literature Cited
Bakelite Limited, Brit. Patent 560,908, April 26, 1944. Belov, P. S., Ch’ao-Ch’iLu, Isagulyants, V. I., Khim. Prom., 1962, p 480. Bondy, H. F., Gregory, A. D., Moore, F. R., Muro, G. E. (to Coalite and Chemical Products Ltd.), Brit. Patent 1,065,337 (April 12, 1967). Burwell, R. L., Archer, S., J . Amer. Chem. SOC.,64, 1032 (1942). Cohen, L. J., ibid., 29, 714 (1907). D’Alelio, G. F. (to Koppers Co.), U.S.Patent 2,802,884 (August 13, 1957). d’Yroire, F., Compt. Rend., 245,531 (1957); 247,297-300 (1958). Hahn, W., (to Farbenfabriken Bayer A.G.), Ger. Patent 1,142,873 (January 31, 1963). Jain, J. R., Pillai, G. N., J . Catal., 9, 332 (1967). blcDonald, G. A. (to Victor Chemical Works), U.S. Patent 2,550,490 (April 24, 1951). Mixer, R. Y., Wagner, J. W. (to Richfield Oil Corp.), Fr. Patent 1,322,509 (March 29, 1963). Ohta, N., J . Chem. SOC.,Japan, Ind. Chem. Sect., 51, 143 (1948). Radzevenchuh, I. F., Kaplan, S. V., Zhur, Obschei Khim., 29, 3945 (1959).
Romadane, I., Stipnieks, G., ibid., 30, 2193 (1960). Ryabov, V. D., Golodnaya, T. S., Zn. Prikl. Khim., 39, 2379 (1966). Rybar, A., Holcik, J., Masen, J., Chem. Pnumysl, 16,533 (1966). Sheffer, H . E., Carbide and Carbon Research Laboratories, South Charleston, W.Va., unpublished data, 1945. Sidorora, N. G., Bokora, A. I., Zh. Organ. Khim., 1 (12), 2176 (1965). Socony Mobil Oil Co., Netherlands Application 6,407,636 (January 7, 1966). Taylor, H. S., Gurney, R. L., Grazier, A. W., Soil. Sci. SOC. A m . Proc., 29 (3), 317-20 (1965). Turova-Polyak, 31. B., Rudenko, N. V., Li-Pan Lin, Zhun. Obshchei Khim., 30, 94 (1960). Union Rheinische Braunkohlen Kraftstoff, A.-G., Xetherlands Application 6,600,159 (July 8, 1966). Venuto, R. B., Hamilton, L. A., Landis, D. S.,Wise, J. J., J . Catal., 5 , 81 (1966). RECEIVED for review October 14, 1970 ACCEPTEDAugust 17, 1971
Aniline Production by Dual Function Catalysis Leon M. Polinski’ and Ernest A. Harvey2 American Cyanamid, Bound Brook, N . J . 08805
Polyfunctional heterogeneous catalysis i s responsible for petroleum reforming of hydrocarbons, and the theory elucidated and developed b y Weisz for these reactions is well established. These same concepts are used to devise a polyfunctional catalyst system for simultaneously steam reforming and hydrogenating nitroaromatic compounds-e.g., nitrobenzene-to aromatic amines-e.g., aniline-in a fixed bed vapor phase catalytic system. This avoids the large exotherms present in a straight hydrogenation (the exotherm is still present but offset b y the endotherm of the reforming reaction) and allows two heretofore separate consecutive catalytic reaction processes to occur simultaneously in a single unit despite unfavorable equilibrium conditions for the steam reforming reaction. The complexities of the kinetics are examined; however, no conclusion on the kinetic steps can be drawn without extensive additional study.
A
one-step process for the manufacture of aniline has proved feasible for utilizing the techniques of dual function catalysis. The commercial methods of aniline manufacture presently involve reduction of nitrobenzene with hydrogen to aniline either in fixed or fluid bed systems. Normally, the hydrogen for this process is generated in a separate reforming plant adjacent to the reduction unit. T h e reforming reaction which involves conversion of methane-rich natural gas streams t o hydrogen and COz, CHI
+ 2Hz0
+
COz
+ 4Hz
(1)
utilizes a nickel reforming catalyst a t high temperatures, 500-10OO0C, necessary to maintain a favorable equilibrium for Reaction 1 which is endothermic. The hydrogen produced is used for reduction of nitrobenzene over a coppersilica fluidized bed catalyst, one of a number of catalysts used to produce aniline a t much lower temperatures below Present address, Givaudan Corp., Clifton, N.J. 07014. To whom correspondence should be addressed. Present address, J. P. Lewis Co., Beaver Falls, N.Y. 13305.
30OoC. This reduction is highly exothermic and requires efficient heat transfer to maintain the desired control. The possibility of manufacturing aniline directly from nitrobenzene, methane (or natural gas), and steam in either a single fixed or fluid bed reactor has been demonstrated in the laboratory. The consecutive steps of Reaction 1 and CsH5NOz
+ 3Hz
-
C6HsNHz
+ 2Hz0
(2)
were combined in the same reactor provided with a uniquely compounded dual functional catalyst system. The resulting overall reaction C6HsNO?
+ CHd
+
CGHJYH2
+ COz + Hz
(3)
actually occurs a t temperatures so low that the equilibrium for Reaction 1 is unfavorable. For instance, under actual experimental conditions in which aniline is produced in 50% conversion and 72% selectivity on nitrobenzene fed, the equilibrium concentration of hydrogen can be calculated. For Reaction 1 the equilibrium constant can be represented b y log1&1
=
-7700/T
+ 7.57 logi,T
- ,00063 T - 12.6 (4)
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 4, 1971
365
Table 1. Criteria for Catalyst Particle Sizing
270 3 . 5 5 X lo-' 7 . 4 7 X 086 > R > ,0086 327 1 . 6 6 X 10-5 1 . 4 2 X lop6 ,119 > R > 0119 In em. b In cmz/sec. c Expressed in mol/sec/cc. Variation in d,\r/dt is assumed between 10-7 and 10-5. 1
2
At reaction conditions of 327OC, the equilibrium constant K1 = 1.66 X lo+, and the maximum possible hydrogen gas concentration in the vapor is estimated a t 0.07 mole fraction. (That is, at a total system pressure of 760 mm, the Hz partial pressure would be 53 torr maximum.) There is no theoretical reason why the reaction could not also proceed a t lower temperatures with even lower H2concentrations. The theoretical concepts of the polyfunctional catalysis mechanism originated with Weisz (1962, 1963) whose admirable descriptions and mathematical model realistically define the dual role of platinum- alumina catalyst in reforming reactions. A brief reexamination of this characterization will help clarify the results of this work. I n a multiple-step reaction of the type A +. B +. C, the overall rate is actually a function of three entities not always considered related. The three interrelated criteria consist of thermodynamic quantities, rate quantities, and purely physical property parameters. The nontrivial case for a poly-step reaction such as the nitrobenzene-methane-H20 reaction is one having the situation: catalyst Y
catalyst X --*
t----AB-+C
-
(5-4)
in which the equilibrium constant for the forward reaction A B is small. The elementary kinetics for a first-order reaction of the type:
K = equilibrium constant for the reaction A B. N = any generalized reaction component. Since dN/dt = k3A, whenever K is large, then k3 is significant if kp >> kl' and has a maximum value approaching kl. I n other words, if the rate of the second reaction B +. C in the series A +. B C is A , then the magnitude of k3 and larger than the rate of B dN/dt is significant. With selection of a proper catalyst, the overall reaction rate for conversion of A a t a lower temperature can be high even though little conversion would result if A is reacted to B in one converter at that same low temperature, and B is reacted to C in a subsequent converter. I n sequential reactions one needs to carry a large finite amount of B to react to C. The mass of B and the flow rate of B thus become limiting. However, the two catalysts need not be placed in separate converters but can be mixed in the same reactor. I n this way intermediate B (in our case, hydrogen) need only move from the surface of catalyst z (reformer catalyst) where it is formed to the surface of catalyst y (hydrogenation catalyst) where it is used up, and the true overall rate is limited only by the transport between z and y of component B. Consider, as first illustrated by Weisz (1962), the parallel plate model with component A reacting on plate z to form B; intermediate B diffuses to plate y and on the plate y B is converted to C. The three equations which result are
-
+ .
dN/dt = kiA - ki'B(z) dN/dt
=
kzB(y)
(12) (13)
and the diffusion rate equation
dN/dt = D/L [B, - B Y ]
(14)
where D is the diffusivity of component B; L is the distance between z and y, and B,, B , are the respective concentrations of B on catalyst plates z and y. This diffusion term leads to a correction constant being added to Equation 11 which becomes (Weisz, 1962)
ki
A e
B
dA/dt dB/dt
=
=
3 C show:
(5B)
k1'B - klA
(6)
- ki'B
kiA
- knB
(7)
dC/dt = kzB
(8)
-
Since we are considering the nontrivial case of this system -+ where A B C or ultimately, A -+ C, me can reason as follows: Obviously, since A must first go through an intermediate state, B, a t least a small amount of B must be present even if the equilibrium conditioiis for B are unfavorable. B at unfavorable equilibrium will not accumulate and, in fact, will remain slightly below its equilibrium concentration, or specifically when in a continuous reactor, a t some positiondependent steady-state concentration never exceeding the equilibrium value. Therefore, +
dB/dt klA - kl'B - kzB dC/dt = kzB
=
kzkiA/(ki - ''A -
+ kz)
-klki'+ -k2kl where kg 366
=
h/(K
2
0
(9)
0; B = klA/(kl'
=
+ kz)
dN/dt E AD/KL = (B)equiiD/L
(16)
For a polyfunctional catalyst system a maximum rate (dN/dt) is determined by the three categories of Equation 16, namely, the equilibrium concentration of component B, the diffusivity of B, and the length of L , the diffusion path from the z catalyst surface to the y catalyst surface. The actual overall rate can in theory be controlled within rather broad limits by variations in Bequll(a function of temperature and (Ao)concentration) and in L (a function of catalyst z and catalyst y particle size). The final relationship established (Weisz, 1962) does not involve parallel plates but, more realistically, spheres or pellets and the equivalent equation to 16 as applied to catalyst spheres is given as
(dN/dt),,,
(10)
6 [BegUlil D/RZ
(17)
or
=
- hA/(K
For values of k3LK/D, which are large compared to 1.0, the limit of Equation 15 becomes (Weisz, 1962, 1963)
+ kz/k1) = k d
+ kp/kl).
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No.
4, 1971
(11)
[R2/D] ( d N / d t ) a c t u a i [1/Bequi1] < 1 (18) for particles of radius R. A number of general orders of magnitude can first be established before the specific system nitrobenzene- methaneH20will be treated.
Table II. Experimental Results
Run no.
Catalyst
Nitrobenzene rate, g/hr
T, OC
H2O liq, cc/hr
CH4(g) rate, cc/min
Time of run, hr
Nitrobenzene in prod, wt ?&
Aniline, wt
Other, wta
Theoretical aniline selectivity based on +NO* recovered)
Ratio feed
HzO/ CH4, mol
%
Converdon of WOz,
%
1 NR-1 335 330 5 30.1 0 2.5 95.0 0.7 4.3 14 0 2 420 79.5 0 330 90.5 2.1 10 1.0 7.4 21 0 3 420 57.8 48 330 1.0 9.4 30 3.0 15.7 85.9 4.7 44.6 4 Combined 81 3.0 3.5 73 12.5 327 47 63.9 32.6 49 5 41.6 4.3 350 47.2 81 2.5 61.5 34.2 72 10.65 47.5 3.2 c 8.95 6 350 2.17 84.0 12.8 16.8 58.1 35 81 7 Catan 350 50.5 42 81 1 No reaction 10.62 0.5 -99 0.0 51 Refers to organic by-products including diphenylamine and azobenzene. * Selectivity based on +NO2 fed is low when compared to selectivity based on total product collected. Losses due to fog formation in the condensing system prevent,ed a total product recovery. Catalyst experienced coking during this run (carbonaceous deposit was 13 wt % of clean catalyst).
Flow resistance to diffusion is lo2 larger between particles than within particles. If the distance between particles is small, in the gas phase, the outside diffusion is short-circuited and Equation 18 is valid. A d N / d t in either laboratory or plant reactions must fall within the limits of lop7to 10-5 mol/sec/cc catalyst volume. If not, either the conversion is not sufficient to get a desired amount of product in a finite catalyst bed, or the rate is so fast that the exotherm destroys the product. The diffusivity of porous materials of high surface area is approximately cm2/sec (Weisz, 1963). With this information the proximity of catalyst z to catalyst y (or the distance R ) can be estimated for a given set of equilibrium conditions of [Begull]to satisfy the above criteria. Table I lists equilibrium constant K l (for Reaction 1) as a function of temperature together with the values of [Hz equilibrium] calculated from Equation 4, the equilibrium for Equation 1 as a function of temperature, and from a material balance on reactants fed to our system. For case 1 the variation in R is calculated between 86 and 860 p . For case 2 the variation is between 119 and 1190 p . The order of magnitude of particle size of the two types of catalyst particles to be mixed together to insure a good reaction rate assuming Equation 19 valid for the estimation is 5 100 mesh (149 p ) a t 327OC and -200 mesh (74 p ) a t 270OC. The catalyst particle size chosen for these reactions was
