T H E PRODUCTION OF CYANOGEN FROM T H E ELEMENTS, USING A PLASMA JET Graphite and Pyrolitic Graphite Successfully Used as Electrode Materials HANS W. LEUTNER Research Institute of Temple University, Philadelphia 44, Pa.
The production of cyanogen under practicable conditions for limited industrial application with yields up to 15%, based on the carbon input, from a consumable carbon cathode with nitrogen and argon as plasma gases, is described. A systematic study of the use of ordinary and pyrolitic graphite as electrode materials is presented.
HE APPARATUS used for the production of cyanogen from Tthe elements was developed from that described earlier (7) and does not differ essentially from it. I n order to have nonconsumable graphite-anode inserts, the water cooling of the copper anode was made more effective and the thickness of the inserts was reduced to '/I6 inch. With these changes, even ordinary graphite inserts were nonconsumable. T h e '/,-inch diameter 2% thoriated tungsten cathodes, used formerly for the production of acetylene, were replaced by ' / c inch diameter graphite cathodes supported by a tight-fitting, water-cooled copper holder. Since the consumption of the cathode was relatively rapid (see Table I ) , a device was used to feed the cathode continuously toward the anode annulus, so that the electrode distance-and in consequence the electrical characteristics-was kept constant. Data on the ranges of these investigations with the apparatus described are given below.
Electrode materials Plasma gases Gas flows Power input Carbon consumption
Ordinary and pyrolitic graphite Argon, nitrogen, and mixtures of argon with hydrogen From 3 to 30 liters per minute From 5 to 14 k w . From 0.25 to 2.25 g.p.m.
Before the final conditions were chosen for the ( C S ) Z synthesis, various forms of graphite \rere tested as both anode and cathode materials. Ordinary and Pyrolitic Graphite as Electrode Materials
Ordinary graphite is used here to mean the natural or commercial graphite bvith random distribution of the graphite crystals. Thermal and electrical properties are the same in all directions. (Ordinary graphite is isotropic.) T h e graphite rods used \vere from the National Carbon Co., "National"Special Graphite Electrodes. Pyrolitic graphite, available only recently, may be considered to be composed of large single crystals of graphite which are arranged in planes forming structures like benzene rings, with carbon-to-carbon distances of 1.42 A. in the crystallographic a-direction. This distance coi responds to the C-C-distance in 166
I & E C PROCESS D E S I G N A N D D E V E L O P M E N T
aromatic compounds and, as there, the coupling is homopolar. Since for the plane-carbon arrangements in graphite only three electrons of the carbon atom are used, the fourth electron belongs to the whole plane (as in benzene to the whole ring) and can move easily in this plane (a-direction = high conductivity direction). I n the direction perpendicular to the plane, the crystallographic c-direction, the carbon-to-carbon distance (distance from layer to layer) is much larger (3.35 .4.) and, therefore, the coupling much weaker (c-direction = lowconductivity direction), For pure single crystals of graphite, the electrical conductivity is 10,000 times larger in the adirection than in the c-direction. (Pyrolitic graphite is anisotropic.) T h e pyrolitic graphite used in these experiments was, of course? not a perfect single crystal; nevertheless, the thermal and electrical properties are completely different in the two crystallographic directions and can he considered as a large imperfect single crystal of graphite. I t showed, for example, a n electrical conductivity 115 times larger in the a-direction than in the c-direction, as was found by measuring the resistivity by commonly known methods. .4s cathode material, ordinary and pyrolitic graphite were used in the form of rods (1/'4-inch diameter). (Anode: copper with a thin ordinary graphite insert.) Ordinary graphite \vas vaporized fairly fast using as plasma gas, pure nitrogen, argon with nitrogen, and argon with hydrogen. Using argon, the carbon consumption is low; it increases substantially upon using nitrogen and mixtures of argon Lrith hydrogen. T h e vaporization rates ranged from 0.05 to 2.0 grams of carbon per minute (under extreme conditions. u p to 3.0 grams per minute). As \vas to he expected, the graphite consumption increased xvith the total poiver input and decreased with increasing gas flows. T h e observed electrical characteristics shoxred a voltage increase of 50 to 7 0 7 , and a n amperage drop of the same magnitude using the same gas flows, po\ver input, and electrode distance, as compared with metal electrodes (2y0 thoriated tungsten cs. copper). Table I gives detailed data on the operating conditions and the results of a series of experiments. As Table I shoirs, using pyrolitic graphite as cathode, a stable plasma jet could he maintained with almost no cathode
consumption? even with argon-hydrogen mixtures as plasma gases. T h e crystallographic a-direction ( =high conductivity direction) was parallel to the axis of the rod and the c-direction perpendicular to the axis. Summarizing, the results of the experiments show that ordinary graphite can be used as a consumable cathode to supply the carbon for compounds to be prepared in the plasma jet: such as carbides, hydrogen cyanide, cyanogen, and eventually organic compounds. I n contrast to ordinary graphite, pyrolitic graphite is practically nonconsumable, since the heat is easily iconducted away in the longitudinal direction (a- or high-conductivity direction) of the rod. Pyrolitic graphite cathodes might be used for reactions where small contaminations of the metal cathode material are undesired, since small arnounts of carbon contaminations will react with the most usual plasma gases to form gaseous compounds: C O ? . (CS)z: CzH2, etc. I n order to find a satisfactory anode material, ordinary and pyrolitic graphite inserts were tested. All the experiments mentioned above were carried out by using a copper anode Ivith a thin ( l 'Ic-inch wall thickness) ordinary graphite insert Ivhich was practically nonconsumable for short periods of running time (2 to 3 minutes). However, in runs over 5 minutes in duration, the graphite inserts vaporized slowly. Pyrolitic graphite inserts with the graphite planes parallel to the copper anode [(c- or low-conductivity direction facing the copper chamber) offered a very high resistance-an arc could hardly be formed and when formed, it was unstable. Because of the impossibility of removing the heat from the
Table 1.
Carbon Consumption in Dependence of Gas Flow and Power Innut Ca,bon Consumed, M g ./ M 1 n .
Gas
USINGORDIXARY GRAPHITE 8 9 470 18 0 8 5 11 9 455 17 5 8 0 17 9 445 19 5 8 7 23 9 435 19 5 8 5 27 4 435 19 5 8 5 17 9 400 17 5 7 0 17 9 445 19 5 8 7 17 9 470 21 0 9 9 17.9 490 24.5 12.0 3.15 280 41.0 11.5 5.1 300 41 5 12 4 7 0 310 42 0 13 0 8 55 300 41 0 12 3 10 8 290 41 0 11 9 14 15 280 43 0 12 0 8 55 220 40 5 8 9 8.55 300 41.0 12.3 7.00 305 43.5 13.2 20.3 9.15 6,5 (3:l) 325 28.0
954 0
Ar Hn
20.3 6,5 (3:l)
650 0
Ar Nz Ar
23.9 5,0 (5:l)
120
45.0
5.40
3 0
23.9 5,0(5:1)
310
37.0
11.47
27 0
'::;(6:1)
150
50
7.5
11 0
'i:; ( 6 : l )
155
50
7.55
10 0
.4r
N 2
.Ai' H2
325
27.25
8.9
342 360 292 154 104 43 292 388 869 938 728 695 568 404 288 395 568 974
0 4 4 0
4 6 4 6 4 4 8 2 4
0
4 2 4 0
U S I N G PYROLITIC G R A P H I T E
Nz Ar Hz Ar Hz
insert surface, the carbon vaporized very rapidly, and a very hot zone developed around the cathode which melted the cathode instantaneously (270 thoriated tungsten was used as cathode). Inserts with the graphite layers perpendicular to the copper anode (a- or high-conductivity direction facing the copper chamber) conduct electricity better than ordinary graphite. Stable plasma jets were obtained. Since the wall thickness of the pyrolitic graphite inserts was only '/le-inch, the difference between ordinary and pyrolitic graphite was not marked; however, the service time of these inserts was twice as long.
The Preparation of Cyanogen
T h e preparation of cyanogen according to the endothermic reaction: 2C
+ N 2 ------+
(CN)2 - 71 kcal.
which was mentioned earlier by Stokes and Knipe (21, was investigated by reacting the carbon vaporized from an ordinary graphite cathode with a nitrogen jet-or an argon jet Lvith nitrogen fed into the 'Ylame" of the jet, Both methods gave the same results-conversions up to l57C based on the carbon input-even when the electrical characteristics and the carbonnitrogen ratios were quite different. T h e unconverted carbon (80%) was collected as very fine soot. I n the soot, no paracyangoen was present as the negative result of the Prussian Blue Reaction proved after intensive treatment with concentrated potassium hydroxide solution. To ascertain if additional quenching affects the results, a water-cooled copper funnel and other quenching devices were introduced through the bottom of the tubing to quench the gases coming out of the jet. Surprisingly, the fast quenching had a not yet explainable negative effect. on the yields, reducing them to one half of the yields obtained without cooling. I t might be due to a too short reaction time or to a catalytic decomposition of cyanogen when contacting the copper of the cooling funnel. T h e reaction time in the plasma flame was calculated to be in the range of 5 to 50 milliseconds, depending on the gas flow rate. T h e reaction temperature was not measured, but was certainly substantially higher than 4000 O C.. since all the carbon was definitely vaporized. Besides cyanogen and soot, only paracyanogen could be observed qualitatively, especially when relatively large quantities of carbon were consumed. Table I1 gives the typical data on the conditions and results of some of the experiments. T h e apparatus used with argon as plasma gas consisted of the electrode arrangement mounted on top of a water-cooled, gas-feeding device which had a round 1,/16-inchwall thickness ordinary graphite insert with eight symmetrically distributed '/s?-inch holes to effect a n even gas distribution to the flame. T h e gas-feeding annulus was sitting on a wide (3-inch diameter and 2-foot length) copper tube which was cooled by circulating water over the walls. T h e lower part of the tubing was packed with glass-wool to hold back the unconverted carbon. \$'hen using nitrogen as plasma gas, the gas-feeding device was omitted. T h e cooled gases were passed through a wide glass column packed with Raschig-rings countercurrently to a 0.5N potassium hydroxide solution. T h e cyanogen formed was hydrolyzed by the potassium-hydroxide : 2KOH
+ (CN)Z ------+
KCN
+ KOCN + HSO
and the solution obtained \vas cdlected and analyzed. ' I h e analysis of the cyanide formed was easily carried out graviVOL. 1
NO. 3 J U L Y 1 9 6 2
167
7.0 7.0 7.0
Operaling Conditions and Cyanogen Yields Obtained 2V‘itrogenCarbon Cyanogen Electrical Characteristics Consumed, Obtained, Carbon -___~__-Volt -1mp . Kw . .Mg./.Win. ‘Wg. /.Win. Ralios NITROGEN AS PLASMA GASWITHOUT COOLING 21.9:l 342 91.5 46.0 240 11 . o 17 : 1 440 130.0 44.0 270 11.9 13.7:l 492 147.6 41 . O 300 12.3
7.0 7.0
42.5 43.0
Table 11.
G a s Flows,
L. /‘Win.
11.9 11.9 17.9
235 280
NITROGEN A S PLASMA GASWITH COOLIKG 15.8:l 474 10.0 12.8:l 542 12.0
ARGON A S PLASMA GASW I T H O U T COOLING.FEEDING RATEFOR I IT ROC EN : 2.0 LITERS/.MIN. 23.5 41 0 9 6 24.0 430 10 3 24.0 400 9 6 A R G O N A S P L A S M A G A S LVITH C O O L I K G .
a
74.5 92.7
22.8 405 17.9 24.0 400 17.9 Feeding rate for nitrogen, 3.15 I./min.
9.25 9.6
FEEDlNC RATEFOR NITROGEN: 2.0 LITERS/MIN. 2.12:l 1007 168 1.4 :1 1509 215
metrically by precipitation as silver cyanide or better, by using the Liebig method of titrating the potassium cyanide with 0.lA’ silver nitrate solution.
Conversions Based on Carbon Input, %
12.25 13.55 13.9 7.3 7.1 14.45 13.8a 14.95 7.7 6.7
of hydrocyanic acid from the elements was accomplished with good yields. Further work is being done on this subject. Acknowledgment
Discussion
T h e economy of the process described depends almost solely on the power consumption. Since cyanogen is a much desired but now rather expensive chemical, the production of cyanogen in a plasma jet may be, even under the low-scale production reported, of industrial interest. (Electrical power cost per pound of cyanogen is approximately $1.40--electrical energy taken a t 6-mills per kw.-hr.) Other reactions were carried out in a plasma jet, using graphite cathodes as the carbon source-e.g., the preparation of acetylene from the elements. A yield u p to 46% of acetylene, based on the carbon input, \vas obtained in contrast to the maximum yield of 33% from hydrogen and carbon in a n argon plasma jet, described earlier ( 7 ) . Also, the preparation
168
I&EC PROCESS DESIGN A N D DEVELOPMENT
T h e author thanks A. V. Grosse and C. S. Stokes for their valuable discussions and theoretical as well as practical assistance in carrying out this work; also R. A. Selson from General Electric Co., Philadelphia, Pa.. for the generosity in preparing and supplying pyrolitic graphite in the form of plates and rods. literature Cited (1) Leutner, H. IV..Stokes,C. S., IND.EYG.CHEM. 53, 341 (1961). ( 2 ) Stokes, C. S.. Knipe, \V. \V., Zbid., 52, 287 (1960).
RECEIVED for review August 28, 1961 ACCEPTED December 11, 1961 This work was financed partially by a contract from the Office of Naval Research, Contract No. 3085 (02), Task No. NR052-429, and partially by a fellowship granted by the NATO Organization to the author.
