DF5 : Catalysis and Cracking Flashcards
Explain two problems with fractions from fractional distillation(4)
- “Straight run” -fractions from primary distillation makes for poor petrol and need to be treated further
- Supply and demand - crude oil contains a surplus of the high boiling fractions e.g gas oil and not enough of the lower boiling fractions e.g gasoline for the demand
How does cracking solve the problem for the demand of petrol?(2)
- Alkanes with larger molecules too large for use in petrol are broken down into alkanes with shorter chains that can be used
- Alkanes are also broken down into cycloalkanes, arenes and alkenes which are blended to produce high grade petrol
Name 4 types of cracking reactions
- alkanes —-> branched alkanes + branched alkenes
- alkanes —-> smaller alkenes + cycloalkanes
- cycloalkanes —-> alkenes + branched alkenes
- alkenes —–> smaller alkenes
Are the products of cracking predictable?
No, they are random, with the same molecule giving different cracking products
Without a catalyst, what would the cracking process need?
It would require extremely high temperatures and pressures and would take far longer, which would be more expensive
What, in short terms, occurs in the catalytic cracking process?
A hydrocarbon vapour is passed over a heated solid catalyst
What temperature does catalytic cracking take place at?
450 - 500 degrees
What catalyst is used normally in catalytic cracking?
Zeolite/platinum mesh
Where does cracking take place?
In a riser reactor: a 60m high vertical tube around 2m in diameter
What is the mixture in the riser reactor like?
A moving fluidised bed where solid particles flow like a liquid
Explain the process of catalyctic cracking in detail (5)
1) The hot vapourised hydrocarbons and the zeolite catalyst are fed into the bottom of the tube and forced upwards by the stream
2) The mixture takes 2 seconds to flow from the bottom to the top of the tube
3) After the riser reactor, the mixture passes into a separator where the steam carries away cracking products leaving behind the solid catalyst
4) The catalyst goes into the regenerator where it takes 10 minutes for coke to burn off in the hot air blown into the regenerator
5) The catalyst is then re-introduced into the base of the riser reactor, ready to repeat the cycle
What does the mixture only taking 2 seconds to flow from the bottom to the top of the tube mean?
The hydrocarbons are in contact with the catalyst for only a very short period of time
Why does the coke need to be burnt off in the regenerator?
Coke forms on the catalyst surface during cracking making it eventually go inactive and unable to catalyse reactions
What does energy released from burning the coke do?
Heats up the catalyst, this energy is then transferred to the feedstock so that the cracking can occur without additional heating
Define “catalyst”
A substance that speeds up a chemical reaction but can be recovered chemically
Define “catalysis”
The process of speeding up a chemical reaction using a catalyst
Do catalysts undergo any permanent chemical change during a reaction?
No
Do catalysts change physically?
Yes, they crumble/become roughened
What does catalysts changing physically suggest?
That they take part somewhat in the reaction, although they can be regenerated
How much of a catalyst is usually required?
Only small amounts
What do catalysts affect in the reaction?
Its rate, not the amount of product formed
Do catalysts appear as reactants in the overall equation?
No
Define “homogeneous catalysis”
Catalysis involving a reactant and catalyst in the same physical state
Give one example of homogeneous catalysis
Enzyme catalysed reactions in cells take part in an aqueous solution
Define “heterogeneous catalysis”
Catalysis where the catalysts and products are in different physical states
What does heterogeneous catalysis usually involve?
A mixture of gases/liquids in the presence of a solid catalyst, providing a surface for the reaction to take part on
