Tank cleaning system removes sludge on-line

Randy Borne, Alan Stobbe
BP Oil Co.
Belle Chasse, La.
Paul Sievert
Landry Service Co. Inc.
Houston

After the manway is removed and covered with a flexible barrier, the robotic tank-entry device is stabbed through the barrier and bolted to the manway flange to form a vapor-tight seal (Fig. 2).

In 1997, British Petroleum Co.'s (BP) Alliance refinery at Belle Chasse, La., successfully removed sludge from a tank without taking it out of service.

The refinery used a modified aboveground storage tank (AST) cleaning process for its wastewater-treatment plant.

By putting the cleaning robot on the roof, rather than on the side of the tank, the process, known as Gas-Tight tank cleaning, successfully desludged and gas-fired the BP tank while it was on-line.

Sludge removal can be combined with on-site waste processing and hydrocarbon recovery in a continuous, closed-loop system that reduces disposal costs and recovers valuable products.

This technology is significant for several reasons:

The tanks remain in service during cleaning.
Personnel need not enter the tank.
The tank is not opened to the atmosphere, which limits hydrocarbon losses and odors while cleaning.
The cleaning process has a camera and video recorder which provide a visual record of the tank floor for preinspection.

Gas-Tight cleaning was developed by Landry Service Co. Inc. (Lansco) of Houston, which also performs the cleaning service.

The cleaning process can help refiners improve safety, reduce costs, shorten tank out-of-service times, and meet increasingly stringent emissions regulations.

Gas-Tight applications

Instead of degassing and cleaning in two separate steps, the Gas-Tight procedure simultaneously scrubs the vapor space as it cleans the tank. Up to 95% of the tank contents are removed while the tank remains sealed. Therefore, volatile organic compounds (VOCs) in the tank are not continuously regenerated into the vapor space as they are with conventional degassing.

By eliminating the need for external degassing, Gas-Tight cleaning provides a substantial cost-savings for the refinery.

Procedure

Major components of the Gas-Tight cleaning system are shown in Fig. 1 [187,754 bytes]. They include a robotic tank-entry device, a remote-control console, a remote-power generator, portable tanks, pumps, and hoses.

The robotic tank-entry device has a camera, a lighting system, and a nozzle for spraying diluent. A technician, called a robot pilot, operates the tank-entry device by video remote control.

If waste processing is needed, a phase-separation system with screeners and high-speed centrifuges is integrated with sludge removal in a continuous system. The equipment is portable, self-contained, and designed to be staged within the tank basin or a nearby location.

Staffing for a typical project requires a Lansco supervisor and a four-person crew.

To begin the cleaning process, a manway hatch is removed and the opening covered with a flexible vapor barrier. A side-mount, tank-entry device (roof-mounted units are also used) is inserted through the flexible barrier and bolted to the manway flange to form a vapor-tight seal ( Fig. 2 [24,577 bytes]). Hydraulic control lines and diluent-supply lines are connected to the device by flange fittings.

The robot pilot operates the tank-entry device by remote control from a climate-controlled command center, which is usually staged within the tank basin. Cameras and lighting systems on the tank-entry device provide video images of the tank interior to the robot pilot.

The tank-entry device sprays water or hydrocarbon-based diluent to scavenge VOCs and to clean tank interior surfaces. With flow rates up to 275 gpm at 250 psi, sludge deposits are liquefied so they can be pumped from the tank for processing.

Liquefied sludge can be removed through a tank sump, a water draw, or a satellite pumping location. A single tank-entry device slurries and removes up to 1,000 bbl of material/10-hr shift.

Commercial application

Faced with rising costs of traditional tank-cleaning methods and waste disposal, the BP Alliance refinery searched for safer, less-costly, and more environmentally responsible ways to clean tanks and control emissions.

The Lansco Gas-Tight process takes less time to clean tanks-typically one third of the time of conventional tank cleaning. Thus, tank owners can save time and money because their tanks are returned to service faster.

For these reasons, in 1995, BP chose the closed-loop, Gas-Tight system for AST cleaning and gas-freeing in its plant.

In the following example, Lansco creatively adapted the Gas-Tight process to clean Tank 15A. Unlike previous tank cleaning, Tank 15A remained in service, which eliminated downtime completely.

In 1997, Lansco was already on site processing waste from two slop-oil tanks in the wastewater-treatment plant. The two 80-ft diameter, fixed cone, internal floating roof tanks are at the tail end of the treatment process. At this stage, solids have been knocked out so that predominantly oil and water remain.

Tank 15A is a 280-ft external floating roof tank that feeds the slop-oil tanks.

Lansco was on-site to remove and separate the sludge (emulsified oil, water, and solids) from the bottom of the two slop-oil tanks. The separation was performed using a patented process that incorporates three high-speed decanting centrifuges. Average throughput was 1,500 b/d.

After several weeks of processing, Lansco personnel noted a substantial decrease in the amount of sludge being produced. Upon further investigation, the BP sludge-management team discovered approximately 12 ft of sludge buildup in Tank 15A.

Desludging Tank 15A presented several problems. First, the volume of material precluded uncovering a manway or cutting a door sheet for access. More importantly, however, the tank could not be removed from service for cleaning, since its capacity was needed in case of heavy rain.

BP needed to desludge the 280-ft tank without opening it, putting people inside, or removing the tank from service-a capability which did not exist at the time.

Technology transfer

Lansco personnel had experience in dealing with the root cause of the problem. Material from Tank 15A was being drawn through a 3-in. line. As the solids content in the tank increased, liquid began to channel, leaving solids-laden sludge to build up in the tank.

The channeling effect is similar to drinking soda through a straw from a cup of finely chopped ice. The liquid takes the path of least resistance, leaving the ice behind.

Lansco suggested using robotic equipment from its Gas-Tight tank cleaning process to break apart the channels and liquefy the sludge. The specifications for using the process are shown in Table 1 65,148 bytes].

Having had success with Gas-Tight cleaning projects in other areas of the refinery, the BP sludge-management team agreed to use the technique for Tank 15A.

Lansco began by installing a roof-mounted version of the tank-entry device. The device periodically jetted water into mounds of sludge and helped move material in the direction of a water draw.

The water used for the jetting operation was recovered water from the processing equipment. Liquefied material exiting the tank was sent to the processing equipment (which includes screeners and centrifuges) where the oil, water, and solids are separated. Recovered water from processing was used by the tank-entry device to liquefy sludge in the tank. By using recovered water, no additional liquid was introduced into the system.

Jetting effectively collapsed the channels and helped move solids out of the tank. Solids content of the material went from 1-2% to 15%. Proof was also seen downstream at the Lansco processing site. Solids production increased from 1-2 tons/day to 8-12 tons/day.

Moving the tank-entry device to various roof hatches and directing sludge toward other water draws enabled Lansco to reach all areas of the tank. Work proceeded around-the-clock, 7 days/week, for 3 months. The project was halted after it reduced the sludge level throughout the tank to roughly 1 ft.

Soon after restoring capacity to Tank 15A, solid buildup was found in a sister-tank, Tank 14A. Lansco performed the same technique there, with equal success.

In 2 years of work on Tanks 14A and 15A, Lansco recovered about 950 tons of solids and 5,714 bbl of oil for reprocessing, all while allowing the wastewater-treatment tanks to remain in service.

The future

BP anticipates broader applications of Gas-Tight cleaning at its refinery. Slurrying and removing sludge using Lansco robotics have proven successful on tanks with waxy crudes, cat fines, and other hard-to-handle materials, as well as with hazardous applications such as sour water tanks.

Using robotics in a Gas-Tight configuration is a more-effective approach for reducing manned entry, eliminating degassing, and controlling emissions.

Bibliography

Reimer, K., Pratt, T., and Sievert, P., "Robots Convert Sludge to Power," Power Engineering, August 1996, pp. 55-57. "Tank cleaning method removes, processes FO37 waste," OGJ, Oct. 9, 1995, pp. 86-87.

The Authors

Randy Borne is solid and hazardous waste coordinator at BP Oil Co. He is responsible for handling solids processing, SARA reporting, and emergency response at the Alliance Refinery. During his 27-year career, Borne has performed various duties at BP, including quality control, production, and environmental and safety activities.

Alan Stobbe is a senior engineer at the BP Oil Co. Alliance refinery. Stobbe joined BP in 1990 from Union Carbide, where he was technical catalyst production consultant. Previously, he worked in the plating and surface finishing industry. Stobbe has extensive experience in the environmental field, including design and construction of waste treatment facilities and regulatory expertise in permit application, modeling, and compliance issues. Stobbe holds a BS in chemistry from Cleveland State University and a BS in chemical engineering from the University of South Alabama.

Paul Sievert is vice-president of marketing for Lansco in Houston. Sievert has 20 years of experience in industrial sludge removal, material processing, resource recovery, and vapor-control services. He was instrumental in bringing to market Lansco's patented processing and robotic tank-cleaning technologies. Sievert holds a bachelors in business administration from Louisiana State University.

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