Abstract:
A cyclonic separator for separating fluids comprises an inlet chamber ( 6 ) having means for inducing fluids flowing through the chamber to swirl around an axis, a cyclonic separation chamber ( 10 ) connected to receive fluids from the inlet chamber, and an outlet chamber ( 8 ) connected to receive fluids from the cyclonic separation chamber. The outlet chamber ( 8 ) has a tangential outlet ( 22 ) for relatively dense fluids and an axial outlet ( 24 ) for less dense fluids. The separation chamber is elongate and has a length L and an inlet diameter D, where L/D is in the range 1 to 10.

Description:
RELATED APPLICATIONS 
     This application is the U.S. National Phase filing under 35 U.S.C. §371 of PCT/GB2007/002759, filed Jul. 19, 2007, entitled “Cyclonic Separator and a Method of Separating Fluids”, which designated the United States and was published in English on Feb. 21, 2008, which claims priority under 35 U.S.C. §119(a)-(d) to Great Britain Application No. 0616101.2, filed Aug. 12, 2006, the entire content of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a cyclonic separator and a method of separating single-phase fluids, as well as an apparatus for separating single-phase fluids. In particular, but not exclusively, it relates to a method and apparatus for separating dissolved gases from liquids (i.e. for degassing liquids), or for separating mixtures of liquids having different vapour pressures. 
     BACKGROUND OF THE INVENTION 
     The phrase “single-phase fluids” as used herein means either liquids with dissolved gases, or mixtures of liquids having different vapour pressures. Such liquids can be separated into their component parts either by taking the dissolved gas out of solution or, in the case of mixtures of liquids having different vapour pressures, by converting one of the liquids to vapour form and then separating it from the remaining liquid. The original single phase fluid can thus be converted into separate gas and liquid phases. It should be noted that while the term “single phase fluid” refers essentially to liquids of the types described above, it is not intended to exclude fluids that include such liquids in combination with some free gas, for example in the form of bubbles. In this latter case, the invention may serve to separate the free gas from the liquid while simultaneously separating the gas portion from the liquid portion of the single phase fluid. 
     Dissolved gases are frequently present in liquids in their natural form. For example, raw crude oil usually contains some dissolved hydrocarbon gas. Air or other gases may also become dissolved in liquids during their production, processing or transportation. For example, chlorine gas may be added to water during treatment. It may be necessary to remove some or all of this dissolved gas prior to processing, transportation or storage. For example, in the case of oil, if the dissolved gas is not removed, it may subsequently be released by agitation during transportation or by a reduction in pressure, leading to a potentially dangerous build-up of explosive gas in containers, tankers or other sources handling such fluids. 
     One widely-used method of degassing liquids is to pass the liquid through a separator vessel in which the pressure of the fluid is reduced to below atmospheric pressure. As the pressure is reduced the dissolved gas comes out of solution and rises to the surface of the liquid as bubbles. The evolved gas can then be removed and separated from the remaining liquid. This method is used in the oil and gas industry to remove dissolved hydrocarbon gases from liquid crude oil before it is sent to storage tanks or to tankers for export. 
     The system described above is however complex and bulky, requiring large separator tanks and vacuum pumps or multi-stage eductors (i.e. ejectors or jet pumps) and compressors to generate the required low pressure. A pumping system is then needed to boost the pressure of the degassed liquid back to the level required for transportation by pipeline to a storage tank or tanker. The pressure of the separated gas phase, which is at or below atmospheric pressure, may also have to be boosted using a compressor or eductor/jet pump, so that it can be transported or flared. 
     A similar method may also be used for separating mixtures of liquids having different vapour pressures. Lowering the pressure of the mixture to below the vapour pressure of one of the liquids causes that liquid to be transformed into a free gaseous phase, which can then be separated from the remaining liquid. This method is commonly used for removing chemicals from mixtures of liquids. 
     A cyclonic separator is described in international patent application No. WO99/22873A. The device is designed primarily for separating dust particles from air in a vacuum cleaner, although it may also be used for separating mixtures of gases and liquids. During use, a vortex is created having a radial pressure gradient with a low pressure at the centre of the vortex and higher pressures at greater radii. A reduction in pressure can thus be achieved along the axis of the separator within its central core. 
     There is no suggestion that the above said cyclonic device can be used for degassing liquids. However, even if the separator could be driven hard enough by increasing the flow rate through it to cause some dissolved gas in the liquid to come out of solution, the separator is not designed for this use and the maximum reduction in pressure that can be achieved (to approximately 0.9 bar) is not sufficient for efficient separation of dissolved gases. The separator is also only able to operate over a relatively narrow range of flow rates. 
     SUMMARY OF THE INVENTION 
     Another type of separator, known as a hydrocyclone, is known from GB 2263077A. This device uses cyclonic action to separate fluids of different densities and has an inlet at one end for mixtures of fluids, a first outlet at the same end for less dense fluids portions and a second outlet at the opposite end of the device for more dense fluid portions. This is a “reverse flow” device, in which the fluid portions flow in opposite directions to the respective outlets and both fluids exist within the original fluid mixture. The disadvantages of this device are that there is a large pressure loss across the unit (i.e. the difference between the inlet pressure and the outlet pressure is large) and no pressure recovery is achieved. 
     It is an object of the present invention to provide a method and an apparatus for separating fluids, which mitigates at least some of the aforesaid disadvantages. 
     According to the present invention there is provided a cyclonic separator for separating single phase fluids, the cyclonic separator comprising an inlet chamber, a cyclonic separation chamber and an outlet chamber, all arranged sequentially such that in use fluids flow substantially uniaxially through the separator, the inlet chamber having means for inducing fluids flowing through the chamber to swirl around an axis, the cyclonic separation chamber being constructed and arranged to receive fluids from the inlet chamber and separate those fluids by cyclonic action into a gas portion and a liquid portion, and the outlet chamber being connected to receive the gas and liquid portions from the cyclonic separation chamber and having a first outlet for liquids and a second outlet for gases, wherein the separation chamber is elongate and has a length L and an inlet diameter D, where L/D is in the range of 1 to 10. Preferably, L/D is in the range 2 to 10, more preferably 5 to 7. The inlet diameter D refers to the internal diameter of the chamber at its inlet point. 
     Using the cyclonic separator, the pressure of fluid passing through the device can be readily reduced to about 0.3 bar absolute if the inlet pressure is between 2 to 3 bar absolute, which provides for rapid and effective degassing of many single-phase fluids containing dissolved gas. The shape and dimensions of the separation chamber provide a stable vortex over a wide range of flow rates, which is not significantly disrupted by fluctuations in the flow rate or inlet pressure. This ensures a good separation of gas and liquid phases, with very little carry over of liquid within the separated gas. 
     The pressure reduction achieved within the vortex is largely recovered in the outlet chamber of both liquid and gas phase by the action of the involute feature of these chambers. Part of the pressure recovery is also achieved by the venturi configuration of the separation chamber where the enlargement of the area near the outlet of the separation chamber reduces the velocity of the fluids and contributes to pressure recovery. The pressure drop across the device is therefore very small, which provides for efficient degassing with minimal energy requirement and may avoid the need for downstream pumps and compressors. 
     The apparatus is also very compact, mechanically simple and reliable, it is capable of continuous operation and requires no active control. It has a large turn-down, typically in the range 5:1, allowing it to maintain acceptable operation even if the flow rate drops to one fifth of its normal value. The separator provides a uni-axial flow regime, with all the fluids flowing from the inlet at one end of the device to the respective outlets at the opposite end of the device. 
     The elongate separation chamber may include a throat portion with a diameter D T , where D T &lt;D. Advantageously, the throat diameter D T  is such that D T /D is in the range of 0.3 to &lt;1.0, preferably 0.5 to 0.9. Advantageously, the throat portion has a length L T , where L T /D T  is less than 3.5, and is preferably in the range 0.1 to 3, more preferably 0.5 to 2.5. The throat increases the rotational speed of the vortex and provides a greater pressure reduction at the centre of the vortex for more effective degassing. It also helps to concentrate the separated gas within the central core of the separation chamber at the outlet end where the vortex finder is located. 
     The elongate separation chamber may include a convergent portion upstream of the throat portion. Advantageously, the convergent portion is enclosed by a wall that is inclined relative to the axis of the separation chamber at an included angle θ C  that is less than 45°, and is preferably in the range 5° to 35°, more preferably 5° to 30°. 
     The elongate separation chamber may include a cylindrical inlet portion upstream of the convergent portion. Advantageously, the inlet portion has a length L i , where L I /D is less than 2 and is preferably in the range 0.1 to 1. 
     The elongate separation chamber may include a divergent portion downstream of the throat portion. Advantageously, the divergent portion is enclosed by a wall, which is inclined relative to the axis of the separation chamber at an included angle θ D  that is less than 30°, and is preferably in the range 2° to 20°, more preferably 5° to 15°. The divergent portion provides for pressure recovery from the vortex, which may reduce or eliminate the need for downstream pumps or compressors. It also contributes to the stability of the vortex, which is necessary for effective separation of the gas and liquid phases at different flow rates. 
     The elongate separation chamber may include a cylindrical outlet portion downstream of the divergent portion. Advantageously, the outlet portion has a length L O  and L O /D is less than 2, and is preferably in the range 0.1 to 1. 
     Advantageously, the inlet chamber includes a curved inlet duct of decreasing radius along the axis of fluid entry, and preferably decreasing cross-sectional area. The curved inlet duct preferably has an involute shape and extends around approximately 360°. The involute inlet duct deflects and accelerates the incoming fluids creating a rapidly rotating vortex within a single turn. 
     Advantageously, the inlet chamber has a substantially tangential inlet and an axial outlet. The inlet chamber may also include another involute. 
     The outlet chamber may include a curved outlet duct of increasing radius and preferably increasing cross-sectional area. The outlet duct preferably has an involute shape and extends around approximately 360°. The outlet duct decelerates and repressurises the swirling fluids and removes the rotation of the fluid. 
     Advantageously, the outlet chamber has an axial inlet, a substantially tangential outlet for liquids and an axial outlet for gases. 
     Preferably, the inlet chamber, the separation chamber and the outlet chamber are substantially coaxial. 
     According to another aspect of the invention there is provided an apparatus for separating fluids, the apparatus including a cyclonic separator according to any one of the preceding claims, and a separator device that is connected to receive fluids flowing through at least one of the outlets. The separator device removes any liquid carried over in the removed gases. The separator device preferably comprises a knock-out vessel. 
     According to another aspect of the invention there is provided a method of separating single-phase fluids, comprising passing the fluids through a cyclonic separator, separating the fluids by cyclonic action into a gas portion and a liquid portion, and capturing through separate outlets any gases and liquids exiting the separator. 
     The method may comprise passing fluids including liquids and dissolved gases through a cyclonic separator to separate at least some of the dissolved gases from the liquids, and capturing the gases and liquids separately as they flow through the respective outlets. 
     Advantageously, the pressure of the fluids is reduced while passing them through a cyclonic separator to a value of less than 0.9 bar absolute, preferably approximately 0.4 bar absolute if the inlet pressure is in the range of approximately 2 to 3 bar absolute. 
    
    
     
       Certain embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic side view showing the general configuration of a cyclonic separator according to an embodiment of the invention; 
         FIG. 2  is a sectional side view of the separator shown in  FIG. 1 ; 
         FIG. 3  is a cross-section on line of  FIG. 2 , and 
         FIG. 4  is a cross-section on line IV-IV of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The cyclonic separator  2  shown in  FIGS. 1 to 4  includes an inlet conduit  4 , involute shaped inlet and outlet chambers  6 ,  8  and an intermediate separation chamber  10  that joins the inlet and outlet chambers along the common axis  12  of the three chambers. 
     The inlet chamber  6  includes an inlet duct defined by a curved wall  13  that extends through 360 degrees around the axis  12 . The involute shape of the inlet chamber  6  may for example be similar to that described in patent application WO99/22873A. The radius of the wall  13  decreases from a maximum radius at  14  to a minimum radius at  16 , and the cross-sectional area of the inlet duct decreases towards its downstream end. The downstream end of the tangential inlet conduit  4  is defined on the outside by the maximum radius portion  14  of the curved wall, and on the inside by the minimum radius portion  16  of the wall. The innermost section of the involute inlet chamber  6  is centred on the normal  18  which passes through the axis  12 . The lower face of the inlet chamber  6  is closed by a plate  19 . The upper face of the inlet chamber  6  opens into the intermediate chamber  10 . 
     The intermediate separation chamber  10  is circular in section and includes an inlet portion  10   a , a convergent portion  10   b , a throat portion  10   c , a divergent portion  10   d  and an outlet portion  10   e . The inlet portion  10   a , the throat portion  10   c  and the outlet portion  10   e  are all cylindrical in shape, while the convergent portion  10   b  and the divergent portion  10   d  are frusto-conical. The radius of the inlet portion  10   a  is slightly smaller than the minimum radius  16  of the inlet involute chamber  6 . 
     The outlet involute chamber  8  includes an outlet duct defined by a curved wall  20  that extends through 360 degrees around the axis  12  and leads to a tangential outlet conduit  22  for heavier phases of the separated fluids. The involute shape of the outlet chamber  8  may for example be as described in WO99/22873A. The radius of the wall  20  increases and the cross-sectional area of the inlet duct increases towards its downstream end. The curvature of the wall  20  thus changes in the opposite manner to that of the inlet involute chamber  6 , the outlet involute chamber  8  being arranged to receive fluids swirling in the same sense about the axis  12  as those exiting the inlet chamber  6 . The outlet involute chamber  8  also includes an axial outlet conduit  24  (or “vortex finder”) for the lighter phases of the separated fluids. The axial outlet conduit  24  comprises a co-axial inner cylinder  26  that extends through the outlet chamber and protrudes at  28  slightly into the intermediate chamber  10 . A frusto-conical wall  30  surrounds the inner cylinder  26 , tapering outwards from the entry of the axial outlet to the far end  32  of the outlet involute. 
     In use, fluids consisting of liquids, dissolved gases and possibly some free gases are introduced into the separator through the inlet conduit  4 . These fluids follow the increasing curvature of the curved wall  13  of the inlet involute chamber  6  and are rapidly rotated through 360° so that they swirl around the axis  12  with increasing velocity. The swirling fluids in the inlet involute chamber  6  create a vortex with a pressure gradient having a low pressure point substantially on the axis  12 . If the fluids include any free gases, these will move inwards towards the centre of the vortex while the denser liquids move outwards towards the wall  13 . 
     The swirling fluids then pass into and through the intermediate separator chamber  10 . As the fluids pass through the convergent portion  10   b  and approach the narrow throat  10   c , the rotational velocity increases and the pressure in the centre of the vortex decreases still further. If the pressure is reduced sufficiently, any dissolved gases in the liquid will come out of solution and form bubbles of gas within the liquid. These bubbles will be less dense than the liquid and so will tend to move inwards towards the axis  12 , while the denser liquid will move outwards towards the outer wall of the separator chamber  10 . This causes a separation of the gas from the liquid. 
     As the swirling fluids leave the throat section  10   c  and travel through the divergent portion  10   d , the rotational velocity decreases and the pressure at the centre of the vortex increases. The divergent portion  10   d  thus provides a pressure recovery stage. Separation of the gases from the liquids is maintained, the gases being located at the centre of the vortex near the axis  12  while the liquids continue to rotate around the wall of the chamber. The length and shape of the separation chamber promote a highly stable vortex during this pressure recovery stage. 
     The swirling vortex of fluids then enters the outlet involute chamber  8 . The less dense gases near the axis  12  leave through the axial outlet conduit  24 , while the denser liquids are guided by the curved wall  20  through the tangential outlet conduit  22 . Good separation of the gas and liquid phases is assisted by the tapered shield  30  of axial outlet conduit  24 . The increasing radius of the wall  20  further reduces the rotational speed and increases the outlet pressure of the liquid phases exiting through the tangential outlet conduit  22 , so that the overall pressure drop across the cyclonic separator is minimal. If required, the pressure drop in the gases can also be reduced by feeding the gases flowing through the axial outlet conduit  24  into a further involute chamber. 
     The gases leaving through the axial outlet conduit  24  may carry with them a small quantity of liquid in the form of droplets. If required, these carried over liquids can be separated by feeding the fluids passing through the axial outlet conduit  24  to a conventional separator or knock-out vessel via an outlet line. 
     In use, fluids are fed to the cyclonic separator  2  and are separated into gas and liquid phases. The gases leave the separator through the axial outlet conduit  24  The liquid phases leave the cyclonic separator  2  through the tangential outlet conduit  22 . 
     The efficiency of the cyclonic separator depends largely on the shape and dimensions of the intermediate separation chamber  10 . In the embodiment shown in  FIGS. 1 to 4 , the diameter D T  of the throat portion  10   c  is approximately half the diameter D of the inlet portion  10   a , while the length L T  of the throat portion  10   c  is approximately equal to the throat diameter D T . The diameter of the outlet portion  10   e  is similar to the diameter of the inlet portion. The total length L of the separation chamber  10  is generally approximately five to ten times the diameter D of the inlet portion  10   a . The length L I  of the inlet portion  10   a  and the length L o  of the outlet portion  10   e  are both approximately one third the diameter D of the inlet portion  10   a . The wall of the convergent portion  10   b  is frusto-conical and is inclined such that the included angle θ C  between opposite sides of the wall is approximately 20°. The wall of the divergent portion  10   d  is also frusto-conical and has an included angle θ D  of approximately 10°. These dimensions are only illustrative: other dimensions and shapes are also possible, preferred ranges being indicated below. 
     
       
         
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Quantity 
                 Good 
                 Better 
                 Best 
               
               
                   
                   
               
             
             
               
                   
                 L/D 
                 1 to 10 
                 2 to 10 
                 5 to 6 
               
               
                   
                 D T /D 
                 0.3 to &lt;1.0 
                 0.4 to 0.9 
                 0.5 to 0.9 
               
               
                   
                 L T /D T   
                 0 to 3.5 
                 0.1 to 3 
                 0.5 to 2.5 
               
               
                   
                 θ C   
                 0° to 45° 
                 5° to 40° 
                 5° to 30° 
               
               
                   
                 θ D   
                 0° to 30° 
                 2° to 20° 
                 5° to 15° 
               
               
                   
                 L I /D 
                 0 to 2 
                 0.1 to 1 
                 0.2 to 0.8 
               
               
                   
                 L O /D 
                 0 to 2 
                 0.1 to 1 
                 0.2 to 0.8 
               
               
                   
                   
               
             
          
         
       
     
     The shape of the intermediate separation chamber  10  may be varied without departing from the scope of the invention. For example, instead of having discrete sections (i.e. the inlet, convergent, throat, divergent and outlet portions) with well-defined joins, those sections can merge into one another through the use of radiused joints or continuously curved walls. 
     We have found that it is possible to achieve a pressure in the centre of the vortex within the throat portion  10   c  ranging from just below atmospheric to as low as 0.3 bar absolute, with an inlet pressure of 2 to 3 bar absolute. This compares with a minimum pressure of 0.9 bar absolute achievable under similar conditions with the cyclonic separator described in WO99/22873A. This provides a much greater degassing effect with a lower energy requirement. The vortex is also much more stable, resulting in a much lower quantity of liquid being carried over in the removed gas (typically less than 10% as compared to 30% previously). 
     The cyclonic separator may be used in various different situations for removing dissolved gases from liquids including, for example, the oil and gas industry, the chemicals and pharmaceutical industries and the water industry. It may also be used to separate two fluids having different vapour pressures.