U.S. patent application number 13/933976 was filed with the patent office on 2015-01-08 for method for reducing rag layer volume in stationary froth treatment.
This patent application is currently assigned to SYNCRUDE CANADA LTD. in trust for the owners of the Syncrude Project. The applicant listed for this patent is SYNCRUDE CANADA LTD. in trust for the owners of the Syncrude Project. Invention is credited to GEORGE CYMERMAN, BRIAN KNAPPER, YIN MING SAMSON NG, TAM TRAN, ALLAN YEUNG.
Application Number | 20150008161 13/933976 |
Document ID | / |
Family ID | 52132082 |
Filed Date | 2015-01-08 |
United States Patent
Application |
20150008161 |
Kind Code |
A1 |
NG; YIN MING SAMSON ; et
al. |
January 8, 2015 |
METHOD FOR REDUCING RAG LAYER VOLUME IN STATIONARY FROTH
TREATMENT
Abstract
A method for reducing rag layer volume in a stationary bitumen
froth treatment process is provided, comprising subjecting dilfroth
having a naphtha diluent to bitumen ratio of about 0.7 to gravity
settling in a splitter vessel to produce an overflow stream of raw
dilbit and an underflow stream of splitter tails; mixing the
splitter tails with a naphtha diluent to give a mixture having a
naphtha diluent to bitumen ratio of less than about 6:1 in a
scrubber feed tank; and subjecting the mixture to gravity settling
and agitation in a scrubber vessel to produce an overhead stream of
scrubber hydrocarbons and an underflow stream of scrubber
tails.
Inventors: |
NG; YIN MING SAMSON;
(Sherwood Park, CA) ; KNAPPER; BRIAN; (Edmonton,
CA) ; CYMERMAN; GEORGE; (Edmonton, CA) ; TRAN;
TAM; (Edmonton, CA) ; YEUNG; ALLAN; (Edmonton,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SYNCRUDE CANADA LTD. in trust for the owners of the Syncrude
Project |
Fort McMurray |
|
CA |
|
|
Assignee: |
SYNCRUDE CANADA LTD. in trust for
the owners of the Syncrude Project
Fort McMurray
CA
|
Family ID: |
52132082 |
Appl. No.: |
13/933976 |
Filed: |
July 2, 2013 |
Current U.S.
Class: |
208/390 |
Current CPC
Class: |
C10G 1/045 20130101 |
Class at
Publication: |
208/390 |
International
Class: |
C10G 1/04 20060101
C10G001/04 |
Claims
1. A method for reducing rag layer volume in a stationary bitumen
froth treatment process comprising: subjecting dilfroth having a
naphtha diluent to bitumen ratio of about 0.7 to gravity settling
in a splitter vessel to produce an overflow stream of raw dilbit
and an underflow stream of splitter tails; mixing the splitter
tails with a naphtha diluent to give a mixture having a naphtha
diluent to bitumen ratio of less than about 6:1 in a scrubber feed
tank; and subjecting the mixture to gravity settling and agitation
in a scrubber vessel to produce an overhead stream of scrubber
hydrocarbons and an underflow stream of scrubber tails.
2. The method of claim 1, further comprising subjecting the raw
dilbit to gravity settling and agitation in a polisher vessel at a
naphtha diluent to bitumen ratio of about 0.7 to reduce and/or
break rag layer and to produce an overflow stream of polished
dilbit and an underflow stream of polisher sludge.
3. The method of claim 1, further comprising diluting bitumen froth
with the produced scrubber hydrocarbons to form the dilfroth.
4. The method of claim 1, wherein the mixture has a naphtha diluent
to bitumen ratio of less than 4:1.
5. The method of claim 1, wherein the mixture has a naphtha diluent
to bitumen ratio of less than or equal to 3:1.
6. The method of claim 1, wherein agitation in the scrubber feed
tank is conducted in the range of about 700 rpm to about 1300
rpm.
7. The method of claim 6, wherein agitation in the scrubber feed
tank is conducted at about 700 rpm.
8. The method of claim 1, wherein agitation in the scrubber vessel
is conducted in the range of 1 rpm to about 188 rpm.
9. The method of claim 8, wherein agitation in the scrubber vessel
is conducted at about 52 rpm to about 188 rpm.
10. The method of claim 1, wherein agitation in the scrubber feed
tank is conducted at about 1300 rpm and agitation in the scrubber
vessel is conducted at about 52 rpm.
11. The method of claim 3, wherein optionally, silicate is added to
the bitumen froth.
12. The method of claim 1, wherein optionally, water is added to
the scrubber vessel.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method for reducing rag
layer volume in a stationary bitumen froth treatment process by
agitating naphtha diluted bitumen froth and using a low naphtha to
bitumen ratio at specific treatment stages in a stationary froth
treatment process.
BACKGROUND OF THE INVENTION
[0002] Oil sand, as known in the Athabasca region of Alberta,
Canada, comprises water-wet, coarse sand grains having flecks of a
viscous hydrocarbon, known as bitumen, trapped between the sand
grains. The water sheaths surrounding the sand grains contain very
fine clay particles. Thus, a sample of oil sand, for example, might
comprise 70% by weight sand, 14% fines, 5% water and 11% bitumen
(all % values stated in this specification are to be understood to
be % by weight).
[0003] For the past 25 years, the bitumen in Athabasca oil sand has
been commercially recovered using a water-based process. In the
first step, the oil sand is slurried with process water, naturally
entrained air and, optionally, caustic (NaOH). The slurry is mixed,
for example in a tumbler or pipeline, for a prescribed retention
time, to initiate a preliminary separation or dispersal of the
bitumen and solids and to induce air bubbles to contact and aerate
the bitumen. This step is referred to as "conditioning".
[0004] The conditioned slurry is then 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,
aerated bitumen rises and forms a froth layer, which overflows the
top lip of the vessel and is conveyed away in a launder. Sand
grains sink and are concentrated in the conical bottom. They leave
the bottom of the vessel as a wet tailings stream containing a
small amount of bitumen. Middlings, a watery mixture containing
fine solids and bitumen, extend between the froth and sand
layers.
[0005] The wet tailings and middlings are separately withdrawn,
combined and sent to 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 flotation in the TOR vessel is recycled to the PSV. The
middlings from the deep cone vessel are further processed in
induced air flotation cells to recover contained bitumen.
[0006] The bitumen froths produced by the PSV are subjected to
cleaning, to reduce water and solids contents so that the bitumen
can be further upgraded. More particularly, it has been
conventional to dilute this bitumen froth with a light hydrocarbon
diluent, for example, with naphtha, to increase the difference in
specific gravity between the bitumen and water and to reduce the
bitumen viscosity, to thereby aid in the separation of the water
and solids from the bitumen. This diluent diluted bitumen froth is
commonly referred to as "dilfroth". It is desirable to "clean"
dilfroth, as both the water and solids pose fouling and corrosion
problems in upgrading refineries. By way of example, the
composition of naphtha-diluted bitumen froth typically might have a
naphtha/bitumen ratio of 0.65 and contain 20% water and 7% solids.
It is desirable to reduce the water and solids content to below
about 3% and about 1%, respectively.
[0007] Separation of the bitumen from water and solids may be done
by treating the dilfroth in a sequence of scroll and disc
centrifuges. Alternatively, the dilfroth may be subjected to
gravity separation in a series of inclined plate separators ("IPS")
in conjunction with countercurrent solvent extraction using added
light hydrocarbon diluent. However, these treatment processes still
result in bitumen often containing undesirable amounts of solids
and water.
[0008] More recently, a staged settling process (often referred to
as Stationary Froth Treatment or SFT) for cleaning dilfroth was
developed as described in U.S. Pat. No. 6,746,599, whereby dilfroth
is first subjected to gravity settling in a splitter vessel to
produce a splitter overflow (raw diluent diluted bitumen or
"dilbit") and a splitter underflow (splitter tails) and then the
raw dilbit is further cleaned by gravity settling in a polisher
vessel for sufficient time to produce an overflow stream of
polished dilbit and an underflow stream of polisher sludge.
Residual bitumen present in the splitter tails can be removed by
mixing the splitter tails with additional naphtha and subjecting
the produced mixture to gravity settling in a scrubber vessel to
produce an overhead stream of scrubber hydrocarbons, which stream
is recycled back to the splitter vessel.
[0009] However, a rag layer tends to form between the bitumen phase
and the tailings phase in the scrubber vessel during gravity
settling of the splitter tails/naphtha mixture, and to a lesser
extent, in the polisher vessel during gravity settling of the raw
dilbit. It is believed that the rag layer may be a result of stable
water-in-oil emulsions persisting, primarily due to the clay solids
present in the diluted bitumen froth. The rag layer is a mixture of
partially oil-wet solids, oil and water-in-oil emulsions. Much of
the clay solids are kaolinite and illite. The formation of such a
rag layer prevents complete separation of the diluted bitumen from
the water and solids, reduces dewatering, and depresses bitumen
recovery.
[0010] Accordingly, there is a need for a method of reducing and/or
breaking the rag layer in stationary bitumen froth treatment
processes.
SUMMARY OF THE INVENTION
[0011] The current application is directed to a method of reducing
rag layer volume in stationary bitumen froth treatment processes.
It was surprisingly discovered that by conducting the method of the
present invention, one or more of the following benefits may be
realized:
[0012] (1) Mixing of the rag layer that forms in a separation
vessel significantly reduces the rag layer volume. In particular,
rag layer mixing alone significantly reduces rag layer volume
compared to feed (e.g., scrubber feed) mixing alone. Gentle or mild
mixing is sufficient. The combined use of rag layer mixing and
scrubber feed mixing is more effective in reducing the rag layer
volume compared to either rag layer mixing alone or feed mixing
alone.
[0013] (2) Use of a low scrubber naphtha to bitumen ratio (less
than about 4:1, preferably less than about 3:1) for the scrubber
feed contributes to a further reduction in rag layer volume by
minimizing the precipitation of asphaltenes which normally
stabilize the rag layer.
[0014] (3) Reduction in rag layer volume is optimally achieved by
combining rag mixing, scrubber feed mixing, and a low naphtha to
bitumen ratio for the scrubber feed, without necessitating silicate
addition to the bitumen froth or rag water addition to the
scrubber.
[0015] (4) Combining rag mixing, scrubber feed mixing, and a low
naphtha to bitumen ratio for the scrubber feed yielded a scrubber
product having a bitumen content greater than about 20 wt % and a
solids content less than about 5 wt %. The enhancement in scrubber
product quality reduces the amount of water and solids recycled to
the splitter feed, thereby, in turn, improving splitter product
quality.
[0016] (5) Agitation of the bitumen froth at various treatment
stages within gravity settlers including for example, the scrubber
feed tank, scrubber and polisher, may reduce the rag layer
volume.
[0017] Thus, use of the present invention may improve bitumen
recovery and product quality by effectively reducing the rag layer
volume.
[0018] In one aspect, a method of reducing rag layer volume in a
stationary bitumen froth treatment process is provided, comprising:
[0019] subjecting dilfroth having a naphtha diluent to bitumen
ratio of about 0.7 to gravity settling in a splitter vessel to
produce an overflow stream of raw dilbit and an underflow stream of
splitter tails; [0020] mixing the splitter tails with a naphtha
diluent to give a mixture having a naphtha diluent/bitumen ratio of
less than about 6:1; and [0021] subjecting the mixture to gravity
settling and agitation in a scrubber vessel to produce an overhead
stream of scrubber hydrocarbons and an underflow stream of scrubber
tails.
[0022] In one embodiment, the method further comprises subjecting
the raw dilbit to gravity settling and agitation in a polisher
vessel to produce an overflow stream of polished dilbit and an
underflow stream of polisher sludge.
[0023] In one embodiment, the naphtha diluent to bitumen ratio of
the mixture is less than 4:1. In another embodiment, the naphtha
diluent to bitumen ratio of the mixture is less than or equal to
3:1.
[0024] In one embodiment, mixing reduces the rag volume in a
polisher vessel at naphtha diluent to bitumen ratio of about
0.7.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] Referring to the drawings wherein like reference numerals
indicate similar parts throughout the several views, several
aspects of the present invention are illustrated by way of example,
and not by way of limitation, in detail in the figures,
wherein:
[0026] FIG. 1 is a graph showing, in general, one embodiment of a
bitumen froth treatment process useful in the present
invention.
[0027] FIG. 2 is a graph showing the rag layer volume (mL) for each
test condition.
[0028] FIG. 3 is a graph showing the rag layer solids content (mass
%) for each test condition.
[0029] FIG. 4 is a graph showing the rag layer water content (mass
%) for each test condition.
[0030] FIG. 5 is a graph showing the rag layer bitumen content
(mass %) for each test condition.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] 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 practiced without these specific
details.
[0032] The present invention relates generally to a method of
reducing and/or breaking rag layer in a stationary bitumen froth
treatment process. The method includes agitating bitumen froth and
using a low naphtha to bitumen ratio at specific stages of the
froth treatment process.
[0033] FIG. 1 is a general schematic of a stationary bitumen froth
treatment process using gravity settlers, which can be used in one
embodiment of the present invention. Bitumen froth 10 is initially
received from an extraction facility which extracts bitumen from
oil sand using a water extraction process known in the art. The
bitumen froth 10, as received, typically comprises about 60%
bitumen, about 30% water and about 10% solids.
[0034] A hydrocarbon diluent 12 is mixed with bitumen froth 10 in a
suitable mixer 14 to provide diluent-diluted bitumen froth
(referred to herein as "dilfroth") 16. In one embodiment, the
hydrocarbon diluent 12 is naphtha. The naphtha is supplied in an
amount such that the naphtha to bitumen ratio of the dilfroth 16 is
preferably in the range of 0.5 to 1.0, most preferably about
0.7.
[0035] As used herein, the term "silicate" refers to any of a wide
variety of compounds containing silicon, oxygen and one or more
metals with or without hydrogen, for example, a sodium silicate
having the general formula xNa.sub.2O.ySiO.sub.2. Silicates are
known to change the surface properties of fine solids, causing them
to associate with the water phase, rather than the oil phase. A
silicate 18 is typically added to the dilfroth 16 at a
concentration ranging between about 0.0001 to about 0.1% wt/wt or
more. However, in the present invention, reduction of the rag layer
volume may be achieved without the addition of silicate 18. In one
embodiment, addition of silicate 18 to the bitumen froth 10 is
optional. The dilfroth 16 may be fed into an agitated feed tank 20,
for example, a splitter feed tank.
[0036] The agitated dilfroth 22 is then pumped into the chamber of
a gravity settler vessel or splitter 24 having a conical bottom 26,
and underflow and overflow outlets 28, 30 at its bottom and top
ends, respectively. The dilfroth 22 is temporarily retained in the
splitter 24 for a sufficient length of time to allow a substantial
portion of the solids and water to separate from the diluted
bitumen. The splitter overflow is referred to as raw dilbit 32.
Line 34 withdraws a stream of splitter tails 36 through the
underflow outlet 28. Splitter overflow line 38 collects an overflow
stream of raw dilbit 32.
[0037] The bottom layer of splitter tails 36 comprises mainly sand
and aqueous middlings, and some hydrocarbons, and the top layer of
raw dilbit 32 comprises mainly hydrocarbons containing some water
and a reduced amount of fines (clay particles).
[0038] The raw dilbit 32 produced through the splitter overflow
outlet 30 routinely comprises less than about 3% solids, and may be
pumped to a second gravity settler vessel or polisher (40)
following optional addition of a demulsifier to enhance water
separation, and subjected to further gravity settling therein. The
polisher is operated at naphtha to bitumen ratio of about 0.7.
Water droplets coalesce and settle, together with most of the
remaining fine solids. Since a rag layer may form during gravity
settling, the raw dilbit 32 is thus agitated while being retained
within the polisher 40 to reduce the rag layer volume. Polisher
dilbit 42, comprising hydrocarbons, typically containing <3.0
wt. % water and <1.0 wt. % solids, is removed as an overflow
stream from the polisher 40. Polisher sludge 44, comprising water,
solids and typically between about 20-70% hydrocarbons, or 12-40%
bitumen, is removed from the polisher 40 as an underflow
stream.
[0039] The splitter tails 36 produced through the splitter
underflow outlet 28 are pumped through line 46, to an agitated feed
tank 48 or scrubber feed tank, where it may be mixed with polisher
sludge 44 and naphtha 12 to produce a scrubber feed 50 preferably
having a naphtha to bitumen ratio less than about 4:1. In one
embodiment, the naphtha to bitumen ratio is less than about 3:1.
The use of a naphtha to bitumen ratio less than about 4:1 prevents
the precipitation of asphaltenes which normally stabilize the rag
layer. The rag emulsion is rendered weaker and easier to break down
through agitation of the scrubber feed 50 with agitator 66. In one
embodiment, agitation is conducted at a speed in the range of about
700 rpm to about 1300 rpm, preferably about 700 rpm.
[0040] The agitated scrubber feed 50 is then introduced to a third
gravity settler vessel or scrubber 52. The scrubber feed 50 is then
temporarily retained in the scrubber 52 (for example for 20 to 30
minutes) and subjected to gravity settling therein. A stable rag
layer typically forms between the diluted bitumen layer and the
water layer in the scrubber 52 during gravity settling of the
scrubber feed 50. The scrubber feed 50 is agitated with agitator 64
while being retained within the scrubber 52. Without being bound to
theory, it is believed that agitation induces shear, which
minimizes rag layer volume and breaks the gel-like rag layer, but
not the water-in-oil emulsion which is present in the oil and water
interface. In one embodiment, agitation is conducted at a speed in
the range of about 52 rpm to about 188 rpm, preferably about 52
rpm.
[0041] Without being bound by theory, it is believed that addition
of water 54 to the rag layer removes fine solids; however, in the
present invention, reduction of the rag layer volume may be
achieved without the addition of water 54 to the rag layer within
the scrubber 52. In one embodiment, addition of water 54 to the rag
layer is optional.
[0042] The scrubber overflow stream 56 of hydrocarbons, mainly
comprising naphtha and bitumen, is removed through an overflow
outlet 58 and in one embodiment may be recycled through line 60 to
the mixer 14. Scrubber underflow stream of scrubber tails 62,
comprising water and solids containing some hydrocarbons, is
removed and forwarded to a naphtha recovery unit (not shown).
[0043] Exemplary embodiments of the present invention are described
in the following Example, which is set forth to aid in the
understanding of the invention, and should not be construed to
limit in any way the scope of the invention as defined in the
claims which follow thereafter.
Example 1
[0044] The flow sheet used for the evaluation of the rag volume
reduction is essentially the same as that shown in FIG. 1 except
that the polisher vessel was omitted to enable timely
experimentation. Five variables including silicates concentration,
water addition to rag layer, rag layer agitation, scrubber feed
agitation and scrubber N/B ratio were evaluated using a 2.sup.5-1
fractional factorial design resulting in 16 different experimental
run conditions. Table 1 summarizes the range of the independent
variables and the test matrix. In addition, Table 1 includes repeat
conditions and an additional run using higher rag layer agitation
(Condition No. 18), resulting in a total of 20 runs completed.
TABLE-US-00001 TABLE 1 Test Matrix Water Rag Layer Silicates
Addition Agitation Scrubber Feed Scrubber Condition wt. % (g/min)
RPM RPM N/B 1 0 0 0 700 >6 2 0 16 52 700 >6 3 0.1 0 52 700
>6 4 0.1 16 0 700 >6 5 0.1 16 52 1300 >6 6 0 0 52 1300
>6 7 0 16 0 1300 >6 8 0 0 52 700 <3 9 0 16 0 700 <3 10
0 0 0 1300 <3 11 0 16 52 1300 <3 12 0.1 0 52 1300 <3 13
0.1 16 0 1300 <3 14 0.1 0 0 1300 >6 15 0.1 0 0 700 <3 16
0.1 16 52 700 <3 17 0 0 0 700 >6 18 0 0 188 700 >6 19 0 0
0 700 >6 20 0.1 16 52 700 <3
[0045] In this evaluation, controlling the rag layer growth in the
scrubber vessel is the primary objective. Quantifying whether the
rag layer has been reduced or changed is done by measuring the rag
volume and rag layer composition. It is desirable for the rag to
occupy less volume in the scrubber, which directly implies that
there is physically less rag layer present in the scrubber. The rag
layer composition is not a concern under steady state conditions,
provided the rag layer is not growing. Therefore, the following
analysis focuses on the rag layer growth, i.e., the rag layer's
volume and not its composition. Thus, the experimental design
evaluates the effect of the five variables on rag layer volume.
[0046] The amount of rag layer produced varied considerably with
the various conditions and, in some instances, there was over an
order of magnitude difference in rag layer volume, from 71 to 780
mL. The volume of rag layer produced in each condition is
summarized in FIG. 2. Five conditions (9, 10, 14, 17 and 19)
produced the largest rag volume, wherein all five conditions did
not have rag mixing. Conditions 2, 4, 8, 16, and 20 produced some
of the lowest amounts of rag volume; these five conditions involved
either rag mixing or rag water addition or both. The variability of
the measured rag volume based on repeats was 31% relative errors.
Based on 95% confidence limits, the maximum and minimum rag volumes
are significantly different.
[0047] Rag layer composition appears to be more variable among the
various conditions tested, as shown in FIGS. 3, 4, and 5. The
results appear to indicate that the rag layer bitumen content
increased when the scrubber N/B was lowered (FIG. 5). Otherwise, no
particular trend of the composition with operating conditions is
observed.
[0048] The effects of the five rag layer mitigation variables on
the rag layer volume reduction were evaluated using an experimental
design software package (Design Expert.RTM. by Stat-Ease). This
software enabled the use of all experimental data, including
repeats, to produce parameter estimates and determine the
significant of the parameter estimates at 95% confidence limits.
The empirical model representing rag volume, Y.sub.1 is:
Y.sub.1=310-57X.sub.2-110X.sub.3+65X.sub.1X.sub.3+47X.sub.1X.sub.4+72X.s-
ub.2X.sub.3-82X.sub.2X.sub.5 R.sup.2=0.91
Note that X.sub.i represents coded value of independent variable i.
The results show that there are two main effects and four
two-factor interaction effects to be significant at 95% confidence
limits. The model has a R.sup.2 of 91%, which means that 91% of the
data variation can be explained by the model. Among these effects,
rag layer mixing (X.sub.3) has the most significant effect on the
rag volume, i.e. high level of mixing reduced the rag volume. The
other main effect is rag water addition, where addition of rag
water reduced rag layer volume. The interpretation of two factor
interaction effects is as follows: [0049] X.sub.1X.sub.3: This term
represents the interacting effect between the silicate and rag
mixing. The interacting effect needs to be minimized to achieve a
reduction in rag volume. Therefore the sign of these two variables
must be opposite. Since the rag mixing has been assigned a positive
value above, the silicate addition will be negative. As a result,
the reduction of rag volume can be achieved through rag mixing and
with no silicate addition. [0050] X.sub.1X.sub.4: This term
represents the interacting effect between the silicate and the
Scrubber feed mixing. Similarly, the sign of the two variables
needs to be opposite for rag volume reduction. Since the sign of
the silicate is negative, the Scrubber feed mixing will be
positive. Therefore, the reduction of rag volume required no
silicate addition and high Scrubber feed mixing. [0051]
X.sub.2X.sub.3: This term represents the interacting effect between
rag water addition and rag mixing. Again, the sign of the two
variables need to be opposite to achieve the rag volume reduction.
However, the main effect of these two variables suggests the sign
to be the same. Under this scenario, magnitude of the three terms
(X.sub.2, X.sub.3, and X.sub.2X.sub.3) required to be evaluated and
optimized. If the rag volume is minimized, the results show that
rag mixing must remain positive, but the sign of rag water addition
will have to change to be negative. [0052] X.sub.2X.sub.5: This
interacting effect is between rag water addition and Scrubber N/B
ratio. The sign of the two variables should be the same to allow
the reduction of the rag. If the sign of rag water addition is
negative, the sign of Scrubber N/B ratio should also be negative.
The results suggest that a N/B ratio less than or equal to 3 should
be to use to decrease the rag volume. This is not unexpected as
N/B>4 would precipitate bitumen asphaltenes, which stabilize
water in oil emulsion and hence the stability of the rag layer.
[0053] Based on the above evaluation, for rag volume reduction, the
recommended rag mitigation variable settings are: no silicate and
rag water addition, high rag and Scrubber feed mixing and low N/B
ratio. Using these variables settings and the developed model
(equation shown above), the rag layer volume can be estimated.
Table 2 focuses on the mixing effects on rag layer volume. The
standard flow sheet conditions were used, which required the N/B
ratio to be greater than or equal to 6 and the scrubber feed mixing
was set at 700 rpm to prevent water and solids settling in the
scrubber feed tank. The rag layer volume without additional mixing
introduced to the system was estimated to be 743 mL. Increasing the
scrubber feed mixing from 700 to 1300 rpm reduced the rag layer
volume to 649 mL. Addition of the rag layer mixing at 52 rpm
significantly decreased the rag layer volume to 249 mL. The use of
rag layer mixing at 52 rpm and an increase of scrubber feed mixing
to 1300 rpm further decreased the rag volume to 155 mL. These
results clearly demonstrated the impact of mixing on rag layer
reduction.
TABLE-US-00002 TABLE 2 Variable Rag volume, mL Base case (no rag
mixing and scrubber feed mixing 743 at 700 rpm) Base case +
Scrubber feed mixing (1300 rpm) 649 Base case + rag mixing (52 rpm)
249 Base case + rag mixing (52 rpm) + Scrubber feed 155 mixing
(1300 rpm)
[0054] Without being bound to theory, a possible explanation as to
why the recommended rag mitigation variable settings worked in the
reduction of rag layer volume is offered as follows. The rag layer
is comprised of multiple emulsions, which are stabilized by solids
and/or bitumen asphaltenes. The majority of solids are hydrophobic
solids as result of surface property change due to the interaction
between the hydrophilic clays and naphthenic acid. The clays and
natural surfactants are present naturally in oil sands and process
water. It was proposed that addition of silicate could change the
solids surface properties from hydrophobic to hydrophilic. However,
it was found that silicate is not required for the rag layer volume
reduction; these results may suggest that solids present in rag
layer may not have originated from hydrophobic solids and they may
be from the organic rich solids like humic matter. Water addition
to the rag layer was hypothesized to remove the converted
hydrophobic clay solids. Since the hypothesized hydrophilic clay
solids do not seem present, addition of rag water is, therefore,
not required. Both the scrubber feed mixing and rag layer mixing
are used to break the emulsion and, hence, reduce the rag layer
volume. The use of low scrubber N/B ratio prevents the
precipitation of asphaltenes, hence, the rag emulsion is weaker and
rag volume can be easier to break down by shear through mixing.
Example 2
[0055] An experimental condition was conducted to determine the
impact of higher rag layer mixing on rag layer volume reduction.
The only variable that changed was to increase the rag layer mixer
speed to 188 rpm. All other rag layer mitigation variables were set
at base case flow sheet conditions, i.e., no silicate or rag water
addition, low scrubber feed mixer speed and high scrubber N/B ratio
of greater than or equal to 6.
[0056] The rag volume comparison for the three rag mixer speeds is
shown in Table 3. The rag layer volume at rag mixer speed of 52 rpm
is not significantly different from the rag layer volume at rag
layer mixer speed of 188 rpm. However, the rag layer volume at base
case condition is significantly different from the rag layer volume
at rag mixer speeds of both 52 rpm and 188 rpm.
TABLE-US-00003 TABLE 3 Rag Variable Volume, ml Base case 740 Base +
rag layer mixing at 52 rpm 249 Base + high rag layer mixing at 188
rpm 273
The results in Table 3 show that rag mixing (52 to 188 rpm) did
significantly reduce the scrubber rag layer volume compared with
the base case. A higher rag mixer speed did not appear further
reduce the rag layer volume. The impact of the shear on the
scrubber rag layer was able to reduce the rag layer volume only to
a certain extent. Other variables to minimize the formation of
emulsion stabilizer, such as scrubber N/B ratio, are also important
in the rag layer volume reduction.
[0057] The scope of the claims should not be limited by the
preferred embodiments set forth in the examples, but should be
given the broadest interpretation consistent with the description
as a whole, wherein 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". All structural and functional equivalents to the elements of
the various embodiments described throughout the disclosure that
are known or later come to be known to those of ordinary skill in
the art are intended to be encompassed by the elements of the
claims. Moreover, nothing disclosed herein is intended to be
dedicated to the public regardless of whether such disclosure is
explicitly recited in the claims.
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