3.2. FOSSIL FUELS
3.2.2. Gasoline
Gasoline, also called gas (United States and Canada), or petrol (Great Britain) or benzine (Europe) is one of several liquid fuels produced from petroleum (Table 3.3) and is a mixture of volatile, flammable liquid hydrocarbons deri-ved from petroleum and used as fuel for internal-combustion engines [2]. Gasoline was
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TABLE 3.3 General Properties of Liquid Products from Petroleum
Chemical
formula Mol. wt
Composition (wt%)
Specific
gravity Boiling point (C)
Autoignition temperature (C)
Flash point (C)
Heating values (kJ/kg)
Flammability limits (vol%)
C H LHV HHV
Gasoline C4eC12 113 85e88 12e15 0.720 35e225 257 43 43,500 46,500 1.4e7.6
Kerosene C10eC16 170 85 15 0.800 150e300 210 37e72 43,100 46,200 0.7e5.0
JP-4 C6eC11 119 86 14 0.751e0.802 45e280 246 23 - 1 42,800 45,800 1.3e8.0
JP-7 C10eC16 166 87 13 0.779e0.806 60e300 241 43e66 43,500 46,800 0.6e4.6
Diesel fuel C8eC25 200 87 13 0.850 180e340 315 60e80 42,800 45,800 1.0e6.0
Fuel oil #1 C9eC16 200 87 13 0.875 150e300 210 37e72 0.7e5.0
Fuel oil #2 C11eC20 0.920 256 52e96
Fuel oil #4 198 0.959 263 61e115
Fuel oil #5 0.960 69e169
Fuel oil #6 0.960 175e345 66
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originally a by-product of the petroleum industry (kerosene being the principal product) and became the preferred automobile fuel because of its high energy of combustion and capacity to mix readily with air in a carburetor of the internal combustion engine.
Gasoline is a mixture of hydrocarbons that usually boil below 180C (356 F) or, at most, below 200C (392F). The hydrocarbon constit-uents in this boiling range are those that have
4e12 carbon atoms in their molecular structure and fall into three general types: alkanes (including the cycloalkanes and branched mate-rials), alkenes, and aromatics (Table 3.4).
Gasoline is manufactured to meet specifica-tions (ASTM D4814) and regulaspecifica-tions and not to achieve a specific distribution of hydrocar-bons by class and size. However, chemical composition often defines properties. For example, volatility is defined by the individual TABLE 3.4 Properties of Hydrocarbons Generally Found in Liquid Products from Petroleum
Molecular
weight Specific
gravity Boiling
point (C) Ignition
temperature (C) Flash
point (C) Flammability limits in air (% v/v)
n-Pentene 70.1 0.641 30 260 49 1.6e7.7
iso-Pentane 72.1 0.621 28 420 51 1.3e9.2
Neopentane 72.1 10 449 61 1.4e7.1
n-Pentane 72.1 0.626 36 260 40 1.4e7.8
Benzene 78.1 0.879 80 560 11 1.3e6.7
Neohexane 86.2 0.649 50 425 48 1.2e7.6
n-Hexane 86.2 0.659 69 225 22 1.2e7.0
Toluene 92.1 0.867 161 533 4 1.2e6.8
n-Heptane 100.2 0.668 215 215 4 1.0e6.0
Xylene 106.2 0.861 138 464 17 1.0e6.0
iso-Octane 114.2 0.702 118 447 12 0.8e5.9
n-Octane 114.2 0.707 126 220 13 0.9e3.2
Naphthalene 128.2 1.162 218 525 79 0.9e5.9
t-butylbenzene 134.2 0.867 169 450 44 0.7e5.7
trans-Decalin 138.2 0.870 187 250 57 0.7e4.9
cis-Decalin 138.2 0.896 193 250 57 0.7e4.9
n-Decane 142.2 0.735 174 210 46 0.8e5.4
1-methylnaphthalene 142.2 1.025 242 529 82 0.7e6.5
n-Dodecane 170.3 0.753 216 203 71 0.6ea
n-Tetradecane 198.4 0.767 250 200 99 0.5ea
n-Hexadecane (n-cetane) 226.4 0.773 287 201 135
iso-Cetane 226.4 0.793 240 102
aNo data available for upper flammability limit (UFL)
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hydrocarbon constituents, and the lowest boiling constituent(s) defines the volatile as determined by certain test methods.
Gasoline was at first produced by distillation, simply separating the volatile, more valuable fractions of crude petroleum [2]. Later pro-cesses, designed to raise the yield of gasoline from crude oil, thermally decomposed (cracked) higher molecular weight constituents into lower molecular weight products.
Thermal cracking, employing heat and high pressures, was introduced in 1913 but was replaced after 1937 by catalytic cracking, the application of catalysts that facilitate chemical reactions producing more gasoline. Other meth-ods used to improve the quality of gasoline and increase its supply include polymerization, alkylation, isomerization, and reforming. In a modern refinery, the product of each of these processes is a blending stock for the production of sales gasoline.
Polymerization is the conversion of gaseous alkenes, such as propylene and butylene, into larger molecules in the gasoline range. Alkyl-ation is a process combining an alkene and alkane such asiso-butane. Isomerization is the conversion of straight-chain hydrocarbons to branched-chain hydrocarbons. Reforming is the use of either heat or a catalyst to rearrange the molecular structure.
3.2.2.1. Automotive Gasoline
Automotive gasoline typically contains about almost 200 (if not several hundred) hydrocarbon compounds and the relative concentrations of the compounds vary considerably depending on the source of crude oil, refinery process, and product specifications. Typical hydrocarbon chain lengths range from C4through Cl2with a general hydrocarbon distribution consisting of alkanes (4e8%), alkenes (2e5%), iso-alkanes (25e40%), cycloalkanes (3e7%), cycloalkenes (le4%), and aromatics (20e50%). However, these proportions vary greatly. Highly branched alkanes, which are particularly valuable constituents of gasoline(s),
are not usually the principal alkane constituents of straight-run gasoline. The more predominant alkane constituents of straight-run gasoline are usually the normal (straight-chain) isomers, which may dominate the branched isomer(s) by a factor of 2 or more. This is presumed to indicate the tendency to produce long uninterrupted carbon chains during petroleum maturation rather than those in which branching occurs.
However, this trend is somewhat different for the cyclic constituents of gasoline, i.e., cycloal-kanes (naphthenes) and aromatics. In these cases, the preference appears to be for several short side chains rather than one long substituent.
Trace amounts of sulfur compounds also occur in gasoline. On January 1, 2006, the sulfur content for gasoline produced at most refineries was lowered to a per-gallon maximum of 80 parts per million (ppm), with an overall maximum annual average of 30 ppm. The per-gallon sulfur limit for some gasoline in the Rocky Mountain area was lowered to 80 ppm on January 1, 2007; this standard does not apply to all gasoline, because there are different regu-lations for some gasoline in the Rocky Mountain area and small refiners.
3.2.2.2. Aviation Gasoline
Aviation gasoline is a form of automotive gasoline that has been especially prepared for use for aviation piston engines. It has an octane number suited to the engine, a freezing point of
60C (76F), and a distillation range usually within the limits of 30e180C (86e356 F) compared to 1e200C (30e390F) for auto-motive gasoline. The narrower boiling range ensures better distribution of the vaporized fuel through the more complicated induction systems of aircraft engines. Aircraft operates at altitudes at which the prevailing pressure is less than the pressure at the surface of the earth (pressure at 5300 m is 0.05 MPa compared to 0.1 MPa at the surface of the earth). Thus, the vapor pressure of aviation gasoline must be limited to reduce boiling in the tanks, fuel lines,
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and carburetors. As a consequence, aviation gasoline does not usually contain the gaseous hydrocarbons (butanes) that give automobile gasoline the higher vapor pressures for starting at cold temperatures.
Aviation gasoline is composed of alkanes and iso-alkanes (50e60%), moderate amounts of cyc-loalkanes (20e30%), small amounts of aromatics (10%), and usually no alkenes, whereas motor gasoline may contain up to 30% alkenes and up to 40% aromatics.
The manufacture of aviation gasoline is dependent on the availability and selection of fractions containing suitable hydrocarbons.
The lower boiling hydrocarbons are usually found in straight-run naphtha from certain crude petroleum. These fractions have high contents of iso-pentanes and iso-hexane and provide needed volatility, as well as high octane number components. Higher boilingiso-alkanes are provided by aviation alkylate, which consists mostly of branched octanes. Aromatics, such as benzene, toluene, and xylene, are obtained from catalytic reforming or a similar source.
3.2.2.3. Gasohol
In the late twentieth century, the rising price of petroleum (and hence of gasoline) led to the increasing use of gasohol, which is a mixture of 90% unleaded gasoline and 10% ethanol (ethyl alcohol). Gasohol burns well in gasoline engines and is a desirable alternative fuel for certain applications because of the availability of ethanol, which can be produced from grains, potatoes, and certain other plant matter. Meth-anol and a number of other alcohols and ethers are considered high-octane enhancers of gaso-line [6]. They can be produced from various hydrocarbon sources other than petroleum and may also offer environmental advantages insofar as the use of oxygenates would presum-ably suppress the release of vehicle pollutants into the air.
3.2.2.4. Octane Rating
Gasoline performance and hence quality of an automotive gasoline are determined by its resistance to knock, for example, detonation or ping during service. The antiknock quality of the fuel limits the power and economy that an engine using that fuel can produce: the higher the antiknock quality of the fuel, the more the power and efficiency of the engine.
In 1922, tetraethyl lead was discovered to be an excellent antiknock material when added in small quantities to gasoline, and gasoline con-taining tetraethyl lead became widely available.
However, the problem of how to increase the antiknock characteristics of cracked gasoline became acute in the 1930s. One feature of the problem concerned the need to measure the anti-knock characteristics of gasoline accurately. This was solved in 1933 by the general use of a single-cylinder test engine, which allowed compari-sons of the antiknock characteristics of gasoline to be made in terms of octane numbers. The octane numbers formed a scale ranging from 0 to 100: the higher the number, the greater the antiknock characteristics. In 1939, a second and less severe test procedure using the same test engine was developed, and results obtained by this test were also expressed in octane numbers.
Octane numbers are obtained by the two test procedures: those obtained by the first method are called motor octane numbers (indicative of high-speed performance) (ASTM D2700 and ASTM D2723) and those obtained by the second method are calledresearch octane numbers (indic-ative of normal road performance) (ASTM D2699 and ASTM D2722). Octane numbers quoted are usually, unless stated otherwise, research octane numbers.
In the test methods used to determine the antiknock properties of gasoline, comparisons are made with blends of two pure hydrocar-bons,n-heptane and iso-octane (2,2,4-trimethyl-pentane). Iso-octane has an octane number of 100 and is high in its resistance to knocking;
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n-heptane is quite low (with an octane number of 0) in its resistance to knocking.
n-Alkanes have the least desirable knocking characteristics, and these become progressively worse as the molecular weight increases. Iso-alkanes have higher octane numbers than the corresponding normal isomers, and the octane number increases as the degree of branching of the chain is increased. Alkenes have markedly higher octane numbers than the related alkanes;
naphthenes are usually better than the corre-sponding n-alkanes but rarely have very high octane numbers; aromatics usually have quite high octane numbers.
3.2.2.5. Additives in Gasoline
Additives are gasoline-soluble chemicals that are mixed with gasoline to enhance certain performance characteristics or to provide char-acteristics not inherent in the gasoline. Additives are generally derived from petroleum-based materials, and their function and chemistry are highly specialized. They produce the desired effect at the parts-per-million (ppm) concentra-tion range.
3.2.2.5.1. ANTIOXIDANTS
Oxidation inhibitors (antioxidants) are aromatic amines and hindered phenols that prevent gasoline components (particularly alkenes) from reacting with oxygen in the air to form peroxides or gums. Peroxides can degrade antiknock quality, cause fuel pump wear, and attack plastic or elastomeric fuel system parts, soluble gums can lead to engine deposits, and insoluble gums can plug fuel filters. Inhibiting oxidation is particularly important for fuels used in modern fuel-injected vehicles, as their fuel recirculation design may subject the fuel to more temperature and oxygen-exposure stress.
3.2.2.5.2. CORROSION INHIBITORS
Corrosion inhibitors are carboxylic acids and carboxylates that prevent free water in the
gasoline from rusting or corroding pipelines and storage tanks. Corrosion inhibitors are less important once the gasoline is in the vehicle.
The metal parts in the fuel systems of today’s vehicles are made of corrosion-resistant alloys or of steel coated with corrosion-resistant coat-ings. More plastic parts are replacing metals in the fuel systems and, in addition, service station systems and operations are designed to prevent free water from being delivered to a vehicle’s fuel tank.
3.2.2.5.3. DEMULSIFIERS
Demulsifiers are polyglycol derivatives that improve the water-separating characteristics of gasoline by preventing the formation of stable emulsions.
3.2.2.5.4. ANTI-ICING
Anti-icing additives are surfactants, alcohols, and glycols that prevent ice formation in the carburetor and fuel system. The need for this additive is being reduced as older-model vehi-cles with carburetors are replaced by vehivehi-cles with fuel injection systems.
3.2.2.5.5. DYES AND MARKERS
Dyes are oil-soluble solids and liquids used to visually distinguish batches, grades, or appli-cations of gasoline products. For example, gaso-line for general aviation, which is manufactured to different and more exacting requirements, is dyed blue to distinguish it from motor gasoline for safety reasons.
Markers are a means of distinguishing specific batches of gasoline without providing an obvious visual clue. A refiner may add a marker to its gasoline so it can be identified as it moves through the distribution system.
3.2.2.5.6. DRAG REDUCERS
Drag reducers are high-molecular-weight polymers that improve the fluid flow character-istics of low-viscosity petroleum products. Drag
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reducers lower pumping costs by reducing fric-tion between the flowing gasoline and the walls of the pipe.
3.2.2.5.7. OXYGENATES (FUEL ADDITIVES) Oxygenates are carbon-, hydrogen-, and oxygen-containing combustible liquids that are added to gasoline to improve performance. The addition of oxygenates gasoline is not new since ethanol (ethyl alcohol or grain alcohol) has been added to gasoline for decades. Thus, oxygenated gasoline is a mixture of conventional hydro-carbon-based gasoline and one or more oxygen-ates. The current oxygenates belong to one of two classes of organic molecules: alcohols and ethers. The most widely used oxygenates in the United States are ethanol, methyl tertiary-butyl ether (MTBE), and tertiary-amyl methyl ether (TAME). Ethyl tertiary-butyl ether (ETBE) is another ether that could be used. Oxygenates may be used in areas of the United States where they are not required as long as concentration limits (as refined by environmental regulations) are observed.
Of all the oxygenates, MTBE is attractive for a variety of technical reasons. It has a low vapor pressure, can be blended with other fuels without phase separation, and has the desirable octane characteristics. If oxygenates achieve recognition as vehicle fuels, the biggest contrib-utor will probably be methanol, the production of which is mostly from synthesis gas derived from methane.
The higher alcohols also offer some potential as motor fuels. These alcohols can be produced at temperatures below 300 C (572 F) using copper oxideezinc oxideealumina catalysts promoted with potassium.Iso-butyl alcohol is of particular interest because of its high octane rating, which makes it desirable as a gasoline-blending agent. This alcohol can be reacted with methanol in the presence of a catalyst to produce MTBE. Although it is currently cheaper to make iso-butyl alcohol from iso-butylene, it can be synthesized from syngas with alkali-promoted
zinc oxide catalysts at temperatures above 400C (752F)[7].
3.2.2.6. Adulteration
Adulteration differs from contamination insofar as unacceptable materials deliberately are added to gasoline for a variety of reasons not to be discussed here. Such activities may not only lower the octane number but will also adversely affect volatility, which in turn also affects performance. In some countries, dyes and markers are used to detect adulteration (e.g., ASTM D86 distillation testing and/or ASTM D2699/ASTM D2700 octane number testing may be required to detect adulteration).