TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method for cleaning a screw conveyor in a system for treatment of biomass material and to a cleaning system for cleaning a screw conveyor provided in a system for treatment of biomass material.

BACKGROUND

In pre-hydrolysis pressure vessel systems for treating biomass, precipitation is build up and may cause problems. The precipitation may stick to for example inner walls and to a transport screw, a screw conveyor, arranged for transporting the biomass through the system. The precipitation may cause friction and may affect the material transport efficiency. Pieces of precipitation material can come loose and cause problems downstream in the system.

Cleaning of the screw conveyor is often performed by applying a cleaning fluid to the screw conveyor. However, it is important to limit the amount of cleaning fluid in order to not affect the biomass material being transported by the screw conveyor negatively.

SUMMARY

An object of the present invention is to provide an improved method for cleaning a screw conveyor in a system for treatment of a biomass material where cleaning efficiency is improved while at the same time an amount of cleaning fluid added can be minimized.

This is achieved in a method, a cleaning system and a computer program product according to the independent claims.

According to one aspect of the invention a method for cleaning a screw conveyor in a system for treatment of biomass material is provided. Said method comprises the steps of: feeding biomass material to be transported by the screw conveyor, into a conveyor housing enclosing a screw blade of the screw conveyor; repeatedly or continuously measuring a value of a parameter being representative of a torque acting on a central shaft of the screw conveyor for rotating it; comparing said measured value with a predefined upper threshold value; and starting a cleaning session when said measured value is larger than said predefined upper threshold value, wherein said cleaning session comprises cleaning at least a part of the screw conveyor with a cleaning fluid.

According to another aspect of the invention a cleaning system for cleaning a screw conveyor provided in a system for treatment of biomass material is provided. Said cleaning system comprises: a cleaning arrangement connected to the screw conveyor for cleaning of the screw conveyor with a cleaning fluid; a measuring device positioned in the system for repeatedly or continuously measuring a value of a parameter being representative of a torque acting on a central shaft of the screw conveyor for rotating it; and a control unit connected to the cleaning arrangement and to the measuring device, said control unit being configured for controlling said cleaning arrangement for cleaning the screw conveyor in dependence of said measured value and according to the method according to the invention.

According to another aspect of the invention a computer program product is provided comprising instructions which, when executed in a processor in a control unit in a cleaning system as defined above, cause the control unit to perform the method according to the invention.

Hereby, by measuring a value of a parameter being representative of a torque acting on the screw conveyor for rotating it, and only clean the material transport screw when this value is above a predefined upper threshold value, the amount of cleaning fluid used for cleaning the material transport screw can be limited. The torque will be indicative of amount of precipitation attached to the material transport screw and hereby a measured value being representative of the torque can indicate when it is time for cleaning. Hereby, there will be no unnecessary cleaning which could be the case if cleaning only is done at regularly time intervals.

In one embodiment of the invention said method further comprises the step of comparing said measured value with a predefined lower threshold value; and ending a started cleaning session when said measured value is lower than said lower threshold value.

In one embodiment of the invention said cleaning session comprises cleaning different sections of the screw conveyor after each other according to predefined cleaning parameters and comparing said measured value with the predefined lower threshold value repeatedly or continuously and possibly end a started cleaning session before all sections have been cleaned, when said measured value is lower than said lower threshold value.

In one embodiment of the invention the step of feeding biomass material into the conveyor housing comprises feeding a limited amount of biomass material into the housing such that 2- 30% of an internal volume of said conveyor housing is filled by biomass material.

In one embodiment of the invention said cleaning session comprises cleaning at least a part of the screw conveyor with a cleaning fluid applied at high pressure.

In one embodiment of the invention the method further comprises the steps of: determining a rate of change of said measured value; comparing said rate of change of said measured value with a predefined rate of change upper threshold value; starting a cleaning session when said measured value is larger than said predefined rate of change upper threshold value, wherein said cleaning session comprises cleaning at least a part of the screw conveyor with a cleaning fluid.

In one embodiment of the invention the method further comprises the steps of: analysing cleaning results from previous cleaning sessions by comparing the measured values of a parameter being representative of a torque acting on the central shaft of the screw conveyor for rotating it, before and after each cleaning session in relation to used cleaning parameters of said cleaning session, said cleaning parameters comprising one or more of: amount of cleaning fluid to be used for each section of the screw conveyor, cleaning time for each section, which sections to clean, in which order sections are to be cleaned, a pressure of the cleaning fluid being applied, a fluid velocity of the cleaning fluid being applied, a cleaning fluid composition such as water and different amounts of additives such as alkalis and levels of the upper threshold, the rate of change upper threshold and the lower threshold; adapting cleaning parameters of coming cleaning sessions based on said analysis, with a purpose to decrease a total amount of used cleaning fluid.

Hereby the cleaning process can be improved over time.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows schematically a cross section of a screw conveyor which can be provided in a system for treatment of biomass material.

Figure 2 shows schematically a cleaning system according to one embodiment of the invention connected to a screw conveyor which can be provided in a system for treatment of biomass material.

Figure 3 is a flow chart of the method for cleaning a screw conveyor according to one embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

According to the invention a method for cleaning a screw conveyor in a system for treatment of biomass material is provided. Furthermore, a cleaning system for cleaning a screw conveyor provided in a system for treatment of biomass material is provided.

In Figure 1 a screw conveyor 11 is shown separate in cross section. The screw conveyor 11 comprises a central shaft 13 provided with a screw blade 15 which is enclosed in a conveyor housing 17. The conveyor housing 17 comprises an inlet 19a in one end and an outlet 19b in the opposite end, whereby biomass material is fed into the screw conveyor 11 via the inlet 19a and is transferred out from the screw conveyor 11 via the outlet 19b. By rotating the central shaft 13 the screw blade 15 is rotated and material is transported by the rotating screw blade 15 through the screw conveyor 11. Precipitation may stick to inner surfaces of the screw conveyor such as inner walls and the screw blade 15. These precipitations will make it harder and harder to rotate the central shaft 13. The screw conveyor 11 needs to be cleaned from time to time. Cleaning can be done by introducing a cleaning fluid into the screw conveyor 11. Cleaning fluid can be introduced into the screw conveyor 11 through the conveyor housing 17 via cleaning fluid inlets 21 which can be provided at different positions along the conveyor housing 17. According to the invention a cleaning system 1 for cleaning a screw conveyor 11 provided in a system for treatment of biomass material is provided. A cleaning system 1 according to one embodiment of the invention is shown in Figure 2. Said cleaning system 1 comprises a cleaning arrangement 3 connected to the screw conveyor 11 for cleaning of the screw conveyor with a cleaning fluid. The cleaning arrangement 3 can comprise cleaning fluids or connections for attaching cleaning fluid sources and fluid connections to one or more fluid inlets 21 which are provided through the conveyor housing 17. In some embodiments of the invention a number of fluid inlets 21 are provided separated along a length extension of the conveyor housing 17 such that cleaning fluid can be provided to an inside of the conveyor housing 17 at many different positions of the screw conveyor 11. Hereby different sections 22 of the screw conveyor can be cleaned at different times and in different orders and according to different cleaning parameters such as amount of cleaning fluid, different pressures for applying the cleaning fluid, different cleaning fluid compositions etc as will be further described below. In some embodiments of the invention the cleaning arrangement is configured for providing a high-pressure cleaning. Hereby a cleaning fluid, for example water, is applied at high pressure, for example between 60-200 bar(g), into the conveyor housing 17 via the fluid inlets 21. The cleaning arrangement 3 may in some embodiments comprise high pressure nozzles which are provided at the fluid inlets 21 for delivering the cleaning fluid at suitable pressure. Hereby the high pressure of the cleaning fluid will contribute mechanically to the cleaning effect and the amount of cleaning fluid needed for the cleaning can be decreased. The cleaning fluid can hereby in some embodiments be water. The cleaning fluid can in other embodiments also comprise water together with cleaning additives of different amounts, such as alkalis, for example NaOH or KOH. For embodiments where the cleaning fluid is applied at a high pressure less additives may need to be added to the cleaning fluid thanks to the mechanical cleaning effect of the high pressure. This may be advantageous because the biomass material may be negatively affected by additives in the cleaning fluid. The cleaning system 1 comprises furthermore a measuring device 5 positioned in the system for repeatedly or continuously measuring a value of a parameter being representative of a torque acting on the central shaft 13 of the screw conveyor 11 for rotating it. The measuring device 5 can for example be provided in connection with a motor which is driving the rotation of the central shaft 13 of the screw conveyor and measuring the effect provided from the motor, for example by measuring the current to the motor. The measuring device 5 could alternatively be for example a strain gauge applied on the central shaft 13 for measuring the torque acting on the central shaft. The cleaning system 1 comprises furthermore a control unit 7 connected to the cleaning arrangement 3 and to the measuring device 5, said control unit 7 being configured for controlling said cleaning arrangement 3 for cleaning the screw conveyor 11 in dependence of said measured value and according to a method for cleaning which will be described further below.

The method for cleaning a screw conveyor 11 according to one embodiment of the invention is schematically illustrated in the flow chart of Figure 3. The method is described below with reference to the different steps in Figure 3.

SI : Feeding biomass material to be transported by the screw conveyor 11, into the conveyor housing 17 which is enclosing the central shaft 13 and the screw blade 15 of the screw conveyor 11. The material is fed into the screw conveyor 11 via the inlet 19a. In some embodiments of the invention the step of feeding biomass material into the conveyor housing 17 comprises feeding a limited amount of biomass material into the conveyor housing 17 such that for example only 2-30% or 2-15% of an internal volume of said conveyor housing is filled by biomass material. Hereby a torque needed for rotate the central shaft 13 of the screw conveyor 11 can be kept smaller than if the conveyor housing 17 were to be filled up more. Hereby, an increase in torque needed for rotating the central shaft when more and more precipitation are being attached to the screw blade of the screw conveyor can be measured with more precision because the increase in torque is a larger part of the total torque required for rotation when less biomass material is transported through the screw conveyor. Hereby this invention, as will be further defined below, can be even more advantageous and efficient when the conveyor housing 17 is filled by biomass material to a lower degree. If a larger part of the internal volume of the conveyor housing is filled by biomass material a change in torque due to precipitation would be more difficult to observe. The control unit 7 is hereby connected to a feeding device 8 which is connected to the inlet 19a of the screw conveyor 11 and which provides biomass material into the screw conveyor. The control unit 7 is configured to control the feeding device 8 to feed biomass material at a certain feeding rate.

S2: Repeatedly or continuously measuring a value of a parameter being representative of a torque acting on a central shaft 13 of the screw conveyor 11 for rotating it. A measuring device 5, as described above can be provided in the system for measuring this value.

Possibly the method further comprises the step of: S3 : Determining a rate of change of said measured value. The control unit 7 can in some embodiments of the invention comprise a determining device which is configured for determining a rate of change of the measured value as measure in step S2. This step is optional. A rate of change of the measured value of a parameter being representative of the torque could be used as a complement to the actual measured value for controlling the cleaning as will be further described below. A high rate of change of the measured value could be a sign that it could be beneficial to start cleaning earlier than usual.

S4: Comparing said measured value with a predefined upper threshold value. A control unit 7 as described above, can be provided in the system connected to the measuring device 5 and to a cleaning arrangement 3 as described above. The step of comparing said measured value with a predefined upper threshold value is performed by the control unit 7, i.e. the control unit 7 is configured for comparing said measured value with the upper threshold value and comprises a comparing function configured for performing the step of comparing. An upper threshold value can be set in the control unit 7 and can also be changed during use or between uses. An operator of the system can set an upper threshold value corresponding to a value for a torque above which cleaning of the screw conveyor is needed. This upper threshold value can also be based on similar systems and/or previous operation of the system. If a rate of change also was determined in the optional step S3, said rate of change of said measured value may also be compared with a predefined rate of change upper threshold value. Said predefined rate of change upper threshold is also set in the control unit 7 and can be changed during use. An operator of the system can possibly also set and change this rate of change upper threshold.

S5: Starting a cleaning session when said measured value is larger than said predefined upper threshold value and/or possibly starting a cleaning session when said measured value is larger than said predefined rate of change upper threshold value. Either only one of these thresholds is used or both can be used and a cleaning session is started when the first threshold is reached. Said cleaning session comprises cleaning at least a part of the screw conveyor 11 with a cleaning fluid. The control unit 7 controls the cleaning arrangement 3 to perform cleaning according to predefined cleaning parameters. Said cleaning session comprises cleaning different sections of the screw conveyor 11 after each other according to the predefined cleaning parameters. A cleaning session can be stopped before all sections have been cleaned, i.e. in a cleaning session only some of the sections of the screw conveyor 11 may be cleaned. This is controlled from the control unit 7 and depends on the cleaning parameters. These cleaning parameters can also in some embodiments of the invention be updated during run of a process or between different processes. Such an update of cleaning parameters can be done manually or automatically depending on previous cleaning results which will be further described below. The cleaning parameters can comprise one or more of for example amount of cleaning fluid to be used for each section, cleaning time for each section, in which order sections are to be cleaned, a pressure of the cleaning fluid being applied, a fluid velocity of the cleaning fluid being applied, a cleaning fluid composition such as water and different amounts of additives such as alkalis and levels of the upper threshold, the rate of change upper threshold and the lower threshold.

S6: Comparing said measured value with a predefined lower threshold value. Said predefined lower threshold value can be stored in the control unit 7 and can possibly be changed by a user or automatically based on cleaning results. This lower threshold value is set such that cleaning can be stopped when this value has been reached, i.e. a sufficient release in torque required for rotating the central shaft has been achieved and cleaning can be stopped. The control unit 7 is configured for comparing the measured value with said predefined lower threshold value. In some embodiments of the invention cleaning and measuring may need to be performed alternating, i.e. not at the same time. In such embodiment cleaning can be performed during a predefined time period or by using a predefined amount of cleaning fluid or during a predefined number of rotations of the central shaft 13 and then cleaning is paused while a value of a parameter being representative of a torque acting on a central shaft 13 of the screw conveyor 11 for rotating it is measured and compared with the predefined lower threshold value. Then, if the measured value still is higher than the lower threshold value, cleaning is started again for another such period before another measuring and comparing takes on and so on.

S7: Ending a started cleaning session when said measured value is lower than said lower threshold value. The control unit 7 is configured to control the cleaning arrangement 3 to stop a cleaning session when said measured value is lower than said predefined lower threshold value.

S8: Analysing cleaning results from previous cleaning sessions by comparing the measured values of a parameter being representative of a torque acting on the central shaft 13 of the screw conveyor 11 for rotating it, before and after each cleaning session in relation to used cleaning parameters of said cleaning session. The cleaning parameters can comprise one or more of for example: amount of cleaning fluid to be used for each section 22 of the screw conveyor, cleaning time for each section 22, which sections 22 to clean, in which order sections are to be cleaned, a pressure of the cleaning fluid being applied, a fluid velocity of the cleaning fluid being applied, a cleaning fluid composition such as water and different amounts of additives such as alkalis and levels of the upper threshold, the rate of change upper threshold and the lower threshold.

S9: Adapting cleaning parameters of coming cleaning sessions based on said analysis made in step S8, with a purpose to decrease a total amount of used cleaning fluid. This method can be provided as a software algorithm which is automatically updated during run of the software in order to optimize the cleaning based on previous cleaning sessions. For example, certain sections 22 of the screw conveyor 11 may not need to be cleaned as much as others or not at all or the level of the predefined upper threshold may be set lower in order to provide a more efficient cleaning requiring a less amount of cleaning fluid in total over a whole process.

A computer program product is also provided according to the invention. Said computer program product comprising instructions which, when executed in a processor 7a in the control unit 7 in the cleaning system 1 as described above, will cause the control unit 7 to perform the method as described above with reference to method steps S1-S9.