Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND SYSTEM OF RECOVERING ENERGY FROM A FLOW OF OIL
SANDS SLURRY
FIELD OF THE INVENTION
The invention generally relates to the field of transporting oil sand
slurries, and
more particularly to a system and method of transporting an oil sand slurry.
BACKGROUND
In traditional mining, slurry pipeline systems are built with significant
reservoir
capacity to maintain a steady flow rate. Oil sands slurry pipeline systems
have
limited reservoir capacity and as a result may have a highly variable flow
rate.
The flow rate may be increased or decreased for a variety of reasons depending
on, for instance, changing upstream availability of oil sands slurry as well
as
other process operating constraints.
Enabling efficient flow of a slurry, such as oil sands slurry, through a
pipeline also
requires some operating conditions that are not normally required for other
liquids. For instance, it is desirable to maintain an adequate flow rate in
the
pipeline when operating at low flow rates to avoid "sanding off', which is
when
some of the oil sands normally suspended in the solvent come out of suspension
thereby hindering the flow and increasing wear on pipeline equipment.
Variable flow rate slurry flowing downhill in undulating terrain, such as in
an oil
sands mine, thus requires a smaller pipeline diameter in order to maintain
adequate line pressure during times of reduced flow rate. Unfortunately, using
a
smaller pipeline diameter results in excessive pipeline wear and system energy
loss at normal or high flow rates. Current industry practice is to accept the
energy loss and install sacrificial wear components such as reduced line size
sections, orifice plates or valves.
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Oil sands mining and transportation are also fraught with harsh conditions and
oils sands slurry can be more damaging on pipeline equipment than other fluids
and suspensions traditionally transported by pipeline.
There is currently a need for a technology that overcomes at least one of the
disadvantages of what is currently known and used in the field.
SUMMARY OF THE INVENTION
The present invention responds to the above-mentioned need by providing a
slurry transportation system and method for transporting slurry.
More particularly, the present invention provides a slurry transportation
system
for transporting an oil sand slurry, comprising:
a pump having an inlet for receiving the slurry and a discharge for
discharging the slurry;
an upstream line in fluid communication with the inlet and a downstream
line in fluid communication with the discharge;
a shaft connectable to the pump;
a driving mechanism connectable to the shaft to drive the same to operate
the pump;
a regulator connectable to the driving mechanism for regulating the torque
applied to the shaft, to allow the driving mechanism to drive the shaft in a
positive torque mode to cause the pump to discharge the slurry at a higher
flow rate, or in a negative torque mode to cause the pump to discharge the
slurry at a lower flow rate.
The present invention also provides a slurry transportation method for
transporting an oil sand slurry, comprising:
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pumping the slurry through a pipeline using a pump driven by a motorised
shaft; and
adjusting the flow rate of the slurry by varying the torque applied to the
motorised shaft between a positive torque mode enabling the pump to
discharge the slurry at a higher flow rate, and a negative torque mode
enabling the pump to discharge the slurry at a lower flow rate.
The oil sand slurry transportation system and method enable positive head
(regular pump action) for normal and high flow rates and negative head (pump
brake action) for low flow rates. This pump brake action can reduce system
energy loss and pipeline wear, for instance due to eliminating the requirement
for
a reduced size section and allowing larger overall line size, while enabling
efficient reduction of vapour breakout and sanding off.
In an optional implementation, the regulator further comprises a control unit
coupled to the variable frequency device to automatically control whether the
shaft is in the positive torque mode or the negative torque mode.
The positive-negative torque regulation allows efficient adaptation to
variable oil
sands processing and transportation conditions. By allowing a negative torque
to
be applied to the shaft, the flow rate can be reduced in a simple and
efficient
manner.
In some implementations, there is provided a method of recovering energy from
a
flow of an oil sand slurry comprising water, bitumen and suspended solids
including sand, the method comprising:
producing the flow of the oil sand slurry from an oil sand mining operation;
applying a negative head to the flow of the oil sand slurry; and
recovering a braking energy from an application of the negative head.
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In some implementations, there is provided a system for recovering energy from
a flow of an oil sand slurry comprising water, bitumen and suspended solids
including sand, the system comprising:
an upstream line in fluid communication with an oil sand mining operation
and a downstream line, the upstream and downstream lines being
configured for conveying the oil sand slurry from the oil sand mining
operation;
a negative head assembly mountable to the upstream and downstream
lines to apply a negative head to the flow of the oil sand slurry; and
an energy receptor electrically connectable to the negative head assembly
for recovering a braking energy from the negative head assembly upon an
application of the negative head.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig 1 is a block diagram schematic of one embodiment of the present invention.
Fig 2 is a block diagram schematic of another embodiment of the present
invention.
Fig 3 is a block diagram schematic of yet another embodiment of the present
invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Preferred embodiments of the oil sand slurry transportation system 10 are
illustrated in Figs 1-3.
The oil sand slurry transportation system 10 is preferably integrated into a
pipeline system for transporting slurry such as oil sands slurry to downstream
reservoirs or processing units. The slurry may include a variety of oil sands
slurries, such as at-face mined oil sand slurry, primary or secondary
middlings or
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tailings slurries, fine tailings or mature fine tailings slurry. Depending on
their
origin and stage of processing, the oil sand slurries contain various
quantities of
sand, bitumen, clay, chemical processing additives and other compounds
inherent to the mined oil sand ore.
5 The slurry transportation system 10 includes a pump 12, an upstream line
14 and
a downstream line 16, a shaft 18 connected to the pump 12, a driving mechanism
20, and a regulator 22.
It should be understood that the upstream and downstream "lines" are
preferably
pipelines but may also be the inlet or outlet of processing equipment such as
tanks, reaction vessels, and the like.
Referring to Fig 1, the pump 12 may be a centrifugal pump having an impeller
(not illustrated) connected to a shaft that is driven by the driving mechanism
20
which in this case consists of a motor 24. The driving mechanism 20 is in turn
coupled to the regulator 22 which regulates the torque applied to the shaft
18, to
allow the driving mechanism 20 to drive the shaft 18 in a positive torque mode
to
cause the pump 12 to discharge the slurry at a higher flow rate, or in a
negative
torque mode to cause the pump 12 to discharge the slurry at a lower flow rate.
It should be understood that "negative torque mode" includes the point at
which
zero torque is applied. At this point, there will only be a pressure drop
across the
pump that amounts to the friction losses within the pump. It should also be
understood that "higher flow rate" and "lower flow rate" are meant relative to
each
other.
The pump 12 has an inlet 26 for receiving the slurry from the upstream line 14
and a discharge 28 for expelling the slurry through the downstream line 16. It
should be noted that the centrifugal pumps 12 used in this system 10 can be
further adapted to improve efficiency of providing negative head, for example
with
tailored impeller design.
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Referring to Fig 1, the driving mechanism 20 is a single motor 24 and the
regulator 22 includes a variable frequency device 32 (hereafter referred to as
a
VFD). The VFD 32 is programmed to operate the motor 24 in such a way that a
positive torque is applied to the shaft 18 during times of high process flow
rates
and a negative torque is applied to the shaft 18 during times of low process
flow
rates. The VFD 32 may be powered by the power grid 34.
Still referring to Fig 1, the regulator 22 may also include a control unit 36
that is
coupled to the VFD 32. The control unit 36 monitors various operating
conditions,
makes calculations to determine the direction and magnitude of torque to be
applied, and provides signals to the VFD 32 to change or otherwise regulate
the
torque applied to the shaft 18 to obtain given flow rates and slurry
pressures.
The control unit 36 can calculate the torque to be applied based on a number
of
variables. For instance, the pressure at the pump inlet, the pressure change
in
the pump, the slurry composition, the friction loss in the pipeline system,
and
other upstream and downstream constraints, may be used to calculate the
applied torque to obtain a given slurry flow rate. There are a number of
empirical
equations and calculation methods to determine the flow rate and corresponding
torque to apply.
In operation, the slurry transportation system 10 can allow high, normal or
low
flow rates, which will be described below.
Normal and high flow rates - positive head
For a normal or high flow rate, a positive torque is applied to the shaft 18
so that
there is a head gain between the pump inlet and the pump discharge. At these
flow rates, it can be said that there is regular pump action.
Flow rate transition
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In response to a requirement for a decreased slurry flow rate, the positive
torque
applied to the shaft is reduced. At zero applied torque, there will only be a
pressure drop across the pump that amounts to the friction losses within the
pump.
Low flow rates - negative head
When the flow rate is to be further decreased, for instance in response to a
requirement for greater back pressure, a negative torque is applied to the
shaft to
resist the flow of fluid through the pump 12. At such low flow rates the
control
unit 36 determines the optimal negative torque to be applied to the shaft 18,
and
communicates this to the motor 24 via the VFD 32.
The negative torque may be set in order to allow the slurry to flow at
sufficient
rate and pressure so as to maintain the oil sands solids in flowable
suspension
and thus reduce or avoid "sanding off'. It is noted that a pressure above the
vapour pressure of the solvent does not impact sanding off. It is also noted
in this
regard that the turbulence of the slurry flow will be a function of the rate
and line
size.
The negative torque may also be set and transitioned to in order to minimize
the
likelihood of vapour breakout, which would occur at higher elevations relative
to
the pump where the pressure is low and may be hundreds of meters from the
pump. Applying negative torque reduces or eliminates vapour breakout, since it
increases the pressure drop across the pump thus increasing the pressure
further upstream to increase the line pressure above the vapour pressure of
the
slurry solvent. Furthermore, the negative torque mode capability of the system
allows controlled and continuous flow rate adjustment for transient conditions
experienced in oil sands mining and pipeline transport.
The slurry transportation system 10 is particularly applicable in downhill
undulating terrain such as in oil sands mining and slurry transport, as
illustrated
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in Fig 3, since the system pressure in such cases is sufficient to allow the
slurry
to flow via gravity to its destination without a pump at a flow rate that may
be
called the no-pump flow rate. Thus, when an even lower flow rate is desired,
the
slurry transportation system 10 causes a braking action in the pump 12 in
negative torque mode. It should be understood that the inlet pressure is
sufficiently greater than the discharge pressure, as a result of the braking
action
of the pump 12 in low flow rate conditions.
In the preferred embodiment of the slurry transportation system 10 illustrated
in
Fig 1, there is a single variable speed and direction motor 24 coupled to the
pump 12 and regulated by the VFD 32 for applying positive or negative torque
to
its shaft 18.
In another optional embodiment of the system 10 as illustrated in Fig 2, there
can
be a motor 24 and a generator 38 coupled to a single pump 12. The coupling of
the motor 24 and the generator 38 may be on different sections of the shaft
18.
The motor 24 can apply a positive torque to the pump 12 and the generator 38
can apply a negative torque. The motor/generator embodiment can be controlled
by one or more regulator 22, which may include a VFD, to enable a positive or
negative torque mode.
Referring to Fig 1, in one optional aspect of the slurry transportation system
10,
when operating in negative torque mode, the system 10 dissipates the braking
energy by either sending it back as reject energy to the power grid 34.
Referring
to Fig 3, the braking energy may be delivered to an energy receptor 40
electrical
load resistor. A variety of regenerative braking techniques may be employed to
recover the braking energy as electricity or as heat for reuse in the system
or the
mining operation at large. Thus, in negative torque mode, the inlet slurry
pressure
is divided into braking energy and the discharge fluid pressure. The discharge
fluid pressure should of course be sufficient to allow the oil sands slurry to
flow
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properly downstream. The control unit 36 may monitor and control the torque
magnitude and direction.
The slurry transportation method of the present invention for transporting
slurry,
includes pumping the slurry through a pipeline using a pump driven by a
motorised shaft; and adjusting the flow rate of the slurry by varying the
torque
applied to the motorised shaft between a positive torque mode enabling the
pump to discharge the slurry at a higher flow rate, and a negative torque mode
enabling the pump to discharge the slurry at a lower flow rate. The
illustrated
embodiments of the system 10 may be used to perform this method.
The embodiments of the slurry transportation system and method enable a
number of advantages. For instance, traditional methods of increasing back
pressure with valves and orifice plates that suffer from excessive wear can be
reduced or avoided. In addition, by using the system of the present invention,
pipeline wear and system energy loss can be reduced during times of normal and
high process flow rates. At normal and high flow rates the pump operates in
the
standard way, the reduction of energy loss and component Wear results from
using larger pipes. In other words, the system exerts continuous and adaptive
control over the flow rate of the slurry so that low flow rates can be
achieved in
larger pipes while respecting the pressure requirements for maintaining the
desirable flow properties of the oil sands slurry. Thus, larger pipeline
diameters
can be used to increase the maximum flow rate, reduce pipeline wear and reduce
system energy loss.
In addition, existing pipeline systems may be retrofitted with the slurry
transportation system 10 of the present invention. In such a case, the slurry
transportation system 10 allows increased adaptability in achieving low flow
rates
by avoiding equipment such as throttling valves and orifice plates. The
continuous control of the flow rate optimizes energy use and minimizes
pipeline
wear in the transient conditions of oil sands mining.
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In one preferred aspect of the slurry transportation system, the pump is
located
in-line in a pipeline and the pipeline does not require any additional units
for
increasing or decreasing the slurry pressure. For instance, the slurry
5 transportation system enables avoiding the necessity of open-ended
cylinders
and the like integrated in the pipeline. The pipeline with in-line pump thus
may be
a closed-system. Alternatively, there may be additional units in combination
with
some embodiments of the present invention to further increase or reduce the
line
pressure, depending on elevation, flow rate requirements (for instance,
outside of
10 preferred flow rate ranges), pump design and other variables.
Furthermore, there
may be one or more additional pumps, each with its corresponding driving
mechanism, shaft, and regulator, arranged in series with the first. One
regulator
could also control the magnitude and direction of both pumps. This in-series
arrangement may be used in situations of high elevation change and of very
high
or low flow rates. It should also be noted that the system may include various
in-
series or in-parallel pump combinations tailored to a given pipeline
topography.
It should be understood that numerous modifications could be made to the
embodiments of the present invention described hereinabove, without departing
from what has actually been invented. For instance, different configurations
of the
system may employ one or more pumps, motors, shaft sections connected to
parts of the driving mechanism, VFDs and control units, in various
configurations,
given the constraints of the oil sands mine and desired operating conditions.
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