Omar Khalid
Plastic is derived from natural, organic materials such as cellulose, coal, natural gas, salt, and, most commonly, crude oil. Crude oil is a mixture of hydrocarbons, compounds made from combinations of carbon and hydrogen atoms that form chains of varying lengths, giving them different properties. These hydrocarbons are the earliest raw materials of plastic. The production of plastics begins with the distillation of crude oil in an oil refinery, which separates the heavy crude oil into lighter components called fractions. One of these fractions, naphtha, is the crucial compound for the production of plastics.
| Characteristics | Values |
|---|---|
| Source of plastic | Crude oil, natural gas, coal |
| Type of plastic | Synthetic plastic, bio-based plastic |
| Process | Extraction, refining, distillation, polymerization, polycondensation |
| Feedstock | Alkanes, olefins (propylene, ethylene, butylenes) |
| Naphtha | A crucial compound for plastic production |
| Steam cracking | A process that breaks down naphtha into ethane and propene |
| Polymerization | The bonding of simple molecules into long molecular chains |
| Environmental impact | High, with synthetic plastics taking over 400 years to degrade |
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What You'll Learn
Crude oil is a mixture of hydrocarbons and other compounds
The specific composition of crude oil depends on its type and origin, and no two crude oils are exactly alike. They are customarily characterised by the type of hydrocarbon compound that is most prevalent in them: paraffins, naphthenes, and aromatics. Paraffins are the most common hydrocarbons in crude oil, and they are highly valued as liquid paraffins are the major constituents of gasoline (petrol). Napthenes are important components of liquid refinery products, but they also contribute to heavy asphalt-like residues. Aromatics, on the other hand, have high freezing points and can form waxy residues.
Crude oil is classified based on its density, sulfur content, and hydrocarbon composition. It ranges in appearance from colourless to black, and its specific gravity is an important physical property that is measured relative to water. Crude oil is typically sent to a refinery, where it undergoes a refining process to transform it into different petroleum products. This process involves heating the oil in a furnace and then sending it to a distillation unit, where it separates into lighter components called fractions. One of these fractions, naphtha, is a crucial compound for making a large amount of plastic.
The long-chain hydrocarbons obtained from distillation are further converted into simpler hydrocarbons through a process called "cracking". This process uses high temperatures and pressures to break down the complex hydrocarbons into simpler alkenes and alkanes. These hydrocarbons are then used as feedstock for the petrochemical industry to produce a wide range of products, including plastics, pharmaceuticals, solvents, lubricants, and detergents.
In summary, crude oil is a mixture of hydrocarbons and other compounds, with a composition that varies depending on its origin. The refining and distillation processes transform crude oil into useful chemicals and feedstock for various industries, including plastic manufacturing.
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Refining separates crude oil into fractions
Plastic is derived from crude oil, natural gas, or coal. Synthetic plastics, in particular, come from petrochemicals. Crude oil is a mixture of hundreds of hydrocarbons, solids, and gaseous hydrocarbons from the alkane family.
Crude oil is refined through a process called fractional distillation, which separates the oil into fractions. This process involves heating the oil in a furnace and sending it to a distillation unit, where it separates into lighter components called fractions. The lightest fractions, including gasoline and liquefied refinery gases, vaporize and rise to the top of the distillation tower, where they condense back into liquids. Medium-weight liquids, including kerosene and distillates, stay in the middle of the tower. Heavier liquids, called gas oils, separate lower down in the tower, and the heaviest fractions settle at the bottom.
The hydrocarbon chains that constitute petroleum fuel products are called alkanes or paraffins, depending on their size. Alkanes are stable and versatile, making them safe for use and storage as a range of fuel products. Fractional distillation can also be used to join or split these chains to create different petroleum products.
One of the crucial fractions for plastic-making is naphtha, which is derived from crude oil refining. Naphtha is further decomposed thermally and separated into ethylene and propylene, which are the raw materials for plastics. These substances are then converted into low-molecular compounds called "monomers," which are the basic building blocks of polymers. The formation of polymers from monomers uses heat, light, and additives such as enzymes.
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Naphtha, a crucial fraction, is a chemical that becomes plastic
Plastic is derived from synthetic or natural sources. Synthetic plastics come from petrochemicals, which are obtained from the Earth's natural oil reserves. Natural plastics, on the other hand, are derived from renewable products such as carbohydrates, fats, and oils.
The process of making synthetic plastic begins with the extraction of raw materials, primarily crude oil and natural gas, but also coal. These materials are composed of thousands of compounds that need to be processed. The refining process involves heating the crude oil in a furnace and then sending it to a distillation unit, where it separates into lighter components called fractions.
One of the crucial fractions obtained from this process is naphtha, a chemical that becomes plastic. Naphtha is a mixture of hydrocarbons obtained from the distillation of crude oil. It is further decomposed at high temperatures of around 800°C in a steam cracker, in the presence of water vapour. This process breaks down the naphtha into light hydrocarbons, specifically olefins and aromatics. The olefins include ethylene (C2), propylene (C3), and butane/butadiene (C4), while the aromatics consist of benzene, toluene, and xylene.
These small molecules are then linked together to form long molecular chains called polymers, which are the building blocks of plastic. However, at this stage, the polymers are still in the form of granules, powders, or liquids. To turn them into usable plastic, they must undergo additional transformations, including kneading, heating, melting, and cooling, to create objects of various shapes, sizes, and colours with precise properties.
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Steam cracking breaks naphtha into ethane and propene
Plastic is derived from crude oil, natural gas, or coal. Synthetic plastics are derived from crude oil, which is extracted from the Earth's surface. Drilling holes through rocks in the ground is one way to extract oil. Oil drilling can also take place under the ocean with the support of platforms. The extracted oil is then transported to an oil refinery through pipelines. Crude oil is a mixture of hydrocarbons, solids, and gaseous hydrocarbons.
The refining process transforms crude oil into different petroleum products, which are then converted into useful chemicals, including monomers, which are the basic building blocks of polymers. During the refining process, crude oil is heated in a furnace and sent to a distillation unit, where heavy crude oil separates into lighter components called fractions. One of these fractions, naphtha, is crucial for making a large amount of plastic.
Naphtha is further decomposed thermally and separated into ethylene and propylene, which are the raw materials for plastics. This process of separating naphtha is called steam cracking, which breaks down saturated hydrocarbons into smaller, often unsaturated, hydrocarbons. Steam cracking is a petrochemical process that uses high temperatures and pressures to break down hydrocarbons without a catalyst. In steam cracking, a gaseous or liquid hydrocarbon feed like naphtha or ethane is mixed with very hot steam and heated in a furnace without oxygen. The reaction temperature is extremely high, reaching around 850°C. This high temperature causes the hydrocarbons to break up into smaller molecules such as small olefins and hydrogen.
Steam cracking is the principal industrial method for producing lighter alkenes (olefins), including ethene (ethylene) and propene (propylene). The presence of steam is vital for processing heavy feeds to achieve better feed dispersion and more effective catalytic cracking. The use of steam cracking to produce ethylene from naphtha is a commonly accepted technology. However, one of the major contaminants in the ethylene product stream is acetylene, which poisons catalysts used in downstream polymerization processes. Acetylene is explosive above 200 kPa (29 psi). To address this, the C2 stream containing acetylene is partially hydrogenated before proceeding to a C2 splitter, where the product ethylene is taken from the overhead of the tower. The ethane from the bottom of the splitter is recycled to the furnaces to be cracked again.
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Polymerisation forms long molecular chains, or polymers
Plastic is derived from crude oil, natural gas, and coal. Crude oil is extracted from the Earth and transported to an oil refinery. The refining process transforms crude oil into different petroleum products, which are then converted into useful chemicals, including monomers. Monomers are the basic building blocks of polymers.
Polymerisation is a chemical process that results in the formation of polymers. It involves reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. The monomers are combined to form larger molecules, which are known as macromolecules or polymers. These polymers can be linear, branched, or cross-linked. Linear polymers are made up of one long continuous chain, without any excess appendages or attachments. Branched polymers have a chain structure consisting of one main chain of molecules with smaller molecular chains branching from it. Cross-linking occurs when primary valence bonds are formed between separate polymer chain molecules.
There are two main classes of polymerisation reaction mechanisms: step-growth and chain-growth. In step-growth polymerisation, pairs of reactants of any length combine at each step to form a longer polymer molecule. The average molar mass increases slowly, and long chains only form late in the reaction. An example of step-growth polymerisation is condensation polymerisation, where a small molecule, such as water, is lost when the polymer chain is lengthened.
In chain-growth polymerisation, the only chain-extension reaction step is the addition of a monomer to a growing chain with an active centre, such as a free radical, cation, or anion. Once the growth of a chain is initiated, chain propagation is usually rapid by adding a sequence of monomers. Chain-growth polymerisation is involved in the manufacture of polymers such as polyethylene, polypropylene, polyvinyl chloride (PVC), and acrylate.
The manner in which polymerisation is conducted is a highly evolved technology. Methods include emulsion polymerisation, solution polymerisation, suspension polymerisation, and precipitation polymerisation.
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Frequently asked questions
Yes, synthetic plastics are derived from refined crude oil, natural gas or coal.
Crude oil is heated in a furnace and sent to a distillation unit where it separates into lighter components called fractions. One of these, called naphtha, is the crucial compound to make a large amount of plastic.
Naphtha is composed of many different hydrocarbons. Two of those compounds, ethane and propene, are the critical components of synthetic plastics.
The next step in the process is called polymerisation, in which simple molecules like ethylene and propylene are chemically bonded into chains. These new, long molecular chains are called polymers.
As of 2019, 9 million barrels of oil are used to make plastic every day worldwide.
