Note: Descriptions are shown in the official language in which they were submitted.
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TNRU AND PROCESS WITH SCRUBBING OF ENTRAINED TAILINGS
PARTICLES FROM AN OVERHEAD NAPHTHENIC SOLVENT STREAM
FIELD OF THE INVENTION
The present invention relates to the treatment of oil sands bitumen froth and
more
precisely to a process for recovering naphthenic solvent from naphtha diluted
tailings.
BACKGROUND OF THE INVENTION
Oil sands extraction processes primarily use hot water mixed with oil sands
ore to
produce a slurry from which is removed a froth fraction containing bitumen.
The
bitumen froth, which contains bitumen, water and fine mineral solids, is
further
processed by adding a diluent solvent to facilitate separation of the bitumen
from
the other components.
In froth treatment operations, the bitumen froth is mixed with diluent and the
diluted
froth is supplied to separation vessels to separate an overflow diluted
bitumen
stream from an underflow solvent diluted tailings stream.
Froth treatment operations thus produce by-products including solvent diluted
tailings. The cost and environmental impact preclude directly discharging
solvent
diluted tailings to tailings ponds. The diluted tailings are thus treated in a
tailings
solvent recovery unit.
Various tailings solvent recovery units have been proposed and each has its
own
set of drawbacks and challenges. Many possible recovery schemes are disclosed
in the literature. In one process, froth treatment tailings from the froth
treatment
plant are introduced into a flash vessel with internal shed decks maintained
at sub-
atmospheric pressures. Steam is introduced below the internals and the major
portion of the diluent vaporizes together with water. The flashed vapours are
removed and cooled to condense diluent and water which separate by gravity
settling. Non-condensed vent gases are withdrawn from the condenser to
maintain
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the sub-atmospheric pressure. The flashed solvent depleted tailings are pumped
from the flash vessel to tailings disposal.
Some challenges encountered by known tailings solvent recovery processes
result
in lower solvent recovery levels than would be desirable. For some processes,
the
lower recovery is attributable to premature flashing at the feed inlet
inducing feed
to bypass the shed decks and negating any addition of steam below the shed
decks. Other processes which operate the flash vessel at near atmospheric
pressures which may permit feed distribution over the shed decks and may
increase the steam addition to maintain vessel temperature to about 100oC can
increase naphtha diluent recovery.
Another diluent recovery process investigation flashes feed to a flash
temperature
such that the enthalpy of vaporized flash components matches enthalpy released
from the flash liquid and the flash temperature governs vapour pressures of
vaporizing components. Given the relative volatility of diluent hydrocarbons,
there
may be an expected direct relationship between feed temperature, flash
temperature and diluent recovery. However, the investigation identified
increased
feed temperatures for the same feed flow did not proportionately translate to
increased diluent recovery due to increased vaporization of water. Stable
operation
for the flash column in terms of flash temperature and pressure was found
marginally below the boiling point of water for the operating pressure and
with
small increases in feed enthalpy resulting in upsets as the water essentially
boils.
Process upsets affect the flash column in at least two ways. Firstly, boiling
on shed
decks results in damage to the extent that frequently the shed decks fail
structurally. Secondly, the vapour velocity in the column increases by an
order of
magnitude exceeding design guidelines, such a set out in "Design Two-Phase
Separators within the Right Limits" W. Svrcek, et al. Chemical Engineering
Progress Oct 1993, to limit entraining solids and bitumen into the overhead
system.
In the overhead of the tailings solvent flash column, bitumen acts a binder
for the
solids to adhere on surfaces in the overhead system. The adherence of solids
to
components of the overhead system restricts vapour flow to the downstream
equipments unit operations such as condensers and separators. The adherence of
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solids on condenser heat transfer surfaces reduce cooling and condensing of
vapours which increases the non-condensed gases to be vented. Directionally,
both effects of solids adhering on surfaces in the overhead system increase
column pressure which reduces feed flashing resulting in actual diluent
recoveries.
The contribution of increased steam to improve diluent recoveries due the
reduced
partial pressure created by the superheated steam can often be largely offset
by
the increased water vapour reporting an overhead system restricted by the
adherence of solids. Over the operating cycle, the deposit of solids causes
column
performance to deteriorate which can only be regained by shutting down the
column and associated systems for repair and cleaning.
Known processes have particular challenges and limitations for achieving high
diluent recoveries while avoiding fouling problems in the overhead systems of
the
tailings flash column. Conventional approaches has either been to either size
the
flash column to minimize the overhead velocity such that particles below a
specific
acceptable diameter settle out or provide demisting pads to protect downstream
equipment. The operating conditions for these vessels are prone to upsets that
entrain particles overhead in quantities higher than typical overhead systems.
There is thus a need for a technology that overcomes at least some of the
drawbacks of what is known in the field, such as the above-mentioned drawback
that may result from the entrainment of solids and their deposition in
overhead
apparatuses. The invention identifies a system for scrubbing entrained
particles
from a flash vessel, to achieve high naphthenic diluent recovery in a tailings
solvent recovery process.
SUMMARY OF THE INVENTION
The present invention responds to the above need by providing a tailings
naphtha
recovery unit (TNRU) comprising a scrubbing system for entrained particles and
a
related process for recovering the naphthenic solvent.
In one embodiment, there is provided a TNRU for recovering a naphthenic
solvent
from a solvent diluted tailings, the TNRU comprising a stripping vessel for
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separating the solvent diluted tailings into a solvent component and a solvent
recovered tailings component, the solvent component comprising a naphthenic
solvent vapour and entrained tailings particles; a scrubbing vessel for
scrubbing
the solvent component, the scrubbing vessel comprising a scrubbing section for
separating the entrained tailings particles from the naphthenic solvent
vapour,
thereby producing a scrubbed solvent vapour; a solvent inlet for providing the
solvent component into the scrubbing section; a fluid inlet for providing a
flushing
fluid to the scrubbing vessel, the flushing fluid removing the entrained
tailings
particles contained in the solvent component, thereby producing a flush media;
a
liquid outlet for releasing the flush media from the scrubbing section; and a
solvent
outlet for releasing the scrubbed solvent vapour from the scrubbing vessel.
The TNRU may also include a heating device for heating the flushing fluid to
or
near the flash temperature of the solvent component before entering the
scrubbing
vessel.
The scrubbing vessel may comprise a removal system for promoting the removal
of the entrained tailings particles from the solvent component. The removal
system
may comprise means for changing the flow pattern, such as cyclonic means or a
grid.
The TNRU may also have a condenser connected to the solvent outlet for
condensing the scrubbed solvent vapour and producing a condensed naphthenic
solvent.
There may also be an overhead separator for receiving the condensed naphthenic
solvent and producing vent gas, recovered naphthenic solvent and produced
water.
The TNRU may also have a recycle line for recycling at least a portion of the
produced water from the overhead separator to the fluid inlet of the scrubbing
vessel.
The TNRU may also have a recycle line for recycling at least a portion of the
produced water from the overhead separator to the stripping vessel.
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The TNRU may also have a first and a second recycle lines for recycling at
least a
portion of the produced water from the overhead separator respectively to the
fluid
inlet of the scrubbing vessel and the stripping vessel.
The TNRU may also have a flush recycle line for recycling at least a portion
of the
5 flush media to the fluid inlet of the scrubbing vessel.
The TNRU may also have a bypass line connecting the stripping vessel to the
condenser for temporally providing the solvent component directly into the
condenser without passing through the scrubbing vessel during maintenance
operations.
The flushing fluid preferably includes water. The flushing fluid may contain
chemical aids to promote the removal of the entrained tailings particles.
The fluid inlet may be located above the solvent inlet.
The flash temperature of the solvent component may range from about 25 C to
about 100 C.
The pressure in the scrubbing vessel may range from about 25 kPaa to about 110
kPaa.
In one embodiment, the scrubbing vessel is a first scrubbing vessel and the
TNRU
further comprises a second scrubbing vessel arranged in series with the first
scrubbing vessel.
The invention also provides a tailings naphtha recovery process for recovering
a
naphthenic solvent from a solvent diluted tailings. The process has the
following
steps:
stripping the solvent diluted tailings, with a stripping fluid within a
stripping
vessel, into a solvent component and a solvent recovered tailings
component, the solvent component comprising a naphthenic solvent vapour
and entrained tailings particles;
introducing the solvent component within a scrubbing vessel;
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introducing a flushing fluid within the scrubbing vessel to contact the
solvent component and remove the entrained tailings particles contained in
the solvent component, thereby producing a scrubbed solvent vapour and a
flush media comprising flushed tailings particles;
withdrawing the scrubbed solvent vapour from an upper section of the
scrubbing vessel; and
withdrawing the flush media from a bottom section of the scrubbing vessel.
The process may include the step of promoting the removal of the entrained
tailings particles from the solvent component with a separator located within
the
scrubbing vessel.
The step of promoting of the removal of the entrained tailings particles may
be
performed by a grid. It may be performed by directional change means. It may
be
performed by cyclonic means.
The process may also include the step of condensing with a condenser the
scrubbed solvent vapour to produce a condensed naphthenic solvent.
The process may also include the step of separating with an overhead separator
the condensed naphthenic solvent into vent gas, recovered naphthenic solvent
and
produced water.
The process may include the step of recycling at least a portion of the
produced
water from the overhead separator to the scrubbing vessel.
The process may include the step of recycling at least a portion of the
produced
water from the overhead separator to the stripping vessel.
The process may include the step of recycling at least a portion of the
produced
water to the scrubbing vessel and the stripping vessel.
The process may include the step of recycling at least a portion of the flush
media
back to the scrubbing vessel.
The process may include the step of bypassing the scrubbing vessel for
maintenance operation of the scrubbing vessel.
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The process may include the step of bypassing the solvent component from the
stripping vessel directly to the condenser for maintenance operation of the
scrubbing vessel.
The flushing fluid may include water.
The process may include the step of promoting the removal of the entrained
tailings particles with chemical aids added to the flushing fluid.
The flush media may contain a weight percentage of flushed tailings particles
ranging from about 0.1 wt% to about 2 wt%.
The process may include heating the flushing fluid to or near the flash
temperature
of the solvent component.
The process may include the step of heating the flushing fluid to a
temperature
ranging from about 25 C to about 100 C.
The process may include the step of pressurizing the scrubbing vessel under a
pressure ranging from about 25 kPaa to about 110 kPaa.
The scrubbing vessel may be a first scrubbing vessel and the process also
comprises scrubbing the solvent component within a second scrubbing vessel in
series operation.
The flush media may recycle back into the flash vessel below an upper liquid
level
of accumulated solvent recovered tailings to create a liquid seal therewith.
Creating
the liquid seal can allow avoiding valves for controlling recycling the flush
media
back into the flash vessel.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the distribution system and the related tailings solvent
recovery
process according to the present invention are represented in the following
figures.
Figure 1 is a block flow plan illustrating a stripping apparatus and a
scrubbing
apparatus according to an embodiment of the present invention.
Figure 2 is a block flow plan illustrating an overall tailings naphtha
recovery unit
(TNRU) according to an embodiment of the present invention.
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While the invention will be described in conjunction with example embodiments,
it
will be understood that it is not intended to limit the scope of the invention
to these
embodiments. On the contrary, it is intended to cover all alternatives,
modifications
and equivalents as may be included as defined by the appended claims.
DETAILED DESCRIPTION
The present invention provides a tailings naphtha recovery unit (TNRU) and a
related process for recovering naphtha from a solvent diluted tailings and
scrubbing entrained particles from a flash or stripping vessel treating the
solvent
diluted tailings.
Referring to Figure 1, the TNRU preferably comprises a stripping apparatus (2)
for
receiving the solvent diluted tailings (4) and separate it into two streams: a
solvent
component (6) and a solvent recovered tailings component (8). The stripping
apparatus (2) comprises a stripping vessel (10) with a stripping section (12)
and a
bottom section (14). The solvent diluted tailings are fed to a tailings inlet
(16)
located in the stripping section (12) where the stripping occurs by action of
a
stripping fluid (18) fed to the stripping vessel through a stripping fluid
inlet (20)
located above the bottom section (14). The stripping fluid (18) may comprise
steam. The produced solvent component (6) is released from the stripping
vessel
through a solvent outlet (22) located at the top of the stripping vessel. The
solvent
component (6) comprises a vaporized naphthenic solvent component and
entrained tailings particles. The stripping fluid (18) entrains the vaporized
naphthenic solvent component (6) and, due to the vapour velocity, tailings
particles
are also entrained out of the stripping vessel. The produced solvent recovered
tailings component (8) is released from the vessel (10) through a tailings
outlet (26)
located in the bottom section (14).
Still referring to Figure 1, the TNRU also comprise a scrubbing vessel (28)
which is
fed by the solvent component (6) containing the entrained tailings particles.
The
scrubbing vessel (28) is used to separate the entrained particles from the
solvent
component (6). The solvent component (6) enters a scrubbing section (30) of
the
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scrubbing vessel (28) through a solvent inlet (32) and is scrubbed by a
flushing
fluid (34) fed to the scrubbing vessel (28) through a fluid inlet (36). The
flushing
fluid is typically water and is preferably heated to flash temperature then
supplied
to a grid within the vessel. The grid promotes the separation of particles
into the
flush fluid by either directional changes in the flow pattern or cyclonic
means. In
some cases, the flush fluid may also use water from the separator to minimize
input of water subject to quality constraints. The fluid inlet (36) is located
above the
solvent inlet (32). A scrubbed solvent vapour (38) is thereby produced and
released from the vessel through a solvent outlet (40). The tailings particles
are
entrained downwards the vessel (28) by the flushing fluid (34) forming a flush
media (42) which is released from the vessel (28) through a tailings outlet
(44). The
flush media with entrained particles collects in the scrubber and preferably
flows by
gravity into the stripper column or to the columns bottom pump. A grid (46)
located
in the scrubbing vessel (28) acts as a separator for promoting the removal of
the
entrained tailings particles from the solvent component. The separator could
also
be a cyclone or any apparatus changing the flow pattern. The flushing fluid
(34) is
heated by a heat exchanger (48) before entering the vessel (28) in order to be
at or
near the flash temperature of the vaporized solvent component (6) and favour
vapour equilibrium. The flash temperature of the solvent component (6) is
preferably ranging from about 70 C to about 100 C, and the pressure in the
scrubbing vessel is ranging from about 25 kPaa to about 110 kPaa. More
preferably, the flushing fluid (34) comprises water.
In another aspect, Figure 2 illustrates a possible configuration for the TNRU.
A
stripping vessel (10), a scrubbing vessel (28) and an overhead separator (50)
operate in series to recover the naphthenic diluent from the solvent diluted
tailings.
As explained in the above description of Figure 1, the scrubbing vessel (28)
separates the entrained tailings particles from the solvent component, thereby
producing a scrubbed solvent vapour (38). This scrubbed vapour (38) is then
sent
to an overhead separator (50) without risking deposition of solids which would
have
damaged the apparatuses. Before being fed to the overhead separator, the
scrubbed vapour (38) is condensed in a condenser (52) thereby producing a
naphthenic condensed solvent (54). The scrubbed gas passes to the condenser
for
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condensing diluent and water by cooling below the dew point. The condenser
(52)
connects the solvent outlet (40) of the scrubbing vessel (28) to the overhead
separator (50). The naphthenic condensed solvent (54) is fed to the overhead
separator (50) through a condensed solvent inlet (56) where it is separated
into a
5 vent gas (58) released through a gas outlet (60); a recovered naphthenic
solvent
(62) which is pumped out through a naphthenic solvent outlet (64) by a first
pump
(66); and a produced water (68) which is pumped out the overhead separator
(50)
through a water outlet (70) by a second pump (72). The water outlet (70) is
connected to the stripping vessel (10) to recycle the produced water back to
the
10 vessel (10) through a recycle inlet (74). The recycle inlet (74) may be
located at or
near the bottom section of the stripping vessel and under the stripping fluid
inlet
(20). A portion (76) of the produced water (68) is recycled back to the heat
exchanger (48) to join the scrubbing fluid (34) and be heated before entering
the
scrubbing vessel. The flush media (42) is recycled back to the bottom section
(14)
of the stripping vessel (10). A portion of the flush media (42) is also
recycled back
to the scrubbing vessel (28) because the solids content of the flush media is
sufficiently low to enable a further flushing action. In one optional aspect,
the flush
media recycle line (42) is in fluid communication with the flash vessel (2),
preferably below the upper liquid level of the solvent recovered tailings pool
in the
bottom of the flash vessel (2). The recycle line (42) may be configured to
enable a
liquid seal with the tailings pool, allowing the flushing media to flow in a
controlled
manner without resorting to a typical valve and level-control setup.
Preferably,
valves can thus be avoided on this recycle line (42), which has benefits
related to
avoiding wear due to particulate-containing fluid flow through valves.
Preferably,
the flush media contains a weight percentage of flushed tailings particles
ranging
from about 0.1 wt% to about 2 wt%. The particle concentration of the flush
media
may be managed within this range or at a desired or predetermined range or
value
by purging excess water or adding make-up water as needed. Finally, the TNRU
comprises a bypass line (80) to feed the solvent component (6) directly to the
condenser (52) without being scrubbed, during maintenance operation of the
scrubbing vessel (28) and its grid (46).
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In one aspect, the scrubber minimizes entrained particles in overhead systesms
to
allow the flash column to be operated at desired operating conditions to
maximize
naphtha recovery. Naphtha recovery and efficiency is also improved over longer
run times. In another aspect, the preferred configuration of external
scrubbers
allows temporary bypassing of the scrubbers through a bypass line for short
time
intervals for online maintenance without seriously affecting operations. In
another
aspect, the scrubber system and any and all configurations described herein
may
also be applied to recovering other types of solvents, such as alkanes from
alkane
diluted tailings derived from a corresponding froth treatment operation. In
another
aspect, the scrubbing systems may be used in connecting with a single stage
flash
vessel or a multi-stage arrangement in which two or more flash vessels are
arranged in series and a scrubber is provided in the overhead system of one or
both of the flash vessels.
Embodiments of the present invention provide a number of advantages. For
instance, in one aspect, the system is external and can be serviced by
bypassing
the unit when required. In addition, the flushing media used in the scrubber
may
aid in removing build up of material and chemical aids can be added to improve
the
removal. The scrubbing device provides a flow path that causes entrained
droplets
to be captured into the flush fluid.
In one aspect, heating of the wash fluid to the flash temperature is
particularly
preferred to maintain the equilibrium with the vapor stream to the unit.
In another aspect, in typical operation the level of particles entrained in
the flush
fluid is relatively low and in such cases it may be desirable that a
significant portion
of the flush media be recycled to minimize flush fluid and heat demands.
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