I
CHARLES S. STOKES, ROBERT P. M. WERNER,l WILLIAM
F. R.
SMITHP2and JOSEPH A. CAHILL
Research Institute, Temple University, Philadelphia, Pa.
f o r Ultra-High f l a m e T e m p e r a t u r e s .
..
Combustion of Cyanogen with Endothermic Oxides of Nitrogen The need for research in the area of high temperature flames is obvious in light of the temperatures reached in exotic fuel exhausts
THE
high positive heat o f formation of cyanogen and the fact its combustion with oxygen produces only carbon monoxide and nitrogen, the most thermally stable molecules known, make possible flame temperatures of the order of 4800' K. ( 5 ) . Under pressure the temperature could be raised considerably because of the decreased dissociation of nitrogen and carbon monoxide (3, 4 ) . Therefore, combustion of other compounds with an even higher chemical heat content would make it possible to obtain even higher temperatures (providing that they produce the same gaseous combustion products). Recently this expectation has been realized by the combustion of carbon subnitride, C4N2, and oxygen (7, 8). The high flame temperatures resulting from the combustion of these endothermic compounds with oxygen may be exceeded by replacing oxygen with oxidizers which have positive heats of formation and form carbon monoxide and nitrogen as the combustion products. These requirements are met by ozone (75) and all the oxides of nitrogen. The heats of formation of these materials are given in Table I.
Table I. Heats of Formation Oxidizers with positive heats of formation may replace oxygen to achieve higher flame temperatures
Material
AH'zss, Kcal./mole 73.84 118.00 33.92 19.55 21.60 8.091 2.30
Ref, (12) (1) (9) (1.4)
(14) (1.4)
(14)
0 -26.413
(18
Present address, Ethyl Gorp., Detroit, Mich. Present address, International Resistance Co., Philadelphia, Pa.
Stoichiometry of Combustions
The Nitric Oxide-Cyanogen Flame. This flame is similar to the cyanogenoxygen flame, but its color is reddish rather than bluish white. The equation is : (CN)z 2 N 0 + 2CO 2N2 AH9298 = -168.88 kcal./mole (1)
+
+
At 80y0 (volume) nitric oxide the equation is: (CN)z 4 N 0 ..-+ 2C02 3Nz (2)
+
+
Between 66.66 and 80% (volume) nitric oxide, carbon monoxide and carbon dioxide plus corresponding amounts of nitrogen can be expected. For this range the general equation is:
+ 2 (a + b)NO
+
(2Q 4- b)Nz (3) The combustion cannot be maintained above 75% (volume) of cyanogen and below 55% (volume) nitric oxide, When burning stoichiometrically 33.33% (volume) cyanogen, and on the cyanogenrich side, the flame forms carbon. The carbon produced at these temperatures becomes incandescent, thus placing the brightness maximum in this range. Equipment and Procedure. The flow rates of the gases were measured with meters of the tri-flat type. T h e meters were calibrated for gases at 294 K. at a pressure of 760 mm. of mercury, The premixing of cyanogen and nitric oxide occurred at the base of a watercooled torch which had an orifice of 0.055 inch. A glass tube 26 inches long and 2 inches in diameter was held by a rubber collar attached to the torch. From the upper end of this tube the combustion gases could flow into a large gas-collecting bottle filled with dilute sulfuric acid, arranged to avoid back pressure. A dry ice or liquid nitrogen trap and a trap containing a concentrated solution of iron(I1) sulfate, which would detect nitrogen dioxide, were incorporated into the system. This was followed by a modified Orsat gas analysis apparatus. The torch was run in an atmosphere of a(CN)z
+ 2 bCOi
+
2 (Q - b)CO
helium which was introduced through the collar that sealed the tube around the torch. The flame was not stable, however, when the tube was placed around the torch. This difficulty was overcome by placing a coil of 0.7-mm. platinum wire (inside diameter approximately 2.5 mm.) 2 mm. above the tip of the torch. The burning then occurred on the glowing wire producing an even, stable flame in the glass tube. Prior to running, helium was flushed through the system for 30 minutes before collecting the combustion products. At least 2000 to 4000 cc. of gas was collected to prevent small variations in flow rate from influencing the composition of the reaction products. A glass tee premixed the gases for one run being attached at the point where the orifice had an inside diameter of 2 mm. The flame was very sooty. The combustion products were passed through a tube filled with glass wool to collect the soot, then through a liquid air trap; no carbon dioxide or cyanogen was detected, The products of combustion never contained nitric oxide or nitrogen dioxide. Results. The results of the combustion analysis are given in Table 11. The percentages of carbon dioxide, carbon monoxide, and nitrogen agree with the theoretical values over the whole range of concentrations of premixed cyanogen and nitric oxide. This concordance shows that the reactions taking place in the flame are quite regular and that the combustion proceeds to completion. NITROUS OXIDE-CYANOGEN FLAME. Cyanogen burns readily with nitrous oxide. The highest temperature of this combustion is reached at the stoichiometric ratio : (CN)2 2N20 + 2CO 3Nz (4)
+
+
AH^^^^
= -165.66 kcal./mole
NITROGEN DIOXIDE-CYANOGEN FLAME. Liquid cyanogen and liquid nitrogen dioxide were first mixed in small quantities in varying proportions. No reaction VOL. 52, NO. 1
JANUARY 1960
75
was observed, so it seemed safe to assume that the two compounds could be premixed in the gaseous state without reaction. Nitrogen dioxide exists as a dimer at the operating temperature of the torch, 25' c. The percentages of nitrogen tetroxide and nitrogen dioxide in the equilibrium mixture a t room temperature (25' C.) were calculated from the data of Verhoek and Daniels (76). I t was found that at 25' C. and 1 atm. 74.8y0 nitrogen tetroxide and 25.2y0 nitrogen dioxide were present. The equation assumed for the combustion yielding carbon monoxide and nitrogen is: (CNj,
Table 11.
3
f2Nz
(CXh
KO,
%
%
25.60 30.00 31.80 33.33 42.00
Table 111.
74.40
70.00 68.20 66.66 58.00
5'01. of Combustion Products, Cc. COz, % Calcd. Exptl. Calcd. Exptl. 2180 2430 5130
... ...
2150 2350 5000
*..
...
20.40 8.00 3.50 0 0
Reactants
++ 2Nz0 ZNO (CN)z + NOz (CN)2 + 0 . 5 NaOi (CN)? + 0.572 (0.25 NO2 + 0.75 NnOi)
(5)
20.00 8.00 3.00 0 0
- co, % Calcd 24.60 40.00 45.60 50.00 45.00
Exptc
Nz, % Calcd. Exptl.
25.00 39.40 46.00 49.80 45.40
55.00 52.00 50.90 50.00 55.00
55.00 52.60 51.00 50.20 54.60
Heats of Reaction and Flame Temperatures Were Calculated from Heats of Formation
(CN)z (CN)z
+ 0.572 (0.75 N 2 0 r + 0.25 N O 2 ) -2CO
Combustion Product Analysis
Close agreement of theoretical and calculated values shows that flame reactions are regular and go to cornpietion
Producti
+ 3 N? + 2 Nr + 1 . 5 Ns + 1 . 5 N? 2 co + 1 . 5 N*
2 2 2 2
CO CO CO CO
AHOzss, 1 others, "Selected Values of Properties of Hydrocarbons," Xatl. Bur. Standards ( U , S.)$Circ. 461 (1947). (14) Zbid.,Circ. 500 (1952). (15) Streng, A. G., Grosse, A. V., J . Am. Chem. SOC. 79, 5583 (1957). 116) Verhoek. F. H.. Daniels, F.. Ibid.. 53, 1250 (1931) RECEIVED for review April 15, 1959 ACCEPTEDSeptember 15, 1959 Supported by the U. S. Air Force through the Air Force Office of Scientific Research of the Air Research and Develoument Command, under Contract No. A F 18(600)-1475.
