Simple distillation will let you pull off the various fractions based on
their boiling points, but for gasoline production you don't get much of
the "good" stuff from raw crude oil. To improve the yield, a process called
catalytic cracking is used, where the long-chain hydrocarbons (like do decane)
are broken down into shorter chains such as octane, nonane and decane, which
are the lengths used in liquid fuels like gasoline, kerosene and diesel oil.
Longer chains make up lubricating oils, bunker fuel and waxes.
Sulphur content will determine whether the crude oil is classed as "light sweet
crude", or "sour crude". Those sulphur compounds don't smell so good. For
refined fuels, the sulphur has to be removed (another separation process) but
for some fixed-point power plant locations they'll use high-sulphur fuels and
scrub the resulting sulphur dioxide emissions from the smokestack gases using
pollution control equipment.
Industrial separation processes
Crude oil is rarely used in its raw form but must instead be processed into its various products. Aside from contaminant minerals such as sulfur and small amounts of trace metals which are removed during refining. Petroleum is composed of hydrocarbons, essentially varying combinations of carbon and hydrogen atoms; any hydrocarbon can be converted into any other given the appropriate application of energy, chemistry and technology. The smaller the molecule and the lower the ratio of carbon to hydrogen, the lighter the hydrocarbon, the lower the evaporation temperature, and, usually, the more valuable the product. Every crude oil contains a mix of these different hydrocarbons, and the two tasks of a refinery are to separate them out into usable products and to convert the less desirable hydrocarbons into more valuable ones.The tall metal towers that characterize petroleum refineries are distillation, or fractionating, towers.
Distillation is the primary method used to refine petroleum. When the heated crude oil is fed into the lower part of a tower, the lighter oil portions, or fractions, vaporize. Losing temperature as they rise, they condense into liquids, which flow downward into the higher temperatures and are revaporized. This process continues until the various fractions have achieved the appropriate degrees of purity. The lighter fractions, like butane, gasoline and kerosene, are tapped off from the top of the tower; heavier fractions, like fuel and diesel oils, are taken from the lower half.
MAJOR REFINERY PRODUCTS
GASOLINE
The most important refinery product is motor gasoline, a blend of hydrocarbons with boiling ranges from ambient temperatures to about 400 degrees F. The important qualities for gasoline are octane number (antiknock), volatility (starting and vapor lock), and vapor pressure (environmental control). Additives are often used to enhance performance and provide protection against oxidation and rust formation.
KEROSENE
Kerosene is a refined middle-distillate petroleum product that finds considerable use as a jet fuel and around the world in cooking and space heating. Kerosene, with less-critical specifications, is used for lighting, heating, solvents, and blending into diesel fuel.
LIQUEFIED PETROLEUM GAS (LPG)
LPG, which consists principally of propane and butane, is produced for use as fuel and is an intermediate material in the manufacture of petrochemicals. The important specifications for proper performance include vapor pressure and control of contaminants.
DISTILLATE FUELS
Diesel fuels and domestic heating oils have boiling ranges of about 400-700 degrees F. The desirable qualities required for distillate fuels include controlled flash and pour points, clean burning, no deposit formation in storage tanks, and a proper diesel fuel cetane rating for good starting and combustion.
RESIDUAL FUELS
Many marine vessels, power plants, commercial buildings and industrial facilities use residual fuels or combinations of residual and distillate fuels for heating and processing. The two most critical specifications of residual fuels are viscosity and low sulfur content for environmental control.
COKE AND ASPHALT
Coke is almost pure carbon with a variety of uses from electrodes to charcoal briquets. Asphalt, used for roads and roofing materials, must be inert to most chemicals and weather conditions.
SOLVENTS
A variety of products, whose boiling points and hydrocarbon composition are closely controlled, are produced for use as solvents. These include benzene, toluene, and xylene.
PETROCHEMICALS
Many products derived from crude oil refining such as ethylene, propylene, butylene, and isobutylene are primarily intended for use as petrochemical feedstocks in the production of plastics, synthetic fibers, synthetic rubbers, and other products.
LUBRICANTS
Special refining processes produce lubricating oil base stocks. Additives such as demulsifiers, antioxidants, and viscosity improvers are blended into the base stocks to provide the characteristics required for motor oils, industrial greases, lubricants, and cutting oils. The most critical quality for lubricating-oil base stock is a high viscosity index, which provides for greater consistency under varying temperatures
