The original Syncrude and Suncor Naphthenic Froth Treatment (NFT) facilities comprised exclusively of centrifuges. Centrifuges are mechanical devices with moving parts that rotate at high speeds, creating high gravitational forces. These high g-forces pull apart the water/solids/hydrocarbon emulsions, leaving behind a relatively clean diluted bitumen product.
The adoption of inclined plate separators (IPS) and cyclones in the NFT process has generally reduced the number of centrifuges required to get a clean product. However, centrifuges remain a very effective method of removing fine solids from diluted bitumen.
DESIGN & THEORY
A centrifuge plant has 3 types of equipment:
- Solid-Bowl Scroll Centrifuges: used as a first stage of cleaning for the removal of coarse solids
- CUNO Filters: used to remove oversized material that may plug-up the disc nozzle centrifuges
- Disc Nozzle Centrifuges: used as a final stage of cleaning for removal of water and residual fine solids.
SOLID-BOWL SCROLL CENTRIFUGES
Solid-bowl scroll centrifuges are used as a first-pass to remove coarse solids from the diluted froth. Sometimes referred to as decanter centrifuges, these units consist of a horizontal revolving shell, or bowl, inside which a screw, or scroll, rotates in the same direction. Feed enters the centre of the bowl, which rotates at approximately 1,000 rpm, pushing the solids to the outer edge. The inner scroll moves the sludge in the opposite direction of the liquid flow, rotating at a slightly lower speed, moving the solids to a discharge hopper. The horizontal bowl has a conical shape on the solids discharge end to encourage beaching and minimize liquid carry-over into the tailings stream.
These centrifuge units recover up to 98% of the hydrocarbons but still leave a solids content of 3-5% in the product. A common manufacturer of solid-bowl scroll centrifuges in the oil sands is Bird Machine Company, now owned by Andritz.
Once the coarse solids are removed from the diluted froth, the de-sanded product must be filtered before it can be fed to the second stage disc nozzle centrifuges. CUNO filters consist of a series of stacked discs spaced approximately 380 µm apart. Any solids greater than 380 µm become lodged between the discs. The solids get continuously removed by a set of stationary blades. The blades are centered between the disc gaps, which clean-out the solids as the discs rotate. The filters are periodically back-washed with hot water to prevent accumulation of solids in the disc housing.
DISC NOZZLE CENTRIFUGES
Disc centrifuges are effective in removing water and finer solids. The feed enters the top of the centrifuge and rotates at high speeds within the bowl, typically greater than 3,500 rpm. High centrifugal forces (over 5,000 g) push the water and solids to the outer edge of the disc stack. The lighter bitumen/naphtha mixture remains in the centre and is forced out the top of the centrifuge. The stacked discs provide ample surface area for the fine solids to settle, which are pushed outwards along with the heavier water phase.
A pressure control valve at the bottom of the centrifuge can either let excess water out or allow hot flush water to enter the centrifuge. The space between the rotating bowl and the outer casing is continuously purged with nitrogen or natural gas. After the bitumen/naphtha mixture is processed through the discs, the solids and water content is typically less than 2%. The most common supplier of disc centrifuges in the oil sands is Alfa Laval.
Due to their rotating mechanical parts, centrifuges are susceptible to a lot of wear and tear. They are also big power-consumers, which increase the electrical load on the overall operation. Some operators have chosen to add capacity through the addition of inclined plate separators (IPS) and cyclones in place of centrifuges to dewater the bitumen and remove fine solids.
Despite their shortcomings, centrifuges are very effective in reducing water content due to the high g-forces imparted onto the diluted bitumen, which is difficult to achieve with cyclones alone. Centrifuges can be found at both the Syncrude Mildred Lake and Suncor Base Plant NFT operations.