Handbook of Solvents - George Wypych - ChemTech - Ventech!

72 George Wypych
Figure 3.3.4. Simplified catalytic cracking flow diagram. [Reproduced from reference 1]
isomerization, polymerization, catalytic reforming, solvent extraction, and other operations.
Thermal cracking (visbreaking), uses heat and pressure to break large hydrocarbon
molecules into lower molecular weight products. Most refineries do not use this process but
use instead its replacement − catalytic cracking which gives a better yield of gasoline.
Feedstock includes light and heavy oils from the crude oil distillation process. The cracking
process occurs at a temperature of 550 o C and under increased pressure. The cracking reaction
is discontinued by mixing with cooler recycle stream. The mixture is stripped of lighter
fractions which are then subjected to fractional distillation. Figure 3.2.4 is a schematic flow
diagram of catalytic cracking. Hydrocracking is a somewhat different process which occurs
under higher pressure and in the presence of hydrogen. It is used to convert fractions which
are difficult to crack, such as middle distillates, cycle oils, residual fuel oils, and reduced
crudes. Alkylation is used to produce compounds from olefins and isoparaffins in a catalyzed
process. Isomerization converts paraffins to isoparaffins. Polymerization converts
propene and butene to high octane gasoline. The application of these three processes has
increased output and performance of gasoline.
Catalytic reforming processes gasolines and naphthas from the distillation unit into
aromatics. Four major reactions occur: dehydrogenation of naphthenes to aromatics,
dehydrocyclization of paraffins to aromatics, isomerization, and hydrocracking.