Cracking (chemistry)
|
|
In petroleum geology and chemistry, cracking is the process whereby complex organic molecules (e.g. kerogens or heavy hydrocarbons) are converted to 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.
| Contents |
Applications
In an 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 for short) 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. 750-900°C or more) which produces valuable ethylene and other feeds 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.
Fluid Catalytic Cracking
Fluid catalytic cracking is a commonly used process and a modern oil refinery will typically include a cat cracker, particularly refineries in the USA due to the high demand for gasoline. The process was first used in around 1942, and employs a powdered catalyst. Initial process implementations were based on a reactor where the catalyst particles were suspended in an ascendant flow of feed hydrocarbons in a fluidized bed.
In newer process variants, the contact time between the catalyst and the feed is greatly reduced in order to reduce the amount of coke deposited on the catalyst. The actual reactor is an ascendant-flow pipe called the "riser" in which pre-heated feed meets hot catalyst particles for just a few seconds before the catalyst is separated from the hydrocarbon using a cyclone, contacted with steam to strip off the remaining hydrocarbon and stop the reaction, and then transported 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 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.
Hydrocracking
Hydrocracking is a catalytic cracking process assisted by the presence of an elevated partial pressure of hydrogen. The products resulted are saturated hydrocarbons; depending on the process severity (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.
Major products from hydrocracking are jet fuel, relatively high octane rating gasoline fractions and LPG. All these products have a very low content of sulfur and contaminants.
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).
In steam cracking, a gaseous or liquid hydrocarbon feed is diluted with steam and then briefly heated in a furnace. Typically, the reaction temperature is very hot—over 900°C—but the reaction is only allowed to proceed for a few tenths of a second before being quenched by contact with a colder fluid.
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 amomunts of propene, C4-hydrocarbons and liquid products.
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.
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.
Catalytic Cracking
Catalytic cracking uses a catalyst to aid the process of breaking down large hydrocarbon molecules into smaller ones. During this process, less reactive and therefore more stabile and longer lived intermediate cations accumulate on the catalysts' active sites generating deposits of carbonaceous products generally (and in many cases inappropriately) 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 are used. 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.
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 major sorts of reactions that take place, with examples, 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.
- 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
History
The first thermal cracking method, the Burton process, was invented by William M. Burton; the oil industry first using it to produce gasoline in 1913.
Catalytic cracking, based upon a process developed by Dr. Alex Golden Oblad at Standard Oil of Indiana has been used from around 1936. Typical catalysts include alumina, silica, zeolites, and various types of clay.
See also
More details about the cracking mechanism are provided in the alkane article.de:Cracken nl:Kraken (chemie) pl:Kraking fi:Krakkaus (kemia) sv:Krackning