Patent Application
20130193077
1. Field of the Invention
Embodiments of the invention relate to methods and apparatuses for the neutralization and precipitation of high pH silica brines.
2. Background of the Related Art
In many industrial situations artisans may find it desirable to neutralize brines. Typically this is done so that the brine or a portion of the brine may be safely and cost effectively disposed of, beneficially reused, or both. This may be the case, for example, in technologies involving extraction of oil from oil sands using steam-assisted gravity drainage. In some situations neutralization of brines may be useful or necessary to prevent scaling during further purification and use of the water stream.
Although a number of chemical species may precipitate during neutralization of brines, silica is one of the most significant. Process design solutions have been designed to handle high silica concentrations in membrane desalination and evaporation systems. Silica solubility in water increases with increase in pH and temperature. These solubility limits, along with concentration limitations, have been well documented. Silica can change its structure as a function of pH. This complicates removal, because silica's solubility additionally depends upon its structure along with the aforementioned temperature dependence. A typical single-step pH reduction is shown in
At 25° C. and a neutral pH the solubility of silica is about 120 ppm as SiO2. At a highly alkaline state, silica is ionized and present in soluble form and can have as much as 3000-8000 ppm silica as SiO2. This high level of silica with high pH in brine cannot be processed for further treatment or disposal until it is neutralized by acid for beneficial reuse or to meet environmental standards cost effectively.
Although silica is very slow to crystallize or precipitate in a macrocrystalline form, in the presence of salt (e.g. NaCl) the dynamics of precipitation change to form a crystalline precipitate instead of a gelatinous substance. The concentration of salt also plays an important role in the gel formation. In the absence of salt the precipitated silica is difficult to filter or separate through settling and decantation.
Typically the residual silica is disposed of by injection in salt caverns or deep wells. The fluffy nature of silica gel causes it to occupy higher volume than other forms of silica, leading to filling the salt caverns prematurely or plugging the underground formation in a deep well during disposal.
From the silica solubility graph (
Depending on feed water such waters may also contain oil and grease and organic contamination. A typical composition of such brines is described in Table 1.
Such concentrated brines need some treatment for further water recovery or disposal, which may involve silica removal by precipitation and pH neutralization. For silica precipitation, pH is typically brought down by neutralization, which involves addition of acid to bring the pH down to desired levels which will be anywhere from 7-9. This may vary based on its subsequent application.
When this high silica brine is neutralized with an acid to a pH of 7-9, the resultant precipitated silica forms a gelatinous substance. This has poor settling properties and is very difficult to handle, separate or filter. It becomes impossible to use this water for any further processing or discharge. Some applications where this water is discharged after conventional neutralization process into salt caverns or deep well injected become very expensive as the caverns fill up very quickly and wells plug, requiring a new well.
We provide a process for the neutralization and precipitation of high pH brines that eliminates or ameliorates the formation of “gelatinous silica ” during neutralization. This results in the creation of a precipitate that can be settled and filtered.
The high pH brine is neutralized in a “two-step” neutralization process. In the first step the salt concentration of a high pH brine is built up to a minimum level of 8-12% and then its pH is reduced to 9.5 to 10. The partially neutralized brine is allowed a reaction period with mild agitation. The hold up time varies based on the type of brine and silica concentration and other contaminants and may vary from 5-10 minutes, 10-30 minutes, more than 30 minutes, or more than an hour. Subsequently the pH is further reduced, typically to 8-8.5 by the same acid. The amount of acid may be the same or different as the amount added in the first step.
A coagulant and/or a polyelectrolyte polymer can optionally be used to enhance the settling or filtration rate of the neutralized stream. The clarified/filtered supernatant liquid is then processed either for disposal in caverns/deep well or utilized with further pH adjustment as required through conventional treatment methods.
One embodiment of the invention is shown in
After the minimum sodium chloride concentration has been reached, the first pH adjustment is conducted. Of course, those of skill in the art will recognize that when terms such as “first pH adjustment” or “second pH adjustment” are used, they are used relative to the neutralization and precipitation process reported herein. Preceding or succeeding steps are not relevant in determining the “first” or “second” adjustment. Typically adjustment of the solution is done in steps. In a first step, given an expected initial pH range of 10.5-13, the pH is reduced by dosing of acid. This acid may be, for example, but is not limited to hydrochloric or sulfuric acid, depending on availability. Acid dosing reduces the pH to between 9.0-9.5.
Following acid dosing a holdup time is given for silica separation and precipitation. Mild agitation should be continued in this period. After the residence time, and without separating the precipitates, the pH of the brine is reduced to around 8-9 by again dosing acid. The residence time may be, for example, between 10-120 minutes, 10-30 minutes, 30-90 minutes, 30-60 minutes, 60-90 minutes, or 60-120 minutes.
Following or during the second acid dosing, a coagulant may be dosed. In one embodiment a coagulant dose of around 50-100 ppm makes the supernatant clearer. Ferric chloride has worked well and is a preferred coagulant, though those skilled in the art will, with the benefit of this disclosure, recognize that other coagulants may be useful.
Residence time for the coagulant is typically from half an hour to one hour to help enable better filtration. After providing some residence time clear water can be separated by decantation/clarification and/or filtration.
In some application addition of a polymer further accelerates the settling properties. Clarified water has a better turbidity value, which reflects the quality of separation and filtration for subsequent purpose.
The brine obtained can be passed through a filter to a storage tank for any subsequent application. The pH of clarified brine could be further adjusted to suit a subsequent use as well. Instead of decanting the water the precipitated solids can also be filtered to remove the sludge and separate clear water for further processing. “Sludge,” generally, describes a residual semi-solid material left over from wastewater purification. The filter can be, for example, a filter press, belt press, centrifuge or any other clarification, decanting or sludge separation and compaction device depending on the application and purpose.
In one embodiment of the invention the produced water from which silica is separated is a concentrated produced water from steam-assisted gravity drainage. Following removal of silica sludge the remaining water, if not re-used, is disposed of by deep well injection or in salt caverns. One example of such an embodiment is shown in
Embodiments of the invention may have one or more advantages over conventional neurtalization process, though this list should not be construed to limit the scope of the claims.
A series of experiments were conducted explore and devise a process for the neutralization and precipitation of silica from high pH brine. To cover a wide range of silica concentration and different salt concentrations, simulated water was used in all of the tests. Sodium chloride (NaCl) was used to increase the salt level of water and sodium meta-silicate nonahydrate (Na2SiO3.9H2O) was used for silica concentration buildup. During water simulation testing, some conditions were maintained as set forth below.
13. pH value: The pH of simulated water was maintained above 10.
In the first phase of the experiments 0.5% to 5% sodium chloride level was maintained and silica was added to get 300-ppm to 3000 ppm in simulated water. Only pH alteration by acid was attempted to get all silica precipitated. At this stage pH was brought down to 7.0 to 8.0 but all cases ended up with the formation of “gelatinous silica.” The characteristics of “gelatinous silica” have been described above. To avoid this gel formation, some other coagulants (Alum & Ferric chloride) were also explored.
Alum was first tried at low pH (7-8) to get a better settling of silica gel, but no significant impact was observed. A combination of alum and polyelectrolyte also did not work to settle fluffy gelatinous silica. At high pH alum was tried to get co-precipitation of silica along with aluminum hydroxide. But extra doses of alum only reduced pH drastically and formation of gelatinous floc occurred. Lesser amounts of alum were not sufficient to reduce silica at higher pH.
Another coagulant (Ferric chloride) was also tried out in a similar fashion but results were more or less the same as with alum. Ferric chloride alone could not reduce the silica at higher pH range. If the pH is lowered with the aid of hydrochloric acid, formation of “gelatinous silica” was observed. Excess dosing of ferric chloride also resulted in formation of the gelatinous floc.
Then in the second phase the salt (sodium chloride) concentration was varied up to 25% and it was observed that a minimum 8% of salt concentration was required to prevent “gelatinous silica” formation and obtain fast settle-able silica precipitation with two step pH reduction. This reduction involved a first adjustment to pH 9.0-9.5 and then an adjustment to pH 8. Adjustments were made through the utilization of acid with the aid of coagulant to get clear supernatant liquid.
Based on these findings an overall process was conceived wherein the salt was added to a threshold level, typically a concentration of between 8-12%, which varied on the brine composition and silica concentration followed by a two step neutralization. The first step involves pH reduction to 9-9.5, followed by a second stage pH reduction and coagulant addition. This enabled achievement of pH in the desirable range and precipitation of silica with good settling particle properties as detailed in Example 3.
A high pH brine (pH 11.95, salt concentration 2% and silica as SiO2 2000 ppm) was prepared by dissolving an appropriate amount of sodium Meta silicate nonahydrate and sodium chloride in de-mineralized water. This high pH brine was neutralized by hydrochloric acid to a pH 8.0 and alum (100 ppm and 200 ppm) was added as a coagulant. A gelatinous formation was observed. The formed gel remained suspended and occupied more than 50% of the volume of brine. The residual silica was checked in supernatant liquid and found to be around 160 ppm. The treated water was difficult to decant and filter and would plug any filtration media. The observation results of the experiment are tabulated in Tables 2 & 3.
Example 1 was repeated, and this time ferric chloride (50 ppm to 200 ppm) was used as coagulant instead of Alum. Ferric Chloride was dosed in simulated water and the pH was reduced by hydrochloric acid to 8.0. Residual silica was checked first at reduced pH 10 and then pH 8.0. After neutralization to pH 8.0, gelatinous silica formation was observed. The results are summarized in Table 4.
As reflected in Table 4, in comparative examples 1 and 2, above, the silica reduction were found to be in the range of 160-170 ppm from 2000 ppm but a gelatinous formation was observed which occupied almost 50-80% of the liquid volume and once it formed could not be separated.
In this experiment the simulated high pH brine (2000 ppm silica and 2% NaCl water) was treated and neutralized as per a process reported in the Detailed Description herein. Different percentages of sodium chloride (8% to 23%) were added to the high pH simulated brine, and pH was reduced first to 9.5 by hydrochloric acid, kept for 2 hours with mild agitation and then pH was further reduced to 8.0. No gel formation was observed.
The silica precipitated and settled easily but the supernatant liquid was not clear, it was slightly turbid. Then 50-ppm of FeCl3 was added to the treated water with polyelectrolyte (1-2 ppm). The supernatant treated water became clear. The residual silica results are summarized in Table 5
In this experiment, simulated high pH brine with 10% salt concentration was prepared with silica concentration of 300 ppm to 2000 ppm as SiO2 and neutralized in two step process by acid as per the devised process.
The residual silica was checked at each step. No gel formation were observed and the residual silica results are summarized in table-6
In this experiment alum was used as a coagulant and sulfuric acid was used for pH reduction. In previous experiment 1 to 4, the brine was simulated by adding only sodium meta-silicate nonahydrate and sodium chloride in dematerialized water but in this experiment the brine was simulated by adding various reagents based on a produced water analysis as listed in Table 7. “Produced water” is water that, for example, results from heavy oil production from steam assisted gravity drainage.
For simulation of the above produced water composition brine various chemicals listed in Table 8 were added in one liter of dematerialized water.
After simulation, the high pH brine was divided into two equal parts (part-1 & part-2)
The part-1 brine was treated as per the method taught in this disclosure. First 8.5% NaCl was added and then the pH was reduced by sulfuric acid to 9.5 and kept for 1-2 hours with mild agitation. After that, the pH was further reduced to 8.0 and 50 ppm of alum was added. No gelatinous floc was formed and silica precipitates settled down easily. The residual silica results of the experiment are tabulated in Table 9. The results were identical when the coagulant was added just before the second lot of acid was added for second step pH adjustment.
The part 2 brine was concentrated 6.5 times by heating and evaporation method to make its salt concentration 10%. When the salt concentration reached 10%, the solution was neutralized in two steps by sulfuric acid as per the disclosed process. 50 ppm alum was used for clear supernatant liquid. The residual silica results and observation are tabulated in table-10.
In part 1 and part 2 brine of Example 5, no gelatinous floc was formed; silica precipitated and settled down easily with a sludge volume around 15% of the brine volume. The treated supernatant liquid can be easily decanted or clarified and then it can be processed either for disposal in caverns/deep well or utilized by further pH adjustment as required through conventional treatment method.
Although various embodiments and aspects of the invention have been reported in the foregoing disclosure and appended drawings and claims, it will be appreciated that those of skill in the art would be able to make various modifications and additions, which would be encompassed within the spirit and the scope of the claims.
This application claims priority to U.S. Provisional Patent Application No. 61/515,672, filed on Aug. 5, 2011, and incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| 61515672 | Aug 2011 | US |
