The formation of crude oil sludge
Most crude oils have a propensity to separate into the heavier and lighter hydrocarbons before refining. Such problem is often exacerbated by cool temperatures, venting of volative components from the crude, and by the static condition of fluid during storage. The heavy ends that separated from the crude oil and are deposited on the bottoms of storage tanks/vessels are known as tank bottoms or sludge.
Sludge are a combination of hydrocarbons, sediment, paraffin and water. It can accelerate corrosion, reduce storage capacity and disrupt operations. For oceangoing marine tankers the problems are twofold. At the end of many journeys the tankers have to go into drydock for maintenance due to corrosion caused by slop oil settling out and coating the walls of the tanks.
Paraffin-based crude oil sludge forms when the molecular orbitals of individual straight chain hydrocarbons are blended by proximity, producing an induced dipole force that resists separation. As the heavier straight chain hydrocarbons flocculate, they tend to fall out of suspension within a static fluid, as in the case of storage tanks/vessels where they accumulate on the bottom as viscous gel commonly known as sludge or wax. This newly formed profile stratifies over time as the volatile components within the sludge are expelled with changes in temperature and pressure. The departure of such volatile components results in a concentrated heavier fractions within the sludge, accompanying with increased in density and viscosity, and decreased fluidity.