Crude oil is a mixture of different hydrocarbons each with a different boiling point. These substances are separated from each other in a distillation tower.
This results in the separation of heavy crude oil into lighter groups called fractions. Each fraction is a mixture of hydrocarbon chains (chemical compounds made up of carbon and hydrogen), which differ in terms of the size and structure of their molecules.
How Stuff Works puts it like this
Hydrocarbons are molecules that contain hydrogen and carbon and come in various lengths and structures, from straight chains to branching chains to rings.
There are two things that make hydrocarbons exciting to chemists:
Hydrocarbons contain a lot of energy. Many of the things derived from crude oil like gasoline, diesel fuel, paraffin wax and so on take advantage of this energy.
Hydrocarbons can take on many different forms. The smallest hydrocarbon is methane (CH4), which is a gas that is a lighter than air. Longer chains with 5 or more carbons are liquids. Very long chains are solids like wax or tar. By chemically cross-linking hydrocarbon chains you can get everything from synthetic rubber to nylon to the plastic in tupperware. Hydrocarbon chains are very versatile!
Find out more about hydrocarbons here.
From crude oil you can distill a whole load of products including;
Naptha a feedstock for plastic
How much in a barrel?
Oil is sold between countries in quantities called barrels.
One barrel of oil is 42 US gallons 159 litres or 35 gallons or 280 pints
The weight of a barrel depends on where the oil comes from. However, there are about 8 barrels in a tonne
A barrel of crude oil can make about
- 7.27 gallons (27.5 liters): Other products (feedstocks for petrochemical plants, asphalt, bitumen, tar, etc.)
- 1.72 gallons (6.5 liters): Liquefied Petroleum Gases (LPG)
- 3.82 gallons (14.5 liters): Jet Fuel
- 1.76 gallons (6.6 liters): Heavy Fuel Oil (Residual)
- 1.75 gallons (6.6 liters): Other Distillates (Heating Oil)
- 9.21 gallons (35 liters): Diesel
- 19.15 gallons (72.5 liters): Gasoline
- Approximate figures because every barrel of crude is different.
- A flight from San Francisco to Tokyo may take about 9,000 US gallons of jet fuel which requires about 2,250 barrels of crude oil to extract. From Econtrader
Definitions of naptha vary but
the fraction that boils between 27 °C and 93 °C (5 – 7 C atoms) is often called light naphtha.
the fraction that boils between 93 °C and 177 °C (6 – 10 C atoms) is heavy naphtha.
Crude oils from different sources contain different percentages of naphtha.
Naptha cannot be refined into gasoline or motor oil.
Naptha is the plastic feedstock of choice for many but in the US, most plastic is made from natural gas.
Google says you can get anything from 27 to 54L of naptha from 1 barrel of crude.
Naptha to Plastic
Cracking & Polymerisation
Hydrocarbon chains can be further refined by cracking and polymerising.
Very basically cracking breaks the existing chains and polymerisation is remixing them into something new. You can read more about it here.
Ethane and Propane are derived from Naptha
Using high-temperature furnaces
Ethane is cracked into ethylene
Propane is cracked into propylene,
Using a catalyst, a reactor and some heat these are now remade into plastic polymers
Ethylene becomes polyethylene also called polythene, the world’s most widely used plastic,
Propylene joins together to create polymers called polypropylene.
Most of the plastics we use are derived from polyethylene and polypropylene
Polypropylene and polyethylene were discovered in 1951 by two chemists working for Phillips Petroleum Company.
There are enough petrochemicals in one barrel of oil to make one of the following
- 39 polyester shirts
- 750 pocket combs
- 540 toothbrushes
- 65 plastic dustpans
- 23 hula hoops
- 65 plastic drinking cups
- 195 one-cup measuring cups
- 11 plastic telephone housings
- 135 four-inch rubber balls
Processing can include the addition of plasticizers, dyes and flame-retardant chemicals – see additives….
The polymers are now melted, cooled then cut into small pellets called nurdles.
These pellets are now shipped to manufacturers who make plastic products by using processes such as extrusion, injection molding, blow molding, etc.
Qualities & Biodegradability
These plastics are chemically inert and will not react chemically with other substances which makes them very useful. It also means that they do not break down chemically so do not biodegrade. This has a huge environmental impact as plastic trash lasts forever. See plastic lifespan.
These plastics can be recycled
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