EP4616954A1

EP4616954A1 - Method for operating a centrifugal separator

Info

Publication number: EP4616954A1
Application number: EP24162752.0A
Authority: EP
Inventors: Per-Gustaf Larsson
Original assignee: Alfa Laval Corporate AB
Current assignee: Alfa Laval Corporate AB
Priority date: 2024-03-11
Filing date: 2024-03-11
Publication date: 2025-09-17
Also published as: WO2025190637A1

Abstract

The present invention provides a method (100) for operating a centrifugal separator (1) for separating at least one liquid phase and a solid phase from a liquid feed mixture. The centrifugal separator (1) comprises a centrifuge bowl (5) arranged to rotate about an axis of rotation and comprising a separation space (9a), in which surface enlarging inserts (10) are arranged, and a sludge space (9b) arranged radially outside the separation space (9a). The centrifuge bowl (5) further comprises an inlet (14) for supply of the liquid feed mixture, at least one liquid outlet (6, 7) for discharge of a separated liquid phase, a sludge outlet (15) arranged for intermittently discharging a separated solid phase from said centrifuge bowl (5), and a first pressure or temperature sensor (61) arranged at a first radial position in the centrifuge bowl (5). The first radial position is within the outer half of the radial extension (ΔR) of the sludge space (9b). The method (100) is comprising the steps of a) rotating (101) the centrifuge bowl (5); b) supplying (102) the liquid feed mixture to the centrifuge bowl (5); c) separating (103) said liquid feed mixture in the centrifuge bowl (5) into at least one liquid phase and a solid phase; d) discharging (105) a solid phase of volume V1; e) measuring (106) a pressure or temperature at least at said first radial position after the sludge discharge of step e); and f) determining (107) if there is still sludge left in the sludge space (9b) from the measurements in step e).

Description

Field of the Invention

The present invention relates to the field of high-speed centrifugal separators, and more specifically to methods for operating a high-speed centrifugal separator.

Background of the Invention

High speed centrifugal separators are generally used for separation of liquids and/or for separation of solids from a liquid. During operation, liquid mixture to be separated is introduced into a rotating centrifuge bowl and heavy particles or denser liquid, usually water, accumulates at the periphery of the rotating bowl whereas less dense liquid accumulates closer to the central axis of rotation. This allows for collection of the separated fractions, e.g. by means of different outlets arranged at the periphery and close to the rotational axis, respectively. Separation members, such as a stack of frustoconical separation discs, are usually used within the rotating bowl in order to enhance the separation performance. An example of a high-speed centrifugal separator is described in patent application EP 3315205 .
During use of a centrifugal separator, a parameter of a liquid feed mixture or its separated light and heavy phase constituents may be measured. The measured parameter may be utilised for monitoring and/or controlling the separation of the liquid feed mixture into the light and heavy phases.
US 7485084 discloses a centrifugal separator and a method of separating a product to a heavy phase and light phase. The separator comprises a sensor for measuring a parameter related to the gas pressure in a central space of the separator.
WO202132353 discloses a centrifugal separation system having a first and second pressure sensor arranged at different radii and positioned to be submerged in the process liquid during operation of the centrifugal separator. The separator further comprises a control unit configured to determine a parameter of the process liquid within the separation space during operation of the centrifugal separator based on measurements from the first and second pressure sensors.
In several separation processes, a solid phase is separated from the liquid feed mixture. Such solid phase may be intermittently discharged during the separation process. This is a process that occurs for a fraction of a second, during which the centrifuge bowl is partially or fully emptied. However, there is a need in the art for improved systems and methods for understanding and controlling the intermittent sludge discharge process.

Summary of the Invention

A main object of the present invention is to provide a method and system for controlling if there is sludge left in the sludge space of the centrifuge bowl after discharge.
As a first aspect of the invention, there is provided a method for operating a centrifugal separator for separating at least one liquid phase and a solid phase from a liquid feed mixture. The centrifugal separator comprises a centrifuge bowl arranged to rotate about an axis of rotation and comprising a separation space in which surface enlarging inserts are arranged, and a sludge space arranged radially outside the separation space. The centrifuge bowl further comprises an inlet for supply of the liquid feed mixture, at least one liquid outlet for discharge of a separated liquid phase, a sludge outlet arranged for intermittently discharging a separated solid phase from said centrifuge bowl and a first pressure or temperature sensor arranged at a first radial position in the centrifuge bowl, wherein said first radial position is within the outer half of the radial extension of the sludge space. The method comprises the steps of

a) rotating the centrifuge bowl;
b) supplying the liquid feed mixture to the centrifuge bowl;
c) separating the liquid feed mixture in the centrifuge bowl into at least one liquid phase and a solid phase;
d) discharging a solid phase of volume V1;
e) measuring a pressure or temperature at least at said first radial position after the sludge discharge of step e); and
f) determining if there is still sludge left in the sludge space from the measurements in step e).

The first aspect of the invention is based on the insight that pressure or temperature readings from a sensor that is arranged in the outer half of the sludge space may provide information for determining if sludge is still left in the sludge space or not. If sludge is left in the sludge space, a higher pressure is expected in the sludge space compared to if a liquid phase is present. In analogy, if sludge is left in the sludge space, a higher temperature is expected in the sludge space compared to if a liquid phase is present. A pressure or temperature reading may also indicate how much sludge that had actually been separated during the separation process and/or how much sludge is ejected from the centrifuge bowl during the discharge. The method of the first aspect may be used to evaluate if the intermittent discharge system is malfunctioning and needs service. Further, the method of the first aspect may be used to evaluate if the intermittent discharge system needs to be adjusted to achieve desirable results.
The "solid phase" may comprise some liquid, such as water, and be in the form of sludge, or a sludge phase.
Step a) of rotating the centrifuge bowl is performed using a drive unit of the centrifugal separator. This may for example be an electrical motor. The centrifuge bowl may be rotated at a speed that is above 3000 rpm, such as above 5000 rpm.
Step b) of supplying liquid feed mixture to the centrifuge bowl may be performed using a pump, as known in the art. Liquid feed mixture may be supplied continuously.
Step c) of separating the liquid feed mixture into a solid phase and at least one liquid phase takes place continuously during the separation process. The liquid feed mixture may be separated into a solid phase and one or two liquid phases, depending on the constituents of the liquid feed mixture and the type of centrifugal separator used.
Step d) of discharging a solid phase of volume V1 may be performed by any known intermittent discharge method. As an example, the centrifuge bowl may comprise a set of intermittently openable sludge outlets in the outer wall of the centrifuge bowl. Such outlets may be opened and closed by means of axially moving a sliding bowl bottom within the centrifuge bowl, as known in the art. Thus, step d) may comprise axially moving a sliding bowl bottom to an open position, in which the set of sludge outlets are open to intermittently discharge the solid phase. Step d) may further comprise closing the sludge outlets after discharging the solid phase. Thus, in embodiments of the first aspect, step d) further comprises continuing the separation of the liquid feed mixture within the centrifuge bowl.
Step e) of measuring a pressure or temperature at the first radial position is performed using the pressure or temperature sensor. For this, the pressure or temperature sensor may be connected to a control unit, such as the separator control unit configured to control the rotational speed of the centrifuge bowl. Such a control unit will be explained in more detail in relation to the second aspect of the invention below. The pressure or temperature in step e) is measured after sludge discharge, such as within 10 s after discharge. If the separation process continues after sludge discharge, the pressure or temperature in step e) may be measured before build-up of any solid phase originating from that continued process.
The first radial position may be arranged within the outer 25 %, such as at within the outer 10 % of the radial extension of the sludge space.
The pressure or temperature in step e) is measured at the first radial position but may in addition be measured at several radial positions within the centrifuge bowl.
Step f) of determining if there is still sludge left in the sludge space may thus comprise determining that there is sludge left in the sludge space or that there is no sludge left in the sludge space. This may be performed e.g. by comparing the measured pressure or temperature with an absolute pressure or temperature levels. For example, as mentioned above, if sludge is still present in the sludge space may give a higher measured pressure than if there were no sludge left, i.e. if the sludge space was empty, filled with separated liquid phase or liquid feed mixture to be separated.
In the context of the present invention, the determining that there is "no sludge" left may comprise that the sludge level is below a certain minimum level, such as below a lower threshold level of sludge within the sludge space.
Further, the method of the first aspect may also involve quantifying the amount of sludge left in the sludge space based on the pressure and/or temperature measurements in step e). Thus, step f) may comprise an additional sub-step of determining the amount, such as a volume or mass, of sludge left in the sludge space.
To determine if sludge is still present in the sludge space in step f), the measured pressure or temperature may also be compared with pressure measurements performed before sludge discharge. Thus, in embodiments of the first aspect, step c) comprises separating said liquid feed mixture in the centrifuge bowl into at least one liquid phase and a solid phase and measuring a pressure or temperature at least at said first radial position before the sludge discharge of step e), and further wherein step f) comprises determining if there is still sludge left in the sludge space from the measurements in step e) and step c).
The determination may then comprise comparing the measured pressure or temperature before and after discharge. If the measured pressure or temperature has not decreased or decreased to a lower extent, one may conclude that there is still sludge left in the sludge space.
If it is determined in step f) that there is still sludge left in the sludge space after sludge discharge the method may comprise increasing the discharge volume for the next discharge. If it instead is determined that there is no sludge left, the discharge volume may instead be reduced. Consequently, in embodiments of the first aspect, the method is further comprising the step g) of discharging a solid phase of volume V2 that is larger than V1 if it is determined in step e) that there is sludge left in the sludge space and discharging a solid phase of volume V3 that is smaller than V1 if it is determined in step f) that there no sludge left in the sludge space.
This is advantageous in that the method allows for determining an optimal discharge volume to minimize product losses and optimize up-time when using intermittent discharge of the separated solid phase. Thus, the risk of discharging a large amount of liquid phase together with the solid phase may be decreased. This could be very useful if for example the centrifugal separator runs at different bowl speeds. The same discharge pressure used in an intermittent discharge system may at different bowl speeds give completely different discharge volumes. This risk may therefore be decreased using the method of the first aspect.
The further discharge of V2 or V3 step g) may be performed after running the separation process for a period of time after the discharge of V1) in step d) has been performed, i.e. such that a new solid phase has time to build up in the sludge space.
The centrifuge bowl may comprise more than one pressure or temperature sensor, such as at least two, such as at least three pressure or temperature sensors.
In embodiments of the first aspect, the first sensor is a pressure sensor and wherein the centrifuge bowl further comprises a second pressure sensor arranged at a second radial position in the sludge space, and wherein step e) further comprises determining the pressure at the first and second radial positions and determining the pressure difference between the first and second radial positions, wherein the first radial position is arranged radially outside the second radial position.
Thus, a second pressure sensor may be arranged radially inside of the first pressure sensor. Measuring the pressure difference between these two sensors may be beneficial since parameters such as flow through the separator, pressure losses, the radial levels of the inlet and liquid outlets, the counter pressure at the liquid outlet or outlets, may be neglected since they affect the first and second pressure sensors within the sludge space to the same degree.
As an example, the second radial position may be within the inner half of the radial extension of the sludge space.
As a further example, step f) may comprise determining that there is still sludge left in the sludge space if the pressure difference as measured in step e) is above a first setpoint. Accordingly, step f) may further comprise determining that there is no sludge left in the sludge space if the pressure difference as measured in step e) is below a setpoint, such as the first setpoint or another setpoint.
The differential pressure between the first and second sensors may also be measured before discharge, such as part of step c), and step e) may further comprise comparing the pressure difference before and after discharge. Based on such comparison, it may be determined in step f) if there is still sludge left in the sludge space or not.
Thus, as an example, step c) comprises separating the liquid feed mixture in the centrifuge bowl into at least one liquid phase and a solid phase and measuring pressure difference between the first and second radial positions before the sludge discharge of step e).
As a further example, the second radial position may be within the outer half of the radial extension of the sludge space. Thus, using two sensors, both of them could be arranged at the outer half of the sludge space.
With to sensors and a measured differential pressure between the sensors, also the density of the phase between the sensors may be determined. As an example, when the second radial position is within the outer half of the radial extension of the sludge space, step e) may further comprises determining the density of the phase in the outer half of the radial extension of the sludge space based on said determined pressure difference. In addition, step f) may comprise determining that there is still sludge left in the sludge space if the determined density is above a threshold value. The threshold value may for example be an absolute density value of a separated liquid phase.
For instance, the density may be calculated utilising the formula: $\frac{p 1 - p 2}{0.5 * w^{2} * (rp 1^{2} - rp 2^{2}) * 10^{- 10}}$ wherein p1 and p2 are the pressures measured by the respective first and second pressure sensors in bar, w is the rotational speed in rad/s, and rp2 and rp3 are the respective radial positions of the second and third pressure sensors in mm.
Further, the density of the phase in the outer half of the radial extension of the sludge space may also be determined before discharge. Thus, step c) may comprise separating the liquid feed mixture in the centrifuge bowl into at least one liquid phase and a solid phase, measuring the pressure difference between the first and second radial positions and determining a density of the phase in the outer half of the radial extension of the sludge space before the sludge discharge of step e).
Step f) may then comprise comparing the measured density of the phase in the outer half of the radial extension of the sludge after discharge with the determined density before discharge. The threshold value used for determining if there is sludge left or not may thus be set depending on the measured density before discharge, i.e. one may conclude that there is still sludge left in the sludge space if the determined density has not decreased at all or just to a small extent.
In embodiments of the first aspect, the first sensor is a temperature sensor and wherein the centrifuge bowl further comprises a second temperature sensor arranged at a second radial position in the sludge space, and wherein step e) further comprises determining the temperature at the first and second radial positions and determining the temperature difference between the first and second radial positions, wherein the first radial position is arranged radially outside the second radial position.
As an alternative, with two temperature sensors, the absolute temperature values of the two sensors may be used to determine if there is still sludge left in the sludge space or not. As discussed in relation to the embodiments with two pressure sensors above, the second radial position may be within the inner or outer half of the radial extension of the sludge space.
In embodiments of the first aspect, the first sensor is a temperature sensor, and step f) comprises comparing the measured temperature with the temperature of the liquid feed mixture to determine if there is still sludge left in the sludge space.
For example, if the temperature sensor within the bowl is covered in sludge or solids, there will be a difference in measured temperature compared to the temperature of the liquid feed mixture. If this temperature difference is above a certain threshold, it may be determined that solids are covering the temperature sensor in the bowl and thus that there is still sludge left in the sludge space.
As a second aspect of the invention, there is provided a centrifugal separator for separating at least one liquid phase and a solid phase from a liquid feed mixture. The centrifugal separator comprises a centrifuge bowl arranged to rotate about an axis of rotation and comprising a separation space, in which surface enlarging inserts are arranged, and a sludge space arranged radially outside the separation space. The centrifuge bowl further comprises an inlet for supply of the liquid feed mixture, at least one liquid outlet for discharge of a separated liquid phase, a sludge outlet arranged for intermittently discharging a separated solid phase from said centrifuge bowl, and a first pressure or temperature sensor arranged at a first radial position in the centrifuge bowl. The first radial position is within the outer half of the radial extension of the sludge space. The centrifugal separator further comprises a control unit that is configured to determine if there is still sludge left in the sludge space after a discharging a solid phase of volume V1 based on a measured pressure or temperature at least at said first radial position after said sludge discharge.
This aspect may generally present the same or corresponding advantages as the former aspect. Effects and features of this second aspect are largely analogous to those described above in connection with the first aspect. Embodiments mentioned in relation to the first aspect are largely compatible with the second aspect.
The centrifugal separator of the second aspect may thus be used for performing the method of the first aspect as discussed above.
The centrifugal separator is for separation of a liquid feed mixture. The liquid feed mixture may be an aqueous liquid or an oily liquid. As an example, the centrifugal separator may be for separating solids and one or two liquids from the liquid feed mixture.
The centrifuge bowl encloses by it walls a separation space and a sludge space. The separation space, in which the separation of the fluid mixture takes place, comprises surface enlarging inserts, i.e. separation members that may be in the form of a stack of separation discs. The separation discs may e.g. be of metal. Further, the separation discs may be frustoconical separation discs, i.e. having separation surfaces forming frustoconical portions of the separation discs. The separation discs may be arranged coaxially around the axis of rotation at a distance from each other such that to form passages between each two adjacent separation discs.
The centrifuge bowl of the separator may be arranged to be rotated around vertical axis of rotation, i.e. the axis of rotation may extend vertically. The centrifuge bowl is usually supported by a spindle, i.e. a rotating shaft, and may thus be mounted to rotate with the spindle. Consequently, the centrifugal separator may comprise a spindle that is rotatable around the axis of rotation (X). The centrifugal separator may be arranged such that the centrifuge bowl is supported by the spindle at one of its ends, such at the bottom end or the top end of the spindle.
The centrifugal separator may further comprise a stationary frame in which the centrifuge bowl is mounted. The frame may comprise an upper hood section that covers the centrifuge bowl.
The centrifugal separator may further comprise a drive member that is arranged to rotate the centrifuge bowl around the axis of rotation. The drive member may comprise an electrical motor arranged to drive e.g. a spindle directly or may for example be provided beside the spindle and rotate the rotating parts of the centrifugal separator by a suitable transmission, such as a belt or a gear transmission.
The centrifugal separator also comprises an inlet for supply liquid mixture to be separated (the liquid feed mixture). This inlet may be arranged for receiving the liquid feed mixture and be arranged centrally in the centrifuge bowl, thus at the rotational axis. The centrifuge bowl may be arranged to be fed from the bottom, such as through a rotating spindle onto which the centrifuge bowl is mounted. However, the centrifuge bowl may also be arranged to be fed from the top, such as through a stationary inlet pipe extending into the bowl to the inlet.
Further, the at least one liquid outlet for a separated liquid phase may be in the form of one or two liquid outlets. Such liquid outlets for separated liquid phase or phases may be arranged at the top or the bottom of the centrifugal separator.
The centrifugal separator is also arranged for discharging a solid phase, i.e. a separated solid phase - that may also contain some liquid - to the surrounding space around the centrifuge bowl. This is performed by the sludge outlet, which may be in the form of a set of ports arranged to be opened intermittently during operation. The sludge outlets may thus be a number of ports arranged at or near the periphery of the centrifuge bowl. Hence, the centrifugal separator may further comprise an intermittent discharge system arranged for intermittently opening and closing the sludge outlet during operation, as known in the art. The centrifugal separator may be arranged for emptying a partial content of the bowl during such an intermittent discharge (partial discharge) or arranged for emptying the whole content of the centrifuge bowl during intermittent discharge (full discharge).
The intermittent discharge system thus controls the opening of the sludge outlets. For this purpose, the intermittent discharge system may comprise an operating slide in, e.g. in the form of a sliding bowl bottom, which is axially movable between a closed position, in which the sludge outlets are closed, and an open position, in which the sludge outlets are open. Keeping the operating slide in a closed position may be effected by supplying water via a channel to a closing chamber in order to hold the operating slide in the closed position. Opening the ports may be affected by supplying opening water to an opening chamber and/or draining water through valve members arranged in the centrifuge bowl.
A pressure or temperature sensor of the present disclosure is operable to generate a sensor output indicative of a pressure or temperature at the position at which it is mounted. Further, a pressure or temperature sensor of the present disclosure is configured to communicate with the control unit. The pressure or temperature sensor is further arranged to rotate with the centrifuge bowl. As mentioned above, the first sensor is a pressure or temperature sensor arranged at a first radial position in the centrifuge bowl that is within the outer half of the radial extension of the sludge space.
The control unit is configured to communicate with the pressure or temperature sensor (or sensors), and with help of such input from the sensor (or sensors) determine if there is sludge left in the sludge space after sludge discharge.
The control unit may be arranged in a stationary part of the centrifugal separator and be configured to communicate with one or several pressure sensors via a wireless connection. As an alternative, the control unit may be and arranged to rotate with the rotor.
The control unit may comprise any suitable type of programmable logical circuit, processor circuit, or microcomputer, e.g. a circuit for digital signal processing (digital signal processor, DSP), a Central Processing Unit (CPU), a processing unit, a processing circuit, a processor, an Application Specific Integrated Circuit (ASIC), a microprocessor, or other processing logic that may interpret and execute instructions. Thus, the control unit may comprise a processor and an input/output interface for communicating with the sensor or sensors for receiving information about a measured pressure or temperature.
Thus, the control unit may be operable to perform the step f) of the method of the first aspect.
The control unit may be configured for controlling a drive unit to rotate the centrifuge bowl at a certain operational speed. The control unit may further be configured for receiving input from one or several pressure or temperature sensors within the bowl.
The control unit may further be configured to control an intermittent discharge system of the centrifugal separator, i.e. to control when to perform an intermittent discharge of a solid phase.
In embodiments of the second aspect, the control unit is configured to determine if there is still sludge left in the sludge space after a discharging a solid phase of volume V1 based on a measured pressure or temperature at least at said first radial position before and after said sludge discharge. The control unit may thus also be configured for receiving input from the pressure or temperature sensor before discharge.
In embodiments of the second aspect, the control unit is further configured to initiate a discharge of a solid phase of volume V2 that is larger than V1 if it is determined that there is sludge left in the sludge space and to initiate a discharge of a solid phase of volume V3 that is smaller than V1 if it is determined that there no sludge left in the sludge space.
The control unit may thus be configured to communicate with an intermittent discharge system of the centrifugal separator and to initiate discharges of different volumes.
In embodiments of the second aspect, the first sensor is a pressure sensor and the centrifuge bowl further comprises a second pressure sensor arranged at a second radial position in the sludge space, and the control unit is configured to determine the pressure at said first and second radial positions and to determine the pressure difference between the first and second radial positions, wherein the first radial position is arranged radially outside the second radial position, and wherein the control unit is further configured to determine if there is still sludge left in the sludge space after a discharging a solid phase of volume V1 based on said determined pressure difference. As discussed in relation to the first aspect above, measuring the pressure difference between these two sensors may be beneficial since parameters such as flow through the separator, pressure losses, the radial levels of the inlet and liquid outlets, the counter pressure at the liquid outlet or outlets, may be neglected since they affect the first and second pressure sensors within the sludge space to the same degree.
As an example, the second radial position may be within the inner half of the radial extension of the sludge space .
Then, as an example, the control unit may be configured to determine that there is still sludge left in the sludge space if the determined pressure difference is above a first setpoint. The control unit may then also be configured to determine that there is no sludge left at the first radial position if the pressure difference is below a second setpoint.
The control unit may further be configured to measure the pressure difference between the first and second radial positions before the sludge discharge and to compare the pressure differences before and after discharge.
As a further example, the second radial position may be within the outer half of the radial extension of the sludge space.
Then, as an example, the control unit may be configured to determine the density of the phase in the outer half of the radial extension of the sludge space based on said determined pressure difference, and further configured to determine that there is still sludge left in the sludge space if the determined density is above a threshold value.
Also the density of the of the phase in the outer half of the radial extension of the sludge space may be measured before discharge and these densities may be compared by the control unit in order to determine if there is still sludge left in the sludge space.
In embodiments of the second aspect, the first sensor is a temperature sensor and the centrifuge bowl further comprises a second temperature sensor arranged at a second radial position in the sludge space, and the control unit is configured to determine the temperature at said first and second radial positions and to determine the temperature difference between the first and second radial positions, wherein the first radial position is arranged radially outside the second radial position, and wherein the control unit is further configured to determine if there is still sludge left in the sludge space after a discharging a solid phase of volume V1 based on said determined temperature difference.
In embodiments of the second aspect, the first sensor is a temperature sensor, and the centrifugal sensor further comprises a second temperature sensor operable to measure the temperature of the liquid feed mixture. Then, the control unit may be configured to determine the temperature difference between the temperature sensor in the bowl and such a second temperature sensor and to determine if there are still solids left in the sludge space based on such comparison.

Brief description of the Drawings

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

Figure 1 shows a schematic drawing of a centrifugal separator.
Figure 2 shows a schematic section drawing of a centrifuge bowl with inlet and outlets.
Figure 3 shows a schematic section drawing of the sludge space of a centrifuge bowl according to an embodiment.
Figure 4 shows a schematic section drawing of the sludge space of a centrifuge bowl according to another embodiment.
Figure 5 shows a schematic section drawing of the sludge space of a centrifuge bowl according to another embodiment.
Figure 6 schematically shows the process steps of the general method of the present disclosure.
Figure 7 schematically shows some process steps in an embodiment of the method.
Figure 8 schematically shows some process steps in an embodiment of the method.
Figure 9 schematically shows some process steps in an embodiment of the method.

Detailed Description

The method and the centrifugal separator according to the present disclosure will be further illustrated by the following description with reference to the accompanying drawings.
Figs.1 and 2 schematically show a centrifugal separator 1 and the centrifuge bowl 5 of the centrifugal separator according to an embodiment of the present disclosure.
The centrifugal separator 1 is configured to separate two liquid phases - a liquid heavy phase and a liquid light phase - as well as a solid phase from a liquid feed mixture. The centrifugal separator 1 has a rotatable part 4, comprising the centrifuge bowl 5 and drive spindle 4a.
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 bowl 5 during operation. The drive motor 3 may hence be an electric motor. Alternatively, the drive motor 3 may be connected to the spindle 4a by transmission means such as a drive belt or the like, and the drive motor may alternatively be connected directly to the spindle 4a.
The centrifuge bowl 5, shown in more detail in Fig. 2, is supported by the spindle 4a, which is rotatably arranged under an upper hood of the stationary frame 2. The bowl 5 is rotatably mounted around the vertical axis of rotation (X) in a bottom bearing 22 and a top bearing 21. The upper hood of the stationary frame 2 surrounds the centrifuge bowl 5.
In the centrifugal separator as shown in Fig. 1, liquid feed mixture to be separated is fed to the bottom to the centrifuge bowl 5 via the drive spindle 4a. The drive spindle 4a is thus in this embodiment a hollow spindle, through which the feed is supplied to the centrifuge bowl 5. However, in other embodiments, the liquid feed mixture to be separated is supplied from the top, such as through a stationary inlet pipe extending into the centrifuge bowl 5.
After separation has taken place within the centrifuge bowl 5, separated liquid heavy phase is discharged through stationary outlet pipe 6a, whereas separated liquid light phase is discharged through stationary outlet pipe 7a. The separated solid phase is intermittently ejected to the space surrounding the centrifuge bowl 5.
Fig. 2. shows a more detailed view of the centrifuge bowl 5 of the centrifugal separator 1.
The centrifuge bowl 5 forms within itself a separation space 9a and a sludge space 9b that is located radially outside the separation space 9a. In the separation space 9a, a stack 10 of separation discs is arranged coaxially around the axis of rotation (X). The stack 10 is arranged to rotate together with the centrifuge bowl 5 and provides for an efficient separation of the liquid feed mixture into at least a liquid light phase and a liquid heavy phase. Thus, in the separation space 9a, the centrifugal separation of the liquid feed mixture takes place during operation. The sludge space 9b is in this embodiment confined between an inner surface of the outer wall 13 of the centrifuge bowl 5 and an axially movable operating slide 16.
The disc stack 10 is arranged under top disc 23 and is further supported at its axially lowermost portion by distributor 11. The distributor 11 comprises an annular conical base portion arranged to conduct liquid mixture from the central inlet 14 of the centrifuge bowl 5 to a predetermined radial level in the separation space 9a.
The centrifuge bowl 5 further comprises an inlet 14 in the form of a central inlet chamber formed within or under the distributor 11. The inlet 14 is arranged for receiving the liquid feed mixture and is thus in fluid communication with the hollow interior 4b of the spindle 4a, through which the liquid feed is supplied to the centrifuge bowl 5.
The inlet 14 communicates with the separation space 9a via passages 17 formed in the base portion of the distributor 11. The passages 17 may be arranged so that liquid mixture is transported to a radial level that corresponds to the radial level of the cut-outs 25 provided in the separation discs of the stack 10. The cut-outs 25 form axial channels within the disc stack and distributes the liquid feed mixture throughout the disc stack 10.
The radially outer portion of the disc stack 10 communicates via a first liquid outlet 6 via channels 24 for discharge of a liquid heavy phase axially over the top disc 23. The radially inner portion of the disc stack 10 communicates with a liquid outlet 7 for a separated light phase of the liquid feed mixture. Separated liquid phases may then be discharged to stationary outlet pipes 6a, 7a that are connected to the centrifuge bowl via mechanical seals 50, 30. As this is an airtight design, they are also often called hermetic seals. The inlet channel 4b is also sealed at lower end of the hollow spindle 4a, thus preventing communication between the inlet channel 4b and the surroundings. The mechanical seal at the inlet is not shown in Fig. 2.
The centrifuge bowl 5 is further provided with sludge outlets 15 at the radially outer periphery of the sludge space 9b. These outlets 15 are evenly distributed around the rotor axis (X) and are arranged for intermittent discharge of a solid phase that is separated from the liquid feed mixture. The solid phase thus comprises solids. The opening of the outlets 15 is controlled by means of an operating slide 16 actuated by operating water channels below the operating slide 16, as known in the art. In its position shown in the drawing, the operating slide 16 abuts sealingly at its periphery against the upper part of the centrifuge bowl 5, thereby closing the sludge space 9b from connection with outlets 15, which are extending through the centrifuge bowl 5. The centrifuge bowl 5 is defined by a surrounding outer wall 13.
During operation of the separator as shown in Fig. 1 and 2, the centrifuge bowl 5 is brought into rotation by the drive motor 3. Via the spindle 4a, liquid feed mixture to be separated is brought into the separation space 9a, as indicated by arrow "A". Depending on the density, different phases in the liquid feed mixture is separated between the separation discs of the stack 10. Heavier component, such as a liquid heavy phase and a solid phase, move radially outwards between the separation discs of the stack 10 to the sludge space 9b, whereas the phase of lowest density, such as a liquid light phase, moves radially inwards between the separation discs of the stack 10 and is forced through the outlet pipe 7a via liquid outlet 7, as indicated by arrow "C". The liquid of higher density is instead discharged over the top disc 23 via discharge channels 24 to another liquid outlet 6 and further out via stationary outlet pipe 6a, as indicated by arrow "B". Thus, during separation, an interphase between the liquid of lower density and the liquid of higher density is formed in the centrifuge bowl 5, such as radially within the stack of separation discs. Solids, or sludge, accumulate at the periphery of the sludge space 9b and is emptied intermittently from within the centrifuge bowl by the sludge outlets 15 being opened, whereupon sludge and a certain amount of fluid is discharged from the separation chamber by means of centrifugal force, as indicated by arrow "D".
In this example, operating water for controlling the position of the operating slide is supplied via operating water module (OWM) 45, that is configured to supply operating water with a certain pressure to the centrifuge bowl 5, as known in the art. This is indicated by arrow "Y3" in Fig. 1. Further, the centrifugal separator comprises a control unit 40. This control unit 40 may be the unit that controls operation, such as rotational speed, of the centrifugal separator 1. The control unit 40 may thus control drive unit 3. According to the present disclosure, the control unit further communicates with one or several pressure sensors that are positioned within the centrifuge bowl 5, as indicated by arrow "Y1" in Fig. 1. The control unit 40 further controls OWM 45 when to initiate a sludge discharge, as indicated by arrow "Y3" in Fig. 1, as well as the extent or amount of such discharge. The pressure sensor setup will further be discussed in relation to Figs. 3-5 below.
Fig. 3 shows a cross-section of the centrifuge bowl 5, more specifically the sludge space 9b that is arranged radially outside of the stack 10 of separation discs (that is arranged in the separation space 9a. In this example, the centrifuge bowl comprises a pressure sensor, but it could as well be a temperature sensor instead. The centrifuge bowl 5 thus comprises a first pressure sensor 61 that is arranged at a first radial position R1. The sludge space 9b extends a radial distance of ΔR, and the first radial position R1 is within the outer half of the radial extension ΔR of the sludge space 9b, more specifically within the outer 25% of the radial extension of the sludge space. The first sensor 61 is arranged on or in the upper inner wall of the centrifuge bowl wall 13 and is in this case arranged just radially inside of the radial extension of the operating slide 16.
During operation of the centrifugal separator, i.e. during a separation process, separated sludge builds up in the outer portion of the sludge space 9b and forms an interface with a liquid phase at position Ri. After discharge of some of solid phase, the radial interphase will move radially outwards. When discharging the whole solid phase, the interface will thus disappear but then grow radially inwards as sludge is yet again built up in the sludge space 9b.
As discussed above, the control unit 40 (not shown in Fig 3) communicates with the first sensor 61. The control unit is further configured to determine if there is still sludge left in the sludge space 9b after a discharging a solid phase based on a measured pressure from the first pressure sensor 61. This determination may for example be comparing the measured pressure with a threshold value. Consequently, with the separator as shown in Fig. 5, the method steps as shown in Fig 6 may be performed, i.e. the steps of a) rotating 101 the centrifuge bowl 5, b) supplying 102 the liquid feed mixture to the centrifuge bowl 5, c) separating 103 the liquid feed mixture in the centrifuge bowl 5 into at least one liquid phase and a solid phase, d) discharging 105 a solid phase of volume V1; e) measuring 106 a pressure at least at the first radial position after the sludge discharge of step e); and step f of determining 107 if there is still sludge left in the sludge space 9b from the measurements in step e).
Based on the measurements, the control unit 40 may be configured to increase or decrease the amount of discharge, e.g. by sending different operational requests to the OWM 45. To discharge a larger amount, the OWM 45 may be controlled by the control unit 40 to supply operating water so that the sludge outlets 15 are open for a longer period of time. In analogy, to discharge a smaller amount, the OWM 45 may be controlled by the control unit 40 to supply operating water so that the sludge outlets 15 are open for a shorter period of time. In this way, the sludge discharge volume may be optimized in order to reduce product losses during sludge discharge. Thus, the control unit 40 may be further configured to initiate a discharge of a solid phase of volume V2 that is larger than V1 if it is determined that there is sludge left in the sludge space 9b and to initiate a discharge of a solid phase of volume V3 that is smaller than V1 if it is determined that there no sludge left in the sludge space 9b. This embodiment is illustrated in Fig. 7, in which the method 100 thereby comprises an additional step g) of discharging 108 a solid phase of volume V2 that is larger than V1 if it is determined in step f) that there is sludge left in the sludge space 9b and discharging 109 a solid phase of volume V3 that is smaller than V1 if it is determined in step f) that there no sludge left in the sludge space 9b.
It is to be understood that the centrifugal separator may comprise other means than an OWM to control intermittent opening of sludge outlets, such as regulating piston valves in the centrifuge bowl 5 electrically to be open for longer or shorter periods of time.
As a complement or alternative, pressure measurements performed prior to sludge discharge may be used together with the pressure measurements performed after sludge discharge for determining if there is still sludge left in the sludge space 9b or not. Consequently, the control unit 40 may be configured to determine if there is still sludge left in the sludge space 9b after a discharging 105 a solid phase of volume V1 based on a measured pressure at least at the first radial position before and after said sludge discharge. This embodiment of the method 100 is shown in Fig. 8, in which step c) comprises separating 103 the liquid feed mixture in the centrifuge bowl 3 into at least one liquid phase and a solid phase and measuring 104 a pressure P_before at least at the first radial position before the sludge discharge of step d). Then, step e) comprises also measuring the pressure P_after when the discharge of volume V1 has been performed. Accordingly, step f) then comprises determining if there is still sludge left in the sludge space from the measurements in step e) and step c).
The control unit may for example be configured to compare the pressure measurements before and after discharge, and if the measured pressure has not decreased or decreased to a lower extent after discharge, the control unit 40 may determine that there is still sludge left in the sludge space 9b.
As an example, the first sensor 61 may be a temperature sensor. Then, the centrifugal separator 1 may comprise a second temperature sensor operable to measure the temperature of the liquid feed mixture that is supplied to the bowl via spindle 4a. Such second temperature sensor may be operable to measure the temperature of the liquid feed mixture in an inlet pipe that is connected to the spindle 4a, i.e. connected to the hollow interior 4b of the spindle (see Fig .2). Then, the measured temperature difference between the first sensor 61 and such a second sensor may be used by the control unit to determine if there are still solids left in the sludge space after an intermittent discharge. If the first sensor 61 is covered in solids after a discharge, there may be a higher difference in temperature compared to the liquid feed mixture, and this may indicate that there are solids left in the sludge space.
Fig. 4 shows another embodiment of a centrifuge bowl 5 for a centrifugal separator, and more specifically a section of the sludge space 9b. In this example, the centrifuge bowl comprises two pressure sensors, but the bowl could as well comprise two different temperature sensors instead. Thus, the difference as compared to the embodiment of Fig. 3 is that the centrifuge bowl 5 further comprises a second pressure sensor 62 that is arranged at a second radial position R2 in the sludge space 9b. This second radial position is within the inner half of the radial extension ΔR of the sludge space 9b. The control unit 40 is configured to communicate with both the first 61 and second 62 pressure sensors and to determine a pressure difference between the two sensors. During a separation operation and when a solid phase has been built up within the sludge space 9b, the pressure measured by the first sensor 61 will be higher than the pressure measured with the second pressure sensor 62. After sludge discharge and if there is still sludge left in the sludge space, the measured pressure difference will be higher than if all sludge has been completely discharged. The measured pressure difference may thus be used to determine if there is still sludge left in the sludge space 9b. Hence, in the embodiment as shown in Fig. 4, the control unit 40 is configured to determine the pressure at the first and second radial positions R1, R2 and configured to determine the pressure difference between the first and second radial positions R1, R2. The first radial position R1 is arranged radially outside the second radial position R2. The control unit 40 is further configured to determine if there is still sludge left in the sludge space 9b after a discharging 105 a solid phase of volume V1 based on the determined pressure difference. As an example, the control unit 40 may configured to determine that there is still sludge left in the sludge space 9b if the determined pressure difference is above a first setpoint.
A method 100 in which the pressure difference between two sensors in the sludge space 9b is measured is illustrated in Fig. 9. In addition to the steps already discussed in relation to Fig. 6, step c) further comprises measuring the pressure P1 at the first radial position, the pressure P2 at the second radial position and the pressure difference P_diff, i.e. P1-P2, before discharge. The first radial position R1 is arranged radially outside the second radial position R2. Moreover, step e) hence further comprises determining the pressures P1, P2 at the first and second radial positions and determining the pressure difference P_diff between the first and second radial positions after discharge. Then, step f) may comprise determining that there is still sludge left in the sludge space 9b if the measured pressure difference is above a certain limit or has not decreased to a large extent.
Measuring the pressure difference instead of the absolute pressure may be an advantage since parameters such as flow through the separator, pressure losses, the radial levels of the inlet and liquid outlets, the counter pressure at the liquid outlet or outlets, may be neglected since they affect the first and second pressure sensors within the sludge space to the same degree.
When using two pressure sensors, both may be arranged within the outer half of the radial extension ΔR of the sludge space 9b. Such an embodiment is illustrated in Fig. 5, in which both radial position R1 of the first pressure sensor 61 and the radial position R2 of the second pressure sensor R2 is within the outer half, such as within the outer 25 % of the radial extension ΔR of the sludge space 9b. However, radial position R1 is still radially outside of radial position R2. With such a setup of the pressure sensors 61, 62, the density of the phase being present between the two sensors may be calculated, e.g. using the formula $\frac{p 1 - p 2}{0.5 * w^{2} * (rp 1^{2} - rp 2^{2}) * 10^{- 10}}$
wherein p1 and p2 are the pressures measured by the respective first and second pressure sensors in bar, w is the rotational speed in rad/s, and rp2 and rp3 are the respective radial positions of the second and third pressure sensors in mm. Consequently, in this embodiment, the control unit 40 is configured to determine the density of the phase in the outer half of the radial extension ΔR of the sludge space 9b based on the determined pressure difference between the first 61 and second 62 pressure sensors. If sludge is left in the sludge space 9b, such determined density between the radial positions R1, R2 may be higher than if there is no sludge left. Thus, as an example, the control unit may be further configured to determine that there is still sludge left in the sludge space 9b if the determined density is above a threshold value.
Accordingly, with the setup as shown in Fig. 5 with two pressure sensors 61, 62 both arranged within the outer half of the radial extension ΔR of the sludge space 9b but at different radii, the method 100 as illustrated in Fig. 10 may be performed. The method 100 may thus comprise the steps of

a) rotating 101 the centrifuge bowl 5;
b) supplying 102 the liquid feed mixture to the centrifuge bowl 5;
c) separating 103 said liquid feed mixture in the centrifuge bowl 5 into at least one liquid phase and a solid phase, measuring 104 the pressure P1 at the first radial position R1, the pressure P2 at the second radial position R2, and determining the pressure difference Pdiff between the first and second radial positions R1, R2 as well as determining 104a the density of the phase between the first and second radial positions R1, R2 before sludge discharge;
d) discharging 105 a solid phase of volume V1;
e) measuring 106 the pressure P1 at the first radial position R1, the pressure P2 at the second radial position R2, and determining the pressure difference Pdiff between the first and second radial positions R1, R2 as well as determining 106a the density of the phase between the first and second radial positions R1, R2 after the sludge discharge of step e); and
f) determining 107 if there is still sludge left in the sludge space (9b) from the measurements in step e); and
g) discharging 108 a volume V2 that is larger than V1 if it is determined that there is still sludge left in the sludge space 9b and discharging 109 a volume V3 that is smaller than V1 if it is determined that there is no sludge left in the sludge space 9b.

The invention is not limited to the embodiments disclosed but may be varied and modified within the scope of the claims set out below. The invention is not limited to the type of separator as shown in the Figures. The term "centrifugal separator" also comprises centrifugal separators with a substantially horizontally oriented axis of rotation.

Claims

A method (100) for operating a centrifugal separator (1) for separating at least one liquid phase and a solid phase from a liquid feed mixture;
wherein the centrifugal separator (1) comprises a centrifuge bowl (5) arranged to rotate about an axis of rotation and comprising a separation space (9a), in which surface enlarging inserts (10) are arranged, and a sludge space (9b) arranged radially outside the separation space (9a);

wherein the centrifuge bowl (5) further comprises an inlet (14) for supply of the liquid feed mixture, at least one liquid outlet (6, 7) for discharge of a separated liquid phase, a sludge outlet (15) arranged for intermittently discharging a separated solid phase from said centrifuge bowl (5), and a first pressure or temperature sensor (61) arranged at a first radial position in the centrifuge bowl (5);

wherein said first radial position is within the outer half of the radial extension (ΔR) of the sludge space (9b);

the method (100) comprising the steps of
a) rotating (101) the centrifuge bowl (5);

b) supplying (102) the liquid feed mixture to the centrifuge bowl (5);

c) separating (103) said liquid feed mixture in the centrifuge bowl (5) into at least one liquid phase and a solid phase;

d) discharging (105) a solid phase of volume V1;

e) measuring (106) a pressure or temperature at least at said first radial position after the sludge discharge of step e); and

f) determining (107) if there is still sludge left in the sludge space (9b) from the measurements in step e).
A method (100) according to claim 1, wherein step c) comprises separating (103) said liquid feed mixture in the centrifuge bowl (3) into at least one liquid phase and a solid phase and measuring (104) a pressure or temperature at least at said first radial position before the sludge discharge of step e), and further wherein step f) comprises determining if there is still sludge left in the sludge space from the measurements in step e) and step c).
A method (100) according to claim 1 or 2, further comprising the step of g) discharging (108) a solid phase of volume V2 that is larger than V1 if it is determined in step f) that there is sludge left in the sludge space (9b) and discharging (109) a solid phase of volume V3 that is smaller than V1 if it is determined in step f) that there no sludge left in the sludge space (9b).
A method (100) according to any previous claim, wherein the first sensor (61) is a pressure sensor and wherein the centrifuge bowl (5) further comprises a second pressure sensor (62) arranged at a second radial position in the sludge space (9b), and wherein step e) further comprises determining the pressure at said first and second radial positions and determining the pressure difference between the first and second radial positions, wherein the first radial position is arranged radially outside the second radial position.
A method (100) according to claim 4, wherein the second radial position is within the inner half of the radial extension (ΔR) of the sludge space (9b).
A method (100) according to claim 5, wherein step f) comprises determining that there is still sludge left in the sludge space (9b) if the pressure difference is above a first setpoint.
A method (100) according to claim 4, wherein the second radial position is within the outer half of the radial extension (ΔR) of the sludge space (9b).
A method (100) according to claim 7, wherein step e) further comprises determining the density of the phase in the outer half of the radial extension (ΔR) of the sludge space (9b) based on said determined pressure difference, and wherein step f) comprises determining (107) that there is still sludge left in the sludge space (9b) if the determined density is above a threshold value.
A method (100) according to any one of claims 1-3, wherein the first sensor (61) is a temperature sensor, and wherein step f) comprises comparing the measured temperature with the temperature of the liquid feed mixture to determine if there is still sludge left in the sludge space (9b).
A centrifugal separator (1) for separating at least one liquid phase and a solid phase from a liquid feed mixture, wherein the centrifugal separator (1) comprises
a centrifuge bowl (5) arranged to rotate about an axis of rotation and comprising a separation space (9a), in which surface enlarging inserts (10) are arranged, and a sludge space (9b) arranged radially outside the separation space (9a);

wherein the centrifuge bowl (5) further comprises an inlet (14) for supply of the liquid feed mixture, at least one liquid outlet (6,7) for discharge of a separated liquid phase, a sludge outlet (15) arranged for intermittently discharging a separated solid phase from said centrifuge bowl (5), and a first pressure or temperature sensor (61) arranged at a first radial position in the centrifuge bowl (5);

wherein said first radial position is within the outer half of the radial extension (ΔR) of the sludge space (9b);

wherein the centrifugal separator (1) further comprises a control unit (40) that is configured to determine if there is still sludge left in the sludge space (9b) after a discharging (105) a solid phase of volume V1 based on a measured pressure or temperature at least at said first radial position after said sludge discharge.
A centrifugal separator (1) according to claim 10, wherein said control unit (40) is configured to determine if there is still sludge left in the sludge space (9b) after a discharging (105) a solid phase of volume V1 based on a measured pressure or temperature at least at said first radial position before and after said sludge discharge.
A centrifugal separator (1) according to claim 10 or 11, wherein the control unit (40) is further configured to initiate a discharge of a solid phase of volume V2 that is larger than V1 if it is determined that there is sludge left in the sludge space (9b) and to initiate a discharge of a solid phase of volume V3 that is smaller than V1 if it is determined that there no sludge left in the sludge space (9b).
A centrifugal separator (1) according to any one of claims 10-12, wherein the first sensor (61) is a pressure sensor and wherein the centrifuge bowl (5) further comprises a second pressure sensor (62) arranged at a second radial position in the sludge space (9b), and the control unit (40) is configured to determine the pressure at said first and second radial positions and to determine the pressure difference between the first and second radial positions, wherein the first radial position is arranged radially outside the second radial position, and wherein the control unit (40) is further configured to determine if there is still sludge left in the sludge space (9b) after a discharging (105) a solid phase of volume V1 based on said determined pressure difference.
A centrifugal separator (1) according to claim 13, wherein the second radial position is within the inner half of the radial extension (ΔR) of the sludge space (9b).
A centrifugal separator (1) according to claim 14, wherein the control unit (40) is configured to determine that there is still sludge left in the sludge space (9b) if the determined pressure difference is above a first setpoint.
A centrifugal separator (1) according to claim 13, wherein the second radial position is within the outer half of the radial extension (ΔR) of the sludge space (9b).
A centrifugal separator (1) according to claim 16, wherein the control unit (40) is configured to determine the density of the phase in the outer half of the radial extension (ΔR) of the sludge space (9b) based on said determined pressure difference, and further configured to determine that there is still sludge left in the sludge space (9b) if the determined density is above a threshold value.