GB2463275A

GB2463275A - Apparatus and method for separating a multiphase fluid

Info

Publication number: GB2463275A
Application number: GB0816266A
Authority: GB
Inventors: Kenneth James Adams; Georgios Dimitrakis; Badrul Huda; Sam Kingman; Edward Henry Lester; John Peter Robinson
Original assignee: BP Exploration Operating Co Ltd
Current assignee: BP Exploration Operating Co Ltd
Priority date: 2008-09-05
Filing date: 2008-09-05
Publication date: 2010-03-10
Also published as: GB0816266D0

Abstract

An apparatus and method for separating a multiphase fluid comprising an aqueous phase, and a liquid hydrocarbon phase wherein at least a portion of the aqueous phase is dispersed in a liquid hydrocarbon phase into its component phases, is described. The apparatus comprises a vessel 1 having an inlet for receiving the multiphase fluid and at least one outlet. An array of one or more element pairs 2, 4 are located in the vessel. A first element 2 in each pair comprises an RF electrode. RF radiation is directed by the RF electrode into the vessel to heat the aqueous phase dispersed in the multiphase fluid and to assist in separation, of the-multiphase fluid into component phases and a second element 4 in each pair is arcuate to reflect RF radiation into the multiphase fluid.

Description

I

APPARATUS AND METHOD FOR ASSISTING IN SEPARATING A

MULTIPHASE FLUID

The present invention relates to apparatus and method for assisting in separating a multiphase fluid comprising an aqueous phase, a liquid hydrocarbon phase, and optionally a gaseous phase in particular, for separating emulsions of produced liquid hydrocarbons and water.

OiL/water separation is a fundamental process in upstream oil production. An oillwater emulsion is produced from wells and transported to a separator tank or vessel through a flowline. The emulsion is in the form of water droplets dispersed in an oil phase.

In practice, produced liquid hydrocarbons, in particular, crude oils, form three layers in a separation vessel, these being an upper liquid hydrocarbon layer, a lower aqueous layer and an intermediate emulsion layer (often referred to as a "rag layer"). It is necessary to separate the water from the oil and typically this is carried out in large separation tanks where, with sufficient residence time, the phases will separate due to density difference, with the water dropping out of the oil phase. The different layers may then be removed from the tank. The emulsion of the intermediate emulsion layer may be stabilized by naturally occurring surfactants in the produced fluid, however, the demulsification of the intermediate emulsion layer can be problematic.

In order to enhance the speed of separation and reduce the size of the separation tanks, the fluid may be heated, chemicals added or electrostatic coalescers used. The use of microwave radiation to aid demulsification of water-in-oil emulsions is known with the microwaves assisting the separation process. The water droplets in the emulsion absorb microwave energy to a much greater extent than the oil or any gas present with the result that the temperature of the water droplets increases to more than that of the surrounding oil. The water droplets drop through the oil, resulting in an increased coalescence of the droplets.

Conventional assemblies for treating emulsions with microwave energy are typically arranged along a flowline, that is, before the separating tank. The fluid is still in the form of an emulsion in the flowline as the flow through the flowline tends to prevent any separation of the water and oil.

US 4,582,629 describes a method for enhancing the separation of oil and water dispersions and emulsions by treating the same with microwave radiation in conjunction with conventional separating devices. The emulsion or dispersion is subjected to microwave radiation, only for a short period of time, and the emulsion or dispersion will be heated to separation temperature by conventional means. The method is said to be applicable to both oil external and water external systems, that is, water-in-oil and oil-in-water emulsions respectively. The emulsion or dispersion is treated at a power level of from about 1 Watt to about 500 Watts per gallon (264.2 to 132,086.4 Watts per m3) of emulsion or dispersion. It is said that this treatment is not sufficient to raise the temperature of the emulsion to levels normally used for separation, but will greatly enhance the resultant separation once heating is completed by conventional means.

The method of US 4,582,629 may be used in combination with conventional treatment vessels such as heat treaters, which are tanks modified to add heat and provide residence time in the system. Microwaves would either be used directly in the tank to generate heat and interfacial disruption, or in a section of a flowline just ahead of the tank.

Likewise, the method could be used in washtanks or gun barrel treaters, either in the tank or in the pipeline leading into the tank. However, it has now been found that microwave irradiation of the entire flow of multiphase fluid in a section of the flowline just ahead of the tank and also microwave irradiation of the entire contents of a tank is uneconomic in terms of the power demands for the microwave generator.

US 4,810,375 relates to a method of and apparatus for enhancing the separation of oil and water emulsions and dispersions by treating the same with microwave radiation, whether alone or in conjunction with additional conventional separating devices. In the method of US 4,810,375, an oil-water emulsion or dispersion is conveyed via an inlet to an applicator where it is heated by microwave energy from a power source through a circulator and waveguide. The heated stream exits the applicator through an outlet to a separator vessel where the stream separates into an upper oil phase and a lower oily water phase. The separated phases are then recovered through outlets on the separator vessel.

The oily water is recirculated through a water load connected to the circulator and returned to the separator vessel. Thus, the entire feed stream to the separator vessel is subjected to microwave irradiation resulting in the separation of the feed into two layers in the separator vessel. As discussed above, microwave irradiation of the entire feed stream to the separator vessel is uneconomic in terms of the power demands for the microwave generator.

US 4,889,639 discloses a similar apparatus to that disclosed in US 4,810,375 but differs therefrom in that the oily water is recirculated through the applicator rather than through a water load.

WO 01/46353 describes a method and apparatus in which a used oil sample having at least 1% (by weight) aqueous substances is heated to a temperature of from about 20°C to about 60°C in a heating section ahead of the separator tank, and the heated oil is extracted by adding super critical CO2 to a demulsification section, whereby a volume of water is removed from the heated used oil sample. The heating step can be accomplished by a microwave heating process or by conventional heating techniques. A microwave generator with a power output of 6kW at 2.45GHz is applied to an emulsion flowing at 8.51 liters per. minute.

US 4,853,507 is directed to an apparatus for treating oil and water emulsions in which microwave energy is radiated into an applicator section having a waveguide with a tapered applicator element which separates the input and output sections of the waveguide.

Emulsion is passed through the waveguide ahead of a settling/separating tank and the emulsion is heated in the waveguide by the microwave radiation. The emulsion is subsequently separated into the constituent components in the settling/separating tank.

US 4,855,695 is directed to an apparatus for automatic tuning of microwave energy in a demulsifier system using a waveguide applicator. A microwave transparent window is placed across the waveguide and the fluid to be treated is passed into the waveguide beyond the window and a surfactant is injected into the waveguide. These fluids are then heated by microwave radiation and exit the waveguide into a separator vessel. The microwave transparent window is designed to inhibit fluid flowing back into the microwave source. The heating of the emulsion in this system is not symmetrical and the microwave transparent window may be subject to damage from inclusions in the emulsion such as sand, gravel or rocks which would affect the performance of the system. As discussed above, microwave irradiation of the entire feed stream to the separator vessel is uneconomic in terms of the power demands for the microwave generator.

EP 1,050,330 is directed to a microwave energy application apparatus to break oil and water emulsions. A stream of hydrocarbon and water emulsion is pumped into a microwave cavity. Dual opposing emulsion flow chambers with a centrally supplied microwave guide form a double-ended resonant chamber with multiple RF energy reflectors to treat the flowing emulsion. The emulsion feedstock enters into the bottom of both flow chambers and exits to a centrifuge or storage vessel for separation.

WO 03/0397706 and WO 2008/007 185 are directed to electrostatic coalescers for use in separating oil/water emulsions. The electrodes in the coalescers are outside a plurality of tubes through which the emulsion flows The electrodes may be in the form of wires wrapped around each tube so that each tube has an associated set of electrodes or in the form of horizontal plates between rows of tubes so that all the tubes in a horizontal row share the same electrodes.

The present invention relates to an apparatus for separating a multiphase fluid comprising an aqueous phase, a liquid hydrocarbon phase and optionally a gaseous phase into its component phases.

According to a first aspect of the present invention there is provided an apparatus for assisting in separating a multiphase fluid comprising an aqueous phase, and a liquid hydrocarbon phase wherein at least a portion of the aqueous phase is dispersed in a liquid hydrocarbon phase into its component phases, the apparatus comprising: a vessel having an inlet for receiving the multiphase fluid and at least one outlet; and an array of one or more element pairs arranged within the vessel, wherein a first element in each pair comprises an RF electrode arranged to direct RF radiation into the vessel to heat the aqueous phase dispersed in the multiphase fluid and to assist in separation of the multiphase fluid into component phases; and wherein a second element in each pair is arcuate to reflect RF radiation into the multiphase fluid.

Preferably, the second element comprises an RF electrode.

The second element may be spaced from the first element in the vessel to permit fluid flow between the elements.

In a further preferred embodiment, the first element in each pair is also arcuate.

The first and second elements may be located adjacent the inner surface of the vessel and in a preferred embodiment, the second element comprises an inner surface of the vessel or an element applied thereto or mounted thereon.

The one or more RF electrodes may be arranged to be driven by RF energy from an external source.

In a preferred embodiment, the vessel has an inlet arranged to direct fluid to be introduced tangentially into the vessel.

The vessel may comprise a flowline, a separator vessel or a pipe locatable in use between a flowline and separator vessel or between a first and second separator vessel that are arranged in series. Preferably, the vessel has a longitudinal axis, and the first RF electrode comprises a rod or a plate extending along the longitudinal axis of the vessel.

The first RF electrode may be movable within the vessel.

According to a second aspect of the present invention there is provided a method of separating a multiphase fluid comprising an aqueous phase dispersed in a liquid hydrocarbon phase into its component phases using the apparatus defined above.

According to a third aspect of the present invention there is provided a method for separating a multiphase fluid comprising an aqueous phase dispersed in a liquid hydrocarbon phase into its component phases, the method comprising: (a) passing the multiphase fluid through aRF radiation treatment zone in a vessel; (b) heating the multiphase fluid by subjecting the multiphase fluid to RE radiation in the vessel, the RF radiation being directed into the multiphase fluid through a first element in a pair, the first element comprising an RF electrode, a second element in the pair being arcuate to reflect the RF radiation, the first and second elements being arranged to direct RF radiation in the vessel to heat the aqueous phase dispersed in the multiphase fluid and to assist in separation of the multiphase fluid into component phases; and (c) withdrawing a liquid hydrocarbon stream and an aqueous stream from the separator vessel.

The present invention will now be described by way of example and withreference to the accompanying drawings in which: Figure 1 is a schematic cross-section through a flowline having an RF electrode configuration therein according to a first preferred embodiment; Figure 2 is an end elevation of the flowline of Figure 1; Figure 3 is a schematic cross-section through a flowline having an RF electrode configuration therein according to a second preferred embodiment; and Figure 4 is a schematic cross-section through a separator vessel having an RF electrode configuration therein according to a third preferred embodiment.

An apparatus according to a first preferred embodiment for assisting in separating a multiphase fluid comprising an aqueous phase dispersed in a liquid hydrocarbon phase is shown in Figures 1 and 2. The apparatus comprises a flowline 1, through which the multiphase fluid is passed, prior to separation of the multiphase fluid into its constituent phases in a separator vessel (not shown) connected to the flowline 1. A plurality of pairs of elements 2, 4 are located in the flowline 1. In the preferred embodiment shown in Figures 1 and 2, both elements 2, 4 are RF electrodes with one electrode in each pair opposing the second electrode in that pair and being spaced therefrom, each electrode preferably being located adjacent the inner side walls of the flowline 1.

In each pair of elements 2, 4, at least one element is an RF electrode 2 which is connected to an RF supply through an RF connection 6. The second element 4 in each pair may be a reflector of RF energy. Alternatively, as shown in Figures 1 and 2, the second elements 4 may also be RF electrodes which are connected to an RF supply through RF connections 8 to increase the strength of the RF field in the flowline 1.

As shown in Figure 2, in a preferred embodime,nt, the elements 2, 4 are both curved to control the direction of reflected RF energy in the flowline 1. In an alternative embodiment (not shown), only one element in each pair 2 or 4 may be curved and in this case it is preferable for the reflector to be.. curved rather than the RF electrode.

A further preferred embodiment is shown in Figure 3. In this embodiment, a single RF electrode 12 in the form of a rod extends along the longitudinal axis of a flowline 14 and is connected to an RE supply through an RF connection 16. The wall of the flowline itself may form the second element in this embodiment for reflecting RF energy.

Alternatively, the inner surface of the flowline 14 may be lined with a second element. In both such embodiments, the second element may be an RF electrode connected to an RF supply through an RE connection 18, or may be a reflector. In both cases, the second element may extend all or partially around the first element 12.

In the embodiments of Figures 1 to 3, when RF energy is applied to the RF electrode or electrodes 2, 4, 12, the energy is absorbed by the water droplets or any macroscopic domains of water that are dispersed in the liquid hydrocarbon phase of the multiphase fluid that is flowing through the flowline 1, 14 causing dielectric heating of the water to improve the coalescence of the water droplets or any macroscopic domains of water. The treated fluid is passed from the flowline 1, 14 into a separator vessel (not shown) where it separates into its constituent phases which may then be removed from the separator vessel. The treatment of the multiphase fluid in the flowline 1, 14 prior to separation enhances the speed and efficiency of subsequent separation in the separator vessel. This effect is explained in more detail below.

In an alternative preferred embodiment (not shown), the RF electrode(s) and element pairs may be arranged in a vertical pipe section or an upwardly inclined pipe section, that is preferably located between the outlet of a flowline and the fluid inlet of a separator vessel. Preferably, the vertical pipe section or upwardly inclined pipe section has a diameter greater than the diameter of the flowline. In such an embodiment, the RF electrode is preferably a rod extending along the vertical axis or inclined longitudinal axis of the pipe and the second element which may also be an RF electrode is either the inner surface of the pipe itself or a separate element attached thereto. The advantage of such an embodiment is that the fluid flow velocity into the separator vessel may be reduced thereby increasing the residence time in the RF treatment zone in the pipe section.

Furthermore, if the fluid be introduced into the pipe tangentially, this may result in a cyclonic (centrifugal) effect being established in the pipe which assists in coalescing the water droplets as the water, being denser and heavier than the oil in the emulsion, will be forced to the side wall of the pipe.

Figure 4 shows a further preferred embodiment of the present invention in which the electrode configuration, instead of being located in the flowline, is located in a separator vessel 20. The apparatus comprises a separator vessel 20 having an inlet 22 for a multiphase fluid, a first outlet 23 for removing gas separated from the multiphase fluid during treatment, a second outlet 24 for removing liquid hydrocarbon separated from the multiphase fluid during treatment, and a third outlet 25 for removing water separated from the multiphase fluid during treatment.

Fluid, in particular, a multiphase fluid comprising a liquid hydrocarbon phase, an aqueous phase and a gaseous phase, is introduced into the separator vessel 20 through the fluid inlet 22. In the separator vessel 20, the fluid separates, under gravity, as it flows through the separator vessel into an upper oil layer 30, an intermediate emulsion layer 32 and a lower water layer 34. A baffle 36 extends into the interior of the separator vessel 20 from the bottom wall thereof, between the water outlet 25 and the oil outlet 24. The baffle 36 is located at a location along the longitudinal axis of the separator vessel 20. at which the multiphase fluid has either substantially separated into its component phases or the thickness of the intermediate emulsion layer 32 has been substantially reduced owing to coalescence of the droplets of aqueous phase under the influence of the RF radiation applied through an RF electrode 40, as described below. The baffle 36 acts to inhibit the separated water from exiting the vessel 20 through the oil outlet 24. Preferably, the baffle plate 36 is arranged across the separator vessel 20, perpendicular to the direction of flow of the multiphase fluid. The top of the baffle plate 36 is located within the liquid hydrocarbon layer 30 in the separator vessel 20 so that the aqueous phase is retained by the baffle plate 36 and the liquid hydrocarbon phase 30 flows over the top of the baffle plate 36. Accordingly, the outlets 25, 24 for the aqueous and liquid hydrocarbon phases are located upstream and downstream of the baffle plate 36 respectively.

An RF electrode 40, in the form of a rod or plate, extends along the length of the separator vessel 20, terminating prior to the baffle 36. The RF electrode 40 is connected to an RF supply through an RF connection 42. In use, the RF electrode 40 is preferably maintained in the emulsion layer 32 and is movable (in an upwards or downwards direction) therein to maintain the RF electrode in the desired position. Alternatively or in addition, the emulsion level in the separator vessel 20 may be controlled by varying the fluid flow rate into the vessel to maintain the RF electrode within the emulsion layer. The second element which acts preferably to reflect RF energy comprises the inner surface of the separator vessel 20. The RF electrode 40 terminates in the separator vessel 20 prior to the baffle 36 which may therefore be considered to be located downstream of the end RF electrode 40 in the separator vessel 20.

In one or more preferred embodiments, although some of the unabsorbed RF radiation may be reflected by the walls of the flowline or separator tank 20, some of it will be wasted. The shape of the, second element which acts as a reflector may be such that it controls the direction of the reflection to reflect the unab sorbed radiation back towards the dispersion of aqueous phase in the liquid hydrocarbon phase that is flowing through the flow line or the emulsion layer that is flowing through the separator vessel so that it may be absorbed by the dispersed aqueous phase of the dispersion or the water droplets of the emulsion respectively to increase efficiency. As mentioned above, the reflector need not be connected to a power supply and it may comprise a sheet of metal. However, in one or more preferred embodiments, the reflector may also be an RF electrode to deliver additional RF energy to the emulsion.

The RF energy may be transmitted to the RF electrode(s) through coaxial cables and therefore embodiments of the present invention may be suitable for application in remote environments such as Arctic or subsea environments.

The dielectric heating of the droplets of aqueous phase of the emulsion layer in the separator vessel greatly increases the separation efficiency compared to gravity settling alone. Similarly, the dielectric heating of the aqueous phase that is dispersed in the liquid hydrocarbon phase of the multiphase fluid in a flowline or pipe section upstream of a separator vessel greatly increases the separation efficiency compared to gravity settling alone. The mechanism, application time and optimum frequencies for the dielectric heating carried out by the application of RF radiation to the dispersed aqueous phase of the multiphase fluid or emulsion are set out below.

In one or more preferred embodiments, either the flowline 1, 14, or separator vessel acts as an electromagnetic radiation treatment zone wherein the dispersed aqueous phase of the multiphase fluid or of an emulsion is selectively heated by subjecting the multiphase fluid to RF radiation having a frequency of around between 1MHz to 300MHz for an exposure time of less than 5 seconds, with a power density in the aqueous phase of at least i07 Wm3. Subjecting the multiphase fluid or emulsion to RF radiation with a power density of at least 1x107Wm3 inthe aqueous phase for a short period of time has been found to result in particularly fast separation times. The high power density causes selective heating of the dispersed aqueous phase which is thought to result.in two mechanisms arising within the multiphase fluid or emulsion. Firstly, the rapid heating of the dispersed aqueous phase, in particular, droplets of aqueous phase, causes localized heating of the oil boundary layer around the droplets. As oil viscosity is highly sensitive to changes in temperature, the viscosity of the oil surrounding the droplets drops considerably, resulting in an increase in droplet settling velocity. Secondly, the rapid heating of the aqueous droplets causes the internal pressure of the droplets to increase, resulting in the expansion of the droplet. This also results in an increase in the droplet settling velocity. These mechanisms ensure that the droplets are more likely to collide and therefore more likely to coalesce with other droplets.

When treating a multiphase fluid in a flowline or pipe section, the aqueous phase being "dispersed" in the liquid hydrocarbon phase is intended to be interpreted to mean a bulk layer of the aqueous phase has not separated from the multiphase fluid prior to passing the multiphase fluid through the RF radiation treatment zone in the flow line. The aqueous phase may be dispersed in thç liquid hydrocarbon phase of the multiphase fluid in the form of emulsified droplets and/or macroscopic aqueous domains.

Preferably, the multiphase fluid is a produced fluid from a hydrocarbon production well. The liquid hydrocarbon phase of the produced multiphase fluid may be selected from crude oils, crude oil blends, and gas condensates. The aqueous phase of the produced multiphase fluid may be selected from connate water, injection water and condensed water.

Crude oils often contain naturally occurring surfactants. Accordingly, emulsified droplets of aqueous phase may arise spontaneously when a mixture of crude oil and water is subjected to shearing, for example, in a production well, choke, or a section of flow line (thereby imparting the energy required to form the emulsified droplets). Macroscopic aqueous domains may arise owing to the multiphase fluid being sufficiently well-mixed that a bulk layer of aqueous phase does not separate from the produced fluid.

Thus, the flow rate of the produced fluid immediately upstream of the electromagnetic radiation treatment zone of a flowline should be sufficiently high that the produced fluid does not adopt a laminar or stratified flow regime. Where the multiphase fluid is passed through a flowline prior to being passed to the separator vessel, the internal diameter of the flowline is preferably less than 36 inches in order to avoid the flow rate reducing to such an extent that the multiphase fluid adopts such a stratified or laminar flow regime. However, in the event that a bulk layer of aqueous phase has separated from the multiphase fluid, a mixing device may be provided upstream of the electromagnetic radiation treatment zone, for re-dispersing the bulk layer of aqueous phase in the liquid hydrocarbon phase.

Optionally, the produced fluid comprises a gaseous hydrocarbon phase, in particular, natural gas. The optional gaseous hydrocarbon phase of the produced fluid may be entrained in the liquid hydrocarbon and aqueous phases. Alternatively, at least part of the optional gaseous hydrocarbon phase may separate into a headspace of the electromagnetic radiation treatment zone.

Where the produced fluid comprises a gaseous hydrocarbon phase, the gaseous hydrocarbon phase may be separated from the multiphase fluid in a gaseous hydrocarbon-liquid hydrocarbon-water separator. Accordingly; agaseous hydrocarbon stream is withdrawn from the separator vessel.

It is also envisaged that the multiphase fluid may arise in a refinery operation and may therefore comprise a crude oil distillate fraction having an aqueous phase dispersed therein. Examples of crude oil distillate fractions include gasoline, gas oil, diesel, jet fuel, kerosene, heating oil and mixtures thereof The amount of thermal energy deposited into a material due to electromagnetic irradiation of the material is dependent upon the electric field of the electromagnetic radiation within the material to be. irradiated, the frequency of the electromagnetic radiation, and the dielectric properties of the material. The liquid hydrocarbon phase and any gaseous hydrocarbon phase of the multiphase fluid are substantially transparent to the electromagnetic radiation, while the aqueous phase has a high dielectric constant and loss.

The aqueous phase therefore strongly absorbs the electromagnetic radiation subsequently dissipating the resulting stored energy as heat. Accordingly, the electromagnetic radiation selectively heats the aqueous phase of the multiphase fluid or the aqueous phase of the emulsion thereby facilitating coalescence of the emulsified droplets of the aqueous phase.

Where the electromagnetic radiation is treating a multiphase fluid in a flow line, the electromagnetic radiation may also selectively heat any macroscopic domains of aqueous phase that are dispersed in the liquid hydrocarbon phase. In a preferred embodiment, at least 90%, and preferably at least 95%, of the electromagnetic radiation is absorbed by the dispersed aqueous phase. of the multiphase fluid or the dispersed aqueous phase of the emulsion thereby selectively heating the aqueous phase. The electromagnetic radiation selectively heats the dispersed aqueous phase to a preferred temperature of at least 50°C, and preferably, to a temperature in the range 60 to 100°C. The temperature differential that develops between the selectively heated aqueous phase and the liquid hydrocarbon phase is preferably at least 20°C, and more preferably, at least 50°C.

The electric field portion of the electromagnetic radiation and the exposure time of the multiphase fluid or emulsion to the electromagnetic radiation are related. Accordingly, for a given multiphase fluid or emulsion, the higher the electric field, the shorter the exposure time required to selectively heat the dispersed aqueous phase of the multiphase fluid or emulsion. The power input required to selectively heat the dispersed aqueous phase also increases with increasing amounts of dispersed aqueous phase. Thus, the power density and/or the exposure time required to selectively heat the dispersed aqueous phase increases with increasing amounts of aqueous phase dispersed in the liquid hydrocarbon phase of the multiphase fluid or emulsion. The amount of aqueous phase that is dispersed in the liquid hydrocarbon phase of the multiphase fluid or emulsion is preferably in the range of 0.5 to 50% by weight (based on the total weight of the aqueous phase and the liquid hydrocarbon phase).

The aqueous phase of the multiphase fluid or emulsion may have salts dissolved therein. This is advantageous owing to the ionic mobility of the dissolved salts enhancing the selective heating of the dispersed aqueous phase. Preferably, the aqueous phase of the multiphase fluid or emulsion has a total salinity of at least 1,000 ppm, for example, at least 5,000 ppm. Examples of dissolved salts include sodium chloride.

The power density in the aqueous phase of the multiphase fluid or emulsion is the energy absorbed per unit volume of the aqueous phase and can be approximated from the following Equation: Pa = 2,r.f.&.e".E,'.

Pd is the power density (watts/rn3) f is the frequency of the applied energy (Hertz) 20. is the permittivity of free space (8.854x10'2 F/rn) e" is the dielectric loss factor of the aqueous phase E is the magnitude of the electric field inside the aqueous phase (volts/rn) The power density in the aqueous phase of the multiphase fluid or emulsion in the RF radiation treatment zone may be at least 1x107 Wrn3, preferably, at least 1x108 Wm3 and optionally at least lOx 10 Wm3.

The power density in the aqueous phase of the multiphase fluid or emulsion may be selected depending on the droplet size. This assessment of droplet size may be important as it has been found that the size of the droplets in the aqueous phase has a significant effect on the speed of separation following irradiation. Increasing the power density in the aqueous phase has been found to be advantageous for smaller droplet sizes, such as droplets having a diameter less than 20microns. Droplets having a diameter of less than 2Omicrons have a high surface area to volume ratio. This has been found to result in a high rate of heat loss to the surrounding oil phase. This rapid cooling of the aqueous phase relative to the oil phase in turn reduces the ability of the droplet to coalesce with nearby droplets. However, subjecting smaller droplets to a greater power density in the aqueous phase can ensure advantageous separation times following irradiation. This assessment can be conducted using a microscope, for example to analyse a slide dosed with a sample of the multiphase fluid, though other suitable techniques may be apparent to the skilled reader.

The multiphase fluid or emulsion may be exposed to the RF radiation in the RF radiation treatment zone for less than 5 seconds, preferably, less than 3 seconds, more preferably, less than 1 second. The exposure time may be achieved by selecting a flow rate of the multiphase fluid or emulsion layer through the RF radiation treatment zone that achieves the desired exposure time. Generally, the exposure time will be fixed by the flow rate of the multiphase fluid through the flow line or the flow rate of fluid through the separator vessel. Accordingly, the electric field strength may be varied to produce the desired treatment regime.

It is believed that the relatively high power density of the RF radiation together with the relatively short exposure time results in rapid and selective heating of the dispersed aqueous phase thereby enhancing separation of the aqueous and liquid hydrocarbon phases.

In the context of the present invention, radio frequency radiation is defined herein as electromagnetic radiation in the frequency range of around 1 MHz to 300MHz.

Preferably, the desired power density is produced by developing a distribution of electric field such that the maxima in the electric field are sufficiently high to enhance the rate of separation of the multiphase fluid into its component phases.

The multiphase fluid may be subjected to RF irradiation in the presence of an additional heating source, an electrostatic field, an acoustic field, or combinations thereof in order to enhance coalescence of the emulsified droplets of the aqueous phase and/or of the macroscopic domains of the aqueous phase.

One or more embodiments of the present invention are particularly advantageous as the configurations permit the RF radiation to target the water content in the emulsion layer that separates under gravity within the separator vessel rather than the oil, enabling targeted heating which raises the emulsion layer water temperature allowing faster and more complete separation than conventional separation techniques using, for example, fired heating with exchangers.

Furthermore, the heating process is volumetric and does not rely on a temperature differential and the heat input is highly controllable thereby enabling the separation process to be highly controllable. Also, the energy can be efficiently transferred over distances in environmentally sensitive areas and the heat application process is less intrusive than conventional processes (as there are, for example, fewer flanges and seals) with lower environmental risks.

Various modifications to the embodiments of the present invention described above may be made. For example, other components and method steps may be added or substituted for those described above. Thus, although the invention has been described using particular preferred embodiments, many variations are possible, as will be clear to the skilled reader, without departing from the invention.

Claims

Claims 1. An apparatus for assisting in separating a multiphase fluid comprising an aqueous phase, and a liquid hydrocarbon phase wherein at least a portion of the aqueous phase is dispersed in a liquid hydrocarbon phase into its component phases, the apparatus comprising: a vessel having an inlet for receiving the multiphase fluid and at least one outlet; and an array of one or more element pairs arranged within the vessel, wherein a first element in each pair comprises an RF electrode arranged to direct RF radiation into the vessel to heat the aqueous phase dispersed in the multiphase fluid and to assist in separation of the multiphase fluid into component phases; and wherein a second element in each pair is arcuate to reflect RF radiation into the multiphase fluid.
2. An apparatus according to claim 1, wherein the second element comprises an RF electrode.
3. An apparatus according to any one of the preceding claims, wherein the second element is spaced from the first element in the vessel to permit fluid flow between the elements.
4. An apparatus according to any one of the preceding claims, wherein the first element in each pair is arcuate.
5. An apparatus according to any one of the preceding claims, wherein the first and second elements are located adjacent the inner surface of the vessel.
6. An apparatus according to any one of the preceding claims wherein the second element comprises an inner surface of the vessel or an element applied thereto or mounted thereon.
7. An apparatus according to any one of the preceding claims, wherein the one or more RF electrodes are arranged to be driven by RF energy from an external source.
8. An apparatus according to any one of the preceding claims wherein the vessel has an inlet arranged to direct fluid to be introduced tangentially into the vessel.
9. An apparatus according to any one of the preceding claims wherein the vessel comprises a flowline.
10. An apparatus according to any one of the preceding claims wherein the vessel comprises a pipe locatable in use between a flowline and separator vessel or between a first and second separator vessel arranged in series.
11. An apparatus according to claim 12, wherein the pipe has a longitudinal axis, and wherein the first RF electrode comprises a rod or a plate extending along the longitudinal axis of the pipe.
12: An apparatus according to any one of claims ito 10, wherein the vessel comprises a separator vessel.
13. An apparatus according to any one of claims 1 to 9 or 12 wherein the vessel has a longitudinal axis, and wherein the first RF electrode comprises a rod or a plate extending along the longitudinal axis of the vessel.15. An apparatus according to any one of the preceding claims wherein the first RF electrode is movable within the vessel.16. A method of separating a multiphase fluid comprising an aqueous phase dispersed in a liquid hydrocarbon phase into its component phases using the apparatus claimed in any one Of the preceding claims.17. A method for separating a multiphase fluid comprising an aqueous phase dispersed in a liquid hydrocarbon phase into its component phases, the method comprising: (a) passing the multiphase fluid through a RF radiation treatment zone in a vessel; (b) heating the multiphase fluid by subjecting the multiphase fluid to RF radiation in the vessel, the RF radiation being directed into the multiphase fluid through a first element in a pair, the first element comprising an RF electrode, a second element in the pair being arcuate to reflect the RF radiation, the first and second elements being arranged to direct RF radiation in the vessel to heat the aqueous phase dispersed in the multiphase fluid and to assist in separation of the multiphase fluid into component phases; and (c) withdrawing a liquid hydrocarbon stream and an aqueous stream from the separator vessel.18. A method as claimed in any one of claims 16 or 17, wherein the multiphase fluid is a produced fluid from a hydrocarbon production well.19. A method as claimed in any one of claims 16 or 17, wherein the multiphase fluid comprises a crude oil distillate fraction having an aqueous phase dispersed therein.20. A method as claimed in any one of claims 16 to 19, wherein the electromagnetic radiation treatment zone is a section of flow line, the array of one or more element pairs being arranged within the flowline in spaced apart arrangement for producing radio frequency iadiation.21. A method as claimed in any one of claims 16 to 19, wherein the electromagnetic radiation treatment zone is a separator vessel, the array of one or more element pairs being arranged within the separator vessel in spaced apart arrangement for producing radio frequency radiation.22. A method as claimed in any one of claims 16 to 19, wherein the electromagnetic radiation treatment zone is a pipe section located in use between a flowline and a separator vessel, the array of one or more element pairs being arranged within the pipe in spaced apart arrangement for producing radio frequency radiation.23. A method as claimed in any one of claims 16 to 22, further comprising driving the one or more RF electrodes by RF energy from an external source.24. A method as claimed in any one of claims 16 to 23, further comprising directing the multiphase fluid tangentially into the vessel.