Reduction of Nitric Oxide with Ethylene on Copper-Silica Catalyst Ahmad Sotoodehnia-Korrani and Ken Nobe School of Engineering and Applied Science, University of California, Los Angeles, Calif. 90024
The reduction of nitric oxide with ethylene on a copper-silica catalyst in the temperature range of 270" to 430°C was investigated. The nitric oxide and ethylene concentrations were varied from 300 to 1500 and 500 to 1500 ppm, respectively. Nitrogen was used as the carrier gas and the gas flow rate was maintained at 400 liters (25"C,
1 atm) per hour. Both integral and differential flow reactors were used to obtain data. The results indicate that the catalytic reduction of nitric oxide with ethylene is essentially complete to COz, HzO, and Nz. An empirical kinetic expression provides a satisfactory correlation of the experimental kinetic data.
A
NUMBER of investigations on reduction of nitrogen oxides with hydrocarbons, carbon monoxide, and hydrogen on copper-containing catalysts have been reported (Ayen and Peters, 1962; Ayen and Yonebayashi, 1967; Baker and Doerr, 1964, 1965). The UCLA group has been concerned primarily with the combustion of hydrocarbons and the dissociation of nitric oxide and nitrogen dioxide on copper oxide catalysts (Accomazzo and Nobe, 1965; Sourirajan and Blumenthal, 1961a,b; Wikstrom and Nobe, 1965). However, for the dissociation of the oxides of nitrogen with carbon monoxide and hydrogen, Sourirajan and Blumenthal (1961a,b) found copper to be a more effective catalyst. The purpose of this investigation was to study the catalytic reaction of low concentrations of nitric oxide and ethylene for the complete reduction of nitric oxide, using ethylene as the reducing agent. Preliminary experiments on the reduction of nitric oxide in an inert carrier gas with alumina-supported ceric oxide catalyst showed no detectable reduction of nitric oxide even a t higher temperatures than those studied by Wikstrom (Wikstrom and Nobe, 1965) in his investigation of nitrogen dioxide dissociation on the same type of catalyst. Other preliminary experiments of the nitric oxide-ethylene reaction with CuO did not provide satisfactory kinetic data under the conditions studied in the experiments. However, for the reaction with a copper-silica catalyst, consistent and reproducible data were achieved. Both differential and integral reactor data were obtained.
Experimental
A schematic diagram of the experimental apparatus is shown in Figure 1. The flow rates of the nitrogen carrier gas, nitric oxide, and ethylene were measured with calibrated flowmeters. The gases were passed through the mixing chamber and the preheater prior to the catalyst
Figure 1. Experimental apparatus 1. 2. 3. 4.
5. 6.
Nitrogen flowmeter Ethylene flowmeter Nitric oxide flowmeter Soap bubble buret Mixing chamber Preheater
7. 8. 9. 10. 11. 12.
Reactor tube Furnace Catalyst bed Flame ionization detector Infrared analyzer Flowmeter
bed. The preheater was made of borosilicate glass tubing (80 cm in length and 1.2 cm in id.). The reactor was borosilicate glass tubing (85 cm in length and 1.5 cm in i.d.); it was equipped with two sampling outlets 20 cm apart and six thermocouple wells. The entering gas mixture was passed through 50 cm of packed glass rings to ensure further mixing. This section also served as a second preheater to minimize the temperature gradients in the bed. The catalyst was placed on top of the glass rings and the catalyst bed height for the differential reactor was about 3 cm. Glass beads (9 cm deep) were placed on top of the catalyst bed to prevent fluidization of the bed. Temperature controls were maintained by heating tapes and Variacs in both the preheater and reactor secInd. Eng. Chern. Process Des. Develop., Vol. 9, No. 3, 1970
455
tions. Four thermocouple wells were located in the catalyst bed to determine the axial and radial temperature gradients. Temperatures were measured with iron-constantan thermocouples. A 12-channel Honeywell recorder was used to record temperatures continuously. Isothermal conditions were maintained in the reactor both axially and radially within *lo C. Ethylene concentrations were determined with a flame ionization detector. NO, HzO, COZ,and CO did not affect the readings of the ethylene concentrations, which were accurate to within 1% of the reading. Nitric oxide concentrations were determined with a Beckman infrared analyzer which included a comparison cell to minimize interference by H20, COz, and CO. The analyzer was not affected by COz, CO, HzO, and ethylene oxide up to 4000, 2000. 4000, and 1000 ppm, respectively. Ethylene interferes with the nitric oxide reading, but correction for it can be made when the ethylene concentrations are known. Gas chromatography was used for detection of ethylene oxide, formaldehyde, acetaldehyde, carbon dioxide, and ethylene. A F&M Model 119 gas chromatograph with a thermal conductivity cell detector was used. The chromatographic column was a 25-foot-long copper tubing of 0.19-inch i.d. packed with 2-methylpropene trimer on Johns-Manville C-22 firebrick. I t was operated a t room temperature with a helium carrier gas flow rate of 25 ml per minute. Samples of 2 ml each were taken from the exit reactor sampling manifold with a hypodermic syringe, and injected into the column. The Nessler reagent (Snell and Snell, 1941) was used to test for the presence of ammonia in the exit gas stream. The copper-silica catalyst was prepared by impregnating silica gel with copper nitrate solution. The catalyst, which was in granular form, was heated to 4OO0C in an air stream to form CuO-SiOz (3 to 7). Two grams of the catalyst mixed with some borosilicate glass distillation helices were placed in the differential reactor. Twenty grams of a second catalyst batch (prepared separately) were placed in the integral reactor. The former catalyst had a surface area of 282 meters' per gram, pore volume of 0.73 ml per gram, and average pore radius of 52 A. The catalyst was then reduced in the reactor with hydrogen at 400°C for 12 hours to Cu-Sios. The surface area, pore volume, and average pore radius of this reduced copper catalyst were 272 meters' per gram, 0.72 ml per gram, and 52 A, respectively. The carrier gas was prepurified nitrogen (American Gas and Welding Co., 99.997% purity). The ethylene and nitric oxide were 99.5 and 98.5% minimum purity, respectively (Matheson Co.). When a run was initiated, the flow rate of the nitrogen carrier gas through the reactor was set a t 400 liters (25"C, 1 atm) per hour and the specified bed temperature was also set. Then, the ethylene was introduced and its initial concentration was set. Subsequently, nitric oxide was introduced in the gas stream and its concentration was fixed a t a specific value. This sequence was important to obtain reproducible experimental data and catalyst activity. After setting the initial concentrations of nitric oxide and ethylene, the analyzers monitored the output concentrations until steady-state conditions were achieved. The output and the input concentrations were then measured and recorded. 456
Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 3, 1970
-
80
z 0 i-
0 3
70-
60-
n W
'
[L S O -
40-
I-
@
30-
E
a
20-
-
IO
0
Figure 2. Catalytic reduction of nitric oxide with ethylene in integral reactor 1:1 Initial concentrations of NO and CZHI, ppm
PN:PE
PN
PE
0
200
200
0
500
500
Ail000
1000 1500
0 1500 IO0
P
90 -
80
-
z
0
70-
I-
o 3 0
60-
#
0
z 50I-
z
40-
W
0
2 30-
[L
20
-
IO
-
'250
350
300
400
1
IO
TEMPERATURE, "C Figure 3. Catalytic reduction of nitric oxide with ethylene in the integral reactor Initial concentrations of NO and CzH4, ppm PN
0
200
0
500
PE
Alooo
2000 2500 3000
0 1500
1500
Results und Discussion
Some typical results of the integral reactor are shown in Figures 2 and 3. Figure 2 shows the degree of nitric oxide reduction on copper-silica as a function of temperature by different concentrations of ethylene but maintaining the ratio of nitric oxide to ethylene at 1 to 1 by varying concentrations from 200 to 1500 ppm. The degree of nitric oxide reduction decreased as the concentrations of nitric oxide and ethylene increased. In Figure 3, the catalytic reduction of nitric oxide, temperature behavior, is shown by varying the ratio of the concentrations of nitric oxide to ethylene from 1 to 1 to 1 to 10. As the ratio of NO to C2H4 decreased, the degree of NO reduction increased. Figures 2 and 3 show that almost complete catalytic reduction of nitric oxide on copper-silica can be achieved in the presence of ethylene a t moderate reactor temperatures. At various conditions, the exit stream was analyzed for ammonia, ethylene oxide, formaldehyde, acetaldehyde, carbon dioxide, and water vapor. Ammonia was determined with the Nessler reagent. The other components were determined with gas chromatography. Under the given operating conditions the gas chromatograph detected concentrations as low as 20 ppm. Ammonia concentrations lower than 1 ppm could be detected. Nevertheless, only water vapor and carbon dioxide were detected in the exit stream. The ratio of the nitric oxide to ethylene reacted was approximately 6 to 1 a t all conditions, indicating the over-all reaction as
CLH, + 6 N 0 -1-2'202
+ 3N2 + 2H20
(1)
Typical experimental data obtained with the differential reactor are given in Table I. The complete set of experimental data is given elsewhere (Korrani, 1966). The initial concentration of nitric oxide was varied from 300 to 1500 ppm and that of ethylene from 500 to 1500 ppm. The nitric oxide reduced varied between 0 and 30"r. As shown in Table I , the ratio of the moles of nitric oxide reduced to the moles of ethylene oxidized was approximately 6, indicating essentially complete reaction to H 2 0 and CO,. This was also confirmed by chemical analysis of the product stream. The rate of nitric oxide reduction was calculated from the design equation,
where A X is the conversion of nitric oxide, W is the catalyst weight, and F is the flow rate of nitric oxide. These experimental rates of nitric oxide reductiori, re, a t various concentrations of nitric oxide and ethylene are given in Table 11. Calculations similar to those made by Accomazzo and Nobe (1965) using the correlations of Yoshida et a2. (1962) indicated that mass transfer effects were negligible. The correlation of the reaction rate data of the differential and integral reactors was attempted with a number of reaction rate expressions. A satisfactory correlation was achieved only with the rate equation shown in Equation 3.
Table I. Experimental Results of Catalytic Reduction of Nitric Oxide with Ethylene 1,
c
P%, P.P.M.
300 500 800 1000 1200 1500
POE
272 351 394 271 350 420 273 353 421 272 355 422 273 349 427 277 358 420
Oh
R
= 500 P.P.M.
POE
270 342 426 271 333 403 277 340 420 275 354 432 215 349 422 271 347 428
x,
5.0 13.3 23.3 4 .O 8.0 15.0 2.5 6.3 10.7 2.0 6.0 9.0 1.3 4.2 7.5 1.3 3.3 6.7
7.5 5.3 5.5 6.3 5.5 6.0 6.3 6.3 5.8 6.3 6.0 6.0 7.5 6.3 6.0 6.3 6.3 5.6
= 1000 P.P.M.
300 500 800 1000 1200 1500
5.0 18.3 22.0 4.0 12.0 19.0 3.0 8.0 12.5 2.5 7.0 10.5 1.7 4.6 7.9 1.7 4.3 6.3
7.5 7.3 6.5 6.7 6.0 5.7 4.8 6.0 6.6 5.0 7.0 5.3 6.6 6.8 6.3 6.1 6.5 6.4
Pi = 1500 P.P.M. 300 336 414 298 351 41 2 270 349 412 276 347 403 287 371 412 272 356 412
300 500 800 1000 1200 1500
10.3 17.0 30.0 8.0 12.0 18.0 3.1 8.8 12.5 2.5 7.0 9.5 2.1 6.3 8.3 1.3 5.3 7.0
6.4 6.3 6.0 5.7 5.0 6.0 5.0 5.8 5.0 5.0 7.0 7.0 5.0 6.3 5.0 5.0 6.6 5.7
The reaction rate parameters of this rate expression were determined from the differential reactor data. Equation 3 was rearranged to obtain Equation 4,
Y = aPN+ bPE + c
(3) Ind. Eng. Chem. Process Des. Develop., Vol. 9,No. 3, 1970 457
Table II. Comparison of Experimental and Calculated Rates of NO Reduction re,
YX
loa
PN,
P.P.M.
PE,
P.P.M.
x io4,
re, x
io4,
Moles/G
MoleslG
Cat.-Hr
Cat.-Hr
2.04 2.30 2.47 2.30 2.13 2.00 2.88 3.23 3.21 3.07 3.09 3.46 3.58 3.61 3.62
2.02 2.32 2.34 2.25 2.14 1.97 2.86 3.18 3.20 3.16 3.04 3.44 3.58 3.63 3.60
T=3WC 2.650 3.238 3.975 4.612 5.273 6.083 4.084 4.907 5.515 6.199 6.913 5.804 6.391 6.989 7.815
288 486 785 986 1187 1488 483 780 981 1181 1481 779 978 1178 1478
498 498 498 498 498 498 997 997 997 997 997 1496 1496 1496 1496
T = 330" C 2.305 2.707 3.264 3.748 4.239 4.880 3.481 4.241 4.622 5.182 5.824 4.876 5.325 5.800 6.463
284 480 778 979 1180 1481 476 774 973 1174 1474 771 969 1168 1468
497 497 496 497 497 497 996 996 995 996 996 1495 1495 1495 1495
2.66 3.26 3.63 3.46 3.26 3.09 3.91 4.29 4.53 4.35 4.33 4.85 5.11 5.19 5.25
2.74 3.28 3.40 3.31 3.18 2.97 3.77 4.36 4.47 4.47 4.37 4.52 4.80 4.95 5.00
3.78 4.37 4.94 5.15 4.70 4.45 5.44 5.77 5.69 5.87 6.20 6.39 6.67 6.80
3.54 4.39 4.70 4.65 4.52 4.25 5.69 5.94 6.03 5.99 5.67 6.15 6.43 6.62
10
c,
p m
Figure 4. Plot of parameter Y in Equation 4 vs.
f B
P% = 5 0 0 ppm Temp., C 320
A
0 340
0 370
T = 360'C 1.906 2.324 2.777 3.051 3.515 4.050 3.745 4.070 4.515 4.979 4.287 4.741 5.093 5.660
277 474 770 969 1171 1473 767 965 1165 1464 762 961 1159 1459
496 496 495 495 495 495 995 994 994 994 1494 1494 1493 1493
0.03
400
600
800
1000 PE,
1200
Figure 5. Plot of parameter Y in Equation 4 vs.
Equation 4 indicates that linear relationships should be obtained for plots of Y against both PN and PE. Some examples of these plots are s,hown in Figures 4 and 5, and both figures show the linearity of these plots for the data obtained in this investigation. Coefficients a, b, and c were determined by a computer program which provided for a least-mean-squares fit of the data. The constants k , K N , and K E were also determined by the computer calculations. The logarithms of these constants were plotted against reciprocal absolute temperatures in Figure 6, and Arrhenius-type linear relationships were obtained. The equations of these curves are given in Equations 5 , 6, and 7:
k = 0.498 exp. ( - 6 2 7 8 / R T ) , moles NO/g catalyst-hr 458
(5)
Ind. Eng. Chern. Process Des. Develop., Vol. 9, No. 3, 1970
1400
16
ppm f E
fi = 800 ppm Temp., ' C
0 300
A 320 0 340
= 0.236 x lo4 exp. ( 3 1 3 / R T ) ,atm-' K E= 7.60 x lo4 exp. ( - 4 2 1 8 / R T ) , atm-'
KN
Ayen and Peters (1962) and Ayen and Ng obtained correlations with the same rate equation for the reduction of NO on Girdler catalysts with H2 and CO, respectively. Unpublished results of Malling (1963) refer to similar correlations for the catalytic reduction of NO with methane. Table I1 gives typical results of the calculated rates
O U
n
0
0
3.0 -
The derivation of this rate equation has been given by Laidler (1954) based on a model of surface reaction between two different adsorbed reactants on adjacent sites. However, the experimental results of this investigation are not interpreted in terms of this model. Rate Equation 3 is merely utilized to provide a satisfactory empirical correlation of the data. The experimental results show that copper-silica is capable of achieving the complete reduction of nitric oxide and the complete oxidation of ethylene and gives promise of a means to eliminate simultaneously the oxides of nitrogen and the unburned hydrocarbons from the exhausts of combustion devices burning hydrocarbon fuels.
U
0
2.5. 0
Nomenclature
a
1
b
2 .00 -
C
8
~
L
1.70
1.60 I
T’
OK-l
Figure 6. Arrhenius plot of parameters k , Equation 3 Solid line represents calculated data:
KN,
and K E in
A
400
SO0
800
PN,
IC00
1200
1400
I!
ppm
Figure 7. Comparison of experimental and calculated rates of catalytic reduction of NO Temp. = 310°C Solid lines represent calculated data Initial CnHr concentrations, ppm
0 500
0 1000
A 1500
Pk PE PN r
OKN,OKE,Ok
of nitric oxide reduction and makes comparisons with the experimental values. An example of a graphical comparison between the experimental and calculated rates at various concentrations of nitric oxide and ethylene is given in Figure 7. As shown in Table I1 and Figure 7, the agreement between experiments and calculations is reasonably good, indicating satisfactory correlation of the data with Equation 3.
200
F k KE KN POE
R T W
X Y
constant in Equation 4 constant in Equation 4 constant in Equation 4 flow rate of nitric oxide, moles/ hr constant in Equation 3, moles NO/g cat.-hr constant in Equation 3, atm-’ constant in Equation 3, atm-’ initial partial pressure of ethylene, atm -’ initial partial pressure of nitric oxide, atm-’ partial pressure of ethylene, atm-’ partial pressure of nitric oxide, atm-’ reaction rate, moles NO/g cat.-hr gas constant temperature, K catalyst weight, g conversion of nitric oxide parameter defined by Equation 4 O
literature Cited
Accomazzo, M. A., Nobe, K., IND.ENG.CHEM.PROCESS DESIGNDEVELOP.4, 425 (1965). Ayen, R. J., Ng, Y., Air Water Pollut. Inter. J . 10, 1 (1966). Ayen, R. J., Peters, M. S., IND.ENG. CHEM.PROCESS DESIGNDEVELOP. 1, 204 (1962). Ayen, R. J., Yonebayashi, T., Atmos. Enuiron. 1, 307 (1967). Baker, R. A., Doerr, R. C., IND. ENG. CHEM.PROCESS DESIGNDEVELOP. 4, 188 (1965). Baker, R. A., Doerr, R. C., J. Air Pollut. Control Assoc. 14, 409 (1964). Korrani, A. Sotoodehenia, thesis, University of California at Los Angeles, December 1966. Laidler, K. J., “Catalysis,” Vol. 1, Chap. 4, P. H. Emmett, Ed., Reinhold, New York, 1954. Malling, G. F., M. S. thesis, University of Illinois, Urbana, Ill., 1963. Snell, F. D., Snell, C. T., “Colorimetric Methods of Analysis,” Vol. 1, p. 653, Van Nostrand, New York, 1941. Sourirajan, S., Blumenthal, J. L., “Actes du Deuxieme Congres International de Catalyse,” Vol. 11, p. 2521, Editions Technic, Paris, France, 1961a. Sourirajan, S., Blumenthal, J. L., Air Water Pollut. Inter. J . 5, 24 (1961b). Wikstrom, L. L., Nobe, K., IND. ENG. CHEM.PROCESS DESIGNDEVELOP. 4, 191 (1965). Yoshida, F., Ramaswami, D., Hougen, 0. A,, A.I.Ch.E. J. 8, 5 (1962). RECEIVED for review August 26, 1969 ACCEPTED March 13, 1970 Work supported by the UCLA air pollution program. Some support received from HEW grant 1-RO 1-AP00913-01 National Air Pollution Control Administration. Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 3, 1970 459
