I N D U S T R I A L A N D ENGINEERING CHEMISTRY
November. 1923
1173
Experiments on the Arc Process for Nitrogen Fixation’ By Edgar D. McCollum and Farrington Daniels UNIVERSITY OF WISCONSIN, MADISON, WIS.
T
HE fixation of nitro-
Liquid nitrogen peroxide may be made on a laboratory scale from The nitrogen peroxide was collected in a weighed tube pro+ gen by the air in an electric arc with the help of silica gel. No improvement of silica gel, X,2 cm, in diamis a most in the chemical efficiency of the arc was obtained by using rapid eter, a tube of glass wool, G, ing problem* It to air currents, by interrupting the arc, or by changing the character the research beof the arc with condensers. Better yields were obtained at high serving to trap particles cause the present limits of voltages. coming from the disintegration of the electrodes.- A efficiency (approximately second tube of silica gel and 1 gram atom of fixed nitrogen per kilowatt-hour) are not established by theoretical a wash bottle were placed after tube X, but they showed a t deductions,2 and because the electrochemistry of gases has all times that the absorption in the first tube was complete. received a new impetus in the recent development of the Most of the nitrogen peroxide was absorbed in the first 10 field of atomic structure. It appeals also to the industrial cm. of the tube, as shown by the brown color. Connections chemist because the raw materials-air, water, and gravitywere made with overlapping glass joints and de Khotinsky cement. The increase in weight of the silica gel tube gave are cheap and because the labor costs are remarkably low. The problem can be solved and the limits of efficiency can directly the number of grams of nitrogen peroxide. This be established only after years of fundamental research in weight was checked occasionally by heating the silica gel in the fields of electricity, photochemistry, and atomic structure. a current of air. The loss in weight was always equal to It seemed worth while, however, to perform a few experi- the gain in weight previously observed. ments in which the variables were carefully limited in order The wattage throughout all the experiments was kept conto test out certain procedures which it was thought might in- stant by adjusting the core of a choke coil, C, until the wheel crease the efficiency. of the watt-hour meter, M , made 6 revolutions per minute. It is important to point out that in the case of stable, This operation required close attention. Since each revoluefficient arcs the practical problem lies in the quick removal tion of the wheel corresponded to 5/24 watt-hour, the energy of the nitric oxide before decomposition can set in, and it is input was 75 watts per hour. Most of the experiments were immaterial whether the nitric oxide is formed in a purely continued for an hour, giving usually about 1 gram of nitrogeh thermal equilibrium, by electronic or ionic impact, or by peroxide, and the total energy consume< was read directly photosynthesis. One of the simplest ways of quickly re- on the watt-hour meter. The current was measured with a moving the products consists in blowing the air through the hot wire ammeter in series with the arc. arc very rapidly. There are two limits to this procedurefirst, the extinguishing of the arc, and, second, the difficulty of recovering the oxidized nitrogen when its concentration is low. The second objection may be overcome by dispensing with the ordinary absorption towers filled with liquids and adsorbing the nitrogen peroxide with silica gel.3 Since the cost of absorption in present practice constitutes about one-quarter of the total cost of fixing nitrogen by the arc process, the experiments seemed particularly worth while.
APPARATUS AXD PROCEDURE The apparatus is shown in Fig. 1. The chamber, A , was made of Pyrex glass 5 cm. in diameter and 100 cm. long. The electrodes, E , were placed in side tubes, fused to the main tubes. The side tubes were long enough so that an airtight joint could be made with rubber tubing without danger of heating the rubber. The air was previously dried through two washing bottles of sulfuric acid and two towers of sodium hydroxide sticks. It was introduced into the arc from below by means of a 5-mm. glass tube a t right angles to the elevtrodes. The end of the glass tube was placed 3 mm. from the electrodes and remained in the same position throughout all the experiments. The electrodes, of 3-mm. steel rods, were touched together to start the arc and then drawn apart to a distance of 5 mm. in all the experiments. The flowmeter, F , was calibrated in liters of air per minute, palsing into the arc. Received April 16, 1923. calculations such as given in Haber’s “Thermodynamic3 of Technical Gas Reactions,” p. 267, are open to considerable suspicion as regards the temperature. I n fact, one cannot speak properly of the temperature of an arc in a quantitative sense. 8 McGavack and Patrick, J. A m Chem Soc , 42, 946 (1920) 1
* Thermodynamical
I
FIG.1-APPARATUSFOR FIXATION OF NITROGEN B Y ARCPROCESS
I n some of the experiments an interrupter, I , was used. It was constructed of four stout wires arranged at right angles on a wooden hub. The wires dipped into a trough of mercury and were so arranged that the circuit was open half the time and closed half the time. The hub made 125 revolutions per minute, interrupting the arc 500 times per minute. The choke coil was set to allow the consumption of 75 watts per hour on continuous current and inasmuch as the current was flowing during only one-half of the time the experiment was allowed to run for a longer time. The actual energy consumption was read directly on the meter. In using
1174
INDUSTRIAL AND ENGINEERING CHEMISTRY
the interrupter the air flow was such as to sweep the products of the arc away from the electrodes while the arc was extinguished. One liter of air was passed through the 5-mm. tube in 1 minute. The cross section of the tube was 0.1963 sq. cm., giving a linear velocity to the gas of 1000/0.1963, or 5102 cm. per minute. The arc was out for 0.002 minute. The gas moved 10 cm. during this time. Since the height of the arc was between 2 and 3 cm. all the gas must have been removed from the region of the arc while the current was off, even allowing for spreading out of the air after leaving the nozzle.
tory, but these variations seem to be a failing common t o all similar investigations, due possibly to slight fluctuations in the arc. Average values are, however, significant. The 110-volt, 1100-volt, and 2200-volt arcs were yellow in color, but the 20,000 volt arc was whitish. The current with the 1100-volt arc was 0.7 ampere and with the 20,000 volt arc, 0.2 ampere. It is obvious that the power factor was low on account of the large amount of inductance in the circuit. I n Table I1 are shown the influence of various changes in the arc. I n all these experiments the energy input was 75 watts per hour and the voltage 20,000. TABLE 11-EFFECT
0.95 0.97 1.00
Large condenser, fat blue spark Large condenser, 115 watts, yellow Ozonized air used, white arc
Volts OF
NITROGEN PEROXIDE
This apparatus made it possible to keep constant the energy input (75 watts per hour), the electrode material, the electrode distance, the air pressure, and the stream lines. With these variables eliminated the effect of the other factors was studied. For 110 volts the ordinary alternating current lighting circuit was connected; for 1100 and 2200 volts a 2-kilowatt, oil-immersed transformer, T, was used, and for 20,000 volts a Thordarson 1-kilowatt transformer served. Corrections were not made for the transformer losses, the energy input being measured on the primary side. This loss probably did not amount to more than 10 per cent. The interrupter was placed ahead of the transformer so that its losses were not included.
RESULTS The experiments of Table I show the influence of voltage and air velocity, with an energy consumption of 75 watts per hour. TABLEI-EFFECTOF VOLTAGE AND AIR VELOCITY ON YIELD Air Flow Yield NaOc I,iters/Minute Grams/Kilowatt-Hour 1.00 1.5 1.00 0.3 7.5 0.48 7.9 0.90 7.6 0.91 4.9 0.94 1.00 4.8 1.00 7.5 1100 Fast 5.0 1100 8.2 Fast 1100 5.0 Fast 1100 12.3 1.00 2200 12.8 1.00 2200 1.00 10.2 2200 1.00 13.3 2200 0.95 9.0 20 000 12.8 1.00 20'000 1.00 15.8 2o:ooo 14.4 1.10 20,000 16.1 1.30 20,000
Volts 110 110 1100 1100 1100 1100 1100
The agreement between experiments performed under supposedly constant conditions is not entirely satisfac-
CHANGES IN ARC
Liters/Minute
Small condenser, blue spark
Interrupted arc, white
OF
Flow
DESCRIPTION O F ARC Spark discharge
FIG.2-RELATION BETWEEN VOLTAGE AND YIELD
Vol. 15, No. 11
0.95 1.00 Fast Very slow 1.00 1.00
1.00 1.00
0.95
1.00 1.00 1.00 1.00
Yield NrOa Grams/ Kilowatt-Hour 2.0 4.8 4.9 12.8 8.9 8.1
Low
Low 4.2 11.4 14.7 17.4 15.9 14.0 13.5 13.4
With large electrodes the yields decreased, and in fact with electrodes of too large cross section the thermal conduction was so great as completely to prevent the maintenance of an arc at the low wattages. If the electrodes were too small, on the other hand, they were melted by the arc. Terminals of stout platinum wire were fused at once. Several experiments with direct current at 110 volts gave low yields which were about the same as those with the alternating current a t 110 volts. Since the second silica gel tube showed no increase in weight, it was assumed that the recovery of the fixed nitrogen was complete. The first silica gel tube returned to its original weight after heating to 350" C., in a slow stream of air (1 or 2 cc. per minute), and no nitric acid was observed either in the connecting tubes or in the tube surrounded by ice and salt in which the nitrogen peroxide was liquefied. These facts showed that all the fixed nitrogen was removed as nitrogen peroxide, even though some nitric acid may have been formed from the water on the silica gel. The most important feature of the investigation consisted in keeping the energy input constant throughout the experiments. The maximum yield of 16 grams of nitrogen peroxide, or 22 grams of nitric acid per kilowatt-hour, is only about a third of the yield frequently obtained with large arcs in industrial practice.
CONCLUSIONS Adsorption with silica gel offers an excellent method for the recovery of oxides of nitrogen from the arc, since in this way nitrogen peroxide may be removed from rapid air currents at low concentrations. By heating the silica gel the concentrated nitrogen peroxide may be recovered ready for liquefaction, or for absorption in water to give concentrated nitric acid. The silica gel is then ready for further adsorption. The yield of nitrogen peroxide per kilowatt-hour increases rapidly with the voltage up to a few thousand volts, after which further increase in voltage does not greatly affect it. The relation between voltage and yield per kilowatt-hour, for an energy input of 75 watts per hour in the apparatus described, is shown in Fig. 2. The curve is roughly of the exponential type and is similar to saturation curves obtained
I N D UETRIAL A N D ENGINEERING CHEMISTRY
November, 1923
in certain ioni~ati’onexperiments. It suggests that possibly a n increase in voltage increases the fraction of energy which can be made t o do chemical work, but that some other influence, such as concentration, or possibly an intermediate step of slow velocity, limits the production for the larger yields. Small variations in the velocity of the air through the arc do not affect the yield within the experimental range studied. When the velocity is high enough to render the arc unstable the yield decreases appreciably. At very low velocities it is to be expected that the yield would decrease. Cramp and Hoyle4 in an exhaustive study of the arc found that increasing the velocity of air through the arc increased the yield of nitrogen peroxide per ampere up to a maximum and then decreased it. It is important to point out, however, that their results were not on the basis of yield per given 4
J . Inst. Elec. Eng., 43, 319 (1909).
1175
energy input, but on the basis of yield for a given current. Haber and Platou5 found that increasing the velocity of air from approximately 0.5 liter per minute to 2 liters per minute increased the yield of nitric acid about one-third. Interrupting the arc so as to sweep out the gases from an extinguished arc does not give increased yields. This fact shows that after the current is shut off the kinetic energy of the molecules is not decreased instantaneously to a condition (temperature) at which the nitric oxide is stable. Spark discharges give less nitrogen peroxide per kilowatthour than arcs. The use of condensers is disadvantageous. No increase in yield results from previously ozonizing the air. Pure nitrogen peroxide may be readily obtained from air on a small scale with the use of silica gel in connection with an electric arc. 6
Z.Elektrochem., 16, 802 (1910).
J
Determination of Nitrate Nitrogen’ I n the Presence of Cyanamide and Some of Its Derivatives By Kenneth D. Jacob FIXEDNITROGEN RESEARCH LABORATORY, WASHINGTON, D. C.
IN
Cyanamide and certain of its derioatioes interfere in the deteralloy, gave in all cases reCONNECTION with investigations on the sults that were from 10 to mination of nitrates by reduction with Deoarda’s alloy. Cyanamide, dicyanodiamide, and guanylurea are quantitatioely 20 per cent above the theorate of nitrification of commercial calcium cyan%removed by precipitation with siloer sulfate. Urea, if present, ~ + c a l . This fact had preis conoerted into ammonia by the action of urease and the nitrogen viousb’ been observed by mide2 and Some of its derivcontained in the urease extract remooed with silver sulfate. C o d e 6 in the course of atives in the Soil an accurate After elimination of the interfering compounds the nitrate is nitrification experiments method for the determinawith cyanamide and dicytion Of nitrate nitroRen in determined by the Deoarda alloy method. the presence of these comanodiamide. ”he high reThese analytical methods were used at this laboratory in conpounds is desired. Of the nection with a series of soil nitrification studies on cyanamide, s d t s for nitrates, in the numerous methods Prodicyanodiamide, guanylurea, and urea, and have gioen entirely Presehce of cyanamide and consistent and satisfactory results on seoeral hundred separate its derivatives, are due to posed for the estimation of nitrate nitrogen, alone or analyses. the gradual decomposition in the presence of other of the latter compounds in forms of nitrogen, those hot alkaline solution with a based upon colorimetric or reduction procedures have as a slow evolution of ammonia, part of which appears as nitrate rule proved the most satisfactory. Colorimetric methods in the final analysis. I n order to obtain accurate results the give accurate results when the quantity of nitrate present interfering nitrogen must be removed before proceeding with is comparatively small, their relative accuracy decreasing as the nitrate determination. the amount of nitrate increases. It is also often difficult For the removal of cyanamide, dicyanodiamide, and guato obtain a colorless solution of the nitrate, especially with nylurea, an adaptation of Caro’s method, as improved by soils containing considerable organic matter and soluble Brioux,6 for the determination of combined cyanamide and salts. Of the reduction methods that using Devarda’s alloy dicyanodiamide was used. Brioux’s method involves the in dilute sodium hydroxide solution has under ordinary con- precipitation of these compounds from a water solution with ditions given consistent and accurate results in the hands silver nitrate and potassium hydroxide. This procedure of different investigators, as shown by the excellent work suggested a means of freeing the solution of cyanamide and of Allen3 and of D a v i ~ s o n . ~While the method gives satis- dicyanodiamide, but silver nitrate could not be used as a factory results in the presence of considerable soil organic precipitant because it introduced additional nitrate nitrogen. matter, it is unreliable in the presence of cyanamide and By using 100 cc. of a saturated solution of silver sulfate many of its transformation products. Modifications of the (containing about 0.75 gram Ag&04) the interfering nitrogen Allen method which eliminate these interfering substances from a solution containing as much as 50 mg. of cyanamide, are here presented. dicyanodiamide, or guanylurea sulfate is satisfactorily rePreliminary experiments on the recovery of added nitrate moved. The interference due to the presence of urea is nitrogen from mixtures containing cyanamide, urea, dicyano- also reduced, but is not completely removed by this prodiamide, or guanylurea sulfate, by reduction with Devarda’s cedure. Urea may be completely hydrolyzed’ to ammonia and IReceived June 18, 1923.
* Hereafter referred to as
cyanamide.
8Tsrs JOURNAL, 7, 521 (1915). 4 I b i d . , 10, 600 (1918).
J . Agr. Sci., 9, 113 (1919). r A n n . sci. agron., 27, Pt. 1, 241 (1910). 7 Fox and Geldard, THISJOUENAL, IG, 743 (1923). 6
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