CA2429295A1 - Method and apparatus for reducing silica in water - Google Patents

Abstract

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.

Description

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.

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.

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 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 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.
-S-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.

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.

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.
_g_ 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 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 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.

Claims (16)

1. A method of reducing silica in water comprising contacting silica-containing water with a high-surface area metal to form a metal/silica complex and silica-depleted water.

2. A method as in claim 1 wherein the high-surface area metal is selected from the group comprising steel wool, aluminium wool, copper wool, zinc wool, stainless steel wool, brass wool, bronze wool and any combination thereof.

3. A method as in claim 2 wherein the metal is steel wool selected from Grades of at least 0000 and no coarser than 4.

4. A method as in claim 1 wherein the reaction temperature is at least 20 degrees Celsius and no higher than 100 degrees Celsius at atmospheric pressure.

5. A method as in claim 1 wherein the reaction pH is greater than 7.

6. A method as in claim 1 wherein the silica-containing water is purged of oxygen prior to contacting the metal.

7. A method as in claim 6 wherein the silica-containing water is purged of oxygen with nitrogen prior to contacting the metal.

8. A method as in claim 1 wherein the reaction is conducted under anaerobic conditions.

9. A method as in claim 3 wherein the steel wool removes silica at a rate of at least 57 milligrams of silica per gram of steel wool.

10. A method as in claim 1 further comprising contacting the metal/silica complex with alkaline water to regenerate the metal from the metal/silica complex.

11. A method of reducing silica in silica-contaminated water comprising:
a. providing water contaminated in silica;

b. passing the provided water contaminated in silica through a reaction vessel containing an effective amount of steel wool to produce silica-depleted water;
c. removing the silica-depleted water.

12. A method as in claim 11 wherein the water contaminated in silica is from a bitumen reservoir and the removed silica-depleted water is returned to a boiler for steam generation and steam injection to the bitumen reservoir.

13. A method as in claim 12 further comprising the step of removing additional hardness ions from the produced water prior to returning the silica-depleted water to the boiler.

14. 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.

15. 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.

16. An apparatus as in claim 15 wherein the first and second vessels are operatively connected to an alkali water source for selectively regenerating the first and second vessels.