Cracking (chemistry)
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/* Thermal cracking */ Fixed an internal WP link
In [[petroleum geology]] and [[chemistry]], '''cracking''' is the process whereby complex [[organic compound|organic]] [[molecule]]s such as [[kerogen]]s or heavy [[hydrocarbon]]s are broken down into simpler molecules (e.g. light hydrocarbons) by the breaking of [[carbon]]-carbon [[chemical bond|bonds]] in the precursors. The [[reaction rate|rate]] of cracking and the end products are strongly dependent on the [[temperature]] and presence of any [[catalyst]]s. Cracking, also referred to as [[pyrolysis]], is the breakdown of a large [[alkane]] into smaller, more useful [[alkenes]] and an [[alkane]]. Simply put, cracking hydrocarbons is when you separate long chain hydrocarbons into short ones.
[[Image:Russian Cracking.jpg|thumb|Factory of [[Shukhov cracking process]], [[Baku]], [[USSR]], [[1934]].]]
== History ==
In 1855, petroleum cracking methods were pioneered by [[United States|American]] chemistry professor, [[Benjamin Silliman, Jr.]], of [[Sheffield Scientific School]] (SSS) at [[Yale University]].
The first thermal cracking method, the [[Shukhov cracking process]], was invented by [[Russia]]n engineer [[Vladimir Shukhov]], in the Russian empire, Patent No. 12926, [[November 27]], [[1891]].<ref>[http://www.shukhov.org/shukhov.html Vladimir Grigorievich Shukhov (Biography)]</ref>
[[Eugene Houdry]], 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.<ref>{{cite journal| author=Tim Palucka|title=The Wizard of Octane: Eugene Houdry|journal=Invention & Technology|volume=20|issue=3| pages=|date=Winter 2005|id= |url=http://www.americanheritage.com/articles/magazine/it/2005/3/2005_3_36.shtml}}</ref><ref>{{cite journal|author=Amos A. Avidan, Michael Edwards and Hartley Owen (Mobil Research and Development)|title=Innovative Improvements Highlight FCC's Past and Future|journal=Oil & Gas Journal|volume=88|issue=2|pages= |date=January 8, 1990|id= |url=http://www.ogj.com/articles/save_screen.cfm?ARTICLE_ID=18257}}</ref><ref>[http://portal.acs.org/portal/acs/corg/content?_nfpb=true&_pageLabel=PP_ARTICLEMAIN&node_id=924&content_id=WPCP_007595&use_sec=true&sec_url_var=region1 The Houdry Process] (from the website of the American Chemical Society)</ref>
==Applications==
[[Oil refinery]] cracking processes allow the production of "light" products such as [[liquified petroleum gas|LPG]] and [[gasoline]] from heavier [[crude oil]] [[fractional distillation|distillation fractions]] such as [[gas oil]]s and residues. Fluid Catalytic Cracking (FCC) produces a high yield of [[gasoline]] and [[liquified petroleum gas|LPG]] while hydrocracking is a major source of [[jet fuel]], diesel, naphtha and LPG.
Thermal cracking is currently used to "upgrade" very heavy fractions ("[[upgrader|upgrading]]", "[[visbreaker|visbreaking]]"), or to produce light fractions or distillates, burner fuel and/or [[petroleum coke]]. Two extremes of the thermal cracking in terms of product range are represented by the high-temperature process called "steam cracking" or [[pyrolysis]] (ca. 750 to 900 °C or more) which produces valuable [[ethylene]] and other feedstocks for the petrochemical industry, and the milder-temperature [[delayed coking]] (ca. 500 °C) which can produce, under the right conditions, valuable [[needle coke]], a highly crystalline petroleum coke used in the production of [[electrode]]s for the [[steel]] and [[aluminium]] industries.
===Fluid catalytic cracking===
{{main|Fluid catalytic cracking}}
Fluid catalytic cracking is a commonly used process and a modern oil refinery will typically include a ''cat cracker'', particularly at refineries in the [[USA]] due to the high demand for [[gasoline]].<ref name=Gary>{{cite book|author=James H. Gary and Glenn E. Handwerk|title=Petroleum Refining: Technology and Economics|edition=4th Edition|publisher=CRC Press|year=2001|id=ISBN 0-8247-0482-7}}</ref><ref name=Speight>{{cite book|author=James. G. Speight|title=The Chemistry and Technology of Petroleum|edition=4th Edition|publisher=CRC Press|year=2006|id=ISBN 0-8493-9067-2}}</ref><ref name=Reza>{{cite book|author=Reza Sadeghbeigi|title=Fluid Catalytic Cracking Handbook|edition=2nd Edition|publisher=Gulf Publishing|year=2000|id=ISBN 0-88415-289-8}}</ref> The process was first used in around [[1942]] and employs a powdered [[catalyst]]. During the Second World War, it provided Allied Forces with plentiful supplies of gasoline and artificial rubber that contrasted with the penury suffered by the Axis Forces. Initial process implementations were based on a low activity [[Aluminium oxide|alumina]] catalyst and a reactor where the catalyst particles were suspended in a rising flow of feed hydrocarbons in a [[fluidized bed]].
Alumina-catalyzed cracking systems are still in use in [[high school]] and [[university]] [[laboratory|laboratories]] in experiments concerning alkanes and alkenes. The catalyst is usually obtained by crushing [[pumice]] stones, which contain mainly [[aluminium oxide]] and [[silicon dioxide|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". Pre-heated feed is sprayed into the base of the riser via feed nozzles where it contacts extremely hot fluidized catalyst at 1230 to 1400 °[[Fahrenheit|F]] (665 to 760 °[[Celsius|C]]). 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 just a few seconds and then the mixture is separated via [[Cyclonic separation|cyclones]]. The catalyst-free hydrocarbons are routed to a main [[Fractionation|fractionator]] for separation into fuel gas, LPG, gasoline, [[naphtha]], 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 [[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.
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 [[olefin]]ic profile. Olefins in gasoline are responsible for the formation of [[polymer]]ic deposits in storage [[Fuel tank|tanks]], fuel ducts and [[Fuel injection|injectors]]. The FCC LPG is an important source of [[carbon|C]]<sub>3</sub>-C<sub>4</sub> olefins and [[isobutane]] that are essential feeds for the [[alkylation]] process and the production of polymers such as [[polypropylene]].
===Hydrocracking===<!-- This section is linked from [[Hydrogen]] -->
Hydrocracking is a catalytic cracking process assisted by the presence of an elevated [[partial pressure]] of [[hydrogen]] gas. Similar to the [[hydrotreater]], the function of hydrogen is the purification of the carbon stream from sulfur and nitrogen hetero-atoms.
The products of this process are [[alkane|saturated hydrocarbons]]; depending on the reaction conditions (temperature, pressure, catalyst activity) these products range from [[ethane]], LPG to heavier hydrocarbons comprising mostly of [[isoparaffin]]s. Hydrocracking is normally facilitated by a bi functional catalyst that is capable of rearranging and breaking [[Hydrocarbon|hydrocarbon chain]]s as well as adding hydrogen to [[aromatics]] and [[olefin]]s to produce [[naphthene]]s and [[alkane]]s. Increased product yields can be obtained using titanium oxide in the fermentation process.
Major products from hydrocracking are [[jet fuel]], [[diesel]], relatively high octane rating gasoline fractions and LPG. All these products have a very low content of [[sulfur]] and [[contaminant]]s.
It is very common in [[India]] because of that country's high demand for diesel and [[kerosene]].
===Steam cracking===
Steam cracking is a [[petrochemical]] process in which saturated [[hydrocarbon]]s are broken down into smaller, often unsaturated, hydrocarbons. It is the principal industrial method for producing the lighter [[alkene]]s (or commonly [[olefin]]s), including [[ethene]] (or [[ethylene]]) and [[propene]] (or [[propylene]]).
In steam cracking, a gaseous or liquid hydrocarbon feed like [[Naphtha]], [[Liquified petroleum gas|LPG]] or [[Ethane]] is diluted with steam and then briefly heated in a furnace (obviously without the presence of oxygen). Typically, the reaction temperature is very high —around 850°C—but the reaction is only allowed to take place very briefly. In modern cracking furnaces, the residence time is even reduced to milliseconds (resulting in gas velocities reaching speeds beyond the [[speed of sound]]) in order to improve the yield of desired products. After the cracking temperature has been reached, the gas is quickly quenched to stop the reaction in a transfer line [[heat exchanger|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 [[ethane]], LPGs or light [[naphtha]]s) 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 hydrocarbon]]s 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 [[coke (fuel)|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. 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 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 [[free radical]]s are formed.
The catalytic cracking process involves the presence of [[acid]] [[catalyst]]s (usually solid acids such as [[silica-alumina]] and [[zeolite]]s) which promote a heterolytic (asymmetric) breakage of bonds yielding pairs of [[ion]]s 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 scission|beta]] (i.e., cracking) and [[intramolecular|intra-]] and [[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 site]]s generating deposits of [[carbonaceous]] products generally known as [[petroleum coke|coke]]. Such deposits need to be removed (usually by controlled burning) in order to restore catalyst activity.
===Thermal cracking===
[[William Merriam Burton]] developed one of the earliest thermal cracking processes in 1912 which operated at 700-750 °F (370-400 °C) and an absolute pressure of 90 psia (620 kPa) and was known as the ''[[Burton process]]''. Shortly thereafter, in 1921, [[C.P. Dubbs]], an employee of the [[Universal Oil Products ]]Company, developed a somewhat more advanced thermal cracking process in 1921 which operated at 750-860 °F (400-460 °C) and was known as the ''[[Dubbs process]]''.<ref>[http://supreme.justia.com/us/322/471/case.html U.S. Supreme Court Cases & Opinions, Volume 322, UNIVERSAL OIL PRODUCTS CO. V. GLOBE OIL & REFINING CO., 322 U. S. 471 (1944)]</ref> The Dubbs process was used extensively by many [[Oil refinery|refineries]] until the early 1940's when catalytic cracking came into use.
Modern high-pressure thermal cracking operates at absolute pressures of about 7,000 kPa. 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 [[Homolysis|homolytic fission]] and produces [[alkenes]], which are the basis for the economically important production of [[polymers]].
A large number of [[chemical reaction]]s take place during steam cracking, most of them based on [[free radical]]s. [[Computer]] [[simulation]]s 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 [[chemical bond]] between two carbon atoms, rather than the bond between a carbon and a [[hydrogen]] atom.
:CH<sub>3</sub>CH<sub>3</sub> → 2 CH<sub>3</sub>•
''Hydrogen abstraction'', where a free radical removes a hydrogen atom from another molecule, turning the second molecule into a free radical.
:CH<sub>3</sub>• + CH<sub>3</sub>CH<sub>3</sub> → CH<sub>4</sub> + CH<sub>3</sub>CH<sub>2</sub>•
''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.
:CH<sub>3</sub>CH<sub>2</sub>• → CH<sub>2</sub>=CH<sub>2</sub> + 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.
:CH<sub>3</sub>CH<sub>2</sub>• + CH<sub>2</sub>=CH<sub>2</sub> → CH<sub>3</sub>CH<sub>2</sub>CH<sub>2</sub>CH<sub>2</sub>•
''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.
:CH<sub>3</sub>• + CH<sub>3</sub>CH<sub>2</sub>• → CH<sub>3</sub>CH<sub>2</sub>CH<sub>3</sub>
:CH<sub>3</sub>CH<sub>2</sub>• + CH<sub>3</sub>CH<sub>2</sub>• → CH<sub>2</sub>=CH<sub>2</sub> + CH<sub>3</sub>CH<sub>3</sub>
Thermal cracking is an example of a reaction whose energetics are dominated by entropy (∆S°) rather than by enthalpy (∆H°) in the Gibbs Free Energy 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 CH<sub>3</sub>-CH<sub>2</sub>-CH<sub>2</sub>-CH<sub>3</sub>
* 1st possibility (48%): breaking is done on the CH<sub>3</sub>-CH<sub>2</sub> bond.
CH<sub>3</sub>* / *CH<sub>2</sub>-CH<sub>2</sub>-CH<sub>3</sub>
after a certain number of steps, we will obtain an alkane and an [[alkene]]:
CH<sub>4</sub> + CH<sub>2</sub>=CH-CH<sub>3</sub>
* 2nd possibility (38%): breaking is done on the CH<sub>2</sub>-CH<sub>2</sub> bond.
CH<sub>3</sub>-CH<sub>2</sub>* / *CH<sub>2</sub>-CH<sub>3</sub>
after a certain number of steps, we will obtain an alkane and an [[alkene]]
from different types: CH<sub>3</sub>-CH<sub>3</sub> + CH<sub>2</sub>=CH<sub>2</sub>
* 3rd possibility (14%): breaking of a C-H bond
after a certain number of steps, we will obtain an [[alkene]] and hydrogen gas: CH<sub>2</sub>=CH-CH<sub>2</sub>-CH<sub>3</sub> + H<sub>2</sub> this is very useful since the catalyst can be recycled.
==References==
{{reflist}}
==External links==
* [http://science.howstuffworks.com/oil-refining5.htm Information on cracking in oil refining] from howstuffworks.com
* [http://canadaconnects.ca/chemistry/10105/ Hydrocarbon Cracking - A Quick Summary for High School Students] from canadaconnects.ca
*[http://www.cpfd-software.com/applications/cfb_riser CFD of dense-phase engagement zone of a FCC riser] from cpfd-software.com
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