Cracking (chemistry)
Cracking (german crack " split " ), rarely cracking, is a method of oil processing, longer chain length are split into shorter chain length hydrocarbons with hydrocarbons. This is necessary because the market is more short-chain hydrocarbons (petrol, diesel, light fuel oil ) calls, as contained in the oil, while long-chain hydrocarbons are increasingly (heavy fuel oil ) less use. The demand for ethylene and propylene for the production of polymers has resulted in that also short chain alkanes, such as ethane, propane, butane, naphtha, but mainly used as a reactant for thermal cracking process.
There are two groups of cracking processes: thermal cracking and catalytic cracking. Since the thermal cracking, no catalysts are used, also residues of petroleum distillation can be supplied that would harm because of their content of heavy metals and sulfur to the catalyst in the catalytic cracking.
- 2.1 Fluid Catalytic Cracking ( FCC)
- 2.2 hydrocracking
Thermal cracking
In the thermal cracking of hydrocarbon Freak tions are heated under pressure to about 450-800 ° C ( depending on the process ). Here come the long hydrocarbon molecules in such strong vibrations that the hydrocarbon chains break. It caused short-chain olefinic hydrocarbon molecules, but also - by recombination - high molecular weight paraffinic compounds, and aromatics, and - as an unwanted byproduct - carbon ( coke). The Russian engineer Vladimir Shukhov developed in 1891 as the first industrial plant for the thermal cracking of petroleum. In the U.S., such a procedure by William M. Burton at Standard Oil was developed and patented in 1913.
Steam cracking
In steam cracking ( C5 - C10), with the addition of water vapor into short-chain olefins (alkenes ), the resulting in a refinery naphtha ( or fractions thereof), which consists mainly of long-chain alkanes split. Even with the natural gas extraction accumulating ethane and propane and butane is used for olefin. Besides even find gas oil fractions and hydrocracker bottoms (see: hydrocracking ) as so-called alternative feedstock use. The steam cracker is used to produce raw materials that are processed mainly for plastics, paints, solvents and pesticides.
Visbreaking
The visbreaking ( for Viscosity breaking, reducing the viscosity ) is mainly used for production of gas oil from residues of petroleum refining (mostly known as vacuum residue, but also atmospheric residue from the distillation of crude oil ). A distinction is made between soaker visbreaking and coil visbreaking. The bituminous residue is first pre-heated to about 200 ° C and then heated in a heating furnace at about 450 ° C ( Soaker ) or 490 ° C ( coil). In the subsequent so-called Soaker ( a several cubic meters of large elongated tank) where the actual cracking place. When the coil visbreaking soaker missing, but the reaction temperatures are higher and thus compensating for the lower residence. The component mixture is behind the soaker / oven separated by distillation (atmospheric and then optionally under vacuum). In addition to the desired main products (gases, naphtha, gas oil, heavy oil components), small quantities of coke, the layers deposited on the furnace tubes in the soaker and in the distillation columns. Therefore, the system must be shut down every few months and are " decoking ". A coil visbreaking the analog method is also suitable for the cracking of vacuum gas oil. Then - conceptual confusing - spoken by a thermal cracker.
Delayed Coking
In the delayed coking ( German: delayed Koksbilden ) is the most violent thermal cracking process, an increase of visbreaking described above. The idea is yet to produce even higher quality products from the bitumen -like residue from the vacuum distillation of a refinery. Here, a similar product mixture of light (atmospheric distillable ) hydrocarbons produced as the visbreaking. However, the yields of light hydrocarbons are considerably higher, no liquid cracked residue is formed, but an integral petroleum coke. The process consists of a furnace and subsequent coke drums (English coke drums ). The delayed ( German: delayed) here has the following meaning: The formation of coke does not take place in the cracking furnace, but in the nachgeschaltenen coke drums. Usually 2 coking chambers with a volume of 500 to 2000 m³ available, which are operated alternately. In the furnace, the mixture is heated to temperatures of around 500 ° C at 20 to 30 bar. The hot mixture from the oven is in the active chamber, where the semi-volatile residue is reflected, while lighter items can be removed in gaseous form, respectively. The residue is cracked by the inflowing hot gas remains only to petroleum coke. The chamber is located out of service then cooled with steam and water. The cured coke is cut with water high pressure up to 300 bar from the respective inactive coke drum. The volatile components are separated by distillation and other parts of the refinery supplied while the coke is salable as a valuable end product or is refined in a subsequent Caciner.
Flexicoking
One on one Exxon Patent BASED thermal cracking process from severe residues ( Visbreaker vacuum residue u.ä ) still produce valuable light hydrocarbons. The hydrocarbon mixtures, however, are highly aromatic and hocholefinisch (such as light coker naphtha, LCN) and require in most cases an additional hydrogen treatment. The process produces very small amounts of coal dust, but a large amount of low calorific gas, which can be used simply as a fuel gas desulfurization and.
Catalytic Cracking
Catalytic cracking processes have the advantage over the thermal process several advantages: they require lower temperatures or lower pressures and run at a higher speed. A distinction is made between two catalytic cracking processes: hydrocracking and Fluidized - Bed Catalytic Cracking ( FCC).
Fluid Catalytic Cracking ( FCC)
( Light Cycle Oil, HCO: Heavy Cycle Oil LCO ) When the FCC heavy vacuum distillate to a refinery gases, liquid gases, gasoline components, two gas oil fractions and a residue fraction (slurry ) is cleaved. The process was developed during the second world war by the United States in order to increase production of aviation fuel. All relevant components are sulfur-and nitrogen-containing, as well as olefinic. The high olefin content is for the fuel components a great advantage (high research octane number ), the sulfur content it is not in the stricter environmental regulations in Germany. The cleavage takes place at temperatures of 450-550 ° C and a reactor pressure of 1.4 bar with the aid of a zeolite catalyst (aluminum silicate). In this method, a considerable amount of coke is formed, a continuous catalyst regeneration is necessary. The cracking process is therefore carried out in a fluidized bed reactor where the catalyst are extracted and in a second reactor ( regenerator ) is burned ( ie the deposited on the catalyst coke is burned ). The regenerated catalyst is returned to the process again. The reactor product is worked up in a allocated to the process plant (distillation, MEROX - Treating, ...). Some "modern" FCC plants can process atmospheric residue (crude oil distillation). Although in this case considerably more coke is produced, but the economy is possibly significantly better.
Hydrocracking
When hydrocracking coke and olefin formation is avoided in contrast to the FCC, since hydrogen is fed to the process. Hydrocracking is a very popular method because a large range of products is possible. However, this method is also very expensive since large amounts of hydrogen are needed, and the process at very high pressure is carried out. Since hydrogen can be very diffuse well through the reactor walls at high pressure, this expensive special steels are required. The catalyst employed is a bifunctional nature metals ( e.g., nickel ) on aluminosilicates. The metals are responsible for the hydrogenation, the acidic aluminosilicate for cracking.
The usual process conditions in the reactor temperatures of 350 to 500 ° C and pressures from 70 to 200 bar; the residence time is a few minutes. Are used vacuum distillates. As products of gases, propane and butane (LPG ), naphtha, kerosene, gas oil and atmospheric residue as so-called HCU -bottoms (also: Hydrowax ) generated. The chemical composition of these components is similar to the corresponding sections of the processing of crude oil (n- and iso- alkanes, cycloalkanes, and - depending on the process - small amounts of aromatics ). Another advantage is the very low nitrogen and sulfur content of the products, as well as the part of high product quality (high cetane number of the gas oil fraction ).
Also for vacuum residues there hydrocracking process (H -Oil, Hycon ).