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Studies of the vapour-phase polymerisation and oxidation of styrene
COMBUSTION A N D FLAME 31, 1-5 (1978)
1
Studies of the Vapour-phase Polymerisation and Oxidation of Styrene C. F. CULLIS and S. KURMANADHAN Department of Chemistry, The City University, London, England
The polymerisation and oxidation of styrene vapour have been studied, both in the absence and presence of the liquid monomer. When liquid styrene is absent, no appreciable polymerisation takes place in the gas phase below 500°C, at which temperature pyrolysis starts to occur; but styrene reacts with oxygen above 300°C, undergoing some polymerisation simultaneously with oxidation. The presence of the polymerising liquid monomer causes styrene vapour-air mixtures to ignite under conditions where they would not normally do so, the promoting influence being more marked at low pressures. On the one hand, the liquid styrene accelerates reaction due presumably to its acting as a source of the chain-carriers involved in the combustion of the monomer. This accelerating effect appears however to be partially offset by the temporary cooling of the reactant vapour caused by introduction of the relatively cold liquid.
INTRODUCTION
EXPERIMENTAL
One of the potential hazards in the chemical industry is that associated with the bulk storage of reactive chemical compounds. This situation arises, for example, in the case of monomers, such as styrene, which readily polymerise in the liquid phase at elevated temperatures, particularly in the presence of certain catalysts. That styrene can also polymerise in the vapour phase at relatively low temperatures is suggested by recent incidents in which clouds of styrene vapour escaping from a storage tank containing the liquid monomer have ignited under conditions where such behaviour would not normally have been expected. It thus appears that polymerising liquid styrene may promote the vapour-phase polymerisation and oxidation of this compound. In order to investigate this observed behaviour, some studies have been made of the reactions of styrene vapour both in the absence and presence of the polymerising liquid monomer. Unlike the liquid-phase 0olymerisation (1) and oxidation (2-4) of this compound, its vapour-phase reactions have hitherto received only little attention (5-8).
Materials Styrene monomer containing 10-15 ppm of ptertbutylcresol as stabiliser was obtained from Ralph N. Emanuel Ltd. and was shown by gas chromatography to be at least 99% pure; in some experiments the inhibitor was extracted with aqueous sodium hydroxide. Finely-ground polystyrene was obtained from Monsanto Chemicals Ltd. Benzoyl peroxide (moistened with water)was incorporated into the monomer without prior removal of the water, a,a'-Azobisisobutyronitrile (>97% pure) was also used without further purification. Oxygen from cylinders was purified by fractional distillation and "white spot" nitrogen (at least 99.98% pure) was dried by passage through Linde 5A molecular sieve. Apparatus and General Procedures
(a) Vapour-phase Studies Styrene vapour, either alone or premixed with the required proportion of oxygen, was admitted at a Copyright © 1978 by The Combustion Institute Published by Elsevier North-Holiand, Inc.
2
C.F. CULLIS and S. KURMANADHAN
known pressure into a heated cylindrical silica reaction vessel (volume, ca. 300 cma), the temperature of which was automatically controlled to within -+0.1°C. Changes in total pressure were measured by means of a Bell and Howell 0-338 kNm - 2 pressure transducer, the signal from which was amplified and recorded on a Honeywell 1706 Visicorder. Specially designed solenoid valves, operated by electronic timing circuits through relays, enabled gas mixtures to be admitted to and withdrawn from the reaction vessel very rapidly and with precision.
~
Pressure transducer
Thermocouplea
(b) Silica-pyrex graded seal
(b) Introduction of Polymerising Monomer In order to introduce polymerising liquid monomer or solid polymer into a reacting styreneoxygen mixture, a modified reaction vessel was used, the design of which is shown diagrammatically in Fig. 1. Narrow glass tubes (of ca. 1 mm dia) containing known amounts of the liquid or finely powdered solid to be added were preheated in the volume between taps T 1 and T 2, the temperature of which was measured by two fine chromel-alumel thermocouples (Fig. 1(a)). After a tube, placed as shown in Fig. l(b), had been heated for a given time, tap T 1 was closed in order to break the seal and immediately afterwards tap T 2 was momentarily opened (Fig. l(c)), thereby allowing the tube and its contents to fall into the reaction vessel. Temperature Measurement The temperature changes taking place during reaction were measured by means of a 0.025 mm dia platinum-rhodium thermocouple, the wires of which were coated with silica (9) and welded onto 0.25 mm dia wires of the same material contained in a twin-bore silica tube. The thermocouple could be placed in any chosen position along the vertical axis of the reaction vessel. The output signals were amplified and recorded in the same way as those from the pressure transducer.
Analysis of Reaction Products The principal gaseous products formed during the vapour-phase reactions of styrene were identified by gas chromatography. A column containing 5% w/w 1,2,3-tris (2-cyanoethoxy) propane supported
Tube containing liquid monomer
(c)
(
~
T2
Silica reaction vessel
Fig. 1. Reaction vessel system for introduction of polymerising monomer. on Chromosorb P was used to separate C6-Clo arqmatic hydrocarbons. Oxygenated products were separated on a column containing 15% w/w Apiezon L supported on Chromosorb P. Formaldehyde was identified by a colorimetric method (10).
RESULTS AND DISCUSSION
Reaction in the Vapour-phase Styrene does not react appreciably in the vapour phase below 300°C at pressures below atmospheric either in the absence or presence of small quanti ties (1-5%) of oxygen. In the absence of oxygen pyrolysis starts to occur only above 500°C, aJ though oxygen induces reaction at temperatures a
OXIDATION OF STYRENE low as 320°C. As the oxygen pressure is increased, oxygen-induced pyrolysis gradually changes to slow oxidation accompanied by some polymerisation (11). Such behaviour then persists until ignition occurs at a temperature in the range 450520°C, depending on the proportion of oxygen present. No region of negative temperature coefficient of oxidation rate was observed. When a 1:5 styrene-air mixture ignites (Fig. 2(a)), both pressure and temperature increase sharply, reach a maximum value and subsequently decrease. In the ignition region and at pressures below 60 kNm - z , all the reactions are preceded by a measurable induction period. Both the pressure and temperature changes accompanying the hot flame increase with the initial pressure of the reactants. The principal products of the slow oxidation of styrene are benzaldehyde and formaldehyde (ca. 10%), which are formed together with toluene and benzene (ca. 2%) and ethylene and ethylbenzene (<1%). At ignition temperatures, acetophenone is also formed in small amounts (<1%). Reaction in the Presence of Polymerising
Liquid Styrene Figure 2(b) shows the variation of pressure and temperature, when polymerising liquid styrene is introduced into a reacting styrene-air vapour mixture which subsequently ignites; and it can be seen that quite a sharp fall in temperature occurs during the introduction process. Similar behaviour was found during experiments involving addition of the solid polymer. Samples of polymerising liquid monomer were subjected to infrared spectrophotometric analysis using the - C H z - band at 3.4 /~m as a measure of the extent of polymerisation at the time of introduction into the reaction vessel. Appreciable polymerisation was found to take place with inhibitor-free styrene only at temperatures above 120°C, although polymerisation occurred to a significant extent at lower temperatures (ca. 90°C) when the monomer contained 1% w/w benzoyl peroxide. Figure 3 shows the variation of the induction period preceding ignition with the initial pressure of a styrene vapour-air mixture. Introduction of
3
Igniti°nj A < ~ ~
i
.-I "
r In{roduct'n. I o~q.id
monomer
~'~', ~..,,..,~. ! ', of
liquid : ~ rnonomer
._I.):T:___S".. .............. Time Fig. 2. The variation with time o f the pressure and temperature changes accompanying t h e s p o n t a n e o u s ignition o f a 1:5 styrene-air mixture. (a) Reaction only in vapour phase, (b) Reaction in presence o f polymerising m o n o mer. , pressure; . . . . . . . , temperature.
the polymerising liquid monomer (Fig. 3(a)) clearly promotes ignition, inhibitor-free styrene being slightly more effective than the inhibited monomer and the promoting effect increasing with the volume of liquid monomer added. Styrene containing benzoyl peroxide (Fig. 3(b)) also promotes ignition; and similar effects are observed when the monomer contains a,a'-azobisisobutyronitrile. Solid polystyrene, however, exerts only a small influence (Fig. 3(b)), suggesting that promotion is caused primarily by a medium which is polymerising at the time of its introduction. In general, the promoting influence of polymerising liquid styrene is smaller at higher pressures (Figs. 3(a) and (b)). The liquid monomer can evidently exert two opposing effects on the vapour-phase reaction. On the one hand, it has an accelerating influence, due presumably to its ability to provide an additional source of the chain-carriers involved in the processes leading to ignition of gaseous styrene-oxygen mixtures. On the other hand, introduction of liquid styrene causes significant cooling of the reactant vapour. With the addition of inert liquid benzene instead
C. F. CULLIS and S. KURMANADHAN (a)
(b)
(c)
6
E
~ 4
2
SO
(a)
g c :o 0)
30
Ignition ~ i)
80--
o
1
40 40 Initial pressure (kNrn- 2 ) Fig. 3. The effect of polymerising styrene, polystyrene and benzene on the induction period preceding the igmtion at ca. 570°C of a 1:5 styrene-air mixture. (a) ©, vapour only.Liquid styrene preheated at 190°C for 15 rain: e, 30 pl inhibited monomer introduced; O, 30 pl inhibitor-free monomer introduced; ®,10 ~l inhibitor-free monomer introduced. (b) o, vapour only; e, 30 pl Liquid styrene, containing 1% w/w benzoyl peroxide and preheated at 75°C for 15 rain,introduced (~, soLid polystyrene preheated at 65°C for 15 rain introduced. (c) (3, vapour only; e, 30 pl liquid benzene introduced.
40
(b)
Ignition d
i) igniti°n'~/l
-
IgnitionI
60--
(c)
Ignition,l
ib
r
40
20
ilntroductionof
- introductionof
liquid monomer . ~
p-
r --20 1
2
I
4
[
6
1
8
0
i
I
I
2 4 6 Induction period (s)
l
8
50
0
I
2
1
4
I
6
Fig. 4. Temperature changes following the introduction of polymerising liquid monomer into a 1:5 styrene-air mixture maintained at 570°C. (a) Initial pressure 47 kNm - 2 . ©, vapour only; e, 30 ~1 inhibitor-free monomer preheated at 170°C for 2 rain introduced. (b) Initial pressure 33 kNm- 2 . ©, vapour only;e, 30 pl inhibitor-free monomer preheated at 170°C for 2 min introduced; (c) Initial pressure 33 kNm - 2 . ©, vapour only; e, 30 pl inhibitor-free monomer preheated at 66°C for 15 min introduced.
OXIDATION OF STYRENE of reacting liquid styrene (Fig. 3(c)), the consequences of such cooling can be seen in isolation. Due to the exponential increase of the rate of polymerisation with temperature, the introduction of a cool liquid, causing a given decrease in temperature, would be expected to retard more significantly the faster reactions taking place at higher pressures. Thus the overall promoting effect of polymerising liquid monomer would tend to be smaller under these conditions. Experimental confirmation of this cooling effect is provided by measurements of the temperature changes taking place within the reaction vessel (Fig. 2(b)). The extent of cooling produced by the introduction of inhibitor-free monomer is shown in Fig. 4, from which it can be seen that the presence of the liquid, which needs to be not only vaporised but also heated to the reaction temperature, suppresses the temperature increase normally observed during slow combustion preceding vapor-phase ignition. In conclusion then, it is clear that the introduction of polymerising liquid monomer into a reacting styrene vapour-air mixture can, despite the associated temporary cooling effect, cause ignition to take place under conditions where it would not normally do so.
5 The authors wouM like to thank Dr. J. H. Burgoyne and partners for valuable financial support.
REFERENCES [1] Ebdon, J. R.,Brit. PolymerJ. 3(1), 9 (1971). [2] Miller, A. A. and Mayo, F. R., J. Am. Chem. Soc. 78, 1017 (1956). [3] Mayo, F. R., J. Am. Chem. Soc. 80, 2465 (1958). [4] Dutka, F., and Gal, D., Int. J. Appl. Rad. Isotopes 13, 27, 35 (1962). [5]
[6] [7] [8] [9] [ 10] [11]
Harkness, J. B., Kistiakowsky, G. B. and Meats,
W. H.,J. Chem. Phys. 5,682 (1937): Mark, H., and Breitenbaeh, J. W., Atti. Congr. Intern. Chim. 2, 335 (1938). Kroger, C., and Bigorajski, G., Erd#'l und Kohle 15(1), 7; 15(2), 109 (1962). Petrella, R. V., and Sellers, G. D., Combust. Flame 16, 83 (1971). Kaskan, W. E., Sixth International Symposium on Combustion, Reinhold, 1957, p. 134. Brieker,C. E., and Johnson, H. R., lnd. Eng. Chem. 17,400 (1945). Medvedev, S., and Zeitlin, P., ActaPhysicochim. USSR 20, 3 (1945).
Received 26 April 1977; revised 31 May 1977
1
Studies of the Vapour-phase Polymerisation and Oxidation of Styrene C. F. CULLIS and S. KURMANADHAN Department of Chemistry, The City University, London, England
The polymerisation and oxidation of styrene vapour have been studied, both in the absence and presence of the liquid monomer. When liquid styrene is absent, no appreciable polymerisation takes place in the gas phase below 500°C, at which temperature pyrolysis starts to occur; but styrene reacts with oxygen above 300°C, undergoing some polymerisation simultaneously with oxidation. The presence of the polymerising liquid monomer causes styrene vapour-air mixtures to ignite under conditions where they would not normally do so, the promoting influence being more marked at low pressures. On the one hand, the liquid styrene accelerates reaction due presumably to its acting as a source of the chain-carriers involved in the combustion of the monomer. This accelerating effect appears however to be partially offset by the temporary cooling of the reactant vapour caused by introduction of the relatively cold liquid.
INTRODUCTION
EXPERIMENTAL
One of the potential hazards in the chemical industry is that associated with the bulk storage of reactive chemical compounds. This situation arises, for example, in the case of monomers, such as styrene, which readily polymerise in the liquid phase at elevated temperatures, particularly in the presence of certain catalysts. That styrene can also polymerise in the vapour phase at relatively low temperatures is suggested by recent incidents in which clouds of styrene vapour escaping from a storage tank containing the liquid monomer have ignited under conditions where such behaviour would not normally have been expected. It thus appears that polymerising liquid styrene may promote the vapour-phase polymerisation and oxidation of this compound. In order to investigate this observed behaviour, some studies have been made of the reactions of styrene vapour both in the absence and presence of the polymerising liquid monomer. Unlike the liquid-phase 0olymerisation (1) and oxidation (2-4) of this compound, its vapour-phase reactions have hitherto received only little attention (5-8).
Materials Styrene monomer containing 10-15 ppm of ptertbutylcresol as stabiliser was obtained from Ralph N. Emanuel Ltd. and was shown by gas chromatography to be at least 99% pure; in some experiments the inhibitor was extracted with aqueous sodium hydroxide. Finely-ground polystyrene was obtained from Monsanto Chemicals Ltd. Benzoyl peroxide (moistened with water)was incorporated into the monomer without prior removal of the water, a,a'-Azobisisobutyronitrile (>97% pure) was also used without further purification. Oxygen from cylinders was purified by fractional distillation and "white spot" nitrogen (at least 99.98% pure) was dried by passage through Linde 5A molecular sieve. Apparatus and General Procedures
(a) Vapour-phase Studies Styrene vapour, either alone or premixed with the required proportion of oxygen, was admitted at a Copyright © 1978 by The Combustion Institute Published by Elsevier North-Holiand, Inc.
2
C.F. CULLIS and S. KURMANADHAN
known pressure into a heated cylindrical silica reaction vessel (volume, ca. 300 cma), the temperature of which was automatically controlled to within -+0.1°C. Changes in total pressure were measured by means of a Bell and Howell 0-338 kNm - 2 pressure transducer, the signal from which was amplified and recorded on a Honeywell 1706 Visicorder. Specially designed solenoid valves, operated by electronic timing circuits through relays, enabled gas mixtures to be admitted to and withdrawn from the reaction vessel very rapidly and with precision.
~
Pressure transducer
Thermocouplea
(b) Silica-pyrex graded seal
(b) Introduction of Polymerising Monomer In order to introduce polymerising liquid monomer or solid polymer into a reacting styreneoxygen mixture, a modified reaction vessel was used, the design of which is shown diagrammatically in Fig. 1. Narrow glass tubes (of ca. 1 mm dia) containing known amounts of the liquid or finely powdered solid to be added were preheated in the volume between taps T 1 and T 2, the temperature of which was measured by two fine chromel-alumel thermocouples (Fig. 1(a)). After a tube, placed as shown in Fig. l(b), had been heated for a given time, tap T 1 was closed in order to break the seal and immediately afterwards tap T 2 was momentarily opened (Fig. l(c)), thereby allowing the tube and its contents to fall into the reaction vessel. Temperature Measurement The temperature changes taking place during reaction were measured by means of a 0.025 mm dia platinum-rhodium thermocouple, the wires of which were coated with silica (9) and welded onto 0.25 mm dia wires of the same material contained in a twin-bore silica tube. The thermocouple could be placed in any chosen position along the vertical axis of the reaction vessel. The output signals were amplified and recorded in the same way as those from the pressure transducer.
Analysis of Reaction Products The principal gaseous products formed during the vapour-phase reactions of styrene were identified by gas chromatography. A column containing 5% w/w 1,2,3-tris (2-cyanoethoxy) propane supported
Tube containing liquid monomer
(c)
(
~
T2
Silica reaction vessel
Fig. 1. Reaction vessel system for introduction of polymerising monomer. on Chromosorb P was used to separate C6-Clo arqmatic hydrocarbons. Oxygenated products were separated on a column containing 15% w/w Apiezon L supported on Chromosorb P. Formaldehyde was identified by a colorimetric method (10).
RESULTS AND DISCUSSION
Reaction in the Vapour-phase Styrene does not react appreciably in the vapour phase below 300°C at pressures below atmospheric either in the absence or presence of small quanti ties (1-5%) of oxygen. In the absence of oxygen pyrolysis starts to occur only above 500°C, aJ though oxygen induces reaction at temperatures a
OXIDATION OF STYRENE low as 320°C. As the oxygen pressure is increased, oxygen-induced pyrolysis gradually changes to slow oxidation accompanied by some polymerisation (11). Such behaviour then persists until ignition occurs at a temperature in the range 450520°C, depending on the proportion of oxygen present. No region of negative temperature coefficient of oxidation rate was observed. When a 1:5 styrene-air mixture ignites (Fig. 2(a)), both pressure and temperature increase sharply, reach a maximum value and subsequently decrease. In the ignition region and at pressures below 60 kNm - z , all the reactions are preceded by a measurable induction period. Both the pressure and temperature changes accompanying the hot flame increase with the initial pressure of the reactants. The principal products of the slow oxidation of styrene are benzaldehyde and formaldehyde (ca. 10%), which are formed together with toluene and benzene (ca. 2%) and ethylene and ethylbenzene (<1%). At ignition temperatures, acetophenone is also formed in small amounts (<1%). Reaction in the Presence of Polymerising
Liquid Styrene Figure 2(b) shows the variation of pressure and temperature, when polymerising liquid styrene is introduced into a reacting styrene-air vapour mixture which subsequently ignites; and it can be seen that quite a sharp fall in temperature occurs during the introduction process. Similar behaviour was found during experiments involving addition of the solid polymer. Samples of polymerising liquid monomer were subjected to infrared spectrophotometric analysis using the - C H z - band at 3.4 /~m as a measure of the extent of polymerisation at the time of introduction into the reaction vessel. Appreciable polymerisation was found to take place with inhibitor-free styrene only at temperatures above 120°C, although polymerisation occurred to a significant extent at lower temperatures (ca. 90°C) when the monomer contained 1% w/w benzoyl peroxide. Figure 3 shows the variation of the induction period preceding ignition with the initial pressure of a styrene vapour-air mixture. Introduction of
3
Igniti°nj A < ~ ~
i
.-I "
r In{roduct'n. I o~q.id
monomer
~'~', ~..,,..,~. ! ', of
liquid : ~ rnonomer
._I.):T:___S".. .............. Time Fig. 2. The variation with time o f the pressure and temperature changes accompanying t h e s p o n t a n e o u s ignition o f a 1:5 styrene-air mixture. (a) Reaction only in vapour phase, (b) Reaction in presence o f polymerising m o n o mer. , pressure; . . . . . . . , temperature.
the polymerising liquid monomer (Fig. 3(a)) clearly promotes ignition, inhibitor-free styrene being slightly more effective than the inhibited monomer and the promoting effect increasing with the volume of liquid monomer added. Styrene containing benzoyl peroxide (Fig. 3(b)) also promotes ignition; and similar effects are observed when the monomer contains a,a'-azobisisobutyronitrile. Solid polystyrene, however, exerts only a small influence (Fig. 3(b)), suggesting that promotion is caused primarily by a medium which is polymerising at the time of its introduction. In general, the promoting influence of polymerising liquid styrene is smaller at higher pressures (Figs. 3(a) and (b)). The liquid monomer can evidently exert two opposing effects on the vapour-phase reaction. On the one hand, it has an accelerating influence, due presumably to its ability to provide an additional source of the chain-carriers involved in the processes leading to ignition of gaseous styrene-oxygen mixtures. On the other hand, introduction of liquid styrene causes significant cooling of the reactant vapour. With the addition of inert liquid benzene instead
C. F. CULLIS and S. KURMANADHAN (a)
(b)
(c)
6
E
~ 4
2
SO
(a)
g c :o 0)
30
Ignition ~ i)
80--
o
1
40 40 Initial pressure (kNrn- 2 ) Fig. 3. The effect of polymerising styrene, polystyrene and benzene on the induction period preceding the igmtion at ca. 570°C of a 1:5 styrene-air mixture. (a) ©, vapour only.Liquid styrene preheated at 190°C for 15 rain: e, 30 pl inhibited monomer introduced; O, 30 pl inhibitor-free monomer introduced; ®,10 ~l inhibitor-free monomer introduced. (b) o, vapour only; e, 30 pl Liquid styrene, containing 1% w/w benzoyl peroxide and preheated at 75°C for 15 rain,introduced (~, soLid polystyrene preheated at 65°C for 15 rain introduced. (c) (3, vapour only; e, 30 pl liquid benzene introduced.
40
(b)
Ignition d
i) igniti°n'~/l
-
IgnitionI
60--
(c)
Ignition,l
ib
r
40
20
ilntroductionof
- introductionof
liquid monomer . ~
p-
r --20 1
2
I
4
[
6
1
8
0
i
I
I
2 4 6 Induction period (s)
l
8
50
0
I
2
1
4
I
6
Fig. 4. Temperature changes following the introduction of polymerising liquid monomer into a 1:5 styrene-air mixture maintained at 570°C. (a) Initial pressure 47 kNm - 2 . ©, vapour only; e, 30 ~1 inhibitor-free monomer preheated at 170°C for 2 rain introduced. (b) Initial pressure 33 kNm- 2 . ©, vapour only;e, 30 pl inhibitor-free monomer preheated at 170°C for 2 min introduced; (c) Initial pressure 33 kNm - 2 . ©, vapour only; e, 30 pl inhibitor-free monomer preheated at 66°C for 15 min introduced.
OXIDATION OF STYRENE of reacting liquid styrene (Fig. 3(c)), the consequences of such cooling can be seen in isolation. Due to the exponential increase of the rate of polymerisation with temperature, the introduction of a cool liquid, causing a given decrease in temperature, would be expected to retard more significantly the faster reactions taking place at higher pressures. Thus the overall promoting effect of polymerising liquid monomer would tend to be smaller under these conditions. Experimental confirmation of this cooling effect is provided by measurements of the temperature changes taking place within the reaction vessel (Fig. 2(b)). The extent of cooling produced by the introduction of inhibitor-free monomer is shown in Fig. 4, from which it can be seen that the presence of the liquid, which needs to be not only vaporised but also heated to the reaction temperature, suppresses the temperature increase normally observed during slow combustion preceding vapor-phase ignition. In conclusion then, it is clear that the introduction of polymerising liquid monomer into a reacting styrene vapour-air mixture can, despite the associated temporary cooling effect, cause ignition to take place under conditions where it would not normally do so.
5 The authors wouM like to thank Dr. J. H. Burgoyne and partners for valuable financial support.
REFERENCES [1] Ebdon, J. R.,Brit. PolymerJ. 3(1), 9 (1971). [2] Miller, A. A. and Mayo, F. R., J. Am. Chem. Soc. 78, 1017 (1956). [3] Mayo, F. R., J. Am. Chem. Soc. 80, 2465 (1958). [4] Dutka, F., and Gal, D., Int. J. Appl. Rad. Isotopes 13, 27, 35 (1962). [5]
[6] [7] [8] [9] [ 10] [11]
Harkness, J. B., Kistiakowsky, G. B. and Meats,
W. H.,J. Chem. Phys. 5,682 (1937): Mark, H., and Breitenbaeh, J. W., Atti. Congr. Intern. Chim. 2, 335 (1938). Kroger, C., and Bigorajski, G., Erd#'l und Kohle 15(1), 7; 15(2), 109 (1962). Petrella, R. V., and Sellers, G. D., Combust. Flame 16, 83 (1971). Kaskan, W. E., Sixth International Symposium on Combustion, Reinhold, 1957, p. 134. Brieker,C. E., and Johnson, H. R., lnd. Eng. Chem. 17,400 (1945). Medvedev, S., and Zeitlin, P., ActaPhysicochim. USSR 20, 3 (1945).
Received 26 April 1977; revised 31 May 1977