the other alcohols, thus tending to bring the parallel lines of Figure 3 closer together. The data are inadequate to permit quantitative corrections, but they suggest that the Arrhenius frequency factors based on the number of sites participating in Reaction 1 may be much closer for the various alcohols than the more usual (and more practical) weight basis.
literature Cited
(1) Adkins, Homer, Perkins, P. P., J . Am. Chem. SOC. 47, 1163-7 (1925). (2) Brown, A. B., Reid, E. E., J . Phys. Chem. 28, 1077-81 (1924). (3) ,Hougen, 0. A., Watson, K. M., "Chemical Process Princiules." Vol. 111. Wilev. New York. 1947. (4) Kabel, R. L.,' Johinson, L. N.; Preprint 48, 54th Annual Meeting A. I. Ch. E., New York, 1961. (5) Komarewsky, V. I., Stringer, J. T., J . Am. Chem. SOC.63, 921-2 (1 941 \ \ - - '-/'
Acknowledgment
T h e authors express appreciation for very generous financial support from the Esso :Educational Foundation. Nomenclature
S = number of carbon atoms t
= reaction rate of individual reaction, gram-moles per hour
per gram of cat,alyst r T = reaction rate for gross decomposition of alcohol, same units as r R = gas constant = 1.98 X lop3 kcal./(gram-mole) (" K.) T = absolute temperature, " K.
(6) Laible, J. R., Dissertation Abstr. 20, 974 (1959). (7) Maurer, J. F., Sliepcevich, C. M., Chem. Eng. Progr. Symposium Ser. 48, No. 4, Reaction Kinetics and Transfer Processes, 31-7 (1952). (8) Miller, D. N., Dissertation Abstr. 16, 926 (1958). (9) Pease, R. N., Yung, C. C., J . Am. Chem. SOC.46, 390-403 (1924). (10) Schwab, G. M., Schwab-Agallidis, E., Zbid., 71, 1806-16
I
11) Senderens, J. B., Ann. chim.phys. 25, 449-529 (1912). 12) (1949). Smith, J. M., "Chemical Engineering Kinetics," McGrawHill. New York. 1956. (13) Topchieva, K. V., Yun-Pin, K., Vestnik Moskou. Univ., 7, No. 12, Ser. Fit.-Mat. i Estestven. Nauauk No. 8, 39-48 (1952); C. A. 47, 9125 (1953). RECEIVED for review June 19, 1961 ACCEPTEDFebruary 7, 1962
T H E REACTION OF AMMONIA WITH CARBON A T ELEVATED TEMPERATURES T H O M A S K . S H E R W O O D A N D
ROBERT 0 . M A A K l
.tfasrachusetls I n d l u l e of Technology, Cambridge, Mass.
A simple heated-filament reactor was employed in studies of the reaction of ammonia with carbon to form HCN, and in the pyrolysis of both liquid and gaseous hydrocarbons. Reaction occurred in a thin layer of fluid in contact with the heated surface; rapid quenching of the reaction products resulted from the flow of cold flluid reactant past the wire. Eleven volume per cent acetylene was obtained b y pyrolyzing methane. Ammonia reacted with both graphite and pyrolitic carbon to form HCN, the reaction being rate is controlled b y the surfirst order in clmmonia. At low temperatures-below about 1600" K.-the face reaction; at higher temperatures-to 2300" K.-mass transfer to the surface controls the rate. Pyrolysis of liquid heptane gave large yields of ethylene, conversion passing through a maximum of 2.4 moles of ethylene per mole of heptane at a wire temperature of 1650" K.
hydrogen cyanide, and other endothermic compounds of cominercial interest can be formed at high temperatures, but recovery requires that the products be quenched rapidly to prevent decomposition. Both the high temperature and the rapid quench can be attained in electric arcs: shock tubes, or by devices equipped to provide adiabatic compression and expansion of gases. S o n e of these are simple to construct and use in experimental investigations of an exploratory character. The present study describes results obtained Lvith a simple reactor employing a heated filament, so arranged as to obtain the necessary rapid cooling of the reaction products. hfost of the data reported are for the reaction of ammonia with carbon; a few results for the pyrolysis of methane and of heptane are included.
A
CETYLEXE,
Present address, Esso Research Laboratories, Baton Rouge, La.
Experimental
T h e reactor, shown in Figure 1, was designed for batch operation. It consisted of a vertical borosilicate glass cylinder having a volume of 680 cu. cm., fitted with a ground glass flange. Gas circulation within the reactor was provided by rotating an annular borosilicate glass cup (rotor) consisting of cylinders 2.5 and 10 cm. in diameter. This was driven a t 0 and 1025 r.p.m. by a vertical shaft passing out through a ground-glass joint holding a Teflon bearing made gas-tight by a rubber 0ring. A second Teflon bearing supported the rotor a t the bottom. The fixed heated filament, 45 mm. long a t a radius of 25 mm., was supported vertically and under slight tension by 4.8-mm. steel rods passing out through seals in the cover. These rods were wound with fine copper wire to reduce their electrical resistance and so hold them below reaction temperatures. Currents up to 60 amp. were used, supplied from a 110-volt source and controlled by a step-down transformer and Variac. ~
VOL.
1 NO. 2 M A Y 1 9 6 2
111
acetylene, ethane, and ethylene by gas chromatography (PerkinElmer Vapor Fractometer, Model 154) with argon a t 100' C. as carrier.
STIRRING MOTOR
K
Results
TEFLON SEAL
LLLY
BEARING
k&y REACTION VESSEL
Figure 1.
Stirred reactor with fixed heated filament
The Ammonia-Carbon Reaction. Hydrocyanic acid appears to have been first synthesized by Clouet ( 3 ) >who passed ammonia over wood charcoal a t red heat. Others who have produced hydrocyanic acid from ammonia and carbon include Bergman (7), Voerkelius (72), and Ing ( 6 ) . The reaction is thermodynamically favorable a t temperatures above 1000° C., but hydrocyanic acid is unstable below about 2000' C.: so the gaseous products must be cooled rapidly if hydrocyanic acid is to be obtained. The hot-wire technique accomplishes this; gas is heated to a high temperature as it passes over the front part of the wire, and is chilled rapidly by mixing with cold ambient gas as it leaves the point of boundary-layer separation. Ing's results were obtained with a tubular graphite flow reactor packed with graphite spheres. He was able to explain his data by assuming ammonia decomposition and hydrocyanic acid formation to be first-order, and hydrocyanic decomposition to be second-order. T h e first two reactions occur on the carbon surface; the third is homogeneous. Ing's rate equation was of the form exp. [ - (k.4
Filament temperatures were measured by means of a n optical pyrometer, calibrated to i6' C. by comparison with a precalibrated Bureau of Standards pyrometer. using a carbon filament lamp. No correction was made for the emissivity of the wire or the sighting through glass; the error involved is estimated to have been of the order of 10' to 15' C. Bulk gas temperatures which never exceeded 200' C., were measured by a mercury thermometer sealed into a ground glass joint in the reactor cover. Another opening in the cover was fitted with a mercury manometer. Filaments of two types were emp1o)ed: tungsten precoated with pyrolytic graphite, and graphite (National Carbon Co., AGSR grade). The pyrolytic carbon was coated on a 0.25-mm. tungsten filament by holding it a t 1920' C . in contact with methane until the desired coating thickness was built up. A s filament diameter is increased the power input at constant voltage increases, but heat loss by radiation and convection also increases. The filament temperature passes through a rather flat maximum at a diameter given by the relation ( 9 )
+ k.v)t] -
kR.vAc.4
(2)
where .\-.i is the number of moles of hydrocyanic acid, iV.,O the initial moles of ammonia, C, the concentration of hydrocyanic acid in the gas, and t is time. The rate constants k,, k,, and k , are for the first-order decomposition of ammonia, formation of hydrocyanic acid at the surface, and homogeneous decomposition of hydrocyanic acid, respectively. Ing's general approach \vi11 be employed in analyzing the present data. The over-all reaction is presumed to involve ( a ) transport of ammonia to the surface; (6) adsorption on the surface; (c) chemical reaction with carbon, involving both ammonia decomposition and hydrocyanic acid formation ; ( d ) desorption of reactants; ( e ) transport of products away from the surface; and (f)partial decomposition of hydrogen cyanide before it is cooled. Steps ( b ) and ( d ) will be assumed to bevery rapid, and step (f) negligible. The following relations may be written:
(4) C,S
where L is the length of the filament, p its specific resistance, and R, is the resistance of the external circuit. Filament diameters of 1.52 to 2.49 mm. in diameter were used, with R, adjusted to operate at the peak temperature a t the voltage used. The small changes in filament diameter then had negligible effect on filament temperature. The reactor was charged with the gas reactants a t room temperature and 0.5 atm. The current was then turned on for 10 to 120 seconds, the temperature was observed, and the reactor was slowly flushed out with argon. The resulting gas mixture was bubbled through 0.1.V sulfuric acid which was later titrated with 0.5*\' sodium hydroxide to obtain an analysis for ammonia. The gas then passed through a bubbler containing sodium hydroxide, which was titrated with 0.01h' silver nitrate for cyanide. The effluent from this bubbler was analyzed for nitrogen, methane, hydrogen, 112
I&EC FUNDAMENTALS
= CYP.YiC,
(5)
Here, k.,., and k,,q are the first-order rate constants for the surface reactions, A is the area of the surface, C.vs is the surface concentration of adsorbed ammonia, (Y is the adsorption equilibrium constant, P,< is the partial pressure of ammonia at the surface, and C,, is the surface concentration of vacant active adsorption sites. The surface is sparsely covered a t the temperatures employed, so that C, is essentially equal to the concentration of total active sites, which will vary with the temperature and the nature of the carbon used. Then
and (7)
I
0.
0
\ I
0 WM 1025 f3P.M.
I IT x
U
91
2 2
104
Figure 2. Over-all rate constant for ammonia dissociation on graphite
0.01
3
0 RPM 1025 RPM
Transport of ammonia to the surface is given by
Eliminating C,.,,
Figure 3. graphite
where C,. is the concentration of ammonia in the ambient gas. Values of I;, and K A were calculated from the experimental data obtained, b y comparison of the data with integrated forms of the rate equation. Arrhenius-type graphs of the data for the two types of carbon are shown in Figures 2-5, covering filament temperatures from 1400’ to 2100O K. These show the data to fall on straight lines and to be independent of rotor speed at the lower tcmperatures. At high temperatures the rates increase with rotor speed, and the rate constants are no longer linear in 1 / T . The interpretation of the results seems clear; the chemical reaction rate is controlling a t the lower temperatures (k,> > kA’ or ,ky’), but mass transfer becomes controlling a t the higher or ,ky’). Similar results have been temperatures (k,:
