University were helpful in discussing individual aspects of the subject matter. Experimental studies were carried out a t American Cyanamid Co. Research Laboratories, Stamford, Conn. LITERATURE C I T E D
(1) Cochran, W. G., and Cox, G. K., “Experimental Designs,” pp. 154-69, 209, New York, John W-iley & Eons, 1950. (2) Eiaenhart, C., Bwmetrics, 3, No. 1, 1-21 (1947).
(3) Mueller, R. K., “Effective Management through Probability Controls,” pp. 132, New York, Funk & Wagnalls Co., 1950. (4) Paull, A. E., “On a Preliminary Test for Pooling Mean Squares in The Analysis of Variance,” Ann. Mathematical Statistics, XXI, 539 (1950). (5) Tukey, J. W., presented at the Middle Atlantic Conference, Am. Soc. Quality Control, Xew York, 1952. RECEIVED for review February 10, 1953.
ACCEPTEDApril 20, 1963.
Phenol Synthesis by Direct Oxidation of Benzene J U C H I N CHU, H. C.AI, A N D DONALD F.O T H M E R POLYTECHNIC INSTITUTE OF B R O O K L Y N . E R O O K L Y N I , N. Y.
T h e work was undertaken t o explore a new method of direct cxidation of benzene t o phenol by electric discharge, free from t h e difficulties usually associated w i t h other direct oxidation methods, such as t h e presence of secondary reactions a t high temperatures and preferential adsorption of phenol on catalysts. W i t h i n t h e limitations of t h e initial design of t h e apparatus, t h e o p t i m u m operating pressure was found t o be 6 t o 12 mm. of mercury a t 3000 t o 4000 volts of t h e discharging circuit, giving a conversion rate of 10 t o 12.5 mole yo of phenol per pass a t a benzene throughput of about 0.1 gram per minute. T h e temperature effect was found t o be unimportant i n t h e range of 100” t o 340” C. investigated. T h e experimental data have been correlated by t h e expression: y = ABz C, where y = phenol (x - 1)-0.21 0.185 ( x - 9)-4, B yield per pass i n 10-6 gram-mole per minute, A = 1.670 2.800 (0.879)‘.3’’ 3.834 (0.726)1.102 - 2.135, C = 0.072 (Z - 0.4)-’ ( X - 6)-2 - 0.620 (Z 0.4)(x - 1)-4, z = benzene pressure in millimeters of mercury and x = oxygen pressure i n millimeters of mercury. Equipment efficiency may be improved by providing a more compact discharge chamber and a larger surface of contact for t h e reactants. T h e yield per pass can be expected t o increase materially for possible commercial application.
+
+
-
T
HE present commercial processes for producing phenol are all
indirect, requiring the preparation, isolation, and purification of intermediate compounds such as benzene sulfonate, chlorobenzene, and cumene. The handling and consumption of critical materials such as chlorine, sulfuric and hydrochloric acids, sodium hydroxide, and propylene have added another disadvantage. The obvious advantage of a direct oxidation process has encouraged many investigations in the past, but so far the yields have been low. X summary of all available pertinent data for the aforementioned direct oxidation processes is t o be found in Table I. For the reaction CdMg)
+ ‘/zOdg) = CdLOWg)
&hefinal standard free energy and equilibrium constant equations are found ( 1 )to be AFT = -41,728
+ 17.88T - 1.082T - 6.45-X1.83 log K T = -AFr/4.576T
(2)
These two equations indicate that practically quantitative equilibrium conversions can be obtained under all ordinary operating temperatures and pressures. I n other words, the thermodynamics of the oxidation reaction imposes no restrictions to process conditions. K I N E T I C S OF THE O X I D A T I O N REACTION
The structure of benzene is a very stable configuration xyith r e q e c t to thermal or chemical attack. According to Jost ( d 7 ) , the C-H bond energy of benzene is around 102 kcal. per mole and the dissociation constant is only 0.0034 even a t 700” C. Amiel ( 2 ) and Hinshelwood and Fort (26) foung the apparent heat of activation for benzene oxidation t o be the second highest among all the hydrocarbons (methane being the highest), 80 to 56 kcal. per mole. As the dissociation of the first single C-H
1266
+
+
bond is realized only a t fairly high temperatures, and such dissociation must precede any chemical reaction, the primary oxidation product, once formed a t such high temperature conditions, is usually less stable than benzene itself and, consequently, is subject to further attack by the reactant. Also, the dissociated phenyl radical may dimerize a t such conditions. Even though the monosubstituted compound may form in quantity, such substituted functional group may often serve to activate other positions on the benzene nucleus, resulting in polysubstituted byproducts. In order t o moderate the oxidation reaction so that it may not proceed too far beyond the phenol stage, the following means could be employed: 1. Using a limited supply of oxygen, or diluting the oxygen with inerts. 2. Providing - means of rapid heat removal from the reaction zone. 3. Using a temperature as low as possible. 4. Usinn a mace velocitv as hiah as Dossible. 5. Provyding some mea&, SUCK as catalyst, a promoter, an electric discharge, or an exposure to actinic light. to activate the initial reversible reactions without exposing the reaction mixture long to high temperatures*
a
At low temperatures and low oxygen concentrations, with high space velocity or short residence tirne in the reaction zone, the oxidation reactions of higher order could be retarded; but, in the absence of the agents as mentioned in paragraph 5 above, the yield per pass of the primaiy oxidation product could also be reduced to insignificant proportions. It is, therefore, obvious that a combination of the methods mentioned in 2 and 8 would give the best results. The right kind of activating agent remains to he chosen. The failure of Keiss and Domms (46)and Charlot ( 9 ) to obtain any significant yield of phenol by catalytic oxidation of benzene in spite of their exhaustive search for a suitable catalyst has been attributed to the preferential adsorption of phenol on the catalyst.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 6
I
Table 1.
*
S u m m a r y of Direct Oxidation Processes for Phenol Synthesis G. Catalyst/
Phase Vap.
Tpp., C. 180-230
Press Atm," Vao.
Vap.
700
1
Vap. Vap.
690-710 300-400
1 10-50
Vap.
700
1-500
Vap.
450-650
55-82
Vap. Liq. Liq.
600 100 25
1 20 1
vrtp. Vap. Vap. Vap.
750-800 325-800 360 600-800
8 1 1
Vap.
371-427
20-34
V oxide
Liq.
200
60-70
HF
Catalyst
...
Promoter
..
Oxides of hy- Oxides of AI, drogen Zr HNO;
....
....
. . a .
Oxides of AI, Si, P
....
Iodine HF-BF; Oxides of V, Cr, Os
....
1
....
....
....
Paraffins olefins, et'hers
.... ....
HaOa
....
....
.... .... ....
Ag, As oxide
100 G . Benzene
% Con-
Type of Reactor Glow glassdischarge, tube
...
... 52.4 12.838.3
Silica, porcelain, clay tube Same as above St. steel vessel
version
1.0
3:l
... ....
0.6
8: 1 5:2
7,200-18,000 5 or 40-50
3.5
1 :5-5: 1
25-1500
6.0
...
8:5
7-25
4-6
30-35
...
68.2
....
.... 0.02 67.0 0.26
1:l
...
400
...
.... ....
...
Batch Batch
...
1:l
13,000
.... ....
1:s 1:4 1:l
4.4-5.0 4.0-8.0
200-400
8-10
12.230.0 57-67 50.0 50.0 50-60
100: 1-1O:l
5:l
....
1-9
...
40-100
...
....
Batch
...
....
....
Marek (34) took the view t h a t such preferential adsorption is due t o the higher boiling point and higher molecular weight of phenol over those of benzene. Schlesman et al. (@) suggested t h a t it is due to the larger dipole moment or the asymmetry of phenol. Coulson and Baldock (11)showed that "surface states" may be induced on the catalyst by the perturbation caused by the approach of a polar or ionic group. Eley (16) calculated the heats of chemisorption of different hydrocarbons and concluded t h a t benzene is only very weakly chemisorbed on the surface metal atoms of the catalyst. Moyer and Klingelhoeffer (S8)reported t h a t the presence of a solid catalyst or even the metallic surface of the reactor wall is detrimental to the production of phenol. Despite all this, no significant improvement can be cited when the reactor is devoid of catalyst and well protected from its surface effects. The promoters of homogeneous catalysts used in the 6, ld,bO, dd,W4,38) are genervapor phase oxidation processes (4, ally insufficient to increase the phenol yield to any sizable extent. Because the catalytic processes have their inherent difficulties and the noncatalytic ones must use elevated temperature and therefore cannot control the reaction at the primary stage, the use of the electric discharge as a means for low temperature activation of the oxidation reaction seems to be a logical choice. The generation of oxygen atoms by glow discharge in oxygen gas or water vapor has been investigated by Glockler andLind(d1) and Lavin and Stewart (30). Both atomic and hydroxyl oxygen were found to be present. Geib and Harteck (18, 19) were the first to study the reaction between benzene and oxygen atoms. They found that when the mixture was made t o react in a vessel cooled by liquid air, 10% of the benzene was converted to a colorless glassy addition compound insoluble in all common solvents. If solid carbon dioxide and acetone were used in place of liquid air, all the benzene was attacked and became the glassy solid. Harteck (d5) in another experiment found t h a t low temperatures, such as around the boiling points of air and oxygen, served t o suppress the formation of the addition compound. On the other hand, the formation of ozone is very much enhanced a t such low temperatures. He pointed out that it was very dangerous t o have a mixture of ozone and organic vapors. Similar observations were made by Brewer and Kueck (8)and Harkins and Jackson (23)in their experiments on oxygen discharges in methane. Based on the findings of Geib and Harteck, Avramenko et al. (3)
June 1953
...
Total
% Yield per Pase 15-27
.... ....
...
Tube and fixed cat. bed St. steel pipe Pyrex tube C-steel bomb Glass vessel Poro. tube Steel vessel St. vessel Regenerated brickwork Countercurrent moving mass end extractor Copper bomb
generated oxygen atoms in glow discharges of oxygen and water vapor, made them react with benzene vapor a t room temperature, and condensed the products in a liquid oxygen trap. They had only three runs with oxygen discharge and three runs with water vapor discharge, under a fixed voltage of 3500. The temperature was not controbled and the benzene pressure and elapsed time were not reported for each run. The purpose of the present work was to study the process in a systematic and quantitative way; to explore the effects of temperature, voltage, and benzene pressure on the phenol yield; to ascertain the range of benzene pressure in the prevailing oxygen pressure; and to put the phenol production on a rate basis. The idea of using electrically discharged oxygen or oxygencontaining gas for the partial oxidation of hydrocarbons was also mentioned in the general claims of the patents of Cotton (10) and Kleinschmidt (d8), which presented no experimental data. EXPERIMENTAL METHOD AND RESULTS
From the viewpoint of the kinetics of the oxidatior. reaction and experimental evidence realized from the survey of previous work, the process variables which may directly or indirectly affect the yield per pass of phenol can be listed a8 follows: Temperature of benzene vapor Pressure of benzene vapor Flow rate of benzene vapor Temperature of oxidant Pressure of oxidant Flow rate of oxidant Dimensions and design of reactor Dimensions and design of discharge tube Rate of cooling of reaction effluents Terminal voltage a t discharge tube Frequency of discharging circuit Nature and/or composition of oxidant
It is obviously not feasible for this study to include all these variables. For an exploratory study of the present scope it is important to limit the variables t o the apparently most significant ones, and to plan the running schedule systematically, so t h a t a maximum of information may result from a reasonable number of runs. Furthermore, the variables must not involve too complicated equipment or procedure. The variables chosen for the present study are as follows:
INDUSTRIAL AND ENGINEERING CHEMISTRY
1267
Table !I. Summary of Experimental and Analytical Results
Run No. 1 2 3
Oxygen Press.
Mm.
Hg
16.0 16.0 16.0 16.0 16.0 16.0 1 6 .0 16.0 12.0 12.0 12.0 12.0 12.0 12.0 12.0 12.0 8.0 8.0 8.0
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
8.0
8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 4.0 4.0 4.0 4.0 4.0 2.0 2.0 2.0 2.0 2.0 2.0
Benzene Press.. h l m . Hg 3.0 3.0 3.0 3.0 2.5 2.0 1.5 1.0 3.0 2.5 2.0 2.0 2.0 2.0 1.5 1.0 3.0 2.5 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 1.5 1.0 0.5 2.5 2.0 1.5 1.0 0.5 1.5 1.0 1.0 1.0 1.0 0.5
React. Tcmp.
c.
255 256 255 255 255 255 255 235 255 2.5; 334 22.5
178 98 255 255 25.5 235 255 334 255 178 98 235 255 25; 255 255 255 255 255 255 25; 255 255 255 255 255 255 255
H. V. Diech. Volts 5300 4600 3950 3280 4600 4600 4600 4600 4600 4600 4600 4603 4600 4600 4600 4600 4600 4600 5300 4600 4600 4600 4600 3950 3280 2620 4800 4600 4600 4600 4600 4600 4800 4600 4600 4600 3960 3280 3620 4600
Benzene Through put,
G./Min 0.1108 0.1108 0.1108 0.1108 0.0928 0.0744 0.0563 0.0369 0.1133 0,0950 0.0736 0.0776 0.0797 0.0809 0,0873 0.0382 0.1163 0.0976 0.0792 0.0777 0.0791 0.0801 0.0809 0.0792 0.0792 0.0792 0.0591 0,0406 0,0209 0.1009 0.0810 0.0617 0.0420 0.0231 0.0662 0.0449 0.0449 0.0449 0.0449 0.0233
Phenol Produced, 10-5 G.-Mole/ Min. 5.66 5.31 4.78 3.81 5.05 4.43 3.54 2.39 9.13 8.15 6.73 6 82 7.09 7.19 5.31 3.63 14.18 12.58 10.71 10.45 10.52 10.82 10.52 10.28 9.66 8.63 8 15 5.49 3.01 15.15 12.50 9.75 6.73 3.63 10.62 7.35 7.26 7.09 7.00 3.90
Reaction temperature Pressure of benzene vapor Pressure of oxygen Voltage of discharging circuit Flow rate of benzene
A schematic flow diagram is shown in Figure 1
. - I %
BENZENE
(q
7
,
TUBE
+ & ,9, I
VARIAC
PUMP
A%,”
I
TRANSF
Figure 1.
Schematic Flow Diagram
The liquid benzene of C.P. grade is admitted from a separatory funnel to a 500-ml. vaporizer fitted with a reflux condenser and a vapor outlet. The pressure in the vaporizer is kept constant by a manostat. The liquid is vaporized a t its boiling temperature by a heating mantle connected to a powerstat. The saturated vapors, after pressure reduction through a control valve, go into an insulated superheater and then into the mixing head of the reactor tube. The oxygen in the gas tank, after pressure reduction, enters the discharge tube through two nozzles near the electrodes. The gaseous mixture after discharge enters the mixing head, where i t joins the benzene stream to the tubular reactor. All the condensables are retained by the trap, and the noncondensable gases pass t o the pump outlet. The discharge tube is made of borosilicate glass 1 inch in diameter and 8 inches long, with two inlet nozzles for oxygen near the ends and one outlet a t the center on the opposite side. The electrode is made of 1/8z-inchaluminum sheet, 0.75 inch in diameter.
1268
0.0818
0.0588 0.0383 0.1180 0.0997 0.0803 0.0791 0.0810 0.0822 0.0825 0.0808 0.0801 0.0799 0 , 0600 0.0414 0.0217 0.1033 0.0828 0.0628 0.0425 0.0225 0.0675 0.0457 0.0462 0.0463 0.0463 0.0241
Yield of Phenol, Mole % 4.00 3.75 3.37 2.68 4.26 4.72 4.91 5.05 6.29 6.70 7.14 6.99 6.95 6.94 7.24 7.43 9.51 10.08 10.58 10.50 10.40 10.28 10.18 10.11 9.53 8.56 10.71 11.03 11.29 11,70 12.02 12.35 12.50 12.91 12.55 12.81 12.64 12.34 12.20 ’ 13.06
Chemical Analysis. A survey of the literature puts the current methods into three main categories-the spectroscope ( I 7 ) , the redox titration (13, 31, 40, 44, 46),and the colorimetric (56). The colorimetric method ( 3 5 )was chosen because it is simple and works satisfactorily in a concentration as low as 0.5 p.p.m. of phenol, There is no uncertainty as to end points, for perfect color matching can always be made by diluting the standard or unknown solutions. The reagent, 4-aminoantipyrine, was prepared from antipyrine as directed by Eisenstedt ( I d ) , with minor modifications. The benzene solution of phenol produced from the reaction, generally about 2 ml., is extracted six or eight times with 50-ml. portions of distilled water. The extracts are combined and diluted to 1000 ml. in a measuring flask, and 50 ml. of this is again diluted to 500 ml., usually corresponding to a concentration of 5 to 20 p.p.m. If the unknonm is found t o be outside the range of the standard solutions, or if it is between two consecutive concentrations, either the unknown or the standard solutions are again diluted in aliquot portions, until the unknown and the standard match perfectly.
NSINOSTAT
VePORlZER
Products plus Unconv. Bene., G./llin. 0.1121 0.1117 0.1117 0.1111 0.0942 0.0754 0.0566 0.0370 0.1141 0.0963 0.0743 0.0776 0.0814
The reactor tube is an 11-mm. borosilicate glass tube about 600 mm. long. A 1.5-inch borosilicate glass sphere serves as the head of the reactor and the mixing chamber for the reactants. A thermometer is fitted into the center of the reactor head. The downstream end of the reactor is connected t o the trap condenser with a bypass t o the vacuum pump. The three-way valves on top can include or bypass the trap, The trap is placed inside a wide mouthed Dewar flask fitted with a large cork ring. The Dubrovin-type gage is dcsigned and constructed with a calibrated sensitivity of 7 to 1. The feed rate of benzene was determined by weighing the condensate in the trap condenser after the temperature and pressure of the two reactants are brought to the desired values, as would he used for the reaction run. After the rate-determining run the syqtem was evacuated for about 5 minutes before the reactant control valves were adjusted to give the same benzene and oxygen rate. As the reaction proceeds, readjustment of the tpmperature and pressure controls may be necessary -4fter the run the trap was turned off. The contents in the trap werc ready for chemical analysis.
Because the presence of small amounts of quinone and polyhydroxy compounds will modify the characteristic phenol coloration considerably, their absence was indicated by the perfect color matching with the pure phenol standard. A negative lead acetate test indicated the absence of maleic acid. Experimental Results. The experimental data are recorded in Table 11. The benzene throughput is based on a prerun calibration under the same conditions as in the run itself. As the gaseous products, if any, are not metered nor analyzed, and the liquid is analyzed for phenol only, the exact conversion of benzene cannot be calculated. The poTver inputs of the primary circuit are recorded in Table 111, and the critical voltages a t various pressures in Table IV. The transfer loss as reflected in the phenol rate nil1 be 0.045 to 0.225%. The error with the chemical analysis will be about &1.28%. The combined error on the phenol rate will be on the order of 3 ~ 1 . 5 % . The error associated with the benzene throughput assumption should be small compared with other errors, since in the reaction run the effluent contains only 15r0phenol a t most, and the molecular weights of phenol and benzene are not too far apart. The
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 6
Table 111. Run NO.
1 2
P
9 10 11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26 27 28 29
30 31 32 33 34 35
36 ..
37 38 39 40
critical values a t which electric meters still register some current with what may be called the dark discharge. The critical voltages for t h e disruption of glow in the oxygen discharge tube, at various pressures, are recorded in Table IV. When benzene was admitted t o the reactor and its pressure reached a certain value, the start of the reaction was indicated by the appearance of a beautiful blue gap in the middle of the pink oxygen discharge tube. The same blue discharge is observed when the gas within the tube is pure water. When the benzene pressure became excessive, a white flash appeared with simultaneous deposition of carbon on the walls and outlet nozzle of the discharge tube. This probably is due to the fact t h a t a traceable amount of benzene leaks into the discharge tube. As a result of reaction between benzene and oxygen, water was formed t o give blue gap during the discharge. Upon cutting off the benzene supply, the carbon deposit on glass surfaces can be completely removed within a short time with the oxygen a t a relatively low pressure, preferably 2 mm. of mercury. This was impossible with ordinary oxygen alone a t a temperature as low as 100" C. The dissociation of molecular oxygen into the atomic species was further indicated by the fact that, when the discharge circuit was opened, there was an instantaneous decrease in pressure. When the circuit was again closed, there was an instantaneous increase in pressure.
Power I n p u t t o Oxygen Discharge Tube Primary Volts 80.6 70.1 60.1 49.9 70.1 70.1 70.1 70.1 70.1 70.1 70.1 70.1 70.1 70.1 70.1 70.1 70.1 70.1 80.6 70.1 70.1 70.1 70.1 60.1 49.9 40.0 70.1 70.1 70.1 70.1 70.1 70.1 70.1 70.1 70.1 70.1 60.1 49.9 40. 0 70.1
Primary Amperes
3 3 3 2 2 1 3 3 3 3 3 3 3 3 3 3 2 2 1 3
00 00 00 50 00 50 00 00 00
Power I n p u t , Kva.
0.210 0.210 0.210 0.150 0,100 0.060 0.210 0.2lb 0.210 0.210 0.210 0.210 0.210 0.210 0.210 0.210 0.150
00 00 00 00
00 00 00 50 00 50 00
DISCUSSION OF RESULTS
0.100
Effect of Temperature. The effect of temperature, as indicated in Figure 2, on the yield of phenol is so slight that i t can be practically neglected. Within the range of the relatively low temperatures used in this study, there would have been no noticeable attack on the benzene if the oxygen had not been subjected to the electric discharge, as ordinary oxidation of the hydrocarbon only begins in the neighborhood of 700" C. T h a t there was a substantial yield of phenol is evidently not due t o a reaction involving thermal dissociation or any simple bimolecular or third body collision. Because there is little chance for the exothermic heat
0.060 0.210
probable error in percentage yield per pass will be on the order of 5=3%. PHENOMENA DURING DISCHARGE AND REACTION
Oxygen Discharge before Reaction. The oxygen glow discharge, like other gaseous discharges, always has a minimum voltage and a maximum pressure beyond which the discharge will not take place. When the voltage and/or pressure begin to fall within the range, violet-colored scintillations become visible near the electrodes. As long as the voltage is higher than the minimum, its effect is gradual and not very pronounced. The pressure effect is far more critical, At the higher pressures, in the neighborhood of 20 mm. of mercury, the pink-colored glow appears only about 1.5 t o 2 inches long a t each end, and the middle section is dark. As the pressure is reduced, the lengths of the two glow sections grow toward the center and the glow becomes continuous but unsteady. It becomes steady when the pressure is down to about 15 t o 16 mm. of mercury. With further reduction in pressure, the cross-sectional area of the glow becomes more and more extended, until it fills the entire discharge tube. Meanwhile, the pink color of the glow becomes fainter and fainter. At this time the violet scintillations appear t o surround the back of the electrodes and the supporting rods inside the tube. When the pressure is gradually increased from the lowest possible one, the above-described phenomenon is reversed, The glow disappears when the pressure and/or voltage have reached certain
x
iz
c
Secondary Volts 854 971 1115 1320 1480 1708 1870 2200 ~~
June 1953
Primary Amperes 0.1 0.5 0.5 0.5 1.0 1.0 1.0 1.0
(4600 V.
2.0 9ENZ.PRES.b
IO
0-
i
E8
ar "0
9-I Id0
o:,
1o:
2;o
2;o
i o
310
TEMPERATURE, 'C.
Table IV. Oxygen Discharge Critical Voltages w i t h Disruption of Glow Primary Volts
2
EFFECT OF TEMPERATURE
i
1
Pressure, Mm. H g
FIG,
f 12
,
of reaction t o accumulate, the dissociation or collision would be enhanced with increase in temperature, and therefore the rate of reaction would be expected t o increase with temperature. As this is not the case, the reaction necessarily involves some highly activated species of the oxygen, atomic or ionic, or both. Such oxidation at the low temperature level practically makes it possible to stop it at the primary stage. Although most of the runs were made a t a temperature of 255' C., the 5atness of the yield us. temperature curve clearly suggests that the reaction can be just as well carried out at room temperature, leaving out all the heating and insulation. Effect of Oxygen Pressure. This is the most important factor, as may be seen from Figure 3. The yield of phenol increases rapidly with decrease in oxygen pressure. The increment generally increases with increase in benzene pressure, but the latter was limited by the oxygen pressure.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1269
The oxidation reaction is probably due to the activity of the dissociated form of the oxygen as a result of the electric discharge. It is conceivable that the phenol yield will increase with increase in concentration of the atomic oxygen in the reaction mixture. Such atomic concentration should be the net value of a n equilibrium between dissociation and recombination of the oxygen configurations. Evidently, a reduction in pressure will
2
4
6 B 10 I2 OXYGEN PRESSURE, MM. HG
14
I6
favor the dissociation side of the reversible reaction, and hence increase the atomic concentration. As the pressure gets down to a certain low value, however, counteracting factors become significant and the increase in yield is retarded, For one thing, the mean free path of the oxygen atoms may be increased to such an extent that the dissociation-recombination process will be largely affected by the kinetic energy of the particles rather than by the pressure. For another, the atomic oxygen concentration in the reaction mixture may not actually increase when the gas becomes highly rarefied. Under such conditione the container wall map yet play a good role, favoring the recombination process. All such countereffects are overshadoir ed by the pressure effect only when the pressure is fairly high. I n the very low pressure range, such as 1 mm. of mercury or below, still higher percentage yield may be obtained by providing more contacting space for the reactants. This means a larger exit duct for the discharged gas. As no benzene should be allowed to enter the discharge chamber, its pressure or flow must be extremely low. As a result, the low production per unit time will more than offset the benefit of a higher percentage yield per pass unless very large vessels are used. W t h a large system and the very low pressure used, the leakage problem T% ill be serious. High pumping speed with large capacity \%illalso be expensive. Thus, it seems desirable to operate a t a moderate pressure and be content with a reasonable percentage yield per pass, without going to the complexities of extreme low pressures. Effect of Benzene Pressure. I t may be seen from Figure 4 that the phenol yield increases rapidly with increase in benzene pressure. However, the benzene pressure is limited by the oxygen pressure to the extent that no benzene may migrate t o the discharge chambers. The rate of increase in phenol yield tapers off as the oxygen pressure is increased, because of the lower percent-
1270
age yield a t higher oxygen pressures. Referring to Table 11, it is readily seen t h a t a higher percentage yield with a smaller benzene throughput is not always desirable. The phenol production per unit time is very much reduced when the throughput is low, although the percentage yield is high. Another factor to be considered is the separation of the product from unrencted benzene and recirculation of the latter in the reacting system. K i t h more unreacted benzene in the mixture, the separation will be more difficult, and the recovery and recirculation of the benzene will be more expensive. This is the case with percentage yield. high throughput and IOTT Effect of Discharge Voltage. I t may be seen from Figure 5 that the effect of oxygen discharge voltage on the phenol yield is only moderate, as long as the voltage is well above the critical value for the prevailing pressure. It may also be expected that as soon as the critical values are approached, the yield will rapidly fall off to zero. The current is proportional to the voltage a t all pressure levels indicating a more or less constant resistance in the high-voltage circuit. The high kilovolt-ampere input per unit production, however, is not inherent in the process. The equipment design can be so improved as to give a higher conductance without going to too high a voltage, and to minimize the dissipation of electrical energy as unusable heat.
BENZENE PRESSURE,
MM. HO
It is interesting to compare the results of this work with those of Avranienko et al. (5). In the oxygen pressure range of 10 to 20 mm. of mercury they obtained slightly better percentage yields (6.6 t o 19%) a t the reduced benzene throughput. Consequently, the production rates were much lower. With water vapor instead of oxygen discharge, a phenol yield of 4.5 to 27% was claimed. However, no detectable amount of phenol was observed under similar conditions in the present work. CORRELATION
O F DATA
Because the present work is only of an exploratory nature, the experimental data cannot be used t o evaluate reaction rate constants and the details of reaction mechanism. However, for practical purposes the data have been correlated to give certain
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 6
empirical equations relating the yield and the important process variables. B y analyzing the variances and applying the method of logarithmic group summation, the effects of the partial pressures of the reactants on the phenol yield have been correlated t o give ( 1 ) y = ABz C (3 1 where y = phenol yield in 10-6 gram-mole per minute A = 1.670 ( Z - 1 ) - 0 . 2 1 0.185 (Z - 9)-4 B = 2.800 (0,879)1.362 3.834 (0.726)'.'0" 2.135 C = 0.072(z - 0.4)-'(x - 6)-* - 0.62O(z - 0.4)(x - 1)-4 z = benzene pressure, millimeters of mercury x = oxygen pressure, millimeters of mercury The data used for the evaluation of Equation 3 were under a constant voltage of 4600 volts and a constant temperature of 255" C. The limitations of this equation may be stated as follows: Temperature. As the temperature effect is very slight, in general, Equation 3 can be used within the entire temperature range of 100" t o 340" C. and may be extended down to room temperature. However, if the temperature is much above the higher limit, secondary reactions may become appreciable and the equation may be expected t o fail. Pressure. Under the constant voltage of 4600 volts, Equation 3 may be used in the entire pressure range of this studynamely, 2 t o 16 mm. of mercury for oxygen, and 0.5 t o 3.0 mm. for benzene. The important limitation is that the benzene pressure must not be too high as compared with the oxygen pressure. The maximum benzene pressure for a certain oxygen pressure is a function of both the equipment dimensions and the oxygen' pressure itself, and in no case should it exceed one half of the oxygen pressure. Voltage. Evidently Equation 3 is applicable only under the specified voltage of 4600. The effect of the oxygen discharge voltage on the phenol yield, as indicated in Figure 5, can be correlated by a quadratic equation
+
+
*
A.
+ +
-
+ +
y =a bv cv2 (4) where v is in volts. The constants a, b, and c, which depend on pressure, are listed in Table V. Generally, under constant presswe conditions, the effect of voltage upon the phenol yield is not pronounced so long as the voltage is well above the critical values.
Table V.
Benzene, Mm Hg
16.0 8.0
3.0 2.0 1.0
2.0
c
Values of Constants a, b, and c, in Equation 4
Oxygen Pressure
a
832 1.654 0.014 ti.
b
-0.0007
3.6170 1,3991
C
0.0253 -0.3596 - 0.0589
Equation 4 or Figure 5 can be looked upon as usable only under the oxygen and benzene pressures specified, and no generalizations can be made when the pressure varies. I n Figure 5 i t is evident t h a t the voltage effect is overshadowed by the pressure effects. The pressure can affect the phenol rate as well as the pattern of discharge even a t constant voltage, resulting in a complicated expression such as Equation 3. OPTIMUM PROCESS CONDITIONS
By optimum process conditions are meant conditions under which the best possible should be expected in yield per pass of the product, its production rate, and costs for equipment and operation, all taken together, based on the experimental findings of this study. An economical balance in dollars and cents for the process is desirable, but, a t this stage, any such figures will be misleading. Equipment and process efficiency can be much improved in a future larger scale experiment, based on the findings of this study. June 1953
The inclusion of additional process variables will also tend to modify the optimum conditions. The follow.ing process conditions are recommended for attaining the over-all optimum result: Temperature, atmospheric for both reacting streams. There is some temperature rise after the oxygen has been subjected t o the electric discharge, but no outside heating is necessary. T h e only heating needed will be for vaporizing the benzene a t the prevailing temperature and pressure. Pressure, 4 t o 8 mm. of mercury for oxygen measured just upstream of the discharge tube, and 2 t o 3 mm. of mercury for benzene measured just upstream of the mixing chamber. Oxygen Discharge Voltage, 3000 t o 4000 volts. These process conditions should correspond t o a benzene throughput of 0.1 gram per minute, a productJion rate of 0.00012 to 0.00014 mole per minute and ayield per pass of 10 t o 13 mole %.
FIG. I
I
OXYGEN DISCHARGE VOLTS
POSSIBLE IMPROVEMENTS I N EQUIPMENT EFFICIENCY
Discharge Chamber. In order t o cut down the wasteful transformation of electrical energy into heat and to provide a large flow of partially ionized or dissociated oxygen gas, the discharge chamber could be made of a number of parallel quartz plates which are thin but large. These plates may have one side etched with hydrofluoric acid, and the other side with aluminum coatings. These metal-coated sides are connected in parallel to the high voltage source, with the etched sides facing each other about a short distance apart. The passage for the discharged gas to enter the mixing chamber should consist of a number of small nozzles instead of a large single one, so t h a t migration of the benzene vapors into the discharge chamber could be prevented. This is essential, as benzene has a much lower ionization voltage than oxygen, and i t would readily decompose in the discharge chamber. Mixing Chamber. The mixing chamber should be in the form of a common header for the oxygen nozzles on one side, and for the benzene vapor nozzles on the other. Some baffles should be built into the chamber t o effect better mixing. The dimensions of the chamber and the interconnecting nozzles should be such as to cause minimum bypassing of the benzene into the exhaust, yet practically no migration of the benzene into the discharge chamber under the prevailing pressure. ACKNOWLEDGMENT
The authors wish to express their appreciation t o Roger Gilmont of the Emil Greiner Co. for his assistance in the design of a part of the apparatus, t o P. T. Huang for his assistance in the preparation of some analytical reagents, and to Wm. I. Denton for his helpful comments.
INDUSTRIAL AND ENG INEERING CHEMISTRY
1271
LITERATURE CITED
Ai, H. C., M.S. thesis in chemical engineering, Polytechnic Institute of Brooklyn, 1953. Amiel, G., C o m p t . rend., 196, 1122, 1896 (1933); A h . ehim., 7, 70 (1937). Avramenko, L. I., Ioffe, I. I., and Lorentso, R. V., D o k l a d y A k a d . Wauk S.S.S.R., 66, 1111 (1949). Bibb, C. H., U. S. Patent 1,547,725 (1925). Bibb, C. H., and Lucas, H. J., IND. ENQ.CHEW,21, 635 (1929). Bone, W.A., and Newitt, D. &’I., Can. Patent 384,682 (1939). Bremner, J. G. M., Brit. Patent 628,503 (1949). Brewer, A. K., and Kueck, W.,J . P h y s . Chem., 35, 1293 (1931). Charlot, G., Ann. chim., 4, 415 (1934). Cotton, W. J., T r a n s . Electrochem. Soc., 91, 407 ff. (1947); U. S . Patents 2,485,476-81 (1949). Coulson, C. A , and Baldock, G. R., Discussions F a r a d a y Soc., No. 8,27 (1950). Denton, W.I., Doherty, H. G., and Krieble, R. H., IND.EXG. CHEW,42, 77 (1950). Duyokaerts, G., B u l l . soc. roy. sei. Liege, 1 8 , 152 (1949). Eisenstedt, E . , J . Org. Chem., 3,153 (1938). Eley, D. D., Discussions Faraday Soc., No. 8, 34 (1950). Faith, W. L., Keyes, D. B.,and Clark, R. L., “Industrial Chemicals,” New York, John Wiley & Sons, 1950. Friedel, R, A., and Pierce, L., A n a l . Chenb., 22, 418 (1950). Geib, K. H., and Harteck, P., Be?., 66, 1824 (1933). Geib, K. H., and Harteck, P., T r a n s . F a r a d a y Soc., 30, 131 (1934). Gill, D., Mardles, E. W.J., and Tett, H. C., Ibid., 24, 574 (1928). Glockler, G., and Lind, S. C., “Elcctrochemistry of Gases and Other Dielectrics,” New York, John Wiley & Sons, 1939. Griffith, R. H., and Hill, S. G., T r a n s . F a r a d a y Soc., 32, 829 (1936). Harkins, W. D., and Jackson, W. F., J . Chem. P h y s . , 1, 37 (1933). Harmon, R.A,, C.S.Patent 2,382,148 (1945). Harteck. P.. T r a n s . F a r a d a v Soc.. 30. 134 (1934) Hinshelwood, C. K,,and Fort, P: J.,’Proc. R o y . Soc. ( L o n d o n ) , A127, 218 (1930). Jost, W., Meuffling, V., and Rohrmann, 2. Elektrochem., 42, 46 (1936). Kleinschmidt, R. T7., C.S. Patent 2,023,637 (1935). Krieble. R. H.. and Denton. W. I.. Ibid.. 2.415.100 (1947). 2,439,812, 2,440,233, 2,440,234 (1948). Lavin, G. I., and Stewart, F. B.,Proc. A‘atl. A c a d . Sei., 15, 829 (1929). Leminger, O., C h e m . Obzor., 24, 88, 100 (1949). Lien, A. P., U. S. Patent 2,499,515 (1950).
Kinetics of the 0 . W.
(33) Mardles, E. W. J., T r a n s . F a r a d a y Sac., 27, 681 (1931). (34) Marek, L. F., “Catalytic Oxidation of Organic Compounds in the Vapor Phase,” New York, Chemical Catalog Go., 1932. (35) Martin, R. W., A n a l . Chem., 21, 1419 (1949). (36) Milas, N. A., J . Am. Chem. SOC.,59, 2342 (1937); U. S. Patent 2,395,638 (1946). (37) Moyer, W.R7.,Ibid., 2,328,920 (1943). (38) Moyer, W. W.,and Klingelhoeffer, W. C., I b i d . , 2,223,383 (1940). (39) Sewitt, D. >I., and Burgoyne, J. H., Proc. R o y . Soc. ( L o n d o n ) , A153. 448 11936). (40) Oshima; K., and Imoto, E., Chem. H i g h P o l y m e r s , 3, 57 (1946). (41) Porter, F., U. S. Patent 2,392,875 (1946). (42) Schlesman, C. H., Denton, W. I., and Bishop, R. B., Ibid., 2,456,597 (1948). (43) Simmonds, J. H., and McArthur, R. E., IKD.ESG. CHEM.,39, 364 (1947). (44) Tomicek, O., and Dolezal, J., Chem. L i s & , 43, 193 (1949). (45) Weiss, J. AI., and Downs, C. R., IND.EM. C H m r . , 12,229 (1920). (46) Zethelius, S., Rea. colombiana quim., 3, 5 (1949). GENERAL REFERENCES
Bone, W. A,, T r a n s . C h e m . Soc., 61, 871; 71, 26, 46; 79, 1042; 81, 535; 85, 693, 1637; 87, 910, 1232, 89, 652. 660. 939: 93. 1198: J . Chem. Soc.. 83. 1074: 89. 660. C h e m . Eng., 58, h o . 10; 232 (1951). Dryer, W. P., I b i d . , 54, KO. 11, 127, 132 (1927). Dushman, S., IND. ENG.CHEM.,40, 778 (1948). Elektrochemische Werke, U. S.Patent 2,015,040 (1935). Gilmont, R., IXD. EKG. CHEM.,ANAL. ED., 18, 633 (1946); A n a l . Chem., 20, 89 (1948); 23, 157 (1951). Hougen, 0. A,, and Watson, K. M., “Chemical Process Principles,” New York, John Wiley & Sons, 1947. I. G. Farbenindustrie, A.-G., Ger. Patent 501,467 (1930). hones, G. W., Chem. Reas., 22, 1 (1938). Jost, W.,“Explosion and Combustion Processes in Gases,” New York, McGraw-Hill Book Co., 1946. Normand, C. E., IND.ENG.CHEM.,40, 783 (1948). O’Connor, J. A., C h e m . Eng., 58, S o . 10, 215 (1951). Perry, J. H., “Chemical Engineers’ Handbook,” 3rd ed., New York, hIcGraw-Hill Book Co., 1950. Schlesman, C. H., U. S. Patent 2,553,944 (1951). Schoch, E. P., Bur. Ind. Chem., Univ. of Texas, B u l l . 5011 (1950). Warren, R. F., Chem. Eng., 58, No.8,279 (1951). Kedge, C., Brit. Patent 2,010 (1901). Weingartner, H. C., IND.ENG.CHEM.,40, 780 (1948). RECEIVED for review J a n u a r y 8, 1953.
ACCEPTED April 3. 1953.
ethane Reaction
N A B O R ’ A N D J. M. S M I T H
P U R D U E UNIVERSITY,. LAFAYETTE, IND.
T
HE kinetics of the homogeneous reaction between sulfur vapor and methane for the production of carbon disulfide were studied by Fisher and Smith ( 3 ) . Forney and Smith (6) reported a specific reaction velocity constant for the silica gelcatalyzed reaction at 600’ C., assuming that equilibrium existed between the sulfur species SZ, SS, and Ss present a t that temperature. The present paper reports data taken over a temperature range 550” t o 700” C. and conclusions regarding the kinetics of the reaction based upon an analysis of these data. It is of particular interest t o know which of the sulfur species plays the prominent part in the reaction with methane. The possibility of reaction of methane and sulfur in a vaporphase process for the production of carbon disulfide, as an alternative t o the high temperature, conventional electrothermal process, was first mentioned by de Simo ( 1 ) . I n two pioneer papers, Thacker and Miller ( 1 0 ) and Folkins, Miller, and Hennig ( 4 ) presented data establishing silica gel as an active and stable catalyst and showing the effects of such operating variables as ratio of temperature and reactants on the conversion to carbon 1
Present address. Magnolia Petroleum Co., P.O. Box 600, Dallas 1, Tex.
1272
disulfide. Folkins, Miller, and Hennig also carried out thermodynamic calculations indicating that the equilibrium conversion of the reaction
CH,
+ U S , = CSZ+ 2HzS
(UZ =
4)
(1)
is essentially 100% for SZ, Sg,or Sa a t temperatures above 500” C. The monatomic species, SI, is not present in appreciable quantities a t temperatures below 800’ C. The results of Forney and Smith (6) indicated that the rates of diffusion of reactants and products to and from the catalyst surface were large in comparison with the rates of the processes of reaction, adsorption, and desorption on the catalyst. Hence, in the present investigation no attempt was made to obtain data under conditions of constant space velocity and varying values of catalyst bed depth and reactants flow rate. Comparison of the results of Fisher ( 3 ) and Forney (6) showed that the rate of the homogeneous reaction, while small, was of sufficient magnitude to necessitate applying a correction procedure in evaluating the catalytic data. An integral-type reactor was used because of the difficulty that
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 45, No. 6
