Crude oils have a propensity to separate into the heavier and lighter phases. The heavy ends that separate from the crude oil and are deposited on the bottoms of storage vessels are known as “sludge.” It is a combination of hydrocarbons, sediment, paraffin and water. Sludge can accelerate corrosion, reduce storage capacity and disrupt operations. The paraffin component of sludge forms when molecules of individual straight-chain hydrocarbons bond together due to weak-induced dipole forces. These dipole forces are called London Dispersion Forces, or van der Waal bonds, and are responsible for like-molecular aggregation.
Agitation is commonly used to combat the formation of sludge in static volumes of crude oil. This comes from the theory it is possible to introduce sufficient kinetic energy into the system to retard or prevent the formation of the induced dipole (the van der Waal bond).
Preliminary investigations into sludge deposition by a major oil company in the 1980s concluded light crude oils require a minimum continuous energy input of 0.4 horsepower per 1,000 barrels (Hp/1,000 Bbl) of volume in order to prevent sludge deposition, increasing to 0.6-0.8 Hp/1,000 Bbl of volume in medium and heavy crudes.
This critical energy minimum can be related to a minimum critical velocity for suspension, or VS, which must be maintained throughout the entire fluid volume in order to prevent sludge formation. Research completed by Filip Hjulstrom, Ake Sundborg and Albert Shields has quantitatively equated fluid velocity and viscosity with its ability to keep particles entrained. Our own calculations and observations show the critical velocity of a typical crude oil required to keep sheared wax particles in suspension to be 2-4 feet per second. The velocity required to initially entrain wax particles can be higher, as the effects of friction and cohesion must be initially overcome.
Conventional electric prop mixers
Traditional practice has been to install electric side-entry prop mixers on tanks to provide the critical energy required to prevent sludge formation. These mixers often have issues with leakage at the packing gland and can be expensive to operate as the price of electricity rises.
In tanks that have been agitated for long periods of time by propeller mixers, it has been often observed only the area near the mixer (e.g., 20–30 feet radius) is free of wax accumulation; beyond this area waxy sludge has continued to be deposited. This observation suggests alternatives to propeller mixers be considered.
Jet mixers
Over the past 15 years, jet mixing has developed as an alternative to conventional prop mixers to combat the buildup of sludge.
Computational fluid dynamics (CFD) simulation research completed by Dr. Siamack Shirazi of the University of Tulsa has been used to understand the behavior and velocity of submerged fluid jets when introduced to the storage tank environment. This work has shown a single, properly de signed, submerged jet can carry a shearing velocity up to 150 feet into a storage tank and maintain the required critical velocity of 2-4 feet per second throughout the tank. Field work with jet mixers has shown them to perform as the CFD simulations predicted, effectively dissolving sludge up to 150 feet away.
Jet mixers are intended to be run for a short duration on an infrequent basis. Duration is dependent on the amount of sludge to be dissolved in the tank, and the frequency is dependent on both the rate of sludge formation and limitations on basic sediment and water (BS&W) levels allowed in the tank contents (more frequent mixes for lower changes in BS&W). A typical jet mix sludge management program would involve jet mixing for a week once every year or two.
The downsides of jet mixing are the clutter from pump, pipe and hoses on the tank lot when the mixer is active and the system needs to be monitored during operation. Considering the short, infrequent nature of active use and the overall effectiveness of the system, these shortcomings should not be seen as significant by the user.
For more information, contact Craig Bell at Craig.Bell@Allerion.com or (519) 862-4200.
