A CENTRIFUGAL SEPARATOR AND A METHOD OF OPERATING A CENTRIFUGAL SEPARATOR

Abstract

A centrifugal separator for separating a liquid heavy phase and light phase from a liquid feed mixture includes a drive member and a rotating part. The rotating part includes a centrifuge rotor enclosing a separation space and a sludge space. The centrifugal separator includes an inlet for receiving the liquid feed mixture into the centrifuge rotor, a first outlet for the liquid heavy phase and a second outlet for the liquid light phase. The centrifugal separator includes a conduit system for recirculating separated liquid heavy phase discharged from a first outlet to the sludge space within the centrifuge rotor without mixing the recirculated separated liquid heavy phase with the liquid feed mixture. Sludge outlets, other than the first and second outlets, discharge sludge separated from said liquid feed mixture. A method for separating a liquid heavy phase and a liquid light phase from a liquid feed mixture is also disclosed.

Claims

1. A centrifugal separator for separating a liquid heavy phase and a liquid light phase from a liquid feed mixture, comprising: a frame; a drive member; and a rotating part, wherein the drive member is configured to rotate the rotating part in relation to the frame around an axis of rotation, wherein the rotating part comprises a centrifuge rotor enclosing a separation space and a sludge space, wherein the separation space comprises a stack of separation discs arranged coaxially around the axis of rotation and wherein said sludge space is arranged radially outside said stack of separation discs, wherein the centrifugal separator further comprises an inlet for receiving the liquid feed mixture into the centrifuge rotor, a first outlet for the liquid heavy phase and a second outlet for the liquid light phase, wherein the centrifugal separator further comprises a conduit system for recirculating separated liquid heavy phase discharged from the first outlet to the sludge space within the centrifuge rotor of the centrifugal separator without mixing the recirculated separated liquid heavy phase with the liquid feed mixture, and wherein said centrifugal separator further comprises sludge outlets, other than the first and second outlets, for discharge of sludge separated from said liquid feed mixture.

2. The centrifugal separator according to claim 1, wherein the conduit system is arranged for recirculating separated liquid heavy phase to a radial inner position of the sludge space.

3. The centrifugal separator according to claim 1, further comprising an inlet pipe arranged for introducing said liquid feed mixture to said inlet, and wherein said conduit system comprises a recirculation inlet pipe arranged within said inlet pipe.

4. The centrifugal separator according to claim 1, wherein the sludge outlets are arranged for intermittent discharge of sludge separated from said liquid feed mixture.

5. The centrifugal separator according to claim 1, wherein said stack of separation discs is arranged axially on top of a distributor, and wherein said conduit system comprises at least one channel arranged axially below said distributor for guiding the recirculated liquid heavy phase radially outwards to said sludge space.

6. The centrifugal separator according to claim 5, further comprising an inlet pipe arranged for introducing said liquid feed mixture to said inlet, wherein said conduit system comprises a recirculation inlet pipe arranged within said inlet pipe, and wherein said recirculation inlet pipe extends to said at least one channel arranged axially below the distributor.

7. The centrifugal separator according to claim 1, wherein the centrifugal separator further comprises a throttle valve arranged for throttling the separated liquid heavy phase, wherein said throttle valve is arranged upstream of the first outlet for the heavy phase.

8. The centrifugal separator according to claim 1, wherein the conduit system further comprises a recirculation inlet and a conduit connecting the first outlet for the liquid heavy phase with said recirculation inlet.

9. The centrifugal separator according to claim 8, wherein said conduit connecting the first outlet for the liquid heavy phase with said recirculation inlet comprises a valve arranged for determining the amount of separated liquid heavy phase being recirculated to said recirculation inlet.

10. A method of separating a liquid heavy phase and a liquid light phase from a liquid feed mixture, comprising the steps of: a) introducing the liquid feed mixture into the centrifugal separator according to claim 1; b) discharging a separated liquid light phase from said centrifugal separator; c) discharging a separated liquid heavy phase from said centrifugal separator; d) recirculating at least a portion of the discharged separated liquid heavy phase to the sludge space without mixing the recirculated separated liquid heavy phase with the liquid feed mixture; and e) ejecting a separated solid phase through a set of radial outlets arranged at an outer periphery of the centrifuge rotor.

11. The method according to claim 10, wherein the centrifugal separator comprises an inlet pipe arranged for introducing said liquid feed mixture to said inlet, and wherein said recirculation is performed via a recirculation inlet pipe arranged within said inlet pipe.

12. The method according to claim 10, wherein said stack of separation discs is arranged axially on top of a distributor, and wherein said recirculation comprises guiding the recirculated liquid heavy phase radially outwards under said distributor.

13. The method according to claim 12, wherein step d) comprises guiding the separated liquid heavy phase to the sludge space at a radial position that is at a radial outer edge of the distributor.

14. The method according to claim 10, wherein step e) comprises intermittently ejecting a separated solid phase through a set of radial outlets arranged at the outer periphery of the centrifuge rotor.

15. The method according to claim 14, wherein step e) further comprises introducing displacement liquid to the centrifuge rotor via the conduit system for recirculating separated liquid heavy phase before ejecting said separated solid phase.

16. The method according to claim 10, wherein the liquid feed mixture is an oily mixture and the liquid heavy phase is an aqueous phase.

17. The method according to claim 10, wherein step d) comprises recirculating separated liquid heavy phase to a radial inner position of the sludge space.

18. The centrifugal separator according to claim 2, further comprising an inlet pipe arranged for introducing said liquid feed mixture to said inlet, and wherein said conduit system comprises a recirculation inlet pipe arranged within said inlet pipe.

19. The centrifugal separator according to claim 2, wherein the sludge outlets are arranged for intermittent discharge of sludge separated from said liquid feed mixture.

20. The centrifugal separator according to claim 3, wherein the sludge outlets are arranged for intermittent discharge of sludge separated from said liquid feed mixture.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0097] The above, as well as additional objects, features and advantages of the present inventive concept, will be better understood through the following illustrative and non-limiting detailed description, with reference to the appended drawings. In the drawings like reference numerals will be used for like elements unless stated otherwise.

[0098] FIG. 1 shows a schematic drawing of a centrifugal separator according to an embodiment of the present invention.

[0099] FIG. 2 shows a schematic drawing of a cross-section of a centrifuge rotor.

[0100] FIG. 3 shows a flow chart of a method according to the present disclosure.

DETAILED DESCRIPTION

[0101] The centrifugal separator and the method according to the present disclosure will be further illustrated by the following description with reference to the accompanying drawings.

[0102] FIG. 1 show a cross-section of an embodiment of a centrifugal separator 1 configured to separate a heavy phase and a light phase from a liquid feed mixture. The centrifugal separator 1 has a rotating part 4, comprising the centrifuge rotor 5 and drive spindle 4a.

[0103] The centrifugal separator 1 is further provided with a drive motor 3. This motor 3 may for example comprise a stationary element and a rotatable element, which rotatable element surrounds and is connected to the spindle 4a such that it transmits driving torque to the spindle 4a and hence to the centrifuge rotor 5 during operation. The drive motor 3 may be an electric motor. Alternatively, the drive motor 3 may be connected to the spindle 4a by transmission means. The transmission means may be in the form of a worm gear which comprises a pinion and an element connected to the spindle 4a in order to receive driving torque. The transmission means may alternatively take the form of a propeller shaft, drive belts or the like, and the drive motor may alternatively be connected directly to the spindle 4a.

[0104] The centrifuge rotor 5, shown in more detail in FIG. 2, is supported by the spindle 4a, which is rotatably arranged in a frame 2 around the vertical axis of rotation (X) in a bottom bearing 22 and a top bearing 21. The stationary frame 2 surrounds centrifuge rotor 5.

[0105] Also shown in FIG. 1 is an inlet pipe 14a extending into the centrifuge rotor 5 axially from the top. After separation has taken place within the centrifuge rotor 5, separated liquid heavy phase is discharged thorough stationary outlet pipe 6a, whereas separated liquid light phase is discharged through stationary outlet pipe 7a.

[0106] FIG. 2. shows a more detailed view of the centrifuge rotor 5 of the centrifugal separator 1.

[0107] The centrifuge rotor 5 forms within itself a separation space 9a and a sludge space 9b, located radially outside the separation space 9a. In the separation space 9a, a stack 10 of separation discs 10a is arranged coaxially around the axis of rotation (X) and axially below a top disc 11 and is thus arranged to rotate together with the centrifuge rotor 5. The separation discs 10a provide for an efficient separation of the liquid mixture into at least a liquid light phase and a liquid heavy phase. Thus, in the separation space 9a centrifugal separation of e.g. a liquid feed mixture to takes place during operation.

[0108] The stack 10 is supported at its axially lowermost portion by distributor 13. The distributor 13 comprises an annular conical base portion 13a arranged to conduct liquid from the center of the centrifuge rotor 5 to a predetermined radial level in the separation space 9, and a central neck portion 13b extending upwards from the base portion 13a.

[0109] The sludge space 9b is in this embodiment confined between the upper part 28 of the centrifuge rotor 5 and an axially movable operating slide 18.

[0110] The centrifugal separator 1 further comprises an inlet 14 in the form of a central inlet chamber formed within or under the distributor 13 into which a stationary inlet pipe 14a extends for supply of the liquid feed mixture to be separated. The inlet 14 communicates with the separation space 9 via passages 20 formed in the base portion 13a of the distributor 13. The passages may be arranged so that liquid mixture is transported to a radial level R1, which may correspond to the radial level of the outer portion of the stack of separation discs. For example, the radial level R1 may be at the radial level of a plurality of cut-outs provided in the separation discs the cut-outs may for example be in the form of slits provided at a radially outer portion of the separation disc 10a, which slits are open towards the outer radius of the separation disc 10a. Such slits may for example be provided in the separation discs when the liquid feed mixture is an oil.

[0111] The top disc 11 and an upper inner wall part 28 of the centrifuge rotor 5 delimits at least one channel 25 extending from the radially inner portion of the sludge space 9b towards a central portion of the centrifuge rotor 5. The first liquid outlet 6 is arranged in a first outlet chamber 15, which is in fluid communication with the at least one channel 25 for discharge of a separated liquid heavy phase. The first liquid outlet 6 is in the form of a stationary paring disc arranged in the outlet chamber 15 for the separated heavy phase for discharging the heavy phase into outlet pipe 6a.

[0112] The radially inner portion of the disc stack 10 communicates with a second outlet 7 for a separated light phase of the liquid feed mixture. The second outlet 7 is arranged in a second outlet chamber 8. The second outlet 7 is in the form of a stationary paring disc for discharge of the light phase into outlet pipe 7a. The paring disc 12 is supported in the second outlet chamber 8 by the stationary inlet pipe 14a

[0113] The centrifuge rotor 5 is further provided with outlets 17 at the radially outer periphery of the sludge space 9b. These outlets 17 are evenly distributed around the rotor axis (X) and are arranged for intermittent discharge of a sludge component of the liquid feed mixture. The sludge component comprises denser particles forming a sludge phase. The opening of the outlets 17 is controlled by means of an operating slide 18 actuated by operating water in channel 19, as known in the art. In its position shown in the drawing, the operating slide 18 abuts sealingly at its periphery against the upper part 28 of the centrifuge rotor 5, thereby closing the sludge space 9b from connection with outlets 17, which are extending through the centrifuge rotor 5.

[0114] However, the centrifugal separator 1 could also be a solid wall bowl centrifuge, i.e. a centrifuge lacking outlets at the periphery of the centrifuge rotor 5.

[0115] As seen in FIG. 2, the centrifugal separator 1 also comprises a conduit system 30 for recirculating separated liquid heavy phase to the sludge space 9b within the centrifuge rotor 5 without mixing the recirculated separated liquid heavy phase with the liquid feed mixture. This is achieved by stationary recirculation inlet pipe 31 that is arranged within the stationary inlet pipe 14a for the liquid feed mixture. The recirculation inlet pipe 31 is at the top of the separator connected to a recirculation inlet 31a, and then extends concentrically with rotational axis X down from the top through the inlet pipe 14a and inlet 14 to a position axially under the distributor 13. The dimension of the recirculation inlet pipe may be selected based on the required recirculation flow rate. As an example, the conduit system 30 may be arranged for recirculating liquid heavy phase with a flow rate that is below 20 m/s in the recirculation inlet pipe 14a.

[0116] The recirculation inlet 31a is connected with conduit 34 to the stationary outlet pipe 6a for the liquid heavy phase. This conduit also comprises regulating valve 35 for determining if liquid heavy phase released from the separator should be recirculated via conduit 34 to recirculation inlet 31a or released to drain via conduit 44. The regulating valve 35 may also be used to stop the discharge of separated liquid heavy phase completely.

[0117] Moreover, the recirculation inlet 31a is also connected to a source of liquid 42. This liquid may be displacement liquid that may be introduced to the centrifuge rotor 5 via the recirculation inlet before ejecting sludge via sludge outlets 17. Valve 41 may be used for controlling if displacement liquid should be introduced into the centrifuge rotor. The source of liquid 42 may for example be part of a water system onboard a ship.

[0118] Moreover, there are a throttle valve 33 arranged upstream of the first outlet 6 for the liquid heavy phase. This throttle valve 33 is arranged for throttling the liquid heavy phase that is being discharged via the first outlet 6. The dimensions of the throttle valve 33 may be selected based on the expected amount of heavy phase present in the liquid feed mixture to be separated.

[0119] The conduit system 30 further comprises at least one channel 32 arranged axially under the distributor 13 for guiding the recirculated liquid heavy phase to the sludge space 9b. These channels 32 under the distributor 13 comprises in this embodiment a radial inner portion 32b that is formed as channels in the cone 40 that secures the centrifuge rotor 5 to the spindle 4a. The channels under the distributor 13 further comprises a radial outer portion 32a arranged for guiding the recirculated heavy phase into the sludge space just under the radial outer brim or portion of the distributor, in this case just under the radial edge of the base portion 13a of the distributor. Thus, the at least one channel 32 under the distributor 13 are arranged for guiding or releasing the recirculated liquid heavy phase to the sludge space at a radial position R2 that is larger than the radial position R1 as discussed above.

[0120] During operation of the separator as shown in FIGS. 1 and 2, the centrifuge rotor 5 is brought into rotation by the drive motor 3. Via the inlet pipe 14a, liquid feed mixture to be separated is brought into the separation space 9a. Depending on the density, different phases in the liquid feed mixture is separated between the separation discs 10a of the stack 10. Heavier component, such as a water phase and a sludge phase, move radially outwards between the separation discs 10a to the sludge space 9b, whereas the phase of lowest density, such as an oil phase, moves radially inwards between the separation discs 10a and is forced through second outlet 7 arranged in the second liquid outlet chamber 8. The liquid of higher density is instead forced out through the passages 25 over the top disc to the liquid outlet 6 for the heavy phase that is arranged at a radial distance that is larger than the radial level of the outlet 7 for the light phase. The throttling valve 33 is used to determine the flow of separated liquid heavy phase to the first outlet 6. However, the valve 35 at downstream of the first liquid outlet 6 may be closed until enough liquid heavy phase has been separated out from the liquid feed mixture. Opening of the valve 35 thus allows for discharge of the liquid heavy phase via the first liquid outlet 6.

[0121] Thus, during separation, an interphase between the liquid of lower density and the liquid of higher density is formed in the centrifuge rotor 5, such as radially within the stack of separation discs. Solids, or sludge, accumulate at the periphery of the sludge space 9b, in the sludge space, and is emptied intermittently from within the centrifuge rotor by the sludge outlets 17 being opened, whereupon sludge and a certain amount of fluid is discharged from the separation chamber 17 by means of centrifugal force. However, the discharge of sludge may also take place continuously, in which case the sludge outlets 17 take the form of open nozzles and a certain flow of sludge and/or heavy phase is discharged continuously by means of centrifugal force.

[0122] Discharged liquid heavy phase is recirculated via recirculation inlet pipe 31a and passages 32 arranged axially below the distributor back to the sludge space. In this way, recirculated heavy phase, such as water is prevented from being mixed with the liquid feed mixture at the inlet, such as an oil.

[0123] FIG. 3 illustrates a method 100 of separating a liquid heavy phase and a liquid light phase from a liquid feed mixture. The method 100 comprises the steps of [0124] a) introducing 101 the liquid feed mixture into a centrifugal separator 1. [0125] b) discharging 102 a separated liquid light phase from the centrifugal separator 1, [0126] c) discharging 103 a separated liquid heavy phase from the centrifugal separator 1, and [0127] d) recirculating 104 at least a portion of the discharged separated liquid heavy phase to the sludge space 9b without mixing the recirculated separated liquid heavy phase with the liquid feed mixture.

[0128] The centrifugal separator may be as discussed in relation to FIGS. 1 and 2 above.

[0129] The method is preferably used when the centrifugal separator 1 comprises an inlet pipe 14a arranged for introducing the liquid feed mixture to the inlet 14 and wherein the recirculation is performed via a recirculation inlet pipe 31 arranged within the inlet pipe 14a. As discussed above, the stack 10 of separation discs 10a may be arranged axially on top of a distributor 13, and the recirculation of step d) may comprise guiding the recirculated liquid heavy phase radially outwards under the distributor 13.

[0130] Moreover, step d) may comprise guiding the separated liquid heavy phase to the sludge space 9b at a radial position that is at the radial outer edge of the distributor 13. Thus, step d) may comprise releasing the recirculated liquid heavy phase to the sludge space at a radial inner portion of the sludge space.

[0131] The method may further comprise a step e) of intermittently ejecting 105b a separated solid phase through a set of radially outlets 17 arranged at the outer periphery of the centrifuge rotor 5. This step e) may further comprise introducing displacement liquid 105a to the centrifuge rotor before ejecting the separated solid phase. This displacement liquid may be supplied e.g. via the conduit system 3 for recirculating separated liquid heavy phase.

[0132] The liquid feed mixture may be an oily mixture and the liquid heavy phase may an aqueous phase, such as water. Consequently, the method may be used for cleaning fuel oil or lubrication oil, e.g. onboard a ship. During such a cleaning of oil in a separator as shown in FIGS. 1 and 2, oil feed is supplied continuously through the inlet pipe 14. After the separating operation has been going on for a period of time, cleaned oil will continuously leave the centrifuge rotor 5 through the second liquid outlet 7. During this period of time an amount of solids and an amount of water is separated from the oil flowing through the stack 10 of separation discs 10a and an interface layer between oil and water may be formed at a certain radial level within the stack. Any water being discharged via the first liquid outlet 6 is recycled back to the sludge space 9b. Before ejecting a sludge phase, i.e. separated solids, recirculation is stopped, e.g. by guiding discharged water to drain via conduit 44, and displacement water is instead introduced via the conduit system 30 to force oil out of the centrifuge rotor to prevent loss of oil during subsequent intermittent discharge. The opening time of sludge outlets 17 is sufficiently long so that all separated solids and displacement water are thrown out of the centrifuge rotor, but sufficiently short to prevent loss of oil. After an intermittent discharge, a new separation period is started, and oil feed is again supplied to the separator 1.

[0133] The invention is not limited to the embodiment disclosed but may be varied and modified within the scope of the claims set out below. The invention is not limited to the orientation of the axis of rotation (X) disclosed in the figures. The term centrifugal separator also comprises centrifugal separators with a substantially horizontally oriented axis of rotation. In the above the inventive concept has mainly been described with reference to a limited number of examples. However, as is readily appreciated by a person skilled in the art, other examples than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.