Thermochemical Process To Remove Sludge From Storage Tanks
- Nelson O. Rocha (Petrobras) | Carlos N. Khalil (Petrobras) | Lúcia F. Leite (Petrobras) | Andre M. Goja (Petrobras)
- Document ID
- Society of Petroleum Engineers
- SPE Projects, Facilities & Construction
- Publication Date
- September 2009
- Document Type
- Journal Paper
- 97 - 102
- 2009. Society of Petroleum Engineers
- 4.2.3 Materials and Corrosion, 4.3.1 Hydrates, 4.2 Pipelines, Flowlines and Risers, 4.3.3 Aspaltenes, 4.1.5 Processing Equipment, 5.3.4 Integration of geomechanics in models, 1.8 Formation Damage, 4.1.9 Tanks and storage systems, 4.1.2 Separation and Treating
- Urucu, storage tanks, wax, sludge removal, thermochemical process
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When crude oil is stored in large tanks, invariably high-molecular-weight organic sediments are deposited. If left to accumulate, these deposits build up to form a sludge, which causes a reduction in the storage capacity. Routine industrial maintenance of storage tanks in vessels, terminals, and refineries unavoidably means the equipment's temporary inoperability. Furthermore, when conventional treatments are used to remove crude oil sludge, there is a potential for high environmental impact.
Petrobras has developed a thermochemical process to remove organic deposits in submersed oil pipelines, wax damage in production reservoirs, and petroleum sludge removal from storage tanks.
Based on a sludge removal case study, this paper describes the laboratory methodology to characterize the organic deposit physical/chemical properties, calculate thermochemical reaction kinetics and treatment dimensioning, and stage a physical simulation. It then reports the treatment's operational application to remove 800 m3 of organic deposit from an oil tanker storage tank.
The process relies on the strong exothermic reaction between two nitrogen salts estimated at 90°C that also produces large volumes of nitrogen generating turbulence. This reaction heats the sludge, which melts, and irreversibly disperses in the organic solvent. This condition was predicted in a previously studied phases diagram.
The method proved to be efficient, safe, and low cost compared to existing methods. The process' financial balance showed that the cost of the chemical reagents and operational facilities was covered by the value of the oil recovered from the organic deposit.
Since 1990, Petrobras has been performing a thermochemical treatment to remove oil sludge from storage tanks and oil and its derivatives in desalinators, separator vessels, flotation units, and related equipment.
The oil tanker Presidente Floriano located on the Rio Negro near Manaus, Brazilian Amazon region, has a 7,000-m3 crude oil storage tank in continuous use. An accumulation of sludge in the tank bottom had reduced this capacity by 800 m3. The objective of the sludge removal case study by thermochemical treatment was to recuperate all the oil in the sludge as well as facilitate compulsory, periodic maintenance of the tank bottom. Operational downtime was a crucial factor.
Crude oil sludge from storage tanks is defined as a multiphase water/oil/solid system with high stability, possessing a semisolid physical state. It is produced under normal storage conditions by the gradual sedimentation of heavy oil fractions. It appears as an emulsion, varying in consistency, density, thickness, and composition across the whole tank bottom.
Inorganic solids in the crude oil, such as clay, silica, calcite, and corrosion-produced residues, contribute to this process. They bring more hardness and higher density to the deposit. The accumulating sediment compacts with organic compound degradation during the storage, resulting in a thick layer of sludge, which is difficult to remove.
The proposed thermochemical treatment, like other sludge removal methods (mechanical, centrifuging, robotic, and solvent dilution), has some application limitations. This method becomes more efficient when the sludges have a predominately wax nature, possess low water percentages, and are devoid of contaminating agents that provoke the alkalinization of the solution.
The shape of the storage tank bottom has no influence on the process. However, the relatively smooth, level surface of the sludge is a fundamental condition for the uniform distribution of the thermochemical reaction over all the sludge. To this end, a floating vessel must also be maintained on a stable, even keel during the treatment.
Furthermore, significant differences between sludge types and their formation make it necessary to establish customized procedures, principally regarding the preselection or mixture of solvents. Therefore, it is important to know the sludge characteristics to define the best form of treatment and removal.
|File Size||512 KB||Number of Pages||6|
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