Cracking

Question 1

What does catalytic cracking use?

Answer 1

• lower temperatures - 720K
• uses zeolites as catalysts
• these are complex aluminoailicates - which are large lattices of aluminium, silicon and oxygen atoms, carrying a negative charge

Question 2

Why are zeolites in catalytic cracking used?

Answer 2

• zeolites used in catalytic cracking are chosen to give high percentages of hydrocarbons - particularly useful for petrol
• also produces high proportions of branches alkanes and aromatic hydrocarbons like benzene

Question 3

What is used in thermal cracking?

Answer 3

• high temperatures (450-750 degrees celsius) and pressures (about 70 atmospheres)
• are used to break the large hydrocarbons into smaller ones

Question 4

Why do we use thermal cracking? What does it produce?

Answer 4

• thermal cracking gives mixtures of products containing high proportions of hydrocarbons
• and double bonds - alkenes

Produces alkanes and alkenes

Question 5

What does steam cracking use?

Answer 5

• uses naptha or sometimes gas oil fractionation as the feedstock
• as well as more simple hydrocarbons like ethane, propane or butane
• these are mixed with steam and passed through a reactor heated to about 800 - 900 degrees celsius - pressure of mixture is about 1 atmosphere

Question 6

Why is steam cracking used?

Answer 6

• it is useful as it produces a high proportion of alkanes in the cracked mixture