Thermochemical process to remove sludge from storage tanks

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. Copyright © 2009 Society of Petroleum Engineers.