Gasoline isomerate fractionation

As a complementary process to reforming, isomerization converts normal paraffins to iso-paraffins, either to prepare streams for other conversions nCi —> /C4 destined for alkylation or to increase the motor and research octane numbers of iight components in the gasoiine pooi, i.e., the C5 or Cs-Ce fractions from primary distillation of the crude, or light gasoline from conversion processes, having low octane numbers. 

Isomerization is a small-volume but important refinery process. Like alkylation, it is acid catalyzed and intended to produce highly-branched hydrocarbon mixtures. The low octane C5/C6 fraction obtained from natural gasoline or from a light naphtha fraction may be isomerized to a high octane product. 

Iso-Kel [Isomerization-Kellogg] A fixed-bed, vapor-phase isomerization process for making high-octane gasoline from aliphatic petroleum fractions. The catalyst is platinum on alumina. Developed by MW Kellogg. 

RCH [Ruhrchemie] A process for increasing the octane rating of gasolines by catalytic isomerization of the olefin fraction, the double bonds migrating from the terminal positions. Developed by Ruhr Chemie in the 1940s. 

While such data are not included in this report, our work has also shown that in FCC operations the acidic matrix is able to better crack and isomerize more n-paraffins, thus reducing the n-paraffin content in middle distillate and thereby the characteristic high cloud point associated with it, while also raising octane number by removing or isomerizing n-paraffins in the gasoline fraction. 

Tn commercial petroleum refining, isomerization of light paraffins has been A applied for many years. Until recently the scope of the process was limited, however, and its main application was isomerization of butane as feed preparation for alkylation processes. Generally, except for a few specific cases, no commercial justification could be found for isomerization of pentane and hexane fractions since in most cases the quality requirements for motor gasoline could be met by alternative processing routes and by addition of various additives, such as lead tetraalkyls, to improve fuel burning characteristics. 

When naphtha or naphthenic gasoline fractions are catalytically reformed, they usually yield a Cx aromatics stream that is comprised of mixed xylenes and ethylbenzene. It is possible to separate the ethylbenzene and o-xylene by fractionation. It is uneconomic to separate the m- and p-xylenes in this manner because of the closeness of their boiling points. To accomplish the separation, a Werner-type complex for selective absoiption of p-xylene from the feed mixture may be used. Or, because of the widely different freezing points of the two xylene isomers, a process of fractional crystallization may be used. To boost the p-xylene yield, die filtrate from the crystallization step can be catalytically isomerized. 

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