Petroleum

James Richard Fromm


Industrial organic chemistry today can be divided roughly into four major areas. In order of their current economic importance they are polymers, petrochemicals, synthetic materials (other than polymers), and miscellaneous organic materials lumped together under the general heading of "fine chemicals". The historic development and present industrial structure of each of these areas are different.

Most modern industrial organic materials are derived from petroleum, whose modern production dates from about 1860, or from natural gas. Less important sources include coal tar, wood, and agricultural waste materials. Oil refineries are generally large installations having some flexibility in both input and output, and traditionally switch output between fuel, lubricating oils, and petrochemicals depending on prices and markets.

Petroleum, or oil, is a naturally occurring liquid with widely different composition of very great complexity. While there are a few surface seepages, the vast majority of petroleum is found well below the surface of the earth and can be reached only by drilling. Oil wells tap into pools of oil, or into porous rock containing the oil, called reservoirs or fields. The oil is sometimes found under sufficiently high pressure to flow to the surface without pumping, but for most wells pumping is required. The amount of oil recoverable from a field by pumping may be only 5%, more frequently 25-30%, of the oil believed to be present. Addition of liquids to the field down wells, often salt water from brine wells or local fresh surface water with a small amount of surfactant added, is used to provide enhanced recovery of oil. In fields where the oil is very heavy steam injection may be used. Complete removal of the oil from a field is not possible even with enhanced recovery methods.

Petroleum as obtained from wells, crude oil, is a complex mixture of hydrocarbons. Its elemental composition is primarily carbon-hydrogen, with variable quantities of oxygen and sulfur, and trace amounts of nitrogen, metals and other elements. Crude oil is classified by the carbon-hydrogen ratio, which is lower for the more desirable crudes containing smaller molecules (light crude) and higher for the less desirable crudes containing primarily larger molecules (heavy crude). Crude oil low in sulfur is called sweet crude while the less desirable crude oil with higher sulfur content is called sour crude.

Crude oil is found in many parts of the world. Major producing areas include the southern United States, western Canada, Mexico, Venezuela, the Middle East, the eastern Soviet Union, Rumania, Nigeria, and Indonesia. Crude oil can be transported economically long distances overland by pipeline and overseas in very large tanker vessels. Refineries are located on seacoasts with tanker docks or are connected to the production areas or tanker ports by pipeline.

Petroleum Refining

Crude oil refining usually begins with washing with water to remove salt and other inorganic impurities, followed by fractional distillation. The fractions into which crude oil is traditionally distilled is shown in the Table below.


Table: Typical Fractions Obtained on Distillation of Crude Oil
Output % Boiling Range (Deg. Celsius) Carbon Atoms Product
       
2 <<30 1 to 4 Light Hydrocarbons
15 to 30 30 - 200 4 to 12 Naptha
5 to 20 200 - 300 12 to 15 Kerosene
10 to 40 300 - 400 15 to 25 Gas Oil
Residue 400+ 25+ Residual Oil

The initial distillation or topping of the crude oil separates the fractions shown in the above Table. Each of these fractions is usually subjected to additional processes and portions of them may be combined to give final desired products. Modern refineries produce primarily fuels, especially motor gasoline, kerosene (jet fuel), fuel oil (heating oil), and heavier oils (residual oil), as well as a variety of minor products such as lubricating oils, paraffin, and asphalt.

The fraction indicated as light hydrocarbons includes methane through the butanes. The fraction or cut designated naptha is also called straight-run gasoline and can be used as a motor fuel, although modern motor gasolines are more sophisticated blends. The kerosene fraction, once in demand for lighting, now finds its primary modern use as jet aircraft fuel. The heavier gas oil fraction is blended into several grades of heating oil and bunker fuel (for ships). Both gas oil and residual oil are feedstocks for further processes. Residual oil is not the only form of residue; paraffins and asphalt are also left undistilled.

Natural gas, on compression, will also condense out heavier alkanes than methane as liquids (liquefied petroleum gases, L.P.G.), and these are a useful source of propanes and butanes for polymers. In a modern refinery, unused "cuts" or fractions can be cycled to cracking units, which heat the material above 230oC at different pressures. Cracking may be done in the presence (catalytic cracking) or absence (thermal cracking) of a catalyst or in the presence of water (steam cracking). The reactions occurring in a cracking process are complex. Most, but not all, convert more complex hydrocarbons to simpler ones of lower boiling point with loss of hydrogen gas. Among them are:

2CH4 rarrow.gif (63 bytes) HC=CH + 6H2,

producing ethyne, and the production of ethene from ethane,

C2H6 rarrow.gif (63 bytes) H2C=CH2 + H2.

Propane also can be cracked to ethene,

C3H8(g) rarrow.gif (63 bytes) H2C=CH2 + CH4,

or to propene,

C3H8(g) rarrow.gif (63 bytes) H2C=CHCH3 + H2.

Heavier fractions of oil are cracked to gasoline. Propene, CH3-CH=CH2, arises as a by-product of the cracking process which yields ethene or is a by-product of gases liberated elsewhere in the refinery. Ethane, propane, and butane obtained from natural gas are cracked to ethene and propene in the same manner. Butenes and butadiene, produced in smaller quantities, are used in the production of synthetic rubber.


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Copyright 1997 James R. Fromm