Hexane, Research Octane Number

The data in Table IV and Figure 1 indicate that most of the paraffin concentration reduction took place in the Ce to Ce range and involved monomethyl paraffins. The blending octane numbers for 2-methyl and 3-methyl hexanes are reported as 40 to 56 by ASTM ( ) and are lower than the blending numbers for light (Cs to C ) olefins and Ce to Cii aromatics. Thus, reduction in concentration of these branched paraffins is expected to improve the research octane number of the gasoline. 

The isomerization process is utilized to convert light paraffins such as butane, pentane, and hexane into higher-octane isoparaffins. Isoparaffins have higher octane numbers than normal paraffins of the same carbon number. For example, n-pentane has a research octane number of about 61, and isopentane has an octane number of approximately 92. 

In view of these considerations, a large amount of effort is reported in the scientific press on the development of a process to produce benzene from n-hexane by combined cyclization and dehydrogenation. w-Hexane has a low Research octane number of only 24.8 and can be separated in fair purities from virgin naphthas by simple distillation. Recently, an announcement was made of a process in the laboratory stage for aromatiza-tion of n-hexane (16). The process utilizes a chromia-alumina catalyst at 900° F., atmospheric pressure, and a liquid space velocity of about one volume of liquid per volume of catalyst per hour. The liquid product contains about 36% benzene with 64% of hexane plus olefin. The catalyst was shown to be regenerable with a mixture of air and nitrogen. The tests were made on a unit of the fixed-bed type, but it was indicated that the fluid technique probably could be used. If commercial application of this or similar processes can be achieved economically, it could be of immense help in relieving the benzene short-age. 

The temperature of isomerization controls equilibrium isomer composition, and thereby product octane. Figure 4.8 is a plot of isopentane in the C5 product as a function of temperature. The data are from pilot plant runs with three types of commercial UOP isomerization catalysts. The feedstock was a 50/50 mixture of normal pentane and normal hexane, containing about 6% cyclics. The 1-8 and I-80 catalysts are very active at a low temperature, where equilibrium isopentane content is highest. The acid functions in 1-8 and 1-80 are chlorided aluminas. The zeolitic catalyst, HS-10 , requires relatively high temperatures of operation. The LPI-100 catalyst contains sulfated zirconia as the acid function and falls in the middle of the temperature range (12). Due to the equilibrium constraints, a lower temperature operation yields a higher octane product. The 1-8 and 1-80 catalysts yielded Research Octane Numbers of 82-84, as compared to 80-82 for LPI-100 catalyst and 78-80 for HS-10. 

Table 7.2 Research Octane Number (RON) of pentanes and hexanes...

Tables 3 and 4 show that isomers have different physical properties. They also can have significantly different chemical properties. For gasoline, one of the most important chemical properties is octane munber. The research octane number (RON) for n-octane is -27 compared to a RON of 100 (by definition) for isooctane (2,2,3-trimethylpentane). For heptane isomers, RON values range from 45 for 2-methyl-hexane to >100 for 2,2,3-trimethylbutane, compared to zero (by definition) for n-heptane. Octane numbers are discussed in more detail in Section 8.2.

In the isomerization of naphtha" using aluminum chloride, the Research octane number in a once-through operation is increased from 65 to 80. Further increase in octane number occurs with recycling (91 at 2 1 recycle ratio), but the economy of recycling is doubtful because of difficulties with fractionation. When processing mixed hexanes" operating conditions are... 

Octane number is measured by research. The octane number of hexane is 24.9, branched alkane wobutane is 101.3 and 2,3,4-trimethylpentane is 102.5, in aromatic compounds benzene is 106, toluene is 110 [28,29]. These compounds having high octane numbers are used for blending agents to gasoline. 

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