Properties and applications of petroleum products
Gases
Gaseous refinery products include hydrogen, fuel gas, ethane, propane, and butane. Most of the hydrogen is consumed in refinery desulfurization facilities, which remove hydrogen sulfide from the gas stream and then separate that compound into elemental hydrogen and sulfur; small quantities of the hydrogen may be delivered to the refinery fuel system. Refinery fuel gas varies in composition but usually contains a significant amount of methane; it has a heating value similar to natural gas and is consumed in plant operations. Periodic variability in heating value makes it unsuitable for delivery to consumer gas systems. Ethane may be recovered from the refinery fuel system for use as a petrochemical feedstock. Propane and butane are sold as liquefied petroleum gas (LPG), which is a convenient portable fuel for domestic heating and cooking or for light industrial use.
Kerosene
Though its use as an illuminant has greatly diminished, kerosene is still used extensively throughout the world in cooking and space heating and is the primary fuel for modern jet engines. When burned as a domestic fuel, kerosene must produce a flame free of smoke and odour. Standard laboratory procedures test these properties by burning the oil in special lamps. All kerosene fuels must satisfy minimum flash-point specifications (49 °C, or 120 °F) to limit fire hazards in storage and handling.
Jet fuels must burn cleanly and remain fluid and free from wax particles at the low temperatures experienced in high-altitude flight. The conventional freeze-point specification for commercial jet fuel is −50 °C (−58 °F). The fuel must also be free of any suspended water particles that might cause blockage of the fuel system with ice particles. Special-purpose military jet fuels have even more stringent specifications.
Diesel oils
The principal end use of gas oil is as diesel fuel for powering automobile, truck, bus, and railway engines. In a diesel engine, combustion is induced by the heat of compression of the air in the cylinder under compression. Detonation, which leads to harmful knocking in a gasoline engine, is a necessity for the diesel engine. A good diesel fuel starts to burn at several locations within the cylinder after the fuel is injected. Once the flame has initiated, any more fuel entering the cylinder ignites at once.
Straight-chain hydrocarbons make the best diesel fuels. In order to have a standard reference scale, the oil is matched against blends of cetane (normal hexadecane) and alpha methylnaphthalene, the latter of which gives very poor engine performance. High-quality diesel fuels have cetane ratings of about 50, giving the same combustion characteristics as a 50-50 mixture of the standard fuels. The large, slower engines in ships and stationary power plants can tolerate even heavier diesel oils. The more viscous marine diesel oils are heated to permit easy pumping and to give the correct viscosity at the fuel injectors for good combustion.
Until the early 1990s, standards for diesel fuel quality were not particularly stringent. A minimum cetane number was critical for transportation uses, but sulfur levels of 5,000 parts per million (ppm) were common in most markets. With the advent of more stringent exhaust emission controls, however, diesel fuel qualities came under increased scrutiny. In the European Union and the United States, diesel fuel is now generally restricted to maximum sulfur levels of 10 to 15 ppm, and regulations have restricted aromatic content as well. The limitation of aromatic compounds requires a much more demanding scheme of processing individual gas oil components than was necessary for earlier highway diesel fuels.
Fuel oils
Furnace oil consists largely of residues from crude oil refining. These are blended with other suitable gas oil fractions in order to achieve the viscosity required for convenient handling. As a residue product, fuel oil is the only refined product of significant quantity that commands a market price lower than the cost of crude oil.
Because the sulfur contained in the crude oil is concentrated in the residue material, fuel oil sulfur levels are naturally high. The sulfur level is not critical to the combustion process as long as the flue gases do not impinge on cool surfaces (which could lead to corrosion by the condensation of acidic sulfur trioxide). However, in order to reduce air pollution, most industrialized countries now restrict the sulfur content of fuel oils. Such regulation has led to the construction of residual desulfurization units or cokers in refineries that produce these fuels.
Residual fuels may contain large quantities of heavy metals such as nickel and vanadium; these produce ash upon burning and can foul burner systems. Such contaminants are not easily removed and usually lead to lower market prices for fuel oils with high metal contents.
Lubricating oils
At one time the suitability of petroleum fractions for use as lubricants depended entirely on the crude oils from which they were derived. Those from Pennsylvania crude, which were largely paraffinic in nature, were recognized as having superior properties. But, with the advent of solvent extraction and hydrocracking, the choice of raw materials has been considerably extended.
When ordinary mineral oils having satisfactory lubricity at low temperatures are used over an extended temperature range, excessive thinning occurs, and the lubricating properties are found to be inadequate at higher temperatures. To correct this, multigrade oils have been developed using long-chain polymers. Thus, an oil designated SAE 10W40 has the viscosity of an SAE 10W oil at −18 °C (0 °F) and of an SAE 40 oil at 99 °C (210 °F). Such an oil performs well under cold starting conditions in winter (hence the W designation) yet will lubricate under high-temperature running conditions in the summer as well. Other additives that improve the performance of lubricating oils are antioxidants and detergents, which maintain engine cleanliness and keep fine carbon particles suspended in the circulating oil.
Gear oils and greases In gear lubrication the oil separates metal surfaces, reducing friction and wear. Extreme pressures develop in some gears, and special additives must be employed to prevent the seizing of the metal surfaces. These oils contain sulfur compounds that form a resistant film on the surfaces, preventing actual metal-to-metal contact. Greases are lubricating oils to which thickening agents are added. Soaps of aluminum, calcium, lithium, and sodium are commonly used, while nonsoap thickeners such as carbon, silica, and polyethylene also are employed for special purposes.
Olefins
The thermal cracking processes developed for refinery processing in the 1920s were focused primarily on increasing the quantity and quality of gasoline components. As a by-product of this process, gases were produced that included a significant proportion of lower-molecular-weight olefins, particularly ethylene, propylene, and butylene. Catalytic cracking is also a valuable source of propylene and butylene, but it does not account for a very significant yield of ethylene, the most important of the petrochemical building blocks. Ethylene is polymerized to produce polyethylene or, in combination with propylene, to produce copolymers that are used extensively in food-packaging wraps, plastic household goods, or building materials.
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