Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02924307 2016-03-15
DOCKET NO.: NS-526
METHOD FOR RECOVERING SOLVENT FROM FROTH TREATMENT TAILINGS
WITH IN-SITU STEAM GENERATION
INVENTORS: WU, Xin Alex and BHATTACHARYA, Sujit
ASSIGNEE: SYNCRUDE CANADA LTD.
Field of the Invention
[0001] The
present invention relates generally to a method for recovering
solvent from froth treatment tailings that are produced during bitumen froth
treatment.
More particularly, froth treatment tailings are allowed to settle in a solvent
recovery
vessel and the settled tailings are heated to generate steam in-situ.
Background of the Invention
[0002] Oil
sand, as known in the Fort McMurray region of Alberta, Canada,
comprises water-wet sand grains having viscous bitumen flecks trapped between
the
grains. The oil sand lends itself to separating or dispersing the bitumen from
the sand
grains by slurrying the as-mined oil sand in water so that the bitumen flecks
move into
the aqueous phase.
[0003] The
bitumen present in oil sand can be recovered using a hot/warm
water process. In general, water extraction of bitumen involves slurrying oil
sand with
heated water, caustic (NaOH) and naturally entrained air. The
slurry is then
conditioned, for example, in tumblers or a hydrotransport pipeline, for a
prescribed
retention time to initiate a preliminary separation or dispersal of the
bitumen and the
solids and to induce air bubbles to contact and aerate the bitumen. The
conditioned
slurry is then subjected to flotation to further separate the bitumen from the
sand.
1
WSLEGAL\053707\00546\13294086v1
CA 2929307 2017-03-21
[0004] Conditioned oil sand slurry may be further diluted with flood
water and
introduced into a large, open-topped, conical-bottomed, cylindrical vessel
(termed a
primary separation vessel or "PSV"). The diluted slurry is retained in the PSV
under
quiescent conditions for a prescribed retention period. During this period,
the aerated
bitumen rises and forms a froth layer, which overflows the top lip of the
vessel and is
conveyed away in a launder. The sand grains sink and are concentrated in the
conical
bottom, They leave the bottom of the vessel as a wet tailings stream.
Middlings, a watery
mixture containing solids and bitumen, extend between the froth and sand
layers.
[0005] The wet tailings and middlings are withdrawn and may be combined
for
further processing in a secondary flotation process. This secondary flotation
process is
commonly carried out in a deep cone vessel wherein air is sparged into the
vessel to
assist with flotation. This vessel is referred to as the TOR vessel. The
bitumen recovered
by the TOR vessel is recycled to the PSV. The middlings from the deep cone
vessel are
further processed in air flotation cells to recover contained bitumen.
[0006] The froths produced by these units are generally combined and
subjected to further processing. More particularly, it is conventional to
dilute the bitumen
froth with a light hydrocarbon diluent, such as naphtha or a paraffinic
diluent, to improve
the difference in specific gravity between the bitumen and water and reduce
the bitumen
viscosity, which aids in the separation of the water and solids from the
bitumen.
Separation of the bitumen from water and solids is commonly achieved by
treating the
froth in a sequence of scroll and disc centrifuges. However, there has been a
recent trend
towards using an inclined plate settling process for separating bitumen from
the water
and solids. Other processes for separating solids and water from diluted
bitumen froth
are known in the art and include stationary froth treatment (SFT).
WSLEGAL\053707\00551 \17650242v1
2
CA 02924307 2016-03-15
[0007] The
primarily water and solids fraction obtained after separation is
commonly referred to as froth treatment tailings. These froth treatment
tailings typically
comprise approximately 2.0 wt. % hydrocarbon diluent, 3 wt. % bitumen, 20 wt.
A solids
and water as the remainder. It is desirable both economically and
environmentally to
recover the hydrocarbon diluent from the froth treatment tailings prior to
disposal.
However, the unique nature of the diluent-containing tailings makes diluent
removal a
challenge to the industry. In
particular, it is believed that some of the diluent is
intimately associated with the solids, making diluent removal from the solids
more
difficult.
[0008]
Canadian Patent No. 1,027,501 discloses a process for treatment of
centrifuge froth treatment tailings to recover hydrocarbon diluent (naphtha).
The process
comprises introducing the tailings into a vacuum flash vessel maintained at
vacuum
conditions (about 35 kPa) in order to flash the naphtha present in the
tailings. The
vessel is also equipped with a plurality of shed decks so that any residual
naphtha
remaining in the tailings stream will be vaporized by the introduction of
steam beneath
these shed decks. In practice, however, this process results in only 60 to 65%
recovery
of the diluent, as the vacuum at the tailings feed inlet of the vessel may
have resulted in
the tailings bypassing the shed decks and pooling near the bottom of the
vessel. In the
alternative, or additionally, the reduction in pressure in the tower to below
atmospheric
resulted in steam condensation and reduced heat transfer to the slurry.
Thus, the
pooled tailings at the bottom of the vessel still contained a substantially
large amount of
diluent. Canadian Patent No. 2,272,035 partially addressed this issue by
introducing
the steam into the tailings pool for vaporizing the residual diluent pooling
near the
bottom of the vessel. However, the naphtha recovery vessel was operating at
sub-
atmospheric pressure (30-35 kPa), which caused operational issues.
[0009]
Canadian Patent No. 2,272,045 discloses a method for recovery of
hydrocarbon diluent from tailings produced in a bitumen froth treatment plant
comprising
introducing the tailings into a steam stripping vessel maintained at near
atmospheric
pressure (e.g. around 95 kPa) in an attempt to avoid the problem of the
tailings
bypassing the shed decks. Without a vacuum, vessel pressure increased to
3
WSLEGAL\053707\0054613294086v I
CA 02924307 2016-03-15
atmospheric, or slightly above, and temperature increased to around 100 C.
This
resulted in increased naphtha recovery. The operating temperature of the
vessel was
preferably maintained at approximately 100 C.
[00010] However, while operating a steam stripping vessel for recovery
of
hydrocarbon diluent from tailings produced in a bitumen froth treatment plant
at about
100 C and at near atmospheric pressure significantly improved diluent recovery
over
previous operations at below atmospheric pressure (e.g., 35 kPa), there still
was a
significant amount of diluent remaining in the tailings pool. As stated in
Canadian
Patent No. 2,272,045, operating the vessel at near atmospheric pressure and at
a
steam to tailings ratio of approximately 9.0 wt. % increased the naphtha
recovery to
about 80%.
[00011] More recently, a steam stripping vessel with built-in stirrers
was
proposed (see Canadian Patent Nos. 2,712,725 and 2,768,852). Steam is
introduced
directly into the slurry pool through spargers. With the mechanical stirrers
and spargers,
the steam-slurry contact is improved. The residual naphtha contents in the
stripped
tailings are general 0.08-0.15 wt%, significantly lower than the naphtha
recovery using
the process of CA 2,272,035. However, addition of mechanical stirrers and
spargers
complicate the vessel design and introduce operational difficulties when
processing
abrasive materials such as the tailings.
[00012] In summary, the majority of the prior-art processes use live
steam
stripping in a vessel. In one case, the main steam-solids contact occurs at
the shed
decks, which is inherently limited due to short contact time and minimal
agitation. Other
processes rely on direct steam sparging, sometimes with additional mechanical
agitation, to improve steam-solids contact. However, they are more challenging
to
operate in the presence of abrasive solids.
4
WSLEGAL1053707 \ 00546 \ 13294086v1
CA 02924307 2016-03-15
Summary of the Invention
[00013] The current application is directed to a process for recovering
a
hydrocarbon diluent from froth treatment tailings. In particular, the present
invention
involves heating the froth treatment tailings in-situ to generate steam
bubbles in-situ.
[00014] It was discovered that the majority of the coarse solids in the
froth
treatment tailings settle by gravity within minutes. A certain amount of water
remains in
the interstices of the settled coarse solids. It was surprisingly discovered
that when the
interstitial water trapped among the settled solids is sufficiently heated to
boil, the in-situ
generated steam bubbles rise through the settled solids layer, vigorously
agitate the
solids without the need of a mechanical stirrer.
[00015] It was further discovered that if heating occurs in the lower
part of the
solids layer, a temperature gradient forms with higher temperature on the
lower part and
lower temperature on the higher part of the solids layer. Thus, some of the in-
situ
generated steam bubbles implode after rising to the top surface of the solids
layer due
to cooling and condensation. The implosion sends out shock waves to vigorously
agitate the tailings above the coarse solids layer without the need of a
mechanical
stirrer. These agitation actions combined with rising steam bubbles
effectively strip
solvent (e.g., hydrocarbon diluent such as naphtha) from the solids. In some
instances,
the resulting tailings may have a naphtha concentration as low as 0.02 wt%. It
is
important that conditions are such that heating and boiling occur below or
within the
settled solids layer, as it was discovered that boiling along the vessel wall
above the
solids layer is relatively ineffective in stripping solvent. Hence, the
present invention
removes solvent from froth treatment tailings with in-situ generated steam
among solids
for both agitation and stripping.
[00016] Thus, broadly stated, in one aspect of the present invention, a
method
for recovering hydrocarbon diluent from froth treatment tailings comprising
bitumen,
coarse solids, fine solids, hydrocarbon diluent and water is provided,
comprising:
WSLEGAL1053707\00546\13294086µ,1
CA 02924307 2016-03-15
= introducing the froth treatment tailings into a vessel having an upper
section, a
middle section and a lower section;
= allowing the coarse solids to settle to the lower section of the vessel
to form a
solids layer having a portion of hydrocarbon diluent and a portion of water
trapped therein; and
= heating the lower section of the vessel such that the water trapped in
the solids
layer generates steam bubbles in-situ to strip the hydrocarbon diluent
associated
with the coarse solids in the solids layer to produce stripped tailings and
hydrocarbon diluent/water vapors.
[00017] In one embodiment, the method further comprises allowing a
portion of
the fine solids and bitumen to remain suspended in the middle section of the
vessel and
a portion of the fine solids and bitumen to rise to the top section of the
vessel to form a
froth layer. In one embodiment, the steam bubbles further strip the
hydrocarbon diluent
associated with the fine solids.
[00018] In one embodiment, the vessel is operated at or near atmospheric
pressure in its overhead space. In one embodiment, the top section of the
vessel is
operated at or near a temperature of 100 C. As used herein, "at or near" means
plus or
minus 5%. In one embodiment, the froth treatment tailings are at a temperature
of
about 80 C when added to the vessel chamber. It is understood that, because of
hydrostatic pressure, the bottom of the vessel has a boiling point
significantly higher
than 100 C.
[00019] Additional aspects and advantages of the present invention will
be
apparent in view of the description, which follows. It should be understood,
however,
that the detailed description and the specific examples, while indicating
preferred
embodiments of the invention, are given by way of illustration only, since
various
changes and modifications within the spirit and scope of the invention will
become
apparent to those skilled in the art from this detailed description.
6
WSLEGAL\053707\00546\13294086v1
CA 02924307 2016-03-15
Brief Description of the Drawings
[00020] The invention will now be described by way of an exemplary
embodiment with reference to the accompanying simplified, diagrammatic, not-to-
scale
drawings:
[00021] Figure 1 is a schematic of one embodiment of the present
invention.
Detailed Description of Preferred Embodiments
[00022] The detailed description set forth below in connection with the
appended drawings is intended as a description of various embodiments of the
present
invention and is not intended to represent the only embodiments contemplated
by the
inventor. The detailed description includes specific details for the purpose
of providing
a comprehensive understanding of the present invention. However, it will be
apparent to
those skilled in the art that the present invention may be practised without
these specific
details.
[00023] As used herein, "froth treatment tailings" means tailings which
are
produced during a bitumen froth treatment process that uses a hydrocarbon
diluent
such as a naphthenic diluent or a paraffinic diluent to dilute the bitumen
froth prior
to/during treatment. Generally, froth treatment tailings comprise water,
solids,
hydrocarbon diluent, and bitumen.
[00024] As used herein, "bitumen froth" refers to primary and/or
secondary
froths produced during extraction of bitumen from oil sand as recognized by
the
industry.
[00025] As used herein, "hydrocarbon diluent" means any substance
containing one or more hydrocarbon compounds and/or substituted hydrocarbon
compounds which is suitable for diluting and/or dissolving bitumen present in
bitumen
froth.
7
WSLEGAL1053707 \ 00546 13294086v1
CA 02924307 2016-03-15
[00026] As used herein, a "naphthenic diluent" means a hydrocarbon
diluent
including naphtha produced from natural gas condensates, petroleum
distillates, and
the distillation of coal tar and peat and generally comprises a mixture of
aromatic and
non-aromatic compounds.
[00027] As used herein, a "paraffinic diluent" means a hydrocarbon
diluent
including a sufficient amount of one or more relatively short-chain aliphatic
compounds
such as C5 to C8 aliphatic compounds.
[00028] As used herein, "coarse solids" mean mineral solid particles
with their
largest dimension larger than 44 pm.
[00029] As used herein, "fine solids" mean mineral solid particles with
their
largest dimension smaller than 44 pm.
[00030] Figure 1 shows one embodiment of the present invention. Froth
treatment tailings 20 are added to a vessel 10 via inlet 15 which is located a
distance
from the bottom section 16 of the vessel 10. In the embodiment shown in Figure
1, inlet
15 is located in the middle section 17 of the vessel 10. The coarse solids
present in
froth treatment tailings will settle to the bottom section 16 of the vessel 10
and form a
solids layer 19. Generally, the froth treatment tailings 20 are fed at a rate
allowing a
residence time in the vessel 10 of about 20-70 minutes to allow solids
settling to a
desired degree.
[00031] In one embodiment, the vessel 10 is a cylindrical tank with an
aspect
ratio (tailings height/diameter) of about 1. In one embodiment, the vessel is
essentially
an empty tank. The solids layer 19 is heated by any means known in the art. In
one
embodiment, saturated steam 30 of any pressure heats the bottom of the unit 10
indirectly through large heat exchanging surfaces, which may be provided by a
network
of tubes/pipes 32. Condensed water 31 exits the heating tubes 32 and may be
reused
for steam generation. In another embodiment, the heating surface is provided
by other
means, for example, by electric heating coils. The heating section may extend
upward
from the bottom 36 of unit 10, but, generally, should not exceed 20% of the
total tailings
8
WSLEGAL\053707\00546\13294086v1
CA 02924307 2016-03-15
height (sum of layer thicknesses of 19, 34 and 39). In another embodiment, in
addition
to steam pipe or electric coil heating, the bottom surface 36 of the vessel 10
is heated
with any external devices or means. In one embodiment, the feed stream 20 may
be
preheated to near boiling temperature of water, in addition to steam pipe or
electric coil
heating.
[00032] With heating, water among the settled solids (tailings layer 19)
boils
sending in-situ generated steam bubbles to the top 35 of the vessel 10, thus,
travelling
across its entire length. These steam bubbles create turbulence among solids
and
slurry, and strip hydrocarbon diluent from the tailings. The steam and
hydrocarbon
diluent vapors rise to the top 35 of the vessel 10 and exit via outlet 33 as
steam and
hydrocarbon diluent vapors stream 21. Stream 21 is then condensed in a heat
exchanger 11 and condensed vapors 22 are separated in a separator 12 to
produce
recovered diluent 23 and water 24.
(00033] After coarse solids settling, the middle section 17 of the
vessel 10
mainly contains a slurry layer 39 comprising water, suspended fine solids and
bitumen
drops. It was further observed that when steam bubbles rise through the
tailings, they
may also carry some bitumen, some residual solvent and fines. As a result, a
froth
layer 34 forms on top of the slurry layer 39 in the upper section 18 of the
vessel 10. The
froth layer 34 overflows and exits the vessel 10 via outlet 37 as froth stream
25.
Generally, the mass flow rate of stream 25 is less than 20% of the mass flow
rate of the
feed stream 20.
[00034] The treated/stripped tailings are removed from the bottom
section 16 of
the vessel 10 via outlet 38 as underflow stream 27. In one embodiment, a waste
heat
recovery device such as a plate exchanger is used to recover heat from stream
27 and
heat the feed stream 20. In one embodiment, froth stream 25 is combined with
underflow stream 27. Alternatively, the froth stream 25 may be further treated
to
remove its bitumen content in a unit 13 and to remove its residual solvent
content in a
unit 14. In one embodiment, the unit 13 includes multiple units for solvent
(e.g. naphtha
diluent) addition/mixing and diluted bitumen separation from water-based
slurry by
9
WSLEGAL\053707\00546\13294086v1
CA 02924307 2016-03-15
gravity. The treated slurry 29 is then sent to the unit 14 for solvent
recovery. In one
embodiment, the unit 14 is settler/stripper unit similar to the vessel 10. The
cleaned-up
fines and water, stream 26, is then combined with the stream 27. Thus, stream
28 is
the combination of either stream 25 and stream 27 or stream 26 and stream 27.
The
combined waste stream 28 is disposed in a tailings pond. In another
embodiment, the
froth stream 25 is sent back to the froth treatment plant for reprocessing. In
this case
stream 27 is the sole waste stream to be disposed.
[00035] In most instances, the in-situ generated steam bubbles agitate
the
solids layer on the bottom of the vessel 10 very vigorously. To further
prevent caking in
the heating section, the solids-rich slurry on the vessel bottom may be pumped
around
to keep solids somewhat mobile in the bottom layer. However, this pumping
action
should not be strong enough to homogenize the slurry in the vessel 10. Thus,
besides
the optional use of pumps, there are no moving parts in the vessel 10, yet
adequate
mixing of steam and solids/trapped hydrocarbon diluent still occurs through in-
situ
steam formation. Furthermore, steam bubbles are generated throughout the
vessel's
cross section, so no sparger is needed to distribute steam, as in the case of
live steam
injection into a slurry pool. Generally, the hydrocarbon diluent recovery with
the present
invention is above 90%. When froth treatment tailings are generated from froth
treatment using naphtha, the naphtha recovery is generally above 95%. The
residual
naphtha content in the treated tailings is generally below 0.1 wt%.
[00036] Thus, as compared to prior art diluent recovery processes, the
present
invention may provide the additional benefit of forming a bitumen-rich froth
stream,
which may be further processed to recover bitumen. Further, when heating is
provided
by steam circulating through a piping system (as opposed to injecting steam
directly into
the vessel), it allows clean water to condense (in stream 31) and be reused
after steam
heating so that the demand for boiler feed water is minimized. This is
especially
valuable in winter months when river water import is near the regulated limit.
WSLEGAL\053707\00546\13294086v1
CA 02924307 2016-03-15
[00037] Example 1
[00038] In this example, a naphtha froth treatment tailings sample of
about 520
g was placed in a cylindrical beaker of 8.3 cm diameter. The slurry height was
about 9
cm. The beaker wall was insulated. The beaker was heated on a hot plate with
various
power settings with its top closed until boiling occurred. The sample was then
boiled for
a given period of time until it lost 10% of its original mass. Naphtha
concentrations in the
feed sample and the treated samples were analyzed and the results are shown in
Table
1.
Table 1
Parameters Boiling Time (min)
0 44 51 73 80
Naphtha Concentration* (wt%) 1.419 0.056 0.018 0.018 0.041
Naphtha Recovery (/o) 0 96.0 98.7 98.7 97.1
*The naphtha concentration was normalized by the original sample weight. The
zero boiling time value
refers to the feed property.
For comparison, the average naphtha concentration after live steam stripping
in
one commercial naphtha recovery unit (N RU) for the same sample was 0.24 wt%
and
the average naphtha recovery in the NRU was 83%.
[00039] Example 2
[00040] Another naphtha froth treatment tailings sample of about 520 g
was
placed in the aforementioned beaker. The sample was either heated directly on
the hot
plate or heated inside a larger beaker filled with silicon oil. A Teflon dish
was placed
below the sample beaker to block its bottom area from being heated directly by
the hot
plate, thus heating was exclusively from the beaker wall through the hot oil.
The boiling
time was about 45 min. The sample mass loss was about 10%. Naphtha
concentrations
in the feed sample and the treated samples were analyzed. The values shown in
Table
2 are average ones of two measurements for each heating condition and four
measurements for the feed.
11
WSLEGAL1053707 \00546\13294086v1
CA 02924307 2016-03-15
Table 2
Parameters Feed Bottom Heating Wall Heating
Naphtha Concentration* 1.591 0.37 0.138 0.01 0.657 0.02
(wt%)
Naphtha Recovery (%) 0 91.3 0.6 58.7 1.4
*The naphtha concentration was normalized by the original sample weight.
[00041] Note that this feed sample is not a typical one, likely
generated under
plant upset conditions. The normalized naphtha concentration after live steam
stripping
in one commercial NRU for this sample was 0.32 wt%. The average naphtha
recovery
in this NRU was 77%. The present method with bottom heating can still achieve
a
naphtha recovery above 90% on this difficult feed. Wall heating is ineffective
since most
of steam bubbles were generated above the settled solids layer and bypassed
the
naphtha containing solids.
[00042] Example 3
[00043] An equilibrium simulation was run using a commercial process
simulator Aspen HYSYS 7.2. The froth treatment tailings feed contains 2.6 wt%
bitumen, 1.7 wt% naphtha, 27.1 wt% solids and 68.5 wt% water. The feed has
been
preheated to 110 C at 220 kPa. Other stream properties are shown in Table 3.
12
WSLEGAL\053707\00546\13294086v1
CA 02924307 2016-03-15
Table 3
Stream # T Absolute Mass Hydrocarbon Water Solids
( C) P flow mass
flows mass flow mass flow
(kPa) (kg/s) (kg/s) (kg/s) (kg/s)
20 110 220 349.1 15.0 239.3 94.8
21 120 114 34.9 5.8 29.1 0
23 85 100 5.8 5.8 0 0
24 85 100 29.1 0 29.1 0
28 122 220 314.2 9.2 210.2 94.8
30 148 446 37.5 0 37.5 (gas) 0
31 148 446 37.5 0 37.5 (liq) 0
Heating 79.5 MW
duty 30,
31
Cooling 71.1 MW
duty @11
Naphtha 99.3%
rec. @ 23
Steam/Feed 0.107
t Refer to Figure 1 for stream #
Sum of bitumen and naphtha flow rates
[00044] The slurry density is 1080 kg/m3. In a hypothetical tank of 10 m in
diameter and 10 m in tailings height, the residence time is 40 min. Assuming
the heat
transfer coefficient, U, for the heating section is 1000 W/m2K, the required
heating area
is 3118 m2. The heating area can be reduced to 1254 m2 if 1136 kPa steam is
used
instead of 446 kPa steam. This smaller area can be provided by a network of
steam
tubes at the bottom of the tank.
[00045] The naphtha recovery here is the thermodynamic limit for one-stage
flashing. If steam/solids contact is ideal, this value can be closely
approached in
operation.
[00046] From the foregoing description, one skilled in the art can easily
ascertain the
essential characteristics of this invention and adapt it to various usages and
conditions.
Reference to an element in the singular, such as by use of the article "a" or
"an" is not intended
to mean "one and only one" unless specifically so stated, but rather "one or
more". Nothing
disclosed herein is intended to be dedicated to the public regardless of
whether such disclosure
is explicitly recited in the claims.
13
WSLEGAL\053707\00546\ 13294086v1
