of Arabian light (a crude oil from Saudi Arabia and widely refined around the world): Once this material has been run through a visbreaker (and, again, there will be considerable variation from visbreaker to visbreaker as no two will operate under exactly the same conditions) the reduction in viscosity is dramatic: Yields The yields of the various hydrocarbon products will depend on the "severity" of the cracking operation as determined by the temperature the oil is heated to in the visbreaker furnace. 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. Fuel oil stability The severity of visbreaker operation is normally limited by the need to produce a visbreaker tar that can be blended to make a stable fuel oil. Stability in this case is taken to mean the tendency of a fuel oil to produce sediments when stored. These sediments are undesirable as they can quickly foul the filters of pumps used to move the oil necessitating time-consuming maintenance. Vacuum residue fed to a visbreaker can be considered to be composed of the following: Asphaltenes: large polycyclic molecules that are suspended in the oil in a coloidal form Resins: also polycyclic but of a lower molecular weight than asphaltenes Aromatic hydrocarbons: derivatives of benzene, toluene and xylenes Parafinic hydrocarbons: alkanes Visbreaking preferentially cracks aliphatic compounds which have relatively low sulphur contents, low density and high viscosity and the effect of their removal can be clearly seen in the change in quality between feed and product. A too severe cracking in a visbreaker will lead to the asphaltene colloid becoming metastable. Subsequent addition of a diluent to manufacture a finished fuel oil can cause the colloid to break down, precipitating asphaltenes as a sludge. It has been observed that a paraffinic diluent is more likely to cause precipitation than an aromatic one. Stability of fuel oil is assessed using a number of proprietary tests (for example "P" value and SHF tests). Economics Viscosity blending The viscosity blending of two or more liquids having different viscosities is a three-step procedure. The first step is to calculate the Viscosity Blending Index (VBI) of each component of