What Is Cracking
Cracking (when applied to crude oil) means splitting up long hydrocarbons to make them smaller. There's only really one reason why we do this and that is because of demand. Demand for some hydrocarbons are far higher than that of others.
Diesel is a long hydrocarbon which is used by a lot of cars. But more cars use petrol. So we crack diesel (makes it a smaller hydrocarbon) and make petrol and another substance/substances.
What Does Cracking Involve
Cracking is basically the thermal decomposition of crude oil (breaking down the molecules down by heating the ).
There are three steps involved in cracking a large hydrocarbon compound. They are:
1) Heat the crude oil until in vaporises (evaporates) and turns into a vapour (gas).
2) Pass the vapour over a hot catalyst (commonly used in 'Aluminium Oxide') at the temperature varying between 400 - 700 degrees celsius (the temperate varies depending on how long the hydrocarbon is).
3) The long-chain molecule splits apart (cracks) giving you the desired product you want. All you do then is separate the two products by fractional distillation.
Most hydrocarbons you crack are Alkanes (a saturated hydrocarbon) or Alkenes (an unsaturated hydrocarbon).
Alkenes : Alkenes are unsaturated hydrocarbons, this means it contains a double bond between two carbon atoms and doesn't haven't the maximum amount of hydrogens it could have.
Alkanes : A saturated hydrocarbon with the full amount of hydrogen atoms connected to the carbon atoms.
Alkenes are hydrocarbons with a double bond between two of the carbon atoms in their chain. They are often referred to as un-saturated because they could fit more hydrogen atoms if the double bond slit often and attached to more hydrogen atoms.
The first three Alkenes are:
Ethene - C2H4
Propene - C3H6
As you can see, there is no hydrocarbon with one carbon atom due to the fact if their is two carbon atoms there can't be a double bond making it methane not methene (which doesn't exist).
Ethene can be reacted with water (hydrated) with the presence of a catalyst creating ethanol. At the moment this process is very common due to the fact that demand for ethanol isn't high enough for excess needed and there is plenty of ethene available at the moment and it's firstly cheap with barely any of the ethene being wasted. Unfortunately, being a product of crude oil, it is a non-renewable resource and will eventually run out.
Ethanol can be produced through a renewable,e resource though! The process is called fermentation. Fermentation requires sugar which is placed in an environment full of yeast which will make this process happen:- sugar ➡️ carbon dioxide + ethanol.
This process requires less scientific equipment and doesn't need to be done in a extremely hot temperatures. An advantage is that because sugar is a major resource for the whole world, it is grown in many countries as a mass production. Another advantage is that it can be used as a cheap fuel fro countries which don't have oil reserves.
An disadvantage is that the concentration if the alcohol isn't very high (about 15%) and if you want to make it stronger you have to distill it.
The most useful and productive thing done to hydrocarbon compounds is something called polymerisation. Polymerisation requires joining hydrocarbon compounds (called monomers) to make long chains of the hydrocarbon making it a polymer.
An example :- Joining large amounts of ethene molecules creates polyethene (mostly referred as polythene which makes most plastic bags found it shops).
A polymers properties depends on not just what it is but also the pressure and heat the polymerisation happened at :
- Poly(ethene) made at 200 degrees celsius and at 2000 atmospheres pressure is flexible and has a low density.
- Poly(ethene) made at 60 degrees celsius and at a few atmospheres pressure with a catalyst is rigid and dense.
Most polymers are cheap and have the ability to be rot (decay) resistant which simp good and bad at the same time. Because most polymers aren't biodegradable they won't be broke down and rot. This makes them difficult to get rid of because they will last forever in a landfill. We combat this by using a bag as many times as possible before trying to recycle it. Recycling is slowly improving but not fast enough!