Patent Application
20140158550
The invention relates to a method for treating produced water using both electrocoagulation and sonication.
In addition to hydrocarbons, a large quantity of produced water is generated during oil and gas production operations. Produced water normally includes natural contaminants originating from the subsurface environment, such as hydrocarbons from the oil- or gas-bearing strata and inorganic salts. In addition, produced water contains chemicals introduced into well treatment fluids such as polymers, breakers, friction reducers, lubricants, acids and caustics; bactericides, defoamers; emulsifiers, filtrate reducers, shale control inhibitors, etc. Such undesirable materials may be removed before produced water may be reused for oilfield operations.
In the past, various methods have been explored to treat produced water. However, because of the wide range of contaminants and the quality of produced water originating from different sources, cost effective treatment systems have not been successful. For example, one method which has been examined is reverse osmosis. This method, however, has not been proven to be effective since it is typically fouled by even trace amounts of friction reduction agents, such as polyacrylamides, and oils. The presence of such materials further contribute to reclaimed produced water being unsuitable as hydraulic fracturing fluids since they inhibit the formation of viscous gels from viscosifying polymers.
More recently, electrocoagulation has been reported to be useful in the removal of a large range of undesirable materials from produced water. In electrocoagulation, an electric current is applied to electrodes through the conductive wastewater. Anodes made of iron or aluminum are consumed to produce positively charged ions in the treatment stream to attract the negatively charged contaminant particles and thereby initiate flocculation, resulting in increased particle size.
Also, during electrocoagulation, hydrolysis of produced water renders oxygen, hydrogen, and hydroxyl ions. As water containing colloidal particulates, oils, metals, or other contaminants move between the electrodes, ionization, electrolysis, oxidation, precipitation, and free radical formation may occur. This alters the physical and chemical properties of the contaminants which, in turn, causes coagulation and flocculation of the contaminants.
Flocculated contaminants may be removed from the treatment stream. Preferred methods for removal of flocculated contaminants include sinking, flotation and filtration.
It is not uncommon, however, for fouling of the electrodes to occur from the build-up of non-reactive material on the surface of the electrodes. This results in an uneven degree of activity over the electrodes and often leads to plugging and blocking of the treatment flow, the build-up of treatment gases, and the pitting, gouging, and uneven wear of electrode plate surfaces. The likelihood of these occurrences increases with increased time duration for the formation of flocculated contaminants. Uncontrolled fouling of the electrodes is a major cause of failure of electrocoagulation cells. The high level of cell maintenance has thus limited the commercial success and the ability to treat produced water on a commercial scale.
Lengthy retention time for contaminant flocculation is also highly undesirable on-the-fly. It might take up to a day for the flocculated material to settle, thus increasing the cost of operation and the footprint of the treatment plan.
There is a need, therefore, for methods to reduce the retention time for flocculant formation.
Further, there is a need for an electrocoagulation process for removing undesirable chemicals from produced water that is energy efficient and cost-effective and which minimizes the effect of fouling of the electrodes.
A method of decreasing the retention time in the formation of flocculants during the treatment of produced water by electrocoagulation consists of coupling electrocoagulation with sonic energy.
The ultrasonic waves may be generated from a sonicator submerged in the electrocoagulation cell.
Coupling electrocoagulation with sonic energy assists in the removal and degradation of scales and minerals from the electrodes of the electrocoagulation cell. This reduced electrode fouling and increased the operational life of the electrodes.
The sonicator may further be in a separate staging container from the electrocoagulation cell, such as a storage tank or frac tank. After the produced water is subjected to electrocoagulation, the treated water is then transferred into the separate staging container for sonication. Reclaimed water is then retrieved from the separate staging container.
The method described herein provides for increased aggregation of flocculants during electrocoagulation and enhanced mass transport of undesirable contaminants through sonication. Retention time for flocculation is thereby reduced. In addition, due to the enhanced mass transport, the method described herein reduces the amount of time required for settling of flocculants prior to the separation of flocculants from the aqueous medium. Reduced settling time is further aided by the increased size of flocculants attributable to the coupling of ultrasonic waves with electrocoagulation.
The ultrasonic waves also cause the formation of microbubbles which accelerate the oxidation and destruction of aromatic compounds in the produced water.
In order to more fully understand the drawings referred to in the detailed description of the present invention, a brief description of each drawing is presented, in which:
Produced water is processed by the combination of electrocoagulation and sonic energy. Untreated produced water contains fine suspended particulates. While electrocoagulation, by itself, increases the particle size of the particulates, the combination of electrocoagulation and sonic energy facilitates the removal of particulates by rendering a top layer of lighter solids and/or a bottom layer of heavy solids.
By subjecting produced water to electrocoagulation and ultrasonic waves, the retention time is decreased. Typically, the retention time, upon the integration of electrocoagulation and sonic energy, is between from about 1 to about 5 hours.
The term “retention time” as used herein refers to the amount of time that produced water is subjected to treatment until the start of flocculation of undesirable chemicals.
As used herein, the term “produced water” shall refer to both water generated during an oil or gas production operation as well as a wastewater stream containing undesirable contaminants produced from a waste treatment process or an industrial water treatment process.
Coupling of sonic energy and electrocoagulation reduces the retention time compared to the treatment method which uses only electrocoagulation. In fact, for a given quantity of produced water of identical composition, the retention time for the sample treated by the combination of electrocoagulation and sonic energy is less than that treated solely by electrocoagulation. Typically, for a sample of produced water, the retention time for the sample treated by the combination of electrocoagulation and sonic energy is at least 30% less than that treated solely by electrocoagulation (where the chemical constituency of the sample, the volume of the sample treated and the time duration of treatment is the same). Thus, a significant increase in efficiency results when electrocoagulation is coupled with sonic energy.
In addition to decreasing retention time, the coupling of electrocoagulation and sonic energy eliminates the need for or reduces the amount of chemical additives required to flocculate the contaminants and degradation products from produced water. In addition to eliminating or reducing the need for coagulants and/or flocculating agents, the method described herein reduces or eliminates the use of biocides. Typically, the presence of biocides is required in the treatment of produced water.
Thus, for a sample of produced water, the requisite amount of coagulants or flocculating agents to effectuate flocculation within a sample when coupling electrocoagulation and sonic energy is less than that required to effectuate flocculation within a sample using only electrocoagulation (where the chemical constituency of the sample, the volume of the sample treated and the time duration of treatment is the same).
In addition, it has been found that the use of ultrasonic waves mitigates and/or reduces the tendencies of scaling of contaminants on the electrode. This enhances the operational lifetime of the electrodes and the electrocoagulation device and thus reduces costs to operators.
In an embodiment, a flocculating reagent, such as aluminum hydroxide, a polyacrylamide, a partially hydrolyzed polyacrylamide or a quaternary ammonium compound like tetramethylammonium bromide, may be added to the electrocoagulation device or cell.
The electrocoagulation device may be any electrocoagulation device commercially available.
For instance, the electrocoagulation device used in the method described herein may include a housing, an electrocoagulation reaction chamber within the housing, and oppositely charged electrodes within the reaction chamber, typically consisting of a plurality of spaced oriented reaction plates/blades, most typically extending substantially vertical within the reaction chamber. The plurality of reaction plates are typically spaced apart from one another to create gaps which extend between adjacent reaction plates.
The housing may be of any shape and is typically designed to facilitate the application of the electric field to the produced water as it flows through the reactor. The housing may have an upper portion and a lower portion, the upper portion defining a development chamber and the lower portion defining the reaction chamber. Representative constructions of the electrocoagulation device may be those described in U.S. Pat. Nos. 6,139,710 and 6,488,835, incorporated by reference herein.
The electrocoagulation device has an inlet port to allow the desired flow of liquid into the reaction chamber and into the gaps or spaces between the blades or plates. An outlet port may also be provided, typically at a higher elevation, and downstream of the inlet port for allowing treated water to flow from the chamber.
The flow of produced water is in a flow direction upward through the gaps between the plurality of reaction plates. In such instances, the outlet may be positioned at the higher level above the inlet.
At least two reaction plate tabs are preferably integral with selected ones of the plurality of reaction plates, the reaction plate tabs having ends that may be isolated from the flow of liquid in the housing.
A source of power provides line voltage to the tabs in order to create an electrical field for the treatment within the reaction chamber. The electric field may be generated from essentially any suitable direct current (DC) or alternating current (AC) source.
In a preferred embodiment, a direct current electric field is used. An electrical potential between the anode and cathode in the electrochemical reactor can be between about 1.5 volts and 12 volts with successful results, however much higher voltages can also be used as desired. The electrical potential is measured in the gaps between the reaction plates.
The anode may be of any material that allows the flow of electrons including, but not limited to metals such as copper, silver, gold, magnesium zinc, aluminum, iron, nickel tin; non-metals or combinations thereof (e.g., alloys or coated or multi-layered structures), such as carbon-based materials like graphite; or combinations of metals and non-metals. The metal may be galvanized or formed in combination with oxygen or oxides. In a preferred embodiment, the anode is made of a copper alloy.
The cathode may be made of any materials to which positively charged ions migrate when the electric current is passed, including copper alloys, stainless steel, platinum, nickel or iron or alloys thereof.
Selected blades may connect to electrical leads which carry an input line voltage. An electrical field is created in the chamber between the electrically connected blades. The electrical leads may be attached to selected blades in order to provide the reaction chamber with the desired voltage and amperage to optimize electrocoagulation of the produced fluid. The ability to vary voltage and amperage within the electrical field of the chamber may be achieved without the use of a separate transformer.
A pump may be placed upstream of the inlet in order to provide additional head for the flow of liquid passing through the apparatus. A series of pre-filters or other preconditioning means may be placed in line with the pump and also upstream of the inlet in order to remove solids or other materials which may otherwise clog the reaction chamber.
A control unit may be used to rectify the incoming AC line voltage to a DC voltage. Electrical leads may interconnect the blades to the DC voltage made available by the control unit. In addition to rectifying the incoming line voltage, the control unit may incorporate a number of other functions which helps to control the apparatus, such as a means to control the speed of the pump and a voltmeter and ammeter to monitor the conditions within the chamber. However, the control unit does not need a transformer as the electrical connections made with the blades may allow the desired voltage and amperage therein to be adjusted, as further discussed below. Additionally, the control unit can be in the form of a programmable logic controller which may only monitor status condition inputs, but also produce outputs to control the electrocoagulation process. For example, the voltage polarity of the electrical leads extending from the control unit may be reversed based upon a timing sequence controlled by the controller. As a further example, the control unit may measure the flow rate of the produced water and adjust it accordingly by either manipulating the pump speed or adjusting the flow rate through a valve positioned upstream of the inlet.
The reactor may further include other equipment to control the flow of reactants and products to and from the reactor. Preferably, the direction of the fluid flow is from the anode to the cathode.
The electro-sonication device which is coupled with the electrocoagulation device is capable of generating ultrasonic sound waves in liquids. Acceptable ultrasonic devices are commercially available. Typically, such devices consist of an ultrasonic generator and an ultrasonic transducer. The transducer, used for converting electrical energy into sound, is preferably a piezoelectric transducer. Any piezoelectric material suitable for converting electrical oscillations into mechanical vibrations are acceptable and include metals, such as barium titanate and lead zirconate titanate, and quartz. The transducer is typically 165 grade.
In an embodiment, an ultrasonic horn transducer or probe may be used. Ultrasound may then be directly delivered to the water stream using a metal horn tip. The horn will act as an amplifier with the shape of the tip determining the mechanical amplification of the piezoelectric vibration. Typically, the tip will have a length corresponding to a multiple of half wavelengths of the ultrasonic wave. The horn is preferably composed of a titanium alloy. Typically, the horn is placed in the container housing the sonic energy device at a depth of not less than 1 to 2 cm.
A cooling coil may further be connected to a thermostat and introduced into the container housing the sonic energy device in order to reduce the amount of heat which typically evolves from high intensity ultrasound.
In a preferred embodiment, ultrasound waves generated from the electro-sonication device causes the formation of micro bubbles which only collapse at the molecular level at extremely high temperatures and pressures. Thus, chemical reactions, such as oxidation, may occur with minimal effect at ambient temperatures.
The formation of the microbubbles occurs more readily in the vicinity of the electrode surface of the electrocoagulation device compared to that in the bulk solution due to typically weaker molecular interactions of the organic molecules in the produced water and wastewater. Upon collapse of the microbubbles, electrolyte is typically cast against the electrode surface. This brings new electroactive material giving an enhanced current to the electrocoagulation device. If the vapor pressure is low within the cell, the microbubbles may grow smaller which may cause an increase in frequency in collapse of the microbubbles. It is preferred that the microbubbles be small at low vapor pressure in order to minimize the likelihood of escape from the surface.
The frequency of the ultrasound waves to which the wastewater is subjected is greater than 16 kHz and is typically from 16 kHz to 500 MHz.
In the embodiment illustrated in
In one embodiment, one or more transducers may be placed beneath the electrocoagulation cell and be separated from the electrocoagulation cell by a physical barrier.
In an embodiment, the electrocoagulation device may be separated from the sonication device such that after the produced water has been subjected to electrocoagulation, the treated water is then transferred into a separate container for sonication. The separate container may be a water storage chamber or a frac tank. The reclaimed or reusable water then exits from the separate tank or chamber without flocculated contaminants. This embodiment may be illustrated by
The method described herein is highly efficient with flocculating aromatic compounds. Such compounds may be oxidized/destroyed by the described sonoelectrochemical procedure at low acoustic wave frequencies. Such compounds agglomerate and the agglomerates become larger as the amount of sonic energy is applied. During the process, free radicals are formed. In the absence of reactive organic compounds with the free radicals, the formation of hydroxyl free radicals from water may be followed by recombination processes that lead to the formation of hydrogen peroxide. The hydrogen peroxide may also be used as a breaker in the destruction of organic polymers present in the production water or wastewater stream.
Waste generated by the combination of electrocoagulation and sonic energy may be readily settable and may be easily dewatered or filtered. Filtration may be carried out by centrifugal mechanical such as use of a filter mesh in a rotary chamber to produce a filtered water stream and a waste filtrate stream. The filtered water stream may then be passed to a media filtration tank capable of collection, filtration and back flushing of post treatment residual and coagulated solids. The filtered water from the media filtration tank may further be passed to a filter mechanical to provide a filtered water discharge having entrained solids level which meets acceptable standards.
Typically, the particle size of the generated flocculants is larger than flocculants generated from electrocoagulation solely.
Further, the tendency of electrode scaling is dramatically reduced by use of the combination of electrocoagulation and sonic energy (versus electrocoagulation solely). Further, scales and minerals are more easily removed from electrode surfaces or plates of the electrocoagulation cell when sonication is coupled with electrocoagulation. As such, the operational lifetime of the electrodes in the electrocoagulation cell is enhanced which results in reduced costs to operators.
Other embodiments within the scope of the claims herein will be apparent to one skilled in the art from consideration of the description set forth herein. It will be observed that numerous variations and modifications may be effected without departing from the true spirit and scope of the novel concepts of the invention.
This application claims the benefit of U.S. patent application Ser. No. 61/669,193, filed on Jul. 9, 2012, herein incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61669193 | Jul 2012 | US |
