Closing the Loop
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THE MAGAZINE DEVOTED TO NICKEL AND ITS APPLICATIONS |
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SUNCOR ENERGY'S Firebag project in northern Alberta. |
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WATER IS RECYCLED in these vertical tube, falling-film, vapour compression evaporators. |
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THE KEY COMPONENT inside the evaporators are tubes made of nickel alloy
N08926. A total of 107 tonnes of this alloy are used in each evaporator. |
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HOW THE EVAPORATORS WORK: As brine flows down the inside surface of the tubes in a thin film, it is
heated by steam on the outside of the tubes. |
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日本語 |
A new water treatment process reduces the environmental footprint of mining oil sands in
northern Alberta, Canada
By Carroll McCormick
Nickel Magazine, November 2005 -- Visiting Fort McMurray in northern Alberta, Canada, one is immediately struck by how the surface mining of oil sands has left its mark on the Boreal forest.
Suncor Energy, one of two major oil producers in the area, has begun addressing the problem: the company is using a low-impact, sub-surface extraction technique that promises to reduce the impact known as environmental footprint. It is the second of six planned developments in Suncor’s Firebag mining lease, which spans 1,000 square kilometres.
The technique employs steam-assisted gravity drainage (SAGD) to pump steam through 700-to-800-metre-long horizontal pipes installed 250 metres deep in the oil sands. Once heated by the steam, bitumen is collected by parallel pipes three to five metres below the steam pipes and brought to the surface for processing, at which stage a nickel alloy plays an essential role.
Firebag 2, which began operating in late July 2005, will use 15,000 cubic metres of water a day during full production. Almost all of the so-called produced water recovered with the bitumen is recycled through three massive evaporators. As a result, less new water needs to be drawn from outside sources. The vertical tube, falling-film, vapour compression evaporators are the largest of their kind in the world. The water treatment technology is licensed from RCCI, a subsidiary of GE Infrastructure, Water & Process Technologies.
Each evaporator contains 107 tonnes of N08926 (containing 25% nickel) in the form of 4,000 pipes. Each pipe is 20 metres long with an outside diameter of 5.1 centimetres and a wall thickness of 1 millimetre.
This technology (unlike the traditional treatment method -- warm or hot lime softening, filtration, and weak acid cation exchange) produces a much higher-quality boiler feed water and minimizes the disposal of water and sludge.
Once the water and oil are recovered from underground, the water is de-oiled. The resulting produced water is concentrated in the evaporators to recover clean, high-quality distillate. A concentrated brine solution, constituting a small percentage of the produced water, is directed to a waste stream, where further treatment occurs. The distillate is used to feed steam generator boilers, and the steam produced is injected into the bitumen deposit, completing the cycle.
The material requirements for the evaporators are strict: RCCI describes typical produced water as "predominantly sodium chloride brine with high silica and minimal calcium and magnesium." The GE subsidiary goes on to explain: "High alkalinity, or carbonate, is present as well. The produced water generally contains about 3,000 micrograms of total dissolved solids per litre but can vary depending on the geological contribution. Dissolved and emulsified organics [oil] are present at a variety of levels, depending on the oil separation processes used."
The alloy used in the evaporators has to be highly resistant to pitting and crevice corrosion in halide media and in hydrogen sulphide-containing (sour) environments. It must also have virtual immunity (under practical conditions) to chloride-ion stress corrosion cracking, and excellent corrosion resistance in a range of oxidizing and reducing media. The evaporators have a service life of 30 years, and there is no provision in their design for tube repair or replacement, according to Associated Tube Industries (ATI), the Ontario, Canada-based company that manufactured the evaporator tubes.
The thin-walled tube required special handling, according to Roger Thomas, ATI’s product sales manager. "Nothing like that had ever been done in alloys of that nature. We made annealing furnace modifications and changed the method for transporting the tubes through the annealing furnace so that they would not be damaged."
ATI continuously roll-formed the tube from coils of raw material slit to a predetermined width. It then welded the tube using GTAW without filler metal, applied localized cold work to the weld, and bright-annealed the tube at 1,149[degrees]C, thereby improving its resistance to corrosion.
Firebag 2 is 52 kilometres from Suncor’s oil sands facility in Fort McMurray. Like Firebag 1, which entered production in 2004, it has a capacity of 35,000 barrels per day (bpcd). Firebag 2 will take about 18 months to attain full capacity.
Suncor will build an 85-megawatt co-generator at Firebag 1 and 2 to increase production by 25,000 bpcd. Starting in January 2009, Firebags 3 to 6 are scheduled to come into production.
How the evaporators work
Produced water returned to the surface with the bitumen is de-oiled, then heated in a plate and frame heat exchanger to about 100[degrees]C. The heated feed is de-aerated to remove non-condensable gasses such as oxygen before being pumped to the evaporators, each of which is about 31 metres high, with a maximum diameter of 7.5 metres.
The condenser tube bundle is arranged vertically on top of the vessel sump. Steam is introduced on the shell side of the condenser, whereas the brine (water) is on the tube side. The brine is pumped from the evaporator sump to the top of the condenser by the re-circulation pump. The brine flows down the inside surface of the tubes in a thin film. A portion of the brine evaporates. The steam that is generated in the tubes is drawn out of the sump and into the compressor. The compressed vapour goes to the shell side of the condenser where it condenses and heats the brine inside the tubes. The condensed steam exiting the evaporator through the distillate line is used to heat the evaporator feed, then goes to feed the steam generator to produce steam, which goes back underground.
The concentrated brine inside the tubes falls back to the evaporator sump and is re-circulated to the top of the condenser for another pass through the tubes. A small portion of the brine is discharged as a waste stream to maintain the proper concentration of the re-circulating brine. The temperature inside the tubes is about 100[degrees]C; the steam outside the tubes is several degrees hotter. The pressure inside the tubes is about two kilopascals above atmospheric; outside the tubes it is 30-35 kilopascals above.
Carroll McCormick is a Montreal-based freelance writer.
PHOTOS: Associated Tube Industries
Roger Thomas Lalit Jain |
