Gasoline
Gasoline (also referred to as motor gasoline, petrol in Britain, benzine in Europe) is a mixture of volatile, flammable liquid hydrocarbons derived from petroleum that is used as fuel for internal combustion engines such as occur in motor vehicles, excluding aircraft The boiling range of motor gasoline falls between –1°C (30°F) and 216°C (421°F) and has the potential to contain several hundred isomers of the various hydrocarbons—a potential that may be theoretical and never realized in practice. The hydrocarbon constituents in this boiling range are those that have 4–12 carbon atoms in their molecular structure and fall into three general types: (1) paraffins (including the cycloparaffins and branched materials), (2) olefins, and (3) aromatics. Gasoline boils at about the same range as naphtha (a precursor to gasoline) but below kerosene.
The various test methods dedicated to the determination of the amounts of carbon, hydrogen, and nitrogen (ASTM D-5291) as well as the determination of oxygen, sulfur, metals, and chlorine (ASTM D-808) are not included in this discussion. Although necessary, the various tests available for composition are left to the discretion of the analyst. In addition, test methods recommended for naphtha may also be applied, in many circumstances, to gasoline.
read also What is Naphtha
PRODUCTION AND PROPERTIES
Gasoline was at first produced by distillation, simply separating the volatile, more valuable fractions of crude petroleum, and was composed of the naturally occurring constituents of petroleum. Later processes, designed to raise the yield of gasoline from crude oil, split higher-molecular-weight constituents into lower-molecular-weight products by processes known as cracking. And, like typical gasoline, several processes produce the blending stocks for gasoline.
By way of definition of some of these processes, polymerization is the conversion of gaseous olefins such as propylene and butylene into larger molecules in the gasoline range.
Reforming is the use of either heat or a catalyst to rearrange the molecular structure. Selection of the components and their proportions in a blend is the most complex problem in a refinery.
Thus gasoline is a mixture of hydrocarbons that boils below 180°C (355°F) or, at most, below 200°C (390°F). The hydrocarbon constituents in this boiling range are those that have 4–12 carbon atoms in their molecular structure. The hydrocarbons of which gasoline is composed fall into three general types: paraffins (including the cycloparaffins and branched materials), olefins, and aromatics. The hydrocarbons produced by modern refining techniques (distillation, cracking, reforming, alkylation, isomerization, and polymerization) provide blending components for gasoline production.
Gasoline consists of a very large number of different hydrocarbons, and the individual hydrocarbons in gasoline cannot be conveniently used to describe gasoline.The composition of gasoline is best expressed in terms of hydrocarbon types (saturates, olefins, and aromatics) that enable inferences to be made about the behavior in service.
TEST METHODS
The test protocols used for gasoline are similar to the protocols used for naphtha. The similarity of the two liquids requires the application of similar test methods. However, knocking properties are emphasized for gasoline and there are several other differences that must be recognized. But, all in all, consultation of the test methods used for the analysis of naphtha can assist in developing protocols for gasoline.
The properties of gasoline are quite diverse, and the principal properties affecting the performance of gasoline are volatility and combustion characteristics. These properties are adjusted according to the topography and climate of the country in which the gasoline is to be used. For example, mountainous regions will require gasoline with volatility and knock characteristics somewhat different from those that are satisfactory in flat or undulating country only a little above sea level. Similarly, areas that exhibit extremes of climatic temperature, such as the northern provinces of Canada, where temperatures of 30°C (86°F) in the summer are often followed by temperatures as low as –40°C (–40°F) in the winter, necessitate special consideration, particularly with regard to volatility.
Because of the high standards set for gasoline, as with naphtha, it is essential to use the correct techniques when taking samples for testing (ASTM D-270, ASTM D-4057, IP 51). Mishandling, or the slightest trace of contaminant, can give rise to misleading results. Special care is necessary to ensure that containers are scrupulously clean and free from odor. Samples should be taken with the minimum of disturbance so as to avoid loss of volatile components; in the case of the lightest solvents it may be necessary to chill the sample.
While awaiting examination, samples should be kept in a cool dark place so as to ensure that they do not discolor or develop odors.
read also Oilfield Paraffin and Asphaltene
Additives
Additives are chemical compounds intended to improve some specific properties of gasoline or other petroleum products and can be monofunctional or multifunctional. Different additives, even when added for identical purposes, may be incompatible with each other and may, for example, react and form new compounds. Consequently, a blend of two or more gasolines, containing different additives, may form a system in which the additives react with each other and so deprive the blend of their beneficial effect.
Thus certain substances added to gasoline, notably the lead alkyls, have a profound effect on antiknock properties and inhibit the precombustion oxidation chain that is known to promote knocking. For a considerable period tetraethyl lead (TEL) was the preferred compound, but more recently tetramethyl lead (TML) has been shown to have advantages with certain modern types of gasoline because of its lower boiling point (110°C/230°F as against 200°C/392°F for tetraethyl lead) and therefore its higher vapor pressure, which enables it to be more evenly distributed among the engine cylinders with the more volatile components of the gasoline.
Some gasoline may still contain tetramethyl lead and tetraethyl lead, whereas others contain compounds prepared by a chemical reaction between tetramethyl lead and tetraethyl lead in the presence of a catalyst.
These chemically reacted compounds contain various proportions of tetramethyl lead and tetraethyl lead and their intermediates, trimethylethyl lead, dimethyldiethyl lead, and methyltriethyl lead, and thus provide antiknock compounds with a boiling range of 110–200°C (230–392°F).The lead compounds, if used alone, would cause an excessive accumulation of lead compounds in the combustion chambers of the engine and on sparking plugs and valves. Therefore “scavengers” such as dibromoethane, alone or in admixture with dichloroethane, are added to the lead alkyl and combine with the lead during the combustion process to form volatile compounds that pass harmlessly from the engine.
The amount of lead alkyl compounds used in gasoline is normally expressed in terms of equivalent grams of metallic lead per gallon or per liter.The maximum concentration of lead permitted in gasoline varies from country to country according to governmental legislation or accepted commercial practice, and it is a subject that is currently under discussion in many countries because of the attention being paid to reduction of exhaust emissions from the spark ignition engine.
The total lead in gasoline may be determined gravimetrically (ASTM D-52, IP 96), polarographically (ASTM D-1269), by atomic absorption spectrometry (ASTM D-3237, IP 428), by the iodine chloride method (ASTM D-3341, IP 270), by inductively coupled plasma atomic emission spectrometry (ASTM D-5185), and by X-ray fluorescence (ASTM D-5059).When it is desired to estimate tetraethyl lead a method is available (IP 116), whereas for the separate determination of tetramethyl lead and tetraethyl lead recourse can be made to separate methods (ASTM D-l949, IP 188).
Other additives used in gasoline include antioxidants and metal deactivators for inhibiting gum formation, surface-active agents and freezing point depressants for preventing carburetor icing, deposit modifiers for reducing spark plug fouling and surface ignition, and rust inhibitors (ASTM D-665, IP 135) for preventing the rusting of steel tanks and pipe work by the traces of water carried in gasoline. For their estimation specialized procedures involving chemical tests and physical techniques such as spectroscopy and chromatography have been used successfully.
Test methods have been developed to measure ethers and alcohols in gasoline-range hydrocarbons, because oxygenated components such as methyl-tert-butylether and ethanol are common blending components in gasoline (ASTM D-4814, ASTM D-4815, ASTM D-5441, ASTM D-5599, ASTM D-5986 ASTM D-5622, ASTM D-5845, ASTM D-6293).
Another type of gasoline sometimes referred to as vaporizing oil or power kerosene is primarily intended as a gasoline for agricultural tractors and is, in effect, a low-volatility (higher boiling) gasoline. For reliable operation, such a gasoline must not be prone to deposit formation (sediment or gum) and the residue on evaporation (ASTM D-381, IP 131) must, therefore, be low. The volatility of vaporizing oil is, as for regular gasoline, assessed by the distillation test (ASTM D-86, IP 123), the requirement normally being controlled by the percentages boiling at 160°C and 200°C (320°F and 392°F).
Because of the lower volatility of vaporizing oil compared with that of gasoline, a relatively high proportion of aromatics (ASTM D-4420) may be necessary to maintain the octane number although unsaturated hydrocarbons may also be used in proportions compatible with stability requirements.
However, the presence of unsaturated constituents must be carefully monitored because of the potential for incompatibility through the formation of sediment and gum. Other tests include flash point (closed cup method: ASTM D-56, ASTM D-93, ASTM D-3828, 6450, IP 34, IP 94, IP 303; open cup method: ASTM D-92, ASTM D-1310, IP 36), sulfur content (ASTM D-1266, IP 107), corrosion (ASTM D-l30, ASTM D-849, IP 154), octane number (ASTM D-2699, ASTM D-2700, ASTM D-2885, IP 236, IP 237), and residue on evaporation (ASTM D-381, ASTM D-1353, IP 131).
Although trace elements such as lead (ASTM D-52, ASTM D-1269, ASTM D-3116, ASTM D-3237, ASTM D-3441, ASTM D-5059, ASTM D- 5185, IP 96, IP 228, IP 270), manganese (ASTM D-3831), and phosphorus (ASTM D-3231) are not always strictly additives, tests for the presence of these elements must be stringently followed because their presence can have an adverse affect on gasoline performance or on the catalytic converter.
Composition
As with naphtha, the number of potential hydrocarbon isomers in the gasoline boiling range renders complete speciation of individual hydrocarbons impossible for the gasoline distillation range, and methods are used that identify the hydrocarbon types as chemical groups rather than as individual constituents.
In terms of hydrocarbon components, several procedures have been devised for the determination of hydrocarbon type, and the method based on fluorescent indicator adsorption (ASTM D-l319, IP 156) is the most widely employed. Furthermore, aromatic content is a key property of low boiling distillates such as gasoline because the aromatic constituents influence a variety of properties including boiling range (ASTM D-86, IP 123),
viscosity (ASTM D-88, ASTM D-445, ASTM D-2161, IP 71), and stability (ASTM D-525, IP 40). Existing methods use physical measurements and need suitable standards. Tests such as aniline point (ASTM D-611) and kauri-butanol number (ASTM D-1133) are of a somewhat empirical nature and can serve a useful function as control tests. However, gasoline composition is monitored mainly by gas chromatography (ASTM D-2427,ASTM D-6296).
A multidimensional gas chromatographic method (ASTM D-5443) provides for the determination of paraffins, naphthenes, and aromatics by carbon number in low olefinic hydrocarbon streams having final boiling points lower than 200°C (392°F). In this method, the sample is injected into a gas chromatographic system that contains a series of columns and switching values. First a polar column retains polar aromatic compounds, binaphthenes, and high-boiling paraffins and naphthenes. The eluant from this column goes through a platinum column that hydrogenates olefins and then to a molecular sieve column that performs a carbon number separation based on the molecular structure, that is, naphthenes and paraffins. The fraction remaining on the polar column is further divided into three separate fractions that are then separated on a nonpolar column by boiling point. A flame ionization detector detects eluting compounds.
In another method (ASTM D-4420) for the determination of the amount of aromatic constituents, a two-column chromatographic system connected to a dual-filament thermal conductivity detector (or two single filament detectors) is used.The sample is injected into the column containing a polar liquid phase. The nonaromatics are directed to the reference side of the detector and vented to the atmosphere as they elute. The column is backflushed
immediately before the elution of benzene, and the aromatic portion is directed into the second column containing a nonpolar liquid phase. The aromatic components elute in the order of their boiling points and are detected on the analytical side of the detector. Quantitation is achieved by utilizing peak factors obtained from the analysis of a sample having a known aromatic content. However, the method may be susceptible to errors caused by alkyl-substituted aromatics where the boiling point increases because of the alkyl side chain and this increase bears little relationship to the aromatic ring.
Other methods for the determination of aromatics in gasoline include a method (ASTM D-5580) using a flame ionization detector and methods in which a combination of gas chromatography and Fourier transform infrared spectroscopy (GC-FTIR) (ASTM D-5986) and gas chromatography and mass spectrometry (GC-MS) (ASTM D-5769) are used.
The accurate measurements of benzene, and/or toluene, and total aromatics in gasoline are regulated test parameters in gasoline (ASTM D-3606, ASTM D-5580,ASTM D-5769,ASTM D-5986).The precision and accuracy of some of these tests are diminished in gasoline containing ethanol or methanol, because these components often do not completely separate from the benzene peak.
Benzene, toluene, ethylbenzene, the xylene isomers, as well as C9 aromatics and higher-boiling aromatics are determined by gas chromatography, and the test (ASTM D5580) was developed to include gasoline containing commonly encountered alcohols and ethers. This test is the designated test for determining benzene and total aromatics in gasoline and includes testing gasoline containing oxygenates and uses a flame ionization detector. Another method that employs the flame ionization technique (ASTM D-1319, IP 156) is widely used for measuring total olefins in gasoline fractions as well as aromatics and saturates, although the results may need correction for the presence of oxygenates. Gas chromatography is also used for the determination of olefins in gasoline (ASTM D-6296).
Benzene in gasoline can also be measured by infrared spectroscopy (ASTM D-4053). But additional benefits are derived from hyphenated analytical methods such as gas chromatography-mass spectrometry (ASTM D-5769) and gas chromatography-Fourier transform infrared spectroscopy) ASTM D-5986), which also accurately measure benzene in gasoline.
The gas chromatography-mass spectrometry method (ASTM D-5769) is based on the Environmental Protection Agency’s gas chromatography/mass spectrometry (EPA GC/MS) procedure for aromatics.
Hydrocarbon composition is also determined by mass spectrometry—a technique that has seen wide use for hydrocarbon-type analysis of gasoline (ASTM D-2789) as well as to the identification of hydrocarbon constituents in higher-boiling gasoline fractions (ASTM D-2425).
One method (ASTM D-6379, IP 436) is used to determine the monoaromatic and diaromatic hydrocarbon contents in distillates boiling in the range from 50 to 300°C (122–572°F). In this method the sample is diluted with an equal volume of hydrocarbon, such as heptane, and a fixed volume of this solution is injected into a high-performance liquid chromatograph fitted with a polar column where separation of the aromatic hydrocarbons from the nonaromatic hydrocarbons occurs.The separation of the aromatic constituents appears as distinct bands according to ring structure, and a refractive index detector is used to identify the components as they elute from the column.The peak areas of the aromatic constituents are compared with those obtained from previously run calibration standards to calculate the % w/w monoaromatic hydrocarbon constituents and diaromatic hydrocarbon constituents in the sample.
Compounds containing sulfur, nitrogen, and oxygen could possibly interfere with the performance of the test. Monoalkenes do not interfere, but conjugated di- and polyalkenes, if present, may interfere with the test performance.
Paraffins, naphthenes, and aromatic hydrocarbons in gasoline and other distillates boiling up to 200°C (392°F) are determined by multidimensional gas chromatography (ASTM D-5443).Olefins that are present are converted to saturates and are included in the paraffin and naphthene distribution. However, the scope of this test does not allow it to be applicable to hydrocarbons containing oxygenates. An extended version of the method can be used to determine the amounts of paraffins, olefins, naphthenes, and aromatics (PONA) in gasoline-range hydrocarbon fractions (ASTM D-6293).
A titration procedure (ASTM D-1159), which determines the bromine number of petroleum distillates and aliphatic olefins by electrometric titration, can be used to provide an approximation of olefin content in a sample. Another related method (ASTM D-2710) is used to determine the bromine index of petroleum hydrocarbons by electrometric titration and is valuable for determining trace levels of olefins in gasoline. Obviously, these methods use an indirect route to determine the total olefins, and the type of olefinic compound present affects the results because the results depend on the ability of the olefins to react with bromine. Steric factors can prove to have an adverse affect on the experimental data.
The compositional analysis of gasoline, up to and including n-nonane, can be achieved using by capillary gas chromatography (ASTM Test Method D5134). Higher-resolution gas chromatography capillary column techniques provide a detailed analysis of most of the individual hydrocarbons in gasoline, including many of the oxygenated blending components.
Capillary gas chromatographic techniques can be combined with mass spectrometry to enhance the identification of the individual components and hydrocarbon types.
The presence of pentane and lighter hydrocarbons in gasoline interferes in the determination of hydrocarbon types (ASTM D-1319 and ASTM D- 2789). Pentane and lighter hydrocarbons are separated by this test method so that the depentanized residue can be analyzed, and pentane and lighter hydrocarbons can be analyzed by other methods, if desired. Typically about 2% by volume of pentane and lower-boiling hydrocarbons remain in the bottoms, and hexane and higher-boiling hydrocarbons carry over to the overhead. In this test (ASTM D-2001) a 50-ml sample is distilled into an overhead (pentane and lower-boiling hydrocarbons) fraction and a bottoms (hexane and higher-boiling hydrocarbons) fraction.The volume of bottoms is measured, and the percent by volume, based on the original gasoline charged to the unit, is calculated.
Sulfur-containing components exist in gasoline-range hydrocarbons and can be identified with a gas chromatographic capillary column coupled with either a sulfur chemiluminescence detector or an atomic emission detector (AED) (ASTM D-5623).The most widely specified method for total sulfur content uses X-ray spectrometry (ASTM D-2622), and other methods that use ultraviolet fluorescence spectroscopy (ASTM D-5453) and/or hydrogenolysis and colorimetry (ASTM D-4045) are also applicable, particularly when the sulfur level is low.
Corrosiveness
Because a gasoline would be unsuitable for use if it corroded the metallic parts of the gasoline system or the engine, it must be substantially free from corrosive compounds both before and after combustion.
Corrosiveness is usually due to the presence of free sulfur and sulfur compounds that burn to form sulfur dioxide (SO2), which combines with water vapor formed by the combustion of the gasoline to produce sulfurous acid (H2SO3). Sulfurous acid can, in turn, oxidize to sulfuric acid (H2SO4), and both acids are corrosive toward iron and steel and would attack the cooler parts of the engine’s exhaust system and its cylinders as they cool off after the engine is shut down.
The total sulfur content of gasoline is very low, and knowledge of its magnitude is of chief interest to the refiner who must produce a product that conforms to a stringent specification.Various methods are available for the determination of total sulfur content. The one most frequently quoted in specifications is the lamp method (ASTM D-1266, IP 107), in which the gasoline is burned in a small wick-fed lamp in an artificial atmosphere of carbon dioxide and oxygen; the oxides of sulfur are converted to sulfuric acid, which is then determined either volumetrically or gravimetrically.
A more recent development is the Wickbold method (ASTM D-2785, IP 243). This is basically similar to the lamp method except that the sample is burned in an oxy-hydrogen burner to give much more rapid combustion. An alternative technique, which has the advantage of being nondestructive, is X-ray spectrography (ASTM D-2622).
Mercaptan sulfur (R-SH) and hydrogen sulfide (H2S) (ASTM D-1219, IP 103, IP 104) are undesirable contaminants because, apart from their corrosive nature, they possess an extremely unpleasant odor. Such compounds should have been removed completely during refining but their presence and that of free sulfur are detected by application of the Doctor test (ASTM D-4952, IP 30).The action on copper of any free or corrosive sulfur present in gasoline may be estimated by a procedure (ASTM D-130, ASTM D-849, IP 154) in which a strip of polished copper is immersed in the sample, which is heated under specified conditions of temperature and time, and any staining of the copper is subsequently compared with the stains on a set of reference copper strips and thus the degree of corrosivity of the test sample determined.
Total sulfur is determined by combustion in a bomb calorimeter (ASTM D-129, IP 61) and is often carried out with the determination of calorific value. The contents of the bomb are washed with distilled water into a beaker after which hydrochloric acid is added and the solution is raised to boiling point. Barium chloride is added drop by drop to the boiling solution to precipitate the sulfuric acid as granular barium sulfate.After cooling, and standing for 24 h, the precipitate is filtered off on an ashless paper, washed, ignited, and weighed as barium sulfate. % by weight sulfur = (wt. of barium sulfate X 13.73)/wt. of sample As an addition to the test for mercaptan sulfur by potentiometric titration (ASTM D-3227, IP 342), a piece of mechanically cleaned copper is also used to determine the amount of corrosive sulfur in a sample (ASTM D-130, IP 112, IP 154, IP 411). The pure sheet copper is placed in a test tube with 40 ml of the sample, so that the copper is completely immersed. The tube is closed with a vented cork and heated in a boiling-water bath for 3 h. The copper strip is then compared visually with a new strip of copper for signs of tarnish. The results are recorded as:
No change: Result negative
Slight discoloration: Result negative
Brown shade: Some effect
Steel gray: Some effect
Black, not scaled: Result positive, corrosive sulfur present
Black, scaled: Result positive, corrosive sulfur present
Thus visual observation of the copper strip can present an indication or a conclusion of the presence or absence of corrosive sulfur.There is also a copper strip corrosion method for liquefied petroleum gases (ASTM D-1838).
References:
1. Handbook of Petroleum Product Analysis.
2. Production Engineering Handbook.
