A visbreaker is a processing unit in an oil refinery whose purpose is to reduce the quantity of residual oil produced in the distillation of crude oil. It also helps to increase the yield of more valuable middle distillates.
The visbreaker thermally cracks the large hydrocarbons molecules by furnace heating to reduce the viscosity. This produces small quantities of light hydrocarbons, (LPG and Gasoline). The process is non-catalytic.
The process reduces the feed stream viscosity. Usually obtained as a residue from vacuum distillation, it reduces the amount of residual fuel oil used by the refinery and increases the output of middle distillates.
Coil visbreaking; the thermal cracking process takes place in furnace tubes. After being heated the oil is quenched and then passed through a fractionator, where it is separated and removed.
Soaker visbreaking; the thermal cracking process occurs in the soaker drum after the furnace. The oil is held at an elevated temperature for a pre-determined time; to allow for cracking, to occur before being quenched. The oil then passes to a fractionator. Lower temperatures in concert with a longer soak time are used.
From the visbreaker it can be fed to another vacuum fractionator.
For both units De-Coking is a problem. However in the coiled unit it will be done more frequently as the smaller diameter of the pipes are easily plugged by less coke material. This system however does not have to be shut down entirely to facilitate the removal of the coke material. Only the affected part need be isolated.
In the soaker unit they need not be done too often as it takes longer to develop. However when it is needed the entire unit needs to be shut down. Also in the soaker unit the lower temperature means that fuel and energy costs can be minimized. Leading to an overall cost saving and better economy.
