Activitv of Solid Catalvsts J J Experimental rate data are presented for two platinum catalysts for the oxidation of sulfur dioxide. Rates of reaction, r1 and ro, for the two sizes of catalysts were correlated by the equation, rl = roFli;o(al/ao),where activity factor F was independent of temperature, composition, and gas velocity. The equation permits the estimation of the effect of physical properties (except porosity) of the catalyst on the rate of reaction from a minimum amount of experimental data. W. B. ARGO’ AND J. XI. SMITH Purdue University, Lafayette, Irzd.
HE problem of designing a fixed-bed catalytic reactor for gas phase reactions has been the subject of considerable investigation during recent years ( I , 3, 4, 7 , 12). Particular emphasis has been given to the fundamental approach based on integrating the differential equations relating composition and temperature to position in the reactor. For a selected catalyst the use of any of these design methods requires a knowledge of the reaction rate as a function of temperature, pressure, and composition. For an optimum design i t is also necessary to know the effects of catalyst properties (such as size and shape of p ~ l l e t ,porosity, concentration of active ingredient on supported catalysts) and mass velocity (if diffusion resistances are important) on the rate of reaction. It is clear that if the rate is t o be obtained b y purely experimental means, the laboratory work required is considerable. It would be expected that the influence of mass velocity could be accounted for by using the available mass transfer data for packed beds. Hougen and Yang (6) have suggested a method of approach, and this procedure has been found to give satisfactory results when compared m-vith experimental data for the oxidation of sulfur dioxide (8). On the other hand, the influence of the catalyst properties has not received its share of attention. Thiele (11) has presented a theoretical approach for predicting the effectiveness of interior surface for porous catalysts. Rase (9) has studied the effectiveness factor of Thiele from an experimental point of view. The most important catalyst properties are probably pellet size and shape, porosity, character of the catalytic surface, and concentration of the catalyst on the carrier if a supported catalyst is used. Because it is difficult to give a quantitative interpretation to some of these properties, particularly the character of the catalytic surface, i t is not likely that a completely theoretical method can be developed for predicting the effect of catalyst properties on the rate. Accepting the fact that experimental data will be necessary, it then becomes desirable to develop methods of determining the effects of these catalyst properties on catalyst activity, with a minimum of experimental work, and in this way reduce the over-all laboratory effort required to obtain complete rate information. The purpose of this paper is to present experimental rate data for the air-oxidation of sulfur dioxide with two sizes of a platinum-on-alumina catalyst. Also a scheme is presented, for correlating the data, which works satisfactorily over the range of conditions studied.
and geometry of these active centers. This concept can be used t o explain differences in activity bctwem two batches of the same catalyst. Thus the concentration and spacing of the active centers may well vary from one batch t o another even though the macroscopic properties and controllable variables in the preparation of the various batches are identical. Expressing this concept mathematically, the rate of reaction in terms of moles of reactant consumed per hour per pound of catalyst can be written in the following way:
r
=
ar1Ca
(1)
where
moles of reactant consumed per “fully effective” active center per hour a = sq. ft. of catalyst surface per pound C = concentration of active centers, number of active centers per sq. ft. of catalyst area a = a specific constant for the particular catalyst r1
=
Constant a is dependent on the characteristics of the catalyst surface, such as the geometry of the active centers. Also if the catalyst is porous, the effect of porosity is also included in a. The method is not as valuable for porous catalysts, however, because the effectiveness of the interior surface depends on rate of reaction. Hence for catalysts with considerable effective interior surface, a would not be a constant but a function of temperature and composition. For such highIy porous catalysts the method would be expected to be valid only over narrow ranges of operating conditions. For catalysts with little or no effective interior surface this restriction on operating conditions is not necessary. Suppose that two catalysts, 1 and 2, differing in size are to be compared for activity. Since the two materials will most likely have been prepared in different batches, the quantities a and C may be different as p r d l as the area per unit mass, a. Then the application of Equation 1 t o each catalyst would yield the expressions ro = a0r10 Co a0 (2) 71 = a1 r11 CI Ul (3) If the catalysts are compared at the same partial pressures of reactants and products a t the gas-solid interface, the rate per active center should be the same for the two sizes. Hence I = and Equations 2 and 3 may he combined to give
?-A
(4) METHOD O F CORRELATION
I n studying the behavior of solid catalysts, Taylor (IO)postulated the existence of active centers on the catalyst surface and proposed that the separate steps involved in the over-all reaction occurred on these active centers. If this thesis is accepted, the activity of a catalyst should depend on the concentration 1
Owing to their complexity, the ratios of a and C are quantities that must be determined at present by actual rate measurements. If the catalyst activity factor, F , is defined as the product of these ratios, Equation 4 may be written
Present address, Monsanto Chemical Co., Annkton, Ala.
298
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Vol. 45, No. 2
Unit Processes
_ _ _ DATA
OF OLSON, EOUATION 7
.9c
-EPUATlON F s 1.405 0
E"
9 ,
El.AL.
WITH
EXPERIMENTAL VALUES
num. Their data covered a temperature range from 360" t o 480" C. at atmospheric pressure and a conversion interval of 0 t o 7oyO,all for a n initial reactants composition of approximately 6.4mole yosulfur dioxide and 93.601, air. It was found, by varying the mass velocity of gas passed the catalyst surface, t h a t the diffusion resistances were important. Hence the results were correlated in terms of the partial pressures existing a t the catalyst surface. These interfacial pressures were computed by using existing information for the mass transfer rates between gases and solid pellets in packed beds (6). The rate expression so developed is as follows:
where B and D are dependent only on the temperature. expression may be rearranged to
.5c
0
This
a.4c
plsoJ ' Figure 1.
ATMOSPHERES.
Correlation of Rate Data of Olson et al. for '/pinch Catalyst Pellets
The data of Olson et al. (8) for three temperatures are shown on Figure 1, plotted as suggested by Equation 7. To test the proposed method of comparing catalyst activity, rates of reaction were measured experimentally under the same conditions except a I/*- X l/*-inch alumina carrier, used for the platinum catalyst. This new catalyst (designated by the subscript 1) also contained 0.2y0 by weight platinum. However, the appearance of the surface was more uneven and the penetration of the platinum greater than for the 1/8-inch catalyst (designated by the superscript, 0). However, for both sizes the catalyst was concentrated near the surface of the alumina pellets. The experimental technique and apparatus employed were the same as used by Olson et al. ( 8 ) and need not be redescribed in
Equation 5 states t h a t the rate of catalyst 1 can be evaluated from the known rate of catalyst 0, provided activity factor F has been determined. The usefulness of Equation 5 depends on whether or not F is independent of temperature and composition. This should be true provided t h a t the reaction mechanism and activation energy are the same for the two catalysts, for under these restrictions both TO and TI are identicaI functions of temperature, pressure, and composition, indicating t h a t F is not dependent on these variables. These restrictions are likely t o be valid for comparing two catalysts of different size, shape, and concentration of active catalyst on an inert carrier. However, as already stated, F would be expected to depend on process variables for catalysts with considerable effective pore surface. Also, for catalysts of different chemical structure, differences in mechanism and activation energy are likely t o exist. For such comparisons Equation 5 is not of real value because F would vary with temperature and composition of the reaction mixture. For similar catalysts it should be possible t o obtain the value of F from a minimum of one r a t e m e a s u r e m e n t with the new catalyst, I, and then be able t o predict the rate at any temperature and c o m p o s i t i o n a n d mass velocity from a knowledge of the rate, ro. for the original c a t a l y d t . T h i s Figure 2. Layout of Experimental Apparatus possibility has been investigated by comparing rates of reaction for two sizes of platinum-on-alumina catalyst pellets used for the oxidetail here. As indicated by the flow diagram (Figure 2) the dation of sulfur dioxide. reactants were pretreated to remove oil and water vapor, metered, and passed through a 2-inch standard pipe (stainless steel) EXPERIMENTAL differential reactor immersed in a constant-temperature bath. Olson, Schuler, and Smith (8) measured the rate of oxidation of The details of the reactor are shown in Figure 3. Provision sulfur dioxide in a differential reactor packed with X 1/8-inch was made for analysis of the gases entering and leaving the reaccylindrical, alumina pellets coated with 0.2% by weight of platitor, and temperatures were measured with thermocouples inFebruary 1953
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~
Run 18 19 20 22 24 25
Mean Bed Temp., O C. 478.4 479.3 479.2 422.0 419.6 419.1
TABLE I. EXPERIMENTAL DATAFOR 1 / 4 - CATALYST 1 ~ ~ ~ PELLETS Yo Conversion Weight
Flow Rates,
G.
Air 0.780 0.782 0.782 1.586 1.586 1.586
SOz 0.051 0.049 0.049 0.110 0.109 0.109
Mole % SOa % Conversion in Reactants in Preoonverter 6.19 0.00 5.91 10.5 5.91 34.3 6.51 0.0 6.40 12.3 6.40 31.2
Maas Velocity, in Differential Lb./(Hr.) (Sq. Ft.) Reactor 147 23.4 147 19.4 147 11.9 300 8.97 300 6.07 300 4.45
serted in the catalyst pellets. These nieasurements were made a t the center and edge of the bed in order to establish the magnitude of radial temperature measurements and to determine a precise, mean bed temperature. I n order to obtain satisfactory results it was necessary t o pretreat the catalyst for a period of 8 hours b y passing sulfur dioxide and air through the reactor a t operating conditions. After this initial interval catalyst life studies indicated constant activity until some error in operating technique caused the activity to decrease. Usually this was due to the admission of water vapor to the reactor through inadequate drying of the entering air. Table I summarizes the results of the experimental investigation of the '/d-inch catalyst. Each run represents the average of three analyses. If the results of the three experiments did not agree within 5% the run was discarded.
of Catalyst,
Grams 10.26 10.26 10.26 10.26 10.26 10.26
Reaction Rate Mean Lb. Moles/(Hr:) Conversion, % (Lb.Catalyet) 11.7 0.0704 0.0568 20.3 0.0365 40.3 0.0577 4.49 0.0385 15.3 0.0281 33.5
point was used to determine a value of (ao/alFlo). The results were as follows: a0
a
Run No. 18 19 20
1.4;
1.32 1.43
Mean Conversion, yo 11.7 20.3 40.3
The solid curve shown through the data is based on a value of ao/alFlo = 1.41 and is adjudged to be the best fit. The ratio ao/al for l/*- and 1/4-inch cylindrical pellets is approximately 0.125 to 0.25 or 2.0. Hence the value of the activity factor is 2/1.41 = 1.42. -4value greater than unity indicates higher values of ( a C ) for the '/*-inch catalyst. This result might have been predicted from the more uneven surface and greater penetration of platinum on the alumina carrier. It should be noted
EVALUATION OF ACTIVITY FACTOR Fto
Assuming t h a t the mechanism of the reaction with the l / q inch catalyst is the same as for the 1/8-inch catalyst, the rate 1'1 for the larger pellet can be expressed in terms of Equations 5 and 6,
-
EXHAUST GASES
- L E V E L OF LEAD BATH
r1
Equation 9 and experimental rate data can be used to evaluate The procedure followed was to determine Fin from the three runs a t 480' C. and a mass velocity of 147 lb./(hr.) (sq. Et.) and then to use this factor to predict the rates a t 420" C. and G = 300 lb,/(hr,) (sq. ft.) for comparison with runs 22, 24, and 25. From the data a t 480" C. and G = 147, values of pig,oa were computed from the expression
8'10.
TI =
koa (Pi - P o ) so3
PREPACKING (WHEN USED)
(10)
Ergun ($3) has recently summarized the available data for mass transfer coefficients (IC,) in fixed beds. It was believed that the correlation of Hougen and Wilke (6),based on relatively thin beds and air as the flowing gas, would be the most nearly applicable to the conditions existing in the differential reactor. Hence k , values were computed from their equations: D G -0.61 = 1.82
(e) 2 /3
The mean diffusivity of sulfur trioxide in the multicomponent mixture was determined by the method of Wilke ( I S ) . A plot of the experimental data is shown in Figure 4 using the coordinates suggested by Equation 9. The correlation of Olson, Sohuler, and Smith (8) at 480' C. from Figure 1 is also shown as a dotted line on the graph. Using the values of B = 0.176 and D = 12.9, established by the dotted line, each experimental
300
DUMMY PELLETS
ENTRANCE FROM COIL-
TO SAMPLING SYSTEM
k s s = = a Figure 3. Differential Reactor
that the use of an area ratio (a,/a,) based on the size of the catalyst pellets is justified only when the catalyst is coated on the outer surfaces of the pellet and when the catalyst on different size pellets is the same-Le., prepared in the same manner. T E S T OF T H E ACTIVITY FACTOR
If the activity factor defined by Equation 5 is to hc useful, it should be independent of temperature, conversion, and mass velocity. Hence the determined value of FlO should be applicable for predicting rates of reaction for the '/l-inch catalyst a t any
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Unit Processes TABLE 11.
/
CALCULATED RATESOF REACTION AT 420" C. G = 300 LB./(HR.)(SQ.FT.)
pisoz, Atm.
Lb. M$Z.'s/(Hi-.) (Lb.of Catalyst)
paso2,
AXD
% Conversion
Atrn.
3.0 14.8
21.5 30.0 40.0
51.6 61 .O e
EFFECT O F PREPACKING u
PI so,
ATMOSPHERES
Figure 4. Correlation of Data Used to Evaluate Catalyst Activity Correction Factor, F temperature, conversion, and mass velocity. This was tested by computing the rate versus conversion curve for 420' C. and G = 300 Ib./hr. (sq. ft). The steps involved in the computation can be summarized as follows:
I n the experimental work presented here and that by 01son, Schuler, and Smith (8) the catalyst bed was held in place a t the entrance by a retaining screen, b u t there was no prepacked section of dummy (plain alumina) pellets preceding the catalyst. Hence the reactant gases, after passing the screen, impinged directly on the active catalyst. Following the catalyst bed were several inches of dummy packing. This type of bed corresponds to that used by Hougen and Wilke (5) in measuring mass transfer coefficients and therefore was best suited to the evaluation of diffusion resistances between the gas and catalyst surface. However, the configuration of the differential bed would be more similar to that in a larger commercial reactor if a section of dummy packing were used both upstream and downstream from the active catalyst. Accordingly, rate measurements were also made with this type of configuration in the differential bed. The rates were about 20% higher than those without the prepacked
L.
1. Using the values of B = 0.499 and
D
,071
I
I
I
I
= 17.5 for the
420' C. straight line shown in Figure 1 for the 1/8-inch catalyst and the value of P = 1.42, Equation 9 was employed to determine the rate, rl, for a series of values of piso, (pi values for sulfur trioxide could have been used equally well). 2. Knowing the rate, r1, for each pisOathe corresponding partial pressures in the main gas stream (that is pose,) were evaluated from the expression
i
a
0
-1 m
I
.os.
\ v)
W -1
. *
300
LW/HR
FT'
4eo -C
\
0 I .04
i
which is a combination of Equations 10 and 11, eliminating kr. 3. The conversion was evaluated from a knowledge of the p , values and the stoichiometry of the reaction. The same 'calculational procedure could be used to evaluate rate curves for the '/(-inch catalyst a t other temperatures and mass velocities. The limitations in the range of these variables are that the rate data for the 1/8-inch pellets must be available a t the t6mperatures required, and the mass transfer coefficients must be represented satisfactorily by the correlations used a t the mass velocity required. The calculations are summarized in Table 11, and the rate curve is shown by the solid line in Figure 5 . The experimentally measured rates from Table I a t 420" C. and G = 300 are also indicated. The agreement (average deviation 7%) is satisfactory, particularly in view of the fact that the rate changes rapidly with mass velocity in the range G = 147 to G = 300 Ib./hr. (sq. ft.). I n fact, above G = 300 the rate for the 1/8-inch pellets is essentially constant. I n summary, it has been possible to correlate the data for the two catalyst sizes by the method of Equation 5. However, before Equation 5 can be considered a general method of approach, data on other reaction systems are necessary. For many reactions the diffusion resistances would be negligible and the partial pressures a t the catalyst surface equal to the bulk gas values. I n these instances the computations would be considerably simplified as step 2 would be eliminated. February 1953
0
9
.-
w.
.03-
Ga \ V c
\ .01
Comparison of predicted and experimental values for '/r-inch catalyst pellets
section a t the same temperature, conversion, and mass velocity. This suggests that mass transfer rates may be higher well within a deep packed bed than near the entrance. It may be that a depth of bed of the order of 50 pellet diameters is necessary to fully develop turbulence within a packed bed, just as a length of about 50 pipe diameters is required for flow in a n empty pipe. CONCLUSIONS
A method of predicting the effect of variatiom of size and surface characteristics on the activity of solid catalysts has been
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30 1
proposed and shown to agrcc well with experimental data for one reaction. T o be applicable the mechanism of the reaction and activation energy must be the same for each catalyst. The method requires complete rate data for the catalyst used as a basis for comparing activities and a minimum of one experimental rate measurement for each new catalyst. The procedure is adapted particularly to predicting rates for catalysts of varying pellet sizes.
Pi ?’
a! P
P
partial pressure of a component a t catalyst surface rate of reaction, Ib. moles of reactant consumed/ (hr.) (Ib. of catalyst) = rate of reaction, Ib. moles of reactant consumed/ (hr.) (active center) = specific activity constant for a catalyst, dimensionless = mean viscosity of gas mixture, lb./(hr.) (it.) = mean density of gas mixture, lb./(cu. ft.) =
=
Subscripts 1 and 0 = catalysts 1 and 0
ACKNOWLEDGMENT
The authors wish to acknowledge T h e Texas Co. for financial assistance in carrying out this work. NOMENCLATURE
a
nominal surface area of catalyst pellet, sq. ft./lb. of catalyst B and D = parameters (functions of temperature) in rate equations c = concentration of active centers on catalyst, number of centers/(sq. ft.) D M = mean diffusivity of a component in gas mixture, sq. ft.!hr. D, = effective catalyst pellet diameter, sq. ft. FlO = activity factor of catalyst 1 with respect to catalyst 0, defined by Equations 4 and 5. G = superficial massvelocity, lb./(hr,) (sq. ft.) j D = mass transfer number, dimensionless k, = mass transfer coefficient, lb. moles/(hr.) (atm.) (sq. f t . ) K = equilibrium constant for oxidation of sulfur dioxide f i l m = mean molecular weight of reaction mixture Pa = partial pressure of a component in bulk gas stream, atm. =
LITERATURE CITED
Baron, T., Chem. Eng. Progr., 48, 118 (1952). Ergun, S., Ibid., 48, 227 (1952). Grossman, L. M., Trans. Am. Inst. Chem. Engrs., 42, 335 (1946). Hall, R. E., and Smith, J. M., Chem. Eng. Progr., 45, 459 (1949). Hougen, 0. A., and Wilke, C. R., Trans. Am. Inst. Chem. E’ngrs., 41,445 (1945). Hougen, 0. A , , and Yang, K. H., Ckem. Eng. Progr. 46, 146 (1950). Irvin, H. B., Olson, R. W7.,and Smith, J. M., Ibid., 47, 287 (1951). Olson, R. W.,Schuler, R. R., and Smith, J. &I., Ibid., 46, 614 (1950). Rase, H., Ph.D. Thesis, Dept. of Chemical Engineering, University of Wisconsin, June 1962. Taylor, €1. A., “Twelfth Report of Committee on Catalysis,” National Research Council, New York, John Wiley & Sons, 1940. Thiele, E. W., IND.ENQ.CHEM.,31, 916 (1939). Wilhelm, R. H., Johnson, W‘. C., and Acton, E’. S., IND.ESG. CHEW.,35,562 (1943). Wilke, C. R., Chem. Eng. Progr., 46,95 (1950). RECEIVED for review May 28, 1952.
ACCEPTED November 14, 1952.
Synthesis of Water-Repellent Dyes h systematic study involving normal long-chain alkyl-substituted dyes showed that the water repellence of these dyes on cotton and wool yarns increases in almost direct proportion to the length of the normal alkyl chain and to the number of such alkyl groups in the dye molecule. Dyed cotton yarn can be made twelve times as water-repellent as the original yarn. Dyed wool yarn has been made sixty times as water-repellent a s the original fiber. Even dyes which have several hydroxyl groups showed marked water repellence when the long-chain normal alkyl group is substituted in the nucleus of the dye molecule. Octyl rosaniline on wool yarn develops a remarkable degree of water repellence. C . C. DEWITT AND P. D. SHROFF’ Michigan State College, East Lansing, Mich.
HE purpose of this investigation was to show that longchain, alkyl-substituted dyes impart nonwetting, waterrepellent qualities to dyed textile fibers. Normal aliphatic alkyl-substituted dyes were used in this work. The chemical structure of the dyes and their intermediates as related to the theory of surface chemistry was kept ever in mind. T h e work reported here had its genesis in a rather unrelated field, mineral flotation. Ludt and DeWitt (20) set forth the use of the principle of selective basic dye adsorption on native copper silicates for the purpose of floating these minerals. T h a t basic dyes adsorb more or less selectively on mineral and crystal surfaces has been shownbysuida (Z8), Dittler ( I I ) , and France ( I S ) . Freundlich and Neuman ( I 4 ) , Dean and Ambrose ( I O ) , and Knect ( 1 8 )have contributed to our knowledge of the adsorption mechanism related to 1
Present address, Research Laboratory, General Electric Go., Pittsfield,
Mass.
302
these dyes. T h e significance of the work of Suida and Dittler is attested b y the fact that most textbooks on mineralogy state that heavy metal silicates may be identified b y the order of the adsorption of certain basic dyes. These basic dyes, while they selectively adsorb on the heavy metal silicates, do not of themselves act as flotation agents or collectors. Actually, according to Taggart (29)) they depress the mineral. However, Ludt and DeWitt found t h a t when in the case of triphenylmethane dyes a normal aliphatic hydrocarbon chain of four to eight carbon atoms is attached t o one of the benzene nuclei of the dye molecule, the dye successfully acts as a flotation collector. Subsequent work along these lines has shown t h a t one may readilv effect the separation of other heavy metal oxygen-containing compounds. Because most longer chain normal alkyl-substituted dyes were not available, i t was necessary to synthesize these compounds. Thus a white cloth, used to mop u p a spilled dye solution, later
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