Catalytic Dehydrogenation of -
Ethane bv SeiectiG Oxidation J
J. P. MCCULLOUGH AND J. S. WALTON Oregon State College, Corvallis, Ore. r .
I he application of selective oxidation to the dehydrogenation of ethane is discussed. On the basis of generally accepted theories of hydrocarbon oxidation a method for selectively burning the hydrogen evolved in a dehydrogenation reaction is proposed. The process consists of adding oxygen to the reacting hydrocarbon at a number of points as it passes through the catalyst chamber. Data are presented which indicate that part of the hydrogen produced in the course of the reaction was oxidized while only a small quantity of oxygenated hydrocarbons was formed. Removal of hydrogen is shown to have resulted in increased olefinic content of the product gases. Yields in excess of that theoretically possible in conventional processes operating without oxygen were obtained. The effect on the degree of conversion and the selectivity of varying reaction conditions-i.e., per cent oxygen added to the reactants, temperature, space velocity, and pressure-is described.
pletely in an empty tube a t 375" C., combustion would not commence in a chamber packed with broken glass until a temperature of 500" C. had been reached and would not go t o completion below 625" C. ($4). Norrish and Reagh have shown both experimentally and theoretically that the reaction rate decreases with the,diameter of the reaction vessel; it is almost completely suppressed in a 5-mm. tube (21). Pease further reported that dissociation began before oxidation, but that the latter aided the former in such a manner as t o increase the unsaturated products without appreciable formation of oxygenated compounds. H e suggested that the oxygen reacted directly with propane to form propene and water. A logical explanation of this surface effect has obtained general acceptance. The first step in hydrocarbon oxidation is the formation of an aldehyde. After an induction period during which this slow reaction occurs, the aldehyde may react a t a surface t o form a peracid (18).
c
ATALYTIC dehydrogenation of hydrocarbons is an important process for converting paraffins to the more reactive olefins and diolefins. Since the early work of Frey and Huppke ( T ) , the technique of dehydrogenation has improved so much that today it is the source of raw materials for the manufacture of many synthetic elastomers, plastics, and other useful chemicals. In this paper the results obtained by the application of selective oxidation t o the conventional procedure are described. It has not been possible to convert ethane to ethylene economically by the present catalytic methods (28). For this reason ethane was chosen for the experiments considered here. In a reaction such as C,HW+Z = C,HZ, Hz, removal of hydrogen from the reacting mixture results in greater formation of the unsaturated product. T o effect the selective elimination of one component from a number of gases while the mixture is in the presence of a catalyst is not easily accomplished. Nevertheless, a number of methods have been patented by which the inventors claim t o realize this end (6, 18-16, 19, 80,SO). Most of these processes consist of passing an oxygen-containing gas into the reacting hydrocarbons in order t o burn the hydrogen evolved; in some the hydrogen is removed from the equilibrium reaction by causing it t o combine with a halogen or carbon dioxide. I n every case employing free oxygen certain precautions for preventing the oxidation of hydrocarbons are observed. These measures are almost entirely based on the ordinary mechanism for the oxidation of paraffins. There is reason t o believe that the combustion of hydrocarbons in a catalytic reactor is not dependent upon customary explosion limits and kindling temperatures. By a knowledge of the kinetics of oxidation under such conditions a simple means for selective oxidation may be postulated.
+
SELECTIVE OXIDATION
The combustion of paraffinic hydrocarbons takes place through a complex chain mechanism (18, 83, 86-,W). While the exact nature of the reaction is subject to conjecture, experimental facts permit a qualitative interpretation of certain pertinent phases. Pease discovered that while propane would burn com-
RCHO
+ O2 surface +
R-
The peracid could conceivably decompose in two ways, in one forming the radicals RCOO and OH, and in the other, the molecules R'CHO, water, and carbon dioxide. The radicals formed by the first alternative are capable of combining with a molecule of the hydrocarbon, thus beginning the chain which leads to complete oxidation. On the other hand, if the stable molecules were formed, the chain would not be initiated. It is believed that a large and active surface area favors the second reaction and thereby suppresses oxidation. Furthermore, many of the active components of the chain proper may be destroyed or deactivated a t a surface; it is plain that such an occurrence would introduce another source of inhibition. In view of the above facts and theories, it seems likely t h a t under the conditions prevailing in a catalytic reaction the oxidation of paraffins would be retarded. The combustion of hydrogen, conversely, is not greatly affected in the situation which prevails in a dehydrogenation vessel (28). Selective oxidation is feasible, therefore, on a theoretical basis, and its effect is demonstrated with experimental findings in the following. EXPERIMENTAL PROCEDURE
A description of the apparatus used in these experiments and the manner of operating can best be presented by reference t o Figure 1. Technical ethane of approximately 92% purity was passed through flowmeter 1 into the top of vertical preheater 2, consisting of a 4-foot length of 1-inch standard iron pipe packed with small porcelain Raschig rings. -4s metallic iron is an active dehydrogenation catalyst, the internal surfaces of both the preheater and reactor were coated with a thin layer of sodium silicate which effectively prevented coking a t the walls. Heat was su plied b two 20-foot coils of 21-gage Nichrome wire, separater! controlgd by rheostats. Steam from boiler 3 was mixed with the hydrocarbon a t the top of the preheater. The pressure was also measured a t this point with a mercury manometer, 4. After leaving the reheater the mixture was a t the proper temperature and passed Zirectly into reactor 5. The last vessel was 16 inches long and of the same material and construction as the preheater; the fwo were joined with a sleeve coupling. Four iron-constantan
1455
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
1456
thermocouples, 12, were spaced evenly in the reactor, and four ports, 6, for the admission of oxygen metered in flowmeter 7 were provided. The reactor was heated by a third Nichrome element. With this chamber com letely filled with catalyst the reaction took place and the pro$)ucts were rapidly cooled in condenser 8. After the liquid products were collected in trap 9, the remaining gas passed to a three-way stopcock from which i t could be sent to the wet test meter 10, or the ammoniacal cuprous chloride bubbler, 11, or the sample bottle. Almost all control was accomplished a t the control panel, 15, and temperatures from five thermocouples tvere recorded with a Tagliabue recording potentiometer, 13. Gas flows were regulated with gas regulating valves attached to the cylinders, and the volume of steam was controlled with a variable electric heater. The quantity of steam was determined by measuring the condensate for a period of time before each run.
A I A
1
-WATER
Vol. 41, No. 7
RESULTS
EFFECT OF OXYGEN. The influence of oxygen on the yield of desired products is shown by curve 8A-SA of Figure 2 and Table I. On this and subsequent graphs the dashed line refers-to the equilibrium concentration of ethylene in a mixture of ethane and ethylene under the conditions of the reaction. These values were calculated from the free energy data of Parks and Huffman (23) neglecting the effect of the oxygen added. Such results indicate that selective oxidation occurred with the anticipated effect. A degree of conversion was attained exceeding the theoretical maximum in ordinary dehydrogenation processes which occur in the absence of oxygen. Furthermore, very little oxidation of the hydrocarbon was observed, the quantity of carbon dioxide formed was always small, and that of carbon monoxide negligible. Presumably any carbon monoxide evolved was oxidized to carbon dioxide. The maximum carbon dioxide formed was 6% of the products, but the average was less than 1%. The quantity of carbon dioxide lost in the condensate was estimated and found to be negligible. The maximum aldehyde formation amounted to only 0.03% of the ethane charged. It is doubtful that methanol, ethanol, or other oxygenated substances were present in significant amounts since the chief liquid oxygen compounds are probably aldehydes (1, 24,2 7 ) .
TABLE I. EFFECTOF OXYGEN
Figure 1.
Flow Diagram of Experimental Apparatus
The data in the tables demonstrating the effects of selective oxidation were taken more or less at random from the various runs. Each of the runs, 4, 6, 8, 9, and 10, depicted by plotted data represents a continuous period of operation. During these runs experimental conditions were allowed to become constant, readings were made, and a snap sample of the effluent stream vas taken. The situation was then changed and the procedure repeated. Because the conditions were constant, composite samples were not necessary. ANALYTICAL.Analysis of the products was accomplished with a modified Bureau of Mines-type analyzer which permitted quantitative determination of carbon dioxide, ethylene, oxygen, hydrogen, carbon monoxide, ethane, and methane assuming no other hydrocarbons were present in appreciable quantities. Continuous qualitative tests for acetylene were made by bubbling the effluent gas through ammoniacal cuprous chloride solution, but a t no time did a precipitate of red copper acetylide indicate the presence of this compound in detectable amounts. The contents of the liquid trap were tested for aldehydes w t h Schiff’s reagent. Titrations using sodium bisulfite and iodine solution showed only a trace of thc oxidized compound in any instance. It was learned subsequently that the amounts of ethane and methane thus determined, while reproducible, were not correct. The sum of the two is correct, however, and the values for the other components are accurate within experimental limits. By the use of a ma.eria1 balance and certain assumptions, calculated values for the relative amounts of ethane and methane have been obtained which are believed t o be qualitative1 correct. MATERIALS.The ethane used in this worz had the followiiig analysis : CzHs CHI I_lluminants ( C ~ H I )
% by Volume 92.0 2.3
u2
2.7 z.a
Na
0.7
Shell Company 105 dehydrogenation catalyst ( 1 1 ) vias employed in all the runs. This material was provided in S/le-inch pellets and was of the following approximate composition: Fez03
CrzOs
70% 30%
CuSOa
KNOs
1% 0.5%
Excellent results have been obtained by the use of this catalyst, particularly in the dehydrogenation of butylene and ethylbenzbne.
Oxygen added, % Volume ethane. ml./min. Volume atesm.’ml./inin. Space velocity, vol./vol./min. Volume product, nil./min.a Mole fraction of ethane Temperature, C. lb
2 3 4 5 Average reactor temp., C. Analysis of product, % by volume GO2 CiH4
a b C
0.0 4.7 6.7 0.0 713 713 2040 2040 1150 1150 2500 2400 9.5 9.5 23.3 21.5 1050 1080 2085 1980 0.37 0.37 0.43 0.46
546 445 550
550 443 553
5io
iii 540
540 0.1 11.4 1.3 11.6 70.0 5.8 15.3 78.5
0.3 12.4 3.0 11.8 65.1 7.4 17.5 73.5
618 477 577 610 615 600 0.9 12.1 4.2 9.7 63.8 9.2 10.6 53.5
610 483 595 622 619 610
0.0 8.6 1.3 5.8 74.7 0.7 0.0 52.5
After condensation of steam. Thermocouples numbered from top t o bottom. Calculated.
EFFECT OF SPACE VELOCITY. More striking results are illustrated by curves 4 4 , 6 8 , and 10A of Figure 3 and the data of Table I1 which depict the effect of space velocity. I n all three examples the conversion falls from a figure greatly above “equilibrium” at low space velocities to a very small amount a t higher rates of flow. “Equilibrium” as used hereafter refers to the value neglecting the effect of oxygen and not to the true equilibrium which would exist with oxygen present. Approximately twice the equilibrium value was obtained in run 10 at the loweat space velocity. When attempts were made to operate at rates below the slowest given, excessive formation of carbon and hydrogen with very little production of ethylene resulted. The effect of oxygen is brought out clearly by these same curves if i t is recalled that the conversion without oxygen would not attain the equilibrium value. Little information is available regarding other attempts at catalytically dehydrogenating ethane. As a comparison, however, from 50 to 80% of the equilibrium conversion has been attained in prior work ( 7 , ,938). Since these earlier experiments were carried out a t 500” C. and without the addition of steam or other diluent, the actual conversions amounted to only 2.3 to 4.0%. EFFECT OF PRESSURE. I n Figure 4 (above) the effect of pressure on conversion is shown. The partial pressure of ethane was varied by using different proportions of steam and ethane
July 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
1457
TABLE 11. EFFECTOF SPACEVELOCITY I-
z
Y a W
a
I t 1 >
t; W -I
W
v)
Space velocity vol./vol./min. Volume ethan; ml./min. Volume ateam,'ml min Volume oxygen Volume produck, m1Jmin.a Temperature, O C.
J/A.
l b
2 3 4 5 Average temperature, C. Mole fraction ethane Analysis of product, % by volume
coz
CZH4
0 . -_
'?3
2
4
6
8 1 0 1 2 1 4 1 6
OXYGEN A D D E D
Figure 2.
- PERCENT
Effect of Oxygen Added
A and B = Conversion and selectivity curves, respectively, for the particular run
3.5 9.5 10.4 18.7 310 750 1000 725 395 1170 1060 1980 15.5 37.5 70.7 121 300 1200 1395 835 513 442 537 493 450 500 0.43
532 422 537 537 498 540 0.38
0.0 28.4 1.2 26.7 33.9 9.8 28.0 66.2
0.0 19.1 1.9 16.8 52.4 10.8 20.4 64.0
HZ CpHscC CHI Conversion to ethylene, % Selectivity, % a After condensation of stesm. b Thermocouples numbered from top to bottom. C Calculated.
600 470 592
...
608 473 577
575 603" 590 590 0.47 0.45 0.4 29.3 2.1 30.0 26.3 11.9 41.7 71.5
1.3 13.3
3.7
12.5 58,l 10.0
12.4 53.5
TABLE111. EFFECTOF REACTION TEMPERATURE while the space velocity and other variables were held constant. As would be expected since the reaction entails an increase in volume, greater conversion was obtained at lower concentrations of ethane. There probably should be a maximum in the curve aa a result of a decreased velocity constant a t low partial pressures, but it was not practical to reach this point with the equipment used. EFFECT OF TEMPERATURE. Figure 4 (below) and Table I11 demonstrate that the role of temperature in selective oxidation is normal. An increased conversion results with increased temperature, but above approximately 600 O C. this effect becomes less pronounced. The negative curvature is undoubtedly due to the increased number and extent of the side reactions which become possible a t more elevated temperatures ( a t constant space velocity). A maximum would occur a t some undetermined temperature since some side reactions, such as the formation of carbon and hydrogen, would eventually predominate. At the temperatures used no acetylene was formed in detectable quantities. It was thought possible that a "hot spot" might develop near the point a t which oxygen was introduced into the hydrocarbon stream. Thermocouple No. 2 was placed directly opposed t o the first oxygen port. I t s consistently low readings (owing t o the cool oxygen stream) show that no such region of unusually high
Average temperature, C. Temperature, e C. 1" 2 3 4 5 Volume ethane, ml. Volume steam ml. Volume oxygeh, ml. Space velocity vol./vol./min. Mole fraction kthane Volume product, d . / m i u . b AnalysiR of product, % by volume
coz
500
560
600
650
622 412 500
550 442 568
573 470 606
602 492 651
487" 558" 600" 651" 1520 1520 1520 1520 2020 1760 1760 1760 57.3 57.3 57.3 57.3 16.0 14.7 14.7 14.7 0.27 0.4 0.4 0.4 1540 1540 1585 1645 0.0 10.7 3.3 9.4 64.8
CZH4 0 2
Hz
CzHsC CHIC 11.8 Conversion, % 8.8 Selectivity, % 42.4 a Thermocouples numbered from top to bottom, b After condensation of steam. C Calculated.
0.4 14.1 2.3 11.5 60.1 11.5 12.5 57.5
0.2 18.3 1.5 13.3 67.7 9.0 17.7 66.5
temperature occurred. Excessive temperatures might develop with larger quantities of oxygen. The point where the retarding effect of the catalyst surface might cease to inhibit oxidation of the hydrocarbon was not investigated because of the danger of explosion. CATALYSTDEACTIVATION. Further information may be ,60
15
!2
lo
8 5
!00 Z
I
03 0.5 0.7 0.9 MOL FRACTION ETHANE
20
80
15
70
8 IO
60
5
- SGCE VELOCITYFigure 3.
~ V ~ M I N
Effect of Space Velocity
A and B = Conversion and selectivity curves, reupectively, for the particular run Run 4 = 544V C., 5$s'O;, no steam Run 6 = 540° C., 59" Oa 0.38 mole fraction CzHa Run lW-= 590° C., 7% 0 2 : 0.58 mole fraction CzHs
0.2 19.7 1.9 15.8 53.5 8.9 20.2 68.5
.!-
e T
1
& W
cn
50
40
0
500
550
600
650
TEMPERATURE- O C . Figure 4. (Above) Effect of Pressure; (Below) Effect of Reaction Temperature A and B = Conversion and selectivity curves. respectively, for the particular run
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INDUSTRIAL AND ENGINEERING CHEMISTRY
derived from a comparison of the curves presented. For instance, the three space velocity curves show the effect of differing conditions upon the process. However, such comparisons of one run against another may not be significant. The catalyst activity was an uncontrollable variable even though Shell 105 is reported to have a long period of constant activity. This was not found to be true in these experiments, probably because of the very low space velocities which were used occasionally. The scattering of points on the conversion curves was largely due to catalyst deactivation during the runs. Nost of the runs weie made with a regular change in conditions-e.g., from low to high space velocities. Check runs were then carried out a t intermediate points, and these data generally showed the extent of deactivation. Consequently, almost all of the curves would lie slightly above those found had the catalyst activity been strictly constant. It can be seen from the scattered points that this effect must be considered in the interpretation of the data.
SELECTIVITY. The amount of ethane and methane in the product gas was deteimined by a catalytic oxidation tube. After the anal ses had been made, it was found from known samples that &e method gave consistently low values for the quantity of ethane. It m-as then decided t o calculate the mole fractions of ethane and methane in the effluent gas by making the C 4assumption that the chief side reaction M-as C2H,-+CH, Hz. On this basis it was possible to arrive at the volume of both saturated hydrocarbons in the products from a hydrogen balance. The selectivity was derived from the figures obtained in this manner. The over-all material balance was generally close, but because of the simplifying assumptions it was usually necessary to change the calculated volumes of ethane and methane slightly in order to make the calculated analysis total 100% without altering the figures for the other constituents, as the latter are correct, For this reason, selectivities derived froin the calculated analyses will not always agree with the tabulated values. T h e selectivities reported here are certainly qualitatively correct I furthermore, from the complete experimental analyses values were obtained which were about 20% lower, but which showed the same trends. Since the ethane analysis was consistently low by about the same order of magnitude, i t appears that the tabulated figures are approximately correct quantitatively. I t will be recalled that the conversion figures do not depend upon any calculated data and hence are accurate within experimental limits . The unusual shapes of the selectivit curves 6B and 10B of Figure 3 are difficult to understand. &he other data show that the selectivity varies in the normal manner and indicate that a fair efficiency was attained. With varying space velocities, however, a maximum selectivity was found at intermediate rates. Using the small amount of kinetic data obtained for the reaction, i t is possible to formulate a mechanism to explain the shape of curves 6B and 10B, but without more abundant information than is available a t present, it would not be -worth yhile to do so here in detail. The actual mechanism of selective oxidation is not known, and extensive kinetic investigation will be necessary before one can be proposed. Pease ( d d ) suggested that propylene was formed by the direct reaction of propane and oxygen, but his proof was not conclusive. Nevertheless, if the mechanism of selective oxidation is actually different from that of ordinary dehydrogenation, i t is possible that the efficiency of the former would not vary in the same manner as that of the latter. From a plot of the data of runs 6 and 10 according to the first order rate equation, i t was found that the reaction appeared to be first order a t high space velocities (low contact time), but as the velocity was lowered the rate of conversion increased abruptly instead of decreasing. This increase in rate of reaction indicates that something such as an induction period or autocatalytic effect must be considered. An inductive effect would probably result in a decreased selectivity a t higher space velocities if there were encountered no such effect in the side reactions. I n other words, at high rates of flow the main reaction would be retarded much more than the side reactions; consequently, a decrease in selectivity could occur. The extent of thermal cracking in the preheater was not determined. That some cracking did occur in the lowest section of the preheater was evidenced by the formation of coke on the packing material over a long period of operation. Such cracking would give low values for both conversion and selectivity because the thermal dehydrogenation of ethane would not be efficient.
-+
Vol. 41, No. 7
CONCLUSIONS
Although a number of conclusions may be drawn from the data presented, the practicability of a process for selective oxidation should be judged from a larger amount of data tha,n was colleci,ed here. At this point there is reason to state that a process for dehydrogenation by selective oxidation is physically capable of realization. Increased yields of unsaturated products may be obtained, in some cases larger than the maximum theoretically possible by conventional means. At the same t,ime the oxidation process is highly selective in that there is little oxidation of an?; hydrocarbons; but because of side reactions t'he over-all selectivity is diminished as the amount of oxygen added is increased. By the proper choice of t,emperature, space velocity, and oxygerl concentration i t is possible to attain a high conversion and a t the same time maintain a high degree of se1ectivit.y. The rcsults of Figure 2 indicate that at, temperatures of 600' to 650 O C., a space velocity of about 10 volumes per volume per minute, lrom 7 i,o 10% oxygen added, and from 2 to 6 moles of per mole of ethane, a conversion perhaps 60% in excess of the equilibrium value can be expected a t a selectivity of 50% or great,er. Such result,s are considered t o show that the process is capable of accomplishing more satisfactory dehydrogenation than processes in commercial use today. There are two principal objections to the use of this pro The first is the afore-mentioned decrease in selectivit,y wit.h increased oxygen added; secondly, thcre is a decrease in selectivity at the higher space velocities. ht present, there does not seem to be any way of eliminating these effects, and they must, be considered a part of the penalty to be paid for the improved conversion. An economic st'udy of t.he process should be made before attempting to select the optimum conditions of operation. Wit,hout such a survey it is not possible to judge the process i n comparison with ordinary dehydrogenation from a practical standpoint, but its experimental performance indicates that, surTh a comparison would be favorable to selective oxidation. While the experimental work of this paper was limited t o t h e use of ethane, there is no reason t o believe that comparable results may not be obtained €or the dehydrogenation of other hydrocarbons-for instance, the same principles might be applied with profit t o such processes as the production of butadiene from butylene or styrene from ethylbenzene. Other uses of controlled and selective oxidation appear to be theoretically possible once the technique of operation has been worked out, to a suficient degree. ACKNOW LEDGlIEh-T
The authors wish t o thank the Shell Chemical Corporatiom tor providing the sample of the Shell 105 catalyst used in these experiments. BIBLIOGRAPHY
, Roy. Soc. (London) 154A, (1) Bone, W. A . , and Gardner, J. B ~Proc. 297-328 (1937). (2) Burgin, J., Groll, H., and Roberta, I t . M., Oil Gas J.,37, No. 17, 48-52 (1938). (3) Corson, B. B.., and C o x , M. W., 0'.S. Patent 2,311,979 (1943). (4) Dunbar, R. E., J. Org. Chem., 3, 242-5 (1938). (6) Egloff, Gustav, Petroleum Eng., 15, 98-112 (1944). (6) Frey, F. E. (to Phillips Petroleum C o . ) , E. S.Patent 2,362,198 (Kov. 7, 1944). (7) Frey, F. E., and I-Iuppke, TV. F., IXD.ENG.CHEM.,25, 54-9 (1933). (8) Grosse, A. V., Zbid., 35, 762--7 (1943). (9) Grosse, A. V., and Ipstieff, K., Zbid., 32, 268-77 (1940). (10) Gutzeit, C. L. (to Shell Development Carp.), U. S. Patent 2,408,139 (Sept. 24, 1946). (11) Ibid., 2,408,140. (12) Hopkins, M. B. (to Standard Oil Development Co.), Zbid.,1,808,168 (June 2, 1931). (13) I. G. Farbenindustrie A.-G., French Patent 837,411 (-4~13 30. 1938).
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1949
(14) I b i d . , 840,519 (April 27, 1939). (15) Kipper, H. B., U. S. Patent 2,274,204 (E’eb. 24, 1942). 116) Klein. H.. Hanbach. F., and Hofedit, W. (to Alien Property Custodian), Ibid., 2,301,727 (1943). (17) Komarewsky, V. I., and Riesr, C. H., Oil Gas J., 42, NO. 7, 90-3 (1943). ~I
(18) Lewis, B., and Von Elbe, G., “Combustion, Flames, and Explosion of Gases,” Cambridge, England, Cambridge University
Press, 1938. (19) Natta, Giulio, Alien Property Custodian, Serial 289, 711 (April 20, 1943). (20) Ibid., 340, 228 (April 20, 1943). (21) Norrish, R. G. W., and Reagh, J. D., Proc. Roy. SOC.(London), 176A,429-48 (1940). (22) Parks, G. S., and Huffman, €I. M., “The Free Energies of Some Organic Compounds,” A. C. S. Monograph 60, New York, Chemical Catalog Co., 1932. (23) Pease, R . N., “Equilibrium and Kinetics of Gas Reactions,”
pp. 148-215, 1942.
1459
Princeton, N. J., Princeton University Presa,
(24) Pease, R. N., J . Am. Chem. Soc., 51, 1839-56 (1929). (25) Pease, R. N., Ibid., 57, 2296-9 (1935). (26) Pease, R. N., and Day, R. A., Zbid., 62, 2234-7 (1940). (27) Pease, R. N., and Munro, W. P., Ibid., 51, 2034-8 (1934). (28) Riesr, C . H., Pelican, T. G., and Komarewsky, V. I., Oil Gas J.,43, NO. 10, 67-9, 96-7 (1944). (29) Russell, R. P., Murphree, E. B., and Asbury, W. C., Trans. Am. Znst. Chem. Engrs., 42,1-14 (1946). (30) Schmidt, Otto, and Stadelman, Sigmund (to Alien Property Custodian), U. S. Patent 2,326,258 (Aug. 10, 1943). (31) Stevenson, D. P., J. Chem. Phys., 10, 291-4 (1942). (32) Turkevich, John, Ibid., 12,345-6 (1944). (33) Watson, C. C., Newton, Fred, iMcCausland, J. W., McGrew, E. H., and Kassel, L. S., Trans. Am. Inst Chem. Engrs., 40, 309-15 (1944).
RECEIVED December 15, 1947.
Flammabilities of Four Chlorosilanes and Methyl Chloride J
E. W. BALIS, H. A. LIEBHAFSKY, AND D. H. GETZ General Electric Company, Schenectady, N . Y .
To aid
in the evaluation of possible explosion hazards involving chlorosilanes and methyl chloride, previously Published data are greatly extended* For each mixture the critical oxygen content is established--that is, the oxygen content below which flame propagation is not possible. The replacement of a flammable group on silicon by chlorine appears to reduce the much less than does a similar change when the central atom is carbon. The only compound containing a silicon-hydrogen linlrage is found to be extremely sensitive to spark ignition; this is attributed to the easy production of active species, such as hydrogen atoms, in the discharge.
R
out the flammability region. The boundaries of the flammability region could then be fixed precisely with the Bureau of Mines apparatus as modified ( I ) to overcome difficulties presented by the chlorosilanes. The two setups did not always give concordant results, and data from the portable apparatus had t o be relied on more heavily than was at first anticipated. As a matter of convenience, the glass plate at the bottom of the explosion tube in the modified Bureau of Mines apparatua (1) was lightly greased and held in place with ball-joint clamps, the mercury seal being abolished. The plate was removed before ignition was attempted. The portable apparatus was unaltered. Except for obvious changes dictated b y the substitution of prepared atmospheres for air, manipulation remained the same. Atmospheres of oxygen and nitrogen, or of oxygen, nitrogen, and carbon dioxide, were prepared by adding the individual gases to a steel tank several cubic feet in volume. After at least 90 minutes had elapsed t o ensure thorough mixing and a sample had been withdrawn far Orsat analysis, the atmosphere waa ready for use.
ESEARCH already published ( I ) describes how standard methods had to be modified in studying the flammabilities of the chlorosilanes and gives lower limits in air for two of these compounds and one of their mixtures, These data do not suffice FLAMMABILITY REGION for the evaluation of possible explosion hazards. For this, it is deThe number of experiments used to determine the flammability sirable to know the critical oxygen content-that is, the oxygen region for each substance is listed in Table I; exploratory testa contentbelow which flame propagation is not possible in a gaseous are not included. Since the individual results are too numeroua atmosphere, no matter what its chlorosilane content. Flammabilfor complete presentation, the original experimental data are ity data establishing this critical oxygen content are given here given only for methyl chloride (Figure 1) and for a seption of the for methyl chloride and for each of the following chlorosilanes: methyltrichlorosilane (NITS), dimethyldichlorosilane (DDS), trimethylchlorosilane (TCS), OF FLAMMABILITY DATA TABLE I. SUMMARY and methyldichlorosilane
(MDS). APPARATUS AND MANIPULATION
‘To fix the critical oxygen content, much of the flammability region must be explored. This requires many more measurements than are needed t o fix only the upper and ,lower flammability limits in air. I n the present work the plan was t o use the portable apparatus. (I), which yields results rapidly, t o block
No. of Expt. in No. of Expt. in Modified Bur. Mines Portable Apparatusb Apparatusb NonFlamNonFlamflammable mable flammable mable
Flammability Limits, % by Vol. Lower Upper limit limit
Content. Diluent %by Atm VOl. 4 5 8.5 Oz N2 15 13 17.2 14.6 1 1 0%: Nz 19 13 7.6 >20 12.9 4 2 3.4 02, Ne 14 9 >9.5 11.5 4 4 0 2 . Nn 22 24 2 0 >6 2 10.3 10 1 00 Ne 42 56 3.4 >24.0 2.9 6 4 0,: Nz. COze 32 13 3 . 4 (?) 28.8 12.4 DDS The reagents were of good quality as follows: methyl chloride, refrigerant grade, better than 99.7% CHaCl; methvltrichlorosilane:d?l.,, found 1.269, pure 1.270n: dimethyldichlorosilane. dZI found 1.066. uure 1.067, : methvldichlorosilane, dz; found 1.104 and 1.103, pure I.fO5; trimethylchlorosilane, di: found 0.853,pure 0.8Si7: O Z , N COP tank gases better than 99% pure. b Numerous experiments in nonflammable region below critical oxygen content were not counted. C Ne/CQs = 5.7. Substance” CHaCl MTS DDS TCS MDS Q
* I
