Visbreaking
The visbreaking (for Viscosity breaking , reducing the viscosity ) is a thermal cracking process in which heavy residue from crude oil processing ( vacuum resid , but also atmospheric residue ) is heated under pressure to approximately 450-490 ° C (depending on the process).
This creates short-chain olefinic hydrocarbon molecules from long-chain saturated hydrocarbon molecules, but also - through recombination - higher molecular weight paraffinic compounds, as well as aromatics and, as a by-product, carbon in the form of coke .
Process variants
Soaker Visbreaking (kettle cracking)
The viscous residue is first preheated to around 200 ° C and then heated to 450 to 460 ° C in a heating oven. The dwell time in the oven is significantly longer than with steam cracking . However, the actual cracking takes place in the downstream soaker (an elongated container several cubic meters in size). The products cool down to around 400 ° C. The residence time here is several minutes.
The component mixture is separated by distillation behind the soaker (atmospheric and then, if necessary, under vacuum). Here, low molecular weight arise hydrocarbons , mainly so-called Visbreaker- gas oil , but also "thermally cracked" naphtha , butane / butene -mixtures, propane / propene blends, as well as small amounts of methane , ethane / ethene and H 2 S . Most components contain olefinic components as well as sulfur and nitrogen compounds, so that they have to be treated with hydrogen before further processing ( hydrotreating , hydrodesulfurization ). The atmospheric “cracked” residue from the bottom of the atmospheric distillation column can be mixed up to produce heavy fuel oil or - if available - separated again in a vacuum column. The so-called flashed distillate (vacuum distillate of the visbreaker) can be used in further processing systems (hydrocracker or FCC, see below). The highly viscous flashed visbroken residue (vacuum residue from the visbreaker) is mixed up to produce heavy fuel oil or can be used as a starting material for residue gasification . In addition to the desired main products, small amounts of coke are produced, which are deposited in layers on the furnace tubes, in the soaker and in the distillation columns. The system must therefore be shut down and "decoked" at intervals of several months.
In the past, all Visbreaker components with an initial boiling point of more than 165 ° C (kerosene to Flashed Visbroken Distillate) were mixed with the cracked residue (so-called "backblending"). The resulting heavy oil had a lower viscosity than the feedstock . Hence the name “viscosity breaker”. Nowadays the light components and the residue are used separately in many cases.
Coil visbreaking (furnace cracking)
A coil (or furnace) visbreaker works like a soaker visbreaker, but the soaker is missing. The resulting reduced residence times are compensated for by higher furnace outlet temperatures (~ 490 ° C). The cracking takes place in the furnace - or better - in the stovepipes (stovepipe coils = "coils"). Behind the furnace, the product stream is suddenly cooled by a quench (cold recycled gas oil from the distillation part of the plant) and the cracking reactions are stopped. Yield structures and qualities are similar to the data of Soaker Visbreaking.
Thermal cracking
A process similar to coil visbreaking is also suitable for cracking vacuum gas oil . Then - confusingly - it is referred to as a thermal cracker.
See also
literature
- Technical chemistry by Manfred Baerns , Arno Behr , Axel Brehm, 757 pages, Verlag Wiley-VCH; Edition: September 1, 2006, ISBN 3-527-31000-2
Web links
Individual evidence
- ↑ Soaker Visbreaking ( Memento of the original from February 7, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 1.2 MB) CBI Chicago Bridge & Iron Company, 2012.
- ↑ Visbreaking (PDF; 70 kB) FWC Foster Wheeler USA Corporation. Clinton, New Jersey 1998.
- ↑ Thermal cracking ( Memento of the original from July 17, 2011 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. .
