U3S2 fractional distillation and cracking

Question 1

How is fractional distillation carried out

Answer 1

• crude oil vaporised at 350 degrees
• vaporised crude oil goes into bottom of fractionating column and rises through trays
• as vapour goes up, it gets cooler creating a negative temperature gradient

Question 2

How are different fractions created

Answer 2

• largest hydrocarbons don’t vaporise as boiling points too high so form gooey residue
• smallest hydrocarbons don’t condense so drawn out of top as gases
• hydrocarbons in middle condense at their boiling point and are drawn out at different levels

Question 3

Why is cracking carried out

Answer 3

• light fractions are in high demand
• breaks heavier fractions into lighter fractions

Question 4

Thermal cracking

Answer 4

• temp up to 1000 degrees and high pressure
• produces lots of alkenes

Question 5

Catalytic cracking

Answer 5

• uses a zeolite catalyst
• slight pressure and temperature
• produces aromatic hydrocarbons and alkanes