Nitrogen Fixation - Encyclopedia




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"NITROGEN FIXATION 19.714).-Important progress was made after 1910 in the commercial fixation of nitrogen for industrial use. The economic importance of nitrogen fixation is to be found in the possibility of preparing fertilizers from the air, in place of being dependent on the nitrate deposits of Chile, or on ammonia obtained by the carbonization of coal. In addition to the artificial production of agricultural fertilizers, the synthetic manufacture of nitric acid becomes of the utmost importance in time of war, by virtue of the part played by this body in almost every explosive. It is certain that the Central Empires, in the World War, could not have continued fighting, by reason of their economic isolation from normal sources of nitric acid, but for the gigantic nitrogen-fixation factories which were erected at Oppau and elsewhere.

Nitrogen, in its ordinary form, combines directly with the following elements :-lithium, calcium, strontium, barium, magnesium, boron, aluminium, various rare earths, titanium, zirconium, cerium, thorium, silicon, vanadium, niobium, tantalum, chromium, uranium, manganese. In each case a nitride is formed, from which ammonia may be produced by the action of steam, but the commercial fixation of nitrogen as a nitride is, for technical and economic reasons, only possible in one or two instances, namely in the case of aluminium, and possibly also of silicon. Aluminium nitride is manufactured on a considerable scale in France by the Societe Generale des Nitrures, by means of the Serpek process (Brit. Pat. 13086/1910). Finely ground alumina (bauxite) is preheated by means of flue gases and, after being mixed with powdered coal, is allowed to pass slowly along an inclined tube containing an electrically heated portion, by means of which the temperature of the charge is raised to about 1800° C. Producer gas, passed through the inclined tube in counter-current to the bauxite, forms the source of nitrogen, and reaction takes place according to the equation: Al203+3C+N2 = 2 AIN-I-3C0. The aluminium nitride is usually subsequently decomposed by means of caustic soda, with production of ammonia and of alumina.

From a commercial aspect, four processes of nitrogen fixation, namely the synthesis of ammonia, the arc process for the manufacture of oxides of nitrogen, the formation of calcium cyanamide (nitrolim) by the interaction of calcium carbide and nitrogen, and the synthesis of alkaline cyanides by the Bucher process, are all of special interest.

Ammonia Process.-The technical synthesis of ammonia, in particular, constitutes one of the great landmarks in chemical technology. The method employed consists in circulating a highly compressed mixture of hydrogen and nitrogen through a heated chamber containing a catalyst, by the action of which a small percentage of ammonia is formed according to the equation 3H2+N2 =2NH 3. This ammonia is subsequently removed from the uncombined gas, either by treatment with water at room temperature or by a process of refrigeration. The gaseous residue, after removal of the ammonia, is circulated once more through the heated catalyst chamber, fresh nitrogen and hydrogen being added to compensate for that transformed into ammonia and to maintain the pressure. In order to economize energy an elaborate system of heat exchangers is provided, by means of which the hot gases leaving the synthesizing bomb are used to heat the incoming gas. The formation of ammonia from its elements is accompanied by a considerable evolution of heat, and the production may, under favourable conditions, become autothermic when once started; that is to say, the heat of reaction may, in the presence of efficient heat exchangers, suffice to maintain the temperature required without the necessity for the supply of extraneous heat. The heat of formation of ammonia increases somewhat with the temperature. Haber, Tamaru and Oeholm (Zeitschr. fur Elektrochem., 1915, 21. 191, 206) give, as the molecular heat of formation at constant pressure, 10,950 calories at o° C., 12,670 cal. at 466° C., 12,900 cal. at 554° C., and 13,150 cal. at 659° C.

By means of this process of circulation alternately through the catalyst chamber and through an ammonia absorption apparatus, the gas mixture treated becomes transformed into ammonia.

The percentage of ammonia formed each time the compressed gas passes the catalyst chamber depends partly on the speed of circulation, that is to say on the time of contact of the gas with the catalyst, and partly on the conditions of equilibrium between nitrogen, hydrogen and ammonia at the temperature and pressure employed, a subject which has been investigated in detail by Haber and his pupils. This equilibrium may be represented by an equation of the usual type, PNH3 Kp P H2 X P N2 obtained by applying the law of mass action to the formation of a gramme molecule of ammonia by the process: 3 - 2 H2 + N NH3. The value of Kp, from which the equilibrium ammonia percentage for any required pressure may readily be calculated from the relation given above, varies with the temperature according to the thermodynamically derived equation log i oKp = ?0982.508810gioT -o000to06T+o186 X t06 T 2 +2. I. For an approximate calculation of Kp, the abbreviated form: logioKp = 2888 may be used.

Temper-

ature

° C.

Equilibrium Percentage of Ammonia

At I atm.

30 atm.

too atm.

200 atm.

200

15.3

67.6

806

85.8

300

2.18

318

52.1

62.8

400

0'44

10.7

25' I

36.3

500

0.129

3.62

10.4

17.6

600

0.049

1.43

4'47

8'25

700

0.0223

o66

2.14

4.11

Boo

0.0117

0.35

1.15

2'24

900

0.0069

0.21

o68

1.34

1000

0.0044

0.13

0'44

o87

The equilibrium ammonia content of a gas mixture, containing nitrogen and hydrogen in the ratio of I: 3 by volume, is given in the following table: From the above figures it will be seen that the equilibrium ammonia percentage decreases rapidly with increase in temperature, but is capable of being raised by working under an increased pressure. The most usual working pressures are from too to 200 atmospheres, but an attempt has recently been made by Claude to operate the process at pressures greatly in excess of this.

Of the catalysts employed, osmium, uranium, and iron are of special importance, the interest of the first two being mainly historical, in that the first successful synthesis of ammonia by Haber and his pupils was carried out with their aid. In the case of iron the activity of the catalyst is usually increased by incorporating secondary constituents termed " promoters." While the formation of ammonia begins at as low a temperature as 360° C., the velocity of the reaction is exceedingly slow under such conditions, in spite of the advantageous effect of a relatively low temperature on the equilibrium ammonia percentage, and a working temperature of 500° or over is usual commercially.

The output of a plant having a catalyst chamber of a given size is obviously governed by the percentage of ammonia formed during the passage of the compressed gas through the catalyst, and by the speed of circulation. It is found advantageous in practice to employ a relatively high speed of circulation, rather than to circulate slowly and to obtain a high ammonia percentage in the gas issuing from the catalyst chamber. The quantity of ammonia produced is measured by the space-time-yield (abbreviated S.T.Y.), this being the number of kilogrammes per hour per litre of catalyst space. For purposes of comparison, it is conventional to express the speed of circulation of the gas in litres per hour, at room temperature and pressure, per litre of catalyst space (space-velocity, abbreviated S.V.).

The efficiency of certain catalysts, under the conditions stated, is exemplified by the figures (see p. 1, 137) for osmium (Haber and Le Rossignol, Zeitschr. fur Elektrochem., 1913, 19. 69), uranium (Haber and Greenwood, ibid. 1915, 21. 241), and iron (Maxted, Ammonia and the Nitrides, p. 34).


Catalyst

Temper-

ature

C.

Pressure

in atmos-

pheres

S.V. of

gas

10 3 X

Percent-

ammonia

formed

S.T.Y.

Osmium. .

585

166

24

7o

146

80

6.2

3.6

"

160

4.2

4.8

Uranium

carbide. .

515

113.6

5.2

7.63

0.28

28.5

6.42

1.3

74.4

174.9

4.7 8

4.18

2.5

5.2

Iron.. .

505

150

10

III

o8

5 30

11

50

6.9

2.5

5.6

4.3

"

200

4.3

6.2

1

300

3.8

8.2

500

3.2

3.o

9.2

10.8

' `

600

2.7

II .6

As already stated, the formation of ammonia in the synthesizing chamber is followed by the elimination of ammonia from the circulating gases either by refrigeration, in which case pure anhydrous ammonia is obtained, or by absorption in water. For use as an agricultural fertilizer, the ammonia has to be converted into a suitable salt, usually the sulphate or chloride. In order to avoid the use of sulphuric acid for neutralization, it has been proposed to allow the ammonia to enter into reaction with calcium sulphate in the presence of carbon dioxide, whereby ammonium sulphate is produced, calcium carbonate being precipitated. For the production of the chloride, several modifications of the well-known Solvay ammonia-soda process have been suggested, by means of which ammonium chloride is formed from common salt, ammonia and carbon dioxide.

The direct synthesis of ammonia constitutes probably the most economical method of fixing nitrogen at present known. The cost of production is regulated principally by that of the hydrogen, the cost of compression being relatively low. On the other hand, the technical difficulties are probably more severe than in any other known industrial chemical operation. The high pressure, combined with a temperature sufficient to render steel of ordinary composition rapidly weakened by the hydrogen contained in the gas employed, has made necessary the construction of furnaces of special design. Further, all raw materials must be of a high degree of purity, by reason of the readiness with which the reaction is impeded and stopped by the presence of traces of catalyst poisons such as sulphur, arsenic, phosphorus, etc. Nitrogen also combines directly with hydrogen at the temperature of the electric arc and, further, under the influence of the silent electric discharge, but these methods have not up to the present given yields sufficient to justify their commercial application.

Immediately previous to and during the World War extensive factories were erected in Germany, at Oppau and Merseburg, for synthesizing ammonia, the tons of nitrogen fixed in this way being reported to have increased from 4,000 in 1913 to Ioo,000 in 1917. In Great Britain, Synthetic Ammonia & Nitrates, Ltd., was registered by Messrs. Brunner, Mond & Co., Ltd., in 1920 with a capital of 5,000,000. In the United States, the synthesis of ammonia has been taken up on a large scale by the General Chemical Co., and plants exist at Sheffield, Alabama.

Cyanamide Process.-A second highly important method of fixing nitrogen consists in forming calcium cyanamide (nitrolim), by the interaction of nitrogen with calcium carbide,CaC 2 +N 2 = Ca :NCN +C. Absorption of nitrogen takes place readily at 1,000° to I,10o° C., with carbide of commercial quality. By the addition of catalysts such as calcium fluoride or calcium chloride, the combination may be carried out at 800° C. The reaction is exothermic, and the temperature of the charge rises considerably owing to the heat produced. Temperatures exceeding 1,400° C. have a marked inhibitive effect on the yield, by reason of the reversible nature of the reaction.

Two types of plant are employed in practice. In those at Odda in Norway, the charge of carbide is reduced to fine powder by grinding and placed in cylindrical firebrick furnaces, which are heated internally to the required temperature by means of carbon resistance rods, nitrogen being admitted under slight pressure. A period of about 36 hours is required for the completion of the reaction, at the end of which time the product contains upwards of 20% of nitrogen. The charge shrinks away from the walls and forms a solid block, which is easily removed. It is the practice at certain other works, particularly those in Germany and Italy, to employ externally heated horizontal retorts. With these, the temperature of reaction is stated to be less easily controlled and trouble is experienced from the adhesion of cyanamide to the walls. Calcium cyanamide, in a finely ground condition, may be used directly as an agricultural fertilizer, ammonia being produced in the soil by hydrolysis: CaNCN+3H 2 0 =2NH3+CaC03.

The above hydrolysis is also effected by the action of superheated steam, as an industrial operation for the manufacture of ammonia. The fixation of nitrogen by the cyanamide process is of considerable extent and importance. Factories exist at Odda (Norway), Piano d'Orta (Italy), Niagara, Wittenberg, Chorzow, Piesterloh and other places. It is stated that the total world production of cyanamide in 1916 amounted to nearly I,000,000 tons, while Germany alone, owing to war-time extensions, is reported to have manufactured 886,000 tons in 1917.

Bucher Process.-The synthesis of sodium cyanide by the interaction of sodium carbonate, carbon and nitrogen in the presence of iron as a catalyst, according to the equation Na2C03+4C+N2 = 2NaCN+3CO 3 constitutes a promising method of nitrogen fixation, the commercial development of which is still in its infancy. The catalytic effect of iron in promoting this formation of cyanides. at relatively low temperatures (800-1,000° C.) was noted by Thompson in 1839. Bucher (Jour. Indust. and Eng. Chem., 1917, 9 . 2 33) drew renewed attention to the process, which has recently been developed industrially in the United States by the Nitrogen Products Co. According to the procedure adopted at Saltville,Virginia (Jour. Indust. and Eng. Chem., 1919, II. Imo), coke is ground to a fineness of 200 mesh, and after the admixture of a small quantity of iron the required quantity of soda ash is added. The charge is moistened slightly, kneaded, and extruded in the form of briquettes, which are dried by the action of flue gases. The briquettes are placed in vertical iron or nichrome retorts, which are heated externally in firebrick furnaces to a temperature of 900 to 1,000° C., a current of nitrogen being led through the retorts. The briquettes, after treatment, contain about 20% to 30% of cyanide, which, in the plant in question, is removed in a somewhat novel manner by subsequent extraction with liquid ammonia, in which sodium cyanide is readily soluble. During this extraction process, the main structure of the briquette remains undestroyed, and the uncombined residue may be used for further treatment with nitrogen. The chief technical difficulty lies in the rapid deterioration of the iron retorts at the temperature employed for fixation, the life of these being about 7 to 12 days. Nichrome retorts last longer, but are more expensive to replace. It has been proposed to use an electrically heated type of furnace in which the charge itself forms the resistance. Further, pure nitrogen, although conducive to a high yield of cyanide, is not essential for commercial success. Ferguson and Manning (Jour. Indust. and Eng. Chem., 1919, II. 94 6), in reviewing the replacement of nitrogen by producer gas containing carbon monoxide, state that at 1,000°C. the presence of 15% of carbon monoxide in the nitrogen reduces the yield of cyanide by about 30%, while, if the producer gas contains 60% of carbon monoxide, the yield is one-half of the value obtained with pure nitrogen. This inhibitive effect of carbon monoxide, the reason for which lies in the reversibility of the equation Na 2 CO 3 +4C+N 2 2NaCN+3CO, is even more pronounced at lower temperatures.

Numerous attempts have been made to synthesize barium cyanide industrially from barium oxide or carbonate, carbon and nitrogen. Margueritte and Sourdeval in 1860 (Brit. Pat.I,027/1860) appear to have been the first to suggest the process which was subsequently improved by Mond (Brit. Pat. 433/1882) and by Readman (Brit. Pat. 6,621/1894). The optimum temperature is about I,400° C., reaction taking place according to the equation: BaO+3C+N2=Ba (CN) 2 +CO. The above synthesis of barium cyanide was at one time worked on a considerable scale, but was not successful commercially, by reason of the deteriorating action of the fused cyanide on the walls of the furnace. It may be noted that the cyanides may readily be hydrolyzed to ammonia by means of superheated steam in an analogous manner to calcium cyanamide. G. W. Heise and H. E. Foote (Jour. Indust. and Eng. Chem., 1920, 12. 331) state that on treating briquettes containing synthetic sodium cyanide with steam at a pressure of 300 to 330 lb., a yield of ammonia amounting to over 90% of that theoretically possible was obtained in 30 to 45 minutes.

Temperature ° C.

Equilibrium Percentage of NO by vol-

ume (from air)

1 ,53 8

0.37

1,604

0.42

1,760

0.64

1,922

0.97

2 ,3 0 7

2.05

2,402

2.23

2,927

5.00


Arc Process.-The final method to be considered consists of the synthesis of nitric acid, either by the action of a high-tension electric arc on air or by the explosion of compressed mixtures of air and a combustible gas in the cylinder of an internal combustion engine (Hausser's process). The production of oxides of nitrogen by either method depends on the reaction of nitrogen with oxygen at a high temperature, according to the reversible equation: N2+02-j>2NO. The equilibrium percentage of nitric oxide formed by heating air to various temperatures has been measured by Nernst, Jellinek and Finckh (Gottinger Nachr. 1904, p. 261), Zeitschr.i f. anorg. Chem., 1905, 45.116; 1906, 49. 212, 229; Zeitschr. f. Elektrochem., 1906, 12. 527), the results being summarized in the following table: Nitric oxide is formed, and consequently also decomposed, at a very high velocity, less than one-thousandth of a second being required for the attainment of equilibrium even at 2,000°, so that, in order to preserve the products formed at arc temperature, these must be removed as quickly as possible from the arc flame; otherwise a less advantageous percentage, corresponding to a temperature lower than the maximum, is obtained. In practice, this rapid cooling is effected by employing a rapid flow of air, which is injected into a specially spread-out arc. Even with these precautions, however, the concentration of nitric oxide preserved in the issuing gases does not usually exceed two parts per cent by volume, nitric oxide being capable of existence in an undecomposed but metastable condition at temperatures below I,000°, by reason of the extreme slowness with which decomposition then proceeds.

Of the commercial plants employed, that due to Birkeland and Eyde has been described in 19.714. In the Schiinherr type, the arc is struck between an internal electrode placed at the base of a tall vertical metal tube, which forms the second electrode, air being injected with a whirling motion vertically through the furnace. The arc is thus blown out into a flame, the length of which may be as much as 7 yards. The yield of nitric acid is stated to be about 75 grammes per kilowatt hour of energy used. The concentration of the nitric oxide in the issuing gases is 2-2.5%. A further type of " blown arc " furnace, due to Pauling, employs special lighting knives for promoting the formation of the arc.

The exit gases from any of the above types of furnace are cooled to about 50°, and passed into a so-called oxidation chamber, in which the excess of oxygen combines with the nitric oxide, forming nitrogen peroxide, 2 NO+O 2 = 2NO 2. The nitrogen peroxide is subsequently absorbed by means of water in large granite absorption towers, nitric acid being produced. The process is operated principally in Norway at Notodden and Christiansand.

In the Hausser process, the requisite high temperature is obtained by the explosion of compressed air with a fuel gas in the cylinder of an engine. Enrichment of the air with oxygen is stated (Greenwood, Industrial Gases, p. 107) to increase the yield of nitric acid. Thus, air containing 26% of oxygen gave a yield of to lb. of nitric acid per I,000 cub. ft. of combustible gas, compared with a yield of 6 . 4 lb. with normal air. (E. B. M.)

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