CARBON AND THE OXIDES OF CARBON 339 



The reaction being reversible, and exothermic in the forward 

 direction, its equilibrium point is displaced towards the left, 

 favoring the backward reaction, as the temperature is raised. 

 It is therefore desirable to work the process at as low a temper- 

 ature as possible. With iron oxide alone as a catalyst, however, 

 the interaction between the gases becomes too slow to be effective 

 at temperatures much below 600. The addition of small quan- 

 tities of other oxides, such as nickel oxide and chromium oxide, 

 has been found to increase the activity of the catalyst very con- 

 siderably. Substances which act in this manner (catalyzing a 

 catalyst, so to speak) are termed promoters. 



In actual practice the reaction, carried out at 450-500, gives 

 a mixture of gases containing only about 2 per cent residual 

 CO. Excess of steam is employed to drive the equilibrium as 

 far as possible towards the right, but the excess of hydrogen 

 present in the original water-gas favors, of course, the opposite 

 reaction (compare p. 234). It should be noted that nearly twice 

 as much hydrogen as was contained in this water-gas is obtained 

 by the process, the second half being derived from the decomposi- 

 tion of the steam. 



The bulk of the carbon dioxide present in the final mixture 

 (approximately 30 per cent by volume) is removed by washing 

 the gas with water under pressure. The last traces oi carbon 

 dioxide are absorbed by means of lime or alkalies. The removal 

 of the 2 per cent residual CO presents difficulties. Absorption 

 of CO by hot caustic soda solutions and by ammoniacal solutions 

 of cuprous salts has been employed. The most efficient method, 

 however, consists of preferential combustion of CO to CO2 with the 

 requisite quantity of air or oxygen in the presence of a second 

 oxide catalyst. If due precautions are taken, CO burns almost 

 quantitatively to CO 2 without any H 2 present burning to H 2 O. 

 The small amount of C02 formed is then removed as already 

 described. 



