Aromatic hydrocarbons octane rating

Reforming (Section 2 16) Step in oil refining in which the pro portion of aromatic and branched chain hydrocarbons in petroleum is increased so as to improve the octane rating of gasoline... 

Source Schauer et al. (1999) reported naphthalene in diesel fuel at a concentration of 600 pg/g and in a diesel-powered medium-duty truck exhaust at an emission rate of 617 pg/km. Detected in distilled water-soluble fractions of 87 octane gasoline (0.24 mg/L), 94 octane gasoline (0.21 mg/L), Gasohol (0.29 mg/L), No. 2 fuel oil (0.60 mg/L), jet fuel A (0.34 mg/L), diesel fuel (0.25 mg/L), military jet fuel JP-4 (0.18 mg/L) (Potter, 1996), and used motor oil (116 to 117 pg/L) (Chen et al, 1994). Lee et al. (1992) investigated the partitioning of aromatic hydrocarbons into water. They reported concentration ranges from 350 to 1,500 mg/L and 80 to 300 pg/L in diesel fuel and the corresponding aqueous phase (distilled water), respectively. Diesel fuel obtained from a service station in Schlieren, Switzerland contained 708 mg/L naphthalene (Schluep et al, 2001). 

Lead. The use of unleaded gasoline is rapidly allowing a solution to this problem. But is the increasing use of aromatic hydrocarbons, necessary for acceptable octane ratings in unleaded gasoline, causing... 

An arbitrary mixture of hydrocarbons is compared to a mixture of these two compounds, with its octane number that equal to the appropriate mixture of these standard compounds. Some molecules and their octane ratings are indicated in Table 2-6. Aromatics have a high octane number (toluene is 120), and some compounds such as tetraethyl lead have a strong octane enhancement when added to other mixtures (blending octane number). Oxygenates such as ethanol and ethers (MTBE) have fairly high octane numbers and supposedly produce less pollution, either alone or blended with hydrocarbons. 

Reforming a naphtha to a higher octane rating must involve at least one of the following chemical reactions (a) production of aromatics, (6) production of highly branched paraffins, (c) production of olefins, or (d) lowering the molecular weight of the hydrocarbons in the naphtha. 

The major role of ZSM-5 is to react with converted products and, in particular, certain gasoline components. The shape selectivity of the zeolite allows only linear and mono-methyl paraffins and olefins ready access to active sites whilst other structures, such as aromatics and multi-branched paraffins, will be restricted [2]. Octane enhancement with ZSM-5 arises from this shape selectivity as the most accessible hydrocarbons are also octane depressants, whilst those structures precluded from the zeolite structure e.g. aromatics have high octane ratings. 

Benzene and toluene, the simplest aromatic hydrocarbons obtained from petroleum refining, are useful starting materials for synthetic polymers. They are two components of the BTX mixture added to gasoline to boost octane ratings. 

Endothermic Reactions. Another example of the need for interstage heat transfer in a series of reactors can be found when upgrading the octane number of gasoline. The more compact the hydrocarbon molecule for a given number of carbon atoms, the higher the octane rating. Consequently, it is desirable to convert straight-chain hydrocarbons to branched isomers, naphthenes, and aromatics. The reaction sequence is... 

Other things equal, branched hydrocarbons have higher octane ratings than unbranched ones. —> Isomerization and aromatization are used to improve octane rating of gasoline Isomerization is used to convert straight chain hydrocarbons to branched ones ... 

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