Hybrid acoustic, centrifugal oil / water separation

Abstract

To provide a device for separating oil from water in generated oil, which does not require a large separation tank and operates with high energy efficiency and a method.SOLUTION: A method for separating oil droplets from water includes the steps of: flowing a mixture through a pipe of a first length having a first outlet orifice; generating a first acoustic standing wave within the interior of the pipe of the first length to produce an acoustic radiation force having a first pressure node, causing oil droplets to aggregate and coalesce at the first pressure node, the acoustic wave having a first frequency; forming a pipe of a second length in a plane having a radius of curvature and an axis when flowing the mixture from the first outlet orifice into the pipe of the second length having a second outlet orifice, and generating an acoustic standing wave within the pipe of the second length, the acoustic wave having a frequency lower than the first frequency; and separating the coalesced oil droplets from the water passing through the second outlet orifice.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] [CROSS REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 053,442, filed July 17, 2020, entitled “Hybrid Acoustic, Centripetal Oil / Water Separation,” the entire contents of which are specifically incorporated by reference herein for all that it discloses and teaches. [Background technology]

[0002] Oil and gas are produced in almost all parts of the world from private wells of 100 barrels per day to large diameter wells of 4000 barrels per day, from reservoirs of 20 m depth to wells of 3000 m depth, and from wells with water depths greater than 2000 m, but many parts of the production process are fundamentally similar. The natural hydrocarbons present in oil reservoirs are mixtures of organic compounds that exhibit multiphase behavior over a wide range of pressures and temperatures, and the mixtures of oil, gas, and water produced from the various wellheads are separated into their individual components in separation facilities. Gas is separated from the mixture at reduced pressure in a storage vessel before the oil and water are separated, which may require a multi-stage thermodynamic separation process in a pressure vessel at different pressures and temperatures, followed by gravity separation of the oil and water into separate oil and water streams. In the case of separation of water and oil, especially where the oil contains heavy oil, the source fluids are heated to change the density of the oil and water so that these fluids can separate.

[0003] An economically attractive alternative for gas-liquid separation is the gas-liquid cylindrical cyclone (GLCC), a simple, compact, low-cost separation device having a vertical pipe section with a nearly centrally located, downwardly inclined tangential inlet. Separation in a GLCC is achieved by centrifugal and gravity effects. Most of the research on liquid-liquid cyclonic separation has focused on the liquid-conical hydrocyclone (LLHC). Summary of the Invention

[0004] In accordance with the objectives of the present invention, as embodied and generally described herein, one embodiment of a method for separating oil droplets from water in a mixture thereof includes the steps of: flowing the mixture through a first length of pipe having a first outlet orifice; generating a first acoustic standing wave within the first length of pipe effective to generate an acoustic radiation force having a first pressure node, whereby the oil droplets coalesce and coalesce at the first pressure node, the acoustic wave having a first frequency; flowing the mixture having the coalesced and coalesced oil droplets and passing through the first outlet orifice through a second length of pipe having a second outlet orifice, whereby the second length of pipe is formed in a planar configuration having a radius of curvature and an axis, whereby the coalesced oil droplets move a greater distance from the axis as a result of centrifugal force; and separating the coalesced oil droplets from the water passing through the second outlet orifice.

[0005] In another aspect of the invention, according to its objects, an embodiment of a method for separating oil droplets from water in a mixture thereof includes the steps of flowing the mixture through a first length of pipe having a first exit orifice, generating a first acoustic standing wave within the first length of pipe effective to generate an acoustic radiation force having a first pressure node, whereby the oil droplets coalesce and coalesce at the first pressure node, the acoustic wave generating the first acoustic standing wave having a first frequency, and flowing the mixture with the coalesced and coalesced oil droplets through a second length of pipe having a second exit orifice, the second length of pipe being formed into a planar configuration having a radius of curvature and an axis, whereby the coalesced oil droplets move a greater distance from the axis as a result of centrifugal force. and flowing the mixture having aggregated and coalesced oil droplets through a third length of pipe vertically oriented and having upper and lower exit orifices and passing through the second exit orifice, whereby the aggregated and coalesced oil droplets separate from the water as a result of the buoyancy of the coalesced oil droplets and pass through the upper exit orifice, and flowing the mixture having aggregated and coalesced oil droplets through an outlet of the second length of pipe vertically oriented and having upper and lower exit orifices, whereby the coalesced oil droplets separate from the water as a result of the buoyancy of the coalesced oil droplets and pass through the upper exit orifice of the third length of pipe and the water passes through its lower exit orifice.

[0006] In another aspect of the invention, according to its objects, one embodiment of an apparatus for separating oil droplets from water in a mixture thereof comprises a first length of pipe through which the mixture is flowed, the first length of pipe having an outer surface, an interior volume, and a first exit orifice, and at least one first acoustic transducer in acoustic contact with an outer surface of the first length of pipe for generating a first acoustic standing wave within the interior volume of the first length of pipe effective to generate an acoustic radiation force having a first pressure node, whereby the oil droplets are separated from the water by a first pressure node. The system includes at least one first acoustic transducer that agglomerates and coalesces at a force node, the acoustic waves having a first frequency; a first waveform generator for actuating the at least one first acoustic transducer; and a second length of pipe having a second exit orifice for receiving the agglomerated and coalesced oil droplets flowing through a first exit orifice of the first length of pipe, the second length of pipe being formed in a planar configuration having a radius of curvature and an axis, whereby the coalesced oil droplets move a greater distance from the axis as a result of centrifugal force.

[0007] In another aspect of the invention, according to its objects, one embodiment of an apparatus for separating oil droplets from water in a mixture thereof includes a first length of pipe through which the mixture is flowed, the first length of pipe having an exterior surface, an interior volume, and a first exit orifice; at least one first acoustic transducer in acoustic contact with an exterior surface of the first length of pipe for generating a first acoustic standing wave effective to generate an acoustic radiation force having a first pressure node within the interior volume of the first length of pipe, whereby the oil droplets condense and coalesce at the first pressure node, the acoustic wave having a first frequency; a first waveform generator for actuating the at least one first acoustic transducer; and a first waveform generator for actuating the at least one first acoustic transducer. a second length of pipe having a second outlet orifice for receiving the coalesced and coalesced oil droplets flowing through the first outlet orifice, the second length of pipe being formed in a planar configuration having a radius of curvature and an axis whereby the coalesced oil droplets move a greater distance from the axis as a result of centrifugal force; and a third length of pipe for receiving the coalesced and coalesced oil droplets flowing through the second outlet orifice, vertically oriented and having an upper outlet orifice and a lower outlet orifice, whereby the coalesced and coalesced oil droplets separate from the water as a result of buoyancy of the coalesced oil droplets and pass through the upper outlet orifice of the third length of pipe and the water passes through its lower outlet orifice.

[0008] Benefits and advantages of the present invention include, but are not limited to, providing an apparatus and method for separating oil from water in their emulsions without the need for large heated separation tanks as currently used, thereby reducing maintenance and improving energy efficiency.

[0009] The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. [Brief description of the drawings]

[0010] [Figure 1] 1A is a schematic diagram of a side perspective view of one embodiment of an apparatus for bulk separation of oil from water showing the horizontal standing wave nodal planes generated in a vertical cylinder by an acoustic transducer located on its bottom, whereby oil droplets in an oil / water emulsion aggregate and coalesce at these planes. 1B shows the resulting formation of an oil layer on the surface of the water due to the increased buoyancy of the coalesced droplets. [Diagram 2] Figure 2A shows the direction of forces on the oil droplets in the apparatus of Figures 1A and 1B of the present application, showing standing wave nodal planes that are parallel circular surfaces with horizontal orientation, with acoustic radiation forces (ARFs) directed towards them from both sides, and gravitational buoyancy forces acting only in an upward vertical direction. Figure 2B shows the progression of this separation process, where the buoyancy forces increase at a later stage as the oil droplets grow in size due to agglomeration. Figure 2C shows the progression of this separation process, where the buoyancy forces increase at a later stage as the oil droplets grow in size due to agglomeration. [Diagram 3] FIG. 2 is a schematic diagram of a side view of a continuous-flow separation system that operates similarly to that of the bulk separation system of FIGS. 1A and 1B, but can be easily scaled to larger dimensions. [Figure 4] Figure 4A is a schematic diagram of a cross-section of a separation chamber with a circular cross-section used as a resonator cavity, where oil droplets are collected in concentric circular planes. Figure 4B is a schematic diagram of a cross-section of a resonator cavity where the symmetry of the cavity is disrupted by making the shape of the pipe slightly elliptical, such that the nodal planes collapse into lines parallel to the surface of the resonator. Figure 4C is a schematic diagram of a side perspective view of the elliptical pipe shown in Figure 4B. [Diagram 5] FIG. 1 is a schematic diagram of a top perspective view of one embodiment of a hybrid acoustic / centrifugal system of the present invention. [Figure 6]Figure 6A is a schematic diagram of a cross section of the pipe shown in Figure 5 of the present application, showing the main forces acting on an oil droplet in water inside a circular pipe section, resulting in a schematic oil collection area, and Figure 6B shows complete separation of the oil and water. [Figure 7] FIG. 13 is a schematic diagram of a side perspective view of multiple stacked separation units operating as a continuous pipe formed in a spiral or helical configuration, providing further separation of oil from water in a compact configuration. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Water found in oil wells is common and burdensome for operators. Current water / oil separation processes are slow, environmentally unfriendly, and consume a lot of energy for heating. On offshore and floating drilling platforms, space is limited and separation equipment with small size and weight, and therefore low energy consumption, is desired. Heating the oil and water mixture in a pressure vessel and using gravity for separation of the components, as well as the need for a series of separation systems (pressure vessels and separation tanks) to achieve the desired separation quality, requires significant space. Weight is also a significant issue for offshore platforms.

[0012] A simple, inexpensive device with a small footprint that could work on the seabed and offshore floating platforms and separate oil from water in real time without the use of large settling tanks could save the industry significant amounts of money in operations and maintenance costs, in addition to saving space and eliminating many environmental problems.

[0013] Briefly, embodiments of the present invention include an apparatus and method for separating oil from water in a generated oil / water mixture without requiring a large tank in which the fluid is heated and without relying on gravity for the separation of the components. Rather, embodiments of the present invention use a simple, energy-efficient separation process that utilizes acoustic radiation forces that function as acoustic gravity, but with some differences. The mixture is flowed through a pipe of any length in which acoustic standing waves are generated that are effective to generate acoustic radiation forces having pressure nodes at which the oil droplets coagulate and coalesce, and then the mixture is flowed through a pipe of any length having a planar circular configuration with any axis, whereby the coalesced oil droplets move a greater distance from the pipe axis as a result of centrifugal forces, and the coalesced oil droplets can be more easily separated from the water.

[0014] Acoustic radiation force is a time-averaged force acting on objects in an acoustic field; objects in the wave path absorb or reflect the sound energy. Acoustic separation of solids (particles) from fluids and liquid-liquid separation is well known and used for small-scale separation processes, with microfluidics being an important application area, but this technology has not been used for large-scale separation, for example in the energy industry. This is because acoustic forces alone cannot be easily scaled to larger dimensions to rapidly separate fluid mixtures with low energy consumption. Acoustic separation processes become inefficient at large dimensions due to the power required to excite large ultrasonic transducers. Furthermore, at high acoustic powers, acoustic streaming becomes dominant, which reduces separation.

[0015] Furthermore, centrifugal, Coriolis, and gravity / buoyancy forces by themselves are not effective in completely separating two-component fluids, especially when the fluids have similar densities, such as heavy oil and water. However, combining both primary and secondary acoustic forces and utilizing the fluid flow results in a synergistic effect where most of the energy of separation comes from the kinetic energy of the flowing fluid itself, while the fluid water dynamics, nucleation, and medium nonlinearity combine to significantly improve the energy efficiency of the separation process. Thus, it is possible to use a section of pipe instead of a large separation vessel to perform the separation process. The acoustic radiation force initiates the mixture separation process and contributes to fluid droplet coalescence, which further enhances the initiation of separation. And the centrifugal force from the flowing liquid in the pipe with a curved shape completes the separation process of the fluid mixtures (both fluid emulsions (oil-water-gas) and suspensions (particle-fluid)) without requiring large acoustic energy input. It is expected that embodiments of the present invention will be effective in subsea operations where it is desired to separate the water from the oil before pumping the oil from the seabed to land. Another important application is floating oil platforms.

[0016] A. Acoustic radiation force All forms of wave motion, including electromagnetic waves, transverse waves on elastic strings, surface waves on liquids, and longitudinal sound waves, exert unidirectional reflective forces on absorbing and reflecting obstacles in their path. Similarly, acoustic radiation force is a physical phenomenon resulting from the interaction of an acoustic wave with obstacles placed in its path and can be interpreted as a time-averaged force acting on objects in an acoustic field. The magnitude of the force depends on the size, density, and compressibility of the object, as well as the properties of the host medium, such as compressibility, density, and speed of sound. Usually, the effect of host medium viscosity is very small. The following equation shows the dependence of the force on various parameters: TIFF2025069307000002.tif2197, where TIFF2025069307000003.tif2097, where Vp = droplet (particle) volume, β = compressibility, ρ = density, λ = wavelength of sound, P0 = peak acoustic pressure, z = distance from the pressure node, m, p = host and droplet (subscripts).

[0017] Equation 2 defines the acoustic contrast factor. The sign of this factor determines in which direction an object will be pushed if it is placed in a resonant sound field inside a cavity. This means that if the frequency of the sound wave is adjusted so that a standing wave is induced in the cavity by reflection from the other side of the acoustic source, small objects will collect quickly at the pressure node. The collection time depends on the residence time, and the acoustic frequency, as well as the power. Furthermore, as fluid droplets or particles approach each other, the attractive Bjerknes force becomes stronger, which makes the pattern more compact and the droplets coalesce.

[0018] Reference will now be made in detail to the present embodiments of the invention, examples of which are illustrated in the accompanying drawings. In the figures, like structures are identified with the same reference numerals. It will be understood that the figures are presented for the purpose of illustrating particular embodiments of the invention and are not intended to limit the invention thereto. Turning now to Figures 1A and 1B, shown is a schematic diagram of a side perspective view of one embodiment of an apparatus 10 for bulk separation of oil from water. A transducer 12 excited by a waveform generator 14 through a power amplifier 15 is disposed on the bottom of a vertical cylinder 16 containing an emulsion 18 of oil in water. The excitation frequency may be in the range of about 200 kHz to about 3 MHz. The power level depends on the depth and shape of the fluid, but is typically between 5 W and 100 W for laboratory applications. The cylinder 16 may have a bottom surface 20 to which the transducer 12 is acoustically coupled, or a bottom surface formed by the transducer 12. The cylinder may be open at a top end 22 or may have a top surface (not shown). When the transducer 12 is operated at a selected frequency, a vertical acoustic field is generated by reflection of the sound waves from the transducer 12 off the top surface 24 of the emulsion 18 due to the large acoustic impedance mismatch between the emulsion and the material (e.g., air) or solid above the liquid. Small oil droplets in the emulsion 18 aggregate and coalesce at the horizontal standing wave node planes 26, increasing in size and rising to the next upper node, and this process becomes an avalanche because larger droplets exhibit a larger buoyancy force, which enhances their gravitational separation. The direction of the force on the oil droplets is shown in Figures 2A-2C, where Figure 2A shows the standing wave node planes 26 as parallel circular surfaces and horizontal in orientation. Acoustic radiation forces (ARFs) 32a, 32b are directed towards these node planes from both sides (shown as top and bottom) and push the oil droplets 34 towards the node planes. In contrast, gravitational buoyancy force 36 acts only in the upward vertical direction. Figures 2B and 2C show the progression of this separation process, where the buoyancy force increases at a later stage as the oil droplets 34 increase in size due to coalescence.

[0019] In addition to these forces, there is also possible acoustic streaming, where sound waves from an excitation source in the liquid induce the suspension to flow, which usually occurs when the sound intensity exceeds a certain threshold that depends on the fluid properties. The ARF, gravity and acoustic streaming forces combine synergistically, which results in rapid oil separation, so that the oil droplets collect in an oil layer 28 on the surface of the water 30, as can be seen in FIG. 1B of the present application. The oil separation process may take place in ∼1 second, and this time can be shortened by varying the frequency applied to the transducer placed at the bottom, starting with a higher frequency (greater than 1 MHz), and lowering it to several hundred kHz as multiple droplets form and their size increases by coalescence. This highly dynamic process requires that the varying frequency be tuned to the process dynamics (e.g., time scale) and the applied frequency. The standing waves and nodal planes remain in the fluid as invisible scaffoldings as long as the acoustic transducer is electrically driven.

[0020] The apparatus shown in Figures 1A and 1B can also accommodate multiple large separation tanks, where several acoustic transducers can be mounted near the bottom side of the tank, the number depending on the size of the tank. The transducers do not need to be located on the bottom, but can be placed to cover a wide range of angles on the outer surface of a typical circular and horizontal separation tank. That is, the transducers can be placed anywhere from half the liquid height to the bottom of the tank, with the purpose of doing so being to significantly speed up the separation process compared to simple gravity separation. Thereby, the size of the separation tank can be reduced, which is a major goal for oil platforms with limited available space. In summary, this approach utilizes acoustics to generate multiple droplets of oil in an oil-water mixture and encourage their coalescence, after which gravity and acoustic fluid forces can complete the final separation of the oil from the water.

[0021] 1A and 1B show a stationary device, devices can be designed with flowing emulsions where an oil-water mixture enters the device at the side near the bottom and separated oil and water streams exit the device at the side near the top, as well as for liquid droplets, gas bubbles, and solid particles.

[0022] FIG. 3 is a schematic diagram of a side view of a continuous flow separation system 50 that operates similarly to that of the bulk separation system of FIGS. 1A and 1B, but can be easily scaled to larger dimensions. The separation chamber 52 has a crude oil-water mixture 54 flowed through it from a source 56 by a pump system 58, a first acoustic piezoelectric transducer 60 driven by a waveform generator 62, and a second acoustic piezoelectric transducer 64 driven by a waveform generator 66, in acoustic contact with its outer surface. Either waveform generator can generate any standard waveform (sine, square, triangular, pulse, chirp, or any shape generated by any desired formula, such as a sine-modulated Gaussian wave). A reflector 68 is located on the opposite side of the separate chamber 52 from the first piezoelectric transducer 60 and the second piezoelectric transducer 64. If the chamber 52 comprises metal, the reflector 68 is not necessary, since the wall 69 is an efficient reflector.

[0023] The first transducer 60 is excited at a frequency in the range between about 500 kHz and about 3 MHz, which performs a so-called pre-treatment step for the oil-water mixture, where very small oil droplets (larger than 10 μm) separate from the mixture by ARF and start to coalesce to produce larger droplets. The second transducer 64 operates at a lower frequency, usually the first transducer frequency divided by any integer. Both transducers then generate standing waves in the fluid mixture, which can separate droplets of different sizes. That is, as gravity becomes dominant, the droplets become larger in size and the buoyancy forces become greater, so the coalescence process continues more efficiently with the lower frequency. Although the two transducers are shown adjacent to each other, they can be slightly offset laterally from each other without affecting the separation process. The acoustic power applied to the transducer is in the range of between about 50 W and about 500 W for a large tank, but can be scaled to higher output powers using liquid-cooled transducers. That is, higher powers can also be applied to the isolation vessel, but may require the transducer to have effective heat sink capability and liquid cooling to prevent overheating of the transducer crystal. If required for design simplicity, the frequency of the transducer 64 is further amplitude and frequency modulated at frequencies in the range of between about 500 Hz and about 10 kHz, the range depending on the size and shape of the chamber, which may be circular rather than rectangular as shown.

[0024] In this manner, an acoustic standing wave 70 is generated by the transducer 60 and the reflector 68 on which the oil droplets may coalesce. A waveform generator 66 modulates the transducer 64, causing the droplets coalescing within the nodes and anti-nodes of the standing wave 70 to further agglomerate (72) and form larger droplets (73), thereby increasing their buoyancy, thereby separating the oil-rich fraction 74 from the water-rich fraction 76 within the flow system, which may then be collected in a separate vessel.

[0025] B. Oil flocculation lines FIG. 4A is a schematic diagram of a cross-section of a separation chamber 80, having a circular cross-section (e.g., a pipe section) used as a resonator cavity driven by a transducer 82 in acoustic contact with its outer surface 84, excited by a waveform generator 86, to separate and coalesce oil droplets collected in a concentric circular plane 88. The transducer 82 may be an acoustic piezoelectric transducer, and an amplifier may be used to increase the output of the waveform generator 86 in some circumstances. FIG. 4B is a schematic diagram of a cross-section of the resonator cavity 80, where the symmetry of the cavity is disrupted by making the shape of the pipe slightly elliptical. When actuated by the waveform generator 86, the once circular nodal plane 88 generated by the transducer 82 (FIG. 4A) collapses into a line 90 parallel to the resonator surface 84. FIG. 4C is a schematic diagram of a side perspective view of the elliptical pipe shown in FIG. 4B.

[0026] There are various ways to excite a pipe other than excitation by an external acoustic transducer (e.g., a piezoelectric transducer) 82 attached to the outer surface 84 of the pipe 80 as shown in Figures 4A-4C. For example, U.S. Patent No. 8,783,109 by Kaduchak et al., entitled "Ultrasonic analyte concentration and application in flow cytometry," introduces the concept of a line-driven tube, where a single rectangular piezoelectric crystal making line contact with the outer surface of a capillary tube can excite the tube such that particles can agglomerate to the axis of the capillary tube through which a suspension of particles flows. The inventors have found that this works well for capillary tubes with diameters smaller than a few millimeters, but for tubes with diameters >2 cm, it is also possible to mount a curved piezoelectric transducer on the outer surface of the pipe to excite the higher frequency vibration modes required to generate multiple concentric rings. The capillary tube excitation is designed to operate in the dipole mode of vibration of the tube, which for a large diameter pipe (e.g., 6 inches), will be within the audible range. Thus, the pipe is driven at frequencies above the audible range, and the inventors have found that this can be done. Depending on the acoustic field strength required for condensation, partial to full coverage of the outer surface of the pipe can be used. Since curved transducers, even flat transducers with curved fronts, are more expensive, it is preferable to use multiple long (about 0.2 to about 1 times the outer diameter of the pipe) piezoelectric transducers, each with a small curvature, that can be easily attached to a surface. The number of collection lines 90 depends on the frequency of the sound applied, since the wavelength of the sound must match the dimension of the resonator width (for a pipe, the pipe diameter) and cannot generate standing waves at all frequencies.

[0027] For circular shapes, the ring structure shown is derived from mathematical Bessel functions. For elliptical configurations, the frequency does not change perceptibly. See, for example, U.S. Patent No. 6,644,118 by Sinha et al., entitled "Cylindrical acoustic levitator / concentrator having non-circular cross section." The standing waves that occur depend on the mechanical structure of the pipe and the elastic properties of the material. The frequency range of interest is tens of kHz, and the pipe itself vibrates along its length to generate standing waves. In conventional configurations, acoustic standing waves are limited to the dimensions of the transducer and the fluid region between the transducer and the reflector. At lower frequencies, structural vibration removes that limitation; i.e., concentric cylinders as nodal planes can be set up over long lengths of pipe. For example, the inventors have found that if the pipe is excited at any location, the effect persists for a length of approximately 20 pipe diameters, and if transducers are placed periodically along the surface of the pipe, this is an efficient way to separate oil and water over long lengths of pipe, since the entire pipe does not have to be excited. In situations where the use of elliptical pipes is impractical, pipes of circular cross section can be effective, albeit at slightly larger lengths. As previously mentioned, external excitation with transducers at predetermined locations on the surface of the pipe (FIGS. 4A-4C) has been found to be effective in producing a flocculation effect that extends beyond 20 tube diameters. See, for example, U.S. Patent No. 7,340,957 by Gregory Kaduchak et al., entitled "Ultrasonic analyte concentration and application in flow cytometry." Thus, only a few piezoelectric sources are required, and the pipe does not have to be driven along its entire length.For further details, see "Acoustic concentration OF particles in piezoelectric tubes: Theoretical modeling OF cavity shape and symmetry breaking" by Shulim Kogan, Gregory Kaduchak, and Dipen N. Sinha, J. Acoustic. Soc. Am., 116(41), 1967-1974 (2004) and U.S. Patent No. 6,644,118 by Gregory Kaduchak and Dipen N. Sinha, entitled "Cylindrical acoustic levitator / concentrator having non-circular cross-section."

[0028] C. Centrifugal hybrid system Centrifugal separators are effective in removing droplets larger than 100 μm in diameter from emulsions, and are often used as a final step in the extraction of small amounts of oil remaining in water after primary separation to a minimum level at which the water can legally be discharged into the ocean. In a centrifuge or cyclone separator, centrifugal forces act on the droplets with a force several times greater than that of gravity as it flows into the cylindrical separator. Separation of immiscible liquids, such as oil, from water in a centrifuge can be accomplished similarly to separation of solids from liquids; however, it is more difficult. The density difference between the different liquids is generally smaller, and the presence of shear can result in breakup rather than coalescence of droplets of the dispersed phase.

[0029] When oil is produced, unintended by-products of oil production are also produced: water, sand, drilling fluids and cuttings, called production wastes. Produced water is the most important of these wastes because it is produced in large quantities. For example, on average, four barrels of water are produced for every barrel of oil during the life of a reservoir. Produced water also contains substances that can be harmful to the environment. Thus, its discharge into the ocean after it is brought to land is subject to stricter regulations, even though separated produced water can be used for water injection.

[0030] For various reasons, there is a need for subsea separation by separating the water and oil before sending them to land. A similar situation exists for offshore oil platforms. For example, there is the cost of bringing the oil-water mixture to land and then separating the oil from the water, since the water then needs to be treated after removing contaminants. Therefore, subsea separation devices constitute a key component to any subsea installation due to their economic and environmental advantages. For both subsea and offshore platform operations, current gravity or centrifuges often require chemical treatments using emulsifiers that suppress the formation of emulsions and prevent foaming, making it more difficult to separate the oil from the water. To reduce costs and make the separation process more efficient, it is important to find a way to separate the oil and water without resorting to such chemical treatments.

[0031] Cyclone separators rely on a single mechanism, namely centrifugal force, which depends on density difference, droplet volume, and velocity. For many types of oil, such as heavy oil, the density difference between oil and water is small. Therefore, the separation force is not high unless the droplet volume is large enough and the velocity is high. In contrast, ARF depends on density difference, compressibility difference, droplet volume, and acoustic frequency, so there is enough force to separate oil from water even when the density difference is minimal and the droplet size is small. Thus, ARF can be effective as a root process where oil is initially separated from water at a microscopic level and begins to coalesce, thereby forming larger droplets, where centrifugal force can become the dominant separation mechanism. Such a hybrid separation system can be formed more compact and more efficient than a centrifuge system alone.

[0032] Turning now to FIG. 5, a schematic diagram of a top perspective view of one embodiment of a hybrid acoustic / centrifuge system 100 of the present invention is shown. A section of hollow pipe 102 is formed in a circular shape 104, arranged horizontally, and has transducers 106 attached to its surface 108 at regular selected intervals. The transducers 106, which may be acoustic piezoelectric transducers, are excited using a waveform generator 110 through an amplifier 112 and are shown attached to the interior surface of the curved pipe 102 at intervals, although they could be located on the outside. The oil-water mixture or emulsion 54 is forced through the pipe 102 using a pump 114, through a horizontal inlet 120, and out through an outlet port 116 in fluid communication with a vertical pipe 118. In some subsea applications, the pump 114 becomes unnecessary as the oil / water mixture is under significant pressure. The pipe 118 has a selected length, not shown to scale, with an upper outlet 122 for oil-rich fluid and a lower outlet 124 for water-rich fluid at either end. The selected length depends on the diameter of the pipe 102, and the outlet locations 122 and 124 may not be in the vicinity of the separation device. Acoustic and centrifugal forces separate the oil micro-droplets from the mixed fluid, and the vertical stage 118 physically preferentially channels the separated fluid to two separate outlets. Because oil is less dense than water, the separated oil reaches the upper outlet location. Once separated, the oil and water streams can be directed, as an example, to a storage tank for the oil and a water treatment unit for the water. A high frequency transducer 126 excited using a waveform generator 128 through an amplifier 130 pre-conditions the oil-water emulsion 54 as it enters the pipe 102, as described above, forming spatially isolated nodes parallel to the direction of flow, as shown in FIG. 3 of the present application. Centrifugal force then causes the agglomerated droplets to move perpendicular to the flow to accomplish the separation process.

[0033] Although a circular cross section of the pipe 102 is expected to be effective in separating oil from water, for maximum efficiency the pipe 102 should be oblate (e.g., elliptical) in cross section. As shown in Figures 4B and 4C, the transducers 82 are placed in acoustic contact with the outer surface 84 of the elliptical pipe, and may be spaced along the pipe as necessary. Typically the major axis of the ellipse should be perpendicular to the radius of the circular loop shown in Figure 5. To further illustrate this shape, in the extreme case a flat pipe is formed into a loop whose width is perpendicular to the radial plane. It is easier to bend the elliptical pipe slightly in that direction. A slight change in the shape of the helix, shown in Figure 7, does not result in a significant change. Typically an oblong pipe is not required. For example, the minor to major axis ratio of the ellipse may range between about 0.8 and about 0.99, and localized aggregation of particles (droplets) still occurs. See, for example, Figure 9A of U.S. Patent No. 6,644,118 (ibid.). It is noted that although the '118 patent describes an elliptical piezoelectric cylinder for flowing fluids, the inventors have found that an elliptical pipe having transducers in acoustic contact with its outer surface produces a similar acoustic force pattern. It is also noted that the pipe 102 need not be precisely circular in its curvature, but simply curved to produce the necessary centrifugal force.

[0034] 6A and 6B are schematic diagrams of a cross section of a pipe 102 showing the major forces acting on a droplet in water inside a circular pipe section. The forces include (1) centrifugal force 132, which depends on density difference, droplet volume, and flow rate; (2) gravity force 134, which depends on density difference; and (3) acoustic force. The acoustic force encourages small droplets to pack together and coalesce. Viscous drag forces oppose any droplet movement, but are not shown in FIG. 6A because the major forces dominate at a later stage. Gravity force 134 provides a buoyancy force that becomes significant as droplets 136 coalesce and grow in size. The magnitude of the centrifugal force depends on the droplet size and the pipe curvature as well as the fluid flow rate. These forces act to move the oil 138 down a selected length of the pipe to one side of the pipe, as shown in FIGS. 6A and 6B. A schematic oil collection area 138 is shown in Figure 6A, while Figure 6B shows complete separation of the oil 140 and water 142. In practice, complete separation of the two phases (oil and water) is not necessary, and usually large oil globs and droplets are pushed to one side. The separation is sufficient for final separation of the oil and water in the vertical separator stage 118, which allows the separator to be more compact.

[0035] At higher temperatures, the density difference is greater, thereby increasing the force on the droplets. Additionally, higher temperatures result in lower viscosity, which encourages the separated droplets to move to one side of the pipe.

[0036] 7 is a schematic diagram of a side perspective view of multiple stacked separation units 102, as shown in FIG. 5 of the present application, formed into a spiral or helical configuration operating as a continuous pipe 144 and having an increased length. This configuration provides further separation of oil from water in a compact configuration. Obviously, other configurations can be envisioned.

[0037] Water moves tangentially through a host medium in a pipe of radius of curvature R and diameter D o and density ρ oA single oil droplet is subjected to a centrifugal force Fc. TIFF2025069307000004.tif1995

[0038] The presence of vortices, nucleation centers, and small bubbles further enhances the separation. In subsea operations, the pressure can be high, which causes the oil-water mixture to have the form of an emulsion with very small droplet sizes. Acoustic separation and droplet coalescence to result in larger droplets significantly aids centrifugation, as larger droplets experience stronger separation forces, as can be seen from FIG. 3.

[0039] The advantages that acoustic separation has over centrifugation can be seen by equations 2 and 3. When the oil and water densities are the same, there is still a strong force acting on the droplets, since the compressibility is usually quite different. The acoustic force depends on both density and compressibility, as well as the droplet size and other factors, whereas for the centrifugal force, it disappears when the oil and water densities are the same, but there may be some secondary effects that act. Therefore, it is more reasonable to acoustically initiate the separation process, thereby generating larger droplets and subsequent centrifugal forces, than to use only centrifugal forces, as is done in hydrocyclones.

[0040] The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed; obviously, many modifications and variations are possible in light of the above teachings. The embodiments have been chosen and described to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to best utilize the invention in various embodiments and with various modifications suited to the particular uses contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.

Claims

1. 1. A method for separating oil droplets from water in a mixture thereof, comprising: flowing the mixture through a first length of pipe having a first exit orifice; generating a first acoustic standing wave effective to generate an acoustic radiation force having a first pressure node within the first length of pipe, whereby the oil droplets aggregate and coalesce at the first pressure node, generating a first acoustic standing wave having an acoustic wave having a first frequency; flowing the mixture having the aggregated and coalesced oil droplets passing through the first exit orifice through a second length of pipe having a second exit orifice, the second length of pipe being formed in a planar configuration having a radius of curvature and an axis; generating an acoustic standing wave within the second length of pipe, the acoustic wave having a frequency lower than the first frequency; separating the coalesced oil droplets from the water passing through the second outlet office; A method comprising:

2. Separating the coalesced oil droplets from the water includes: flowing the mixture, with the coalesced and combined oil droplets passing through an outlet of the second length of pipe, through a third length of pipe vertically disposed and having an upper exit orifice and a lower exit orifice, whereby the coalesced and combined oil droplets separate from the water as a result of the buoyancy of the combined oil droplets and pass through the upper exit orifice and the water passes through the lower exit orifice. The method of claim 1 , comprising:

3. the first frequency and the second frequency are between 500 kHz and 3 MHz; The method of claim 1.

4. the first frequency is greater than the second frequency; The method of claim 1.

5. the second length of pipe is formed into a helical configuration having a radius of curvature and an axis; The axis is vertical. The method of claim 1.

6. The first length of pipe and the second length of pipe have an oblate cross section. The method of claim 1.

7. 1. A method for separating oil droplets from water in a mixture thereof, comprising: flowing the mixture through a first length of pipe having a first outlet; generating a first acoustic standing wave effective to generate an acoustic radiation force having a first pressure node within the first length of pipe, whereby the oil droplets aggregate and coalesce at the first pressure node, generating a first acoustic standing wave having an acoustic wave having a first frequency; flowing the mixture having aggregated and coalesced oil droplets passing through the first outlet through a second length of pipe having a second diameter and a second outlet, the second length of pipe being formed in a planar circular configuration having a radius of curvature and an axis; generating an acoustic standing wave within the second length of pipe, the acoustic wave having a frequency lower than the first frequency; flowing the mixture, with the aggregated and coalesced oil droplets passing through a second outlet orifice, through a third length of pipe vertically disposed and having an upper outlet orifice and a lower outlet orifice, whereby the aggregated and coalesced oil droplets separate from the water as a result of buoyancy of the coalesced oil droplets and pass through the upper outlet orifice of the third length of pipe and the water passes through the lower outlet orifice. The method includes:

8. the first frequency and the second frequency are between 500 kHz and 3 MHz; The method of claim 7.

9. the first frequency is greater than the second frequency; The method of claim 8.

10. the second length of pipe is formed into a helical configuration having a radius of curvature and an axis; The axis is vertical. The method of claim 7.

11. The first length of pipe and the second length of pipe have an oblate cross section. The method of claim 7.

12. 1. An apparatus for separating oil droplets from water in a mixture thereof, comprising: a first length of pipe through which the mixture is flowed, the first length of pipe having an exterior surface, an interior volume, and a first exit orifice; at least one first acoustic transducer in acoustic contact with the exterior surface of the first length of pipe for generating a first acoustic standing wave effective to generate an acoustic radiation force having a first pressure node within the interior volume of the first length of pipe, whereby the oil droplets aggregate and coalesce at the first pressure node, and an acoustic wave having a first frequency; a first waveform generator for activating the at least one first acoustic transducer; a second length of pipe having a second outlet orifice for receiving the coalesced and coalesced oil droplets flowing through the first outlet orifice of the first length of pipe, the second length of pipe being formed in a planar configuration having a radius of curvature and an axis whereby the coalesced oil droplets move a greater distance from the axis as a result of centrifugal force; at least one acoustic transducer in acoustic contact with the exterior surface of the second length of pipe for generating acoustic standing waves within the interior volume of the second length of pipe, wherein acoustic waves have a frequency lower than the first frequency; a waveform generator for actuating the at least one acoustic transducer; and An apparatus comprising:

13. a third length of pipe for receiving the coalesced and combined oil droplets flowing through the second outlet orifice, the third length of pipe being vertically disposed and having an upper outlet orifice and a lower outlet orifice, whereby the coalesced and combined oil droplets separate from the water as a result of buoyancy of the combined oil droplets and pass through the upper outlet orifice of the third length of pipe and the water passes through the lower outlet orifice thereof. The apparatus of claim 12 further comprising:

14. the first frequency and the second frequency are between 500 kHz and 3 MHz; 13. The apparatus of claim 12.

15. the at least one first acoustic transducer and the at least one second acoustic transducer comprise piezoelectric transducers.

13. The apparatus of claim 12.

16. the first frequency is greater than the second frequency; 15. The apparatus of claim 14.

17. the second length of pipe is formed into a helical configuration having a radius of curvature and an axis; The axis is vertical.

13. The apparatus of claim 12.

18. The first length of pipe and the second length of pipe have an oblate cross section.

13. The apparatus of claim 12.

19. 1. An apparatus for separating oil droplets from water in a mixture thereof, comprising: a first length of pipe through which the mixture is flowed, the first length of pipe having an exterior surface, an interior volume, and a first exit orifice; at least one first acoustic transducer in acoustic contact with the exterior surface of the first length of pipe for generating a first acoustic standing wave effective to generate an acoustic radiation force having a first pressure node within the interior volume of the first length of pipe, whereby the oil droplets aggregate and coalesce at the first pressure node, and an acoustic wave having a first frequency; a first waveform generator for activating the at least one first acoustic transducer; a second length of pipe having a second outlet orifice for receiving the coalesced and combined oil droplets flowing through the first outlet orifice of the first length of pipe, the second length of pipe being formed in a planar configuration having a radius of curvature and an axis, whereby the combined oil droplets move a greater distance from the axis as a result of centrifugal force; at least one acoustic transducer in acoustic contact with the exterior surface of the second length of pipe for generating acoustic standing waves within the interior volume of the second length of pipe, wherein acoustic waves have a frequency lower than the first frequency; a waveform generator for actuating the at least one acoustic transducer; and a third length of pipe for receiving the coalesced and combined oil droplets flowing through the second outlet orifice, the third length of pipe being vertically oriented and having an upper outlet orifice and a lower outlet orifice, whereby the coalesced and combined oil droplets separate from the water as a result of buoyancy of the combined oil droplets and pass through the upper outlet orifice of the third length of pipe and the water passes through the lower outlet orifice thereof; An apparatus comprising:

20. the first frequency and the second frequency are between 500 kHz and 3 MHz; 20. The apparatus of claim 19.

21. the at least one first acoustic transducer and the at least one second acoustic transducer comprise piezoelectric transducers.

20. The apparatus of claim 19.

22. the first frequency is greater than the second frequency; 21. The apparatus of claim 20.

23. the second length of pipe is formed into a helical configuration having a radius of curvature and an axis; The axis is vertical.

20. The apparatus of claim 19.

24. The first length of pipe and the second length of pipe have an oblate cross section.

20. The apparatus of claim 19.

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