Thermal cracking encompasses a
range of non-catalytic processes used to enhance the heavy fraction of crude
oil by conversion and separation.
In order for a refinery to produce
more valuable products, the heavy "residue" fractions can be processed into
lighter fractions. Thermal cracking produces gas, naphtha and distillate
(kerosene and diesel).
Examples of this separation method are:
- Vis-breaking, which improves fuel oil quality
- Delayed coking, which also produces coke
- Flexi-coking, which includes coke gasification
Vis-breaking entails the use of mild cracking, whereby
the residue is converted into lighter products so that the viscosity is
reduced. Typical yields from this process are 25% diesel and 75% fuel oil.
Operating conditions vary, but are typically; temperature 450° C and
pressure 5 - 10 barg.
Delayed coking completely converts residue into
gas, naphtha, diesel and coke. It is normally a semi-continuous process.
Operating parameters are typically; temperature 450° - 500° C in the
coke drum and 2 - 3 barg pressure.
Flexi-coking produces a range of
lighter products and coke. Typical yields are 67% light products (high energy
LPG, naphtha and diesel) and 33% low energy fuel gas. Flexi-coking does not
produce fuel oil residue.