PRODUCTION OF HYDROGEN CYANIDE USING A PLASMA JET HANS W.
LEUTNER
Research Institute, Temple University, Philadelphia 44,Pa.
Four methods of using a DC plasma jet io obtain HCN from the elements and from hydrogen-, nitrogen-, and carbon-containing compounds are described. Better than carbon input. Acetylene appears to be the main by-product.
NCOURAGED
by the production of acetylene ( 3 ) from the
E elements and from methane, and of cyanogen (2,5)using a
plasma jet as the heat source lvith either a n inert o r a reactive plasma gas, the author investigated the production of another simple endothermic compound, hydrogen cyanide (HCN). Hydrogen cyanide is likely to be formed from the elements or from simple carbon-, hydrogen-, and nitrogen-containing compounds, since the heat of formation of gaseous hydrogen cyanide from the elements is -30.1 kcal. per mole, similar to !he heat of formation of acetylene (- 54.3 kcal. per mole) and since any cyanogen, (CX)2, formed intermediately, should react in the presence of hydrogen a t the high temperatures of plasma jet to form hydrogen cyanide.
5070 conversion
is achieved, based on the
16',/~~-inch holes drilled a t a n angle of 30" to the flame axis, with the gases reaching the plasma flame outside the feeding device approximately 3/4 inch below the anode. The reaction products were cooled by a water-cooled brass tubing 2 feet long and 2l/2 inches in diameter attached to the gas feeding ring, provided with four observation windows. Through the brass bottom plate a water-cooled brass funnel (I1/2-inch O.D., l/Z-inch I.D., 6-inch length) was introduced for several experiments a t a distance of 1 inch from the feeding ring, allowing a rapid quenching of the products. Figure 2 shows the plasma jet with the feeding and quenching devices used.
Description of Apparatus
The apparatus used was described earlier (2, 3). Figure 1 shows the design of the electrode assembly used in most of the experiments The cathode consisted of a ','s-inch 2% thoriated tungsten sod strongly water-cooled o n its upper end. The plasma gas (argon o r nitrogen) was fed through a n annulus around the ,cathode with six equally spaced holes. T h e anode consisted of a water-cooled removable copper insert pressed into a stainless-steel holder using O-ring seals. LVhen nitrogen was used as plasma gas, it was necessary to place a graphite inseri: '/I6 inch thick ('/le inch in diameter) in the copper anode to help distribute the arc discharge over the whole anode surface, preventing the arc from striking o n a small area. TYhen the noble gases were used, this precaution .\vas not necessary. The electrodes were separated by a nylon case which served a s insulator and contained the cathode assembly. When elementary carbon was used as one of the starting materials, the nonconsumable 2yGthoriated tungsten cathode .\vas replaced by a corisumable graphite electrode '/4 inch in diameter. (National special graphite electrodes from Xational Carbon lvere used.) The graphite rod serving as the cathode \vas held in a tight-fining water-cooled copper liner and was pushed mechanically toward the anode, according to the vaporization rate of the graphite (0.5 to 2.0 grams per minute). The anode was essentially the same for the graphite cathode assembly? except that the diameter was opened to 5/,3 inch because of the thicker graphite cathode. The reactive gases, with the exception of nitrogen when used as plasma gas, were introduced into the plasma flame through t\vo types of water-cooled devices, circular feeders attached directly to The plasma generator: (1) with eight equally distributed '/sl-inch holes in a removable graphite o r stainless steel insert introducing the gas perpendicularly into the flame about inch helow the anode of the jet; and (2) with
Figure 1.
Design of electrode assembly
1. 2. 3.
4. 5. 6.
7. 8.
9. 10. 11. 12.
Cathode assembly Cathode top plate Gas distributor plate Cathode bus bar Cathode bus bar stud Gas entry extension nipple Gas tube Anode assembly Anode assembly cover Copper anode nozzle Nylon insulator Cathode
VOL. 2
NO. 4
OCTOBER
1963
315
CATHODE
coo
-m HOOD
QUENCHING CHAMBER
GLASS WOOL
,
II jI 1-1
IN WATER OUT
WATER
GASOMETER
~
LIQUID COLLECTOR
Methods of Analysis
The products obtained were analyzed by chemical methods. The effluent gases from the cooling chamber were led through a glass tube packed with glass wool to retain the finely divided soot formed during the reaction, and then passed through a glass column 3 inches in diameter and 3 feet in length, packed with Raschig rings, countercurrently to a flowing 0.5-1- potassium hydroxide solution. The compounds dissolved in the solution were collected and analyzed. The undissolved gases were collected over water for further analysis. HCN Determination. H C N reacted with the basic (KOH) solution according to the equation KOH HCN .-t K C K HzO The cyanide formed was determined by the Liebig-Dtnig6s ( 7 ) method of titrating the potassium cyanide with 0.1,V silver nitrate solution. C2Hz Determination. Acetylene was determined by gas analysis and by titration with a 0.1 N sodium thiosulfate solution of the previously oxidized copper acetylide obtained by precipitation from a basic cuprous acetylide solution (6). Gas Analysis. T h e collected gases were analyzed in an Orsat apparatus with th: commonly used reagents. Hydrogen was determined by passing the gas over copper oxide a t 650 O C. ; the hydrocarbon values were obtained by measuring the C O z absorption after combustion. Carbon dioxide, oxygen, acetylene, hydrogen, and hydrocarbons were determined for all the listed experiments.
+
+
Preparation of Hydrogen Cyanide
Four reactions were carried out in a plasma jet by using compounds of carbon, hydrogen, and nitrogen in different ratios as starting materials, in order to obtain hydrogen cyanide. From t h e Elements. According to the reaction 2C Hz NZ 2 H C N - 60.2 kcal.
+ +
-
hydrogen was fed through the gas feeding ring into the flame of the nitrogen plasma, using a consumable graphite cathode as the carbon source. Hydrogen and nitrogen were both used in large excess. Table I shows the experimental data and the results obtained in these experiments. Over 50% conversion into HCN, based o n the carbon input, was obtained. The only significant by-product of the reaction was acetylene. The analysis of the collected gases showed that other hydrocarbons were formed, but in yields of less than 2% based o n the carbon input. T h e remaining carbon input 316
Figure 2. Plasma jet with feeding and quenching devices
=@
RASCHIG RINGS
I & E C PROCESS D E S I G N A N D DEVELOPMENT
was collected as finely divided soot in the cooling chamber. Xeither ammonia (SHa) nor hydrazine (NzH4) as a possible reaction product of the excess hydrogen and nitrogen was detected. A limiting factor of the method described is the relatively small vaporization rate of the graphite cathode (approximately 1 gram per minute). When quenching was not so pronounced (omitting the water-cooled funnel below the plasma flame), H C N yields were higher. From Carbon a n d Ammonia. Ammonia instead of hydrogen was fed through the gas feeding ring into the flame of the nitrogen plasma gas and the vaporization rate of the graphite cathode was forced (1.0 to 1.5 grams per minute) T h e results were similar to those obtained with hydrogen. Ammonia was decomposed quantitatively into nitrogen and hydrogen while passing through a plasma flame. The hydrogen determined by gas analysis was in agreement with the calculated values for complete decomposition of ammonia. Table I1 shows the experimental conditions and results. I n accordance with the results obtained from the elements, the yield increased o n omitting the cooling funnel. The higher carbon input did not increase the yield, contrary to expectations. The amount of product formed per time unit remained the same. From Methane a n d Nitrogen. T h e carbon vaporization rate in the preceding experiments was low. O n feeding methane into the plasma flame, decomposition into carbon and hydrogen was expected. This would increase the carbon supply and provide the hydrogen necessary for the H C N formation. Both graphite and nonconsumable 2% thoriated tungsten electrodes were used. Methane was added through the gas feeding ring, into the pure nitrogen plasma flame. As the gas
Hydrogen Cyanide from Elements Operating power, HCN Formationa C2H2 Formation" Starting Ratio C:N C:H Kw. % G./min. % G . / Z 1.0 13.9 0.18 13.3 43.3 1:7.8 1:9.6 1.0 12.9 0.1Zb 1 2 . 8 51.1 1:4.6 1:9.2 Product formation based on carbon input. * Without using cooling funnel for fast quenching of products formed. Table 1.
Table II.
Hydrogen Cyanide from Carbon and Ammonia Optzrating power, E V Formationa C2H2 Formationa Starting Ratio C:N C:HKw. % G./min. % G./m=.1:2.8 1:7.8 13.3 33.8 1.0 KO values 1:lO 1:6 13.6 37.3 1.0 6.9 0.1 1:9 1:6.7 12.9 39.0 1.0 17.8 0.2* Product formation based on carbon input. Without using cooling urine1for fast quenching ofproducts formed.
analysis of the collected gases showed, 92 to 95% of the methane input was decomposed into the elements. I n various cases, after 2 to 3 minutes of reaction, the anode and/or the gas feeding ring was blocked by heavy soot formation. By using the circular gas feeder, with a 30' angle to the plasma flame axis, this difficulty was overcome and large excesses of methane could be used. Table 111 gives the experimental data a n d the results obtained in these experiments. All these reactions were carried out without a cooling funnel for fast quenching of the products formed. A s shown in Table 111, higher H C N yields a r e obtained by using a large excess of' nitrogen. With a pyrolytic graphite cathode and nitrogen in the C : S proportion of 1 to 7.7, 91.3y0 of the total carbon input was converted into H C N and C2H2. T h e amount of product:; formed is relatively low compa.red with 1.9 grams per minute of H C N and 1.5 grams per minute of C2H2 when a stoichiometric C:N ratio, high gas flows, and a 2 7 , thoriated tungsten cathode are used. No hydrocarbons besides CzH2 were present, and cyanogen was found in trace amounts. The experiments listed 2.re also important from the viewpoint of nitrogen fixation. I n some experiments more than 20% of the nitrogen input was converted into hydrogen cyanide. However, the main product from methane and nitrogen was acetylene. Compared to the nitrogen fixation rates for producing the oxides of r.itrogen from air, yields of 2 mole were reported by Phillips and Ferguson (4); similar values were repeatedly obtained a t .:his institute by introducing oxygen into the flame of a nitrogen plasma jet. From Ammonia a n d Methane. Stoichiometric mixtures of ammonia and methane were fed into the flame of either a n argon or a nitrogen plasma jet. Since the previously described experiments showed that CH, is almost quantitatively decomposed into its e1emeni.s in the plasma flame, methane was chosen as carbon and hydrogen source. Ammonia was used as nitrogen source instead of pure nitrogen in order to avoid hear losses in disassociating the strong nitrogen molecule into atomic nitrogen ; hydrogen was therefore used in large excess (from NH, and CHI). Essentially the same results should be
Table 111.
.liethane Inpui, L ./ M i n .
__ Starting Ratio
c..N
2.0
1 :4
4.0
1:3.1 1:-7.7
2.0
C: H 1:2.5 1:2.9
1:3.6
obtained by feeding the nitrogen-methane mixture instead of a n ammonia-methane mixture into the plasma flame. Higher electrical power would be necessary. All the experiments listed were carried out by using the circular gas feeding device with gas input perpendicular to the flame axis. As Table I V shows, the conversion to hydrocyanic acid and acetylene based on the carbon input (from methane) ranged between 60 and 75%, when argon o r nitrogen was used as plasma gas. By using nitrogen as pure nitrogen plasma or adding small amounts to a n argon plasma (N2 in excess), H C N was formed in yields u p to 50% based o n the carbon input (from methane). \\Tith argon as plasma gas, CzH2 formation has the preference. \Yith the reaction conditions and the apparatus described, fast quenching with the cooling funnel introduced 11/2 inches underneath the anode end favored the C2Hz formation when argon was used as plasma gas. With the same quenching devices, higher power input through the argon plasma jet also favored the formation of acetylene (because of the higher temperature), in accordance with the theory. Highest H C N conversions were obtained in a nitrogen plasma jet with gas flows between 5 and 7 liters per minute. \Vhen the gas flow rates were lowered, causing higher plasma jet temperatures, acetylene formation was again favored. The residence time of the particles in the plasma was calculated to range between 1 and 20 milliseconds, depending o n the gas flow rates. Besides HCN and CQH?, small quantities of cyanogen (CN)z were formed; maximum amount of (CS)2 found was 0.8%, based o n the carbon input. No paracyanogen was present in the soot (average yield 20% of carbon input) as the negative result of the Prussian Blue reaction proved after intensive treatment with concentrated potassium hydroxide solution. The gases collected contained, besides C2H2,between 2 and 5% hydrocarbons, based o n the carbon input. I t is assumed that unreacted methane was collected and that no other hydrocarbons were formed. Discussion of Results
The formation of hydrogen cyanide or any compound, such as acetylene (3) and cyanogen ( Z ) , using a plasma jet as the high temperature source, may be considered as occurring in two principal steps: decomposition of the molecules of the reactive plasma gas or the molecules fed into the plasma flame, into atoms or activated atoms; and freezing out by using fast quenching methods of the chemical equilibrium obtained a t these high plasma temperatures, which is far different from the equilibria a t ordinary temperatures. The four methods described yield acetylene as by-product. T h e higher yield of either H C N o r C2Hz is dependent upon the
Hydrogen Cyanide from Methane and Nitrogen
Operating HCN Formationa Power, Kw . % G./min. Graphite Cathodes 31.5 1.5 12.7 21.1 1.4 12.2 4 5 . 7 1 .2 12.5
CzH2 Formationa % G./min. 0.9 1.3 0.6
40.3 39.4 45.6
27, Thoriated Tungsten Cathodes 3.25 l:Y!.5 1:3.17 11.5 23.2 1 .o 62.3 8.Olc 1: 1:4 12.5 19.5 1.9 31.75 Pyrolytic graphite was used as cathode material. a Product formation based on total (cathode and methane) carbon input. used.
NO. 4
7.17 6.82 5 . 8b
21.9 20.3 Circular, 30' angle-gas feeder
1.4 1.5
I!
VOL. 2
Conversion Based on Nitrogen, 7 0
6
OCTOBER 1 9 6 3
317
~~
Table IV.
Hydrogen Cyanide from Ammonia and Methane
Stoich.
14.35 15.75 11.9 11.9
11.9 11.9”
1:l 1:l 1:l 1:O.V
1:0.97 1:1.56
1:8 1:7.1 5.0 1:4.3 1:4.7 4.15 1:2.6 1:6.8 1.7 1.lmin. nitrogen added to plasma gas. 7.0
a
1:7.25 1:6.7 1:7.0 1:6 8 1:6.8 1:6.7
8.8
8.6 9 9
9 2 5 6 1 2
Conversion Based on
Argon as Plasma Gas 4.0 32.2 6.0 31.9 10 0 30 8 10 0 28 55 10 0 25 0 10 0* 36 0
0,78 1.15 1 86 1 74 1 67 2 2
42.9 30.0 39 75 44 1 48 7 25 7
0.45
0.52
N O
1 18
NO
1 3 1 6 0’
Yes
Nitrogen as Pla5ma Gas 5 5 4 0 47 2 1 13 21 6 0 25 4 8 6 0 51 1 88 20 7 0 38 14.0 10.0 40.0 2.4 32.9 1. o Gas mixture fed through circular gasjeeding ring with 30” angle us. plasmajame.
temperature achieved in the plasma jet and upon the quenching rates. If the same cooling devices are used, the quenching rate obviously varies with the electrical power input, the plasma gas and flow used, and the feeding rate of the reactive gases. Converting solid carbon into hydrogen cyanide or feeding the carbon as a gas in the form of methane into the nitrogen plasma does not affect the yields of formation of HCN, the latter method being technically easier. Considering the yields obtained o n the basis of electrical power consumed, 1 gram-mole of H C N requires approximately 2.7 kw.-hr., yielding as by-product approximately 0.5 grammole of CtHz. With the actual maximum electrical power available for the plasma jet unit described (15 kw.), it was not possible to increase the yields of H C N o r CzHz by increasing the amount of starting material. By using higher power units (in the megawatt order) it is assumed that even better conversion based o n the electrical power input would be obtained. IYith present production costs o n the order of 8 cents per pound, the plasma jet method would appear to be beyond consideration. But these experiments were carried out in a small scale.
Yes
X O S O
Yes No
NO
6 5 113 5 18.4
If the unit were scaled up, much more favorable production costs could be obtained. Acknowledgment
The author thanks A. V. Grosse for valuable discussions and support given to this work and C. S. Stokes for assistance in resolving the many engineering problems involved. literature Cited (1) Dtnigts, Compt. Rend. 117, 1078 (1893). (2) Leutner, H. W., IND.ENG.CHEM.,PROCESS DESIGN DEVELOP. 1, 166 (1962). (3) Leutner, H. W., Stokes, C. S., Znd. En?. Chem. 53,341 (1961). (4) Phillips, R. C., Ferguson, F. A., “High Temperature Tech-
nology,” p. 192, McGraw-Hill, New York, 1960. (5) Stokes, C. S., Knipe, W. I V . , Znd. Eng. Chem. 52, 287 (1960). (6) Treadwell, F. E., Hall, W. T., “Analytical Chemistry,“ Vol. 2, 9th ed., p. 695, Wiley, New York. 1957. RECEIVED for review September 20, 1962 ACCEPTED July 29: 1963 LVork financed by a contract from the Office of Naval Research,
Contract Nonr 3085(02), Task No. NR052-429.
STARCH VINYLATION Determination of Optimum Conditions by Response Suface Designs JAMES W.
Department
of
B E R R Y , H E N R Y T U C K E R , A N D A R C H I E J. D E U T S C H M A N , J R .
Agricultural Biochemistry, University of Arizona, Tucson, Ariz.
This report describes a statistical study of vinylation, partly in the unexplored range of 350 to 750 p.s.i.g. acetylene pressure. Starch, the polyhydric material under study, has been vinylated to produce vinyl starch, a new material. The interrelationship of five variables has been determined by employing a central composite rotatable second-order response surface design. A comparison of predicted and observed values for degree of substitution indicates that the response surface design is a good characterization of the relation between the variables and degree of substitution. INYLATION involving the addition of alcohols to acetylene in Vthe presence of base is a well established reaction which has been utilized on a n industrial scale (3, 70). Recently, the vinylation of polyfunctional alcohols and carbohydrates has received considerable attention (5. 7-9, 7 7-74). The study of vinylation reported here is significant in three aspects. First, starch, the polyhydric material under study,
318
I&EC PROCESS D E S I G N A N D DEVELOPMENT
has been vinylated to produce vinyl starch, a new material. Second, previously unexplored regions of high pressure have been studied. Third, the study has been made in a statistical manner, yielding information about optimum reaction conditions for vinylation. Because initial vinylations of starch showed the reaction to be complex, with five variables being recognized (time, temperature, pressure. concentration of
