Isopentane, Research Octane

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. 

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. 

Mechanism 1 is basically the reaction of 1 mole of isobutane (or isopentane) with 1 mole of olefin to give a C7, Cg, or C9 isoparaffin (or Cg, C9, or Cio isoparaffin) depending on the carbon number of the olefin. As already indicated, the olefin reacts significantly faster than the isobutane (or isopentane) and there are many more reaction steps than equation (2) and (3). The approximate octane numbers of isoparaffins produced by mechanism 1 vary greatly depending on the olefin and isoparaffin feed (isobutane or isopentane) (18). The best quality alkylates are produced when 2-butenes and isobutylene are reacted to form TMPs, which have research octane number (RON) averaging about 102-102.5. Much lower quality alkylates are produced, however, when dimethyUiexanes are the major isoparaffins produced RON values in such cases average less than 70. 

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