ABSTRACT
One of the most important quality criteria for gasoline is its resistance against autoignition(engineknock),thatis,octanequality(Dabelsteinet al.2012).The octanequalityofgasolineisdescribedbytheoctanenumber.Theoctanenumberofgasolineisdeterminedbycomparativemeasurementsofitsoctanequality andthatof binary mixtures having variable concentrations of n-heptane (low octane quality) and 2,2,4-trimethylpentane (isooctane, high octane quality). By de‚nition, theoctanenumberofn-heptaneis0andthatofisooctaneis100.Theoctanenumbersofmixturesaregivenbytheirpercentagebyvolumeofisooctane.Octanenumbers>100canbedeterminedwithlead-containingisooctaneortoluene-containing mixtures(Dabelsteinet al.2012).Octanenumberdeterminationiscarriedoutin single-cylinder, four-stroke test engines specially developed for the purpose, which areusedundertwodifferentrunningconditions:Theresearchoctanenumber(RON) describestheknockingperformanceatlowandmediumenginespeeds,whereas themotoroctanenumber(MON)de‚nestheknockbehaviorunderhighspeedand load(Dabelsteinet al.2012).Asdeterminingoctanenumbersintestengineswas not feasible in our laboratory, we calculated them. Since true octane numbers do not blendlinearly,itisnecessarytouseblendingoctanenumbersinmakingcalculations.Blendingoctanenumbersarebaseduponexperienceandarethosenumbers that,whenaddedonavolumetricaveragebasis,willgivethetrueoctanenumberof theblend(Garyet al.2007).Becauseourproductcompositionsaredeterminedin moles, we tested the in¸uence of calculating octane numbers in molar fractions or on avolumetricbasisandfoundthatthedifferencewasonlyindecimalplaces.Thus, we calculated the RON of our liquid product using Equation 13.1:
RON (blending RON)= = ∑ xi i i
× 1
(13.1)
where i is the number of liquid components from 1 to n xi is the molar fraction of component i
In Table 13.1, the main constituents of the products from the reaction of aromatics with ethane and propane are listed with their boiling points, their true or actual octane numbers, and their blending octane numbers. It is obvious that it is dif‚cult to determine the true octane numbers of aromatics. When regarding the blending RON numbers, it is noticeable that benzene has a blending RON of 99, which is far lower than the values of all alkylaromatics. Alkylaromatics have blending RON values of 120-146 (Owen 1984; Satter‚eld 1980). Thus, the alkylation of benzene with mixtures of ethane and propane produces high-octane fuel components and at the same
time reduces the toxicity of benzene. The next sections will go into more detail and show the different parameters that in¸uence the octane number of the liquid mixture.
For preliminary experiments, only two reactants were used (Table 13.2). The alkylation of benzene with ethane is very selective: mainly the primary alkylation product ethylbenzene (see Figure 13.1) is formed with small amounts of the secondary product toluene, which probably originates from the hydrogenolysis of ethylbenzene (Rezai et al. 2009; Vazhnova et al. 2013). Thus, the RON of the liquid product phase without benzene is with a value of 124 rather high. However, since the benzene conversion is low, the RON of the total liquid with unconverted benzene (blending RON, 99; Table 13.1) is only 102. By contrast, the alkylation of benzene with propane is not very selective: propylbenzenes, the primary alkylation products, are only formed in small amounts. Secondary products such as xylenes, ethylbenzene, and toluene (see Figure 13.1) are formed in larger amounts. The largest product fraction consists of butanes besides ethane, pentanes and some methane, which is due to disproportionation/cracking of propane (Ivanova et al. 1999; Wang et al. 2004). This reaction is thermodynamically favored (Traa 2008), which becomes also evident, if ethane and propane are converted without benzene (Table 13.2). In this case, also small amounts of ethylbenzene and toluene are produced by ethane/propane aromatization (Ono 1992). Thus, the higher reactivity of propane as compared to ethane leads to a higher
