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In the vacuum-steam distillation process the crude oil is heated and is introduced into a large cylindrical still.
Steam is introduced into the still to aid in the vapourisation of the more
volatile constituents of the petroleum and to minimise decomposition of the
distillates and residues.
The volatile constituents are collected, condensed, and the various fractions
stored for further refining, if needed.
The residues from this distillation are then fed into a vacuum distillation
unit, where residue pressure and steam will further separate out heavier gas
oils.
The bottom fraction from this unit is the vacuum-steam-refined asphalt cement.
The consistency of asphalt cement from this process can be controlled by the
amount of heavy gas oil removed.
Normally, asphalt produced by this process is softer.
As the asphalt cools down to room temperature, it becomes a semi solid
viscous material.
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