Engineering Chemistry S Datta

394 ENGINEERING CHEMISTRY
Flue gases
Cyclone
Cracked
vapour
Light
fraction
Cooler
Gases
Catalyst
regenerator
600°C
500°C
Gases
Feed
oil
Regenerated
catalyst
Catalyst
+oil
Reactor
Air + spent catalyst
Spent
catalyst
Fraction
matching
column
Air
Heavy
oil
Gasoline
+gas
Stabilizer
Gasoline
Blower
Fig. 18.8 Moving-bed type catalytic cracking.
Advantages of catalytic cracking:
(i) The quality and yield of petrol is better.
(ii) External fuel is not necessary since the coal embedded in the catalyst supplies the
heat.
(iii) Operating pressure is lower.
(iv) Byproduct gas evolution being low, yield of petrol is higher.
(v) Due to higher aromatics content, the anti-knocking properties are higher.
(vi) Greater portion of S escapes as H 2
S so residual S content of the oil is low.
(vii) Gum-forming compounds are very low.
(viii) In presence of specific catalysts preferentially the cracking of naphthenic materials
takes place, so it becomes richer in paraffinic compounds.
(ix) Only the high-boiling hydrocarbons and the side chain of the aromatics are decomposed
preferentially.
Highlights:
• Gasoline is the fuel for motor engines.
• Gasoline contains alkanes C 5
—C 10
atoms.
• Enough gasoline is not produced during purified petroleum distillation.
• Oil refineries use catalytic cracking to make gasoline.
• For smooth running of the motors, gasoline has to burn smoothly without knocking.
• The octane number scale was devised by Thomas Midgley (1889-1944) and he
discovered anti-knocking additives based on lead i.e., T.E.L. (Tetra Ethyl Lead)
• Leaded fuel is now avoided and the oil companies produce high octane fuel by
increasing the proportions of both branched alkanes and arenes and blending some
oxygen compounds.
(contd.)