The present invention relates to processes for treating wastewater and more particularly to a process for treating oil sands process water and other industrial wastewater employing an evaporator.
Oil sands are deposits that are generally made up of bitumen, clay, sand and water and form a large part of the oil reserves in North America. In order to recover oil from oil sands, the bitumen is extracted from the oil sands. There are various processes used to extract bitumen, depending on geographical locations. For example, bitumen maybe recovered by surface mining or in-situ thermal processes such as stream-assisted gravity discharge, which is sometimes referred to as a SAGD process.
Oil sands process affected water (sometimes referred to as OSPW or simply oil sands process water) refers to water that has been in contact with oil sands or released from oil sands deposits. For example, in a SAGD process, an oil-water mixture is removed from an oil-bearing formation and the oil is separated from the mixture leaving what is referred to as produced water. As used herein, produced water is an oil sands process water. In this example, the resulting produced water typically includes suspended solids and dissolved solids that must be removed from the produced water prior to the produced water being used, for example, to generate steam that is used in the SAGD process. Another example of oil sands process water is tailings pond water. Tailings pond water is found in ponds or lagoons and generally results from the surface mining of bitumen in oil sands regions such as those in Alberta, Canada. Like produced water, tailings pond water must be treated prior to discharge or being used to generate steam or used as process makeup water. Another type of oil sand process waters is basal water. Basal water comes from a basal aquifer, situated underneath the oil sands geological layer. In mining operations, depressurizing the basal aquifer is one of the key goals for achieving a safe and productive mining operation. As a result of depressurization of the basal aquifer, basal water is collected at the surface of the oil formation. Basal water must be treated prior to discharge. Basal water can also be blended with other oil sands process waters (such as produced water) and used to generate steam through an evaporation process.
Many oil sands process waters and other industrial wastewater include significant concentrations of alkalinity (bicarbonate and carbonate), as well as calcium hardness. The presence of calcium and alkalinity in the oil sands process water often leads to the precipitation of calcium which is a scaling compound. In processes that employ evaporators for example, calcium carbonate scaling can be a serious problem. This is because the calcium carbonate precipitants scale the heat transfer tubes of the evaporator, substantially decreasing the efficiency of the evaporator and requiring expensive and time-consuming cleaning and maintenance.
In the past, calcium carbonate scaling of evaporator and process equipment has been dealt with upstream of the evaporator or process equipment by acidification and degasification. This process reduces the alkalinity concentration in the wastewater stream. However, acidification and degasification have serious drawbacks. The acid cost can be prohibitive if the wastewater has a high alkalinity concentration. In addition, in some applications where organics are present in the wastewater, the organics precipitate in response to acidification. In other cases, an ion exchange softening unit is employed upstream of the evaporator or other process equipment. This process reduces the concentration of calcium and magnesium in the wastewater. This process also has some drawbacks. The use of an ion exchange softening unit adds extra cost and generates a liquid waste stream which often requires treatment. Also, as a practical matter, there is a limitation on the calcium concentration to be treated by ion exchange resin. Another option for dealing with wastewater streams having a relatively high alkalinity concentration and hardness is employing a chelant inside the evaporator or other process equipment. This chelant maintains calcium in a soluble form and hence prevents precipitation. The drawback here is that the efficiency is limited and chelants are expensive.
The present invention provides a process for treating oil sands process water and other industrial wastewater with an evaporator wherein the process is designed to inhibit calcium carbonate scaling of the evaporator heat transfer tubes and other process equipment that might be used.
The present invention entails a process that reduces the tendency of calcium to scale process equipment. Fundamentally, the process entails adding a source of magnesium to a feedwater to form magnesium hydroxide, which in turn allows calcium to precipitate without scaling.
In one embodiment, the method entails treating oil sands process water having alkalinity and calcium hardness. Oil sands process water is directed to an evaporator that produces steam and a concentrated brine. Calcium scaling of the evaporator is inhibited by mixing magnesium oxide with the oil sands process water and precipitating magnesium hydroxide . The precipitated magnesium hydroxide may act as a seed for calcium compounds such as calcium carbonate to precipitate. Magnesium hydroxide and calcium-based crystals, along with the oil sands process water, is directed to the evaporator where the magnesium hydroxide and calcium-based crystals are mixed with the concentrated brine. The method includes circulating the concentrated brine, magnesium hydroxide and calcium-based crystals through the evaporator. Further, the method includes circulating at least some of the concentrated brine and mixed crystal slurry therein to a point upstream of the evaporator and mixing the concentrated brine and mixed crystal slurry with the oil sands process water. This crystallization process incorporated into the overall process of treating the oil sands process water inhibits or reduces calcium carbonate scaling of the evaporator heat transfer tubes.
Other objects and advantages of the present invention will become apparent and obvious from a study of the following description and the accompanying drawings which are merely illustrative of such invention.
The present invention, in one embodiment, relates to treating what is referred to as oil sands process affected water. The term “oil sands process affected water” comprises various water that has been in contact with oil sands. Examples of these waters are produced water that results in a SAGD process and tailings pond water resulting from the surface mining of bitumen. With water being relatively scarce in many parts of the world, coupled with the need to protect the environment, it has become necessary to treat and purify oil sands process water in order that the water can be productively re-used or safely discharged.
As will be discussed below, the oil sands process water is treated or purified by an evaporator. The evaporator evaporates the oil sands process water, generating steam and a concentrated brine. The steam is condensed and forms a distillate. The distillate can be used to generate steam or can be used as process makeup water or can be discharged. Typically, oil sands process water includes a relatively high concentration of alkalinity (sometimes as high as 2000 ppm) in the form of bicarbonate and, depending on the pH, carbonate. Along with the alkalinity, it is not uncommon for oil sands process water to include a significant concentration of hardness (for example, 50 ppm) in the form of calcium and, in some cases, magnesium. Because of the presence of alkalinity and calcium hardness, the oil sands process water is prone to scaling process equipment. Calcium carbonate precipitates from the water and scales equipment surfaces, such as the heat transfer surfaces of evaporators. This is a problem because calcium carbonate scaling substantially decreases the efficiency of the evaporator and requires significant cleaning and maintenance. As discussed herein, the process described adds a magnesium source, such as magnesium oxide, to a feedwater stream and this results the precipitation of magnesium hydroxide which acts as a seed for calcium carbonate and allows calcium carbonate to precipitate without scaling downstream process equipment.
With reference to the drawings, particularly
From the pre-heater 12, the oil sands process affected water is directed to a deaerator 14 that removes oxygen, carbon dioxide and other non-condensable gases from the water passing through the deaerator. It should be noted that pre-heating and de-aerating the oil sands process water is optional.
After treatment in the deaerator 14, the oil sands process water is directed to a crystallization reactor 15. Crystallization reactor 15 can include one or more mixed tanks. In the embodiment illustrated herein, magnesium oxide is added to the oil sands process water and mixed therewith. The magnesium oxide will react with the water and form magnesium hydroxide, among other compounds. This results in the precipitation of magnesium hydroxide which acts as a seed for calcium compounds and allows calcium compounds to precipitate in a manner that does not result in significant scaling of downstream process equipment. As presented herein, the calcium species or compounds that is discussed is calcium carbonate. It should be understood, however, that the process described herein is directed at precipitating calcium species or calcium compound that are prone to scale process equipment, such as evaporator heat transfer tubes. Thus, the process of the present invention entails utilizing magnesium oxide or a source of magnesium to precipitate calcium species or compounds and to prevent these calcium species or compounds from scaling process equipment. Calcium carbonate, discussed throughout this application, is one example of a calcium species or calcium compound that is precipitated as a result of mixing magnesium oxide or a magnesium source with the water being treated. Adding the magnesium oxide also increases the pH of the oil sands process water. In one embodiment, it is contemplated that the pH of the oil sands process water is raised to 10.2 to 11.5. This increase in pH lowers the solubility of calcium carbonate and result in calcium carbonate precipitating. In some cases, a caustic is added to assure an appropriate pH range. For example, sodium hydroxide can be added to the oil sands process water in the crystallization reactor 15 to assure that the pH is raised to approximately 10.2 to approximately 11.5. Thus, in the crystallization reactor, calcium carbonate will precipitate from the oil sands process water. The resulting calcium carbonate precipitants will begin to crystallize in the crystallization reactor 15.
The oil sands process water, along with the magnesium hydroxide and calcium carbonate crystals, are transferred from the crystallization reactor 15 to a downstream evaporator 16. More particularly, the water, along with the mixed crystals, are directed into the sump of the evaporator 16. There the oil sands process water and crystals are mixed with concentrated brine produced by the evaporator 16. The concentrate brine having the magnesium hydroxide and calcium carbonate crystals is pumped by the pump 16C through a brine recirculation line 16A to a top portion of the evaporator 16. Here the concentrated brine is dispersed, in this exemplary evaporator, into vertical heat transfer tubes. The concentrated brine forms a thin film on the inside surfaces of the heat transfer tubes and falls downwardly therethrough. The heat transfer tubes are heated with steam causing the concentrated brine having the crystals therein to be further concentrated and to produce a vapor. In this example, the evaporator 16 is a mechanical vapor recompression evaporator. Accordingly, the vapor produced from the concentrated brine is directed to a compressor 16B which compresses the vapor and produces steam which is directed against the outside of the heat transfer tubes, again heating the thin film brine falling downward along the inside of the tubes. The steam condenses and produces a distillate that is directed from the evaporator. The distillate can be used in a variety of ways. It may be directed to a steam generator for producing steam that is utilized in a SAGD operation. On the other hand, the distillate may be used as a makeup process water or may be appropriately discharged.
A portion of the concentrated brine is directed from the evaporator 16 as evaporator blowdown. The evaporator blowdown maybe directed to an evaporator blowdown treatment unit 18 for further treatment. Here the evaporator blowdown can be further concentrated or subjected to a crystallizing process in a crystallizer which can produce a zero liquid discharge. In other cases, the evaporator blowdown can be subjected to a suspended solids removal process that removes suspended solids therefrom, after which the treated evaporator blowdown stream can be disposed of through deep well injection.
Another portion of the concentrated brine, along with the crystals therein, is recirculated and mixed with the oil sands process water at various points upstream of evaporator 16. In the embodiment illustrated in
In some cases, the oil sands process water includes a significant concentration of silica. Silica is also susceptible to scaling the heat transfer tubes of the evaporator 16. To reduce silica scaling, the magnesium oxide added to the crystallization reactor 15 reacts with the water to form magnesium hydroxide crystals.
The magnesium hydroxide crystals effectively adsorb or pull silica out of solution. The elevated pH, due to adding magnesium oxide and possibly a caustic, reduces the solubility of magnesium hydroxide. Substantial amounts of the silica in the form of silica-based compounds in the oil sands process water is drawn out of solution via adsorption by the magnesium hydroxide crystals. The dissolved silica concentration in the oil sands process water is thus reduced to a relatively low level. The concentration of the remaining silica in solution is typically below the solubility limits and thus does not scale the evaporator heat transfer tubes.
Another form of oil sands process water is tailings pond water. Oil sand is typically comprised of 80-85% minerals, 10-15% bitumen and 3-4% water by weight. The mineral component of the oil sand contains both sand and fines. Bitumen is separated from oil sand using what is sometimes referred to as a hot water extraction process. The remaining sand, fines and water combine to form “tailings”. Tailings are typically discharged and contained in large tailings ponds. The larger sand particles in the tailings settle to form a stable deposit fairly quickly while the finer clay particles sometimes take years to settle and are known as fluid fine tailings. Through treatment, tailings pond water can be reclaimed and the water reused.
In the above discussions, the process has discussed treating oil sands process water including, for example, produced water and tailings pond water. However, the process of the present invention is also applicable to other industrial wastewater streams, particularly those wastewater streams that are treated with an evaporator and which include a significant concentration of alkalinity and calcium hardness. For example, the process of the present invention can be used to treat coal seam gas produced water, gas field produced water and other industrial waste streams. The same process is employed. By mixing magnesium oxide with the wastewater stream, magnesium hydroxide is precipitated and also due to a pH increase calcium carbonate is precipitated and crystallized. The crystallization process involving the magnesium hydroxide assures that the calcium carbonate solids do not scale the evaporator heat transfer tubes or even other process equipment that is utilized in the treatment of such wastewater streams.
From the foregoing specification and discussion, it is appreciated that the process of the present invention is directed at treating oil sands process water and other wastewater streams by evaporating the water and employing upstream processes, prior to evaporation, that are designed to prevent calcium carbonate from scaling the heat transfer tubes of the evaporator. This is accomplished by adding magnesium oxide, and in some cases sodium hydroxide, in order to establish conditions that promote the precipitation and crystallization of calcium carbonate. Once in a crystallized form, the calcium carbonate will not significantly scale the heat transfer tubes of the evaporator 16.
The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.
Number | Date | Country | |
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62016127 | Jun 2014 | US |