Due to their high reactivity, olefins, if present at all, only occur in very limited amounts in natural gas and crude oil. They must be manufactured in cleavage or cracking processes. Refinery technology employs essentially three different approaches in converting the range of products naturally occurring in crude oil to those meeting market requirements. These are the catalytic, the hydrocatalytic and the thermal cracking processes.

In catalytic cracking, higher boiling distillation fractions are converted into saturated branched paraffins, naphthenes (cycloparaffins), and aromatics. As the proportion of olefins is small, catalytic cracking is primarily used for producing motm fuels. Various technologies are used, but generally a fluidized bed (FCC=fluidized catalytic cracking) or a reactor with rising catalyst (riser cracking) is used. The usual process conditions are 450- 500 °C and a slight excess pressure. Formerly, aluminum silicates with activators such as Cr₂O₃(TCC catalyst=Thermofor catalytic cracking process of Mobile Oil) or MnO (Houdry catalyst) were used. Mixtures of crystalline aluminum silicates (in the form of zeolites) with amorphous, synthetic, or naturally occurring aluminum silicates are now employed, either in the acidic form or their cations exchanged with rare earths; those exchanged with rare earths have a higher thermal stability. The zeolite catalysts increase the gasoline yield through shape selectivity (product selectivity determined by the geometry of the pore system) and lowered coke deposition. As in all catalytic cracking processes, the catalyst must be reactivated by burning off the deposited coke layer; cracking catalysts therefore often contain small amounts of platinum metal to promote C/CO oxidation to CO₂.

Catalytic cracking is important worldwide and has become the largest industrial user of zeolites. ing processes have been developed by such companies as Exxon, Gulf, Kellogg, Stone and Webster, Texaco, and UOP.

With hydrocracking (catalytic cracking in the presence of hydrogen), residues, as well as higher boiling distillation fractions, can be converted into lower boiling products by various processes. The product does not contain olefins, and its composition can be determined over a wide range by the choice of feed, type of catalyst, and process conditions. With an LPG feed (liquefied petroleum gas), the process can be optimized for production of isobutane, gasoline, naphtha, or fuel oil. Bifunctional catalyst systems, consisting of metallic hydrogenation-dehydrogenation (e.g., Co-Mo or Pd, Pt) and acidic (e.g., A1₂O₃^.SiO₂, preferably as zeolite) cracking components, are employed in the presence of hydrogen. Relatively high investment costs are required for the process which operates at 270-450°C and 80-200 bar. Additionally, between 300-500 m³ of hydrogen must be supplied per tonne of oil feed. This must be manufactured separately as it is not available from the refinery.

Thermal cracking plays an important role in olefin manufacture. This process which involves a radical cleavage of hydrocarbons takes place under pressure and starts at about 400-500°C.

The basic mechanism of a cracking reaction can be envisaged as follows, using n-octane as an example. Thermal cracking is initiated by homolysis of a C-C bond to form two free radicals, e. g.,



Each alkyl radical can abstract a hydrogen atom from an n-octane molecule to produce an octyl radical and a shorter alkane, e. g.,



Abstraction of secondary hydrogen is favored over primary hydrogen due to the lower C-H bond energy, with an equal probability for removal of each secondary hydrogen. Any of these radicals can also undergo [β-cleavage to form ethylene or propene and a shorter alkyl radical:

The cracking reactions thus involve changes in the H₂ content as well as in the carbon skeleton. Dehydrogenation and H₂ transfer from H₂-rich hydrocarbon fractions (low boiling) belong to the former, and chain cracking of H₂-deficient higher molecular fractions to the latter category.

Primary reactions involving the carbon skeleton include not only chain shortening, but also isomerization and cyclization. Secondary reactions include olefin polymerization, alkylation, and condensation of aromatics to form polynuclear aromatic compounds.

Thermodynamically, all saturated and unsaturated hydrocarbons are unstable with respect to their elements at the applied cracking temperatures. That is, if pyrolysis were allowed to go to thermodynamic equilibrium, all hydrocarbons would completely decompose into graphite and molecular hydrogen.

Accordingly, in a commercial cracking process large amounts of energy must be transferred at high temperatures within a time period sufficient to allow cracking to occur but insufficient for decomposition into the elements.

Hydrocarbon cracking is optimized by regulating three kinetic parameters:
1. Cracking temperature
2. Residence time
3. Partial pressures of the hydrocarbons

To 1:

Temperature affects the cracked gas composition. At about 400°C the carbon chains are preferentially cracked in the center of the molecule. With increasing temperature, the cracking shifts towards the end of the chain, leading to formation of more low molecular weight olefins.

Also, the cracking rate increases with temperature, as higher radical concentrations are generated.

To 2:

The residence time affects the ratio of primary to secondary products for a constant cracking temperature. With a short residence time, primary reactions resulting in olefin formation dominate. A longer residence time allows the increase of secondary reactions such as oligomerization and coke deposition.

To 3:

In the desired cracking reactions, there is an increase in the number of moles, and so the partial pressure of the hydrocarbons has a powerful effect. A high partial pressure favors polymerization and condensation reactions and a low partial pressure improves the olefin yield. In order to lower the partial pressure of the hydrocarbons, a foreign gas - usually steam - is mixed with the hydrocarbon fraction for pyrolysis (steam cracking or reforming). As the steam content is increased, the yield of olefin rises, while carbon deposition diminishes.

In conclusion, the manufacture of low molecular weight olefins is favored by high temperature, short residence time, and low partial pressures.

Essentially two processes, differing in the severity of conditions employed, have been developed for steam cracking:
1. Low-severity cracking below 800°C with 1 second residence time.
2. High-severity cracking approaching 900 °C with roughly 0.5 second residence time.
The C₂/C₃/C₄ olefin distribution can be controlled with these variables.