I
G. BRYANT BACHMAN and HERBERT I. BERMAN' Department of Chemistry, Purdue University, Lafayette, Ind.
Carbon Black f r o m Flames of Oxidanfs in Hydrocarbons Different types of carbon blacks are obtained by burning hydrocarbonoxidant systems. Halogen- and nitrogen-containing blacks are attractive for commercial usage
N
EW TYPES of carbon blacks have resulted from attempts to synthesize organic compounds in hydrocarbon flames. Yields are higher when the combustion oxidant used is chlorine rather than oxygen. Blacks containing substantial amounts of chemically bonded chlorine may be obtained. They appear potentially interesting for applications in the rubber and plastics industries. Physical evidence for carbon-containing radicals in hydrocarbon flames is considerably better than chemical evidence (7). Thus, spectral studies have conclusively demonstrated the presence of C? radicals, but so far little is known about the chemistry of these and similar radicals. With adequate amounts of oxygen they yield oxides of carbon, but they should be capable of a variety of other reactions and synthetic applications. Ordinary flames of hydrocarbons in air or oxygen are scarcely conducive to preservation of such radicals. The material movement is toward regions of high oxidizing power and any radicals formed would be expected to convert promptly to carbon monoxide and carbon dioxide. M'ith reversed flames of oxygen in hydrocarbons better opportunities exist for capture of carbonaceous radicals, because the material movement is away from regions of high oxidizing power. For lack of other descriptive terms to designate these two types of flames, the terms exoxidant (oxygen outside) and endoxidant (oxygen inside) will be employed. This report covers a study
Present address, Armstrong Cork Go., Research Center, Columbia Pike, Lancaster, Pa. 1
which began with comparisons of these types of flames. Experimental The combustion experiments \\ere performed in the apparatus diagrammed below. A metered flow of hydrocarbon was introduced into the combustion chamber through one of the outer orifices of the inlet tube. A metered flow of the oxidant was next started through the inner orifice, and was immediately ignited by an electric spark. Carbonaceous material was collected on the glass wool packing of the collecting tube. Any liquid products were collected below the water conden-
sers or in the low temperature trap. Gases were metered through a wet test meter and collected for analysis in an Orsat apparatus. The carbonaceous products \%ere washed with water until neutral, dried at 54' C. and 5 mm. of mercury pressure, and ana1)zed by combustion. Solvent extractions ivere performed in a Soxhlet extractor. Heat treatments were carried out in a borosilicate glass tube inserted in a stainless steel block heated by a gas burner. For the mass spectrographic studies, the sample was placed in a similar glass tube, sealed to the spectrograph, immersed in liquid nitrogen, pumped out under high vacuum, and
This flame apparatus was used to obtain carbon blacks 1.
2. 3.
4. 5.
Flow meters Backflash prevention trap Combustion chamber, 6 X 60 cm. Ignition spark Carbon collection chamber lightly packed with glass wool, 5 X 25 cm.
6. 7. 8.
9. 10.
W a t e r condensers Dry ice condenser Acid neutralizer or scrubbing tower G a s sampling W e t test meter
VOL. 52, NO. 7
e
JULY 1960
621
Table I. Flames of Chlorine in Hydrocarbons Give More Carbon
Table II.
Carbon from Flames of Chlorine in Hydrocarbon Contains Up to 45% Chlorine Composition by Analysis, yo Carbon Hydrogen Chlorine
Cln/Hydro-
Hydrocarbon
Ratio
version
Sample
Hydrocarbon
carbona
s-1
Natural gas Natural gas Natural gas Methane Ethylene Propane
1.3 2.0 1.6 1.4 0.4 0.5
5-2 5-3 s-4 s-5 S-6
Yield
Endoxidant Flames of Chlorine
Natural gas Methane Propane Ethylene
1.49 1.50 1.50 1.00
15 50 9 30
90 90 90 90
a
83.0 50.9 56.8 66.8 74.7 65.8
1.6 1.1 1.7 2.2 1.5 1.7
9.4 45.1 34.2 30.6 18.3 27.8
Chlorine t o hydrocarbon mole ratios: values A0.2.
Commercial Processes
Channel black Gas black, thermatomic Lamp black, Rubber grade Higher grade
10 60 60 20
A
A-
4 c2H4
XCH4
/-
Endoxidant Flames of Oxygena
Ethylene
1.20
9
Exoxidant Flames of Oxygen
Ethylene
1.20
1
Other yields. %: H P 48; CO, 66; C O X , 14. Other yields, %: Hz, 6; CO, 39; 12.
con,
then heated gradually and the off gases were analyzed. T h e chlorination of carbon blacks was accomplished in a I-liter flask fitted with a stopper containing the chlorine inlet tube and an exit tube. It was heated on a hot plate and stirred magnetically. Results and Discussion
The endoxidant flame of oxygen in ethylene differed from the corresponding exoxidant flame of ethylene in oxygen in that the former was redder, less poorly defined a t the top, and yielded more carbon and hydrogen. There was, however, no evidence of formation of volatile organic materials from any carbon-containing radicals present in the flame. In fact, such radicals appeared to lose substantially all of their hydrogen and polymerized to carbon black before they could be captured by the excess ethylene present. As hydrogen atoms are removed from hydrocarbons the residual molecules become increasingly unstable relative to elementary carbon and hydrogen from a thermodynamic standpoint. Hence it would be expected that with lower hydrocarbons only a few hydrogen atoms per molecule would need to be removed before the rest would leave practically spontaneously a t flame temperatures. The residual all-carbon radicals would then polymerize immediately, or after disintegration, into smaller all-carbon radicals. If the rate of polymerization is faster than the rate of combination of such radicals with ethylene, then the
622
Figure 1. More carbon i s formed as the chlorine-hydrocarbon mole ratio in creases in halogen-hydrocarbonflames
results of the above experiments are explained. Oxygen has been studied so frequently as the oxidant in hydrocarbon flames that it is easy to lose sight of its peculiarities relative to other oxidizing agents. Chlorine, for example, exhibits marked differences from oxygen in its attack on hydrocarbons. Thus, of the following reactions, 1 is more probable than 2. but 4 is more probable than 3, on the basis of the estimated free energy changes a t flame temperatures. AFlb000,
Kcal.
Reaction
+ COz + 2H2 -124 + Oy C + 2H?O -87 + 2Cl2 ccI4 + 2Hx -66 CH4 + 2Cl3 -,C + 4HC1 -100
CHa CH, CHd
0 2
+
+
+
(1)
(2) (3) (4)
Consequently, as soon as possible in combustion, oxygen begins to exert its
Table 111. Temp., Samplea D-1 D-2 D-3
c-1
C-2 c-4
c. 22 100 425 22 200 400
preference for combination with carbon rather than with hydrogen, while chlorine begins to exert its preference for combination with hydrogen rather than with carbon. Carbon black is therefore at best a by-product when hydrocarbons are burned with oxygen, while it is the major product when hydrocarbons are burned with submaximal quantities of chlorine. Thermal dissociation probably accounts for most of the carbon produced in present commercial processes. Carbon production in chlorine-hydrocarbon flames increases with the chlorine-hydrocarbon ratio, at least up to the stoichiometric ratio (Figure I), and also varies with the hydrocarbon used, being greatest for methane (Figure 2). Chlorine is present in the blacks in substantial amounts (Table 11). I t is probably introduced during rather than after the polymerization process, because a num-
Chlorine Treatment of Carbon Blacks Time, Hr. 26 23 21 72 23 26
Carbon
Product Composition, 5% Hydrogen Chlorine
88.4 91.8
0.60
97.8
0.55
...
...
...
...
0.85
...
...
1.10 2.90 1.97 0.81 0.72 0.68
Sample Identification: D , Darco G-60 Carbon, Atlas Powder Co., Wilrnington, Del.; C, SRF Carbon, Thermatomic Carbon Co., Sterlington, La.
INDUSTRIAL AND ENGINEERING CHEMISTRY
a
CARBON BLACKS ber of attempts to chlorinate commercial blacks led to products containing only a few per cent of chlorine (Table 111). The chlorine is apparently chemicall!bonded to the carbon rather than physically absorbed on it. because chlorine molecules could not be removed by any method tried. Even heating to 700' C. in a high vacuum gave gases which sho\ved no peaks for chlorine molecules in a mass spectrograph. Boiling with acetone gave no detectable trace of the highly lachrymatory chloroacetone which would be formed if chlorine molecules were adsorbed on the carbon. The removal of chlorine in any form was hard and became increasingly difficult. Various solvents, reactants, and heat treatments were only partially successful in removing the chlorine and ultimately left about 10% or more in the sample (Table IV). Chlorocarbon blacks resemble ordinary carbon blacks in appearance. The)are black in color, finely powdered in texture, have no appreciable odor, and resist burning, especially those with a high chlorine content. Ordinary carbon blacks probably contain layers of carbon atoms arranged in a honeycomb structure of hexagonal rings ( 2 ) . There are, on the average, 3.5 layers per crystallite and 35 rings per layer. The layers are nearly circular rather than linear in shape. Such a structure cannot account for the high chlorine content of some chlorocarbons, because there are not enough peripheral positions available for bonding all the chlorine (Figure 3). This appears especially true when it is considered that a large hexagonal network of carbon atoms with every peripheral position occupied by chlorine could only result from a remarkably fortuitous concatenation of collisions of the proper atoms at the proper times. Possible modifications in structure which might occur to accommodate more chlorine include: smaller layers containing fewer rings, more linear and less circular layers, and internal or nonperipheral bonding of chlorine to carbon. Of these possibilities, the last is considered least likely from energy considerations, while the first two seem equally probable. Experimental evidence in support of the first modification was found by examining the benzene extracts of chlorocarbons. Hexachlorobenzene was isolated from them in a pure form in small amounts. The calculations upon which the curves in Figure 3 are based were derived as follows: A single hexagonal ring may be completely surrounded by a shell of 6 hexagonal rings, by a second shell of 12 rings, a third shell of 18 rings, and so on. I n such systems, the following relationships hold between the number of shells S, the total number of rings R, the num-
*
CH4
\I
S
I
IO moles of chlorine'mle of
I5 hydrocarbon
20
Figure 2. Methane yields more carbon than either ethylene or propane, thus showing variation with hydrocarbons used in chlorine-hydrocarbon flames
W
I
0
I
I
20
40
No
of
c
u
l
a
r
Network
1
1
60
BO
Rm9s
I
Figure 3. Maximum peripheral chlorine content of hexagonal networks of chlorocarbons distinguishes between linear and circular arrangements
- Bored
on C
0 Mole
% C12 in
Oxidant
Figure 4. Oxygen suppresses carbon formation in chlorine-hydrocarbon endoxidant flame systems
Figure 5. Oxygen lowers the chlorine content of chlorocarbons more than expected in halogen-alkane flames VOL. 52, NO. 7
JULY 1960
623
ber of peripheral positions P,and the total number of carbon atoms C:
+ 3S + 3S2 P = 6(S + 1 ) C = 6(S + 1 ) 2
R = 1
Table V.
35.457P 12.01c 35.457P
+
-
(7)
Sample
Hydrocarbon
1 2 3
Methane Methane Ethylene
For a strictly linear system of benzene rings
+
=
1
0.3387(S
“‘
+ 1) + 1 1
-1 0.17 _ _ + 1.17 4R
(9)
For networks containing u p to 5 shells or a total of 91 rings, S R P C x 0 1 6 6 74.7 1 7 12 24 59.5 2 19 18 54 49.5 3 37 24 96 42.4 4 61 30 150 37.0 5 91 36 216 32.9
Table IV.
s-1 s-2
s-3
benzene
s-4
s-4
Ct=2+4R
(11)
+ 70.91R + 118.95R
141.83 = 165.85
(12)
and R 1 7
P’
61
91
‘Y
C’
6 18
6 30
74.7 64.3
126 186
246 368
60.0
Higher values for these quantities may easily be calculated from the above equations. For networks intermediate
C = 89.6
C = 86.5 t H = 1.3 c1 = 10.2
acetone
--+-
H = 2.9
45 hours
c1 =
water 25 hours
+
C = 57.2 H = 1.6 C1 = 38.4
+
C = 76.5 H = 1.7 C1 = 18.2
420’ C. 10 hours
C = 78.3 = 1.7 C1 = 1 5 . 8
llOa C., 5 mm. Hg. 22 hours
C = 78.9 C1 = 13.6
l l O o C., 5 mm. Hg. 22 hours
79.7
l l O o C., 5 mm. Hg.
acetone acetone
s-4
(10)
Solvent Extraction and Heat Treatment of Chlorocarbon Blacks
30 hours s-4
+ 2R
P‘ = 4
+
but % = S 1 (from Equations 5 and 6 P above), hence
x
0.50 0.54 0.70
24 hours
7.2
+H
benzene 24 hours
4
carbon disulfide
0.69 0.57 2.55
Composition. 5% N
c1
5
11 9 4
7 12
1
o.3387p C
(8) I
Reactant Ratio Cl,/hydrocarbon Cls/”H3
(6)
Similarly the percentage of chlorine, X, possible in such networks if every peripheral position is occupied by a chlorine atom is
X =
Nitrogen and Chlorine Contents of Carbonaceous Material from Flames between Hydrocarbons, Ammonia, and Chlorine
(5)
H = 1.8
+H
=
24 hours
C1
=
30% aq. K O H 24 hours
C = 62.0 ----f
=
1.6 12.8 2.2
C1 = 16.9
20 hours
500’ C.
-+-
10 hours
>
22 hours
llOo C., 5 mm. Hg. 22 hours
f
+
C = 69.5
H = 3.3 C1 = 26.5 C = 81.4 H = 1.2 c1 = 11.2 C = 78.7 2,1 c1 = 1 4 . 7 C = 80.1 =
1,5
C1 = 14.6
>
C = 78.8 H = 1.4 C1 = 13.2
in shape between circular and linear, the values of P,C, and X are also intermediate for similar values of R. Additives to the chlorine-hydrocarbon endoxidant flame system have varying effects. Thus, nitrogen acts as an inactive diluent. Oxygen added in the chlorine diminishes both the yield of carbon (Figure 4) and the chlorine content of the carbon (Figure 5 ) more than would be expected from the amount introduced. The preference of oxygen for carbon is apparently not altered by the presence of chlorine, and good yields of chlorocarbon blacks could only be obtained with chlorine-oxygen ratios of 4 or more. Ammonia added in the hydrocarbon leads to nitrogen carbon blacks (Table V) in which the nitrogen content varies inversely with the chlorine content. Carbon production is diminished by ammonia and ceases entirely at moderate rates of ammonia flow (Figure 6). These blacks resemble the chlorocarbon blacks in both appearance and properties. The nitrogen was not removed by solvent action or by heating a t 400’ C. for 5 hours. Aqueous suspensions of the blacks \vere neutral and contained no titratable acidity or basicity. T h e addition of nitrogen dioxide to the chlorinehydrocarbon flame suppressed carbon formation completely at low concentrations of the additive and extinguished the flame itself a t higher concentrations. Small amounts of liquid products were formed. These were not identified, but possessed an esterlike odor and evolved nitrogen dioxide and hydrogen chloride on attempted distillation.
C = 60.0 = 1,4 C1 = 17.7
Literature Cited (1) Lewis, B., von Elbe, G., “Combustions, Flames, and Explosions of Gases,”
.4cademic Press, New York. 1951. (2) Sweitzer, C. W., Heller, G. L., Rubber U o i l d 134,855 (1956).
-
Figure 6.
Ammonia
suppresses formation at moderate rates of ammonia flow
NH3/CH4
624
INDUSTRIAL AND ENGINEERING CHEMISTRY
RECEIVED for review November 16, 1959 ACCEPTED April 14, 1960 Division of Industrial and Engineering Chemistry, Symposium on Chemicals from Flames, 136th Meeting, ACS, Atlantic City, N. J., September 1959. Abstracted from dissertations submitted by Herbert I. Berman to the Graduate School of Purdue University in partial fulfillment of the requirements for the degrees of M.S., February 1956, and Ph.D., June 1958.
