\vhich functions by promoting hydrogen dissociation is predicted to increase the demethylation reaction to the degree observed in thr runs \vith heptane. This difference is equivalent t o a change in the reaction temperature of about io' C.
very difficult to obtain in practice. However, the benefits of a somervhat lo\ver reaction temperature resulting from the use of a catalyst might justify the catalyst cost. Nomenclature
Conclusions
Toluene reacts with hydrogen rradily at elevated temperatnrrs. producing benzene and methane as the main products. ' l h r reaction is clean and unaffected by carbon monoxide. thiophene. or pyridine. T h e aromatic nucleus is not hydrogenated a t temperatiires u p to 9.30' C. Carbon and byproduct formation are minor unless the reaction temperature and conversion are high and the amount of rxccss hydroqen is small. l ' h e toluene decomposition and benzene formation reactions are first-order \.iith respect to toluene and half-order Lvith respect to hydrogen. Hydrogen reacts \\-ith toluene by means of a chain mechanism involving hydrogen atoms. 'Phe activation energy calculatrd for the reaction indicates that the hydrogen atom concentration ma>- not reach i t ? equilibrium value. From the runs Tvith heptane i t may be concluded that heptane acts as a free radical catall-st and increases the h>-drogen atom concentration to its equilibrium value. h t the same time side reactions are increased oiving to the reaction of toluene and benzrne Lvith the heptane. ?'he ultimate yield of benzene predicted for thermal demeth>-lationis greater than 9570. Since this yield is high. the use of a catalyst may not be justified. 'I'he increase in selecivity required to compensate a modest catall-st cost might be
rate constant: (moles 'liter) -l'p(sec.) -1 toluene fped rate, ml. min. reaction temperature, OK. reaction pressurr. inches Hg absolute equivalent volume of reactor. liters conversion of toluene or yield of benzene, mole per mole of feed hydrogen-toluene ratio. moles 'mole )-ield of by-products. Ib. 'lb. feed actual gas residence time in reactor, sec. equilibrium constant for dissociation of hydrogen, H, e 2 H literature Cited (1) Rrtts, i V . D.. Popprr. F., Silsby. R. I., J..4pp/. Chrm. 7 , 497-503
(1957). (2) Hougrn. 0. A , . i2'atson. K. M.,.'Chemical Procrss Principlrs." Vol. 3, p. 884. \Vilry. Sew York. 1947. (3) Hydrocarbon Process. Petrol. Refinrr 42, No. 3. 121-4 (1963). (4) I d . Enq. Chrni. 54, 28-32 (February 1962). (5) Matsui. H.. ;\mano. .i..Tokuhisa; H.. Bull. J o j . Petroi. I n i t . 1, 67-72 (March 1959). (6) Rossini. F. D.. et ai.."Selected Values of Physical and Thermodynamic Properties of Hydrocarbons and Related Compounds." p. 747. Carnegie Press, Pittsburgh, 1953. (7) Silsby. K. I.. Sawyer, E. i V . . J.Appl. C h m . 6, 347-56 (1956). (8) Tsuchiya, ..\.. Hashimoto. A , : Tominaga, H.. Masamune, S.. Ritil. Jop. Petrol. Inst. 1 , 7 3 ~ ' ('March 1959). RECEIVED for review August 9. 1963 ACCEPTED November 26. 1963
REACTIONS OF CARBON VAPOR WITH HYDROGEN AND WITH METHANE IN A HIGH INTENSITY ARC R A Y M O N D F. B A D D O U R A N D JEAN
L. B L A N C H E T '
.Massachusrtts Institute 01Technolog), Cambridgf 39, .Mass.
Theoretical calculations show that appreciable acetylene concentrations can b e obtained when carbon vapor reacts with hydrogen and with methane a t temperatures between 3500" and 4500
K.
A reactor was
built to study these systems in a high intensity arc using a consumable graphite anode as the source of carbon vapor, The acetylene output was determined to b e mostly dependent on the quench temperature and the carbon-hydrogen ratio, It appears that high concentrations of acetylene might b e obtained a t competitive power inputs for the carbon-methane system.
EII-c-ryr: arcs have been used recently in a variety of new devices to create high entxrgl- environments, In particular.. plasina jets. lo\< intencity arc?. and high intensit); arcs have bec.n used rxten~ivrlyI O producr remprrature? in the range of abont 3000" to 20.00O0 K. Rccrnrly. a hish intencity arc reactor \va? used (2) to make carbon react \\-it11 hydrogen. .I'he major product obtained \vas acet!-lene. up to 18% a t I-atm. prewu-e. Because of limitation\ in dmign of the original rractor. only the heterogeneous region. \\-here carbon exictr in thr solid phahe. could be annlyzrcl. -1'he present ctudy is aimed a t drtermining the
258
I&EC PROCESS DESIGN AND DEVELOPMENT
composition of a hot gas mixture obtained by reaction of carbon and hl-drogen or carbon and methane in an arc struck between t\vo carbon electrodes. In contrast \vith the previous \\-ark. an attempt has been made to study the homogeneous region, \There carbon exists in the gas phaie only. to determine acetylene yields. and also to help determine rvhich important compounds may be expected in the equilibrium gas at high temperatures ( I ) . Carbon-Hydrogen Reactions
Reliable thermodynamic data for C-H compounds a t temperatures above 1500' K. are almost nonexistent in the literature. To estimate thermodynamic properties in the high temperature range \\.here measurements have not been madr.
t\vo techniques are commonly used. In one, lo\v temperature data are extrapolated EO a higher range. In the other, the data are based on estimates of interatomic distances and vibrational energies of the molecules. T h e first method may give erroneous values because of the uncertainty involved in extrapolating to a region where no d a t a exist, whereas the second is still onlj- approximate because of the assumptions involved ( 7 7 ) . Spectroscopic d a t a taken a t high temperatures are needed in order to provide more reliable values of thermodynamic properties of species containing carbon and hydrogen (5,7 ) . T h e carbon-hydrogen system has been studied theoretically betv-een 200Oo a n d 6 O 0 O o K.. taking into account C , . C?. C3, H. Hs. CH,. CeH2. CH. CH2: and CH3 as the predominant species ( 9 ) . Theoretical ( 3 ) and experimental (2. 72) w.orks, ho\vever, point to the existence of radicals of the general form C,H. I n particular, experimental determinations of acetylene formation in the carbon-hydrogen system Ivere explained by assuming that C n H scrved as precursor to C 2 H z . Other species. such as C3H and C4H. may exist in appreciable concentrations a t high temperatures ( 3 )and affect CZH2 formation in a n important way. Under equilibrium conditions. a reacting mixture of solid carbon. C,. and hydrogen a t 3000' to 4000' K. would probably consist predominantly of C1: Cz: C3. H, He. CzHz, CnH. C3H, C,H, C H . C H y .a n d C&I2. Of the polyatomic forms of carbon, only species containing from one to three atoms of carbon are important a t high temperatures (7). Hydrogen gas dissociates between 2000' and 600~0' K.! \vhile ionization begins around 8000" to 9000' K. T h e inclusion of the C,H radical stems from the \vorks of Plooiiter and Read ( 7 2 ) and Baddour and Iwasyk ( 2 ) . A recent analysis ( 3 )indicates that C3H a n d C4H might be present in concentrations comparable to those of C r H between 3000' and 5000' K.. and that CH and CH2 become significant above 4000' K . T h e diacetylene molecule has been included because recombinations of C,H radicals (7.1) or reaction with CnH2 (70) can give C4Hs. Calculations of the equilibrium composition of the carbonhydrogen system in both the heterogeneous and homogeneous regions have been performed on the IBM 7090 computer a t the M . I . T . Computation Center. taking into account the 12 species listed above. For the heterogeneoums region, the following reaction scheme was adopted:
cs 2
-
c1
c, c2 -+
3 C,
C3
Ki
=
pcl
(1)
Kz
=
pcZ
(2)
K3
=
Pc3
(3)
For the homogeneous region, the following reactions wrre involved : He
+
K5
2H
C . ? H l + CzH
+H
K6
Values of the enthalpy of formation for various components in the ideal state appear in Table I . Values of the free energy functions and of the equilibrium constants for the 16 reactions as calculated by the method of Bauer a n d Duff ( 3 )are tabulated by Blanchet ( 4 ) . I n the heterogeneous region, the sum of the partial pressures must equal the total pressure. I n the homogeneous region, besides this condition, the carbon-hydrogen ratio must remain the same throughout the whole temperature range, thus involving a rearrangement of the various species over this range. T h e solution of the first system of equations results in a quadratic equation with the partial pressure of H atoms to be determined, from which all other component concentrations can be easily derived by manipulating Equations 1 to 9. T h e homogeneous system gives two quadratic equations that must check simultaneously, a situation most efficiently handled by a computer. T h e results of such calculations are presented in Figure I , for a carbon-hydrogen ratio of 1 a t a total pressure of 1 atm. O n this plot, the dashed line represents the sublimation temperature for the particular carbon-hydrogen ratio-i.e., the maximum temperature a t which solid carbon exists and the trmperature a t which the homogeneous region begins. Results of calculations for other carbon-hydrogen ratios are similar to those shown in Figure 1 . For any carbon-hydrogen ratio, those portions of the curve which lie in the heterogeneous region coincide. T h e homogeneous region is characterized by a rearrangement of the different species obtained a t the sublimation point and the relative distribution of the various components depends on the carbon-hydrogen ratio. T h e predominant components in this zone are hydrogen atoms: carbon vapors, a n d radicals CTH, C3H, and C ? H , the radicals reaching a maximum concentration near the sublimation point and having approximately the same order of magnitude. T h e carbon species except C I decrease a t temperatures above 4700' K . Above 6000' K . only H and CL remain in appreciable amount a n d their relative concentration approaches that calculated from the fixed carbon-hydrogen ratio. T h e transition from the heterogeneous to the homogeneous region is smooth a t low C/H2 ratio, but breaks sharply a t
Table I.
Hz
AH',, of Components, Kilocalories per Mole
0 0
H 51 62
54 329
189 0
121 1
CIH 154 0
C1
C>H?
CLH C, C1 116 7 0 0 163 58 C,Hl CH28 111 3 CH 140 4 66
C2 138 0 C H I 38 -15
(6) VOL. 3
NO. 3
JULY
1964
259
higher ratios ( 4 ) . This provides a means of defining the sublimation point for various conditions. Since both regions are calculated independently) draLving the curves for the heterogeneous region and for the homogeneous region from the low and high ends of their range, respectively. should bring the concentration of each component to the same value a t the sublimation point, as was done in Figure 1. Another way to determine this temperature is to approach it from the heterogeneous region, by calculating a t each temperature the C , H P ratio from the gas phase composition. Plotting these values resulted in Figure 2 : which shows that the heterogeneous region can be extended by either providing a larger proportion of carbon or operating a t higher pressures. -4cetylene concentration as calculated from a series of equilibrium diagrams goes through a maximum mole fraction of 0.07 near 3300' K. in the heterogeneous region ( 4 ) . I t decreases steadily in the homogeneous region. These equilibrium values of C ~ Hconcentration P are considerably lower than those obtained experimentally by sampling the hot gas through a water-cooled probe, which was explained in terms of C2H and H combination in the probe to yield additional CyHn (2). This illustrates the importance of considering all important high temperature species when attempting to predict the composition of the quenched gas from a high temperature reactor. Since C,H: C 4 H , C H , and CH2 were not all considered in previous works (2, 9? 72), the maximum C ~ H concentrations P obtainable under optimum quenching conditions were computed from the results of the calculations disrussed above. T h e quench mechanism adopted was the following: All C P H ~ , HP, and C I H z present remain unchanged, C P H recombines with H atoms to yield more C 2 H 2 ;the remaining H not used by CPH forms molecular hydrogen; and C3H, C 4 H , C H , CH2, C1, CP, and C3 go to solid carbon and H2. Under these conditions, a perfectly quenched sample would contain only CZHP and H2, and also a minute amount of C ~ H P .The results of these calculations are presented in Figure 3 for a total pressure of 1 atm. and carbon-hydrogen ratios from 0.5 to 15. T h e curve calculated for the heterogeneous region is common to all cases u p to the specified C/HZ ratio as inferred from the equilibrium diagrams. The maximum acetylene concentration occurs in the homogeneous region for a carbonhydrogen gaseous mixture quenched from about 4000' to 4500' K. '4s the relative amount of carbon is increased. more ace'tylene can be obtained. T h e effect of increasing the C,'Hs ratio from 1 to 5 is more pronounced. however. than from 5 to 10. For instance. for C i H 2 = 100 a t 4500' K.. the CPH? concentration is only SOTo.as compared Lvith the same concentration for C 'H2 = 15 a t 4300' K. Above 500Oo K., acetylene concentration is low for all mixtures of carbon and hydrogen, and operation between 3500' and 4500' IC. would provide maximum acetylene concentrations.
TEMPERATURE,
O K
Equilibrium diagram at 1 atm
Figure 1 .
C/Hz = 1.0
CARBON
TO
HYDROGEN RATIO, MOLES C / M O L E H1
Figure 2. Carbon sublimation temperature as a function of C/H2 ratio at various total pressures
-
__~___
60T-
-
TOTAL PRESSURE = I O A T M .
L
I" N
0
Carbon-Methane Reactions
,\"
Since the reaction of carbon vapor with hydrogen to form acetylene is exothermic. it is interesting to consider a reaction scheme whereby the endothermic heat of methane cracking would be furnished by this exothermic heat of reaction. Consider the following reactions : 2 C,
+ H,
-+
C?Hy(at T)
(17)
2 C H , (298' K.) + 2 CH, (at T)
(18)
+. C2H?+ 3
(19)
2 C H r (7.)
H ? (at 7')
Reaction 17 may be uved to furnish the sensible heat required to bring CH, to rraction tt-mprrature (Rt-action 18) and to 260
I&EC PROCESS DESIGN A N D DEVELOPMEN1
2500
3000
3500
4 0 0 0 4500 5000 TEMPERATURE, " K
Figure 3. Calculated maximum C/H, ratios
C2H2
5500 6000
content for various
Table II.
CHd
1650
2000
2 500
3000
3500
4000
52 55 172.10 -22 06
52.33 171.95 -22 16
52.10 171.64 -22 16
51.38 171.09 -22 04 0 43 5
50.48 170.47 -21 89
49.46 169.87 -21 85 0 67 7
48.46 169.16 -21 85
0
H2
CHr ( s r n ~ .heat)
AHT,', Kilocalories per Mole
1TO0
0
0
21 75
18 73
31 95
carry out the cracking reaction at that temperature (Reaction 19). Below 2000' K.. however, Reaction 19 is equilibrium limited as discussed by Anderson and Case ( 7 ) . The energy required for making one mole of acetylene as a function of temperature \vas calculated taking into account the heat requirement for Reactions 18 and 19 in the range from 1000' to 2000' K. The parameter used was the yield or per cent of methane decomposed that is converted to acetylene. Above 2000' K. all methane can be decoinposed to yield acetylene and hydrogen: but a region determined by the equilibrium limits of Equation 19 is not thermodynamically possible below 2000' K. Reaction 17 can also produce acetylene and the over-all reaction scheme can give a higher acetylene concentration in the product gas than the 25% possible by Reaction 19 only. T h e heats of reaction of the different reactions have been calculated for the temperature range 1500' to 4000' K.: using the data from Rosvini (13) when available and extrapolating to high temperatures by the method of Bauer and Duff ( 3 ) . Table I1 shows the 4HT,' for the various components as obtained by the above procedure, and also the sensible heat (H,' - H,') for methane. The over-all reaction may be written as:
C,
+ C H I -,C2Hz + Hz
(20)
The heat of reaction for this reaction-Le., for the sum of Reactions 1 7 , 18. and 19-has been calculated, taking into account the equilibrium limitation on methane cracking. The results are plotted in Figure 4. From 2000' to 4000' K.: the over-all heat of reaction decreases steadily with increasing temperature. but rises sharply near 1500' K.? where equilibrium limits the react.iion. This graph also shows that an excess of exothermic hea.t at 4000' K. is still present and can be used to make more methane react. Another \Yay of analyzing the results Lvould be to determine the number of moles of carbon vapor necessary to carry out Reactions 18 and 19 by using all the exothermic heat available from Reaction 17. As deduced from Figure 4 for the entire temperature range. it will take fewer moles of carbon vapor reacted than moles of methane to be processed.
140r--120
t-
2000
Figure 4.
I
I
I 3000 TEMPERATURE,OK
4000
Net heat of reaction
1
0 55 5
0
80 1
Figure 5 shows this effect for a net AH,' equal to zero, the bend in the curve at temperatures below 2000' K. being due again to the equilibrium limitation imposed on Reaction 19. Also represented on this plot is the theoretical concentration of C L H as ~ calculated for the ratio of C1 to C H I that gives a null over-all heat of reaction. For the temperature range 2000' to 4000' K., a CrH2 concentration of 40 to 47% could be theoretically obtained, and more is possible at higher temperatures. Although other processes such as the endothermic dissociation of hydrogen
Hz
-+
2H
or the highly exothermic reaction of atomic hydrogen with carbon vapor
2 C1
+2H
+
C2H2
which also produces more acetylene, or the formation of higher carbon species which can also react with hydrogen to form further acetylene, or degradation of methane to solid carbon and hydrogen, were not included in this analysis, the results nonetheless point out that it is possible to obtain appreciable concentrations of acetylene by reaction of carbon vapors with methane, much more than the theoretical 25% possible by Reaction 19 alone ( 7 ) or the 12% obtained for reaction of solid carbon with methane as disclosed in Weir's patent (75). For example, a 50 to 75% CgHz gas may very well be possible from the cracking of methane in a high intensity arc reactor. As a point of economic interest, a quantity defined as the specific energy (6) might be examined. It is a measure of the energy used to produce a given amount of C2H2:
where fi is the coefficient of volume expansion, which can be obtained from analysis of the output gas ( 4 ) , G is the reactant gas flow rate i n liters per minute at 70' F., Pis the power input in kilowatts. u ' is the outlet CnH2 concentration in mole per cent. and k is a conversion factor-e.g., k = 702 for A in kw.-hr. Ib. of C 2 H 2 .
1.0)
I
I
I
I
1
TEMPERATURE ,OK
Figure
5. Carbon requirement for null reaction heat VOL. 3
NO. 3
JULY
1964
261
For Reactions 4: involving solid carbon and hydrogen only, and 1 9 , .,,3,/ = 1 and p,, = 2> respectively. Thus the methane-carbon system has the potential of a considerably lolver specific energy because of the higher maximum value of 6. In order to relate the fraction of carbon in the feed that is found in the output gas as CPH? after reaction a t higher temperatures, a last quantity might be considered. This quantity, termed 0, is defined by
n=
/
801
C/H, in output gas C;"?
in feed
(22)
s SYMBOL GAS ANODE DIAM,IN H2 I /4
20
where C/H? in the feed includes the carbon vaporized from the anode. I t is a measure of the efficiency of carbon utilization for acetylene production in either the carbon-hydrogen or the carbon-methane system, since essentially all the carbon in the quenched gas is present as acetylene.
H2 CH4
0
A
3/8 318
Apparatus and Procedure
T h e reactor for this study was built using a modification of the basic design used by Iwasyk ( 2 . 8), T h e anode holder section and the screw drive mechanism for the anode feed were left unchanged. T h e main reactor shell was made of 4-inch steel pipe and cooled by water flowing through three coils wrapped around the outside walls; a rectangular Lvindow covered with a Vycor glass plate and a dark blue glass plate were placed in the main shell to allow observation of the arc and to permit visual control of the anode feed. T h e cathode holder section was also made of a single pipe \vith a coil Lvrapped around the exterior. A probe holder section was added a t the back of the cathode holder to isolate the probe electrically from the cathode section and thus prevent arcing between the anode and probe. This section also contained the exhaust line from the reactor. Reactant gas could be fed through either the anode holder flange or hollow anodes. Consumable anodes were l l q - and 3/8-inch 0 . d . graphite electrodes; cathodes were made from a 1' 2- to 2-inch 0.d. solid graphite rod through which a 1-inch center hole was bored with threads a t one end in order to accept screlved-on caps. These caps had a n inside diameter of I,'Z or 5,;s inch, depending on which anode size was used. A gap spacing of ' 1 8 inch \vas kept between the outside diameter of the anodes and the inside diameter of the cathode caps. Thus blocked cathodes were easily replaced. Electrical insulation was provided by means of Teflon and rubber gaskets with plastic washers. The probe was inserted through a special fitting containing two rubber O-rings that made a good seal, and it was positioned before each run. The probe could be kept close to the arc reaction zone, the closest distance being 0.5 inch. '4 5-hp. pump provided cooling water to the main reaction shell and cathode holder a t 7 gallons per minute under 200p.s.i. pressure while a n independent 1-hp. pump circulated water a t the rate of 0.3 to 1.25 gallons per minute through the probes under 125 p.s.i. Probes were made of three concentric tubes. The inner one consisted of a stainless steel hypodermic needle with inside diameter varying from 0.043 to 0.135 inch where the hot gases from the arc zone were quenched in a few milliseconds (8). A middle tube, made of brass or stainless steel, divided the water flow. The outside shell was made of copper. A coil of enameled copper wire was wrapped around the reactor over a 1-inch portion a t the reaction zone. Direct current passing through this coil generated a magnetic field a t right angles to the electron path. providing a swirling action for mixing the hot gases. T h e magnetic field strength was about 70 gauss. Power for the arc was furnished by two Lincoln welding generators hooked in series and capable of delivering 30 kw. to the electrodes. A Miller arc starter and stabilizer was used in series with the generators. The procedure consisted in assembling the reactor with proper positioning of the probe, inserting the consumable anode and adjusting the gap between the electrodes by viewing 262
l&EC PROCESS D E S I G N A N D DEVELOPMENT
r 20, I
I
I
I
SYMBOL
GAS H2
FLOW RATE 3 0 L I TT.I/M 3.0 MI N . 1 5 L I TT.//M 1.5 MI N .
CH,
0
U [L
z
0
5
IO
1 15
I
20
25
POWER IN KW.
Figure 7.
Effect of power on carbon vaporization rate Hydrogen runs. '/,-inch anodes Methane runs. 3/s-inch anodes
I - SYMBOL
0
w
PROBE D l A M
0.043 ( 1 9 G A G E ) 0.063 [ 14GAGE)
0
N
, IN.
20
r
N
0
101 2 w
i
A
-
-
3 -I
Figure
I
8.
C2Hz
concentration
vs.
Hz flow rate, liters per minute
0
+
3.0 1.5
'/a-inch anodes
I
power
input
from the opened window. then closing the \vindow and setting thr anode feed rate for the po\ver setting on the generators. \7acuum was then applied to the reactor to remove the air. and €3?or C:H4 Ro\v was started. follo\ved by cooling Lvater f l o ~ v . A small fraction of the feed gas \vas directed through thr probe for purging and preventing the probe from plugging brforr a gas sample of the arc zone was \vithdra\vn. Potver \vas applied to the electrodes and the arc starter and the magnetic field \vere s\\-itched on. T h e anode feeder \vas started. and vacuum lell on a sampling bottle. .Just prior to \vithdra\val of a sample. the purge flow \\;as stopped, the probe liiir evacuated. and the sampling bottlr under vacuum conrircrcd lor about 2 seconds \vith the rraction zone via the quenching Iirobe. .Vter a ixn. the reacl.or \vas purged with helium and the sainplr taken to a vapor c,hromatography unit for analysis. Results for Carbon-Hydrogen System
I he arc characteristics \vcrr drtrrniined from the data of various runs and are sho\i.n in Figure 6. T h e rising voltage-
d vr
r
Figure
9.
C2HS concentration vs. C/H, ratio HB flow rate, liters per minute 0 H :3.0 I .5
+
I" N
501
0
T O T A L PRESSURE = I O A T M . T = 4000" K
w
TV,
3
O*
>
n 20
Ilh/ ON TO HYDROGEN RATIO, MOLE/MOLE
Figure 10. Maximum ClH2 content as a function of C/H2 ratio for various probe inlet temperatures a t 1 .O atm.
+
0
C
a -L
G a s Flow Rate, L./Min.
1.5 3.0 6.0
10.0 16.0
Probe Diam., Inch
0.1 0.063 0.1 0.1 0.1
Probe Distance, Inches
0.5 0.5 2
3 5
rising current pointi. characteriTtic of a high intensity arc. rshibit rather \\id? scattering. ~ l ' h evoltagr-current charactcri\rics depend on the sizes of the anode u.;ed or on the arc length. Inci.easing the anode s i x . hcnce the arc l e n g t h , reduced the burning voltage somr\vhat. but there \vas no detectablr influence of either the frrd gas or the gai floiv rate. It \ v a \ obsrrved during actual runs that the voltage and current varird in oppositr dirrctioris. so t h a t the product C J ~L- X I w a s almost constant---i.e.. the l)o\ver fed to the electrodes \vas constant despite fluctuations of volrage and c'urrent. '['his is sho\vn in Figure 7 , I t was u5ually found that about half the condensed carbon formed hard deposits iiisidr thr cathode cap. reducing the cathode opening to about half the initial diameter. Most of the rrmaindrr \\-as found 011 the cold reactor tvalls. 'l'he probe \vas coated for a length of 3 to 4 inches from the tip and a cmall amount of carbon \vas deposited inside the probe. carbon deposit> werc found in the sampling bottle after sampling. A I shoivn in 1:igur.e 7 , the carbon vaporization rate depends only o n the size of thr gap bettveen the electrodrs, since the upper line is for 8-inch anodes and 5,'a-inchgap and the lo\vrr line for I 4-inch anodrs and I r-inch gap and thr slope is steeper for 8-inch than for I, 4-inch anodes. Larger flow rates reduced somewhat the carbon vaporization rate. .4t flow r a t e of 5 to 6 liters per minute of either feed gas for 3;a-inch anodes, the carbon vaporization rate line cuts the po\ver axis a t 4 instead of 2 . 5 k\v., but runs parallel to the ;a-inch anode cize line. T h e effect. of larger flo\v rates is to arch the arc flame and increase its length. If it is assumed that the plasma is contained in the same volume, then the area is larger and heat losseb due to radiation are larger, thus requiring a larger po\ver for starting vaporization of the carbon anodes. 'l'he slope ratio was found to be approximately the same as the area ratio of the arc. T h e concentrations of acetylene obtained in the quenched samples are shown in Figure 8 as a function of the power fed to the arc. Data for the smaller. 19-gage probe was difficult to duplicate. T h e hypodermic tubing used plugged in each r u n and thus good samples (25 cc. or more) were rather the exception. T h e 14-gage probe results are more reliable. since plugging did not occur and any size of sample could he collected. 'I'his probe also gave high acetylene concentrations a4 compared \vith the other t\vo probes; the probe position \vas 2 inch from the anode tip in each run. For the smaller probe. this distance \vas varied from 1 to l , ' 2 inch. Data taken \vith the 0.100-inch probe a t higher flo\v rates indicate that lines parallel to that of Figure 8 for this probe size are obtained, but lo\ver C:?Hr concentrations result from the same po\ver input. The output concentration of C ? H 2 is thus directly proportional to the po\ver, P. and inversely proportional to the gas flow rate, G . or to P G. Since the carbon vaimrizarion rate is directly related to the poxver. P G i h a measure of the tio. 'l'his is 5hou.n in Figure 9 \\.here thr above rcrults output are plotted against the C: I{? ratio. 'I'hr corrclations for both larger probes secm to presrnt a dirrct relationjhip bet\veen the t\vo quailtitie\: the 19-gage probe result\ are. ho\vevcr. a little rrratic. At thih stage i n the course of the precent stildy. the c o m p u t c r results were analyzed more carefull)- in a n cirorr 1 0 tIcrrrn1ine a way of comparing t h e o r y \villi csprritlicnt~. fieferring back to Figure i. a c r o h b plot \ \ - a i Inadc \vir11 rhr .> “Reactions of Carbon I’apor \vith Hydrogcn and \vith Methane in a High Intensity Arc,” Sc.1). thesis in chemical engineering, Mass. Inst. Tcchnoiogy, Cambridge, Mass., 1963. (5) Chupka, L \ . A , , Meschi. D. J., Berko\\itz, J., ’.Kractions of Graphite with €Iz. Heat of Formation of the .Methylme
Properties of Gases. Liquids and Solids,’’ hSMC Symposium on Thermal Properties. McGraLv-Hill. New York, 1959, (10) I,eroux, P. J., Mathieu. P. M., Chern. E I ~ A-ogr-. . 57 ( l l ) , 54 (1961). (1 1) Maryno\vski. C:. LV., Phillips, K. C.. Philli s, J. K., Hiester, N. K.. IND.ESG. CmW. FUNDAMENTALS 1, 52 P(1962). (12) Plooster. M. N.. Reed, T. B.. J . Chem. Phys. 31, 66 (1959). (13) Kossini. F. D.. Tables of Thermochemical Data. Natl. Bur. Std.. Circ. 500 (February 1. 1952). (14) Slvsh. I t . S.. Kinnev. C. K.. J . Phvs. Chpm. 65. 1044 11961). (l5j \ceir. H . M., U . S. Patent 2,731,410 (Jan. 17,’1956) RECEIVED for review September 3, 1963 ACCEPTED November 19. 1963 49th Meeting, American Institute of Chemical Engineers. New Orleans. La.: March 1963.
PRODUCTION OF URANIUM HEXAFLUORIDE IN A FLUIDIZED BED REACTOR BY T H E REACTION OF URANIUM TETRAFLUORIDE WITH OXYGEN C H A R L E S D . S C O T T , JOSEPH B . A D A M S , ’ A N D J A M E S C. BRESEE Onk Ridge .\htioual Laborator>, Ool; Ridge, Tenn.
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The production o f UFS b y the reaction 2UF1 02 = UFs 4- U O Z F ~was investigated to determine the engineering feasibility after initial laboratory studies indicated small-scale workability. The process can b e used for the production of UFSwithout the use of elemental fluorine. Engineering feasibility was demonstrated in a continuous, 4-inch diameter fluidized-bed reactor in which either d r y air or oxygen was used as the oxidizing agent-fluidizing gas with refined UF, feed. Greater than 90% o f the theoretical amount o f UF6 formed was collected in cold traps, and the remainder was accounted for b y side reactions. To demonstrate the feasibility of continuous operation, the fluidized-bed reactor was operated in one test for 56.8 hours, and 33.8 kg. of UFSwere collected in the system cold traps. Methods were derived for determining reaction rates in the fluid bed and the resulting first-order dependence of the reaction rate showed good agreement with earlier laboratory thermogravimetric studies.
conventional method for producing UF6 from UF, is F, = UF,; the use of elemental fluorine, UF, ho\vrver, the reaction of uranium tetrafluoride with dry oxygen O? = L-F6 UO?F,. represents a method for or air. 2UF, large-icale production of uranium hexafluoride from uranium tetrafluoride Lvithout use of elemental fluorine (7, 7, 8). T h e bayic cheniistq of this reaction has been studicd by several \\orkers (o‘. 9,/O).and the kinetics of the reaction as determined by laboratory thermogravimetric methods has been reported ( I ) . ‘l‘he engineering development of a 4-inch diameter fluidized bed rractor for producing YF6 from UF, by use of oxy’gen is presented her? (72). HE
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’ Prcsent
addrcss. O a k Ridge Gaseous Diffusion Plant, Union C:ar.t)idr ( h r p . . Nuclrar Division. Oak Ridge, Tenn. 266
l&EC P R O C E S S DESIGN AND DEVELOPMENT
Process Description
A complete process for producing UFB from UF, by the use of oxygen must include provisions for both the primary
oxidation reaction 2LF4
+ 0,
UOZFZ
=
+ UF6
(1)
and a method of recycling the solid uranyl fluoride by-product. A possible method for the recycle is by- reducing the L-O?E‘S to UO?with hydrogen
COnFr
+ H?
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LO:,
+ 2HF
(2)
and hydrofluorinating the resulting LO, to UFI
UOr
+ 4HF
-+
UF,
+ 2HZ0
(1)
