Chapter 6 - Thermal Cracking and Coking

Abstract

This chapter explores thermal cracking and coking in the process of petroleum refining. Thermal cracking is the cracking of heavy residues under severe thermal conditions. The liquid products of this process are highly olefinic, aromatic, and have high sulfur content. They require hydrogen treatment to improve their properties. Coking is the process of carbon rejection from the heavy residues producing lighter components lower in sulfur, since most of the sulfur is retained in the coke. The thermal treatment of hydrocarbons follows a free radical mechanism where cracking reactions take place in the initiation step. The reactions in the final step result in the formation of heavy fractions and products like coke. There are three classes of industrial thermal cracking processes. (1) Mild cracking (as in visbreaking) in which mild heating is applied to crack the residue just enough to lower its viscosity and also to produce some light products; (2) Delayed coking in which moderate thermal cracking converts the residue into lighter products, leaving coke behind; (3) The final process involves severe thermal cracking: part of the coke is burned and used to heat the feed in the cracking reactor, as in fluid coking. In other version of the process, steam is used to gasify most of the coke (flexicoking).