Gasoline, composition octane number

The addition of an organolead compound to motor gasolines raises octane numbers, but not uniformly. The incremental increase depends on the composition of the base stock fuel, the particular lead compound employed, the increasing amount of lead compound added, the method of testing, etc. As an extremely crude indication, it may be assumed that 2 ml of tetraethyllead per gallon will result in roughly 10 octane numbers appreciation in the fuel. 

Modern automobiles with spark ignition engines need high-octane gasoline. The octane number ON), which is a measure of a fuel s antiknock properties, depends on the gasoline composition. The ONs of aromatics are highest followed by naphthenes, branched alkanes, and olefins. Normal paraffins have... 

The analyst now has available the complete details of the chemical composition of a gasoline all components are identified and quantified. From these analyses, the sample s physical properties can be calculated by using linear or non-linear models density, vapor pressure, calorific value, octane numbers, carbon and hydrogen content. 

To the refiner, the question of octane numbers in future gasolines is of primary importance because it determines the course of operations, the development or on the contrary the stagnation of such and such a process. Table 5.12 thus gives an example of the typical composition by origin and concentration of different base constituents of three grades of the most common motor fuels distributed today in Europe conventional premium gasoline at 0.15 g Pb/1, Eurosuper and Superplus. 

Figure 9.9. Gasoline composition and research octane numbers (RON). The RON scale is set by n-heptane (RON = 0) and 2,2,4-trimethylpentane (RON = 100). The RON of a...

Gasoline varies widely in composition, and even those with the same octane number may be quite different. The variation in aromatics content as well as the variation in the content of normal paraffins, branched paraffins, cyclopentane derivatives, and cyclohexane derivatives all involve characteristics of any one individual crude oil and influence the octane number of a gasoline. 

There are three different kinds of octane catalysts in current use. Some are based in part on an active non-zeolite matrix composed of a porous silica/alumina component. Others are based on low cell size (2.425-2.428 nm) ultra stable faujasite (USY), a catalyst composition developed in 1975 (2) for the purpose of octane enhancement. A third catalyst system makes use of a small amount (1-2%) of ZSM-5 as an additive. While the net effect in all cases is an increase in the measured octane number, each of the three catalytic systems have different characteristic effects on the composition and yield of the gasoline. The effects of the ZSM-5 component on cracking is described in other papers of this symposium and will not be discussed here. 

In this paper we will discuss the detailed compositional analyses generated in riser pilot plant studies with and without ZSM-5. In particular, we note the relative yields of both the branched and normal paraffins and the iso and normal olefins as a function of carbon number and relate these to the increase in motor octane. Two different base catalysts were used in this study to provide a wide range in the base gasoline composition. 

The main objective in FCC catalyst design is to prepare cracking catalyst compositions which are active and selective for the conversion of gas-oil into high octane gasoline fraction. From the point of view of the zeolitic component, most of the present advances in octane enhancement have been achieved by introducing low unit cell size ultrastable zeolites (1) and by inclusion of about 1-2 of ZSM-5 zeolite in the final catalyst formulation (2). With these formulations, it is possible to increase the Research Octane Number (RON) of the gasoline, while only a minor increase in the Motor Octane Number (MON) has been obtained. Other materials such as mixed oxides and PILCS (3,4) have been studied as possible components, but there are selectivity limitations which must be overcome. 

Often, calibration of natural products and materials is a desirable goal. In these kinds of assays, it is usually not feasible to control the composition of calibration and validation standards. Some well-known examples include the determination of protein, starch, and moisture in whole-wheat kernels and the determination of gasoline octane number by NIR spectroscopy. In cases such as these, sets of randomly selected samples must be obtained and analyzed by reference methods. 

After the addition of tetraethyllead to gasoline was prohibited, the oil companies were forced to make unleaded gasolines. In order to prevent engine knocking, the car manufacturers lowered the compression ratio of the engine, and oil companies changed the hydrocarbon composition of gasolines to incorporate more branched alkanes and aromatics to increase the octane number. Benzene and toluene were some additives, as well as ethanol in some cases. 

The introduction of catalytic converters has had a tremendous impact on the composition of gasoline. The catalysts used became poisoned by small amounts of impurities in particular the lead compounds present in high octane gasoline were detrimental. Processes which produce high octane number compounds were therefore stimulated. First, cracking and reforming increased in importance. More recently, the aromatics content is also expected to have to decrease and alternative processes are in use or under way, e.g. the production of MTBE (methyl tertiary-butyl ether). 

Research and motor octane numbers are calculated for the gasoline fraction using a GC-based compositional octane model. 

Figure 7 shows calculated octane numbers from hexadecane cracking as a function of gasoline yield. Calcined and steamed zeolites are represented by open and closed symbols, respectively. The calculated octane number reflects changes in the gasoline molecular weight distribution and, to a lesser extent, composition effects. 

M. E. Myers, Jr, J. Stollsteimer, and A. M. Wims, Determination of gasoline octane numbers from chemical composition. Anal Chem., 47, 2301-2304 (1975). 

In some cases the octane number has a maximum, or a minimum, value at a particular composition. Figures 7.5 and 7.6 show some examples in which the effect is particularly apparent, because here the component gasolines have the same octane number, although the compositions are different. Alkene fuels blended with alkanes tend to produce positive deviations from linear blending for RON (represented by equality in equation (7.1)), and possibly maxima, as in Fig. 7.5. Figure 7.6 shows strong negative effects in MON when alkanes and aromatics are blended. 

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