The DS Jet Mixing System offered by Allerion Oilfield Services can prevent the build-up of crude oil sludge
CRUDE oils have the propensity to separate into the heavier and lighter hydrocarbons from which they are composed. This is often aggravated by cool temperatures, the venting of volatile components from the crude, and by the static condition of fluid during storage.
The heavy ends that separate from the crude oil and are deposited on the bottoms of storage vessels are known as ‘tank bottoms’, or ‘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 that it is possible to introduce sufficient kinetic energy into the system to retard or prevent the formation of sludge.
Preliminary research into sludge deposition by a major oil company concluded that light crude oils require a minimum continuous energy input of 0.4 Hp/1000 bbl of volume in order to prevent sludge deposition which increases to 0.6–0.8 Hp/1,000 bbl of volume in medium and heavy crudes. In order to prevent sludge formation, the minimum critical velocity for suspension, or Vs, must be maintained throughout the entire fluid volume.
Conventionally, side-entry electric propeller mixers are installed on the tanks for agitation to prevent sludge formation, but these mixers are not enough to maintain the critical energy throughout the volume and results in sludge-free areas only surrounding the mixer. The areas beyond a specific radius (about 20–30’, or 6–9 m) accumulate substantial or severe sludge deposition, which generally results in operational problems and lost inventory.
THE DS JET MIXING SYSTEM
For more than 20 years, the DS Jet Mixing System, developed by Allerion Oilfield Services, has been the best method for recovering aboveground crude oil storage tank sludge. This technique, developed as an alternative to conventional prop mixers, is safe, efficient and cost-effective in combating sludge build-up.
The DS Jet Mixer is an oscillating nozzle mounted permanently on the tank manway. It introduces a high-pressure stream of diluent into the sludge, shearing and resuspending the wax molecules back into a useable state. This reclaims service capacity and drastically reduces cleaning costs and outage time. This smart technology de-sludges without removing the tank from service or disrupting operation. A powerful diesel-driven pumping system and the inline nature of the DS Jet Mixer gives the DS Jet Mixing System an effective cleaning radius of more than 150’, with a focussed, high-velocity jet that moves through a 120° arc.
The use of this method greatly reduces the personnel exposure risks and hazards associated with tank cleaning. It provides an economical and permanent method of managing tank bottom accumulation. When not active, it rests in dormant configuration on the tank ready for easy activation.
The standard DS Jet Mixer is designed to mount on a standard 24” (60.96 cm) diameter storage tank manway. Adapter rings are available to mount the mixer on manways with a diameter as small as 18” or as large as 48”. Custom adapters can be fabricated for unusually sized or shaped manways.
Other related products are the DS Articulating Lance, DSM2 Mixer, and DS Rapid Jet Mixer. Since the inception of this concept, Allerion has successfully provided tank bottoms management and cleaning services to major oil and petrochemical companies reclaiming thousands of barrels of useable hydrocarbons worth millions of dollars that would have been disposed of, if cleaned conventionally.
Jet mixers are intended to be activated 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 SUCCESSFUL EXAMPLE
Two 50,000 m3 capacity crude oil tanks were cleaned using the DS Jet Mixing System to resuspend and shear the accumulated sludge. The jetting resulted in the recovery of 1,200 m3 of oil from the tank bottoms and elimination of the need for incineration of sludge. Shown in Figures 6 and 7 are 3D sludge profiles illustrating the deposition of sludge in the tank before and after mixing. After the success of this project, five more tanks were jet mixed and cleaned resulting in 3,847 m3 of oil from the tank bottoms.
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