The oil sands of Alberta
probably contain more petroleum than the reserves of Arabia
In Alberta province, Canada, oils sands hold as much oil as Saudi Arabia. In 1893, the Canadian government was investigating the possibility of producing petroleum from the "tar sands" of Alberta.
Commercial mining of the oil sands started about 30 years ago. Today two companies are treating these sands to extract petroleum products. We will look at the process used by Suncor.
The oil sands are a mixture of bitumen, sand and shale. This material is extracted using conventional opencut methods. The feasibility of melting the bitumen with high pressure steam is being tested.
What are advantages of melting the bitumen in the sand?
The molten bitumen can be pumped out for treatment.
The surface of the ground is little affected.
The expense of opencut mining is avoided.
The oil sand is mined as huge lumps. It must be crushed before it travels from the mine to the treatment plant. This is process which will be redundant if the bitumen can be liquified underground.
The separation of sand and bitumen is based on a discovery by Karl Clark in 1920. He tipped a bucket of oil sand into the family washing machine added hot water and caustic soda and turned the machine on. A few minutes later he saw a froth of bitumen floating on the water. History does not tell us how his wife and family received this experiment. Winter in Alberta is long and cold.
In the modern process the crushed ore is fed in rotating drums where it mixes with hotwater and steam.
The separation of sand and bitumen is a two stage process.
The superheated water, bitumen, sand and shale separate on standing.
The bitumen, thinned with naphtha, is pumped to centrifuges which remove any remaining water and solid material.
The water containing sand and shale is pumped to settling ponds. The fine shale particles are very slow to settle. If the settling ponds were not treated they would be like farm dams where the clay remains suspended indefinitely.
How is the silt removed?
Calcium sulfate is added to the water. Although nearly insoluble, it causes the silt particles to clump and sink.
Why do they use calcium sulfate?
They produce it on site using sulfur removed from the oil.
Sulfuric acid, manufactured from the sulfur, is reacted with limestone.
The bitumen thinned with naphtha (C4 to C10 hydrocarbons) is now distilled.
The naphtha is reused for bitumen thinning.
The very hot, liquid bitumen moves to the separation towers.
Why is naphtha such a good thinner?
Naphtha is a mixture of light hydrocarbons which are all non-polar molecules. Bitumen is a more complicated mixture of (non-polar) hydrocarbons. The naphtha/bitumen mixture is a solution.
The hydrocarbon vapours separate from the solid coke in tall towers.The coke is used as a fuel for plant processes. The vapours are fractionated.
The hydrocarbon output is separated into three fractions;
naptha, C5 to C9 hydrocarbons
kerosene, C10 to C13 hydrocarbons
gas oil, larger hydrocarbons (diesel fuel, lubricating oil)
see Scientific American March 1998 p 66
