Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR REDUCING SILICA IN WATER
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for reducing silica in
water by
contacting silica-containing water with a high surface-area metal to form a
metal/silica
complex and silica-depleted water. The process of the invention is suitable
for many
applications where silica-depleted water is required including steam-based
bitumen recovery
operations.
BACKGROUND OF THE INVENTION
In many industries and industrial applications, removal of silica from water
is
required. In particular, silica removal is desired in those industries and
applications when
silica-contaminated water comes into contact with equipment in a manner that
results in
scaling of equipment or those industries where such water is returned to the
environment.
In the oil industry, silica removal is required for water used in steam-based
heavy oil
recovery operations. In steam-based heavy oil recovery operations, steam is
injected into an
oil-bearing reservoir to reduce the viscosity of the heavy oil. Once in the
reservoir, the
condensed steam reacts with the reservoir sand and minerals, and becomes rich
in silica and
hardness ions including calcium and magnesium. The condensed steam is
recovered as
produced water with the heavy oil as an oil-in-water mixture. The heavy oil is
separated from
the produced water and the produced water is returned to a boiler for further
steam generation.
The silica and hardness minerals must be removed prior to steam generation to
prevent
scaling in the boiler. At the present time, silica-removal processes using
magnesium oxide are
the processes of choice in a commercial cyclic steam stimulation (CSS) process
for recovering
bitumen from Alberta Oil Sands. However, this silica-removal method represents
a significant
portion of the total cost of water treatment.
In other examples, such as in the geothermal power industry, water is
similarly cycled
between underground systems where the water becomes contaminated with silica
and above
ground power generation equipment. Accordingly, it is also required that this
water be
cleaned of silica prior to contact with the power generating equipment.
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In the nuclear power industry, low-silica water is also required for water
coming into
contact with the power generating equipment and in the semi-conductor
industry, low silica
water is required to wash silicon wafers.
In addition to the standard method of removing silica from water as presently
used in
steam-based heavy oil recovery operations, other methods have been proposed to
reduce silica
in water. For example, Iler (Iler, R.K., The Chemistry of Silica, Wiley-
Interscience
Publication, 1979) and Betz (Betz Handbook of Industrial Water Conditioning,
8'h Edition,
Betz Laboratories, 1980) describe the use of hydroxides of magnesium,
aluminium and
calcium to remove silica. Wibowo et al ("The Measurement and Removal of
Dissolved and
Colloidal Silica in Ultrapure Water", Semiconductor Pure Water and Chemicals
Conference,
1997) teach a reverse osmosis technique with a semi-permeable membrane that
prevents
dissolved silica from passing across the membrane and allows water molecules
to pass
through. Leaf ("Silica Removal with Iron Shavings", J. Am Water Works Assoc.
(1948), 40,
980-988) describes a process using iron shavings to reduce silica content in
water and Mindler
et al. (Mindler, A. B., and Bateman, S. T. "Sidestream Treatment of High
Silica Cooling
Water and Reverse Osmosis Desalination in Geothermal Power Generation",
Department of
the Interior, Office of Water Research and Technology, Contract No. 14-34-0001-
9527,
1981) also describe a process of reducing silica content by rusting iron
shavings.
A review of the patent literature also indicates that various methods for
silica removal
have been utilized in various industries and for various applications.
For example, US Patent 3,458,438 discloses the use of strong-base, anion-
exchange
resin to remove silica, US Patent 5,351,523 discloses the use of a filter to
remove colloidal
silica from ultra-pure water used in semi-conductor wafer cleaning and US
Patent 4,046,684
describes a process for the treatment of a colloidal suspension. Other
examples are US Patent
4,405,463 that describes a process for stabilizing silica-rich geothermal
brine to prevent silica
scaling using ferric ions to precipitate the siliceous material and US Patent
5,453,206 that
describes a process for removing silica from aqueous liquors using a
precipitant or absorbent.
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US Patent 1,860,781 discloses the use of metal oxide gels to remove silica
from water.
In this patent, the gels are in dried granular form and water is percolated
through a pervious
bed of granular hydrated metal oxides. When the bed becomes charged with
silica it must be
regenerated by treatment with a weak alkali solution to extract the silica and
render the
granules ready for re-use.
US Patent No. 1,992,532 discloses the removal of silica from alkaline brines
by
adding the brine to a suitable quantity of magnesium compound. The solution is
agitated at a
high temperature and the magnesium compound reacts with the silica to form a
precipitate.
The precipitate is then separated out by filtration or sedimentation.
US Patent No. 2,194,524 discloses the treatment of water with a gelatinous
precipitate
of aluminum hydroxide or hydrate. This patent discloses regenerating aluminum
hydrate and a
cyclic method of continuously operating the process. The freshly precipitated
aluminum
hydroxide is added as a gelatinous mass to the water, the water is agitated
and then the water
is removed by filtration or decantation.
US Patent No. 4,016,075 discloses a method of water treatment whereby aluminum
hydroxide is added to a mixture of high temperature, high pressure, geothermal
steam and
brine to raise the pH of the brine and precipitate out aluminum hydroxide
sludge which then
sweeps out enough dissolved silica to prevent silica scale formation.
US Patent No. 4,276,180 discloses a method for industrial wastewater reuse.
Silica is
removed from the water by chemically conditioning water over active alumina
and then
returning the water to an existing cooling water system. A continuously
diverting sidestream
is used to allow for constant circulation. The alumina is reactivated by
washing with a dilute
base, followed by washing with a dilute acid.
US Patent No. 4,378,295 discloses the desilication of geothermal water using a
fluidized particle bed. Silica is deposited on the bed and then the coated
particles are removed
from the system. The coatings are removed and the particles are recycled to
the fluidized bed.
The fluidized particle bed can be made up of any hard material with a specific
gravity of less
than that of water and one that allows the continuous flow of geothermal water
through the
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fluidized bed and the continuous treatment of the particles to remove the
silica coating that
forms.
US Patent No. 4,405,463 discloses a process for stabilizing silica-rich
geothermal
brine. This patent teaches the use of dissolved ferric ions that combine with
the silica to form
an insoluble material. The insoluble material is then separated from the
liquid.
US Patent No. 5,609,765 discloses a steam stripping method for softening of
water. A
feedstream of water is passed into intimate contact with uncondensed steam
that elevates the
temperature of the water and increases the pH leading to the production of
uncondensed steam
byproducts and liquid byproducts.
US Patent No. 5,965,027 discloses a method to treat wastewater with a chemical
coagulant to create spherical clusters having a diameter of greater than 5
microns. After the
addition of the chemical coagulant, the water is passed through a
microfiltration membrane
that physically separates coagulated silica particles.
US Patent No. 6,051,141 discloses a method of silica removal whereby the water
is
contacted with a fine powder of anhydrous alumina cement that entrains silica-
containing
impurities. The water is subsequently separated out by filtration.
US Patent No. 6,416,672 discloses the removal of dissolved and colloidal
silica
through the use of small amorphous silica particles. The amorphous particles
are incorporated
into a sidestream reactor in an existing cooling tower flow-through system.
Accordingly, there is a need for an improved process for removing silica from
water.
Specifically, there is a need to remove silica from silica-contaminated water
by passing the
water through a vessel that removes at least part of the silica. This
invention satisfies that
need.
SUMMARY OF THE INVENTION
The invention is directed to a process for removing silica from the water by
passing
contaminated water through a vessel full of a high surface area metal with the
result that water
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passing from the vessel has a lower level of silica contamination than the
water flowing into
the vessel.
The high surface area metal is preferably a thin fiber metal matrix such as
steel wool.
The silica reduction system can be incorporated into existing water treatment
plants and can
be applied to manufacturing fields and other fields where low silica content
water is desirable.
In particular, the process of the invention can be applied to the petroleum
industry,
particularly in steam-based bitumen recovery, as well as potentially in the
nuclear, geothermal
and semiconductor fields.
More specifically, the invention provides a method of reducing silica in
water. In one
embodiment, the method comprises contacting silica-containing water with a
high-surface
area metal to form a metal/silica complex and silica depleted water.
In a second embodiment, the invention provides a method of reducing silica in
silica-
contaminated water from a steam-based bitumen recovery process comprising:
providing
water contaminated in silica; passing the provided water contaminated in
silica through a
reaction vessel containing an effective amount of steel wool to produce silica-
depleted water;
and, removing the silica-depleted water. In this embodiment, the water
contaminated in silica
may be from a bitumen reservoir and the removed silica-depleted water may be
returned to a
boiler for steam generation and steam injection to the bitumen reservoir.
In a third embodiment, the invention provides an apparatus for reducing silica
in
silica-contaminated water comprising at least one vessel packed with steel
wool, the vessel
having an inflow port and an exit port at opposing positions in the vessel
whereby silica-
contaminated water flowing between the inflow port and exit port contacts the
steel wool.
In a fourth embodiment, the invention provides an apparatus for reducing
silica in
silica-contaminated water comprising: at least two vessels packed with steel
wool, each vessel
having an inflow port with an inflow valve and an exit port with an outflow
valve whereby
silica-contaminated water may be selectively passed through each vessel by
selective
operation of respective inflow and outflow valves.
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BRIEF DESCRIPTION OF THE DRAWINGS
The invention is described with reference to the following drawings wherein:
Figure 1 is a schematic diagram of the reaction chemistry in accordance with
the
invention.
Figure 2 is a graph showing silica-reduction of silica-contaminated water
through
successive treatments.
Figure 3 is a schematic diagram of a plant providing a continuous flow
treatment of
silica-contaminated water.
Figure 4 is a schematic diagram of the integration of the silica-reduction
equipment in
accordance with the invention into a steam-based bitumen recovery operation.
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DETAILED DESCRIPTION OF THE INVENTION
With reference to the figures and the following detailed description, a method
and
apparatus for removing silica from water is described in connection with its
preferred
embodiments. However, to the extent that the following description is specific
to a particular
embodiment or a particular use of the invention, this is intended to be
illustrative only.
Accordingly, the invention is not limited to the specific embodiments
described below, but
rather, the invention includes all alternatives, modifications, and
equivalents falling within the
true scope of the appended claims.
Process Overview
Generally, the method of the invention contacts water contaminated with
soluble or
colloidal silica with a high surface area metal matrix, such as steel wool,
with the result that
the silica within the contaminated water reacts with the steel wool according
to the reaction as
shown in Figure 1. More specifically, it is believed the metal reacts at basic
pH levels with
hydroxide to form a metal hydroxide, which then reacts with silica to form a
metal/silica
complex and water with the result that water flowing from the matrix is lower
in silica
contamination. The metal is preferably steel wool but may include other
reactive metals
including aluminum, copper and zinc, and metal alloys including brass, bronze
and any
combination thereof. Furthermore, the reaction is reversible enabling the
metal matrix to be
regenerated.
As shown in Figure 1, the reaction is a condensation reaction catalyzed by the
hydroxyl ("OH") groups present in alkaline water. In the case of steel wool,
the iron oxides on
the surface of the steel wool pick up OH groups from the water enabling the
surface to react
with soluble silica. Once a receptive surface is formed, more deposition of
silica on silica will
take place. The silica is therefore removed from the water by surface capture
with the silica
being deposited on the solid surface of the steel wool by collision and
combination with the
solid surface. Fine woofs including commercial grades 0000, 000, 00, 0, 1, 2,
3 and 4 are all
acceptable wherein the finest grade (Grade 0000) containing no fibers greater
than 0.00127
centimeters (0.0005 inches) thick has the highest surface area and is the most
effective for
silica removal. The steel wool is available from commercial suppliers.
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The reaction kinetics are affected by temperature, pressure, and pH.
Experimentally, it
has been shown that in order to maximize the amount of silica removal, the
temperature of the
water should be within the preferred range of 20 degrees Celsius to 100
degrees Celsius at
atmospheric pressure. The upper limit of the temperature may be increased with
a
corresponding increase in pressure. Also, for most effective silica removal,
the water should
preferably be slightly alkaline having a pH in the preferred range of 8 to 9.
Furthermore, silica
is removed more efficiently under anaerobic conditions and thus this method is
preferably
performed under anaerobic conditions.
After time, the silica removal reaction will slow down as the number of
reaction sites
or surface areas of the reaction sites is reduced. The steel wool may then be
regenerated by a
regeneration process wherein the steel wool/silica complex is flushed with
alkaline solution to
dissociate concentrated colloidal and dissolved silica from the steel wool for
removal from the
system. The steel wool remains intact.
Examples
Example 1
1 S grams of steel wool was placed in a glass beaker containing 400 grams of
produced/feed water obtained from a CSS well at Cold Lake, Alberta. The water
and the steel
wool were heated for 2 hours at 97 degrees Celsius. The pH of the solution was
8.
After 2 hours of treatment, the steel wool was removed from the water and the
water
was analyzed for silica using Inductively Coupled Plasma (ICP). The steel wool
was then
placed in contact with a fresh batch of produced water for the same amount of
time and at the
same temperature as the first treatment. Each subsequent treatment was
labelled a batch as
shown in Figure 2. A second series of treatments/batches was conducted where
the produced
water or feed was purged with nitrogen in order to remove dissolved oxygen
from the water.
Figure 2 shows the silica reduction ability of steel wool from 15 batches and
shows the
difference between experiments where no attempt was made to removed dissolved
oxygen
and the nitrogen-purged feed water.
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The feed water contained 192 parts-per-million (ppm) silica. Typical boilers
require
less than 50 ppm silica. As shown in Figure 2, the non-purged batches met the
50 ppm
standard within 3 treatments whereas the nitrogen-purged series met the target
within the first
treatment. Therefore, removal of silica is more effective in the absence of
dissolved oxygen in
the feed water.
In this example, steel wool removed 57 milligrams of silica per gram of steel
wool or
192 milligrams silica per liter of water. Alternatively, 15 grams of steel
wool treated 15
batches of water with 400 milliliters water in each batch from 192 ppm to 50
ppm.
Example 2
A pilot plant 10 was built to demonstrate the process of the invention as
shown in
Figure 3. Produced water 12 from a bitumen recovery operation (Cold Lake field
in Alberta,
Canada)) was introduced into a mixing vessel 14 and pumped by pump 15 through
a 12 liter
heated vessel 16 packed with 592 grams of steel wool (packing density = 49.3
grams per
liter). A nitrogen stream 14a was optionally bubbled through the vessel 14 to
purge the
produced water of oxygen. The residence time in the vessel 16 was 2.7 hours.
The
temperature of the vessel 16 was maintained at 97 degrees Celsius. The
produced water had a
silica content of 170 ppm and entered the bottom of the vessel 16 at room
temperature and
exited the top of the vessel at 97 C. Water samples exiting the top of the
vessel 16 were
collected in vessel 18 and analysed for silica content every two hours. In
this experiment, the
process was a continuous flow process where valves V3, V4 and V5 were closed
and valves
V 1 and V2 were open.
During 28 hours of operation, the silica level in the treated water was 25
ppm, which
was much lower than the field target of 50 ppm.
In other embodiments, the produced water may be re-circulated depending on the
desired level of silica removal and the reactor 16 properties. In a re-
circulation operation,
valves V1 and V3 are open and valves V2, V4 and V5 are closed until the
desired level of
silica removal is achieved whereupon valve V3 is closed and V2 is opened.
After the reaction vessel 16 becomes saturated, flushing an alkaline solution
through
the vessel regenerates the reaction vessel 16. In the regeneration process,
valves V5 and V4
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are opened, valves V1, V2 and V3 are closed and alkaline solution 22 from a
vessel 20 is
pumped through pump 24 and then through the reaction vessel 16 into vessel 26.
This example demonstrates that the silica reduction by steel wool works in a
continuous flow system using commercially available steel wool and equipment.
Applications
The methodology in accordance with the invention can be incorporated into
existing
water treatment systems where low-silica water is desirable. In one
embodiment, as shown in
Figure 4, the silica removal process and apparatus may be incorporated within
an existing
water treatment plant associated with a steam-based bitumen recovery
operation.
In Figure 4, a steam-based bitumen recovery process 50 using hot lime
softening 66 is
shown. As shown in Figure 4, steam 52 is produced by a steam generation
process in a boiler
54 and injected into a reservoir 56 to recover bitumen 58 and produced water
60 by methods
known to persons skilled in the art. The produced water 60 through the process
of contacting
reservoir sand and minerals downhole after steam injection has become enriched
in hardness
ions and silica. The bitumen 58 and produced water 60 are separated 62 in a
separator and the
bitumen 58 is removed. The produced water is de-oiled 63 in a de-oiling system
wherein the
produced water 60 is subjected to various treatments to remove contaminants so
as to enable
its reintroduction into the boiler. These treatments include a silica
reduction process 64 in
accordance with the invention, a hot lime softening process 66 and an ion
exchange process
67 as are known conventionally to persons skilled in the art.
The silica removal system 64 is upstream of the hot lime softening system and
preferably includes two vessels 64a, 64b packed with steel wool. The two
vessels 64a, 64b are
in parallel in order that each may be alternatively selected as a silica
reduction vessel and a
silica regeneration vessel. That is, in a preferred mode of operation, at
least two vessels are
alternately selected as the silica reduction vessel to remove silica from the
produced water and
as the silica regeneration vessel for treatment with an alkaline stream to
regenerate the steel
wool for silica removal. Therefore, the vessels may be selectively switched
whereby the silica
reduction vessel (which has been saturated with silica) becomes the silica
regeneration vessel
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and the former silica regeneration vessel becomes the silica reduction vessel.
Additional
vessels may be added as needed.
The alkaline solution obtained from existing softening processes can be
utilized for the
regeneration process. That is, the alkali waste stream 68 may be directed
through reaction
vessels 64a, 64b to regenerate each vessel to produce a silica-enriched waste
stream 69.
The process and apparatus can be incorporated into existing water treatment
systems.
Persons skilled in the art will recognise that the silica reduction process in
accordance with
the invention has applications in many diverse fields where low-silica water
is desirable,
including manufacturing fields, petroleum industry, as well as potentially in
the nuclear and
semiconductor fields.
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