Patent Publication Number: US-2005115273-A1

Title: Multistage fluid separation assembly and method

Description:
BACKGROUND OF THE INVENTION  
      The invention relates to a multistage fluid separation assembly and method.  
      In many oil and/or gas production wells the well effluents may comprise complex fluid mixtures which contain crude oil, natural gas (methane), water, brine, condensates, sulphur, hydrogen sulphide and other components.  
      During production the well effluents expand and generally cool down from reservoir temperatures which may be between 100 and 200 degrees Celsius to significantly lower atmospheric or submarine temperatures.  
      This causes condensation and/or solidification of various components and formation of hydrate, wax and/or asphaltene deposits in the production tubulars and equipment.  
      It is known from U.S. Pat. No. 4,026,120 to remove condensable and/or solidifiable components from the produced well effluents at the wellhead by cooling down the well effluents in a choke and injecting the cooled well effluents into a so called ‘LTX’ (Low Temperature expansion) vessel in which the liquified and/or solidified components, such as water, wax, asphaltenes and hydrates drip down to the bottom, where they are heated to about 20 degrees Celsius to create a pumpable liquid slurry which is then pumped into a liquid discharge conduit at the bottom part of the LTX vessel. The gaseous components are removed from the LTX vessel via a gas outlet conduit near the upper end of the LTX vessel.  
      U.S. Pat. No. 4,208,196 discloses an LTX vessel into which well effluents are injected without prior expansion in a choke. The known LTX vessel is provided with a vertically oriented tubular inlet section into which the well effluents are injected tangentially to enhance segregation of liquefied and/or solidified components from the gaseous components by centrifugal forces. The tubular inlet section is capped and provided with a grating structure at its lower end to inhibit extension of the swirl induced in the inlet section into the liquid collecting region at the bottom of the LTX tank. The tubular inlet section is located inside a cylindrical and horizontal separation tank in which water and oil are collected and separated from each other by gravity segregation, and are subsequently tapped off via separate water and oil discharge tubes near the bottom of the tank. The gaseous components are induced to flow through the grating down from the tubular inlet section into the separation tank and removed from the top of the separation tank at a significant distance from the inlet section.  
      International patent application PCT/NL00/00382 discloses a separation vessel for separating heavy, such as liquid or solid, components from a gaseous mixture in which vessel countercurrent inner and outer swirls are induced by countercurrent swirl imparting vanes arranged near the center and outer periphery of the vessel. A disadvantage of this known device is that the swirl imparting vanes are prone to fouling.  
      International patent application PCT/EP98/04178 discloses a supersonic cyclonic inertia separator in which the produced well effluents are drastically cooled down by adiabatic expansion as a result of their acceleration to a supersonic velocity in a supersonic nozzle. In the supersonic nozzle a swirl is created to segregate the thus condensed and/or solidified heavy components from the lighter gaseous components. The gaseous condensables depleted components are discharged from the separator through a central primary gas outlet conduit whereas the condensables enriched components are discharged from the separator through one or more secondary outlet conduits which extend away from a central axis of the nozzle.  
      It has been found that the secondary condensables enriched fluid outlet of a supersonic cyclonic inertia separator may be connected to an LTX vessel, but that the high velocity of the injected liquefied and/or solidified condensables enriched fluid mixture resulted in a reduced gravity separation efficiency of the LTX vessel. The multistage fluid separation assembly and method according to the preamble of claims  1  and  17  are known from U.S. Pat. No. 2,825,423. In the known assembly fluid is injected into a tubular section of a secondary separation vessel via a single secondary fluid injection conduit, which may lead to instability of the vortex in, and separating performance of, the secondary separation vessel if the pressure and velocity of the injected fluid fluctuates.  
      It is an object of the present invention to provide a hybrid multistage fluid separation assembly in which an LTX type separation vessel is connected to liquefied and/or solidified condensables enriched fluid outlet of one or more gas cooling devices such as supersonic and/or subsonic cyclonic inertia separators such that a synergetic effect is obtained between the performance of the gas cooling device, such as a cyclonic inertia separator, and the LTX separation vessel.  
      It is a further object to provide a hybrid multistage fluid separation assembly, which is more compact than a combination of a gas cooling device, such as a cyclonic inertia separator, and a conventional LTX vessel.  
      It is yet a further object to provide a hybrid multistage fluid separation assembly in which a plurality of gas cooling devices, such as cyclonic inertia separators, can be connected to a single compact LTX vessel by relatively short liquefied and/or condensables enriched fluid outlet conduits such that the risk of solids, wax and/or hydrate deposition in these secondary outlet conduits is minimized.  
     SUMMARY OF THE INVENTION  
      In accordance with the invention there is provided a multistage fluid separation assembly comprising: 
          a primary gas cooling devices which has a liquefied and/or solidified condensables enriched fluid outlet; and     a secondary fluid separation vessel having a tubular section of which a central axis has a substantially vertical or tilted orientation, which vessel is connected to said condensables enriched fluid outlet of at least one of said gas cooling devices, wherein during normal operation of the vessel the condensables enriched fluid is induced to swirl around the central axis of the tubular section of the vessel such that a tertiary stream of liquefied and/or solidified condensables is induced by gravity and centrifugal forces to swirl in downward direction alongside the inner surface of the tubular section of the vessel into a liquid collecting tank at or near the bottom of the vessel for collecting a tertiary mixture of liquefied and/or solidified condensables, which tank is provided with one or more heaters for heating the tertiary mixture to reduce the amount of solidified condensables and with one or more outlets for discharging the tertiary mixture from the tank, wherein a plurality of secondary fluid injection conduits of a plurality of primary gas cooling devices are connected at regular circumferential intervals to the tubular section of the secondary separation vessel, which conduits inject in use condensables enriched fluid in an at least partially tangential direction into the interior of the secondary separation vessel.        

      It is preferred that the tubular section of the secondary separation vessel is equipped with a tertiary gas outlet conduit having an inlet which is located at or near the central axis of the tubular section and which tertiary gas outlet extends through an upper end of the tubular section of the secondary separation vessel.  
      Suitably, the secondary separation vessel has a dome- or disk-shaped top, which is mounted on top of the tubular section and the tertiary gas outlet conduit is arranged substantially co-axial to the central axis of the tubular section and passes through the center of the top.  
      Preferably, the liquefied and/or solidified condensables enriched fluid outlet of at least one primary cooling device, such as a cyclonic fluid separator, is connected to a secondary fluid injection conduit which injects the condensables enriched fluid in an at least partially tangential direction into the tubular section of the secondary separation vessel.  
      It is also preferred that the central axis of the tubular section of the secondary separation vessel has a substantially vertical orientation and that the secondary fluid injection conduits inject in use liquefied and/or solidified condensables enriched fluid in an at least partially tangential and partially downward direction into the interior of the secondary separation vessel.  
      Suitably, the liquid collecting tank is formed by a cup-shaped tubular lower portion of the second stage separation vessel which is substantially co-axial to the central axis and has a similar or larger internal width than the upper portion of the vessel and a vortex breaker is arranged in the interior of the secondary separation vessel between the lower end of the tubular section and the liquid collecting tank.  
      The assembly may be provided with one or more ultrasonic vibration transducers for imposing ultrasonic vibrations at a frequency between 20 and 200 KHz on one or more components of the assembly, such as the secondary fluid injection tubes and the vortex breaker, to inhibit deposition of solidified condensables, such as ice, wax and/or hydrates, within the assembly.  
      The liquid collecting tank may be provided with a grid of heating tubes, which are designed to heat the liquid and solid fluid mixture in the tank to a temperature of at least 15 degrees Celsius.  
      One or more primary gas cooling devices may comprise cyclonic inertia separators which comprise an expansion nozzle in which the fluid mixture is cooled to a temperature lower than 0 degrees Celsius by a substantially isentropic expansion and in which one or more swirl imparting vanes induce the fluid to swirl into a diverging outlet section which is equipped with a central primary condensables depleted fluid outlet conduit and an outer secondary condensables enriched fluid outlet conduit.  
      Suitably each primary gas cooling device, such as a cyclonic inertia separator, comprises an expansion nozzle, which is designed to accelerate the fluid mixture within the nozzle to a supersonic speed, thereby cooling in use the temperature of the fluid passing through the nozzle to a temperature lower than −20 degrees Celsius.  
      The fluid separation assembly according to the invention may comprise a plurality of primary cyclonic inertia separators of which the expansion nozzles are substantially parallel and equidistant to the central axis of the tubular section of the secondary separation vessel and of which the secondary condensables enriched fluid outlets are connected to secondary fluid injection conduits which intersect the wall of the tubular section of the secondary separation vessel at regular circumferential intervals and in an at least partially tangential direction, and which secondary fluid injection conduits each have a length less than 4 meter.  
      The gas cooling devices may comprise chokes known as Joule-Thompson valves in which the gas is accelerated and cooled by expansion such that a liquefied and/or solidified condensables enriched fluid is generated, which is subsequently fed into the secondary fluid separation vessel.  
      The invention also relates to a method of separating condensable components from a fluid mixture in a multistage fluid separation assembly, which method comprises: 
          injecting the fluid mixture into a primary gas cooling devices in which the fluid mixture is expanded and cooled and condensable components are liquefied and/or solidified and optionally separated from the gaseous components by centrifugal force, and in which a stream of condensables enriched fluid components is fed into a secondary fluid outlet; and     injecting the stream of condensables enriched fluid components into a secondary fluid separation vessel having a tubular section of which a central axis has a substantially vertical or tilted orientation and in which the condensables enriched fluid stream is induced to swirl around the central axis of the tubular section of the vessel such that a tertiary mixture of liquefied and/or solidified condensables is induced by gravity and centrifugal forces to swirl in downward direction alongside the inner surface of the tubular section of the vessel into a liquid collecting tank at or near the bottom of the vessel, in which tank the tertiary mixture of liquefied and/or solidified condensables is collected and heated to reduce the amount of solidified condensables and from which tank liquid and/or solidified components are discharged through one or more outlets, wherein a plurality of secondary fluid injection conduits of a plurality of primary gas cooling devices are connected at regular circumferential intervals to the tubular section of the secondary separation vessel, which conduits inject condensables enriched fluid in an at least partially tangential direction into the interior of the secondary separation vessel.        

    
    
     DESCRIPTION OF SUITABLE EMBODIMENTS  
      Suitable embodiments of a multistage fluid separation assembly according to the present invention will be described in more detail with reference to the accompanying drawings wherein:  
       FIG. 1  is a schematic vertical split sectional view of a first suitable embodiment of a multistage fluid separation assembly according to the invention wherein four primary cyclonic inertia separators discharge a condensables enriched fluid mixture into a vertically oriented tubular secondary fluid separation vessel;  
       FIG. 2  is a top view of the multistage fluid separation assembly of  FIG. 1 ;  
       FIG. 3  is a schematic vertical split sectional view of another suitable embodiment of a multistage fluid separation assembly according to the invention wherein two primary cyclonic inertia separators discharge a condensables enriched fluid mixture into a horizontally oriented secondary fluid separation vessel;  
       FIG. 4  is a horizontal cross-sectional view of the multistage fluid separation assembly of  FIG. 3 , taken across the primary cyclonic inertia separators and seen from above;  
       FIG. 5  is a schematic horizontal sectional view of a simplified multistage fluid separation assembly of which the primary gas cooling devices consist of chokes known as Joule Thompson valves; and  
       FIG. 6  is a schematic vertical sectional view of the assembly shown in  FIG. 5 . 
    
    
      Referring to  FIG. 1  there is shown a primary cyclonic inertia separator  1  which is connected to a secondary LTX fluid separation vessel  2  by a secondary condensables enriched fluid injection conduit  3 .  
      The primary cyclonic inertia separator  1  comprises a fluid inlet  4  which is connected to a wellhead (not shown) of a natural gas production well via which a mixture of natural gas, water vapours, condensates, hydrates and other condensable components are produced.  
      The fluid mixture flowing into the primary separator  1  is first accelerated to a high, preferably supersonic, velocity in a nozzle  5  and thereby cooled down preferably to a temperature below −20 degrees Celsius, such that water vapour and/or other condensables condense and/or solidify as a result of adiabatic expansion and subsequently induced to swirl by a tilted wing  6  so that the cooled fluid mixture is separated by centrifugal forces into a central flux of condensables depleted natural gas and an annular outer flux of condensables enriched fluid.  
      The central flux of condensables depleted natural gas is discharged through a central primary condensables depleted outlet  7  into a gas distribution conduit (not shown) and the annular outer flux of condensables enriched fluid is discharged through an annular condensables enriched fluid outlet chamber  8  into the secondary condensables enriched fluid injection conduit  3  as illustrated by arrow  9 .  
      The condensables enriched fluid injection conduit  3  feeds the condensables enriched fluid into a vertical section  10  of the secondary LTX fluid separation vessel  2  for further separation into substantially dry gas, water and liquefied hydrocarbons and hydrates.  
      As illustrated in  FIG. 2 a  series of four primary inertia separators  1 ,  1 B,  1 C and  1 D feed condensable enriched fluid tangentially into the vertical tubular section  10  of the secondary LTX separation vessel  2  via a series of four condensables enriched fluid injection conduits  3 ,  3 B,  3 C and  3 D.  
      In  FIG. 1 a  schematic vertical split sectional view is given of the first primary inertia separator  1  and of the secondary LTX separation vessel  2  and the contours of other three primary inertia separators  1 B,  1 C and  1 D are illustrated in dotted lines.  
      As illustrated in  FIG. 1  the secondary LTX separation vessel  2  comprises below the vertical tubular section  10  a large diameter bottom section  11  in which a tertiary stream of liquids is collected and further separated, a vortex breaker  12  which is located between the tubular and bottom sections  10  and  11  and a dome-shaped top  13  through which a tertiary gas outlet conduit  14  extends into the tubular section  10 .  
      The entrance  15  of the tertiary gas outlet conduit  14  is located below the level at which the condensables enriched fluid inlet conduit  3  debouches into the tubular section  10  of the secondary separation vessel  2 .  
      The fluid injected via the tangential fluid injection conduits  3  into the tubular section  10  will circulate, when seen from above in an anti-clockwise direction, alongside the inner wall of the tubular section  10  as illustrated by arrows  17 . A tertiary stream of liquid and/or solid components will be concentrated in the outer periphery of the interior of the tubular section  10  and will as a result of gravitational forces drop through the annular gap  16  alongside the outer circumference of the vortex breaker  12  into a liquid collecting tank  19  within the large diameter bottom section  11  of the secondary fluid separation vessel  2 .  
      Low density gaseous components of the fluid mixture injected via the tangential fluid injection conduits  3 ,  3 B,  3 C and  3 D into the tubular section  10  will be concentrated near the centre of the tubular section  10  and will flow up into the tertiary gas outlet  14 . The arrangement of the entrance  15  of the tertiary gas outlet  14  below the level at which the tangential fluid injection conduits  3 ,  3 B,  3 C and  3 D inject fluid into the tubular section  10  will generally induce the gaseous components to swirl as an eddy current  18  in an opposite, when seen from above, clockwise direction. The clockwise circulating eddy current  18  of substantially dry gas, which circulates as a result of the Hirsch-Rankin effect in a direction opposite to the outer stream  17  of condensables further enhances the separation of dry gas from the liquid and/or solid components. The vortex breaker  12  has a conical shape and the top of the vortex breaker  12  performs as a vortex seeker for the clockwise rotating central eddy current  18  of dry gas.  
      The tubular section  10  is a substantially open vessel without swirl imparting vanes, in order to minimise the risk of plugging and/or contamination of the interior of the vessel.  
      The liquid collecting tank  19  is equipped with heating coils  20  which maintain the collected liquid and solid components at a substantially constant temperature between 15 and 25 degrees Celsius. At this temperature waxes and hydrates will melt and will float up above the water in the liquid collecting tank, such that a top layer  21  of wax, hydrates, condensate and other hydrocarbon liquids will float above a lower layer  22  of water.  
      Water (H 2 O) is discharged through a water discharge conduit  23  at the bottom of the liquid collecting tank  19 , whereas the mixture of wax, hydrates, condensate and other hydrocarbons will flow over the upper rim  24  of the liquid collecting tank  19  into an annular hydrocarbon liquids collecting zone  25  which is equipped with a hydrocarbon liquids discharge conduit  26 . The water discharge conduit  23  is equipped with a valve (not shown), which is controlled such that the water/hydrocarbon liquids interface  27  is maintained at a substantially constant level.  
      Optionally the vortex breaker  12  and the walls of the vertical tubular section  10  of the secondary separation vessel  2  and the conduits  3 ,  3 B,  3 C and  3 D are heated and/or equipped with ultrasonic vibration transducers (not shown) to inhibit deposition of wax, scale and hydrates and other fouling on these components. The ultrasonic vibration transducers preferably vibrate the walls of these fouling sensitive components at a frequency between 20 and 200 KHz to shake off any fouling.  
      The compact configuration of the multistage separation assembly according to the invention makes it very suitable for use on offshore platforms where deckspace is limited and the presence of small surface areas within the short conduits  3 ,  3 B,  3 C and  3 D and within the secondary separation vessel,  2  which are vulnerable to fouling and which can be heated and vibrated to remove any fouling allows the use of primary cyclonic separators in which the wet gas is adiabatically cooled to a very low temperature, which may be below minus 20 or even below minus 40 degrees Celsius, such that a maximum amount of water, hydrates, waxes and/or other condensable and/or solidifiable components are condensed and/or solidified without obstructing the associated peripheral conduits  3 ,  3 B,  3 C and  3 D and secondary LTX separation vessel  2 .  
      Referring now to  FIGS. 3 and 4  there is shown another suitable configuration of a multistage fluid separation assembly according to the invention wherein two primary cyclonic inertia separators  31  and  31 A discharge a condensables enriched fluid mixture via conduits  33  and  33 A tangentially into a vertical tubular top section  30  of a secondary LTX fluid separation vessel  32 .  
      The two primary cyclonic inertia separators  31  and  31 A have a horizontal orientation but are otherwise similar to the primary cyclonic inertia separators  1 - 1 D shown in  FIGS. 1 and 2  and also comprise a wet gas inlet  34 ; a nozzle  35 ,  35 A in which the wet natural gas stream is accelerated to a preferably supersonic or near sonic velocity and thereby adiabatically cooled to minus 20 to minus 40 degrees Celsius such that water and other condensable and/or solidifiable components condense and/or solidify and the cooled fluid mixture is swirled around by one or more swirl imparting vanes  36 ,  36 A and separated by centrifugal inertia forces into a central flux of substantially dry gas which is discharged through a central gas outlet  37 ,  37 A and an annular flux of a condensables enriched fluid which is discharged via an annular collecting zone  38 ,  38 A into the conduits  33  and  33 A.  
      The secondary LTX separation vessel  32  comprises below the vertical tubular top section  30  an elongate horizontally oriented tubular bottom section  41  in which liquids are collected and further separated, a vortex breaker  42  which is located between the tubular top and bottom sections  30  and  41  and a dome-shaped cap  43  through which a tertiary gas outlet conduit  44  extends into the tubular top section  38 .  
      The entrance  45  of the tertiary gas outlet conduit  44  is located below the level at which the condensables enriched fluid inlet conduits  33  and  33 A debouch into the tubular top section  30  of the secondary separation vessel  32 .  
      The fluid injected via the tangential fluid injection conduits  33  and  33 A into the tubular top section  30  will circulate, when seen from above in a clockwise direction, alongside the inner wall of the tubular top section  30  as illustrated by arrows  47 . Liquid and/or solid components will be concentrated in the outer periphery of the interior of the tubular section  30  and will as a result of gravitational forces fall down through the annular gap  46  alongside the outer circumference of the vortex breaker  42  into a liquid collecting tank  49  within the elongate tubular bottom section  41  of the secondary fluid separation vessel  32 .  
      Low density gaseous components of the fluid mixture injected via the tangential fluid injection conduits  33  and  33 A into the tubular top section  30  will be concentrated near the centre of the tubular top section  30  and will flow up into the tertiary gas outlet  44 . The arrangement of the entrance  45  of the tertiary gas outlet  44  below the level at which the tangential fluid injection conduits  44  inject fluid into the tubular top section  30  will generally induce the gaseous components to swirl as an eddy current  48  in an opposite, when seen from above, anti-clockwise direction. The anti-clockwise circulating eddy current  48  of substantially dry gas, which circulates in a direction opposite to the outer stream  47  of condensables further enhances the separation of dry gas from the liquid and/or solid components. The vortex breaker  42  has a conical shape and the top of the vortex breaker  42  performs as a vortex seeker for the anti-clockwise rotating central eddy current  48  of dry gas.  
      The liquid collecting tank  49  is equipped with heating coils  50  which maintain the collected liquid and solid components at a substantially constant temperature between 15 and 45 degrees Celsius. At this temperature waxes and hydrates will melt and will float up above the water in the liquid collecting tank, such that a top layer  51  of wax, hydrates, condensate and other hydrocarbon liquids will float above a lower layer  52  of water. The foamy top layer  51  will also minimize re-evaporation of the liquid hydrocarbons and will therefore keep the hydrocarbon dewpoint in the upper part of the interior of the LTX vessel  2  low.  
      Water (H 2 O) is discharged through a water discharge conduit  53  at the bottom of the liquid collecting tank  49 , whereas the mixture of wax, hydrates, condensate and other hydrocarbons will flow over the upper rim  54  of a bulkhead  58  at the right side of the liquid collecting tank  49  into a hydrocarbon liquids collecting zone  55  which is equipped with a hydrocarbon liquids discharge conduit  56 . The water discharge conduit  53  is equipped with a valve (not shown), which is controlled such that the water/hydrocarbon liquids interface  57  is maintained at a substantially constant level.  
      Optionally the vortex breaker  42  and the walls of the vertical tubular top section  30  of the secondary separation vessel  32  and the conduits  33  and  33  are heated and/or equipped with ultrasonic vibration transducers (not shown) to inhibit deposition of wax, scale and hydrates and other fouling on these components. The ultrasonic vibration transducers preferably vibrate the walls of these fouling sensitive components at a frequency between 20 and 200 KHz to shake off any fouling.  
      The large volume and horizontal surface area of the liquid collecting tank  49  facilitate low liquid velocities of the separated water and liquefied hydrocarbon components such that an optimal gravity separation of the aqueous and hydrocarbon phases takes place within the tank  49 .  
      It will be understood that the bulkhead  58  may be removed from the tubular bottom section  41  of the vessel  32  and that the hydrocarbon liquid outlet  56  may pass through the sidewall of said tubular bottom section  41  just below the upper surface of the foamy top layer  51  of hydrocarbon liquids.  
      Referring to  FIGS. 5 and 6  there is shown a set of four primary cooling devices  61 ,  61 B,  61 C, and  61 D which are connected to a secondary LTX fluid separation vessel  62  by a set of four tangential secondary condensables enriched fluid injection conduits  63 ,  63 B,  63 C and  63 D.  
      Each primary cooling device comprises a fluid inlet  64  which is connected to a wellhead (not shown) of a natural gas production well via which a mixture of natural gas, water vapours, condensates, hydrates and other condensable components is produced.  
      The fluid mixture flowing into each primary cooling device  61 ,  61 B,  61 C and  61 D is accelerated to a high, preferably supersonic, velocity in a flow restriction  65 , such as a venturi, choke, or Joule Thompson valve and thereby cooled down preferably to a temperature below −20 degrees Celsius, such that water vapour and/or other condensables condense and/or solidify as a result of adiabatic expansion whereupon a liquefied and/or solidified condensables enriched fluid mixture flows into the narrow secondary fluid injection conduits  63 ,  63 B,  63 C and  63 D.  
      Each condensables enriched fluid injection conduit  63 ,  63 B,  63 C and  63 D feeds the liquefied and/or solidified condensables enriched fluid at high speed into a vertical section  70  of the secondary LTX fluid separation vessel  62  for separation into substantially dry gas, water and liquefied hydrocarbons and hydrates.  
      As illustrated in  FIG. 5 a  series of four primary inertia separators  61 ,  61 B,  61 C and  61 D feed a liquefied and/or solidified enriched fluid mixture tangentially into the vertical tubular section  70  of the secondary LTX separation vessel  62  via a series of four condensables enriched fluid injection conduits  63 ,  63 B,  63 C and  63 D.  
      In  FIG. 6 a  schematic vertical sectional view is given of the primary inertia separator  61  and  61 C and of the secondary LTX separation vessel  62 .  
      As illustrated in  FIG. 6  the secondary LTX separation vessel  62  comprises below the vertical tubular section  70  a bottom section  71  in which a tertiary stream of liquids is collected and further separated, a vortex breaker  72  which is located between the tubular and bottom sections  70  and  71  and a dome-shaped top  73  through which a tertiary gas outlet conduit  74  extends into the tubular section  70 .  
      The entrance  75  of the tertiary gas outlet conduit  74  is located below the level at which the condensables enriched fluid inlet conduits  63 ,  63 A,  63 B and  63 C debouch into the tubular section  70  of the secondary separation vessel  62 .  
      The fluid injected via the tangential fluid injection conduits  63  into the tubular section  70  will circulate, when seen from above in an anti-clockwise direction, alongside the inner wall of the tubular section  70  as illustrated by arrows  77 . A tertiary stream of liquid and/or solid components will be concentrated in the outer periphery of the interior of the tubular section  70  and will as a result of gravitational forces drop through the annular gap  76  alongside the outer circumference of the vortex breaker  72  into a liquid collecting tank  79  within the bottom section  71  of the secondary fluid separation vessel  62 .  
      Low density gaseous components such as methane (CH 4 ) etc. of the fluid mixture injected via the tangential fluid injection conduits  63 ,  63 B,  63 C and  63 D into the tubular section  70  will be concentrated near the centre of the tubular section  70  and will flow up into the tertiary gas outlet  74 . The arrangement of the entrance  75  of the tertiary gas outlet  74  below the level at which the tangential fluid injection conduits  63 ,  63 B,  63 C and  63 D inject fluid into the tubular section  70  will generally induce the gaseous components to swirl as an eddy current  78  in an opposite, when seen from above, clockwise direction. The clockwise circulating eddy current  78  of substantially dry gas, which circulates as a result of the Hirsch-Rankin effect in a direction opposite to the outer stream  77  of condensables further enhances the separation of dry gas from the liquid and/or solid components. The vortex breaker  72  has a conical shape and the top of the vortex breaker  72  performs as a vortex seeker for the clockwise rotating central eddy current  78  of dry gas.  
      The bottom section  71  of the vessel  62  is equipped with heating coils  80  which maintain the collected liquid and solid components at a substantially constant temperature between 15 and 25 degrees Celsius. At this temperature waxes and hydrates will melt and will float up above the water in the liquid collecting tank, such that a top layer  81  of wax, hydrates, condensate and other hydrocarbon liquids (C x H y ) will float above a lower layer  82  of water.  
      Water (H 2 O) is discharged through a water discharge conduit  64  at the bottom of the vessel  62 , whereas the mixture of wax, hydrates, condensate and other hydrocarbons (C x H y ) are discharged via a central hydrocarbon liquids discharge conduit  83 . The discharge conduits  83  and  84  may be equipped with valves (not shown), which may be controlled such that the water/hydrocarbon liquids interface  87  is maintained at a substantially constant level.  
      If desired water and hydrocarbons may be discharged from the vessel  62  via a single liquid outlet at the bottom of the vessel  62  for further separation in a separation unit (not shown) downstream of the vessel  62 .  
      Optionally the vortex breaker  72  and the walls of the vertical tubular section  70  of the secondary separation vessel  62  and the conduits  63 ,  63 B,  63 C and  63 D are heated and/or equipped with ultrasonic vibration transducers (not shown) to inhibit deposition of wax, scale and hydrates and other fouling on these components. The ultrasonic vibration transducers preferably vibrate the walls of these fouling sensitive components at a frequency between 20 and 200 KHz to shake off any fouling.  
      The multistage fluid separation assembly according to the invention is also useful for separating solid particles from a gaseous fluid stream. In the gas cooling devices the solid particles will serve as nucleation sources which promote droplet growth of liquefied condensables. The solid particles will flow together with the liquefied condensables into the liquid collecting tank of the secondary fluid separation vessel and are thereby separated from the tertiary stream of condensables depleted, substantially dry gas, that is discharged from the top of the secondary fluid separation vessel.