Air Humidification Coefficients in Spray Towers GAS-FILM MASS TRANSFER COEFFICIENTS AT LOW AIR VELOCITIES FOR USE IN SCRUBBER DESIGN CHARLES F. BONILLA, JOSEPH R. MO'ZTES, AND MURRAY WOLF2 Columbia University, New York, N. Y.
A
study was made of the performance of three small hollow-cone spray nozzles of different orifice diameters in spraying water into air in an empty tower. Mass transfer coefficients for the system air-water were determined over a range of water and low air flow rates-rates that might be employed in a scrubbing tower. The tower used for the investigations was large enough (3 feet in diameter) that the wall effect was believed to be negligible. Empirical equations indicating the effect of flow rates and tower height on the mass transfer coefficient at 75" F. were developed. Equations were also developed for koa in terms of nozzle water pressure for variations i n nozzle size and for other vapors or gases. Thus, the results should be helpful in the design of spray scrubbers for the scrubbing of any gas from air with water in the ranges of conditions covered, providing the gas film i s the controlling factor.
spray nozzles show variations in droplet size with liquor flow rate as well as nonuniformity of droplet size. The only studies found on spray nozzles are those by Kiederman et al. (ZI), and Boelter and Hori ( 9 )on the cooling of water. APPARATUS
The spray chamber consisted of a steel tank 3 feet in diameter and 7 feet high with an upward dished bottom. Air was metered by a standard 1-inch orifice, employing A.S.M.E. factors (1). It was then blown in a t the bottom through a %foot horizontal pipe (3 inches in diameter) which was drilled with 04 */pinch holes with 1-inch centers along a horizontal line on each side of the pipe. The air blew out at the top of the chamber to the atmosphere through a 3.5inch opening. An Eco doubleim eller ump forced water from a storage drum through a cagbrate8 orifice to the nozzle, which w a s centrally mounted and directed downward in the chamber. The water outlet, which contained a U-bend liquid seal, was located as low as possible in order to keep the bottom of the chamber well drained. The inlet and outlet liquid and gas temperatures were read on calibrated thermometers graduated in 0.1O C. intervals. The
T
HE spray tower is one of the oldest types of absorption equipment. It is frequently employed in industry, particularly for the scrubbing of obnoxious impurities from large volumes of
air. Nevertheless, mass transfer coefficients for commercial nozzles, which would be required for the rational design of scrubbing towers, have not been available in the literature or in the files of the manufacturers of these nozzles. Accordingly, a study of t h e operation of a typical series of commercial nozzles was instituted; standard hollow-cone spray nozzles which employ a side inlet whirl chamber (manufactured by the Buffalo Forge Company) were used. Nozzles with 1 / ~ ,a / 3 T , and '/pinch orifices were tested; larger sizes require a larger spray chamber in order to avoid heavy wetting of the walls. An independent determination of the liquid and gas film resistances as functions of gas and liquid flow rates and column height was planned. For any given system the separate film resistances .could be estimated and combined by standard methods (19)t o give the over-all absorption coefficient. It is planned to obtain subsequently the liquid film coefficient for the absorption of pure oxygen and/or carbon dioxide by water. The determination of the gas film coefficient by t h e humidification of air is discussed in this paper. These results should be of value in designing air humidification or water-cooling spray chambers a%the low air flow rates covered.
0
2
4 6 8 1 T I M E HOURS
0
Figure 1. Analysis of Manganese Dioxide A.
E.
Reaction of m a n g a n e s e tetraohloride and acetylacetone on potasaium iodide Reaction of chloroacetylacetone on potassium iodide
P R E V l O U S STUDIES
Previous investigations on absorption in spray towers have concerned single drops (6, 8, 1 4 ) and perforated plate distributors (4, 9). Cyclone spray towers have also been considered as absorbers (6). However, results with perforated plate distributors .do not necessarily apply to commercial spray nozzles, because 1 2
Present address, Merck & Company, Inc., Rahway, N. J. Present address, Criterion Paper Company, Wagsaic, N. Y.
air thermometers (wet and dry bulb) were in the blower intake and in the spray chamber outlet wheqe the air velocity always exceeded 15 feet per second. k o appreciable change in air temperature occurred between the blower intake and the spray chamber. The characteristics of the nozzles were measured in the open, and are qiven in Table I and Figure 1. The slopes of the lines are exact y 0.5, which differs appreciably from Boelter's value of 0.56 for similar but larger nozzles (8). The equations of the 2521
Vol. 42, No. 12
I N D U S T R I A L. A N D E N G I N E E R I N G C H E M I S T R Y
2522
CHARhCTERISTICs O F NOZZLES T E S T E D TABLE 1. FLOW Maximum Diameter of Spray Cone 6 Ft. from Nozzle, Inches
AP,
I,b./Sq. Inch
L,
Actual, Lb./Hr.
L Rated by Manufacturer, Lb./Hr.a
l/s-Inch Diameter Nozzle 34 34
%
59
141.0 177.0 223 2 287 5 404 0
(153) 1202) 1267) 330 460
A wide variation of liquor rate is not feasible with this type of spray nozzle because of orifice flow characteristics. The liquid rates'employed corresponded to pressures near the usual operating range of 10 to 20 pounds per square inch and could be varied by about 15 t o 30YG. The gas flow rate was usually varied over the range from 4 to 20 pounds per hour per square foot which corresponds to 1inea.r velocitirs of approximately 1 to 5 fvet per minute, based on the circular cross section of thc spray c1i:mtm. ,MATHEMATICAL T R E A T M E K T O F R E S U L T S
The value of koa for each run was computed by the equation:
3/wInch Diameter Nozzle 3.25 5.50 7.25 13.90 24.00 4 . 5 (Spray develops)
32 38
38
40 42
92.7 116.8 133.4 187.5 251.0
(94.5) (124) (142) 198 260
1/is-Incli Diameter Nozzle 24 24 25 40 42 42 a
61.0 79.5 01.5 107.5 116.0 126.0
(42.5)
(50.5) (67.0) 83.7 90.0 102.0
Vnlues in parentheses are extrapolated.
lines for the nozzles are given in Table TI (Equations 1, 2 , and 3). -4deviation up to 25% from the manufacturer's nominal capacity is evident. The measurements of diameter were made with a micrometer microscope. The particular '/le-inch orifice employed had a jagged perimeter and its average diameter waa slightly uncertain. The nozzle orifices had a constant diameter and sharp edges, and the thickness of the plate was approximately 0.040 inch. The discharge coefficients ( b / d a )for the larger nozzles, however, are similar; thus, b can be estimated from d for other similar sizes. The spray cone at 6 feet exceeded the diameter of the spray chamber. However, trial runs showed that most of the spray remained within the chamher for these nozzles. It is believed that the spray which wet the wall produced enough wet surface on the wall to make up substantially for the fact that i t did not travel the full height in the form of droplets; the wall effect is relatively minor and yields conservative design data for larger scrubbers with relatively small wall effect. If the chamber diameter had been significantly larger, by-passing of air around th.j spray might have become important. The construction of the nozzles is shown in Figure 2 . TEST PROCEDURE
The inlet air was maintained a t room temperature, and the water was kept several degrees cooler, approaching more or less the wet-bulb temperature. Water and air temperatures were read frequently until they were constant over a IO-minute period; the constant temperatures were recorded. For each nozzle, height, and liquor rate, six to eight gas rates were employed. These runs were repeated for four or five liquor rates. All three nozzles were tejted in this manner a t heights of 38, 48, arid 60 inches above the air inlet pipe. In all, 258 runs were carried out, the data for which are available (IO).
The extreme ranges of the. j v a t r r vapor pressures were as follows: p , = 5 to 12 nim. ((Irpeiiding on atmospheric humidity); p 2 = 17 to 23 mm.; (p2 - p j ) = I 1 to 15 mm.; ( p * - p)im = 5 to 9 mm.; and ( p 2 * - p t r ) = , I t o :3 mm. In view of the small variation in liquid tempPtvtturct :mcl ththi,rforr> p * over the t,oiver, by the use of the logztno important, error should hr iiitro~1u(*ed rithmic mean driving force. (htphival integration of l/(p* - p ) agAinst p would have, been prder:hle; however, this piocedure is tinmconsuming and must, br tmed o n incremental heat balances. In view of the low flow r:ttes, over-all heat balances were poor. The heat losses varied from near zero to about 600 I3.t.u. per hour into the tower, and (,heck roughly with magnitudes basrd on the outer surface o f the tower and the temperature differentialbetween the outside and t,he inside of the tower. As a check on the use of t,he logarithmic mean driving force, three typical runs were graphically intrgrated, starting wit,h the conditions a t the top of the column and assuming adiabatic humidification. The average driving forw exceeded the logarithmic mean by 3.0, 4.3, and -1.5% for the three runs. This small discrepancy agrees wit,h prrvious authors ( 2 , 7 ) who theorized that the humidification zone is close to the nozzle and adiabatic in nature. The minor effect of height on total huniidification is additional verificat,ion. Owing to the heat losses into the column, the computed inlet air temperature ranged to as much as 8" F. below the observed inlet temperature; however, this does not cause any important error in the calculated mean A p . I s view of the low air velocity and the heat unbalance, i t seems. evident that the air in the colunin is well mixed by the spray. Calculated and actual nutlet water temperaturea agreed closely for the t>hreeruns computed.
TABLE IT. SIZECHARACTERISTICS OF NOZZLES TESTED Nominal Size, Inch '/I6 9/91 '/8
Equation L = b( A P j l / Z L = 28.4(AP)1/2 (1) L = 5 0 , O ( A P ) 1 / * (2) L = 8 3 . 1 ( A P ) " f (3)
Av. Diam., d , Inah 0.0661 0.0938 0,1251
(b/&) 6500 5685 5315
Bide Inlet Diameter,. Inch '/a? O/ar :/IS
The average film temperature varied between approximately 73' and 78" F. In view of the abundant evidence (15) that gasfilm mass transfer coefficients vary with only a fractional power of absolute temperature, i t is evident that no correction for temperature is necessary before the results of different runs are compared. These coefficients may hr considered to hold unifoi mly for 75' F. The humidities were obtainrd from a Goff chart ( 3 ) and the p * values Rere obtained from Perry ( I d ) . RESULTS
Figure 2.
Standard Buffalo Hollow Cone Spray Nozzle
For each nozzle, k d was plotted against G on log-log paper as shown in Figure 3. The best slope through the points for each combination of constant 2 and L was computed by linear re-
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1 9 9
0.3 0.2 m a P
0.1 0.09
0.07
3
4
5
6
paper. The slopes of the resulting least-square lines in Figure 5 were -1.03, -0.88, and -0.63, respectively, for the 1 / , ~ , 3 / 2 T 1 and l/ginch nozzles. The value of - 1.03 is not plausible because it indicates that, under otherwise identical conditions, the total mass transfer would be greeter with a smaller height. Although this is unreasonable, the other nozzles give evidence that additional height beyond 38 inches does not contribute to the capacity, and that the correct slope is not much above -1.00. The observed value of -LO3 has been retained for the same reasons as the high exponent of L.
30
SO
8 1 0
2523
0, Lb./Hr. X Sq. FI.
Variation of koa w i t h Gas Flow Rate
Figure 3. 1 ;',
0.9
ineh nozzle, height 5 feet
A L = a61.5 L 234.3
0 L
=.2w
gression. For the 38 slopes involved, the lowest slope obtained was 0.590 and the highest was 0.805. There was no regular trend with Z or L, although there was some consistent effect of nozzle size. The arithmetic mean of the slopes for each size waa computed as follows: 0.71 for the l/l&nch nozzle, 0.68 for the a13p inch nozzle, and 0.64 for the '/cinch nozzle. Lines were plotted with the corresponding average slope through the geometric mean of each group of points, as shown in Figure 3. The variation of kGa with L was obtained by plotting the value of koa a t G = 10 against L on log-log paper, as shown in Figure 4. For each nozzle size substantially the same slopes were obtained by least squares, regardless of nozzle height. The arithmetic means of the slopes for the three heights were 5.5 for the
0.1
,s
0.09
0.07 0.04
0.05
7 - 7 3
I
OS3
P
3
I
I
I
4 2. Ft.
5
6
0.04 7
Figure 5. Variation of koa w i t h Height a t Constant Gas and Liquor Rates G = 10 l/a-inch nozzle, L = 250 0 = z/a-ineh nozzle, .I 150 A = I/v-inch nozzle, L = 100
0=
*
From Figure 5 the equations for the three orifices are obtained aa follows: inch: koa = 5.72 X 10-1aG0J1L6.6Z-1.03
0.1
100
150
200
300
L, Lb./Hr.
Figure 4. Variation of koa w i t h Liquor Rate
-
G = 10 :/:+inch nozzle 0 2 2 3.167 feet A 2 4feet 0 2 9 Sfwt
'/lsinch nozzle, 0.80 for the 8/s2-inch nozzle, and 0.68 for the l/ginch nozzle. The unusual magnitude for the smallest nozzle is due to a large variation in spray particle size over the narrow range of L, and would not be significant beyond this range. However, none of these results should be employed beyond the ranges covered. Thus, the best procedure in representing these coefficients by formulas would be to employ in all cases the slopes observed even if they differ significantly. The lines in Figure 4 were drawn with the average . slope (0.80) through the geometric mean point in each case. The variation of koa with height was obtained by taking values of koa from the curve for each height u t a convenient value of L within the range of points, and plotting against height on log-log
-
'/a
(5)
inch; koa = 0.00174C~~~8LO~802-~.88
(6)
inch: koa = 0.00237GO.04Ije.0aZ--O.ba
(7)
Eighty per cent of the experimental points agree within 5% with the corresponding formula above. In case equations in terms of water pressure at the nozzles are desired, L can be eliminated between Equations 1, 2, 3 (Table 11),and 5,6, 7, respectively. The following equations result: '/I6
'/at
inch: koa = 0.0000563G0.71( L ~ P ) * J ~ Z - ~ . O ~ (8) inch: koa = 0.0398G0.68(AP)o.40Z-0.8S inch: koa = 0.0478G0.64(AP)o.s4Z".e3
(9)
(10)
Because the water pressure drop across the orifice is the cause of atomization, it was anticipated that (neglecting the different effects of surface tension, chamber diameter, etc., as nozzle diameter is changed) a rough correlation of the three nozzles could be obtained. It might be expected that a t the same pressure drop, G, and 2, the average spray velocity and particle size would be roughly constant and that koa would be proportional to L, or to the cross section of the nozzle orifice. Dividing Equa-
INDUSTRIAL AND ENGINEERING CHEMISTRY
2524
tions 8, 9, and 10 by Equations 1, 2, and 3, respectively, yields the following: inch: k a a / L = 0.00000198G0~7~( AP)a.*6Z-1.03
(11)
3/82
inch: kGa/L = 0.000796G0.68(AP)-o.10Z-0.8s
(12)
'/8
inch : k o a / L = 0.000575G0.6*( AP)-0.1KZ-o.63
(13)
'/I6
These equations are plotted in Figure 6 over the range of water pressure employed, for two sets of conditions as follows: G = 20, 2 = 5, and G = 5, 2 = 3.167. I t is evident that the smaller the nozzle the more efficient it is under the test conditions. However, agreement within ahout ZOOj, is obtained between the two larger nozzle sizes. I t seems probable therefore, that Equations 12 and 13 are satisfactory for computing the humidification
I
I
herein for vwtical flow, by basing G on the horizontal cross section. The variation of koa with height to the -0.63 to -1.03 power checks Boelter and Hori's (2) exponent of -0.76 with a spray nozzle, but differs somewhat more from Hixson and Scott's value of -0.5 for large uniform drops ( 5 ) . The variation of koa with L t o the 0.68 to 0.80 power for the two larger nozzles agrees with Xirderman's (11) 0.76 power and Boelter's (2) 0.82 to 1.03 power for '/32- to 7/3pinch hollow-cone nozzles operated in a similar pressure and height range. Ever1 less agreement than in exponents might be expected in comparing observed values of koa with values computed from equations obtained for other conditions. Thus, for one set of Boelter's conditions-Figure 6 in (2)--one 7/8rinch orifice with Z = 6 feet, S = 6 sq. feet, G = 767, and L = lo00 a t 90" F., a value of kQu = 33/29 = 1.14 was observed. However, Equation 13 gives kQa = 9.5 for the same conditions. If Equation 13 employed GO.14, koa = 1.09 would have been obtained. Thus, ivhile Equations 11 and 13 correctly describe the conditions in the range of G covered, a much smaller exponent for G,similar to that of Niederman, would be necessary to bring the two sets of results into a simple equation, and would presumably be better for interpolating. The equations presented herein should therefore be limited, for reliable results, t o the design of nozzle tested and to the ranges of each variable covered, as follows: Nozele Sine, Inch
\ 0.4 4
PO
5 6 7 8 1 0
A
Figure 6.
30
40
P, Lb./Sq. Inch
Variation of ( k o a / L ) w i t h At' at Constant G and Z 1. 2.
C = 20, 2 = 5 G
_ -_-I
Z = 3.167 '/winch nozzle a/rr-inch nozzle r-inrh nozzle
= 5,
coefficients for other nozzles of the same nominal size but somewhat different orifice size and flow characteristics. They may even be suitable for roughly astimating the coefficients for larger nozzles in the same ranges of G, 2, and A P . It is readily shown that k ~ a / L ~ or .?~ kea/ Ap'.a7 would be almost independent of AP and L for the two larger nozzles. Also, kca/L0.6*and k ~ a / d 0 . ~ 9 would be practically the same for both of these nozzles a t any given A P . Any one of these groupings might therefore be preferable in predicting koa for other nozzle sizes. The negative exponents of A P in Equations 12 and 13 indicate that increasing the nozzle pressure decreases the efficiency; therefore, droplet interference and coalescence decrease the effectiveness of the spray a t high pressures more than it is increased by finer droplet size and higher velocity. DISCUSSION OF RESULTS
The proportionality of kaa with the 0.64 to 0.71 power of G found for these low values of G does not agree with the results of Niederman (11),who obtained a 0.17 power variation for a range of G from 280 to 1400, and no variation with G for the spray alone, corrected for wetting of the walls. However, it agrees fairly well with workers who employed single drops (5, 1 4 ) , and obtained 0.8 as the exponent of G. It therefore seems possible to conclude that the effect of G on koa does not depend strongly on the droplet size, a t least in the low range of G employed in this work. This suggests that the direction of the air flow a t these low air velocities may not affect koa a t constant air transit time. Thus, until more evidence is forthcoming, it would seem that approximate coefficients for horizontal flow of air through rectangular spray chambers may also be computed by using the formulas obtained
Vol. 42, No. 12
0.Lb./
(Hr.)(Sq. Ft.) 4-20 4-20 4-20
AP Lb./Sq.'Inch 6-13 8-23 15-19
z.
L Lb&. 205-290 145-240 110-120
.Ft. 3.176-5 3,167-5 3.167-5
I n all cases, the volumetric mass transfer coefficients were based on the %foot diameter cylinder. If nozzles are located so that they cover somewhat more or less floor area per nozzle, S,yet do not significantly wet the walls or interfere with each other, the value for koa based on the total volume per nozzle should probably be multiplied by the ratio 7.07/5. Niederman ( I f ) found that the nozzle effectiveness decreased somewhat as the number of nozzles in the column was increased; however, in his case the walls were considerably wetted with one nozzle and the nozzles were closer together and interfered with each other. ABSORPTION O F OI'HER MATERIALS
If the humidification coefficients are corrected for changes in physical properties of the gas phase, they should be applicable as over-all coefficients for the absorption of other vapors and gases by spray scrubbers-assuming the gas phase resistance predominates, and neglecting the uncertain effect of evaporation or nonevaporation of the smallest droplets. Additional tests with these same nozzles would be required to determine the proper form of this correction. The Bernoulli theory velocity is of the order of 50 feet per second a t the operating pressures, and 100 microns is a typical droplet size (8); hence, the average Reynolds number is of the order of 100. It is evident that much of the mass transfer will take place in the turbulent range, in which kG and koa have usually been taken to be proportional to the -*/a power ( 7 ) of the Schmidt number. Since the diffusivity, D, would be the most widely varying factor, and the highest power reported for it is 1 (15),and the lowest is 0.5 (6, 8), the -2/3 power assumption seems a reasonable rough average. D for air-water is about 0.00027 square foot per second under these conditions. Introducing the diffusivity and floor area corrections into Equations 8,9, and 10 yields: inch: kca = 0.0054f~1/3Go ? l ( AP)2.7s/(SZ1.03)
(14)
3/32
inch: koa = 67.4112/3G0.68( AP)o.40/(SZo.8s)
(15)
l/g
inch: koa = 81.01>2/3G0.61( iP)0.34/(,SZ0.83) (16)
l/,6
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1950
These coefficients are based on total scrubber volume per nozzle. In view of the assumptions made, Equations 14 to 16 should be used only for rough estimation until they can be checked in actual use in scrubbing columns. A C K N O I LEDGMENT
Thanks are due the Buffalo Forge Company, Buffalo, N. Y.,for supplying the spray nozzles employed in this study. NOMENCLATURE
2525
LITERATURE C I T E D
(1) American Society of Mechanical Engineers, “Fluid Meters, Their Theory and Applications,” 4th ed., Part 1, 1937. (2) Boelter, L. M. K., and Hori, S., Trans. Am. SOC.Heating Ventilating Engrs., 49, 309 (1943). (3) Goff, J. A., “Goff Diagram for Moist Air,” American Society of Heating and Ventilating Engineers, 1945. (4) Haslam, R. T., Hershey, R. L., and Keen, R. H., IND.ENG. CHEM.,16, 227 (1924). (5) Hixson, A. W., and Scott, C. E., Ibitl., 27,307 (1935). (6) Johnstone, H. F.,and Kleinschmidt, R. V., Chem. & Met. E ~ Q . , 45, 370 (1958). (7) Johnstone, H. F.,and Silcox, H. E., IND.ENG. CHEM.,39, 808 (1947). (8) Johnstone, H.F., and Williams, G. C., Ibid., 31, 993 (1939). (9) Kowalke, D. L., Hougen, 0. A., and Watson, K. M., Bull. Univ. Wis. Eng. Expt. Sta., No. 68 (June 1925). (10) Mottes, J. R.,and Wolf, M., M.S. thesis in chemical engineering, pp. 55-74, 87-103, Columbia University, 1949; available on loan or as microfilm or photostats through Interlibrary
b = constant in the equation L = 6( A P ) l for nozzle d = spray nozzle orifice diameter D = diffusivity of solute in stagnant solvent gas, sq. ft./second G = gas superficial mass velocity, pounds of dry air/(hr.)(sq. ft. of tower cross section) koa = gas-film mass transfer coefficient, lb. moles/(hr.) (cu. ft.)(atm.) L = water rate, pounds/hour for the nozzle p = partial pressure of water in the gas phase, mm. of mercury p * = vapor tension of pure water, mm. of mercury P = total pressure in the spray column, absolute atm. AP = water pressure differential across the nozzle, pounds/sq. inch S = floor area per nozzle, sq. feet Z = height of spray nozzle above bottom of column, feet
(11) Niederman, H.H., Howe, E. D.. Longwel1,’J. P., Seban, R. A., and Boelter, L. M. K., Trans. Am. SOC.Heating Ventilating Engr,?., 47, 413 (1941). (12) Perry, J. H., “Chemical Engineers’ Handbook,” 2nd ed., p. 391, New York, McGraw-Hill Book Co., 1941. (13)Ibid:, p. 1148. (14) Whitman, W. G.,Long, L., J r . , and Wang. H. Y., IND.ENG. CHEM.,18, 363 (1926). (15) Wilhelm, R. H., Chem. Eng. Progress, 45,208 (1949).
Subscripts 1 = tower conditions, bottom 2 = tower conditions, top 1m = log mean
RECEIVEDJanuary 16, 1950. Presented at the 16th Annual Chemical SOCIETY a t the Ohio Engineering Symposium of the AWEXICANCHEMICAL State University, Columbus, Ohio, December 29, 1949. Contribution No. 4 from the Chemical Engineering Laboratories, Engineering Center, Columbia University, N. Y.
Loan, Columbia University, New York.
HUMIC ACIDS FROM COAL Controlled Air-Oxidation of Coals and Carbons at lSOo to 400° &I. LOUIS D. FRIEDMAN AND CORLISS R. KINNEY Pennsylvania State College, State College, Pa. Alkali-soluble coal acids were produced in better than 90% yields from bituminous coals by air oxidation, and humic acidlike oxidation products were obtained in yields amounting to 80 to 85% of the original coal. The optimum temperature of oxidation was about 200” C., and the time required to oxidize -60 mesh bituminous coal was about 120 to 180 hours. Temperaturesabove 200’ C. increased the rate of oxidation, but the over-all yields of acids were not 80
good. The best yields were 90.2, 87.5, 92.7, and 96.5%, respectively, for the high volatile A, medium volatile No. 1, medium volatile No. 2, and low volatile bituminous coals oxidized at 200’ C. Lignitic and subbituminous coals, although easily oxidized, gave low yields of acid products. As was expected, anthracite and various carbons required higher oxidation temperatures, and the oxidation products had low solubilities in alkali.
I
to 75 hours of oxidation by much slower rates, and a large excess of air could then be passed through the coals with little tendency for the temperature t o rise. Under these conditions, lignitic and subbituminous coals were oxidized a t temperatures as high as 250°, bituminous coals up to 300°, anthracite, carbon black, and lampblack up to 350°, and graphite up to 400’ C. The source of the coals and carbons together with their fixed carbon contents are given in Table I . Data for carbon and hydrogen on a moisture- and mineral matter-free basis are shown in Figure 6. The percentage of carbon in the medium volatile No. 2 coal (Sewell), which is low for coal of this rank, was redetermined a t three different times with a range of the six determinations from 81.30 to 81.96% on a moisture- and mineral matter-free basis. Volatile matter (moisture- and mineral matter-free basis) was also redetermined twice with a range of 25.3 to 26.2% ou the four determinations. Since the behavior of the coal is that of a medium
T IS well known that bituminous coals undergo atmospheric
e
oxidation and that thr rate increases rapidly with tempereture ( 1 4 ) ; the reaction can be controlled to about 150’ C. by spreading the coal in thin layers and stirring it often (10, 17). In a few instances coals havr hem subjected to oxidation temperatures higher than 150” C. bnt usually only after preoxidation a t loww temperatures (6, 7, 6 Since air oxidation a t moderate temperatures produces gooa elds of alkali-soluble, humic acidlike oxidation products which ay have value as a source of organic chemicals, a survey of (+I rolled air oxidation was made a t temperatures higher than 150’ C., primarily for the purpose of shortening the time of obtaining these acids. The difficulty of controlling oxidation rates at the higher temperatures was solved by admitting only as much air as the coal would take a t a given temperature without raising the temperature, Rapid initial rates of oxidation were replaced after some 25 ~
