Tar ands

1. CHAPTER 11 – Tar Sands
Introduction
“Tar sands” is a unique term. The word tar has been defined as oily, dark-
colored bituminous products obtained by the distillation of various petroleum
feedstocks, including crude oil. Tars are typically rich in organic compounds
that are related to benzene. High molecular tars (or pitches) are used in the
manufacture of roofing papers and for making coal briquettes.
Tar sands (also alternately referred to as bituminous or oil sands) are located in
certain beds. They contain a somewhat dense, viscous form of petroleum, with
the assigned technical term of bitumen.

2. Introduction
The term tar sands was first employed around the turn of the twentieth
century. The interstices between the grains of sand are largely filled with a
thick tarry hydrocarbon mixture that can be extracted. This extract can be
processed to yield 65 to 75 percent synthetic crude oil. The sand was initially
mined and transported to a separation plant.

3. Early History
The first commercial plant at the Athabasca region came online for operation
in 1978. The plant used a hot-water separation method. Hot water and steam
separated the sticky oil, called bitumen, from the sand and other sol- ids. The
bitumen was then upgraded and treated with hydrogen to remove impurities,
producing a relatively high-grade synthetic crude oil. This plant produced one
barrel of oil from two barrels of tar sand feedstock. The commercial plant
followed a pilot plant built earlier in 1967.

4. Availability/Distribution
At the international level, over 50 percent of the world’s tar sands are in North
America.
It is estimated that 90 percent of U.S. tar sand deposits are located in Utah.
Both surface mining and in situ recovery are now being used to obtain oil from
tar sands. Thus, considerable efforts are being made today to further develop
processes to exploit these considerable reserves. However, it is difficult to
estimate the percentage of these reserves that are economically recoverable
while complying with environmental regulations.

5. Characterization
The oils from tar sands are characterized by an unfavorable hydrogen/car- bon
ratio that essentially requires that hydrogen must be added during the refining
process.
Three classifications of the Athabasca tar sands have been reported in the
literature:
1- Good-grade oil sand. Under 200-mesh sand is at a low percentage, typically
less than 20 percent by weight, but usually under 10 per- cent. Oil is over 10
percent by weight with an average of 13.5 to 15 percent; clay is about 1 percent,
and water is 2 to 5 percent.

6. Characterization
2. Interbedded oil sand and shale. Under 200-mesh material is at a medium
percentage. Oil is 4 to 10 percent with an average 6.5 percent, and water
averages 9 percent.
3. Shale. There is a high percentage of under 200-mesh material; oil is less than
4 percent, usually 1 to 2 percent, and water is 10 to 15 percent.

7. Extraction
In many respects surface mining of tar sands is similar to strip mining of coal
and the potential for fugitive air emissions from material handling and transfer
has created problems with increased environmental impacts.
There is also the problem of organic loading of runoff water, which is not
usually present with coal mining. A problem similar to that present in the oil
shale industry is the disposal of spent tar sands after they have been processed
through the bitumen extraction plant.

8. Extraction
Because of the high viscosity of the bitumen, the sands must be extracted by
strip mining or in situ techniques, which reduce the viscosity by injecting
steam, solvents, and/or hot air into the sands. These processes require more
water and energy than conventional oil extraction. This heavy crude feed-
stock then needs preprocessing before it is sent to a conventional refinery. This
preprocessing is called upgrading, which necessitates the removal of water,
sand, physical waste, and lighter products.

9. Processing
Catalytic purification and hydrocracking are together known as
hydroprocessing. The big challenge in hydroprocessing is to deal with the
impurities found in heavy crude as they can poison the catalysts over time.
The extracted oil obtained before treatment is naphthenic and slightly more
dense than water; its specific gravity at 25°C (77°F) is 1.002 to 1.027. The lighter
oil will flow at ordinary temperatures; the heavier oil has a high viscosity. It
contains about 5 percent sulfur.

10. Processing
The overall processing sequence for a typical plant consists of the following:
1. Large rotary cutters, loading onto belts
2. Conveyor belts to the separation plant
3. Hot water—steam application to separate the oil from most of the tar sand
4. Fractionation to complete the separation from sand and water, and to coke
the residue
5. Desulfurization by mild hydrogenation
6. Pipeline to assorted destinations

11. Transportation/Transmission
The heavy crude oil or bitumen extracted from the tar sands is a viscous, solid,
or semisolid form that does not easily flow at normal oil pipeline
temperatures. This makes it difficult to transport to processing plants.

12. Environmental Issues
Environmental problems include: the siltation of streams and lakes from any
destruction of natural drainage patterns; any destruction of salmon- spawning
areas; the contamination of groundwater; and air pollution.
Heavy metals, including lead, mercury, chromium, cadmium, arsenic, etc., can
also be present in tar sands.
The disposal of spent tar sands after they have been processed through the
bitumen extraction plant is another major problem.

13. Environmental Issues
To summarize, tar sands operations, like all fossil fuel activities, can have an
adverse effect on the environment. Tar sands projects can potentially affect
1. The land when the bitumen is initially mined potentially with large
deposits of toxic chemicals.
2. The water during the separation process and through the drainage of
rivers.
3. The atmosphere due to the release of gaseous and particulate emissions.

14. Future Prospects and Concerns
The deposits of tar sands are no longer restricted to only a few regions. A
systematic, worldwide exploration for these sands has taken place since the
start of this century.
The economics of oil extraction from tar sands have become more attractive
this past decade. Support for this once unfavorable alternate energy source
continues to increase. Further support is expected in both the near and distant
future. It is now evident that considerable efforts will be made in the future to
explore the considerable potential of tar sands systematically and to develop
technologies for society’s utilization.