Abstract
Hydrocarbons are used to manufacture petrochemicals, including ammonia, methanol, ethylene, propylene, benzene, toluene, and xylene (BTX). About 14% of the world’s oil production and 8% of the world’s gas production are used to make petrochemicals. Compounds from oil and gas serve two purposes: they are the feedstock used to create the chemical products, and they provide energy needed for the transformation from feedstock to petrochemicals. Coal can also be used as a feedstock.
Ammonia is widely used as an agricultural fertilizer. Methanol is transformed into many other products such as plastics, plywood, paints, explosives, and textiles. Ethylene and propylene are polymerized into polyethylene and polypropylene. The former is used to make plastic bags and plastic pipes. The latter is used to make plastic containers, furniture, and clothing. One xylene isomer, paraxylene, is the feedstock for making polyethylene terephthalate (PET) which is widely used to make plastic bottles for drinks. The triangular recycling symbols with a number in the center refer to various types of plastic.
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Notes
- 1.
This is achieved with two sequential chemical reactions. The formula for the steam-methane reforming reaction is CH4 + H2O = 3 H2 + CO. The formula for the water-gas shift reaction is CO + H2O = CO2 + H2.
- 2.
In the UK, people refer to plastic bags in trees fluttering with the wind as “witch’s britches.”
- 3.
Source: IEA (2018). The Future of Petrochemicals. All rights reserved.
- 4.
- 5.
The method for creating the free radicals is beyond the scope of this book.
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Appendices
Appendix: Polymerization
Polyethylene (PE) is the most common plastic in use today. You probably use polyethylene every day. You use it if you get a plastic bag. PE is made from ethylene which has two carbon atoms and is a lightweight gas. By contrast, PE has hundreds or thousands of carbon atoms and is a solid. This appendix gives an introduction to polymerization: the process of combining monomers, like ethylene, to produce polymers like polyethylene.
One method to achieve polymerization is to convert some of the monomers into “free radicals.”Footnote 5 A chemical bond has two electrons. A free radical is a molecule that had a double bond, but has lost one of its electrons. They are very reactive. If a free radical encounters another monomer, they combine and continue to be a free radical. If an ethylene monomer with two carbon atoms becomes a free radical and encounters another ethylene monomer, then they combine to form another highly reactive free radical with four carbon atoms. If it encounters yet another ethylene monomer, then they combine to form another highly reactive free radical with six carbon atoms.
If one free radical encounters another free radical, then they combine, but the combined compound is no longer a free radical, and is no longer highly reactive. It may encounter another free radical if they are still present in the mixture. In this case they will combine and cease being a free radical. This process continues until all of the free radicals have paired up with other free radicals.
Due to the intrinsic randomness of interactions between the molecules, the resulting polymer at the end of the polymerization reaction is a mixture of molecules with different numbers of carbon atoms. Furthermore, there will be many different geometries for each polymer molecule. Some will have several branches, as well as branches with other branches. Others will have fewer branches.
So there is a distribution of the number of carbon atoms in the molecules in the final polymer mixture. The distribution can be affected by the number of free radicals that are introduced into the monomer mixture initially. The nature of the distribution, as well as the degree to which the polymer is branched or more of a straight chain (see Sect. 2.5.1), determines the mechanical properties of the polymer, such as its density and its mechanical strength. Differences in these properties are reflected in the recycling codes of Fig. 8.1. LDPE is low-density polyethylene, and HDPE is high-density polyethylene. Mechanical properties can also be affected by using a mixture of different monomers, such as butadiene and styrene. The polymerization reaction will be similar to that described above, but there will be a random mixture of the monomers in the polymer.
Mechanical properties can also be adjusted by including additives in the polymer. Polymers are a fascinating topic. Perhaps you will be inspired to study them further in another course.
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8.1.1 Figures
8.1.2 Table
Number | Source |
|---|---|
8.1 | IEA (2018). The Future of Petrochemicals. All rights reserved |
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Cronshaw, M. (2021). Petrochemicals. In: Energy in Perspective. Springer, Cham. https://doi.org/10.1007/978-3-030-63541-1_8
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