Petroleum refining and processing

Petroleum is separated into a variety of fuels—gasoline, kerosene, and diesel fuel—and into feedstocks for the chemical industry. Petroleum is first distilled, then each of the “cuts” is further treated or blended to provide the various marketed products. A significant effort is devoted to gasoline production, in order to obtain the quantities needed and the desired engine performance.

Background

Petroleum, or crude oil, is found in many parts of the world. It is not a chemically pure substance of uniform properties. Rather, is a complex mixture of hundreds of individual chemical compounds that occur in various proportions, depending on the source and geological history of the particular sample. As a result, various kinds of petroleum range in properties and appearance from lightly colored, free-flowing liquids to black, tarry, odiferous materials. It would be impractical to design furnaces or engines capable of efficient, reliable operation on a fuel whose characteristics varied so widely. Therefore, to provide products of predictable quality to the users, petroleum is separated into specific products that, through treating, blending, and purification, go on the market as the familiar gasoline, kerosene, and diesel and heating oils. Some petroleum supplies also contain impurities, most notably sulfur compounds, that must be removed for environmental reasons. The sequence of separation, blending, treating, and purification operations all make up the processes of petroleum refining.

Distillation

The first major step in refining petroleum is distillation, the separation of components based on boiling point. In principle, it would be possible to separate petroleum into each of its component compounds, one by one, producing many hundreds of individual pure compounds. Doing so would be so laborious that the products would be too expensive for widespread use as fuels or synthetic chemicals. Instead, petroleum is separated into boiling ranges, or “cuts,” such that even though a particular distillation cut will still be composed of a large number of compounds, its physical properties and combustion behavior will be reasonably constant and predictable.

Many crude oils contain dissolved gases, such as propane and butane. These are driven off during distillation and can be captured for sale as liquefied petroleum gas (LPG). The first distillation cut (that is, the one with the lowest boiling temperature) that is a liquid is gasoline. Products obtained in higher boiling ranges include, in order of increasing boiling range, naphtha, kerosene, diesel oil, and some heating oils or furnace oils. Some fraction of the crude oil will not distill; this is the residuum, usually informally called the resid. The resid can be treated to separate lubricating oils and waxes. If the amount of resid is large, it can be distilled further at reduced pressure (vacuum distillation) to increase the yield of the products with higher boiling ranges and a so-called vacuum resid.

Catalytic Cracking

The product that usually dominates refinery production is gasoline. Gasoline produced directly by distillation, called straight-run gasoline, is not sufficient in quantity or in engine performance to meet modern market demand. Substantial effort is devoted to enhancing the yield and quality of gasoline. The yield of straight-run gasoline from very good quality petroleum is not more than 20 percent; from poorer quality crudes, it may be less than 10 percent. About 50 percent of a of petroleum needs to be converted to gasoline to satisfy current needs. Gasoline engine performance is measured by octane number, which indicates the tendency of the gasoline to “knock” (to detonate prematurely in the engine cylinder). Knocking causes poor engine efficiency and can lead to mechanical problems. Most regular grade gasolines have octane numbers of 87; straight-run gasolines may have octane numbers below 50.

Increasing the yield of gasoline requires producing more molecules that boil in the gasoline range. Generally the boiling range of molecules relates to their size; reducing the boiling range is effected by reducing their size, or “cracking” the molecules. Octane number is determined by molecular shape. The common components of most crude oils are the paraffins, or normal alkanes, characterized by straight chains of carbon atoms. These paraffins have very low octane numbers; heptane, for example, has an octane number of 0. A related family of compounds, isoparaffins, have chains of carbon atoms with one or more side branches; these have very high octane numbers. The compound familiarly referred to as iso-octane (2,2,4-trimethylpentane) has an octane number of 100. Increasing the yield and engine performance of gasoline requires both cracking and rearranging the molecular structures.

Both of these processes can be performed in a single step, using catalysts such as zeolites. For this reason, the overall process is known as catalytic cracking. The feedstock to a catalytic cracking unit is a high-boiling cut material of low value. Different refineries may choose to use different feeds, but a typical choice would be a vacuum gas oil, which is produced in the vacuum distillation step. Much effort has gone into the development of catalysts and into evaluating appropriate choices of temperature, pressure, and reaction time. Catalytic cracking is second only to distillation in importance in most refineries. It can produce gasolines with octane numbers above 90 and increases the yield of gasoline in a refinery to about 45 percent.

Catalytic Reforming

Straight-run gasoline and naphthas have acceptable boiling ranges but suffer in octane number. Treating these streams does not require cracking, only altering the shapes of molecules—re-forming them—to enhance octane number. This process also relies on catalysts, though of different types than those used in catalytic cracking. Reforming catalysts usually include a metal, such as nickel or platinum. Catalytic reforming can produce gasolines with octane numbers close to 100.

Hydrotreating

Other distillation cuts, such as kerosene and diesel fuel, require less refining. Two processes of importance for environmental reasons are the removal of sulfur and removal of aromatic compounds. Since both involve the use of hydrogen, they are referred to as hydrotreating.

Sulfur removal—hydrodesulfurization—is done to reduce the amount of sulfur emissions that would have been produced when the fuel is burned. Additionally, sulfur compounds are corrosive and can have noxious odors. Hydrodesulfurization is performed by treating the feedstock, such as kerosene, with hydrogen using catalysts containing cobalt or nickel and molybdenum. As environmental regulations become more stringent, hydrodesulfurization will become increasingly important.

Aromatic compounds also have several undesirable characteristics. Some compounds, such as benzene, are carcinogens. Larger aromatic molecules, which might be found in kerosene or diesel oil, contribute to the formation of smoke and soot when these fuels are burned. Soot formation is unpleasant in its own right, but in addition, some soot components are also carcinogens. Aromatic compounds are reacted with hydrogen to form new compounds—naphthenes or cycloalkanes—of more desirable properties.

Resid Treating

Resids can be treated with solvents to extract lubricating oils (these oils can also be made during the vacuum distillation of resid), waxes, and asphalts. Although lubricating oils are produced only in low yield (about 2 percent of a barrel of crude may wind up as lubricating oil), they are commercially valuable products. Asphalts are of great importance for road paving. Resid is also converted by heating into petroleum coke, a solid material high in carbon content. High-quality petroleum cokes are used to manufacture synthetic graphite, which has a range of uses, the most important of which is for electrodes for the metallurgical industry. Poorer quality petroleum cokes can be used as solid fuels.

Petrochemicals

Petroleum is the source not only of liquid fuels but also of most synthetic chemicals and polymers. Some products having low value as fuels, such as naphtha or even waxes, can be decomposed to produce ethylene, the most important feedstock for the chemical industry. Ethylene is converted to polyethylene, polyvinyl chloride, polyvinyl acetate, and polystyrene, which together make up a large share of the total market for plastics. Another petroleum product of great use in the chemical industry is propylene, the starting material for making polypropylene and polyacrylonitrile.

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