to 10 cm.; the screen analyses were different and in some cases all the coal sample was used, whereas in other cases the finer screen sizes were rejected; and the rates of heating used by various investigators were in the range 1' to 5" C. per minute. If comparisons are to be made of the results obtained by different investigators, it is necessary to know the extent to which changes in the values of the variables altered the results. It will also be desirable to standardize the apparatus
TEST NO. 13 alOO
0 I
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TEST NO. 13-A TEST NO. 13-8
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I I
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40
20
370
410
450
TEMPERATURE
- "C.
490
FIGURE3. TEMPERATURE-GAS PRESSURE DATA FROM TRIPLICATE RUNS
itself if it is to be continued in use, for it appears fairly obvious from a study of the arrangements of parts adopted by the several investigators that the temperatures recorded were not in all cases those of the coal charge. In the present work, the apparatus adopted was one which Ball and Curtis had shown to be reasonably satisfactory. It was necessary to adopt a set of values for the factors listed above, and to keep all these the same from run to run except only the factor purposely changed t o study its influence on the gas pressure-temperature curve. After a considerable amount of preliminary work in adjusting the various factors to give smooth operation of the apparatus, the following were adopted as tentative standard values: Rate of gas flow: 6 cc. per minute per sq. cm. cross section of fusion tube Packing: as described below Length of charge: 2.5 cm. (4.3 grams of coal)
Effect of Rate of Gas Flow With conditions "standard" as specified above, and using the same medium volatile coal in successive tests, the rate of gas flow through the coal charge was varied from 2.9 to 12 cc. per minute per square centimeter cross section of tube, with results shown in Table I. TABLEI I. EFFECTOF VARYING RATEOF GASFLOW Test No.
Rate of Gas Flow Cc./min./ sq. em.
Temp. of Temp. at Initial Max. PresFusion sure C.
O
C.
Max. Pressure
11-c
2.9 2.9 2.9 2.9
405 400 405 405
445 445 445 445
19.0 67.6 67.7 73.0
12
4.4
405
440
77.0
10
6.0
400
440
83.0
14
9.3
400
430
72.0
12.0 12.0 12.0
400 395 400
440 430 435
58.8 117.0 114.0
11 11-A
11-B
13 134 13-B
Remarks
Cm. Ha0
Note large vari? tions in max!mum pressure in duplioate runs
Same remark as above
It will be observed in Table I that neither the temperature of initial softening (first increase in pressure) nor the temperature of maximum resistance to gas flow is changed notably by a fourfold increase in rate of gas flow. A rate of 6 cc. per minute per square centimeter cross section was retained as a standard rate for further work. From the known facts regarding coal carbonization it is probable that a very high rate of gas flow would change the observed temperatures. The maximum gas pressure reached varied widely, even for the same rate of gas flow in successive experiments. These conclusions are similar to observations of Bunte and Lohr (8, 3 ) . As an indication of the type of gas pressuretemperature curves obtained, the data for triplicate tests 13, 13-A, and 13-B are shown in Figure 3. It will be noted that the temperature a t which softening of the coal begins is fairly reproducible, but there is considerable variation in the maximum gas pressure reached and in area under the curve. Additional evidence as to the lack of reproducibility in the values of the maximum gas pressures is found in the quadruplicate tests, 11, 11-A, 11-B, and 11-C in Table I. Effect of Packing the Coal Charge It would appear desirable in a procedure of the kind under study to have the coal charge uniformly packed from test to test. The resistance offered to gas flow a t a constant rate through the coal charge was used as a criterion in determining which of several methods gave the most reproducible packing. It was found that the following relatively simple procedure gave a closely reproducible packing of the charge and was adopted as standard: The stock sample was thoroughly mixed and then approximately 1-gram portions of coal were added to the fusion tube, the tube being gently tapped meanwhile. After each 1-gram addition the surface of the coal in the tube was leveled to prevent sizing of later additions through rolling of larger particles t o the wall of the tube. 0
SEPTEMBER 15, 1935
ANALYTICAL EDITION
T E S T NO. 2 3
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TEST NO. 2 3 - A
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T E 5 r NO. 23-8
1 1
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329
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FIGURE 4. TEMPERATURE-GAS PRESSURE DATA From runs 10,22! ZZ-A, and 22-B, Bhowinz effect of eliminating fines from coal charge
410
450
TEMPERATURE - O C .
490
FIGURE5. TEMPERATURE-GAS PRESSURE DATA From rune 23,23-A, and 23-B, showing results with fine coal present
In order to determine whether the density of packing had any effect on the pressure-temperature curves obtained, three runs were made with charges in which the density of packing had been increased by prolonged tapping of the tube. The temperatures of initial fusion and the temperature at maximum pressure were not changed; the maximum values reached by the gas pressure were somewhat increased, but these pressures are variable even under the most constant conditions which could be set up.
Effect of Length of Coal Charge :Duplicate tests were made with charges of lengths 2.5 cm. (4.3 grams), 3.8 cm. (6.5 grams), and 5.0 cm. (8.6 grams). In all cases the temperatures of initial fusion checked those of Table I within experimental error, Likewise the temperature a t maximum gas pressure was the same for the 2.5-cm. and the 3.8-cm. charges. The 5.0-cm. charge swelled so much as to overflow the fusion tube and lift the thermocouple, so that no significant reading of temperature was had. In view of the above results a standard charge of 2.5 cm. (4.3 grams) was retained as specified in the table of tentative standard values.
Screen Analysis of Coal Charge It was early found that the results of the Foxwell procedure were influenced considerably by the screen analysis of the coal charge. Some of the investigators cited above rejected the finer sizes. For many coals this rejection of the particles below, say, 80-mesh may not change the results, but the fact that most bituminous coals are made up of layers of unequal hardness, and actually show more or less concentration of the component parts in the various screen sizes, makes it appear obvious that all the screen sizes in a given sample should be used. The standard sample of coal used in the present study of the variables of the Foxwell procedure was a coal (No. 56) from the Lower Kittanning seam, of the following proximate analysis: volatile combustible matter 22.3 per cent; fixed carbon 73.0 per cent; ash 4.7 per cent. It was ground to the following screen analysis : Cumulative Per Cent through Tyler Standard Screen Sizes 28 48 80 100 Screen 10 20 Percent through 100 77.2 61.7 36.2 22.9 17.2
Test 10 (Figure 4) was made with this standard sample. I n tests 22, 22-A, and 22-B, also shown on Figure 4, all the coal passing a 50-mesh screen (30.6 per cent) was rejected in making up the coal charge. The comparison of these three with test 10 gives the effect of eliminating the fines-i. e., a slight increase in the temperatures of initial softening. The small variation in the magnitude of maximum pressure in the tripli-
370
410
TEMPERATd:E -"C.
490 P
FIGURE6. TEMPERATURE-GAS PRESSURE DATA From rum 26 and 26-A, with coal ground to pass 80-mesh screen
cate tests 22, 22-A, and 22-B, as compared with the wide variations shown in Figure 3, should be noted and explains, possibly, why previous investigators who eliminated fines were able to obtain reasonably reproducible results in respect to maximum pressures reached. The standard coal sample was next reground to the following screen sizes: Cumulative Per Cent through Tyler Standard Screen Sizes Screen 10 20 28 48 80 100 Per cent through 100 99.2 97.9 74.3 46.0 36.0
With this finer ground sample tests 23, 23-A, and 23-B (Figure 5) were made. The temperature of initial softening was increased somewhat for two of the three tests. The gas pressure-temperature curve in all three tests passed through a first maximum in the temperature range 435' to 445" C., but rose again in all cases and passed through a second maximum in the temperature range 465" to 480" C. The amount of finely ground coal present evidently influenced the results considerably. The standard sample was next ground so that all would pass 80-mesh and tests 26 and 26-A (Figure 6) were made with this fine material. The gas pressure-temperature curves in these tests were different from any of the previous ones. At about 390" C. in both cases the pressure began to decrease until in both cases a temperature of about 450' to 460' C. was reached. The curves then rose sharply to maxima in the temperature range 465' to 475' C. The large influence of the fine material was again apparent. Realizing that the large influence of screen analysis of the coal charge would be a serious deterrent toward standardizing the Foxwell procedure, an attempt was made to avoid the difficulty. The coal sample was first ground to pass 150-mesh,
INDUSTRIAL AND ENGINEERING CHEMISTRY
330
then moistened with 10 per cent of water and briquetted without binder, the briquet being then crushed and ground, and the ground material separated into various screen sizes. This procedure presumably gave a product of the same composition in all ,the screen sizes, so that, by reblending the fractions in suitable proportions, any desired standard screen analysis could be obtained. In test 24, the sample was made up in this manner with a screen analysis approximately the same as that of the standard sample used in test 10. I n test 25,
5
5 &
u 0 5
TEST NO. 24 * TEST NO. 25
0
1
I
FIGURE 7. TEMPERATURE-GAS PRESSURE DATA From runs 24 and 26, showing effect of fine grinding, briquetting, and recrushing coal
.
a blend was made of the -20 to +28-mesh fractions of the ground, briquetted, and reground coal. The temperaturepressure data obtained with these two samples are shown in Figure 7 and are entirely different from those of the standard sample (test 10). The fusing characteristic of the coal had been almost destroyed. One thinks immediately of oxidation, since it is well known that oxidation does decrease the degree of fusibility of coal. The coal used, however, was a high-rank coal in which oxidation proceeds slowly. Moreover, no vidation effect was noted in the other finely ground samples. From the shape of the plastic curve it would appear that raising the temperature of the coarse particles caused them to disintegrate and, consequently, the charge finally consisted of very fine particles when the plastic range was reached. The two experiments showed that the difficulty of duplicating screen analyses in the Foxwell method could not be avoided by grinding, briquetting, and regrinding, logical as this procedure may seem. The large influence of screen analysis of the coal charge raised what appears to be a serious difficulty. If, however, a given coal be ground to pass 10-mesh and the amount passing SO-mesh be kept below 25 per cent by suitable grinding, the Foxwell procedure gives fairly consistent values for the temperatures of initial softening and the temperature of maximum gas pressure.
VOL. 7, NO. 5
Standardization of Foxwell Procedure The temperature which can be most certainly determined in the Foxwell procedure is that a t which the coal softens sufficiently to begin to close the gas passages through the charge of crushed coal. With an apparatus such as shown in Figures 1 and 2, using purified nitrogen, and the standard conditions outlined above, this temperature may be determined within about 5" C. in successive runs. Having adopted some suitable set of standard conditions, such as given above, it will be found that the observed temperature of initial softening of the coal is not appreciably affected by small changes in rate of gas flow, in length of coal charge, in rate of heating, or in method of placing the coal charge in the vertical fusion tube. The rather large effect of the screen sizes of the coal charge raises the main difficulty, but if the coal be ground to pass 10-mesh and the fines (below 80-mesh) be kept less than 25 per cent, duplicate runs on the same coal will give about the same observed initial softening temperature. The temperature at which the gas pressure ahead of the coal charge reaches a maximum cannot be determined as certainly as can the temperature of initial softening of the coal, but a consideration of all the data on tests mentioned above indicates that this temperature is probably also a characteridtic of the coal, although not so well defined, and of less certain significance in the :carbonization process.
Effect of Rate of Heating From the known behavior of coal during carbonization, one would anticipate that either a very slow or a very rapid rate of heating would tend to increase the observed temperature of initial softening. At a very slow rate fused material accumulates more slowly, whereas a t a high rate of heating the relatively slow coal decomposition processes, which give rise to fused material, cause a lag between the time the coal reaches the necessary temperature for softening and the time when the gas passages through the coal charge close sufficiently t o show increased pressure in the apparatus. Foxwell, and later Davidson, found rate of heating, within the range they used, to be without appreciable effect on the temperature of initial softening. Ball and Curtis observed, however, a small increase in softening temperature when the rate of heating was increased from 2" t o 5" C. and to 8" C., while a t 0.5" C. per minute the coal became nonfusing. It was observed in the present work that between rates of 1" and 4" C. per minute there is only a small tendency to obtain higher temperatures of initial softening and of maximum pressure with the higher rates of heating. A rate of 2" C. per minute was retained as a standard.
FIGURE8. EXTRUSIVITY APPARATUS U.
Piston Compressed air
Not drawn to scale L. Recorder M . Extrusion apparatus
I. J . Pressure-reducing valve K. Vent valve
T. Lever system X. Cylinder
The maximum gas pressure varies in successive runs, in spite of attempts to keep all conditions the same. It is obvious that the slope of the gas pressure-temperature curve, to which Layng and his associates attempted to give significance, will depend on a number of factors, such as (1) rate of gas delivered to the apparatus compared with volume of the gas purification train, tubing, etc., ahead of the coal charge, (2) rate of closing of the gas passages in the coal charge (3) rate of heating of that part of the gas which is passing through the heating coil in the lead bath, etc. Even with conditions as nearly the same as may be in duplicate runs, the
SEPTEMBER 15, 1935
ANALYTICAL EDITION
331
t
magnitude of the pressure developed seems to be determined by a fortuitous combination of events in the coal. The slope of the gas pressure-temperature curve has evidently no such significance as given in some of the papers of Layng and his associates, nor is the area under the pressure-temperature curve or the maximum pressure developed a characteristic of the coal. During the present investigation, evidence was secured showing that the height of the gas pressure-temperature curve as a measure of the degree of plasticity reached by a coal during carbonization was not in agreement with the degree of plasticity as indicated by the amount of swelling observed when small briquets of the coal were carbonized.
Condition of Coal While Taking PressureTemperature Data The initial increase in gas pressure observed in the pressuretemperature curve is certainly had when the coal has softened so that it begins to flow under its own weight and thus closes the gas passages through the charge. It has been shown by Ball and Curtis (I) that coal is somewhat plastic a t temperatures considerably below the so-called “softening point” as determined by the Foxwell method. It appears, however, that the increase of plasticity with temperature increase is high in the neighborhood of this softening point and that the temperature of the softening point so determined is truly a characteristic of the coal. The condition of the coal during the balance of the period during which the gas pressure-temperature data are taken in the Foxwell method is by no means so evident. The divergent views on this point have already been noted. Inasmuch
as Layng assumed that the maximum in the gas pressuretemperature curve obtained in the Foxwell procedure marked the temperature a t which the coal ceased to be plastic, this point was subjected t o direct observation in an apparatus described below. It was easily shown that the coal charge was plastic a t temperatures above that of maximum gas pressure observed in the gas-flow procedure, and that Layng’s conclusion was therefore in error.
Extrusivity of Coals i n Plastic Condition
In order to secure further information as to the condition of a “fusing” coal in the temperature range above the softening point, and also in the hope that a test procedure might be developed which would give additional data serving to characterize coals, attention was turned to measurement of what may be termed the “ex t r u s i v i t Y” of L 0. ? 1 coals in the plastic condition. Briefly, this term is used to express the rate a t which coal in the plastic s t a t e m a y b e extruded through an orifice or short tube under fairly closely c o n t r o l l e d conditions of temperature and pressure in a given apparatus. It was recognized, ‘ 4 I FIGURE 10. ORIFICE FITTING of course. ..~ that the N . Plug term “extrusivity,, F . Orifice proper M . Thermocouple R . Spring support well for plug a s here applied could not begiven any such respectable scientific standing as adheres to the term “viscosity” (quite aside from the violence done etymology in coining the term “extrusivity”). Coal in the so-called plastic condition is not strictly a homogeneous body; it is a t the time undergoing chemical changes and concomitant physical changes; it is liberating gases and vapors. Any attempt to measure a rheologic property of such a material must appear bizarre. Whether or not the conditions could be controlled sufficiently and the inevitable causes of inaccuracy reduced to a point where measurements characteristic of the coal could be made was the question to be answered. Porter (IO)has published some data bearing on this problem. ~
~
4
T
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Apparatus for Measuring Extrusivity
FIGURE 9. EXTRUSION CYLINDER D. Plunger
Electric furnace 0. Steel cylinder E.
P. Orifice fitting
8. Supporting frame for cylinder 0
The apparatus is shown diagrammatically (not to scale) in Figure 8. Gas from high-pressure cylinder I was passed through pressurereducing valve J, through vent valve K , and used in cylinder X to operate a piston, G, whose position was recorded continuously through the lever system, T, on the recorder, L. The pressure in the gas line was also recorded continuously on L. The motion of piston G was transmitted t o the plunger in the extrusion apparatus, M , shown in detail in Figure 9. In this apparatus steel cylinder 0 received plunger D. The lower end of the plunger rested on the coal charge. The plunger was cut away a short distance from the lower end t o provide a space in which any small amount of plastic coal which pushed past the head of the plunger would be caught. The steel cylinder, 0, was surrounded by an electrically heated furnace, E, and was supported independent of the furnace by the frame, 8. The lower end of the steel cylinder carried a steel orifice, P, shown in cross section in Figure 10. The orifice proper is shown at F in Figure 10 and had a thickness of 0.318 em. (0.125 inch). The following sizes of orifices were used:
INDUSTRIAL AND ENGINEERING CHEMISTRY
332 Orifice No.
-Diameter-Inch 0.313 0.156 0.0938
Cm.
3 4 5
0.795 0.397 0.238
The orifice body was drilled to receive a thermocouple at M . The plug, N , served to hold the coal in place while it was being heated preliminary to extrusion, during which period a thermocouple was kept in the coal charge through the hole drilled in the plug. The plug was held in place by the spring support, R. During extrusion a beam of light was thrown on the orifice and the extrusion process observed in a suitably placed mirror. The pressure recorder was calibrated and from time to time during the experiments the pressure necessary to overcome the friction between the moving piston and the packing in cylinder X was measured. This value and the weight of the piston were used to obtain the actual pressure applied t o the coal charge from the pressure indicated on the recorder. Five grams of coal were taken for each test and placed in the apparatus as shown in Figure 9. The temperature of the coal was then raised nearly to that desired, the plunger inserted, plug N removed, and the fusion tube and furnace were moved quickly into place as shown in Figure 8. Pressure in air cylinder X had meanwhile been brought to the desired value, movement of the piston being prevented mechanically until the extrusion apparatus was in place. As soon as pressure had been applied to the coal, the orifice was observed. When extrusion began, electrically driven recorder L was started, giving a continuous record of time, gas pressure in cylinder X, and movement of piston G . In all work with such a material as coal, there is difficulty in determining the temperature of a sample a t any moment, and for this reason much of the published research on coal does not correctly correlate the actual temperature of the coal with other recorded observations. In the present work it was not possible to measure accurately the temperature of the coal a t the moment of extrusion, but a fairly close approximation was made in the following manner: With plug N in place, a thermocouple was passed through the hole in the plug into the coal charge for a distance of 0.635 cm. (0.25 inch). Another couple was inserted in hole M in the orifice head. TABLE11. SOURCES AND PROXIMATE ANALYSES OF COALS USED Sample No.
Seam Deepwater Black Creek
1
Volatile Fixed Matter Carbon
Location Nauvoo Mine, Nauvoo. Ala.
Ash
%
%
%
39.1
56.9
4.0
VOL. 7, NO. 5
The temperature of the orifice head was observed during extrusion. Preliminary measurements indicated that, a t the beginning of the extrusion period, there was, on the average, a temperature difference of 8" between the steel orifice head and the coal 0.635 cm. (0.25 inch) above the orifice. Furthermore, there was a decreasing gradient of about 8" per cm. in the coal upward from the orifice. During extrusion this steady condition was disturbed somewhat, but the rise of temperature of the apparatus during extrusion tended to offset the temperature gradient in the coal column. The temperature of the coal passing through the orifice was therefore known only approximately, but was probably within a few degrees of that of the coal 0.635 cm. (0.25 inch) above the orifice a t the beginning of the extrusion period. The description,. proximate analysis, and size composition of coals used in this investigation are given in Tables I1 and TTT
111.
Relation of Extrusivity to Temperature Preliminary experiments revealed that extrusivity changes rapidly with temperature and that during an extrusion test the coal temperature must be confined to a relatively short interval. In Table IV are presented extrusion data on five coals. In each series of tests the temperature of the coal a t the start of extrusion was increased by intervals of 10' until the entire temperature range of plasticity of the coal had been covered. A standard pressure of 2.43 kg. per sq. em. was applied to the piston except in the case of coal 52. The degree of plasticity developed by this low-volatile bituminous coal was so small that extrusion could only be secured by application of a pressure of 7.24 kg. per sq. em. and by very rapid preliminary heating of the charge. The temperatures listed in Table IV show the temperature range of plasticity for each coal; a t temperatures lower than those listed the coal had not softened sufficiently to permit extrusion and a t higher temperatures the formation of hard coke developed. TABLEIV. RELATION OF EXTRUSIVITY AND TEMPERATURE Coal
Orifice Number
Initial Coal Temperature O
c.
Extrusivity G./Min.
52
B or Lower
Mine No. 37, Windber, Pa.
17.5
75.2
7.3
54
No. 2 Gas
Bridge Fork Mine, Anstead, W. Va.
36.1
61.0
2.9
55
Pittsburgh
Banning No. 1 Mine, Van Meter, Pa.
34.8
59.4
5.8
55
5
410 420 430
0.85 1.17 0.75O
56
Lower Kittanning
Colver, Pa.
22.3
73.0
4.7
56
4
57
Imboden
Stonega, Va.
34.0
60.4
5.6
430 440 450
0.38 0.59 0.37=
57
4
410 420 430
0.62 1.20 1.05
52
3
500
1.15a
Kittanning
ANALYSESOF COALS USED TABLE111. SCREEN Coal No,
On 20 Mesh
1 52 54 55 56 57
40.8 33.4 36.8 36.5 22.8 50.3
%
On28 Mesh % 15.2 13.3 14.1 14.7 15.5 13.5
On48 Mesh % 18.6 18.7 19.8 19.7 25.5 16.2
On80 Mesh % 1;:; 10.0 10.4 13.3 7.3
On 100 Through Mesh
100 Mesh
%
% 12.7 19.5 14.9 14.9 17.0 9.1
!:;
4.3 3.6 5.7 3.0
I n running tests, the temperature of the apparatus was brought up to 300" C. a t a rate of about 10" per minute and the rate of increase, as read in the orifice head, was then adjusted to 2" C. per minute. The temperature in the coal a t 0.635 cm. (0.25 inch) above the orifice was taken just before extrusion pressure was applied, and the couple then removed.
a Formation of hard coke prevented complete extrusion of the charge.
It is apparent that these coals attained sufficient plasticity to permit extrusion in a range of temperature that did not exceed 40 Furthermore, the extrusivity varied rapidly with temperature and attained a maximum value for each coal. Attempts were made to measure extrusivities a t 1-minute intervals as the temperature of the tube was raised in a single test through the entire temperature range of plasticity. The impossibility of correctly recording the temperature of the coal as it passed through the orifice, however, prevented the successful completion of this project.
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ANALYTICAL EDITION
SEPTEMBER 15, 1935
333
Correlation of Extrusivities and Other Data
Acknowledgments
Before comparing extrusivities, it was necessary to secure a factor that would place extrusivities determined by the No. 4 and No.5 orifices on a comparable basis. The extrusivity of coal 57 was measured at the temperature of maximum plasticity, 420' C., with both the No. 4 and No. 5 sizes of orifice. From these tests it was found that extrusivities measured by the No. 4 orifice should be reduced by the factor 0.4 to make them comparable to extrusivities measured by the No. 5 orifice. No attempt was made to correct the one value with the No. 3 orifice because of its large diameter and the high pressure that was used with it. In Table V the coals have been arranged in order of decreasing percentage of volatile matter. The volatile matter gives only a rough indication of the degree of extrusivity. Coal 1 with the highest percentage of volatile matter is almost nonfusing and coal 57 develops much less extrusivity than coal 54, although coal 57 has a higher volatile content. There is an approximate correlation between extrusivity and the temperature of initial softening and the temperatures maximum pressure as measured by the gas-flow test; extrusivity decreases as these temperatures rise.
The authors wish to express their appreciation of a fellowship to the junior author granted by the National Fuels Corporation, New York, N. Y. Thanks are due G. A. Berry, vice president, Calco Chemical Company, Bound Brook, N. J., for helpful aid in the design and construction of the extrusivity apparatus.
Literature Cited (1) Ball. A. M.. and Curtis. H. A.. IND. ENG.CHEM..22.137 (1930). (2) Bunte, K., and Lohr, H.,Gas'u. Wusserfuch,77; 242 (1934). (3)Ibid., 77,261 (1934). (4) Davidson, W., Fuel, 9, 489 (1930). (5) Foxwell, G.E., Ibid., 3, 122 (1924).
(6) Layng, T. E., and Coffman, A. W., IND.ENG. CHEM., 19, 924 (1927).
(7) Ibid., 20, 165 (1928). (8) Layng, T. E., and Hathorne, W. S., Ibid., 17,165 (1925). (9) Lloyd, T. C., Chem. & Met. Eng., 37, 169 (1930). (10) Porter, H.C., Proc. Intern. Conf. Bituminous Coal (1931). (11) Van Brunt, C., J. Am. Chem. SOC.,36, 1448 (1914). RECEIVED May 31, 1935. The experimental data used in the present paper are taken from a dissertation submitted by J. H. Lum to the Graduate School of Yale University in 1932,in partial fulfillment of the P6.D. requirements.
Plastic Condition at Maximum Pressure
It will be noted in Table V that the temperatures of maximum pressure in the gas-flow test are in most cases less than the temperatures a t which maximum extrusion occurs. This difference is important in view of the fact that the point of maximum pressure has been stated by some investigators to be the:end of plasticity and the beginning of coke formation.
Estimation of Chloramine in Water Supplies
TABLTO V. CORRELATION OF EXTRUSIVITIES AND OTHER DATA Coal No. 1 54 57 55 56 52
--Extrusion Tests--Gas-Flow TestInitial temp. Volatile Initial Maximum for max. Average matter fusion pressure extrusion extrusion % O C . oc. 0. G./min. 39,l 400 415-35 Not su5ciently plaatic 430 1.59 415 36.1 385 0.48 425 420 36.1 395 1.17 415 420 34.8 375 440 0.24 440 22.3 400 500 1.15" 490 17.5 440
a Uncorrected value obtained from the No. 3 orifice and by application of high pressure.
The difficulty of obtaining true coal temperatures in the extrusion test has been pointed out. To eliminate the possibility that the methods of measuring coal temperatures in the gas-flow and extrusion tests were not comparable, the extrusion apparatus was changed to permit carrying out both tests in the same apparatus in a single experiment. A length of brass tubing was wrapped around the outside of the fusion tube, 0 (Figure 9) to serve as a preheating coil for nitrogen. The lower end of the preheating coil was fitted tightly into the orifice below the coal charge and the usual procedure for the gas flow test was then followed. When the gas pressure had reached a maximum value and had begun to fall, the nitrogen tube was pulled away from the orifice, the pistoninserted, and extrusion allowed to take place. By these combined gas-flow and extrusion tests, in which the possibility of error from the method of measuring coal temperature had been eliminated, the following data were secured and clearly show that coals are in a plastic condition at temperatures higher than that of the maximum gas pressure in the gas-flow test: Coal Temperature of maximum gaa pressure, Temperature of start of extrusion, C. Time of extrusion minutes Amount of extrusion, grams Extrusivity, grams per minute
C.
55 415 425 4.5 3.9 0.87
54 427 427 5.0 4.3 0.86
PAUL D. McNAMEE United States Public Health Service, Cincinnati, Ohio
T
HE widespread use of chloramine instead of chlorine for
the disinfection of water supplies renders a distinctive test for chloramine desirable. At present, the chloramine content is measured by the o-tolidine test for residual chlorine. The limitations of this test are well known and interference by nitrite ions is especially notable. AS pointed out by Hulbert (S), nitrite is usually formed in chloramine-treated water. The hydrogen-ion concentration determines the type of chloramine present. According to Chapin (a), a t pH 8.5 or above only monochloramine is formed and below pH 4.4 only nitrogen trichloride is produced. Between pH 4.4 and 8.5, mono- and dichloramine coexist in a ratio fixed by the pH of the solution. When sufficiently acidified, a solution of monochloramine is converted t o nitrogen trichloride according to the equation 3Ji"zCl
+ 2H+ + 2NH4+ + NCli
Marckwald and Wille place in two steps: 2NHzC1 NHzCl
(4) infer that the above reaction takes
+ 4HC1 +2NHaC1 + 2Cla + 2Cla + NCls + 2HC1
(1)
(2)
and that the reaction is catalytically accelerated by liberated acid. In dilute solutions of chloramine, this reaction is very rapid, less than 1 minute being required to convert monochloramine to nitrogen trichloride a t 15' C. As indicated by the above equations, the chloramine content of a solution may be measured by determining the amount of ammonium ion formed on acidification. When the pH of a monochloramine solution is lowered to 4.4 or
