Petroleum Flashcards
What is crude oil?
Mostly alkanes- they range from smallish alkanes, like propane, to massive alkanes with more than 50 carbons
How does fractional distillation work?
The crude oil is vaporised at about 350 degrees C.
Vaporised -> fractionating column.
Largest hydrocarbons don’t vaporise at all, because their boiling points are too high- they just run to the bottom.
As it rises it gets cooler, because of the different chain lengths each fraction condenses at a different temperature. The fractions are drawn off at different levels in the column.
The hydrocarbons with the lowest boiling points don’t condense they’re drawn off as gases at the top of the column.
What is a petrochemical?
Any compound that is made from crude oil or any of its fractions and is not a fuel.
What is cracking?
Breaking long chain alkanes into smaller hydrocarbons that are more useful by breaking the C-C bonds.
Explain the process of catalytic cracking.
Heavier fractions are passed over a catalyst at a high temperature and a moderate pressure.
This breaks them up into smaller molecules.
Using a catalyst cuts costs, because the reaction can be done at a lower temperature and pressure and it also speeds up the reaction.
What is isomerisation?- straight chain to branched chain.
Occurs when you heat straight chain alkanes with a catalyst stuck on inert aluminium oxide.
The alkanes break up and join back together as branched isomers.
A molecular sieve (zeolite) is used to separate the isomers.
Straight chain molecules go through the sieve and are recycled.
What is reforming?- straight chain to cyclic.
Reforming converts alkanes into cyclic hydrocarbons.
It uses a catalyst made of platinum and another metal. You need to stick the catalyst on inert aluminium oxide.
How does a petrol engine work?
- The fuel/air mixture is squashed by a piston and ignited with a spark.
- This drives the piston up again, turning the crankshaft.
- Multiple pistons (often four) work one after the other, so that the engine runs smoothly
What is auto igniting?
Straight chain alkanes in petrol tend to auto ignite which means they ignite by themselves- when the fuel/air mixture is compressed they explode without being ignited by the spark.
This extra explosion causes knocking in the engine and can damage the pistons.
What does the octane rating of a petrol tell you?
How likely it is to auto ignite.
The higher the number, the less likely it is to auto ignite
What conditions are required for catalytic cracking?
A catalyst, high temperature and moderate pressure
Why do we crack heavier petroleum fractions?
There is more demand for lighter petroleum fractions
Hexane has a very low octane rating, what does this tell you about it?
It easily auto ignites in a petrol engine and doesn’t combust very efficiently.
How could the octane rating of a fuel containing hexane be increased?
By adding in hydrocarbons with higher octane ratings, e.g shorter branched chain alkanes, cycloalkanes or arenes.
Why is a high octane rating beneficial?
High octane fuels can be burnt more efficiently than low octane fuels because they are less likely to auto ignite.
