Note: Descriptions are shown in the official language in which they were submitted.
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SILICA INHIBITION AND BLOWDOWN EVAPORATION (SiBE) PROCESS
FIELD OF THE INVENTION
Embodiments of the invention are related to processes for the
treatment of boiler feed water to minimize scaling and more particularly to
the
removal of silica from boiler feed water used for steam generation for use in
heavy
oil extraction.
BACKGROUND OF THE INVENTION
As water is heated and converted into steam, contaminants brought
into a boiler tend to be left in the boiler. The boiler functions as a
distillation unit,
taking pure water out as steam, and leaving behind concentrated minerals and
other contaminants in the boiler. Scale forms as a result of the precipitation
of
normally soluble solids that become insoluble as temperature increases. Some
examples of boiler scale are calcium carbonate, calcium sulfate, and calcium
silicate. Scale acts as an insulator, reducing boiler efficiency. Scaling can
lead to
boiler tube failure due to overheating.
A variety of different methodologies have been employed to attempt to
remove contaminants from boiler feed water including chemical precipitation
such
as lime softening, ion exchange, reverse-osmosis, electrodialysis,
distillation and
freezing.
Typically, large amounts of very high quality steam are required for in-
situ heavy oil and oil sands recovery processes, such as steam assisted
gravity
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drainage (SAGD). There is an ever-increasing concern regarding the effects of
such
techniques on water supplies and on the discharge therefrom of water which may
be heavily laden with said contaminants, such as large quantities of silica.
There is
great interest in recycling of water for use in the steam generation processes
such
as using boiler blowdown as at least a partial feed stream to a steam
generator for
the steam generation process.
Canadian Patent Application 2475048 to Total SA teaches use of
silica inhibitors to reduce the need for a warm lime softening (WLS) process
used to
treat feedwater by about 25%. The reduction results in reduced costs due to
reduced chemical requirements and increased boiler efficiency, particularly
related
to a once-through steam generator (OTSG). The addition of inhibitors to the
water
output from the WLS permits the product water from the WLS to have a higher
silica
content, the inhibitors being added to the product water prior to being
introduced to
the boiler. The silica inhibitors act to inhibit amorphous silica from
polymerising into
colloidal silica (groups of SiO2(OH),,; where n>8) which adheres to the boiler
tubes
creating silica scaling that may lead to tube failure.
There is an ongoing interest in the industry to reduce the capital
expenditure and the overall operating costs associated with improving the
quality of
water used for steam generation for processes such as SAGD and which would
permit at least some recycling of water in the processes, which would
otherwise
contain unacceptable levels of silica and other contaminants.
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BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1A is a schematic illustrating a method of steam generation
according to en embodiment of the invention;
Figure 1 B is a schematic illustrating an alternate method according to
Fig. 1 A;
Figure 2 is a schematic illustrating an embodiment according to Fig.
1A further comprising a crystallization unit;
Figure 3 is a graph illustrating silica composition evolution in the
produced water of Example B of Table A over time;
Figure 4 is a schematic illustrating an embodiment of an
evaporation/crystallization process for treating the blowdown stream; and
Figure 5 is a schematic illustrating a mass balance according to an
embodiment of the invention.
SUMMARY OF THE INVENTION
The invention relates to a method for producing steam comprising:
- providing feedwater containing silica;
- supplying the feedwater to a boiler;
- adding an inhibitor of silica deposition to the feedwater prior to
supplying the feedwater to the boiler;
- generating steam and a liquid blowdown in the boiler;
- treating at least part of the liquid blowdown to produce a diluent
water stream;
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- adding at least part of the diluent water stream to the feedwater
before supplying the feedwater to the boiler;
- wherein the silica content of the diluent water stream is below the
silica content of the feedwater to which it is added.
According to an embodiment, the inhibitor of silica deposition is
selected from the group consisting of high molecular weight organic polymers
such
as derivatives of phosphovinyl-sulfonic acids and their salts, organic acids
such as
citric acid, maleic acid, formic acids, lactic acids, phosphino-carboxylic
acids and
their salts, inorganic compounds such as boric acid, hydrofluoric acid and
their
salts, borax, sodium aluminates and sodium chlorates, as well as mixtures
thereof.
According to an embodiment, the boiler is a once through steam
generator.
According to an embodiment, the boiler generates approximately 1-50
wt% liquid blowdown and 50-99 wt% steam, preferably approximately 5-40 wt%
liquid blowdown and 60-95 wt% steam, most preferably approximately 15-30 wt%
liquid blowdown and 70-85 wt% steam.
According to an embodiment, the treatment of the liquid blowdown is
performed by decantation and/or by evaporation and/or crystallization.
According to an embodiment:
- the silica content of the feedwater prior to addition of the diluent
water stream is equal to or less than approximately 400 mg/L,
preferably is approximately comprised between 300 and 400 mg/L,
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most preferably is approximately comprised between 333 and 400
mg/L; and/or
- the silica content of the liquid blowdown prior to treatment is
above approximately 400 mg/L, preferably above approximately
5 1000 mg/L, most preferably above approximately 1600 mg/L;
and/or
- the silica content of the diluent water stream is below
approximately 200 mg/L, preferably below approximately 50 mg/L,
and most preferably the diluent water stream is substantially silica-
free.
According to an embodiment, there is no reduction of the silica content
of the feedwater prior to adding the diluent water stream, and there is no
reduction
of the silica content of the feedwater after adding the diluent water stream.
According to an embodiment, the abovementioned method comprises
storing the feedwater in a storage tank prior to supplying it to the boiler.
According to an embodiment, the inhibitor of silica deposition is added
to the feedwater after storing the feedwater in the storage tank and before
supplying
to the boiler.
According to an embodiment, the inhibitor of silica deposition is added
to the feedwater before storing the feedwater in the storage tank.
According to an embodiment, the abovementioned method comprises
reducing the hardness of the feedwater prior to supplying it to the boiler,
and
preferably prior to storing the feedwater in the storage tank.
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According to an embodiment, the treatment of the liquid blowdown is
performed by decantation and the diluent water stream is added to the
feedwater
before reducing the hardness of the feedwater.
According to an embodiment, the treatment of the liquid blowdown is
performed by evaporation and/or crystallization and the diluent water stream
is
added to the feedwater after reducing the hardness of the feedwater.
According to an embodiment, the feedwater is produced water from
heavy oil recovery processes, such as steam assisted gravity drainage, said
produced water being preferably de-oiled.
The invention also relates to an installation for producing steam
comprising:
- a boiler;
- a feedwater supply system connected to an inlet of the boiler;
- an inhibitor of silica deposition supply system, an outlet of which is
connected to the feedwater supply system;
- a steam conduit connected to an outlet of the boiler;
- a liquid blowdown conduit connected to an outlet of the boiler; and
- a treatment unit for reducing the silica content of water, an inlet of
which is connected to the liquid blowdown conduit and an outlet of
which is connected to the feedwater supply system.
According to an embodiment, the boiler is a once through steam
generator.
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According to an embodiment, the treatment unit comprises a decanter
and/or an evaporator and/or a crystallizer.
According to an embodiment, the abovementioned installation does
not comprise any lime softening unit.
According to an embodiment, the feedwater supply system comprises
a hardness reducing unit, which is preferably an ion exchange system.
According to an embodiment, the treatment unit comprises a decanter
and the outlet of the treatment unit is connected to the feedwater supply
system
upstream the hardness reducing unit.
According to an embodiment, the treatment unit comprises an
evaporator and/or a crystallizer and the outlet of the treatment unit is
connected to
the feedwater supply system downstream the hardness reducing unit.
According to an embodiment, the feedwater supply system comprises
a feedwater storage tank.
According to an embodiment, the outlet of the inhibitor of silica
deposition supply system is connected to the feedwater supply system upstream
the feedwater storage tank.
According to an embodiment, the outlet of the inhibitor of silica
deposition supply system is connected to the feedwater supply system
downstream
the feedwater storage tank.
The invention also relates to a process for extracting hydrocarbons
from a subterranean formation comprising:
- producing steam according to the abovementioned method;
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- injecting the produced steam into at least one injection well;
- recovering hydrocarbons and produced water from at least one
extraction well;
- de-oiling the produced water and using the de-oiled produced
water as feedwater in the steam production method.
Hydrocarbons which may be extracted according to the present
invention include in particular bitumen and heavy oils.
DESCRIPTION OF EMBODIMENTS
The invention will now be described in more detail without limitation in
the following description.
Embodiments of the invention permit boiler feedwater having an
increased silica content to be used to produce steam in a boiler such as a
conventional boiler or a once through steam generator (OTSG) as a result of
the
combination of the addition of silica inhibitors to the water entering the
conventional
boiler or the OTSG and the effect of adding a substantially silica free or
reduced
silica stream of water to the feedwater. An OTSG has a lower steam yield than
a
conventional boiler. Embodiments of the invention eliminate the prior art
requirements for treatment of the feedwater with warm lime softening and also
reduce scaling in the boiler. One source of feedwater for steam production is
produced water from heavy oil recovery processes, such as steam assisted
gravity
drainage (SAGD), the produced water having been de-oiled following production
using conventional technology. A SAGD recovery process typically comprises at
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least one injection well and at least one extraction well. Steam is injected
though
the at least one injection well for recovering hydrocarbons and produced water
from
the at least one extraction well.
In an embodiment of the invention, the permissible silica content of the
feedwater directed to the conventional boiler or to the OTSG can be increased
from
an upper limit of about 100 mg/L known in the prior art for conventional
boiler
processes, to about 333 mg/L in embodiments utilizing a recycling of about 20%
blowdown from an OTSG. Further, Applicant believes the maximum permissible
silica content, according to some embodiments of the invention, may be as high
as
about 400 mg/L with a 20% to 25% blowdown. Increased dilution with silica-free
water from the evaporator or increased amounts of inhibitor or a combination
of the
two may be used if silica concentrations in the produced water exceed 400
mg/L.
In an embodiment of the invention (Fig. 1A), an installation comprising
a feedwater supply system is connected to the inlet of a conventional boiler
or
OTSG. The feedwater supply system can comprise a de-oiling system, an ion
exchange unit or ion exchange system, and a boiler feedwater storage tank. The
ion exchange unit can serve as a hardness reducing unit.
Blowdown from the conventional boiler or OTSG has a high silica
content and can be flowed to a silica treatment unit, such as a blowdown
evaporator, for reducing the silica content of water. An inlet of the silica
treatment
unit is connected to the liquid blowdown and an outlet of which is connected
to the
feedwater supply system. A distillate water produced from the evaporator
provides
a substantially silica free diluent water stream which is recycled into the
feedwater
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stream following an ion exchange unit or before the ion exchange unit and
prior to
or after the addition of the silica inhibitors. Thus, embodiments of the
process permit
recycling of the blowdown water.
In another embodiment of the invention (Fig. 1 B), a substantially silica-
5 free water from a source other than the evaporator is added to the feedwater
stream
for diluting the feedwater stream.
Optionally, as shown in Fig. 2, a combined evaporation and
crystallization unit may be used to permit land-filling of the silica and
other
contaminants produced by the evaporation process.
10 As an alternative to the evaporation and optionally crystallization unit,
or in addition to that unit, use may be made of a decanter wherein solid
silica, as
well as other suspended solids, is separated from the bulk of the liquid
blowdown
owing to gravity. When only a decanter is used for converting the liquid
blowdown to
the diluent water stream, the diluent water stream must generally be added to
the
feedwater upstream the ion exchange unit. When an evaporation (and optionally
crystallization) unit is used for converting the liquid blowdown to the
diluent water
stream, the diluent water stream can generally be added to the feedwater
downstream the ion exchange unit, since the evaporator removes hardness as
well
as silica.
Having reference to Figs. 1A and 113, and in an embodiment of the
invention, such as in a process for extracting hydrocarbons from a
subterranean
formation, a high quality steam is produced, for injection through at least
one
injection well and into a heavy oil or bitumen deposit using the conventional
boiler or
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the once-through steam generator (OTSG). Steam is produced from a steam
conduit connected to a first outlet of the boiler. Desired steam quality is
typically
greater than 70% and more preferably greater than 75%. Feedwater for the
process is typically de-oiled produced water from a recovery wellbore. The
produced water is typically de-oiled using conventional technology.
Hydrocarbons
and produced water are recovered from at least one extraction well.
Table A illustrates typical ionic constituents of samples of produced
water from three different SAGD operations.
TABLE A
EXAMPLE A EXAMPLE B EXAMPLE C
CATIONS m /1 m /I m /I
Sodium, Na+ 2200.0 706.0 780.0
Calcium, Ca + 2.7 3.9 22.0
Magnesium, M+ 6.1 0.3 11.0
Barium, Ba + 0.0 0.2 0.0
Potassium, K' 62.0 28.7 13.0
iron, Fe 1.9 0.7 0.3
otal Cations 2273.0 740.0 826.3
NIONS m /I m /I m /I
Chloride, CI- 1697.0 930.0 962.0
Sulfate, SO4 - 56.0 1.0 58.0
Carbonate, C03 108.0 1.0 0.0
Bicarbonate, HCO " 2677.0 332.0 489.0
otal Anions 4538.0 1264.0 1509.0
otal Dissolved Solids: 6951.0 2370.0 2585.0
Hardness Ca ', M 32.0 11.0 100.9 m/I as CaCO3
H 25 C : 7.9 8.0 7.7
Silica 260.0 260.0 250.0 m/I
Fig. 3 illustrates the silica composition evolution in the produced water
of Example B over time. According to this graph, the maximum value of silica
achieved is about 400 mg/I.
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Having reference to Figs. 1A, 1B and 2, the produced, de-oiled water
is mixed with sufficient make-up water to meet the feedwater demands of the
conventional boiler or the OTSG. The combined stream of produced water and
makeup water is passed through the ion exchange unit comprising an ion
exchange
resin, in many cases a weak acid cation exchange resin (WAC), for
demineralization of the feed water and typically also for removal of hardness
which
may aid in reducing the amount of blowdown required from the boiler. Removal
of
Ca2+ and Mg2+ ions further prevents the formation of silicates therewith which
form
deposits in the boiler. The diluted, treated feedwater is typically stored in
a storage
tank for use in the conventional boiler or the OTSG.
An injection skid or other suitable means, as an inhibitor of silica
deposition supply system, is typically positioned downstream from the diluted,
treated feedwater storage and upstream from the conventional boiler or the
OTSG
for the addition of silica inhibitors as described in Canadian Patent
Application
2,475,048 to Total SA. Alternatively, this injection skid may be positioned
anywhere
upstream the boiler, i.e. either upstream or downstream the feedwater storage,
either upstream or downstream the ion exchange unit (if present), either
upstream
or downstream the addition of diluent water stream. A liquid blowdown conduit,
connected to a second outlet of the boiler, typically from a separator used to
separate water and steam produced from the conventional boiler or OTSG, is
directed to an evaporator where the blowdown water is distilled to produce a
substantially silica free distillate stream and a concentrated silica waste
stream
which is discharged therefrom.
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Certain low molecular weight organic acids, certain alcohols, and ions,
such as fluorides, borates and the like, as well as specific conditions such
as
alkaline or very acidic pH are known to deteriorate glass and silicates. In
embodiments of the invention, the inhibitors of silica deposition used are
selected
from among compound families known to inhibit the deposition of silica
carbonates.
Preferably, the compounds used include high molecular weight organic polymers
such as derivatives of phospho-vinyl-sulphonic acids and their salts; organic
acids
such as citric acid, maleic acid, formic acids, lactic acids, phosphino-
carboxylic
acids, and their salts; inorganic compounds such as boric acid, hydrofluoric
acid
and their salts; borax, sodium aluminates and sodium chlorates. The compounds
are used alone or in mixtures thereof.
In an embodiment of the invention, the inhibitor comprises a
combination of organic inhibitors utilizing phosphor-vinylsulfonic acid or a
polyvinylsulfonate as the principal active ingredient in combination with a
mixture of
one or more organic acids, typically citric acid, maleic acid, formic acid or
lactic acid.
Further, salts of the organic acids may be used to maintain a pH closer to
neutrality.
Ammonium salts of formic acid and citric acid are not used.
In one embodiment of the invention, citric acid and maleic acid are
added to phosphovinylsulfonic acid to obtain a thermally stable inhibitor. In
one
embodiment, the weight ratio of citric acid to maleic acid is about 10/1, the
% by
weight of citric acid being between about 3% to about 10% and preferably about
5%.
The inhibitors, used alone or in combination, act to prevent the
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coalescence of monomers of (OH)3-Si-O-Si(OH)3, and leave polymers of
SiO2(OH)n,
wherein n is equal to less than 8, in suspension. The inhibitors are capable
of
performing inhibition functions at temperature of 300 C or greater which is
particularly advantageous when used for steam generation in amounts ranging in
some embodiments from between about 2 ppm to about 2000 ppm, in some
embodiments from between about 3 ppm to about 100 ppm and in some
embodiments from about 5 ppm to about 50 ppm. Use of the inhibitors, according
to
embodiments described in Canadian Application 2475048 to Total SA, permits the
use of feedwater recycled from blowdown and having silica concentrations which
are 4 to 5 times the permissible concentration known in the prior art without
substantial silica deposition in the conventional boiler or the OTSG.
The amount of inhibitor used may be determined depending upon the
pH of the feedwater as pH controls the solubility of silica, the silica being
more soluble
in very alkaline conditions or in certain acidic conditions, such as in the
presence of
some acids, such as hydrofluoric acid. Having reference to Figs. 1A and 2 and
in
embodiments of the invention, approximately 20 to 25% of the total water
output
from the boiler is recycled as a substantially silica-free distillate stream
and is added
to the feedwater stream downstream from the ion exchange. In embodiments of
the
invention, the amount of total water output from the boiler that is recycled
may be
increased to about 40% as required.
Having reference to Fig. 1 B, substantially silica free water from a
source other than a recycle evaporator is added to the feedwater stream
downstream from the ion exchange.
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The feedwater is diluted so as to reduce the silica concentration in the
feedwater such that, in combination with the addition of inhibitors according
to
embodiments of the invention and without the need for warm lime softening,
there is
a significant reduction in deposition of silica in the conventional boiler or
in the
5 OTSG. Thus, embodiments of the invention achieve substantial inhibition of
silica
deposition in a cost effective manner.
In embodiments of the invention therefore, a method is provided for
producing steam from feedwater containing silica comprising receiving a
substantially oil-free feedwater stream, treating the feedwater stream with
ion
10 exchange for substantially reducing the concentration of ions contributing
to
hardness of the feedwater, introducing a substantially silica-free stream to
the
feedwater for diluting the feedwater for reducing the silica concentration
therein,
adding an effective amount of inhibitors for minimizing formation of colloidal
silica
and generating steam and a blowdown water from the feedwater in a conventional
15 boiler or in a once through steam generator OTSG, the blowdown water being
subsequently evaporated or concentrated for producing at least a concentrated
silica stream for disposal thereof.
In embodiments of the invention, the substantially silica-free stream
which is recycled to the feedwater stream is produced by the evaporation or
concentration of the blowdown water.
Applicant believes that the maximum silica concentration in produced
water is typically about 400 mg/L or less and the dilution of the feedwater,
being the
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substantially de-oiled produced water, results in a silica concentration
entering the
boiler of about 333 mg/L or less.
In embodiments of the invention, the effective amount of inhibitor is
typically determined dependent upon the pH of the feedwater.
For example, as shown in Figs. 1A, 1B and 2, the effect of the dilution
of the feedwater stream reduces the silica concentration in a produced/de-
oiled
water from a maximum of about 400 mg/L to a maximum of about 333 mg/L and
therefore the inhibitors added downstream are required to inhibit a maximum of
about 333 mg/I of silica in the boiler feedwater. Additional WLS is therefore
not
required as the process according to embodiments of the invention, which
combine
inhibitors and dilution of the feedwater, is capable of handling initial feed
streams
having higher silica concentrations without scale buildup. Similarly,
according to the
present invention, there is preferably no treatment of the feedwater in an
evaporator
either.
Further, water can be routed directly from the ion exchange
regeneration to the evaporator for distillation for forming at least a portion
of the
substantially silica-free stream which is subsequently recycled to the
feedwater
stream downstream from the ion exchange.
Embodiments of the invention are limited to produced and source or
make-up water having a hardness below about 200 mg/L as CaCO3 and more
preferably below about 100 mg/L. Preferably, the produced water also has a low
salinity, having a total dissolved solids (TDS) content of less than about
10,000
mg/L.
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As shown in Fig. 2 and in an embodiment of the invention, the
concentrated silica stream from the evaporator may be directed to a
crystallization
unit wherein the concentrated silica stream, produced by evaporation, is
crystallized
for disposal in landfill or other suitable disposal site.
Having reference to Fig. 4, a conventional evaporation/crystallization
process is shown for processing the blowdown stream.
Having reference to Fig. 5 and in an example of an embodiment of the
invention, a feedwater from an ion exchange, having a silica concentration of
less
than or equal to 400 mg/L, is diluted with a substantially silica-free diluent
stream
derived from the evaporator as evaporator blowdown, the resulting diluted
feedwater stream having a concentration of less than or equal to 333 mg/L
silica.
The diluted feedwater is stored for use in the conventional boiler or the
OTSG.
Inhibitors are added upstream from the conventional boiler or the OTSG in an
amount sufficient to inhibit about 270 mg/L of silica for preventing
deposition in the
conventional boiler or the OTSG, the remaining 63 mg/L silica being within the
tolerance limits of the conventional boiler or the OTSG. The produced steam
and
water from the boiler is passed to a separator which separates the steam from
the
water containing impurities. Approximately 20-25% of the water blown down to
an
evaporator wherein the blowdown stream is further distilled to a silica-free
distillate
for recycling to the feedwater as diluent and a highly silica-concentrated
stream
which is crystallized or otherwise treated for disposal.
Optionally, a freezing crystallization unit (not shown) may replace the
evaporation crystallization unit.
