Video explaining petroleum refining basics
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PRESENTER: For crude oil to be used effectively by modern industry, it has to be separated into its component parts and have impurities like sulfur removed. The most common method of refining crude is the process of fractional distillation. This involves heating crude oil to about 350 degrees Celsius, to turn it into a mixture of gases. These are piped into a tall cylinder, known as a fractional tower. Inside the tower, the very long carbon chain liquids, such as bitumen and paraffin wax, are piped away to be broken down elsewhere. The hydrocarbon gases rise up inside the tower, passing through a series of horizontal trays and baffles called bubble caps. The temperature at each tray is controlled so as to be at the exact temperature that a particular hydrocarbon will condense into a liquid. The distillation process is based on this fact. Different hydrocarbons condense out of the gas cloud when the temperature drops below their specific boiling point. The higher the gas rises in the tower, the lower the temperature becomes.
The precise details are different at every refinery, and depend on the type of crude oil being distilled. But at around 260 degrees, diesel condenses out of the gas. At around 180 degrees, kerosene condenses out. Petrol, or gasoline, condenses out at around 110 degrees, while petroleum gas is drawn off at the top. The distilled liquid from each level contains a mixture of alkanes, alkenes, and aromatic hydrocarbons with similar properties, and requires further refinement and processing to select specific molecules. The quantities of the fractions initially produced in an oil refinery don't match up with what is needed by consumers. There is not much demand for the longer chain, high molecular weight hydrocarbons, but a large demand for those of lower molecular weight-- for example, petrol.
A process called cracking is used to produce more of the lower molecular weight hydrocarbons. This process breaks up the longer chains into smaller ones. There are many different industrial versions of cracking, but all rely on heating. When heated, the particles move much more quickly, and their rapid movement causes carbon-carbon bonds to break. The major forms of cracking are thermal cracking, catalytic, or cat cracking, steam cracking, and hydrocracking. Because they differ in reaction conditions, the products of each type of cranking will vary. Most produce a mixture of saturated and unsaturated hydrocarbons. Thermal cracking is the simplest and oldest process. The mixture is heated to around 750 to 900 degrees Celsius, at a pressure of 700 kilopascals That is, around seven times atmospheric pressure. This process produces alkenes, such as ethane and propane, and leaves a heavy residue.
The most effective process in creating lighter alkanes is called catalytic cracking. The long carbon bonds are broken by being heated to around 500 degrees Celsius in an oxygen-free environment, in the presence of zeolite. This crystalline substance, made of aluminum, silicon, and oxygen, acts as a catalyst. A catalyst is a substance that speeds up a reaction or allows it to proceed at a lower temperature than would normally be required. During the process, the catalyst, usually in the form of a powder, is treated and reused over and over again. Catalytic cracking is the major source of hydrocarbons, with 5 to 10 carbon atoms in the chain. The molecules most formed are the smaller alkanes used in petrol, such as propane, butane, pentane, hexane, heptane, and octane, the components of liquid petroleum gas. In hydrocracking, crude oil is heated at very high pressure, usually around 5,000 kiloPascals, in the presence of hydrogen, with a metallic catalyst such as platinum, nickel, or palladium. This process tends to produce saturated hydrocarbons, such as shorter carbon chain alkanes, because it adds a hydrogen atom to alkanes and aromatic hydrocarbons. It is a major source of kerosene jet fuel, gasoline components, and LPG.
In one method, thermal steam cracking, the hydrocarbon is diluted with steam and then briefly heated in a very hot furnace, around 850 degrees Celsius, without oxygen. The reaction is only allowed to take place very briefly. Light hydrocarbons break down to the lighter alkenes, including ethane, propane, and butane, which are useful for plastics manufacturing . Heavier hydrocarbons break down to some of these, but also give products rich in aromatic hydrocarbons and hydrocarbons suitable for inclusion in petrol or diesel. Higher cracking temperature favors the production of ethene and benzene. In the coking unit, bitumen is heated and broken down into petrol alkanes and diesel fuel, leaving behind coke, a fused combination of carbon and ash. Coke can be used as a smokeless fuel. Reforming involves the breaking of straight chain alkanes into branched alkanes. The branched chain alkanes in the 6 to 10 carbon atom range are preferred as car fuel. These alkanes vaporize easily in the engine's combustion chamber, without forming droplets and are less prone to premature ignition, which affects the engine's operation.
Smaller hydrocarbons can also be treated to form longer carbon chain molecules in the refinery. This is done through the process of catalytic reforming, When heat is applied in the presence of a platinum catalyst, short carbon chain hydrocarbons can bind to form aromatics, used in making chemicals. A byproduct of the reaction is hydrogen gas, which can be used for hydrocracking. Hydrocarbons have an important function in modern society, as fuel, as solvents, and as the building blocks of plastics. Crude oil is distilled into its basic components. The longer carbon chain hydrocarbons may be cracked to become more valuable, shorter chain hydrocarbons, and short chain molecules can bind to form useful longer chain molecules. [MUSIC PLAYING]
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Crude oil refineries employ some of the United States' top scientists, engineers, and safety professionals to ensure that products are produced efficiently and safely. US refineries process about 17 million barrels of crude oil a day. Refinery configurations vary, but US refineries are undeniably some of the world's most sophisticated.
Much like a simple still, in a distilling column, liquid is heated to a vapor and lifted upward to be distilled again into separate substances. This is the beginning of the refining process. Distilling exploits the characteristic of the chemicals in crude oil to boil at different temperatures, a phenomenon that engineers chart along distillation curves. Unlike a still, a distilling column contains a set of trays that allow heated vapors to rise and collect at different levels, separating out the various liquids derived from crude oil. The top of the column is cooler than the bottom, so as liquids vaporize and rise, they condense again, collecting onto their respective trays. Butane and other light products rise to the top of the column, while straight-run gasoline, naphtha, kerosene, diesel, and heavy gas oil gather on the trays, leaving straight run residue at the base of the column. Liquids are considered 'heavy' or 'light' based on their specific gravity, which is determined based on its weight and density compared to that of water.
Because there is more demand for some distilled products like gasoline, refiners have an incentive to convert heavy liquids into lighter liquids. The term cracking comes from the process of breaking up long hydrocarbon molecules into smaller, more useful molecules. The cracking process converts heavy straight run liquids into gasoline. There are multiple versions of the cracking process, and refiners use the process extensively. Cracking is a highly controlled process, so cracking units exist separate from distillation columns. The most common type of cracking is 'cat cracking,' named for the use of catalysts, substances added to a chemical reaction to speed up the process.
The process of reforming was developed to raise both the quality and volume of gasoline produced by refiners. Using a catalyst again, after a series of reforming processes, substances are converted into aromatics and isomers, which have much higher octane numbers than the paraffins and napthenes produced by other refinery processes. Most simply, reforming rearranges the naphtha hydrocarbons to create gasoline molecules. The reforming process produces reformate, which is needed to increase the octane for today's cleaner burning fuels. Interestingly, hydrogen is also produced by the catalytic reforming process - this hydrogen is then used in other refining processes such as hydrotreating.
Crude oil naturally contains contaminants such as sulfur, nitrogen, and heavy metals, which are undesirable in motor fuels. The treating process, primarily hydrotreating, removes these chemicals by binding them with hydrogen, absorbing them in separate columns, or adding acids to remove them. The recovered molecules are then sold to other industries. Refineries that process sour crudes produce more sulfur than refineries that process sweet crudes. Following the treatment, blending, and cooling processes, the liquids finally look like the fuels and products you're familiar with: gasoline, lubricants, kerosene, jet fuel, diesel fuel, heating oil, and petrochemical feedstocks that are needed to create the plastics and other products you use every day.
The last major step of the refining process is blending various streams into finished petroleum products. The various grades of motor fuels are blends of different streams or 'fractions' such as reformate, alkylate, catalytically cracked gasoline, etc. Refineries blend compounds obtained either from their internal refining process operations as noted above, or externally, to make gasoline that meets specifications for acceptable motor vehicle performance. A typical refinery may produce as many as 8 to 15 different streams of hydrocarbons that they then must mix into motor fuels. Refiners might also mix in additives like octane enhancers, metal deactivators, anti-oxidants, anti-knock agents, rust inhibitors, or detergents into their hydrocarbon streams. Blending can take place at the refinery along the pipelines and tanks that house processed fuel or even at off-site locations or on ships or terminals once the fuel has left the refinery gate.
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