When hazardous wastes are involved, aboveground storage tank cleaning and waste processing must be handled carefully to ensure environmental compliance and cost control.
An unpublished report by Rachele Grasso Smith of the RGS Group, Nutley, N.J., and Paul Sievert of Landry Service Co. Inc. (Lansco), Houston described a sludge removal and processing project at a major refinery on the U.S. East Coast. In this project, almost 5,200 bbl of oil were reclaimed from 7,500 bbl of tank sludge.
The problem
A 117-ft diameter floating-roof tank had to be cleaned in preparation for repairs. Timely completion of the task was critical to overall refinery operations.
The tank was in effluent-water sludge service. It contained about 7,500 bbl of sludge, which reached a level more than 3 ft above the manway.
Preliminary testing of samples indicated that the material contained 90% bs&w, comprising 60% solids and 30% water. After opening the tank, it was determined that the material contained 70% oil, 5% solids, and 25% water. The material was classified as EPA F037 hazardous waste.
Objectives
All tank cleaning and waste processing had to meet the standards published in the latest edition of API Bulletin No. 2015, Cleaning Petroleum Storage Tanks. The refiner considered two other processes for the project, both involving dewatering of the sludge using filter presses.
To operate efficiently, filter presses require added solids such as filter media and stabilizers. These solids increase the volume of solids requiring disposal.
Project requirements included:
- Sludge removal and tank cleaning. All visible sludge was to be removed from the tank, including sludge covering the tank wall up to the current roof line. The tanks floating roof was to be cribbed (temporarily supported) and 27 roof legs and 84 rafters were to be cleaned.
The tank seal was to be removed and roof pontoons freed of gas. Upon completion of the cleaning, the tank floor, shell walls, and internal floating roof were to be completely free of residue and the tank was to be certified gas-free and ready for hot work permitting. - Processing. Sludge was to be separated into three phaseswater, oil, and solids--with specifications established for each phase. Water was to be essentially free of solids, clear, and suitable for recycling through the refinery effluent treatment plant.
A maximum quantity of oil was to be recovered for reuse by the refinery. This oil was to meet a maximum 5% bs&w specification. - Disposal. Solid waste was to be disposed of at an appropriate facility. Because of the high cost of disposing F037 hazardous waste, the volume and water content of the solids were key to minimizing overall costs.
The solid material had to: contain at least 40 wt % net dry solids and no free liquids; pass EPA 40 CFR 260, 264, 265, and 270 paint filter tests; and be acceptable for landfill disposal without further treatment.
The process
Lanscos continuous separation process is based on centrifugation, and thus does not require the addition of solids. Fig. 1 (92469 bytes) shows a schematic of the process.
Pumpable material is removed from the tank by vacuum suction. The remaining solid material is diluted with heated cutter stock to produce a pumpable liquid. Any light oil can be used as cutter stock, but No. 2 fuel oil or diesel fuel typically are used.
A remotely operated robotics system is used to apply the diluent and manipulate the sludge in the tank.
The diluent is heated to about 140 F. and pumped at a pressure of 100-120 psi. Blending is controlled carefully to maintain optimum viscosity of the mixture. The mixture should be thick enough to hold solids, but thin enough to be pumped from the tank efficiently.
The blended sludge is removed from the tank using vacuum pumps. The sludge is then processed in portable centrifugation equipment located in the tank basin.
The blended sludge is diluted, then superheated to more than 200 F. and processed in a high-speed centrifuge. Solid particles are separated from the effluent and dropped from the centrifuge into roll-off boxes. At this point, the solids are dry, stable, and free of oil.
Clarified oil is pumped to a location specified by the refiner, and recovered water is sent to the effluent treatment plant.
The process continues until virtually all of the sludge is removed. Residual material in the tank is removed using a high-pressure (3,000 psi) water wash.
It is sometimes necessary to sandblast portions of the tank interior for API 653 inspection, although this was not required in the case of the East Coast refiner.
Removal
The liquid material in the tank was pumped to portable standby tanks in preparation for sludge removal and processing. Lansco used its manway box to open the manway and insert the robotic system without leaking any of the material from the tank.
Two centrifuges were used for this project, as were screeners, generators, and boilers. All equipment was portable, enabling work to be performed within the tank basin.
A 30-day work schedule was devised, based on six 10-hr days per week. A special roving air permit arrangement between Lansco and the State Department of Environmental Protection allowed Lansco to obtain permits and begin on-site preparations quickly after the contract was awarded. Without such an arrangement, permitting can take as long as 6 weeks.
On Day 30, refinery officials inspected the tank and determined that it met all requirements.
The process recovered about 5,200 bbl of oil (excluding cutter stock), clarified to refinery specifications, and 1,800 bbl of water. Only 100 cu yd of solids were produced from the original 7,500 bbl of sludge. Copyright 1995 Oil & Gas Journal. All Rights Reserved.
