The first refinery, which opened in 1861, produced kerosene by simple atmospheric distillation. Its by-products included tar and naphtha. With the advent of mass production and the First World War, the number of gasoline-powered vehicles increased dramatically, and the demand for gasoline grew accordingly. However, only a certain amount of gasoline could be obtained from crude oil through basic distillation processes. Other processes needed to be developed to maximize the amount of gasoline drawn from crude.
Thermal cracking, which was developed in 1913, subjected heavy fuels (which had been separated from the distillation process) to both pressure and intense heat, physically breaking their large molecules into smaller ones, producing additional gasoline and distillate fuels. Distillates are simply the resulting component parts of oil after being distilled.
As higher-compression gasoline engines were developed, there was a demand for higher-octane gasoline with better anti-knock characteristics. The introduction of catalytic cracking and polymerization processes in the mid- to late 1930s met this demand by providing improved gasoline yields and higher octane numbers. As decades passed and engine performance increased, so did the need to enhance the yield from crude oil and increase the octane of gasoline.
The numbers and types of different processes used in modern refineries depend primarily on the nature of the crude oil being refined and finished product requirements. Not all oil is alike and thus, not all refineries are alike. Below is a description of some of the basic processes used in a typical refinery.
How Refineries Work see companion infographic The Processes
There are three basic processes refineries utilize to create a multitude of end products
Separation. Crude oil is physically separated by fractionation (explained later) in atmospheric and vacuum distillation towers, into groups of hydrocarbon molecules with various boiling-point ranges, called “fractions” or “cuts”.
Conversion Conversion processes change the size and/or structure of hydrocarbon molecules through
• dividing molecules through distillation, cracking, coking and breaking
• combining molecules though alkylation and polymerization
• rearranging molecules through isomerization and catalytic reforming treatment
Treating and Blending. Once separated, component parts are combined in different degrees to create a variety of finished products.
Step-by-Step There are a myriad of processes performed at refineries. As complex as they may seem, their basic functions often rely on simple natural processes.
Distilling Processes The first step in petroleum refining is the fractionation, or separating oil into fractions (also known as cuts) of crude oil in atmospheric and vacuum distillation towers. Fractionation works because the differences in temperature from the bottom to the top of the distillation tower causes the higher-boiling-point components to condense first, while the lower-boiling-point fractions rise higher in the tower before they condense.
Atmospheric distillation is a rather basic process in that as the crude oil is heated, the various fractions, or cuts the crude that is intermixed, begins to separate at different boiling points. As each separates at different temperatures, fractions rise as steam gas up the tower.
Fractions with the lowest boiling points, such as fuel gas and light naphtha, are removed from the top of the tower by an overhead line as vapor. Naphtha, or straight-run gasoline, is taken from the upper section of the tower as an overhead stream. Intermediate fractions, including gas oil, heavy naphtha and distillates, are removed from the middle section of the tower as side streams. The heavier, higher-boiling-point fractions (called residue, bottoms or topped crude) condense and remain at the bottom of the tower and are sent into a vacuum distillation tower for further fractionation. Vacuum distillation reduces the pressure in the sealed unit to separate heavier compounds.
Each component separate from the atmospheric distillation process is then ferried through piping systems to other processes for further refining.
There are several other smaller distillation towers in refineries (not shown in the infographic), called columns, designed to separate specific and unique products, which all work on the same principles as atmospheric towers. For example, a depropanizer is a small column designed to separate out propane.
Conversion Processes Dividing/Cracking Cracking breaks (or cracks) heavier petroleum fractions into more valuable products such as gasoline blending stocks, gas oil and fuel oil. The basic types of cracking are thermal cracking, catalytic cracking and hydro-cracking.
Thermal cracking heats distillate fuels and heavy oils under pressure in large drums until they crack (divide) into smaller molecules with better anti-knock characteristics. This early method, which produced large amounts of solid, unwanted coke, has evolved into modern thermal cracking processes including visbreaking, steam cracking and coking.
Visbreaking is a mild form of thermal cracking which significantly lowers the viscosity of feedstock. Steam cracking produces olefins (which boosts octane) by thermally cracking large hydrocarbon molecule feedstocks at moderate pressures and at very high temperatures.
Coking is a severe form of thermal cracking used to obtain straight-run gasoline (coker naphtha) and various middle distillate fractions used as catalytic cracking feedstocks. This process so completely reduces hydrogen from the hydrocarbon molecule, that the residue is a form of almost pure carbon called coke.
Catalytic cracking breaks up complex hydrocarbons into simpler molecules in order to increase the quality and quantity of lighter, more desirable products. Heavy hydrocarbons are exposed at high temperature and low pressure to catalysts which promote chemical reactions. Catalysts in a refinery basically induce a chemical reaction and are made up of a fine powder of solids like clay, silica, and/or aluminum.
Two combining processes, polymerization and alkylation, are used to join together smaller molecules, called olefins, recovered from thermal and catalytic cracking, in order to create more desirable gasoline blending stocks. Polymerization converts olefins into heavier, more complex, higher-octane molecules, including naphtha and petrochemical feedstocks. Alkylation combines the molecules of olefins produced from catalytic cracking with those of isoparaffins (a synthetic solvent) in order to increase the volume and octane of gasoline blends.
Catalytic reforming and isomerization are processes which rearrange hydrocarbon molecules to produce products with different characteristics. Catalytic reforming processes convert low-octane heavy naphthas into aromatic hydrocarbons for petrochemical feedstocks and high-octane gasoline components, called reformates, by molecular rearrangement or dehydrogenation.
Isomerization converts butane, pentane and hexane into synthetic solvents (iso-paraffins.) while also increasing the octane of these components.
Other Processes Lubricating oils and waxes are refined from various fractions of atmospheric and vacuum distillation. With the invention of vacuum distillation, it was discovered that the waxy residue made a better lubricant than any of the animal fats that were then in use. This sparked the beginning of modern lubricant refining technology which includes a series of processes including de-asphalting, solvent extraction and separation and treatment processes such as dewaxing and hydrofinishing.
Sweetening and treating processes
The products created by separating and converting crude oil often require the final step of treatment, sweetening, and blending in order to be used in gasoline and fuel oil blends.
Treating is a process that uses a variety of substances (acids, solvents, clays) to remove contaminants like sulphur, nitrogen, and dissolved metals that had not been removed by other processes. Sweetening removes sulfur which can foul the smell of a final product and produce dangerous emissions when burned.
Blending The last step in the refining process is blending. Blending mixes fractions from 10 to 15 different streams into the desired final product, including various octane rated gasoline blends. treating, sweetening, and blending processes are also used to produce lubricants, greases, and waxes.
Distillation products, including kerosene and other distillates, may contain trace amounts of aromatics and naphthenes and lubricating-oil base stocks may contain wax. These undesirables are removed either at intermediate refining stages or just prior to sending products to blending and storage, by refining processes such as solvent extraction and solvent dewaxing. A variety of intermediate and finished products, including middle distillates, gasoline, kerosene, jet fuel and sour gases need to be dried and sweetened.
Treating is performed either at an intermediate stage in the refining process or just before sending finished products to blending and storage. Treating removes contaminants from oil, such as organic compounds containing sulphur, nitrogen and oxygen, dissolved metals, inorganic salts and soluble salts dissolved in emulsified water. Treating materials include acids, solvents, alkalis and oxidizing and adsorption agents. Acid treatments are used to improve the odor, color and other properties of lube base stocks, to prevent corrosion and catalyst contamination, and to improve product stability. Hydrogen sulphide which is removed from “dry” sour gas by an absorbing agent (diethanolamine) is flared, used as a fuel or converted to sulphur. The type of treatment and agents depends on the crude feedstock, intermediate processes and end-product specifications.