Method and system for monitoring a polyolefin composition

Abstract

The invention relates to a method for monitoring the preparation of a polyolefin composition by means of an extruder device (9), comprising the method steps of: feeding polyolefin raw material into the extruder device (9); metering at least one additive into the extruder device (9); extruding the polyolefin composition from the polyolefin raw material and the at least one additive; granulating the polyolefin composition to form polyolefin granules by means of a granulating device (27); feeding the polyolefin granules into an equalisation tank (29); measuring at least one measurement variable of the polyolefin granules in and / or on the equalisation tank (29) by means of at least one measuring sensor (33, 34, 40) and calculating the mass flow of the polyolefin granules on the basis of the at least one measured measurement variable.

Description

[0001] Method and equipment for monitoring the production of a polyolefin composition

[0002] The content of the German patent application DE 10 2024 207 742.9 is incorporated herein by reference.

[0003] The invention relates to a method and a system for monitoring the production of a polyolefin composition.

[0004] EP 3 186 051 Bl discloses a process for producing a polyolefin composition. In an extruder, polyolefin is extruded with at least one additive and pelletized into polyolefin pellets. To maintain the correct formulation of the polyolefin composition, the flow rate of the polyolefin fed to the extruder or the flow rate of the polyolefin pellets produced by the extruder is measured in order to adjust the dosage of the polyolefin and / or the at least one additive. Measuring the flow rate is complex. The flow rate is measured, in particular, using a so-called solids flowmeter. Such a device is large and expensive. Its calibration is time-consuming.

[0005] It is an object of the present invention to simplify the monitoring of the production of a polyolefin composition.

[0006] This problem is solved by a method with the features specified in claim 1, by a system with the features specified in claim 7, and by a buffer tank with the features specified in claim 14. The core of the invention is that a solids flow meter can be dispensed with if the flow rate, i.e., a mass flow, is calculated and not measured. According to the invention, it has been found that the mass flow of polyolefin granules can be calculated based on measured variables that are easily and, in particular, directly measured by means of at least one measuring sensor. The measurement takes place in and / or on a buffer tank in which polyolefin granules, which have been extruded in an extruder, can be received and temporarily stored. The buffer tank has a standard volume of at least 2 m³. 3 , in particular at least 10 m 3 , in particular at least 20 m3 , in particular at least 30 m 3 , in particular at least 60 m 3 , in particular at least 100 m 3 and in particular at least 200 m 3 In particular, the expansion tank has a standard volume of no more than 1000 m³. 3 The polyolefin granules can be temporarily stored in the buffer tank for a buffer time of at least 2 minutes, in particular at least 5 minutes, in particular at least 10 minutes, in particular at least 30 minutes, and in particular at least 45 minutes. In particular, the polyolefin granules can be temporarily stored for a buffer time of up to 5 hours.

[0007] A metering device is arranged on the expansion tank to enable the metered dispensing of the polyolefin granules from the expansion tank. The metering device is specifically designed as a rotary valve.

[0008] In the extruder, a polyolefin composition is extruded from a polyolefin raw material to which at least one additive is added. The polyolefin raw material comprises, in particular, polyolefin powder and / or polyolefin granules. The polyolefin powder has a mean particle size of 70 pm to 2500 pm, particularly from 100 pm to 2200 pm, and especially from 120 pm to 2000 pm. The polyolefin granules have a mean particle size of 2000 pm to 7000 pm, particularly from 2500 pm to 6000 pm, and especially from 2500 pm to 5000 pm.

[0009] Polyethylene (PE), especially high-density polyethylene (HDPE), low-density polyethylene (LDPE) and / or linear low-density polyethylene (LLDPE), polypropylene (PP), polyolefin plastomer (POP), polyolefin elastomer (POE), and all related copolymers are used as polyolefin raw materials.

[0010] The polyolefin raw material is melted and homogenized with at least one additive in the extruder. The resulting melt is then granulated to produce polyolefin granules. It is advantageous to dry the polyolefin granules before feeding them into the expansion tank, particularly using a drying device. Drying can be mechanical and / or thermal, i.e., by applying heat.

[0011] Based on the calculated mass flow rate, the current production output of the plant, i.e., its actual output, can be determined. Material flows and operating modes of individual plant components can be optimized. In particular, the dosing of the at least one additive can be adjusted. A method according to claim 2 is particularly easy to implement. The mass of the polyolefin granules in the expansion tank can be measured directly and easily with at least one weighing element. The at least one weighing element is arranged on the expansion tank. In particular, the expansion tank is supported on the at least one weighing element. In particular, mass measurement is ensured by means of at least one load cell and, in particular, several load cells.

[0012] Additionally or alternatively, the fill level can also be considered as a measured variable according to claim 3. A fill level sensor serves as the measuring sensor. Using a fill level signal determined by the fill level sensor, the volume of the polyolefin granules in the expansion tank can be calculated. Using a known bulk density of the polyolefin granules, the mass of the polyolefin granules in the expansion tank can be calculated from the volume.

[0013] Additionally or alternatively, according to claim 4, the rotational speed of a metering drive of a metering device can also serve as a measured variable. It has been found, in particular, that the rotational speed of the metering device is representative of the outflow of the polyolefin granules from the expansion tank. A rotational speed sensor on the metering drive of the metering device serves as the measuring sensor. The rotational speed can be measured directly with the rotational speed sensor. Alternatively, the rotational speed can be measured indirectly using a frequency converter. The metering device is, in particular, designed as a rotary valve. Alternatively, the metering device can also be designed as a screw conveyor. The rotational speed of the screw conveyor can be taken into account accordingly. In a simple embodiment, the measuring sensor merely detects whether the metering device is in operation, i.e., whether the metering drive is rotating or stationary.In another embodiment, the speed sensor serves to detect the actual rotational speed of the metering drive.

[0014] If the dosing device is designed as a positioning slide, a position signal from the positioning slide can be used. From the position signal and the geometry of the positioning slide, the remaining cross-sectional area through which the polyolefin granules are discharged from the expansion tank can be determined. This remaining cross-sectional area is representative of the flow of polyolefin granules from the expansion tank.

[0015] A method according to claim 5 is particularly easy to implement. In particular, the method enables two-point measurement, in which a measured quantity is measured at at least two different times and the change in the measured quantity over time is determined from the measurement results.

[0016] A method according to claim 6 enables a straightforward and cost-effective adjustment of the outflow rate of the polyolefin granules from the expansion tank. This is particularly advantageous when the outflow of the polyolefin granules from the expansion tank is effected by means of a rotary valve or a screw conveyor, the drive motor of which is connected to a frequency converter. Alternatively, the outflow rate can be adjusted via a positioning slide. This allows for a change in the outflow rate, particularly stepless, by changing the rotational speed of the rotary valve, the screw pitch of the screw conveyor, or the position of the positioning slide. Additionally or alternatively, the outflow can also be stopped, particularly at least temporarily, by a closing element, in particular a gate valve, arranged upstream of the rotary valve.The closing element is arranged, in particular in the conveying direction of the bulk material, between the equalization tank and the rotary valve.

[0017] An installation according to claim 7 has essentially the same advantages as the corresponding method, to which reference is hereby made. In particular, the conveying of the polyolefin raw material from the storage container to the extruder device and / or from the metering unit to the extruder device is carried out gravimetrically, in particular by means of a gravity feed line.

[0018] A system according to claim 8 simplifies the targeted, in particular controlled, outflow of the polyolefin granules from the compensation tank.

[0019] A system according to claim 9 simplifies, in particular stepless, adjustment of the outflow rate of the polyolefin granules from the compensation tank.

[0020] A system according to claim 10 improves the monitoring of the quantity of polyolefin granules discharged from the expansion tank.

[0021] A system according to claim 11 enables advantageous connection of the system to a pneumatic conveying system. A system according to claim 12 is of uncomplicated design. The measurement of various measured variables, particularly direct measurement, is uncomplicated. The implementation of a measuring sensor as a load cell, level sensor and / or tachometer is uncomplicated.

[0022] A system according to claim 13 improves the accuracy of measuring the at least one measurand. In particular, measurement errors resulting from internal and / or external forces such as pressure superposition, pressure fluctuations in pipelines, thermal stresses, and / or other mechanical influences on the expansion tank are avoided. "Decoupled" means that force changes, which can occur particularly in and / or on pipelines, especially bulk material lines, leakage air lines, and / or exhaust air lines, are decoupled from the expansion tank. Such force changes do not affect the expansion tank or the measuring sensors arranged therein.

[0023] A compensation tank according to claim 14 enables the advantageous intermediate storage of the polyolefin granules produced in the extruder device as well as the uncomplicated acquisition of measured values, which is the basis for calculating the mass flow rate of the polyolefin granules.

[0024] Both the features specified in the claims and those specified in the exemplary embodiments of the invention are suitable, individually or in combination, for further developing the subject matter of the invention. The respective combinations of features do not constitute a limitation with regard to the further development of the subject matter of the invention, but are essentially merely exemplary. Further features, advantages, and details of the invention will become apparent from the following description of exemplary embodiments with reference to the drawings. The drawings show:

[0025] Fig. 1 shows a system according to the invention for producing a polyolefin composition with a compensation tank,

[0026] Fig. 2 is an enlarged schematic representation of the expansion tank in Fig. 1,

[0027] Fig. 3 Fig. 2 corresponding representation of an expansion tank according to a second embodiment with level sensors,

[0028] A system, designated as a whole by 1 in Fig. 1, is used to produce a polyolefin composition. A first storage container 2 holds polyolefin raw material in the form of polyolefin powder. The polyolefin powder is fed to the first storage container 2 via a feed line.

[0029] 3. A first drain line is connected to the first storage tank 2.

[0030] 4 connected, which leads into a first rotary valve 5. The first rotary valve 5 is rotatably driven by a first drive 6, in particular a motor and especially an electric motor. Downstream of the first rotary valve 5, a bulk material line 7 is connected, which, according to the illustrated embodiment, is designed as a gravity line.

[0031] The bulk material line 7 leads into an inlet hopper 8 of an extruder device 9. The extruder device 9 has a homogenizing element 10, in particular an extruder screw, which can be driven by means of an extruder drive 11, in particular in the form of an electric motor.

[0032] The system 1 includes a metering unit 19. The metering unit 19 serves to add an additive to the extruder 9 in a metered manner. The metering unit 19 comprises a metering device 21, which is designed in particular as a screw conveyor. The metering device 21 is driven by a metering device drive 22. The metering device drive 22 is designed as a motor, and in particular as an electric motor. By means of the metering device 21, the additive is dispensed into an additive line 23 and fed to the inlet hopper 8 of the extruder 9.

[0033] The additive is fed into the dosing device 21 via a feed line 26.

[0034] The extruder 9 produces a melt of the polyolefin composition, comprising the polyolefin raw material and at least one additive. The melt is granulated into polyolefin granules in a granulating device 27 and, in particular, dried by means of an optional drying device 26. The granulating device 27 is, in particular, arranged directly downstream of the extruder 9.

[0035] A granule conveying line 28 connects a buffer tank 29 to the drying device 26. Conveying along the granule conveying line 28 is primarily gravimetric. The granule conveying line 28 is primarily designed as a gravity feed line. A second rotary valve 30 is connected to the buffer tank 29 and serves to feed the polyolefin granules into a pneumatic conveying line 31. The second rotary valve 30 is a metering device that serves to dispense the polyolefin granules from the buffer tank 29 in a metered manner.

[0036] The second rotary valve 30 is rotatably driven by a second rotary valve drive 32. The pneumatic conveying line 31 is only partially shown in Fig. 1. In particular, essential components of the pneumatic conveying line, especially a compressed air source, as well as target containers connected to this pneumatic conveying line 31 at a destination location, are not shown. The polyolefin granules can be conveyed to a destination location using the pneumatic conveying line 31.

[0037] At least one measuring sensor 34 is arranged on and / or in the expansion tank 29, which serves to measure at least one parameter of the polyolefin granules in and / or on the expansion tank 29. The at least one measuring sensor 34 is in signal communication with an evaluation unit 35. The signal connection can be wired or wireless. The evaluation unit 35 is designed to calculate the mass flow rate of the polyolefin granules produced by the extruder 9 from measurement data, in particular from the at least one measuring sensor 34.

[0038] The specific design of the expansion tank 29 is explained in more detail below with reference to Fig. 2.

[0039] The expansion tank 29 is a storage tank. The expansion tank 29 enables at least temporary storage, in particular intermediate storage, of the polyolefin granules before they are fed into the pneumatic conveying line 31. The expansion tank 29 is a buffer tank. All pipelines, in particular bulk material lines, especially the granule conveying line 28, a second drain line 37, an exhaust air line 36, and a leakage air line 38, are each decoupled and connected to the expansion tank 29. A compensator 39, designed as a flexible connecting element, serves this purpose, enabling a mechanically flexible connection between the respective lines 28, 36, 37, and 38 and the expansion tank 29. This minimizes and, in particular, prevents disruptive forces on the expansion tank 29 resulting from stresses, movements, and / or fluctuations.

[0040] According to the illustrated embodiment, several, in particular different, measuring sensors are arranged on the expansion tank 29. Several load cells 34 are suitable for measuring the mass of the filled expansion tank 29. The load cells 34 are arranged in a vertically lower region of the expansion tank 29, in particular at the transition from a cylindrical to a conical region of the expansion tank 29.

[0041] A level sensor 40 is arranged on the expansion tank 29 as an additional measuring sensor. The level sensor 40 is located in an upper area of ​​the expansion tank 29, in particular on an upper end cap of the expansion tank 29. The level sensor 40 is a continuous level sensor, designed as a so-called level transmitter.

[0042] The level transmitter 40 generates a continuous measurement signal that can be used to determine the actual fill level of the expansion tank 29. In this respect, the measurement signals from the level sensor 40 and the load cells 34 are redundant, thereby increasing measurement accuracy and reliability.

[0043] The measuring sensors, i.e. the load cells 34 and the level sensor 40, are in signal communication with the evaluation unit 35, in particular wirelessly.

[0044] A closing element 51 in the form of a gate valve is arranged at the outlet 41 of the expansion tank 29, and in particular between the expansion tank 29 and the second rotary valve 30. The gate valve 51 allows the second discharge line 37 to be shut off quickly and, in particular, immediately, so that the outflow of polyolefin granules from the expansion tank 29 can be stopped immediately.

[0045] The second rotary valve 30 is rotatably driven by means of the second rotary drive 32. A speed sensor 33, which is in signal communication with the second rotary drive 32, serves to monitor the rotational speed of the second rotary valve 30.

[0046] The following describes in more detail a process for operating plant 1, i.e., for the continuous production of a polyolefin composition. In a first process step, the polyolefin raw material, in particular polyolefin powder and / or polyolefin granules, is fed from the storage container 2 into the extruder device 9.

[0047] In a second process step, the at least one additive is added to the extruder device 9. In a third process step, a melt of the polyolefin composition consisting of the polyolefin raw material and the at least one additive is extruded.

[0048] In a fourth process step, the polyolefin composition is granulated to form polyolefin granules using the granulating device 27. In particular, drying is also carried out using the drying device 26.

[0049] In an optional next process step, the granules can be sieved and / or classified.

[0050] In the next process step, the polyolefin granules are conveyed into the compensation tank 29.

[0051] In a next process step, at least one measured quantity is measured using the at least one measuring sensor 34, 40 and the mass flow rate of the polyolefin granules is calculated from this in a further process step.

[0052] In a subsequent process step 50, the dosage quantities for the polyolefin raw material and / or the additive can be adjusted based on the calculated mass flow of the polyolefin granules.

[0053] The calculation of the mass flow rate using various calculation methods is explained in more detail below.

[0054] In a first calculation method, the expansion tank 29 is empty in its initial state and, in particular, in its operating state. In this state, there is no polyolefin granulate in the expansion tank 29. Starting from this state, the outflow of polyolefin granulate from the expansion tank 29 is first stopped by either stopping the rotary drive of the second rotary valve 30 by means of the second rotary valve drive 32 and / or by closing the closing element 51 at the outlet 41 of the expansion tank 29. If the closing element 51 is used, the still-filled chambers of the second rotary valve 30 can be emptied by a controlled rotary movement.

[0055] The polyolefin granules conveyed via the granule conveying line 28 into the expansion tank 29 cause an increase in the mass of the expansion tank 29. This increase in mass is measured and recorded by means of the load cells 34. Because the expansion tank 29 is decoupled, this measurement is very accurate. The mass flow rate riiE of the polyolefin granules produced in the extruder 9 can be determined, for example, by a two-point measurement, by relating the mass increase to the corresponding time interval of the measurement.

[0056] A more precise measurement of the mass flow rate is possible if the rate of mass increase is determined by linear regression of individual measurement points within the measurement time interval. With a time interval of 60 s and an assumed mass flow rate of 60 t / h through extruder 9, 1,000 kg of polyolefin granules are extruded in one minute, specifically with a volume of 2 m³.3 at a bulk density of 500 kg / m³ 3 The measurement signal, which is primarily a weighing signal, could be recorded every second, allowing the slope to be determined across 61 measurement points within the specified time interval. Regardless of whether a two-point measurement or a linear regression of the measurement points is used, calculating the mass flow rate is straightforward, especially when based on directly measured quantities.

[0057] It is possible to repeat the mass flow calculation procedure regularly, i.e., at fixed time intervals, and / or on an ad-hoc basis. Occasions that may necessitate a recalculation of the mass flow include: significant changes in the computationally monitored extruder mass flow riiE, particularly due to a change in the bulk density of the polyolefin raw material, especially the polyolefin powder, and / or build-up in the metering powder feeder and / or powder screw.

[0058] Changes to monitored extruder parameters such as torque and / or current consumption of the extruder drive 11 and / or an optional melt pump, in particular its melt temperature and / or the pressure in front of the die plate,

[0059] Change in the rotational speed of the extruder device 9, especially when the extruder device 9 is not operated with pre-metering,

[0060] Changing a target container that is connected to the pneumatic conveying line 31, i.e., when the pneumatic conveying is switched from a first target container to a second target container;In this case, mass flow measurement of the extruder device 9 is particularly advantageous because, due to the switching of a pipe diverter, the pneumatic conveying line 31 must be emptied and the supply of polyolefin granules into the pneumatic conveying line 31 can be stopped by stopping the third rotary valve 30 or by closing the closing element 51. A desired change in extruder output, for example, due to a required backflow in a powder silo upstream, emptying of a powder silo, a reduction in extruder output due to problems downstream, shutdown of the plant 1, a color change and / or an adjustment of the extruder output for quality reasons, as well as maximization of the extruder output, is recommended after starting the extruder device 9 and completing the start-up process.

[0061] Another calculation method is based on the assumption that the expansion tank 29 filled with polyolefin granules has a constant mass. This is particularly the case if the fill level in the expansion tank 29 is constant, especially under the assumption that the bulk density of the polyolefin granules is constant.

[0062] Starting from a defined fill level in the reservoir 29, the rotational speed for the second rotary valve 30 can be determined at the current extruder output so that the fill level in the reservoir 29 remains constant. This can be done, for example, using a PID controller or based on historical data, such as from a process parameter database. When the second rotary valve 30 operates at the rotational speed determined in this way, the rotational speed n S0 When the system is in operation, the fill level in the expansion tank 29 is constant. This means that the measurement signal from the load cells 34 and the fill level sensor 40 does not change.

[0063] The mass flow rate of the polyolefin granules can be calculated from the rotational speed of the second rotary valve 30, the volume of polyolefin granules conveyed from the expansion tank 29 by means of the rotary valve 30 and on the basis of its bulk density.

[0064] A second embodiment of an expansion tank is described below with reference to Fig. 3. Structurally identical parts are given the same reference numerals as in the first embodiment, to whose description reference is hereby made. Structurally different, but functionally equivalent, parts are given the same reference numerals with the suffix "a" appended.

[0065] The essential difference compared to the previous embodiment is that the expansion tank 29a is equipped with two level sensors 40, 40a in addition to the load cells 34. The level sensors 40, 40a are designed as level switches. The level switches 40, 40a are arranged laterally, particularly on an outer wall of the expansion tank 29a. The level sensors 40, 40a are arranged at different height positions on the expansion tank 29a. The lower level sensor 40, facing the outlet 41, is referred to as the first level switch, and the upper level sensor 40a, facing away from the outlet 41, is referred to as the second level switch. The level sensors 40, 40a are suitable for detecting different fill levels in the expansion tank 29a.The level sensors 40, 40a are in signal communication with the evaluation unit 35 and transmit a signal indicating whether the respective level has been reached or not.

[0066] In the illustrated embodiment, the operation of the second rotary valve 30 can be detected by means of the second rotary valve drive 32. For this purpose, the second rotary valve drive 32 is in signal communication with the evaluation unit 35. This allows it to be detected whether the second rotary valve 30 is rotating. The exact rotational speed is not required for the operation of the expansion tank 29a and, in particular, for calculating the mass flow rate of the polyolefin granules.

[0067] The expansion tank 29a enables a further calculation method in which further measurement signals are taken into account, in particular the measurement signals of the at least two level sensors 40, 40a.

[0068] The calculation is performed, in particular, when the first level sensor 40 measures a lower limit. The closing element 51 is closed, and the level of the polyolefin granules in the expansion tank 29a rises. As soon as the second level sensor 40a measures an upper limit, the time difference elapsed since the measurement signal of the first level sensor 40 is recorded. The closing element 51 is then opened again. Because the volume of polyolefin granules between the level sensors 40 and 40a, as well as the bulk density of the polyolefin granules, are known, the mass flow rate of the polyolefin granules from the extruder 9 can be calculated using the recorded time difference.

[0069] Alternatively, this method can also be carried out using a continuous level transmitter according to the first embodiment instead of the level sensors 40, 40a. This transmitter outputs a signal, depending on the volume of the polyolefin granules in the expansion tank 29a, indicating a fill level between 0% and 100%. Corresponding switching points, which, for example, correspond to the fill levels of the level sensors 40, 40a in the embodiment shown in Fig. 3, can be predefined.

[0070] Alternatively, the described procedure can also be carried out with the signal provided by the load cells 34, whereby the corresponding switching points are defined by suitable polyolefin granulate masses.

[0071] The mass flow calculation for the extruder device 9, based on a continuous measurement signal, can be started at any time. It is advantageous if the calculation starts when the fill level in the expansion tank 29a is as low as possible, in particular at most 50%, 40%, 30%, 20%, or 10% of the maximum fill quantity.

[0072] After the initial calculation of the mass flow rate riiE, the level sensors 40 and 40a can be used to operate the second rotary valve 30 at fixed rotational speeds m and n2 such that the fill level lies between the level sensors 40 and 40a. Upon reaching the upper limit, the second rotary valve 30 switches from the lower rotational speed m to the higher rotational speed m. The decreasing mass flow rate in the expansion tank 29a can then be calculated using the signal from the load cells 34 and the procedure described above. With a constant extruder mass flow rate riiE, the decreasing mass flow rate from the expansion tank 29a is constant. This value can be compared with the previously calculated decreasing mass flow rates. If a deviation is found, the extruder mass flow rate riiE can be checked again as already explained.In the present example and with the described calculation method, the mass flow rate can be calculated for both decreasing and increasing fill levels.

[0073] Reference symbol list

[0074] 1 Annex

[0075] 2 storage containers

[0076] 3 Supply line

[0077] 4 Drain pipe

[0078] 5 1. Rotary valve

[0079] 6 1. Rotary valve drive

[0080] 7 Bulk material pipeline

[0081] 8 inlet funnels

[0082] 9 Extruder device

[0083] 10 Homogenizing element

[0084] 11 Extruder drive

[0085] 19 dosing units

[0086] 21 Dosing device

[0087] 22 Metering drive

[0088] 23 Additive line

[0089] 26 Drying device

[0090] 27 Granulation device

[0091] 28 Granule conveying line

[0092] 29 expansion tanks

[0093] 30 2. Rotary valve

[0094] 31 Pneumatic conveying line 32 2. Rotary valve drive

[0095] 33 Measuring sensor (speed sensor)

[0096] 34 Measuring sensor (weighing sensor)

[0097] 35 evaluation unit

[0098] 36 Exhaust duct

[0099] 37 2. Drain pipe

[0100] 38 Leakage air line

[0101] 39 Compensator

[0102] 40, 40a Measuring sensor (level sensor)

[0103] 41 Outlet

[0104] 51 Locking element

Claims

Patent claims 1. Method for monitoring the production of a polyolefin composition using an extruder apparatus (9) comprising the process steps Feeding polyolefin raw material into the extruder device (9), Adding at least one additive to the extruder device (9), Extrusion of the polyolefin composition from the polyolefin raw material and the at least one additive, Granulating the polyolefin composition to produce polyolefin granules using a granulating device (27), Feeding the polyolefin granules into a compensation tank (29; 29a), Measuring at least one parameter of the polyolefin granules in and / or on the expansion tank (29; 29a) using at least one measuring sensor (33, 34, 40, 40a), Calculating the mass flow rate of the polyolefin granules based on at least one measured parameter.

2. Method according to claim 1, characterized in that the mass of the polyolefin granules serves as the measuring parameter.

3. Method according to one of the preceding claims, characterized in that the fill level of the polyolefin granules in the compensation tank (29; 29a) serves as the measuring variable.

4. Method according to one of the preceding claims, characterized in that the rotational speed of a metering drive (32) of a metering device (30) at the outlet (41) of the expansion tank (29; 29a) serves as the measuring variable.

5. Method according to one of the preceding claims, characterized in that the change over time of the at least one measured quantity is determined.

6. Method according to claim 5, characterized in that the outflow rate of the polyolefin granules from the compensation tank (29; 29a) is changed or stopped in order to selectively generate the temporal change of the at least one measured quantity.

7. System for monitoring the production of a polyolefin composition comprising: a. at least one storage container (2) in which raw polyolefin material is stored; b. an extruder device (9) conveyingly connected to the at least one storage container (2); c. at least one metering unit (19) conveyingly connected to the extruder device (9) for adding additive to the extruder device (9); d. a granulating device (27) for granulating the polyolefin composition produced in the extruder device (9) into polyolefin granules; e. a compensation container (29; 29a) conveyingly connected to the granulating device (27) for receiving the polyolefin granules. f. at least one measuring sensor (33, 34, 40, 40a) arranged in and / or on the compensation container (29; 29a) for measuring at least one measured quantity of the polyolefin granules, g. an evaluation unit (35) designed to calculate the mass flow rate of the polyolefin granules on the basis of the measured measured quantity.

8. System according to claim 7, characterized in that a metering device, in particular a rotary valve (30), is arranged at the outlet (41) of the expansion tank (29; 29a).

9. System according to claim 8, characterized in that a drive motor (32) of the rotary valve (30) is connected to a frequency converter.

10. System according to claim 8 or 9, characterized by a speed sensor (33) in signal communication with a drive motor (32) of the rotary valve (30) for measuring the speed of the drive motor (32) 11. Plant according to one of claims 7 to 10, characterized in that a pneumatic conveying line (31) is connected to the rotary valve (30) for feeding the polyolefin granulate.

12. System according to one of claims 7 to 11, characterized in that the at least one measuring sensor (33, 34, 40, 40a) is designed as a weighing element, in particular as a load cell, as a level sensor and / or as a tachometer.

13. System according to any one of claims 7 to 12, characterized in that all pipelines (28, 36, 37, 38) connected to the expansion tank (29; 29a) are decoupled.

14. Expansion tank for use in a system according to any one of the Claims 8 to 14, wherein the expansion tank (29; 29a) has at least one measuring sensor (33, 34, 40, 40a) for measuring at least one measured quantity of the polyolefin granules stored in the expansion tank (29; 29a), wherein pipes (28, 38) leading into the expansion tank (29; 29a) and pipes (36, 37) leading away from the expansion tank (29; 29a) are connected in a decoupled manner.

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