So welcome to part 5 of the petroleum refining article series by chemo concept. In this part, I have covered thermal conversion processes. These processes are the first ad the oldest conversion processes mostly replaced by other energy-efficient processes such as catalytic conversion processes. But learning these methods is important because in some refineries these processes are still carried out.
Conversion properties are very important processes in petroleum refining. These processes are carried out to produce light valuable products from heavy bottom products such as Vacuum residue.
What are Thermal Conversion Processes?
Thermal conversion processes involve the cracking of higher molecular weight hydrocarbons in severe conditions (high temperatures). These higher molecular weight hydrocarbons are from the bottoms of distillation units, especially from vacuum distillation units.
Cracking means these long-chain molecules of hydrocarbon are broken down into smaller size molecules. And thermal cracking is done by The main goal of these processes is to produce valuable light components from heavy residues.
Mechanism of Thermal Conversion Processes
Thermal conversion processes are based on a free radical mechanism. Free radicals are molecules containing carbon with one unpaired electron. And due to this unpaired electron, these free radicals are highly reactive.
So thermal conversion (cracking) processes take place in two steps:
- Initiation: In these steps due to high temperatures heavy residues get split into two parts (mostly uneven) and a free radical is formed.
- Final Conversion: A pie bond is formed between two nearby carbon atoms having free radicals and an unsaturated compound is formed.
Types of Thermal Conversion Processes
There are major two types of thermal conversion (cracking) processes:
- Visbreaking: Visbreaking is a mild thermal conversion (cracking) process that takes place at relatively low temperatures. The main goal of visbreaking is to reduce the viscosity of heavy bottoms to increase their flowing ability. There are major two types of visbreaking processes categorized based on types of equipment used for the operation: Coil visbreaking and Soaker visbreaking.
- Coking: Coking is a severe thermal conversion (cracking) process that takes place at relatively high temperatures. The main goal of coking is not only to produce light components but also produce petroleum coke which can be utilized in other industries such as in blast furnace in iron production. There are major three types of visbreaking processes categorized based on the process taking place: Delayed, Fluid, and Flexi Coking.
Products of Thermal Conversion Processes
The main products of thermal conversion processes are streams of light hydrocarbons mostly consist of olefins and aromatic content. Before blending these components to other components hydrogenation is carried out to saturate these hydrocarbons. (Saturation means the removal of pie bonds from hydrocarbons like alkenes and alkynes which are olefins and unsaturated cyclic content which are aromatics to saturated components also called alkanes).