Ind. Eng. Chem. Res. 1992,31, 987-994
987
KINETICS AND CATALYSIS Comparison and Analysis of Intrinsic Kinetics and Effectiveness Factors for the Catalytic Reduction of NO with Ammonia in the Presence of Oxygen John Marangozis School of Chemical Engineering, Ryerson Polytechnical Institute, 350 Victoria Street, Toronto, Ontario, Canada M5B 2K3
Literature data for the selective catalytic reduction of NO with NH3 in the presence of O2 on 27 catalysts of supported and unsupported metal oxides and zeolites are critically reviewed and analyzed. Intrinsic catalyst activities, apparent activities, and catalyst effectiveness factors are calculated in the light of a postulated mechanism for the reaction. Intrinsic activation energies and preexponential factors were obtained and correlated in terms of the catalytic compensation effect. The catalysts are compared and rated in terms of their activity to reduce nitric oxide from flue gas of stationary power plants. Introduction Controlling power plant NO, emissions has become inevitable around the industrial world. In Japan, Germany, Canada, the USA,and elsewhere, selective catalytic reduction of NO, has gained increasing importance as an effective, although costly, process for the extensive removal of NO, from stationary power sources. The technological aspects and the economics of the process have been extensively discussed (e.g., a recent overview has been presented by Boer et al. (1990)). The process involves reacting flue gases (preferably dust- and SOz-free),containing NO, and stoichiometric amounts of NH3 and oxygen, over selective catalysts of metal oxides loaded on supports (pelleted or monolithic) or of catalytic synthetic zeolites. Monolithic supports are gaining importance due to low pressure drop and other advantageous considerations. The activity and kinetic performance of some of these catalysts have been reported (Kiovsky et al., 1980; Wong and Nobe, 1984; Beeckman and Hegedus, 1991; Inomata et al., 1982; Bauerle et al., 1978a). The stoichiometry of the reaction on metal oxide catalysts appears to be agreed upon to be 4 N 0 + 4NH3 + O2 = 4Nz 6Hz0 (R1) while on zeolites the stoichiometry is slightly different (Kiovsky et al., 1980): 4 N 0 + 3NH3 + 0.2502 = 3.5Nz + 4.5H2O (R2) The difference shown in stoichiometry is a result of the difference in actual reaction mechanism, as will be discussed later. Catalysts containing noble metals (Pt, Rh, Pd, etc.) on supports have been studied to a limited extent (e.g., Bauerle et al., 1975a; Boer et al., 1990). Although such catalysts have been widely in use in catalytic converters for automobile-exhaust NO, reduction, they are usually not considered for use for coal- or oil-fired power service due to several drawbacks. Among those are that they are susceptible to sulfur oxide poisoning and, at higher tem-
+
peratures, their activity is reduced due to oxygen inhibition. Also, they tend to promote formation of N20, they require a high ",/NO, ratio (from 1.6 to 2.4), and they operate at lower efficiencies. Catalyst efficiencies or what is better known as catalyst effectiveness factors have not been too widely studied and reported in the literature. Perhaps this was due to the fact that the intrinsic reaction rates are not well-known yet nor are the reaction kinetics thoroughly understood. It has been the purpose of this investigation to analyze the reported data in order to extract information on the intrinsic kinetics and effectiveness factors of pelleted and monolithic catalysts of metal oxides and zeolites for the selective catalytic reduction of NO in the presence of NH3 and oxygen. Such information may prove useful in the further evaluation of the process. Also, it has been an aim of the present study to compare the apparent activities and to correlate the intrinsic activation energies and preexponential factors of all reported catalysts according to the compensation effect (Schwab, 1983). Mechanism of t h e Reaction 1. Metal Oxide Catalysts (VZO5/TiOz,VzO5/Al2O3). Vanadium oxide catalysts on various types of supports have been used as effective catalysts for NO selective reduction with NH3 in the presence of oxygen. Experimental rate data have been reported (further analyzed in this paper), as summarized in Table I. Table I shows the various types of metal oxide catalysts used by the various investigators. Bauerle et al. (1975b) studied the reaction in ideal mixtures of NO-NH3-02-N2 and in simulated flue gas (containing 14% COz, 5% HzO, and trace SOz) on V205/Alz03and on FeZO3-CrzO3/Al2O3 catalysts. On vanadium oxide catalyst in the absence of Oz,conversion of NO with NH3to Nz increased with temperature up to 510 "C, with the simultaneous production of small amounts of N,O. When Ozwas added to the mixture, NO reduction increased sharply, exhibiting a maximum at 400 O C , and simultaneously NzO production was increased, when the oxygen concentration was increased from 1100 to 9100 ppm
oaaa-5aa5/92/ 2631-09a7~03.00/0 0 1992 American Chemical Society
988 Ind. Eng. Chem. Res., Vol. 31, No. 4, 1992
02.In simulated flue gas mixtures it was found that NO conversion increased with increasing ratio of ",/NO from 0.5 to 1.1and with increasing O2mole fraction from 0 to 3% 02.Little or no N 2 0was detected in the product stream of simulated flue gas. The presence of C02 and H 2 0 had little effect on the temperature of maximum conversion, which was found again to be at 400 "C. Also, the effect of SO2added into the mixture on NO removal was found to be negligible up to 1500 ppm SOz. It should be noted that in a comparison study (Bauerle et al., 1975a) it was found that, on noble metal catalysts, SO2 had a deleterious effect on NO reduction. Thus, it appears that V205 is not inhibited by SO2. Bauerle et al. (1978a) discussed some limited aspects of the reaction mechanism on V2O5 catalysts and presented an intrinsic rate equation. Inomata et al. (1980, 1982) studied the NO reduction reaction using 100% V2O5, V205/A1203,and V205/Ti02catalysts as summarized in Table I. They found that the rate of reduction was markedly increased with increasing O2mole fraction at 200 OC up to 1mol % O2and then it remained almost constant. Nitrogen was almost selectively formed as the reaction product, and the activity of the catalysts changed significantly with the content of V205and the kind of support. The rate was found to be a linear function of the number of V-0 surface bonds, as measured by IR and ESR spectroscopy. They suggested the following reaction mechanism:
'I
t
readion2 + 0 2
H
I
I
0
I
-o-v-o-v5-o-
'. .' ;
yON*. "
H
I
0
I
-W2 +
HzO)
I
0
II
0
I
v-0-v5-ocomplex
The suggested mechanism allows for the formation of a surface activated complex between NH, and NO which is decomposed to N2and H20. Oxygen's role is to regenerate the surface V = O bonds. It accounts for the facts that the rate is proportional to the surface V = O bonds (V5+),that NH3is strongly and rapidly chemisorbed, and that the rate is proportional to NO partial pressure, but it does not account for the formation of N 2 0intermediate as reported by many investigators including Inomata et al. (1980) and it does not predict the observed effect of oxygen partial pressure. To account for all these observed phenomena, a new postulated mechanism is needed for the reduction of NO in the presence and in the absence of oxygen. Such a plausible mechanism is discussed below. The effect of the support (Ti02,Al2O3, Si02)is related to the possibility of enhancing or not the surface concentration of V=O bonds. Thus, Ti02(anatase)appears to be more beneficial than Si02 or even A1203. Wong and Nobe (1984) studied the reaction with V205/Ti02(anatase or rutile Ti02 support chemically mixed or impregnated) and V205/A1203catalysts. Their kinetic data indicated a reaction of fmt order with respect to NO, zero order with respect to NH,, and 0.25 and 0.50
orders with respect to O2 for "chemical mixed" and impregnated catalysts, respectively. Odenbrand et al. (1985) studied the reaction with V205/Ti02-Si02catalyst. They investigated in detail pore diffusion phenomena to interpret the intrinsic reaction kinetics, and they concluded that the reaction orders are fractional for NO, NH,, and O2 partial pressures. No mechanism was advanced by those authors. Postulated Mechanism for the NO + NH3 O2 Reaction. To account for all the observed effects in this reaction, the following mechanism is postulated: step 1: chemisorption of NO and O2 (rate limiting)
+
2N0
+ 2e- = 02-+ N20(ads)
(1/2)0,
+ 2e- = 02-
step 2: reaction of N20(ads)with adsorbed NH3 (fast) 3V,05 + N20(ads) + 2NH3(ads) + 2e- = 3V204+ 2N2 step 3: oxidation of V204to V205(fast)
+ 3H20 + 02-
3V204+ 302- = 3V205+ 6eoverall: 2 N 0 + 2NH3 + (1/2)02 = 2N2 + 3H20 This mechanism suggests that N 2 0 is an intermediate reaction product at steady state, that the activity of the catalyst is proportional to the number of V205surface sites (Le., the V5+=0 surface bonds), and that the rate may be first-order with respect to NO, half-order with respect to 02,and zero-order with respect to NH,. The overall stoichiometry agrees with what has been published in the literature. The molar ratio of ",/NO should be 1.0 or better. Postulated Mechanism in the Absence of Oxygen. step 1: chemisorption of NO (rate limiting) 2(2NO + 2e- = 02-+ N20(ads)) step 2: reaction of N20(ads)with ",(ads) on V205 sites (fast) 3V205+ NzO(ads) + 2NH3(ads) + 2e- = 3v@4 + 2Nz + 3H20 step 3: oxidation of V204to V205(fast)
+ 02-
3V2o4+ 302- = 3V205+ 6eoverall: 4N0 2NH3 = N 2 0 + 2N2 3H20 Again, this mechanism suggests that N20 is now a product of the reaction, that the activity of the catalyst is proportional to V205sites, and that the rate is zero-order with respect to NH3 and proportional to NO partial pressure. The molar ratio of ",/NO should be 0.5 or better. 2. Synthetic Zeolite Catalysts. Synthetic zeolite catalysta have been used commercially for the removal of NO with NH, from nitric acid plant tail gas, which is high in NO, concentration but normally free of SO2 and particulates. The use of natural or synthetic zeolites in flue gas purification, with and without the presence of SO2, is under investigation. Kiovsky et al. (1980) have reviewed the subject and have reported on their experimental work. A commercial synthetic zeolite catalyst designated as NC-300 was used, having the following composition: Si02 72-80%; A120317-22%, Fe2031-4%, and K 2 0 less than 1.5%. It was found that this catalyst does not catalyze the reduction of NO with NH, in the absence of OF However, reduction of NO was dramatically increased with O2 mole fraction increasing from 0 to 21 % , leveling off somewhat
+
+
Ind. Eng. Chem. Res., Vol. 31, No. 4,1992 989 Table I. Reaction Orders for the Selective Catalytic Reduction of NO with NHa and O2 (Stoichiometric NHJNO Ratio; Oxygen less than 3 vol %) apparent reaction rate order NHq NO 0, reference catal no. catal type" 1 5% V2O5-TiO2*(A) 0 1 0.25 Wong and Nobe (1984) 2 0 1 5% V205-Ti02(A) 0.50 Wong and Nobe (1984) 3 0 1 10% V205-Ti02(A) 0.50 Wong and Nobe (1984) 10% v p 0 5 - ~ 4 l 2 0 3 0 1 4 0.50 Wong and Nobe (1984) 5 10% V205-Ti02*(R) 0 1 0.25 Wong and Nobe (1984) 6 10% V205-Ti02(R) 0 1 0.40 Wong and Nobe (1984) 7 zeolite NC-300 (Norton) 0 1 0.22 (this work) Kiovsky et al. (1980) 8 0 1 0.8% V205-Ti02monolith 0.25 (this work) Beeckman and Hegedus (1991) 0 1 9 21% V205-Ti02(A) 0.36 (this work) Inomata et al. (1982) 10 21% V205-Ti02(A-R) 0 1 Inomata et al. (1982) 0.25 (this work) 11 16.5% V205-Al203 0 1 0.38 (this work) Inomata et al. (1982) 12 0 1 0.50 (this work) 100% V2O6 Inomata et al. (1980) 13 10% V205-Al203 0.05 Bauerle et al. (1978a) 0 (1) 14 15% V205-Al203 0.05 Bauerle et al. (1978a) 0 (1) 15 0 1 20% V205-Al203 0.05 Bauerle et al. (1978a) 0 1 16 25% v205-&03 0.05 Bauerle et al. (1978a) 10% VzO5-Al2O3(Harahaw) 17 0 1 0.05 Bauerle et al. (1978a) 18 9/1 (Fe203/Cr203),10% Fe-Cr/A1203 0 1 0.15 Bauerle et al. (1978b) 19 20/1 (Fe203/Cr203),10% Fe-Cr/A1203 0 1 0.15 Bauerle et al. (1978b) 50/1 (Fe2O3/Cr2O3),10% Fe-Cr/A1203 20 0 1 0.15 Bauerle et al. (1978b) 21 5/1 (Fe203/Cr203),11% Fe-Cr/A1203 0 1 0.15 Bauerle et al. (1978b) 10% FezO3/Al2O3 0 1 22 0.15 Bauerle et al. (1978b) 20% FezO3/Al2O3 23 0 1 0.15 Bauerle et al. (1978b) 24 etched metal screens 0 1 0.28 Kittrell and Eldridge (1985) 0 1 16.7% V205-Si02 25 0.50 (assumed) Shikada et al. (1983) 16.7% V205-3.2% TiOz-SiOz 26 0 1 0.50 (assumed) Shikada et al. (1983) 20% V205-46% TiO2-34% Sio2 27 0 1 (0.30) Odenbrand et al. (1985) "A = anatase; R = rutile; A-R = mixed anatase-rutile;
* = chemical mixing technique; % is w t %.
between 10 and 21%. This is a much increased tolerance for oxygen compared to the metal oxide catalysts. For NH3/N0 molar ratios of 0.761.0 the reduction efficiency was increased with increasing temperatures from 260 to 370 OC. Conversions of better than 90% were obtained when the NH3/N0 was maintained above the stoichiometric ratio of 0.75 up to a ratio of 1.0. The presence of 10 mol % water in simulated flue gas reduces the NO conversion at the lower temperatures. At temperatures higher than 340 "C conversions greater than 90% were obtained, comparable to "dry" gas, at space velocity of 6OOO h-l. A by-product, NzO,was detected in the effluent in small amounts. Also it was found that the addition of NO2 in the gas assisted the reduction of NO to N2, even in the absence of oxygen. Thus, Kiovsky et al. concluded that NO2must be a reactive species in a postulated mechanism for this reaction. They proposed that the rate-limiting step may be the chemisorption of NO and O2 to form NO2. However, no NOz was detected in the effluent gas. Thus, it can be argued that a postulated mechanism may be devised with N20 as a reactive species which has been detected as a by-product. Postulated Mechanism for Zeolite Catalyst. step 1: chemisorption of NO and Oz (rate limiting) 8 N 0 + 8e- = 4N20(ads) + 402-
+
(1/2)02 2e- = 02step 2: chemisorption of NH3 on zeolite (H+Z-) (fast) 6NH3 + 6H+Z- = 6NH4+Zstep 3: reaction between NH4+Z-and N 2 0 (fast) 6NH4+Z-+ 4N20(ads) + 502- = 10e- + 7N2 + 9H@+ 6H+Zoverall: 8N0 + 6NH3 + (1/2)02 = 7N2 + 9H20
The mechanism suggests that the reaction rate is zeroorder with respect to NH,, first-order with respect to NO, and half-order with respect to oxygen partial pressure. It also suggests that the activity of the catalyst is proportional to zeolite acid sites and that the stoichiometric NH3/N0 ratio should be 0.75 or better. It would now be of great interest to examine the published rate data for all reported catalysts, in comparison with the suggested reaction mechanisms, to obtain global reaction rate equations, activation energies, intrinsic reaction-rate coefficients, and effectiveness factors. Reaction Rates. As implied by the mechanisms discussed, the general form of the intrinsic rate equation should be rate = keNoOo,
(1)
where 8, the surface coverage due to chemisorption on single sites, may be represented by a Langmuir-type isotherm:
Ko,'/2Po,'/2 1 + KN&0
+ Ko:/2Po,'/z
(3)
then rate =
kKNOK0:/2PNOP0;/2 (1 + KN#No Ko,'/2Po,'/2)2
+
(4)
It is suggested by the data that the adsorption of NO is much weaker than the adsorption of O2due to low values of PNO. If this is the case, the rate equation may degenerate to the form rate = k'PN#o,m
(5)
990 Ind. Eng. Chem. Res., Vol. 31, No. 4, 1992 Table 11. k, and r) Values Calculated from Literature Data on NO Reduction with NH3 and O2 catal De, no. catal type T,K kv&,,m(predicted), s-l 109 cm2/s Wonn and Nobe (1984) 41617.0 6.7 1 5% V205-Ti02*(A) 477 98.2 7.4 533 7.9 330 577 8.3 791 12.4 430 12.0 2 5% V205-Ti02(A) 40.7 13.3 466 14.7 525 224 572 15.6 686 1.6 13.0 3 10% V205-Ti02(A) 366 427 14.9 15.0 461 56.2 16.1 489 16.9 150 427 3.9 4 10% V205-Al203 3.9 4.1 461 39.8 4.2 489 206 4.5 544 807 48.0 5 10% V205-Ti02*(R) 439 4.3 10.8 59.0 494 21.9 69.8 544 83.4 602 48.6 47.4 6 10% V206-Ti02(R) 436 2.9 55.5 477 5.5 66.1 527 10.8 81.5 25.1 594 36.4 627 89.6 7
zeolite NC-300 pellets
Kiovsky et al. (1980) 533 6.4 593 11.4 643 13.5 700 13.5 550 6.2 605 11.4 666 13.5 548 5.6 558 6.7 603 9.4 558 6.7 573 7.7 608 9.4 638 10.3 673 11.4
kV
0
6.1 X 4.1 x 1.2 x 3.0 x 1.5 x 5.4 x 3.1 X 1.0 x 1.8 X 1.9 x 7.4 x 2.0 x 5.0 x 5.2 x 2.8 X 1.2 x 1.6 X 4.0 X 8.3 X 1.9 x 8.8 X 1.8 x 3.6 x 8.7 x 1.3 x
lo2
10.5 11.3 11.9 12.6 10.7 11.4 12.2 10.7 10.8 11.4 10.8 11.0 11.5 11.9 12.2
1.4 X 2.9 X 3.9 x 4.3 x 1.3 X 3.0 X 3.8 X 1.6 X 2.1 x 3.3 x 1.7 X 2.0 x 2.7 X 3.1 X 3.6 X
1ff
6.6 6.6 6.6 6.6 6.6 6.3 6.3 6.2 6.2 6.0 6.0 6.3 6.3 6.2 6.2 6.0 6.0 6.2 6.2 6.5 6.5 6.7 6.7 6.3 6.5 6.7 6.2 6.3 6.5 6.6 6.7 6.8
103 104 104 104 104 106 108
103 104 104 105 103 104 106 108
lo2
lo2 lo2 103
lo2 103 103 103 104
0.35 0.16 0.09 0.06 0.50 0.32 0.15 0.09 0.85 0.50 0.32 0.20 0.48 0.19 0.08 0.04 0.90 0.80 0.70 0.58 0.92 0.90 0.80 0.70 0.65
lo2
0.68 0.58 0.52 0.52 0.70 0.57 0.53 0.74 0.68 0.61 0.68 0.65 0.60 0.60 0.58
2.6 x 2.6 x 2.3 x 2.8 x 2.6 x 1.3 x 1.2 x 7.8 x 6.6 X 3.5 x 2.6 X 1.1 x
104 104 104 104
1.0 1.0 1.0 1.0
6.0 X 6.0 x 3.3 x 3.0 x 6.7 x 6.7 x 2.6 x 2.6 x 4.6 x 4.6 x 9.9 x 2.4 x 4.8 x 7.0 x
103
lo2 101 102 1ff
lo2 lo2
lo2 102 102 lo2 102 lo2 lo2
Beeckman and Hegedus (1991)
8
0.8% V205-Ti02 8C (crushed particles)
monolith slab (0.125-cm slab)
8 M (monolith shape) integral reactor
623 623 623 623 623 576 575 552 549 525 522 573 573 549 546 525 521 554 554 614 614 653 653 571 611 651 553 573 598 623 648 668
712 712 683 832 772 394 364 239 203 109 81 30.2 18.8 18.4 18.4 13.4 11.1
12.3 12.3 23.2 23.2 27.1 27.1 15.0 21.5 28.0 15.0 17.4 20.4 22.6 25.9 27.0
104
104 104 103 103 103
103 104
1.1x 104
103 103 103 103
103 104 104 104 104 103 104
104
103 1.1x 104 2.3 x 104 3.8 x 104 4.4 x 104 9.2 x 104
1.o
1.0 1.0 1.0 1.0 1.0 1.0 0.09 0.09 0.10 0.10
0.13 0.12 0.06 0.06 0.03 0.03 0.02 0.02 0.05 0.03 0.02 0.07 0.05 0.03 0.02 0.02 0.01
Ind. Eng. Chem. Res,, Vol. 31,No.4,1992 991 Table I1 (Continued) catal no.
catal type
D.. T,K k,qCo,m(predicted), s-l lo3 c&/s
11
21% V,05-Ti0,(A) 21 % V,O,-TiO,(A-R) 16.5% v205-&03
Inomata et al. (1982) 473 63.5 473 80.8 473 76.9
12
100% VzO5
Inomata et al. (1980) 473 10.8 118 586
9 10
13 14 15 16 17
18
9/1 (Fez03/Crz03),10% Fe-Cr/Alz03
19
20/1 (Fe,03/Cr,03), 10% Fe-Cr/Alz03
20
50/1 (Fe2O3/Cr2O3),10% Fe-Cr/Al&
21
5/1 (Fe203/Cr203), 11% Fe-Cr/Al,O,
22
10% Fe203/A1203
23
20% FeZO3/Al2O3
24
etched metal screens
25
16.7% V,O,-SiO,
26
16.7% V&-3.2%
27
20% V205-46% Ti0,-34% Sioz
Ti0,-SiO,
13.4 13.4 3.8
k"
7
2.0 x 104 5.1 X 103 6.2 x 104
0.75 0.70 0.40
1.9 x 104 22.6 x 104
(1.0) (1.0)
Bauerle et al. (1978a) 473 4.6 573 24 673 48.5 473 4.8 573 32 673 63 473 4.6 28 573 673 56 493 10 573 22 673 44 473 5.6 573 20 673 47
3.2 3.5 3.8 3.5 3.8 4.2 3.9 4.3 4.6 4.2 4.6 4.9 3.5 3.8 4.2
49 1040 5030 50 1720 7720 45 1460 5550 175 683 3200 55 682 3090
0.20 0.05 0.02 0.20 0.036 0.017 0.22 0.04 0.021 0.12 0.06 0.03 0.22 0.06 0.035
Bauerle et al. (1978b) 486 2.1 573 20.8 673 39.2 473 3.1 573 12.6 673 32.9 473 2.5 573 11.2 673 32.9 473 5.6 573 18.7 673 32.9 473 0.98 573 3.00 673 6.03 473 2.5 573 8.4 673 23.8
4.5 4.8 5.2 4.4 4.8 5.2 4.4 4.8 5.2 4.4 4.8 5.2 4.4 4.8 5.2 4.8 5.3 5.7
39 2840 8520 75 992 7150 52 790 7150 232 2240 7150 12 64 194 52 476 3280
0.44 0.06 0.04 0.34 0.11 0.04 0.40 0.12 0.04 0.20 0.07 0.04 0.70 0.70 0.27 0.40 0.15 0.06
100 87 82 71 214 198 206 187 361 357 369 336 496 520 545 565
(1.0)
6.55 X 103 12 x 103 59.9 x 103 1.72 x 104 3.67 x 104 8.90 x 104
0.96 0.95 0.77 0.92 0.85 0.72
103 109
0.80 0.65 0.52 0.40 0.35
Kittrell and Eldridge 600 600 600 600 625 625 625 625 650 650 650 650 675 675 675 675
(1985) 2.5 2.2 2.1 1.8 5.4 5.0 5.2 4.7 9.0 8.9 9.2 9.6 12.2 12.8 13.4 13.9
Shikada et al. (1983) 453 7.3 473 12.9 523 51.1 453 18.3 473 35.2 523 69.3 Odenbrand et al. (1985) 450 28.8 480 54.3 515 103 550 161 580 218
11.3 11.5 12.1 12.4 12.7 13.4 7.0
7.3 7.7 7.9 8.2
2.25 X 5.22 X 1.32 X 2.68 x 4.15 X
10' 104 10'
992 Ind. Eng. Chem. Res., Vol. 31, No. 4, 1992 Table 111. Summary of Calculated Values of Intrinsic Activation Energies and Preexponential Factors for All Catalysts Studied catal no. E. kcal/mol A. s-l (rnol/cm3)-" 1 14.39 9.75 x 109 2 4.12 X 10" 14.62 1.05 X lo1* 15.01 3 4 5.54 x 10'' 27.34 1.50 X lo1 5 10.67 7.98 8.03 X lo6 6 5.74 3.10 x 104 7 1.19 x 1010 15.74 8 na na 9 10 na na 11 na na 12 12.1 7.64 x 109 4.06 X lo8 14.78 13 14 1.41x 109 15.65 15 2.10 x 109 16.47 16 1.35 x 107 11.10 17 12.95 5.47 x 10' 3.63 X lo6 18 10.90 4.13 X lo8 19 14.66 9.47x loe 20 15.80 2.42 X lo1 21 10.70 1.38x 105 22 8.70 5.77 x 107 23 13.16 7.25 X 10s 24 18.78 2.57 X 1O'O 25 13.60 3-64x 109 26 10.90 1.08 x 109 27 11.63
where the exponent rn should be equal or leas than 0.5, and k' is the combined chemical/adsorption rate constant: k ' = kKN&011/2
(6)
It is of further usefulness to express the rate per unit volume of catalyst and to use molar reactant concentrations outside the catalyst particles instead of partial pressures. Thus R, = kvCNOCOzm, mol of NO/(s.cm3 ofcatalyst) (7) For pelleted or monolithic catalysts exhibiting pore diffusion effects, the intrinsic reaction rate R, can be related to the apparent integral rate and the effectiveness factor q by a reactant (NO) mass balance: -F dCNo = R,q dVR(1 - et,) = (k,d(l - eb) dVR CNOCoZm(8) where VR is the reactor-bed volume, tb is the bed void fraction, C's are surface concentrations, and F is the total volumetric feed flow rate. If the surface oxygen concentration remains practically constant, due to excess oxygen used and small oxygen consumption, then eq 8 may be integrated with respect to NO to yield k,q for isothermal operation and plug flow:
10 1A
1.0
1.11
2.0
d/T
7
2.2
2.4
2.6
K-l
Figure 1. Comparison of Arrhenius plots: intrinsic reaction-rate constanta for all Catalysts reviewed for the catalytic reduction of NO with NH3 and 02.
Experimental rate data of the literature cited here were treated and analyzed in view of this theory, and k, and 9 values were determined and tabulated in Table 11. Care was taken to utilize data obtained with stoichiometric ratios of NH3/N0 and O2 content less than 3 vol %. Arrhenius plots were prepared for k,, as shown in Figure 1,and activation energies and preexponential factors were obtained from the plots and tabulated in Table 111. To compare the apparent activities of the catalysts, the pseudo-first-order apparent reaction-rate constants (kvqCOzm) were plotted in Figure 2 as a function of temperature. In all, 27 catalysts were compared. Their reported (or estimated) characteristics were tabulated in Table IV. Effective diffusivities De were predicted with a tortuosity factor assumed at a value of 2.
Discussion of Results The Arrhenius plots of Figure 1demonstrated excellent correlationsbetween the intrinsic rate constants k, and the temperature. As can be seen from the plots, the catalysts exhibit a wide span of intrinsic activities over 5 orders of magnitude. The best catalysts appear to be those conwhere F/VR is the reactor space velocity, X N O is the reactaining V206supported on TiOa or A1203.Si02 does not tant conversion, and k,V is the apparent rate constant appear to be such an effective support. This is better including pore-diffusion effects. The product (kv~COzm) demonstrated in Figure 2, when the apparent catalyst may be considered as a pseudo-first-order apparent rate activity, including the effectiveness factor, is plotted vs constant with respect to NO, at constant oxygen concen1/T. This plot demonstrates two discrete ranges of cattration, equal to (R,),iJCNO, where (RJObe= R,q. alysts, the superior catalysts containing V206and the less The pseudu-first-order reaction-rate constant k,C effective catalysts containing Fe203/Cr203,zeolites, or gethe effectiveness factor q are interrelated through t eand etched metal screens. Perhaps the beneficial effect of the ometry of the catalyst pellet (or the monolith) and the support is to increase the surface concentration of active effective diffusivity De, according to standard techniques V5+=O sites, as discussed by Inomata et al. (1982). of chemical reaction engineering (e.g., Odenbrand et al., Catalyst 12 contained 100% V205,and yet it was less 1985; Beeckman, 1991; Beeckman and Hegedus, 1991). effective than catalyst 25, which contained 16.7% V206on
%m
Ind. Eng. Chem. Res., Vol. 31, No. 4,1992 993 Table 1V. Summary of Reported (or Estimated) Catalyst Characteristics catal no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
synthesis, % w/w 5% V,Ok-TiO9*(A) (mixed) 5% V;O,-TiO;(A) .(impre;) 10% V2O5-TiOz(A) (imprg) 10% V206-Al203 10% V205-TiOz*(R)(mixed) 10% V205-TiOz(R)(imprg) zeolite NC-300 0.8% V205-Ti02(crushed) 0.8% V2O5-TiO2(monolith) 21% Vz05-Ti02(A) 21% V2O5-TiO2(A-R) 16.5% VZO~-A~& 100% VzO5 10% VZO~-A&O~ 15% V205-A1203 20% V206-Al203 25% V&A1203 10% V206-A1203(Harshaw) 9/1 (FezO3/CrzO3),10% Fe-Cr/Alz03 20/1 (Fez03/Cr203),10% Fe-Cr/A1203 50/1 (FezO3/Cr2O3),10% Fe-Cr/Alz03 5/1 (Fe203/Cr203),11% Fe-Cr/Alz03 10% Fe203/A1203(Harshaw) 20% Fe203/A1203 etched metal (stainless steel) screen (no promoter) 16.7% -Vz05-Si02 16.7% V205-3.2% TiOz-SiOz 20% V205-46% TiO2-34% SiOz
particle size 3.2 X 3.2 mm cylinders 3.2 X 3.2 mm cilinders 3.2 X 3.2 mm cylinders 3.2 X 3.2 mm cylinders 3.2 X 3.2 mm cylinders 3.2 X 3.2 mm cylinders 1.5 mm diam X 6 mm 50/80 mesh 2.32 X 2.32 X 15 cm; 9 channels; 0.60 X 0.60; 0.13 wall d , = 0.06 cm d , = 0.06 cm d = 0.06 cm (&, = 0.06 cm) 3.2 X 4.8 mm 3.2 X 4.8 mm 3.2 X 4.8 mm 3.2 X 4.8 mm 3.2 X 3.2 mm 3.2 X 4.8 mm cylinders 3.2 X 4.8 mm cylinders 3.2 X 4.8 mm cylinders 3.2 X 4.8 mm cylinders 3.2 X 3.2 mm cylinders 3.2 X 4.8 mm 0.43-mm wire etched
surf. area, m2/g 77
-
14
-
21
-
bed void fraction (0.40) (0.44 (0.44) (0.50) (0.50) (0.50) (0.43) (0.50) 0.60
pore vol, cm3/g 0.34 0.32 0.32 0.46 0.26 0.25 na na na
(1.5) (1.5)
(1.1) (1.1) (1.1) (1.2) (na)
(0.50) (0.50) (0.50) (0.50) (0.50) (0.50) (0.50) (0.50) (0.50) (0.5) (0.5) (0.5) (0.5) (0.5) (0.5) (na)
na na na na na na na na na na na na na na na (na)
0.69 (0.68) 1.37
0.48 0.48 (0.5)
1.02 (1.0) 0.313
(1.1)
(1.7) (1.1)
(1.1) (1.2) (1.25) (1.1) (1.1) (1.1)
20/40 mesh 20/40 mesh 0.568-mm granules
27
particle density, g/cm3 1.48 1.58 1.57 1.08 1.96 2.01 (1.25) na na
16-
0
515-
8 W
12
-
S-
1
1 .e
*.I
2.0
2.2
id/r ,
2.4
2.8
2.
f'
Figure 2. Comparison of apparent activities of all catalysts. Catalytic reduction of NO with NH3 and 02.
Si02. Catalyst 8, which contains 0.8% V205supported on Ti02 monolith, correlates very well in Figure 1,for both the crushed particles and the shaped monoliths and slabs, not withstanding the fact of the very low effectiveness factors calculated for the monolith. However, in Figure 2 catalyst 8M (the monolith) has a very low apparent activity due to ita low effectivenessfactors of 1-109'0 (Table 111, compared to catalyst 8C (crushed). Intrinsic activation energies and preexponential factors determined from Figure 1and tabulated in Table I11 were
6-
3t, / ;
,
,
,
,
24
30
sb
0
0
6
12
InA
Figure 3. Compensation effect for catalytic reduction of NO with NH3 and 02:correlation between intrinsic activation energy and preexponential factor for all catalysts.
correlated in Figure 3. Activation energies calculated here were generally higher in value than those previously re-
994 Ind. Eng. Chem. Res., Vol. 31, No. 4, 1992
ported in the literature. At any rate, the intrinsic activation energy was plotted vs In A , of the preexponential factor, and a good correlation was obtained in Figure 3 of the compensation effect (Schwab, 1983). This means that the catalytic reaction studied exhibits an intrinsic reaction-rate constant, for the types of catalysts used, which can be described by a single-parameter equation, to be used with extreme caution:
k" = ex.[
(;
-
f)]
The isokinetic temperature (or "theta" temperature) is about 313 K. The catalyst effectiveneas factors q calculated according to standard methods and tabulated in Table I1 were found to be small indeed, even for some of the small-size catalyst granules, but particularly for the pelleted and the monolith catalysts. Thus, pore diffusion is very important due to the high intrinsic activity of most catalysts. External mass-transfer phenomena are of no significance for the catalytic reduction of NO with NH, and 02.
Conclusions The review of 27 different catalysts reported in the literature for the catalytic reduction of NO in a flue gas with NH, in the presence of O2has revealed that the best conditions for the reaction are to operate at nearly 1:l mole ratio of ",/NO, in the presence of 2-3% 02,at temperatures below 600 K (to avoid oxidation of NHJ and above 450 K (to obtain good catalyst activities) in the presence of catalysts containing up to 15% V205supported on Ti02(anatase)or A1203. The reaction is strongly influenced by pore-diffusion phenomena; therefore, an optimum catalyst structure and geometry are required to maximize the catalyst effectiveness factor. Under the above reaction conditions, the global rate of the chemical reaction can be described by a simple rate law (eq 9). Intrinsic reaction rate constants can be described by a single parameter equation (eq 10) which involves only the activation energy. The catalysts reviewed were compared and related according to their intrinsic and apparent catalytic activities. Nomenclature A = Arrhenius preexponential factor, s-l (mol/cm3)-" C = gas-phase concentration, mol/cm3 De = effective diffusivity in catalyst pores, cm2/s E = activation energy, cal/mol F = total volumetric feed flow rate, cm3/s K = Langmuir adsorption constant, atm-l k = reaction-rate constant defined by eq 4 k' = reaction-rate constant defined by eq 5 k , = intrinsic reaction-rate constant defined by eq 7, s-l (mol/cm3)-" m = exponent, reaction order with respect to oxygen R, = intrinsic reaction rate, mol s-l (cm3of catalyst)-' R = universal gas constant, cal mol-l K-' ?k= temperature, K
VR = reactor volume, cm3 xNO
= reactant NO conversion factor, fractional
Greek Symbols = reactor bed voids, fractional q = catalyst effectiveness factor defined by eq 8, fractional 0 = chemisorption coverage, fractional tb
Subscripts b = bed e = effective g = gas R = reactor v = catalyst volume Registry No.
NO,10102-43-9;NH,,7664-41-7.
Literature Cited Bauerle, G. L.; Wu, S. C.; Nobe, K. Reduction of Nitric Oxide with Ammonia on Noble Metal Catalysts. Znd. Eng. Chem. Prod. Res. Dev. 1975a,14, 123-130. Bauerle, G. L.; Wu, S. C.; Nobe, K. Catalytic Reduction of Nitric Oxide with Ammonia on Vanadium Oxide and Iron-Chromium Oxide. Znd. Eng. Chem. Prod. Res. Dev. 1975b,14, 268-273. Bauerle, G. L.; Wu, S. C.; Nobe, K. Parametric and Durability Studies of NO, Reduction with NH, on V2O5 Catalysts. Znd. Eng. Chem. Prod. Res. Dev. 1978a,17, 117-122. Bauerle, G. L.; Wu, S. C.; Nobe, K. Parametric and Durability Studies of NO, Reduction with NH3 on Fe-Cr Oxide Catalysts. Znd. Eng. Chem. Prod. Res. Dev. 1978b,17, 123-128. Beeckman, J. W. Measurement of the Effective Diffusion Coefficient of Nitrogen Monoxide through Porous Monolith-Type Ceramic Catalysts. Znd. Eng. Chem. Res. 1991,30,428-430. Beeckman, J. W.; Hegedus, L. L. Design of Monolithic Catalysts for Power Plant NO, Emission Control. Znd. Eng. Chem. Res. 1991, 30,969-97a. Boer, F. P.; Hegedus, L. L.; Gouker, T. R.;Zak, K. P. Controlling Power Plant NO, Emissions: Catalytic Technology. Economics and Prospects. CHEMTECH 1990,May, 312-319. Inomata, M.; Miyamoto, A.; Murakami, U. Mechanism of the Reaction of NO and NH3 on Vanadium Oxide Catalyst in the Presence of Oxygen under the Dilute Gas Condition. J. Catal. 1980, 62, 140-148. Inomata, M.; Miyamoto, A.; U1, T.; Kobayaehi, K.; Mutakami, Y. Activities of V205/Ti02and v205/A&03 Catalysts for the ReacZnd. Eng. Chem. Prod. tion of NO and NH3 in the Presence of 02. Res. Dev. 1982,21,424-428. Kiovsky, J. R.;Koradia, P. B.; Lim, C. T. Evaluation of a New Zeolitic Catalyst for NO, Reduction with NH3. Znd. Eng. Chem. Prod. Res. Dev. 1980,19,218-225. Kittrell, J. A,; Eldridge, J. W. Novel Catalyst Preparation Method for NO, Control. Environ. Prog. 1985,4 (2),78-84. Odenbrand, C. U. I.; Lundin, S. T.; Andersson, L. A. H. Catalytic Reduction of Nitrogen Oxides I. The Reduction of NO. J.Appl. Catal. 1985,18,335-352. Schwab, G.-M. On the Apparent Compensation Effect. J. Catal. 1983,84,1-7. Shikada, T.; Fujimoto, K.; Kuhugi; Tominaga, H. Reduction of Nitric Oxide with Ammonia on Silica-supported Vanadium Oxide Catalysts. J. Chem. Technol. Biotechnol. 1983,33A,446-454. Wong, W. C.; Nobe, K. Kinetics of NO Reduction with NH3 on "Chemical Mixed" and Impregnated Vz05-TiOz Catalysts. Znd. Eng. Chem. Prod. Res. Dev. 1984,23,564-568.
Receiued for review September 10,1991 Revised manuscript receiued December 13, 1991 Accepted January 13,1992
