Recently, Clark et al. ( 2 ) , who studied COS adsorption on reduced iron catalysts impregnated with small quantities of KOH, observed that COz adsorption increased with increasing KOH content. Their results are consistent with those obtained in this investigation. Because the temperature scanning experiments, and the adsorption kinetic experiments with carbon monoxide and carbon dioxide, seem to indicate that the surface was heterogeneous, the desorption of carbon dioxide as well as the adsorption of carbon monoxide probably takes place at different rates on different sites. Thus, it seems reasonable that, at a lower temperature, the catalytic oxidation reaction would take place a t an appreciable rate only on a fraction of the total possible reaction sites. On the other sites, either the rates of adsorption of carbon monoxide was too slow because of large activation energies, or, as is more reasonable from the experimental data, the rate of desorption of carbon dioxide was too slow to allow those sites to contribute significantly to the total rate of oxidation. The experiments which showed that C O Z inhibited the adsorption and oxidation rates of C O suggest that adsorbed COS renders sites inactive for C O adsorption and oxidation. If this were the case, raising the temperature would increase the number of sites available for reaction by the desorption of COS. Therefore, the increase in the rate of oxidation with increasing temperature is due to an increase in not only the carbon monoxide adsorption rate (sites of larger activation energies
become accessible), but also the number of sites which were previously rendered inactive by adsorbed carbon dioxide. Nomenclature
r = rate of adsorption, molecules/sec. S = number of molecules adsorbed on sample t = time, sec. a = constant, molecules/sec.-mm. Hg P = pressure, mm. H g CY = constant, molecule-‘ literature Cited
(1) Accomazzo, M. A., Nobe, K., Chem. Eng. Progr. Symp. Ser. 59, No. 45, 71 (1963). (2) Clarke. J.. Drv. M. E.. Montano.’ J. J.. Van Zvl. -, W. J.. Trans. Faraday SoC.‘57, 2239 (1961). (3) Low, M. J., Chem. Revs. 6 0 , 267 (1960). (4) Schwab, G., Drikos, G., Z. Physik. Chem. 52, 234 (1942). (5) Sourirajan, S., Accomazzo, M. A., Can. J . Chem. 38, 1990 (1960). (6)’ Souhajan, S., Accomazzo, M. A., Nobe, K., “Actes du Deuxierne Congres International de Catalyse,” Vol. 11, p. 2497, Editions Technic, Paris, France, 1961, (7) Taylor, H. S., Liang, S. C., J . Am. Chem. Soc. 69, 1306 (1947). ( 8 ) Trapnell, B. N., “Chemisorption,” p. 104, Academic Press, New York, 1955. (9) Yoshida, F., Ramaswami, D., Hougen, 0. A., A.Z.Ch.E. J . 8, 5 (1962). RECEIVED for review June 30, 1965 ACCEPTED October 25, 1965 \ ,
I
Work supported by funds from the University of California’s air pollution research program. Partial support received from an Institute of Geophysics grant-in-aid.
CATALYTIC CONVERSION OF A N ORGANOPHOSPHATE VAPOR OVER
PLAT I N U M I A LU M I NA W. M . GRAVEN, S. W. WELLER,‘ AND D. L. PETERS Aeronutronic Division, Philco Corp., Newport Beach, Cal;f.
s PART of a broad program to investigate catalytic methods
A for the removal of toxic chemical agents from air, a study has been made of the catalytic conversion of dimethyl methylphosphonate, CH,PO(OCHs)? [DMMP]. D M M P has many of the structural features which characterize toxic organophosphates. D M M P itself is relatively nontoxic, however, and its physical properties make it a convenient simulant for use in laboratory studies. A considerable number of catalysts, both commercial and laboratory-prepared, were initially screened. Platinum supported on high-area alumina gave the most promising results, as it had both high initial activity and relatively high resistance to deactivation. This paper summarizes the results obtained for D M M P conversion over two types of platinum-alumina catalyst, one a commercially available material, the other prepared by laboratory impregnation of gamma-alumina with chloroplatinic acid. 1 Present address, Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, N. Y .
Experimental
The kinetics experiments were carried out in a flow system of conventional design. Compressed air or nitrogen from cylinders was passed through a purifying train and metered by capillary float flowmeters; flow rate was precisely measured by a wet-test meter at the exit end of the analytical train. D M M P vapor was introduced by bubbling the carrier gas through saturators immersed in a bath with temperature constant to 10.1’ C. Variation of bath temperature permitted a range of D M M P concentration of 0.2 to 3.5 mg. of D M M P per liter of carrier gas. A 37-mm. i.d. cylindrical reactor was developed which had an integral helical preheater. The preheater consisted of 17 turns of 5-mm. i.d. tubing covered with a resistive platinum coating to permit independent electrical heating. A perforated plate supported the catalytst bed. Separate axial thermocouple wells permitted simultaneous monitoring of inlet gas temperature and catalyst bed temperature. Evacuated portions at both ends of the reactor minimized the “dead space” and reduced heat losses to the ends of the furnace. With furnace temperature at 400’ C., the inlet gas temperature remained constant within 1’ C. when the gas flow rate was varied from 0 to 10 liters per minute. The axial temperature gradient across the bed (in the absence of reaction) varied from VOL. 5
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The conversion of dimethyl methylphosphonate (DMMP) vapor in a stream of air, or nitrogen, has been studied over platinum-alumina catalysts. A commercial catalyst and a number of laboratory-prepared catalysts were investigated over a range of temperatures from 300"to 500' C., residence times from 0.1 5 to 2.7 seconds, and average catalyst particle sizes from 0.31 to 2.4 rnm. The activity of fresh commercial catalyst was too high to permit kinetic studies, but after some hours on stream, deactivation occurred to the point that measurable quantities of DMMP appeared in the effluent. Pseudo-steady-state kinetics over deactivated commercial catalyst were approximately first order with respect to DMMP, with an activation energy of 7 to 8 kcal. per mole. Over-all DMMP conversion proceeded about as rapidly in Nz as in air, and product analyses indicated that the initial reaction over deactivated catalyst was principally hydrolysis to give methanol and a phosphorus acid. lntraparticle pore diffusion had an important influence on the catalytic conversion rate, Catalyst deactivation, as measured by the decrease of first-order rate constant, k, with time on stream, f, was empirically correlated by the expression k = k,e-"'.
9' C. at a flow rate of 1 liter per minute to 5' C. a t 10 liters per minute. The effluent DMMP was originally determined by trapping at -78' C., extracting with acetone, and measuring infrared absorbance a t 1040 cm.-l Substantial improvement in sensitivity and time requirement resulted from the development of a gas chromatographic method (flame ionization detector) for direct analysis of an aliquot of the reactor effluent. No commercial packed columns gave really satisfactory results; however, a 22-foot stainless steel capillary column (0.02-inch i.d.) coated with DC704 silicone oil or, preferably, Carbowax 4000 and operated at 60' C. gave results reproducible within 5%. The sensitivity was estimated to be about 0.2 mg. of D M M P per cu. meter, or 2 X mg. of D M M P per liter. For a typical inlet concentration of 2 mg. per liter, this sensitivity corresponds to a maximum measurable D M M P conversion of 99.9%; higher conversions would result in effluent concentrations below the limit of reliable detection. The D M M P was obtained from the Virginia-Carolina Chemical Co. and was redistilled prior to use. The commercial Pt-A1203catalyst used in these studies was Houdry Type 3D, obtained through the courtesy of Houdry Process and Chemical Co. This is a pelleted material of 3/32-in~h (2.4-mm.) diameter; it contains 0.5 weight % Pt and has a specific surface area of about 80 sq. meters per gram. T o permit study of catalyst particle size, a number of laboratory-prepared catalysts were made by impregnating ~-A1203with sufficient H2PtC1, solution to give, after drying, 0.5 weight yo Pt. The base alumina was Houdry HA 100, having a surface area of about 80 sq. meters per gram. The as-received 3/32-in~halumina pellets were crushed and sieved to the desired size range prior to impregnation. After the catalyst had been loaded into the reactor, it was reduced with Hz for 4 hours at 440' C., cooled to room temperature in H2, purged with prepurified N2 and then with air, and brought to reaction temperature in flowing air. D M M P was then introduced into the air stream by means of the saturators.
known for the empty and the bead-filled reactor, the apparent area is, in fact, roughly threefold higher for the reactor containing glass beads. This result, along with the relatively low value for the activation energy, strongly suggests that the "uncatalyzed" reaction is rather a glass-catalyzed reaction. Although no further experiments were carried out to elucidate this point, this conclusion would be consistent with the nature of the reaction observed over platinum-alumina catalyst. Commercial Catalyst. STEADY-STATE KINETICS. Figure 2 shows the type of deactivation typically observed in a D M M P run over Houdry Type 3D (3/32-inchpellets), in this case at 390' C. and 0.53-second residence time. During the 100hour run, the conversion was too high to be measured for the first few hours; it then fell off with increasing time on stream and leveled off at a poorly defined "steady-state" level. The steady-state kinetics were studied as a function of temperature, inlet concentration, bed residence time, and nature of the carrier gas (air, 0 2 , N2). These steady-state results represent data for catalysts which had been deactivated by prior running for several days on DMMP at 400' C.; they do not measure the activity of a fresh catalyst. I
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I
Results and Discussion
Uncatalyzed Reaction. T o have some basis for comparison with the results from experiments with catalysts, the conversion of D M M P vapor was first studied with an empty reactor over the temperature range 393' to 494' C. The extent of decomposition at 393' C. varied from l l to 3% when the residence time was varied from 2.7 to 0.64 second; at 494' C. the corresponding range was 35 to 10% for a variation in residence time of 2.5 to 0.57 second. The data were found to fit first-order plots, as shown in Figure 1. An Arrhenius plot of the first-order rate constants yields an apparent activation energy of 14 to 15 kcal. per mole. A control run was carried out in which the reactor was filled with 50 ml. of 3-mm. diameter borosilicate glass beads. At a reactor temperature of 392O C., a first-order rate constant of 0.11 set.-' was obtained, almost threefold higher than the value of 0.043 set.? observed a t 393' C. for the empty reactor (Figure 1). Although the true areas of heated glass are not 184
I h E C P R O C E S S DESIGN A N D D E V E L O P M E N T
\k
I
1.0
1
= 0.178 SEC:'
I 2.0
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RESIDENCE TIME (SEC.)
Figure 1.
Conversion of DMMP in empty reactor
1
DMMPINLET 1 . 6 MG/L AIR FLOW RATE 2.5 L/MIN. TEMPERATURE 39OoC, BED RESIDENCE TIME 0 . 5 3 SEC
97t
h, TIME (HOURS)
Figure 2.
Deactivation of commercial platinum-alumina catalyst
Figure 3 shows typical results, in this case at a reactor temperature of 245' C., for steady-state conversion us. residence time; inlet concentration, C,, was varied over a fivefold range. If the reaction were truly first order in D M M P concentration, the semilog plot for each C, should be a straight line, and the same line should be obtained independent of C,. It is clear that the first condition is approximately satisfied, but there appears to be a slight trend toward steeper slope-i.e., higher rate constant-at lower C,. I n view of the difficulty in guaranteeing that the catalyst activity is stable during such series of runs, further data would be required to establish unambiguously this effect of varying C,. The effect, if real, is small; a fivefold variation in C, resulted in only a 15 to 20% change in rate con-
1.30
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TE hi1 PE R A T UR E 2 4 5OC.
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Y
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i
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BED RESIDENCE TIME (SECONDS)
Figure 3. lyst
-
1610 ( O K.)
(11
___ T
Of particular interest is the fact that in two of the three runs made with the use of prepurified N2 (Matheson, said to be 99.996% pure) as carrier, the over-all rates of D M M P con-
\ \I
0 DMMPINLET 0.7 MG/L. 1.20-
loglo k(sec.-') = 3.125
I
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A DMMPINLET 1 . 6 MG/L.
-
stant. The major conclusion is that the over-all behavior is approximately first order. An Arrhenius plot, based on a number of such runs at temperatures from 200' to 400' C., is shown in Figure 4. The slope of the "least-squares" line, drawn through the points for air as carrier gas, corresponds to an unusually low activation energy of 7 to 8 kcal. per mole. The equation of the line is
Steady-state kinetics over commercial cata-
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1.6
1.7
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1 0 ~( O1 K) ~
Figure 4. Effect of temperature and carrier gas on steady-state kinetics VOL. 5
NO. 2 A P R I L 1 9 6 6 185
version were as great as those obtained with air as carrier. The implications of this result for the reaction mechanism are discussed below. Table I summarizes the major and minor vapor products, determined by gas chromatography, found in a large number of runs with either air or prepurified NPas carrier. With air, the principal products are methanol and carbon dioxide with trace amounts of dimethyl ether. With Ks,the only major product is methanol; no carbon dioxide is formed, and small amounts of methane, dimethyl ether, and formaldehyde a.re observed. In both cases the methanol is formed in molar amounts comparable with the amount of DMMP converted. The carbon dioxide, produced in the runs with air, is formed in relatively small amounts a t low temperature (200’) and short residence time (0.15 second) but becomes comparable, on a molar basis, with the D M M P converted as the temperature and residence time are increased. It is plausible to suggest that the methanol formed in the presence of prepurified NS as carrier results from a hydrolytic reaction between D M M P and residual catalyst “water,” present as surface hydroxyl groups even on alumina dried a t 500’ C. ( 7 ) . When air is used as carrier, the methanol may result either by this route or by the reaction of D M M P with water formed as an oxidation product from DMMP. With an air carrier, a sirupy condensate is obtained at the exit of the reactor. The infrared spectrum, pH titration curve, and qualitative tests for phosphate ion indicate that this condensate is largely H3P04. Laboratory-Prepared Catalysts. As in the case of the commercial catalyst, the initial activity of all the laboratory catalysts was too high to be measured, even at a reaction temperature of 200’ C. After some hours on stream, however, a X mg. per liter) of DMMP detectable amount (-2 appeared in the effluent stream; this time may be called the “breakthrough” time for DMMP. I t was found to be a function of DMMP concentration, air flow rate, reaction temperature, and catalyst particle diameter. Figure 5 shows the deactivation behavior for 20/35-mesh (0.63-mm.) catalyst as a function of temperature, over the range 299’ to 486’ C. These tests were made at the relatively high C, of 3.5 mg. per liter and room temperature flow rate of 8.9 liters per minute. The breakthrough times a t 299’, 396’, and 444’ C. were approximately 8, 12, and 16 hours, respectively. At 496’ C. no effluent D M M P was observed even when the run was terminated, after 30 hours, because of excessive accumulation of reaction products in the exit lines. Under these conditions, if no catalyst had been present the amount of thermal decomposition at 396’, 444’, and 496’ C. would have been only 1, 3, and 5%, respectively. The effect of particle size on deactivation at 299’ C. is shown in Figure 6. As the average particle diameter is decreased in twofold increments, from 1.24 mm. (10/20-mesh) to 0.63 mm. (20/35-mesh) to 0.31 mm. (35/65-mesh), the approximate breakthrough times increase from 5 to 8 to 14 hours. Since there is some variability between replicate runs, the effect of particle size has been examined more quantitatively by analyzing the results of a total of 22 runs, made with
Table 1. Carrier
Air NP
186
Vapor Reaction Products of DMMP Conversion over
Type 3D Catalyst Major
CHaOH, COS CHPOH
Minor
(CH3)20 (CHs)20, HCHO, CHI
l & E C PROCESS DESIGN AND DEVELOPMENT
w 0
c
U
94 -
INLET DMIVIP 3 . 5 hlG/L. AIR FLOlV RATE 8.85 L/MIN. 5 0 M L . P T (1 PERCENT)-ON-ALUhilNA (HA100j; 20135 MESH
496’C. 444OC. A 396’C X
0
93 -
92
0 299OC.
-
4
8
12
16
20
24
25
32
TIME ( H O U R S )
Figure 5. Effect of temperature on deactivation of laboratory catalyst
these three particle sizes, all a t temperatures near 300’ C. Figure 7 shows the mean “protection time” as a function of average particle diameter. T o permit quantitative treatment of the results, the protection time has been arbitrarily defined as the time on stream when the first-order rate constant decreases to a value of 50 sec.-I The protection times so defined correspond closely to the breakthrough times obtained by observation of the first detectable D M M P in the reactor effluent. The horizontal bars in Figure 7 represent the range of sizes included in the sieve fraction; the vertical bars show the standard deviation from the mean of the protection time. For the smaller particle sizes, the difference in mean values of the protection time (7.9 hours for 0.63-mm., 10.6 hours for 0.31mm. particles) was shown by the Student’s t test to be statistically significant even at the 1% level ( 3 ) . A small number of runs a t 400’ C. indicated that the increase in “protection time” with decrease in particle size was, if anything, slightly greater than at 300’ C. The apparent linearity shown in Figure 7 is not taken as theoretically significant, but the empirical relationship shown does permit a rough extrapolation in order to predict probable behavior at still smaller particle sizes. The data suggest that although some improvement a t 300’ C. will result from further size decrease, no dramatic increase in protection time is likely. The existence of a particle size effect is excellent evidence that the over-all reaction rate is significantly limited by the rate of diffusion through the pores of the catalyst to the interior surface. Wheeler ( 2 ) has given a simplified method for estimating f, the fraction of total catalyst surface which is effectively utilized when diffusional limitations occur. The results of such calculations are summarized in Table 11. An average pore radius of 100 A. has been used for the catalyst, along with a bulk density of 0.86 gram per cc. and a pore volume of 0.46 cc. per gram. k,,, is the over-all first-order rate constant, arbitrarily computed on the basis of the first measurable D M M P concentration appearing in the effluent.
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All of the kinetic data and rate constants presented in Table
I1 pertain to catalyst which has been deactivated by extended
i INLET [DtvlblP] 3.5 MG/L. RESIOENCE TIME 0.18 SEC. TEhlPERATURE 299OC. PT (1 PERCENT)-ON-ALUMINA (HA100); 50 ML 0 1 0 / 2 0 MESH D 20135 MESH A 3 5 / 6 5 MESH
k = k,e-ai
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running. Although it is obviously important to characterize catalyst activity quantitatively during the first hours on stream, the conversions over fresh catalyst have been so high that we have not been able to measure reaction rates at temperatures of interest. I n order to have some means of characterizing this initial period and to permit some prediction of catalyst performance, a n empirical correlation has been established for the deactivation behavior of laboratory catalyst. As illustrated by Figure 8, which is a plot of log k us. t for a typical run, reasonable fit of the rate data can be obtained by the use of the equation
I
16
where k is the apparent first-order rate constant after time t on stream, k, is the extrapolated rate constant at zero time, and CY is a deactivation rate constant. The data for a limited number of runs indicate that both CY and k, vary exponentially with 1/T, a decreasing and k, increasing with increasing temperature. Furthermore, a appears to vary directly as C, the inlet concentration, and inversely as T , the residence time. For the case of 0.31 mm. of catalyst (for which the greatest amount of data was obtained), the empirical correlations are :
Figure 6. Effect of particle size on deactivation of laboratory catalyst
(This is obviously an underestimate of the k,,, which would characterize the catalyst during the initial period of very high activity.) k , is a rate constant characterizing the mass transfer of reactant to the external particle surface, and kD is a rate constant characterizing the diffusion of reactant within the pores of the catalyst ( 2 ) . The fact that k,,,/k, is very small indicates that mass transfer to the external surface is not limiting. The ratio k e x p / k Dis not small, however, indicating that intraparticle diffusion is significant; from this ratio the values off shown in the last column may be computed by Wheeler's method. I n agreement with the qualitative conclusions from Figure 7 , it is again found that serious limitation by intraparticle diffusion occurs in 1.24-mm. particles, but that most of the internal surface is effectively utilized when the average particle diameter is 0.31 mm.
(2)
-3250 _
k,
=
1.96 X l O 5 e
c --1200 T
a = 1.41 X 1 0 - 6 y e T
(3) (4)
The factor C,/r in Equation 4 corresponds to the weight of DMMP entering the reactor in unit time, per unit volume of catalyst. Attempts to incorporate catalyst particle diameter into the correlation indicated serious scatter in the computed values of a and k,. Sufficient data were available to attempt this only for the set of 22 runs a t 300' C. shown graphically in Figure 7 . The results for the deactivation parameters are summarized in Table 111. Minor variations in C, and r were corrected for by ~, in column 4 of Table computing the product C Y T / C shown 111, in place of a. The standard deviation for each of the three parameters is shown after the mean value.
4 0 3. PT-A1203 CATALYST
-Ln 1
BED TEMPERATURE 3 0 0 ' 3.52 MG. DMMP/L. AIR
CL
0 3
5
BED RESIDENCE TIME 0.18 SECONl
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a W CL
2
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0.75
1.00
1
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(MM) Variation of protection time with particle diameter AVERAGE PARTICLE DIAMETER
Figure 7.
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600
Table II.
Rate Parameters for Various Particle Diameters C, = 3.52 mg. DMMP/l. T = 0.15 sec.) Rate Constants. Sec. -l Av. Particle f> Diameter, M m . kexp k, kD %
-
( T = 400' C.
-
400-
-
T = 30OoC. = 0.18 SEC.
T
c0
200-
\
\
10080-
60-
\
= 3 . 5 mg.
11'
1.242 (10/12-mesh) 0.625 (20/35-mesh) 0.313 (35/63-mesh)
AVERAGE DIAMETER = 1 . 2 4 mm,
\
\
w 0
10.2
21
1250
40.1
62
3530
160.0
88
Table 111.
Deactivation Parameters for Various Particle Diameters at 300" C. Av. No. Diameter, of Protection w / C O ,Hr.+ M m . Runs Time, Hr. See. Mg.-' L . k,, Set.-' 10 2 . 7 f 1 . 2 0.0155 f 0.0041 141 f 115 1.24 0.63 7 7.9 f 1 . 8 0.0126 zk 0.0035 349 f 186 0.31 5 10.6 f 2 . 3 0.0118 f 0.0033 543 f 429
-
lA
1 0
449
\
7 .40-
-
49.3 at t = 6 . 5 hr. 51.5 at t = 12.5 hr. 56.9 at t = 17 hr.
2
mm. and 0.62-mm. particles satisfies this requirement ( 3 ) . (In this case, significance is observed even at the 1% level.) In the other comparisons the standard deviations are too large, relative to the differences in mean values, to permit accurate evaluation of the particle size effect. The utility of the deactivation correlations is illustrated in Figure 9. The curves show the outlet concentrations which would be computed for a first-order reaction, carried out at 260" C. over 0.31-mm. catalyst, where the first-order rate constant decreases with time on stream according to Equations 2, 3, and 4. Two different inlet concentrations are considered, 100 and 1000 mg. of DMMP per cu. meter of air, for bed residence times ranging from 0.02 to 1.O second. Use of these curves is illustrated by the following example: If a steady inlet concentration of 100 mg. per cu. meter is supplied to the reactor a t a residence time of 0.2 second, it will require slightly more than 300 hours for the catalyst to deactivate to the point that the effluent concentration reaches 0.001 mg. per cu. meter.
L TIME (HOURS)
Figure 8. Deactivation correlation (Equation 2) for typical run
The protection times have been exhibited in Figure 7. As mentioned above, the variation of protection time with particle diameter is meaningful even at the 1% level. The situation for (Y (or (YT/C,)and k, is less clear-cut. The mean values indicate a tendency of (YT/C,to decrease, and k, to increase, with decreasing particle diameter. However, application of the Student's t test shows that, if the difference in mean values is arbitrarily required to be statistically significant a t least a t the 10% level, only the difference in k, values for the 1.24-
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Figure 9. Predicted effluent concentrations at 260" C. for deactivating catalyst (0.31 mm.) 188
I & E C PROCESS D E S I G N A N D DEVELOPMENT
If catalyst deactivation did not occur, the curves shown in Figure 9 would simply be a set of horizontal lines, with ordinates corresponding to the steady effluent concentrations determined by the rate constant and the bed residence times. Ac knowledgment
The authors express their gratitude to the Houdry Process and Chemicals Co. for furnishing the Type 3D catalyst, and to Leonard Jonas and Joseph Epstein for many helpful discussions of this work.
Literature Cited
(1) Lee, J. K., Weller, S. W., Anal. Chem. 30, 1057 (1958). (2) Wheeler, A., “Catalysis,” P. H. Emmett, ed., Vol. 2, Chap. 2, Reinhold, New York, 1955. (3) p u d e n , W. J., “Statistical Methods for Chemists,” Chap. 3, b i l e y , New York, 1951. RECEIVED for review August 13, 1965 ACCEPTEDNovember 1, 1965 Work supported by the U. S. Army Chemical Research and Development Laboratories, Edgewood Arsenal, Md., under contract DA 18-108-CML-6671(A).
CATALYTIC OXIDATION OF COKE ON A LU M I NOS I LICATES M A R V I N S. GOLDSTEIN
American Cyanamid Co., Stamford, Conn.
Differential thermal analyses were used to study the oxidation of supported cokes. The supports included a Type-X molecular sieve in three cationic forms, an amorphous silica-alumina cracking catalyst which had been promoted with the same three cations, and the unpromoted amorphous catalyst. The DTA curves for the oxidation reactions peaked at lower temperatures for cokes deposited on supports that contained cations. The peak temperatures were related to the molar concentrations of the cations but were independent of either the nature of the support or the cations.
c
are important in catalytic hydrocarbon conversion processes. Almost invariably catalysts become deactivated after a period of use because coke deposits have accumulated on them. Regeneration procedures which remove the coke, usually by oxidative gasification, have therefore become standard operations in the over-all scheme for many of these processes. Several laboratory scale studies have been concerned with the rates of coke burning on supported catalysts (2, 4, 9, 70). Other studies have dealt with unsupported carbon blacks and graphites (5, 7, 8). I n all cases, two important aspects have been to describe the nature of the coke or carbon and to determine if the rate of the reaction was limited by the rate of the chemical changes which occurred or by the rate of masstransfer. I n this study, differential thermal analysis (DTA) was used to monitor the oxidation of cokes which had been deposited by a standardized procedure on different aluminosilicate supports. T h e supports were promoted with representative cations from the alkali, alkaline earth, and rare earth groups. Catalytic effects by these cations were observed in the rate of coke oxidation. In general, information about the reaction rates was simply taken from the temperature locations of the exothermic peaks in the DTA curves. OKE BURNING RATES
Table 1.
Concentration of Cations in the Molecular Sieves Meq. Cation per Gram Silica-Alumina Sieve Type Na Ca RE Total NazX 8 8 CaX 3.6 4.4 8 REz/& 1 7 8
Experimental
Materials.
Aluminosilicate Supports.
A . An amorphous silica-alumina cracking catalyst, American Cyanamid’s 13% alumina Aerocat catalyst. B. A , promoted with 4 meq. of sodium per gram of silicaalumina from an aqueous NaCl solution. This support, and C and D, were all promoted by impregnating the calcined catalysts with the corresponding solutions. C. A , promoted with 4 meq. of calcium per gram of silicaalumina from an aqueous Ca(NOJ2 solution. D. A , promoted with 4 meq. of rare earth per gram of silica-alumina from an aqueous RE nos)^ solution. E. A sodium exchanged Type-X molecular sieve, Linde 13X, subsequently designated Na2X. F. A calcium exchanged Type-X molecular sieve, Linde lox, subsequently designated CaX. G. A rare earth exchanged Type-X molecular sieve, subsequently designated RE2,,X. This sieve was prepared by ion exchange of Na2X with an aqueous solution of REC13. Supports E, F, and G were not pure with respect to their designated cations. T h e actual cation compositions are estimated in Table I. NaCl and Ca(N03)2 were reagent grade. Rare earth chloride and nitrate salts were purchased from the Lindsay Division of American Potash and Chemical Corp. They were cerium-rich salts, Lindsay code numbers 340 and 350. T h e oil used as the source for the coke deposits was a virgin, midcontinent, gas oil having an initial boiling point of 282’ and a 90% point of 416’ C. Prepurified nitrogen was used in the thermogravimetric analyses (TGA). The same nitrogen was used in the carbonhydrogen analyses, but it was further purified over hot reduced VOL. 5
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