Alexei Volkov
Plastic is derived from natural, organic materials such as cellulose, coal, natural gas, salt, and crude oil. Crude oil is a complex mixture of thousands of compounds and needs to be processed before it can be used to make plastic. The first step in this process is to heat the oil in a furnace and then feed the resulting vapour into a fractional distillation tower, which separates the mixture into different compartments called fractions. These fractions are then used to create an \alien\ material unfamiliar to microbes in Earth's water and soil, which can be used to make plastic.
| Characteristics | Values |
|---|---|
| Main ingredient | Crude oil and natural gas derivatives |
| Other ingredients | Natural gas, coal, cellulose, salt |
| Process | Polymerisation or polycondensation |
| First step | Breaking hydrocarbons into monomers |
| Cracking | Turning any hydrocarbon into another hydrocarbon |
| Reforming | Turning heavier molecules into lighter molecules |
| Naphtha | A crucial compound for plastic production |
| Polymers | Long molecular chains |
| Additives | Toughness, flexibility, elasticity, colour, hygiene |
| Types | Clear, cloudy, solid colour, flexible, rigid, soft |
| Biobased plastics | Made from renewable products like carbohydrates, fats and oils |
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What You'll Learn
Crude oil is heated and separated into fractions
Crude oil is a source of raw material for making plastics, but it is not the major source of feedstock for plastic production in the United States. Other sources include natural gas and feedstocks derived from natural gas and crude oil refining.
Crude oil is heated to around 600 degrees Celsius (1112 Fahrenheit) and it boils to form a vapour. This vapour enters the bottom of the fractional distillation column. The distillation column is filled with trays or plates that collect the liquids as the vapour condenses. The trays collect the various liquid fractions, which then follow pipework outside the column. The collected liquid fractions may pass to condensers, which cool them further, and then go to storage tanks or other areas for further chemical processing.
The process of separating crude oil into its constituent parts is known as fractional distillation. When heat is applied to the crude oil, its chemical compounds separate, causing the fractions of the mixture to vaporize and distil. The vapour rises through the vertical column, and as the gases rise, the temperature decreases. As the temperature decreases, certain hydrocarbons begin to condense and run off at different levels. Each fraction that condenses off at a certain level contains hydrocarbon molecules with a similar number of carbon atoms. These boiling point 'cuts' allow several hydrocarbons to be separated out in a single process.
The fractions can be divided into three categories: light, middle, and heavy fractions. Heavier components condense at higher temperatures and are removed at the bottom of the column. The lighter fractions are able to rise higher in the column before they are cooled to their condensing temperature, allowing them to be removed at slightly higher levels.
The light distillate is one of the more important fractions, and its products have boiling points around 70-200°C. Useful hydrocarbons in this range include gasoline, naphtha (a chemical feedstock), kerosene, jet fuel, and paraffin. These products are highly volatile, have small molecules, low boiling points, flow easily, and ignite easily. Medium distillates have boiling points of 200-350°C, and products in this range include diesel fuel and gas oil, which are used in the manufacturing of town gas and commercial heating. Fuel oil is another product of this process, with large molecules, low volatility, poor flow, and a low capacity for ignition.
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Hydrocarbons are broken down into monomers
Hydrocarbons are organic compounds consisting entirely of hydrogen and carbon. They are hydrophobic, usually odourless, and can be gases (such as methane and propane), liquids (such as hexane and benzene), low-melting solids (such as paraffin wax and naphthalene), or polymers (such as polyethylene and polystyrene). They are the basis of petroleum fuels and may be either linear or branched species. One or more of the hydrogen atoms in hydrocarbons can be replaced with other atoms, such as chlorine or another halogen, in a process called a substitution reaction. An example of this is the conversion of methane to chloroform.
Hydrocarbons are classified as saturated or unsaturated. Saturated hydrocarbons, also known as alkanes, are composed entirely of single bonds and are saturated with hydrogen. The formula for acyclic saturated hydrocarbons (i.e. alkanes) is CnH2n+2. The most general form of saturated hydrocarbons, whether linear or branched species, and whether with or without one or more rings, is CnH2n+2(1-r), where r is the number of rings. Those with exactly one ring are the cycloalkanes.
Unsaturated hydrocarbons are divided into alkenes and alkynes. Alkenes contain a carbon-carbon double bond, while alkynes contain a carbon-carbon triple bond. Aromatic hydrocarbons, which are a type of unsaturated hydrocarbon, are significantly more stable than their Lewis structures would suggest. They are classified as either arenes, which contain a benzene ring, or non-benzenoid aromatic hydrocarbons, which lack a benzene ring.
In the context of plastic production, hydrocarbons are isolated and broken down from different sources, including crude oil and natural gas, and then reconstituted into new formations. This process creates an "alien" material unfamiliar to microbes in Earth's water and soil, which cannot digest and convert it into water and carbon dioxide.
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Polymers are formed from hydrocarbons
Hydrocarbons are organic compounds composed of only carbon and hydrogen. The simplest organic compounds are hydrocarbons, which, despite being composed of only two types of atoms, exhibit a wide variety of structural forms. This variety arises from the ability of carbon atoms to form up to four strong bonds with other carbon atoms, resulting in chains, branches, and rings of various sizes, shapes, and complexities.
The process of polymerization involves converting monomers, which are simple molecules, into polymers, which are large, chain-like molecules. Monomers are the building blocks of polymers, and they combine to form dimers (two monomers), which can further combine to form trimers (three monomers), and so on, eventually yielding polymers. This process can be facilitated by catalysts, such as acids or peroxides, and it is known as addition polymerization. Another type of polymerization is condensation polymerization, where two or more different monomers combine and release a byproduct, such as water.
In the context of plastic production, hydrocarbons are isolated and broken down from crude oil, natural gas, or coal, and then reassembled into new structures to create synthetic plastics. This process, known as cracking, involves applying high temperatures of around 800 °C in the presence of steam to break down hydrocarbons into smaller hydrocarbons, such as ethylene, propylene, and butylene. These monomers are then linked through chemical polymerization to form polymers, which are thick, viscous substances used to create plastic products.
One example of hydrocarbon cracking is the conversion of decane into propylene, which can then be used to make poly(propylene). Another example is the conversion of crude oil or natural gas into naphtha, a mixture of volatile hydrocarbons, through distillation. Naphtha is then further processed to produce the building blocks for plastics.
It is important to note that not all plastics are derived from fossil fuels. Bioplastics, which make up a small percentage of plastics, are produced from renewable sources such as carbohydrates, fats, oils, and sugars from plants. These bio-based plastics offer an alternative to traditional fossil fuel-based plastics, contributing to a more sustainable future.
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Natural gas is used to make plastic
Natural gas is used as a raw material in plastic extrusion. It is a fossil fuel that undergoes a process called steam cracking to become plastic polymers. In this process, natural gas is heated to high temperatures, breaking apart or cracking molecules to form new ones. For example, ethane (C2H6) molecules lose two hydrogen molecules, forming a separate, stable hydrogen molecule (H2) and leaving molecules that are about 80% ethylene (C2H4). Ethylene is a gas and needs pressure and a catalyst to turn it into polyethylene, a resin. This process of converting ethylene into polyethylene is known as polymerization.
The availability of natural gas sources from shale has led to an increase in ethane and propane production, impacting the plastics industry significantly. The low cost of feedstock derived from unconventional sources has revitalized the petrochemicals industry in North America, attracting foreign investment and creating high-quality jobs.
Natural gas is not the only source of feedstock for plastics production. In the United States, the majority of Hydrocarbon Gas Liquids (HGLs) produced are byproducts of natural gas processing, while the rest come from crude oil/petroleum refineries. The HGLs from petroleum refineries contain alkanes and olefins, which can be used as feedstock for petrochemical crackers or direct inputs into plastics manufacturing.
Crude oil is another important raw material for plastic production. It undergoes an extensive refining process to create the chemicals used to make plastic. However, the process of refining crude oil specifically for plastics would leave much of the barrel unused, which could lead to environmental concerns. Therefore, the current practice is to refine crude oil for multiple purposes, including fuels and plastics.
In summary, natural gas plays a crucial role in plastic production, particularly through the process of steam cracking to create plastic polymers. The availability of natural gas sources has influenced the plastics industry, while crude oil also serves as a significant feedstock for plastics production through refining processes.
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Bioplastics are an alternative to oil-based plastics
Plastic is made by isolating hydrocarbons, breaking them down into their component parts, and then reconstituting these parts into entirely new formations. These hydrocarbons are sourced from crude oil and natural gas. As of 2019, 9 million barrels of oil are used to make plastics every day worldwide. This has led to a search for alternatives, such as bioplastics.
Bioplastics are made from biomass sources like plants, algae, and microorganisms. They are also derived from corn starch, sugar cane, and wood pulp. The sugar from these plants is extracted, dissolved, and combined with other materials to make plastics. For example, corn is submerged in sulfur dioxide and hot water, breaking the plant down into its individual components. The resulting materials then combine with citric acids and bind together, creating a polymer.
Bioplastics are generally compostable, decaying into natural materials that blend harmlessly with soil. Some bioplastics can break down in a matter of weeks, while others take longer. Unlike traditional plastics, bioplastics generally do not produce a net increase in carbon dioxide gas when they break down. This is because the plants used to make bioplastics had absorbed the same amount of carbon dioxide to begin with. One of the most widely used bioplastics, PLA, produces almost 70% less greenhouse gas when it degrades in landfills.
Bioplastics have the potential to be carbon-neutral and decrease dependence on fossil fuels. However, bioplastics are not without their limitations. They are more expensive to produce and are more fragile and vulnerable to heat and water. They also face challenges in terms of land use and environmental impact, as growing bioplastic feedstock can divert land from food production.
Despite these challenges, bioplastics are becoming increasingly popular due to high consumer demand for sustainable alternatives to traditional plastics. The global bioplastics market is estimated to reach $29.8 billion by 2030. While bioplastics may not be the final answer to sustainability issues in the plastics industry, they are a step in the right direction.
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Frequently asked questions
Plastic is made from crude oil, natural gas, or coal. Crude oil is a complex mixture of thousands of compounds and needs to be processed before it can be used.
Crude oil is heated in a furnace and fed as a vapour to a fractional distillation tower. The tower separates the mixture into different compartments called fractions. Each fraction is a mixture of hydrocarbon chains, which are then used to create polymers.
Polymers are long molecular chains that are formed by linking small molecules together. They are not yet plastic but are in the form of granules or powders. Polymers are then mixed with additives to give them properties such as toughness, flexibility, elasticity, and colour.
Bioplastics are made from renewable products such as carbohydrates, fats, and oils. They can also be made from the sugars in plants like corn, beets, or potatoes.
