Aromatics octane number

The preceding information indicates the paths to follow in order to obtain stocks of high octane number by refining. The orientation must be towards streams rich in aromatics (reformate) and in isoparaffins (isomerization, alkylation). The olefins present essentially in cracked gasolines can be used only with moderation, considering their low MONs, even if their RONs are attractive. 

Other organo-metallic structures (based on manganese in particular), based on the chemistry of -rr complexes with aromatic structures, can also be used to improve the octane number (Guibet, 1987, p. 276). 

A key process in the production of gasoline, catalytic reforming is used to increase the octane number of light crude fractions having high paraffin and naphthene contents (C7-C8-C9) by converting them to aromatics. 

The octane numbers of many pure compounds have been measured and reported in the Hterature. Probably the most comprehensive project was carried out under the auspices of the American Petroleum Institute (18). Table 2 Hsts RON and MON values for a number of representative compounds. Some aromatic compounds cannot be tested neat in the knock engine, so these are evaluated at levels of 20%, and the equivalent octane number is calculated. The values for oxygenates in Table 2 have been reported elsewhere (19). 

As can be seen from Figure 4, LBVs for these components are not constant across the ranges of composition. An iateraction model has been proposed (60) which assumes that the lack of linearity results from the iateraction of pairs of components. An approach which focuses on the difference between the weighted linear average of the components and the actual octane number of the blend (bonus or debit) has also been developed (61). The iadependent variables ia this type of model are statistical functions (averages, variances, etc) of blend properties such as octane, olefins, aromatics, and sulfur. The general statistical problem has been analyzed (62) and the two approaches have been shown to be theoretically similar though computationally different. 

All lation. The combination of olefins with paraffins to form higher isoparaffins is termed alkylation (qv). Alkylate is a desirable blendstock because it has a relatively high octane number and serves to dilute the total aromatics content. Reduction of the olefins ia gasoline blendstocks by alkylation also reduces tail pipe emissions. In refinery practice, butylenes are routinely alkylated by reaction with isobutane to produce isobutane—octane (26). In some plants, propylene and/or pentylenes (amylenes) are also alkylated (27). 

Benzene, toluene, and xylene are made mosdy from catalytic reforming of naphthas with units similar to those already discussed. As a gross mixture, these aromatics are the backbone of gasoline blending for high octane numbers. However, there are many chemicals derived from these same aromatics thus many aromatic petrochemicals have their beginning by selective extraction from naphtha or gas—oil reformate. Benzene and cyclohexane are responsible for products such as nylon and polyester fibers, polystyrene, epoxy resins (qv), phenolic resins (qv), and polyurethanes (see Fibers Styrene plastics Urethane POLYiffiRs). 

The cumene product is 99.9 wt % pure, and the heavy aromatics, which have a research octane number (RON) of 109, can either be used as high octane gasoline-blending components or combiaed with additional benzene and sent to a transalkylation section of the plant where DIPB is converted to cumene. The overall yields of cumene for this process are typically 97—98 wt % with transalkylation and 94—96 wt % without transalkylation. 

Conventional Hydrofining of diesel oils does not improve octane number because octane number improvement, like smoke point improvement in kerosenes, requires samration of aromatics. Higher pressures are needed to gain appreciable aromatics samration and cetane number improvement. 

The naphthenes and aromatics both have cyclic (or ring-like) molecular structures and both possess high octane numbers. Napthenes are saturated and aromatics contain alternate double bonds on their ring. They are typically found in gasoline. The naphthenes also are an important part of kerosene. 

Increasing the octane number of a low-octane naphtha fraction is achieved by changing the molecular structure of the low octane number components. Many reactions are responsible for this change, such as the dehydrogenation of naphthenes and the dehydrocyclization of paraffins to aromatics. Catalytic reforming is considered the key process for obtaining benzene, toluene, and xylenes (BTX). These aromatics are important intermediates for the production of many chemicals. 

Gasoline End Point. The effect of gasoline end point on its octane number depends on the feedstock quality and severity of the operation. At low severity, lowering the end point of a paraffinic feedstock may not impact the octane number however, reducing gasoline end point produced from a naphthenic or an aromatic feedstock will lower the octane. 

A tank containing 1500 m3 of naphtha is to be blended with two other hydrocarbon streams to meet the specifications for gasoline. The final product must have a minimum research octane number (RON) of 95, a maximum Reid Vapor Pressure (RVP) of 0.6 bar, a maximum benzene content of 2% vol and maximum total aromatics of 25% vol. The properties and costs of the three streams are given in the Table 3.5. 

The gasolines used in industrialized countries display high octane numbers while having no more than 43 % aromatics and up to 2 % benzene. As regards Ukrainian products, the respective values may be at 62 % and 4.6 % for gasolines produced by oil refineries and as high as 77 % and 26 % for products of coke plants. 

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