Note: Descriptions are shown in the official language in which they were submitted.
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RECOVERY OF HYDROCARBONS, ENERGY AND WATER FROM TAILINGS
SOLVENT RECOVERY UNIT UNDERFLOW
FIELD OF THE INVENTION
The present invention relates to the recovery of solvent from solvent diluted
tailings derived from a bitumen froth treatment operation and more
particularly to
treatment of the underflow from a tailings solvent recovery unit (TSRU).
BACKGROUND
In bitumen froth treatment processes, solvent or diluent is added to a bitumen
froth to separate a diluted bitumen stream for further processing. In a
paraffinic
bitumen froth treatment process, for example, bitumen froth derived from oil
sands is combined with paraffinic solvent and then supplied to a settling
vessel in
which a bitumen rich fraction is separated from a bottoms fraction rich in
asphaltenes, water, solvent and solids as well as residual amounts of bitumen.
This bottoms fraction is often referred to as solvent diluted tailings or
froth
treatment tailings.
Solvent diluted tailings are preferably treated to recuperate the paraffinic
solvent,
which is subject to environmental discharge regulations and a valuable
commodity, prior to disposal of the resulting solvent recovered tailings
containing
primarily water and solids. Solvent diluted tailings may be treated in
tailings
solvent recovery units that include flash vessels.
Flash vessels conventionally used to recover diluent from froth treatment
tailings
are specified for a feed flow and feed temperature so that, at the stage
column
pressure with optional stripping, steam vaporizes the diluent for recovery in
the
overhead condensing system.
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The tailings solvent recovery unit produces an overhead solvent stream which
is
further processed before returning for reuse as diluent in the froth settling
vessels
and solvent depleted underflow. The solvent depleted underflow is often
discarded as tailings and sent to tailings ponds.
Some processes have been proposed for treating the underflow from a tailings
solvent recovery unit. Some processes have been described in US patent
application published under No. 2010/0258478, international PCT patent
application published under No. WO 2007/102819 and US patent application
published under No. 2010/0126395. These processes have a number of
challenges and drawbacks including complexity, inefficiency and applicability
mainly to tailings derived from naphthenic solvent treated froth.
There is indeed a need for a technology that overcomes or responds to the at
least some of the challenges or drawbacks of known techniques.
SUMMARY OF THE INVENTION
The present invention responds to the above-mentioned need by providing a
process and system for recovering hydrocarbons from TSRU underflow.
In one embodiment, there is provided a process for recovering asphaltenes from
an tailings underflow stream of a tailings solvent recovery unit associated
with a
paraffinic bitumen froth treatment operation, the process comprising
subjecting
the tailings underflow stream to floatation to produce an asphaltene-rich
froth and
an asphaltene-depleted stream; filtering the asphaltene-rich froth to remove a
first portion of gas and water therefrom to produce a wet asphaltene
concentrate
and an aqueous filtrate stream; drying the wet asphaltene concentrate to
remove
residual moisture therefrom to produce an asphaltene concentrate fuel; and
combusting the asphaltene concentrate fuel in a fluid bed combustor to
generate
energy.
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In one aspect, the process includes the step of reusing at least a portion of
the
energy in the paraffinic bitumen froth treatment operation.
In another aspect, the process includes the step of thickening the asphaltene-
depleted stream to produce a thickened tailings and recovered hot water.
In another aspect, the process includes the step of reusing the recovered hot
water in the paraffinic bitumen froth treatment operation.
In another aspect, the floatation is performed in a sealed vessel.
In another aspect, the floatation is performed using an inert gas.
In another aspect, the inert gas comprises nitrogen, C02 or a combination
thereof.
In another aspect, the process includes the step of subjecting flue gas
generated
by the combusting of the asphaltene concentrate fuel to desulphurization.
In another aspect, the process includes the step of combining the aqueous
filtrate
stream with the asphaltene-depleted stream.
In another aspect, the tailings underflow stream contains about 10 wt% to
about
15 wt% of hydrocarbons comprising asphaltenes and bitumen, and about 25 wt%
to about 35 wt% mineral solids, about 55 wt% to about 65 wt% water.
In another aspect, the tailings underflow stream also contains up to about 0.5
wt% of residual paraffinic solvent.
In another aspect, the tailings underflow stream is supplied to the floatation
at
about 60 C to about 70 C.
In another aspect, the floatation is performed in one or more floatation cells
configured in series or parallel or a combination thereof.
In another aspect, the process includes the step of recuperating released gas
from the floatation, the filtering or the drying or a combination thereof.
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In another aspect, the process includes the step of recovering paraffinic
solvent
contained in the released gas.
In another aspect, the process includes the step of recovering floatation gas
contained in the released gas.
In another aspect, the floatation comprises agitation.
In another aspect, the filtering comprises belt filtering.
In another aspect, the asphaltene concentrate fuel is transported via a
conveyor
system.
In another aspect, the process includes the step of feeding the asphaltene
concentrate fuel into a hopper prior to the combusting.
In another aspect, the combusting comprises addition of limestone for control
of
sulphur dioxide.
In another aspect, the asphaltene concentrate fuel comprises at least about 20
wt% asphaltenes on a dry basis.
In another aspect, the asphaltene concentrate fuel comprises at least about 25
wt% asphaltenes on a dry basis.
In another aspect, the asphaltene concentrate fuel comprises at least about 30
wt% asphaltenes on a dry basis.
In another aspect, the asphaltene concentrate fuel comprises at least about 50
wt% asphaltenes on a dry basis.
In another aspect, the asphaltene concentrate fuel comprises up to about 50
wt%
minerals on a dry basis.
In another aspect, the asphaltene concentrate fuel comprises an amount of
minerals sufficient to facilitate operation of the fluid bed combustor.
In another aspect, the asphaltene concentrate fuel comprises an amount of sand
sufficient to act as a fluidizing medium for fluid bed combustor.
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In another aspect, the process includes the step of collecting gases emitted
from
the floatation, the filtering or the drying or a combination thereof.
In another aspect, the process includes the step of recycling at least a
portion of
the collected gases as flotation gas for the floatation.
5 In another aspect, the process includes the step of adding a flocculent to
the
asphaltene-depleted stream.
In another aspect, the process includes the step of disposing of the thickened
tailings in a tailings pond.
In another aspect, the process includes the step of subjecting the thickened
tailings to a belt filtering treatment or a drying treatment or a combination
thereof
to produce a dried tailings material.
In another aspect, the process includes the step of segregating the dried
tailings
material into subcomponents.
In another aspect, the floatation is conducted between about 4 minutes and
about 20 minutes equivalent residence time.
In another aspect, the floatation is conducted between about 7 minutes and
about 16 minutes equivalent residence time.
In another aspect, the floatation is conducted between about 10 minutes and
about 13 minutes equivalent residence time.
The present invention also provides a corresponding system comprising a
floatation unit, a filtration unit, a drying unit and a fluid bed combustion
unit.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig 1 is a block flow diagram of a hydrocarbon recovery process from TSRU
tailings, according to an embodiment of the present invention.
Fig 2 is a graph of asphaltene recovery versus floatation time over different
runs.
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Fig 3 is a graph of asphaltene grade versus asphaltene recovery over different
runs.
DETAILED DESCRIPTION
Referring to Fig 1, in a preferred embodiment of the present invention, the
process allows recovery of asphaltenes from TSRU tailings a boiler fuel that
produces energy and hot water from TSRU tailings for reuse in the plant.
The process treats TSRU tailings 10 that are derived from a tailings solvent
recovery unit from a paraffinic froth treatment operation. The TSRU tailings
thus
contain water, particulate mineral material, residual amounts of paraffinic
solvent
and residual bitumen of which asphaltenes are the major hydrocarbon
component.
As illustrated in Fig 1, the TSRU tailings 10 are supplied to an asphaltene
floatation apparatus 12. The asphaltene floatation apparatus 12 has a tailings
inlet 14 for supplying the TSRU tailings 10 and a gas inlet 16 for supplying a
floatation gas 18 to at least one floatation chamber 20. The floatation gas 18
enters the chamber 20 and contacts the TSRU tailings 10 preferably counter-
currently to contact asphaltenes and cause them to rise and separation from a
predominantly water and mineral fraction of the TSRU tailings. The asphaltene
floatation apparatus 12 also has an underflow outlet 22 for releasing
floatation
tailings 24. The asphaltene floatation apparatus 12 also has an overflow
system
26 comprising a froth outlet 28 for releasing an asphaltene-rich froth 30. The
asphaltene floatation apparatus 12 also has a gas outlet 32 for releasing vent
gas 34 which may contain residual paraffinic solvent.
In one aspect, the asphaltene floatation apparatus 12 is preferably sealed and
the floatation gas 18 is inert, e.g. preferably nitrogen though CO2 may also
be
used. The floatation gas 18 performs two main functions: firstly, the
asphaltenes/bitumen attach to the rising gas bubbles for recovery thereof and,
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secondly, the gas further aids stripping residual solvent from the TSRU
tailings
stream.
The TSRU tailings 10 are preferably derived from flash or stripping vessel
used
in the upstream TSRU. TSRU tailings 10 may contain about 9 wt% to about 15
wt% bitumen which is primarily asphaltenes. For instance, without the
asphaltenes the bitumen content would be about 1 wt%. The TSRU tailings also
contain about 55 wt% to about 75 wt% water, optionally 55 wt% to about 65 wt%;
about 15 wt% to about 35 wt% mineral solids, optionally about 16 wt% to about
22 wt%; and up to about 0.5 wt% paraffinic solvent, optionally less than about
0.1
wt% or between about 0.01 wt% and about 0.03 wt%. The TSRU tailings may be
provided at various flowrates depending on the upstream froth treatment and
TSRU conditions and throughputs, optionally about 1000 t/hr to about 2000
t/hr,
or between 1350 t/hr and about 1550 t/hr. The TSRU tailings 10 are pumped at a
temperature of about 60 C to about 80 C, preferably from about 74 C to about
77 C, into the asphatlene floatation apparatus 12, which may include one or
more flotation vessels, arranged in series or in parallel. The configuration
of the
flotation vessels between series and parallel depends on the capacity of the
given flotation vessels.
Referring to Fig 2, asphaltene recoveries of 80% may be achieved with
floatation
times up to about 15 minutes, which may vary due to variations in gas
injection
rates, agitation rates and mixing effectiveness that are attributes of
specific
flotation apparatuses.
It is also noted that the TSRU tailings may contain some unrecovered maltenes
due to non optimal bitumen recovery performance in the froth settling vessel.
The
maltene component of bitumen can act as a binder in certain circumstances.
More regarding this will be discussed herein-below.
The floatation unit may be configured, sized and operated according to TSRU
tailings characterization including bitumen, solids, water, asphaltene,
particle
sizes and distribution, water chemistry, settling rates and clay activity, for
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example. The floatation unit or its floatation vessels may be periodically
cleaned
using water wash with optional rougher. The feed density, temperature and pH
of
the TSRU tailings may also be controlled to optimize the floatation
performance.
For example, the feed density may be modified by water addition to achieve a
desired density in the floatation vessels for bubble floatation performance.
The
flow rate of inlet floatation gas, agitator speed, additives, and the like,
may be
used and controlled to enhance performance.
It is noted that the floatation step helps not only to recover asphaltenes but
can
also enable recovery of additional fugitive solvent unrecovered in the TSRU.
Referring back to Fig 1, the overflow of asphaltene-rich froth 30 is further
processed by feeding it to a filtration unit 36 to produce a filtered
asphaltene-rich
concentrate 38 which is gas and water depleted along with a filter gas stream
40
and a filter slurry stream 42. The filter slurry stream 42 may be returned and
combined with the floatation tailings 24 for further processing as will be
described
herein-below. The filtration unit 36 thus removes gas and water from the froth
to
produce the filtered asphaltene-rich concentrate 38, which is fed to a drying
unit
44. The drying unit 44 performs final water removal and produces a dried
asphaltene-rich concentrate 46 and a drying gas stream 48. The drying unit 44
is
preferably operated at drying temperature and conditions to remove the
residual
water from the asphaltene concentrate without inducing pre-mature combustion,
gasification, reaction or physiochemical changes in the concentrate that would
disturb or aggravate operations.
The drying gas stream 48 may be combined with one or more other vent gas
streams including the floatation gas stream 34 and/or the filter gas stream 42
to
form a combined vent gas 50, if desired. The individual or combined vent gas
stream may be subjected to processing to separate various components thereof
for reuse in the process of the PFT plant. For instance, the inert floatation
gas
may be recovered for recycling back as inlet floatation gas 18 and the
paraffinic
solvent may be recovered for reuse as diluent in the bitumen froth treatment
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vessel in the PFT plant. Gases from the flotation, filtering and drying
operations
may be collected and may be subjected to cooling to condense solvent for
recycle to the PFT plant, recompression to minimize floatation gas import with
purge provisions to ensure the system remains well outside the explosive
envelope due to oxygen accumulation.
A variety of specific equipment may be used to filter and dry the asphaltene
concentrate. During processing, the asphaltene concentrate develops limited
fluidity and thus conveyor systems may be used in conjunction with the
filtering
and drying, e.g. employing belt filtering and drying to a hopper (not
illustrated)
which would feed the combustion unit.
The filtration unit may be provided with a filtering surface or structure
having pore
sizes in accordance with the particle sizes of the asphlatene-mineral
composite
in the asphaltene-rich froth.
Still referring to Fig 1, the dried asphaltene-rich concentrate 46 is supplied
to a
fluidized bed combustion unit 52. The fluidized bed combustion unit 52 allows
burning the asphaltene-rich concentrate 46 as a fuel. The asphaltene-rich
concentrate 46 has relatively high mineral content and thus the fluidized bed
combustion unit 52 is advantageously used. In one preferred aspect, the bed
combustion unit 52 has a fluid bed 54 where the combustion takes place, an
exhaust outlet 54 for releasing combustion gas 55, a desulphurization module
56
coupled to the exhaust 54 or in fluid communication therewith for capturing
sulphur evolved from burning the asphaltene-based fuel and a solid water
outlet
58 for releasing combustion solid waste 60. The fluidized bed combustion unit
52
thus produces energy 62 which can be used in a number of ways, for heating
process streams and reactors in oil sands processing plants and units and the
power may also be sold to the grid.
In some optional aspects, the power generated by the combustion is used to
create steam which may be used in heat exchangers, for heating water for
mining oil sands, for in situ hydrocarbon recovery purposes such as a SAGD
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operation, for electrical power used in the in situ, mining, extraction,
upgrading or
other uses. In a preferred aspect, the fluid bed combustion unit is located
proximate to where the power is used. In one aspect, the power is used in a
PFT
plant to power various pieces of equipment. It is noted that in the field of
oil
5 sands processing, many process streams are slurries requiring powerful pumps
at operating flow rates. For instance, slurry pumps that transport oil sands
tailings
from extraction or PFT operations may be required to pump the tailings many
kilometers for disposal or further treatment, often about 5 Km to 10 Km or
more.
The high dilbit pumps used to pump the overflow from froth settling vessel of
the
10 PFT plant also have high power requirements, often in the range of about
2000
to about 5000 horsepower. The bitumen froth feed pumps of the PFT plant also
have high power requirements to provide pressure of about 800 to about 900
kPa. PFT plants also have high steam requirements at various stages and units,
including heat exchangers and direct steam injection e.g. into bitumen froth
for
pre-heating. In addition, there may also be power requirements for steam
tracing
for pipelines or electrical tracing for pipelines. The recuperation of the
energy
contained in the rejected asphaltenes provides advantageous operation and
efficiency by minimizing the external energy imported into the PFT plant or
oil
sands operation in general often derived by natural gas sources.
In one aspect, the asphaltene combustion is coupled with a carbon capture and
storage system in order to comply with emissions regulations or recuperate
combustion gases for reuse.
The combustion unit may be constructed, designed and operated in a number of
ways depending on the desired combustion and the properties of the asphaltene
concentrated to be used as fuel. In one aspect, limestone may be added for
control of sulfur dioxide and the sand used in the fluid bed combustor is
largely
mineral from the asphaltene concentrate itself, e.g. on a dry basis about
50/50
asphaltene/mineral.
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In another aspect, the combustion unit may be configured or comprise a module
for capturing C02 and/or other gases, which could be reused or sequestered
underground.
Referring to Fig 3, the asphaltene-rich froth recovered from the floatation
unit is
relatively consistent at about 30 wt% asphaltene with asphaltene recoveries up
to
about 85%. The "grade" of the asphaltene-rich froth is used to indicate its
asphaltene content and thus its usefulness and properties as a fuel. The
asphaltene-rich froth is subsequently dehydrated and the asphaltene
concentrate
fuel thus has a higher asphaltene concentration often around 50 wt%.
Referring back to Fig 1, the asphaltene-rich concentrate 46 may have a variety
of
applications. The asphaltene-rich concentrate 46 may be used for alternative
processing such as gasification for hydrogen production, marketed directly as
an
asphalt product or back blending into heavy crude to produce on-spec material
for certain asphalt products.
The asphaltene concentrate to be used as fuel may be conditioned, handled or
stockpiled prior to subjecting to combustion. The properties of the asphaltene
concentrate is dependent on the upstream PFT plant operating conditions and
bitumen froth properties, including asphaltene rejection, bitumen mixing and
bitumen recovery levels.
In one aspect, the asphaltene concentrate contains a sufficient amount of
minerals to facilitate processing in the fluidized bed combustion unit.
The asphaltene concentrate fuel, which may also be referred to as an
asphaltene-mineral composite, preferably has a bitumen component which
comprises over 80%, optionally over 83%, asphaltene. Asphaltene is an
advantageous secondary energy source due to its high calorific content. The
asphaltene-mineral composite fuel may contain about 70% asphaltene and about
30% mineral solids on a wet basis. The process may be adapted for improved
handling of the asphaltene-mineral composite fuel in accordance with several
characteristics. In one aspect, the asphaltene-mineral composite has a
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flowability, cohesive strength, wall friction properties and compressibility
that
determine the handling strategy. For instance, the minimum outlet size may be
provided based on the cohesive strength of the asphaltene-mineral composite to
prevent arching and ratholing. In addition, critical hopper angles may be
provided
to achieve required or preferred mass flow based on the wall friction
properties of
the asphaltene-mineral composite. Furthermore, critical chute angles may be
provided to maintain flow after impact and prevent pluggage. In addition, in
terms
of storage issues, the handling and storing of the asphaltene-mineral
composite
may be provided to minimize stagnation and degradation potential. The
compressibility of the asphaltene-mineral composite may also drive the
configuration and operation of the process steps.
In one aspect, the asphaltene concentrate fuel is combusted to produce solid
waste ash 60 into which certain compounds are present and may be extracted if
desired. For instance, various metals are bound with the asphaltene composite
and require thermal breakdown of the asphaltenes to be removed. After
combustion the metals have been removed from the asphaltene fuel and report
to the solid waste stream. It may be preferable to recuperate the metals from
the
solid waste stream to other recuperation methods. It is also noted that the
asphaltene rejection from bitumen froth which is enabled by PFT improves
downstream operations and upgrading. For instance, refineries benefit from
asphaltene rejection since the asphaltenes remove the metals from the bitumen
froth to produce a cleaner bitumen product that can be upgraded. Having a
lower
amount of metals in the bitumen reduces poisoning of refining catalysts. Thus,
the metals removed from the bitumen froth and carried with the asphaltenes do
not enter the refining processes but may be rejected as part of a solid waste
stream for optional recuperation. The ash may also contain gypsum which could
be separated for use in various applications.
As briefly mentioned above, the maltene component of the TSRU tailings may
have downstream effects on embodiments of the process. For instance, part of
the maltene component may be carried over with the asphaltene-rich froth and,
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through the subsequent process steps, the maltenes may act as a binder
between asphaltene concentrate which may be in a generally particulate form.
It
is worth mentioning at this juncture that in the PFT process, the asphaltenes
first
precipitate out of the bitumen froth in the form of flocs and make up a
component
of the solvent diluted tailings which are supplied to the TSRU. Upon injection
into
the TSRU, the asphaltene floc structures are broken down due to the nozzles as
they release solvent. Thus, the asphaltenes in the TSRU tailings are broken
down aggregates. The asphaltene-rich froth 46 thus contains asphaltenes in the
form of such broken down aggregates. Through the subsequent filtration and
drying steps, maltenes present in the froth may act as a binder of the
asphaltene
aggregates which can create difficulties for handling. In one aspect, the
maltene
or bitumen component in the TSRU tailings is maintained or controlled below a
level sufficient to reduce or avoid binding of the asphaltene aggregates in
the
asphaltene concentrate. This can improve materials handling as well as
filtration
and drying performance.
Referring now to Fig 1, the tailings underflow 24 from the floatation unit 12
and
possibly the filtering tailings stream 42 from the filtration unit 36, are
further
treated in a thickening unit 64. The thickening unit 64 includes a tailings
inlet 66
for receiving the floatation underflow 24, a bottoms outlet 68 for releasing
thickened tailings 70 and a liquid outlet 72 for releasing recovered hot water
74.
Thus, the thickening unit 64 recovers hot water from the asphaltene-stripped
TSRU tailings stream and the dense tailings are disposed. The disposal of the
dense tailings may include further dewatering, flocculation, beaching and
drying
operations, if desired, disposal into a tailings pond or other consolidation
or
disposal methods. The thickened tailings may be disposed through the overall
tailings system of an oil sands facility. Optionally, the thickened tailings,
which
may have limited flowrates, may be subjected to a belt filter and drying to
facilitate production of a dry tailings product. This dry product may be
further
processed to recover titanium, zirconium, vanadium or other compounds for
their
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value or to enhance environmental reclamation activities. Accordingly, the dry
tailings stream may be produced so as to be segregatable.
The thickening unit 64 permits an additional efficiency option by producing
hot
water 70 for reuse. In addition, the thickening unit 64 can be operated with
greater efficiency due to the asphaltene-depleted feed supplied thereto. With
a
lower amount of asphaltenes, the minerals separate easier, settling reate
increase and the effect of flocculents for flocculating clays is increased as
they
are not competed with, deactivated or adsorbed by the hydrocarbon phase.
The thickening unit may be configured and operated to have a given flow rate,
rake speed, feed and underflow compositions, pH, bed height, temperature and
settling rate. It is noted that the thickener as described in Canadian patent
application No. 2,454,942 may be used, preferably as the separate thickener 64
and alternatively as a floatation unit and thickener combination.
In addition, various thickening and flocculating agents may be used in
conjunction with the thickening unit, with addition either upstream of the
thickening unit or within it.
It is also noted that various recirculation and/or return lines may be
incorporated
for the underflow streams of the process. For instance, a portion of the
underflow
of the floatation unit may be recycled back as a recycled underflow stream
into
the floatation vessel itself or into the floatation feed which can stabilize
flow. A
portion of the underflow of the thickening unit may also be recycled back as a
recycled underflow stream into the thickening unit or into the feed thereto
which
can stabilize flow.
Finally, it should be noted that the aspects and embodiments described or
illustrated herein should not limit the scope of the present invention.
