Lower-boiling naphtha fractions

The principal class of reactions in the FCC process converts high boiling, low octane normal paraffins to lower boiling, higher octane olefins, naphthenes (cycloparaffins), and aromatics. FCC naphtha is almost always fractionated into two or three streams. Typical properties are shown in Table 5. Properties of specific streams depend on the catalyst, design and operating conditions of the unit, and the cmde properties. 

These materials are extracted from the dephenolized oil with aqueous solutions of mineral acids. This operation may be carried out on the entire neutral oil, or it may be done on the solvent naphtha fraction. In the latter case, only the lowest-boiling bases (picolines and lutidines) are recovered, and die higher-boiling bases (mostly qninoline and isoquinolinc) can be recovered firom postnapthalene fractions or left in the residue for disposal. In European practice, the topping is carried out so that several fractions are obtained carbolic oil, which yields the phenols and lower-boiling bases and naphthalene oil, from which naphthalene is recovered by crystallization. 

The lower-boiling fraction of the neutral oil is a very powerful solvent, particularly for coatings containing coal tar and pitch. The material also is a source of unsaturated compounds, such as indene and, in a lesser amount, coumarone and homologues of these compounds. Resins are formed in situ from these compounds when solvent naphtha is treated with Friedel-Crafts type catalysts. See also Friedel-Crafts Reaction. These resins are useful in the manufacture of inexpensive floor tiles and coatings. The remaining solvent is recovered by distillation and used as a solvent. 

The fraction of diesel-oil-like hydrocarbons had also a triple sequence and the main aliphatic componnds may be characterized with carbon numbers 12, 15, 18, 21, 24, 27. In contrast with the experimental results of naphtha diesel oil had considerably lower concentration of aromatics. In the case of each MPW sample its concentration was not more than 1%, because aromatics with lower boiling point stayed in the naphtha-like fraction. Similarly, as mentioned above, the diesel-oil-like fraction also had favourable properties for further fuel-like application. The olefin content was a bit smaller than in case of naphtha-like fractions, because cracking reaction resulting olefins (e.g. P-scission) produced hydrocarbons with a shorter length of carbon chain. Both cetane numbers and diesel indexes of products were high enough, while the CFPP was rather low. 

Atmospheric distillation also yields lower-boiling heavy straight-run (HSR) gasoline (or HSR naphtha). Often after hydrotreating, this fraction is processed further at the catalytic reformer. 

Note A2.2—The boiling range correction is needed for cracked naphthas since it is an empirical fact that the percentage by volume of olefins is higher in the lower boiling fractions and that these olefins are also of lower relative molecular mass (molecular weight). 

When gasoline is the main product required, it is usual to remove the Cs-Ce straight-mn naphtha cut before reforming the 100-160°C fraction. This is because separate isomerization of the Cs-Ce paraffins (Section 6.10) gives an overall improvement in octane number. If aromatics are required, a lower-boiling fraction, in the range 80-120°C, is reformed. The heavy naphthas are not reformed because coke forms more readily and quickly leads to deactivation of the catalyst. 

Rerunning is a second distillation step applied to distillate stocks in order to remove undesirable higher boiling materials from the product. These materials may be present because of poor fractionation in the primary distillation step more frequently they are heavy polymers formed in treating operations. Rerunning may be combined with solvent removal, as in the case of heavy lube distillates which are diluted with naphtha to lower their viscosity before being chemically treated. 

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