REACTIONS BETWEEN ELEMENTAL CARBON AND HYDROGEN A T TEMPERATURES ABOVE 2800"K. RAYMOND F. BADDOUR AND JOHN M . IWASYK'
Massachusetts Institute of Technology, Cambridge 39, Mass.
A high intensity arc reactor was designed and built to react carbon vapor and hydrogen.
The hot gaseous
reaction products were quenched and sampled with a water-cooled probe. Under all conditions, acetylene was the mujor reaction product in the quenched gas. Its concentration in the quenched sample reached 18.6% a t I-atm. pressure and 23.8% (diluent-free basis) with 66.7% helium diluent. The results a r e consistent with the hypothesis that C2H radicals exist in large Concentrations in the hot reaction mixture and react to form additional acetylene in the quench probe. u RECEKT years, research in high temperature chemistry has
I-become increasingly active (3, 9, 72).
The advantages of operating a t high temperature are high reaction rates and improved equilibrium concentrations for some species. Furthermore. by rapid quenching of the hot gases, it is possible in some cases to form a desired material in favor of other species more highly favored by equilibrium, because of differences in rates of the various quench reactions. O n a long-range basis. commercial processes involving very high temperatures should become more competitive. since the cost of electric power has remained relatively stable while material and labor costs have generally been increasing rapidly. Limitations on commercial exploitation of high temperature chemical processes include ( 9 ): Better heat-resistant materials of construction are desired. The rapid quench from high temperatures required to "freeze" chemical compositions a t the high temperature equilibrium values and to promote the desired quench reactions is wasteful of high temperature heat. Fundamental high temperature data, especially on the concentration of high temperature radicals in the hot equilibrium gas mixture, and recombination mechanisms during quenching are almost nonexistent. Additional research in these areas \vi11 accelerate the development of industrial high temperature processes. Carbon-Hydrogen Reactions
At low temperatures pure carbon does not react readi1)- xvith hydrogen. but a t 800' K . the reacticn C(S)
+ 2Hdg.)
-L
CHi(g)
(1 j
\vi11 occur to an appreciable extent. especially with a cobalt or nickel catalyst. As the temperature is raised, this reaction reverses. The next significant reaction bettveen eleaental carbon and hydrogen begins to occur at 2500' K . ?
+
~ C ( S ) H d g ) + C?H?(g)
(2)
where the equilibrium yield of acetylene increases with temperature for a w i l e rarige of temperatures. Early investigators (I, 1)were able to obtain acetylene steady-state concentrations Present address, Engineering Research Laboratory, E. I. du Pont de Nemours & C o . , Inc., Wilmington 98, Drl.
up to 8 volume % using a low intensity arc as a high temperature source. I n the most recent investigations of the carbonhydrogen system (7, 73); graphite tube resistance furnaces were heated u p to about 2800' K. and the products were quenched with a water-cooled probe. Plooster and Reed found good agreement between experimental and thermodynamic equilibrium concentrations of acetylene (Figure 3), assuming that the following reactions occurred :
+
~ C ( S ) Hn(g)
+
HS + 2H C2H2 + C2H
CSHdg)
(2)
(3)
+H
(4)
The unique feature of Plooster and Reed's work was the Fostulation of the existence of the C2H radical. They assumed that the reverse of Reactions 3 and 4 occurred in the quench probe. Thus: the presence of C2Hin the reaction mixture bvould permit an acetylene content in the quenched gas higher than the acetylene content in the hot equilibrium gas. Kroepelin and Winter ( 7 7) calculated the equilibrium coniposition diagram for the C H ? and the C 2H2 systems between 1000' and 6000' K . accounting for the species C(S), C1, C f , Cs, H?, H, CH, CH2, CH3, CH4,C?Hg, and C?HI in the hot equilibrium gas: but did not consider the e>-istenceof C2H. Their calculations show that C(s), C , , C2,( 2 3 , H.: H, and C2H2 are the only species present in significant concentrations (greater than 0.1 volume %). They also point out that:
+
+
Maximum hydrocarbon concentrations occur in the heterogeneous region. where solid carbon exists a t 1-atm. pressure. At temperatures abotre 6000' K., C1 and H are the only species present to any significant extent. Acetylene content reaches a maximum near 3800' K., the temperzture at ivhich the solid carbon phase disappears (called the sublimation point). Yields of acetylene in excess will be formed Lvhen the hot reaction mixture is quenched, because of carbon condensation, hydrogen atom recombination. combination of hydrocarbon radicals to form hydrocarbon molecules. and other kinetic processes which occur during the quench. Acet)-lene content in the equilibrium gas rises as the carbonhydrogen ratio in the gas increases. Equilibrium Calculations
Plooster and Reed's ( 7 3 ) thermodynamic calculations are limited because they extend only u p to 3500' K . and do not take carbon vapor into account, while Kroepelin and IYinter's VOL.
1
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169
I
'
j
l
t
l
-
-
0 20
E
e
+
-
-
-
-
-
U
-
? VI 0
In c i
- 0.10-
.-
0 .L
0
a
0 2600
3000
3500
Figure 1.
Equilibrium diagram
4 000
3750
Temperature,
for system C
4500
OK.
+ Hz a t 1 -atm. pressure
Calculation of Quenched Gas Compositions Table 1.
Comparison of Thermodynamic Calculations
Sublimation temp., K. Maximum C2H2 concn., vol.
%
Maximum C2H concn., vol.
Kroepelin and W n t u ( 7 7) Figure 1 3860 3750 11 at 3600" K. 8 at 3500" K.
yo 0
13.8 at 3750" K.
Since the heterogeneous carbon-hydrogen system contains the maximum concentration of hydrocarbons, and thermodynamic equilibrium calculations are considerab1)- simplified for this region: the remainder of this discussion is concerned with this region only. Using the previously cited thermodynamic data, the following equilibria may be considered for the carbon-hydrogen system up to the sublimation temperature:
calculations ( 7 7 ) are limited because they do not take C?H into account. The equilibrium diagram for the C HS system has been calculated a t temperatures between 2500' and 4200' K. in Figure 1. taking into account C2H and the most recent experimental data published by Drowart et al. (4) on the composition and pressure of carbon vapor. Thermodynamic functions for C(s). C2H?.C2H. H2.and H are taken from Plooster and Reed and Kroepelin and IVinter. The results in Figure 1 differ from Kroepelin and \+'inter's results, as shown in Table I. Figure 1 shows that as the concentration of carbon increases, concentrations of hydrocarbon vapors decrease, in agreement with the results of Kroepelin and Winter's calculations. As the temperature is increased bevond the sublimation point, the concentrations of polyatomic species decrease until CI and H become the predominant species.
Finally, the calculations on which Figure 1 are based must be considered as only approximate because of the lack of reliable thermodynamic data on the C2H radical. A variation of 5% in the value used for the enthalpy of formacion of C2H will cause a 2.5-fold variation in the predicted maximum concentration of the C P H radical. I n addition. a recent calculation of the C / H system (5) indicates tha a large number of other C-H molecules, as well as C2H, may be important at high temperatures. 170
I & E C P R O C E S S DESIGN A N D DEVELOPMENT
-c2
ZC(S1 -.c
Kz
3C(S) + c3
Ks
C(S1
+
+
~C(S) H
+
c 1
C?H1
Ki
= =
-
Pcl
(5)
Pcz
(6)
Pc,
(7)
PCaHr
(8)
I - -
PH
H ? + 2H
(9)
The value of P, is then substituted back into Equations 8, 9. and 10 to solve for PRz, P C Z H , and PC2A2. If Reaction 10formation of C2H-is not considered to occur, the following equation applies:
- 1+ 4 1 PH =
+ 4 [ -~ (Ki iK2 + K I)
(~f) I + K
(13,
Now with Equation 12 or 13, the effects of temperature, pressure, diluent, and carbon vapor on product composition can be calculated for the heterogeneous system. Since only the quenched reaction products were analyzed, it is assumed that upon quenching the following reactions occur: 2H
CBH
+H ca
HB 4
+
(14)
C2H2 3C(S)
-
cz + 2C(S) c1 C(S)
c 0
m
i'
o
N
c -
ae -
>
-
5 1I 0
It is also assumed th.at the acetylene present in the hot mixture is so rapidly quenched that the reverse reaction back to carbon and hydrogen does not occur and that no side reactions occur. With the above asisumptions quenched gas compositions, which would consist of hydrogen and acetylene only, may be calculated from Equations 12 and 13. The results are plotted in Figures 2 and 3. Two important conclusions may be deduced. First, Figure 2 shows that an acetylene content of 14% may be achieved a t 1 atm.. while Figure 3 shows that acetylene contents as high as 34% may be achieved in the quenched gas at 1 atm. Secondly, the presence of a diluent or a vacuum \vi11 raise the acetylene content, if CZH is assuined to be present. Moreover, going to high pressure (which in effect increases the sublimation temperature and extends the heterogeneous regicn) increases acetylene yield. For example, at 100 atm. and 5000' K. and a H2/C ratio of 0.56, the quenched gas could theoretically contain 86.5%, acetylene. The preceding theoxtical results suggest experimental means for indirectly checking the assumption about the existence of C2H. If the acetylene content in the quenched gas from the high temperature reactor should exceed 14% a t 1 atm., this would be evidence in support of the C2H hypothesis. Increase in the acetylene content with a decrease in pressure (addition of a diluent) would be furi:her supporting evidence.
-
1
I
I II
2600
I]
I I I I I I I
3000
In these studies. the reactions occurring in the hot plasma gases were deduced from analyses of the cold quench gases. Thus. it was ver) important to select a good probe design. Ideally, the hot gases entering the probe \vould be so quenched that all the C2H radicals would react ivith H atoms to produce acetylene, no acetylene tvould decompose, and no side reactions such as hydrogenation of acetylene would occur. Information in the literature indicates that one of the fastest, simple quench devices is a water-cooled probe containing a stainless steel hypodermic tubing. This device has been used b\ previous investigators for sampling flames and the carbonhvdrogen system ( 7 7 , 73). Plooster and Reed concluded that under conditions of choked flow there is an optimum probe size for maximum acetylene content in the quenched gases. Probes smaller than the optimum size had gas flow rates too low for good heat transfer and probes larger than the optimum size had inadequate surface area for fast quenching. T h e probe material or the probe inlet design had no detectable effect on the quench gas composition, although corrosion effects were significant for stainless steel and silver, while platinum and alumina-silica painted tubes resisted corrosion. A change in quench composition was noted when sampling rates were so high as to sample cooler sections of the flame.
]
I
II
-
I
I
.
3500
Equilibrlum Temperature o f Arc Gases, O K .
Figure 2. Calculated acetylene content in quenched gas, neglecting
CzH
P i o o s t e r 8 R e e d ' s Data 0
2500
C . 2 5 arm.
3000
3500
Equilibrlum Temperature Gcses,
Probe Theory a n d Calculations
,I
O
4000 o f Arc
K.
Figure 3. Calculated acetylene content in quenched gas, considering
C2H
I t was concluded from these studies that a probe size near the 26-gage optimum recommended by Plooster and Reed should be satisfactory for the carbon-hydrogen s) stem. Calculations were made to relate probe size to quench times. taking into account the various probe phenomena for the case of a water-cooled tube sampling a 4000" K. mixture, quenching to room temperature, and filling an evacuated flask (70). These calculations sho\v that quench times from 10-6 to lo-' second could be obtained with average flow rates varying from several to several hundred milliliters per second as the probe size was varied from 26 to 14 gage. These calculations also point out the complexity of the probe phenomena and suggest that, with so many effects to consider, it is extremely speculative to predict what will happen for a given system during the quench. Experimental Apparatus
Three schemes were tried for achieving temperatures above 2800' K.; only the high intensity arc reactor was superior in terms of carbon available for reaction and minimal operational VOL. 1
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171
Reactor Off-Gas
Variable Speed D.C.
Reaction Zone
Coo
Cathode Terminal Starting Wire
Insulating Gasket
Figure 4. High intensity arc reactor
difficulties ,4 plasma jet reactor failed to provide sufficient carbon at temperatures in excess of 2800' K. for significant reaction with hydrogen. and an induction-heated reactor failed to achieve temperatures above 2800' K. The theory and operational characteristics of the high intensity arc have been thoroughly discussed by Finkelnberg ( 6 ) and Sheer ( 7 1 ) . Briefly. as the anode current density of an arc exceeds a critical value, its operational characteristics change and it becomes a high intensity arc The anode rapidly vaporizes and the anode vapor streams away from the anode surface at a high velocity. The anode flame extends several inches from the anode and leaves a luminous trail as the vapors cool. Up to 90Y0 of the input power may be transferred to the vaporizing anode. I n addition, the anode vapors may be superheated to temperatures of the order of 8000" to 10,000" K. The high intensity arc displays rising voltage-rising current characteristics as compared with falling voltage-rising current characteristics of the low intensity arc. These and the other characteristics of a high intensity arc indicate that it could be incorporated into a convenient hightemperature research reactor for the carbon-hydrogen system. Consequently, a vacuum-tight reactor of convenient size was designed with sufficient anode capacity for a t least a 30-second run and sufficient water cooling of the walls for full power dissipation in the event of a power short circuit. The details of the high-intensity arc reactor are shown in Figure 4 . The main reactor parts are the anode, the cathode. the sample probe, and the water-cooled reactor shell.
the anode flame and the reaction mixture, and showed only slight deterioration after 2 hours of operation. The cathode was purposely designed with a large cross-sectional area to avoid consumption and to keep the section downstream of the arc relatively cool during operation. Carbon normally condensed 2 to 1 inch back along the cathode from the arc. After 5 grams of carbon had been evaporated from the anode, the cathode was usually 7.57" blocked and the cathode \vas then drilled out. The deposits were whitish gray or black; sometimes nodules 16 inch high were deposited on the other cooler regions of the cathode face. To start the arc a wire was inserted through the reactor shell and through the cathode until it just touched the anode. The sample probes quenched a sample of the hot reaction products down to room temperature in times of the order of 10-5 to second. They also cooled the remainder of the product gases, so that hot, flammable, and potentially explosive gases would not be expelled to the atmosphere. The probes were inserted into the cathode pipe as shown in Figure 4, with the probe tip at various distances from the arc core.
9 t
Anode Diameter Hydrogen Feed R a t e , liters/rnin.
E
0 0 t o 0.25 0 2 75 5 5
- 1/4 in.
/
The anodes in all runs were graphite rods, hollow or solid, inch in diameter and 7 to 12 inches long. The hollow rods had '/*-inch holes drilled through them axially. The anodes were mounted in a brass piston fitted with three silicone O-rings. This piston fitted into a water-cooled copper tube 3/4-inch i.d. The purpose of the plunger assembly was to prevent air leakage through the anode holder. T h e anode assembly was driven by a variable-speed d.c. screw drive. Speeds could be varied between 1 and 6 inches per minute, which allowed power inputs up to 10 kw. Above 10 kw., the anode consumption rate was in excess of the anode feed rate. which resulted in the arc's being extinguished because of the evcessive arc gap. Electrical current was fed through both the piston-anode rod assembly and the water-cooled copper cylinder. Examination of the anode rods showed that most of the current followed the cylinder path, so that the anode rod actually in the cylinder remained relatively cool. Gas could be introduced either through the three gas ports in the anode holder or through the piston and hollow anode. The cathode for all runs consisted of a 9-inch length of graphite pipe 11/2-inch-o.d. by '12-inch-i.d., held in a watercooled brass electrode holder. The cathode actually contained 172
I & E C P R O C E S S DESIGN AND D E V E L O P M E N T
Figure 5. Anode consumption rate as a function of power input
Each probe consisted of three concentric stainless steel tubes. n i t h water circulating through the outer two to cool the inner tube. Runs were made with inner tube sizes of 11, 14, 19>and 22 gage. In all cases the outer tube diameter was l;’4 inch. T h e connections were silver-solder brazed. T o prevent plugging of the probe inlet by carbon condensation from the anode flame or by soot formation from the decomposition of acetylene, the inner tube of the probe was purged with hydrogen until the sample was taken. This prevented plugging under all the conditions of this study. A l’la-inch layer of oily soot, presumably from acetylene decomposition. was always found on the outside of the probe for distances of 3 inches back from the tip. It was determined experimentally that with a 200-ampere current, the probe could be brought up to 11’2 inches from the arc core before probe failure occurred. The 3l 4-inch-i.d.: 17-inch-long reactor shell consisted of double-jacketed water-cooled brass tubing. T h e anode holder was bolted and sealed by O-rings to the reactor shell flange. The cathode holder was electrically isolated from the reactor shell by a rubber gasket inch thick and plastic washers. Ample space was allowed between the electrode and the reactor shell to prevent short circuits from broken electrode parts and carbon deposits. ’The reaction products were expelled through the exhaust tube to the outside of the building. ,4check valve in the exhaust line prevented air entry into the reactor in case a vacuum developed in the reactor. The entire reactor was contained in a barricaded enclosure with a blow-out window. T h e reactor was operated by remote control from outside the barricade. T h e barricade was ventilated by exhaust fans to prevent toxic gas accumulation. In preliminary runs detectable concentrations of H C N were produced when air leaked into the reactor. T h e H C N condensed on the reactor shell walls, thus introducing a potential safety hazard whenever the reactor was disassembled. I n the experiments carried out in this investigation no attempt was made to measure the temperature of the hot reaction mixture entering the probe.
Operational Characteristics of High Intensity Arc Reactor
No data have been published on arcs obtained with the electrode geometry used in this study. The electrode geometry as well as the type of electrode carbon and electrode cooling used is important in determining the anode consumption rate as a function of arc power input. For various reasons, the arc voltage was not constant during a run or from run to run, the variation being about 5 volts. With the hollow graphite cathode and a hydrogen atmosphere the average arc voltage was approximately 50 volts through the 100- to 250-ampere range. Finkelnberg reports 50 volts for a conventional high intensity carbon arc operating a t 100 amperes in air, with an anode consumption rate of 1.96 grams per minute. For the geometry used in this study the high intensity effect disappeared in an argon atmosphere (no anode consumption) with an arc operating a t 25 volts at 175 amperes. A high intensity arc was maintained at 32 volts in a helium atmosphere. but the anode consumption rate was reduced by 707,. Helium-hydrogen atmospheres gave arc voltages and anode consumption rates close to those of hydrogen atmospheres. I n Figure 5 the data on anode consumption rates as a function of arc power input are summarized. The linear increase in anode consumption rate with increase in arc power is in a5reement with the results of other investigators working with conventional high intensity arcs. The reduction in anode consumption rate with an increase in hydrogen flow rate is presumably caused by a lowering of the electrode temperatures and convection cooling of the arc core.
Experimental Results
The experimental results presented here are for the heterogeneous carbon-hydrogen system, since it was not found possible to approach the homogeneous region of the anode flame without experiencing probe failure. Since reactor temperatures were not measured, experiments were limited to studying the effect of reactor variables on the composition of the quenched gas for this system. The independent reactor variables were: Hydrogen flow rate. Arc power input. This varies the carbon vaporization rate and the enthalpy content of the hot reaction mixture. T h e arc polver input and the hydrogen flow rate also determine the input HZ/C ratio. Probe position, probe diameter, and sample flask volume. Inerts content in the feed gas. Effect of P r o b e Size on Gas Composition. From Figure 6 it is seen that the acetylene content reaches a maximum a t 14 gage as probe size is increased. Previous investigators have found that smaller probes yield faster quenches. which in this case should result in higher acetylene contents ( 7 3 ) . There is a reported exception for the case where surfaces may catalyze side reactions (8). It is proposed that a major portion of the acetylene in the quenched gas may be formed by reaction betLveen H and C2H radicals in the probe. If the recombination of H radicals were surface-catalyzed, the higher surface-volume ratio of the smaller probes could result in a high rate of H radical recombination and a deficiency in H radicals for combination with the CZH radical to form acetylene. The excess CZH could react to form higher acetylenes or decompose to carbon and hydrogen. Thus, increasing the probe diameter could actually result in increased acetylene yield, despite the increase in quench time. Since the 14- and 11-gage probes gave the highest acetylene yields: they were used in subsequent experiments. These larger probes were also more convenient to work with. Figure 7 shows the effect of probe diameter on ethylene content. The appearance of ethylene in such high concentrations was surprising, since equilibrium calculations indicated the presence of ethylene in the hot reaction mixture only in trace
i
I 4 t
s
‘t 4
2.75 l i t e r s H 2 / m i n . 7.5 kw. 5 0 c c sample f l a s k
0 1.4
22goge 1 4 1 I
0
Iller5
Hq/min.
7.5 kw. 270 c c sample f l a s k
199
149
I4 1
It
llg I 0.050 0.100 Sornple Probe Inner Diameter, hcher i
I
It
Figure 6. Effect of sample probe diameter on acetylene content of quenched gas VOL. 1
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JULY 1962
173
:I I
c 0
0 f
1.0
P
r
-
-
-
I
I
I
0@-,
0
0 4 ' -
0 1.4 l i ters H2/ mi n. 7.5 kw.
270-cc. Sample Flask
-
5
229
19g
149
-
E
8c
k-Probe
Probe
-
-
IIg
I
It
I 4
0.
--
\\
'e
2 . 7 5 liters H2 /min. 7.5 k w . 50-m.Sample F l a s k
-
N
V
z
I
Follure Zone Due
Power
amounts (10--6%). Ethylene could appear by the following reactions, probably surface-catalyzed :
+ H2 CzHz + 2H CzHz
+
C2H4
Favorable below 1400" K.
+
C2H1
Favorable below 2500' K.
The absence of a strong trend in ethylene content as a function of probe size in Figure 7 suggests that a major part of the acetylene hydrogenation occurs outside the probe. Effect of Probe Position on Gas Composition. Figure 8 shows a definite decrease in acetylene content with increased sampling distance from the arc core. This effect is presumably due to the decomposition of acetylene on the cool cathode walls and the cooling of the gas downstream of the arc. If it is assumed that acetylene composition is a n indicator of temperature, then the highest temperature sampled was 3470 K. at 1'12 inches from the arc and the lowest was 3280' K . at 4l/2 inches from the arc. which is a surprisingly loiv temperature gradient in the holloiv cathode. This low gradient is a result of the anode flame's filling the hollo\z cathode, since the anode flame was observed to issue 2 to 3 inches from the exit hole when the exit flange was removed. Probe position also affects ethylene content, as shown in Figure 9. The increase in ethylene content with distance from the anode face is taken as an indication that the major portion of the ethylene is formed by hydrogenation of acetylene on the cathode walls (and perhaps on the outer probe walls) rather than in the probe itself.
H 2 Flow R o t e l i ters / m m . 14
2 2 2 5
75 75 75 5
_-___I 0 S 0 0
1 2 3 4 5 Probe Tip Position, Inches from Anode Face
0
a 0
I
oc
.-E
Figure 8. Effect of probe position on acetylene content of quenched gas
N
I N
u
5 t
t
UI
0
W
I]
U
0 c
a 0
.-C
Probe Goge Sire
P
rN
Power Input. kw.
ti2 Flow Rote I i ters/min.
Probe Gage Size
I. 4
u
75 11
-
I1
0
75
12 6 75
2 75 2 75 2 75 5. 5
> 0
15
5 IO H2 Feed Rate, Iiters/min.
Figure 10. Effect of hydrogen feed rate and power input on acetylene content of quenched gas
81 0 -
I
2 3 4 5 Probe Position, Inches from Anode Face
Figure 9. Effect of sample probe position on ethylene content of quenched gas 174
0
I&EC PROCESS DESIGN A N D DEVELOPMENT
V
A
w e
0 X
11 14 11 11 19 19 19
Probe Position, Inches from Arc
1 'I8 1 2.
3
3 3 3
Power Input,
Kw. 10 7.5 12.6 12.6 7.5 4.8 4.8
Anode Type Solid Solid Solid Solid Solid Hollow Solid
Effect of Hydrogen Flow Rate and Power Input on Acetylene Content. As shown in Figure 10, decreasing the hydrogen flow rate (decreasing the H*/C ratio), decreasing the
24t
*e
probe distance, and increasing the power input all tend to increase the acetylene content in the quenched gas. The effects are all in agreement with the equilibrium calculations discussed previously.
1
Four runs in which acetylene contents greater than 14% were achieved provide evidence for the existence of C2H (or other) radicals capable of producing acetylene in the quench probe. All runs in Figure 10 had diluent contents below 7 % : so that this effect is insignificant.
Effect of Diluents on Acetylene Content. Figure 11 shows a definite increase in acetylene content with increasing helium content. Moreover, a t the apparent sampling temperature for the conditions in Figure 11, the increase from 18.6 to 237, acetylene with increased helium content is in satisfactory agreement with the calculations shown in Figure 3. This is the second piece of evidence in support of the theory for the existence of a n appreciable CZH concentration in the hot gas mixture. T h e C O formed by reaction of leakage water lvith carbon also acted as a diluent and increased the acetylene content.
i
1 Diluenl
0
20 Vol.
Conversion to Hydrocarbons. The fraction of the carbon vaporized that appeared in the product as hydrocarbons was estimated from a hydrogen balance and the weight loss of t h t anode. The results of these calculations (Figure 12) show a remarkable increase in the fraction of carbon vaporized in the product \vith an increase in hydrogen flow rate.
Power
40 O/O
t o r Feed
60
80
100
Diluent i n Hydrogen Feed
Figure 1 1 , Effect of diluents on acetylene content of quenched gas
At low hydrogen floiv rates, carbon vapor is present in excess and much of it condenses on the cathode walls rather than mixing and reacting Ivith the slow-moving hydrogen stream. This condition represents the highest reaction temperatures and the lowest HZ/C ratios which result in high acetylene content but low conversion of carbon vaporized to hydrocarbons. .4t higher hydrogen flow rates? mixing and reaction of the carbon vapor with the hydrogen feed are more complete. Figure 12 also shows that mixing is enhanced by introducing the hydrogen through a hollow anode. The better mixing and higher hydrogen feed rates result in higher H?/C ratios and lower temperatures in the reaction mixture. T h e net result is higher conversion of vaporized carbon to hydrocarbons but a loiver acetylene content in the gas.
At very high hydrogen flow rates, the hydrogen could actually quench the vaporized carbon to solid rather than react with it. At this point both acetylene content and conversion of vaporized carbon to hydrocarbons would be low.
I
I
-
P o w e r Input - 4 8 kw
19 gage P r o b e 3 i n f r o m Arc H o l l o w Anode
e'
-
Mixing is thought to be especially important in these studies, since the highest hydrogen flow rate (15 liters per minute) corresponds to a Reynolds number in the tube of about 500.
P o w e r Input - 2 5 k w . I I g a g e P r o b e I V 2 in. f r o m A r c Solid A n o d e
0
Composition of Quenched Gas. A mass spectrometric analysis of the sample with the highest acetylene content (18.6 volume %) showed the presence of diacetylene (0.4y0): vinyl acetylene (0.047,): and a trace of benzene. Carbon monoxide and methane were formed whenever water leaked from the probe or anode holder.
-
Power I n p u t - 4 8 kw. 19 g a g e P r o b e 3tn. f r o m A r c Solid Anode
Conclusions 0
The main products of the reaction between carbon vaporized in a high-intensity arc reactor and hydrogen passing through the arc were acetylene, hydrogen? and condensed carbon when the hot reaction mixture was sampled under fast-quench conditions. .4cetylene contents as high as 18.6 volume 70 in the
I 5 Hydrogen
I
I IO F l o w R a t e , l i t e r s / rnin.
15
Figure 12. Effect of hydrogen flow rate on conversion of carbon vapor to hydrocarbons VOL. 1
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JULY 1962
175
quenched gas were obtained without diluent and as high as 23.8 volume % (diluent-free basis) with 63.6% helium diluent. These acetylene concentrations are two to three times higher than any previously reported in the literature for reactions between the elements. Acetylene concentrations in the quenched gas greater than 147, and the increase in acetylene concentration with a n increase in diluent concentration support the proposal of Plooster and Reed (73) that C2H radicals exist in the hot gas mixture in appreciable concentrations. T h e high-intensity arc reactor proved to be far superior to the plasma jet reactor, induction furnace, resistance furnace, or combustion reactor for studying reactions between carbon and hydrogen a t high temperatures.
l i t e r a t u r e Cited
(1) Berthelot, M., Comfit. rend. 54, 640-4 (1862). (2) Bone, LV. A.: Jordan, D. C., J . Chem. Sac. 71, 41 (1897); 79, 1042 (1901). (3) Brewer, L., Searcy. A. \V., ilnn. Rer,. Phys. Chem. 7 , 259 (1956).
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RECEIVED for review October 26, 1961 ACCEPTED March 5, 1962
ROLE OF THE PACKING IN A SCHEIBEL EXTRACTOR J. R. HONEKAMP’ AND L. E. BURKHART Institute Jor Atomic Research and Department oJ Chemical Engineerin,
