Little, R. C., J. Colloidlnterface Sci.. 37, 81 1 (1971). Little, R. C., Patterson, R. L., and Ting, R. Y., J. Chem. Eng. Data, 21, 281 (1976). Lumley, J. L., Ann. Rev. FluidMech., 1, 367 (1969). Lumley, J. L., J. Polym. Sci., Macromol. Rev., 7, 263 (1973). Lumley, J. L., in "Symposia Mathematica", Voi. 9, Academic Press, New York, N.Y.. 1972, p 315. MacWilliarns. D. C., Rogers, J. H., and West, T. J., in "Water Soluble Polymers", Bikales, Ed., Plenum, New York, N.Y., 1973, p 105. Marrucci, G., and Astarita, G., lnd. Eng. Chem., Fundam., 6, 471 (1967). Melton. L. L., and Malone, W. T., SOC. Petrol. Eng. J., 56 (1964). Metzner, A. B., and Astarita, G., AlChE J. 13, 550 (1967). Mysels, K. J., Chem. Eng. Progr., Symp. Ser., 67, 45 (1971). Offen, G. R.. and Kline, S. J., J. FluidMech., 62, 223 (1974). Paterson, R. W., Ph.D. Dissertation. Harvard University, Cambridge, Mass., 1969. Peterlin, A,, Pure Appl. Chem., 12, 563 (1966). Poreh, M., Zakin, J. L., Brosh, A., and Warshavsky, M., ASCEJ., Hydraulics Div., 96, 903 (1970). Pruitt, G. T., Whitsitt, N. F., and Crawford, H. R., NASA Contract No. NAS 7-369, The Western Co. Reserch Division, Dallas, Texas, 1966. Rouse, P. E., J. Chem. Phys., 21, 1272 (1953). Sanders, J. V., Henderson, L. H., and White, R. J., J. Hydronautics, 7 , 124
(1973). Scott, D., Science, 164, 1466 (1969). Seyer, F. A,, and Metzner, A. B., AlChE J., 15, 426 (1969). Smith, K. A,, Keuroghlian, G. H., Virk, P. S.,and Merrill. E. W., AiChEJ., 15, 294 (1969). Ting, R. Y., AiChEJ., 20, 810(1974). Ting, R. Y., and Little, R. C., J. Appl. Polym. Sci., 17, 3345 (1973). Toms, E. A,. Proc. lnt. Congr. Rheol., 2, 135 (1948). Treiber, K. L., and Sieracki, L. M., Columbia Research Corporation Report No. 101-2, 1970. Virk, P. S.,Ph.D. Dissertation, MIT Cambridge, Mass., 1966. Wells, C. S., AlChEJ., 14, 406 (1968). Western Co., The, Federal Water Pollution Control Administration Report WP20-22, 1969. Wetzel, J. M., and Tsai, F. Y., AlChE J., 14, 663 (1968). Zimm, E. H., J. Chem. Phys., 24, 269 (1956).
Receiued for reuieu! January 14, 1977 Accepted March 3,1977 Presented a t the Applied Rheology for Industrial Chemists Workshop, Kent, Ohio, June 28-July 2, 1976.
Reduction of Nitric Oxide with Ammonia on Vanadium Pentoxide S. C. Wu and Ken Nobe' School of Engineering and Applied Science, University of California, LOS Angeles, California 90024
The rate of reduction of NO with NH3 on a 1 5 % V205-AI2O3 catalyst has been investigated between 180 to 380 OC in a flow reactor under plug-flow conditions (space velocity = 20 000 h-'). The reaction rate can be ex1 2 , mol/(g of cat. h), which takes into account pressed by the equation r = 13.68 exp[-(9300/RT)]PN00~2PNH30 the effects of pore diffusion. Catalytic activities of alumina-supported SrRu04 perovskite, NiCt-204 spinel, Fe-Cr oxide, Harshaw V2O5, and a laboratory-prepared V2O5 for the reduction of NO with NH3 in the absence and presence of oxygen, and with NH3 in simulated flue gas are compared. V2O5 showed the most promise as a practical catalyst for removal of NO from stack gas. Alkali metal promoters for the SO2 oxidation reaction were shown to poison the NO-NH3 reaction on V2O5-AI2O3. Adsorption kinetic studies show that NH4 adsorption on unsupported V205 follow Elovich behavior with a first-order dependence on pressure. Similar behavior was observed for NO adsorption on reduced V205. The adsorption of NO on unreduced V2O5 was not measurable.
Introduction A promising approach for the control of nitrogen oxide emissions from stationary sources is selective catalytic reduction. Thus far, ammonia is the only reductant found effective for the selective removal of NO, in the presence of a large excess of 0 2 over a wide range of NH3/NO ratios. Recently, Bauerle e t al. (1975a) have provided a brief review of previous work on catalytic reduction of NO with "3. In a subsequent paper, alumina-supported V2O5 and Fe-Cr oxide catalysts have been shown to be effective for the selective removal of NO with NH3 in simulated flue gas containing SO2 during short-term durability studies (Bauerle e t al., 1975b). This paper reports on further kinetic studies of the NONH3 reaction on V ~ O Scatalysts. In addition, catalytic activities of Fe-Cr oxide and two catalysts, SrRu03 perovskite and NiCr204 spinel, which have been shown to be effective for the removal of NO from auto exhausts (Bauerle and Nobe, 1974), are compared with V205, and the effect of various preparation or pretreatment procedures on the catalytic ac136
Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 2, 1977
tivity of VZOS have been studied. Adsorption rates of NO and NH:{ on VZOj were also measured in order to provide additional information concerning the nature of the catalytic reaction.
Experimental Section Catalysts. 1. Fe-Cr Oxide. Dry alumina cylindrical pellets ('18 in. X 'h in., Filtrol grade 86) were soaked with an aqueous solution containing dissolved iron nitrate and chromium trioxide (Mallinckrodt analytical reagents), in the Fe/Cr atomic ratio of 5 . The impregnated pellets were then calcined in air a t 500 "C for 16 h. 2. NiCrzOd Spinel. NiCOn and CrCl3 (1:2 mole ratio) were dissolved in an HC1 solution. Addition of excess KOH precipitated a mixture of nickel and chromium hydroxide. T h e precipitate was filtered and washed four times with demineralized water. After washing, it was heated in an electric oven a t 1000 "C for 24 h. The compound structure was verified by x-ray diffraction. The NiCrzOd was ground with alumina
VENT 6 A S CHROMATOGRAPH GAS MIXER
r
STOPCOCKS
FLOWMETERS-
NEEDLE VALVES
T
-4
J
4 1
Np
NO
SOA P BUBBLE BLRET "3 ABSORBER
Figure 1. Apparatus for dry gas system.
powder (Filtrol grade 90) with stearic acid (about 10%of the total mixture) added as a die lubricant. The thoroughly mixed powder was compressed into l/S in. X 3/R in. cylindrical pellets in a stokes single-tablet machine. Then, the pellets were calcined in air a t 450 "C for 20 h. 3. SrRu03 Perovskite. The perovskite, SrRuO3, was prepared by mixing and grinding SrC03 with RuO2 (1:l molar ratio) and heating the mixture a t 1000 "C for 24 h. The compound structure was verified by x-ray diffraction. The SrRuO:i was mixed with alumina powder and compressed into pellets following the same procedure described for the NiCr204 catalyst. 4. VzOj. One of the catalysts was a commercial (Harshaw Co.) 10% V205-Alz03, in. X in. cylindrical pellets. The laboratory-prepared V205 catalysts were made by soaking alumina cylindrical pellets (% in. X % in., Filtrol grade 86) in an oxalic acid solution containing dissolved ammonium metavanadate in the NH4V03to oxalic acid ratio of 2:l. The pellets were dried in an oven at 150 "C for 1 h and then heated in air a t 450 "C for 17 h. Two different weight percentages of V2Os (10 and 15%) on alumina carrier were prepared. In addition to the above V205 catalysts four other V205A120:i catalysts (15 wt %) were prepared following different pretreatment procedures as described: sample A was prepared by soaking 25 g of dried AlzO:{ pellets in 25 ml of 2.5 NHCl solution; sample B was prepared by soaking 25 g of A1203 pellets in 20 ml of 5.0 N Ca(OH)2 solution; sample C was prepared by soaking 25 g of A1203 pellets in 10 ml of 18 N Na2SiO:i solution; sample D was prepared by soaking 25 g of A1203 pellets in 20 ml of 3.1 N NaOH solution. These pretreated alumina pellets were dried in an oven a t 200 "C for -17 h and then impregnated with the vanadiumcontaining solution, dried, and calcined in air a t 450 "C following the same procedure as described above for the first two laboratory-prepared V ~ O Fcatalysts. , Reactor Apparatus. Studies of the reduction of NO in nitrogen carrier gas were conducted in a fixed bed integral flow reactor. Figure 1 shows the apparatus which consisted of a gas mixing manifold, a Pyrex upflow reactor (85 cm in length and 2.78-cm i.d.), two sampling tubes (20 cm apart) placed before and after the catalyst bed, and six thermocouple wells within the catalyst bed (chromel-alumel thermocouples). Isothermal conditions in the catalyst bed were monitored by an API Compack I meter-relay control to within f 2 "C. The NO
(99.0%) and NH3 (99.999%) gases were introduced to the manifold through individual metering Nupro precision needle valves and rotameters. Inlet and outlet gas samples were withdrawn from sampling tubes for analysis. The catalyst bed was supported by 5-mm-diameter glass beads. Nitrogen carrier gas flow rate was maintained a t 300 L (STP)/h (S.V. -20 000 h-1). The NO and N20 concentrations were determined by a Beckman Model 315A nondispersive infrared analyzer and a Perkin-Elmer Model 990 gas chromatograph (containing Porapack Q), respectively. Ammonia analysis was performed by absorption in boric acid and subsequent titration. Figure 2 shows the simulated flue gas system which consisted of a stainless steel reactor tube (12-mm i.d. and 13 cm in length) and a 4-mL stainless steel preheater coil (5-mm id.), both of which were immersed in a n electrically heated fluidized alundum powder sand bath to maintain isothermal conditions. The simulated flue gases contained 14%Con, 5% H20, and 3% 0 2 . Water was injected with a pump through a flow meter into an electrically heated (380 "C) vaporization tube (14-mm i.d. and 30 cm in length) located before the preheater. Ammonia was introduced before the vaporizer. The gas flow rate was 283 L (STP)/h, which gave a space velocity of about 20 000 h-l. The NO and NzO were analyzed by nondispersive infrared spectroscopy and gas chromatography, respectively; NO2 was analyzed by a Beckman continuous flow colorimeter; ammonia was analyzed as described above. The details of the constant volume apparatus and experimental procedure for the adsorption kinetic studies have been described elsewhere (Chien et al., 1975).
Results and Discussion Comparison of Catalytic Activity. Reduction of NO with NH3 in the absence and presence of oxygen (1.2%) in nitrogen carrier gas on alumina-supported SrRuO3 (ti%), NiCr204 (lo%),Fe--Cr oxide (lo%),Harshaw V205 (10%) and the laboratory-prepared VZOh (10%) catalysts was examined a t temperatures from 150 to 500 "C. Twelve grams of each catalysts were used in these experiments. Figure 3 shows that, except for SrRu0.7, which had by far the the greatest activity for the reduction of NO with other four catalysts, although much less active, did not differ too greatly in their effectiveness for removal of NO with NH,j in the absence of 0 2 . On the other hand, in the presence of' Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 2, 1977
137
n
FLOW METER
Figure 2. Apparatus for simulated flue gas system.
0
,
I
A IMPREGNATE0 V205
SrRu03
vp5
A 0 NiC iM&'&NATED
I
0 HARSHAW V205 8
V
Fe-Cr
1000 ppm NO 800 ppmNH3
"100
m
300
400
5 00
TEMPERATURE (OC)
1
I
I
I
200
300
400
500
TEMPERATURE ('C)
Figure 3. Catalytic reduction of NO with NH3 in the absence and presence of 0 ~ N carrier 2 gas.
gas.
oxygen (1.2%),SrRuO3 was the least active. The reduction of NO with NHs on the other four catalysts was accelerated by the presence of oxygen below -400 "C. The enhancement effect was greatest for the laboratory-prepared V205 catalyst followed by the Harshaw V205, Fe-Cr, and NiCrzO4. Below -350 OC, the Harshaw V2O5 and Fe-Cr catalysts had essentially equivalent activity in the presence of oxygen. In simulated flue gas, Figure 4 shows that the two V205 catalysts had the same activity and were the most effective Below 350 "C, Fe-Cr was the for the removal of NO with "3. next most effective, followed by NiCr204. SrRuO3 was the least active for removal of NO in simulated flue gas. These results show that the SrRuO3 perovskite, which is highly effective for the removal of NO in the reducing atmosphere of auto exhausts (Bauerle et al., 1974),has poor activity in the oxidizing atmosphere of flue gas of stationary sources. In contrast to 0 2 enhancement of the activity of Fe-Cr oxide, NiCrzO4, and the two V205 catalysts, the reduction of NO with NH3 on SrRu03-Al203 is inhibited by the presence of 0 2 . Similar results were obtained previously for the same reaction on Ru-Al203 catalysts (Bauerle et al., 1975a).
Although differences in activity of the two V2O5 catalysts in the COz-free,dry nitrogen carrier gas are apparent, it should be noted that the activities were essentially equivalent in the presence of the substantial amounts of CO2 and HzO in simulated flue gas. Effect of Catalyst Pretreatment on Activity. In addition to the Harshaw and the laboratory-prepared V2O5 catalysts, a commercial VzO5 catalyst (Girdler-Sudchemie type G-l01), which was specifically developed for SO2 oxidation in contact sulfuric acid plants, was examined for activity for the NONH3 reaction in simulated flue gas. Manufacturer specifications indicate that the catalyst is a silica carrier impregnated with vanadium and potassium salts (K20:V205 ratio of -2.7:l). The G-101 catalyst had practically no activity for NO reduction with NH3 in simulated flue gas between 200 and 460 "C. The temperature of maximum conversion (
