Asphalt Manufacturing Processes

Asphalt Manufacturing Processes

Distillation

Normally this takes place in two stages: atmospheric distillation at normal pressure and vacuum distillation under reduced pressure. In atmospheric distillation, crude oil, after desalting, is heated to a temperature usually not exceeding 385°C and introduced into a fractionating column. Volatile fractions such as gasoline, kerosine and gas oil components, are separated and drawn off at selected levels of the column. The heavier hydrocarbons, known as the atmospheric residue and having the consistency of fuel oil, are removed at the bottom of the column.

This atmospheric residue is normally fed, at a temperature up to approximately 380°C, into a second fractionating column. Pressure is reduced to a very low level to permit distillation at lower temperatures and avoid undesirable thermal cracking. Lubricating oil distillate fractions are separated and drawn off. A vacuum residue is removed from the bottom of the column.
With heavy bituminous crudes, the vacuum residue is often a "commercial" bitumen. With lighter crudes it is a feedstock for further processing.

Air Blowing

This refining process consists of introducing air under pressure into a bitumen feedstock usually heated to between 220 and 300°C in a reactor. Both continuous and batch processing are used, sometimes in the presence of a catalyst. Oxidation and condensation reactions occur resulting in formation of higher molecular weight compounds which give a harder and less temperature susceptible bitumen. Moderate blowing is used to obtain hard road bitumens whilst severe treatment produces oxidized bitumens suitable for a wide range of building and industrial applications.

Thermal Conversion

This process results in the reduction of large paraffinic molecules to smaller ones. To a lesser degree a condensation occurs increasing asphaltenes and resin. Thus the process may be utilized to modify the ratio between paraffins, resins and asphaltenes using residues from lighter crudes than conventional ones. Temperatures up to 450°C are used but at a pressure of 15 to 20 atmospheres. During the cracking process some PAC generation occurs, and the thermal residue obtained is then distilled in a vacuum unit, to remove volatiles including the PACs, and the residue of this subsequent distillation can then be used as a component for bitumen.

Solvent Precipitation or deasphalting

Many vacuum residues are the source of valuable high viscosity base oils for lubricants generally known as bright stocks. Part of the process of refining bright stocks is the removal of asphaltic compounds by solvent treatment. Liquid propane or propane/butane mixture is generally used (at around 60°C and under sufficient pressure to maintain it as liquid) to dissolve the oil. The asphaltic fraction is precipitated and drawn off from the bottom of the tower and may be used in the manufacture of bitumens.
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Blending

Components are blended to achieve required specifications, for example the blending of two distillation residues of different penetration levels. Blending may be of the batch type in storage tanks with mixing facilities or continuous in-line blenders which ensure homogeneous mixing of two or even three components with a high degree of precision.

Ancillary Processes

Additional blending and fluxing processes may be used to provide further flexibility by use of fluxes and/or additives to manufacture bitumen derivatives. Auxiliary processing may also include the manufacture of bitumen emulsions.

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