Gasoline additives, octane numbers

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. 

Addition Reactions of Alkynes A WORD ABOUT... Petroleum, Gasoline, and Octane Number... 

This section reviews examples of results obtained from the catalytic cracking runs conducted in the Riser Simulator. These runs show the ability of the Riser Simulator to assess catalyst performance. First, the trends in conversion of gas oils, yields of products and gasoline research octane numbers will be discussed for both the commercial feedstocks and for the pure light oil mixtures used. Then the kinetic parameters obtained from the 3-lump model and an 8-lump model using various decay functions are presented. Additional details about product distribution are provided in Kraemer (1991). 

In relatively small doses (see Chapter 5), additives made it possible for the refiner to gain several points in octane number and thereby to allow the premium gasoline to meet specifications. 

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. 

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. 

Table 1 shows the major gasoline additives that were introduced from the 1920s through the 1980s. The increase in octane number of gasoline with use of these additives is shown. 

Benzol and other alcohol-hased additives improved octane number, up to a point. Experiments using alcohol (ethanol, methanol) as a replacement for gasoline began as early as 1906. In 1915, Hetu-y Ford announced a plan to extract alcohol from grain to power his new Fordson tractor, an idea that never achieved commercial success. 

Unbumt gasoline and cracked hydrocarbons such as ethylene and propylene are also substantial constituents of exhaust. Gasoline contains additives such as benzene, toluene and branched hydrocarbons to achieve the necessary octane numbers. The direct emission of these volatile compounds, e.g. at gas stations, is a significant source of air pollution. Leaded fuels, containing antiknock additions such as tetra-ethyl-lead, have been abandoned because lead poisons both human beings and the three-way exhaust catalyst, especially for the removal of NO by rhodium. 

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... 

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. 

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. 

Isomerization of paraffins using current octane catalysts under current conditions is favorably away from equilibrium. Additional isomerization activity would make more normal paraffins and a lower octane at FCC temperatures. A much more olefinic gasoline is a possibility. However, additional olefins above the current olefin levels of 10-30% would have decreased effectiveness, especially on the motor octane number. 

The efficacy of an additive, such as tetraethyl lead on the octane number of gasoline, shows a concave curve so that the first cubic centimeter of additive would achieve much more than the second cubic centimeter, and the third cubic centimere would do even less. This can also be described by the term diminished return, which is described by d f/dx < 0, which is the characteristic of a function with a diminishing value of df/dx with increase in x. 

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