Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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NS-408
METHOD FOR REDUCING RAG LAYER VOLUME IN STATIONARY FROTH
TREATMENT
FIELD OF THE INVENTION
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
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).
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".
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
"PS V"). 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
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containing a small amount of bitumen. Middlings, a watery mixture containing
fine solids and
bitumen, extend between the froth and sand layers.
The wet tailings and middlings are separately withdrawn, combined and sent to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.
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.
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.
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. Patent 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
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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.
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.
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
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:
(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.
(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.
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(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.
(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.
(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.
Thus, use of the present invention may improve bitumen recovery and product
quality by
effectively reducing the rag layer volume.
In one aspect, a method of reducing rag layer volume in a stationary bitumen
froth
treatment process is provided, comprising:
')0
= 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/bitumen ratio of less than about 6:1; 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.
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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.
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.).
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
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:
FIG. 1 is a graph showing, in general, one embodiment of a bitumen froth
treatment
process useful in the present invention.
FIG. 2 is a graph showing the rag layer volume (mL) for each test condition.
FIG. 3 is a graph showing the rag layer solids content (mass %) for each test
condition.
FIG. 4 is a graph showing the rag layer water content (mass %) for each test
condition.
FIG. 5 is a graph showing the rag layer bitumen content (mass %) for each test
condition.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
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.
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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.
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
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.
A hydrocarbon diluent 12 is mixed with bitumen froth 10 in a suitable mixer 14
to
10 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.
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 xNa20.ySi02. 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.
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.
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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).
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 clemulsifier 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.
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.
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
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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.
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.
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).
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
The flow sheet used for the evaluation of the rag volume reduction is
essentially the same
as that shown in Figure 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 25-1
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.
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Table I: Test Matrix
Water Rag Layer
SilicatesScrubber Feed Scrubber
Condition Addition Agitation
RPM N/B
(g/min) RPM
_______________ 1 0 0 0 700 >6
7 0 16 52 700 >6
3 0.1 0 52 700 >6
4 _ 0.1 16 0 700 >6
_
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
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
_ , _
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
j 0.1 ' 16 52 700 <3
In this evaluation, controlling the rag layer growth in the scrubber vessel is
the primary
5 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
10 layer's volume and not its composition. Thus, the experimental design
evaluates the effect of the
five variables on rag layer volume.
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
Figure 2. Five
15 conditions (9, 10, 14, 17 and 19) produced the largest rag volume,
wherein all five conditions did
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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.
Rag layer composition appears to be more variable among the various conditions
tested,
as shown in Figures 3, 4, and 5. The results appear to indicate that the rag
layer bitumen content
increased when the scrubber N/B was lowered (Figure 5). Otherwise, no
particular trend of the
composition with operating conditions is observed.
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
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 I is:
Y1 = 310 ¨57 X? ¨110 X3+ 65 XIX3 + 47 XiX4 + 72 X?X3 ¨ 82 X2X5 R2 = 0.91
Note that Xi 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 R2 of 91%, which means that 91% of the data variation
can be explained
by the model. Among these effects, rag layer mixing (X3) 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:
X1X3: 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.
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X X4:
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.
X2X3: 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 (X2, X3, and X2X3)
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.
X2X5: 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.
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
70010 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
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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 2
Variable Rag volume,
mL
Base case (no rag mixing and scrubber feed mixing at 700 rpm) 743
Base case + Scrubber feed mixing (1300 rpm) 649
Base case + rag mixing (52 rpm) 249
Base case + rag mixing (52 rpm) + Scrubber feed mixing (1300 rpm) 155
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
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
1?
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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.
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 3
Variable Rag
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.
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
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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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