petroleum refining

petroleum refining

Introduction
 conversion of crude oil into useful products.

History

Distillation of kerosene and naphtha
      The refining of crude petroleum owes its origin to the successful drilling of the first oil well in Titusville, Pa., in 1859. Prior to that time, petroleum was available only in very small quantities from natural seepage of subsurface oil in various areas throughout the world. However, such limited availability restricted the uses for petroleum to medicinal and specialty purposes. With the discovery of “rock oil” in northwestern Pennsylvania, crude oil became available in sufficient quantity to inspire the development of larger-scale processing systems. The earliest refineries employed simple distillation units, or “stills,” to separate the various constituents of petroleum by heating the crude oil mixture in a vessel and condensing the resultant vapours into liquid fractions. Initially the primary product was kerosene, which proved to be a more abundant, cleaner-burning lamp oil of more consistent quality than whale oil or animal fat.

      The lowest-boiling raw product from the still was “straight run” naphtha, a forerunner of unfinished gasoline. Its initial commercial application was primarily as a solvent. Higher-boiling materials were found to be effective as lubricants and fuel oils, but they were largely novelties at first.

      The perfection of oil-drilling techniques quickly spread to Russia, and by 1890 refineries there were producing large quantities of kerosene and fuel oils. The development of the internal-combustion engine in the later years of the 19th century created a small market for crude naphtha. But the development of the automobile at the turn of the century sharply increased the demand for quality gasoline, and this finally provided a home for the petroleum fractions that were too volatile to be included in kerosene. As demand for automotive fuel rose, methods for continuous distillation of crude oil were developed.

Conversion to light fuels
      After 1910 the demand for automotive fuel began to outstrip the market requirements for kerosene, and refiners were pressed to develop new technologies to increase gasoline yields. The earliest process, called thermal cracking, consisted of heating heavier oils (for which there was a low market requirement) in pressurized reactors and thereby cracking, or splitting, their large molecules into the smaller ones that form the lighter, more valuable fractions such as gasoline, kerosene, and light industrial fuels. Gasoline manufactured by the cracking process performed better in automobile engines than gasoline derived from straight distillation of crude petroleum. The development of more powerful aircraft engines in the late 1930s gave rise to a need to increase the combustion characteristics of gasoline and spurred the development of lead-based fuel additives to improve engine performance.

      During the 1930s and World War II, sophisticated refining processes involving the use of catalysts (catalyst) led to further improvements in the quality of transportation fuels and further increased their supply. These improved processes—including catalytic cracking of heavy oils, alkylation, polymerization, and isomerization—enabled the petroleum industry to meet the demands of high-performance combat aircraft and, after the war, to supply increasing quantities of transportation fuels.

      The 1950s and '60s brought a large-scale demand for jet fuel and high-quality lubricating oils. The continuing increase in demand for petroleum products also heightened the need to process a wider variety of crude oils into high-quality products. Catalytic reforming of naphtha replaced the earlier thermal reforming process and became the leading process for upgrading fuel qualities to meet the needs of higher-compression engines. Hydrocracking, a catalytic cracking process conducted in the presence of hydrogen, was developed to be a versatile manufacturing process for increasing the yields of either gasoline or jet fuels.

Environmental (environment) concerns
      By 1970 the petroleum-refining industry had become well established throughout the world. Demand for refined petroleum products had reached almost 2.3 billion tons per year (40 million barrels per day), with major concentrations of refineries in most developed countries. As the world became aware of the impact of industrial pollution on the environment, however, the petroleum-refining industry was a primary focus for change. Refiners added hydrotreating units to extract sulfur compounds from their products and began to generate large quantities of elemental sulfur. Effluent water and atmospheric emission of hydrocarbons and combustion products also became a focus of increased technical attention. In addition, many refined products came under scrutiny. By the mid-1970s petroleum refiners in the United States were required to develop techniques for manufacturing high-quality gasoline without employing lead additives, and by 1990 they were required to take on substantial investments in the complete reformulation of transportation fuels in order to minimize environmental emissions. From an industry that produced a single product (kerosene) and disposed of unwanted by-product materials in any manner possible, petroleum refining had become one of the most stringently regulated of all manufacturing industries, expending a major portion of its resources on the protection of the environment.

Raw materials

hydrocarbon chemistry
      Petroleum crude oils (crude oil) are complex mixtures of hydrocarbons, chemical compounds composed only of carbon (C) and hydrogen (H).

Saturated molecules
      The simplest of the hydrocarbon molecules is methane (CH4), which has one carbon atom and four hydrogen atoms per molecule. The next simplest, ethane (C2H6), has two carbon atoms and six hydrogen atoms. A whole class of hydrocarbons can be defined by expanding upon the relationship between methane and ethane. Known as the paraffins (paraffin hydrocarbon), this is a family of chainlike molecules with the chemical formula CnH2n + 2. These molecules are also referred to as saturated, since each of the four valence electrons on a carbon atom that are available for bonding is taken up by a single hydrogen or carbon atom. Because these “single” bonds leave no valence electron available for sharing with another atom, paraffin molecules tend to be chemically stable.

 Paraffins can be arranged either in straight chains (normal paraffins, such as butane; see figure—>) or branched chains (isoparaffins). Most of the paraffin compounds in naturally occurring crude oils are normal paraffins, while isoparaffins are frequently produced in refinery processes. The normal paraffins are uniquely poor as motor fuels, while isoparaffins have good engine-combustion characteristics. Longer-chain paraffins are major constituents of waxes.

      Once a hydrocarbon molecule contains more than four carbon atoms, the carbon atoms may form not a branched or straight chain but a closed-ring structure known as a cyclo-compound. Saturated cyclo-compounds are called naphthenes. Naphthenic crudes tend to be poor raw materials for lubricant manufacture, but they are more easily converted into high-quality gasolines than are the paraffin compounds.

Unsaturated molecules
      Two other chemical families that are important in petroleum refining are composed of unsaturated molecules. In unsaturated molecules, not all the valence electrons on a carbon atom are bonded to separate carbon or hydrogen atoms; instead, two or three electrons may be taken up by one neighbouring carbon atom, thus forming a “double” or “triple” carbon-carbon bond. Like saturated compounds, unsaturated compounds can form either chain or ring molecules. Unsaturated chain molecules are known as olefins (olefin). Only small amounts of olefins are found in crude oils, but large volumes are produced in refining processes. Olefins are relatively reactive as chemicals and can be readily combined to form other longer-chain compounds.

 The other family of unsaturated compounds is made up of ring molecules called aromatics. The simplest aromatic compound, benzene (C6H6), has double bonds linking every other carbon molecule (see figure—>). The double bonds in the benzene ring are very unstable and chemically reactive. Partly for this reason, benzene is a popular building block in the petrochemical industry.

      Unsaturated hydrocarbons generally have good combustion characteristics, but their reactivity can lead to instability in storage and sometimes to environmental emission problems.

Types of crude oil
      The above description of hydrocarbons refers to simpler members of each family, but crude oils are actually complex mixtures of very long-chain compounds, some of which have not yet been identified. Because such complex mixtures cannot be readily identified by chemical composition, refiners customarily characterize crude oils by the type of hydrocarbon compound that is most prevalent in them: paraffins, naphthenes, and aromatics. Some crude oils, such as those in the original Pennsylvanian oil fields, consist mainly of paraffins. Others, such as the heavy Mexican and Venezuelan crudes, are predominantly naphthenic and are rich in bitumen (a high-boiling semisolid material frequently made into asphalt for road surfaces).

 The proportions of products that may be obtained by distillation of five typical crude oils, ranging from heavy Venezuelan Boscan to the light Bass Strait oil produced in Australia, are shown in thefigure—>. Given the pattern of modern demand (which tends to be highest for transportation fuels such as gasoline), the market price of a crude oil generally rises with increasing yields of light products. It is possible to process heavier crudes more intensely in order to improve their yield of light products, but the capital and operating costs required to support such high conversion processes are much greater than those required to process lighter crudes into the same yield of products.

      In addition to the hydrocarbons, compounds of sulfur, nitrogen, and oxygen are present in small amounts in crude oils. Also there are usually traces of vanadium, nickel, chlorine, sodium, and arsenic. These elements may affect the safety of oil-transport systems, the quality of refined products, and the effectiveness of processing units within a refinery. Minute traces can usually be tolerated, but crudes with larger amounts of these materials must be extensively treated in order to restrict their harmful effects.

Conventional measurement systems
       Measurement systems employed in petroleum refiningPetroleum refining is a continuous manufacturing process that is highly dependent on careful measurement of operating variables to influence product qualities and to control operating expenses. The conventional practice for the industry in the United States is to measure capacity by volume and to employ the English system for other operating measurements. Most refiners in other areas of the world define capacity by the weight of materials processed and record operating measurements in metric units. Since many refiners throughout the world have U.S. shareholders, international results are often reported on both bases, which are shown in the Table (Measurement systems employed in petroleum refining). In this section, all measurements will be presented in international terms with the U.S. equivalent indicated in parentheses.

Basic refinery processes
      Each refinery is uniquely designed to process specific crude oils into selected products. In order to meet the business objectives of the refinery, the process designer selects from an array of basic processing units. In general, these units perform one of three functions: (1) separating the many types of hydrocarbon present in crude oils into fractions of more closely related properties, (2) chemically converting the separated hydrocarbons into more desirable reaction products, and (3) purifying the products of unwanted elements and compounds.

      The primary process for separating the hydrocarbon components of crude oil is fractional distillation. Crude oil distillers separate crude oil into fractions for subsequent processing in such units as catalytic reformers, cracking units, alkylation units, or cokers. In turn, each of these more complex processing units also incorporates a fractional distillation tower to separate its own reaction products.

      Modern crude oil distillation units operate continuously over long periods of time and are much larger than the fractional distillation units employed in chemical or other industries. Process rates are commonly delineated in American barrels; units capable of processing 100,000 barrels per day are commonplace, and the largest units are capable of charging more than 200,000 barrels per day.

 The principles of operation of a modern crude oil distillation unit are shown in the figure—>. Crude oil is withdrawn from storage tanks at ambient temperature and pumped at a constant rate through a series of heat exchangers in order to reach a temperature of about 120° C (250° F). A controlled amount of fresh water is introduced, and the mixture is pumped into a desalting (desalination) drum, where it passes through an electrical field and a saltwater phase is separated. (If the salt were not removed at this stage, it would be deposited later on the tubes of the furnace and cause plugging.) The desalted crude oil passes through additional heat exchangers and then through steel alloy tubes in a furnace. There it is heated to a temperature between 315° and 400° C (600° and 750° F), depending on the type of crude oil and the end products desired. A mixture of vapour and unvaporized oil passes from the furnace into the fractionating column (column chromatography), a vertical cylindrical tower as much as 45 metres (150 feet) high containing 20 to 40 fractionating trays spaced at regular intervals. The most common fractionating trays are of the sieve or valve type. Sieve trays are simple perforated plates with small holes about 5 to 6 millimetres (0.2 to 0.25 inch) in diameter. Valve trays are similar, except the perforations are covered by small metal disks that restrict the flow through the perforations under certain process conditions.

      The oil vapours rise up through the column and are condensed to a liquid in a water- or air-cooled condenser at the top of the tower. A small amount of gas remains uncondensed and is piped into the refinery fuel-gas system. A pressure control valve on the fuel-gas line maintains fractionating column pressure at the desired figure, usually near atmospheric pressure (about 1 kilogram per square centimetre, or 15 pounds per square inch). Part of the condensed liquid, called reflux, is pumped back into the top of the column and descends from tray to tray, contacting rising vapours as they pass through the slots in the trays. The liquid progressively absorbs heavier constituents from the vapour and, in turn, gives up lighter constituents to the vapour phase. Condensation and reevaporation takes place on each tray. Eventually an equilibrium is reached in which there is a continual gradation of temperature and oil properties throughout the column, with the lightest constituents on the top tray and the heaviest on the bottom. The use of reflux and vapour-liquid contacting trays distinguishes fractional distillation from simple distillation columns.

 As shown in thefigure—>, intermediate products, or “sidestreams,” are withdrawn at several points from the column. In addition, modern crude distillation units employ intermediate reflux streams. Sidestreams are known as intermediate products because they have properties between those of the top or overhead product and those of products issuing from the base of the column. Typical boiling ranges for various streams are as follows: light straight-run naphtha (overhead), 20°–95° C (70°–200° F); heavy naphtha (top sidestream), 90°–165° C (195°– 330° F); crude kerosene (second sidestream), 150°–245° C (300°–475° F); light gas oil (third sidestream), 215°–315° C (420°–600° F).

      Unvaporized oil entering the column flows downward over a similar set of trays in the lower part of the column, called stripping trays, which act to remove any light constituents remaining in the liquid. Steam is injected into the bottom of the column in order to reduce the partial pressure of the hydrocarbons and assist in the separation. Typically a single sidestream is withdrawn from the stripping section: heavy gas oil, with a boiling range of 285°–370° C (545°–700° F). The residue that passes from the bottom of the column is suitable for blending into industrial fuels. Alternately, it may be further distilled under vacuum conditions to yield quantities of distilled oils for manufacture into lubricating oils or for use as a feedstock in a gas oil cracking process.

      The principles of vacuum distillation resemble those of fractional distillation (commonly called atmospheric distillation to distinguish it from the vacuum method), except that larger-diameter columns are used to maintain comparable vapour velocities at reduced operating pressures. A vacuum of 50 to 100 millimetres of mercury absolute is produced by a vacuum pump or steam ejector.

      The primary advantage of vacuum distillation is that it allows for distilling heavier materials at lower temperatures than those that would be required at atmospheric pressure, thus avoiding thermal cracking of the components. Firing conditions in the furnace are adjusted so that oil temperatures usually do not exceed 425° C (800° F). The residue remaining after vacuum distillation, called bitumen, may be further blended to produce road asphalt or residual fuel oil, or it may be used as a feedstock for thermal cracking or coking units. Vacuum distillation units are essential parts of the many processing schemes designed to produce lubricants.

Superfractionation
      An extension of the distillation process, superfractionation employs smaller-diameter columns with a much larger number of trays (100 or more) and reflux ratios exceeding 5:1. With such equipment it is possible to isolate a very narrow range of components or even pure compounds. Common applications involve the separation of high-purity solvents such as isoparaffins or of individual aromatic compounds for use as petrochemicals.

      Absorption processes are employed to recover valuable light components such as propane/propylene and butane/butylene from the vapours that leave the top of crude-oil or process-unit fractionating columns within the refinery. These volatile gases are bubbled through an absorption fluid, such as kerosene or heavy naphtha, in equipment resembling a fractionating column. The light products dissolve in the oil while the dry gases—such as hydrogen, methane, ethane, and ethylene—pass through undissolved. Absorption is more effective under pressures of about 7 to 11 kilograms per square centimetre (100 to 150 pounds per square inch) than it is at atmospheric pressure.

      The enriched absorption fluid is heated and passed into a stripping column, where the light product vapours pass upward and are condensed for recovery as liquefied petroleum gas (LPG). The unvaporized absorption fluid passes from the base of the stripping column and is reused in the absorption tower.

Solvent extraction
      Solvent extraction processes are employed primarily for the removal of constituents that would have an adverse effect on the performance of the product in use. An important application is the removal of heavy aromatic compounds from lubricating oils. Removal improves the viscosity-temperature relationship of the product, extending the temperature range over which satisfactory lubrication is obtained. The usual solvents for extraction of lubricating oil are phenol and furfural.

      Certain highly porous solid materials have the ability to select and adsorb specific types of molecules, thus separating them from other materials. Silica gel is used in this way to separate aromatics from other hydrocarbons, and activated charcoal is used to remove liquid components from gases. Adsorption is thus somewhat analogous to the process of absorption with an oil, although the principles are different. Layers of adsorbed material only a few molecules thick are formed on the extensive interior surface of the adsorbent; the interior surface may amount to several hectares per kilogram of material.

      Molecular sieves (molecular sieve) are a special form of adsorbent. Such sieves are produced by the dehydration of naturally occurring or synthetic zeolites (crystalline alkali-metal aluminosilicates). The dehydration leaves intercrystalline cavities that have pore openings of definite size, depending on the alkali metal of the zeolite. Under adsorptive conditions, normal paraffin molecules can enter the crystalline lattice and be selectively retained, whereas all other molecules are excluded. This principle is used commercially for the removal of normal paraffins from gasoline fuels, thus improving their combustion properties. The use of molecular sieves is also extensive in the manufacture of high-purity solvents.

      The crystallization of wax from lubricating oil fractions is essential to make oils suitable for use. A solvent (often a mixture of benzene and methyl ethyl ketone) is first added to the oil, and the solution is chilled to about −20° C (−5° F). The function of the benzene is to keep the oil in solution and maintain its fluidity at low temperatures, whereas the methyl ethyl ketone acts as a wax precipitant. Rotary filters deposit the wax crystals on a specially woven cloth stretched over a perforated cylindrical drum. A vacuum is maintained within the drum to draw the oil through the perforations. The wax crystals are removed from the cloth by metal scrapers, after washing with solvent to remove traces of oil. The solvents are later distilled from the oil and reused.

Conversion
      The separation processes described above are based on differences in physical properties of the components of crude oil. All petroleum refineries throughout the world employ at least crude oil distillation units to separate naturally occurring fractions for further use, but those which employ distillation alone are limited in their yield of valuable transportation fuels. By adding more complex conversion processes that chemically change the molecular structure of naturally occurring components of crude oil, it is possible to increase the yield of valuable hydrocarbon compounds.

Naphtha reforming
      The most widespread process for rearranging hydrocarbon molecules is naphtha reforming. The initial process, thermal reforming, was developed in the late 1920s. Thermal reforming employed temperatures of 510°–565° C (950°–1,050° F) at moderate pressures (about 43 kilograms per square centimetre, or 600 pounds per square inch) to obtain gasolines with octane numbers of 70 to 80 from heavy naphthas with octane numbers of less than 40. The product yield, although of a higher octane level, included olefins, diolefins, and aromatic compounds. It was therefore inherently unstable in storage and tended to form heavy polymers and gums, which caused combustion problems.

      By 1950 a reforming process was introduced that employed a catalyst to improve the yield of the most desirable gasoline components while minimizing the formation of unwanted heavy products and coke. (A catalyst is a substance that promotes a chemical reaction but does not take part in it.) In catalytic reforming, as in thermal reforming, a naphtha-type material serves as the feedstock, but the reactions are carried out in the presence of hydrogen, which inhibits the formation of unstable unsaturated compounds that polymerize into higher-boiling materials.

      In most catalytic reforming processes, platinum is the active catalyst; it is distributed on the surface of an aluminum oxide carrier. Small amounts of rhenium, chlorine, and fluorine act as catalyst promoters. In spite of the high cost of platinum, the process is economical because of the long life of the catalyst and the high quality and yield of the products obtained. The principal reactions involve the breaking down of long-chain hydrocarbons into smaller saturated chains and the formation of isoparaffins, made up of branched-chain molecules. Formation of ring compounds (technically, the cyclization of paraffins into naphthenes) also takes place, and the naphthenes are then dehydrogenated into aromatic compounds (ring-shaped unsaturated compounds with fewer hydrogen atoms bonded to the carbon). The hydrogen liberated in this process forms a valuable by-product of catalytic reforming. The desirable end products are isoparaffins and aromatics, both having high octane numbers.

      In a typical reforming unit the naphtha charge is first passed over a catalyst bed in the presence of hydrogen to remove any sulfur impurities. The desulfurized feed is then mixed with hydrogen (about five molecules of hydrogen to one of hydrocarbon) and heated to a temperature of 500°–540° C (930°–1,000° F). The gaseous mixture passes downward through catalyst pellets in a series of three or more reactor vessels. Early reactors were designed to operate at about 25 kilograms per square centimetre (350 pounds per square inch), but current units frequently operate at less than 7 kilograms per square centimetre (100 pounds per square inch). Because heat is absorbed in reforming reactions, the mixture must be reheated in intermediate furnaces between the reactors.

      After leaving the final reactor, the product is condensed to a liquid, separated from the hydrogen stream, and passed to a fractionating column, where the light hydrocarbons produced in the reactors are removed by distillation. The reformate product is then available for blending into gasoline without further treatment. The hydrogen leaving the product separator is compressed and returned to the reactor system.

      Operating conditions are set to obtain the required octane level, usually between 90 and 100. At the higher octane levels, product yields are smaller, and more frequent catalyst regenerations are required. During the course of the reforming process, minute amounts of carbon are deposited on the catalyst, causing a gradual deterioration of the product yield pattern. Some units are semiregenerative facilities—that is, they must be removed from service periodically (once or twice annually) to burn off the carbon and rejuvenate the catalyst system—but increased demand for high-octane fuels has also led to the development of continuous regeneration systems, which avoid the periodic unit shutdowns and maximize the yield of high-octane reformate. Continuous regeneration employs a moving bed of catalyst particles that is gradually withdrawn from the reactor system and passed through a regenerator vessel, where the carbon is removed and the catalyst rejuvenated for reintroduction to the reactor system.

      The use of thermal cracking units to convert gas oils into naphtha dates from before 1920. These units produced small quantities of unstable naphthas and large amounts of by-product coke. While they succeeded in providing a small increase in gasoline yields, it was the commercialization of the fluid catalytic cracking process in 1942 that really established the foundation of modern petroleum refining. The process not only provided a highly efficient means of converting high-boiling gas oils into naphtha to meet the rising demand for high-octane gasoline, but it also represented a breakthrough in catalyst technology.

      The thermal cracking process functioned largely in accordance with the free-radical theory of molecular transformation. Under conditions of extreme heat, the electron bond between carbon atoms in a hydrocarbon molecule can be broken, thus generating a hydrocarbon group with an unpaired electron. This negatively charged molecule, called a free radical, enters into reactions with other hydrocarbons, continually producing other free radicals via the transfer of negatively charged hydride ions (H−). Thus a chain reaction is established that leads to a reduction in molecular size, or “cracking,” of components of the original feedstock.

      Use of a catalyst in the cracking reaction increases the yield of high-quality products under much less severe operating conditions than in thermal cracking. Several complex reactions are involved, but the principal mechanism by which long-chain hydrocarbons are cracked into lighter products can be explained by the carbonium ion theory. According to this theory, a catalyst promotes the removal of a negatively charged hydride ion from a paraffin (paraffin hydrocarbon) compound or the addition of a positively charged proton (H+) to an olefin compound. This results in the formation of a carbonium ion, a positively charged molecule that has only a very short life as an intermediate compound which transfers the positive charge through the hydrocarbon. Carbonium transfer continues as hydrocarbon compounds come into contact with active sites on the surface of the catalyst that promote the continued addition of protons or removal of hydride ions. The result is a weakening of carbon-carbon bonds in many of the hydrocarbon molecules and a consequent cracking into smaller compounds.

 Olefins crack more readily than paraffins, since their double carbon-carbon bonds are more friable under reaction conditions. Isoparaffins and naphthenes crack more readily than normal paraffins, which in turn crack faster than aromatics. In fact, aromatic ring compounds are very resistant to cracking, since they readily deactivate fluid cracking catalysts by blocking the active sites of the catalyst. The Table—> illustrates many of the principal reactions that are believed to occur in fluid catalytic cracking unit reactors. The reactions postulated for olefin compounds apply principally to intermediate products within the reactor system, since the olefin content of catalytic cracking feedstock is usually very low.

      Typical modern catalytic cracking reactors operate at 480°–550° C (900°–1,020° F) and at relatively low pressures of 0.7 to 1.4 kilograms per square centimetre (10 to 20 pounds per square inch). At first natural silica-alumina clays were used as catalysts, but by the mid-1970s zeolitic (zeolite) and molecular sieve-based catalysts became common. Zeolitic catalysts give more selective yields of products while reducing the formation of gas and coke.

 A modern fluid catalytic cracker employs a finely divided solid catalyst that has properties analogous to a liquid when it is agitated by air or oil vapours. The principles of operation of such a unit are shown in the figure—>. In this arrangement a reactor and regenerator are located side by side. The oil feed is vaporized when it meets the hot catalyst at the feed-injection point, and the vapours flow upward through the riser reactor at high velocity, providing a fluidizing effect for the catalyst particles. The catalytic reaction occurs exclusively in the riser reactor. The catalyst then passes into the cyclone vessel, where it is separated from reactor hydrocarbon products.

      As the cracking reactions proceed, carbon is deposited on the catalyst particles. Since these deposits impair the reaction efficiency, the catalyst must be continuously withdrawn from the reaction system. Unit product vapours pass out of the top of the reactor through cyclone separators, but the catalyst is removed by centrifugal force and dropped back into the stripper section. In the stripping section, hydrocarbons are removed from the spent catalyst with steam, and the catalyst is transferred through the stripper standpipe to the regenerator vessel, where the carbon is burned with a current of air. The high temperature of the regeneration process (675°–785° C, or 1,250°–1,450° F) heats the catalyst to the desired reaction temperature for recontacting fresh feed into the unit. In order to maintain activity, a small amount of fresh catalyst is added to the system from time to time, and a similar amount is withdrawn.

      The cracked reactor effluent is fractionated in a distillation column. The yield of light products (with boiling points less than 220° C, or 430° F) is usually reported as the conversion level for the unit. Conversion levels average about 60 to 70 percent in Europe and Asia and in excess of 80 percent in many catalytic cracking units in the United States. About one-third of the product yield consists of fuel gas and other gaseous hydrocarbons. Half of this is usually propylene and butylene, which are important feedstocks for the polymerization and alkylation processes discussed below. The largest volume is usually cracked naphtha, an important gasoline blend stock with an octane number of 90 to 94. The lower conversion units of Europe and Asia produce comparatively more distillate oil and less naphtha and light hydrocarbons.

      The light gaseous hydrocarbons produced by catalytic cracking are highly unsaturated and are usually converted into high-octane gasoline components in polymerization or alkylation processes. In polymerization, the light olefins propylene and butylene (butene) are induced to combine, or polymerize, into molecules of two or three times their original molecular weight. The catalysts employed consist of phosphoric acid on pellets of kieselguhr, a porous sedimentary rock. High pressures, on the order of 28 to 80 kilograms per square centimetre (400 to 1,100 pounds per square inch), are required at temperatures ranging from 175° to 230° C (350° to 450° F). Polymer gasolines derived from propylene and butylene have octane numbers above 90 and, with the addition of lead additives, above 100.

      The alkylation reaction also achieves a longer chain molecule by the combination of two smaller molecules, one being an olefin and the other an isoparaffin (usually isobutane). During World War II, alkylation became the main process for the manufacture of isooctane, a primary component in the blending of aviation gasoline.

      Two alkylation processes employed in the industry are based upon different acid systems as catalysts. In sulfuric acid alkylation, concentrated sulfuric acid of 98 percent purity serves as the catalyst for a reaction that is carried out at 2° to 7° C (35° to 45° F). Refrigeration is necessary because of the heat generated by the reaction. The octane numbers of the alkylates produced range from 85 to 95.

      Hydrofluoric acid is also used as a catalyst for many alkylation units. The chemical reactions are similar to those in the sulfuric acid process, but it is possible to use higher temperatures (between 24° and 46° C, or 75° to 115° F), thus avoiding the need for refrigeration. Recovery of hydrofluoric acid is accomplished by distillation. Stringent safety precautions must be exercised when using this highly corrosive and toxic substance.

      One of the most far-reaching developments of the refining industry in the 1950s was the use of hydrogen, made possible in part by the availability of hydrogen as a by-product of catalytic reforming. Since 1980 hydrogen processing has become so prominent that many refineries now incorporate hydrogen-manufacturing plants in their processing schemes.

      Though hydrocracking processes a similar feedstock to the catalytic cracking unit, it offers even greater flexibility in product yields. The process can be used for producing gasoline or jet fuels from heavy gas oils, for producing high-quality lubricating oils, or for converting distillation residues into lighter oils. The jet fuel and distillate oil products are of high quality and low sulfur content and may be blended into final products without further processing. Hydrocracked naphtha, on the other hand, is often low in octane and must be catalytically reformed to produce high-quality gasoline.

      Hydrocracking is accomplished at lower temperatures than catalytic cracking—e.g., 260° to 425° C (500° to 800° F)—but at much higher pressures—70 to 280 kilograms per square centimetre (1,000 to 4,000 pounds per square inch). The design and manufacture of large, thick-walled vessels for operation under these conditions has been a major engineering achievement.

      Hydrocracking catalysts vary widely. The cracking reactions are induced by materials of the silica-alumina type. In units that process residual feedstocks, hydrogenation catalysts such as nickel, tungsten, platinum, or palladium are employed. The activity of the catalyst system can be maintained for long periods of time, so that continuous regeneration is not necessary as in catalytic cracking.

      The demand for aviation gasoline became so great during World War II and afterward that the quantities of isobutane available for alkylation feedstock were insufficient. This deficiency was remedied by isomerization of the more abundant normal butane into isobutane. The isomerization catalyst is aluminum chloride supported on alumina and promoted by hydrogen chloride gas.

      Commercial processes have also been developed for the isomerization of low-octane normal pentane and normal hexane to the higher-octane isoparaffin form. Here the catalyst is usually promoted with platinum. As in catalytic reforming, the reactions are carried out in the presence of hydrogen. Hydrogen is neither produced nor consumed in the process but is employed to inhibit undesirable side reactions. The reactor step is usually followed by molecular sieve extraction and distillation. Though this process is an attractive way to exclude low-octane components from the gasoline blending pool, it does not produce a final product of sufficiently high octane to contribute much to the manufacture of unleaded gasoline.

Visbreaking, thermal cracking, and coking
      Since World War II the demand for light products (e.g., gasoline, jet, and diesel fuels) has grown, while the requirement for heavy industrial fuel oils has declined. Furthermore, many of the new sources of crude petroleum (California, Alaska, Venezuela, and Mexico) have yielded heavier crude oils with higher natural yields of residual fuels. As a result, refiners have become even more dependent on the conversion of residue components into lighter oils that can serve as feedstock for catalytic cracking units.

      As early as 1920, large volumes of residue were being processed in visbreakers or thermal cracking units. These simple process units basically consist of a large furnace that heats the feedstock to the range of 450° to 500° C (840° to 930° F) at an operating pressure of about 10 kilograms per square centimetre (140 pounds per square inch). The residence time in the furnace is carefully limited to prevent much of the reaction from taking place and clogging the furnace tubes. The heated feed is then charged to a reaction chamber, which is kept at a pressure high enough to permit cracking of the large molecules but restrict coke formation. From the reaction chamber the process fluid is cooled to inhibit further cracking and then charged to a distillation column for separation into components.

      Visbreaking units typically convert about 15 percent of the feedstock to naphtha and diesel oils and produce a lower-viscosity residual fuel. Thermal cracking units provide more severe processing and often convert as much as 50 to 60 percent of the incoming feed to naphtha and light diesel oils.

      Coking is severe thermal cracking. The residue feed is heated to about 475° to 520° C (890° to 970° F) in a furnace with very low residence time and is discharged into the bottom of a large vessel called a coke drum for extensive and controlled cracking. The cracked lighter product rises to the top of the drum and is drawn off. It is then charged to the product fractionator for separation into naphtha, diesel oils, and heavy gas oils for further processing in the catalytic cracking unit. The heavier product remains and, because of the retained heat, cracks ultimately to coke, a solid carbonaceous substance akin to coal. Once the coke drum is filled with solid coke, it is removed from service and replaced by another coke drum.

      Decoking is a routine daily occurrence accomplished by a high-pressure water jet. First the top and bottom heads of the coke drum are removed. Next a hole is drilled in the coke from the top to the bottom of the vessel. Then a rotating stem is lowered through the hole, spraying a water jet sideways. The high-pressure jet cuts the coke into lumps, which fall out the bottom of the drum for subsequent loading into trucks or railcars for shipment to customers. Typically, coke drums operate on 24-hour cycles, filling with coke over one 24-hour period followed by cooling, decoking, and reheating over the next 24 hours. The drilling derricks on top of the coke drums are a notable feature of the refinery skyline.

      Cokers produce no liquid residue but yield up to 30 percent coke by weight. Much of the low-sulfur product is employed to produce electrodes for the electrolytic smelting of aluminum. Most lower-quality coke is burned as fuel in admixture with coal. Coker economics usually favour the conversion of residue into light products even if there is no market for the coke.

      Before petroleum products can be marketed, certain impurities must be removed or made less obnoxious. The most common impurities are sulfur compounds (organosulfur compound) such as hydrogen sulfide (H2S) or the mercaptans (“R”SH)—the latter being a series of complex organic compounds having as many as six carbon atoms in the hydrocarbon radical (“R”). Apart from their foul odour, sulfur compounds are technically undesirable. In motor and aviation fuels they reduce the effectiveness of antiknock additives and interfere with the operation of exhaust-treatment systems. In diesel fuel they cause engine corrosion and complicate exhaust-treatment systems. Also, many major residual and industrial fuel consumers are located in developed areas and are subject to restrictions on sulfurous emissions.

      Most crude oils contain small amounts of hydrogen sulfide, but these levels may be increased by the decomposition of heavier sulfur compounds (such as the mercaptans) during refinery processing. The bulk of the hydrogen sulfide is contained in process-unit overhead gases, which are ultimately consumed in the refinery fuel system. In order to minimize noxious emissions, most refinery fuel gases are desulfurized.

      Other undesirable components include nitrogen compounds, which poison catalyst systems, and oxygenated compounds, which can lead to colour formation and product instability. The principal treatment processes are outlined below.

      Sweetening processes oxidize mercaptans into more innocuous disulfides, which remain in the product fuels. Catalysts assist in the oxidation. The doctor process employs sodium plumbite, a solution of lead oxide in caustic soda, as a catalyst. At one time this inexpensive process was widely practiced, but the necessity of adding elemental sulfur to make the reactions proceed caused an increase in total sulfur content in the product. It has largely been replaced by the copper chloride process, in which the catalyst is a slurry of copper chloride and fuller's earth. It is applicable to both kerosene and gasoline. The oil is heated and brought into contact with the slurry while being agitated in a stream of air that oxidizes the mercaptans to disulfides. The slurry is then allowed to settle and is separated for reuse. A heater raises the temperature to a point that keeps the water formed in the reaction dissolved in the oil, so that the catalyst remains properly hydrated. After sweetening, the oil is water washed to remove any traces of catalyst and is later dried by passing through a salt filter.

Mercaptan (thiol) extraction
      Simple sweetening is adequate for many purposes, but other methods must be used if the total sulfur content of the fuel is to be reduced. When solutizers, such as potassium isobutyrate and sodium cresylate, are added to caustic soda, the solubility of the higher mercaptans is increased and they can be extracted from the oil. In order to remove traces of hydrogen sulfide and alkyl phenols, the oil is first pretreated with caustic soda in a packed column or other mixing device. The mixture is allowed to settle and the product water washed before storage.

clay treatment
      Some natural clays, activated by roasting or treatment with steam or acids, have been used for many years to remove traces of impurities. The phenomenon is similar to that described under the adsorption process: the clay retains the longer chain molecules within its highly porous structure.

      Clay treatment removes gum and gum-forming materials from thermally cracked gasolines in the vapour phase. A more economical procedure, however, is to add small quantities of synthetic antioxidants to the gasoline. These prevent or greatly retard gum formation. Clay treatment of lubricating oils is widely practiced to remove resins and other colour bodies remaining after solvent extraction. The treatment may be by contact—that is, clay added directly to the oil, with the mixture heated and the clay filtered off—or by percolation, in which the heated oil is passed through a large bed of active clay adsorbent. The spent clay is often discarded, although it can be regenerated by roasting. However, the problem of dealing with spent clay, now designated as a hazardous waste in many places, has led many refiners to replace clay treatment facilities with a mild hydrogenation (hydrogen) process.

Hydrogen treatment
      Hydrogen processes, commonly known as hydrofinishing, hydrofining, or hydrodesulfurization, are the most common processes for removing sulfur compounds. The oil is combined with high-purity hydrogen, vapourized, and then passed over a catalyst such as tungsten, nickel, or a mixture of cobalt and molybdenum oxides supported on an alumina base. Operating temperatures are usually between 260° and 425° C (500° and 800° F) at pressures of 14 to 70 kilograms per square centimetre (200 to 1,000 pounds per square inch). Operating conditions are set to facilitate the desired level of sulfur removal without promoting any change to the other properties of the oil.

      The sulfur in the oil is converted to hydrogen sulfide, which is removed from the circulating hydrogen stream by absorption in a solution such as diethanolamine. The solution can then be heated to remove the sulfide and reused. The hydrogen sulfide recovered is useful for manufacturing elemental sulfur of high purity.

Molecular sieves (molecular sieve)
      Molecular sieves are also used to purify petroleum products, since they have a strong affinity for polar compounds such as water, carbon dioxide, hydrogen sulfide, and mercaptans. Sieves are prepared by dehydration of an aluminosilicate such as zeolite. The petroleum product is passed through a bed of zeolite for a predetermined period depending on the impurity to be removed. The adsorbed contaminants may later be expelled from the sieve by purging with a gas stream at temperatures between 200° and 315° C (400° and 600° F). The frequent cycling of the molecular sieve from adsorb to desorb operations is usually fully automated.

Petroleum products and their uses

Gases
      Gaseous refinery products include hydrogen, fuel gas, ethane, and propane or LPG (liquefied petroleum gas). Most of the hydrogen is consumed in refinery desulfurization facilities; small quantities may be delivered to the refinery fuel system. Refinery fuel gas usually has a heating value similar to natural gas and is consumed in plant operations. Periodic variability in heating value makes it unsuitable for delivery to consumer gas systems. Ethane may be recovered from the refinery fuel system for use as a petrochemical feedstock. Liquefied petroleum gas, or LPG, is a convenient, portable fuel for domestic heating and cooking or light industrial use.

      Motor gasoline, or petrol, must meet three primary requirements. It must provide an even combustion pattern, start easily in cold weather, and meet prevailing environmental requirements.

Octane rating
      In order to meet the first requirement, gasoline must burn smoothly in the engine without premature detonation, or knocking. Severe knocking can dissipate power output and even cause damage to the engine. When gasoline engines became more powerful in the 1920s, it was discovered that some fuels knocked more readily than others. Experimental studies led to the determination that, of the standard fuels available at the time, the most extreme knock was produced by a fuel composed of pure normal heptane, while the least knock was produced by pure isooctane. This discovery led to the development of the octane (octane number) scale for defining gasoline quality. Thus, when a motor gasoline gives the same performance in a standard knock engine as a mixture of 90 percent isooctane and 10 percent normal heptane, it is given an octane rating of 90.

      There are two methods for carrying out the knock engine test. Research octane is measured under mild conditions of temperature and engine speed (49° C [120° F] and 600 revolutions per minute, or RPM), while motor octane is measured under more severe conditions (149° C [300° F] and 900 RPM). For many years the research octane number was found to be the more accurate measure of engine performance and was usually quoted alone. Since the advent of unleaded fuels in the mid-1970s, however, motor octane measurements have frequently been found to limit actual engine performance. As a result a new measurement, road octane number, which is a simple average of the research and motor values, is most frequently used to define fuel quality. Automotive gasolines generally range from research octane number 87 to 100, while gasoline for piston-engine aircraft ranges from research octane number 115 to 130.

      Each naphtha component that is blended into gasoline is tested separately for its octane rating. Reformate, alkylate, polymer, and cracked naphtha, as well as butane, all rank high (90 or higher) on this scale, while straight-run naphtha may rank at 70 or less. In the 1920s it was discovered that the addition of tetraethyl lead would substantially enhance the octane rating of various naphthas. Each naphtha component was found to have a unique response to lead additives, some combinations being found to be synergistic and others antagonistic. This gave rise to very sophisticated techniques for designing the optimal blends of available components into desired grades of gasoline.

      The advent of leaded, or ethyl, gasoline led to the manufacture of high-octane fuels and became universally employed throughout the world after World War II. Lead is still an essential component of high-octane aviation gasoline, but, beginning in 1975, environmental legislation in the United States restricted the use of lead additives in automotive gasoline. Similar restrictions have since been adopted in most developed countries. The required use of lead-free gasoline has placed a premium on the construction of new catalytic reformers and alkylation units for increasing yields of high-octane gasoline ingredients and on the exclusion of low-octane naphthas from the gasoline blend.

High-volatile and low-volatile components
      The second major criterion for gasoline—that the fuel be sufficiently volatile to enable the car engine to start quickly in cold weather—is accomplished by the addition of butane, a very low-boiling paraffin, to the gasoline blend. Fortunately, butane is also a high-octane component with little alternate economic use, so its application has historically been maximized in gasoline. Another requirement, that a quality gasoline have a high energy content, has traditionally been satisfied by including higher-boiling components in the blend. However, both of these practices are now called into question on environmental grounds. The same high volatility that provides good starting characteristics in cold weather can lead to high evaporative losses of gasoline during refueling operations, and the inclusion of high-boiling components to increase the energy content of the gasoline can also increase the emission of unburned hydrocarbons from engines on start-up. As a result, since 1990 gasoline consumed in the United States has been reformulated to meet stringent new environmental standards. Among these changes are the inclusion of some oxygenated compounds (methyl or ethyl alcohol or methyl tertiary butyl ether [MTBE]) in order to reduce the emission of carbon monoxide and nitrogen oxides.

Gasoline blending
      One of the most critical economic issues for a petroleum refiner is selecting the optimal combination of components to produce final gasoline products. Gasoline blending is much more complicated than a simple mixing of components. First, a typical refinery may have as many as 8 to 15 different hydrocarbon streams to consider as blend stocks. These may range from butane, the most volatile component, to a heavy naphtha and include several gasoline naphthas from crude distillation, catalytic cracking, and thermal processing units in addition to alkylate, polymer, and reformate. Modern gasoline may be blended to meet simultaneously 10 to 15 different quality specifications, such as vapour pressure; initial, intermediate, and final boiling points; sulfur content; colour; stability; aromatics content; olefin content; octane measurements for several different portions of the blend; and other local governmental or market restrictions. Since each of the individual components contributes uniquely in each of these quality areas and each bears a different cost of manufacture, the proper allocation of each component into its optimal disposition is of major economic importance. In order to address this problem, most refiners employ linear programming, a mathematical technique that permits the rapid selection of an optimal solution from a multiplicity of feasible alternative solutions. Each component is characterized by its specific properties and cost of manufacture, and each gasoline grade requirement is similarly defined by quality requirements and relative market value. The linear programming solution specifies the unique disposition of each component to achieve maximum operating profit. The next step is to measure carefully the rate of addition of each component to the blend and collect it in storage tanks for final inspection before delivering it for sale. Still, the problem is not fully resolved until the product is actually delivered into customers' tanks. Frequently, last-minute changes in shipping schedules or production qualities require the reblending of finished gasolines or the substitution of a high-quality (and therefore costlier) grade for one of more immediate demand even though it may generate less income for the refinery.

      Though its use as an illuminant has greatly diminished, kerosene is still used extensively throughout the world in cooking and space heating and is the primary fuel for modern jet engines. When burned as a domestic fuel, kerosene must produce a flame free of smoke and odour. Standard laboratory procedures test these properties by burning the oil in special lamps. All kerosene fuels must satisfy minimum flash-point specifications (49° C, or 120° F) to limit fire hazards in storage and handling.

      Jet fuels must burn cleanly and remain fluid and free from wax particles at the low temperatures experienced in high-altitude flight. The conventional freeze-point specification for commercial jet fuel is −50° C (−58° F). The fuel must also be free of any suspended water particles that might cause blockage of the fuel system with ice particles. Special-purpose military jet fuels have even more stringent specifications.

Diesel oils
      The principal end use of gas oil is as diesel fuel for powering automobile, truck, bus, and railway engines. In a diesel engine, combustion is induced by the heat of compression of the air in the cylinder under compression. Detonation, which leads to harmful knocking in a gasoline engine, is a necessity for the diesel engine. A good diesel fuel starts to burn at several locations within the cylinder after the fuel is injected. Once the flame has initiated, any more fuel entering the cylinder ignites at once.

      Straight-chain hydrocarbons make the best diesel fuels. In order to have a standard reference scale, the oil is matched against blends of cetane (normal hexadecane) and alpha methylnaphthalene, the latter of which gives very poor engine performance. High-quality diesel fuels have cetane ratings of about 50, giving the same combustion characteristics as a 50-50 mixture of the standard fuels. The large, slower engines in ships and stationary power plants can tolerate even heavier diesel oils. The more viscous marine diesel oils are heated to permit easy pumping and to give the correct viscosity at the fuel injectors for good combustion.

      Until the early 1990s, standards for diesel fuel quality were not particularly stringent. A minimum cetane number was critical for transportation uses, but sulfur levels of 0.3 to 0.5 weight percent by weight were common in most markets. With the advent of more stringent exhaust emission controls, however, diesel fuel qualities came under increased scrutiny. In the United States, diesel fuel is generally restricted to a maximum sulfur level of 0.05 weight percent, and regulations have restricted aromatic content as well. The limitation of aromatic compounds requires a much more demanding scheme of processing individual gas oil components than was necessary for earlier highway diesel fuels.

Fuel oils (fuel oil)
      Furnace oil consists largely of residues from crude oil refining. These are blended with other suitable gas oil fractions in order to achieve the viscosity required for convenient handling. As a residue product, fuel oil is the only refined product of significant quantity that commands a market price lower than the cost of crude oil.

      Because the sulfur contained in the crude oil is concentrated in the residue material, fuel oil sulfur levels naturally vary from less than 1 to as much as 6 percent. The sulfur level is not critical to the combustion process as long as the flue gases do not impinge on cool surfaces (which could lead to corrosion by the condensation of acidic sulfur trioxide). However, residual fuels may contain large quantities of heavy metals such as nickel and vanadium; these produce ash upon burning and can foul burner systems. Such contaminants are not easily removed and usually lead to lower market prices for fuel oils with high metal contents.

      In order to reduce air pollution, most industrialized countries now restrict the sulfur content of fuel oils. Such regulation has led to the construction of residual desulfurization units or cokers in refineries that produce these fuels.

Lubricating oils
      At one time the suitability of petroleum fractions for use as lubricants depended entirely on the crude oils from which they were derived. Those from Pennsylvania crude, which were largely paraffinic in nature, were recognized as having superior properties. But, with the advent of solvent extraction and hydrocracking, the choice of raw materials has been considerably extended.

       viscosity is the basic property by which lubricating oils are classified. The requirements vary from a very thin oil needed for the high-speed spindles of textile machinery to the viscous, tacky materials applied to open gears or wire ropes. Between these extremes is a wide range of products with special characteristics. Automotive oils represent the largest product segment in the market. In the United States, specifications for these products are defined by the Society of Automotive Engineers (SAE), which issues viscosity ratings with numbers that range from 5 to 50. In the United Kingdom, standards are set by the Institute of Petroleum, which conducts tests that are virtually identical to those of the SAE.

      When ordinary mineral oils having satisfactory lubricity at low temperatures are used over an extended temperature range, excessive thinning occurs, and the lubricating properties are found to be inadequate at higher temperatures. To correct this, multigrade oils have been developed using long-chain polymers. Thus, an oil designated SAE 10W40 has the viscosity of an SAE 10W oil at −18° C (0° F) and of an SAE 40 oil at 99° C (210° F). Such an oil performs well under cold starting conditions in winter (hence the W designation) yet will lubricate under high-temperature running conditions in the summer as well. Other additives that improve the performance of lubricating oils are antioxidants and detergents, which maintain engine cleanliness and keep fine carbon particles suspended in the circulating oil.

Gear oils and greases
      In gear lubrication the oil separates metal surfaces, reducing friction and wear. Extreme pressures develop in some gears, notably those in the rear axles of cars, and special additives must be employed to prevent the seizing of the metal surfaces. These oils contain sulfur compounds that form a resistant film on the surfaces, preventing actual metal-to-metal contact.

      Greases (grease) are lubricating oils to which thickening agents are added. Soaps of aluminum, calcium, lithium, and sodium are commonly used, while nonsoap thickeners such as carbon, silica, and polyethylene also are employed for special purposes.

Other petroleum products
      Highly purified naphthas (naphtha) are used for solvents in paints, cosmetics, commercial dry cleaning, and industrial product manufacture. Petroleum waxes (petroleum wax) are employed in paper manufacture and foodstuffs.

      Asphaltic bitumen is widely used for the construction of roads and airfields. Specialized applications of bitumen also include the manufacture of roofing felts, waterproof papers, pipeline coatings, and electrical insulation. carbon black is manufactured by decomposing liquid hydrocarbon fractions. It is compounded with rubber in tire manufacture and is a constituent of printing inks and lacquers.

Petrochemicals (petrochemical)
      By definition, petrochemicals are simply chemicals that happen to be derived from a starting material obtained from petroleum. They are, in almost every case, virtually identical to the same chemical produced from other sources, such as coal, coke, or fermentation processes.

Olefins (olefin)
      The thermal cracking processes developed for refinery processing in the 1920s were focused primarily on increasing the quantity and quality of gasoline components. As a by-product of this process, gases were produced that included a significant proportion of lower-molecular-weight olefins, particularly ethylene, propylene, and butylene. Catalytic cracking is also a valuable source of propylene and butylene, but it does not account for a very significant yield of ethylene, the most important of the petrochemical building blocks. Ethylene is polymerized to produce polyethylene or, in combination with propylene, to produce copolymers that are used extensively in food-packaging wraps, plastic household goods, or building materials.

      Ethylene manufacture via the steam cracking process is in widespread practice throughout the world. The operating facilities are similar to gas oil cracking units, operating at temperatures of 840° C (1,550° F) and at low pressures of 1.7 kilograms per square centimetre (24 pounds per square inch). Steam is added to the vaporized feed to achieve a 50-50 mixture, and furnace residence times are only 0.2 to 0.5 second. In the United States and the Middle East, ethane extracted from natural gas is the predominant feedstock for ethylene cracking units. Propylene and butylene are largely derived from catalytic cracking units in the United States. In Europe and Japan, catalytic cracking is less common, and natural gas supplies are not as plentiful. As a result, both the Europeans and Japanese generally crack a naphtha or light gas oil fraction to produce a full range of olefin products.

Aromatics
      The aromatic compounds (aromatic compound), produced in the catalytic reforming of naphtha, are major sources of petrochemical products. In the traditional chemical industry, aromatics such as benzene, toluene, and the xylenes were made from coal during the course of carbonization in the production of coke and town gas. Today a much larger volume of these chemicals are made as refinery by-products. A further source of supply is the aromatic-rich liquid fraction produced in the cracking of naphtha or light gas oils during the manufacture of ethylene and other olefins.

Polymers (polymer)
      A highly significant proportion of these basic petrochemicals is converted into plastics, synthetic rubbers, and synthetic fibres. Together these materials are known as polymers, because their molecules are high-molecular-weight compounds made up of repeated structural units that have combined chemically. The major products are polyethylene, polyvinyl chloride, and polystyrene, all derived from ethylene, and polypropylene, derived from monomer propylene. Major raw materials for synthetic rubbers include butadiene, ethylene, benzene, and propylene. Among synthetic fibres the polyesters (polyester), which are a combination of ethylene glycol and terephthalic acid (made from xylenes), are the most widely used. They account for about one-half of all synthetic fibres. The second major synthetic fibre is nylon, its most important raw material being benzene. Acrylic fibres, in which the major raw material is the propylene derivative acrylonitrile, make up most of the remainder of the synthetic fibres.

Inorganic chemicals
      Two prominent inorganic chemicals, ammonia and sulfur, are also derived in large part from petroleum. Ammonia production requires hydrogen from a hydrocarbon source. Traditionally, the hydrogen was produced from a coke and steam reaction, but today most ammonia is synthesized from liquid petroleum fractions, natural gas, or refinery gases. The sulfur removed from oil products in purification processes is ultimately recoverable as elemental sulfur or sulfuric acid. It has become an important source of sulfur for the manufacture of fertilizer.

Refinery plant and facilities

Processing configurations
      Each petroleum refinery is uniquely configured to process a specific raw material into a desired slate of products. In order to determine which configuration is most economical, engineers and planners survey the local market for petroleum products and assess the available raw materials. Since about half the product of fractional distillation is residual fuel oil, the local market for it is of utmost interest. In parts of Africa, South America, and Southeast Asia, heavy fuel oil is easily marketed, so that refineries of simple configuration may be sufficient to meet demand. However, in the United States, Canada, and Europe, large quantities of gasoline are in demand, and the market for fuel oil is constrained by environmental regulations and the availability of natural gas. In these places, more complex refineries are necessary.

Topping and hydroskimming refineries
      The simplest refinery configuration, called a topping refinery, is designed to prepare feedstocks for petrochemical manufacture or for production of industrial fuels in remote oil-production areas. It consists of tankage, a distillation unit, recovery facilities for gases and light hydrocarbons, and the necessary utility systems (steam, power, and water-treatment plants).

 Topping refineries produce large quantities of unfinished oils and are highly dependent on local markets, but the addition of hydrotreating and reforming units to this basic configuration results in a more flexible hydroskimming refinery, which can also produce desulfurized distillate fuels and high-octane gasoline (see figure—>). Still, these refineries may produce up to half of their output as residual fuel oil, and they face increasing economic hardship as the demand for high-sulfur fuel oils declines. Indeed, few older hydroskimming refineries survived the precipitous reduction in worldwide demand for petroleum products that followed the sharp rise in crude oil prices in 1973 and 1979. Those that were not retired from service found it economical to invest in more sophisticated processing facilities in order to increase their yield of gasoline, jet fuel, and diesel oils and to curtail the production of residual fuels.

Conversion refineries
 The most versatile refinery configuration today is known as the conversion refinery (see figure—>). A conversion refinery incorporates all the basic building blocks found in both the topping and hydroskimming refineries, but it also features gas oil conversion plants such as catalytic cracking and hydrocracking units, olefin conversion plants such as alkylation or polymerization units, and, frequently, coking units for sharply reducing or eliminating the production of residual fuels. Modern conversion refineries may produce two-thirds of their output as unleaded gasoline, with the balance distributed between high-quality jet fuel, LPG, low-sulfur diesel fuel, and a small quantity of petroleum coke. Many such refineries also incorporate solvent extraction processes for manufacturing lubricants and petrochemical units with which to recover high-purity propylene, benzene, toluene, and xylenes for further processing into polymers.

Off-sites
      The individual processing units described above are part of the process-unit side of a refinery complex. They are usually considered the most important features, but the functioning of the off-site facilities are often as critical as the process units themselves. Off-sites consist of tankage, flare systems, utilities, and environmental treatment units.

      Refineries typically provide storage for raw materials and products that equal about 50 days of refinery throughput. Sufficient crude oil tankage must be available to allow for continuous refinery operation while still allowing for irregular arrival of crude shipments by pipeline or ocean-going tankers. The scheduling of tanker movements is particularly important for large refineries processing Middle Eastern crudes, which are commonly shipped in very large crude carriers (VLCC) with capacities of 250,000 tons (1,600,000 barrels) or more. Generally, intermediate process streams and finished products require even more tankage than crude oil. In addition, provision must be made for short-term variations in demand for products and also for maintaining a dependable supply of products to the market during periods when process units must be removed from service for maintenance.

      Nonvolatile products such as diesel fuel and fuel oils are stored in large-diameter cylindrical tanks with low-pitched conical roofs. Tanks with floating roofs reduce the evaporative losses in storage of gasolines and other volatile products, including crude oils. The roof, which resembles a pontoon, floats on the surface of the liquid within the tank, thus moving up and down with the liquid level and eliminating the air space that could contain petroleum vapour. For LPG and butanes, pressure vessels (usually spherical) are used.

      One of the prominent features of every oil refinery and petrochemical plant is a tall stack with a small flame burning at the top. This stack, called a flare, is an essential part of the plant safety system. In the event of equipment failure or plant shutdown, it is necessary to purge the volatile hydrocarbons from operating equipment so that it can be serviced. Since these volatile hydrocarbons form very explosive mixtures if they are mixed with air, as a safety precaution they are delivered by closed piping systems to the flare site, where they may be burned in a controlled manner. Under normal conditions only a pilot light is visible on the flare stack, and steam is often added to the flare to mask even that flame. However, during emergency conditions the flare system disposes of large quantities of volatile gases and illuminates the sky.

Utilities
      A typical refinery requires enough utilities to support a small city. All refineries produce steam for use in process units. This requires water-treatment systems, boilers, and extensive piping networks. Many refineries also produce electricity for lighting, electric motor-driven pumps, and compressors and instrumentation systems. In addition, clean, dry air must be provided for many process units, and large quantities of cooling water are required for condensation of hydrocarbon vapours.

Environmental treatment
      The large quantity of water required to support refinery operations must be treated to remove traces of hydrocarbons and noxious chemicals before it can be disposed of into waterways or underground disposal wells. In addition, each of the process units that vent hydrocarbons, flue gases, or particulate solids must be carefully monitored to ensure compliance with environmental standards. Finally, appropriate procedures must be employed to dispose of spent catalysts from refinery processing units.

Bulk transportation
      Large oceangoing tankers have sharply reduced the cost of transporting crude oil, making it practical to locate refineries near major market areas rather than adjacent to oil fields. To receive these large carriers, deepwater ports have been constructed in such cities as Rotterdam (Neth.), Singapore, and Houston (Tex.). Major refining centres are connected to these ports by pipelines.

      Countries having navigable rivers or canals afford many opportunities for using barges, a very inexpensive method of transportation. The Mississippi River in the United States and the Rhine and Seine rivers in Europe are especially suited to barges of more than 5,000 tons (37,000 barrels). Each barge may be divided into several compartments so that a variety of products may be carried.

      Transport by railcar is still widely practiced, especially for specialty products such as LPG, lubricants, or asphalt. Cars have capacities exceeding 100 tons (800 barrels), depending on the product carried. The final stage of product delivery to the majority of customers throughout the world continues to be the familiar tanker truck, whose carrying capacity is about 150 to 200 barrels.

      The most efficient mode of bulk transport for petroleum is the network of pipelines (pipeline) that are now found all over the world. Most crude-oil-producing areas are connected by pipeline either to refining centres or to a maritime loading port. In addition, many major crude-oil-receiving ports have extensive pipeline distribution networks to inland refineries. Centrifugal pumps usually provide the pumping power, with booster stations installed along the line as necessary. Most of the major product lines have been converted to fully automated operation, with the opening and closing of valves carried out by automatic sequence controls initiated from remote control centres.

      Natural gas is often found in close association with crude oil. In fact, in many instances it is the pressure of natural gas exerted upon the subterranean oil reservoir that provides the drive to force oil up to the surface. Such “associated” gas is often considered to be the gaseous phase of the crude oil and usually contains some light liquids—hence the term wet gas. However, there are also instances of “dry gas” reservoirs that are not connected with any known source of liquid petroleum.

Composition
      Natural gas components are mostly saturated light paraffins such as methane, ethane, and propane that exist in the gaseous phase, depending on the pressure in the reservoir. When pentane and heavier compounds coexist, they are usually found as liquids. Often natural gases contain substantial quantities of hydrogen sulfide or other organic sulfur compounds.

      When a natural gas reserve contains substantial amounts of ethane and the higher paraffinic compounds, these are usually extracted at the production site and produced as natural gas liquids (liquefied petroleum gas) (NGL). The NGLs can be separated into fractions, ranging from the heaviest condensates (butanes, pentanes, and hexanes) through LPG (essentially propane and butane) to ethane. This source of light hydrocarbons is especially prominent in the United States, where natural gas processing provides a major portion of the ethane feedstock for olefin manufacture and the LPG for heating and commercial purposes.

      Several nonhydrocarbon gases also are found in natural gas mixtures. nitrogen and carbon dioxide are noncombustible and may be found in substantial proportions. Nitrogen is inert, but, if present in significant amounts, it reduces the heating value of the mixture; it must therefore be removed before the gas is suitable for the commercial market. Carbon dioxide is removed in order to raise heating value, reduce volume, and sustain even combustion properties. hydrogen sulfide is generally removed by treatment with ethanolamine in a process similar to that used in petroleum refining.

      Commercial natural gas stripped of NGL and sold for heating purposes usually contains 85 to 90 percent methane and the remainder mainly nitrogen and ethane. It usually has a heating value of approximately 40 megajoules per cubic metre (about 9,300 kilocalories per cubic metre, or about 1,050 British thermal units per standard cubic foot of gas). While sulfur compounds are removed in processing, a minute quantity of a noxious mercaptan odorant is always added to commercial natural gas to ensure the rapid detection of any leakage that may occur in transport or use.

Field processing
      Field-production gas is often available at very low pressures, 1 kilogram per square centimetre (14 pounds per square inch) or less being common. Most end uses of gas require it to be available at a pressure of 35 to 70 kilograms per square centimetre (500 to 1,000 pounds per square inch), so it usually will be processed through multiple stages of compression. In a simple compression gas-processing plant, field gas is charged to an inlet scrubber, where entrained liquids are removed. The gas is then successively compressed and cooled to remove condensed liquids and to reduce the temperature of the fluid in order to conserve compressor power requirements.

      In plants of this type, water vapour in the gas condenses as the pressure is increased and the temperature reduced. If liquid forms in the coolers, the gas may be at its dew point with respect to water or hydrocarbons. This may result in the formation of gas hydrates, which can cause difficulty in plant operation and must be removed from the gas in order to avoid problems in subsequent transportation. Hydrate removal is accomplished by injecting a glycol solution into the process stream to remove any dissolved water. Liquid products from a compression plant have a very high vapour pressure and are therefore difficult to store without further processing.

      If market economics warrant the recovery of heavier liquids from the gas stream, a more complex refrigerated absorption and fractionation plant may be required. The compressed raw gas is processed in admixture with a liquid hydrocarbon, called lean oil, in an absorber column, where heavier components in the gas are absorbed in the lean oil. The bulk of the gas is discharged from the top of the absorber as residue gas (usually containing 95 percent methane) for subsequent treatment to remove sulfur and other impurities. The heavier components leave with the bottoms liquid stream, now called rich oil, for further processing to remove ethane for plant fuel or petrochemical feedstock and to recover the lean oil. Some gas-processing plants may contain additional distilling columns for further separation of the gas liquids into propane, butanes, and heavier NGLs.

      Many older gas-absorption plants were designed to operate at ambient temperature, but more recent facilities usually employ refrigeration to lower processing temperatures and increase the absorption efficiency.

Transport
      The growth of the natural gas industry has largely depended on the development of efficient pipeline systems. The first metal pipeline was constructed between Titusville and Newton, Pa., in 1872. This 6.3-centimetre- (2.5-inch-) diameter cast-iron system supplied some 250 residential customers with natural gas at a pressure of about 5.7 kilograms per square centimetre (80 pounds per square inch). By 1970 more than 400,000 kilometres (250,000 miles) of pipelines were operating in the United States, servicing some 42 million customers. Modern gas pipelines operate at about 70 kilograms per square centimetre (1,000 pounds per square inch), with diameters up to 1.4 metres (56 inches). Large automated compressor stations are located along the pipelines to boost system pressure and overcome friction losses in transit.

      The discovery of natural gas fields in remote areas of the world gave rise to an interest in developing an efficient means of long-distance transport. Since liquefied natural gas would occupy only 0.16 percent of the gaseous volume, an international trade has naturally developed in LNG. Modern liquefaction (thermal fusion) plants employ autorefrigerated cascade cycles, in which the gas is stripped of carbon dioxide, dried, and then subjected to a series of compression-expansion steps during which it is cooled to liquefaction temperature (−161.5° C [−258.7° F]). The compression power requirement is usually supplied by consuming a portion of the available gas. After liquefaction the gas is transported in specially designed and insulated tankers to the consuming port, where it is stored in refrigerated tanks until required. Regasification requires a source of heat to convert the liquid back into vapour. Often a low-cost method is followed, such as exchanging heat with a large volume of nearby river water. All methods of liquefaction, transport, and regasification involve a significant energy loss, which can approach 25 percent of the original energy content of the gas.

Applications
      The largest single application for natural gas is as a domestic or industrial fuel. However, several specialized applications have developed over the years. The clean-burning characteristics of natural gas have made it a frequent choice as a nonpolluting transportation fuel. Buses and commercial automotive fleets now operate on compressed natural gas in many areas of the United States. carbon black, a pigment of colloidal dimensions, is made by burning natural gas with a limited supply of air and depositing the soot on a cool surface. It is an important ingredient in dyes and inks and is used in rubber compounding operations.

      More than half of the world's ammonia supply now is manufactured via a catalytic process from methane. It is used directly as a plant food or converted into a variety of chemicals such as hydrogen cyanide, nitric acid, urea, and a range of fertilizers.

      A wide array of other chemical products can be made from natural gas by a controlled oxidation process—for example, methanol, propanol, and formaldehyde, which serve as basic materials for a wide range of other chemical products. Methanol can be used as a gasoline additive or gasoline substitute. A mixture of 85 percent methanol and 15 percent gasoline entered the commercial market in California in 1992 as an alternative to conventional gasoline. In addition, methyl tertiary butyl ether (MTBE), an oxygenated fuel additive added to gasoline in response to environmental regulations in the United States, is produced via chemical reaction of methanol and isobutylene over an acidic ion-exchange resin. Much of the world's supply of MTBE is dependent on the availability of isobutylene from refinery catalytic cracking units or olefin-manufacturing units in petrochemical plants. However, it is possible to base the process entirely on natural gas by processing NGLs through isomerization units and butane dehydrogenation facilities in order to produce isobutylene and then separately convert methane from the dry gas to methanol. Then the process would proceed as described above, reacting the methanol and isobutylene over an acidic ion-exchange resin to produce the MTBE product.

John E. Carruthers A.L. Waddams Lee H. Solomon

Additional Reading
General references include Bill D. Berger and Kenneth E. Anderson, Modern Petroleum: A Basic Primer of the Industry, 3rd ed. (1992), a nontechnical introduction to the entire petroleum industry that provides a good understanding of the interaction between exploration, drilling, transportation, refining, petrochemicals, and marketing; Daniel Yergin, The Prize: The Epic Quest for Oil, Money, and Power (1991), a popular history of the petroleum industry from 1850 to 1990, highlighting the development of major refinery processes and describing their impact on the evolution of the entire industry worldwide; and the International Petroleum Encyclopedia (annual), summarizing the activities of all phases of the international petroleum and natural gas industries, with statistics, maps, and tables.A simple introduction to petroleum refinery processes is William L. Leffler, Petroleum Refining for the Non-Technical Person, 2nd ed. (1985), with chapters on each major process. More detailed descriptions of processes and typical operating considerations are provided in G.D. Hobson (ed.), Modern Petroleum Technology, 5th ed., 2 vol. (1984); Robert A. Meyers (ed.), Handbook of Petroleum Refining Processes (1986); James G. Speight, The Chemistry and Technology of Petroleum, 2nd ed., rev. and expanded (1991); John J. McKetta (ed.), Petroleum Processing Handbook (1992); and James H. Gary and Glenn E. Handwerk, Petroleum Refining: Technology and Economics, 3rd ed. (1994). Purification processes are described in James G. Speight, The Desulfurization of Heavy Oils and Residua (1981).Peter H. Spitz, Petrochemicals: The Rise of an Industry (1988), offers a comprehensive history of the development of the petrochemical industry from its origin in the 1920s to the late 1980s. Robert A. Meyers (ed.), Handbook of Chemicals Production Processes (1986), provides detailed descriptions of a broad range of chemical processes, including those for the major petrochemicals, with process descriptions and diagrams, chemical reactions, and brief process economics.A detailed text on the origin of natural gas and the major natural gas basins of the world is E.N. Tiratsoo, Natural Gas, 3rd ed. (1979), which includes a brief treatment of transportation, storage, and liquefaction of natural gas. Arlon R. Tussing and Connie C. Barlow, The Natural Gas Industry (1984), provides a history of the industry in the United States beginning with coal gasification in the 19th century through the development of government regulatory programs in the 1980s; in addition, the structural evolution of the domestic gas industry is described, with special emphasis on the impact of industry regulations. H. Dale Beggs, Gas Production Operations (1984), is a highly technical publication on the entire gas-processing industry from reservoir engineering through production operations to pipeline transport systems.Lee H. Solomon

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Universalium. 2010.

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