Note: Descriptions are shown in the official language in which they were submitted.
1
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. 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
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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-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.
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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.
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.
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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
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
a screen unit in fluid communication with the downstream compartment of
the feed unit for receiving the bitumen froth therefrom to remove debris
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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.
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;
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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.
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.
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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.
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
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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
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.
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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:
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
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;
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.
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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
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.
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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.
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.
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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.
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.
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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.
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.
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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.
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
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 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.
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.
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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.
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 (PET) operation.
Processes and systems are described for pre-treating the bitumen froth in
preparation for the PET 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 permits 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
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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 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' 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
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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.
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.
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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 (DSI), sufficient to heat the froth 22 above 75 C. The DS! 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 PET. 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
CA 2971324 2017-06-20
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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-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 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 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
includes the cross-flow de-aeration apparatus 48, having a construction and
configuration offering advantageous functionality for preparing bitumen froth.
In
one optional implementation, the cross-flow de-aeration apparatus 48 can act
as
20 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 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 30 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 chamber 58. The spacing of the
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plates 62 in the pack 58 may be provided so as 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 DS! 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 maximum debris particle size that can pass through the oil sand
ore
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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.
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
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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 32is 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 include 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 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
CA 2971324 2017-06-20
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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.
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 to minimize re-entrainment of gas into the de-aerated froth 32.
The
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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 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 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.
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.
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 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 than with pores of about 8 mm. Therefore, by adjusting the
pore
CA 2971324 2017-06-20
25
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 reduces 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 2-46, 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 PET. 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.
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
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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. 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
CA 2971324 2017-06-20
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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 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.
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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 tronnmel or
rotatable
drum screen, the debris can be conveyed by helical screws in the trammel
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 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 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.
Enhanced screening operational aspects
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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 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).
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 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 appreciated that varying the screen
speed
permits adjusting the screen area so as to 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
CA 2971324 2017-06-20
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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 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
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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 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 (DS!) 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.
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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 DS! 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 DS! 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 DS! 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 DS! pre-heater 38 may also be controlled in accordance with a froth
heating control device 128 so achieve a 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.
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