DF5 : Catalysis and Cracking

Question 1

Explain two problems with fractions from fractional distillation(4)

Answer 1

• “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

Question 2

How does cracking solve the problem for the demand of petrol?(2)

Answer 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

Question 3

Name 4 types of cracking reactions

Answer 3

• alkanes —-> branched alkanes + branched alkenes
• alkanes —-> smaller alkenes + cycloalkanes
• cycloalkanes —-> alkenes + branched alkenes
• alkenes —–> smaller alkenes

Question 4

Are the products of cracking predictable?

Answer 4

No, they are random, with the same molecule giving different cracking products

Question 5

Without a catalyst, what would the cracking process need?

Answer 5

It would require extremely high temperatures and pressures and would take far longer, which would be more expensive

Question 6

What, in short terms, occurs in the catalytic cracking process?

Answer 6

A hydrocarbon vapour is passed over a heated solid catalyst

Question 7

What temperature does catalytic cracking take place at?

Answer 7

450 - 500 degrees

Question 8

What catalyst is used normally in catalytic cracking?

Answer 8

Zeolite/platinum mesh

Question 9

Where does cracking take place?

Answer 9

In a riser reactor: a 60m high vertical tube around 2m in diameter

Question 10

What is the mixture in the riser reactor like?

Answer 10

A moving fluidised bed where solid particles flow like a liquid

Question 11

Explain the process of catalyctic cracking in detail (5)

Answer 11

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

Question 12

What does the mixture only taking 2 seconds to flow from the bottom to the top of the tube mean?

Answer 12

The hydrocarbons are in contact with the catalyst for only a very short period of time

Question 13

Why does the coke need to be burnt off in the regenerator?

Answer 13

Coke forms on the catalyst surface during cracking making it eventually go inactive and unable to catalyse reactions

Question 14

What does energy released from burning the coke do?

Answer 14

Heats up the catalyst, this energy is then transferred to the feedstock so that the cracking can occur without additional heating

Question 15

Define “catalyst”

Answer 15

A substance that speeds up a chemical reaction but can be recovered chemically

Question 16

Define “catalysis”

Answer 16

The process of speeding up a chemical reaction using a catalyst

Question 17

Do catalysts undergo any permanent chemical change during a reaction?

Answer 17

No

Question 18

Do catalysts change physically?

Answer 18

Yes, they crumble/become roughened

Question 19

What does catalysts changing physically suggest?

Answer 19

That they take part somewhat in the reaction, although they can be regenerated

Question 20

How much of a catalyst is usually required?

Answer 20

Only small amounts

Question 21

What do catalysts affect in the reaction?

Answer 21

Its rate, not the amount of product formed

Question 22

Do catalysts appear as reactants in the overall equation?

Answer 22

No

Question 23

Define “homogeneous catalysis”

Answer 23

Catalysis involving a reactant and catalyst in the same physical state

Question 24

Give one example of homogeneous catalysis

Answer 24

Enzyme catalysed reactions in cells take part in an aqueous solution

Question 25

Define “heterogeneous catalysis”

Answer 25

Catalysis where the catalysts and products are in different physical states

Question 26

What does heterogeneous catalysis usually involve?

Answer 26

A mixture of gases/liquids in the presence of a solid catalyst, providing a surface for the reaction to take part on