BULK HEATING CLEANS PARAFFINIC BOTTOMS FROM CRUDE TANKS

Feb. 20, 1995
John Badrock Cooperheat Inc. Houston Robert Coutu Cooperheat Inc. Mississauga, Ontario Norman Johnson Statia Terminals N.V. St. Eustatius, Netherlands Antilles Andrea Martin Chicago Bridge & Iron Co. Oak Brook, Ill.
John Badrock
Cooperheat Inc.
Houston

Robert Coutu
Cooperheat Inc.
Mississauga, Ontario

Norman Johnson
Statia Terminals N.V.
St. Eustatius, Netherlands Antilles

Andrea Martin
Chicago Bridge & Iron Co.
Oak Brook, Ill.

It is often challenging to remove from service crude oil tanks that have been in use for many years. Bulk heating, as opposed to localized heating, has been used to successfully liquefy heavy paraffinic or asphaltic bases in crude oil tanks. The process provides economical product recovery, while minimizing waste production and reducing human exposure to hazardous working conditions.

Statia Terminals Point Tupper Inc., Point Tupper, N.S., used bulk heating to remove 2 ft of weathered, paraffinic tank bottoms from six 450,000 bbl tanks that held Cabinda crude from West Africa. Immersion tube heaters were installed through the tank manholes and a diluent was added. Two 10-million BTU/hr propane burners supplied heat, and tank mixers operated continuously to aid in heat transfer

The tank contents were heated to 135 F. and the temperature was held constant for 3 days. The resulting hot liquid was a pumpable and recoverable product.

About 10 days were required to clean each tank. All six tanks were cleaned in a total of 12 weeks.

Statia Terminals retained Cooperheat Inc.'s thermal engineering services to assist in evaluating the tank heating requirements. Cooperheat designed the immersion tube heaters, and Statia Terminals fabricated and installed them.

TANK CLEANING

Concern for the general condition of tanks, especially tank bottoms, has motivated many tank owners to implement tank inspection programs based on API Standard 653, "Tank Inspection, Repair, Alteration, and Reconstruction." Tanks also are removed from service for repairs and product changes.

Tank cleaning can be the most expensive part of an inspection and repair program, especially for tanks that contain heavy sludge bottoms. A fast, economical tank cleaning method can help in keeping tanks operational.

In the past, tank owners sometimes removed heavy sludge bottoms by methods that involve high worker risk and hazardous material disposal problems. Mechanical removal of sludge bottoms, whether by shovel, pick ax, or bulldozer, meant that personnel and equipment in the tank were exposed to potentially hazardous conditions. Cleaning often required weeks or months to complete. And damage to the tank, in the form of bottom gouging and door sheet damage, was common.

Recently, robotics and portable processing equipment have addressed some of the manual labor problems. These methods also have the added benefit of recovering product. But even these more sophisticated approaches have their limitations. The robots are small in relation to the amount of sludge in a large tank, and the cleaning process can be arduous. In fact, at Statia Terminals, robotics. operations were used on three tanks during a 3-month period, without success.

Tank owners and operators need a variety of tank cleaning methods from which to choose.

The choice of a tank cleaning method is based on several factors, including sludge characteristics, the cost and time involved, and the amount of recoverable product.

Bulk heating has broad applicability for tanks containing sludges that respond well to heat and diluent, and for tanks that are equipped with one or more manholes and either permanent mixers or nozzles for mixer installation. The bulk heating process can be done safely from outside the tank; it generates a minimum of hazardous waste if any, it recovers most or all of the product in the sludge, and it requires little time to complete the process.

STATIA PROJECT

Six 450,000-bbl crude oil tanks at the Statia terminal had collected varying amounts of sludge and had been standing unused for several years. Previous attempts to clean the tanks included shoveling them out with the aid of cold diluent, using remote-control robots and a high-pressure spray of hot diluent, and adding diluent with localized heating.

None of these operations were successful and all involved lengthy downtime. And the cost of these unsuccessful attempts was significant. In fact, the use of robotics was halted because of the projected costs to complete the job. The paraffinic sludge had the consistency of shoe polish and a pour/flash point of 130 F. The sludge went into stable solution at 1300 F. with the addition of diluent and agitation. The resulting mixture had a 20 F. pour point and was a salable product.

Before the unsuccessful attempts to clean three of the tanks, the addition of diluent and heat had been identified correctly as the best course of action. But the diluent and heat had been applied locally. To be successful, the tank contents as a whole had to be heated and mixed to the required temperature.

HEATING OPERATION

The task of uniformly heating a 240-ft diameter tank is formidable, especially when the product is combustible. The ideal heating operation applies a large quantity of heat to the tank contents in a short time, hence the term "bulk heating.

A thorough thermal analysis of the sludge is needed to operate safely and to remove the maximum amount of sludge in the most efficient manner. Hot gas immersion tube heaters were chosen because they have several advantages. The tubes could be installed through the tank manholes and they reached well into the product. And, in comparison with steam heating, hot gas immersion uses less energy and equipment to achieve the same effect.

Work done by British Gas on recirculation of hot gases showed immersion tube heaters to be a safe, efficient way to apply large quantities of heat to fluids." Heat-transfer rates from an immersion tube can vary from 15,000 BTU/sq ft-hr with low-velocity burners to 30,000 BTU/sq ft-hr with high-velocity burners.

Cooperheat Inc. had a suitable burner for this application.

Although the presence of a gas burner next to a storage tank storing flammable material seems risky, the immersion tube heaters are completely safe. In this heating method, the flame is contained within the combustion chamber and never contacts the flammable product.

Although the gases entering the heater reach very high temperatures, the temperature of the secondary heater pipe that contacts the sludge never surpasses the flash point of the sludge. Ignition, therefore, cannot occur.

The immersion tube heaters were fabricated as a 16-in. pipe within a 24-in. pipe, with a connection for the burner and an exhaust vent. A side view of the installation is shown in Fig. 1 (115510 bytes).

This heater configuration takes advantage of the mixing of the hot gases leaving the burner with the returning gases in the annular gap. Because the gases make more than one pass through the system, there is an increase in volumetric flow down the inner tube and a reduction in the temperature of the inner tube.

During the heating operation, the maximum temperatures of the burner and exhaust gases were, respectively, 1,800 F. and 700 F. This temperature difference produces a efficiency of 61%.

CLEANING

The six tanks were cleaned in series. For each tank, two heaters, each with a 10 million BTU/hr high-velocity propane burner, were installed in manholes 180 apart. The tanks already were equipped with two permanently installed mixers each. The configuration is illustrated in Fig. 2 (102832 bytes).

Diluent No. 2 (cat gasoline) was added to a level of 7 ft. (Although only a 60% diluent mixture was necessary, the roof needed to float at the 7-ft height for the mixers to operate.) To minimize heat loss, I in. of polystyrene insulation was temporarily installed on the tank roof and shell up to the liquid level.

The tanks reached 1351 F. in about 4 days. The burners were throttled and the temperature was held at 135 F. for 3 days. Continuous agitation helped the diluent dissolve the sludge. Pumping of the resultant mixture began at the end of the seventh day and all product was transferred to a holding tank.

Once the product had been removed from one tank, the heaters and insulation were moved to the next tank. Upon opening each tank, inspection showed that the only areas in which product still remained were those "dead" areas that the mixers did not reach. This product was removed by vacuum truck.

All of the transferred product stored in the holding tank was sold during the summer of 1994.

ECONOMIC ANALYSIS

Table 1 (40292 bytes) compares the cost of the robotic cleaning services with the cost of the bulk heating operation. The comparison is not completely valid, however, because robotics was not used to complete cleaning of any tanks. But the comparison is helpful as an "order of magnitude" evaluation.

Both methods enable recovery of some costs by the sale of the useable end product. But because, for comparison purposes, both methods presumably reclaim the same sludge, this recovered cost is not reflected in the summaries in Table 1 (40292 bytes).

It should be noted that the partial cleaning of the three tanks by robotics did not provide any advantage for the follow-up cleaning by bulk heating. The heaters and insulation still had to be installed, the diluent added, and the tank contents heated and mixed.

The cost and duration of the bulk heating process are fairly independent of the amount of sludge to be removed; whereas the cost and duration of cleaning by robotics are highly dependent on the amount of sludge present. In evaluating cleaning methods, bulk heating's lack of dependence on sludge volume is an advantage.

Bulk heating also enjoys the economy of multiples. The same heater can be used for several tanks, and savings in analysis, management, and setup are realized. Even for cleaning one tank, bulk heating costs about $624,000, compared to $1,161,000 for robotics.

REFERENCES

  1. Francis, W.E., "Forced Recirculation in Industrial Gas Appliances," 2nd Autumn Meeting of the Institute of Gas Engineers, November 1956.

  2. Industrial Development Committee (U.K.) Report 730/6OU171, "Injectors, Ejectors, and Jet PUMPS."

THE AUTHOR

John Badrock is a combustion engineer with Cooperheat Inc. in Houston. He has worked for 24 years in the on site treatment industry, domestically and overseas. He graduated from Manchester University in fuel technology and is a chartered engineer in the U.K.
Robert Coutu is Cooperheat's Eastern Canada district manager. Based in Mississauga, Ont., he is responsible for sales and operations in the area. He has 18 years' experience in the thermal processing industry and has worked for Cooperheat for 8 years.
Norman Johnson is a project manager for Statia Terminals N.V., St. Eustatius, Netherlands Antilles, a wholly owned subsidiary of CBI Industries Inc. He has 32 years of experience in engineering and construction with Chicago Bridge & Iron Co., domestically and overseas. He has a BS in civil engineering from the University of Idaho.

Andrea Martin is an environmental specialist in Chicago Bridge & Iron's business development department. She has worked for CBI for 13 years. She is a professional engineer and has a BS in chemical engineering from Michigan Technological University.

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