PETROLEUM — FIELD 249 



The important development of catalytic cracking enabled the pro- 

 duction of superior aviation base stocks. In these processes, the pres- 

 ence of a catalyst results in the formation of appreciable quantities of 

 isoparaffins and aromatics. Only a very small amount of olefins, 

 which are undesirable for aviation gasoline, are formed in contrast 

 to older thermal cracking processes which produce large amounts of 

 olefins. 



Three chief sources of isoparaffins were utilized. The most im- 

 portant was the product known as "alkylate," which is produced by 

 combining low molecular weight olefin and isoparaffin hydrocarbons. 

 The second was the polymerization of two low molecular weight olefins 

 (product known as codimer) followed by hydrogenation to obtain an 

 isoparaffin. The third was the rigorous fractional distillation of 

 straight-run stocks. To meet military demands all these processes 

 were perfected and expanded. 



The need for aromatic hydrocarbons immediately imposed another 

 synthesis problem since benzene, the most readily available aromatic, 

 freezes at 40 "^ F. and cannot be used to any great extent because avia- 

 tion gasoline must meet low-temperature requirements for high- 

 altitude flymg. Consequently, cumene (f. p., —141° F.) was imme- 

 diately synthesized by alkylating propylene (a waste refinery gas) 

 and benzene and became the first important source of aromatics for 

 aviation fuel. It is interesting to note that cumene had never been 

 produced commercially before the war but by the end of the war about 

 500,000 gallons per day were in production. Other sources of aro- 

 matics were xylenes and toluene both synthesized from petroleum and 

 excess ethyl benzene from the synthetic-rubber program. The fact 

 that some of these aromatics were produced by alkylating benzene 

 with olefins (cumene and ethyl benzene) while others were made by 

 dehydrogenation of cycloparaffins illustrates the versatility used to 

 achieve results. 



The old source of pentanes used for volatility did not escape exami- 

 nation and these ultimately were separated and only the isopentanes 

 used. The chemical "aviation gasoline" near the close of the war had 

 the following composition, listed in approximate order of increasing 

 boiling range : 



Isopentane 

 Isoparaffins : 



Alkylate 



Hydrogenated codimer 



Isohexanes and isoheptanes (fractioued from straight-run naphtha) 

 Aromatics : 



Toluene 



Xylenes 



Ethyl benzene 



Cumene (isopropyl benzene) 

 Catalytic cracked base stock 



