T
H E coking power of bituminous coal can b e d e s t r o y e d completely (f7)by oxidation a t elevated temperatures or by weathering under ordinary storage c o n d i t i o n s for extended periods. The changes on storage are i m p o r t a n t because under the operating c o n d i t i o n s of many coke plants it is necessary to store large quantities of coal for long times. It would be advantageous to have a better understanding of what is occurring during this storage period. Studies of the oxidation of coking coals would also be of value to those interested in the pretreatment of coal before carbonization where preliminary oxidation of the coal is practiced to modify the c a r b o n i z i n g properties. The purpose of the work described in this paper was to d e v e l o p a m e t h o d for evaluating the changes in properties of coking coals due to oxidation and to aaalv the
the coal is g r e a t e r t h a n t h e weight of carbon and hydrogen appearing as gaseous products. The rate of oxidation of coal a t o r d i n a r y temperatures increases rapidly with increasing temperature; the effect amounts in some c a s e s t o d o u b l i n g
Oxidation of Coal
at Storage Temperatures Effect on Carbonizing Properties
L. D. SCHMIDT, J. L. ELDER, A m J. D. DAVIS U. S. Bureau of Mines Experiment Station, Pittsburgh, Pa.
makes it difficult to use the experience gained in storage of coal t o d e t e r m i n e t h e effect of oxidation on carbonization properties. Since the character of the products of oxidation vary with the temperature, it is probable that the total effect of a given amount of oxygen consumed, on the properties of the coal may also vary with the temperature of oxidation. Consequently, even in the oxidation of coal on a laboratory scale, great care must be taken to avoid local overheating. EFFECTOF OXIDATIONON C O K I N G PROPERTIES. At present little is known about the effect of oxidation on the coking p r o p e r t i e s of coal. Some idea m a y b e g a i n e d from the fact that preliminary oxidation is practiced in low-temperature carbonization (12) where the stickiness of the charge is t o be reduced so that it can be carbonized in a rotary retort. Also for m e d ium-temperature car-
properties of the coal, there is considerable doubt as to its reliability as a practical measure of coking power (f0, 15), when we define strong coking power as the ability of a coal to produce a good commercial coke.
NOVEMBER, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
scale large enough to permit 2-kg. samples to be taken periodically for actual carbonization tests. Some authors (9,11) have indicated that moist coal oxidizes much more rapidly than dry coal. I n this work it was decided to t r y to simulate storage conditions more closely by oxidizing moist coal, but to keep the relative humidity of the oxygen in contact with the coal constant.
Oxidation Apparatus and Procedure Figure 1 shows the oxidatJionapparatus in plan and elevation: Essentially, it consists of a drum 30 cm. (1 foot) in diameter and 122 cm. (4 feet) long, half-filled with crushed coal and rotating at 20 revolutions per hour while oxygen is recirculated through it. The drum and the entire recirculating system are immersed in a water bath maintained at 30" * 0.01' C. by means of a (1-kw.)knife heater controlled by a toluene bulb thermostat and a relay. The water bath is well insulated on all sides, top, and bottom to reduce the power required to maintain this t,emperature and to make it possible t o maintain higher temperatures if required later. The entire mechanism is driven by a 0.25-horsepower electric motor (not shown) connected by a series of pulleys to a main shaft which passes through a packing gland in the wall of the water bath. Positive circulation of water in the bath is obtained by means of a four-bladed propeller on the main shaft and the baffle arrangement shown. Oxygen recirculation is accomplished by means of a sliding-vane rotary pump directconnected to the main shaft. The drum is rotated by means of a chain-and-sprocket connection to the main shaft. The carbon dioxide generat,ed during oxidation of the coal is removed from the recirculating oxygen by passing the gases through an absorption bottle containing powdered calcium hydroxide. The water formed by the reaction of oxygen with the coal is condensed out of the recirculated gases in the dew-point bat,h which is shown clearly in the elevation in Figure 1. This bath is 33 cm. (13 inches) in diameter and 66 cm. (26 inches) high and is placed in a 46 cm. X 66 cm. (18 X 26 inch) container immersed
1
Cnns!aol-tem~erature bath
FIGURE1. APPARATCSFOR OXIDIZINGCOAL AT TEMPERATURES
1347
Mild oxidation of Pittsburgh bed coal at 30" C. followed by actual carbonization tests on 2-kg. samples shows the following correlation between carbonizing properties and extent of oxidation up to 4-month exposure to oxygen: (1) A progressive increase in strength of coke ; (2) a regular decrease in the yield of tar, accompanied by an equal increase in the yield of coke; (3) a regular increase in the amount of carbon dioxide evolved with the gaseous products of carbonization; (4) a change in plastic properties characteristic of increased coke strength. These changes are appreciable before the oxidation has progressed sufficiently far to affect significantly other properties such as agglutinating value, proximate and ultimate analyses, and heating value.
in the large bath. The annular space betvieen the inner and outer containers is filled with insulation. The temperature of the dewpoint bath, maintained several degrees colder than the main bath, is kept constant by means of a heater connected through a relay with another toluene-bulb thermostat. The recirculating oxygen passes down through the glass condenser tubing, which is formed in a helix, and then out through a vertical riser and back to the drum. At the bottom of the helical condenser tube is a glass bulb which collects the water condensed out of the recirculating oxygen. The condensed water can be removed periodically by the drain tube shown. Two filters in the exit of the drum remove any dust from the recirculating oxygen. All pipe connections of the recirculating system are kept below the water level of the large bath to avoid condensation of moisture in the lines. As the oxygen in this system is consumed by reaction with the coal, make-up oxygen is metered in through a wet test meter with a capacity of 2.8 liters (0.1 cubic foot) per minute. As shown in Figure 1, a check valve arrangement is inserted between the meter and the rest of the system. This has been found necessary to keep oxygen from forcing its way back through the meter when the atmospheric pressure is decreasing rapidly. In operation, the drum was charged by removing one of the ends and introducing 27 kg. of coal, crushed in a nitrogen atmosphere to pass a 0.635-cm. (0.25-inch) screen. The system was thoroughly flushed out with nitrogen and the large bath brought up to 30" C. It was maintained at that temperature throughout the remainder of the run. The temperature in the dew-point bath was then lowered progressively until the formation of dew in the helical glass condenser was noted. The thermoregulator was set to maintain the small bath at this temperature for the remainder of the run. After a preliminary period in which equilibrium was established, no more water was condensed out, although a visible film of moisture remained on the inside of the condenser tubing. For the work described in this report, the dew-point bath was maintained at 27.3" C., which corresponds t o a relative humidity of 86 per cent in the oxygen over the coal. The system was then flushed out with oxygen, the meter connected, and the apparatus allowed to run continuously until the desired amount of oxygen had been consumed by the coal. After periods ranging from several weeks to a month, the large bath was drained and a sample of coal taken for carbonization. Special precautions had to be taken to obtain a representative sample. Previous experiments had shown that rotation of the drum containing crushed coal resulted in pronounced segregation of fines into sharply defined zones. In one test where the drum was half full of 0 to 0.635 cm. (0 to 0.25 inch) Pocahontas coal and was rotated for 4 hours at a rate of 1.8revolutions per minute, two very sharp bands of fines were formed adjacent to bends which were to a large extent free of fines. The following screen CONBTAKT analysis of two grab samples taken after 7650 revolutions shows the large extent to which t'his segregation had taken place.
INDUSTRIAL AND EXGINEERING CHEMISTRY
1348
13 mm inside dia g18~5luhing Rubber tube connections
;r;:;!r
VOL. 28, NO. 11
(1-inch) angle irons riveted to the shell, 90" apart and parallel to the axis of rotation of the drum, It is charged with coke and rotated a t 24 r. p. m. for measured periods. I n operation, the furnace was heated to 950' C. and the retort was put into place and connected with the recovery train. The cold retort lowered the furnace temperature to a minimum of about 720 O C. About 35 minutes were required to raise the furnace temperature to 900" C. The autoTo meter contacts
eatlng coil 24 turns nchmme w e
drious
0 5827 cm.dja.
Gas s m ~ l ep u m p
~nsulal#on,SIIO.Cel and insulat~ogbrick
FIGURE 2. CARBONIZATION APPARATUS I
U. S. Sieve Series
c
On
4-mesh Sample 1 (fine band) Sample 2 (coarse band)
8-mesh
14-mesh
100-mesh
%
%
%
%
2.3 0.3
6.8 70.8
2.3 22.0
76.0 0.83
7
Through 100-mesh
% 11.9
0.07
These results indicate that, in general, rotating drums should not be depended upon to mix conglomerates of various particle sizes preparatory to sampling, unless due allowance is made for this tendency for segregation of fines. In the actual oxidation runs segregation was not pronounced enough to be visible, perhaps because of the slow speed of revolution. However, since the tendency was known, suitable precautions were taken to obtain a good sample. After removing the end of the drum, a trough (5 X 5 X 122 cm.) was inverted and placed on top of the coal and the drum rotated through a half-revolution, thus filling the trough. It was then removed and carefully weighed, and the coal was ke t in tightly sealed containers under nitrogen until needed i%r carbonization. The oxidation a paratus was then put into operation again and the oxidation aiowed to proceed until another sample was taken. At the time each sample was taken the moisture was drained from the collector in the dew-point bath and measured. Likewise, the solid calcium hydroxide was removed from the adsorption bottle, and its carbon dioxide and moisture contents determined.
Carbonization Apparatus and Procedure The apparatus used for carbonizing the samples of oxidized coal (Figure 2) is a modification of that used by Davis and Hanson (7). A 1950-gram (4.3-pound) sample of coal is carbonized in a cylindrical iron retort, 12.7 X 22.86 cm. (5 X 9 inches) in size, by means of a n electrically heated furnace maintained a t the desired temperature with a Leeds & Northrup automatic temperature controller. Figure 2 shows the retort in place in the furnace, the glass reflux condenser and Cottrell precipitator for recovery of liquor and tar, and the caustic tower for removal of hydrogen sulfide and carbon dioxide. The gas is measured with a wet test meter. Figure 3 shows a new design for a sampler pump, which proved very satisfactory. A mercury contact on the gas meter dial actuates the pump which removes about 10 cc. of the gas for every 2.8 liters (0.1 cubic foot) passing. The sample is stored over mercury in the gas holder shown. The tumbler for testing the coke was the one used by Davis and Hanson (7). It consists of a drum 45.7 cm. (18 inches) in diameter and 15.2 cm. (6 inches) deep, with four 2.54-cm.
FIGURE 3. GAS SAMPLER AND HOLDER
matic controller was then set to maintain this temperature for the remainder of the test. The furnace temperatures were checked with an auxiliary chromel-alumel thermocouple and a potentiometer, and care was taken to reproduce the same temperature-time curves in the various tests. The outlet pipe from the retort was kept clear of pitch coke by frequent use of the ejector or ram attachment shown in Figure 2. The device eliminated the possibility of building up an excessive back pressure in the retort. The rate of gas evolution reached a maximum of about 9.9 liters (0.35 cubic foot) per minute after 35 minutes. The test was stopped when this rate had fallen to 0.8 liter (0.03 cubic foot) per minute, the total time of carbonization being approximately 1.75 hours. Upon completion of the test, the bottom of the retort was taken off in a lathe, the coke carefully removed and weighed, and a screen analysis made. A 900-gram sample of coke sized to pass a 5.08-cm. (2-inch) and be retained on a 3.18-cm. (1.25-inch) square-hole screen was placed in the tumbler, which was rotated a t 24 r. p. m. for one hour. The coke was taken out and a screen analysis was made; it was then put back into the tumbler for another 30 minutes. The results of both the hour tumbler test and the 1.5-hour test are reported.
Results Obtained i n Oxidation Apparatus The data in this report refer to Pittsburgh bed coal from the Bureau of Mines Experimental Mine a t Pittsburgh. It is classified as a high-volatile A, coking coal (2) and falls toward the low-rank boundary of this class. A special sample of this coal was taken from a section of the mine where a large amount of coal had just been removed. The rate of oxygen consumption by this coal a t 30" C. is shown in Figure 4 and Table I. Here, under the heading "total oxygen used" is shown the total amount of make-up oxygen metered in to the drum after due allowance was made
NOVEMBER, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
for differences in barometric pressure a t the beginning and end of a run. The rate of oxygen consumption is slow a t this temperature (30' C.), amounting t o 0.879 per cent of the weight of the coal in 2722 hours or about 4 months. The rate is relatively high a t first and falls off slowly to a steady value. Gas analysis of the recirculation oxygen indicated that carbon dioxide and water are the only gases evolved by the reaction in appreciable quantities a t this temperature. The lower curve in Figure 4 s h o w the increase in the total amount of carbon dioxide evolved in the oxidation process. Unfortunately, an accurate value for the total amount of n-ater formed during the run was not obtained because of experimental difficulties. The figure given in Table I for water evolved is only approximate. However, this does not affect the accuracy of the values for total oxygen used.
TABLE I. RESULTS OBT.4INED IN OXIDIZING PITTSBURGH BED COALAT 30" C. Sample
Time Hours
Total Oxygen Used per Kg. Dry Coal Litersa Grams
A-1 0 0 0 A-2 105 0.602 0.86 A-3 297 1.458 2.083 A-4 847 3.310 4.73 A-5 1244 4.080 5.83 A-6 1929 5.026 7.18 A-7 2722 6.155 8.79 a Standard temperature and pressure.
COz Evolved Hz0 Evolved Per kg. Total Per kg. Total 0 2 dry evolved dry 02 as Con coal as water coal Gram
%
Grums
%
0 0.133 0.292 0.501 0.620 0.755 0.883
0 11.3 10.2 7.7 7.7 7.6 7.3
0
.. ..
. .. ...
...
3.9
. ,. .. .
.. 59:5
.. ..
As shown in Table I, the percentage of the total oxygen used which appears as carbon dioxide decreases from 11.3 t o 7.3 per cent as the oxidation proceeds. The relatively high initial rate of evolution of carbon dioxide may be due to desorption of carbon dioxide which was originally adsorbed in the coal in the mine. Francis and Wheeler (8), working with 40 to 60 mesh coal oxidized a t 60' C., found that about 11.0 per cent of the total oxygen used appeared as carbon dioxide. Their somewhat higher value may have been due t o the type of coal tested or to the increased temperature.
1349
TABLE11. YIELDSOF CARBONIZATION PRODUCTS (DRYBABIB) AND PHYSICAL PROPERTIES OF COKE Sample Grams Or/kg. dry coal Coke, % Tar. % Liquor, % Gas,
%
A-1 0.0 Yields 65.1 13.5 4.6 16.1
A-2
-1-3
-1-4
A-5
0.86 2.'083 4 . 7 3 5.83 from Carbonization Tests 65.0 65.6 65.9 66.2 13.6 12.9 12.7 12.6 4.7 4.3 4.4 4.5 15.7 15.9 15.2 16.0 Coke Tests
1-hour tumbler: 44.0 42.1 34.9 42.8 40.6 Stabilitya 67.7 68.8 Hardnessb 64.2 64.2 66.3 54.4 52.6 53.4 % friabilitye 57.9 55.7 1.5-hour tumbler: 3 4 . 0 3 3 . 0 3 5.5 2 7 . 0 3 4 . 5 Stabilitya 60.2 62 0 63.0 57.9 57.9 Hardnessb 59.7 58.6 Yofriabilityc 62.6 61.3 59.6 0 Cumulative per cent on 2.54-cm. (I-inch) screen. b Cumulative er cent on 0 47 cm (4 mesh) screen. Per cent reauction in s i s i [ I S ) :
A-6
A-7
7.18
8.79
66.0 11.9 4.2 15.9
67.0 11.5 4.1 15.8
45.9 69.3 53.2
39.6 72.4 53.0
35.4 62.7 59.6
27.4 66.3 59.4
average size in the tumbler, as calculated by the "volume mean" method (18) decreased. This trend parallels the hardness and stability results. I n general, the results of the 1.5hour tumbler test given in Table I1 show the same trends as those of the 1-hour tumbler. The increased time appears to decrease the sensitivity of the test somewhat and to result in a corresponding increase in reproducibility. Also, it should be mentioned that as the oxidation of the coal progressed, the time required for the carbonization decreased regularly. Unoxidized coal required 117 minutes for carbonization, but, as the amount of oxidation increased, this time regularly decreased until with sample A-7 only 109 minutes were required for carbonization. The decrease in carbonizing time was accompanied by a regular increase in the maximum rate of gas evolution during a carbonization test. Thus the maximum rate of gas evolution for unoxidized
Carbonization Tests EFFECT OF OXIDATION ON YIELDASD ON COKEPROPERTIES. The effects of oxidation of the coal on the yields of carbonization products and on the strength of coke are given in Table I1 and plotted in Figures 5 and 6 , respectively. The yield of coke increased from 65.0 to 67.0 per cent, owing to the oxidation of the coal, and its hardness (cumulative per cent on 4-mesh screen after one hour in the tumbler) increased regularly from 64 to 71.6 per cent. Oxidation of the coal resulted in progressive decrease in the yield of tar from 13.7 per cent of the weight of the coal to 11.6 per cent. It is interesting to note that the increase in the yield of coke due to oxidation of the coal corresponds closely to the decrease in the yield of tar. The indications are that oxidation affects the tar-forming constituents of the coal in such a way that they are cracked on carbonization to form coke instead of coming off with the other distillation products. The increased hardness of the coke may be related to this increased cracking of the tar. The stability of the coke--that is, the cumulative per cent retained on a 2.54-cm. (1-inch) screen after the tumbler tests-also increased owing to the oxidation. Because of the small amounts of coke tested, the experimental error in this test is rather large. However, the general upward trend is clear. The per cent friability or the per cent reduction in
Tme hours
FIGURE 4. RATEOF OXIDATION OF PITTSBURGH BED COALIN 4s ATMOSPHEREOF OXYGEN AT 30" C.
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VOL. 28, NO. 11
TABLE 111. EXAMINATION OF DEHYDRATED TARS Sample
A-1
A-2
A-3
A-4
A-5
A-6
01per kg. dry coal rams Sp. gr. of t a r , 15.6'/g15.6' C.
0.0 1.108
0.86 1.120
2.083 1.126
4.73 1.118
5.83 1.124
7.18 1.133
7.4 24.7 33.1 53.8 1,006
7.8 25.3 37.6 57.0 1.003
9.0 26.3 36.8 56.0 1.006
10.8 28.3 38.0 58.9 1.007
6.6 25.4 36.7 50.2 0.996
8.0 26.5 40.1 54.1 1.009
0.06 4.1 14.7 35.0
0.03 3.9 18.7 34.4
0.04 3.4 16.5 36.1
0.05 4.1 18.3 36.5
0.03 3.3 13.8 33.1
0.17 4.0 14.6 35.5
Distn., cumulative % ' b y vol.: 0-170' C. 170-235O C. 235-270' C . 270-320' * 15'. C. Sp. gr. of dist., solid-free Analysis of t a r , % by vol.: Solids Bases Acids Neutral Oil3
----.--C c . / k p . Gal./ton
Yields on basis of dry coal:a Tar Bases Acids Neutral oils
122.1 5.0 18.0 42.7
Ultimate analysis of t a r , %: Hydrogen Carbon Nitrogen Oxygen Sulfur Calorific value: Cal./gram B. t. u./lb. a Ton of 2000 pounds
29.29 1.20 4.31 10.25
---
--A__
C c . / k g . Gal./ton 29.19 121.7 4.8 1.14 22.8 5.46 10.04 41.9
--A-
C c . / k p . Gal./ton 114.8 27.54 3.9 0.94 18.9 4.54 41.4 9.94
C c . / k g . Gal./ton 113.9 27.31 4.7 1.12 20.8 5.00 41.6 9.97
6.9 85.1 1.0 6.4 0.6
7.1 84.7 1.1 6.5 0.6
6.9 85.6 1.1 6.7 0.7
7.0 85.3 1.1 6.0 0.6
8,912 16,040
8,956 16,120
8,984 16,170
8,951 16,110
---
C c . / k y . Gal./ton 112.4 26.95 3.7 0.89 15.5 3.72 37.2 8.92
----
C c . / k g . Gal./ton 105.3 25.25 4.2 1.01 15.4 3.69 37.4 8.96
6.8 1.2 5.4 0.6
6.7 85.9 1.2 5.6 0.6
8,978 16,160
8,912 16,040
86.0
coal was 9.6 l i t e r s (0.34 The proximate and ultimate analyses of the coal cubic foot) per minute; for and coke are s h o w n in sample A-7, the most highly Table IV. There is no very oxidized sample tested, this significant change in these rate had increased to 11.0 values with t8he oxidation liters (0.39 cubic foot) per of the coal. The fact that minute. the analyses do not change The increase in strength a p p r e c i a b l y shows that and in yield of coke and the they do not constitute a corresponding decrease in very sensitive measure of the yield of tar with the the extent of oxidation or mild o x i d a t i o n t o which we a t h e r i n g of coal-at this coal was exposed conleast not as sensitive as stitute the major changes in the strength of coke procarbonizing p r o p e r t i e s duced. The total decrease observed. The yields of in heating value due to oxiliquor and gas show but dation, 95 calories per gram little change. (170 B. t. u. per pound), Table I11 and Figure 7 appears to be of signifishow the properties of the cant magnitude, but, when tar produced. Except for the figures are reduced to the decrease in yield, the I I , , : I ! , , # the ash-free basis, the total 1 1 1 1 I d l l d l b v a r i a t i o n s are not very 3 4 5 6 7 8 decrease is found to be 80 large and do not constiOxygen grams per kilogram of dry coal c a l o r i e s (144 B. t. u.). tute as sensitive a gage of OF OXIDATION OF COALON YIELDSOF CARFIGURE 5. EFFECT Considering the fact that the extent of oxidation of BONIZ.4TIOK PRODUCTS two different samples are inthe coal as does the variavolved, this variation is close tion in coke strength. The to the experimental error and would not be considered proof u k k , t e analyses-of the tars given in Table 111 show little of oxidation of the coal unless supported by other evidence. change with oxidation. I - )
AND ULTIMATE ANALYSES, DRY BASIS TABLEIV. PROXIMATE
.~
Samole
Coal
Sample No,
Proximate Analysis Volatile Fixed matter carbon
Ash
%
%
A-7
0.0 0.86 2.08 4.73 5.83 7.18 8.79
38.7 38.1 37.7 37.8 37.3 38.1 37.8
57.7 58.3 58.7 58.5 58.8 58.4 58.4
A-1 A-7
0.0 8.79
1.2 0.5
93.2 93.5
6.0
A- 1 A-2 A-3 A-4 A-5 A-6
Coke
0%Used per Kg. Drv Coal Grams
%
Ultimate Analysis Hydrogen Carbon Nitrogen Oxygen
Sulfur
%
%
%
%
%
3.6 3.6 3.6 3.7 3.9 3.5 3.8
5.5 5.5 5.3 5.5 5.4 5.4 5.4
81.5 81.5 82.0 81.0 80.6 81.5 80.6
1.6 1.6 1.6 1.6 1.6 1.6 1.7
6.5 6.4 6.1 6.7 7.0 6.7 7.0
1.3 1.4 1.4
5.6
1.0 0.9
89.7 89.8
1.7 1.6
0.9 0.5
1.1 1.2
1.5
1.5 1.3 1.5
Calorific Value (Dry) B. t . u./lb. Cal./gram 14.690 8,162 14,680 8,156 8.151 14,670 14,580 8,101 14,570 8,095 14,610 8,117 14,520 8,067 13,740 13,700
7,634 7,612
NOVEMBER, 1936 Table V summarizes the carbonization t e s t s a n d p r e s e n t s t h e results in greater detail. The first three columns show the results obtained with three samples of the same unoxidized coal and indicate the reproducibility of the carbonization results. Column 4 is the average of the three check determinations. The screen analyses of coal as charged were made to ascertain whether the rotation of the drum in the o x i d a t i o n apparatus res u l t e d in an appreciable grinding action on the coal. S o appreciable change in the screen analyses can be noted. For example, the rotation of the drum during a period of a b o u t 4 months did not produce any appreciable change in the amount of coal that would pass a 240-mesh screen (1.8 per cent). In general, the analyses of the gas produced on carb o n i z a t i o n show little change with the extent of oxidation of the coal. However, the total carbon dioxide (grams per kilogram of dry coal carbonized) produced in the carbonization did show a definite increase. This increase is indicated in Figure 8. The amount of oxygen evolved as carbon monoxide and as water during the carbonization did n o t s h o w an appreciable increase. It is interesting to conipare t h e i n c r e a s e i n t h e amount of oxygen coming off as carbon dioxide on c a r b o n i z a t i o n with the amount of oxygen remaining in the coal during oxidation. I n the o x i d a t i o n a p p a r a t u s about 67 per cent of the total oxygen used is evolved immediately as water and carbon dioxide (Table I). The oxygen remaining in the coal amounts to about 2.9 grams per kg. of dry coal for s a m p l e A-7. Using the values given in Figure 7 for the increase in carbon dioxide evolved on carbonization that is due to oxidation of the coal, we find that the i n c r e a s e amounts to 2.4
INDUSTRIAL AND ENGINEERING CHEMISTRY
1351
grams of oxygen per kg. of coal. Thus the increase in the oxygen given off as carbon dioxide on carbonization accountsfor most of the oxygen remaining in the coal during oxidation. The difference is easily within the p o s s i b l e error in the value of 59.5 per cent found for the percentage of total oxygen coming off as water during oxidation. These results indicate that the amount of carbon dioxide given off during carbonization can be used as a measure of the extent of oxidation of the coal. This condition will hold only if the carbonization conditions are such that but little of the carbon dioxide evolved on carbonization is reduced to $ carbon monoxide by incang 38 descent coke before escaping from the retort. These fig234 ures indicate that little reOxygen, grams per kilogram of dry coal duction of carbon dioxide to FIGURE 6. EFFECT OF OXIDATION OF COAL ON PHYSICAL carbon monoxide took place PROPERTIES OF COKE under the c a r b o n i z a t i o n conditions used in this work. It is difficult t o obtain a balance of the total oxygen in the coal with t h a t in the @A -x products of carbonization. k 8 I n ultimate analysis procedure, oxygen is given by the 108 4 difference between 100 per cent and the total per cent 81001 of all constituents d e t e r E E41 7 mined directly; hence this -0 figure may include all the experimental errors of the 5 334 other determinations. n However, it may be worth $EO while to give approximate n values showing how the total p oxygen in the coal entering 2 167 the retort divides up into . the various carbonization i 83 p r o d u c t s . U s i n g the average of all the carbonization tests, we find that the 0 total oxygen in the products Oxygen, grams per kilogram 01 dry coal checks the total oxygen enFIGURE 7. EFFECT OF OXIDATION O F COAL ON TARAND TAR tering the coal within 6 PRODUCTS grams per kg. of coal (corresponding to an increase of 0.6 per cent in the ultimate analysis figure for oxygen in L B eL" the coal). The totaloxygen gP in the carbonization prodE% % ucts is d i s t r i b u t e d as 0"B follows: 10.9, 18.4, 53.9, x 10.2,and 6.6 per cent, in the Total oxygen grams Der kllogram of dry coal carbon d i o x i d e , carbon monoxide, water, tar, and FIQURE 8. EFFECT OF OXIDATIOXOF COALON YIELDOF CARBON DIOXIDE ON CARBONIZ4TION coke, respectively.
-
Y)
Y)
-
1352
INDUSTRIAL AND ENGINEERING CHEMISTRY
VOL. 28, NO. 11
, TABLE V Sample
A-1-L
Extent of oxidation, days 0.0 On per kg. dry coal, grama 0.0 Screen analysis of coal charged, cumulative %: On 4 mesh On 8 mesh On 14 mesh On 40 mesh On 60 mesh . . On 100 mesh On 240 mesh Through 240 mesh . . Av. size, cm.5 ... Charge in retort, grams?? Yield+ Coke, %
SUMMARY O F CSRBONIZATION A-1-hl
A-1-N
0.0 0.0
0.0 0.0
. . ...
. . ...
. .
Av. Unoxidined 0.0 0.0
24.0 59.2 77.8 91.2 94.7 96.4 98.2
100.0
0.353
TEBTS AT 900'
c.
A-2
A-3
A-4
-4-5
A-6
A-7
4.4 0.86
12.4 2.08
35.3 4.73
51.8 5.83
80.0 7.18
113 8.79
18.6 46.2 65.5 84.5 90.9 93.9 97.1 100.0 0.295
13.3 43.4 68.1 87.6 92.8 95.0 97.4 100.0 0.274
13.0 52.9 69.0 89.1 93.7 95.8 98.0 100.0 0,292
18.0 62.0 80.0 90.9 94.5 96.2 98.3
19.3 50.8 70.4 87.2 93.0 95.6 98.2
0.337
0.310
... , . .
...
. .
. .
. . ... ...
. .
100.0
100,0
1950
1950
1950
1950
1950
1950
1950
1950
1950
1950
63.7 12.9 6.2 16.6 99.4
63.9 14.0 6.3 14.9 99.1
64.3 13.0 6.2 99.5
64.0 13.3 6.2 15.8 99.3
63.9 13.1 5.9 14.9 98.1
64.5 12.7 5.9 15.7 100.0
64.7 12.5 6.2 15.4 98.8
65.3 12.4 6.0 15.7 99.4
65.0 11.7 5.6 15.7 98.0
66.1 11.3 5.4 15.0 98.4
303 9710
284 9090
306 9820
298 9540
279 8960
297 9510
290 9310
298 9540
297 9520
298 9560
8.45
9.26
10.07
9.26
10.75
9.70
10.95
11.15
12.31
11.70
1.6 5.6 0.3 51.0 6.1 29.7 3.5 2.2 3.i 0.434
1.7 6.3 0.4 48.1 6.7 32.1 2.5 2.2 3.7 0.449
1.6 6.1 0.3 48.7 6.6 31.4 2.9 2.5 3.7 0.445
2.0 5.2 0.3 51.3 6.3 29.3 3.6 2.0 3.7 0.441
1.7 5.9
Nr HIS, grams/kg. coal Sp. gr. of gasd
1.4 6.5 0.3 47.0 6.7 32.3 2.8 3.0 3.6 0,453
1.9 5.6 0.3 49.5 6.5 30.7 3.4 2.1 3.7 0.440
1.9 5.8 0.3 49.5 6.8 30.3 3.2 2.2 3.8 0.437
2.1 6.4 0.4 46.7 6.8 31.9 2.8 2.9 3.9 0.437
2.0 6.2 0.5 48.4 6.8 31.2 3.5 1.4 4.0 0.435
Bv. size,-cm." % friability"
9.9 33.0 54.6 63.0 64.4 100.0 1.77 58.5
3.7 16.9 44.0 62.2 66.9 67.7 100.0 2.02 52.6
21.1 42.1 59.2 67.8 68.8 100.0 1.99 53.4
:i%2 % Gas, 7%
Total Gas:= Liters/kg. coal Cu. ft./tond coal Total COz evolved in carbonizing, grams/kg. dry coal Gas analysis, %:
co,
Illuminants 01
Hz
co
CH4
CzHa
,..
...
12.1 34.3 52.8 61.7 62.9 100.0 1.76 58.7
16.0
...
11.9 37.5 59.4 64.3 65.2 100.0 1.86 56.4
Screen analysis of coke after 1 . 5 h r . i n tumbler, cumulative %: On 1.5 in. (3.81 cm.) ... ... ... 7.2 On 1.25 in. (3.18 em.) 9.1 5.3 On 1 in. (2.54 om.) 25.8 25.0 30.2 46.4 52.6 47.0 On 0.75 in. (1.91 cm.) 56.6 55.3 58.2 On 2 mesh (0.91 cm.) 58.3 56.5 59.0 On 4 mesh (0.47 om.) Through 4 mesh (0.47 em.) 100.0 LOO. 0 100.0 1.57 1.56 1.65 Av. size 63.2 63.3 61.4 % friabhitya 4 Yaneey and Zane (18). b As-charged basis (1.7 per cent HzO). 0 Saturated a t 60a F. and 30 inches Hg, (15.56' C. and 762 mm. Hg). d 2000-pound ton. 8 Specific gravity air = 1.
Effect of Oxidation on Plastic Properties of Coal
...
11.3 34.9 55.6 63.0 64.2 100.0 1.80 57.9
...
7.2 27.0 48.7 56.7 57.9
100.0
1.59 62.6
...
17.1 42.8 56.0 63.0 64.2 100.0 1.89 55.7
...
7.0 34.5 49.6 57.0 57.9 100.0
1.65 61.3
0.6
46.5 6.9 31.3 3.0 4.1 3.7 0.439
...
21.6 40.6 58.8 64.7 66.3 100.0 1.94 54.4
...
12.2 34.0 52.5 58.5 60.2 100.0 1.72 59.6
. .
7.3 33.0 53.0 61.2 62.0 100.0 1.71 59.7
...
, , .
11.1
35.5 52.6 62.1 63.0 100.0 1.76 58.6
...
12.0 45.9 63.0 68.9 69.3 100.0 1.99 53.2
...
5.4 35.4 51.8 62.3 62.7 100.0 1.72 59.6
...
13.2 39.6 64.3 71.4 72.4 100.0 2.00 53.0
...
3.0 27.4 56.8 65.2 66.3 100.0 1.73 59.4
9 shows plastic-range curves for fresh coal (sample A-1) and for Coal which Was oxidized for 1244 hours, COnSUming 5.83 The strength of the coke measured by the tumbler test grams of o v g e n Per kg. dry coal (sample A-5). This amount gradually increases as oxidation a t 300c. progresses. Figure of oxidation raised the initial temperature of plasticity from 389" to 400' C. and iowered the end temperature from 507' t o 500" C., thus shortening the total range by 18". Furthermore, the range of maximum fluidity, which began at 415" C. in the fresh coal and a t 428" in the oxidized coal, was reduced from 35" t o 19' C. -4paper ( S ) , correlating the length of these ranges with the strength of the coke, shows that for high-rank coking coals, such as the one used in this work, the shorter these ranges are, the stronger is the coke. Changes in the expansion characteristics of the coal due to oxidation were measured by the modified Agde'Damm method ( 3 ) . It was found that the contraction interval for the unoxidized coal (sample A-1) was 79" C. compared with a value of 71" C. for the oxidized coal (sample A-5). Temperature 9: OF OXIDATION OF COALOK PLASTIC PROPERTIES AS SHOWN I n general, this change can be correlated with FIGURE 9. EFFECT increased strength of coke (5). BY DAYIS PLASTOMETER
INDUSTRIAL AND ENGINEERING CHEMISTRY
NOVEMBER, 1936
*
The agglutinating values of the coal as determined by the Bureau of Mines method (16) are shown in Table VI. These values were not changed significantly by the amount of oxidation to which this coal was subjected. It would appear, therefore, that the plastic range intervals furnish a more sensitive measure of the extent of oxidation of this coal than do the agglutinating values. 1
OF TABLE VI. EFFECT
Sample
OXIDATION O F COAL ON AQOLUTINATIXG
VALUE(16) -4-1
used per kg. d r y coal, grams
A-2
A-3
0.86
2.08
0 2
0.0
A-4
A-5
A-6
A-7
4.73 5.83 7.18 8.79
I n general, oxidation of the coal has an effect very similar t o that obtained by Davis and Hanson ( 7 ) by mixing fine inert material with Pittsburgh bed coal, thereby reducing the fusibility and increasing the strength of the coke produced. Mild oxidation under the conditions used in this work with its attendant increase in coke strength, decrease in tar yield, and fusibility of the coal, apparently destroys some of the fusible coking constituents, of which there is an excess.
1353
Literature Cited Agde, G., and Winter, A,, Brennstof-Chem., 15, 46-50 (1934). (2) Am. Soo. Testing Materials, Proc. 35, Pt. I, 847-53 (1935). (3) Brewer, R. E., and Atkinson, R. G., “Plasticity of Coals, Its Measurement and Relation to Quality of Coke Produced” (in manuscript, 1936). (4) Bunte, K., and Buchner, H., 2. angew. Chem., 47, 84-6 (1934). ( 5 ) Coles, G., and Graham, J., Fuel, 7,21 (1928). (6) Davis, J. D., and B.wne, J. F., J. Am. Ceram. Soc., 7, 809-16
(1)
(1924). (7) Davis, J. D., and Hanson, 0. G., IND. ENG.CHEM.,Anal. Ed., 4, 328 (1932). ( 8 ) Francis, W., and Wheeler, R. V., J. Chem. Soc., 1927, 2955. (9) Haldane, J. S., and Makgill, R. H., J. Soc. Chem. I n d . , 53, 359T (1934). (10) Jenkner, A., Kuhlwein, F. L., and Hoffman, E., Gluckauf, 70, 473-81 (1934). (11) Michaelis, P., Ibid., 71, 413-23 (1935). (12) Pamart, C., Chaleur & ind., 15, 329-33 (1934). (13) Parr, S. W., and Milner, R. T., IKD.ENG. CHEM..17, 115 (1925); Fuel, 5, 295 (1926). (14) Porter, H. C., and Ralston, 0. C., U. S. Bur. Mines, Tech. Paper 65 (19143. (15) Rose, H. J., and Sebastian, J. J. S., Fuel, 11, 284-97 (1932). (16) Selvig, TT. h.,Beattie, B. B., and Clelland, J. B., Proc. Am. Soc. Testing Materials, 33, Pt. 2, 741-57 (1933). (17) Strache, H., and Lant, R., ”Kohlenchemie,” Leipsig, rlkademische Verlagsgesellschaft m. b. H., 1924. (18) Yancey, H. F., and Zane, R. E., Bur. Mines, Rept. Investigations 3215 (1933). RECEIVED .4ugust 3, 1936. Presented before t h e Division of Gas and Fuel Chemistry at t h e 92nd Meeting of t h e American Chemical Society, Pittsburgh, P a . , September 7 to 11, 1936. Published by permissi0.n of the Director, U. S. Bureau of Mines. ( K o t subject t o copyright.)
LIQUID-LIQUID EXTRACTION E x a c t Q uant it at ive Relations’
L
IQUID-liquid extraction is constantly employed both in the laboratory and in industry. The ether extraction of organic substances from their aqueous solutions, and the commercial production of absolute ethyl alcohol (9) and of glacial acetic acid (8) are a few examples. With the recent introduction of solvent refining of lubricating oils in the petroleum industry this type of extraction becomes of considerable technical interest. I n industrial work, optimum conditions govern the choice between several alternatives of solvent, of method, and of equipment. Not always will a combination of the most efficient solvent, the most efficient method, and the most efficient equipment, constitute optimum conditions; in fact, this condition may seldom be true. Given a solvent, the method of extraction will largely determine the type of equipment t o be used; the relative advantage of one method over another will mostly be governed by the choice of the solvent; and, for a definite degree of extraction, the necessary amount of solvent will depend on the method used. Yet, no intelligent choice of the proper combination can be made until the interrelations of these different variables are known quantitatively.
Methods of Extraction When a liquid to be submitted to extraction is treated with a suitable liquid solvent, and two layers are formed, one of the 1
T h e first article in this series appeared in August, 1936, pages 928 t o 933
K. A. VARTERESSIAN AND M. R. FENSKE The Pennsylvania State College, State College, Pa.
layers will usually contain a large proportion of solvent and a small proportion of the liquid to be extracted; the other layer will contain a large proportion of the liquid to be extracted and a small proportion of solvent. After such treatment, when the two layers are separated and the solvent is removed and recovered from these layers, what is left of the layer consisting of a large part of the solvent mill be called the “extract,” and what is left of the layer consisting of a large part of the liquid to be extracted Fill be called the “raffinate.” The two layers before the removal of the solvent will be called the “extract layer” and the “raffinate layer,” respectively. Depending upon the relative densities of the extract layer and of the raffinate layer, either one may constitute the top or “light layer,” the other being termed the bottom or “heavy layer.” The various methods of extraction are arranged for convenience as follows: 1. Single-stage 2. Cocurrent contact: a. Multiple-stage b Infinite-stage
3. Countercurrent contact: a. Multiple-stage 6. Infinite-stage
