(2) Dintses. A. I., Compt. Rend. Acad. Sci. C'.S.S.R. 2, 153 (1933). (3) Dintses, .A. I., C'spPkhi Khim. 7 , 404 (1939). (4) Dintses, A. I.. Frost, A . V., J . Gen. Chem. V.S.S.R. 3, 747 (1933). (5) Dixon, \V. J., Massey, F. J., Jr., "Introduction to Statistical Analysis," McGraw-Hill. New York, 1957. (6) Fabuss. B. M., Smith, J. O., Lait, R . I., Borsanyi, A. S., Satterfield, C. N., IND.ENG.CHEM.PROCESS DESIGN DEVELOP. 1, 293 (1962). (7) Frey, F. E., Ind. Eng. Chem. 26, 198 (1929). (8) Frost, A. V., J . Phyr. Chem. U.S.S.R. 8, 290 (1936). 191 Johns. I. B.. McElhill. E. X.. Smith. J. 0.. IND.ENG.CHEM. PROD.RES.DEVELOP. 1,'2 (1962). (10) .Jost, LV., Mufflng, L., Z . Elektrochem. 47, 766 (1941). (11) Kasansky, B., Plate, A. F., Ber. 6 7 , 1023 (1934). (12) Konovalov, D. S., Migotina, E. N., Zh. Prikl. Khim. 26, 332 (1953). (13) Kuchler. L.. Trans. Faraday Soc. 35, 874 (1939). (141 Levenspiel. 0.. "Chemical Reaction Engineering," LYiley, h e w York. 1962. (15) MalinoLskii, M . S.. Stoyanovskaya, Ya. I., J . Appl. Chem. l'.S.S'.R. 29, 1369 (1956). \
I
(16) Morris, H . E., Fabuss. B. M.: Smith. J. O., Satterfield, C. N., in "hdvances in Petroleum Chemistry and Refining," J. J. McKetta, ed., Vol. IX, \Vile!.. N c w York. in press. (17) Pease, R . N., Morton, J. M.. J . d m . Chrm. Soc. 5 5 , 3190 (1934). (18) Rosen, R., Oil Gas J . 39, Fcb. 13. p. 49; Feb. 20, p. 45 (1941). (19) Schultze, G. R., LVassermann, G.. Z. Ekektrochem. 47, 774 (1941). (20) Sherwood, T. K . , Reid, R . C.. '.The Propertirs of Gases and Liquids," McGraw-Hill, New York; 19%. (21) Steacie, E. I V . R., "Atomic and Free Radical Reactions," p. 122, Reinhold, New York, 1954. (22) SundgrCn..'i.,.lnn. Ofice .Vat/. Combustibles L q u i d e s 5,44 (1930). (23) Tilicheev, M . D., Z h . Przk/. K h m . 12, 735 (1939). RECEIVED for review July 25, 1963 ACCEPTED November 4. 1963 \Vork performed under the direction of Materials Central, Directorate of Advanced Svstems Technolow. LVricht .\ir Development Dibision, \Vright-Pa;terson Air For& Base," Ohlo, on Contract AF 33(657)-8193.
THERMAL DEMETHYLATION OF TOLUENE C
.
C
.
ZIM M ERM AN
A ND R0 BE RT Y 0 R K
, Cornr// Iniae?sztj. Ithaca, S.Y .
The thermal demethylation of toluene at 700" to 950" C. and atmospheric pressure has been studied in a continuous flow reactor. The reaction was found to be first-order with respect to toluene and half-order with respect to hydrogen. The rate constants for the decomposition of toluene and formation of benzene are identical within experimental error and have values in units of (moles/liter)-"2 (sec.)-l of k , = 3.5 X 10'" exp (-50,90O/RT) and kh = 3.0 1 O'O exp (-50,90O/RT). The yield of by-products follows the
x
where W = the yield of b y approximate empirical relation: W = 1 .O X 1 O'O O/b exp (-51,00O/RT), products, weight per cent of feed, b = hydrogen-toluene rotio, and 0 = actual gas residence time, seconds. The reaction of toluene to form benzene i s increased b y the presence of large amounts of n-heptane, but i s unaffected b y thiophene, pyridine, or carbon monoxide. A possible reaction mechanism is discussed.
I
past few )-ears there has been much interest in utilizing the large supply of petrochemical toluene as a raw material for chemical synthesis rather than as a fuel. At elevated temperatures, toluene will react with hydrogen to produce benzene and methane as the main products. N T m
CHa
Processes for both the thermal and catalytic demethylation of toluene have appeared in the recent literature (3:4 ) . Silsby and Sawyer (7) have proposed a reaction mechanism and presented some kinetic d a t a o n the thermal demethylation reaction. Several other articles on the thermal demethylation reaction ( 7 . 5. 8) have also appeared. However, few pertinent data of use to the chemical engineer have been published. 'This study was undertaken to obtain kinetic a n d engineering d a t a on the thermal demethylation reaction. Of particular interest \vas the determination of the yield of benzene as a function of temperature, residence time, and the hydrogentbluene ratio of the feed. Some of the problems of commercial application were investigated by determining the effects of various impurities in the toluene and hydrogen feed streams. 1 Present address. Denver Research Center. Marathon Oil Co., Littleton. Colo.
254
I&EC PROCESS DESIGN A N D DEVELOPMENT
Experimental
Reactor System. T h e flow reactor is shown schematically in Figure 1. T h e main part of the reaction system was a vaporizer, a preheater, and a reactor. T h e vaporizer consisted of 10 feet of '/,-inch stainless steel tubing immersed in a molten salt bath. T h e vaporized toluene, after mixing \vith hydrogen, passed into the preheater tube made of 11 feet of Vycor tubing formed into a coil. T h e preheater section was designed for rapid heat transfer and a short residence time. T h e reactor, or soaker, section was also constructed of Vycor glass tubing and provided the required residence time for conversion to take place. T h e reactor section was 24 inches long and had a volume of 2.25 liters. Temperature was measured with five Chromel-.4lumel thermocouples located a t intervals inside the reaction chamber. Residence time in the reactor was varied by changing the reactant feed rates. T h e reaction product was quenched in a short. double-pipe heat exchanger which was cooled with circulating cold water. T h e reaction product passed through the annular space between the thermocouple lead protection tube and the inner wall of the condenser. T h e condenser effluent, in the form of a dense aromatic fog, was then passed through a glass x.001 filter to separate the gas and liquid. T h e gas was then passed through a cold trap (acetone and dry ice) where additional liquid product condensed. Toluene was fed into the reaction system by a metering pump, and the hydrogen flow was regulated by means of a calibrated rotameter. T h e liquid product from the reactor was determined by weighing the filter collection flask and cold trap aftrr each experimental r u n . Gaseous products were metered by a wet-test meter prior to being vented to a flare.
Reaction Temperature and Volume. During the course of c.ac.11 rLin the tc.mpemtures of the five thrr.InocoLrples in the rr;ictor ivcre measured and recorded a t regular intervals. C;c.nt.raIIb-. tlie maxitniim temperature \,ariation ainong the tipprr f o ~ thcrniocou~~1i:s r \cas 6 ' C , or less during the course 01' each run. T h e avcrage of the periodic readings of the u p p e r ibur thermocoul)le> \vas used as the correct reaction te I n pe ra t tire . 1 tic. portion of the x a c t o r Lvhich included the upper four thc.rinocouplrs had a volume of 1,?5 liters and comprised 7Xc/, of rhc. total rt'actor volun1e. 111 the 1o~vc.rportion of the reactor the temperarure rapidly di.oi)l)c.d until the exit remperature was about 300" C. below [tic. rc.actiorr temperature. .l'he loicer .section of the reactor had ii volume of 500 mi. #:)rZ 2ccof the total reactor volume. I'hc contribution \vhich this lower yection m a d e to the rt.;ic.tioii \\-;is calculated by the method of equivalent reactor voliitne a \ oiitlincd by flougen and \\'atson ( 2 ) . An activation erit.rgy of i0 kcal. per gram-mole \cas assumed for the calculation\. 'l'he 500-mi. section \vas found to be equivaleiit to 220 r 20 nil. a t the temperature of the reaction. T h e c.otitrib\itiori of the preheater section, with a nominal volume o f 120 inl.. way calculated in the Tame manner a n d found to be 10 rnl. rqiiivalent to 30 'rhe reactor volume uied for calculation purposes was equal to the volume of the upper section plus the equivalent volumes of the lower section and the preheater. Thus, the reactor volume wa? 2000 i 30 ml. for all runs. Analysis. Analysis o1^the liquid product was made by gasliquid chromatography using a '-meter T y p e A column (diisodecyl phthalate on diatomaceous earth). Analyses were conducted a t 125' a n d 150' C. for each sample of liquid product. No analysis of the product gas was made.
*
Feed Materials
'Toluene Hydrogen Nitroqen Carhon monoxide Thiophene Pyridine n-Heptane
Mallinckrodt Chemical Co. analytical reagent grade. 99.3cE purity by weight Air Reduction Co. standard electrolytic yrade, guaranteed purity 99.557 A i r Reduction Co. oil pump grade Matheson Co. C . P . grade. 99.5Vc purity Matheson Coleman & Bell, boiling point, 82-84" C. Allied Chemical purified grade T b v o grades: Analytical grade (boiling point 98-99" (2,): practical grade: 5-heptane 8OC2. methylcycloliexane 107,. isoheptanes 9(;;, toluene 17;
Rotameters
hn
1 a 1
-?
7 - 1
p
Furnace1
Reactor
Nitrogen Manometer Corbonmonoxide
r
T m e Feed t a n k
v
Q
~
1
~
~
~
Manometer
Ti
flare
1
To f l a r e
Wet l e s t m i t e r Toluene vaoor izei
4
Filter collection tank
Collection flask
Metering pump
Figure 1.
Cold trap
Gas
warmer
Diagram of apparatus
LVhen nitrogen was added as a diluent, the hydrogentoluene ratio varied from 0.11 to 3.73. Temperature varied from 714' to 859' C . , and residence time from 1.8 to 5.0 seconds. I n the series of runs without nitrogen, the hydrogen-toluene ratio varied from 0.76 to 1.38. Temperature varied from 755' to 863' C . . and residence time from 2.4 to 4.4 seconds. T h e compositions of the feed streams used to determine the effects of various impurities are found in Table 11. 'I'he hydrogen-toluene ratio varied from 1.2 to 4.3, temperature varied from 725" to 859" C . , and residence time varicd from 2.5 to 5.8 seconds. Reaction Kinetics. T h e kinetic model of Silsby a n d Saivyer (7) \vas found to fit the d a t a best. This model utilizes the reaction mechanism
H.
+ C6H5CH3 CbHj.
+ Hs
+ CHI CsHs + H .
+ -+
(2)
CsH5'
(3)
.4ssuming that Reaction 1 is a t equilibrium leads to kinetic equations of the form,
Experimental Runs. T h e experiments were divided into four main parts:
and
1 . X series of runs with a large excess of hydrogen in which the toluene f l o ~ v rate and temperature \cere varied. T h e initial hydrogen-toluene ratio w a s kept greater than 4 to 1. 2. I : series of runs cirnilar to the first series where successively l a y e r portions of the hydrogen \vere replaced by nitrogen. For these esperirnents. the total gas-toluene ratio was approximately 4 to 1. 3. :I srrim of runs \\:here the hydrogen-toluene ratio \vas varicd. but no nitrogen \vas added. 4. A series of runs to determine the effect of such impurities as carbon monoxide. thiophene. pyridine, analytical grade nheptane. a n d practical grade 71-heptane.
Operating Conditions. T h e range of experimental operating conditions is sho\cn in Table I . For the series of runsl Lvith a large excess of toluane) the temperature varied from 602' to 949" C . and the residence time from 1 . 3 to 4.7 seconds.
Theoretically: the rates of toluene decomposition and benzene formation are equal. T h e reaction rate constants were evaluated in the following manner. From the analysis of the liquid product. both the conversion of toluene a n d the yield of benzene were computed. These values. along with the temperature, pressure. feed rate, a n d the ratio of the reactants. were used to calculate the rate constants, using the equation
Table II.
Feed Compositions for Runs 62 to 8 2
7'uluPnf Feed.
Table I.
Minimum Masim (1m
Operating Conditions If2 ' f ' d i ~ i p Rpsiiipncp Temp.. F P P .Ifole ~. Time. c. Prpssure Ratio src. 692 I\tm. 0.11 1 3 949 Atm. 5.5 5 8
H y h o y r n Fprd. ilddrrl Chniponent. .\laif 7;
.Iddrci Component. b*'t.
'2
4 . 6 ? ; pyridine 4 . 9"; thiophene 4 . 8 ' analytical qradc 71-heptane 205, practical qradr n-heptane Pure toluene
Pure hydroqen Pure hyclroqrn Pure hydroqen Pure hytirogrn 8 5-20' carbon monoxidc ~~~
VOI. 3
NO. 3
JULY
1964
255
~
~
60
50
a9
40
0 c
.-
I L n
a >
5
V
30
q%P
0)
m
3 -
;20
0 -@OC
72YC
IO
0
0
I
I
I
I
I
2
3
4
$ (Seds) Figure 2.
Effect of hydrogen-toluene ratio on conversion
Results
T h e experimental results are shown in Figures 2 through 6 . Figure 2 shoxvs the effect of hydrogen-toluene ratio on the conversion a t a fixed temperature. Figure 3 shows the effect of temperature on the conversion a t a fixed hydrogen ratio. Figures 4 and 5 are standard Arrhenius plots showing the temperature relationships of the rate constants. T h e reaction rate constants may be expressed by the equations,
kr
=
3.5 X 10'0 exp (-50.900
RT)
kh = 3.0 X 1010 exp (-50,900:'RT)
(4)
(5)
These equations are valid from 700' to 950' C. and a t hydrogen-toluene ratios above 0.2. .4bove 850' C., the observed rate of formation of benzene will be slightly lower than predicted by Equation 5 if the hydrogen-toluene ratio is less than 1.5 because of increased by-product formation. T h e benzene ring showed good stability under the reaction conditions. Carbon formation was low, and the liquid product recovered was 95% of the liquid feed or better, except nnrler the most extreme conditions. Carbon was observable on the reactor filter a n d in the product only when the reaction temperaturr was above 850' C. and the hydrogen-toluene ratio was less than 1.5 to 1. By-products. Three types of by-products were observed : alkylated benzenes, diphenyl, and condensed ring structures. By-product yields varied from a negligible amount to more than 5 .tieight $G of the feed. Generally. by-product yields exceeded 0.5 weight yc of the feed only when the temperature was above 850' C.. the hydrogen-toluene ratio was less than 2 to 1. or the conversion was greater than SOYG, itJhen the temperature. the hydrogen-toluene ratio, and the conversion simultaneously exceeded these limits. the by-product formation \vas ?. weight 76 of the feed or graater. 255
l & E C PROCESS D E S I G N A N D D E V E L O P M E N T
Figure 3.
Effect of temperature on conversion of toluene
T h e yield of by-products follows the appro.ximate empirical relation,
TI'
=
1.0 X 10'O
e
- exp
b
(-51,000
RT)
I n Figure 6 the data are plotted from which Equation 6 was determined. Effect of Feed Composition. T h e demethylation reaction was unaffected by thiophene, pyridine. or carbon monoxide. However. the addition of n-heptane to the toluene feed produced a marked increase in both the demethylation reaction rate and the yield of by-products. T h e heptane also thermally cracked and formed low boiling compounds which were not recovered in the liquid product. T h e increase in the reaction rate can be seen in Figures 4 and 5 %where the runs with heptane are also plotted. T h e activation energy calculated from the runs with heptane is 55 kcal. per mole. Discussion
T h e toluene decomposition and benzene formation rates shown in Equations 4 and 5 differ by about 15% for all values of temperature. This difference is largely due to errors in the material balance and not to the formation of by-products. Statistical analysis of the d a t a shoivs that pre-exponential factors differ by less than one standard deviation. \Vith confidence limits of 95%. the pre-exponential factors lie betLveen 1.8 X loio and 6 . 6 X 10'O for toluene and 1.7 X 1O'O and 5.4 X 10'0 for benzene. 'The selective yield of benzene was 95% or better for the thermal demethylation. T h e actual yield of by-products found in the reaction product was generally less than 1% of the feed. Examination of Figure 6 shows that the yield of byproducts reaches lYc only above 840' C. if the hydrogentoluene ratio and the residence time in the reactor are equal.
40
30
20
30
\'
HZ/Toluene r a t i o 2 4 t o 1 H2/Toluene r a t i o < 4 t o 1 0 Nitrogen added as diluent x Carbon monoxide in hydrogen v Pyridine or thiophene in the feed toluene
L
0 A
0
-
L
-
IO
IO
8
8
-
-
6
-IN-
0
$
4
3
L
L
c
0)
-1 al c
2
1 ul
-
0
E
E
hi
2
u)
0
c
4
-IN
al
al -
H2/Toluene r a t i o 2 4 to 1 Hz/Toluene r a t i o < 4 to 1 Nitrogen added as diluent
6
I
v
$
0 A 0
20
n
10 08
1
08
06
06
04
04
03 02
02 1
0
1
i
1
DO
,
I
0
O
01
80
85
90
UT Figure 4.
Toluene decomposition rate constant
T h e kinetic model of Silsby and Sawyer is based on the assumption of h!-drogeri dissociation equilibrium. From d a t a taken from the literatiire (6) the dissociation constant of hydrogen benveen 900' and 1250 K. may be expressed by the equation.
K
=
39'70 exp ( - 104,40O/R T )
or
K1'2 = 63 exp (-52.200,'RT) 'rhr activation energy o f Reaction 2 may be calculated using
Figure 5.
95
0
-
r105
loo",
( 1 0 )( O~K ) - '
Benzene formation r a t e constant
IO
8 6 4
W
Yield of by-products as wt % o f feed b = Hydrogen-to-toluene
3
8 = Residence time in reactor in seconds
2
the hydrogen equilibrium constant and the experimentall?. drtrrmined rate constant,
E2
=
50,900 - 52,000 = - 1300 cal
IO
08
'Phis negative activation energy indicates t h a t hydrogen dissociation equilibrium may not be reached during the reaction. 'The increase in the reaction rate constants for the runs with 20% heptane are significantly different from the toluene decomposition and benzene formation rate constants. T h e d a t a for the 20% heptane runs fall outside the upper 95% confidence limits for both the toluene decomposition a n d benzene formation rate constants. T h e increase in the reaction rate constant noticed i n the runs with 20'35 heptane is probably caused by the increase in the free radical concentration brought on by the cracking of the heptane. T h e higher activation energy of 55.000 cal. is also more consistent with the assumption of hydrogen dissociation equilibrium, since this higher value predicts a positive activation energy for Fkeaction 2. I t would appear. then, that the increase in free radical concentration has the effect of promoting hydrogen dissociation equilibrium. A catalyst
SI- 0 6 04
03 02
01 -
0 08
----
-
006 -
004 80
85
90
100
95
IO 5
I/T ( 1 0 ) ~ PK)-~ Figure 6.
By-product formation VOL.
3
NO. 3
JULY
1964
257
\vhich functions by promoting hydrogen dissociation is predicted to increase the demethylation reaction to the degree observed in thr runs \vith heptane. This difference is equivalent t o a change in the reaction temperature of about io' C.
very difficult to obtain in practice. However, the benefits of a somervhat lo\ver reaction temperature resulting from the use of a catalyst might justify the catalyst cost. Nomenclature
Conclusions
Toluene reacts with hydrogen rradily at elevated temperatnrrs. producing benzene and methane as the main products. ' l h r reaction is clean and unaffected by carbon monoxide. thiophene. or pyridine. T h e aromatic nucleus is not hydrogenated a t temperatiires u p to 9.30' C. Carbon and byproduct formation are minor unless the reaction temperature and conversion are high and the amount of rxccss hydroqen is small. l ' h e toluene decomposition and benzene formation reactions are first-order \.iith respect to toluene and half-order Lvith respect to hydrogen. Hydrogen reacts \\-ith toluene by means of a chain mechanism involving hydrogen atoms. 'Phe activation energy calculatrd for the reaction indicates that the hydrogen atom concentration ma>- not reach i t ? equilibrium value. From the runs Tvith heptane i t may be concluded that heptane acts as a free radical catall-st and increases the h>-drogen atom concentration to its equilibrium value. h t the same time side reactions are increased oiving to the reaction of toluene and benzrne Lvith the heptane. ?'he ultimate yield of benzene predicted for thermal demeth>-lationis greater than 9570. Since this yield is high. the use of a catalyst may not be justified. 'I'he increase in selecivity required to compensate a modest catall-st cost might be
rate constant: (moles 'liter) -l'p(sec.) -1 toluene fped rate, ml. min. reaction temperature, OK. reaction pressurr. inches H g absolute equivalent volume of reactor. liters conversion of toluene or yield of benzene, mole per mole of feed hydrogen-toluene ratio. moles 'mole )-ield of by-products. Ib. 'lb. feed actual gas residence time in reactor, sec. equilibrium constant for dissociation of hydrogen, H, e 2 H literature Cited (1) Rrtts, i V . D.. Popprr. F., Silsby. R. I., J..4pp/. Chrm. 7 , 497-503
(1957). (2) Hougrn. 0. A , . i2'atson. K. M.,.'Chemical Procrss Principlrs." Vol. 3, p. 884. \Vilry. S e w York. 1947. (3) Hydrocarbon Process. Petrol. Refinrr 42, No. 3. 121-4 (1963). (4) I d . Enq. Chrni. 54, 28-32 (February 1962). (5) Matsui. H.. ;\mano. .i..Tokuhisa; H.. Bull. J o j . Petroi. I n i t . 1, 67-72 (March 1959). (6) Rossini. F. D.. et ai.."Selected Values of Physical and Thermodynamic Properties of Hydrocarbons and Related Compounds." p. 747. Carnegie Press, Pittsburgh, 1953. (7) Silsby. K. I.. Sawyer, E. i V . . J.Appl. C h m . 6, 347-56 (1956). (8) Tsuchiya, ..\.. Hashimoto. A , : Tominaga, H.. Masamune, S.. Ritil. Jop. Petrol. Inst. 1 , 7 3 ~ ' ('March 1959). RECEIVED for review August 9. 1963 ACCEPTED November 26. 1963
REACTIONS OF CARBON VAPOR WITH HYDROGEN AND WITH METHANE IN A HIGH INTENSITY ARC R A Y M O N D F. B A D D O U R A N D JEAN
L. B L A N C H E T '
.Massachusrtts Institute 01Technolog), Cambridgf 39, .Mass. Theoretical calculations show that appreciable acetylene concentrations can b e obtained when carbon vapor reacts with hydrogen and with methane a t temperatures between 3500" and 4500
K.
A reactor was
built to study these systems in a high intensity arc using a consumable graphite anode as the source of carbon vapor, The acetylene output was determined to b e mostly dependent on the quench temperature and the carbon-hydrogen ratio, It appears that high concentrations of acetylene might b e obtained a t competitive power inputs for the carbon-methane system.
EII-c-ryr: arcs have been used recently in a variety of new devices to create high entxrgl- environments, In particular.. plasina jets. lo\< intencity arc?. and high intensit); arcs have bec.n used r x t e n ~ i v r l yI O producr remprrature? in the range of abont 3000" to 20.00O0 K. Rccrnrly. a hish intencity arc reactor \va? used (2) to make carbon react \\-it11 hydrogen. .I'he major product obtained \vas acet!-lene. up to 18% a t I-atm. prewu-e. Because of limitation\ in dmign of the original rractor. only the heterogeneous region. \\-here carbon exictr in thr solid phahe. could be annlyzrcl. -1'he present ctudy is aimed a t drtermining the
258
I&EC PROCESS DESIGN AND DEVELOPMENT
composition of a hot gas mixture obtained by reaction of carbon and hl-drogen or carbon and methane in an arc struck between t\vo carbon electrodes. In contrast \vith the previous \\-ark. a n attempt has been made to study the homogeneous region, \There carbon exists in the gas phaie only. to determine acetylene yields. and also to help determine rvhich important compounds may be expected in the equilibrium gas at high temperatures ( I ) . Carbon-Hydrogen Reactions
Reliable thermodynamic d a t a for C-H compounds a t temperatures above 1500' K. are almost nonexistent in the literature. To estimate thermodynamic properties in the high temperature range \\.here measurements have not been m a d r .
