Cracking (chemistry)
[1] More loosely, outside the field of petroleum chemistry, the term "cracking" is used to describe any type of splitting of molecules under the influence of heat, catalysts and solvents, such as in processes of destructive distillation or pyrolysis.
An overall process of disproportionation can be observed, where "light", hydrogen-rich products are formed at the expense of heavier molecules which condense and are depleted of hydrogen.
The actual reaction is known as homolytic fission and produces alkenes, which are the basis for the economically important production of polymers.
750 °C to 900 °C or higher) which produces valuable ethylene and other feedstocks for the petrochemical industry, and the milder-temperature delayed coking (ca.
It is the principal industrial method for producing the lighter alkenes (or commonly olefins), including ethene (or ethylene) and propene (or propylene).
In modern cracking furnaces, the residence time is reduced to milliseconds to improve yield, resulting in gas velocities up to the speed of sound.
[citation needed][8] The products produced in the reaction depend on the composition of the feed, the hydrocarbon-to-steam ratio, and on the cracking temperature and furnace residence time.
Since coke degrades the efficiency of the reactor, great care is taken to design reaction conditions to minimize its formation.
[citation needed] In newer designs, cracking takes place using a very active zeolite-based catalyst in a short-contact time vertical or upward-sloped pipe called the "riser".
The hot catalyst vaporizes the feed and catalyzes the cracking reactions that break down the high-molecular weight oil into lighter components including LPG, gasoline, and diesel.
Olefins in gasoline are responsible for the formation of polymeric deposits in storage tanks, fuel ducts and injectors.
The FCC LPG is an important source of C3–C4 olefins and isobutane that are essential feeds for the alkylation process and the production of polymers such as polypropylene.
Hydrocracking is normally facilitated by a bifunctional catalyst that is capable of rearranging and breaking hydrocarbon chains as well as adding hydrogen to aromatics and olefins to produce naphthenes and alkanes.
[14] The major products from hydrocracking are jet fuel and diesel, but low sulphur naphtha fractions and LPG are also produced.
The hydrocracking process depends on the nature of the feedstock and the relative rates of the two competing reactions, hydrogenation and cracking.
Also, unit internals can often be patented by the process licensors and are designed to support specific functions of the catalyst load.
Currently, the major process licensors for hydrocracking are: Outside of the industrial sector, cracking of C−C and C−H bonds are rare chemical reactions.
In principle, ethane can undergo homolysis: Because C−C bond energy is so high (377 kJ/mol),[18] this reaction is not observed under laboratory conditions.
