Visbreaking (from the English vis (cosity) - viscosity , stickiness, ductility and breaking - breaking, breaking) is one of the types of thermal cracking . It is used to obtain mainly boiler fuels (heating oil ) from tar . Also in order to reduce the viscosity of heavy oil residues.
Content
Conduct Method
The process is carried out in the liquid phase under relatively mild conditions: 430-500 ° C, 0.5-3.0 MPa, the residence time of the raw material in the reaction zone is from 2 to 30 minutes or more. The main reactions are the splitting of paraffin and naphthenic hydrocarbons with the formation of hydrocarbon gases and gasoline , as well as liquid fractions boiling within 200-450 ° C, and secondary asphaltenes (the highest molecular weight components of oil).
Application of visbreaking products
Gasoline and gas (total yield 7-12% of the mass of raw materials) are separated from the vapor-liquid mixture by distillation; the cracking residue boiling above 200 ° C is a liquid boiler fuel (yield about 90%). Gases are sent to a gas fractionation unit, gas after refining using deep hydrogenation and catalytic reforming is used as a component of automobile fuel
In some cases, gas oil fractions are isolated from the cracked residue in a special evaporator (boiling limits 200–360 ° C and 360–450 ° C; yield 20–45% by weight). The first fraction after hydrotreatment serves as diesel fuel. Moreover, to ensure a given viscosity of boiler fuel, the remaining portion of the cracking residue is diluted, for example, with catalytic cracking gas oil.
Kerosene-gas oil fraction (50-55%) - is a valuable component of naval fuel oil ; after hydrotreating it can be used as a component of diesel fuels;
Cracking residue (38-42%) - used as boiler fuel , has a higher calorific value, low pour point and viscosity than straight-run fuel oil .
Hardware
The main reaction apparatus of the visbreaking unit is a tube furnace. In the case of the process at low temperatures (440-460 ° C), when the desired degree of conversion of the feed cannot be achieved in the furnace, an additional reaction chamber is provided. Due to the significant volume of the latter (30-50 m 3 ), the vapor-liquid mixture “ripens” in it for a predetermined time, which makes it possible to deepen visbreaking.
Literature
- Smidovich E.V. Cracking of crude oil and processing of hydrocarbon gases. 3 ed., M., 1980 .
- Kapustin V.M., Gureev A.A. Oil refining technology. At 2 hours. Part two. Destructive processes. - M.: KolosS, 2007 .-- 334 p.