Normal pentane, octane number

Isomerization. Isomerization is a catalytic process which converts normal paraffins to isoparaffins. The feed is usually light virgin naphtha and the catalyst platinum on an alumina or zeoflte base. Octanes may be increased by over 30 numbers when normal pentane and normal hexane are isomerized. Another beneficial reaction that occurs is that any benzene in the feed is converted to cyclohexane. Although isomerization produces high quahty blendstocks, it is also used to produce feeds for alkylation and etherification processes. Normal butane, which is generally in excess in the refinery slate because of RVP concerns, can be isomerized and then converted to alkylate or to methyl tert-huty ether (MTBE) with a small increase in octane and a large decrease in RVP. 

Octane number is a measure of a fuel s abiUty to avoid knocking. The octane number of a gasoline is deterrnined in a special single-cylinder engine where various combustion conditions can be controlled. The test engine is adjusted to give trace knock from the fuel to be rated. Various mixtures of isooctane (2,2,4-trimethyl pentane) and normal heptane are then used to find the ratio of the two reference fuels that produce the same intensity of knock as that by the unknown fuel. 

The octane number of a fuel is a measure of the tendency of the fuel to knock. The octane scale has a minimum and maximum based on the performance of reference fuels. In the laboratoi"y, these are burned under specific and preset conditions. One reference fuel is normal heptane. This is a very poor fuel and is given an octane rating of zero. On the opposite end of the scale is iso-octane (2,2,4 trimethyl pentane). Iso-octanc is a superior fuel and is given a rating of 100. 

The isomerization process is utilized to convert light paraffins such as butane, pentane, and hexane into higher-octane isoparaffins. Isoparaffins have higher octane numbers than normal paraffins of the same carbon number. For example, n-pentane has a research octane number of about 61, and isopentane has an octane number of approximately 92. 

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. 

With all of the catalysts in this study, the ratio of isopentane to normal pentane varies directly as the ratio of isohexanes to n-hexane. (In this paper, the branched C6 paraffins are collectively referred to as isohexanes.) Therefore, the iso/normal ratio of the paraffins of either carbon number can be correlated with octane number for C5-l80°F product of a given cycloparaffin content and serve as a convenient indication of catalyst performance. 

The temperature of isomerization controls equilibrium isomer composition, and thereby product octane. Figure 4.8 is a plot of isopentane in the C5 product as a function of temperature. The data are from pilot plant runs with three types of commercial UOP isomerization catalysts. The feedstock was a 50/50 mixture of normal pentane and normal hexane, containing about 6% cyclics. The 1-8 and I-80 catalysts are very active at a low temperature, where equilibrium isopentane content is highest. The acid functions in 1-8 and 1-80 are chlorided aluminas. The zeolitic catalyst, HS-10 , requires relatively high temperatures of operation. The LPI-100 catalyst contains sulfated zirconia as the acid function and falls in the middle of the temperature range (12). Due to the equilibrium constraints, a lower temperature operation yields a higher octane product. The 1-8 and 1-80 catalysts yielded Research Octane Numbers of 82-84, as compared to 80-82 for LPI-100 catalyst and 78-80 for HS-10. 

Normal pentane has an oetane number of 62, while iso-pentane has an octane number of 90 The n-pentane adsorbs onto the platinum, where it is dehydrogenated to form n-pentene. The n-pentene desorbs from the platinum arid adsorbs onto the almnina, where it is isomerized to i-pentene, which then desorbs and subsequently adsorbs onto platinum, where it is hydrogenated to form i-pentane. That is,... 

Reforming and Isomerization Feedstocks. Some saturated fractions contain substantial amounts of low octane-number normal pentane or normal hexane. These chemical species can be isomerized for antiknock performance enhancement. It is, however, useful to remove branched compounds from the feedstock by sharp distillation. A de-isohexanizer, for example, may need 60 or more actual valve plates and a high reflux ratio. The modeling of equipment for this purpose follows standard distillation design procedures, but because of the narrow relative volatilities, special care is needed with the basic data. 

The various types of hydrocarbons in gasoline behave differently in their preflame reactions and thus, their tendency to knock. It is difficult to find any precise relationship between chemical structure and antiknock performance in an engine. Members of the same hydrocarbon series may show very different antiknock effects. For example, normal heptane and normal pentane, both paraffins, have antiknock ratings (octane numbers) of 0 and 61.9, respectively (Table 5.5). Very generally, aromatic hydrocarbons (e.g., benzene and toluene), highly branched iso-paraffins (e.g., iso-octane), and... 

As we have seen, isomerization occurs as a side-reaction in all conversion processes. But in refining, when we say isomerization process, we are referring specifically to the on-purpose isomerization of n-butane, n-pentane, and n-hexane. The main purpose of n-paraffin isomerization is to produce isoparaffins with substantially higher octane numbers. An isomerization reaction for normal hexane was shown in Figure 17. 

Normal pentane has an octane number of 62. while /so-pentane, which is nK/re Cs 10% compact, has an octane number of 90 The n-pentane adsorbs onto the platt-... 

The main commercial applications during World War II and in a limited way thereafter were the isomerization of butane to isobu-tane, of pentane to isopentane,and of naphtha or normal hexane fraction into isohexanes (or a naphtha of higher octane number). In these processes aluminum chloride is used in several ways, (1) with anhydrous hydrochloric acid - as a liquid slurry or complex, (2) on a granular alumina or bauxite support, or (3) dissolved in molten antimony trichloride. In all processes the feed must be dried so that moisture is not carried into the acid zone, but corrosion may be severe. 

The value k n decreases for normal alkanes with increasing n. This effect reflects the growth of steric barriers as a result of the hydrophobic molecule rolling itself up into a coil . For n-octane, almost half the carbon atoms do not participate in the reaction. It should be noted that the rate constant of the oxidation remains practically unchanged despite the increase of the number of carbon atoms from pentane to octane. This may mean that only the C-H bonds of the primary and secondary carbon atoms from the hydrocarbon are reactive, i.e., ti)- and CO-1-carbon atoms. If this is so, then steric factors in the alkane oxidation by platinum complexes produce the same effect as they do in the selective oxidation under the action of monooxygenase with participation of cytochrome P450 and methane monooxygenase. 

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