Asphalt is a black, sticky, and very viscous semi-solid or liquid form present in many crude petroleums and also in some natural deposits, commonly referred to as asphaltum. Asphalt refining mostly starts with the separation of crude oil into different fractions available through the process of distillation. Once separation is complete, the fractions are further refined into various other products including kerosene, paraffin, asphalt, gasoline, diesel oil, and naphtha. Because asphalt is the heaviest constituent of crude petroleum, it doesn't boil off or evaporate during this process of distillation.
The asphalt refining process begins once crude petroleum is piped into a tube heater or heat exchanger and the temperature raised for the distillation process to begin. It then goes to an atmospheric distillation tower where light and volatile fractions of the crude petroleum are removed through a series of condensers and coolers. The end product after this process is normally referred to as topped crude, which is what is further refined to make asphalt.
Through a process known as vacuum distillation, high boiling fractions are drawn off to produce what is known as 'straight run' asphalt. Note however that the topped crude can still have low volatile components which cannot be drawn off through the simple distillation process, hence solvent de-asphalting, or solvent extraction, is further used to produce asphalt.
The resulting asphalt can then be 'cut back' or blended with a volatile substance to give a soft and easy to use pure asphalt. Note that the main use of asphalt is cementing and once the blended asphalt is used for construction or paving, the volatile elements used to blend it will evaporate once exposed to heat and air, leaving pure hard asphalt. Asphalt refining also entails a process known as emulsifying where asphalt is emulsified to give a liquid that will be easily sprayed through different sized nozzles, be able to mix easily with aggregate, and also be able to get pumped easily through different sizes of pipes.
Be advised though that asphalts will mislay their plasticity and hence harden and start to crumble or even crack once they lose their volatile molecular weight components or when these components become oxidized. Asphalts will also be affected by external things such as moisture from rain and even from other sources as asphalts have a big number of polar components to attract water molecules. You can modify the performance of asphalt by say blowing air on hot liquid asphalt to get rid of more volatile compounds and end up with highly viscous asphalt in the end.
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