510 



PARAFFIN 



Among the many processes patented may be named the use of hypochlorite of 

 lime, the application of chlorine gas to the vapours of the oils, of naacent chlorine 

 to the liquid oil by the decomposition of hydrochloric acid or chloride of sodium 

 with black oxide of manganese, the use of bichromate of potash and other chromates, 

 and of free chromic acid, of the permanganates, and almost every known oxidising 

 agent. 



The sulphuric-acid treatment is the only method which has been found commer- 

 cially successful. The proportion of acid required depends upon the oil, and must be 

 determined by experiment. Generally speaking, the heavier oils require more acid 

 than the lighter ; but even this rule is subject to special exceptions. 



Many forms of agitators have been adopted, from simple tubs with a paddle to very 

 complex devices worked by steam-power ; the problem, however, is merely to keep a 

 heavy liquid sulphuric acid stirred xip amidst a lighter one the crude, or ' once- 

 run* oil. A cast-iron vessel, fitted with paddles working on an axle similar to a 

 common churn, is the apparatus generally used. Upright cylindrical vessels, with 

 an Archimedean screw working vertically to raise the acid from the bottom, have 

 been used, but not extensively. 



In some refineries the crude oil is treated with the acid and alkali, but more com- 

 monly it is ' once run,' as already described. 



In all the stages of distillation for the refinery the use of superheated or dry steam 

 is found very advantageous, not as the primary source of heat, but as an auxiliary. 

 In accordance with the well-established law of diffusion of gases, the vapour from the 

 boiling oil will diffuse much more readily into an atmosphere of steam than into 

 one of its own vapour ; and thus the superheated, or simply the dry steam, enables 

 the distillation to be conducted at a lower temperature than would be necessary 

 without it, which is a matter of considerable importance, not merely as regards 

 the quantity of work done with given fuel, but also as affecting the quality of the 

 oil produced. 



The form of still most commonly used is shown in Jig. 1610, usually of a capacity 

 of 1,000 to 2,000 gallons. Much difference of opinion prevails among refiners as 



1610 



regards the merits of cast or wrought iron. Many serious conflagrations have lately 

 occurred from the sudden cracking of cast-iron stills, which have led to a much more 

 extensive use of wrought iron than formerly. 



It is a common practice in the first running of crude oil to use a wrought-iron still, 

 or still of cast iron with bottom outlet-pipe, like that in fig. 1610, for the first part 

 of the process, until the heavier products begin to run over ; then to run out the 

 residue from this still into a small round bottom still, without bottom outlet, and then 

 ' coke down,' that is, carry on the distillation to dryness. By this means the most 

 dangerous stage is carried on in the smaller still, which is far less liable to rupture. 

 Wrought-iron stills cannot well be used for coking down, the great heat required 

 destroying the rivets. 



In the refining of the solid paraffin considerable progress has been made. 



In the first place, the crude oil is distilled over to dryness, ' coked down,' as already 

 described. The heavy oil obtained from the latter stages of distillation is found to 

 contain largo quantities of bright crystalline scales of solid paraffin in suspension. 



