78 CHEMICAL FERTILIZERS 



column. The empty spaces are packed with wool and the 

 outside of the machine is similarly protected. The liquid 

 air is then led to a point near the top of the rectifying tower, 

 down which it trickles, nitrogen coming off as gas and pure 

 oxygen passing away from the bottom. Compressed oxygen 

 is a valuable by-product of this process. Some moisture and 

 carbon dioxide are usually condensed in the apparatus, 

 which needs cleaning from time to time. 



Nitrogen may also be separated from air by the Pictet 

 process, according to which air, cooled down to liquefying 

 point, is injected in the gaseous condition at a little over 

 atmospheiic pressure into a separating column over the plates 

 of which liquid nitrogen is allowed to flow. This liquid 

 nitrogen allows the air to be separated, as it permits the 

 passage upwards of gaseous nitrogen, whilst gaseous oxygen 

 is condensed by the cold produced by evaporating the liquid 

 nitrogen. The mixture of liquid oxygen and nitrogen flows 

 into the lower part of the fractionating column, where it con- 

 tinues to undergo fractionation in such a way that it reaches 

 the reservoir at the bottom as liquid oxygen nearly free from 

 nitrogen. In this process only one-fifth of the air to be 

 separated is liquefied, nitrogen being kept in a gaseous 

 condition during the separation ; it is only a small fraction 

 that is subsequently liquefied to maintain a sufficient flow 

 of liquid nitrogen through the column. This subsequent 

 liquefaction of a small proportion of the liquid nitrogen can 

 be carried out by slight compression in closed coils placed 

 on the oxygen reservoir and on the plates of the column, 

 the pressure needed for liquefaction being obtained by a small 

 compressing plant, so arranged that the pressures and 

 temperatures are adjusted to condensing point according to 

 the situation in the column. For example, a coil in the top 

 of the column, where the temperature is 196 C., requires 

 only very slightly increased pressure to obtain liquefaction 

 of the nitrogen contained therein, whilst a coil at the bottom 

 must be supplied with a higher pressure. The heat evolved by 

 liquefaction in the coils supplies the heat by means of which 

 the mixture on the lower plates of the column is fractionated. 



