Cracking (chemistry)

History

In 1855, petroleum cracking methods were pioneered by American chemistry professor, Benjamin Silliman, Jr. (1816-1885), of Sheffield Scientific School (SSS) at Yale University.

The first thermal cracking method, the Shukhov cracking process, was invented by Russian engineer Vladimir Shukhov (1853-1939), in the Russian empire, Patent No. 12926, November 27, 1891.

Eugene Houdry (1892-1962), a French mechanical engineer, pioneered catalytic cracking and developed the first commercially successful process after emigrating to the United States. The first commercial plant was built in 1936. His process doubled the amount of gasoline that could be produced from a barrel of crude oil.

Applications

steel and aluminum industries.

Fluid catalytic cracking

Fluid catalytic cracking, developed by American engineers pumice stones, which contain mainly aluminium oxide and silicon(IV) oxide into small, porous pieces. In the laboratory, Aluminum oxide (or porous pot) must be heated.

In newer designs, cracking takes place using a very active fractionator for separation into fuel gas, LPG, gasoline, light cycle oils used in diesel and jet fuel, and heavy fuel oil.

During the trip up the riser, the cracking catalyst is "spent" by reactions which deposit coke on the catalyst and greatly reduce activity and selectivity. The "spent" catalyst is disengaged from the cracked hydrocarbon vapors and sent to a stripper where it is contacted with steam to remove hydrocarbons remaining in the catalyst pores. The "spent" catalyst then flows into a fluidized-bed regenerator where air (or in some cases air plus endothermic reaction. The "regenerated" catalyst then flows to the base of the riser, repeating the cycle.

The gasoline produced in the FCC unit has an elevated octane rating but is less chemically stable compared to other gasoline components due to its polypropylene.

Hydrocracking

Hydrocracking is a catalytic cracking process assisted by the presence of an elevated hydrotreater, the function of hydrogen is the purification of the carbon stream from sulfur and nitrogen hetero-atoms.

The products of this process are alkanes.

Major products from hydrocracking are kerosene.

Steam cracking

Steam cracking is a propylene).

In steam cracking, a gaseous or liquid hydrocarbon feed like heat exchanger.

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. Light hydrocarbon feeds (such as aromatic hydrocarbons and hydrocarbons suitable for inclusion in gasoline or fuel oil. The higher cracking temperature (also referred to as severity) favours the production of ethene and benzene, whereas lower severity produces relatively higher amounts of propene, C4-hydrocarbons and liquid products.

The process also results in the slow deposition of carbon, on the reactor walls. This degrades the efficiency of the reactor, so reaction conditions are designed to minimize this. Nonetheless, a steam cracking furnace can usually only run for a few months at a time between de-cokings. Decokes require the furnace to be isolated from the process and then a flow of steam or a steam/air mixture is passed through the furnace coils at 950 -1050 C . This converts the hard solid carbon layer to carbon monoxide and carbon dioxide. Once this reaction is complete, the furnace can be returned to service.

Chemistry

"Cracking" breaks larger molecules into smaller ones. This can be done with a thermic or catalytic method. The thermal cracking process follows a homolytic mechanism, that is, bonds break symmetrically and thus pairs of intermolecular hydrogen transfer or hydride transfer. In both types of processes, the corresponding reactive intermediates (radicals, ions) are permanently regenerated, and thus they proceed by a self-propagating chain mechanism. The chain of reactions is eventually terminated by radical or ion recombination.

Catalytic cracking

Catalytic cracking uses a zeolite catalyst and moderately-high temperatures (400-500 °C) to aid the process of breaking down large hydrocarbon molecules into smaller ones. During this process, less reactive, and therefore more stable and longer lived intermediate cations accumulate on the catalysts' active sites generating deposits of carbonaceous products generally known as coke. Such deposits need to be removed (usually by controlled burning) in order to restore catalyst activity.

Thermal cracking

In thermal cracking elevated temperatures (~800^oC) and pressures (~700kPa) are used, a process first developed by William Merriam Burton. 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.

A large number of chemical reactions take place during steam cracking, most of them based on free radicals. Computer simulations aimed at modeling what takes place during steam cracking have included hundreds or even thousands of reactions in their models. The main reactions that take place include:

Initiation reactions, where a single molecule breaks apart into two free radicals. Only a small fraction of the feed molecules actually undergo initiation, but these reactions are necessary to produce the free radicals that drive the rest of the reactions. In steam cracking, initiation usually involves breaking a hydrogen atom.

CH₃CH₃ → 2 CH₃•

Hydrogen abstraction, where a free radical removes a hydrogen atom from another molecule, turning the second molecule into a free radical.

CH₃• + CH₃CH₃ → CH₄ + CH₃CH₂•

Radical decomposition, where a free radical breaks apart into two molecules, one an alkene, the other a free radical. This is the process that results in the alkene products of steam cracking.

CH₃CH₂• → CH₂=CH₂ + H•

Radical addition, the reverse of radical decomposition, in which a radical reacts with an alkene to form a single, larger free radical. These processes are involved in forming the aromatic products that result when heavier feedstocks are used.

CH₃CH₂• + CH₂=CH₂ → CH₃CH₂CH₂CH₂•

Termination reactions, which happen when two free radicals react with each other to produce products that are not free radicals. Two common forms of termination are recombination, where the two radicals combine to form one larger molecule, and disproportionation, where one radical transfers a hydrogen atom to the other, giving an alkene and an alkane.

CH₃• + CH₃CH₂• → CH₃CH₂CH₃

CH₃CH₂• + CH₃CH₂• → CH₂=CH₂ + CH₃CH₃

Thermal cracking is an example of a reaction whose energetics are dominated by entropy (∆S°) rather than by enthalpy (∆H°) in the free equation ∆G°=∆H°-T∆S°. Although the bond dissociation energy D for a carbon-carbon single bond is relatively high (about 375 kJ/mol) and cracking is highly endothermic, the large positive entropy change resulting from the fragmentation of one large molecule into several smaller pieces, together with the extremely high temperature, makes T∆S° term larger than the ∆H° term, thereby favoring the cracking reaction.

Here is an example of cracking with butane CH₃-CH₂-CH₂-CH₃

• 1st possibility (48%): breaking is done on the CH₃-CH₂ bond.

CH₃* / *CH₂-CH₂-CH₃

after a certain number of steps, we will obtain an alkane and an alkene: CH₄ + CH₂=CH-CH₃

• 2nd possibility (38%): breaking is done on the CH₂-CH₂ bond.

CH₃-CH₂* / *CH₂-CH₃

after a certain number of steps, we will obtain an alkane and an alkene from different types: CH₃-CH₃ + CH₂=CH₂

• 3rd possibility (14%): breaking of a C-H bond

after a certain number of steps, we will obtain an alkene and hydrogen gas: CH₂=CH-CH₂-CH₃ + H₂