3.3.2.1 Fractional Distillation And Cracking

Question 1

Define a petroleum fraction

Answer 1

A mixture of hydrocarbons with similar chain length and boiling point range

Question 2

Describe the process of fractional distillation

Answer 2

1) Crude oil vapourised then passed into a column
2) Fractions i.e. groups of specific chain lengths of alkanes condense at different heights of the column because…
3) The BPs of these fractions decrease, the shorter the length of the alkane chain
4) And the temperature gradient up the column is negative
5) So shorter fractions condense higher up the column and bigger fractions condense at the bottom of the column

Question 3

From “ground” up, list the uses of the fractions siphoned off of a fractioning column (8)

Answer 3

1) Bitumen
2) Lubricating oils
3) Fuel oil
4) Diesel oil
5) Kerosene
6) Naptha
7) Petrol/gasoline
8) Fuel gas

Question 4

Define cracking?

Answer 4

Conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of C-C bonds

Question 5

Lost the economic benefits of cracking (3)

Answer 5

1) Shorter C chains are in larger demand
2) Save (i.e. not waste) excess larger hydrocarbons
3) Products of cracking are more valuable

Question 6

What are the conditions required for thermal cracking?

Answer 6

1) High pressure: 7000 kPa

2) High temp: 400 - 900 degrees

Question 7

What are the products of thermal cracking?

Answer 7

1) Alkenes

2) Hydrogen

Question 8

What are the conditions required for catalytic cracking?

Answer 8

1) Slight pressure
2) High temperature
3) Zeolite catalyst

Question 9

What are the products of catalytic cracking?

Answer 9

1) Fuels

2) Aromatic hydrocarbons

Question 10

Why do branched and cyclic hydrocarbons improve motor fuels?

Answer 10

1) Make them burn cleaner

2) Give them higher octane numbers

Question 11

Why is catalytic cracking less expensive than thermal cracking?

Answer 11

1) Saves energy: lower temps needed

2) Lower pressure