To converts the heavy residue to lighter distillate.
To decreases the boiling point, pour point and Viscosity.
To increases the octane number.
To increases the unsaturation and aromatization.
Effect of temperature and pressure on cracking:
Pressure retards cracking reactions. But in practice a positive pressure of 10 to 15 Kgs/cm2is used to minimize coke formation. Recycling increases refractory nature of stocks and hence recycling should not exceed 2 to 3 times of fresh stock for economic operation.
Increase in cracking time and temperature increases the lighter fraction and decreases the fuel oil yield.
Pressure has negligible effect on velocity of reaction. At high pr. gas / gasoline ratio is less
Increases in pressure also increase the coke production.
For high pressure more resistance time is required which results in formation of more hydrocarbons.
With higher pressures gas to gasoline ratio becomes less.Thermal cracking:
Disassociation of high molecular wt. Hydrocarbons into smaller fragments through agency of heat alone is called Thermal cracking.
It is an endothermic and gas phase reaction and here all feed stock vaporized.
Thermal cracking is currently used to "upgrade" very heavy fractions to produce light fractions or distillates, burner fuel and/or petroleum coke.
In this process long chain hydrocarbons broken to smaller fragment one is
saturated and other is unsaturated and also formation of coke and hydrogen.
Process variable: feed stock property, temperature, pressure and resistance
