The separation of petroleum into fractions and the treating of these fractions to yield marketable products. Petroleum is a mixture of gaseous, liquid, and solid hydrocarbon compounds that occurs in sedimentary rock deposits throughout the world. In the crude state, petroleum has little value but, when refined, it provides liquid fuels (gasoline, diesel fuel, aviation fuel), solvents, heating oil, lubricants, and the distillation residuum asphalt, which is used for highway surfaces and roofing materials. See also Petroleum.
Crude petroleum (oil) is a mixture of compounds with different boiling temperatures that can be separated into a variety of fractions. Since there is a wide variation in the composition of crude petroleum, the proportions in which the different fractions occur vary with origin. Some crude oils have higher proportions of lower-boiling components, while others have higher proportions of residuum (asphaltic components).
Petroleum processing and refining involves a series of steps by which the original crude oil is converted into products with desired qualities in the amounts dictated by the market. In fact, a refinery is essentially a group of manufacturing plants that vary in number with the variety of products in the mix. Refinery processes must be selected and products manufactured to give a balanced operation; that is, crude oil must be converted into products according to the demand for each. For example, the manufacture of products from the lower-boiling portion of petroleum automatically produces a certain amount of higher-boiling components. If the latter cannot be sold as, say, heavy fuel oil, these products will accumulate until refinery storage facilities are full. To prevent such a situation, the refinery must be flexible and able to change operations as needed. This usually means more processes, such as thermal processes to change excess heavy fuel oil into gasoline with coke as the residual product, or vacuum distillation processes to separate heavy oil into lubricating oil stocks and asphalt.
Distillation
In a petroleum distillation unit, a tower is used for fractionation. The feedstock of crude oil flows through one or more pipes arranged within a large furnace where it is heated to a temperature at which a predetermined portion of the feed changes into vapor. The heated feed is introduced into a fractional distillation tower where the nonvolatiles or liquid portions pass downward to the bottom of the tower and are pumped away, while the vapors pass upward through the tower and are fractionated into gas oils, kerosine, and naphthas.
Vacuum distillation is used in petroleum refining to separate the less volatile products, such as lubricating oils, from petroleum without subjecting the high-boiling products to cracking conditions. Operating pressure for vacuum distillation is usually 50–100 mm of mercury (6.7–13.3 kilopascals) [atmospheric pressure = 760 mm of mercury]. By this means, a heavy gas oil that has a boiling range in excess of 315°C (600°F) at atmospheric pressure may be obtained at temperatures of around 150°C (300°F); and lubricating oil, having a boiling range in excess of 370°C (700°F) at atmospheric pressure may be obtained at temperatures of 250–350°C (480–660°F). Atmospheric and vacuum distillation are major parts of refinery operations, and no doubt will continue to be used as the primary refining operation.
Thermal processes
One of the earliest conversion processes used in the petroleum industry was the thermal decomposition of higher-boiling materials into lower-boiling products. This process is known as thermal cracking. The majority of the thermal cracking processes use temperatures of 455–540°C (850–1005°F) and pressures of 100–1000 psi (690–6895 kPa). For example, the feedstock (reduced crude) is preheated by direct exchange with the cracking products in the fractionating columns. Cracked gasoline and heating oil are removed from the upper section of the column. Light and heavy distillate fractions are removed from the lower section and are pumped to separate heaters. Higher temperatures are used to crack the more stable light distillate fraction. The streams from the heaters are combined and sent to a soaking chamber where additional time is provided to complete the cracking reactions. The cracked products are then separated in a low-pressure flash chamber where a heavy fuel oil is removed as bottoms. The remaining cracked products are sent to fractionating columns. The thermal cracking of higher-boiling petroleum fractions to produce gasoline is now virtually obsolete. The antiknock requirements of modern automobile engines together with the different nature of crude oils (compared to those of 50 years ago) has reduced the ability of the thermal cracking process to produce gasoline on an economic basis. See also Distillation column.
Visbreaking (viscosity breaking) is a mild thermal cracking operation that can be used to reduce the viscosity of residua to allow the products to meet fuel oil specifications. Alternatively, the visbroken residua can be blended with lighter product oils to produce fuel oils of acceptable viscosity. By reducing the viscosity of the residuum, visbreaking reduces the amount of light heating oil that is required for blending to meet fuel oil specifications.
Delayed coking is a thermal process for converting residua into lower-boiling products, such as gases, naphtha, fuel oil, gas oil, and coke. It is a semicontinuous process in which the heated charge is transferred to large soaking (or coking) drums, which provide the long residence time needed to allow the cracking reactions to proceed to completion. The feedstock is introduced into a product fractionator where it is heated and the lighter fractions are removed as a side streams. Gas oil, often the major product of a coking operation, serves primarily as a feedstock for catalytic cracking units. The coke obtained is typically used as fuel; but specialty uses, such as electrode manufacture, and production of chemicals and metallurgical coke are also possible, increasing the value of the coke. For these uses, the coke may require treatment to remove sulfur and metal impurities. See also Coke; Cracking; Naphtha.
Catalytic cracking is basically the same as thermal cracking, but differs by the use of a catalyst, which directs the course of the cracking reactions to produce more of the desired higher-octane hydrocarbon products. Catalytic cracking is regarded as the modern method for converting high-boiling petroleum fractions, such as gas oil, into gasoline and other low-boiling fractions. The usual commercial process involves contacting a gas oil faction with an active catalyst at a suitable temperature, pressure, and residence time so that a substantial part (>50%) of the gas oil is converted into gasoline and lower-boiling products, usually in a single-pass operation.
Hydroprocesses
The use of hydrogen in thermal processes was perhaps the single most significant advance in refining technology during the twentieth century. The process uses the principle that the presence of hydrogen during a thermal reaction of a petroleum feedstock will terminate many of the coke-forming reactions and enhance the yields of the lower-boiling components, such as gasoline, kerosine, and jet fuel. See also Hydrogenation.
Destructive hydrogenation (hydrogenolysis or hydrocracking) is characterized by the conversion of the higher-molecular-weight constituents in a feedstock to lower-boiling products. Such treatment requires severe processing conditions and the use of high hydrogen pressures to minimize the polymerization and condensation reactions that lead to coke formation. See also Hydrocracking.
Nondestructive hydrogenation is used for improving product quality without appreciable alteration of the boiling range. Nitrogen, sulfur, and oxygen compounds undergo reaction with the hydrogen, forming ammonia, hydrogen sulfide, and water, respectively. Unstable compounds that might lead to the formation of gums or insoluble materials are converted to more stable compounds.
