PETEOL 289 



to friction and differences of level it is not practicable to force 

 the oil beyond a distance of forty or fifty miles. At intervals, 

 therefore, the oil is delivered into a tank at each station where 

 an auxiliary pump passes it along to the next stage, and so on, 

 through the entire distance which commonly reaches several 

 hundred miles. The illustration (Fig. 95 facing page 293) 

 shows a view of the storage tanks near to an oil field, together 

 with one of the pumping stations referred to. 



The conveyance of oil in the Baku district has been already 

 sufficiently described. Fig. 96 facing page 293 gives a view of a 

 steamer of the American river type, used for towing tank barges. 



Arrived at the refineries the oil has to undergo the process of 

 distillation. As everyone knows, this process consists in heating 

 the liquid in some kind of boiler or " still " with a head which 

 confines the vapour given off and conducts it into a pipe or 

 series of pipes, cooled, if necessary by water, where the vapour 

 is condensed into the liquid state, and is run into a receiver. In 

 operating on such a mixture as petroleum the distilled liquid 

 differs in properties and composition from the original, for it 

 consists of those ingredients the boiling points of which are 

 lower than the boiling points of those portions which are con- 

 verted into vapour at a later stage. 



The stills actually in use differ in different countries both as 

 to form and capacity. One form commonly used in the United 

 States is shown in the Figs. 97 and 98 facing page 294. 



It consists of a cylindrical iron vessel, capable of holding 

 between 50,000 and 60,000 gallons, connected by a wide pipe 

 which delivers the vapour into a box furnished with a large 

 number of pipes surrounded by water for the purpose of con- 

 densation. Many modifications have been introduced with a 

 variety of objects, such as making the process continuous by 

 causing crude petroleum to flow into the first of a series of con- 

 nected stills, the temperature of each successive still being higher 

 in proportion as the more volatile portions pass off. In more 

 recent years, with the object of " cracking " the oil and obtaining 

 a larger yield of certain light portions, the upper part of the still 

 is kept comparatively cool, so that the less volatile portions 

 condense and drop back into the hot boiler. 



In order to explain the process of cracking it is necessary to 

 give a brief account of the nature and constitution of natural 

 petroleum, and the chief products obtained from it. 



