Abstract:
A separator for separating a multiphase mixture comprising a pressure vessel supported for rotation within a casing containing a gas which may be held at an elevated temperature or pressure. A plurality of vanes is disposed within the pressure vessel. The pressure vessel has an inlet, a first phase outlet and a plurality of second phase outlets disposed radially outwardly of the first phase outlet with respect to a separator axis. A regulator is provided in the form of pressure-activated nozzles to regulate flow through the second phase outlets. In use, a mixture of solids and liquid is fed into the pressure vessel and the pressure vessel is spun within the gas causing solids to accumulate in the vicinity of the second phase outlets. The pressure-activated nozzles are repeatedly opened and closed to expel the accumulated solids.

Description:
RELATED APPLICATION 
       [0001]    This U.S. patent application is a continuation application of U.S. Ser. No. 12/765,520 filed on Apr. 22, 2010. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to a separator, and is particularly, although not exclusively, concerned with a rotary separator for separating phases of a multiphase mixture. 
       BACKGROUND OF THE INVENTION AND PRIOR ART 
       [0003]    Centrifugal separators for separating multiphase mixtures into their component phases are well known. 
         [0004]    Existing centrifugal separators often rely on a batch separation process. This involves separating phases of a mixture into different regions of the separator. Once separation is complete, the separator is stopped and each phase can be removed from the separator. A batch process is often undesirable since it involves periodic interruption of the separation process. 
         [0005]    Alternatively, each phase may be removed continuously via separate outlets from a separator. With such methods, removal rates of each phase need to be constantly monitored to ensure that the separation process remains effective. Furthermore, solids and emulsion can build up during the separation process and fill the separator and swamp the rotor. 
         [0006]    The term “phase” may refer, in the context of this specification, to the particular state of a substance, for example, whether a substance is a solid, liquid or gas. The term “phase” may also be used to distinguish different substances, for example, immiscible liquids or solids from liquids. 
       SUMMARY OF THE INVENTION 
       [0007]    According the present invention there is provided a separator for separating a multiphase mixture comprising a pressure vessel, which defines a separator axis, a support for supporting the pressure vessel for rotation about the separator axis, at least one vane disposed within and coupled for rotation with the pressure vessel and a flow regulator, wherein the pressure vessel has an inlet, a first phase outlet and a plurality of second phase outlets disposed radially outwardly of the first phase outlet with respect to the separator axis and the flow regulator is arranged to regulate flow through the second phase outlets. 
         [0008]    The flow regulator may comprise a plurality of pressure-activated nozzles disposed respectively at the second phase outlets. 
         [0009]    Each pressure-activated nozzle may comprise a non-return valve for preventing flow into the pressure vessel. The non-return valve may comprise a bias which biases the non-return valve towards a closed position. 
         [0010]    The pressure-activated nozzles may be provided in a radially outer wall of the pressure vessel. 
         [0011]    A plurality of accumulators may be disposed within the pressure vessel adjacent respective second phase outlets. The accumulators may comprise funnels which converge in a radially outward direction towards the respective second phase outlets. 
         [0012]    The separator may further comprise a pressure regulator for regulating pressure within the pressure vessel. The pressure regulator may comprise a flow controller for controlling flow through the first phase outlet. 
         [0013]    The separator may comprise a plurality of vanes. The vanes may be flat circular discs that are coaxial with, and extend radially outwardly from, the separator axis. Alternatively, the vanes may be cone shaped discs that are coaxial with, and extend radially outwardly from, the separator axis. 
         [0014]    Each disc may have an array of apertures arranged circumferentially about the separator axis, wherein the apertures of adjacent discs are angularly offset with respect to one another. The apertures may be perforations. 
         [0015]    Spacer fins may extend between adjacent discs and the spacer fins may be arranged with respect to the apertures to form staggered and/or interconnected flow passages from the pressure vessel inlet to the first phase outlet. 
         [0016]    At least one emulsion outlet may be disposed radially outwardly of the first phase outlet and radially inwardly of the second phase outlets. The or each emulsion outlet may comprise a tube which extends radially outwardly with respect to the separator axis, wherein the or each tube is in fluid communication with an emulsion discharge passage which extends along the separator and which exhausts through an end of the separator for removing emulsion from the separator. 
         [0017]    The separator may further comprise a rotor shaft provided with spray nozzles for supplying fluid into the interior of the pressure vessel. The spray nozzles may be arranged such that they are directed towards the second phase outlets. 
         [0018]    The separator may further comprise a third phase outlet disposed radially outwardly of the first phase outlet and radially inwardly of the second phase outlets. 
         [0019]    The separator may further comprise a sealable casing within which the pressure vessel is rotatably mounted. The casing may comprise a sump in the lower region of the casing from which the second phase is discharged. 
         [0020]    Means may be provided for introducing fluid under pressure between the casing and the pressure vessel. The fluid may be a gas. 
         [0021]    The separator may comprise a pressure regulator for regulating pressure between the casing and the pressure vessel. 
         [0022]    The present invention also provides a method of separating a mixture comprising a first phase and a second phase using a separator for separating a multiphase mixture comprising a pressure vessel, which defines a separator axis, a support for supporting the pressure vessel for rotation about the separator axis, at least one vane disposed within and coupled for rotation with the pressure vessel and a flow regulator, wherein the pressure vessel has an inlet, a first phase outlet and a plurality of second phase outlets disposed radially outwardly of the first phase outlet with respect to the separator axis and the flow regulator is arranged to regulate flow through the second phase outlets comprising the steps:
       (a) generating a positive pressure difference across the second phase outlets such that flow through the second phase outlets is prevented;   (b) spinning the pressure vessel such that the second phase accumulates in the vicinity of the second phase outlets;   (c) generating a negative pressure difference across the second phase outlets such that flow through the second phase outlets is permitted.       
 
         [0026]    Step (a) may comprise the step of restricting or preventing flow though the first phase outlet to increase pressure within the pressure vessel. 
         [0027]    Step (a) may comprise increasing the external pressure on the pressure vessel. The external pressure may be sufficient to counteract the internal pressure of the pressure vessel and the centrifugal force acting on the pressure vessel. 
         [0028]    Steps (a) to (c) may be repeated to remove accumulated second phase through the second phase outlets. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the following drawings, in which: 
           [0030]      FIG. 1  is a perspective view of a separator; 
           [0031]      FIG. 2  is perspective sectional view of the separator shown in  FIG. 1 ; 
           [0032]      FIG. 3  is an enlarged perspective sectional view of an end of the separator shown in  FIG. 1 ; 
           [0033]      FIG. 4  is an enlarged sectional view of the end of the separator shown in  FIG. 1  opposite the end shown in  FIG. 3 . 
           [0034]      FIG. 5  is a cut-away perspective view of part of a rotor of the separator shown in  FIG. 2 ; 
           [0035]      FIG. 6  is a radial sectional view of the part of the rotor shown in  FIG. 2 ; 
           [0036]      FIG. 7  is an enlarged partial sectional view of the region VI in  FIG. 6 ; 
           [0037]      FIG. 8  is a perspective view of part of a shaft and vane section of the rotor shown in  FIG. 2 ; 
           [0038]      FIG. 9  is a further perspective view of part of a drum section of the rotor shown in  FIG. 2 ; 
           [0039]      FIG. 10  is a partial perspective view of the rotor according to a variant of the invention in the region of an accumulator; 
           [0040]      FIG. 11  is a perspective sectional view of a further embodiment of the separator; 
           [0041]      FIG. 12  is an enlarged perspective sectional view of an end of the separator shown in  FIG. 11 ; 
           [0042]      FIG. 13  is an enlarged sectional view of the end of the separator shown in  FIG. 11  opposite the end shown in  FIG. 12 ; 
           [0043]      FIG. 14  is a radial sectional view of the part of the rotor shown in  FIG. 11 ; and 
           [0044]      FIG. 15  is a perspective view of part of a shaft and vane section of the rotor shown in  FIG. 11 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0045]      FIGS. 1 and 2  show a separator  2  comprising an outer casing  4  which supports a rotor  6  for rotation therein. The outer casing  4  comprises a cylindrical section  8  which is closed at each end by an inlet flange  10  and an outlet flange  12 . 
         [0046]    The rotor  6  comprises a pressure vessel in the form of a cylindrical drum  7 , carried by a shaft  14 . The shaft  14  is supported by bearings  18  in the respective flanges  10 ,  12  for rotation about a separator axis  16 . The drum  6  is provided with a drum inlet  20 , a first phase outlet  22 , a plurality of second phase outlets  24  and a third phase outlet  26 . 
         [0047]    Referring to  FIG. 3 , the drum inlet  20  comprises four arcuate and circumferentially spaced apertures which extend circumferentially about the axis  16 . 
         [0048]    The first phase outlet  22  is at the end of the drum  7  opposite the drum inlet  20 . The first phase outlet  22  comprises an annular aperture which extends circumferentially about the axis  16 . The second phase outlets  24  are formed through a radially outer wall  49  (see, e.g.,  FIGS. 6 and 7 ) of the pressure vessel/drum  7 . The second phase outlets  24  are arranged in an axially and circumferentially spaced array. The third phase outlet  26  is disposed adjacent the first phase outlet  22  and comprises a plurality of apertures arranged circumferentially about the axis  16 . The third phase outlet  26  is coaxial with the first phase outlet  22  but is spaced radially outwardly of the first phase outlet  22  and radially inwardly of the second phase outlets  24 . 
         [0049]    A stack of discs  28  (the embodiment shown in the Figures comprises eighteen discs  28 ) is arranged along the length of the shaft  14 . The discs  28  extend perpendicularly to the separator axis  16  and are secured to the shaft  14 . The discs  28  are thus coupled for rotation with the drum  7 . 
         [0050]    As shown in  FIGS. 2 ,  6  and  8 , each disc  28  has a plurality of radially extending slots  30  spaced equally about the separator axis  16 . The embodiment shown has twenty slots  30  in each disc  28 . The discs  28  are arranged such that the slots  30  of adjacent discs  28  are angularly offset about the axis  16  with respect to each other and so that the slots  30  of alternating discs  28  are angularly aligned. Fins  32  are disposed between, and adjoin, adjacent discs  28 . The fins  32  extend both axially and radially. Each fin  32  is aligned with a respective slot  30  of a forward disc—i.e. a disc closer to the drum inlet  20 —and bisects the slot  30  along its length. The slots  30  and fins  32  thus define a series of staggered and interconnected flow passages along the length of the drum  7 . Each fin  32  has profiled edges  34  which fit with corresponding locating notches  35  provided in the discs  24  at the ends of the slots  30 . 
         [0051]    As shown in  FIG. 2 , an annular weir plate  29  is provided adjacent the first and third phase outlets  22 . The radially inner periphery of the weir plate  29  is offset from the outer surface of the shaft  14 . An annular plate  31  extends from the radially inner periphery of the weir plate  29  to the end wall of the drum  7  so as to define an annular flow passage between the weir plate  29  and the first phase outlet  22 . 
         [0052]    As shown in  FIGS. 2 ,  5 ,  6  and  7 , accumulators in the form of pyramid-shaped funnels  36  are arranged about the inside of the radially outer wall of the drum  7 . The funnels  36  are disposed radially outwardly of the discs  28  and fins  32 . Each funnel  36  converges in a radially outward direction towards a respective second phase outlet  24 . 
         [0053]    The funnels  36  are constructed from an arrangement comprising a corrugated plate  38  and a plurality of funnel plates  40 . The corrugated plate  38  extends circumferentially within the outer wall of the drum  7  such that the corrugations  42  of the corrugated plate  38  extend parallel with the separator axis  16 . The corrugated plate  38  shown in the embodiment has eight corrugations  42 , and so has the shape, in cross-section as seen in  FIG. 6 , of an eight-pointed star. A funnel plate  40  is disposed along the length of each corrugation  42  on the radially inward side of the corrugated plate  38 . Each funnel plate  40  is corrugated along its length and has six corrugations  44 . The profiles of the funnel plates  40  correspond to the profile of the corrugations  42  along which they are disposed. The corrugated plate  38  and the funnel plates  40  cooperate to define forty-eight funnels  36  in total. Each funnel  36  has two opposite sides formed by opposite sides of one of the corrugations  42  of the corrugated plate, and two opposite sides formed by opposite sides of one of the corrugations  44  of the respective funnel plate  40 . In the embodiment shown, the radially inner edges of each funnel  36  are conterminous with radially inner edges of adjacent funnels  36 . This ensures that the funnel structure on the inside of the drum  7  provides inclined surfaces over a large proportion of the interior of the rotor  6 . 
         [0054]    Each funnel  36  has an aperture  46  at the convergence of the funnel  36  which aligns with a corresponding second phase outlet  24 . In an implementation, as seen in, for example,  FIGS. 6 and 7 , the separator  2  includes the pressure vessel  7  and a flow regulator  47 . The flow regulator  47  comprises a plurality of pressure-activated nozzles. Each pressure-activated nozzle  47  comprises a non-return valve  48  (see, e.g.,  FIG. 7 ) that is disposed at each of the second phase outlets  24  to control flow through the respective outlets  24 . The plurality of pressure-activated nozzles  47  are provided in the radially outer wall  49  of the pressure vessel/drum  7 . 
         [0055]      FIG. 7  shows an enlarged sectional view of the vertex of one of the funnels  36  and the corresponding section of the cylindrical wall of the drum  7  in the region of a second phase outlet  24  and non-return valve  48 . The non-return valve  48  comprises a cylindrical body  50  having a screw-threaded outer surface. The body  50  is screwed into a tapped hole  68  in the outer wall of the drum  7 . The hole  68  has a convergent portion  52  which communicates with the second phase outlet  24 . The body  50  has a central bore  54  which extends along its length. The bore  54  has a screw threaded portion  56  at the end opposite the convergent portion  52  of the hole  68 . A plurality of flow passages  58  are arranged circumferentially about the central bore  54 . The flow passages  58  extend along the length of the body  50  and provide fluid communication between the second phase outlet  24  and the outer region between the separator casing  4  and the drum  7 . A spring  66  is accommodated within the bore  54  and abuts an adjustment screw  64 . The spring  66  biases a ball  60  into the convergent portion  52  to close the second phase outlet  24 . 
         [0056]    When the valve  48  is closed, the ball  60  is seated on the periphery of the second phase outlet  24  and is held in contact with the periphery of the second phase outlet  24  by the spring  66 . Displacement of the ball  60  against the action of the spring  66  creates a flow path from the second phase outlet  24  about the ball  60  and through the flow passages  58  thereby opening the valve  48 . 
         [0057]    Referring to  FIGS. 2 ,  3  and  4 , the shaft  14  comprises a tubular section  70  into which solid end sections  72 ,  74  are partially inserted at each end. The tubular section  70  thus defines an elongate cavity between the solid end sections  72 ,  74 . The solid end sections  72 ,  74  are supported by the bearings  18 . The bearings  18  are housed in respective chambers formed by end walls of the flanges  10 ,  12 . Mechanical seals  19  seal the shaft  14  in the casing  4  and define separation zones between the mechanical seals  19  and the bearings  18  which prevent liquid contamination of the bearings  18 . The mechanical seals  19  are double mechanical seals comprising a lubricant held at a higher pressure than the process pressure between the mechanical seals  19  to prevent solids ingression. The bearings  18  are open to the atmosphere to prevent pressurization of the bearings  18  during operation of the separator  2 . A motor (not shown) is provided to drive the shaft  14 . 
         [0058]    Emulsion tubes  76  project in a radial direction from the solid end section  74  at the outlet flange  12  to a region which is radially outwards of the first phase outlet  22  and radially inwards of the outer periphery of the weir plate  29 . The emulsion tubes  76  are in fluid communication with a discharge passage  78 . The discharge passage  78  comprises a tube which extends axially along the length of the shaft  14  and exits through the solid end section  72  at the inlet flange  10 . 
         [0059]    The cylindrical section  8  of the casing  4  has flanges  80 ,  82  at each end which are welded to the casing and attached to the respective flanges  10 ,  12  by fasteners such as bolts or studs. 
         [0060]    The outer casing  4  defines a chamber within which the drum  7  is disposed. A sump  84 , formed in the wall of the cylindrical section  8 , extends radially downwardly from the bottom of the separator  2 . A solids outlet port  86  is provided at the bottom of the sump  84 . A solids flow regulator (not shown) for regulating flow from the solids sump  84  through the solids outlet port  86  and a level control (not shown) for controlling the level of liquid in the sump  84  are also provided. 
         [0061]    The inlet flange  10 , shown in  FIGS. 2 and 3 , comprises an inlet chamber  88  disposed adjacent the drum inlet  20 . The inlet chamber  88  is in fluid communication with the interior of the drum  7  through the drum inlet  20 . A seal  90 , for example a labyrinth seal, is disposed about the periphery of the drum inlet  20  between the inlet flange  10  and the drum  7 , thereby sealing the inlet chamber  88  and the interior of the drum  7  from the chamber defined by the outer casing  4 . The inlet chamber  88  has an inlet port  92  which is arranged tangentially with respect to the separator axis  16 . 
         [0062]    The outlet flange  12 , shown in  FIGS. 2 and 4 , comprises a first phase outlet chamber  94  disposed adjacent the first phase outlet  22  and a third phase outlet chamber  96  disposed adjacent the third phase outlet  26 . The drum  7  is in fluid communication with the first and second phase outlet chambers  94 ,  96  through the respective first and third phase outlets  22 ,  26 . 
         [0063]    The first phase outlet chamber  94  comprises a smaller diameter portion  98  adjacent the first phase outlet  22  and a larger diameter portion  100  spaced away from the first phase outlet  22  in an axial direction. A first phase outlet pipe  102  projects radially downward from the lower region of the larger diameter portion  100 . The first phase outlet pipe  102  is perpendicular to the separator axis  16 . 
         [0064]    A gas outlet pipe  104  extends from a region axially adjacent the larger diameter portion  100  of the first phase outlet chamber  94  in an upward direction. A cartridge seal is disposed between the shaft  14  and the outlet flange  12  in the region of the gas outlet pipe  104 . A flow path between the larger diameter portion  100  and the gas outlet pipe  104  is defined across the cartridge seal. 
         [0065]    The third phase outlet chamber  96  is annular and surrounds the smaller diameter portion  98  of the first phase outlet chamber  94 . A partition  106  is disposed at the third phase outlet  26  between the drum  7  and the third phase outlet chamber  96 . The partition  106  is formed integrally with the radially inner wall of the third phase outlet chamber  96  and extends radially outwardly with respect to the separator axis  16 . A third phase outlet pipe  108  (shown in  FIG. 1  and in outline in  FIG. 4 ) projects radially outwardly from the third phase outlet chamber  96 . The third phase outlet pipe  108  is perpendicular to the separator axis  16  and the first phase outlet pipe  102 . 
         [0066]    An annular first seal  110  is disposed between the drum  7  and the outlet flange  12  about the periphery of the first phase outlet  22  thereby sealing the first phase outlet chamber  94  from the chamber defined by the outer casing  4  and also from the third phase outlet chamber  96 . A second seal  112  is disposed between the drum  7  and the outlet flange  12  about the outer periphery of the third phase outlet  26 . The second seal  112  is also annular and is coaxial with, and disposed radially outwardly of, the first seal  110 . The second seal  112  thus seals the third phase outlet chamber  96  from the chamber defined by the outer casing  4 . The seals  110 ,  112  allow rotation of the drum  7  with respect to the flanges  10 ,  12 . In the present embodiment the seals  110 ,  112  are labyrinth seals. 
         [0067]    Ducts  114 ,  116  and  118  are formed within walls of the inlet flange  10  and the outlet flange  12  to supply sealing fluid to the respective labyrinth seals  90 ,  110  and  112 . The sealing fluid may, for example, be pressurized oil, water or gas. 
         [0068]    A pressure release valve (not shown) is provided in the outer casing  4 . 
         [0069]    Means (not shown) for independently controlling the back pressure at the first phase outlet  22  and the third phase outlet  26  are provided. This may, for example, be flow regulators. 
         [0070]    In use, an influent mixture comprising two immiscible liquids, such as oil and water, a solid particulate, such as sand, and a gas is supplied through the inlet port  92  into the inlet chamber  88 . The tangential arrangement of the inlet port  92  promotes circulation of the influent within the inlet chamber  88  before flowing through the drum inlet  20  into the drum  7  which is rotated at high speed by the motor driving the shaft  14 . The rotor  6  may, for example, be driven at speeds which are not less than 1750 rpm and not more than 10000 rpm. 
         [0071]    The influent mixture flows from the drum inlet  20  towards the first and third phase outlets  22 ,  26  by passing through the slots  30  in the discs  28 . As the mixture progresses along the drum  7 , the rotating discs  28  exert shear forces (e.g. laminar drag) on the mixture which accelerate and maintain rotation of the flow. The fins  32  assist with promoting and maintaining rotation of the mixture in synchronization with rotation of the rotor  6 . High-speed rotation of the mixture generates a centrifugal force which causes the denser components, i.e. the water and the sand, to migrate radially outwardly which, in turn, displaces the oil and gas radially inwardly. Thus, as the mixture progresses along the drum  7  it separates into stratified layers of the individual components or phases. The staggered flow passages  30  inhibit flow from the drum inlet  20  directly to the first and third phase outlets  22 ,  26 . Inhibiting the flow increases the residence time of the mixture in the drum  7  so that the oil and water of the original mixture are substantially separated upon arrival at the first and third phase outlets  22 ,  26 . An interface between the water and oil is therefore formed. The radial position of the interface can, for example, be controlled by varying flow rates through the first and third phase outlets  22 ,  26 , although it will be appreciated that alternative methods are possible. The water flows over the outer periphery of the weir plate  29  towards the third phase outlet  26 . The position of the interface is controlled so that it remains radially outward of the first phase outlet  22  and radially inward of the outer periphery of the weir plate  29 . This ensures that the separated oil is prevented from exiting through the third phase outlet  26  and instead flows along the passage defined by the annular plate  31  towards the first phase outlet  22 . An emulsion, or rag layer, forms at the interface of the oil and water and/or the interface of the water and solids. 
         [0072]    The centrifugal forces cause the solid particulates to “settle” within the flow which, in effect, causes them to migrate radially outwardly towards the funnels  36 . 
         [0073]    The separation process comprises two stages: an accumulation stage and a discharge stage. During the accumulation stage the pressure in the outer casing  4  is increased to a pressure which may be at least equal to the pressure inside the rotating drum  7 . The pressure across the second phase outlets  24  during the accumulation stage is a positive pressure difference. The pressure in the outer casing, supplemented by the spring loading of the non-return valves  48 , is sufficient to keep the non-return valves  48  closed against the pressure exerted by the rotating fluid on the internal surface of the drum  7 . The pressure within the outer casing  4  is generated by introducing a fluid, preferably a gas such as nitrogen, to the outer casing  4 . The pressure in the outer casing  4  may, for example, be held at 220 psi (approximately 1500 kPa). The introduced gas has a low viscosity with respect to the influent mixture. By surrounding the drum  7  with a low viscosity fluid, the drag acting on the drum  7  during the accumulation stage can be reduced. Furthermore, the effects of boundary layers, eddy flows and frictional forces are also decreased. The torque, and hence power, required to rotate the rotor  6  is reduced, thus improving operating efficiency. Pressurization of the outer casing  4  generates an external pressure on the drum  7 , and hence a radially inwardly acting force on the outer wall of the drum  7 . The radially inwardly acting force partially balances the centrifugal force acting on the drum  7  and thus reduces radial loading on the drum  7  for a particular operating speed of the rotor  6 . The rotor  6  can therefore be operated at speeds which are greater than would otherwise be possible owing to structural limitations of the material of the rotor  6 . The elevated speeds enhance separation of the mixture, for example, by reducing the separation time or improving the quality of the separated phases. 
         [0074]    During the accumulation stage, oil and water are discharged from the drum  7  through the first and third phase outlets  22 ,  26  respectively into the first and third phase outlet chambers  84 ,  86 . Oil exits the separator  2  through the first phase outlet pipe  102 . Water exits the separator  2  through the third phase outlet pipe  108 . Solid particulates entrained by the flow move radially outwardly and accumulate as a slurry or caked solid within the funnels  36 . The inclined surfaces provided by the funnels  36  inhibit solids build-up in regions other than the convergences of the funnels  36 . 
         [0075]    The discharge stage begins once a desired quantity of solid particulates has accumulated in the funnels  36 , or a set period of time has elapsed. One or both of the first phase and third phase outlets  22 ,  26  is/are restricted or closed and the pressurization of the outer casing  4  is maintained. This generates a back pressure within the drum  7 . The back pressure is increased until it exceeds the pressure in the outer casing  4  and is sufficient to overcome the spring bias of the non-return valves  48  to force the valves  48  open. Alternatively, the valves may be forced open by introducing a higher pressure gas into the drum  7 . At this point the pressure across the second phase outlets  24  is a negative pressure difference. The increased back pressure expels the accumulated solids from the drum  7  through the second phase outlets  24  into the region between the rotor  6  and the outer casing  4 . It will be appreciated that the solids may be flushed through the second phase outlets  24  by discharging a proportion of the water in the radially outward region of the drum  7  with the solids. The expelled solids collect in the sump  84  from where they are discharged through the solids outlet port  86  either continuously under the control of the solids flow regulator, or in batches. A minimum liquid level is maintained in the sump  84  to provide a plug to maintain pressure in the casing  4  and to prevent gas blow-by. 
         [0076]    The emulsion layer which forms at the interface of the oil and water is continuously, or periodically, extracted through the emulsion tubes  76  and discharged from the separator  2  through the discharge passage  78 . The radial position of the emulsion layer may be controlled by varying the pressures at the first and third phase outlets  22 ,  26 . For example, an increase in the back-pressure at the first phase outlet  22  would create a build up in the quantity/depth of oil retained in the drum  7  with respect to the quantity of water, thus displacing the emulsion layer radially outwardly. Control of the emulsion layer may be carried out with a timer on a programmable logic controller. 
         [0077]    An emulsion layer may form at the interface of the water and sand. The emulsion layer comprises very fine particles (e.g. particles of sand) covered by a thick film of oil and a further film of water such that the coated particle has neutral buoyancy in water and so resides at the interface of the water and sand. Build up of the emulsion layer may be identified by a change in differential pressure or change in the balance of the rotor  6 . This emulsion layer may be expelled through the second phase outlets  24  during the discharge phase. 
         [0078]    Gas collects in the larger diameter portion  100  of the first phase outlet chamber  94 , in the region adjacent the shaft  14 . The gas flows around the cartridge seal and exits the flange  12  through the gas outlet pipe  104 . This ensures that the separator  2  is degassed at all times. 
         [0079]    It will be appreciated that opening of the valves  48  and expulsion of solids from the drum  7  could also be achieved by decreasing pressure in the outer casing  4  or altering the bias acting on the balls  60  in the valves  48  during operation, or by increasing the rotational speed of the drum  7 . Combinations of these may also be used. Other suitable means for opening the valves could also be used. 
         [0080]    It will be appreciated that the positive pressure difference generated during the accumulation stage can refer to embodiments in which a pressure difference in which the region between the casing  4  and the drum  7  is equal to or less than the pressure in the drum  7 , provided that the valve bias is sufficient to close the valve  48 . 
         [0081]    The pressure in the outer casing  4  may, during the accumulation stage, be held at not less than 150 psi (approximately 1000 kPa), and not more than 600 psi. Embodiments in which the pressure in the outer casing  4  is held respectively at 150 psi (approximately 1000 kPa), 300 psi (approximately 2000 kPa) and 600 psi (approximately 4100 kPa) are possible. 
         [0082]    The rate of flow through the separator  2  may be not less than 100 US gallons per minute (approximately 18.9 liters per second) and not more than 1000 US gallons per minute (63.1 liters per second). 
         [0083]    During use, the fluid in the outer casing  4  may be held at an elevated temperature. For example, the fluid may be hotter than the influent mixture. 
         [0084]    Although the discs  28  are shown to be flat circular discs, it will be appreciated that they could be a different shape, for example cone shaped. The flow passages may, for example, be formed by perforations in the discs  28 . 
         [0085]    The first and third phase outlet pipes  102 ,  108  can be arranged tangentially with respect to the separator axis  16 . 
         [0086]    It will be appreciated that a single set of circumferentially arranged funnels  36  could be used. 
         [0087]      FIG. 10  shows an embodiment in which a baffle  120  extends across a mid-portion of each funnel  36  in a direction which is parallel with the separator axis  16 . The baffle  120  has a radially inner edge which is adjacent the divergent end of the respective funnel  36  and a radially outer edge which is spaced away from the second phase outlet  24 . 
         [0088]    A variant of the present invention comprises a rotor having high pressure spray nozzles arranged along the shaft which are oriented to spray cleaning fluid radially outwardly towards the funnels. The spray nozzles are in communication with the emulsion discharge passage. When the separator is not in operation, or following the discharge stage, a washing fluid can be supplied through the discharge passage and sprayed through the nozzles against the inside of the funnels to clean the funnels. An alternative function of the spray nozzles is to introduce a solution to dilute the influent mixture within the drum during the separation process, or to break-up compacted solids and to slurry the solids before discharge. 
         [0089]    A further embodiment of the invention is shown in  FIGS. 11 to 16 . The main differences with respect to the embodiment shown in  FIGS. 1 to 10  are described. 
         [0090]    The discs  28  are spaced axially so that two adjacent discs  28 , and corresponding fins  32 , are disposed adjacent each funnel  36 . 
         [0091]    Each disc  28  has notches  122  along the inner peripheral edge of the disc  28  adjacent the shaft  14 . Each notch  122  defines an aperture  124  with the radially outer surface of the shaft  14 . In use, gas which has migrated to the region adjacent the shaft  14  flows through the apertures  124  towards the first phase outlet  22 . 
         [0092]    Spray nozzles  126 , for example high pressure spray nozzles, extend radially outwardly from the shaft  14 . The spray nozzles  126  are spaced axially and circumferentially along the shaft  14 . The number of spray nozzles  126  is equal to the number of funnels  36 , and the spray nozzles  126  are arranged such that each spray nozzle  126  extends towards the convergence of a respective funnel  36  and corresponding second phase outlet  24 . 
         [0093]    The spray nozzles  126  are in communication with the interior of the tubular section  70  of the shaft  14 . A bore  128  is provided in each solid end section  72 ,  74  of the shaft  14 . The respective bores  128  extend along the separator axis  16  and exhaust through opposite ends of the shaft  14 . In the regions in which the tubular section  70  overlaps the solid end sections  72 ,  74 , the spray nozzles  126  are in direct communication with the bores  128  via passages provided in the solid end sections  72 ,  74 , which extend perpendicularly to the bores  128 . 
         [0094]    In use, a high pressure fluid can be supplied through the spray nozzles  126 . The fluid is used to perform two functions: cleaning of the funnels  36  and the region surrounding the second phase outlets  24 , and fluidizing of compacted solids to create a slurry prior to expulsion of the solids through the second phase outlets  24 . When the solids content in the flow is low, the separator may be run for a longer period of time between expulsion stages to allow the solids to accumulate. However, the accumulated solids are more likely to become compacted against the inner surface of the funnel  36  by the centrifugal forces. Compacted solids can reduce the effectiveness of the expulsion stage. Therefore, fluidization of the solids prior to expulsion improves the efficiency of the expulsion process. 
         [0095]    The number of fins  32  exceeds the number of spray nozzles  126 . In the present embodiment, there are twelve fins  32  and eight spray nozzles  126 . The fins  32  and the nozzles  126  are arranged so that they are angularly offset from each other about the separator axis  16 . 
         [0096]    As shown in  FIGS. 11 and 15 , auxiliary vanes  130  are disposed between the weir plate  29  and the end wall of the drum  7 . The auxiliary vanes  130  are secured to the weir plate  29  for rotation therewith. The auxiliary vanes  130  extend radially outwardly from the annular plate  31  to the outer periphery of the weir plate  29 . Each auxiliary vane  130  is perforated. In use, the auxiliary vanes  130  maintain rotation of the flow and so inhibit vortex flow in the region between the weir plate  29  and the third phase outlet  26 . The perforations in the auxiliary vanes  130  allow water to pass through the auxiliary vanes  130  during operation of the separator  2 , and so ensure that the water levels, measured with respect to the axis  16  in the radial direction of the separator  2 , in the regions between the auxiliary vanes  130  remain equal. Rotor imbalance resulting from uneven distribution of water about the rotor shaft  14 , particularly during start-up and shut-down of the separator  2 , is therefore prevented. 
         [0097]    Referring to  FIG. 13 , the smaller diameter portion  98  of the first phase outlet chamber  94  is provided with stator fins  132 . The stator fins  132  extend in an axial direction along the radially outer inner surface of the smaller diameter portion  98 . The height of each stator fin  132  increases in the direction away from the first phase outlet  22 . The stator fins  132  are fixed with respect to the first phase outlet chamber  94 . 
         [0098]    The third phase outlet chamber  96  is provided with stator fins  134 . The stator fins  134  extend in an axial direction along the radially outer surface of the third phase outlet chamber  96 . The stator fins  134  extend from the third phase outlet  26  to midway along the third phase outlet chamber  96 . The stator fins  134  are tapered along their length and are arranged so that their height, with respect to the outer surface of the third phase outlet chamber  96 , increases in the direction away from the third phase outlet  26 . The stator fins  134  are fixed with respect to the third phase outlet chamber  96 . 
         [0099]    In use, the stator fins  132 ,  134  arrest flow rotation within the respective outlet chambers  94 ,  96 . 
         [0100]    It will be appreciated that the spray nozzles  126  may be fitted, or retrofitted, to the separator described with reference to  FIGS. 1 to 10 . 
         [0101]    The respective arrangements of the notches  122  in the discs  28 , fin  32  spacing, auxiliary vanes  130  and/or tapered fins  132 / 134  described with respect to the second embodiment could be incorporated separately, or as combinations thereof, into the other embodiments and variants described. 
         [0102]    A further embodiment of the separator is used to separate algae in an effluent or backwash treatment process. With such an embodiment, the influent will be a two phase mixture comprising algae entrained by a liquid. The separator in this embodiment will not necessarily require a third phase outlet. 
         [0103]    In use, the algae accumulates in the funnels either as a solid or as a concentrate. A centrifugal force may be generated which is sufficient to ‘burst’ the algal cells as they are compressed against the inner surfaces of the funnels. The algae may, however, be burst before or after the separation process. The accumulated algae are expelled through the second phase outlet and the remaining fraction of the influent mixture is expelled through the first phase outlet. The flow rate may be controlled in response to the algae density. For example, a desired algae density could be 60 000 ppm, or, for example, 6% solids by volume. 
         [0104]    Following separation or concentration, the algae may be transplanted for further processing, for example in the manufacture of biofuel.