Visbreaker

Material exiting the furnace is quenched to halt the cracking reactions: frequently this is achieved by heat exchange with the virgin material being fed to the furnace, which in turn is a good energy efficiency step, but sometimes a stream of cold oil (usually gas oil) is used to the same effect.

The extent of the cracking reaction is controlled by regulation of the speed of flow of the oil through the furnace tubes.

Here the oil is held at an elevated temperature for a pre-determined period of time to allow cracking to occur before being quenched.

However, each offers significant advantages in particular situations: The quality of the feed going into a visbreaker will vary considerably with the type of crude oil that the refinery is processing.

At the low end of the scale, a furnace heating to 425 °C would crack only mildly, while operations at 500 °C would be considered as very severe.

Arabian light crude residue when visbroken at 450 °C would yield around 76% (by weight) of tar, 15% middle distillates, 6% gasolines and 3% gas and LPG.

These sediments are undesirable as they can quickly foul the filters of pumps used to move the oil necessitating time-consuming maintenance.

Subsequent addition of a diluent to manufacture a finished fuel oil can cause the colloid to break down, precipitating asphaltenes as a sludge.