Petroleum

VIII

Offshore Drilling

Another method to increase oil-field production has been the construction and operation of offshore drilling rigs. The drilling rigs are installed, operated, and serviced on an offshore platform in water up to a depth of several hundred meters; the platform may either float or sit on legs planted on the ocean floor, where it is capable of resisting waves, wind, and—in Arctic regions—ice floes.

As in traditional rigs, the derrick is basically a device for suspending and rotating the drill pipe, to the end of which is attached the drill bit. Additional lengths of drill pipe are added to the drill string as the bit penetrates farther and farther into Earth’s crust. The force required for cutting into the earth comes from the weight of the drill pipe itself. To facilitate the removal of the cuttings, mud is constantly circulated down through the drill pipe, out through nozzles in the drill bit, and then up to the surface through the space between the drill pipe and the bore through the earth (the diameter of the bit is somewhat greater than that of the pipe). Successful bore holes have been drilled right on target, in this way, to depths of more than 6.4 km (more than 4 mi) from the surface of the ocean. Offshore drilling has resulted in the development of a significant additional reserve of petroleum—in the United States, about 5 percent of the total reserves.

IX

Refining

Once oil has been produced from an oil field, it is treated with chemicals and heat to remove water and solids, and the natural gas is separated. The oil is then stored in a tank, or battery of tanks, and later transported to a refinery by truck, railroad tank car, barge, or pipeline. Large oil fields all have direct outlets to major, common-carrier pipelines.

A

Basic Distillation

The basic refining tool is the distillation unit. In the United States after the Civil War (1861-1865), more than 100 still refineries were already in operation. Crude oil begins to vaporize at a temperature somewhat less than that required to boil water. Hydrocarbons with the lowest molecular weight vaporize at the lowest temperatures, whereas successively higher temperatures are required to distill larger molecules. The first material to be distilled from crude oil is the gasoline fraction, followed in turn by naphtha and then by kerosene. The residue in the kettle, in the old still refineries, was then treated with caustic and sulfuric acid, and finally steam distilled thereafter. Lubricants and distillate fuel oils were obtained from the upper regions and waxes and asphalt from the lower regions of the distillation apparatus.

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In the late 19th century the gasoline and naphtha fractions were actually considered a nuisance because little need for them existed. The demand for kerosene also began to decline because of the growing production of electricity and the use of electric lights. With the introduction of the automobile, however, the demand for gasoline suddenly burgeoned, and the need for greater supplies of crude oil increased accordingly.

B

Thermal Cracking

In an effort to increase the yield from distillation, the thermal cracking process was developed. In this process, the heavier portions of the crude oil were heated under pressure and at higher temperatures. This resulted in the large hydrocarbon molecules being split into smaller ones, so that the yield of gasoline from a barrel of crude oil was increased. The efficiency of the process was limited, however, because at the high temperatures and pressures that were used, a large amount of coke was deposited in the reactors. This in turn required the use of still higher temperatures and pressures to crack the crude oil. A coking process was then invented in which fluids were recirculated; the process ran for a much longer time, with far less buildup of coke. Many refiners quickly adopted the process of thermal cracking.

C

Alkylation and Catalytic Cracking

Two additional basic processes, alkylation and catalytic cracking, were introduced in the 1930s and further increased the gasoline yield from a barrel of crude oil. In alkylation small molecules produced by thermal cracking are recombined in the presence of a catalyst. This produces branched molecules in the gasoline boiling range that have superior properties—for example, higher antiknock ratings—as a fuel for high-powered engines such as those used in today’s commercial airplanes.

In the catalytic-cracking process, the crude oil is cracked in the presence of a finely divided catalyst. This permits the refiner to produce many diverse hydrocarbons that can then be recombined by alkylation, isomerization, and catalytic reforming to produce high antiknock engine fuels and specialty chemicals. The production of these chemicals has given birth to the gigantic petrochemical industry, which turns out alcohols, detergents, synthetic rubber, glycerin, fertilizers, sulfur, solvents, and the feedstocks for the manufacture of drugs, nylon, plastics, paints, polyesters, food additives and supplements, explosives, dyes, and insulating materials. The petrochemical industry uses about 5 percent of the total supply of oil and gas in the United States.

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