Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02882609 2015-02-20
Removal of Residual Bitumen From
Oil Sands Tailings
Background
The Athabasca bituminous sand or oil sand deposits are presently being
surfaced mined for oil
extraction. Those deposits close to the surface are physically removed and the
bitumen is extracted by
making a slurry of oil sands and hot water. Two tonnes of oil sands are mined
for each barrel of oil
produced. Approximately 135,000 cubic metres of oil are recovered daily. Three
times the volume of
oil produced or 405,000 cubic meters of water are sent to tailings ponds where
the fine material settles
out over time. The tailings ponds are estimated to cover 180 square
kilometers. The fine tailings
produce a somewhat stable suspension that may take decades to consolidate.
Regulations are in effect
that require the volume of fluid fine tailings be reduced and the ponds be
ready for reclamation no
longer than five years after the ponds cease to be in service.
Canberra (2008) suggests that the stable suspensions are largely the result of
the residual bitumen
content. The bitumen specific gravity is slightly less than that of water. The
bitumen is viscous and
once mixed with the sediments it is virtually locked into the sediments. The
consolidation of fine
sediments requires the migration of water out of the inter-particle voids. If
the residual bitumen blocks
the pore throats, water cannot move and the particles remain in suspension.
Diagram 1 shows the
structure of fine tailings. The sand size fraction separates rapidly from the
finer fraction is deposited in
close to the tailings discharge point. Removal of some or all of the residual
bitumen will therefore
allow the pore water to migrate upwards and the sedimont to consolidate.
The amount of residual bitumen in the fine tailings varies between 0 and 5%.
The residual bitumen is
an economic loss to the oil sands producer. Apart from the obvious loss of
revenue from the sale of the
oil, there is a costly environmental impact. Some free bitumen floats to the
surface as the tailings are
discharged into the ponds. Heavy fines have been levied against producers for
allowing water fowl to
land on the tailings ponds and become oil coated. Methane generated by the
anaerobic decomposition
of the residual bitumen contributes to the overall greenhouse gas emissions.
The volume of tailings produced per day is in the order of 400,000 cubic
meters or approximately
62,000 imperial gallons per minute. Process upgrades have increased the
bitumen recovery to between
90% and 100%. Therefore the amount of residual bitumen has decreased with
increasing efficiency.
CA 02882609 2015-02-20
For the study conducted in connection with the present invention the residual
bitumen content of the
tailings was 0.57%. Given the volume of tailings produced, this small residual
translates into
approximately two cubic meter (12.6 bbl) of oil being discharged per minute.
There is therefore an economic and environmental reason to remove the
remaining bitumen from the
tailings. Faster consolidation extends the life of the pond as the volume will
reduce faster. Since the
inter pore water will be released, more water is available for recycling. Less
bitumen should also
reduce the amount of greenhouse gasses being released from the ponds.
If the residual bitumen is to be removed from the fine tailings, plant
processes have to be installed to
capture the bitumen prior to its release into the tailings ponds. Given the
volume of water and bitumen
being disposed of, any new process has to carefully consider the magnitude of
the materials
management issues. The novel approach in the present invention is to mix the
tailings with a
hydrophobic, organic based oil absorbent. The absorbent will capture some or
all of the residual
bitumen and then processed to recover the absorbed bitumen. The absorbent will
then be available for
reuse.
Brief Description of Disclosure
The present invention provides a novel means of removing residual bitumen from
oil sands tailings
using biochar or a like hydrophobic, oleophillic absorbent. The hydrophobic,
oleophillic biochar is
mixed with the tailings and some or all of the residual bitumen is absorbed on
the biochar. The
absorbent particle size is large enough relative to the fine tailings to allow
the absorbent and absorbed
bitumen to be removed by a separator. The remaining water, fine particles and
remaining unabsorbed
bitumen are then sent to the tailings pond where the fines will compact at an
increased rate because
fewer pores are blocked by the bitumen droplets.
In a preferred form of the present invention, the absorbent is manufactured
from organic material such
as biomass. The biomass includes, but is not restricted to, forest and
agricultural waste materials. The
biomass is carbonized into biochar char in a reactor. The choice of reactor
depends on the required
mass of biochar required for the volume of tailings produced per unit time.
Typically biochar is
manufactured at temperatures between 300 degrees centigrade to 600 degrees
centigrade in an oxygen
reduced atmosphere. The hot biochar may be either cooled and stored in silos
or mixed immediately
with the tailings. The ratio of biochar to tailings is set according to the
desitred aborption of the
bitumen. The example presented below uses 2.5% biochar by mass, relative to
the mass of the fine
tailings although higher ratios may be effective in removing more biochar.
CA 02882609 2015-02-20
The spent biochar is returned to the reactor where it is reheated to between
400 degrees and 600
degrees. The absorbed bitumen is cracked into lighter weight oil and recovered
in a condenser. Most
oil sands contain sulphur and prior to condensation of the hydrocarbon, a
sulphur removal system may
be required. The recarbonized bitumen is reused as an absorbent and the
cracked hydrocarbon is stored
in an appropriate tank or immediately transferred for blending into the
ongoing bitumen recovery
process.
Description of Drawings
Drawing 1 shows an idealized sketch of a clay, silt and bitumen mixture. The
void space is assumed to
be full of water and any chemicals that have been entrained by the recovery
process or otherwise.
Drawing 2 is a flow diagram of the process showing the process from the
addition of wood chips to the
reactor to the recovery of upgraded bitumen. A single reactor is used to both
carbonize the biomass and
recover the bitumen.
Drawing 3 shows a process whereby the biochar is produced separately from the
recovery of the
bitumen.
Drawing 4 shows the increased consolidation rate through the addition of
biochar.
Detailed Description
The embodiments of this invention will now be described with reference to
Drawings 2 and 3
Wood chips from any source can be used. The size and geometry will be a
function of the equipment
used to prepare the wood chips. Ideally the chips should be equi-dimensional
with a side length of
5mm to lOmm. Uniform chips are rarely available, therefore the aspect ratio
for the chips should be as
close to being equal as practically possible. The prepared wood chips are
placed in a hopper or storage
bin (1) where upon the chips are augered into a pyrolysis reactor (2). Drying
the wood chips to less
than 20% prior to carbonization may be desirable as lower moisture content
wood may make the
subsequent parts of the process more efficient.
Drawing 2 shows a process where a single reactor is used to prepare the
absorbent as well as recovery
the bitumen. The reactor (2) can be any pyrolysis unit whereby the absorbent
and bitumen can be
heated to between 300 centigrade and 600 centigrade in an oxygen reduced
environment.
CA 02882609 2015-02-20
Hydrophobicity is normally imparted to the biomass at temperatures above 250
centigrade. The
minimum temperature is dictated more by the subsequent cracking of the bitumen
during the bitumen
recovery stage than the requirements for hydrophobicity of the biochar.
Drawing 3 presents a separate carbonization stage. The advantage of a separate
carbonization system
(13) is the gases and tars produced during carbonization are separated from
the gases and oils produced
during bitumen recovery. There will be some attrition of the biochar and the
separate carbonization
process means the carbonizer can be sized to simply add make up char when
required. Alternatively
the carbonizer does not have to be located in proximity to the bitumen
recovery system. The
carbonizer can be located closer to the source of the biomass and only the
biochar delivered to the chip
hopper (1) or other storage device.
The difference between the two flow sheets has been described above. Apart
from these differences,
the remainder of the process is the same for both approaches. The biochar (3)
is conveyed to a
continuous flow mixing chamber (4) where tailings (6) from the primary oil
recovery process are
blended together in a predetermined ratio. The weight of biochar is adjusted
to a ratio of 1% to 10% by
weight of tailings. Following mixing the absorbent is separated (5) from the
tailings. Separation can
be accomplished by any method typically used in industrial process. These
process can include, but not
limited to hydrocyclones or screens. The liquid fraction is conveyed to the
tailings ponds as is
currently the practice. The reclaimed biochar (7) is conveyed to the pyrolysis
reactor (2) where the
mass is heated to a temperature sufficiently high to crack and vapourize the
bitumen. The hot pyrolysis
vapours then conveyed to the condensing system (10). Since bitumen contains
approximately 5%
sulphur, sulphur removal (9) may be required prior to condensation. Sulphur
removal is a mature
technology and units can be sized and purchased to fit into the system. The
cracked and partially
upgraded bitumen is conveyed to storage (11), while the non-condensing gas
(12) is returned to the
pyrolysis reactor (2) where the gas provides some of the heat required to
crack and recover the
bitumen.
When a single reactor system is used (Drawing 2) there will be some tars and
water produced during
carbonization of the biomass. This water and tar will form a separate phases
and will require removal
from the recovered bitumen. The recovery of the water phase and tar phase have
not been included in
Drawing 2.
Example
Laboratory scale experiments were conducted to evaluate the potential for
removal of bitumen from the
tailings. A know volume of raw fine tailings was mixed with 2.5% coarse
biochar. The liquid was
separated from the fine tailings and the carbon content measured. It was
concluded that 2.5% biochar
CA 02882609 2015-02-20
absorbed approximately 33% of the bitumen. Slow recarbonizing the
bitumen/biochar mixture showed
that 10% of the volatiles were removed. The vapours were not condensed and
therefore the yield of oil
and non-condensing gas is not known at this time.
Approximately 1 liter samples of tailings with no biochar, 1% biochar and 2.5%
biochar were mixed in
Erlenmeyer flasks for 30 minutes and then poured into 1.2 litre graduated
cylinders. Clear water was
soon visible at the top of the cylinder. The height of sediments was measure
on regular intervals and the
percent solids in the sediments was measured. Drawing 4 shows that 1% biochar
improves the rate of
consolidation, but 2.5% significantly improves the rate of consolidation. The
starting solids content for
the three samples was 14.7%. After 33 days the sample with no biochar saw an
increase in solids
content from 14.7% to 16.7%. During the same interval the 2.5% biochar sample
increased from
14.7% to 21.7%. While the rate of increase in solids content is decreasing for
all three samples, the
2.5% biochar continues to outperform the 1% biochar and no bioichar. It is
evident from Drawing 4
that even small percentages of biochar can dramatically improve the
consolidation rate. It is also
concluded that the assumption made by Canberra with respect to the bitumen
retarding consolidation
appears to be valid.
Claims
1) A process for recovery of bitumen from fine tailings produced in oil sands
processing, in which a
hydrophobic/oleophillic absorbent is mixed with oil sands tailings and a
portion or all of the residual
bitumen in the tailings is absorbed.
2) An absorbent for use in the process of Claim 1 that is capable of
withstanding a temperature of 700C
in an oxygen reduced environment.
3) An absorbent as claimed in Claim 2 that is prepared from biomass have a
dimension of 0.25 to 5 cm.
4) The process of claim 1, wherein upon separation of the absorbent and
absorbed bitumen from the
tailings, the absorbent and bitumen are carbonized in an oxygen reduced
atmosphere at a temperature
between 300C and 700C where upon the absorbed bitumen is cracked and
vapourized.
5) Vapourized bitumen produced by the process of Claim 4 in a cooled and
condensed state.
6) The absorbent of Claim 2 or 3, regenerated for reuse for the further
absorption of bitumen in tailings.
References
tp:,/,1v,,w. w.nrcan.gc.ca/sites/w
w.nrcan.gc.ca/files/energv/pdfleneene/pubpub/pdf/OS Tailings Mana
i4ement-erw.,.pdf
Additional references to follow
