Petrochemistry

Definition:Petrochemistry concerns itself with the production of petrochemicals and their intermediaries, which are used in the chemical industry in a diverse variety of products and applications. The traditional sources of petrochemicals are petroleum (crude oil) and natural gas. Contemporary petrochemicals include inorganic chemicals such as ammonia and even pure sulfur, gained from crude-oil refining processes and natural-gas processing. Additional petrochemicals are derived from synthesis gas.

Petrochemistry occupies an intermediary position between the petroleum or oil-and-gas industry and the manufacture of end products in the chemical industry. Petrochemical plants are generally integrated at petroleum refineries, from where they derive their primary feedstock, or built in close proximity to them.

Basic Principles

In the early twentieth century, chemists realized that they could manufacture chemicals from the byproducts of crude-oil refining. In 1857, the world’s first commercial oil refinery was commissioned in Romania, and the oil age began in earnest with the 1859 success of American Edwin Drake’s oil drilling in Pennsylvania. In 1872, the first petrochemical was manufactured. It was carbon black, gained from partial combustion of natural gas in the air, and was used as ink and pigment.

With the inventions of the gasoline and diesel engines, in 1876 and 1892, and the automobile in 1885, petroleum refineries focused on the manufacture of engine fuels from crude oil. Since 1913, thermal cracking of crude oil increased gasoline and diesel yields, leading to an increase of very light hydrocarbons. Chemists soon took advantage of this new source from refineries to replace the manufacture of these chemicals from coal or chalk. Butadiene, unwanted at refineries, was used to manufacture synthetic rubber following a process invented in 1909 in Germany. After World War II, the rise of the plastics industry created an exploding market for petrochemicals as their feedstock. By 2012, basic, specialty, and pharmaceutical chemicals created from petrochemicals had increased their value. This trend was reinforced by use of petrochemicals in nanotechnology.

Contemporary petrochemicals derive from core primary sources. Aromatics are extracted from the naphtha stream after they undergo catalytic reforming at a refinery. Olefins, or unsaturated hydrocarbons, are manufactured by cracking hydrocarbon molecules with more carbon atoms into lighter molecules with fewer carbon atoms. This is done at refinery units or at designated plants, such as an ethylene cracker operating apart from the refinery processes of manufacturing hydrocarbon fuels. Paraffins, or saturated hydrocarbons, are typically sourced from refinery processes. Natural gas is an important source of petrochemicals, including ammonia. Because of this source, ammonia is considered a petrochemical even though it lacks a carbon atom. The same is true for sulfur, a byproduct of desulfurization of gasoline and diesel at refineries.

Core Concepts

Petrochemical production is closely linked to petroleum refining and processing and natural-gas processing, as these are the sources of its raw materials and products. In consequence, petrochemistry relies on some key refining processes that can be adjusted to yield more petrochemicals instead of fuel and fuel additives if so desired. The vast majority of petrochemicals serve as raw materials for the chemical industry to create end products from them.

Steam Cracking. Steam cracking is the primary mode of producing the two most common petrochemicals, ethylene (ethene) and propylene (propene). The term crackingrefers to the process of using heat to break up larger hydrocarbon molecules into smaller ones. Steam cracking is a form of thermal cracking, which was invented independently in Russia in 1891 and in the United States in 1908. In steam cracking, a stream of hydrocarbons in either gas or liquid form is mixed with steam and sent into the furnace of the steam cracker. Temperatures inside the furnace range from about 760 to 840 degrees Celsius (1,400 to 1,544 degrees Fahrenheit), but the hydrocarbons stay in the furnace for only 0.3 to 0.8 seconds. They are cooled down, or quenched, immediately after leaving the furnace to stop further reactions and to extract the desired cracked petrochemical products.

The hydrocarbon mix for the steam cracker can come from a variety of sources. A straightforward, stand-alone ethylene cracker would get most of its hydrocarbon feedstock in the form of ethane, a component of natural gas. Steam crackers can also be fed with hydrocarbons in the form of straight-run naphtha or gas oil from the distillation towers of an oil refinery. In this case, the share of propylene and precursors of butadiene as well as aromatics is increased, and multiple quenching and distillation processes are needed to extract the desired petrochemical products.

Catalytic Reforming; Aromatics Extraction and Separation. The majority of the petrochemicals known as aromatics are a byproduct of the catalytic reforming of straight-run heavy naphtha streams at a petroleum refinery. The process of catalytic reforming was invented by American chemist Vladimir Haensel for the Universal Oil Products company, which put the process into commercial operations in 1949. Catalytic reforming is done to increase the octane number of the resulting raffinate (material from which a component has been removed), which is blended into gasoline to improve its quality. Catalytic reforming can be used to produce aromatics, but not every refinery does so. Sometimes, aromatics extraction from the raffinate is done at a different refinery.

To extract the aromatics—benzene, toluene, and xylenes—a solvent is added to the raffinate leaving the catalytic reformer. This solvent binds only the aromatics, which can then be separated from the rest of the raffinate in an extraction tower at the refinery. Next, the solvent is removed from the aromatics in an extraction-stripper column. Here, steam strips the aromatics off the solvent, which leaves the unit at the bottom while aromatics depart from the top. The aromatics are typically washed with water of any remaining impurities before being sent to a gas-fractionating plant. At the plant, benzene and toluene are separated by distillation. Separating the remaining aromatics presents a technological challenge because their boiling points are very close; thus, a system of super-distillation and crystallization is used. Meta-xylene and para-xylene are separated either by being chilled into crystal form, using their separate solidification points, or through a molecular sieve adsorbent designed to adsorb only para-xylene molecules.

Vapor-Phase Adsorption. Normal paraffins, which are saturated hydrocarbons, are typically extracted from refinery product streams vaporized into the gaseous stage. The hydrocarbon feedstock is sent across a bed of molecular sieves that adsorb the paraffin molecules. In a complementary unit, ammonia is sent across the filled molecular sieves, and the paraffin molecules are desorbed and leave the unit separate from the ammonia.

Synthesis—Ammonia. Industrially, ammonia is manufactured through the reaction of nitrogen with hydrogen, which comes from a hydrocarbon source. The most common hydrogen source is methane, the biggest component of natural gas. Hydrogen can also come from refinery gases, from steam reforming of naphtha refinery streams, or from partial oxidation of very-long-chained hydrocarbon molecules forming residues at a refinery. Because of this connection to hydrocarbons, ammonia can be considered a petrochemical.

Desulfurization. Desulfurization of gasoline and diesel has been mandated in developed countries, including the United States and the European Union, for environmental reasons. As a result, refineries have modernized their units so that gasoline and diesel are treated with hydrogen to extract sulfur in the form of hydrogen sulfide. In a subsequent sulfur-recovery unit using the Claus process, invented in 1883, sulfur is extracted in elementary form.

Oxidation, Nitration, Halogenation, Hydroxylation. Raw petrochemicals are subjected to a series of chemical reactions that add other atoms to their molecules. This is done to create intermediary products from the petrochemicals for further use by the chemical industry. Ethylene is primarily oxidized to form ethylene oxide of great purity. Ethane and propane are nitrated, adding nitrogen dioxide to their molecules, to create nitromethane and nitroethane, as well as 1- and 2-nitropropane. These products are used as solvents or feedstock for further synthesis processes. Similar processes subject petrochemicals to the addition of chlorine in halogenation and of alcohols in hydroxylation reactions.

Applications Past and Present

Petrochemicals and their intermediaries provide the raw materials for a vast array of products created by the chemical industry that have become essential elements of contemporary life. The production of plastics, synthetic rubber, synthetic fibers and fiberglass, polycarbonates, solvents, adhesives, detergents, herbicides, pesticides, and pharmaceuticals all begins with petrochemicals.

Applications for petrochemicals have risen dramatically since the manufacture of the first petrochemical, carbon black, in 1872. In the 1920s, chemists discovered that petrochemicals could serve as a more economical source of hydrocarbons than coal. Butadiene from refinery steam crackers became an important element of synthetic rubber in both North America and Germany. One of the first plastics, Bakelite, began using phenol for its phenol resins, which ultimately derived from benzene extracted after catalytic reforming at refineries after 1949. In general, petrochemical applications rose together with the plastics industry in the second half of the twentieth century, with further innovative applications developed since. To look at contemporary applications, it is useful to order these by the primary petrochemical from which they derive.

From Ethylene. Ethylene (chemical name ethene) is one of the most widely used petrochemicals. In its raw form, it acts as a ripening agent for fruits and plants, but this use is dwarfed by the applications derived from the chemical processing of ethylene. When ethylene is polymerized under pressure and with a catalyst, it forms polyethylene, which is the most common plastic. It is used for packaging, plastic foils, and plastic bottles, as well as for injection molding parts and household plastic items.

Oxidation of ethylene delivers ethylene oxide, which is further processed, particularly into ethylene glycol. About half of ethylene glycol is used as antifreeze for engines; the other half is processed to form polyesters, among them polyethylene terephthalate, abbreviated as PET. PET is used for synthetic fibers and plastic bottles. (PET does not contain polyethylene; the prefix poly refers to the compound itself.) Washing powders, paints, and dyes are also made on the basis of ethylene oxide.

Chlorination of ethylene creates ethylene dichloride (also known as 1,2-dichlorethane), which is a feedstock in the manufacture of PVC (polyvinylchloride), a plastic widely used in the construction industry for piping, for clothing and toys, or as a rubber substitute.

Ethanol can be produced by hydrating ethylene. Made this way, ethanol is used for solvents or as an intermediary for pharmaceuticals. This ethanol has a different petrochemical origin compared to the bio-ethanol used as a gasoline additive.

Combining the two petrochemicals of ethylene and benzene delivers ethylbenzene. This is used to manufacture styrene, from which the plastic polystyrene is created.

From Propylene. Propylene is processed to manufacture chemicals with a wide variety of applications. It is commonly oxidized to form propylene oxide. Propylene oxide can be used as an intermediary for polyol, out of which polyurethanes are made. Polyurethanes provide flexible and hard foams, seals, adhesives, fibers, and garden hoses. Propylene oxide leads to the manufacture of propylene glycol, an engine coolant also used to deice aircraft.

Propylene is polymerized to manufacture acrylonitrile, which is then used to make acrylic fibers that can replace wool in clothing. Another application is the thermoplastic acrylonitrile butadiene styrene (ABS). ABS is used for its impact resistance and toughness, for example in luggage manufacture or for car components.

Propylene is oxidized to form acrylic acid. Acrylic acid leads to acrylic polymers, which are widely used to create transparent and elastic plastics, among them the trademark product Plexiglas. Other products include paints, elastomers, strong adhesives, or flocculants for waste water treatment.

Epxoy resins are created in a series of chemical manufacturing steps that begin with propylene. Hydrated propylene forms isopropyl alcohol, commonly known as rubbing alcohol.

From Butadiene. Butadiene is a component of the so-called C4 cut of petrochemicals with four carbon atoms in their molecule. Its primary use is as part of synthetic rubbers, of which there are many different kinds. In 2021, 8.27 million metric tons of styrene-butadiene was produced. Butadiene is also processed further to form isoprene and chloroprene synthetic rubbers. One important use of synthetic rubber is in the manufacture of car tires, where it is favored because of its compatibility with petroleum products, such as oil or fuel, that may leak from the car or be present on the road. In hot climates, tires made from synthetic rubber endure better than those made from natural rubber.

From Benzene. Benzene is an aromatic prime petrochemical from which many products are created through multiple processing steps. It is also combined with ethylene to form ethylbenzene for plastics production. Alkylation of benzene with propylene in the presence of a zeolite-based catalyst creates cumene (chemical name isopropylbenzene). Cumene is then turned into cumene hydroperoxide, which is the feedstock for the industrial synthesis of phenol and acetone. Phenol is used to manufacture plastics like Bakelite, synthetic fibers like nylon, epoxies for coatings and adhesives, detergents, pharmaceuticals, and herbicides. A special phenol compound, bisphenol A, is used to manufacture polycarbonates, which are used to make hard but lightweight plastic shells, such as in mobile telephones or for the touch screens of smart phones. Acetone is used either directly as a thinner, including in nail polish remover, or as a solvent.

When benzene is made to react with hydrogen, it forms cyclohexane. Cyclohexane is used to produce adipic acid and caprolactam, both raw materials for the production of nylon.

Nitration of benzene leads to nitrobenzene, which is hydrated to form aniline. From this, after another intermediary step, polyurethanes are produced. Alkylation of benzene creates the material for a variety of detergents. Historically, the pesticide DDT was made from chlorobenzene. Because of its vast environmental damage, DDT has been generally phased out of use. Benzene itself is carcinogenic; this limits its direct applications, such as its use as an octane booster in fuel.

From Toluene. Toluene is a petrochemical directly used as powerful solvent for paints, glues, ink, and lacquers, as well as in paint thinner. It is also used to generate carbon nanotubes in a solution. Commonly, toluene is nitrated twice with nitric acid to form toluene diisocyanate, which is used to manufacture polyurethanes in an alternative to the polyurethane-production process via propylene.

Toluene is used in the manufacture of explosives like TNT (trinitrotoluene). It is also an ingredient of jet and racing fuel, added to boost octane.

Some toluene is dealkylated to form benzene. Toluene, like benzene, can be used in the production of nylon. For this, toluene is partially oxidized to form benzoic acid, which can then be used to create caprolactam, a precursor to the synthesis of nylon.

From Xylol. Para-xylene, which is a petrochemical with a particularly challenging extraction process, is almost exclusively used to manufacture terephthalic acid and dimethyl terephthalate. Both substances are used in the production of PET bottles and polyester yarns and fibers.

Ortho-xylene is used to manufacture terephthalic anhydride. This chemical is used as plasticizer for plastics, increasing their fluidity and ability to be molded. It is commonly used for PVC pipes to increase their plasticity.

Meta-xylene does not have many uses and is often converted to either para- or ortho-xylene. Mixtures of all three of these xylene isomers are typically referred to as xylol. Xylol is used as solvent, as varnish or paint thinner.

From Synthesis Gases. The two most important products created from synthesis gases are methanol and ammonia. Methanol is used as solvent, fuel, and antifreeze. From methanol, formaldehyde is produced via catalytic oxidation. Formaldehyde is used to make inexpensive and durable furniture by bonding wood veneer with plastic laminate. Methanol is part of the chemical processes that lead to the production of PET bottles, through its role in creating acetic acid. Methanol has become important for biodiesel through production of methyl esters for this alternative fuel. The controversial fuel additive MTBE (methyl tertiary butyl ether) is also based on methanol. Because of concerns over groundwater contamination with MTBE, its use was banned in California in 2004, and other states followed suit.

Ammonia is used to manufacture fertilizers. Ammonia-based fertilizers have been of great importance as providers of nitrogen to agricultural crops to enhance their production. Ammonia is also used to manufacture explosives, via nitric acid, and to create intermediary products for the pharmaceutical industry.

From Sulfur. Desulfurization of gasoline and diesel has, in effect, turned sulfur into a petrochemical. Virtually all sulfur produced has been extracted from crude oil–based fuels, in which sulfur is an unwanted ingredient. Sulfur is converted into sulfuric acid in two alternative processes using oxygen and water. Sulfuric acid is used in the chemical manufacture of fertilizers, such as superphosphates and ammonium sulfates. It is also used for detergents, resins, and pharmaceuticals. Some sulfuric acid is used to manufacture catalysts for the petroleum industry. This use returns sulfur atoms to the location of their previous extraction.

Social Context and Future Prospects

The shape of contemporary life would be vastly different without the products derived from petrochemicals and their intermediaries. The quest to find more lightweight-yet-sturdy, elastic-yet-tough, clear, and scratch-resistant synthetic materials has culminated in such innovative products as polycarbonate films for smart-phone touch screens and crash helmets for motorcyclists. The packaging and bottling industries have been revolutionized by plastics from petrochemicals, as have the fiber and textile industries by polyester yarns. New paints, adhesives, and solvents support production of ever-more-useful end products.

The dramatic development of petrochemicals since the 1950s has made exploitation of natural hydrocarbon resources such as crude oil and natural gas vastly more efficient. A contemporary refinery gives off very little of its production as unwanted and unusable byproducts. The value creation of raw hydrocarbons is actually much greater for petrochemicals than for fuels.

Grave social problems have been created by the extraction of hydrocarbons without concern for the people living at the extraction sources. A negative example is the plight of the Ogoni people of Nigeria, whose land has been massively degraded by oil extraction without any local benefits. To keep more of the value created from petrochemicals in their own countries, many oil-rich nations of the Middle East, including Saudi Arabia and the United Arab Emirates (UAE), built up their own petrochemical industries during the first decades of the twenty-first century.

There are many research-and-development efforts in the field of petrochemicals to find new applications and improve existing processes. However, as the petrochemical industry has consolidated in the United States and is not expected to expand, a future petrochemist may consider an international career in the industry more rewarding than facing a challenging job market at home. Scientific work at universities is very dependent on public funding of research and thus is linked to domestic budget policies.

Bibliography

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"The Future of Petrochemicals." International Energy Agency, Oct. 2018, www.iea.org/reports/the-future-of-petrochemicals. Accessed 28 Aug. 2024.

"What Is Petrochemistry?" Petro Online, 2023, www.petro-online.com/news/fuel-for-thought/13/breaking-news/what-is-petrochemistry-nbsp/35676. Accessed 28 Aug. 2024.