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 CITED
(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.
e
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 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 Since air oxidation a t moderate loww temperatures (6, 7, 6 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 ~
Vol. 42, No. 12
INDUSTRIAL AND ENGINEERING CHEMISTRY
2526
volatile coal, the anomaly of low carbon was not inyestigated further . PROCEDURE
Vertical, electrirally heated glass tubes, 48 mm. inside diameter by 30 cm., were used for the oxidations. A piece of wire gauze was wedged in the lower end of the tube about 8 cm. from the bottom. A layer of glass wool was laid on the gauze and above this the coal sample, crushed to -60 mesh. The weight of sample was 100 grams. Glass wool was placed in the top of the tube to prevent h e s from being blown out. The ends of the tube were closed with rubber stoppers carrying air inlet and outlet tubes Three thermometers were also inserted in the upper stopper with their bulbs buried in the coal a t approximately one quarter, one half, and three quarters of the height of the sample. At 350' and 400" C. thermocouples were used. Compressed air, which averaged 0.3 to 0.4y0moisture calculated on a volume basis, was uwd from the laboratory line. OF COALS AND CARBONS TABLE I. SOURCE
Legend Used in Figures L S HV MV No. 1 MV No. 2 LV
Coal or Carbon Lignite Subbituminous High volatile A Medium volatile No. 1 Medium volatile No. 2 Low volatile
Fixed Carbona,
%
52.7 55.1 58.9
Bed or Name Velva Monarch Pittsburgh
69.1
U ;:;
Location or Source North Dakota Wyoming Pennsylvania
\
25
LIGNITE 50
73.6 80.1
-
Free-
Sewell Po$aho_ntas
KO
450
200
250
loor a c
25
':f so
50
=--
2%.
I
O
I00
150
200
2%
Pennsylvania West Virginia West Virginia
NO. 3
A CB
Anthracite Carbon blaok
95.7 94.1
LB
Lamp black
96.9
Martin
Pennsylvania Continental Carbon Co. Spear Carbon
G
Graphite
99.4
Spectroscopic grade
?iat,ional Carbon Co.
*' Moisture- and iitineral matter-free
Southern field Witco No. 1
CO.
basin.
All coal samples were heated t o 10" to 20" C. below the oxidation temperature before admitting air; the temperature was controlled by a variable voltage regulator. Usually, depending on the rank of the coal and the temperature of oxidation, the temperature of the coal rose to approximately the desired level when air was passed into the coal. Minor adjustments of the voltagr regulator were then made to obtain n steady oxidation. Temperatures within the coal mass ofteii varied 5" to 10". I n the beginning, the temperature of the lower portion of the tube, where the air first encountered the coal, began to rise, and on continuing the oxidation this was transhted to upper portions. After about 25 to 75 hours the oxidation temperature became steady and oxidation proceeded smoothly. At this stage, an exces8 of air could be passed through the coal with little tendency for the temperature to rise, and 0.05 cubic foot per minute was used as the standard rate of air flow. This rate was the maximum that could be used without excessive loss of fine coal. Periodically during the oxidation, the charge was removed, thoroughly mixed, and returned to the oxidation tube, The marked initial reactivity of the coals was directly related t o their rank, and made considerable difference in obtaining steady oxidation. The lignitic and subbituminous coals could take the standard air-flow rate of 0.05 cubic foot per minute as soon as the temperature had been raised to 150", but not a t 200" or 250" C. The bituminous coals could take the full flow in the beginning up to 200"; the anthracite, carbon black, and lampblack up to 350"; and graphite up to 400' C. All oxidations above the indicated temperatures required cautious admission of air during the early stages. The course of the oxidation of each coal and carbon was followed by deternlining the weight of the oxidized product when the reaction was interrupted to mix the sample. Curves showing
8
25
e s
LOW VOLATILE
o
50
ANTHRACITE 0
50
I
I
I
I
(00
150
200
250
J
300.
I
1
I
I
100
IS0
200
2M
3W I
100 150 200 TIME OF O X l M T i O l i , n&RS
250
ya.
I
I
I
I
25
CARBON B L I C K 0
so
I
:-b .im.
4
'.
I
25-
L I M P BLICU
I
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
December 19%
I
L O 04
40
-
350'
400.
TEMPERATURE OF OXIOATION
Figure 2.
Yields of Oxidized Coals and Carbons
these changes in weight are given in Figure 1. The final weights of the coals and carbons oxidized for about 250 hours (150 hours for the lignitic and subbituminous coals) are compared in Figure 2. So]ubilities of the oxidized coals were determined in sodium hydroxide as follows: Analytical samples weighing about i gram were boiled with 50 ml. of 1 N sodium hydroxide for 30 minutes, cooled, allowed to settle, and filtered; the residue was washed with water. Thisprocess was repeated, including the water washing, until a nearly colorless filtrate was obtained. The final residue was dried for an hour at 105" C. and weighed. Percentage solubilities (by difference) were then calculated and are in Figure 3. The coal material obtained in this way was about 85% recipitated by acidifying the alkaline extract with dilute acid. Tge precipitated acids were hygroscopic and, for purposes, were not easily dried to constant weight. On carbonization gases were evolved but no tar was formed. Equivalent weights of the oxidized coals were first obtained by the calcium precipitation method (IO),but later these values were found to be about half of those obtained electrometrically with a glass electrode (4). Since it appeared probable that the former were low owing to the precipitation of a basic calcium salt they were discarded. Typical electrometric equivalent weights from the high volatile A coal, oxidized a t different temperatures and for different lengths of time, are shown in Figure 4, E uivalent weights were also taken a t the end of each oxidation (&out 250 hours) for all coals except the lignitic and subbituminous coals which were stopped a t about 150 hours (Figure 1). These results are given in Figure 5. The oxidized carbons gave high values of over io0 2000 indicating that practically no carboxyl groups were introduced. For this reason they were not included in Figure 5. eo Analyses of the final oxidation products for carbon and hydrogen were obtained and are compared in Figure 6. The 3 4 balance between the sum of the percentages of carbon and hydrogen and 100% is given in the figure as oxygen. All data used in constructing the figures were calculated on a moisture40 = ! . and mineral matter-free basis.
in introducing the full flow of air (0.05 cubic foot per minute) a t the higher temperatures in the early stages. This resulted in certain irregularities-for example, the curves for both the lignitic and subbituminous coals a t 150' fall below those at 200" C . This waq because more oxidation occurred a t 150" C. with full air flow than took place a t the higher temperature with only small quantities of air admitted. A similar situation occurred at 200' and 250" C., for the medium volatile No. 1 coal, compared with the curve at 300' C. I n the latter case the full flow of air could not be passed through the coal until over 100 hours of carefully controlled oxidation had destroyed the structures which were very reactive a t this temperature. Even then the oxidation continued a t a very high rate for another 50 hours. I n general, ease of oxidation decreased with rank (percentages of k e d carbon, Table I), and consequently higher temperatures were required to obtain active oxidation of the higher rank coals. The coals and carbons fdl into four natural. groups depending on the temperature required to induce relatively rapid oxidatioii. Thus the lignitic and subbituminous coals were rapidly oxidized a t 250" (Figures 1 and 2); the remaining bituminous coals a t 300" ; anthracite, carbon black, and lampblack a t 350'; and graphite was resistant a t 400" C. The oxidations of graphite at 3.50" and 400' C. shown in Figure 1 were repeated with air dried with b c a r i t e to check the higher yields observed a t 400" C . , and almost identical curves were obtained. It aPPeam, themfore, that a t 400' graphite combines with and retains more oxygen than a t 350' Similarly, higher yields were obtained for the 'tow volatile coal a t 200' compared with 150' C.; yields greater than 100% were obtained for the high volatile A and medium volatile No. 2 coals a t 150' and 200' C. Without doubt these increased weights were due to the retention of more oxygen than the weight of oxidation products given off. Solubilities of the oxidized coals in 1 N sodium hydroxide (Figure 3) show that all the coals oxidized at 200' C. were at leas 90% soluble with the exception of the anthracite; this appears to set it apart from the other coals. The oxidized carbons were even less soluble than the anthracite and therefore were not included in Figure 3. Only the lignitic and subbituminous coals oxidized a t 150' C. were over 90% soluble, but the solubility of the oxidized high volatile A coal was up to 78%. Above 2 0 0 ~C. of the oxidized bituminous coals rose slightly with the exception of the medium volatile No. 1coal and the low volatile coal a t 300"C. Although the percentagesof tended to rise at the higher temperatures, maximum yields of soluble mal, based on the origind weight of coal, were obtained a t 150' C. for the lignitic
c.
?
2
-
z
5
DISC USSION
Oxidation of the coals and carbons under the conditions used appears most frequently to be a direct function of time (Figure 1). With the lower rank cods, considerable difficulty was experienced
2521
51 2 0 0
TEMPERATURE OF OXIDATION
Figure 3. Alkali Solubilities
2528
INDUSTRIAL AND ENGINEERING CHEMISTRY
400
E 350 In a m W W U K I
5
30C
i Q
-r:
'5Y50 5? w 3 t-
y
3
200
a
0 W
I50
100
1
!m
I
I
I00 I50 TIME OF OXIDATION, HOURS
I 200
1 250
Figure 4. Equivalent Weights of Oxidized High Volatile Coal and subbituminous coals arid a t 200' C. for all the higher rank coals excepting the anthrwite. These rcsults were obtained by comparing the data in Figure 3 with that in Figure 2. The results are given in Table 11. The maximum over-all yields of soluble coal were 73.5 and 59.5% for the lignitic and subbituminous coals a t 150' C., respectively, and 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. Consequently, for the manufacture of carboxylic acids from coal, a variety of bituminous rank coals will give over 90% yields of alkali-soluble acids. Precipitation with dilute acid yields humic acidlike products amounting t o 80 to 85% of tha original coal. These results also indicate that the best choice of coal would be one 600 of low volatile rank. However, it is very -I likely that improved yields could be ob4tained from sperific coals by raising or 2- 5oolowering the temperature from,200° C. B s and by shortening the time of oxidation, If this is correct, it is possible that higher g4Ooyields may be obtained from coals of low 3 rank rather than low volatility. Without k doubt improved yields would also be oby g30'3tained from finely pulverized coal using 0 a fluidized technique. The solubility of the oxidized coals is primarily due to the formation of care00 boxyl groups. The low solubility of the oxidized anthracite is probably due to a I low ratio of acid groups to molecular IO0
Vol. 42, No. 12
equivalent weight of the oxidized coals, determined electrometrically, was characteristic of the rank of the coal and the temperature of oxidation. I n general, the equivalent weights fell rapidly during the initial reactive stages and then leveled off as the main part of the cod molecules was being oxidized. A typical pxample is shown in Figure 4 where the results obtained from the high volatile A coal are given. The equivalent weights obtained from the coal oxidized a t 150" C. did not fall as low as those a t the higher temperatures. This accounts for the loivc~ solubility in alkali observed for this coal (78%) when oxidized a t 150" C. At 200°, 250°, and 300" C. the equivalent weights fell to a surprisingly constant level which accounts for the uniformly high alkali solubilities observed a t these temperatures. At 300' C. the ourve passed through a minimum. This was observed with other coals a t high oxidation temperatures where the oxidation was complete except for a small residue (lignite and medium volatile No. 1) and emphasizes the necessity o'f controlling the oxidations, either to obtain the maximum yield of alkali-soluble coal or the minimum equivalent weight. A comparison of the average equivalpnt weights from the various coals at the end of the oxidations is given in Figure 5. The convergence of the equivalent weights from all ranks of coal except anthracite and at oxidation temperatures of 200" to 300' C. is very significant. A t 200' C. the range is 185 to 236, and a t 250' C. i t is 164 to 194, excepting for the lignite a t the latter temperature which is out of line because the oxidation was continued until only 15% of the original weight of coal substance remained. The shape of the anthracite curve suggests that osidation at slightly higher temperatures than 350' C. might lower the equivalent weight to the level of the bituminous coals and thereby produce high alkali solubility. However, i t is doubtful whrther this would yield an alkali-soluble product since there n a s little in-
TABLE11. MAXIMUM YIELDSOF SOLUBLE ACIDS Temp. of Maximum Yield,
Coal Lignite Subbituminous High volatile A Medium volatile KO.1 Medium volatile No. 2 Low volatile Anthracite
150 150 200 200 200 200 250
I
I 200.
I
I 250'
Maximum Yield, % 73.5 59.5 90.2 87.5 92.7 96.5 7 ,5
-
150'
C.
I
-043
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1950
so 2 0 m
10
I 200-
I
250.
I
I
300'
350.
COAL
to W L
5?
tl k= Y 6 Q
/
P
IO
0
ORIOINAL
I 150.
1 200.
I
I
I
250.
300'
SO'
COAL
f: 4
2529
The behavior of the coal8 on oxidation with air is significant in respect to their structure. The initial reactive stage, exhibited by all coals below anthracite, but with decreasing intensity as the rank of the coals increases, is due to the oxidation of strurtures which on carbonization yield tar and hydrocarbon gas ( I ) , whereas the slower but steady oxidation which follows is the oxidation of the nuclei or oxidation-resistant portion of the coal molecules. The complete destruction of the structures whirh yield tar on carbonization might be expected from the work of Donnelly, Foott, and Reilly (6) who found that tar yields decreased with oxidation of the coal. Because coals up to and including the low volatile rank yield humic acidlike oxidation products (alkali-soluble, acid-insoluble) and have similar oxidation curves, the similar equivalent weights and analyses of the oxidation products suggest a marked similarity in the constitution of the nuclei of these coals. The failure of the equivalent weights to continue to fall as oxidation was continued indicates that the nuclei of these humic acids are resistant to further oxidation and that when a nucleus was oxidiaed it was completely broken down to carbon dioxide and water. This was observed particularlywith the mediumvolatile No. 1coal whichwasoxidized to 14% of the original weight of coal substance with no apparent drop in the equivalent weight. On continuing the oxidation at 325' C. to speed up the rate of oxidation, i t waa oxidized within 1% of the ash content. At no time were crystals of benzene oarboxylic acids sublimed out of the oxidation zone t~ cooler portions of the tube as observed by Fischer, Peters, and Cremer (7) or as obtained with aqueous oxidizing reagents (8, 3, 8, 11, 13, 16). These results are particularly signifirant and are being investigated in greater detail. The similarity in the behavior of anthracite, carbon black, and lampblack emphasizes the more graphitic structure of anthracite shown previously by Turner and Anderson (16)from x-ray studies. Graphite is probably still less reactive because of more complete graphitization including the elimination of hydrogen, oxygen, nitrogen, and othrr atoms.
k3
82 I
I
ORIOINAL COAL
Figure 6.
160°
I
I
200e 2500 OXIDATION TEMPERATURE
ACKNOWLEDGMENT
I
300'
I
350.
Analysis of Coals and Oxidized Coals
crease in the solubility of the coal oxidized a t 350' compared with 300" C., although there was a large drop in the equivalent weight. Since only 26% of the coal remained a t 350' C., oxidations a t higher temperatures were not investigated. The degree of oxidation of the coals a t the various oxidizing temperatures is shown in Figure 6 in which the oxygen percentages are calculated as the balance between tho sum of the carbon and hydrogen percentages and 100yo. There is a marked tendency for each of the three values to converge and level off a t oxidation temperatures above 200' C. This corresponds to the region of maximum alkali solubility shown in Figure 3 and minimum equivalent weight, Figure 5. For the range of coals excluding the anthracite, the carbon percentages fell within 65 to 71 %, oxygen within 27 to 33%, and hydrogen about 2%. This is close to tho amount of oxidation that was obtained with concentrated nitric acid (5),for example. Likewise the alkali solubility and size of the average equivalent weights (1%) are about the same. Figure 6 shows that the lignite at 250' C. has less oxygen than a t 200" or even 150' C.; this probably accounts for the high equivalent weight observed a t 250' C. The oxidation of the anthracite showed a sharp drop in the equivalent weight a t these temperatures (Figure 5). At 350' C. the equivalent weight had fallen to 239 and, assuming that the acid character was due to carboxyl groups, 56y0 of the oxygen in the oxidized coal was in this form. With the lower rank coals 59 to 7Oy0of the oxygen was in the formof carboxyl groups.
The authors are indebted to Carl Neubling and H. T. Darby, analysts in the Mineral Industries Experiment Station, for the ultimate and proximate analyses reported in this investigation. The authors are also indebted to the donors of the coal and carbon samples ueed. LlTERATURE CITED
(1) Ahmad and Kinney, J . A m . Chem.Soc., 72,556 (1950). (2) Ibid., p. 559. (3) Bone, Parsons, Sapiro, and Groocock, Proc. Rov. SOC.( L m d o n ) , 148A,492 (1935). (4) B r o w and Collett, Fuel, 17, 356 (1938). (5) Charmbury, Eckerd, LaTorre, and Kinney, J . A m . Chem. SOC., 67,625 (1945). (6) Donnelly, Foott, and Reilly, J . Chew. Soc. Ind., 48, lOlT (1929). (7) Fischer, Peters, and Cremer, Brennstof-Chem., 14, 184 (1933). ( 8 ) Francis and Wheeler, J . Chem. Soc., 127, 112 (1925). (9) Francis and Wheeler, Safety in Mines Research Board, Paper No. 28 (1926). (10) Fuchs, Polansky, and Sandhoff, IND.ENG.CHEM.,35, 343 (1 943). (11) Fuchs and Stengel, Ann., 478, 267 (1930). (12) Herbert, Charmbury, and Kinney, Fuel, 27, 168 (1948). (13) Juettner, Smith, and Howard, J . A m . Chem. SOC.,59,236 (1937). (14) Lowry, "The Chemistry of Coal Utilization," Vol. I, p. 346, New York, John Wiley & Sons, 1945. (15) Smith, Tomarelli, and Howard, J . A m . Chem. SOC.,61, 2398 (1939). (16) Turner and Anderson, IND. ENG.CHEM.,23, 811 (1931). (17) Yohe and Harman, Trans., IZLinois State Acad. Sci., 32, No. 2, 134 (1939). RECEIVED March 7, 1850. Presented before the Divioion of Gas and Fuel CHEMICAL SOCIETY, St. Chemistry at the 114th Meeting of the AMERICAN Louie, Mo.
