Those processes used to convert, with limited cracking, petroleum liquids into higher-octane gasoline. Due to the demand for higher-octane gasoline, thermal reforming was developed (from thermal cracking processes) to improve the octane number of fractions within the boiling range of gasoline. See also Cracking; Octane number.
Upgrading by reforming may be accomplished, in part, by an increase in volatility (reduction of molecular size) or by the conversion of n-paraffins to isoparaffins, olefins, and aromatics, and of naphthenes (cycloalkanes) to aromatics. The nature of the final product is influenced by the structure and composition of the straight-run (virgin) naphtha (hydrocarbon mixture) feedstock. In thermal reforming, the reactions resemble those in the cracking of gas oils. The molecular size is reduced, while olefins and some aromatics are synthesized. For example, hydrocracking of high-molecular-weight paraffins yields lower-molecular-weight paraffins and an olefin; dehydrocyclization of paraffin compounds yields aromatic compounds; isomerization of n-paraffins yields isoparaffins; and isomerization of methylcyclopentane yields cyclohexane.
In the presence of catalysts and in the presence of the hydrogen available from dehydrogenation reactions, hydrocracking of paraffins to yield two lower-molecular-weight paraffins takes place, and olefins that do not undergo dehydrocyclization are dehydrogenated so that the end product contains only traces of olefins. See also Dehydrogenation; Hydrocracking; Isomerization; Paraffin.
Thermal reforming was a natural development from thermal cracking, since reforming is also a thermal decomposition reaction. Cracking converts heavier oils into gasoline constituents, whereas reforming converts these gasoline constituents into higher-octane molecules. The equipment for thermal reforming is essentially the same as for thermal cracking, but higher temperatures are used. The higher octane number of the product (reformate) is due primarily to the cracking of longer-chain paraffins into higher-octane olefins. See also Distillation.
The products of thermal reforming are gases, gasoline, and residual oil. The amount and quality of the reformate are very dependent on the temperature. As a rule, the higher the reforming temperature, the higher the octane number of the product but the lower the reformate yield. Adding catalysts increases the yield for higher-octane gasolines at a given temperature.
Thermal reforming is less effective and less economical than catalytic processes and has been largely supplanted. The octane number was changed by the severity of the cracking, and the product had increased volatility, compared to the volatility of the feedstock.
Modifications of the thermal reforming process due to the inclusion of hydrocarbon gases with the feedstock are known as gas reversion and polyforming. These are essentially the same but differ in the manner in which the gases and naphtha are passed through the heating furnace. In gas reversion, the naphtha and gases flow through separate lines in the furnace and are heated independently of one another. In naphtha reforming, the C3 and C4 gases are premixed with the naphtha and pass together through the furnace.
Like thermal reforming, catalytic reforming converts low-octane gasoline into high-octane gasoline (reformate). Although thermal reforming can produce reformate with a research octane number of 65–80 depending on the yield, catalytic reforming produces reformate with octane numbers on the order of 90–95. Catalytic reforming is conducted in the presence of hydrogen over hydrogenation-dehydrogenation catalysts. Depending on the catalyst, a definite sequence of reactions takes place, involving structural changes in the feedstock. See also
