WO2008082291A1

WO2008082291A1 - Device and method for separating a flowing medium mixture into fractions

Info

Publication number: WO2008082291A1
Application number: PCT/NL2007/050601
Authority: WO
Inventors: Raichel Elton Taciano Leito; Jozef Johannes Hubertus Brouwers
Original assignee: Romico Hold A.V.V.
Priority date: 2006-12-01
Filing date: 2007-11-28
Publication date: 2008-07-10
Also published as: NL2000350C2

Abstract

The invention relates to a device for separating a flowing medium mixture into at least two fractions with differing mass density. The device comprises rotation means in the form of a rotating assembly of feed channels in which the medium mixture is separated, and a feed and discharge for at least one of the fractions of the separated medium mixture connecting to the rotation means. Upstream of the rotation means the device comprises a rotating vane unit with vanes running in radial direction from a central shaft, with an inlet for the medium mixture running in axial direction and with a discharge for at least one of the pre-separated fractions of the medium mixture. The invention also relates to a method for separating a flowing medium mixture using the claimed device.

Description

Device and method for separating a flowing medium mixture into fractions

The invention relates to a device for separating a flowing medium mixture into at least two fractions with differing mass density, comprising rotation means for rotating the flowing mixture for separating. The invention also relates to a method for separating a flowing medium mixture into at least two fractions with differing mass density.

The separation of a (flowing) medium mixture has very diverse applications. Medium mixture is here understood to mean a mixture of solid and/or liquid and/or gas particles of micron or sub-micron size dispersed in at least one liquid or gas. Examples are a gas/gas mixture, a gas/liquid mixture or aerosol, a liquid/liquid mixture, a gas/solid mixture, a liquid/solid mixture, or such a mixture provided with one or more additional fractions. The separation of a medium mixture is for instance known from various applications of liquid cleaning, (flue) gas cleaning and powder separation. Separation of fractions with a great difference in particle size and/or a great difference in mass density is relatively simple. Large-scale use is made for this purpose of processes such as filtration and screening. In the separation of fractions with a smaller difference in mass density, as is for instance the case for gas/gas mixtures, use is made of chemical separating techniques and/or separating techniques such as sedimentation and centrifugation. Certainly when processing large volumes of medium mixture, chemical separating techniques are less economic and usually also less environmentally-friendly. Separating fractions by means of sedimentation requires time and, when processing larger volumes of medium mixture, makes it necessary to make use of voluminous reservoirs, which is, among other things, expensive. Another per se known technology makes use of the differences in mass density of the fractions for separating by applying a centrifugal force to the mixture by causing the mixture to rotate in a centrifuge or a cyclone. This technique is not usually sufficiently selective to realize a separation of the desired level in a short time.

A known device of the type stated in the preamble is described in NL 8700698 (or in the corresponding patent EP 0286160). The known device comprises rotation means accommodated in a tubular housing and having the form of a rotating assembly of feed channels for rotating the flowing mixture for separating, a feed for the medium mixture to be separated connecting to the rotation means, a discharge connecting to the rotation means for discharging the fractions of the separated medium mixture. In a rotating feed channel the heavier fraction of the mixture for separating is moved further outward toward the wall thereof than the lighter fraction under the influence of the centrifugal force occurring therein, thus resulting in a separation. The selectivity of the separation of the device known from NL 8700698 is capable of improvement. There is likewise a need for a more rapid separating process.

The present invention has for its object to provide a device and method which has an increased selectivity for the fractions for separating, and which also enables a more rapid separation than has heretofore been usual.

The invention provides for this purpose a device of the type stated in the preamble with the features stated in claim 1. Provided according to the invention is a device for separating a flowing medium mixture into at least two fractions with differing mass density, which device comprises rotation means in the form of a rotating assembly of feed channels for rotating the flowing mixture for separating, a feed for the medium mixture to be separated connecting to the rotation means, and a discharge connecting to the rotation means for discharging at least one of the fractions of the separated medium mixture, and which device is characterized in that upstream of the rotation means the device comprises a first vane unit with vanes running in a radial direction from a central shaft, with an inlet for the medium mixture running in axial direction and with a discharge for at least one of the fractions of the medium mixture. According to the invention the rotating first vane unit disposed upstream of the rotating assembly of channels ensures that secondary flows which can adversely affect the separating process are suppressed. According to the invention the first vane unit is approached substantially axially by the medium mixture to be separated. The first vane unit further acts as pre-separator for at least a part of the fractions of the medium mixture. Secondary flows comprise local rotations or swirls in the medium mixture. These disrupt the migration of the fractions to be separated to the outer wall of the device and therefore result in a lower degree of separation. It has been found that the first vane unit suppresses such secondary flows. The migration of the fractions to be separated to the outer wall will hereby also proceed better and more quickly, whereby the first vane unit acts as a pre-separator. The heavier fraction will accumulate relatively quickly on the outer wall of the first vane unit and can already be discharged via the likewise provided discharge before the medium mixture enters the rotating assembly of channels. A further advantage of the device according to the invention is that the first vane unit also ensures that the shear forces in the flow of the medium mixture can be kept relatively low. This has particular advantages with medium mixtures in which a gaseous or liquid phase is present in a gaseous or liquid carrier medium. Because the shear forces exerted by the carrier medium on the dispersed phase are relatively low, the gas or liquid bubbles taken up in such medium mixtures will be less likely to break up into smaller bubbles. This further enhances the separation of such medium mixtures. It is advantageous in this respect that the vanes of the first vane unit are provided with a curvature on at least the upstream outer end such that the entry of the medium mixture into the first vane unit progresses relatively gently, Le. without the flow being disrupted too much.

A preferred embodiment of the device according to the invention has the feature that during use the first vane unit co-rotates with the rotation means. Adverse secondary flows are further avoided by properly adapting to each other the rotation speed of the medium mixture just upstream of the rotation means and the rotation speed of the rotating assembly of feed channels. The present feature can be achieved in simple manner by arranging the first vane unit on the rotation shaft of the rotating assembly of feed channels, in other words by having the central shaft of the first vane unit coincide with the rotation shaft of the rotating assembly of feed channels. The first vane unit hereby also acts as drive for the rotating assembly of feed channels.

It is advantageous to characterize the device according to the invention in that the first vane unit comprises a peripheral wall provided with a number of openings along which at least (a part of) one of the fractions of the medium mixture can be discharged. This preferred variant provides for an even more uniform flow, and in the case of a liquid mixture has for instance the advantage of a more uniform and better controlled discharge.

The radially outward running vanes of the first vane unit divide the space in this vane unit into a number of compartments in similar manner to the division of a pie. The number of vanes (and so the number of compartments) can in principle be selected within wide limits. It is however advantageous if the first vane unit comprises at least two vanes distributed uniformly in peripheral direction of the first vane unit. More preferably the first vane unit comprises a maximum of thirty-five vanes distributed uniformly in peripheral direction of the first vane unit. Still more preferably the first vane unit comprises between five and twenty-five vanes distributed uniformly in peripheral direction of the first vane unit. The number of vanes distributed uniformly in peripheral direction of the first vane unit most preferably lies between eight and fifteen vanes. Thus selecting the number of vanes achieves that turbulent flow in the compartments is suppressed. This further enhances the pre-separation of the medium mixture. According to the invention a substantially axial flow will thus occur in the compartments, at least in a co-rotating coordinate system. The flow is in other words substantially the superimposition of a solid body rotation and a purely axial flow. Such a flow pattern is highly advantageous for the separation process.

The form of the vanes of the first vane unit can in principle be freely chosen. It is thus possible to give the vanes a curvature, for instance in the rotation direction of the vane unit. Preferably however, the device according to the invention is characterized in that the vanes of the first vane unit run linearly in radial direction. Such a form of the vanes is favourable for the pre-separation and makes the operation of the vane unit independent of the rotation direction thereof. It is also simple to manufacture and robust in use.

The length of the first vane unit in axial direction, this being the direction of throughput of the medium mixture in the device, can vary, for instance as a function of the composition of the medium mixture to be separated. Because the first vane unit rotates, the tangential speed of the medium mixture from the central shaft of the vane unit will increase almost linearly with the radius. This means that on average a larger amount of fraction, for instance in the form of particles, will accumulate at a larger radius. It is therefore advantageous that the axial length of the vanes of the first vane unit varies in radial direction, and more preferably increases with the radius. A larger fraction of the medium mixture will hereby be collected by the vanes of the first vane unit. The average axial length (average along the radius) of the vanes of the first vane unit is preferably almost equal to the length of the rotating assembly of feed channels, more preferably almost equal to twice the length, and most preferably a maximum of five times the length of the rotating assembly of feed channels. The longer the form of the first vane unit, the larger the amount of fraction pre-separatedfiom the medium mixture and the more efficiently it will operate as pre-separator.

In a further preferred embodiment of the device according to the invention, wherein the first vane unit is provided with a peripheral wall, the radius of the peripheral wall of the first vane unit increases in axial direction from an Initial radius to a final radius. The initial radius of the first vane unit preferably amounts here to between 15% and 85% of the final radius, more preferably to between 25% and 75%, most preferably to between 35% and 65%. This preferred variant provides a gentler flow in the first vane unit, which further enhances the separation of the medium mixture. A further advantage of this embodiment is that a positive pressure difference is created between the outlet of the assembly of feed channels and the inlet of the first vane unit. This pressure difference avoids leakage of untreated medium mixture along the walls in downstream direction.

According to the invention the first vane unit must preferably rotate, and moreover rotate coaxially with the rotating assembly of feed channels. It is advantageous to characterize the device according to the invention in that the first vane unit is driven by a turbine and/or axial centrifugal pump disposed upstream thereof. Such a centrifugal pump comprises a spiral-shaped feed channel running tangentially to the peripheral wall thereof and a rotor with blades. Because the medium mixture is supplied tangentially, the rotor with blades will rotate, whereby the medium mixture is set into pre-rotation and will thus drive the first vane unit. The first vane unit is suspended for this purpose for substantially free rotation from the central shaft. In another preferred embodiment of the device according to the invention the first vane unit is driven separately by means of a motor. This has the advantage that the rotation speed of the first vane unit can be independently chosen, whereby the pre-separating action thereof can be optimally adjusted.

The separation of the device according to the invention is based on the rotation of the assembly of feed channels. In a rotating feed channel the heavier fraction of the medium mixture to be separated is moved further outward toward the wall thereof than the lighter fraction under the influence of the centrifugal force occurring therein, thus resulting in a separation. The assembly of feed channels must therefore be driven rotatingly. In the device known from EP 0286160 this takes place by means of for instance a cyclone or centrifugal pump disposed upstream of the feed channels, as shown in figures 8-10 of EP 0286160. It is on the other hand desirable to perform a pre- separation in order to in any case remove the fraction differing most in mass density prior to entry into the assembly of feed channels. If the centrifugal pump of EP 0286160 has any pre-separating action, it is very limited, among other reasons because the inlet and the outlet are situated at the same location in axial direction and the incoming medium covers only half the periphery before it reaches the outlet. An additional advantage of the device according to the invention is that said two basic functions - driving and pre-separation - are separated so that both functions can be performed optimally. It is thus possible for instance to select the dimensions and/or the number of vanes of the first vane unit such that the pre-separation proceeds optimally, or to separately adjust the residence time. The cyclone disposed upstream of the first vane unit can be dimensioned such that the driving proceeds as well as possible.

It is noted that in the context of the invention the separating of the fractions is understood to mean the at least partial separation of the fractions such that a significant difference in the average mass density of the two fractions results; it will not always be possible to realize a complete (100%) separation in practice. As a result of the rotation of the mixture of the fractions for separating, the lighter fraction will migrate at least substantially to the inner side of the rotation and the heavier fraction will migrate at least substantially to the outer side of the rotation. A possible application is a separation which increases the options for use of at least one of the fractions relative to the mixture. This usable ("cleaned") fraction may still have a part of another undesired fraction even after separation ("be contaminated with another fraction"), but the presence of this other fraction is significantly smaller than the presence of this undesired fraction in the original mixture. The rotating separator in the form of a rotating assembly of feed channels has the advantage that the average distance of the medium from a wall (in radial direction) remains limited, whereby a desired degree of separation can be achieved in relatively short time (this corresponding to a relatively limited length in axial direction of the rotating separator in combination with a high axial speed, so that more flow rate can be processed). The flow speeds to be applied can be varied or optimized according to the situation. A further improved degree of separation is achieved by applying the first vane unit. For the best possible operation it is desirable to have the mass density of the fractions for separating differ as much as possible. If desired, the device according to the invention can be provided for this purpose with means influencing mass density which are situated in front of the rotation means, and are therefore disposed upstream in the flow direction of the medium.

The means influencing mass density can for instance comprise expansion means. By means of (optionally isotropic) expansion the temperature of a medium can be decreased within a very short time, for instance by applying an expansion cooler of the "joule thomson" type or a turbine. Another option is that the cooling is brought about by a cooling medium, which is for instance expanded in a separate circulation system so as to be thus brought to the desired low temperature level. The density of the fractions is affected by temperature decrease. Particularly favourable effects can thus be achieved if the mixture consists of fractions with the same phase (for instance gas/gas mixture or a liquid/liquid mixture), at least one fraction of which undergoes a phase change due to the temperature change such that the phases of the fractions for separating differ from each other (whereby for instance a gas/liquid mixture, a gas/solid mixture or a liquid/solid mixture results). This phenomenon of phase change of a substance as a result of temperature change is of course a generally known phenomenon. It is however expressly noted that for the separation by means of the rotation means it is not essential to create a phase difference between the components for separating; the device is equally applicable to a mixture of fractions which are in the same phase (for instance liquid/liquid mixtures such as a dispersed liquid and gas/gas mixtures). Some examples of possible applications of the present invention are the separation of an air/nitrogen mixture, de-aerating or degassing of water, dehydrating of air, cleaning of natural gas and the like.

In yet another preferred embodiment the device also comprises a second rotating vane unit disposed downstream of the rotation means with vanes running in radial direction from a central shaft, and a discharge for at least one of the fractions of the medium mixture. After leaving the rotating assembly of feed channels the medium mixture has a tangential rotation, wherein the rotation speed increases with the radius. This creates a pressure difference in both radial and axial directions, whereby swirls occur in the flow. This makes discharge of the separated fractions more difficult. This swirl formation in the flow is at least partially prevented by now providing a second rotating vane unit. The device is preferably characterized in that during use the second vane unit co-rotates with the rotation means. This enhances the efficiency of the collection of the fractions separated in the assembly of feed channels.

The second vane unit of the inventive device can, if desired, have an open periphery. This is the appropriate configuration for separating solid particles present in a medium mixture. If such particles are heavier than the matrix of the medium mixture (which will generally be the case), they are then displaced practically radially outward in the second vane unit, wherein they are discharged in a discharge channel arranged in the peripheral wall of the device. In yet another preferred embodiment of the device according to the invention the second vane unit comprises a peripheral wall provided with a number of openings along which at least one of the separated fractions of the medium mixture can be discharged. Such a configuration is particularly suitable for discharging a liquid fraction. In a further preferred embodiment the second vane unit comprises a discharge disposed at the position of the central shaft thereof. Such configuration is particularly suitable for discharging a fraction dispersed in the medium mixture which is lighter than the matrix of the medium mixture. Such a situation occurs for instance in the case of a medium mixture with a liquid matrix incorporating gas bubbles, wherein the gas bubbles have to be separated.

In order that the discharge of the fractions to be separated proceeds as efficiently as possible the second vane unit comprises at least two blades distributed uniformly in peripheral direction of the second vane unit. More preferably the second vane unit comprises a maximum of thirty-five vanes distributed uniformly in peripheral direction of the second vane unit. Still more preferably the second vane unit comprises between five and twenty-five vanes uniformly distributed in peripheral direction of the second vane unit. The number of vanes distributed uniformly in peripheral direction of the second vane unit most preferably lies between eight and fifteen vanes. The vanes of the second vane unit preferably run linearly in radial direction. It is further advantageous that the axial length of the second vane unit varies in radial direction. For fractions which must be discharged at the position of the periphery of the second vane unit the axial length preferably increases with the increase in radius. For fractions which have to be discharged at the position of the central shaft of the second vane unit the axial length preferably increases with the decrease in radius.

The invention also relates to a method for separating a flowing medium mixture into at least two fractions with differing mass density. The method for separating a flowing medium mixture into at least two fractions with differing mass density comprises according to the invention the processing steps of A) supplying a medium mixture for separating, B) rotating the flowing mixture for separating in rotation means provided for this purpose which comprise a rotating assembly of feed channels, and C) discharging at least one of the separated fractions, and is characterized in that prior to processing step B) the flowing medium mixture is guided through a first rotating vane unit with vanes running in radial direction from a central shaft and a discharge for at least one of the fractions of the medium mixture. The advantages of applying such a first vane unit have already been discussed above in respect of the description of the device, and will not therefore be repeated here.

A further preferred method for separating a flowing medium mixture into at least two fractions with differing mass density comprises the processing steps of A) supplying a medium mixture for separating, B) rotating the flowing mixture for separating in rotation means provided for this purpose which comprise a rotating assembly of feed channels, and C) discharging at least one of the separated fractions, and is characterized in that after processing step B) the flowing medium mixture is guided through a second rotating vane unit with vanes running in radial direction from a central shaft and a discharge for at least one of the fractions of the medium mixture. The advantages of this method have also already been discussed above in respect of the description of the device, and will not therefore be repeated here.

The method according to the invention is applied particularly for purifying natural gas. In such a preferred application of the method natural gas is supplied during processing step A), during an additional processing step the temperature of the natural gas is lowered as a result of expansion to a temperature lower than -50⁰C, or less than -60⁰C, whereby the fractions of contaminating substances present in the natural gas, such as for instance CO₂ and H₂S, at least substantially change phase, which contaminating fractions (for instance CO₂ and H₂S) are separated from the fraction of hydrocarbons during processing step B) such that the fraction of hydrocarbons with the contaminants at least partly removed is discharged during processing step C). Prior to processing step B) the natural gas is guided through a first rotating vane unit with vanes running in radial direction from a central shaft and a discharge for at least one of the fractions of the medium mixture. At least a part of the fractions of contaminating substances present in the natural gas, such as for instance CO₂ and H₂S, are hereby separated from the fraction of hydrocarbons and discharged. According to a further preferred method according to the invention, after processing step B) the flowing natural gas is guided through a second rotating vane unit with vanes running in radial direction from a central shaft and a discharge for at least one of the fractions of the medium mixture. The second rotating vane unit provides for a more efficient discharge of the fractions of contaminating substances present in the natural gas, such as for instance CO₂ and H₂S. Using this preferred method technically recoverable natural gases contaminated with unwanted gases can be separated in economically cost-effective manner from the hydrocarbons present therein to a degree which is improved relative to the prior art.

The method and device according to the invention can also be used to separate condensable liquid-like constituents in natural gas, such as for instance the water vapour present in natural gas or the heavier fractions thereof. By cooling the medium mixture for instance prior to the separating step, such constituents condense into droplets. The thus formed droplets are then separated from the other constituents in the feed channels. Natural gas can thus be dried in an efficient manner. It is also possible, for instance in the case the heavier fraction is being separated, to collect the thus formed petrochemical product.

The present invention will be further elucidated on the basis of the non-limitative exemplary embodiments shown in the following figures. Herein: figure 1 shows a schematic side view of a device according to the invention, figure 2 shows a schematic perspective view of a first or second vane unit according to the invention, and figure 3 shows a schematic side view of another preferred variant of the device according to the invention. Figure 1 shows a device 1 for cleaning a contaminated gas such as for instance natural gas. The contaminated gas is supplied as according to arrow Pi by a feed 2 under a pressure which can vary from 100 to 500 Bar and higher (a typical pressure amounts for instance to about 250 Bar) and a temperature of more than or about 100⁰C. The gas supplied as according to arrow Pi is then cooled in accordance with the shown preferred method in a heat exchanger 3, for instance by means of cooling into the atmosphere. The cooled gas flows from heat exchanger 3 as according to arrow P₂ to a throttle valve 4. The gas supplied as according to arrow P₂ is expanded by means of throttle valve 4, preferably in isotropic manner, to a lower pressure of between 5 and 20 Bar. As a result of the sudden fall in pressure the temperature of the gas will fall (for instance to a temperature lower than -50⁰C) such that a part of the fractions present in the gas changes phase. A gas/vapour mixture 5 (aerosol) is created as a result. This gas/vapour mixture 5 is guided through a first vane unit 12 via an inlet running in axial direction. Vane unit 12 is disposed rotatably around a central shaft 8 which is suspended by means of a bearing 17. Vane unit 12 has a peripheral wall with an axial length (in the direction of arrow P₄) which increases with the radius S. In first vane unit 12 at least a part of the substantially axially flowing vapour is condensed in the form of liquid droplets 13, consisting for instance of liquid CO₂ and H₂S. At least a part of the liquid droplets 13 are collected in a basin 14 which can be emptied via discharge conduit 15 by means of activating a pump 16. This part of the liquid CO₂ and H2S fraction is discharged as according to arrow P₅. The gas leaving first vane unit 12 thus has CO₂ and H₂S at least partly removed before the gas enters the assembly 7 of feed channels 6. In feed channels 6 of rotor 7 the vapour condenses as a result of the rotation R against the sides of feed channels 6 of rotor 7 which are remote from rotation shaft 8. For pressures lying between 5 and 20 bar the kinematic viscosity < of the gas/vapour mixture 5 will be in the order of magnitude of 10^'6 m²/s. Although it is apparent that the throughfeed speed of the gas/vapour mixture 5 through feed channels 6 can be chosen within broad limits, the axial throughfeed speed wo usual for economic operation will generally be in the order of magnitude of 5 m/s. On the basis of the normal process conditions the diameters of feed channels 6 are chosen such that the flow in channels 6 is substantially laminar. This is not however essential. The condensed vapour leaves rotor 7 as liquid droplets 9 on the side remote from throttle valve 4. In order to enable efficient discharge of these liquid droplets 9 a second vane unit 18 is situated downstream of rotor 7. This vane unit 18 is also disposed rotatably around central shaft 8. Vane unit 18 has a peripheral wall with an axial length (in the direction ot arrow P₄) which increases with the radius S. Droplets 9, which consist for instance of liquid CO₂ and H₂S, are carried along by the vanes of second vane unit 18 and collected in a basin 10 which can be emptied by means of activating a pump 11 such that the liquid CO₂ and H₂S are discharged as according to arrow P₃. The gas leaving rotor 7 thus has at least a considerable part of the CO₂ and H₂S removed and leaves device 1 as according to arrow P₄ as cleaned gas. It is expressly stated that, while the use of a heat exchanger 3 and a throttle valve 4 is recommended, these components are not however essential to the invention. Although in figure 1 the axial length of the first and second vane unit is shown as being smaller than the axial length of rotor 7 with feed channels 6, this length is preferably at least equal to the length of the rotor.

The particle material collected in the feed channels can be removed therefrom by removing the rotating assembly of feed channels from its housing, subsequently cleaning and re-placing it, or by replacing it with a cleaned rotating assembly. The rotating assembly of feed channels can also be cleaned in situ - if necessary during operation - by subjecting it for instance to vibrations, by producing sound waves or, preferably, by spraying the feed channels under high pressure or by spraying with air or another gaseous or liquid medium.

Figure 2 shows a first (or second) vane unit 12 according to the invention. Vane unit 12 comprises a central shaft 8 with vanes 120 running in radial direction and a discharge 14 (shown in figure 1) for at least one of the fractions of the medium mixture. Central shaft 8 is coaxial with the shaft of rotor 7, whereby during use the first vane unit 12 co-rotates with rotor 7. In the shown embodiment first vane unit 12 comprises a peripheral wall 121 provided with a number of openings 122 along which at least one of the fractions of the medium mixture can be discharged via discharge 14. Vanes 120 are distributed uniformly in peripheral direction of vane unit 12 and run substantially linearly in radial direction (they are therefore substantially flat). In the shown variant the axial length of vane unit 12 does not depend on the radius thereof.

Figure 3 finally shows yet another preferred embodiment of a device 1 according to the invention. The shown embodiment variant differs from that shown in figure 1 due to a different embodiment of first vane unit 12. The medium mixture 5 to be separated, for instance a solid/liquid/gas mixture, is guided through a first vane unit 12 which has a peripheral wall with a radius S which increases with the axial length (in the direction of arrow P4). The peripheral wall of first vane unit 12 is provided on the outer end (with the largest radius) with openings 122 along which a part of the fraction for separating, for instance solid particles, can be discharged. Because the radius S of first vane unit 12 increases in downstream direction, the peripheral speed of first vane unit 12 likewise increases in downstream direction. Because the cross-section of first vane unit 12 likewise increases in downstream direction, the axial speed will however decrease in downstream direction. This results in a gentler flow with a lower swirl ratio (defined as the ratio of tangential speed to axial speed), which further enhances separation. A further advantage of this embodiment is that the average pressure in the flow will increase from the entry to the first vane unit up to the outer end where the radius S is largest. By having the radius S of first vane unit 12 increase in the axial direction it is possible to ensure that the pressure increase in first vane unit 12 is greater than the pressure drop over the assembly 7 of feed channels 6. This results in a positive pressure difference between the outlet of assembly 7 and the inlet of first vane unit 12. This pressure difference avoids leakage of untreated medium mixture along the walls in downstream direction. At least a part 13 of the separated fractions is collected in a basin 14 which can be emptied via discharge conduit 15 by means of activating a pump 16. This part is discharged as according to arrow P₅. The medium leaving first vane unit 12 thus has contaminants at least partially removed before the medium enters assembly 7 of feed channels 6. The medium is then further purified in assembly 7 as already described above.

Claims

1. Device for separating a flowing medium mixture into at least two fractions with differing mass density, comprising: - rotation means in the form of a rotating assembly of feed channels for rotating the flowing mixture for separating, a feed for the medium mixture for separating connecting to the rotation means, and a discharge connecting to the rotation means for discharging at least one of the fractions of the separated medium mixture, characterized in that upstream of the rotation means the device comprises a first rotating vane unit with vanes running in radial direction from a central shaft, with an inlet for the medium mixture running in axial direction and with a discharge for at least one of the fractions of the medium mixture.

2. Device as claimed in claim 1, characterized in that during use the first vane unit co-rotates with the rotation means.

3. Device as claimed in claim 1 or 2, characterized in that the first vane unit comprises a peripheral wall provided with a number of openings along which at least one of the fractions of the medium mixture can be discharged.

4. Device as claimed in any of the foregoing claims, characterized in that the first vane unit comprises a number of between five and twenty-five vanes distributed uniformly in peripheral direction of the first vane unit.

5. Device as claimed in any of the foregoing claims, characterized in that the vanes of the first vane unit run linearly in radial direction.

6, Device as claimed in any of the foregoing claims, characterized in that the axial length of the vanes of the first vane unit varies in radial direction.

7. Device as claimed in any of the foregoing claims, characterized in that the radius of the peripheral wall of the first vane unit increases in axial direction from an initial radius to a final radius.

8. Device as claimed in claim 7, characterized in that the initial radius of the peripheral wall of the first vane unit amounts to between 25% and 75% of the final radius.

9. Device as claimed in any of the foregoing claims, characterized in that the axial length of the first vane unit amounts to a maximum of five times the length of the rotating assembly of feed channels.

10. Device as claimed in any of the foregoing claims, characterized in that the axial length of the first vane unit is substantially equal to the length of the rotating assembly of feed channels.

11. Device as claimed in any of the foregoing claims, characterized in that the first vane unit is driven by a turbine and/or centrifugal pump disposed upstream thereof which sets the medium mixture into pre-rotation.

12. Device as claimed in any of the foregoing claims, characterized in that the first vane unit is driven by means of a motor.

13. Device as claimed in any of the foregoing claims, characterized in that it also comprises a second rotating vane unit disposed downstream of the rotation means with vanes running in radial direction from a central shaft, and a discharge for at least one of the fractions of the medium mixture.

14. Device as claimed in any of the foregoing claims, characterized in that during use the second vane unit co-rotates with the rotation means.

15. Device as claimed in claim 13 or 14, characterized in that the second vane unit comprises a peripheral wall provided with a number of openings along which at least one of the separated fractions of the medium mixture can be discharged.

16. Device as claimed in any of the foregoing claims, characterized in that the second vane unit comprises a number of between five and twenty-five vanes uniformly distributed in peripheral direction of the second vane unit.

17. Device as claimed in any of the claims 13-16, characterized in that the vanes of the second vane unit run linearly in radial direction.

18. Device as claimed in any of the claims 13-17, characterized in that the axial length of the second vane unit varies in radial direction.

19. Method for separating a flowing medium mixture into at least two fractions with differing mass density, comprising the processing steps of:

A) supplying a medium mixture for separating, B) rotating the flowing mixture for separating in rotation means provided for this purpose which comprise a rotating assembly of feed channels, and

C) discharging at least one of the separated fractions, characterized in that prior to processing step B) the flowing medium mixture is guided through a first vane unit as claimed in any of the claims 1-12.

20. Method for separating a flowing medium mixture into at least two fractions with differing mass density, comprising the processing steps of:

A) supplying a medium mixture for separating,

B) rotating the flowing mixture for separating in rotation means provided for this purpose which comprise a rotating assembly of feed channels, and

C) discharging at least one of the separated fractions, characterized in that after processing step B) the flowing medium mixture is guided through a second vane unit as claimed in any of the claims 13-18.