Low-Temperature Oxidation of Carbon W. K. LEWIS, E. R. GILLILAND, AND R . R. PAXTON’ Massachusetts Znstitute of Technology , Cambridge, Mass.
T
HE carbon-oxygen reaction is highly exothermic and the
principal experimental difficulty of earlier investigators has been that of obtaining a uniform temperature in the reaction zone. Lewis (7‘) has suggested a fluidized bed of powdered carbon to minimize localized overheating. This technique has been successfully used by Lewis, Gilliland, and McBride (8) to study the kinetics of carbon dioxide reduction by carbon. The present investigation (9) is a continuation of this experimental program. EXPERIMENTAL TECHNIQUE
APPARATUS.The apparatus used in this work is shown in Figure 1. The reactor was similar to that described by Lewis, Gilliland, and McBride ( 8 ) , a Type 310 alloy steel tube 10 feet high and 1.78 inches in inside diameter. It was electrically heated and equipped with sealed thermocouple wells reaching into the axis of tube. Gases entering the reactor were passed through their individually calibrated glass capillary “orifices,” into an unheated mixing manifold and thence to the gas injection tube a t the base of the reactor. Gases leaving the reactor passed in turn through an unheated cyclone, a filter packed with glass wool, an orifice, and finally the exit manifold where a sample tap was located. Three %arboris” were used in this investigation: PROCEDURE. hardwood charcoal, metallurgical coke, and Ceylon graphite. The coke and charcoal were purchased as 2-inch lumps, which were crushed, screened, and then blended to the desired size distribution. The graphite, purchased from the Joseph Dixon Crucible Co. as Ceylon flake graphite No. 1, was suitable for use as received.
Oxygen was admitted when the desired bed temperatures were achieved. It was then necessary to cut back the heating power, as the problem became that of removing heat rather than adding heat. Power supply to the reactor above the dense bed was also cut back, so that no part of the equipment would be hotter than the reacting bed. The total pressure in the reactor was constant, averaging 1.1 atmospheres. Oxygen partial pressure at the inlet was controlled by dilution, using nitrogen or carbon dioxide. The total volume of gas injection was adjusted to give an internal gas velocity of 0.5 foot per second based on an empty tube at the reaction temperature and pressure. This velocity was nearly three times that needed to fluidize the bed. It produced turbulent solid mixing in the bed with a solids loss from the “boiling” fluidized bed of from 1 to 2% per hour. With graphite, however, a gas velocity of 0.2 foot per second was used. To ascertain the effect of gas velocity upon reaction rates and products for coke, a few data were also obtained a t gas velocities varging from 0.05 to 1.0 foot per second.
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F I SEPARATOR
Table I lists properties of composite samples of the virgin carbons. The same batch of coke was often used for several runs, but the cumulative carbon burnoff never exceeded 33%.
OF CARBONS USED TABLE I. PROPERTIES
Hardwood Charcoal
Total Ultimate analysis, (samples dried at 105’ C.) Carbon Hydrogen Nitrogen Sulfur Residue (ash) Oxygen (by difference) Total Bulk density, , g . / c c Screen analysis, %
1.71 21.4 74.95 1.94 100.00
0.17 2.56 87.93 9.34 100.00
82.39 3.69 0.34 0.08 2.41 11.09 100.00 0.46
87.46 0.50 0.92 0.76 9.91 0.46 100.00 0.98
... , . .
95 5
t
100
I I Figure 1. Experimental Apparatus
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++ 80 100 -100 + 140 -140 + 200 - 200 -60 -80
100.0
100.0
... ... ... ... ... ...
1.24 0.1 0.1 6.7 15.1 38.9 18.4 20.7 100.0
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Prior to dumping in a coke charge, the reactor was preheated and purged with nitrogen. The bed of coke, fluidized with nitrogen, was then brought to the desired temperature level by the external heating coils. This nitrogen pretreatment commonly lasted 1.5 hours, during which time small amounts of water, carbon dioxide, and carbon monoxide were evolved from the bed. 1
EFFLUENT SAMPLE
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+-4040 + 60 Total
I
Netallurgi cal Coke Graphite
Present address, Stanford University, Calif.
ANALYTICAL METHODS AND CALCULATIOSS. Relative calibration of the inlet orifices was occasionally checked by analyzing a sample of inlet gas. Samples of the effluent gas were analyzed for carbon dioxide, carbon monoxide, oxygen, hydrogen, and occasionally methane, using a Fisher precision gas analysis a p paratus. (No more than traces of methane and hydrogen were ever found.) Rate of carbon gasification a a s computed from the exit gas analysis and the known rate of nitrogen or oxygen input. The exit orifice reading permitted one to compare the output of nitrogen and oxygen (as oxygen or as carbon oxides) with the input. Output was within &lo% of input for 93% of the gas analyses. Table I1 is a specimen sheet of tabulated data covering 3 of the 30 runs. A similar material balance waB made for carbon. Carbon input was taken as the weight of charge minus the ashcontained therein. At the end of a run, the bed was withdrawn from the reactor, weighed, and analyzed for ash content. To the carbon content of this residue, calculated as above, was added the carbon carried out in the gas stream as carbon oxides plus the carbon content of
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C A R B O N TEMPERATURE,
Figure 2.
Vol. 46, No. 6
' F.
Effect of Carbon Temperature on Fraction of Carbon Gasified to Produce Carbon &Ionoxide Dotted lines are for equilibrium C
the solids recovered from the cyclone and filter. Carbon output was within 5% of input for 28 out of 30 runs. In calculating the fraction of carbon in the bed gasified per dW minute, - -, it is necessary to know the actual carbon content It'd8 of the bed at the time the carbon gasification rate was measured. The value of TV was taken as the carbon content of the charge less the carbon estimated to have been carIied out with the gas up to the time of the rate measurement. As carbon removal from the bed seldom exceeded 10% for a whole run, estimation of W was accurate. Some data were taken using carbon diovide rather than nitrogen as a diluent. The carbon gasification rate was taken as the excess of carbon (as oxide) leaving over that entering the reactor. The carbon dioxide value for the net CO/(CO CO,) ratio of the product gas was similarly adjusted for the carbon dioxide content of the feed gas. OPERATIOX OF APPARATUS.Operation of the unit was generally satisfactory. At low inlet oxygen concentrations, it was always possible to maintain seven of the eight thermocouples in the reaction zone at a temperature within 4' F. of their average. Under these conditions the bottom thermocouple-only 2 inches above the gas inlet-sometimes indicated a temperature as much as 10" F. lower than the average of the other seven. Most of the data were obtained with temperature variations in the reacting bed less than these. The reacting fluidized bed with internal temperature variations equal to or less than those described above is hereafter referred t o as an "isothermal" bed. With this apparatus there was, for any given reactor temperature, a limiting rate of heat dissipation. When heat generation exceeded this limit, the temperature of the reactoi and its contents started t o rise. An isothermal bed could be maintained for a while with the temperature of the bed as a n-hole rising. As the bed temperature rose, however, heat release became more localized near the oxygen inlet. A temperature was reached a t which solid mixing was unable to carry upward all the heat liberated a t the bottom of the bed. Isothermal conditions could no longer be maintained. The temperature of the bottom portion of the bed rose faster and faster. Unless oxygen concentration was dropped back to 25% or less, the bottom of the reactor would be burned away. With pure (99%) oxygen feed, the highest temperature at which an isothermal bed could be maintained was 340", goo", and 1100" F. for charcoal, coke, and graphite, respectively. At lower oxygen concentrations in the feed, iso-
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thermal conditions could be held at considerably higher bed temperatures. All the data herein reported xvere obtained under isothermal conditions, COMPOSITION OF COMBUSTION PRODUCTS
Searly every sample of effluent gas from the "burning" fluidized carbon bed contained oxygen, carbon dioxide, and carbon monoxide. Production of carbon monoxide appeared to be related to production of carbon dioxide. Figure 2 shows the fraction of carbon gasified to produce carbon monoxide-viz., CO/(CO i- CQ+plotted against reaction temperature. The most precise data were obtained on coke at temperatures COz)ratio averaged between 700" and 900" F. The CO/(CO 0.24 in this temperature range. Between 291 O and 700 F., the points scatter badly from a low of 0.09 to a high of 0.62. Most of these data were obtained on charcoal which contained 2.30 weight % ' of net hydrogen. (Net hydrogen is hydrogen in excess of that required to tie up the oxygen, determined by an ultimate analysis, as water.) The few points on coke in this region had poor precision, as the carbon monoxide content of the off gas was less than 0.7%. The apparent upward trend in the CO/(CO COS) ratio with decreasing temperature must be viewed in the light of these experimental uncertainties. A t temperaturea above 1010" F. the CO/(CO CQ1) ratio for coke drops sharply. With graphite this ratio appears to start
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041
02
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I O
LOG M E A N A V E R A G E O X Y G E N P A R T I A L PRESSURE, A T M .
Figure 3. Effect of Oxygen Partial Pressure on Fraction of Carbon Gasified to Produce Carbon Monoxide Coke, 800' to 1O1Oo F.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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declining at a somewhat lower temperature, although here again is the problem of poor precision owing to low carbon monoxide concentrations in the off gas. Obviously a t high temperatures secondary carbon monoxide oxidation by residual oxygen precent in the product gas is bound to reduce the CO/(CO COZ) ratio found in the product gas. From the rather limited data it cannot be determined whether the decline in the CO/(CO COZ) ratio a t higher temperatures is due to secondary carbon (monoxide oxidation or to a change in the primary reaction mechanism. Figure 3 shows the fraction of carbon gasified to produce carbon monoxide plotted gainst the average oxygen partial pressure in the reaction zone. The average oxygen partial pressure was varied by diluting the feed gas with nitrogen or carbon dioxide. There is a definite trend toward higher values of this ratio a t the lower oxygen partial pressures. These data throw important light on the question of the primary products of the reaction between carbon and oxygen. Earlier investigations in this temperature range were all carried out using fixed, stationary beds of carbon (1, 2, 4 , 6, 10). Because of the low thermal conductivity of such beds it has been recognized that there may have been localized zones of high temperature in the beds. Secondary rpactions, either the oxidation of carbon monoxide by residual oxygen or the reduction of carbon dioxide by hot carbon, could occur in such zones. In the runs plotted in Figure 2, the temperatures indicated by the thermocouples immersed in the bed of fluidized carbon particles never differed in any one run by more than 14' F. Fluidization conditions are felt to have been sufficiently good so that heat distribution over the relatively short distances between thermocouples was excellent. In other words, it seems unlikely that significant secondary reactions caused by local temperature variations in the bed could have occurred. This conjecture was tested by replacing the diluent nitrogen in the gas entering the reactor by carbon dioxide in eight of the runs plotted in Figure 2. The COz) ratios, the resulting CO/(CO carbon dioxide in the ratio being that over and above what was added to the inlet gas, averaged 0.27. Had the carbon monoxide been formed by reduction of carbon dioxide rather than being a primary product of the oxidation of carbon itself, the injection of carbon dioxide in the entering gas would have been expected markedly to increase carbon monoxide formation, and consequently the value of this ratio. But the CO/( CO Cog) ratio of the carbon that was
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N O M I N A L G A S V E L O C I T Y IN R E A C T O R , F O O T PER S E C O N D
Figure 4.
Effect of Gas Velocity on Fraction of Carbon Gasified to Produce Carbon Monoxide Coke, 716' to 890' F. L O G M E A N A V E R A G E O X Y G E N P A R T I A L PRESSURE, A T M .
gasified was not significantly increased by the large increase in carbon dioxide partial pressure throughout the coke bed. (This substitution had no effect upon the rate of gasification either.) If, on the other hand, carbon dioxide were to be formed by carbon monoxide oxidation, one would expect the CO/(CO Cog) ratio in the product gas to be affected by the residence time of the oxygen-carbon monoxide-carbon dioxide mixture in the reactor. Figure 4 shows that a tenfold change in gas velocity had no effect upon the fraction of carbon gasified t o produce carbon monoxide. Loiv bed temperatures were used for the low velocity points, so that the oxygen concentration would not fall off unduly. Two more interesting facts in connection Kith the off-gas composition are that a 100-fold variation in the rate of carbon gasification produced no significant change in the CO/(CO COz) ratio, and the difference between the gas composition and that for the carbon-carbon monoxide-carbon dioxide equilibrium did not influence this ratio. The equilibrium values of the CO/(CO CO,) ratio, shown as a dotted line on Figure 2, are generally below the experimentally found values. [The equilibrium values were calculated for carbon in the form of graphite. Carbon in the form of charcoal probably has a somewhat higher free energy and consequently would give a dightly higher equilibrium CO/( CO COZ) ratio.] The conclusion seems inescapable that both carbon monoxide and carbon dioxide are primary products of the oxidation of carbon. The ratio of their generation seems t o be relatively independent of the carbon type and of the temperature level between 300" and 950" F. This fraction of carbon gasified a s c a r b o n monoxide, may, however, increase sharply as the oxygen 5 partial pressure becomes very ? low a t the carbon interface.
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Figure 5 .
Effect of Oxygen Partial Pressure o n Carbon Gasification Rate Coke, 950' F.
tial pressure of oxygen in any given run would be the logarithmic mean of the inlet and outlet values. Figure 5 shows that, at a given temperature, the fraction of carbon gasified per minute was proportional t o the log mean average of the inlet and outlet oxygen partial pressures. Strictly speaking, change in volume due to carbon monoxide formation makes the use of such an average inexact, but study of the data shows that the error of using this simplification is not serious for these data because of the limited amount of carbon monoxide formed and of the fact that an inert diluent gas was so generally used. Calling W the weight of carbon (not total solids) in the bed, one rrould expect that a t a given temperature - d v / d e = c w ( p 0 , )Bv.
Data are available for a number of runs carried out a t essentially the same temperature and for these runs the Feight of carbon in the reactor varied sixfold and the average oxygen pressure varied twentyfold. The C values from these runs together with the values from nearly 100 other determinations are all plotted as a function of temperature in Figure 6. The results bespeak the suitability of this equation for correlating the data over the experimental range involved The reaction rate con-
s
REACTION RATES
f
$5
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Earlier work (5, IO) has led to the conclusion that the rate of gasification of carbon by oxygen is proportional t o the oxygen partial pressure. This may be a c c e p t e d f o r t h e moment. Gilliland and Mason (3) have shown that in a fluidized bed of high dense bed depth relative to column diameter the gases rise through the bed with little vertical mixing. Were the reaction product t o be carbon dioxide alone, there would be no change in gas volume on isothermal reaction, and the effective par-
2' $g
gi .a gz
~ i c *
Qn
E2 0
5!i 6
7
8
9 10 11 R E C I P R O C A L TEMPERATURES, iO,OOO/T,
Figure 6.
' R-*
Reaction Rates of Carbon
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Tune 1954
stants (C values) for each of the three carbons are correlated by lines having the familiar Arrhenius-type formula:
C
= Ae-EIRT
where A and E are constants depending upon the carbon type. Carbon Type Hardwood charcoal Metallurgical coke Ceylon graphite
A , Fraction C Gasified/ Min. Atm. Oz Partial Pressure 1 . 2 x 104 4 . 5 x 106 8.1 X 108
E, B.t.u./Lb. Mole 28,600 52,200 88,500
Nearly nine tenths of the data lie within 2 ~ 5 0 %of the values predicted by the above equations and constants. The tendency of the data to scatter a t the lower end of each line is probably due to analytical difficulties caused by low concentrations of the reaction products in the off gas. The effect upon the reaction rate constant of gas velocity and of fraction carbon burned off was also investigated by suitable cross plots of the data. There was a considerable scatter among the points for both plots. There was, however, some indication that a fivefold increase in gas velocity halved the reaction rate constant, C. This decrease may have been caused by less efficient gas-solid contact at the higher gas flow rates. No correlation was obtained between the cumulative per cent carbon burned off and the rate constant, C.
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bon type and temperature between 300' and 950' F. The fraction of carbon gasified as carbon monoxide, CO/(CO COz), averaged 0.24. This fraction tended to rise as the oxygen partial pressure was reduced below 0.2 atmosphere. The rate of carbon gasification per unit weight was proportional to the oxygen partial pressure.
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ACKNOWLEDGMENT
The authors wish to thank the Standard Oil Development Co. for the financial support which made this work possible. LITERATURE CITED
(1) Bolland, C. B., and Cobb, J. W., J. SOC. Chem. Ind., 52, 1953-91
SUMMARY AND CONCLUSIONS
(1933). (2) Cobb, J. W., Chaleur et ind., 15, 377-86 (1934). ENG.CHEM.,41, 1191-6 (3) Gilliland, E. R., and Mason, E. A., IND. (1949). (4) Kullgren, K. F., Gas u. Wasserfuch, 67, 226-9 (1924). (5) Lambert, J. D., Trans. Faraday SOC.,32,452-62, 1584-91 (1936). (6) Letort, M., et al., J. chim. phys., 47,548-56 (1950). (7) Lewis, W. K., Chem. Eng. News,25,2815-18 (1947). (8) Lewis, W. K., Gilliland, E. R., and McBride, G. T., IND. ENG. CHEM.,41, 1213-26 (1949). (9) Paxton, R. R., Sc.D. thesis, Massachusetts Institute of Technolonv. 1949. (10) Rhead:y. F. E., and Wheeler, R. V., J. Chem. Soc., 101,84&60 (1912); 103, 461-88 (1913).
Both carbon monoxide and carbon dioxide were primary products of the low temperature oxidation of carbon. The ratio of their production was relatively independent of car-
RECEIVEDfor review October 16. 1953. ACCEPTEDFebruary 24, 1954. Presented before the Division of Gas and Fuel Chemistry, Symposium on Properties and Reactions of Carbons, a t the 124th Meeting of the AXERICAN CHEMICAL SOCIETY, Chicago, Ill.
Wastes from Potato Starch Plants TOMMY W. -4MBROSEl AND CASTLE 0. REISER2 University of Idaho, Moscow, Idaho
A
LTHOUGH many references may be found which give the approximate volume and strength of industrial wastes, such data for plants processing potato starch are not readily available. These data are helpful to public health authorities and others in estimating the approximate pollution attributable to a plant. Furthermore, if the nature of the various wastes i s known, the economic recovery of valuable materials from these wastes may follow. The wastes from two starch plants which were discharged into a stream used by the Presque Isle Air Base in Maine during World War I1 are reported to have created a serious pollution problem. U. S. Army Engineers made a study of this condition and the results were given in a report published April 4, 1945. Although copies of this report are not readily available, it covered possible methods of treating the waste pulp and protein water ( l a ) . Consideration was given to dewatering and saving the waste pulp by means of vacuum filtration and pressing. Studies on the protein water included chemical coagulation Kith and without aeration, heat coagulation, and biofiltration. A high rate biofilter for treatment of the protein water was recommended but was economically unattractive. White potato starch has been manufactured in Idaho, Maine, and to a small extent in Minnesota. Sweet potatoes have been processed for starch in some of the southern states (9). In 1949, Idaho was reported to have taken the lead in white potato starch production with a daily capacity of 192 tons (3). A 1 Present 2
address, General Eleotrio Co., Richland, Wash. Present addreas, Food Machinery and Chemical Corp., San Jose, Calif.
plant of approximately 30 tons capacity has been built a t Idaho Falls since. Most of the Idaho plants are located along the Snake River or its tributaries where the stream flow is large and other industries are small. Hence the pollution of the river by these plants does not appear too serious. This investigation was undertaken to determine the order of magnitude of the contamination and the nature of the streams wasted. DESCRIPTION OF STARCH PROCESS
Starch production in Maine by both batch and continuous settling processes has been described by Howerton and Treadv,-ay (6). In general, the Idaho plants have a larger capacity and operate with a combination batch and continuous settling process. The manufacture of starch from potatoes is mainly physical in nature. It involves the grinding of the whole potato followed by water extraction of the soluble materials. Nonstarch solids are separated from starch by screening and selective settling of water slurries. Generally, sulfur dioxide gas is added to the slurry to aid in preventing undesirable chemical and biological actions. Hypochlorite solutions may be used also to sterilize the equipment and improve the product quality. Processing conditions differ among the various plants and some operating data that have been accumulated as a result of experimentation are withheld as trade secrets. Although the composition of potatoes varies with type, age, and locality, an average composition would show approximately
