Thermal cracking - high-temperature processing (heating without air) of hydrocarbons in order to obtain, as a rule, products with a lower molecular weight. Thermal cracking of oil fractions is carried out at a temperature of 500-540 Β° C and a pressure of 2-5 MPa to obtain components of fuels and raw materials for the chemical and petrochemical industries.
Along with the decomposition of heavy hydrocarbons during thermal cracking, polymerization and condensation occur, the products of which are polycyclic and polyaromatic compounds. Thermal cracking also produces unsaturated hydrocarbons that are absent in natural oil and have relatively low chemical stability. These two factors are the main disadvantages of thermal cracking and the reason that this process is being replaced by other, more advanced methods of oil refining, in particular catalytic cracking .
The first scientific studies of high-temperature transformations of oil belong to the domestic chemical engineer A. A. Letniy , who first discovered that at temperatures above 300 Β° C heavy oil residues partially decompose into lighter products - gasoline, kerosene, gases ("Dry distillation of bituminous minerals" , 1875). This discovery formed the basis for the development of the cracking process.
The world's first industrial continuous thermal cracking unit was created and patented by Russian engineer V. G. Shukhov in 1891.
Classification
Thermal cracking is divided into three main types [1] :
- thermal cracking of liquid petroleum feedstock under high pressure;
- thermal cracking of liquid oil residues at low pressure ( coking , destructive distillation );
- pyrolysis of liquid and gaseous petroleum feedstocks.
Physico-chemical basis of the process
As a rule, thermal cracking proceeds according to a radical chain mechanism with the breaking of Π‘ β Π‘ bonds in paraffinic , naphthenic , alkylaromatic and high boiling unsaturated hydrocarbon molecules of crude oil and Π‘ β H bonds in low molecular weight paraffin and other hydrocarbons. In parallel with the breaking of bonds, polymerization and condensation reactions occur, leading to the formation of a high-boiling tar-asphaltene cracking residue and coke [2] .
The main factors of cracking are the thermal stability of the feedstock, temperature, pressure, process duration, as well as the heat of reaction and coke formation [1] .
Notes
- β 1 2 Smidovich E.V. Destructive processing of oil and gas // Technology of oil and gas processing. - M .: Chemistry, 1968 .-- 376 p.
- β Thermal Cracking - Chemical Encyclopedia