PETROLEUM — FIELD 245 



only if, and where, imsaturation with respect to hydrogen exists ; thus 

 the boiling range of the product is substantially the same as that of 

 the charge to the process. In the latter, operations are carried out 

 under conditions which result in rupture of some of the hydrocarbon 

 chains (cracking), the hydrogen adding on, in general, where the 

 breaks in the chain have occurred. A lowering of boiling range 

 generally results from this type of hydrogenation, the degree of 

 change depending on the operating conditions. Nondestructive hy- 

 drogenation is generally a low-temperature, low-pressure operation, 

 whereas destructive hydrogenation operates at high temperature and 

 high pressure. Catalysts are employed in almost all hydrogenation 

 processes. 



Dehydrogenation. — The general use of this term is so broad that 

 it is practically useless. For example, the cracking of gas oil in an 

 early batch still involved dehydrogenation. For the purpose of this 

 discussion, the term will be limited to operations on material in the 

 gasoline boiling range, or lighter, and considered in two categories, as 

 was done for hydrogenation, i. e., nondestructive and destructive. 

 Nondestructive dehydrogenation is defined as the removal of hydrogen 

 from the hydrocarbon molecule without the cracking which would 

 significantly change its boiling range. Destructive dehydrogenation 

 removes hydrogen from the molecule, but is accompanied by chain 

 rupture to some degree. Catalysts are employed for the nondestruc- 

 tive dehj^drogenation, but the destructive dehydrogeneration may be 

 carried out either with or without catalyst, depending on the degree 

 of selectivity desired. 



Polymerization. — Broadly speaking, polymerization can be consid- 

 ered as the linking of two or more hydrocarbon molecules to form one 

 molecule having a longer carbon chain and a liigher boiling point. 

 Here again the definition must be narrowed to make it useful, because 

 the tar produced when gas oil is cracked in a conventional cracking 

 coil is the result of polymerization. Polymerization, as practiced 

 intentionally, at the present time is confined to hydrocarbons having 

 four carbon atoms or less to the molecule (butane and lighter), and 

 links them together under conditions which result in the product being 

 predominantly within the gasoline boiling range. The operation can 

 be carried out either catalytically or thermally. The catalytic process 

 operates at relatively high pressures, but at moderate temperatures, 

 and only the unsaturates in the charge react. The thermal process 

 operates at high pressures and high temperatures, but both unsaturates 

 and saturates react, owing in part, but not entirely, to thermal dehydro- 

 genation taking place in the heating coil as the charge is brought to 

 operating temperature. 



Alhylation. — If any saturated hydrocarbon molecule is deprived 

 of a hydrogen atom and then united with another hydrocarbon molecule 



