This process involves crude oil being heated to temperatures of between 300 and 350 degrees Celsius, to separate lighter fractions from the non-boiling component, known as an atmospheric residuum. The lighter fractions are drawn off and sent to other refinery units. To remove the last traces of the lighter fractions and avoid thermal transformation of the molecules, the atmospheric residue is then introduced into a vacuum distillation unit, where the pressure reduction lowers boiling temperatures and avoids unwanted thermal cracking of the molecules. The pressure and temperature conditions within the vacuum process determine the hardness of the residuum and, as such, the grade of bitumen produced.
Specific solvents can also be used to separate the lubricant and bitumen components of crude, without changing their chemical structure. Depending on the solvent used – propane or butane – different categories of bitumen can be obtained. By controlling the variables in the vacuum distillation process, bitumens with varying degrees of hardness can be produced. Needlepenetration and softening point Ring and Ball are the two fundamental defining characteristics of bitumens produced to engineering specifications.
Bitumen can be further processed by blowing air through it at elevated temperatures to alter its physical properties for specific applications. Two different types of bitumen can be produced in this way, depending on the degree of oxidation: air rectified bitumen and oxidised bitumen. Oxidised bitumen is used in roofing applications, while air rectified bitumen is used in paving applications and some roofing applications.
Oxidised bitumen has a distinctive consistency at room temperature and a rubbery nature that affects how it responds to stress. The process of oxidation increases the stiffness and softening point Ring and Ball of the bitumen and considerably alters key physical properties. Varying the length of the oxidation or air blowing process varies the extent of the reaction and produces distinctive end products.