Visbreaker

Visbreaking ( Viscosity for breaking, reducing the viscosity ) is a thermal cracking process in which heavy residues resulting from the processing of crude oil ( vacuum residue, atmospheric residue but also ) is heated under pressure to approximately 450-490 ° 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 ( mainly all the gas oil range ), and - as an irritating but inevitable by-product - carbon ( coke).

Soaker visbreaking (boiler - cracking)

The bituminous residue is first pre-heated to about 200 ° C and then heated in a heating furnace at 450 to 460 ° C. The residence time in the furnace is a significantly longer than in the steam cracking. In the subsequent so-called Soaker however ( several cubic meters a large elongated tank) where the actual cracking place. The products cool to approximately 400 ° C. The residence time is several minutes here. The component mixture is behind the soaker separated by distillation (atmospheric and then optionally under vacuum). This produces low molecular weight hydrocarbons, primarily so-called visbreaker gas oil, as well as " thermally cracked " naphtha, butane / butene mixture, propane / propylene mixtures and small amounts of methane, ethane / ethene and H 2. Most components contain olefinic units as well as sulfur and nitrogen compounds, so that they must be treated before further processing with hydrogen ( hydrotreating, hydrodesulfurization ). The atmospheric " cracked " residue from the bottom of the atmospheric distillation column can be mixed to heavy fuel oil or - are separated in a vacuum column again - if any. The so-called flashed distillate (vacuum distillate of the vis-breakers ) can be used in further processing facilities ( hydrocracker or FCC, see below). The highly viscous flashed visbroken residue (vacuum residue of the vis-breakers ) is mixed up to heavy fuel oil and can be used as starting material for a residue gasification. In addition to the desired main product, 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 ".

In the past, all components were Visbreaker with an initial boiling point greater than 165 ° C (kerosene to Visbroken Flashed Distillate ) mixed with the cracked residue ( so-called " backblending "). The thus created heavy oil had a lower viscosity than the feedstock. Hence the name "viscosity breaker " comes. Nowadays, light components, and the residue can find a separate use in many cases.

Coil visbreaking ( oven - cracking)

A coil (or furnace ) visbreaking works like a soaker visbreaking, but lacks the soaker. The resulting reduced residence times are compensated by higher furnace exit temperatures ( ~ 490 ° C). So the cracking takes place in the oven - or better - in the furnace tubes ( furnace coils = " Coils" ) instead. Behind the furnace is ( cold recycled gas oil from the distillation section of the plant ), the product stream abruptly cooled by a quench, it stops the cracking reactions. Yield structures and qualities are similar to the data of the soaker visbreaking.

Thermal Cracking

A coil visbreaking the analog method is also suitable for the cracking of vacuum gas oil. Then - conceptual confusing - spoken by a thermal cracker.