Thermal Cracking and Decomposition Processes

Although these exact reactions probably do not take place, they are representative of the over-all reaction that occurs during cracking. 

Thus two general types of reactions take place (1) primary reactions in which decomposition of large molecules into small molecules takes place and (2) secondary reactions by which active products polymerize to form heavy tarry materials. At the same time the products of polymerization may be again decomposed into smaller molecules. The nature of the primary reactions may be illustrated by the experimental work of Hurd and Spence on the decomposition of n-butane at 600°C (1112 F). 

Their work also indicates that the tendency to dehydrogenate, leaving an olefin with the same number of carbon atoms as the original paraffin hydrocarbon, rapidly diminishes as the series is ascended. Thus the production of large amounts of hydrogen by cracking gas-oil stocks should not be expected. 

The olefin hydrocarbons are not usually found in raw petroleum stocks, and hence the decomposition of olefins should properly be considered as a secondary decomposition reaction. Both hydrogenation and dehydrogenation may occur during the decomposition of olefins. Diolefins, paraffins, polymers, new olefins, hydrogen, and probably many other materials are produced. As an example of diolefin and paraffin formation, pentene-1 reacts thus  

The secondary reactions by which polymers are formed may be illustrated thus  

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THERMAL CRACKING AND DECOMPOSITION PROCESS Recycle ratio ... 

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