RA = hydraulic radius defined by Equation 2
N, = modified Reynolds number for spherical packed 6Dc(1 - t ) 150p(1 - t)
DPG
+1
40p + 11 + 1.75 [ 6D,(1 -
(6)
t)
This includes the effect of the wall on the hydraulic radius as a parameter. Data for pressure drop us. flow rate through packed beds are correlated more accurately when the friction due to the wall surface area or the effect of the wall on the hydraulic radius is taken into account, particularly when the column to particle diameter ratio is less then 50 to 1. With the help of the modified Ergun equation (Mehta, 1966) laboratory data obtained under conditions where the wall effect is important can be scaled to size where the wall effect is not important. Since the experimentation was limited to values of D,/ D, > 7 , Equation 5 should be used only for D,/D, > 7. Nomenclature
D, D, G g, L M
= = = = = =
diameter of column diameter of glass beads mass flow rate gravitational constant length of the bed correction factor defined by Equation 4
bed, wall effect neglected NRe= modified friction factor for spherical packed bed, wall effect neglected Ap = pressure drop c = void fraction p = viscosity of fluid p = density of fluid literature Cited
Bird, R . B., Stewart, W. E., Lightfoot, E. N., “Transport Phenomena,” pp. 44-6,180-200, Wiley, New York, 1965. Blake, S,P., Trans. A m . Inst. Chem. Eng. 14, 415 (1922). Burke, S. P., Plummer, W. B., Ind. Eng. Chem. 20, 1196 (1928). Carman, P. C., Trans. Inst. Chem. Eng. (London) 15, 150 (1937). Ergun, S., Chem. Eng. Progr. 48, 89 (1952). Ergun, S., Orning, A. A., Ind. Eng. Chem. 41, 1179 (1949). Lea, F. M., Nurse, R. W., Trans. inst. Chem. Eng. (London) 25, Supplement, 47 (1947). Mehta, Devendra, M. S. thesis, Michigan State University, 1966. Morcom, A. R., Trans. Inst. Chem. Eng. (London) 24, 30 (1946). Morse, R. D., Ind. Eng. Chem. 41, 1117 (1949). Perry, J., ed., “Chemical Engineer’s Handbook,” p. 198, McGraw-Hill, New York, 1965. RECEIVED for review July 22, 1968 ACCEPTED January 7, 1969
CATALYTIC COMBUSTION OF CARBON MONOXIDE ON COPPER OXIDE Effect of Carbon Dioxide N . T . T H O M A S , 1. S. C A R E T T O ’ , A N D K E N N O B E School of Engineering and Applied Science, Uniuersity of California, Los Angeles, Calif. 90024
The effect of carbon dioxide on the catalytic combustion of carbon monoxide on copper oxide was investigated. The carbon dioxide content of the feed stream was varied from 0 to 10% with the total gas flow maintained constant a t 400 liters (25OC., 1 atm.) per hour. The initial concentration of carbon monoxide was varied from 300 to 1150 p.p.m., and reaction temperatures ranged from 60’ to 134OC. Correlation of the experimental data with the simple power l a w rate equation showed that the reaction order increased from 0.3 to 0.8 for carbon dioxide concentrations ranging from 0 to 10%. A more general correlation of the kinetic data with a single rate equation for the entire range of carbon dioxide concentration was achieved.
THISinvestigation is a quantitative
study of the effect of carbon dioxide on the rate of catalytic oxidation of carbon monoxide a t low concentrations. It is a part of the continuing University of California program on air pollution control by heterogeneous catalysis.
’ Present address, University of California, Berkeley, Calif. 282
I & E C PROCESS D E S I G N A N D D E V E L O P M E N 1
There have been considerable literature on the catalytic oxidation of carbon monoxide and reviews by Katz (1953) and Dixon and Longfield (1960), but few investigations of the inhibiting effect of carbon dioxide on the activity of catalysts for oxidation of CO. Almquist and Bray (1923) studied the oxidation of carbon monoxide on copper oxide, manganese dioxide, and
mixtures of these two oxides, and found carbon dioxide in the feed stream had no marked effect on the activity of the catalyst. Jones and Taylor (1923) studied the oxidation of carbon monoxide on copper oxide using a static system and postulated a reduction-oxidation mechanism. They concluded that the rate of carbon monoxide oxidation was negligible until copper nuclei were formed on the copper oxide. Carbon dioxide in small concentrations inhibited the reduction of copper oxide to copper nuclei, decreasing the rate of oxidation. Schwab and Drikos (1942) studied the catalytic oxidation of carbon monoxide on copper oxide from 280" to 400°C. and observed no effect of C 0 2 in this range. Previous studies (Blumenthal and Nobe, 1966) on the adsorption of carbon dioxide and carbon monoxide on copper oxide showed that preadsorbed carbon dioxide markedly decreased the adsorption of carbon monoxide. Furthermore, carbon dioxide in the feed stream inhibited carbon monoxide oxidation. Carbon dioxide also decreased the activity of CuO-A1203catalysts for oxidation of carbon monoxide (Accomazzo, 1963). Parravano (1953) studied the oxidation of carbon monoxide on pure nickel oxide and found the activity of the catalyst reached a minimum constant value after an initial period of high activity. The rate of the reaction after the activity of' the catalyst became constant was 0.5-order from 106' to 154°C. and first-order from 205" to with respect to the carbon monoxide. Although he reported no effect of C o r on the rate of oxidation in the high temperature range, there was no mention of effects observed in the low temperature range. Winter (1955) found that carbon dioxide did not affect the oxidation of carbon monoxide on cuprous oxide but did affect it on nickel oxide. Dell and Stone (1954) investigated the adsorption of carbon monoxide and carbon dioxide on nickel oxide and concluded that adsorbed carbon dioxide on the surface of the nickel oxide inhibited the oxidation of carbon monoxide a t low temperatures. They concluded that below 160" C. the catalytic oxidation of carbon monoxide proceeds through adsorbed species without the direct participation of the oxygen ions of the catalyst. Rogensky and Teselinskaya (1948) observed that during the oxidation of carbon monoxide on nickel oxide below 160°C. the catalyst is poisoned by adsorbed carbon dioxide but is not, poisoned above this temperature. Teichner and Marcellini (19571, Courtois and Teichner (1964),and Gravelle and Teichner (1964), investigated the effect of carbon dioxide on the adsorption of CO and oxygen on nickel oxide. They surmised that adsorbed carbon dioxide and oxygen formed a carbonate complex and attributed the poisoning of the catalyst for CO oxidation to the fraction of this complex which was strongly adsorbed. Experimental
A schematic drawing of the reactor has been given (Cohen and Xobe, 1966). The properties of the catalyst are given in Table I. Compressed air was first passed through a water trap to remove oil and then through a drying tube containing molecular sieve (Linde, 1 3 ~ to ) remove water vapor and COz. The drying tube (40 cm. in height and 5.5 cm. in i.d.1 contained 1.2 pounds of the desiccant. The desiccant was changed every 6 hours and regenerated by passing hot air a t 600°F. through the used molecular sieve. The
Table I. Physical Properties of CuO Catalyst
Diameter, cm. Height, cm. Wt. per pellet, mg. BET surface area, sq. m./ g. Pore vol., ml.1 g. Mean pore radius, A. Wt. of catalyst bed, g.
0.2 0.2 9.4 12.5 0.47 752 20
flow rate of the air was controlled by a pressure regulator and a precision needle valve, and metered with a calibrated Fischer and Porter glass Flowrator before it entered the mixing chamber. The flow rate of Matheson carbon dioxide (Coleman grade, 99.995 purity) was controlled by a pressure regulator and a very fine precision Nupro needle valve. Two calibrated Fischer and Porter Flowrators were used for metering the flow of carbon dioxide, one for low and the other for high flow rates. The initial concentration of carbon dioxide in the feed stream was determined by the ratio of the flow rate of carbon dioxide t o the total flow rate of the feed stream. When very low concentrations of carbon dioxide were required, the flow rate was determined by a soap bubble buret. The flow rate of Matheson carbon monoxide (c.P. grade, 99.sLcminimum purity) was controlled by a pressure regulator and fine precision needle valve. The carbon monox ide concentration was measured accurately by a MSA Model Lira 300 infrared analyzer. The inlet and exit concentrations of CO were determined and monitored with the infrared analyzer connected to a Leeds & Northrup recorder. The infrared analyzer scale reading was from 0 to 2000 p.p.m. of carbon monoxide and had a nonlinearity less than 2% of full scale. The zeroing with dry air and the calibration with MSA span gas (1860 p.p.m. of carbon monoxide) were checked before each run. The analyzer was not sensitive to carbon dioxide. During a run the exit stream was monitored continuously until steady state was reached. After the temperature of the bed and the outlet concentration reached a steady state, the inlet concentration of carbon monoxide was measured. The procedure for preparing the copper oxide catalyst was given by Cohen and Kobe (1966). The catalyst was activated by passing air a t 400°C. through the bed for 48 hours. After activation, the reactor was in operation continuously. T o obtain reproducible data after the reactor was shut down for a prolonged time the catalyst bed had to be reactivated. Prior to each run, the catalyst bed was heated to 210°C. for a t least 10 minutes. The flow rate of the feed stream was maintained a t 400 liters (25"C., 1 atm.) per hour ( S V = 22,000 hr.-') for all runs. Previous studies (Koutsoukos and Nobe, 1965) indicated that plug flow conditions were achieved a t this gas flow rate. The concentration of the carbon dioxide was varied from 0 to lo%, and the concentration of carbon monoxide was varied from 300 t o 1150 p.p.m. Results and Discussion
Typical experimental results for the effect of carbon dioxide on the rate of oxidation of carbon monoxide are given in Figures 1 and 2. The degree of conversion increased with temperature and decreased with increase in VOL. 8 NO. 2 APRIL 1969
283
All the carbon monoxide oxidation data were correlated initially with the power law rate equation,
IOO-
80 -
-
8
2
60-
K Y
-
w
>
20-
1
I
I
I
I
1
BED TEMPERATURE
Figure 1. Catalytic oxidation of carbon monoxide with 2500 p.p.m. of COS in feed stream Initial CO concentration, p.p.m.
A
0 A
300 500 800
0
1150
100-
1
60-
40-
20-
I
I
BO
90
I 100
BED
I
1
I IO
120
1
I30
1 135
TEMPERATURE
Figure 2. Effect of carbon dioxide content on CO oxidation Initial C O concentration 1150 p.p.m. Concentration of carbon dioxide in feed stream, p.p.m.
10,000 25,000
0 0 0 650
A
2,500
0 5,000
+
A
50,000 100,000
the initial concentration of carbon monoxide and the concentration of carbon dioxide in the feed stream (Figure 3). The poisoning effect of carbon dioxide on copper oxide in the oxidation of carbon monoxide was reversible (Figure 4). T o demonstrate this reversibility, runs of carbon monoxide oxidation in the absence of C 0 2 were made after the runs with 5000 p.p.m., 2.5%, and 5% COZ in the feed stream and compared with the initial run with 0% COe. After each run the catalyst bed was heated to 210" C. for 10 minutes. The initial activity of the catalyst was regained subsequent t o this reactivation procedure. 284
k = k oe- E . ? ]
(2)
r=
BO 8
(1)
The complete set of experimental data is given elsewhere (Thomas, 1966). The kinetic parameters for each concentration of COzwere determined with the computer program developed by Accomazzo and Nobe (1965). Values are given in Table 11. Figure 5 compares typical experimental and calculated values and shows very good agreement. The apparent reaction order increased from 0.3 to 0.8 and the apparent activation energy increased from 1'7.4 to 25.3 kcal. per mole when the concentration of carbon dioxide in the feed stream increased from 0 to 10% (Table 11). Claude1 et al. (1964) investigated the oxidation of carbon monoxide on thoria. Their results indicated a reaction order of 0.3 with respect to carbon monoxide and an activation energy of 17.7 kcal. per mole which was comparable to the 17.4 kcal. obtained in this investigation (in the absence of C 0 2 in the feed stream). Although their results were rather sketchy, they indicated that carbon dioxide inhibited the carbon monoxide oxidation reaction and that the rate of oxidation (in the presence of excess oxygen) could be expressed as
40-
1(
r = kPE, where
l&EC PROCESS DESIGN A N D DEVELOPMENT
UP;:, 1 + bP,,
(3)
Maxted (1951) attributed catalyst poisoning to selective adsorption of the inhibitor on previously active sites. These sites were then inactive for further reaction. For such poisoned catalysts, the total number of active sites was decreased, and the catalyst exhibited a lesser activity for reaction. An increase in concentration of the poisoning substance in the gas phase increased adsorption and thus increased the fraction of the catalyst surface which was inactive. Blumenthal and Nobe (1966) observed that the copper oxide surface was heterogeneous for the adsorption of carbon dioxide-that is, the adsorption sites had different activation energies for adsorption. Furthermore, the adsorption rate of carbon monoxide was considerably lower on catalyst with preadsorbed carbon dioxide than on catalyst free of adsorbed CO,. Thus, sites active for CO, adsorption were also active for CO adsorption. I t is reasonable that in the oxidation of carbon monoxide on CuO, COz in the feed stream adsorbs on part of the active catalyst surface and subsequently decreases its activity for the oxidation reaction. Figure 2 shows the inhibiting effect of carbon dioxide on the catalytic combustion of carbon monoxide. Although the experimental data show that carbon dioxide poisons the CuO catalyst for carbon monoxide oxidation, the original activity may be regained by simply heating the catalyst above 210" C. for 10 minutes (Figure 4 ) . Lower temperatures would reactivate the catalyst, but longer heating times would be required. Reactivation of the catalyst by heating is just the desorption of the carbon dioxide from the surface sites which are active for the oxidation reaction. The above observations substantiate those of others that for the oxidation of carbon monoxide the metallic oxides are not poisoned by C 0 2 a t the higher temperatures.
o
I
I
1
1 1 1 ' 1 1
I
I
I 1 1 1 1 1
I
1
I
I
1
1 Ill1
I
J
1 I 1 1111
Table II. Effect of Carbon Dioxide on Power l a w Kinetic Parameters in Catalytic Oxidation of Carbon Monoxide
CO2 Concn. in Feed, Reaction P.P.M. Order
60
20
0 650 2,500 5,000 10,000 25,000 50,000 100,000
1.43 x 4.15 X 29.95 x 46.25 x 25.91 x 48.27 x 10.62 x 23.00 x
13.00 x 10.05 X 6.15 x 5.35 x 4.05 x 2.35 x 2.00 x 1.53 x
17.40 23.20 24.53 24.45 24.24 24.59 24.89 25.30
10' 10" 10" 10'' 10" 10" 10" 10l2
Oxidation Rate 100" C., 1150 P.P.M. CO, Mole CO/ Hr.-G. Cat. lo-' lo-'
io-' io-' 10-~ 10 -' 10 '
, r t-
0
0.30 0.35 0.45 0.55 0.55 0.65 0.75 0.80
Frequency Actiuation Factor, Energy, Moles per G. Kcal. i Cat.-Hr.-Atm. Mole
65
70
eo
90
100
io-'
110
BED TEMPERATURE
eo -
Figure 4. Reversibility of carbon dioxide poisoning of CuO in catalytic oxidation of CO Initio1 CO concentration 1150 p p m .
$
0% co1
0 Initial run 0 After run with 5000 p,p.m. h After run with 2 . 5 % CO2 0 After run with 5 % CO?
i 11
2W
CO2
>
60-
40-
Correlation of all the experimental data obtained in this investigation with a rate equation and a single set of kinetic parameters was attempted. Several rate equations which are frequently used to correlate data of inhibited catalytic reactions were examined with the assistance of digital computers, but satisfactory correlation was not achieved. However, a reasonably good correlation of the data was obtained with the rate equation,
t 01 e5
I
I
I
95
IO5
115
I 125
BED TEMPERATURE
Figure 5 . Catalytic oxidation of carbon monoxide with 2.5% COn in feed stream tines represent calculated dota (power law rate equation) Points represent experimental data Initial CO concentration. p.p.m.
A
A 300
gram moles per gram of catalyst per h o u r
(4)
0 500
800
01150 VOL. 8
N O . 2 APRIL 1 9 6 9
285
-
100
8 20
-
2 > 0'
90
-
80
-
70
-
6050-
Y
V
40-
30 20
-
IO
-
OL 70
I
I
I
I
I
80
90
100
IIO
120
TEMPERATURE
('C)
Figure 6. Correlation of experimental kinetic data with rate equation AP;; r =
PU 1 + B' ;
co Initial C O concentration 800 p.p,m, tines represent calculated data Points represent experimental data Concentration of CO1 in feed stream, ,p.p.m. A 1o,ooo 0 0 650 25,000 0 2,500 0 50,000
A
5,000
where
Nomenclature 6,410
gram moles hr. gram -atm."'
T,
A = 2.54 x 10'e
for T I 373°K.
_ _1,532 _ = 0.532 e
T
for T 2 373°K.
and 13,100
B = 9.8 x 10-16e
, atm.?
A = constant in Equation 4, moles/ hour-gram cat.atm." a = constant in Equation 3, moles/ hour-gram cat.atm." B = constant in Equation 4, atm.-"' b = constant in Equation 3, atm.-' E = apparent activation energy, kcal. / mole k = rate constant, moles/ hour-gram cat.-atm." k , = frequency factor, moles/ hour-gram cat.-atm." n = reaction order pco = partial pressure of CO, atm. pc, - partial pressure of COz, atm. R = gas constant r = reaction rate, moles/ hour-gram cat. sv = gas space velocity, hour-' T = temperature. K O
Figure 6 gives a typical comparison between experimental and calculated data obtained with the above equation for the oxidation of carbon monoxide a t various concentrations of carbon dioxide in the feed stream. A reasonably satisfactory correlation was achieved for the kinetic data. Previous results (Cohen and Nobe, 1966) compared to those obtained in this investigation indicate that water vapor is a better inhibitor of carbon monoxide oxidation on copper oxide than carbon dioxide. 286
I&EC PROCESS DESIGN A N D DEVELOPMENT
Literature Cited
Accomazzo, M. A., thesis, University of California a t Los Angeles, June 1963. Accomazzo, M. A., Nobe, Ken, IND.ENG.CHEM.PROCESS Design Develop. 4,425-30 (1965). Almquist, J. A . , Bray, W. C., J . A m . Chem. SOC.45, 2305-322 (1923). Blumenthal, J. L., Nobe, Ken, IND.ENG. CHEM.PROC. DESIGNDEVELOP.5, 177-83 (1966). Claudel, B., Juillet, F., Trambouze, Y ., Veron, J.,
Proceedings of Third International Congress on Catalysis, Vol. 1, pp. 214-26, Xorth Holland Publ. Co., Amsterdam, Holland, 1965. Cohen, M., Nobe, Ken, IND. ENG. CHEM.PROC.DESIGN DEVELOP. 5, 214-17 (1966). Courtois, M., Teichner, S. J., J . Chim. Phys. 61, 62531 (1964). Dell, R . M., Stone, F. S., Trans. FaradaJ SOC.50, 50110 (1954). Dixon, J. K., Longfield, J. E., Catalysis 7, 281-343 (1960). Gravelle, P., Teichner, S. J., J . Chim. Phys. 61, 62531 (1964). Jones, H. A., Taylor, H. S., J . Phys. Chem. 27, 62351 (1923). Katz, M., Aduan. Catalysis 5, 177-216 (1953). Koutsoukos, E . P., Nobe, Ken, Ind. Eng. Chem. Prod. Res. Develop. 4, 153-7 (1965).
Maxted, E. G., Advan Catalysis 3, 129-77 (1951). Parravano, G., J . A m . Chem. SOC.75, 1448-51 (1953). Rogensky, S. G., Teselinskaya, T. F., J . Phys. Chem. USSR 22, 1360 (1948). Schwab, G.M., Drikos, G. Z., 2 . Phys. Chem. 52B, 23452 (1942). Teichner, S. J., Marcellini, R. P., Aduan. Catalysis 9, 458-71 (1957). Thomas, N . T., thesis, University of California a t Los Angeles, January 1966. Winter, E. R . S., J . Chem. SOC.1955, Pt. 3, 2726-40. RECEIVED for review July 24, 1968 ACCEPTED December 9, 1968 Work supported by University of California Air Pollution Program. The authors appreciate the use of the facilities a t the UCB and UCLA Computer Centers.
VOL. 8 NO. 2 APRIL 1 9 6 9
287
