OXIDATION OF PROPYLENE WITH AIR OVER A COPPER OXIDE CATALYST D. S. BILLINGSLEY1 AND C. D. HOLLAND Department of Chemical Engineering, A&M College of Texas, College Station, Tex.
A differential reactor was used to investigate the stoichiometry and kinetics of the oxidation of propylene with air over a copper oxide catalyst. The reactor was maintained a t very nearly isothermal conditions by use of three individually controlled heaters. The output of these heaters was controlled by a system of relays and a Geneva mechanism. All streams were analyzed with a gas chromatograph. The usual variables such as feed rate and composition were varied over wide ranges. If the data for catalytic reactions are correlated by use of mass transfer equations, it is not necessary to propose a reaction mechanism. This method is demonstrated by the correlation of the experimental results obtained for the catalytic oxidation of propylene.
of the kinetics of the catalytic air oxidation of propylene, it was found that the experimental data could be correlated by using mass transfer equations without proposing a reaction mechanism. Oxidation of propylene with air over a copper oxide catalyst has been studied by Woodham and Holland (70) and others (7, 4-6 8, 9). T o provide accurate experimental data, a temperature controller suitable for highly exothermic reactions was designed and constructed, and the reactant and product streams were analyzed with gas chromatography. Two competing primary reactions (the formation of acrolein and the formation of COz) were found to occur. It was possible to correlate the experimental results by use of the appropriate rate expressions for mass transfer along with two postulates: the rate of mass transfer of oxygen was “controlling,” and the ratio of the production rate of acrolein to that of COSvaried linearly with the ratio of the partial pressure of propylene to that of oxygen. URING AN INVESTIGATION
Experimental
A flow-type, tubular, fixed-bed differential reactor was used. A flow diagram showing the arrangement of the major pieces of equipment is presented in Figure 1. The feed to the reactor was a mixture of propylene and air. The catalyst was prepared as described by Woodham and Holland (70).
Feed System. Bottles of technical grade propylene were the source of this component of the feed. Each bottle as used was fitted with a pressure regulator set to maintain a pressure of approximately 45 p.s.i.g. T h e line from this regulator contained a check valve and led to a rotameter calibrated a t line pressure. Air was supplied to the system a t 75 to 100 p.s.i.g. I t passed successively through a check valve, a trap, and a pressure regulator set to maintain a pressure of 44 p.s.i.g., and subsequently was split into two streams. One supplied air to the instruments and the other supplied air for the reactor feed. The latter stream was next passed through a bed of activated alumina to remove most of the remaining moisture. T h e alumina was contained in a laboratory Lectrodryer designed to reduce the dew point of air passing through it to between -70’ and -40’ C. This stream was next passed through another check valve, a rotameter, and then into the feed manifold. Reactor System. The feed flowed next to the reactor system. Total pressure was measured a t the reactor entrance.
There was a sample port between the feed manifold and the reactor system. The reactor itself consisted of a 23-inch length of nominal 0.75-inch stainless steel pipe which had a stainless steel flange welded to each end. The bottom of the reactor was closed by a blind flange to which a thermocouple well was attached. The upper end of the reactor was closed by another blind flange to which was welded a cylinder containing a spiral groove; this served as a preheater for the feed. T h e top of the catalyst bed was located about 0.75 inch below the preheat cylinder and extended downward for approximately 1 inch. The remainder of the steel pipe was filled with stainless steel filler blocks. Detailed drawings of the feed systems and the reactor have been presented (2, 70). T h e temperature of the catalyst bed was regulated by a control system. Temperature Controller. Temperatures within the reactor were measured by iron-Constantan thermocouples located a t approximately 0.5-inch intervals along the reactor below the preheat section. Thermocou les were housed in stainless steel wells made of nominal 3 L i n c h tube. T h e catalyst bed, 1 inch long, surrounded two thermocouples, as indicated in Figure 2. A Geneva mechanism was used to connect each thermocouple successively to the temperature recorder. A pair of mercury switches mounted on the temperature recorder was set to open and close a t particular temperatures. The Geneva mechanism also connected the poles of the mercury switches to successive relay circuits. Each relay circuit was designed to supply one of three voltages, “high,” “low,” and zero, to the heating coil attached to it and wrapped around a short length of the reactor tube. T h e
E F F L U E N T FROM VAPOR FRbCTDHETER SbHPLINQ D E V I C E
TO VALOR
-
FRACTOYETER SbMPLING DEVICE
3-WbY VALVE
OIbPHRbGM
I
1
MOTOR VALVE
= T
1
EFFLUENT fROM REACTOR
1 Present address, International Business Machines Gorp., Houston, Tex.
252
l&EC FUNDAMENTALS
Figure 1 .
Sampling and pressure control systems
voltage actually supplied depended upon the positions of the mercury switches a t the: time they were connected to the relay circuit. This voltage was maintained after mercury switches were disconnected and until they were again connected to that relay circuit. “High” and “low” voltages were set manually for each relay circuit by a n adjustable transformer and a resistor connected in series with each relay circuit. T h e over-all effect was to decrease the voltage to a heating coil when the temperature in the section it surrounded exceeded a preset value and to increase it when this temperature fell below a lower preset value. Figure 3 shows the recorded temperatures from start-up through R u n 46. T h e reaction commenced a t about 460 F. as shown by the slightly more erratic behavior of the bed temperature profile. Ehen after the reaction started, the control system was able to restrict the variation to about 3’ F. Sample collection required 1 to 2 minutes per r u n ; the samples constituting R u n 46 were withdrawn about 1 hour and 45 minutes after start-up. Sampling and Analytical Procedures. Each sample was analyzed on a Perkin-Elmer (Model 154-C) Vapor Fractometer. Two meters of Perkin-Elmer S column were used to separate nitrogen and oxygen, ethane, CO,, ethylene, propane, and propylene. A temperature of 65 C. and a helium eluent rate of 15 cc. per minute were employed. Two meters of Perkin-Elmer W column were used to separate acrolein and water from each other and from the remaining constituents of the sample. For these separations. a temperature of 60’ C. and a helium eluent rate of 15 cc. per minute were employed. Identification was made by comparing the chromatograms produced by the samples of feed and eWuent from the reactor Lvith the chromatograms of known pure samples. The area corresponding to each component on the chromatogram was corrected for the thermal conductivity of that component, and the amount of that component was taken to be proportional to the corrected area. The nitrogen to oxygen ratio was taken as 4.0 for both feed and effluent. Calculations for the latter were repeated twice, each time using the nitrogen to oxygen ratio obtained from the preceding calculations. In most cases, however, the amount of oxygen reacted was small enough so that the ratio of nitrogen to oxygen in the effluent was close to 4.0. The conditions selected for the chromatographic runs were those which gave the most nearly symmetrical and distinct peaks for all components of interest. In determining the best temperatures a t which to make the chromatographic runs, a slight variation of the relative areas with temperature was observed. T h e feed-air, feed-propylene, and effluent streams were split, and in each case p a r t of the stream was sent directly (but not simultaneously) to the sampling device attached to the Vapor Fractometer, and thereupon analy7ed. I n the case of the feed streams this procedure was adopted for convenience, but in the case of the etfluent stream it lessened the likelihood of appreciable condensation of acrolein and water before these compounds could be separated. I n addition, 400-ml. effluent samples were withdrawn into dry borosilicate glass sample bombs by displacing acidified salt solution and used for the separation of all components other than acrolein and water. Figures 4 and 5 show the chromatograms for the analysis of the effluent from R u n 34. Figure 4 represents essentially an analysis of the first peak shown in Figure 5 . Suitable qualitative organic spot tests taken from Feigl (3) were used to detect the presence of methylene ketones, enols, alcohols. esters, and fcirmaldehyde. In no case were these compounds found. Solutions for testing were obtained by bubbling reactor effluent through distilled water until the solution became saturated with acrolein, as indicated by formation of a precipitate in a saturated solution of 2,4-dinitrophenylhydrazine. This solution was contained in a scrubber placed immediately after and in series with the vessel containing the distilled water. Runs 32, 51, and 52 (Table I ) were made a t essentially the same set of operating conditions to determine the over-all reproducibility. Figures 6 and 7 indicate this was about 77,.
THERMOCOUPLE TO BOTTOM OF P R E H E 4 T S E C T I O N
VOLTAGE CONTROL (HlGH. LOW, OFF I
GENEV4 H E C 3 4 N l S H
p
,
*EATER NO PREHEATI
HEATER
1I I
TEMPERATURE
CONTROLLERPNO STRIP C H I R T RECORDER
I
HEATER
F R OEMF FREACTOR LUENT
I 1
I ,I I
NO 3
\ I i ’ I
THERMOCOUPLE T O B O T T O M OF C C I T A L Y S T B E 0
I
i
THERMOCOUPLE T O TOP OF C I T h L Y S T B E D
Figure 2.
Closed-loop temperature control system
6ED -EMPERbTLSt PREHEbT TEMPERATURE
BELOW BED
TEMPERbTURE TEMPERATURE
1-112”BELOW BED
RUN 4 6 WADE 4 T T H
C
POIYT
2 BELOW BED TEMPERbTURE 2 1IP”BELOW B E D
05
I 0
300
400
500
TEMPERATU-E
600
700
(OF)
Figure 3. Temperature profiles from start-up through Run 46
Correlation of Results
The use of a differential reactor simplifies the mathematical analysis of catalytic reactions. A differential reactor is one in
Figure 4. Chromatogram for separation of acrolein and water in Run 34 VOL. 2 NO. 4 N O V E M B E R 1 9 6 3
253
which the conversion per pass is small enough that the rate of reaction for any component may be represented with good accuracy by :
An j
zu
=
ocooccooocooocccoooocco
\I
3l
: -E u 3
of component j reacted (or produced) per unit time per unit mass of catalyst An, = change in molal flow rate of component j-molal flow rate of component j at reactor exit minus molal flow rate of component j at reactor inlet W = mass of catalyst contained in reactor
r1
= rate of reaction for component j-moles
0 0 0 0 - c 0 - ~ 0 - - N c ~ ~ ~ o o - N c o
-Nrcit.*rO*y
o c c c c o o ~ o ~ o o c c o c occoooocoo~occ0ccocoooo ooooooooo~6ooooooocoooo
For any given feed composition, the over-all mechanism was independent of the residence time (or FIT,'+), as shown in Figures 6 and 7. If secondary reactions such as the further oxidation of acrolein to COz or the polymerization of acrolein to the dimer were significant, the curves in Figure 6 would have exhibited negative slopes, and those in Figure 7 would have had positive slopes. The horizontal lines shown in these figures, plus the fact that other possible products did not appear in the effluent in significant amounts, suggest that the reactions are adequatelv described bv the folloiring stoichiometric equations.
+
CIHB
C3H6 u)
(1)
where
a 1-
z *
w
1.4
0 2
+ H20 + 3Hz0
e CHzCHCHO
+ 4.5
0 2
3Coz
(2)
(3)
these compounds are denoted as follo\vs : o o ~ ~ o o o o o o o c ~ ~ o o o o o o o For o osimplicity .
+B 2 C +D + 4.5B @ 3E + 3D
.4 A
(4) (5)
These reactions may not go precisely as written; the mechanisms by which these reactions proceed may differ appreciably from these simple equations. These t\ro equations are to be regarded as representing the stoichiometric relationships between two competing mechanisms. The complete process involves the mass transfer of the reactants (A and B) from the flo\iing stream to the interface of the catalyst, the adsorption of one or both of these by the catalyst. reaction of A and B. and the desorption of the products from the catalyst to the interface, followed bv the mass transfer of the products from the interface to the flowing stream. For reactants ,4and B the rates rA and yB are defined in terms of the disappearance of A and B. Similarly. for products C, D, and E, the rates rc, rD, and rE are defined in terms of the appearance of C, D, and E. Furthermore. for convenience rates r 1 and r2 are introduced. These are the rates at which the reactions given by Equations 2 and 3 (or Equations 4 and 5) proceed : rl
= rc
(6)
Also, for these two stoichiometric reactions, it is readily shown that the rates are related by three independent equations:
At steady state, the rate of mass transfer of each reactant from the flowing stream to the interface is equal to the rate a t which it is consumed by reaction. Likewise, the rate of production of each product by reaction is equal to its rate of mass transfer 254
l&EC FUNDAMENTALS
NOTE
ORDINATE ATTENUATION FKTOR
F
/PROPYLENE F-32
-APPROXIMATION
TART ?UN
TATION
34
E N D RUN
Figure 5. Chromatogram for separation of components other than acrolein and water in Run 34
from the interface to the flowing stream. mass transfer :
Thus, in terms of
= k d j A - pi%)
(11)
rA
- $Br)
(12)
IC =
kc(Pct - P c )
(13)
rD =
ko(pD, - p n )
(14)
TB
kB($B
(15)
T E = kE(.bE% - P E )
where p is partial pressure, k is the rate constant, and subscript i distinguishes the value of the partial pressure of a component a t the interface from its bulk values in the flowing stream. When the expressions given by Equations 11 to 15 are substituted in Equations 8. 9, and 10, Equations 16, 17, and 18 in five unknowns (the interfacial partial pressures) are obtained : k4(p.4
AD(FD,
- $A>) =
kC(fiCt
+3
- PD) = kc(Pci - P c )
+
I O
-";
kE
- ,bc)
00
(PEa
- $E)
~ E ( P E $-$E)
(16)
20
30
40
, G R A M S OF C A T A L Y S T P E R G R A M - M O L E OF F E E D P E R HOUR
Figure 6. Relative amount of primary product acrolein increased as ratio of partial pressures of propylene to oxygen was increased
(18)
To solve these equations, two further postulates are required, such as a mechanism for each reaction, the assumptions that the two reactions are a t equilibrium, the assumptions that mass transfer controls (pAi = psi = 0), or any other pair of conditions thought to represent the experimental results. In attempting to correlate the results, the linear relationship shown in Figure 8 was a n early observation. The equation of the best straight line through the points is: (19)
This was taken as a n additional relationship, which in view of Equations 6 and 7 gives four equations (Equations 16 to 19) in five unknowns. T h e further postulate that the rate of mass transfer of B was "controlling," that is: $Si
0
(20)
0 41 00
IO
-
reduces the number of unknowns to four. This postulate (pEi = 0) led to expressions for r1 and r2 that fitted the experimental results better than the rate expressions based on certain other postulates, as discussed later.
2 0
I 30
40
G R A M S OF C A T A L Y S T PER G R A M - M O L E O F FEED PER HOUR
"T,
Figure 7. Relative amount of CO:! formed decreased as ratio of partial pressures of propylene to oxygen was increased VOL. 2 NO. 4 N O V E M B E R 1 9 6 3
255
T h e postulates represented by Equations 19 and 20 are readily utilized to obtain expressions for r l and r2 as follows. Equations 6, 7, and 9 may be combined to give: rB
= I1
+ 4.5 -(3 3
(21)
r2)
Thus :
Combining this result with Equation 19 produces :
I n view of Equations 12, 19, and 20, Equation 21 reduces to:
~ B P B=
(0.72@ PB
+ 4.5)
The value of the mass transfer coefficient k , was computed b>use of Equation 24 and correlated as a function of the total molal flow rate. This result:
12
kg
os
I O
20
40
6 0
8 0 100
PcsH66/Pop, RATIO OF T H E P A R T I A L P R E S S U R E OF P R O P Y L E N E T O THE P A R T I A L P R E S S U R E OF O X Y G E N
Figure 8. Ratio of rates of reactions producing acrolein and COSvaried linearly with ratio of partial pressures
= 0.94
x
10-3(nr)056a
(25)
was used to compute the calculated values of r l and r2 shown in Figures 9 and 10. Comparison of these curves with those in Figures 6 to 8 suggests that the accuracy of the correlation is about as good as the experimental accuracy. The mass transfer coefficient, k,. is a function of the Reynolds and Schmidt numbers. Of the quantities that appear in these numbers, only the total flow rate was varied to any appreciable extent. Because of this, k, was correlated as a function of the single variable. total flow rate. The exponent 0.555 on the total flow rate is in general agreement with exponents reported by Hougen and LVatson ( 7 ) . This agreement lends support to the postulates represented by Equations 19 and 20. Runs 51. 52. 55, and 56 show that the effect of total pressure was negligible over the range investigated. Discussion
Other sets of postulates considered in the correlation of the data are worthy of mention. One set was that the rates of mass transfer of both A and B were controlling (bA, = 0, pB, = 0). O n the basis of this set of postulates, it is readily shown that:
80-
v 0
w
For this expression to reduce to the experimental result given by Equation 19, it would be necessary that:
IO
20 40 6 0 e o 100 EXPERIMENTAL VALUE OF r, x l o 4
Figure 9. Correlation of acrolein producing reaction
ISO
rate
of
be negligible with respect to unity and that unity be negligible with respect to: 4.5
(k)@)
In view of these requirements. the set of postulates pSZ= 0, = 0 was discarded. = 0 Instead of postulating that pB, = 0, the postulate could have been made. The latter postulate plus the one given bv Equation 19 yielded :
pB,
EXPERIMENTAL VALUE OF rz x
io4
Figure 10. Correlation of rate of Coz producing reaction 256
l&EC FUNDAMENTALS
HoMever, this postulate (pat = 0) was discarded because these expressions did not correlate the experimental results nearly as well as Equations 23 and 24. The well-known approach of assuming the reactions given by Equations 2 and 3 to be in dynamic equilibrium was considered. It was abandoned because the resulting equations were so complex that explicit expressions for r l and r2 as func-
tions of p, and pB could not be obtained. The stoichiometric coefficients of 4.5 and 3 in Equation 2 appear as exponents on the partial pressures in the expressions for the equilibrium constants. Actually, either a nonnegative number or a function of pB could have been selected for psi as required to minimize the error between the calculated and experimental rates of reaction. I n view of the simplicity of the method for treating the particular system investigated, it is believed that this same general approach could be successfully applied to other catalytic reactions. T o apply it, only the stoichiometry of the reaction must be established experimentally. Additional postulates may be required to produce a n equality between the numbers of unknowns (the interfacial partial pressures) and the number of equations. The use of the mass transfer relationships requires far fewer postulates than does the usual approach of proposing a reaction mechanism. For example, a t the outset the five postulates, pAi = pA,psi = pB>pci = pcj Pni = pD, and pEi = p,, would have been required for a system such as the one considered herein. Sext, it is generally assumed that the rate of diffusion to and from the interior of the catalyst is fast. Finally, the assumption of a homogeneous catalytic surface is commonly employed. After all of these postulates have been made, the postulates pertaining to the reaction mechanism must be made and justified. The necessity for making this multiplicity of postulates is eliminated by use of the simple method employed herein.
In principle, this method of correlation may be employed for the correlation of integral reactor data. The corresponding procedure differs from the one shown only in the calculation of values for the rj’s. I n the case of a n integral reactor, r j a t any given W’/noT is the slope a t this value of the abscissa of a plot of nJnoT us. W/noT. literature Cited
(1) Andrianona, T. I., RoginshiY, S. C., Zhur. Obshchei. Khim. 24, 605 (1954). (2) Billingsley, D. S., Ph.D. Dissertation, A&M College of Texas, College Station, Tex., 1961. ( 3 ) Feigl, F., “Qualitative Analysis by Spot Tests,” 3rd ed., Elsevier, New York, 1946. (4) Goodings, E. P., Hadley, D. J. (to Distillers Co.: Ltd.), Brit. Patent 625,330 (June 27, 1949). (5) Zbid., 658,179 (Oct. 3, 1951). (6) Hearne, G. W., Adams, M. L. (to Shell Development Co.), U. S. Patent 2,486,842 (Nov. 1, 1949). (7) Hougen, 0. A,, Watson, K. M., “Chemical Process Principles,” p. 985, Wiley, New York, 1948. (8) Margolis, L. Ya., RoginshiY, S. Z., Gracheva, T. A., Zhur. Obshchei Khim. 26, 1368-71 (1956). (9) N. V. de Bataafsche Petroleum Maatschappij, Brit. Patent 640,383 (July 19, 1950). (10) Woodham, J. F., Holland, C. D., Ind. Eng. Chem. 52, 985 (1960). RECEIVED for review January 4, 1963 ACCEPTEDAugust 19, 1963 A.1.Ch.E. South Texas Section Meeting, Galveston, Tex., October 1962. Work supported by the Dow Chemical Co. and the Texas Engineering Experiment Station of the A & M College of Texas.
DIFFUSION AND HETEROGENEOUS REACTION IN A TUBULAR REACTOR Decomposition of Hydrogen Peroxide Vapor CHARLES N. SATTERFIELD A N D REGINALD S. C. YEUNG’ Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Mass. The heterogeneous decomposition of HzOz vapor on platinum or 304 stainless steel was studied in a 1/4-inch i.d. tubular reactor a t temperatures of 130” to 460” C., Reynolds numbers of 370 to 41 00,H102 log mean concentrations up to 2.1 mole % H z 0 2 , and atmospheric pressure. Under most conditions mass transfer and surface reaction are both significant. Intrinsic surface rate constants are presented as a function of temperature for both catalysts. An unexpected finding is that the true surface rate goes through a maximum with increased temperature.
N STUDYING
a heterogeneous reaction under conditions in
I which diffusion and surface reaction may each be a significant rate-limiting step, it is necessary to choose a geometrical arrangement for which the mass transfer characteristics are known o r readily predictable. T h e packed bed may have uncertain temperature and concentration gradients axially and radially and the point mass transfer coefficient varies with position around each particle. I t was anticipated that the rotating cylinder, while useful for many investigations, would cause experimental difficulties in this case, a n d especially that undesired decomposition would occur on Present address, Cabot Gorp., Concord Road, Billerica, Mass.
the walls of the containing vessel. The tubular reactor having a smooth nonporous wall, chosen for these studies, provides a “uniformly accessible” geometry which is subject to fundamental theoretical analysis and for which a substantial amount of mass-transfer information is available. There is little difficulty in maintaining its surface essentially isothermal. T h e present investigation was concerned with a study of the heterogeneous decomposition of hydrogen peroxide vapor on two different catalyst surfaces, platinum and 304 stainless steel. The study was stimulated by a practical problem in the catalytic decomposition of concentrated hydrogen peroxide in catalyst beds. This is the basis of a method frequently used VOL. 2 N O . 4 N O V E M B E R 1 9 6 3
257
