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Encyclopedia > Cracking (chemistry)
Factory of Shukhov cracking process, Baku, USSR, 1934
Factory of Shukhov cracking process, Baku, USSR, 1934

In petroleum geology and chemistry, cracking is the process whereby complex organic molecules such as kerogens or heavy hydrocarbons are broken down into simpler molecules (e.g. light hydrocarbons) by the breaking of carbon-carbon bonds in the precursors. The rate of cracking and the end products are strongly dependent on the temperature and presence of any catalysts. Cracking, also referred to as pyrolysis, is the breakdown of a large alkane into smaller, more useful alkanes and an alkene. Image File history File linksMetadata Russian_Cracking. ... Image File history File linksMetadata Russian_Cracking. ... Vladimir Grigorievich Shukhov Vladimir Grigorievich Shukhov (Russian: ), (August 28 [O.S. August 16] 1853 - February 2, 1939) was a great Russian engineer renowned for his pioneering works on new methods of analysis for civil engineering that led to breakthroughs in industrial design of hyperboloid structures, shell structures, tensile structures, oil... Municipality: Baku Area: 1000 km² Altitude: -28 m Population: 2,074,300 census 2003 Population density: 1280 persons/km² Postal Code: AZ10 Area code: +99412 Municipality code: BA Latitude: 41° 01 52 N Longitude: 21° 20 25 E Weather types: 9 of 11 Mayor: Hajibala Abutalybov The Baku region. ... 1934 (MCMXXXIV) was a common year starting on Monday (link will take you to calendar). ... Petroleum geology is a term used to refer to the specific set of geological disciplines that are applied to the search for hydrocarbons (oil exploration). ... This article or section includes a list of works cited or a list of external links, but its sources remain unclear because it lacks in-text citations. ... Benzene is the simplest of the arenes, a family of organic compounds An organic compound is any member of a large class of chemical compounds whose molecules contain carbon and hydrogen; therefore, carbides, carbonates, carbon oxides and elementary carbon are not organic (see below for more on the definition controversy... In science, a molecule is a group of atoms in a definite arrangement held together by chemical bonds. ... Kerogens are chemical compounds that make up a portion of the organic matter in sedimentary rocks. ... Hydrocarbons are refined at oil refineries and processed at chemical plants A hydrocarbon is a chemical compound that consists only of the elements carbon (C) and hydrogen (H). ... General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Standard atomic weight 12. ... A chemical bond is the physical process responsible for the attractive interactions between atoms and molecules, and that which confers stability to diatomic and polyatomic chemical compounds. ... Iron rusting - a chemical reaction with a slow reaction rate. ... Fig. ... It has been suggested that this article or section be merged into Catalysis. ... Simple sketch of pyrolysis chemistry Pyrolysis usually means the chemical decomposition of organic materials by heating in the absence of oxygen or any other reagents, except possibly steam. ... Chemical structure of methane, the simplest alkane Alkanes are chemical compounds that consists only of the elements carbon (C) and hydrogen (H) (i. ... An alkane in organic chemistry is a type of hydrocarbon in which the molecule has the maximum possible number of hydrogen atoms and so has no double bonds (they are saturated). ... The chemical structure of ethylene, the simplest alkene. ...

Contents

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. Silliman and his father were Skull and Bones members, and SSS professors of chemistry. Benjamin Silliman, Jr. ... The Sheffield Scientific School was founded as Yale Scientific School in 1854 and renamed in 1861 in honor of Joseph E. Sheffield. ... “Yale” redirects here. ... Emblem of the Skull and Bones society The Order of Skull and Bones, once known as The Brotherhood of Death[1], is a secret society based at Yale University, in New Haven, Connecticut, and one of the earliest-established of student secret societies that rival Phi Beta Kappa, also originally...


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. Vladimir Grigorievich Shukhov Vladimir Grigorievich Shukhov (Russian: ), (August 28 [O.S. August 16] 1853 - February 2, 1939) was a great Russian engineer renowned for his pioneering works on new methods of analysis for civil engineering that led to breakthroughs in industrial design of hyperboloid structures, shell structures, tensile structures, oil... November 27 is the 331st day (332nd in leap years) of the year in the Gregorian calendar. ... Year 1891 (MDCCCXCI) was a common year starting on Thursday (link will display the full calendar) of the Gregorian calendar (or a common year starting on Saturday of the 12-day slower Julian calendar). ...


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. Eugene Houdry (1892-1962) was a French mechanical engineer who invented catalytic cracking of petroleum feed stocks. ... Mechanical engineering is the application of physical principles to the creation of useful devices, objects and machines. ...


Applications

Oil refinery cracking processes allow the production of "light" products such as LPG and gasoline from heavier crude oil distillation fractions such as gas oils and residues. Fluid Catalytic Cracking (FCC) produces a high yield of gasoline and LPG while hydrocracking is a major source of jet fuel, gasoline components and LPG. Thermal cracking is currently used to "upgrade" very heavy fractions ("upgrading", "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 electrodes for the steel and aluminum industries. View of Shell Oil Refinery in Martinez, California. ... 45 kg LPG cylinders Spherical Gas Container typically found in Refineries. ... Gasoline or petrol is a petroleum-derived liquid mixture consisting mostly of hydrocarbons and enhanced with benzene or iso-octane to increase octane ratings, used as fuel in internal combustion engines. ... Pumpjack pumping an oil well near Sarnia, Ontario Petroleum (from Greek petra – rock and elaion – oil or Latin oleum – oil ) or crude oil is a thick, dark brown or greenish liquid. ... Fractional distillation is the separation of a mixture into its component parts, or fractions, such as in separating chemical compounds by their boiling point by heating them to a temperature at which several fractions of the compound will evaporate. ... Kerosene, gas oil, paraffin (not the waxy solid of that name) is a colorless flammable hydrocarbon liquid. ... Gasoline or petrol is a petroleum-derived liquid mixture consisting mostly of hydrocarbons and enhanced with benzene or iso-octane to increase octane ratings, used as fuel in internal combustion engines. ... 45 kg LPG cylinders Spherical Gas Container typically found in Refineries. ... Jet fuel is a type of aviation fuel designed for use in jet-engined aircraft. ... A visbreaker is a chemical plant based on a techology whereby via thermal cracking in a furnace reactor (at high temperature) heavy hydrocarbons (e. ... A visbreaker is a chemical plant where thermal cracking in a furnace reactor (at high temperature) is used to transform heavy hydrocarbons (e. ... Petroleum coke (often abbreviated pet coke) is a carbonaceous solid derived from oil refinery cracking processes. ... Simple sketch of pyrolysis chemistry Pyrolysis usually means the chemical decomposition of organic materials by heating in the absence of oxygen or any other reagents, except possibly steam. ... Ethylene (or IUPAC name ethene) is the chemical compound with the formula C2H4. ... Petroleum coke is a carbonaceous solid derived from oil refinery cracking processes. ... Petroleum coke is a carbonaceous solid derived from oil refinery cracking processes. ... An electrode is a conductor used to make contact with a nonmetallic part of a circuit (e. ... The steel cable of a colliery winding tower. ... Aluminum is a soft and lightweight metal with a dull silvery appearance, due to a thin layer of oxidation that forms quickly when it is exposed to air. ...


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. 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 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 laboratories in experiments concerning alkanes and alkenes. The catalyst is usually obtained by crushing 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. Gasoline or petrol is a petroleum-derived liquid mixture consisting mostly of hydrocarbons and enhanced with benzene or iso-octane to increase octane ratings, used as fuel in internal combustion engines. ... Year 1942 (MCMXLII) was a common year starting on Thursday (the link is to a full 1942 calendar). ... It has been suggested that this article or section be merged into Catalysis. ... Alumina redirects here. ... A fluidized bed is formed when a quantity of a solid particulate substance (usually retained in a holding tank) is forced to behave as a fluid; usually by the forced introduction of pressurised gas through the particulate medium. ... Main article: Secondary education High school is a name used in some parts of the world, and particularly in North America, to describe the last segment of compulsory education. ... Representation of a university class, 1350s. ... For other uses of lab, see Lab. ... Specimen of highly porous pumice from Teide volcano on Tenerife, Canary Islands. ...


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 °F (665 to 760 °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 cyclones. The catalyst-free hydrocarbons are routed to a main fractionator for separation into fuel gas, LPG, gasoline, light cycle oils used in diesel and jet fuel, and heavy fuel oil. Zeolite The micro-porous molecular structure of a zeolite, ZSM-5 Zeolites (Greek, zein,to boil;lithos,a stone) are minerals that have a micro-porous structure. ... Fahrenheit is a temperature scale named after the German physicist Daniel Gabriel Fahrenheit (1686–1736), who proposed it in 1724. ... Celsius is, or relates to, the Celsius temperature scale (previously known as the centigrade scale). ... A Cyclone Separator Cyclonic separation is a method of removing particulates from an air (or gas) stream without the use of filters. ... Fractional distillation is the separation of a mixture of compounds by their boiling point, by heating to high enough temperatures. ...


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. General Name, Symbol, Number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Standard atomic weight 15. ... In Chemistry an endothermic reaction is one in which the reactants have less energy than the products, and thus a net input of energy, usually in the form of heat, is required. ...


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 olefinic profile. 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. A synonym for the more widely accepted term, alkene. ... A polymer is a substance composed of molecules with large molecular mass consisting of repeating structural units, or monomers, connected by covalent chemical bonds. ... // Fuel injection is a means of metering fuel into an internal combustion engine. ... General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Standard atomic weight 12. ... Butane is an alkane hydrocarbon with the molecular formula C4H10. ... Alkylation is the transfer of an alkyl group from one molecule to another. ... Polypropylene lid of a Tic Tacs box, with a living hinge and the resin identification code under its flap Micrograph of polypropylene Polypropylene or polypropene (PP) is a thermoplastic polymer, made by the chemical industry and used in a wide variety of applications, including food packaging, textiles, plastic parts and...


Hydrocracking

Hydrocracking is a catalytic cracking process assisted by the presence of an elevated partial pressure of hydrogen. The products of this process are saturated hydrocarbons; depending on the reaction conditions (temperature, pressure, catalyst activity) these products range from ethane, LPG to heavier hydrocarbons comprising mostly of isoparaffins. 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. In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... Chemical structure of methane, the simplest alkane Alkanes are chemical compounds that consists only of the elements carbon (C) and hydrogen (H) (i. ... Ethane is a chemical compound with chemical formula C2H6. ... An alkane in organic chemistry is a type of hydrocarbon in which the molecule has the maximum possible number of hydrogen atoms and so has no double bonds (they are saturated). ... Hydrocarbons are refined at oil refineries and processed at chemical plants A hydrocarbon is a chemical compound that consists only of the elements carbon (C) and hydrogen (H). ... In chemistry, an aromatic molecule is one in which electrons are free to cycle around circular arrangements of atoms, which are alternately singly and doubly bonded to one another. ... A synonym for the more widely accepted term, alkene. ... Cycloalkanes are chemical compounds with a one or more rings of carbons to which hydrogens are attached according to the formula CnH2n. ... An alkane in organic chemistry is a type of hydrocarbon in which the molecule has the maximum possible number of hydrogen atoms and so has no double bonds (they are saturated). ...


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 contaminants. It is very common in India because of the high demand for diesel and kerosene. Jet fuel is a type of aviation fuel designed for use in jet-engined aircraft. ... Rudolf Christian Karl Diesel (1858-1913), inventor of the diesel engine. ... General Name, Symbol, Number sulfur, S, 16 Chemical series nonmetals Group, Period, Block 16, 3, p Appearance lemon yellow Standard atomic weight 32. ... The Lachine Canal, in Montreal, is badly polluted Pollution is the release of harmful environmental contaminants, or the substances so released. ... Rudolf Christian Karl Diesel (1858-1913), inventor of the diesel engine. ... Kerosene or paraffin oil (British English, not to be confused with the waxy solid also called paraffin wax or just paraffin) is a colorless flammable hydrocarbon liquid. ...


Steam cracking

Steam cracking is a petrochemical process in which saturated hydrocarbons are broken down into smaller, often unsaturated, hydrocarbons. It is the principal industrial method for producing the lighter alkenes (or commonly olefins), including ethene (or ethylene) and propene (or propylene). Petrochemicals are chemical products made from raw materials of petroleum (hydrocarbon) origin. ... Hydrocarbons are refined at oil refineries and processed at chemical plants A hydrocarbon is a chemical compound that consists only of the elements carbon (C) and hydrogen (H). ... The chemical structure of ethylene, the simplest alkene. ... A synonym for the more widely accepted term, alkene. ... Ethylene or ethene is the simplest alkene hydrocarbon, consisting of two carbon atoms and four hydrogens. ... Ethylene (or IUPAC name ethene) is the chemical compound with the formula C2H4. ... Propylene, also known as propene, is a colorless flammable gas with chemical formula C3H6 having a garlic odor. ... Propylene, also known by its IUPAC name propene, is an organic compound having the chemical formula C3H6. ...


In steam cracking, a gaseous or liquid hydrocarbon feed like Naphtha, 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 hot —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 exchanger. Naphtha is a group of various volatile flammable liquid hydrocarbon mixtures used primarily as feedstocks in refineries for the reforming process and in the petrochemical industry for the production of olefins in steam crackers. ... 45 kg LPG cylinders Spherical Gas Container typically found in Refineries. ... Ethane is a chemical compound with chemical formula C2H6. ... The speed of sound is a term used to describe the speed of sound waves passing through an elastic medium. ... A heat exchanger is a device built for efficient heat transfer from one fluid to another, whether the fluids are separated by a solid wall so that they never mix, or the fluids are directly contacted. ...


The products produced in the reaction depend on the composition of the feed, the hydrocarbon to steam ratio and on the cracking temperature & furnace residence time. Light hydrocarbon feeds (such as ethane, LPGs or light naphthas) give product streams rich in the lighter alkenes, including ethylene, propylene, and butadiene. Heavier hydrocarbon (full range & 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 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. Ethane is a chemical compound with chemical formula C2H6. ... Naphtha is a group of various volatile flammable liquid hydrocarbon mixtures used primarily as feedstocks in refineries for the reforming process and in the petrochemical industry for the production of olefins in steam crackers. ... Butadiene can refer to either one of two hydrocarbon chemical compounds which are alkenes that are isomers of each other. ... An aromatic hydrocarbon (abbreviated as AH) or arene [1] is a hydrocarbon, the molecular structure of which incorporates one or more planar sets of six carbon atoms that are connected by delocalised electrons numbering the same as if they consisted of alternating single and double covalent bonds. ... Gasoline or petrol is a petroleum-derived liquid mixture consisting mostly of hydrocarbons and enhanced with benzene or iso-octane to increase octane ratings, used as fuel in internal combustion engines. ... Fuel oil is a fraction obtained from petroleum distillation, either as a distillate or a residue. ...


The process also results in the slow deposition of coke, a form of carbon, on the reactor walls. This degrades the effectiveness 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. Coke Coke is a solid carbonaceous material derived from destructive distillation of low-ash, low-sulfur bituminous coal. ... General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Standard atomic weight 12. ...


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 radicals are formed. 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 (i.e., cracking) and 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. In chemistry free radicals are uncharged atomic or molecular species with unpaired electrons or an otherwise open shell configuration. ... Acidity redirects here. ... It has been suggested that this article or section be merged into Catalysis. ... Silica-alumina, also known as alumino-silicate(s). ... Zeolite The micro-porous molecular structure of a zeolite, ZSM-5 Zeolites (Greek, zein,to boil;lithos,a stone) are minerals that have a micro-porous structure. ... “Multivalent” redirects here. ... A carbocation (IPA pronunciation: ) is an ion with a positively-charged carbon atom. ... A hydride is a compound of hydrogen with more electropositive elements. ... An anion is an ion with negative charge. ... The initial step in the chemistry of thermal cracking of hydrocarbons is the formation of free radicals. ... Intramolecular describes a process or characteristic limited within the structure of a single, or each molecule; a property or phenomenon limited to the extent of a single, or each molecule. ... Intermolecular describes a process or characteristic that extends from one molecule to an adjacent one; a property or phenomenon that extends from one molecule to another. ...


Catalytic cracking

Catalytic cracking uses a solid acid catalyst (Most commonly Aluminium Oxide and Silicon Dioxide) and moderately-high temperatures (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. Acidity redirects here. ... It has been suggested that this article or section be merged into Catalysis. ... The active site of an enzyme is the binding site where catalysis occurs. ... This article belongs in one or more categories. ... Petroleum coke (often abbreviated pet coke) is a carbonaceous solid derived from oil refinery cracking processes. ...


Thermal cracking

In thermal cracking elevated temperatures (~800oC) 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. William Merriam Burton (November 17, 1865 - December 29, 1954) was an U.S. chemist who developed the first thermal cracking process for crude oil. ... In chemistry, homolysis is chemical bond dissociation of a neutral molecule generating two free radicals. ... An alkene is one of the three classes of unsaturated hydrocarbons that contain at least one carbon-carbon double bond and have the general molecular formula of CnH2n (the other two being alkynes and arenes). ... A polymer is a long, repeating chain of atoms, formed through the linkage of many molecules called monomers. ...


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: Vapours of hydrogen chloride in a beaker and ammonia in a test tube meet to form a cloud of a new substance, ammonium chloride A chemical reaction is a process that results in the interconversion of chemical substances. ... In chemistry free radicals are uncharged atomic or molecular species with unpaired electrons or an otherwise open shell configuration. ... A BlueGene supercomputer cabinet. ... Look up simulation in Wiktionary, the free dictionary. ...


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. A chemical bond is the physical process responsible for the attractive interactions between atoms and molecules, and that which confers stability to diatomic and polyatomic chemical compounds. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ...

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.


See also

Chemical structure of methane, the simplest alkane Alkanes are chemical compounds that consists only of the elements carbon (C) and hydrogen (H) (i. ... The chemical structure of ethylene, the simplest alkene. ...

External links

  • Information on cracking in oil refining
  • Hydrocarbon Cracking - A Quick Summary for High School Students

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