Increasing Octane Number

Increase in the reactor temperature. In general, one research octane number increase per 17°F (10°C) increase in the reactor temperature. 

The octane number increases as the amount of branching or number of rings increases. Example 2,2,4-trimethylpentane causes a higher octane number than n-octane raethylcyclohexane causes a higher octane number than n-heptane. 

To the extent that motor octane is dependent on aromatic and isoparaffin content, motor octane increases in an FCC unit will be difficult to achieve. Thermodynamics does not allow the production of highly branched isoparaffins in an FCC unit, and blending studies show that motor octane is relatively insensitive to the aromatic content of an FCC gasoline in the range of 35-507e aromatics. However, a more aromatic gasoline may have better blending characteristics in that less aromatics from other sources will be required to achieve the same motor octane number increase. 

Pressure development due to preflame reactions of paraffin hydrocarbons is affected little by the presence of tetraethyllead even though autoignition is suppressed 25, 115). In one series of experiments 25), increasing fuel quality 10 octane numbers reduced the pressure development 10 pounds per square inch, while addition of tetraethyllead to achieve the same 10 octane number increase resulted in a decrease of only 2 pounds per square inch. These findings indicate that tetraethyllead is specific in its activity, inhibiting only certain types of reactions leading to the final autoignition step. 

Figure 7-13 shows the FT-Raman spectra of blended gasolines of various octane numbers. It may be observed that as the octane number increases there are increases in the methyl/methylene ratio (3,053/2,870 cm-1) and 1,000/2,870 cm-1 ratio, the latter indicative of aromatic additives. Table 7-2 shows the relative intensities of the aromatic bands with the grade of gasoline. The intensities at 3,053 and 1,000 cm-1 increase with octane number. The 780 cm-1 band intensity increases, and this is indicative of the substituted phenyl ring (e.g., toluene). The 743 cm-1 band increases as well, indicating a t-butyl group (iso-octane). 

The data show that methanol improved the RON for all base fuels. The effect of methanol on the MON is not nearly as pronounced as on RON. Only base fuels with a MON less than 83 showed any significant octane number increase. Base fuels with MONs above 83 showed substantial improvement or even loss in MON with the addition of methanol. 

Isomerization Reactions. In petrochemical production, the conversion of normal hydrocarbon chains to branched chains is important, since the latter has a higher gasoline octane number. When tt-pemane is isomerized to t-peniane, the octane number increases from 62 to 90 Acid-promoted AUOy is a catalyst used in such isomerization reactions.. Although this and other acid catalysts are used in isomerization reactions, it has been found that the conversion of normal paraffins to isoparaffins is easiest when both acid sites and hydrogenation sites are present, such as in the catalyst Pt on AlvOj. 

The octane number increases as the amount of branching or the number of rings increases. 

Then came the idea to treat an entire gasoline fraction, well desulfurized, over a platinum catalyst. Various supports were made up and, as expected, we did convert a part of the naphthenes into aromatics, but the octane number increase was nothing sensational. When we moved up in temperature, the catalyst lay down and died. So we ran with hydrogen and applied a moderate pressure. The results were not particularly startling but at least the catalyst survived this ordeal. So we kept moving up and, sure enough, we did get better conversions. 

Tetraethyllead (bp about 200 °C [31], 91 C/19 mmHg [32]) is the representative antiknock agent the octane number increases by 10-15 when 0.8—1.0 g per one liter of gasoline is added to gasoline. It is the lowest price substance as an antiknock agent [29], Tetramethyllead (Me4Pb, bp 100 C [32]) is a lower boiling point... 

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