recovered. Soaker visbreaking In soaker visbreaking, the bulk of the cracking reaction occurs not in the furnace but in a drum located after the furnace called the soaker. Here the oil is held at an elevated temperature for a pre-determined period of time to allow cracking to occur before being quenched. The oil then passes to a fractionator. In soaker visbreaking, lower temperatures are used than in coil visbreaking. The comparatively long duration of the cracking reaction is used instead. Process options Visbreaker tar can be further refined by feeding it to a vacuum fractionator. Here additional heavy gas oil may be recovered and routed either to catalytic cracking, hydrocracking or thermal cracking units on the refinery. The vacuum-flashed tar (sometimes referred to as pitch) is then routed to fuel oil blending. In a few refinery locations, visbreaker tar is routed to a delayed coker for the production of certain specialist cokes such as anode coke or needle coke. Soaker visbreaking versus coil visbreaking From the standpoint of yield, there is little or nothing to choose between the two approaches. However, each offers significant advantages in particular situations: De-coking: The cracking reaction forms petroleum coke as a byproduct. In coil visbreaking, this deposits in the tubes of the furnace and will eventually lead to fouling or blocking of the tubes. The same will occur in the drum of a soaker visbreaker, though the lower temperatures used in the soaker drum lead to fouling at a much slower rate. Coil visbreakers therefore require frequent de-coking. This is quite labour-intensive, but can be developed into a routine where tubes are de-coked sequentially without the need to shut down the visbreaking operation. Soaker drums require far less frequent attention but their being taken out of service normally requires a complete halt to the operation. Which is the more disruptive activity will vary from refinery to refinery. Fuel Economy: The lower temperatures used in the soaker approach mean that these units use less fuel. In cases where a refinery buys fuel to support process operations, any savings in fuel consumption could be extremely valuable. In such cases, soaker visbreaking may be advantageous. Quality and yields Feed quality and product quality The quality of the feed going into a visbreaker will vary considerably with the type of crude oil that the refinery is processing. The following is a typical quality for the vacuum distillation residue