Cracking is the process in which large molecules are broken into smaller molecules.
C10H22 C5H12 + C5H10
C8H18 C5H12 + C3H6
The Cracking process can be used in making gasoline from heavy oil. There are two methods of cracking.
1. Thermal cracking.
2. Catalytic cracking.
When higher hydrocarbon molecules are subjected to high temperature and pressure it decomposes to form smaller molecules. It can be of two types.
(a) Liquid phase thermal cracking: this is carried out at a temperature at 470 - 5300C and high pressure of 100 kg/cm2. The yield of petrol by this method from heavy oil is about 60%.
(b) Vapour phase thermal cracking: it is carried out at a temperature about 6000C and a low pressure about 10-20 kg/cm2. The yields of gasoline is about 50-60%. But it is of poor stability. The petrol may have better octane number.
Here the cracking is carried out in presence of catalysts. The use of catalyst increases the rate of cracking at lower temperature & pressure. The yield of gasoline is higher. Better quality gasoline of higher octane number is produced. Harmful sulphur is removed as H2S.
The catalysts generally uses are alumina, aluminium chloride, zirconium oxide, hydrated aluminium silicate etc.
Two methods used for catalytic cracking are
1. Fixed bed catalytic cracking
2. Moving bed (fluid-bed) catalytic cracking.
(i) Fixed-bed catalytic cracking
The heavy oil is preheated to 425-4500C and then forced to catalytic chamber. Artifical clay mixed with zirconium oxide is used as catalyst which is fixed in catalyst towers. Catalyst chamber is maintained at a temperature of 425-4500C and a pressure of 1.5 kg/cm2. Cracking take place here. The carbon formed is adsorbed on catalyst bed. The catalyst bed when exhausted due to deposition of carbon. The deposited carbon may be burned off to reactivate the catalyst. The vapours produced on cracking are passed into fractionating column where the heavy oil condenses and is removed. The vapours are then passed into cooler where gasoline vapours along with some gases get condensed. The gasoline is then sent to a stabilizer where dissolved gases are removed and gasoline is recovered.
(ii) Moving bed (fluid-bed) catalytic cracking
In this process, the solid catalyst is powdered to behave almost like a fluid and mixed with heavy oil. It is sent to reactor at 5000C. Cracking takes place here. The carbon gets de posited over catalyst particles becomes heavy and settle down. These are then sent to regenerator where they are heated to 6000C. The fuel gas formed is removed and catalyst retained is brought back to the reactor.