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Humphreys independently developed and patented a similar process as U. Among its advantages was the fact that both the condenser and the boiler were continuously kept under pressure. Sinclair Oil apparently wished to suggest that the patent of Burton and Humphreys, in use by Standard Oil, was derived from Shukhov’s patent for oil cracking, as described in the Russian patent.
If that could be established, it could strengthen the hand of rival American companies wishing to invalidate the Burton-Humphreys patent. In the event Shukhov satisfied the Americans that in principle Burton’s method closely resembled his patents, though his own interest in the matter was primarily to establish that “the Russian oil industry could easily build a cracking apparatus according to any of the described systems without being accused by the Americans of borrowing for free”.
The replacement was not complete; many types of cracking, including pure thermal cracking, still are in use, depending on the nature of the feedstock and the products required to satisfy market demands. Thermal cracking remains important, for example in producing naphtha, gas oil, and coke, and more sophisticated forms of thermal cracking have been developed for various purposes.
These include visbreaking , steam cracking , and coking. 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.
Two extremes of the thermal cracking in terms of product range are represented by the high-temperature process called “steam cracking” or pyrolysis ca. Shortly thereafter, in , C. It is the principal industrial method for producing the lighter alkenes or commonly olefins , including ethene or ethylene and propene or propylene. Steam cracker units are facilities in which a feedstock such as naphtha, liquefied petroleum gas LPG , ethane , propane or butane is thermally cracked through the use of steam in a bank of pyrolysis furnaces to produce lighter hydrocarbons.
The products obtained depend on the composition of the feed, the hydrocarbon-to-steam ratio, and on the cracking temperature and furnace residence time. In steam cracking, a gaseous or liquid hydrocarbon feed like naphtha , LPG or ethane is diluted with steam and briefly heated in a furnace without the presence of oxygen. In modern cracking furnaces, the residence time is reduced to milliseconds to improve yield, resulting in gas velocities up to the speed of sound.
After the cracking temperature has been reached, the gas is quickly quenched to stop the reaction in a transfer line heat exchanger or inside a quenching header using quench oil. Light hydrocarbon feeds such as ethane , LPGs or light naphtha give product streams rich in the lighter alkenes, including ethylene, propylene, and butadiene.
Heavier hydrocarbon full range and heavy naphthas as well as other refinery products feeds give some of these, but also give products rich in aromatic hydrocarbons and hydrocarbons suitable for inclusion in gasoline or fuel oil. The process also results in the slow deposition of coke , a form 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.
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. The overhead stream from the demethanizer tower consists of all the hydrogen and methane that was in the cracked gas stream. Methane recovery is critical to the economical operation of an ethylene plant. The overhead stream from the deethanizer tower consists of all the C 2’s that were in the cracked gas stream. The C 2 stream contains acetylene, which is explosive above kPa 29 psi.
The C 2’s then proceed to a C 2 splitter. The product ethylene is taken from the overhead of the tower and the ethane coming from the bottom of the splitter is recycled to the furnaces to be cracked again; the bottom stream from the de-ethanizer tower goes to the depropanizer tower.
The overhead stream from the depropanizer tower consists of all the C 3’s that were in the cracked gas stream. Before feeding the C 3’s to the C 3 splitter, the stream is hydrogenated to convert the methylacetylene and propadiene allene mix. This stream is then sent to the C 3 splitter. The overhead stream from the C 3 splitter is product propylene and the bottom stream is propane which is sent back to the furnaces for cracking or used as fuel.
The bottom stream from the depropanizer tower is fed to the debutanizer tower. The overhead stream from the debutanizer is all of the C 4’s that were in the cracked gas stream. The bottom stream from the debutanizer light pyrolysis gasoline consists of everything in the cracked gas stream that is C 5 or heavier. Most of the energy recovered from the cracked gas is used to make high pressure psig steam. This steam is in turn used to drive the turbines for compressing cracked gas, the propylene refrigeration compressor, and the ethylene refrigeration compressor.
An ethylene plant, once running, does not need to import steam to drive its steam turbines. A typical world scale ethylene plant about 1. Catalytic methods[ edit ] The catalytic cracking process involves the presence of acid catalysts usually solid acids such as silica-alumina and zeolites which promote a heterolytic asymmetric breakage of bonds yielding pairs of ions of opposite charges, usually a carbocation and the very unstable hydride anion.
Carbon-localized free radicals and cations are both highly unstable and undergo processes of chain rearrangement, C-C scission in position beta as in cracking, and intra- and intermolecular hydrogen 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.
Fluid catalytic cracking Schematic flow diagram of a fluid catalytic cracker Fluid catalytic cracking is a commonly used process, and a modern oil refinery will typically include a cat cracker , particularly at refineries in the US, due to the high demand for gasoline.
During WWII, the Allied Forces had plentiful supplies of the materials in contrast to the Axis Forces which suffered severe shortages of gasoline and artificial rubber. Initial process implementations were based on low activity alumina catalyst and a reactor where the catalyst particles were suspended in a rising flow of feed hydrocarbons in a fluidized bed.
The catalyst is usually obtained by crushing pumice stones, which contain mainly aluminium oxide and silica into small, porous pieces. In the laboratory, aluminium oxide or porous pot must be heated. 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. The catalyst-hydrocarbon mixture flows upward through the riser for a few seconds, and then the mixture is separated via cyclones.
The catalyst-free hydrocarbons are routed to a main fractionator for separation into fuel gas, LPG, gasoline, naphtha , light cycle oils used in diesel and jet fuel, and heavy fuel oil. The “spent” catalyst is disengaged from the cracked hydrocarbon vapors and sent to a stripper where it contacts 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 oxygen is used to burn off the coke to restore catalyst activity and also provide the necessary heat for the next reaction cycle, cracking being an endothermic reaction. The “regenerated” catalyst then flows to the base of the riser, repeating the cycle.
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 C 3-C4 olefins and isobutane that are essential feeds for the alkylation process and the production of polymers such as polypropylene. Unlike a hydrotreater , hydrocracking uses hydrogen to break C-C bonds hydrotreatment is conducted prior to hydrocracking to protect the catalysts in a hydrocracking process.
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. It is very common in Europe and Asia because those regions have high demand for diesel and kerosene. In the US, fluid catalytic cracking is more common because the demand for gasoline is higher. In principle, ethane can undergo homolysis: More common examples of cracking reactions involve retro- Diels-Alder reactions.
Illustrative is the thermal cracking of dicyclopentadiene to give cyclopentadiene.
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