Lanewala, M. A , , Pickert, P. E., Bolten, A. P., J. Catal., 9,95 (1967). Receiued for reuiew October 11.1973 NL Industries, Baroid Division Brochure Introducing Barasyrn SMM. Accepted F e b r u a r y 28,1974 Thomas, C . C., Barrnby, D. S . ,J. Catal., 12,341 (1968). . Prod, R ~ D ~ ~ , ~ ~ / Presented ~ ~ , , a t t h e D i v i s i o n of P e t r o l e u m C h e m i s t r y , 1 6 7 t h N a t i o n a l Voorhies, A , , Jr., Beecher, R . G.,/nd. ~ n g Chem, M e e t i n g o f t h e A m e r i c a n C h e m i c a l Society, Los Angeles, Calif., 8,366 (1969). Wright, A . C., Granquist, W. T., Kennedy, J. V., J. Catal., 25,65 (1972). April 1974.
Catalyst Evaluation for the Simultaneous Reduction of Sulfur Dioxide and Nitric Oxide by Carbon Monoxide Vernon N. Goetz, Ajay Sood,* and J. R. Kittrell Department of Chemical bgineering, University of Massachusetts, Amherst, Mass. 07002
A catalyst test procedure has been developed which provided a comparison of the activity and selectivity of several commercial and experimental catalysts for the simultaneous reduction of SO2 and NO by
CO in tubular flow reactors using dry cylinder gas mixtures. Copper-alumina was an effective catalyst, when 12% copper was impregnated on alumina and calcined at 1000°F. Although iron-alumina and chromium-alumina are also active catalysts, an iron-chromium catalyst was found which was 10 times as active as the best copper-alumina catalyst tested. Catalyst selectivity, as measured by COS yields, was unaltered by all preparational changes on copper-alumina catalysts. However, the COS production of the iron-chromium catalyst was as much as four times that of the copper catalysts.
Two major air pollutants, sulfur dioxide and nitric oxide, are emitted in large quantities by coal and oil fired power plants (Bartok, e t al., 1971; Chilton, 1971). Numerous processes for the abatement of sulfur dioxide and nitric oxide emissions from stationary sources have been suggested (Bartok, e t al., 1969; CEP Technical Manual, 1971). Most of these processes are directed toward fossilfired power plants since they represent large localized sources of air pollution. One process, which appears attractive from an economic standpoint (Duffy, 1972; Quinlan and Kittrell, 1973) and which produces sulfur in the easily handled elemental form, utilizes the catalytic reduction of sulfur dioxide and nitric oxide with carbon monoxide. Carbon monoxide appears to be an attractive reductant since it can be readily produced by steam-methane reforming or partial oxidation processes, and some evidence indicates that carbon monoxide levels can be controlled by low excess air firing (Munroe, 1964). Carbon monoxide reduction of sulfur dioxide has been studied by Khalafalla e t al. (1971), over catalysts comprised of binary mixtures of iron and alumina and a peak activity with a 43% iron catalyst was demonstrated at 932°F; Ryason and Harkins (1967) investigated the simultaneous reduction of SO2 and NO with CO over impregnated copper, silver, and palladium catalysts supported on alumina, as well as manganese, nickel, silver, and copper catalysts supported on silica gel. Maximum SO2 removal efficiencies of 97% a t 1000°F and a space velocity of 10,000 hr-1 were demonstrated, with a virtually complete removal of NO, on the copper-alumina catalyst. Querido and Short (1973) screened five catalysts for the CO-SO2 reduction and concluded that the Harshaw copper on alumina catalyst (Cu 0803) had the maximum activity of those tested. Quinlan, et al. (1973a,6) and Okay and Short (1973) studied the effect of various process variables on the activity and selectivity of this Harshaw copper catalyst, with and without the presence of nitric oxide and water. The addition of nitric oxide and water to the CO-SO2 feed 110
Ind. Eng. Chem., Prod. Res. Develop., Vol. 13, No. 2 , 1974
was shown to depress the activity of the catalyst for SO2 conversion. The activity loss was reversible, suggesting an adsorption effect. If the carbon monoxide reduction process is to be competitive with other processes for S02-NO removal from stack gases, improved catalyst compositions must be developed. Accordingly, the major goal of the present study was to develop a suitable catalyst testing procedure and to conduct an initial catalyst screening program to isolate active catalysts for the simultaneous reduction of sulfur dioxide and nitric oxide with carbon monoxide. Both laboratory prepared and commercially available catalysts were evaluated and the results are reported in this paper. Specifically, the activity of catalysts is reported as a function of the catalytic metal and the effect of different preparational variables such as impregnation technique, copper level, calcining temperature, and duration of calcining is described for the copper-alumina catalyst. The important reactions that occur during the carbon monoxide reduction of sulfur dioxide and nitric oxide are
+
+ 2co + so2= 2C02 + xs2 co + %s2=c0s 2 c o s + so, = 2c0, + 3AS, CO
NO=COz
f/2N2
(1)
(2)
(3) (4)
Equations 1 and 2 are the reduction of NO and SO2 to nitrogen and elemental sulfur, respectively. Equation 3 represents the formation of carbonyl sulfide from CO and sulfur, as noted by several researchers previously (Quinlan, 1972; Ryason and Harkins, 1967); the formation of COS is undesirable since it is a toxic gas. Equation 4 is the direct reaction of COS and SO2 t o produce elemental sulfur (Querido and Short, 1973). In the present study, dry, pure cylinder gases were blended to obtain a feed gas containing approximately 6200 ppm of CO, 2000 ppm of SO2, 600 ppm of NO, and
the balance Nz. These levels of SO2 and NO are typical of actual stack gases; carbon monoxide in slight excess of the stoichiometric requirements was used. No water, ash, or unburned hydrocarbons were added to the feed gas. Experimental Section The equipment used in these catalyst screening studies is shown schematically in Figure 1. Pure, compressed gas cylinders of N2, CO, S02, and NO were used and the gases were blended to the desired composition with the help of micrometering valves and rotometers. The gas mixture was split into several streams, one being used to sample the upstream gas composition while the other six were fed to six titanium reactors in parallel. The flow to each reactor was carefully monitored by a rotometer and the upstream reactor pressure measured by a 0-300 mm pressure gauge. The reactor pressure was nearly atmospheric, the total pressure drop across the system being approximately 200 mm. A safety pressure relief valve was installed in each line to release the gases to a venting hood in case of any plugging in the reactor or any of the downstream lines. Also, as a safety precaution, the entire apparatus was enclosed and vented to an exhaust fan. Each of the six catalyst beds was contained in individual ?&-in.I.P.S. titanium pipes with an internal diameter of approximately y4 in. Titanium was chosen as the reactor material since Quinlan, et al. (1973b), have shown that it does not catalyze the reduction of SO2 or NO by CO. The titanium reactors were arranged in a circle of 1314in. internal diameter and suspended in a vertically mounted Lindberg Heavi-Duty three-zone, hinged tube furnace. The catalyst bed was supported by a 40-mesh stainless steel screen. All catalyst charges weighed 3 g and ranged from 33/4 to 4y4 in. in height. The average catalyst particle size used in all runs was 20-/30+ U. S. Sieve Series mesh since it provided a particle to internal reactor diameter ratio slightly smaller than the minimum recommended by Dowden and Bridger (1957) in order to ensure plug flow through the catalyst bed. Calculations and experimental tests for internal diffusion restrictions indicated that the minimum effectiveness factor of the catalyst particles is 0.7 and that it is likely to be near 1.0. It was also found that film diffusion limitations were negligible. Temperature control of the three-zone furnace was provided by a Lindberg Heavi-Duty controller and the temperature measured with three chromel-alumel thermocouples mounted on the outer wall of each of the titanium reactors at 2-in. intervals over the catalyst bed. A Leeds and Northrup potentiometer was used to monitor the thermocouple output. The settings of the three-zone furnace were adjusted to operate the catalyst bed isothermally within *3"F. After contacting the catalyst, the product gas from each reactor was passed successively through lines traced with heating tapes (to prevent line plugging due to premature sulfur precipitation), room temperature sulfur precipitators, and 32°F cold traps (to condense elemental sulfur) and a sampling manifold. The sampling manifold permitted sampling of the upstream gases or any of the six reactor downstreams without disturbing the flow system. A Varian Aerograph Model 1860-30 chromatograph was used for CO, COS, and SO2 analyses. The carrier gas was helium and a hot wire detector operating a t 115°C detector temperature and 235 mA detector current was used for the analyses. Details of the chromatographic analysis are listed in Table I. Analysis for NO was performed using a Dynasciences Pollution Monitor NX 130 equipped with a SO2 scrubber. Due to the corrosive nature of SO2 and NO, type 304 stainless steel tubing and stainless steel valves were generally employed.
Figure 1. Schematic flow diagram of experimental equipment. Table I. Details of Chromatographic Analysis for CO, COS, and SO?
Column A Gas analyzed Column packing Packing size Column Helium flow rate, cm3/min Column temperature
Column B
co
cos, so*
50
30
68OC
68OC
Molecular Sieve 5A Porapak QS 80/100 mesh SO/lOO mesh 6 ft X in. S.S. 6 ft X in. S.S.
In the preparation of catalysts the support was impregnated with the metal nitrate without prior evacuation, and a Fisher forced draft oven was used for drying. An Ohaus moisture determination balance was employed for measuring moisture remaining in dried catalysts. The nitrate was decomposed in a Lindberg box type calcining furnace before charging the catalyst to the reactor. The calcining furnace temperature was automatically controlled and programmed by a Leeds and Northrup Speedmax H indicating and recording controller used in conjunction with a Trendtrak program control unit, thus permitting unattended operation. Results Catalyst Screening Test. The catalyst screening test consisted of start-up and catalyst activation, changing to actual test conditions, measuring steady-state conversions, and shut-down. The catalysts were activated with a feed gas containing approximately 6000-6500 ppm of CO and 1900-2100 ppm of SO2 and balance nitrogen (no NO present), at a temperature of about 825°F. At these conditions, the catalysts generally required 3-5 hr to be totally activated; during this period, conversion of SO2 would rise to 100%. Such a start-up procedure has been shown to provide a reproducible catalyst activity (Quinlan and Kittrell, 1973). After this activation period, the conditions were changed to the test conditions, which were normally 745°F catalyst temperatures and a space velocity of about 40,000 hr-1 for copper catalysts. At this time, 600-640 ppm of NO was also added to the feed gas. After the introduction of NO, a transient in the activity of the catalyst for SOa conversion due to preferential NO adsorption was observed as shown in Figure 2. Hence all activity data were taken after the catalyst activity stabilized, or about 16 hr after NO addition. Each analysis for CO, COS, SOz, and NO took approximately 15 min and the seven lines (one upstream and six reactor downstreams) could be sampled in approximately 2 hr. Usually, two or three sets of data Ind. Eng. Chem., Prod. Res. Develop., Vol. 13,No. 2, 1974
111
I
0" l90oo P h cn
I
I
\"
SYMBOLS DENOTE DIFFERENT PREPARATIONS OF 12 X Cu ON KAISER ALUMINA, KA-201
I
v,
f
0 COPPER ON KAISER ALUHlNACB4LYSTS
CALCINE0 AT 1 W . F - 0803,
HARSHAW cu
COFPER
1
CONDITIONS
1 0
60
7 4 5 . F . 6 3 0 c c / g m min.S.V 2000 ppm 502 6 M ppm NO 6M)O ppm CO Feed
3 20
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745' F . 6 3 0 cc/pm min S V 2 O l J p SO2 , 6 2 0 ppm NO
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,
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z
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WEIGHT PERCENT COPPER IN CATALYST
TIME ON S T R E A M WITH NITRIC OXIDE,HOURS
Figure 2. Approach to steady-state SO2 activity after nitric oxide addition.
I
2
/
Figure 3. Effect of catalyst copper level on SO2 activity. TEST CONDITIONS 745'F, 630 cc/gmmin S V
Table 11. Properties of Kaiser Alumina KA 201"
Typical Chemical Analysis
62M)wm CO Feed N
Component
Weight Per Cent, Dry Basis
SOz Fea03 Ti02
NapO A1203 Loss on ignition
0.002 0.30 93.6 6.058
Data provided by Kaiser Chemicals.
were taken to ensure that steady-state conditions had been reached. Most runs were of 30-40 hr duration. Among the several runs, there were small variations in reaction conditions such as catalyst temperature and upstream CO level. The SO2 conversions obtained with different catalysts were corrected to a common catalyst temperature and CO level by using the procedure recommended by Quinlan, et al. (1973a), for the Harshaw copper on alumina catalyst, Cu 0803. This correction was considered justified since the response of the laboratory prepared copper on alumina catalysts to different process variables was similar to that observed for the Harshaw catalyst. In most cases, the corrections involved were quite small. Effect of Catalyst Preparational Variables. The effect of several preparational variables on the activity of the catalyst was studied by preparing a series of copper on alumina catalysts in the laboratory. All catalysts were prepared by impregnation of a high surface area Kaiser alumina KA 201 with the desired metal salt solution (e.g., copper nitrate) with subsequent drying and calcining. The characteristic properties of this alumina support are shown in Table 11. Table I11 contains activity data for a series of 12% (by weight) copper on Kaiser alumina catalysts which were dried, calcined, and tested under almost identical conditions. The catalysts differ in the size of the catalyst batch prepared, the number of impregnations used, the temper112
0
0.02 0.02
Typical Physical Properties Surface area 380 m2/g Pore volume 0.51 cm3/g Average pore diameter 50 A Bulk density, packed 47-48 lb/ft3 Static adsorption at 60% relative humidity 21-22 % Abrasion loss 0.2% Crushing strength 50 lb force a
0
Ind. Eng. Chem., Prod. Res. Develop., Vol. 13, No. 2 , 1974
I
100'
I
I
900.
1
I I 100.
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I 1300'
T E R M I N A L CALCINATION TEMPERATURE,OF
Figure 4. Effect of calcination temperature on SO2 activity. ature to which the cupric nitrate solution was heated before mixing with the alumina, and the pH of the solution at the time of impregnation. There does not appear to be any trend in catalyst activity for SO2 reduction with any of these variations in the method of preparation, since the 1Wo scatter observed in the SO2 conversion is considered to be due to experimental error. The effect of copper level on the activity of the catalyst is presented in Figure 3. It is apparent that the optimal copper level for the Kaiser alumina is about 12% by weight. Although the peak is not well defined, it is unlikely that significant activity improvements can be achieved by higher copper loadings. The peak activity obtained from a 12% copper catalyst is slightly over 60% SO2 conversion, compared to less than 20% conversion for the Harshaw Cu 0803, which is the catalyst studied by Querido and Short (1973), Quinlan, et al. (1973a,b), and Quinlan and Kittrell (1973). Using the first-order rate constant as a comparative basis of catalyst performance leads to the conclusion that the 12% copper catalyst is five times as active as the Harshaw Cu 0803. The NO conversion for all these catalysts is over 90%. Different calcination conditions also have a slight effect on the activity of the catalyst. In Figure 4, a broad maximum in catalyst activity is shown between about 800 and 1000°F calcination temperature. This temperature range corresponds to the range of peak acidity for alumina (Tanabe, 1970). In Figure 5, calcination time is shown to have only a slight effect on SO2 conversion activity of the catalysts. In all the catalysts reported in Figures 4 and 5, nitric oxide was reduced a t greater than 90% conversion levels.
Table 111. Effects of Methods of Preparation on the Activity of 12% Copper on Alumina Catalysts.
Inlet Catalyst identification
Wt of No. of imalumina pregnations in cat. batch used
N-21
10 g
2
N-25
12 g
2
N-31
10 g
2
N-32
10 g
2
N-33 N-34
15 g 10 g
1
Temp ( O F ) of soln a t time of impreg
Soln pH a t time of impreg
(1) 180 (2) 160 (1) 78 (2) 76 (1) 76 (2) 75 (1) 188 (2) 180 78 77
(1) 3 . 0 (2) 3 . 0 (1) 2 . 6 1 (2) 2 . 7 0 (1) 2 . 7 0 (2) 2 . 7 5 (1) 2 . 3 0 (2) 2 . 3 8 1.70 1.70
1
Corrected % of inlet SO2 reduced
Inlet NO converted,
SOacon-
%
verted t o cos, %
66
100
7
65
100
5
76
100
9
72
100
2
68 69
100
100
4 3
5Tested at: 745OF, 40,000 hr-1 space velocity; 2000 ppm of SO2, 620 ppm of NO, 6200 ppm of CO in feed gas. Table IV. Comparison of Activity of Selected Catalystsa
Catalyst
Composition
% SO2 conv.
% NO' conv.
% COSh prod.
Harshaw Cu 0803 n'arshaw P d 0501 Girdler G3A Girdler G49A Girdler G13 Girdler G22
8% Cu on alumina 21 93 6 P d on alumina 0 5 3 Cr promoted iron oxide 100 96 50 Ni on Kieselguhr 21 90 19 Copper chromite 59 66 6 Barium promoted cu chromite 30 ... 4 Laboratory 12% Cu on Kaiser 60 97 8 alumina Tested at: 745'F, 40,000 hr-l space velocity, 2000 ppm of SO*,620 ppm of NO, 6200 ppm of CO in feed gas. b Percentage of SO, in feed.
