Page 136 - Synthetic Fuels Handbook

122 CHAPTER FOUR

of tar sand bitumen boils above 540°C (1004°F). Thus, a product of acceptable quality
could be obtained by distillation to an appropriate cut point but the majority of the bitumen
would remain behind to be refined by whichever means would be appropriate, remember-
ing, of course, the need to balance fuel requirements and coke production. It is therefore
essential that any bitumen-upgrading program convert the nonvolatile residuum to a low-
boiling, low-viscosity, low-molecular-weight, high hydrocarbon/carbon ratio oil.
Bitumen is hydrogen-deficient that is upgraded by carbon removal (coking) or hydro-
gen addition (hydrocracking). There are two methods by which bitumen conversion can
be achieved: (a) by direct heating of mined tar sand and (b) by thermal decomposition of
separated bitumen. The latter is the method used commercially but the former deserves
mention here since there is the potential for commercialization.
An early process involved a coker for bitumen conversion and a burner to remove carbon
from the sand. A later proposal suggested that the Lurgi process might have applicability to
bitumen conversion. A more modern approach has also been developed which also cracks
the bitumen constituents on the sand. The processor consists of a large, horizontal, rotat-
ing vessel that is arranged in a series of compartments. The two major compartments are a
preheating zone and a reaction zone. Product yields and quality are reported to be high.
Direct coking of tar sand with a fluid-bed technique has also been tested. In this process,
tar sand is fed to a coker or still, where the tar sand is heated to approximately 480°C (896°F)
by contact with a fluid bed of clean sand from which the coke has been removed by burning.
Volatile portions of the bitumen are distilled. Residual portions are thermally cracked, result-
ing in the deposition of a layer of coke around each sand grain. Coked solids are withdrawn
down a standpipe, fluid with air, and transferred to a burner or regenerator [operating at
approximately 800°C (1472°F)] where most of the coke is burned off the sand grains. The
clean, hot sand is withdrawn through a standpipe. Part (20–40 percent) is rejected and the
remainder is recirculated to the coker to provide the heat for the coking reaction. The prod-
ucts leave the coker as a vapor, which is condensed in a receiver. Reaction off-gases from the
receiver are recirculated to fluidize the clean, hot sand which is returned to the coker.

4.6.1 Primary Conversion
The overall upgrading process by which bitumen is converted to liquid fuels is accom-
plished in two steps. Initially, at the time of opening of both the Suncor and Syncrude
plants, the first step is the primary conversion process or primary upgrading process (coking)
that involves cracking the bitumen to lighter products that are more easily processed down-
stream. The secondary upgrading process involves hydrogenation of the primary products
and is the means by which sulfur and nitrogen are removed from the primary products.
The synthetic crude oil can then be refined to gasoline, jet fuel, and homes heating oil by
conventional means.
There are two coking processes that have been applied to the production of liquids from
Athabasca bitumen. Delayed coking is practised at the Suncor plant, whereas Syncrude
employs a fluid-coking process which produces less coke than the delayed coking in
exchange for more liquids and gases.
Thus, coking became the process of choice for bitumen conversion and bitumen is cur-
rently converted commercially by delayed coking (Suncor) and by fluid coking (Syncrude).
In each case the charge is converted to distillate oils, coke, and light gases. The coke frac-
tion and product gases can be used for plant fuel. The coker distillate is a partially upgraded
material in itself and is a suitable feed for hydrodesulfurization to produce a low-sulfur
synthetic crude oil.
Delayed coking is a semibatch process in which feed bitumen is heated before being fed
to coking drums that provide sufficient residence time for the cracking reactions to occur. In