Low octane number

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. 

The naphtha fraction is dorninated by saturates having lesser amounts of mono- and diaromatics (Table 2, Eig. 4). Whereas naphtha (ibp to 210°C) covers the boiling range of gasoline, most raw petroleum naphtha molecules have a low octane number and most raw naphtha is processed further, to be combined with other process naphthas and additives to formulate commercial gasoline. 

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. 

On account of its faint odour, leakage is not easily identified, (2) Handling must be under pressurized conditions, (3) Suitability under high compression ratios, (4) Exhibits a low octane number and a high load sensitivity, (5) Shows very poor response to blending. Overall LPG seems suitable as a fuel for trucks and tractors. 

The main components of FCC catalysts are Zeolite Y, e.g., REY orUSY as the major active component (10 to 50%), and a binder that is typically an amorphous alumina, silica-alumina, or clay material. In addition to these main components, other zeolite components, e.g., ZSM-5, and other oxide or salt components are quite frequently used additives in the various FCC catalysts available on the market. The addition of 1 to 5% ZSM-5 increases the octane number of the gasoline. ZSM-5 eliminates feed compounds with low octane numbers because it preferentially center-cracks n-paraffins producing butene and propene [14], These short-chain olefins are then used as alkylation feedstocks... 

Reforming the conversion of hydrocarbons with low octane numbers into hydrocarbons having higher octane numbers (e.g., the conversion of an n-paraffin into an isoparaffin). 

In addition to this, solid acid catalysts can also be used in the hydroisomerization cracking of heavy paraffins, or as co-catalysts in Fischer-Tropsch processes. In the first case, it could also be possible to transform inexpensive refinery cuts with a low octane number (heavy paraffins, n-Cg 20) to fuel-grade gasoline (C4-C7) using bifunctional metal/acid catalysts. In the last case, by combining zeolites with platinum-promoted tungstate modified zirconia, hybrid catalysts provide a promising way to obtain clean synthetic liquid fuels from coal or natural gas. 

Namral gasoline a gasoline obtained by recovering the butane, pentane, and hexane hydrocarbons present in small proportions in certain natural gases. Used in blending to produce a finished gasoline with adjusted volatility, but low octane number. Do not confuse with natural gas (q.v.). 

Napththa Isomerization. The only commercial isomerization of light naphtha was carried out in two plants employing the isomate process developed by the Standard Oil Co. (Indiana) (20). In this process, a feed containing normal pentane and low octane number hexanes is converted to isopentane and to hexanes of higher octane number. Pentanes and hexanes in any ratio may be processed. By recycle of selected fractions of the product, concentrates of isopentane or of neohexane and diisopropyl can be obtained as the ultimate products. 

The reactor-outlet stream contains a dispersion of hydrocarbons in sulfuric acid. The first separation step is therefore a liquid-liquid split The sulfuric-acid phase contains some amounts of sec-butyl acid sulfate, which decomposes at higher temperature (15 °C) to produce conjuct polymers dissolved in the acid and a mixture of C4-C1( isoparaffins with low octane number (pseudoalkylate) that separates as a second liquid phase. The hydrocarbon phase contains a small amount of di-isoalkyl sulfates. These need to be removed before entering the distillation units otherwise they will decompose and release sulfuric acid. The sulfates are removed by washing with either dilute caustic or sulfuric acid. In the first case, sulfates are converted to salts that are discarded. With sulfuric acid, sulfates are converted to isoalkyl acid sulfates that can be recycled to the alkylation reactor [15, 10]. 

The purpose of catalytic reforming is to boost the octane number by converting molecules with a low octane number into molecules with a high octane number (see Table 2.1). 

The effect of water in HF alkylation catalyst is an Important one it appears to be that of slowing polymerization. The production of low-octane-number residue can be reduced by 50 percent when catalyst water content is optimized. 

Thus, good dispersion or mass transfer favors olefin isomerization (to isobutene), isobutene dimerization, and maximizes hydrogen transfer and primary alkylation reactions, i.e., yielding the greatest amount of high-octane-number trimethylpentanes, and minimizing low-octane-number byproducts from secondary reactions such os excess polymerization. 

Reactions of Butyl Sulfates. At 0 C or higher, sec-butyl sulfate is unstable (5). When it decomposes in the absence of isobutane, both a low quality (i.e. low octane number) hydrocarbon mixture and acid-soluble hydrocarbons are produced. The products obtained by such decompositions are very similar to those produced when pure C4 olefins are contacted with sulfuric acid at 10 C (1,3). It seems safe to conclude that the Initial reaction is the reverse of Reaction 1-1 as shown next ... 

The difference between the RON and MON values for a fuel is called the sensitivity. The sensitivity for most practical paraffinic (e.g., alkylate) fuels is close to zero (and exactly so, by definition, for n-heptane and isooctane), but Fig. 7.2 shows that some low octane number alkanes have values of MON > RON. However, in the practical range of ON (>70) for aromatics and especially alkenes, the MON is usually less than the RON. For commercial gasolines the sensitivity is about 10 octane numbers, depending on the aromatic and alkene content. Although the RON of fuels is more usually quoted and is determined in the CFR test more... 

Both types of anti-knock are more effective in paraffinic fuels then in olefinic or aromatic fuels, and can even promote knock when added to some alcohols. In Fig. 7.9 the response of some pure hydrocarbons to the addition of 3ml/US gal of tetra-ethyl lead is shown, in terms of Performance Number. Almost all the alkanes lie on a steeper line than the alkenes. The exceptions are low octane number alkenes, which are largely straight alkane chains, and a few highly-branched alkanes (which also have high sensitivity, see Section 7.2.3). Notwithstanding the subtleties of lead additives, a broad explanation in chemical kinetic terms is that the antiknock acts to increase radical termination rates and, consequently, has proportionately less effect in those fuels where the termination rates are already high. 

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