Page:EB1922 - Volume 31.djvu/1190

1136 BIBLIOGRAPHY. The best list of books and reports relating to Nigeria will be found in Prof. A. B. Keith's Historical Geography of the British Colonies (vol. iii., West Africa). See also Capt. C. W. J. Orr, The Making of Northern Nigeria (1911) ; C. Martin, Les Posses- sions Britanniques en Afrique Occidentals (1917); Nigeria Handbook (1917) ; Sir F. Lugard, Report on the Amalgamation of Nigeria (1919) ; Red Book of West Africa ( 1920) ; Reports in the Blue Book of Nigeria (1914-20). The annual addresses of the governors of Nigeria con- tain much that is of interest. (H. CL.) NILSSON, CHRISTINE (1843-1921), Swedish singer (see 19.701), died at Copenhagen, Denmark, Nov. 22 1921. NITROGEN FIXATION (see 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, tanta- lum, 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 in- stances, 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: Al 2 O3+3C-r-N 2 = 2 A1N+3CO. 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 manu- facture 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 par- ticular, constitutes one of the great landmarks in chemical tech- nology. 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 per- centage of ammonia is formed according to the equation 3H 2 +N 2 = 2NH 3. This ammonia is subsequently removed from the un- combined 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 main- tain 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 cat- alyst 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 circu- lation, that is to say on the time of contact of the gas with the cat- alyst, and partly on the conditions of equilibrium between nitro- gen, 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,

PNH, P P 3 XP J

r H 2 A r Nj

obtained by applying the law of mass action to the formation of a gramme molecule of ammonia by the process: - H 2 + ?N 2 ^_ NH.

The value of K p, from which the equilibrium ammonia percentage for any required pressure may readily be calculated from the rela- tion given above, varies with the temperature according to the thermodynamically derived equation: -

- 2-5o881ogi T-o-oooioo6T+o-i86X io-T 2 +2-i. For an approximate calculation of K p, the abbreviated form: logioKp = -- 6-134 ma X

The equilibrium ammonia content of a gas mixture, containing nitrogen and hydrogen in the ratio of 1 : 3 by volume, is given in the following table :

used.

Temper- ature C.

Equilibrium Percentage of Ammonia

At I atm.

30 atm.

100 atm.

200 atm.

200

15-3

67-6

80-6

85-8

300

2-18

31-8

52-1

62-8

400

0-44

10-7

25-1

36-3

500

0-129

3-62

10-4

17-6

600

0-049

i-43

4-47

8-25

700

0-0223

0-66

2-14

4-ii

800

0-0117

o-35

I'lS

2-24

900

0-0069

O-2I

0-68

1-34

IOOO

0-0044

0-13

0-44

0-87

From the above figures it will be seen that the equilibrium ammo- nia 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 100 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 his- torical, 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 " prompters."

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 work- ing 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 circu- late slowly and to obtain a high ammonia percentage in the gas issuing from the catalyst chamber. The quantity of ammonia pro- duced 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 tem- perature 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. 1137) for osmium (Haber and Le Rossignol, Zeitschr. filr Elektrochem., 1913, 19. 69), uranium (Haber and Greenwood, ibid. 1915, 21. 241), and iron (Maxted, Ammonia and the Nitrides, p. 34).

As already stated, the formation of ammonia in the synthesizing chamber is followed by the elimination of ammonia from the circu- lating ^ases either by refrigeration, in which case pure anhydrous ammonia is obtained, or by absorption in water. For use as an agri- cultural 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