Natural Resources Canada team investigates solvent extraction process for oil sand bitumen; non-aqueous alternative to hot water processing with reduced environmental impacts
A team of two researchers from Natural Resources Canada’s CanmetENERGY reports on a new process for solvent extraction of bitumen from mineable Athabasca oil sands (i.e., surface mining, not in situ well-based production) in a paper published in the ACS journal Energy & Fuels. Wide-scale deployment of such a non-aqueous process could mitigate some of the negative environmental impacts of the mining method of bitumen production, such as high water consumption; the resultant tailing ponds; energy consumption; and greenhouse gas emissions, they suggest.
In this process, oil sand is mixed with light hydrocarbon solvents, and the solvent–bitumen solution is separated from the mineral solids by centrifugal filtration or regular pressure filtration. The solvent left in the filtration cake is recovered by evaporation under vacuum at room temperature.
Their results show that, for both high-grade and low-grade ores, using appropriate solvent, the bitumen recovery and product quality are comparable to those from the currently used hot water extraction followed by naphtha froth treatment. The recovery of light hydrocarbon solvent is relatively easy, so this novel process has great potential to reduce solvent losses compared to existing technologies, the team suggests.
The Clark hot water extraction (CHWE) process has been applied in commercial bitumen extraction from the Athabasca oil sands for more than 40 years. However, Jiangying Wu and Tadeusz Dabros note in their paper, the water-based technology now faces numerous challenges:
High water consumption. 3 to 4 barrels of water are required for every barrel of bitumen produced.
Tailing ponds. The hot-water process generates huge tailing ponds that have serious, long-term environmental impacts.
Treatment challenges. If naphthenic froth treatment is utilized, bitumen product quality is low; if paraffinic froth treatment process is utilized, the asphaltenes rejection is high.
Energy and GHG. The high thermal capacity of water results in high energy consumption and high greenhouse gas (GHG) emissions.
Non-aqueous bitumen extraction is being investigated as an alternative to the CHWE process. By eliminating water use in bitumen extraction, all water-related issues would be resolved. For example, elimination of the fresh water draw from the Athabasca River would help preserve the ecology. The need for wet tailing ponds would be eliminated; instead, dry tailings suitable for continuous backfilling of the open pit would make it possible to reclaim the mined-out area in a much shorter period of time than is currently feasible. The non-aqueous process also has the potential to greatly improve energy efficiency and reduce GHG emissions.—Wu and Dabros
Overall, two types of non-aqueous bitumen recovery processes have been explored, they note: pyrolysis and solvent extraction. One drawback of the pyrolysis process is the high operating temperatures required. Several solvent extraction processes have been investigated. However, Wu and Dabros note, the major problem with solvent extraction processes is that they require large amounts of solvent to form a slurry with the oil sand feed, and high-boiling-point solvents make it difficult to recover the solvent from the large volumes of sand.
Combined solvent-and-water extraction processes have also been investigated, however, “once water is introduced into the extraction process so are most of the problems associated with conventional water-based extraction.”
Wu and Dabros set out to obtain bitumen recovery of more than 90 wt % and to limit solvent losses to less than 4 bbl of solvent per 1,000 bbl of extracted bitumen (0.4%). Low-boiling-point solvents were used to facilitate recovery; five different solvents were investigated at relatively low solvent-to-bitumen ratios (S/B), which is on a mass basis.
Their findings were:
Cyclopentane is a good solvent for extracting bitumen from oil sand. Cyclopentane gives higher bitumen recovery than toluene. To reduce solvent cost, cyclopentane can be mixed with n-pentane.
For both high-grade and low-grade ores, bitumen recovery using the investigated solvent extraction process is comparable to that of the water-based bitumen extraction process. E.g., using a three-stage extraction process, more than 95 wt % of the bitumen from high-grade ore can be recovered.
The quality of solvent extracted bitumen is similar to that of bitumen from water-based extraction followed by naphtha froth treatment.
Centrifugal filtration has a higher efficiency than regular pressure filtration. However, if high S/B is used, regular pressure filtration can also achieve over 90 wt % bitumen recovery.
Owing to its higher bitumen recovery and low boiling point, cyclopentane is much easier to recover than toluene. For high-grade ore, solvent loss can be easily limited to less than 4 barrels per 1,000 barrels of bitumen production; for low-grade ore, this could be achieved at slightly elevated temperatures.
Jiangying Wu and Tadeusz Dabros (2012) Process for Solvent Extraction of Bitumen from Oil Sand. Energy & Fuels doi: 10.1021/ef201457m