Summary
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.
Introduction
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.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
20 December 2006
- Meeting paper published:
28 February 2007
- Revised manuscript received:
30 March 2009
- Manuscript approved:
7 May 2009
- Published online:
28 August 2009
- Version of record:
21 September 2009
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