Engineering Chemistry S Datta

FUELS AND COMBUSTION 393
CH 3
(CH 2
) 8
CH 2
⎯⎯→ CH 3
(CH 2
) 4
CH 3
+ CH 3
CH 2
CH = CH 2
n-decane n-hexane butene-1
The olefins formed isomerizes and polymerizes.
CH3
CH3
⏐ ⏐
2CH 3
CH 2
CH = CH 2
CH 3
—CH 2
— C— C=CH
⏐
CH
This cracking taking place at 475°C–530°C temperature leads to liquid phase thermal
cracking. The products are separated by fractional distillation. When the cracking oil is vaporized
and then cracked at 600°C–750°C at a low pressure, it is called vapour phase thermal cracking.
The products have better anti-knock properties.
2. Catalytic Cracking. The use of catalyst during cracking accelerates the reactions
and at the same time modifies the yield and the nature of product. Catalyst used are synthetic
composition of silica and alumina, zeolites in the form of beads or pellets.
There are two main types of catalytic cracking:
(a) Fixed-bed catalytic cracking: The catalysts are fixed in towers, through which the
hot oil (500°C) flows from the top and passes down. 40% of the charge is converted to gasoline
and 2–4% carbon is formed. This carbon deposits on the catalyst beds. The bottom liquid is
reboiled and recycled to the fractionating column and ultimately gas oil is obtained having
high octane value. The gasoline is stripped off dissolved gases and purified. The carbon deposits
on the catalysts are burnt by compressed air in one chamber for reactivation while the other
catalyst chambers are active (Fig. 18.7).
3
2
Catalyst
Cracked
vapour
Cooler
Stabilizer
Gases
Vapour
Heavy
oil charge
Pre-heater
(425°C –
450°C)
Heater
Catalyst
chamber
(425°C –
450°C)
Fractionating
column
Heavy
oil
Gasoline
Gasoline
+
some dissolved gases
Fig. 18.7 Fixed-bed catalytic cracking.
(b) Moving bed or fluidized bed catalytic cracking: The finely powdered catalyst behaves
as a fluid when suspended in gas or oil vapour. The preheated heavy oil is forced through
tower along with the fluidised catalyst. At the top of the tower a cyclone separator is active to
separate the cracked oil vapour and passes it to the fractionating column. The catalyst powder
is retained and sent back. The catalyst becomes deactivated by a deposition, which is reactivated
by burning away the deposits with compressed air in a regenerator (Fig. 18.8).