6.1 Introduction
A critical observation of the overall refinery process block diagram indicates that the straight run gasoline (this is the gasoline obtained from the CDU) does not have good octane number (40 – 60) and needs to be upgraded to obtain the desired octane number (85 – 95).
Typically, cracking, reforming and isomerisation are regarded as the three most important processes that contribute towards upgradation of the octane number.
In this lecture, we present an overview of the cracking operation in the refinery.

Typically cracking involves the thermal or catalytic decomposition of petroleum fractions having huge quantities of higher molecular weight compounds. Since heat is required, typically cracking reactions are carried out in furnaces that are supplied with either fuel oil or fuel gas or natural gas or electricity as heat source. Cracking facilitates initiation, propagation and termination reactions amongst the hydrocarbon themselves. However, when steam cracking is carried out, in addition to the energy supplied by the direct contact of steam with the hydrocarbons, steam also takes part in the reaction to produce wider choices of hydrocarbon distribution along with the generation of H₂ and CO.

6.2 What is cracking?

• • Cracking involves the decomposition of heavier hydrocarbon feedstocks to lighter hydrocarbon feed stocks.

• Cracking can be carried out to any hydrocarbon feedstock but it is usually applied for vacuum gas oil (VGO)

• Cracking can be with or without a catalyst.

• When cracking is carried out without a catalyst higher operating temperatures and pressures are required. This is called as thermal cracking.  This was the principle of the old generation refineries.

• Now a days, cracking is usually carried out using a catalyst.  The catalyst enabled the reduction in operating pressure and temperature drastically.

6.3 Cracking chemistry

• • Long chain paraffins converted to olefins and olefins

• Straight chain paraffins converted to branched paraffins

• Alkylated aromatics converted to aromatics and paraffins

• Ring compounds converted to alkylated aromatics

• Dehydrogenation of naphthenes to aromatics and hydrogen

• Undesired reaction: Coke formation due to excess cracking

• Cracking is an endothermic reaction

Therefore, in principle cracking generates lighter hydrocarbons constituting paraffins, olefins and aromatics.  In other words, high boiling low octane number feed stocks are converted to low boiling high octane number products.