Petroleum, major fractions obtained from distillation

TABLE 3-3 C Major Fractions Obtained from Distillation of Crude Petroleum... 

The first step in refining petroleum is a careful fractional distillation. The products of that distillation are not pure alkanes but mixtures of alkanes with useful ranges of boiling points. Table 3-3 shows the major fractions obtained from distillation of crude petroleum. 

Petroleum crude and its refinery products have two major component based on distillation. The portion that can be distilled under refinery conditions can be called volatiles and the nondistillables are the nonvolatiles. The volatiles can be analyzed by GC or GC-MS. The crude has both components. The distillate as the names applied, such as naphtha and kerosene contain only volatiles. When GPC is used for analyzing various distillates, the fractions separated by GPC can be characterized by GC or GC-MS. These data can be used to verify the nature of components present in various distillation cuts as a function of GPC elution volume. If the samples such as crude contains both volatiles as well as nonvolatiles, the samples should be separated into volatiles and nonvolatiles. The GPC of both components should be used to calibrate the GPC of the total crude. The parameter that can be obtained from GPC is effective molecular length. It can be used to relate other molecular parameters of interest after calibration. 

The saturates remain the major component in the mid-distillate fractions of petroleum but aromatics, which now include simple compounds with up to three aromatic rings, and heterocyclic compounds are present and represent a larger portion of the total. Kerosene, jet fuel and diesel fuel are all derived from middle distillate fractions and can also be obtained from cracked and hydropro-cessed refinery streams. 

Butadiene is obtained directly by the thermal cracking of petroleum fractions in the naphtha to gas oil range (section 2.2). This process is directed mostly towards the production of ethylene and propylene but butadiene is always a co-product. The butadiene is separated from the C4 stream by extractive distillation, in which distillation is carried out in the presence of a solvent (such as acetonitrile, furfural or N-methylpyrrolidone) which decreases the volatility of the butadiene relative to the other C4 compounds. Thermal cracking represents the major source of butadiene in Europe and Japan and is increasing in importance in the USA. 

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