Note: Descriptions are shown in the official language in which they were submitted.
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PROCESSES AND SYSTEMS FOR PRE-TREATING BITUMEN FROTH FOR
FROTH TREATMENT
FIELD OF THE INVENTION
The present invention generally relates to the field of bitumen froth
treatment
operations and, more particularly, to pre-treating bitumen froth before
undergoing
froth treatment, such as paraffinic froth treatment.
BACKGROUND
During oil sands processing, oil sands ore containing bitumen is mixed with
heated
water to form a slurry of oil sand and water. This slurry is then fed into a
primary
separation vessel (PSV), or separation cell, where the more buoyant bitumen
component floats to the surface to form a froth which is commonly referred to
as
bitumen froth. Due to their density and affinity to hydrocarbons, debris such
as coal,
petrified wood, green wood, and plastics are found in varying quantities in
the slurry,
and are also present in the bitumen froth.
This debris floats in the bitumen froth and often consists of large particles
which can
clog equipment used in froth treatment and downstream process operations.
Furthermore, the debris typically has a density between that of water and
diluted
bitumen, which results in the debris tending to accumulate at the
hydrocarbon/water
interface. Smaller sized particles, such as mineral fractions, with a diameter
greater
than 44 microns, are generally referred to in the industry as "sand". Even
smaller
mineral fractions with a particle diameter less than 44 microns are generally
referred
to as "fines".
It is desirable to separate this debris from the bitumen froth before the
froth is further
treated. Debris screening is used in various processes, but the properties of
bitumen
froth are such that application of general known techniques is fraught with
difficulties
and disadvantages.
One reason for this is that bitumen froth is highly viscous, which tends to
limit the
direct separation of debris to relatively large particles, e.g. greater than 8
mm in size.
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Diluents can be added to reduce the viscosity of the bitumen froth, but these
can
negatively affect downstream processing and may need to be removed before or
during froth treatment. Handling the large debris particles that may be coated
with
bitumen and/or diluent is also difficult, labour intensive, and can involve
attendant
safety risks resulting from the handling of hot, viscous, adherent materials
as well as
variable compositions and flow rates of such materials.
Smaller screens, e.g. having grates of 1 inch spacing, tend to require the use
of the
entire perimeter of a separation cell, and a dedicated work crew to clear the
grating in
order to prevent it from plugging up. While other screening techniques using
rotating
trommels or linear screens can advantageously reduce the clogging in certain
applications, there are disadvantages related to handling periodic surges and
debris
exceeding the screen's capacity, which is sensitive to density and viscosity.
Attempts have been made to grind or strain large debris particles after a
diluent is
added, but this tends to lead to the debris interfering at the oil/water
interface, which
makes oil/water separation more difficult. Furthermore, handling diluent-
contaminated
debris is best avoided.
Bitumen froth can also have a high gas content. Usually the gas essentially
consists
of air. When bitumen froth exits the PSV, it typically includes 30-50% vol.
gas. This
volume of gas needs to be significantly reduced so as to prevent vapour lock
and
other pumping problems when the froth is pumped downstream for treatment. It
is
also desirable to reduce the gas content of the froth prior to paraffinic
froth treatment.
Upon performing de-aeration using some of these known techniques, the gas
content
of the bitumen froth may be reduced to 10-15% vol., which has generally been
considered sufficiently de-aerated for the purposes of froth pumping. However,
flow
variations or surges from separation cells can adversely influence de-aeration
operations, which may interrupt the flow of de-aerated bitumen exiting the de-
aerator
and heading towards the pump, which can cause vapour lock in the froth pump,
further causing flow problems in the operations of downstream units.
There are also other disadvantages with known de-aeration techniques. Static
de-
aeration only removes about 50% vol. of gas from the froth, and does not
prevent re-
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entrainment of air or other gases into the froth, resulting in bitumen froth
often having
at least 9.5% vol. of gas. The process of steam de-aeration typically consists
of froth
cascading against rising steam and condensing on shed decks. However, the
viscosity and surface tension of the froth can result in unstable flows over
the shed
decks. Mechanical de-aeration can involve installation of impellers in a
launder.
Although certain data indicate that de-aerated froth having less than 5% vol.
can be
achieved by using mechanical de-aeration, the commercial viability of such a
technique is not known.
SUMMARY OF THE INVENTION
The present invention provides processes and systems for pre-treating bitumen
froth
containing debris for a froth treatment operation, such as a paraffinic froth
treatment
operation.
In one implementation, there is provided a process for pre-treating a variable
flow of
bitumen froth prior to a froth treatment operation, comprising:
providing the variable flow of bitumen froth to a feed unit;
adjusting the feed unit in response to the variable flow to provide a
regulated
flow of the bitumen froth;
subjecting the regulated flow of the bitumen froth to screening to remove
debris from the regulated flow of the bitumen froth to produce screened
bitumen froth and separated debris; and
providing the screened bitumen froth into the froth treatment operation.
In one optional implementation, the feed unit comprises an upstream
compartment
and a downstream compartment and the feed unit is adjusted to provide the
regulated flow of the bitumen froth from the upstream compartment into the
downstream compartment.
In another optional implementation, the feed unit comprises a surge tank
comprising
a regulator device separating the upstream compartment and the downstream
compartment.
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In another optional implementation, the surge tank retains the bitumen froth
for a
duration ranging from about 1 minute to about 10 minutes.
In another optional implementation, the adjusting of the feed unit comprises
displacement of the regulator device.
In another optional implementation, the regulator device comprises a partition
defining the upstream compartment and the downstream compartment.
In another optional implementation, the partition is vertically displaceable.
In another optional implementation, the adjusting comprises:
gradually increasing the regulated flow of bitumen froth in response to a flow
surge of the variable flow of bitumen froth; or
gradually decreasing the regulated flow of bitumen froth in response to a
large decrease of the variable flow of bitumen froth.
In another optional implementation, the process further comprises:
dispending the regulated flow of the bitumen froth so as to have a
consistent quantity for the screening.
In another optional implementation, the process further comprises:
spraying a wash fluid onto the bitumen froth or screen unit during the
screening.
In another optional implementation, the froth treatment operation is a
paraffinic froth
treatment operation.
In another implementation, there is provided a system for pre-treating a
variable flow
of bitumen froth prior to a froth treatment operation, comprising:
a feed unit comprising:
an upstream compartment comprising an inlet for receiving the
variable flow of the bitumen froth;
a downstream compartment; and
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a regulator device separating and regulating transfer of the bitumen
froth from the upstream compartment to the downstream
compartment, to provide a regulated flow of the bitumen froth into
the downstream compartment; and
5 a screen unit in fluid communication with the downstream compartment of
the feed unit for receiving the bitumen froth therefrom to remove debris from
the bitumen froth to produce screened bitumen froth and separated debris.
In one optional implementation, the feed unit comprises a surge tank.
In another optional implementation, the regulator device comprises a partition
provided within the surge tank and separating the upstream compartment from
the
downstream compartment.
In another optional implementation, the wall comprises a plate arranged
transversely
with respect to the flow of the bitumen froth.
In another optional implementation, the partition has a top edge and is
vertically
displaceable for adjusting the flow rate of the bitumen froth flowing over the
top edge
into the downstream compartment.
In another optional implementation, the regulator device comprises a weir.
In another optional implementation, the system further comprises:
a spraying apparatus arranged with respect to the screen unit for spraying
wash fluid over the bitumen froth and/or the screen unit.
In another optional implementation, the screen unit comprises a moving screen
device.
In another optional implementation, the moving screen device includes a speed
adjuster for adjusting speed of the moving screen device.
In another optional implementation, the moving screen device comprises a
linear
screen.
In another optional implementation, the froth treatment operation is a
paraffinic froth
treatment operation.
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In another implementation, there is provided a system for pre-treating bitumen
froth
comprising solid debris prior to froth treatment, comprising:
a moving screen device comprising:
an upstream section;
a downstream section;
means to advance the bitumen froth from the upstream section
toward the downstream section while a portion of the bitumen froth
passes through the screen as screened bitumen froth and the solid
debris is advanced toward the downstream section;
a feed inlet arranged at the upstream section of the moving screen device
for feeding the bitumen froth thereto;
a washing device for providing washing fluid to the moving screen device;
and
a froth launder below the upstream section of the moving screen device for
receiving the screened bitumen froth.
In one optional implementation, the washing device is configured in relation
to the
downstream section of the moving screen to enable washing the debris present
on
the downstream section and to produce washed debris and spent washing fluid.
In another optional implementation, the system further comprises a debris
launder
arranged proximate the downstream section for receiving the washed debris.
In another optional implementation, the system further comprises a wash fluid
launder for receiving the spent washing fluid.
In another optional implementation, the wash fluid launder is configured for
spanning
the length below the moving screening unit.
In another optional implementation, the moving screen device comprises a
linear
screen or a drum screen.
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In another optional implementation, the system further comprises a feed unit
comprising an upstream compartment, a downstream compartment, and a regulator
device separating and regulating transfer of the bitumen froth from the
upstream
compartment to the downstream compartment, the downstream compartment being
in fluid communication with the feed inlet for feeding the bitumen froth to
the
upstream section of the moving screen device.
In another optional implementation, the froth treatment operation is a
paraffinic froth
treatment operation.
In another implementation, there is provided a process for pre-treating
bitumen froth
comprising solid debris prior to froth treatment, comprising:
feeding the bitumen froth to an upstream section of a moving screen
device;
screening the bitumen froth to produce screened bitumen froth and
separated solid debris;
advancing the separated solid debris toward a downstream section of the
moving screen device;
receiving the screened bitumen froth below the upstream section of the
moving screen device; and
washing the moving screen device.
In one optional implementation, the washing comprises spraying the downstream
section of the moving screen device to enable washing the separated solid
debris
present on the downstream section and to produce washed debris and spent
washing
fluid.
In another optional implementation, the washing comprises spraying a screen of
the
moving screen device to produce additional spent washing fluid.
In another optional implementation, the process further comprises receiving
the spent
washing fluid in a wash fluid launder.
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In another optional implementation, the process further comprises recycling
the spent
washing fluid for reuse in spraying.
In another optional implementation, the process further comprises recuperating
energy from the spent washing fluid.
In another optional implementation, the froth treatment comprises a paraffinic
froth
treatment operation.
In another implementation, there is provided a process for pre-treating
bitumen froth
prior to froth treatment, comprising:
subjecting the bitumen froth to de-aeration in a de-aerator comprising a
cross-flow plate pack to produce a de-aerated bitumen froth; and
subjecting the de-aerated bitumen froth to screening to remove solid
debris therefrom and to produce screened bitumen froth and debris.
In one optional implementation, the bitumen froth is pre-heated and introduced
into
the cross-flow plate pack.
In another optional implementation, subjecting the bitumen froth to de-
aeration
comprises slowing bitumen froth flow across the de-aerator.
In another optional implementation, subjecting the bitumen froth to de-
aeration further
comprises minimizing the depth that gas bubbles within the bitumen froth need
to rise
to be released from the bitumen froth.
In another optional implementation, subjecting the bitumen froth to de-
aeration further
comprises increasing turbulence in the bitumen froth to augment release of gas
bubbles.
In another optional implementation, the froth treatment comprises a paraffinic
froth
treatment operation.
In another implementation, there is provided a system for pre-treating bitumen
froth
prior to froth treatment, comprising:
a de-aerator unit having a cross-flow plate pack for producing a de-aerated
bitumen froth; and
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a screening unit for receiving de-aerated bitumen froth from the de-aerator
unit so as to remove solid debris therefrom to produce screened bitumen
froth and rejected debris
In one optional implementation, a heater is provided upstream of the cross-
flow plate
pack for supplying a pre-heated bitumen froth to the cross-flow plate pack.
In another optional implementation, the cross-flow plate pack comprises an
inlet
defining an expanding nozzle so as to slow bitumen froth flow across the de-
aerator
unit.
In another optional implementation, the inlet defines a substantially frusto-
conical
shape.
In another optional implementation, the de-aerator unit further comprises a de-
aeration chamber having opposed side walls, a bottom wall and a top wall, the
de-
aeration chamber promoting cross-flow of the bitumen froth.
In another optional implementation, the cross-flow plate pack further
comprises
multiple plates arranged in spaced-apart relationship, thereby defining flow
channels
through which the bitumen froth is conveyed.
In another optional implementation, the length of each plate is about ten
times the
width of the same plate.
In another optional implementation, the cross-flow plate pack further
comprises a weir
for stabilising bitumen froth flow.
In another optional implementation, the de-aerator unit further comprises
water
sprayers positioned above the bitumen froth for spraying gases released from
the
bitumen froth so as to remove mists of bitumen collecting above the bitumen
froth.
In another optional implementation, the froth treatment comprises a paraffinic
froth
treatment operation.
In another implementation, there is provided a treatment process comprising:
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subjecting the bitumen froth to pre-treatment to produce a pre-treated
froth,
wherein the bitumen froth comprises bitumen, water, air and solids, the
solids
comprising fines and debris;
adding a paraffinic solvent to the pre-treated froth to for a diluted froth
5 comprising precipitated water-solid-asphaltene aggregates;
separating the diluted froth into an upper hydrocarbon rich fraction and a
lower tailings fraction containing the water-solid-asphaltene aggregates,
wherein a water-hydrocarbon interface forms between the upper fraction
and the lower fraction;
10 wherein the pre-treatment comprises:
removing from the bitumen froth at least a portion of the debris
having a non-aggregatable size so as to prevent or reduce
accumulation of the debris at the water-hydrocarbon interface.
In one optional implementation, the step of removing the portion of the debris
comprises subjecting the bitumen froth to screening.
In another optional implementation, the process further comprises:
monitoring the water-hydrocarbon interface to determine the accumulation
of debris therein; and
adjusting the pre-treatment of the bitumen froth in response to the
accumulation of the debris in the water-hydrocarbon interface.
In another optional implementation, the monitoring comprises measuring a
thickness
of accumulated debris in the water-hydrocarbon interface.
In another optional implementation, the adjusting of the pre-treatment
comprises:
adjusting the screening; and/or
adjusting a temperature of the bitumen froth;
In another optional implementation, the adjusting of screening comprises:
adjusting a speed of a moving screen device; and/or
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modifying a pore size of a screen.
In another optional implementation, the moving screen device comprises a
linear
screen.
In another optional implementation, in response to an increase in the
accumulation of
debris in the water-hydrocarbon interface, the speed of the moving screen
device is
increased.
In another optional implementation, in response to an increase in the
accumulation of
debris in the water-hydrocarbon interface, the pore size of the moving screen
device
is decreased.
In another optional implementation, the screening is performed through pores
having
a size of about 1 to about 4 orders of magnitude greater than the size of
aggregatable
solids in the bitumen froth.
In another optional implementation, the screening is performed through pores
having
a size of about 2 to about 3 orders of magnitude greater than the size of
aggregatable
solids in the bitumen froth.
In another implementation, there is provided a process for pre-treating
bitumen froth
prior to a froth treatment operation, comprising feeding the bitumen froth
onto a
screen having pores with different sizes, the pores being positioned with
respect an
area of the screen and a supply of the bitumen froth in order to promote even
distribution of the bitumen froth over the screen.
In one optional implementation, the pores comprise a first set of pores each
defining
a small opening and a second set of pores each defining a larger opening, the
first
set of pores being arranged to receive a main feed flow of the bitumen froth
and the
second set of pores being arranged to receive an outward spreading flow of the
bitumen froth.
In another optional implementation, the screen comprises a moving screen
device.
In another optional implementation, the process further comprises adjusting
the
speed of the moving screen device.
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In another optional implementation, the froth treatment operation is a
paraffinic froth
treatment operation.
In another implementation, there is provided a system for pre-treating bitumen
froth
prior to a froth treatment operation, comprising:
a screen unit having a plurality of pores that are disposed with respect to
the screen to promote even distribution of the bitumen froth over the
screen and to produce a screened bitumen froth;
a feed inlet in fluid communication with the screen unit for feeding the
bitumen froth to the screen unit; and
a froth launder positioned below the screen unit for receiving the screened
bitumen froth.
In one optional implementation, the pores comprise a first set of pores each
defining
a small opening and a second set of pores each defining a larger opening, the
first
set of pores being arranged to receive a main feed flow of the bitumen froth
and the
second set of pores being arranged to receive an outward spreading flow of the
bitumen froth.
In another optional implementation, the first set of pores are disposed about
along a
middle line of the screen and the second set of pores are disposed near the
extremities of the screen.
In another optional implementation, the screen unit comprises a moving screen
device.
In another optional implementation, the moving screen device includes a speed
adjuster for adjusting speed of the moving screen device.
In another optional implementation, the moving screen device comprises a
linear
screen.
In another optional implementation, the froth treatment comprises a paraffinic
froth
treatment operation.
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In another implementation, there is provided a process for pre-treating
bitumen froth
prior to a froth treatment operation, the bitumen froth comprising bitumen,
fines,
water, air and debris, the process comprising:
subjecting the bitumen froth to heating and de-aeration to produce a
heated de-aerated bitumen froth;
subjecting the heated de-aerated bitumen froth to screening to remove the
debris therefrom and produce screened bitumen froth and separated
debris; and
regulating the heating, the de-aeration and the screening such that the
screened bitumen froth comprises a consistent air content prior to being
introduced into the paraffinic froth treatment operation.
In one optional implementation, the de-aeration and the heating of the bitumen
froth
is performed concurrently.
In another optional implementation, the heating is performed before the de-
aeration in
a separate heater.
In another optional implementation, the heating is performed after the de-
aeration in a
separate heater.
In another optional implementation, the bitumen froth is heated to a
temperature
greater than about 65 C.
In another optional implementation, the heating is performed by a direct steam
injector.
In another optional implementation, the screening is performed by a moving
screen
device.
In another optional implementation, the regulating includes adjusting speed of
the
moving screen device.
In another optional implementation, the screening further comprises spraying
wash
fluid over the heated de-aerated bitumen froth and the regulating includes
adjusting
flow rate, direction or composition of the wash fluid or a combination
thereof.
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In another optional implementation, the process further comprises subjecting
the
separated debris to washing with washing liquid to produce a washed debris and
spent washing liquid.
In another optional implementation, the washing of the debris is at least
partly
performed during the screening.
In another optional implementation, the froth treatment operation is a
paraffinic froth
treatment operation.
In another implementation, there is provided a system for pre-treating bitumen
froth
prior to a froth treatment operation, the bitumen froth comprising bitumen,
fines,
water, air and debris, the system comprising:
a de-aeration unit for de-aerating the bitumen froth and a heater for
heating the bitumen froth, in order to produce a heated de-aerated
bitumen froth;
a screening unit for removing the debris from the heated de-aerated
bitumen froth to produce screened bitumen froth and separated debris; and
the de-aeration unit and the screening unit are coupled and regulated such
that the screened bitumen froth comprises a consistent air content prior to
being introduced into the paraffinic froth treatment operation.
In one optional implementation, a heater is provided upstream of the de-
aeration unit
for supplying a pre-heated bitumen froth to thereto.
In another optional implementation, a heater is provided downstream of the de-
aeration unit for heating bitumen froth after de-aeration in the de-aeration
unit.
In another optional implementation, the bitumen froth is heated to a
temperature
greater than about 65 C.
In another optional implementation, the heater is a direct steam injector.
In another optional implementation, the de-aeration unit comprises a de-
aeration
chamber having opposed side walls, a bottom wall and a top wall, the de-
aeration
chamber promoting flow of the bitumen froth.
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In another optional implementation, the de-aeration unit further comprises
water
sprayers positioned above the bitumen froth for spraying gases released from
the
bitumen froth so as to remove mists of bitumen collecting above the bitumen
froth.
In another optional implementation, the screening unit comprises a moving
screen
5 device.
In another optional implementation, the moving screen device includes a speed
adjuster for adjusting speed of the moving screen device.
In another optional implementation, the moving screen device comprises a
linear
screen.
10 In another optional implementation, the screening unit comprises washing
sprays for
spraying wash fluid over the heated de-aerated bitumen froth.
In another optional implementation, the system further comprises a washing
unit for
washing the separated debris with a washing liquid to produce a washed debris
and
spent washing liquid.
15 In another optional implementation, the froth treatment comprises a
paraffinic froth
treatment operation.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig 1 is a process flow diagram illustrating production and treatment of
bitumen froth.
Fig 2 is a process flow diagram of a bitumen froth pre-treatment operation.
Fig 3 is another process flow diagram of a bitumen froth pre-treatment
operation with
a pre-heating unit.
Figs 4A and 4B are additional process flow diagrams of a bitumen froth pre-
treatment
operation.
Fig 5 is another process flow diagram of a bitumen froth pre-treatment
operation with
a de-aeration apparatus.
Fig 6 is a top view schematic of a screen.
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DETAILED DESCRIPTION
Referring to Fig 1, a general process overview is illustrated. Mined bitumen
ore is
broken down, mixed with water and processed to form a slurry which is fed to a
bitumen extraction step 10. Bitumen extraction generates bitumen froth which
is fed
to a froth de-aeration step 12. The resulting de-aerated froth is then
subjected to an
optional froth heating step 14 followed by a froth screening step 16 to remove
debris.
Once the debris is removed from the bitumen froth, the screened froth is
provided to
a bitumen froth treatment step 18, e.g., a paraffinic froth treatment (PFT)
operation.
Processes and systems are described for pre-treating the bitumen froth in
preparation for the PFT operation. In some implementations, the pre-treatment
system for pre-treating bitumen froth containing debris is able to regulate
and
smooth-out the flow of froth upstream of a screen so as to optimise debris
separation
at the screen, and is also able to stabilize the flow of froth to downstream
froth-
treatment units, such as PFT separation vessels. The process and system by
which
this is achieved permit adaptability in response to froth or debris flow
surges, while
removing debris particles of various sizes in an efficient manner.
Referring to Figs 2-5, implementations of the froth pre-treatment system 20
are
illustrated. The pre-treatment system 20 may comprise various units for pre-
treating
the bitumen froth containing debris in order to reduce the gas content and
debris
content of bitumen froth 22.
Still referring to Figs 2-5, the bitumen froth may be received from what is
commonly
called a primary separation vessel (PSV) 24 as illustrated. It should
nevertheless be
understood that the pre-treatment system 20 may receive bitumen froth from a
number of units and sources depending on the source and upstream processing of
the froth. For instance, in some implementations, the bitumen froth is derived
from an
oil sands mining and extraction operation where the bitumen containing ore is
processed to produce the bitumen froth 22. In other implementations, the
bitumen
froth may be derived from an in situ recovery process in which bitumen and/or
heavy
oil are recovered using underground wells. The bitumen froth 22 may also be a
combination of froth derived from an oil sands mining and extraction operation
and an
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in situ recovery operation. The bitumen froth may also be fed to the pre-
treatment
system 20 from a holding tank or a unit other than the PSV 24 depending on the
infrastructural design integrating the extraction step with the pre-treatment
step.
In some implementations as illustrated in Fig 2, in standard operating mode
the pre-
treatment system 20 receives bitumen froth 22 from a PSV 24, which separates
oil
sand ore slurry 26 into an underflow tailings stream, a middlings stream, and
an
overflow of the bitumen froth 22. The oil sand ore slurry 26 has a composition
dependant on the slurry preparation operation, as well as on the geological
body from
which the ore was obtained. Thus, the oil sand ore slurry 26 and, in turn, the
bitumen
froth 22, may vary in composition and flow rate. These variations may occur
gradually
or as a step change, often reflecting the nature of the oil sand ore body. The
variations may also derive from upsets in upstream unit operations in oil sand
mining
and extraction operations. As noted above, the bitumen froth 22 provided to
the pre-
treatment system 20, rather than coming from an oil sands mining and
extraction
operation, may be derived from an in situ heavy hydrocarbon operation. In situ
operations involve subterranean wells located in bitumen containing reservoirs
and
use heat, steam, hot water, solvent or various combinations thereof to
mobilize the
bitumen so that it can be withdrawn through a production well. In situ bitumen
containing streams may be subjected to bitumen froth treatment, such as
paraffinic
froth treatment (PFT), to improve bitumen quality by reducing asphaltene
content.
After being treated in the PSV 24, the bitumen froth 22 is typically highly
aerated with
gas contents in the range of 30-50% vol. In addition, as a result of the
density of
various types of debris and the debris's affinity to hydrocarbons, particles
such as
coal, petrified wood, green wood, and plastics are found in varying quantities
in the
bitumen froth 22. Furthermore, the gas content of the froth 22 is often
unstable, with
large froth vapour bubbles breaking as the froth 22 is transported through the
system 20.
In order to improve the pumping of bitumen froth 22 through the system 20
using
conventional pumps 28, the gas content of the froth 22 is reduced.
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Referring to Figs 2-4B, after being treated in the PSV 24, the froth 22 is
sent to a de-
aerator 30 so as to reduce its gas content to a level that will prevent vapour
locking in
downstream equipment. The de-aerator 30 produces a de-aerated froth 32 in
which
the gas content can be reduced below 15% vol., for instance in a range of 10-
15%
vol. The gas content of the de-aerated froth 32 may be sufficiently low such
that there
is reduced or no vapour locking when the de-aerated froth 32 is pumped by
downstream pumps 28.
Bitumen froth de-aeration and heating
The general operation of some implementations of the de-aerator 30 is now
described, with reference to Figs 2-4B. The steam de-aerator 30 receives the
bitumen froth 22 at an upper portion and produces the de-aerated froth stream
32
from a lower portion. The bitumen froth 22 which arrives from the PSV 24 may
be de-
aerated by condensing on structures, which may be called shed decks 34, and
cascading down a series of these shed decks arranged in vertical columns. Each
shed deck 34 can be configured in the form of an inverted "V" or a "W". The
bitumen
froth 22 increases its liquid content by condensing in the center of the "W"-
shaped
shed deck 34. The outer arms of the "W" act as weirs that allow the fluid to
pool
inside the "W" until there is enough that it spills over these outer arm weirs
as a thin
curtain to the next shed deck 34 below. In order to consolidate the bitumen
froth 22
by reducing its gas content, steam is typically injected below the shed decks
34. The
steam rises counter-currently up the columns of shed decks 34. As the froth 22
is
heated by the steam, its gas is released.
Conventional steam de-aerators may lead to the viscosity and surface tension
of the
bitumen froth 22 causing unstable flows over weirs and localised streams,
which can
reduce pumping efficiency after de-aeration. Steam de-aerators may also heat
the
bitumen froth 22 to temperatures that are too low to optimise the release of
gas
bubbles from the froth 22.
Referring to Fig 5, in some implementations, the pre-treatment system 20
includes
pre-heating the bitumen froth 22 before undergoing de-aeration. This pre-
heating
step may be performed with a pre-heater 36 using direct steam injection (DSO,
CA 02947081 2016-10-31
19
sufficient to heat the froth 22 above 75 C. The DSI pre-heater unit 36 may
heat the
froth 22 to between 70-95 C, and further optionally around 90 C. This heating
step
can provide certain advantages. The vapour pressure of water in the froth 22
can
increase the size, i.e. volume, of the bubbles contained in the froth 22,
which allows
for these large bubbles to be burst more easily and the gas contained in the
bubbles
to be more easily released during downstream de-aeration. In addition, the pre-
heating reduces the viscosity of the froth 22, thus allowing a more even and
continuous flow over the shed decks or other structures in the downstream de-
aeration step, which reduces vapour locking in the downstream pump and, in
turn,
the resulting downstream flow upsets. Heating the froth 22 at this stage can
also
minimize the use of additional heating in downstream processes for PFT. The
froth 22 thus emerges from the de-aerator as a heated and de-aerated bitumen
froth.
The pre-heater 36 may be a DSI unit using steam 38 to heat the bitumen froth.
The
pre-heater 36 may also use other DSI methods or other direct or indirect
heating
methods. The pre-heater 36 produces a heated froth 40 which may be provided to
downstream processing units for de-aeration and screening. More regarding the
de-
aeration shown in Fig 5 will be discussed below.
Referring to Fig 3, the pre-treatment system 20 may include a heating step
after de-
aeration using a de-aerated froth heater 38. This heater 38 may be a DSI
heater
receiving steam 42 from common steam source 44 as the de-aerator steam 46
provided to the upstream de-aerator 30. The common steam source 44 may be
regulated and split into the DSI heater steam 42 and the de-aerator steam 46
to
advantageously adjust the heating programme of the bitumen froth without
requiring
additional steam generating infrastructure.
Referring back to Fig 5, the pre-treatment system 20 may include a cross-flow
de-
aeration apparatus 48 configured in between the pre-heater 36 and a downstream
screening unit, which will be discussed in further detail below. It should
also be noted
that the cross-flow de-aeration apparatus 48 as illustrated in Fig 5 and
described
herein may be used upstream of a heater or without a separate heater if
desired. The
pre-heated froth 40 may be introduced at the normal liquid level into the
cross-flow
de-aeration apparatus 48. The pre-heated froth 40 may enter the cross-flow de-
CA 02947081 2016-10-31
aeration apparatus 48 via an entrance comprising an expanding nozzle or inlet
50 so
as to slow the froth feed velocity sufficiently to substantially conform to
the froth flow
velocity across the cross-flow de-aeration apparatus 48. This allows for an
initial gas
content to disengage from the heated froth 40 without creating slug flow,
which is an
5 unstable flow resulting from the separation of gas and liquids in piping
networks
which can lead to process upsets and/or equipment failures by vibration. The
de-
aerator inlet 50 may be constructed to have a generally frusto-conical shape
tapering
outwardly toward de-aeration apparatus 48. The de-aerator inlet 50 may
alternatively
be box-shaped with generally flat side, bottom and top walls. The de-aerator
inlet 50
10 may be positioned and configured relative to certain internals of the
cross-flow de-
aeration apparatus 48, such that the flow of froth distributes over the height
and/or
width of the internals to enhance de-aeration.
In another possible implementation, as shown in Fig 5, the pre-treatment
system 20
includes the cross-flow de-aeration apparatus 48, having a construction and
15 configuration offering advantageous functionality for preparing bitumen
froth. In one
optional implementation, the cross-flow de-aeration apparatus 48 can act as a
separate surge buffer. The cross-flow de-aeration apparatus 48 can include
vessel
side walls 52, a bottom 54 and a top 56 together defining a de-aeration
chamber 58.
The de-aeration chamber 58 may have an internal structure promoting cross-flow
of
20 the bitumen froth or minimizing the depth the gas bubbles need to rise
in order to be
released from the froth. In one optional implementation, the de-aeration
chamber 58
has a cross-flow plate pack 60 which promotes cross-flow and minimises the
depth
the gas bubbles need to rise in order to be released from the froth. The cross-
flow
plate pack 60 includes a plurality of plates 62 which can be arranged in
generally
vertical or horizontal and spaced-apart relationship. When arranged in the
generally
vertical spaced-apart relationship, the plates 62 can form an angle of between
about
to about 60 degrees above the horizontal. The plates 62 are arranged to define
flow channels 64 therebetween and are oriented such that the flow channels
provide
fluid flow from the inlet 50 toward an outlet 66 at an opposed side of the de-
aeration
30 chamber 58. The spacing of the plates 62 in the pack 58 may be provided
so as
CA 02947081 2016-10-31
21
optimise or increase turbulence in the chamber to thereby augment gas
separation
from the froth.
The de-aerator inlet 50 may be positioned and configured relative to the cross-
flow
plate pack 60, such that the flow of froth from the inlet is directed toward
the
channels 64 and is distributed over the entire height of the cross-flow plate
pack 60,
thereby dividing the froth into separate flow components through respective
channels 64, to enhance de-aeration. The de-aerator inlet 50 may be provided
outside of the side wall 52, as illustrated; the expanding part of the inlet
50 may
alternatively be located partially or fully inside the de-aeration chamber 58.
Still referring to Fig 5, the cross-flow de-aeration apparatus 48 may also
include an
underflow weir 68 and an overflow weir 70 for stabilising froth fluid flows.
The
overflow weir 70 may be positioned at the discharge outlet 66 of the de-
aeration
apparatus 48, arranged just upstream of a screening step, and limits sudden
flow
surges from the pump or the DSI unit 36 from affecting the efficient operation
of the
screening step. To prevent or reduce the froth from bypassing the cross-flow
de-
aeration apparatus 48 and heading over the overflow weir 70 to the screening
unit 76,
the froth may be required to flow below the underflow weir 68 prior to flowing
over the
overflow weir 70. This underflow weir 68 may be positioned upstream of the
flow
crest of the overflow weir 70. In one implementation, the underflow weir 68
comprises
a rectangular weir having sufficient width and having a surface oriented in a
generally
perpendicular relation to the cross-flow plate pack 60. The surface of such a
rectangular weir can also be oriented in a general normal relation to the
froth flow
through the channels 64, thus allowing for dampening of froth feed surges.
In another implementation, the underflow weir 68 and the cross-flow plate pack
60
may be sized and configured such that underflow weir 68 spans at least the
entire
height of the cross-flow plate pack 60 and thus ensures dampening of froth
flow
through all of the channels 64. In another implementation, the overflow weir
may be
located so as to be at or above the height of the top plate in the cross-flow
plate
pack 60. In yet another optional implementation, the cross-flow plate pack 60
is
inclined which provides for an increased separating area where gas can be
released
from the froth. The spacing of the plates 62 depends, among other things, on
the
CA 02947081 2016-10-31
22
maximum debris particle size that can pass through the oil sand ore
preparation and
which floats in the bitumen froth after processing in the PSV 24, which is
typically
about 100 mm to about 150 mm. The length of plates 62 can be in the order of
ten
times plate spacing, with the top of the plate 62 optionally placed just below
the feed
nozzle 50. In a further implementation, the overflow weir 70 or the underflow
weir 68
or both are adjustable to vary the froth flow characteristics. The overflow
weir 70 may,
for example, be vertically adjustable to vary the flow rate of froth released
from the
de-aeration apparatus 48.
In some implementations, water sprayers (not illustrated) may be used in the
cross-
flow de-aeration apparatus 48. These water sprayers may be positioned in the
top
section of the de-aeration chamber 58, above the released gas that collects
above
the froth liquid. The water sprayers may advantageously be configured,
positioned
and operated to remove potential entrained mists of bitumen in the vapour
stream,
further adding to the recovery of bitumen. One technique for controlling
bitumen mist
is to maintain a low mist velocity above the froth liquid. The water sprayers
can
mitigate sudden gas/steam upsets. In addition, the water sprayers may also
enable
cooling of the released gas, and the recovered water/bitumen mixture may be
directed to a wash water pump box 106, such as the one described below, for
recycle/reuse in extraction. Some entrained water and mineral solids from the
froth
may settle in the de-aeration chamber 58. In order to prevent the accumulation
of
solids building up to a level that blocks the de-aerator 48, these may be
collected in a
hopper and periodically flushed by the injection of flush water to the wash
water
pump box 106. By recycling the bitumen in this manner, a portion of froth can
be
recovered.
The cross-flow de-aeration apparatus 48 de-aerates the froth so that it can be
efficiently pumped, while also achieving stable froth flows at desired
temperatures by
maintaining a consistent gas content, the combined effect of which can
mitigate
upsets or surges from the PSV 24 before the froth flow reaches downstream
processing such as a screening step.
CA 02947081 2016-10-31
23
Bitumen froth debris removal and surge buffering
Referring to Figs 2-4B, after de-aeration the de-aerated bitumen froth 32 may
be
transported or conveyed via the pump 28 to a debris separator 72. Note that
there
may be one debris separator 72 as illustrated in Figs 2-4A, or two or more
debris
separators 72a, 72b as illustrated in Fig 4B.
Referring to Fig 2 in particular, the debris separator 72 may include a surge
tank 74
followed by a screening unit 76.
Referring to Fig 5, in some implementations the de-aeration apparatus 48 may
be
directly upstream of the debris separator 72 and configured and located such
that the
de-aerated froth 32 is conveyed or transported by gravity alone. It should
also be
noted that the de-aeration apparatus 48 having enhanced surge buffering
capacity
may be configured directly upstream of the screening unit 76 with no
intermediate
separate surge tank 74.
Referring back to Fig 2, before being fed to the screening unit 76, the de-
aerated
froth 32 may be first conveyed to a surge tank 74, which may also be referred
to as a
"screen feed tank" or "screen feed surge tank". The surge tank 74 may have an
upstream compartment 78, which receives a variable flow of the de-aerated
froth 32,
and a downstream compartment 80 that is separated from the upstream
compartment 78 by a regulating device 82. The regulating device 82 is
constructed
and configured to regulate the flow of froth from the upstream compartment 78
to the
downstream compartment 80. Alternatively, the regulating device 82 may have
other
forms and structures to aid in flow regulation of the froth.
The flow regulating device 82 includes a partition within the cavity of the
surge
tank 74, spanning across the entire width of the cavity and extending from the
bottom
toward the top of the cavity terminating in an upper edge 84 in spaced-apart
relation
with the top of the cavity of the surge tank 74. The upper edge 84 and the top
of the
cavity thus define a froth flow opening 86 through which the froth is allowed
to flow
from the upstream compartment 78 to the downstream compartment 80. The
partition
of the flow regulating device 82 may take the form of a plate-shaped member
arranged transversely with respect to the flow of the bitumen froth. The
partition may
CA 02947081 2016-10-31
24
be vertically displaceable for adjusting the height of its upper edge 84 and
thus the
size of the froth flow opening 86 which, in turn, enables adjustment of the
flow rate of
the bitumen froth flowing over the upper edge 84 into the downstream
compartment
80. The partition could also be laterally or otherwise displaceable to adjust
froth flow.
In one optional implementation, the flow regulating device 82 operates as an
adjustable weir, which may be controlled manually or automatically. Hereafter,
the
flow regulating device 82 is referred to generally as a surge tank weir 82.
The surge tank weir 82 allows for control of the transfer of de-aerated froth
32 from
the upstream compartment 78 to the downstream compartment 80 so as to provide
a
regulated flow of de-aerated froth 32 to subsequent processing equipment, such
as
the screening unit 76. The surge tank weir 82 is displaceable so as to
regulate the
flow of de-aerated froth 32 in response to periodic surges. In some
implementations,
the surge tank 74 may include multiple surge tanks, each one feeding the de-
aerated
froth 32 in a regulated fashion to a corresponding screening unit, or multiple
surge
tanks in series or parallel all feeding a screening step.
Different known surge tank weirs 82 can be used in the surge tank 74 such as
rectangular weirs, V-notch weirs, or a combination of differently-shaped
weirs, all of
which can assist in dampening de-aerated froth flow by permitting level
changes in
the surge tank 74. In normal operation, the surge tank weir 82 maintains a
normal de-
aerated froth level in the surge tank 74. When the de-aerated froth flow is
interrupted,
such as when the froth pump 28 vapour locks, the surge tank weir 82 permits
both
the de-aerated froth level in the surge tank 74 and de-aerated froth flow to
the
screening unit 76 to decline. Conversely, when the de-aerated froth flow
surges, the
surge tank weir 82 permits the de-aerated froth level in the tank 74 and the
de-
aerated froth flow to the screening unit 76 to increase. The change in flow
through or
over the surge tank weir 82 is stable and may be controlled and regulated in a
predictable fashion, and reduces the likelihood of sudden flow surges
adversely
impacting froth screening and other downstream processing steps.
The de-aerated froth feed surges are suppressed and regulated by the surge
capacity of the surge tank 74 coupled with the surge tank weir 82.
CA 02947081 2016-10-31
The discharge of de-aerated froth 32 into the surge tank 74 may be configured
either
below the normal liquid level of the surge tank 74, or by drop pipe, so as to
prevent
the de-aerated froth 32 plunging directly into a pool of fluid in the upstream
compartment 78. It is desirable to avoid having froth flows plunge into the
pool so as
5 to minimize re-entrainment of gas into the de-aerated froth 32. The surge
tank 74
provides a working retention time of froth flow and acts as a buffer against
periodic
surges of froth flow to the screening unit 76. The surge tank 74 retains a
volume of
de-aerated froth 32 and can accommodate periodic surges in froth flow by
permitting
the froth levels within the surge tank 74 to vary. This allows the surge tank
74 to
10 accommodate additional volumes while still regulating the amount of de-
aerated
froth 32 transmitted to the screening unit 76.
The term "retention time" as used in the context of the surge tank signifies a
response
time to surges that may emanate from the froth pump. This retention time can
range
from 1 minute, where froth pumps have vapour locking sensors to cycle the
froth
15 pump for clearing the vapour lock, to up to 10 minutes for manual vapour
lock control
systems. An optional range of retention time is between about 2 and about 5
minutes,
and further optionally about 3 minutes.
It is also noted that the upstream compartment 78 functions as a froth feed
surge box
and the downstream compartment 80 functions as a froth screen feed box.
20 Referring still to Fig 2, after exiting the surge tank 74, the de-
aerated and flow-
regulated froth is then fed to the screening unit 76. The screening unit 76
may have a
variety of constructions and configurations. In some implementations, the
screening
unit is a moving screen, for example a moveable linear or trommel screen which
may
have adjustable screen speeds or adjustable apertures or a combination
thereof.
25 The de-aerated and flow-regulated froth is fed from the froth screen
feed box 80 to
the screening unit 76 in order to remove debris contained in the froth. The
screening
unit 76 may include a screen 86 which may be a linear screen which is
connected to
a conveyor system 88 for moving the screen 86 with respect to the discharge of
the
froth screen feed box 80. The debris in the froth is removed by passing the
froth
through the screen 86 and retaining the debris on the surface of the screen
86. The
CA 02947081 2016-10-31
26
area of the screen 86, i.e. essentially the total surface area through which a
designated froth flow rate will be maintained during screening, can vary
depending in
part on the size of the screen apertures or pores and the flow rate and nature
of froth
to the treatment. It is understood that for smaller pores, i.e. around 2 mm, a
greater
screen area is required to screen the same volume of froth as with pores of
about
8 mm. Therefore, by adjusting the pore size, the system operator can determine
the
screen area available to screen a given volume of bitumen froth.
In some implementations, an example of which is shown in Figure 6, the size of
the
pores 75 in the screen 86 can vary from the center of the screen 86 to its
extremities.
In one example of such a configuration, the pores 75a of the screen 86 in its
middle
are smaller than the pores 75b,75c near the screen's 86 sides. This
configuration and
pore size distribution reduce the open area at the center of the screen 86,
thereby
forcing the froth 32 outward toward the pores at the sides and thus enhancing
froth
distribution over the surface of the screen 86.
Referring to Figs 2-4B, the de-aerated and flow-regulated froth passes through
the
screen 86 such that debris remains on the surface of the screen 86 while
screened
froth 90 passes through after being collected by a screened froth launder 92.
Optionally, the screened froth 90 is collected by a sloped under-pan type
launder.
The screened froth launder 92 may be positioned just below the lower surface
of the
screen 86 and may span a given section of the screen 86 under which a
substantial
portion of the screened froth passes. The screen 86 may have a downstream
section
94 which is not covered by the screened froth launder 92. The screened froth
launder
92 collects the screened froth 90 and conveys it to a screened froth pump box
96,
which may further treat the screened froth 90 before conveying it, via a
screened
froth pump 98, to additional downstream treatment such as PFT. The screened
froth
pump box 96 may include a perforated distribution pipe 100 or like equipment
to
avoid re-entrainment of gas into the screened and de-aerated froth 90. The
screened
froth 90 may thus be transferred to the screened froth pump box 96 either
through an
inlet below the froth liquid level, or via a perforated stand pipe 100 to
avoid re-
entrainment of gas into the screened froth 90.
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27
Referring to Figs 2-5, the screening unit 72 may also include a froth screen
spray 102. The froth screen spray 102 sprays wash fluid, which can be warm or
hot,
to wash the screen. Spent wash fluid 103 may be collected by a wash fluid
launder
104 and sent for processing. The spent wash fluid 103 may be transported to a
wash
water pump box 106 for treatment so as to recycle the spent wash fluid 103 for
reuse
in the screen spray 102 or other unit operations, for example in oil sands
extraction,
and/or to recover the bitumen contained therein. The spent wash fluid 103 is
pumped
using wash pump 108 to downstream units.
Each of the Figs illustrates a different froth screen spray configuration. The
screen 86
as illustrated is a linear conveyor type screen which is displaceable in a
linear
rotational manner by a conveyor system 88 including several conveyor rollers.
Depending on the setup, there may be two or four or more conveyor rollers
associated with the conveyor system 88. The screen 86 is translated around by
the
conveyer rollers and at any given point there is at least an upper screen
section and
a lower screen section. There may also be an upstream side screen section and
a
downstream side screen section. The froth screen spray 102 may be arranged to
spray and clean different sections of the screen, which will be further
described
below.
Referring to Fig 2, froth screen spray 102 may be provided to spray the lower
section
of the screen 86. In this configuration, the screen spray 102 is able to
effectively
"back flush" the screen 86 since the direction of spray is opposite to the
direction the
froth passes through the upper section of the screen 86. Backwashing can
provide
several advantages such as dislodging debris that may have become wedged in
the
pores of the screen 86. A screen wash launder 104 may be provided below the
lower
section of the screen 86 to collect the wash water that passes through the
screen 86.
The screen wash launder 104 may span the entire length of the screen setup as
illustrated. The screen spray 102 may be static or displaceable and may be
manually
or automatically operated to spray and cleanse the entire area of the screen
86 as in
passes below. This screen spraying technique is particularly useful for
removing
smaller debris from the screen 86 by back washing, optionally with hot wash
fluid.
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28
This can aid in removing accumulated smaller debris and also in eliminating
the build-
up of bitumen on the screen 86 itself.
Referring to Fig 3, the froth screen spray 102 may be provided with its outlet
in
between the upper and lower sections of the screen 86 and it may be operated
to
spray onto the lower and/or side sections of the screen 86.
Referring to Fig 4A, the froth screen spray 102 may be split into multiple
spray
streams for spraying onto different parts of the screen 86. A first stream may
be a
debris wash 110 that is provided above the downstream section 94 of the screen
86
where the debris is exposed prior to collection of the debris. A second stream
may be
a screen wash 112 similar or identical to the example shown in Fig 2. The wash
launder 104 in this scenario can be arranged to collect the spent wash fluid
from both
of the spray streams which enable debris washing and screen washing.
Collection of
the combined wash fluid enables further recuperation of bitumen that coated
the
debris and the screen. The combined wash stream (identified by character 103
in Fig
4A) may be advantageously processed for residual bitumen recovery or recycled
to
unit operations in oil sands extraction to help recuperate the bitumen in the
wash
stream while making use of its heat in extraction units. The length of the
screen 86
may be provided or increased compared to other implementations to allow for
the
screen to be sprayed with two or more hot wash fluids.
Referring to Fig 4B, the froth screen spray 102 may be split into multiple
spray
streams for spraying two different screens 86a, 86b. In this illustrated
scenario, a first
stream 102a is used similarly to the configuration of Fig 2, while a second
stream 102b is used for spraying in the second screening unit 76b in a
different
manner. The second stream 102b is sprayed to wash the first stage debris that
is fed
to the second stage screed 86b. The second screen enables additional washing
of
screened debris to advantageously recover more bitumen and also provide
contingency for mitigating the effects of surges that may affect the first
screen. Hot
water or another liquid, which may be aqueous or solvent based, can be sprayed
over the debris to reduce bitumen adherence to the debris, and the run-off
water is
collected as bitumen contaminated water, which may be used for hot water and
CA 02947081 2016-10-31
29
bitumen recovery in extraction. Work crews are thus no longer needed to remove
bitumen from debris because the debris is rinsed with hot water.
Referring to Fig 5, the froth screen spray 102 may be provided for spraying in
a
generally parallel orientation relative to the surface of the screen 86. In
this illustrated
scenario, the froth screen spray 102 sprays downwardly against the side
section of
the screen 86 as it too moves in a downward direction. Fig 5 also shows a
debris
wash line 114 feeding a debris wash distribution box 116 which provides wash
fluid to
the debris.
For revolving trommel type screening units (not illustrated), in which the
screen takes
the form of a rotating perforated drum, the debris outlet may have wash water
assisting debris transport. For a linear screen 86, as illustrated in the
Figs, bitumen
froth passes through a substantially horizontally moving screen 86 which
transports
removed debris to a discharge point prior to returning to the initial froth
feed zone.
The de-aerated froth that is transported by the screen 86 along with the
debris can be
collected by a pan collection system between multiple screens.
Bitumen froth debris treatment and collection
Referring to Figs 2-5, the debris separated from the screen 86 is collected by
a
debris launder 118 and then fed to a froth debris pump box 120. There, the
debris
can be rinsed with debris flush liquid 122 so as to recover bitumen still on
the debris
by producing washed debris and spent flush liquid. The temperature of the
debris
flush liquid 122 is at least 45 C and optionally about 80 C. The debris flush
liquid 122
may be recycled water, and this washing action can be performed continuously
with
the screening of the froth. In the case of a trommel or rotatable drum screen,
the
debris can be conveyed by helical screws in the trommel towards an outlet, and
then
via a launder to the debris pump box 120. When either sensors or closed-
circuit-
television (CCTV) identifies the debris pump box 120 as full, a froth debris
pump 125
hydraulically transfers the debris to an extraction tailings disposal.
Alternatively, the
debris can be collected in trash box for disposal, and the spent washing
liquid can be
recycled. This recycling back into a bitumen extraction process permits the
reuse of
CA 02947081 2016-10-31
energy and the recovery of bitumen. The minimal amount of spillage also
promotes a
cleaner oil sands plant.
Still referring to Figs 2-5, the debris pump box 120 may allow transport of
the washed
debris for further treatment, or may direct the washed debris directly to
disposal in
5 tailings ponds. Optionally, the launders 92, 118 handling screened froth
90 and/or
debris are configured to minimize re-entrainment of gases into the screened
froth 90.
Referring to Figs 2-5, the debris pump box 120 may also have a return line 123
which returns at least a portion of the debris stream back into the debris
pump
box 120.
10 Enhanced screening operational aspects
The screening implementations described and illustrated herein may also
involve
enhanced operational aspects. In one optional implementation, the screening
involves specifying or designating a portion of the screen surface likely to
be plugged
by debris during operation of the screen. During the design process it is
possible to
15 determine that amount of the screen that is likely to become plugged
during
operation. This amount of plugging can be accommodated by adding extra area to
the screen. In anticipation of these blockages and so as to accommodate them,
the
designated screen area can be increased by about 25% to about 100% to reflect
the
variable flow of debris for a given screen speed (linear or rotational).
20 Referring to Figs 2-4B, in another optional implementation the screening
involves
varying the screen speed to maintain a desired or determined froth
level/volume on
the screen at a reference location. A sensor 124, such as a non-contact sensor
like
an ultrasonic level detector, may be provided to measure the level/volume of
de-
aerated froth dispensed onto the screen 86. The sensor 124 may be configured
to
25 signal the screening unit to vary the screen speed, for example by
signalling a motor
to vary the screen speed, in response to changes in the level/volume.
Similarly,
CCTV may be used to aid operators monitoring the screen 86 to override the
sensor 124, if necessary, or to adjust screen speeds themselves. The froth
level may
be indicative of the amount or type of debris plugging the screen 86. Thus, it
can be
30 appreciated that varying the screen speed permits adjusting the screen
area so as to
CA 02947081 2016-10-31
31
optimise de-aerated froth flow therethrough, even with small screen apertures,
thus
minimizing upsets discharging from the screening unit 76 and affecting
downstream
processes.
For both trommel and linear screens, the depth of the de-aerated froth on the
screen
surface is reduced as the de-aerated froth flows through the screen until
effectively all
the de-aerated froth passes through the screen further down its length, or as
the de-
aerated froth is distributed to the sides and ends of the screen, as explained
above.
This length depends on the screen speed, and the time required for the de-
aerated
froth to pass through the screen. In some implementations, allowance is made
to
permit residual bitumen to drip from the screen and into the screened froth
launder.
Enhanced coupling of pre-treatment heating and screening to PFT operations
The process and system of pre-treating bitumen froth can be coupled to a PFT
operation. In a typical PFT operation, bitumen froth which has undergone a de-
aeration treatment is combined with paraffinic solvent and fed into a
separation
apparatus which may include multiple counter-current gravity settlers. One
reason
froth treatment is performed is to remove or substantially reduce the aqueous
and
solids components that are still present in the bitumen froth. One
implementation of
PFT involves adding paraffinic solvent to the bitumen froth in an amount
sufficient to
cause precipitation of asphaltenes present in the froth and this precipitation
forms
aggregates or "flocs" comprising water, solids and asphaltenes. These water-
solid-
asphaltene aggregates settle to the lower fractions in the counter-current
gravity
settlers and become part of the tailings stream, whereas the bitumen rich
upper
fraction is withdrawn as overflow.
In various implementations, the pre-treatment system and process are coupled
to
PFT so as to enhance the efficiency of the operation. Prior to pre-treatment,
the
bitumen froth includes bitumen, fines, water, air and debris. As noted above,
the
paraffinic froth treatment operation includes adding a paraffinic solvent to
the bitumen
froth to precipitate asphaltenes in the form of water-solid-asphaltene
aggregates, and
then separating the bitumen froth into an upper hydrocarbon rich fraction and
a lower
tailings fraction containing the water-solid-asphaltene aggregates. In the PFT
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separation step, there is a water-hydrocarbon interface that forms between the
upper
fraction and the lower fraction during separation. The pre-treatment system
and
process may include promoting the removal of debris having a non-aggregatable
size¨that is, the larger debris having a size too large for entrapment within
the water-
solid-asphaltene aggregates to be formed in the PFT operation¨so as to reduce
accumulation of debris at the water-hydrocarbon interface. The removal of this
debris
can be accomplished using screening units that are configured and operated for
promoting non-aggregatable debris removal.
In some implementations, removal of non-aggregatable debris is performed by a
screening unit having pores with a maximum size that promotes substantial
removal
of the non-aggregatable debris for a given PFT operation. The size of the
aggregatable solids present in the bitumen froth may be determined by various
techniques including empirical testing, measurements, estimations and
modelling
techniques. For example, in some PFT operations, aggregatable soilds have a
size
between about 1 pm and about 10 pm, and thus the screen is configured, sized
and
operated to promote removal of larger solid debris. It should be noted that
the
maximum pore size of the screen may have a size that is greater than the
maximum
size of the aggregatable solids. In some scenarios, the screening unit can
have pores
sized to be about 1 to 4 orders of magnitude larger than the aggregatable
solids, or 2
to 3 orders of magnitude larger than the aggregatable solids. For example, the
screening unit can have pores of about 10 mm, which amounts to about 3 to 4
orders
of magnitude larger than the aggregatable solids of 1 pm to 10 pm. PFT
operations
may vary depending on the operating temperatures and pressures in the settler
vessels, the type of paraffinic solvent used (for example pentane, hexane,
heptane,
octane, mixtures thereof and/or isomers thereof), the solvent-to-bitumen ratio
(S/B)
that is employed in different stages of the PFT process, as well as process
configuration and other operational parameters. In another implementation, the
bitumen froth is sufficiently pre-heated prior to screening so as to reduce
the viscosity
of the bitumen component in the froth to enable the froth to pass through the
screen
pores while the solid debris within the froth, which does not undergo
viscosity
reduction by heating, is screened out. The froth may be heated to a relatively
high
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33
temperature, for instance between about 70 C and about 95 C. In this regard,
elevated froth temperatures have additional advantages in PFT operations. For
instance, froth temperature entering the PFT has a number of effects on froth-
paraffin
mixing, separation stability and separation performance. Bitumen froth is a
viscous
material having a high viscosity and heating the froth prior to screening can
reduce its
viscosity and facilitate its processing.
In some optional implementations, the bitumen froth undergoes direct steam
injection
(DSI) pre-heating before being subjected to a screening step. The pre-heating
reduces the viscosity of the bitumen froth and facilitates passage of the
froth through
the screen and removal of the debris. The heated screened froth is then fed to
PFT
operation which is enhanced both by the pre-heating and the pre-screening of
the
input froth.
More regarding certain bitumen froth heating strategies for PFT may be found
in
Canadian patent application No. 2,740,935 many of which may be combined with
the
implementations and implementations described herein.
In one implementation, the temperature of the bitumen froth is provided as
high as
possible without exceeding the flash temperature of the solvent and the flash
temperature of water which is in the froth. The froth may be pre-heated to a
temperature of at least 65 C. The froth may be pre-heated to a temperature
between
about 70 C and about 95 C. The froth may be pre-heated to a temperature of
about
90 C.
Referring to Fig 3, in some implementations of the pre-treatment system and
process,
the DSI pre-heating unit 38 is provided to maintain a consistent de-aerated
froth feed
temperature to the downstream debris separator 72. The de-aerated froth 32 is
transported via the froth pump 28 through the DSI pre-heater 38 and is heated
with
the steam 42 to produce a de-aerated pre-heated bitumen froth stream 126,
which
can be provided to the surge tank 74 and then the screening unit 76. The DSI
pre-
heater 38 may heat the froth such that the de-aerated pre-heated bitumen froth
stream 126 has a temperature of about 65 C or more. The DSI pre-heater 38 may
also be controlled in accordance with a froth heating control device 128 so
achieve a
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consistent froth stream 126 temperature. With a consistent froth temperature,
the
range of froth viscosity variation is minimized, which allows for more
consistent debris
screening and higher temperatures also enable enhanced screening treatments.
Still referring to Fig 3, the froth 32 may be heated using the pre-heater 38
as
described above to a temperature at which the vapour pressure of the water in
the
heated froth 126 increases, thus forming larger bubbles, which, in combination
with
the reduced froth viscosity from the added heat, aids in further de-aeration
of the
froth. The froth may be heated before being processed by the de-aerator 30.
After
heating, the gas content of the froth may be reduced and maintained at a
consistent
level. This reduction in gas content may be achieved with a de-aerator vessel
30 as
described above.
Finally, it should be noted that features of various implementations may be
combined
with implementations and features of other implementations described herein.
