METHOD AND SYSTEM FOR CLEANING OILY WASTE

Abstract

A method for cleaning oily waste including water, solids and at least one fuel oil includes removing water from the oily waste to be cleaned so as to produce dried oily waste; introducing dried oily waste into a two-phase centrifugal separator through a liquid inlet; separating the dried oily waste in the two-phase centrifugal separator into a cleaned oil phase and a sludge phase; discharging the sludge phase through a sludge outlet of the two-phase centrifugal separator, and discharging the cleaned oil phase through a liquid outlet of the two-phase centrifugal separator. A system for cleaning oily waste including water, solids and at least one fuel oil is also disclosed.

Claims

1. A method for cleaning oily waste comprising water, solids and at least one fuel oil, wherein the method comprises the steps of: providing oily waste to be cleaned, said provided oily waste to be cleaned comprising water, solids and at least one fuel oil; removing water from said provided oily waste to be cleaned so as to produce dried oily waste; introducing the dried oily waste produced in said step of removing water into a two-phase centrifugal separator through a liquid inlet; separating said dried oily waste in said two-phase centrifugal separator into a cleaned oil phase and a sludge phase comprising solids from said dried oily waste; discharging said sludge phase through a sludge outlet of said two-phase centrifugal separator, and discharging said cleaned oil phase through a liquid outlet of said two-phase centrifugal separator.

2. The method according to claim 1, wherein said at least one fuel oil constitutes at least 80 wt % of all oil of said provided oily waste to be cleaned.

3. The method according to claim 1, wherein said provided oily waste to be cleaned comprises a single oil, said single oil being a fuel oil.

4. The method according to claim 1, wherein said two-phase centrifugal separator comprises a screw conveyor and a stack of separation discs and wherein said step of discharging said sludge phase comprises discharging said sludge phase through said sludge outlet of said two-phase centrifugal separator by means of said screw conveyor.

5. The method according to claim 1, further comprising a step of transferring said discharged cleaned oil phase to a boiler fuel tank.

6. The method according to claim 1, further comprising a step of transferring said discharged cleaned oil phase to a fuel oil bunker tank or a fuel oil settling tank.

7. The method according to claim 1, wherein said step of removing water from said provided oily waste to be cleaned comprises heating said provided oily waste so as to boil off water from said provided oily waste and/or so as to facilitate evaporation of water from said provided oily waste.

8. The method according to claim 1, wherein said step of removing water from said provided oily waste to be cleaned comprises settling water of said provided oily waste and removing the settled water.

9. A system for cleaning oily waste comprising water, solids and at least one fuel oil, wherein the system comprises: at least one fuel oil treatment device for pre-treatment of fuel oil before use in an engine installation on board a ship or in a power plant, each fuel oil treatment device providing fuel oil waste comprising water, solids and at least one fuel oil during use for pre-treatment of fuel oil; a water removal tank arranged to receive fuel oil waste provided by said at least one fuel oil treatment device and optionally further waste material provided by one or more other waste sources such that fuel oil waste and optionally further waste material are collected in said water removal tank and thereby form said oily waste to be cleaned, said water removal tank further being arranged to accommodate said formed oily waste to be cleaned, whereby said oily waste to be cleaned comprises water, solids and at least one fuel oil; a first transferring arrangement for transferring fuel oil waste from said at least one fuel oil treatment device to said water removal tank and optionally transferring further waste material provided by one or more other waste sources to said water removal tank; a water removal arrangement for removing water from said oily waste accommodated in said water removal tank and thereby producing dried oily waste; a two-phase centrifugal separator for separation of dried oily waste produced in said water removal tank into a cleaned oil phase and a sludge phase comprising solids from said dried oily waste, whereby the centrifugal separator comprises a liquid inlet for the dried oily waste, a sludge outlet for the sludge phase and a liquid outlet for the cleaned oil phase; and a second transferring arrangement for transferring dried oily waste from the water removal tank to said liquid inlet of said two-phase centrifugal separator.

10. The system according to claim 9, wherein said two-phase centrifugal separator comprises a screw conveyor and a stack of separation discs, wherein said screw conveyor is arranged to transport said sludge phase towards and out of said sludge outlet.

11. The system according to claim 9, wherein said first transferring arrangement comprises: at least one collecting tank, wherein each collecting tank is arranged to receive fuel oil waste provided by at least one of said at least one fuel oil treatment device and optionally further waste material provided by one or more other waste sources such that fuel oil waste and optionally further waste material are collected in each collecting tank; a third transferring arrangement for transferring fuel oil waste from each of said at least one fuel oil treatment device to at least one of said at least one collecting tank and optionally transferring further waste material from one or more other waste sources to one or more of said at least one collecting tank; and a fourth transferring arrangement for transferring collected fuel oil waste and optionally collected further waste material from said at least one collecting tank to said water removal tank.

12. The system according to claim 9, wherein said water removal arrangement comprises a heating arrangement arranged to supply heat to said oily waste to be cleaned accommodated in said water removal tank so as to boil off water from said oily waste or so as to facilitate evaporation of water from said oily waste.

13. The system according to claim 12, wherein said heating arrangement comprises heating coils provided in said water removal tank.

14. The system according to claim 13, wherein said heating arrangement comprises: steam coils provided in said water removal tank; a device for generating heated steam to be supplied to the steam coils; a steam inlet in said water removal tank; and a condensed steam outlet in the water removal tank.

15. The system according to claim 9, wherein said water removal arrangement further comprises a water vapour outlet in said water removal tank and an evacuation fan arranged to evacuate water vapour from said water removal tank.

16. The system according to claim 9, wherein said system further comprises a fifth transferring arrangement for transferring said discharged cleaned oil phase to a boiler fuel tank.

17. The system according to claim 9, wherein said system further comprises a sixth transferring arrangement for transferring said discharged cleaned oil phase to a fuel oil bunker tank or a fuel oil settling tank.

18. The method according to claim 2, wherein said provided oily waste to be cleaned comprises a single oil, the single oil being a fuel oil.

19. The method according to claim 2, wherein said two-phase centrifugal separator comprises a screw conveyor and a stack of separation discs and wherein said step of discharging said sludge phase comprises discharging said sludge phase through said sludge outlet of said two-phase centrifugal separator by means of said screw conveyor.

20. The method according to claim 3, wherein said two-phase centrifugal separator comprises a screw conveyor and a stack of separation discs and wherein said step of discharging said sludge phase comprises discharging said sludge phase through said sludge outlet of said two-phase centrifugal separator by means of said screw conveyor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0086] In the drawings, wherein like reference characters denote similar elements throughout the several views:

[0087] FIGS. 1a-c show schematic views of different embodiments of the system for cleaning oily waste according to the present disclosure;

[0088] FIG. 2 shows a schematic view of another embodiment of the system for cleaning oily waste according to the present disclosure;

[0089] FIG. 3 shows a schematic view of one example of a two-phase centrifugal separator that may be utilized in the method and system according to the present disclosure, and

[0090] FIG. 4 shows a schematic view of another embodiment of the system for cleaning oily waste according to the present disclosure.

DETAILED DESCRIPTION

[0091] The method and system according to the present disclosure will be further illustrated by the following description of some embodiments with reference to the accompanying drawings.

[0092] FIG. 1a shows a schematic view of one embodiment of a system 1 according to the present disclosure. The system 1 is arranged for cleaning oily waste comprising water, solids and at least one fuel oil. The system 1 shown in FIG. 1a comprises four different fuel oil treatment devices 2a-d, which are arranged to treat fuel oil before use in an engine installation on board a ship or in a power plant. As shown in FIG. 1a, the fuel oil treatment devices 2a-d are comprised in a fuel oil treatment system in which they are connected in series. Each of the fuel oil treatment devices 2a-d provides fuel oil waste comprising water, solids and one or more fuel oil(s) during use for pre-treatment of fuel oil. Thus, the fuel oil treatment devices 2a-d constitute fuel oil waste sources.

[0093] In the system 1 shown in FIG. 1a, the first fuel oil treatment device 2a is a settling tank, the second fuel oil treatment device 2b is a separator for cleaning fuel oil, the third fuel oil treatment device 2c is a day tank and the fourth fuel oil treatment device 2d is a fuel oil filter. The settling tank 2a provides fuel oil waste in the form of sediment in the tank during use. The separator 2b provides fuel oil waste in the form of discharged sludge during use. The day tank 2c provides fuel oil waste in the form of sediment in the tank during use. The fuel oil filter 2d provides fuel oil waste in the form of a filter reject during use.

[0094] In the system 1 of FIG. 1a, the settling tank 2a is arranged to be provided with untreated fuel oil from a fuel oil bunker tank 6a. The fuel oil bunker tank 6a is shown with dashed lines in FIG. 1a and may optionally be comprised in the system 1.

[0095] Furthermore, the system of FIG. 1a comprises a first transferring arrangement 3 and a water removal tank 7. The first transferring arrangement 3 is arranged for transferring fuel oil waste from the four fuel oil treatment devices 2a-d and an engine 6b to the water removal tank 7. The engine 6b is shown with dashed lines in FIG. 1b and may optionally be comprised in the system 1. The engine 6b provides fuel oil waste in the form of a leakage product during use. The water removal tank 7 is arranged to receive fuel oil waste from the four fuel oil treatment devices 2a-d and the engine 6b such that the transferred fuel oil waste are collected/pooled in the water removal tank 7. The fuel oil waste collected/pooled in the water removal tank 7 form thereby the oily waste to be cleaned by the system 1, which oily waste to be cleaned comprises water, solids and at least one fuel oil. The water removal tank 7 is further arranged to accommodate the oily waste to be cleaned.

[0096] More specifically, in the embodiment shown in FIG. 1a the first transferring arrangement 3 comprises a third transferring arrangement 5, a collecting tank 4 and a fourth transferring arrangement 8.

[0097] The third transferring arrangement 5 is arranged for transferring fuel oil waste from the four fuel oil treatment devices 2a-d and the engine 6b to the collecting tank 4. In the embodiment shown in FIG. 1a, the third transferring arrangement 5 comprises tank inlet pipes 5a-e. The third transferring arrangement 5 may further comprise one or more pumps for performing the transfer, one or more further pipes, and/or one or more intermediate collecting tanks (not shown). In further alternatives, the third transferring arrangement 5 comprises another number of tank inlet pipes and/or one or more further pipes. For example, in alternatives the third transferring arrangement 5 comprises only one tank inlet pipe, but further transfer pipes that transfer fuel oil waste from the fuel oil treatment devices and merge with each other and/or with the tank inlet pipe.

[0098] The collecting tank 4 is arranged to receive fuel oil waste from the four fuel oil treatment devices 2a-d via tank inlet pipes 5a-d and to receive fuel oil waste from the engine 6b via the tank inlet pipe 5e such that fuel oil wastes are collected in the collecting tank 4. Thus, fuel oil wastes from the fuel oil treatment devices 2a-d and the engine 6b are pooled/collected in the collecting tank 4.

[0099] The fourth transferring arrangement 8 is arranged for transferring collected fuel oil waste from the collecting tank 4 to the water removal tank 7. In the embodiment shown in FIG. 1a, the fourth transferring arrangement 8 comprises a pump 8a and a transfer pipe 8b.

[0100] Thus, in the embodiment shown in FIG. 1a, the water removal tank 7 is arranged to collect fuel oil waste provided by the four fuel oil treatment devices 2a-d and the engine 6b via the collecting tank 4, i.e. fuel oil waste from the fuel oil waste sources 2a-d and 6b are first collected in the collecting tank 4 and the collected fuel oil waste is there after transferred to the water removal tank 7.

[0101] The system 1 shown in FIG. 1a comprises four different types of fuel oil treatment devices 2a-d. In alternatives of the system 1 shown in FIG. 1a, the number and/or types of fuel oil treatment devices may differ. In addition, in alternatives the system 1 may comprise two or more of the same type of fuel oil treatment device, e.g. in alternatives the system 1 may comprise two separators, two filters etc. In these alternatives the third transferring arrangement 5 is arranged to transfer fuel oil waste from the alternative number and/or types of fuel oil treatment devices to the collecting tank 4. In alternatives the collecting tank 4 may also be arranged to receive further waste material such as further water and/or solids and/or oils provided by one or more other waste sources such that the further waste material is pooled with the fuel oil waste in the collecting tank 4. The third transferring arrangement 5 is then arranged to also transfer the further waste material to the collecting tank 4.

[0102] Furthermore, the system 1 of FIG. 1a comprises a water removal arrangement 10 for removing water from the oily waste accommodated in the water removal tank 7 and thereby producing dried oily waste. In the system 1 shown in FIG. 1a the water removal arrangement 10 comprises a heating arrangement 9, a water vapour outlet 10a and a water vapour outlet pipe 10b. The heating arrangement 9 is arranged to supply heat to the oily waste in the water removal tank 7 so as to boil off water from the oily waste or facilitate evaporation of water from the oily waste. In the system 1 shown in FIG. 1a the heat supply means 9 comprises heating coils 9a, e.g. electric coils. As shown in FIG. 1a the heating coils 9a may be positioned in the bottom of the water removal tank 7, but may alternatively be positioned at any other suitable position in the water removal tank 7. In another alternative the heat supply means comprises any other heating means positioned in or connected to the water removal tank 7. As indicated with dashed lines in FIG. 1a, the system 1 may further optionally comprise a water tank 10d for collecting water originating from water vapour that has been removed from the water removal tank 7 via the water vapour outlet 10a and water vapour pipe 10b and has been condensed.

[0103] In addition, the system 1 of FIG. 1a comprises a two-phase centrifugal separator 12 and a second transferring arrangement 11.

[0104] The two-phase centrifugal separator 12 is arranged to separate dried oily waste produced in the water removal tank 7 and introduced into the separator 12 into a cleaned oil phase and a sludge phase and comprises a liquid inlet 13 for dried oily waste, a sludge outlet 14 for the sludge phase and a liquid outlet 15 for the cleaned oil phase. In addition, in the embodiment shown in FIG. 1a the centrifugal separator 12 comprises a screw conveyor 16 and a stack of separation discs 17 (see FIG. 3). The screw conveyor 16 is arranged to transport the sludge phase towards and out of the sludge outlet 14. A more detailed schematic description of the working principles of one example of the two-phase centrifugal separator 12 is disclosed in relation to FIG. 3 below.

[0105] The second transferring arrangement 11 is arranged for transferring dried oily waste from the water removal tank 7 to the liquid inlet 13 of the two-phase centrifugal separator 12. In the embodiment shown in FIG. 1a, the second transferring arrangement comprises a pump 11a and a transfer pipe 11b.

[0106] Furthermore, the system 1 shown in FIG. 1a comprises a sludge phase tank 18 arranged to receive the discharged sludge phase from the separator 12 and an oil phase tank 19 arranged to receive the discharged cleaned oil phase from the separator 12.

[0107] As indicated with dashed lines in FIG. 1a the system 1 may optionally further comprise a fifth transferring arrangement 20 for transferring the discharged cleaned oil phase from the oil phase tank 19 to a boiler fuel tank 21 for reuse as boiler fuel. In the embodiment shown in FIG. 1a, the fifth transferring arrangement 20 comprises a pump 20a and a transfer pipe 20b. Alternatively or additionally, the system 1 may optionally further comprise a sixth transferring arrangement 22 for transferring the discharged cleaned oil phase from the oil phase tank 19 to the fuel oil bunker tank 6a for reuse as fuel. In the embodiment shown in FIG. 1a, the sixth transferring arrangement 22 comprises a pump 22a and a transfer pipe 22b.

[0108] As shown in FIG. 1a, a recirculation loop may be formed in case the system 1 comprises the sixth transferring arrangement 22, whereby the discharged cleaned oil phase may be recirculated to the fuel oil bunker tank 6a.

[0109] A method for cleaning oily waste according to the present disclosure will now be described in connection with the system of FIG. 1a.

[0110] When the system of FIG. 1a is utilized for cleaning oily waste, fuel oil waste from the fuel oil treatment devices 2a-d and the engine 6b are transferred to the collecting tank 4 via the tank inlet pipes 5a-e of the third transferring arrangement 5 such that fuel oil waste is pooled/collected in the collecting tank 4. Fuel oil waste collected/pooled in the collecting tank 4 is thereafter transferred from the collecting tank 4 to the water removal tank 7 via the transfer pipe 8a by means of the pump 8b of the fourth transferring arrangement 8. The fuel oil waste collected in the water removal tank form the oily waste to be cleaned, which comprises water, solids and at least one fuel oil.

[0111] Accordingly, oily waste to be cleaned is provided in the water removal tank 7 by means of two sub-steps, i.e. fuel oil wastes are collected in the collecting tank 4 in a first sub-step and collected fuel oil waste is thereafter transferred from the collecting tank 4 to the water removal tank 7 in a second sub-step so as to pool/collect fuel oil waste in the water removal tank 7 and thereby form the oily waste to be cleaned in the water removal tank 7.

[0112] In the water removal tank 7 the oily waste to be cleaned is heated by the heating coils 9a so as to remove water from the oily waste and thereby produce a dried oily waste. The oily waste to be cleaned may be heated such that water is boiled off from the oily waste. Thus, the oily waste is then heated to a temperature at which water boils off, but at which the oil(s) comprised in the oily waste is/are not vaporized. Alternatively, the oily waste to be cleaned may be heated such that evaporation of water is facilitated, i.e. heated to a temperature at which evaporation of water is facilitated. Produced water vapour exits the water removal tank 7 via the water vapour outlet 10a and the water vapour outlet pipe 10b. Optionally, water vapour may thereafter be collected after condensation in the water tank 10d.

[0113] Dried oily waste produced in the water removal tank 7 is thereafter introduced into the separator 12 via the transfer pipe 11b and the liquid inlet 13 by means of the pump 11a of the second transferring arrangement 11. The separator 12 separates the dried oily waste into a cleaned oil phase and a sludge phase comprising solids from the dried oily waste. The sludge phase is discharged through the sludge outlet 14 into the sludge phase tank 18 by means of the screw conveyor 16 (see FIG. 3) of the separator 12. The cleaned oil phase is discharged through the liquid outlet 15 into the oil phase tank 19.

[0114] Optionally, the discharged cleaned oil phase may then be transferred from the oil phase tank 19 via the transfer pipe 20b by means of the pump 20a of the fifth transferring arrangement 20 to the boiler fuel tank 21 and reused as boiler fuel. Alternatively, in case the discharged cleaned oil phase comprises one or more fuel oils and less than an acceptable level of other oils, the discharged cleaned oil phase may then optionally be transferred from the oil phase tank 19 via the transfer pipe 22b by means of the pump 22a of the sixth transferring arrangement 22 to the fuel oil bunker tank 6a.

[0115] The content of the sludge tank 18 can be landed ashore as dry waste.

[0116] FIG. 1b shows a schematic view of another embodiment of the system 1 according to the present disclosure. The system 1 of FIG. 1b corresponds to the system 1 of FIG. 1a except for concerning the heating arrangement 9. In the system 1 of FIG. 1b the heating arrangement 9 comprises a steam inlet 9c in the water removal tank 7, a steam inlet pipe 9d, a condensed steam outlet 9e in the water removal tank 7 and a condensed steam outlet pipe 9f. In addition, the heating arrangement 9 of the system 1 of FIG. 1b comprises a device 9b for generating heated steam and steam coils 9a provided in/at the bottom of the water removal tank 7. The steam inlet 9c is then arranged to transfer heated steam from the device for generating heated steam 9b to the steam coils 9a and the condensed steam outlet 9e is then arranged to transfer condensed steam from the steam coils 9a back to the device 9b for generating heated steam. The device 9b for generating heated steam may comprise a boiler.

[0117] Thus, when the system of FIG. 1b is utilized for cleaning oily waste provided in the water removal tank 7, the removal of water is performed by means of heating the oily waste by heated steam introduced into the steam coils 9a provided in the water removal tank 7. More specifically, heated steam is generated by the device 9b and introduced into the water removal tank 7 via the steam inlet pipe 9d and the steam inlet 9c. After heating the oily waste, condensed steam is led out of the water removal tank 7 via the condensed steam outlet 9e and the condensed steam outlet pipe 9f and returned to the device 9b.

[0118] FIG. 1c shows a schematic view of still another embodiment of the system 1 according to the present disclosure. The system 1 of FIG. 1c corresponds to the system 1 of FIG. 1a except for concerning the water removal arrangement 10. In the embodiment shown in FIG. 1c, the water removal arrangement 10 comprises further an evacuation fan 10c arranged to evacuate water vapour from the water removal tank 7. The evacuation fan 10c is connected to the water vapour outlet 10a via the water vapour outlet pipe 10b.

[0119] Thus, when the system of FIG. 1c is utilized for cleaning oily waste provided in the water removal tank 7, water vapour evaporated or boiled off from the oily waste is evacuated from the water removal tank 7 via the water vapour outlet 10a and the water vapour outlet pipe 10b by means of the evacuation fan 10c.

[0120] FIG. 2 shows a schematic view of another embodiment of the system 1 according to the present disclosure. The system 1 of FIG. 2 corresponds to the system 1 of FIG. 1a except for concerning the fact that the system 1 of FIG. 2 does not comprise the collecting tank 4 and the fourth transferring arrangement 8 of the system 1 of FIG. 1a.

[0121] In the embodiment of FIG. 2, the water removal tank 7 is arranged to receive fuel oil waste provided by the four fuel oil treatment devices 2a-d and the engine 6b via the tank inlet pipes 5a-e of the third transferring arrangement 5. In the embodiment shown in FIG. 2 the first transferring arrangement 3 is constituted by the third transferring arrangement 5.

[0122] Thus, fuel oil waste from the fuel oil treatment devices 2a-d and the engine 6b are pooled/collected in the water removal tank 7 and form thereby oily waste to be cleaned in the water removal tank 7. In other words, oily waste to be cleaned, which oily waste comprises water, solids and at least one fuel oil, is provided in the water removal tank 7 by pooling/collecting fuel oil waste from the fuel oil treatment devices 2a-d and the engine 6b in the water removal tank 7.

[0123] In alternatives the water removal tank 7 may be arranged to receive fuel oil waste from another number of fuel oil waste sources and/or other types of fuel oil waste sources and/or other combinations of fuel oil waste sources and optionally further waste material, such as further water and/or solids and/or oil from one or more other waste sources.

[0124] When the system of FIG. 2 is utilized for cleaning oily waste, oily waste to be cleaned, which comprises water, solids and at least one fuel oil, is provided in the water removal tank 7 by collecting/pooling fuel oil wastes provided by the fuel oil treatment devices 2a-d and the engine 6 in the water removal tank 7. Thus, oily waste to be cleaned is formed in the water removal tank 7 and comprises fuel oil waste generated by the four fuel oil treatment devices 2a-d and the engine 6.

[0125] FIG. 3 shows a schematic view of one example of a two-phase centrifugal separator 12 that may be utilized in the method and system according to the present disclosure, for example in the embodiments shown in FIGS. 1a-c and 2.

[0126] The two-phase centrifugal separator 12 comprises a rotor 23 which is rotatable at a certain speed around a vertical axis of rotation R, and a screw conveyor 16 which is arranged in the rotor 23 and rotatable around the same axis of rotation R but at a speed which differs from the rotation speed of the rotor 23.

[0127] Two-phase centrifugal separator 12 is intended to be suspended vertically in a manner indicated by WO 99/65610 A1. The device necessary for suspending and driving two-phase centrifugal separator 12 is therefore not described here.

[0128] The rotor 23 has an essentially cylindrical upper rotor portion 23a, and an essentially conical lower rotor portion 23b, the rotor portions 23a and 23b being connected to one another by screws. Alternative connection members can of course be used. The cylindrical rotor portion 23a includes an extension axially upwards in the form of a hollow rotor shaft 24, which is connected to a drive device (not shown) for rotating the rotor 23 around the axis of rotation R.

[0129] A further hollow shaft 25 extends into the rotor 23 through the interior of the hollow rotor shaft 24. This hollow shaft 25 supports the screw conveyor 16 by means of screws 26, is drivingly connected to the screw conveyor 16, and is hereinafter called the conveyor shaft 25. The screw conveyor 16 comprises a cylindrical upper conveyor portion 16a which extends axially inside the cylindrical rotor portion 23a, a conical lower conveyor portion 16b which extends axially inside the conical lower rotor portion 23b, and a conveyor flight 16c which extends in a helical manner along the upper cylindrical portion 16a and the lower conical portion 16b of the screw conveyor 16. The screw conveyor 16 may of course have more than one conveyor flight 16c, e.g. two or three conveyor flights, which all extend in a helical manner along the inside of the rotor 23.

[0130] The separator 12 comprises a liquid inlet 13, which in turn comprises a separator inlet pipe 27a and an inlet chamber 28. The separator inlet pipe 27a for dried oily waste which is to be treated in the rotor 23 extends through the conveyor shaft 25 and leads on into a central sleeve 27b in the interior of the screw conveyor 16. The central sleeve 27b delimits the inlet chamber 28 for the dried oily waste, wherein the inlet chamber 28 communicates with a separation chamber 29 via radially extending distribution channels 30.

[0131] The separation chamber 29 is an annular space that surrounds the inlet chamber 28 and comprises a stack of truncated conical separation discs 17. The stack is fitted radially inside the cylindrical portion 16a of the screw conveyor 16 and arranged coaxially with the axis of rotation R. The conical separation discs 17 are held together axially between an upper truncated conical support plate 31 and a lower truncated conical support plate 32. As can be seen, the lower support plate 32 is formed in one piece with the central sleeve 27b. The separation discs 17 comprise holes which form channels 33 for axial flow and distribution of liquid through the stack of separation discs 17. The lower support plate 32 comprises a corresponding hole (not shown), whereby the distribution channels 30 communicate with the channels 33 for axial flow of liquid in the stack of separation discs 17. The upper support plate 31 comprises a number of holes 34 which connect a radially inner annular space 35, within the stack of separation discs 17, with a liquid outlet chamber 36, which is connected to the liquid outlet 15. The liquid is the separated oil phase. A so called paring disc 37 for discharging the cleaned oil phase is disposed within the outlet chamber 36. The paring disc 37 is stationary and firmly connected to the separator inlet pipe 27a, wherein the paring disc 37 is communicating with an outlet channel 38 extending in an outlet which surrounds the separator inlet pipe 27a.

[0132] The cylindrical portion 16a of the screw conveyor 16 radially surrounds the stack of separation discs 17, wherein the cylindrical portion 16a comprises a number of axially extending apertures 39 which are distributed round the axis of rotation R. The axially extending apertures 39 are provided to allow for the separated solids to pass through and deposit on the inside wall of the cylindrical portion 23a of the rotor 23. Liquid will of course also be able to pass through the apertures 39 in the cylindrical portion 16a of the screw conveyor 16.

[0133] The rotor 23 has at its lower end a sludge (solids) outlet 14. In connection with this sludge outlet 14, the rotor could be surrounded by a container (not shown in FIG. 3) for intercepting and collecting the sludge which leaves the sludge outlet 14. The solids are transported by means of the conveyor flight 16c towards and out of the sludge outlet 14. Accordingly, during operation, the screw conveyor 16 is arranged to rotate at a different speed than the rotor 23, whereby the sludge is discharged by the conveyor flight 16c of the screw conveyor 16. This differential speed between the rotor 23 and the screw conveyor 16 may be constant or varied in a known manner; depending on for instance a desired dryness on the discharged sludge and/or the amount of torque needed to drive the screw conveyor 16 for the discharge of the sludge. If the sludge is very difficult to discharge, the rotor 23 may even be operated in a cycle comprising a solids discharge phase of lower rotational speed than a separation phase. In this way, the sludge is discharged more easily as the centrifugal forces inside the rotor 23 are reduced at the lower speed. A known way of operating the centrifugal separator in such a cycle is further described in WO 2011/053224 A1. Furthermore, after an interrupted (or paused) operation, the centrifugal separator may need to be cleared before a restart. Hence, there could be some remaining mixture of solids and liquid inside the stack of separation discs 17, which need to be cleared prior to resuming a separating operation. This may be achieved by bringing the screw conveyor 16 with the stack of separation discs 17 into rotation within a stationary (and empty) rotor 23. In this way, the remaining mixture will be thrown out of the stack 17 and into the inside wall of the stationary rotor 23, whereon it is easily discharged out of the sludge outlet 14 by means of gravity and the rotating screw conveyor 16.

[0134] The screw conveyor 16 may be made in one piece of a polymer material, such as plastic or nylon, which could be fibre-reinforced. The conical portion 16b has a hollow interior or cavity, which is either sealed or open to the surrounding. If desired, the cavity could be filled with some material having a relatively low density, such as cellular plastic or the like. Furthermore, the lower conical portion 23b of the rotor 23 is arranged with a support device 40 for the screw conveyor 16.

[0135] FIG. 4 shows a schematic view of yet another embodiment of the system 1 according to the present disclosure. The system 1 of FIG. 4 corresponds to the system of FIG. 1a except for concerning the water removal arrangement 10 and the fact that the water removal tank 7 is arranged for settling of water of oily waste accommodated in the water removal tank 7. In the embodiment shown in FIG. 4, the water removal arrangement 10 comprises the heating arrangement 9, a water outlet 10e and a water outlet pipe 10f. In the system of FIG. 4 the heating arrangement 9 is arranged to supply heat to the oily waste in the water removal tank 7 so as to facilitate gravitational settling of water of the oily waste, i.e. facilitate split into an oil phase and a water phase. The water outlet 10e and water outlet pipe 10f are arranged for drainage of settled water from the water removal tank 7. As indicated with dashed lines in FIG. 4, the system 1 may further optionally comprise a water tank 10g for collection of water that has been removed from the water removal tank 7 via the water outlet 10e and the water outlet pipe 10f. The system 1 may further optionally comprise a device (not shown) for addition of a demulsifier to the oily waste before introduction into the water removal tank 7. In the system of FIG. 4, the heating arrangement 9 comprises heating coils 9a as in the embodiment of FIG. 1a. However, the heating arrangement 9 may alternatively be constituted by any other suitable heating arrangement. Furthermore, the collecting tank 4 may alternatively be omitted in the system corresponding to the system shown in FIG. 2.

[0136] When the system of FIG. 4 is utilized oily waste provided in the water removal tank 7 is heated by the heating arrangement 9 so as to facilitate gravitational settling of water in the water removal tank 7, i.e. so as to facilitate split into an oil phase and a water phase. Settled water is removed by drainage via the water outlet 10e and the water outlet pipe 10f. Optionally, the removed water may thereafter be collected in the water tank 10g. The oil phase (i.e. dried oily waste), from which water has been removed by settling, is introduced into the separator 12 as described in connection with the embodiment of FIG. 1a.

[0137] While the invention has been described in connection with various exemplary embodiments, it is to be understood that the invention is not to be limited to the disclosed exemplary embodiments, on the contrary, it is intended to cover various modifications and equivalent arrangements within the appended claims. Furthermore, it should be recognized that any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefor, to be limited only as indicated by the scope of the claims.