I
G. BEDJAI, H. K. ORBACH, and F. C. RIESENFELD Research Division, The Fluor Corp., Ltd., Whittier, Calif;
Reaction of Nitric Oxide with Activated Carbon and Hydrogen Concentrations of nitric oxide of the order of 5000 p.p.m. may be quantitatively removed from an inert gas b y reaction with activated carbon at 700' C. and a space velocity of 1500 hr.-l, and with hydrogen on a carbon catalyst at about 600' C. and a space velocity of 1500 hr-l. The exact temperature at which hydrogen will react quantitatively with nitric oxide is dependent on the hydrogen-nitric oxide ratio and the surface area of the carbon. The activity of a given carbon decreases with time, eventually stabilizing, probably through formation of a layer of carbon-oxygen complex on the surface of the carbon. The use of hydrogen greatly decreases the consumption of carbon. The beneficial effect of hydrogen is thought to be due to its reaction with the carbonoxygen surface complex. Evidence of nitrous oxide in the reaction products at low temperatures indicates a more complex mechanism than heretofore assumed. The reactions have value in reducing nitric oxide on a commercial scale in certain industriaI instaIIations.
H
AACEN-SMIT has advanced the theory ( 2 ) that the active ingredient in smog which causes eye smarting and crop damage is formed from pollutants in the atmosphere under the influence of sunlight. Leighton and Perkins ( 3 ) have
shown theoretically that nitrogen dioxide is the only substance capable of absorbing solar energy in sufficient quantity to act as a catalyst for the reaction. The ability of nitrogen dioxide to act in that role has been verified experimentally by irradiating air containing common hydrocarbon pollutants with and without nitrogen dioxide ( 2 ) . When nitrogen dioxide was present, a typical smog resulted. A mechanism for the complete series of reactions
leading to a likely smog substance has been proposed (5). Elimination of nitrogen oxides from the atmosphere should therefore be an effective means of smog abatement. Nitric oxide is known to be formed when a mixture of nitrogen and oxygen is subjected to a high temperature, followed by rapid cooling. This occurs in almost all combustion processes such as internal combustion engines, heat generation plants, and steam power
1oc
ao
0 450
500
550
600
650
700
750
TEMPERATURE "C.
Figure 1. Essentially all nitric acid is removed at 700' C. and space velocity, of 1500 hr.-' VOL. 50, NO. 8
AUGUST 1958
1 1 65
100
combustion process exhaust gases. It was felt that the presence of hydrogen in the gas should also be investigated, because of the expectation of higher reaction rates and lower carbon consumption.
90
Pertinent literature Ref. Chemisorption occurs in place of simple adsorption Removal of nitric oxide from coke-oven gases by activated carbon Reduction of nitric oxide by carbon activated by inorganic salts Nitrous oxide is less reactive than nitric oxide at lower temperatures and thermally decomposed at higher temperatures Catalytic hydrogenation of nitric oxide By nickel By iron By platinum
80
/
70
0
4.2
Conc. NO Vol. a/.. 0.22
4.3
0.16
(9, 1 1 ) (6, 7 )
(8)
(4,12, I S )
(10)
(14) (11
Exper imenta I
I
60
S.V. 1400-1600 Hr.-' Carbon C
I 50 500
450
600
550
650
750
700
TEMPERATURE "C.
Figure 2.
Hydrogen appreciably increases the rate of nitric oxide removal
plants. Nitrogen dioxide is the stable form a t low temperature, however, and nitric oxide is slowly converted to this compound in the atmosphere by reaction with oxygen. The investigations described are preliminary studies aimed a t discovering means of removing nitric oxide from combustion process exhaust gases by chemical reaction. Specifically, the discussion includes reduction of
Temp., O
c.
25-375a 375 485 540 5956 595b 650b 675 705b a
Heating.
1 166
nitric oxide by activated carbon and by hydrogen in the presence of activated carbon. No quantitative data were available on the rate of reaction of nitric oxide with commercially available high surface area carbons in a dynamic system. This work was initiated to obtain that information under conditions of concentration and temperature common to
Table I. Reaction of Nitric Oxide with Carbon C (Feed gas composition, % by vol. He, 99.47; NO, 0.53) Removal b f NO is 99% complete a t 705' C. Space Velocity, Volume yo in Exit Gas Hr.-1 co NZO coz NO
-
1490 1600 1530 1540 1440 1520 1535 1540 1570
0
0 0.003 0.007 0.043 0.044 0.054 0.066 0 * 091
Average values.
0.0230 0.0009 0 0 0 0 0
0 0
0.123 0.0026 0.013 0.045 0.139 0.188 0.334 0.259 0.265
K O
Removal
0.379 0.530 0.463 0.347 0.198 0.197 0.040 0.026 0.005
Gas saturated with water at room temperature.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
28.5
0 12.7 34.4 62.0 63 .Oc 92.5 95.0 99.0
The reaction chamber consisted of a quartz tube 1 inch in diameter in an electrically heated combustion tube furnace. The tube was held vertically with the gas flow downward. The temperature was measured by a thermocouple and controlled by a Powerstat. Ten grams of granular activated carbon were supported in the ccnter of the furnace by a plug of granular silica gel. The gas flow was measured on a wettest meter and reduced to dry standard conditions (0' C. and 760 mmi of mercury). Space velocities are calculated as volumes of gas a t standard conditions per bulk volume of carbon per hour. Various commercially available activated carbons were used. The nitric oxide was obtained from the Matheson Co.! Inc., with a nominal purity of 98 to 99%. Infrared analyses indicated approximately lyO nitrogen dioxide. Tiitrogen was obtained from the Kational Cylinder Gas Co., and was the food processing grade. Mass spectrometer analyses showed: nitrogen and argon, 99.370, oxygen 0.0037c, and hydrogen 0.70y0 by volume. The helium was from Air Reduction Co., Inc., and was essentially pure. All analyses on the actual runs were by infrared absorption spectroscopy. Samples were taken into a I-meter path length cell and analyzed in a PerkinElmer Model 21 infrared spectrophotometer. The analysis for nitric oxide is estimated to be reliable to 2k40 p.p.m. Amounts as low as 40 p.p.m. were easily detected. The analysis for nitrous oxide is estimated to be reliable to 4 p.p,rn., for carbon dioxide 10 p.p,m., and for carbon monoxide 40 p.p.m. All concentrations are given in parts per million by volume. T h e carbon was heated in an inert
atmosphere while being brought to temperature, The gas mixture was then passed over the catbon until the activity of the carbon remained constant. Samples of the outlet gas were taken for analysis at 1-hour intervals.
100
80
Reaction of Nitric Oxide with Carbon The results of a series of tests on the reaction of nitric oxide in helium with activated carbon (Table I and Figure 1) indicate that the reaction starts at about 450' C., and essentially all of the nitric oxide is removed a t 700' C. and space velocity of 1500 hr.? Saturation of the gas with water prior to entering the reaction zone has little effect on the rate of nitric oxide reaction. However, a small amount of reaction of water with the carbon is evidenced by increase in carbon dioxide in the exit gas.
Reaction of Nitric Oxide with Carbon in Presence of Hydrogen
Discussion Most previous workers have interpreted the reaction of nitric oxide with carbon as starting with chemisorption, probably due to the unpaired electron in nitric oxide. The next 'step is a release of nitrogen, with the oxygen remaining on the surface as a carbonoxygen complex:
+ C +. (C-0) + l/zNz
(1)
Shah suggests that it may be the complex itself which reacts with nitric oxide as the reaction progresses and the surface becomes saturated with complex: NO
+ (C-0)
-+ COz f l/zlNz
60
p: Lu
0
5
x
5
Surface Area
Sq. M / g
40
!-
3 %in.
The results of a series of tests designed to evaluate the effect of hydrogen on the reaction are presented in Table I1 and Figures 2 and 3. Hydrogen increases the rate of nitric oxide removal appreciably. Increasing the hydrogen-nitric oxide ratio (Figure 2) and the surface area of the carbon (Figure 3) is beneficial. Nitric oxide can be completely removed a t 600' C. and a space velocity of 1400 to 1600 hr.-I With an expensive highsurface-area carbon removal can be accomplished a t temperatures as low as 540' C. Measurements of loss in weight of the carbon as well as analyses for carbon oxides in the exit gas indicate that the use of hydrogen greatly decreases the consumption of carbon, as predicted.
NO
5
(2)
In a static system the gas in equilibrium with the carbon contains nitrogen, carbon monoxide, and carbon dioxide a t a lower total pressure than the original
+
20
low
NO 0.22%, HZ 0.90% S.V. 1400-1500 Hr.-J 0 400
450
500
550
650
600
700
TEMPERATURE "C. Figure 3.
Increasing the surface area of the carbon is beneficial
nitric oxide. Some of the oxygen, therefore, remains on the surface as a complex. According to these theories a steady state might be achieved in a dynamic system with part of the reaction occurring by Equation 1 and part by Equation 2, for Equation 2 releases a site for Reaction 1 to occur. I t might also be argued, however, that the reaction occurs only by Equation 1, with the surface being freed for further reaction by simply having the oxides swept off. I n the present work carbon oxides were carried off the surface of a saturated carbon when pure helium was swept over it. Table I11 shows that a considerable period was required to attain equilibrium, during which time the reactivity of the carbon dropped appreciably. At higher temperatures equilibrium was attained more quickly. This decrease in activity supports the assumption that the surface is being altered. If a mixture of hydrogen in nitrogen remains in contact with the heated carbon overnight, a -deactivated surface can be
restored to its original activity. If oxygen is fixed on the surface-and there is considerable evidence that it isthese observations would lead to the conclusion that the oxided surface reacts much less readily with nitric oxide than does carbon itself. This assumption would explain the effect of hydrogen on the reaction as a means of keeping the surface free of oxygen and thus more reactive. Hydrogen does not react directly with nitric oxide except a t much higher temperatures. Contact surface alone is not sufficient, for the authors failed to obtain any appreciable reaction when the carbon was replaced with silica gel. The mechanism is more complicated than originally supposed, however, for at low temperatures nitrous oxide is found in the exit gas. This would suggest that nitrous oxide, which would be immediately decomposed on carbon a t the higher temperatures (70),is an intermediate in the mechanism. This offers the intriguing possibility of a termolecular collision between two VOL. 50, NO. 8
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AUGUST 1958
1167
Table 11.
Reaction of Nitric Oxide with Hydrogen
Presence of hydrogen increases rate of NO removal
Space Velocity,
Temp., O
c.
Volume yo in Exit Gas GO2
co
Hr.-1
NO
% No Removal
0,093 0.097 0.046 0.037 0.029 0.000 0.000 0.013
55.0 55.4b 78.8 83.0 86.6 100.0 100.0 93.9
0.046 0.0
71.0 100.0
0.322 0.052
50.5 92.3
0.03 0* 003
84.7 100.0
Carbon C, Gas Mixture B 485Q 485” 545 545 545 595” 595 595
1650 1570 1840 1570 1430 1940 2550 3170
540a 625
1490 1620
540a 625
1585 1580
540 595
1460 1430
literature Cited
0.009 0.010 0.007 0.007 0.007 0 * 008 0.011 0.008
Carbon C, Gas Mixture C 0 0
0.006 0.016
Carbon C, Gas Mixture A 0.014
(1) Andrusov, L., Ber. 60, 536 (1927). (2) Haagen-Smit, A. J., Bradley, C. E., Fox, M. M., IND. END. CHEM. 45,2086 (1953). (3) Leighton, P. A,, Perkins, W. A , , Air Pollution Found. Refit. No. 14, (March 1956). (4) Madely, D. G., Strickland-Constable, R. F., Trans. Faraday SOC. 4 9 , 1312 (1953’1. RLnze&, N. A., Romanovsky, J. C., (5) J . Air. Pollution, Control Assoc. 6 , 154 (1956). Riese, W., Brennstoff-Chem. 20, 301 , _ ^ ^ ^ I
0.047 0.079
0.047
higher space velocities. Such a process would be practical only in a n industrial installation where the heat value of the natural gas consumed could be recovered.
(lY5Y).
Carbon B, Gas Mixture B
... ...
0.013 0.022
(10)
Carbon A, Gas Mixture B 425 485 545
... ...
1580 1540 1620
0.113 0.062 0.000
47.8
71.3 100.0
Feed Gas Composition, Val. %
Gas Mixture A
a
0.032 0.032 0.020
I . .
B C Average values.
N2
H 2
KO
98.62 98.88 99.14
0.70 0.90 0.70
0.68 0.22 0.16
* Gas saturated with water at room temperature.
Table Ill. Activity Decreases with Time (Feed gas composition, 1701. %, Kz, 99.88; Hz,0.90; NO, 0.22. Temperature, 485’ C.)
Time,
Space Velocity,
Hr.
Hr.-I
coz
Volume % in Exit Gas NO
%so Removal
1490 1490 1480 1700 1700 1600
0.008 0.010 0.010 0.009 0,009 0.008
0.017 0.046 0.088 0.093 0.093 0.093
92.1 78.8 59.4 56.8 55.0 55.0
molecules of nitric oxide (or N202) and carbon. Termolecular collisions involving nitric oxide and chlorine, bromine, oxygen, and hydrogen have been described ( 3 ) . The present work is a preliminary investigation and as such was not designed to give information on the exact mechanism. Any attempts to assign a mechanism on the basis of these experiments would be presumptuous. Interpretation of over-all results from experiments such as these must proceed with caution, especially when attempting to predict the results in a complex system such as is found in a n exhaust gas. The equilibrium existing at the surface of the carbon may be sensitive to the presence of the other exhaust constituents. Further work is needed to
1 168
establish the effect of these other substances. A major problem in attempting to use activated carbon for removing nitric oxide from exhaust gases is the fact that most of these gases contain oxygen. T h e oxygen will readily oxidize the carbon a t 600’ C.; therrfore, some means of oxygen removal must be provided. One method in industrial use is to inject natural gas into the exhaust and use up the oxygen by combustion. This reaction, if carried out under the proper conditions, will produce hydrogen a t the same time. This scheme would be beneficial in two ways: (1) Hydrogen would be produced to aid the speed of reaction and decrease consumption of carbon, and (2) the temperature of the gas would be raised, allowing
INDUSTRIAL AND ENGINEERING CHEMISTRY
(9)
(11) (12) (13)
(14)
Zbid., 25, 25 (1940). Riesz, C. H., Morritz, F. L., Franson, K. D., Air Pollution Found. Refit. No. 20 (May 1957). Shah. M. S.. J . Chem. SOC.1929. 2661. 2676.’ Shapleigh, J. H., U. S. Patent 2,381,699 (Aug. 7, 1945). Smith, R. N., Lesnini, D., Mooi, J., J.Phys. Chem. 60, 1063 (1956). Zbid., 61,81 (1957). Strickland-Constable, R. F., Trans. 34, 1374 (1938). Faraday SOC. Usachev, P. V., Ilinskaya, 0. V., Russian Patent 53,543 (July 31, 1938). RECEIVED for review August 15, 1957 ACC~PTED January 29, 1958
Divisions of Industrial and Engineering and Water, Sewage, and Sanitation Chemistry, Symposium on Air Pollution, 132nd Meeting, ACS, New York, PI’. Y . , September 1957.
CORRECTION Engineering Design on a Computer I n the article on “Engineering Design on a Computer” by E. J. Higgins, J. W. Kellett, and L . T. Ung [IND.ENG. CHEM.50, 712 (1958)], the following corrections should be made: O n page 726, in the box on Furnace Coil Procedure, this heading and statement should follow the third item or listing: CALCULATE Outlet conversion by rrial and error. O n page 717, Table I, item 6, the second portion of the second line should read : If the shell side resistance is less than O n page 718, the title of the first box should be : How the Cost Estimation Program Operates
