OIL 

PETROLEUM REFINING PROCESSES

I.            INTRODUCTION.                              A.            The petroleum industry began with the successful drilling of the first commercial oil well in 1859, and the opening of the first refinery two years later to process the crude into kerosene. The evolution of petroleum refining from simple distillation to today's sophisticated processes has created a need for health and safety management procedures and safe work practices. To those unfamiliar with the industry, petroleum refineries may appear to be complex and confusing places. Refining is the processing of one complex mixture of hydrocarbons into a number of other complex mixtures of hydrocarbons. The safe and orderly processing of crude oil into flammable gases and liquids at high temperatures and pressures using vessels, equipment, and piping subjected to stress and corrosion requires considerable knowledge, control, and expertise.                               B.            Safety and health professionals, working with process, chemical, instrumentation, and metallurgical engineers, assure that potential physical, mechanical, chemical, and health hazards are recognized and provisions are made for safe operating practices and appropriate protective measures. These measures may include hard hats, safety glasses and goggles, safety shoes, hearing protection, respiratory protection, and protective clothing such as fire resistant clothing where required. In addition, procedures should be established to assure compliance with applicable regulations and standards such as hazard communications, confined space entry, and process safety management.                              C.            This chapter of the technical manual covers the history of refinery processing, characteristics of crude oil, hydrocarbon types and chemistry, and major refinery products and by-products. It presents information on technology as normally practiced in present operations. It describes the more common refinery processes and includes relevant safety and health information. Additional information covers refinery utilities and miscellaneous supporting activities related to hydrocarbon processing. Field personnel will learn what to expect in various facilities regarding typical materials and process methods, equipment, potential hazards, and exposures.                              D.            The information presented refers to fire prevention, industrial hygiene, and safe work practices, and is not intended to provide comprehensive guidelines for protective measures and/or compliance with regulatory requirements. As some of the terminology is industry-specific, a glossary is provided as an appendix. This chapter does not cover petrochemical processing.      II.            OVERVIEW OF THE PETROLEUM INDUSTRY.                              A.            BASIC REFINERY PROCESS: DESCRIPTION AND HISTORY. Petroleum refining has evolved continuously in response to changing consumer demand for better and different products. The original requirement was to produce kerosene as a cheaper and better source of light than whale oil. The development of the internal combustion engine led to the production of gasoline and diesel fuels. The evolution of the airplane created a need first for high-octane aviation gasoline and then for jet fuel, a sophisticated form of the original product, kerosene. Present-day refineries produce a variety of products including many required as feedstock for the petrochemical industry.                                                       1.            Distillation Processes. The first refinery, opened in 1861, produced kerosene by simple atmospheric distillation. Its by-products included tar and naphtha. It was soon discovered that high-quality lubricating oils could be produced by distilling petroleum under vacuum. However, for the next 30 years kerosene was the product consumers wanted. Two significant events changed this situation: (1) invention of the electric light decreased the demand for kerosene, and (2) invention of the internal combustion engine created a demand for diesel fuel and gasoline (naphtha).                                                       2.            Thermal Cracking Processes. With the advent of mass production and World War I, the number of gasoline-powered vehicles increased dramatically and the demand for gasoline grew accordingly. However, distillation processes produced only a certain amount of gasoline from crude oil. In 1913, the thermal cracking process was developed, which subjected heavy fuels to both pressure and intense heat, physically breaking the large molecules into smaller ones to produce additional gasoline and distillate fuels. Visbreaking, another form of thermal cracking, was developed in the late 1930's to produce more desirable and valuable products.                                                       3.            Catalytic Processes. Higher-compression gasoline engines required higher-octane gasoline with better antiknock characteristics. The introduction of catalytic cracking and polymerization processes in the mid- to late 1930's met the demand by providing improved gasoline yields and higher octane numbers. Alkylation, another catalytic process developed in the early 1940's, produced more high-octane aviation gasoline and petrochemical feedstock for explosives and synthetic rubber. Subsequently, catalytic isomerization was developed to convert hydrocarbons to produce increased quantities of alkylation feedstock. Improved catalysts and process methods such as hydrocracking and reforming were developed throughout the 1960's to increase gasoline yields and improve antiknock characteristics. These catalytic processes also produced hydrocarbon molecules with a double bond (alkenes) and formed the basis of the modern petrochemical industry.                                                       4.            Treatment Processes. Throughout the history of refining, various treatment methods have been used to remove nonhydrocarbons, impurities, and other constituents that adversely affect the properties of finished products or reduce the efficiency of the conversion processes. Treating can involve chemical reaction and/or physical separation. Typical examples of treating are chemical sweetening, acid treating, clay contacting, caustic washing, hydrotreating, drying, solvent extraction, and solvent dewaxing. Sweetening compounds and acids desulfurize crude oil before processing and treat products during and after processing. Following the Second World War, various reforming processes improved gasoline quality and yield and produced higher-quality products. Some of these involved the use of catalysts and/or hydrogen to change molecules and remove sulfur. A number of the more commonly used treating and reforming processes are described in this chapter of the manual. TABLE IV: 2-1. HISTORY OF REFINING Year Process name Purpose By-products, etc. 1862 Atmospheric distillation Produce kerosene Naphtha, tar, etc. 1870 Vacuum distillation Lubricants (original) Cracking feedstocks (1930's) Asphalt, residual coker feedstocks 1913 Thermal cracking Increase gasoline Residual, bunker fuel 1916 Sweetening reduce sulfur & odor Sulfur 1930 Thermal reforming Improve octane number Residual 1932 Hydrogenation Remove sulfur Sulfur 1932 Coking Produce gasoline basestocks Coke 1933 Solvent extraction Improve lubricant viscosity index Aromatics 1935 Solvent dewaxing Improve pour point Waxes 1935 Cat. polymerization Improve gasoline yield & octane number Petrochemical feedstocks 1937 Catalytic cracking Higher octane gasoline Petrochemical feedstocks 1939 Visbreaking reduce viscosity Increased distillate,tar 1940 Alkylation Increase gasoline octane & yield High-octane aviation gasoline 1940 Isomerization Produce alkylation feedstock Naphtha 1942 Fluid catalytic cracking Increase gasoline yield & octane Petrochemical feedstocks 1950 Deasphalting Increase cracking feedstock Asphalt 1952 Catalytic reforming Convert low-quality naphtha Aromatics 1954 Hydrodesulfurization Remove sulfur Sulfur 1956 Inhibitor sweetening Remove mercaptan Disulfides 1957 Catalytic isomerization Convert to molecules with high octane number Alkylation feedstocks 1960 Hydrocracking Improve quality and reduce sulfur Alkylation feedstocks 1974 Catalytic dewaxing Improve pour point Wax 1975 Residual hydrocracking Increase gasoline yield from residual Heavy residuals   B.      BASICS OF CRUDE OIL. 1.      Crude oils are complex mixtures containing many different hydrocarbon compounds that vary in appearance and composition from one oil field to another. Crude oils range in consistency from water to tar-like solids, and in color from clear to black. An "average" crude oil contains about 84% carbon, 14% hydrogen, 1%-3% sulfur, and less than 1% each of nitrogen, oxygen, metals, and salts. Crude oils are generally classified as paraffinic, naphthenic, or aromatic, based on the predominant proportion of similar hydrocarbon molecules. Mixed-base crudes have varying amounts of each type of hydrocarbon. Refinery crude base stocks usually consist of mixtures of two or more different crude oils. 2.      Relatively simple crude oil assays are used to classify crude oils as paraffinic, naphthenic, aromatic, or mixed. One assay method (United States Bureau of Mines) is based on distillation, and another method (UOP "K" factor) is based on gravity and boiling points. More comprehensive crude assays determine the value of the crude (i.e., its yield and quality of useful products) and processing parameters. Crude oils are usually grouped according to yield structure. 3.      Crude oils are also defined in terms of API (American Petroleum Institute) gravity. The higher the API gravity, the lighter the crude. For example, light crude oils have high API gravities and low specific gravities. Crude oils with low carbon, high hydrogen, and high API gravity are usually rich in paraffins and tend to yield greater proportions of gasoline and light petroleum products; those with high carbon, low hydrogen, and low API gravities are usually rich in aromatics. 4.      Crude oils that contain appreciable quantities of hydrogen sulfide or other reactive sulfur compounds are called "sour." Those with less sulfur are called "sweet." Some exceptions to this rule are West Texas crudes, which are always considered "sour" regardless of their H2S content, and Arabian high-sulfur crudes, which are not considered "sour" because their sulfur compounds are not highly reactive.   TABLE IV: 2-2. TYPICAL APPROXIMATE CHARACTERISTICS AND PROPERTIES AND GASOLINE POTENTIAL OF VARIOUS CRUDES (Representative average numbers) Crude source Paraffins (% vol) Aromatics (% vol) Naphthenes (% vol) Sulfur (% wt) API gravity (approx.) Napht. yield (% vol) Octane no (typical) Nigerian   -Light 37 9 54 0.2 36 28 60 Saudi   -Light 63 19 18 2 34 22 40 Saudi   -Heavy 60 15 25 2.1 28 23 35 Venezuela   -Heavy 35 12 53 2.3 30 2 60 Venezuela   -Light 52 14 34 1.5 24 18 50 USA   -Midcont. Sweet - - - 0.4 40 - - USA   -W. Texas Sour 46 22 32 1.9 32 33 55 North Sea   -Brent 50 16 34 0.4 37 31 50   C.     BASICS OF HYDROCARBON CHEMISTRY. Crude oil is a mixture of hydrocarbon molecules, which are organic compounds of carbon and hydrogen atoms that may include from one to 60 carbon atoms. The properties of hydrocarbons depend on the number and arrangement of the carbon and hydrogen atoms in the molecules. The simplest hydrocarbon molecule is one carbon atom linked with four hydrogen atoms: methane. All other variations of petroleum hydrocarbons evolve from this molecule. Hydrocarbons containing up to four carbon atoms are usually gases, those with 5 to 19 carbon atoms are usually liquids, and those with 20 or more are solids. The refining process uses chemicals, catalysts, heat, and pressure to separate and combine the basic types of hydrocarbon molecules naturally found in crude oil into groups of similar molecules. The refining process also rearranges their structures and bonding patterns into different hydrocarbon molecules and compounds. Therefore it is the type of hydrocarbon (paraffinic, naphthenic, or aromatic) rather than its specific chemical compounds that is significant in the refining process. 1.      Three Principal Groups or Series of Hydrocarbon Compounds that Occur Naturally in Crude Oil. a.  Paraffins. The paraffinic series of hydrocarbon compounds found in crude oil have the general formula CnH2n+2 and can be either straight chains (normal) or branched chains (isomers) of carbon atoms. The lighter, straight-chain paraffin molecules are found in gases and paraffin waxes. Examples of straight-chain molecules are methane, ethane, propane, and butane (gases containing from one to four carbon atoms), and pentane and hexane (liquids with five to six carbon atoms). The branched-chain (isomer) paraffins are usually found in heavier fractions of crude oil and have higher octane numbers than normal paraffins. These compounds are saturated hydrocarbons, with all carbon bonds satisfied, that is, the hydrocarbon chain carries the full complement of hydrogen atoms. b.  Aromatics are unsaturated ring-type (cyclic) compounds which react readily because they have carbon atoms that are deficient in hydrogen. All aromatics have at least one benzene ring (a single-ring compound characterized by three double bonds alternating with three single bonds between six carbon atoms) as part of their molecular structure. Naphthalenes are fused double-ring aromatic compounds. The most complex aromatics, polynuclears (three or more fused aromatic rings), are found in heavier fractions of crude oil. c.  Naphthenes are saturated hydrocarbon groupings with the general formula CnH2n, arranged in the form of closed rings (cyclic) and found in all fractions of crude oil except the very lightest. Single-ring naphthenes (monocycloparaffins) with five and six carbon atoms predominate, with two-ring naphthenes (dicycloparaffins) found in the heavier ends of naphtha. 2.      Other Hydrocarbons. a.  Alkenes are mono-olefins with the general formula CnH2n and contain only one carbon-carbon double bond in the chain. The simplest alkene is ethylene, with two carbon atoms joined by a double bond and four hydrogen atoms. Olefins are usually formed by thermal and catalytic cracking and rarely occur naturally in unprocessed crude oil. b.  Dienes and Alkynes. Dienes, also known as diolefins, have two carbon-carbon double bonds. The alkynes, another class of unsaturated hydrocarbons, have a carbon-carbon triple bond within the molecule. Both these series of hydrocarbons have the general formula CnH2n-2. Diolefins such as 1,2-butadiene and 1,3-butadiene, and alkynes such as acetylene, occur in C5 and lighter fractions from cracking. The olefins, diolefins, and alkynes are said to be unsaturated because they contain less than the amount of hydrogen necessary to saturate all the valences of the carbon atoms. These compounds are more reactive than paraffins or naphthenes and readily combine with other elements such as hydrogen, chlorine, and bromine. 3.      Nonhydrocarbons. a.  Sulfur Compounds. Sulfur may be present in crude oil as hydrogen sulfide (H2S), as compounds (e.g. mercaptans, sulfides, disulfides, thiophenes, etc.) or as elemental sulfur. Each crude oil has different amounts and types of sulfur compounds, but as a rule the proportion, stability, and complexity of the compounds are greater in heavier crude-oil fractions. Hydrogen sulfide is a primary contributor to corrosion in refinery processing units. Other corrosive substances are elemental sulfur and mercaptans. Moreover, the corrosive sulfur compounds have an obnoxious odor. Pyrophoric iron sulfide results from the corrosive action of sulfur compounds on the iron and steel used in refinery process equipment, piping, and tanks. The combustion of petroleum products containing sulfur compounds produces undesirables such as sulfuric acid and sulfur dioxide. Catalytic hydrotreating processes such as hydrodesulfurization remove sulfur compounds from refinery product streams. Sweetening processes either remove the obnoxious sulfur compounds or convert them to odorless disulfides, as in the case of mercaptans. b.  Oxygen Compounds. Oxygen compounds such as phenols, ketones, and carboxylic acids occur in crude oils in varying amounts. c.  Nitrogen Compounds. Nitrogen is found in lighter fractions of crude oil as basic compounds, and more often in heavier fractions of crude oil as nonbasic compounds that may also include trace metals such as copper, vanadium, and/or nickel. Nitrogen oxides can form in process furnaces. The decomposition of nitrogen compounds in catalytic cracking and hydrocracking processes forms ammonia and cyanides that can cause corrosion. d.  Trace Metals. Metals, including nickel, iron, and vanadium are often found in crude oils in small quantities and are removed during the refining process. Burning heavy fuel oils in refinery furnaces and boilers can leave deposits of vanadium oxide and nickel oxide in furnace boxes, ducts, and tubes. It is also desirable to remove trace amounts of arsenic, vanadium, and nickel prior to processing as they can poison certain catalysts. e.  Salts. Crude oils often contain inorganic salts such as sodium chloride, magnesium chloride, and calcium chloride in suspension or dissolved in entrained water (brine). These salts must be removed or neutralized before processing to prevent catalyst poisoning, equipment corrosion, and fouling. Salt corrosion is caused by the hydrolysis of some metal chlorides to hydrogen chloride (HCl) and the subsequent formation of hydrochloric acid when crude is heated. Hydrogen chloride may also combine with ammonia to form ammonium chloride (NH4Cl), which causes fouling and corrosion. f.  Carbon Dioxide. Carbon dioxide may result from the decomposition of bicarbonates present in or added to crude, or from steam used in the distillation process. g.  Naphthenic Acids. Some crude oils contain naphthenic (organic) acids, which may become corrosive at temperatures above 450° F when the acid value of the crude is above a certain level.   D.     MAJOR REFINERY PRODUCTS. 1.      Gasoline. The most important refinery product is motor gasoline, a blend of hydrocarbons with boiling ranges from ambient temperatures to about 400 °F. The important qualities for gasoline are octane number (antiknock), volatility (starting and vapor lock), and vapor pressure (environmental control). Additives are often used to enhance performance and provide protection against oxidation and rust formation. 2.      Kerosene. Kerosene is a refined middle-distillate petroleum product that finds considerable use as a jet fuel and around the world in cooking and space heating. When used as a jet fuel, some of the critical qualities are freeze point, flash point, and smoke point. Commercial jet fuel has a boiling range of about 375°-525° F, and military jet fuel 130°-550° F. Kerosene, with less-critical specifications, is used for lighting, heating, solvents, and blending into diesel fuel. 3.      Liquified Petroleum Gas (LPG). LPG, which consists principally of propane and butane, is produced for use as fuel and is an intermediate material in the manufacture of petrochemicals. The important specifications for proper performance include vapor pressure and control of contaminants. 4.      Distillate Fuels. Diesel fuels and domestic heating oils have boiling ranges of about 400°-700° F. The desirable qualities required for distillate fuels include controlled flash and pour points, clean burning, no deposit formation in storage tanks, and a proper diesel fuel cetane rating for good starting and combustion. 5.      Residual Fuels. Many marine vessels, power plants, commercial buildings and industrial facilities use residual fuels or combinations of residual and distillate fuels for heating and processing. The two most critical specifications of residual fuels are viscosity and low sulfur content for environmental control. 6.      Coke and Asphalt. Coke is almost pure carbon with a variety of uses from electrodes to charcoal briquets. Asphalt, used for roads and roofing materials, must be inert to most chemicals and weather conditions. 7.      Solvents. A variety of products, whose boiling points and hydrocarbon composition are closely controlled, are produced for use as solvents. These include benzene, toluene, and xylene. 8.      Petrochemicals. Many products derived from crude oil refining, such as ethylene, propylene, butylene, and isobutylene, are primarily intended for use as petrochemical feedstock in the production of plastics, synthetic fibers, synthetic rubbers, and other products. 9.      Lubricants. Special refining processes produce lubricating oil base stocks. Additives such as demulsifiers, antioxidants, and viscosity improvers are blended into the base stocks to provide the characteristics required for motor oils, industrial greases, lubricants, and cutting oils. The most critical quality for lubricating-oil base stock is a high viscosity index, which provides for greater consistency under varying temperatures.   E.      COMMON REFINERY CHEMICALS. 1.      Leaded Gasoline Additives. Tetraethyl lead (TEL) and tetramethyl lead (TML) are additives formerly used to improve gasoline octane ratings but are no longer in common use except in aviation gasoline. 2.      Oxygenates. Ethyl tertiary butyl ether (ETBE), methyl tertiary butyl ether (MTBE), tertiary amyl methyl ether (TAME), and other oxygenates improve gasoline octane ratings and reduce carbon monoxide emissions. 3.      Caustics. Caustics are added to desalting water to neutralize acids and reduce corrosion. They are also added to desalted crude in order to reduce the amount of corrosive chlorides in the tower overheads. They are used in some refinery treating processes to remove contaminants from hydrocarbon streams. 4.      Sulfuric Acid and Hydrofluoric Acid. Sulfuric acid and hydrofluoric acid are used primarily as catalysts in alkylation processes. Sulfuric acid is also used in some treatment processes.

III.            PETROLEUM REFINING OPERATIONS.                              A.            INTRODUCTION. Petroleum refining begins with the distillation, or fractionation, of crude oils into separate hydrocarbon groups. The resultant products are directly related to the characteristics of the crude processed. Most distillation products are further converted into more usable products by changing the size and structure of the hydrocarbon molecules through cracking, reforming, and other conversion processes as discussed in this chapter. These converted products are then subjected to various treatment and separation processes such as extraction, hydrotreating, and sweetening to remove undesirable constituents and improve product quality. Integrated refineries incorporate fractionation, conversion, treatment, and blending operations and may also include petrochemical processing.                               B.            REFINING OPERATIONS. Petroleum refining processes and operations can be separated into five basic areas:                                                       1.            Fractionation (distillation) is the separation of crude oil in atmospheric and vacuum distillation towers into groups of hydrocarbon compounds of differing boiling-point ranges called "fractions" or "cuts."                                                       2.            Conversion processes change the size and/or structure of hydrocarbon molecules. These processes include:                                                                                  §            Decomposition (dividing) by thermal and catalytic cracking;                                                                                  §            Unification (combining) through alkylation and polymerization; and                                                                                  §            Alteration (rearranging) with isomerization and catalytic reforming.                                                         3.            Treatment processes are intended to prepare hydrocarbon streams for additional processing and to prepare finished products. Treatment may include the removal or separation of aromatics and naphthenes as well as impurities and undesirable contaminants. Treatment may involve chemical or physical separation such as dissolving, absorption, or precipitation using a variety and combination of processes including desalting, drying, hydrodesulfurizing, solvent refining, sweetening, solvent extraction, and solvent dewaxing.                                                       4.            Formulating and Blending is the process of mixing and combining hydrocarbon fractions, additives, and other components to produce finished products with specific performance properties.                                                       5.            Other Refining Operations include: light-ends recovery; sour-water stripping; solid waste and wastewater treatment; process-water treatment and cooling; storage and handling; product movement; hydrogen production; acid and tail-gas treatment; and sulfur recovery. Auxiliary operations and facilities include: steam and power generation; process and fire water systems; flares and relief systems; furnaces and heaters; pumps and valves; supply of steam, air, nitrogen, and other plant gases; alarms and sensors; noise and pollution controls; sampling, testing, and inspecting; and laboratory, control room, maintenance, and administrative facilities.

TABLE IV:2-3. OVERVIEW OF PETROLEUM REFINING PROCESSES.

Process name Action Method Purpose Feedstock(s) Product(s) FRACTIONATION PROCESSES Atmospheric distillation Separation Thermal Separate fractions Desalted crude oil Gas, gas oil, distillate, residual Vacuum distillation Separation Thermal Separate w/o cracking Atmospheric tower residual Gas oil, lube stock, residual CONVERSION PROCESSED—DECOMPOSITION Catalytic cracking Alteration Catalytic Upgrade gasoline Gas oil, coke distillate Gasoline, petrochemical feedstock Coking Polymerize Thermal Convert vacuum residuals Gas oil, coke distillate Gasoline, petrochemical feedstock Hydro-cracking Hydrogenate Catalytic Convert to lighter HC's Gas oil, cracked oil, residual Lighter, higher-quality products *Hydrogen steam reforming Decompose Thermal/ catalytic Produce hydrogen Desulfurized gas, O2, steam Hydrogen, CO, CO2 *Steam cracking Decompose Thermal Crack large molecules Atm tower hvy fuel/ distillate Cracked naphtha, coke, residual Visbreaking Decompose Thermal reduce viscosity Atmospheric tower residual Distillate, tar CONVERSION PROCESSES—UNIFICATION Alkylation Combining Catalytic Unite olefins & isoparaffins Tower isobutane/ cracker olefin Iso-octane (alkylate) Grease compounding Combining Thermal Combine soaps & oils Lube oil, fatty acid, alky metal Lubricating grease Polymerizing Polymerize Catalytic Unite 2 or more olefins Cracker olefins High-octane naphtha, petrochemical stocks CONVERSION PROCESSES--ALTERATION OR REARRANGEMENT Catalytic reforming Alteration/ dehydration Catalytic Upgrade low-octane naphtha Coker/ hydro-cracker naphtha High oct. Reformate/ aromatic Isomerization Rearrange Catalytic Convert straight chain to branch Butane, pentane, hexane Isobutane/ pentane/ hexane TREATMENT PROCESSES *Amine treating Treatment Absorption Remove acidic contaminants Sour gas, HCs w/CO2 & H2S Acid free gases & liquid HCs Desalting Dehydration Absorption Remove contaminants Crude oil Desalted crude oil Drying & sweetening Treatment Abspt/ therm Remove H2O & sulfur cmpds Liq Hcs, LPG, alky feedstk Sweet & dry hydrocarbons *Furfural extraction Solvent extr. Absorption Upgrade mid distillate & lubes Cycle oils & lube feed-stocks High quality diesel & lube oil Hydrodesulfurization Treatment Catalytic Remove sulfur, contaminants High-sulfur residual/ gas oil Desulfurized olefins Hydrotreating Hydrogenation Catalytic Remove impurities, saturate HC's Residuals, cracked HC's Cracker feed, distillate, lube *Phenol extraction Solvent extr. Abspt/ therm Improve visc. index, color Lube oil base stocks High quality lube oils Solvent deasphalting Treatment Absorption Remove asphalt Vac. tower residual, propane Heavy lube oil, asphalt Solvent dewaxing Treatment Cool/ filter Remove wax from lube stocks Vac. tower lube oils Dewaxed lube basestock Solvent extraction Solvent extr. Abspt/ precip. Separate unsat. oils Gas oil, reformate, distillate High-octane gasoline Sweetening Treatment Catalytic Remv H2S, convert mercaptan Untreated distillate/gasoline High-quality distillate/gasoline

* Note: These processes are not depicted in the refinery process flow chart.

other materials...

Crude Oil

Crude oil is the term for "unprocessed" oil, the stuff that comes out of the ground. It is also known as petroleum. Crude oil is a fossil fuel, meaning that it was made naturally from decaying plants and animals living in ancient seas millions of years ago -- anywhere you find crude oil was once a sea bed. Crude oils vary in color, from clear to tar-black, and in viscosity, from water to almost solid.

Crude oils are such a useful starting point for so many different substances because they contain hydrocarbons. Hydrocarbons are molecules that contain hydrogen and carbon and come in various lengths and structures, from straight chains to branching chains to rings.

There are two things that make hydrocarbons exciting to chemists:

• Hydrocarbons contain a lot of energy. Many of the things derived from crude oil like gasoline, diesel fuel, paraffin wax and so on take advantage of this energy.
• Hydrocarbons can take on many different forms. The smallest hydrocarbon is methane (CH₄), which is a gas that is a lighter than air. Longer chains with 5 or more carbons are liquids. Very long chains are solids like wax or tar. By chemically cross-linking hydrocarbon chains you can get everything from synthetic rubber to nylon to the plastic in tupperware. Hydrocarbon chains are very versatile!

The major classes of hydrocarbons in crude oils include:

• Paraffins
• general formula: C_nH_2n+2 (n is a whole number, usually from 1 to 20)
• straight- or branched-chain molecules
• can be gasses or liquids at room temperature depending upon the molecule
• examples: methane, ethane, propane, butane, isobutane, pentane, hexane
• Aromatics
• general formula: C₆H₅ - Y (Y is a longer, straight molecule that connects to the benzene ring)
• ringed structures with one or more rings
• rings contain six carbon atoms, with alternating double and single bonds between the carbons
• typically liquids
• examples: benzene, napthalene
• Napthenes or Cycloalkanes
• general formula: C_nH_2n (n is a whole number usually from 1 to 20)
• ringed structures with one or more rings
• rings contain only single bonds between the carbon atoms
• typically liquids at room temperature
• examples: cyclohexane, methyl cyclopentane
• Other hydrocarbons
• Alkenes
• general formula: C_nH_2n (n is a whole number, usually from 1 to 20)
• linear or branched chain molecules containing one carbon-carbon double-bond
• can be liquid or gas
• examples: ethylene, butene, isobutene
• Dienes and Alkynes
• general formula: C_nH_2n-2 (n is a whole number, usually from 1 to 20)
• linear or branched chain molecules containing two carbon-carbon double-bonds
• can be liquid or gas examples: acetylene, butadienes

On average, crude oils are made of the following elements or compounds:

• Carbon - 84%
• Hydrogen - 14%
• Sulfur - 1 to 3% (hydrogen sulfide, sulfides, disulfides, elemental sulfur)
• Nitrogen - less than 1% (basic compounds with amine groups)
• Oxygen - less than 1% (found in organic compounds such as carbon dioxide, phenols, ketones, carboxylic acids)
• Metals - less than 1% (nickel, iron, vanadium, copper, arsenic)

Salts - less than 1% (sodium chloride, magnesium chloride, calcium chloride)

From Crude Oil
The problem with crude oil is that it contains hundreds of different types of hydrocarbons all mixed together. You have to separate the different types of hydrocarbons to have anything useful. Fortunately there is an easy way to separate things, and this is what oil refining is all about.

The oil refining process starts with a fractional distillation column.

Different hydrocarbon chain lengths all have progressively higher boiling points, so they can all be separated by distillation. This is what happens in an oil refinery - in one part of the process, crude oil is heated and the different chains are pulled out by their vaporization temperatures. Each different chain length has a different property that makes it useful in a different way.

To understand the diversity contained in crude oil, and to understand why refining crude oil is so important in our society, look through the following list of products that come from crude oil:

• Petroleum gas - used for heating, cooking, making plastics
• small alkanes (1 to 4 carbon atoms)
• commonly known by the names methane, ethane, propane, butane
• boiling range = less than 104 degrees Fahrenheit / 40 degrees Celsius
• often liquified under pressure to create LPG (liquified petroleum gas)
• Naphtha or Ligroin - intermediate that will be further processed to make gasoline
• mix of 5 to 9 carbon atom alkanes
• boiling range = 140 to 212 degrees Fahrenheit / 60 to 100 degrees Celsius
• Gasoline - motor fuel
• liquid
• mix of alkanes and cycloalkanes (5 to 12 carbon atoms)
• boiling range = 104 to 401 degrees Fahrenheit / 40 to 205 degrees Celsius
• Kerosene - fuel for jet engines and tractors; starting material for making other products
• liquid
• mix of alkanes (10 to 18 carbons) and aromatics
• boiling range = 350 to 617 degrees Fahrenheit / 175 to 325 degrees Celsius
• Gas oil or Diesel distillate - used for diesel fuel and heating oil; starting material for making other products
• liquid
• alkanes containing 12 or more carbon atoms
• boiling range = 482 to 662 degrees Fahrenheit / 250 to 350 degrees Celsius
• Lubricating oil - used for motor oil, grease, other lubricants
• liquid
• long chain (20 to 50 carbon atoms) alkanes, cycloalkanes, aromatics
• boiling range = 572 to 700 degrees Fahrenheit / 300 to 370 degrees Celsius
• Heavy gas or Fuel oil - used for industrial fuel; starting material for making other products
• liquid
• long chain (20 to 70 carbon atoms) alkanes, cycloalkanes, aromatics
• boiling range = 700 to 1112 degrees Fahrenheit / 370 to 600 degrees Celsius
• Residuals - coke, asphalt, tar, waxes; starting material for making other products
• solid
• multiple-ringed compounds with 70 or more carbon atoms
• boiling range = greater than 1112 degrees Fahrenheit / 600 degrees Celsius

You may have noticed that all of these products have different sizes and boiling ranges. Chemists take advantage of these properties when refining oil. Look at the next section to find out the details of this fascinating process.

The Refining Process
As mentioned previously, a barrel of crude oil has a mixture of all sorts of hydrocarbons in it. Oil refining separates everything into useful substances. Chemists use the following steps:

1. The oldest and most common way to separate things into various components (called fractions), is to do it using the differences in boiling temperature. This process is called fractional distillation. You basically heat crude oil up, let it vaporize and then condense the vapor.
2. Newer techniques use Chemical processing on some of the fractions to make others, in a process called conversion. Chemical processing, for example, can break longer chains into shorter ones. This allows a refinery to turn diesel fuel into gasoline depending on the demand for gasoline.
3. Refineries must treat the fractions to remove impurities.
4. Refineries combine the various fractions (processed, unprocessed) into mixtures to make desired products. For example, different mixtures of chains can create gasolines with different octane ratings.

The products are stored on-site until they can be delivered to various markets such as gas stations, airports and chemical plants. In addition to making the oil-based products, refineries must also treat the wastes involved in the processes to minimize air and water pollution.

In the next section, we will look at how we separate crude oil into its components.

Fractional Distillation

The various components of crude oil have different sizes, weights and boiling temperatures; so, the first step is to separate these components. Because they have different boiling temperatures, they can be separated easily by a process called fractional distillation. The steps of fractional distillation are as follows:

1. You heat the mixture of two or more substances (liquids) with different boiling points to a high temperature. Heating is usually done with high pressure steam to temperatures of about 1112 degrees Fahrenheit / 600 degrees Celsius.
2. The mixture boils, forming vapor (gases); most substances go into the vapor phase.
3. The vapor enters the bottom of a long column (fractional distillation column) that is filled with trays or plates.
• The trays have many holes or bubble caps (like a loosened cap on a soda bottle) in them to allow the vapor to pass through.
• The trays increase the contact time between the vapor and the liquids in the column.
• The trays help to collect liquids that form at various heights in the column.
• There is a temperature difference across the column (hot at the bottom, cool at the top).
4. The vapor rises in the column.
5. As the vapor rises through the trays in the column, it cools.
6. When a substance in the vapor reaches a height where the temperature of the column is equal to that substance's boiling point, it will condense to form a liquid. (The substance with the lowest boiling point will condense at the highest point in the column; substances with higher boiling points will condense lower in the column.).
7. The trays collect the various liquid fractions.
8. The collected liquid fractions may:
• pass to condensers, which cool them further, and then go to storage tanks
• go to other areas for further chemical processing

Fractional distillation is useful for separating a mixture of substances with narrow differences in boiling points, and is the most important step in the refining process.

Very few of the components come out of the fractional distillation column ready for market. Many of them must be chemically processed to make other fractions. For example, only 40% of distilled crude oil is gasoline; however, gasoline is one of the major products made by oil companies. Rather than continually distilling large quantities of crude oil, oil companies chemically process some other fractions from the distillation column to make gasoline; this processing increases the yield of gasoline from each barrel of crude oil.

In the next section, we'll look at how we chemically process one fraction into another.

Chemical Processing
You can change one fraction into another by one of three methods:

• breaking large hydrocarbons into smaller pieces (cracking)
• combining smaller pieces to make larger ones (unification)
• rearranging various pieces to make desired hydrocarbons (alteration)

Cracking
Cracking takes large hydrocarbons and breaks them into smaller ones.

There are several types of cracking:

• Thermal - you heat large hydrocarbons at high temperatures (sometimes high pressures as well) until they break apart.
• steam - high temperature steam (1500 degrees Fahrenheit / 816 degrees Celsius) is used to break ethane, butane and naptha into ethylene and benzene, which are used to manufacture chemicals.
• visbreaking - residual from the distillation tower is heated (900 degrees Fahrenheit / 482 degrees Celsius), cooled with gas oil and rapidly burned (flashed) in a distillation tower. This process reduces the viscosity of heavy weight oils and produces tar.
• coking - residual from the distillation tower is heated to temperatures above 900 degrees Fahrenheit / 482 degrees Celsius until it cracks into heavy oil, gasoline and naphtha. When the process is done, a heavy, almost pure carbon residue is left (coke); the coke is cleaned from the cokers and sold.
• Catalytic - uses a catalyst to speed up the cracking reaction. Catalysts include zeolite, aluminum hydrosilicate, bauxite and silica-alumina.
• fluid catalytic cracking - a hot, fluid catalyst (1000 degrees Fahrenheit / 538 degrees Celsius) cracks heavy gas oil into diesel oils and gasoline.
• hydrocracking - similar to fluid catalytic cracking, but uses a different catalyst, lower temperatures, higher pressure, and hydrogen gas. It takes heavy oil and cracks it into gasoline and kerosene (jet fuel).

After various hydrocarbons are cracked into smaller hydrocarbons, the products go through another fractional distillation column to separate them.

Unification
Sometimes, you need to combine smaller hydrocarbons to make larger ones -- this process is called unification. The major unification process is called catalytic reforming and uses a catalyst (platinum, platinum-rhenium mix) to combine low weight naphtha into aromatics, which are used in making chemicals and in blending gasoline. A significant by-product of this reaction is hydrogen gas, which is then either used for hydrocracking or sold.

Alteration
Sometimes, the structures of molecules in one fraction are rearranged to produce another. Commonly, this is done using a process called alkylation. In alkylation, low molecular weight compounds, such as propylene and butylene, are mixed in the presence of a catalyst such as hydrofluoric acid or sulfuric acid (a by-product from removing impurities from many oil products). The products of alkylation are high octane hydrocarbons, which are used in gasoline blends to reduce knocking (see "What does octane mean?" for details).

Now that we have seen how various fractions are changed, we will discuss the how the fractions are treated and blended to make commercial products.

Treating and Blending the Fractions
Distillated and chemically processed fractions are treated to remove impurities, such as organic compounds containing sulfur, nitrogen, oxygen, water, dissolved metals and inorganic salts. Treating is usually done by passing the fractions through the following:

• a column of sulfuric acid - removes unsaturated hydrocarbons (those with carbon-carbon double-bonds), nitrogen compounds, oxygen compounds and residual solids (tars, asphalt)
• an absorption column filled with drying agents to remove water

sulfur treatment and hydrogen-sulfide scrubbers to remove sulfur and sulfur compounds After the fractions have been treated, they are cooled and then blended together to make various products, such as:

• gasoline of various grades, with or without additives
• lubricating oils of various weights and grades (e.g. 10W-40, 5W-30)
• kerosene of various various grades
• jet fuel
• diesel fuel
• heating oil
• chemicals of various grades for making plastics and other polymers