Conveyor system and process for conveying plastic granules

Abstract

Conveying system for plastic granules comprising: a. several degassing silos (2) for storing and degassing the plastic granules, b. an air connection (5, 13) at each degassing silo (2) for supplying degassing air (23, 24), c. a feed unit (8) connected to the respective degassing silo (2) by means of a drop line (7) for feeding the plastic granules into a conveying line (10), d. a conveying pressure unit (11) for generating a conveying pressure (pF) in the conveying line (10) that is greater than a silo pressure (pS), e. a leakage air discharge unit (18) for discharging leakage air from the feed unit (8) as degassing air (24) into the respective degassing silo (2), characterized by: f. a retention element (25) arranged at the air connection (13) to prevent the plastic granules from flowing back into the degassing air (23),g.a separate air supply unit (14) for supplying at least one air connection (5, 13) with degassing air (23, 24), h. a degassing air silo distributor (16) for the controlled supply of degassing air from a degassing air line (15) connected to the air supply unit (14) into the respective degassing silo.

Description

The invention relates to a conveying system for plastic granules and a method for conveying plastic granules. In the production of plastic granules, degassing serves to separate volatile components that can, for example, cause unpleasant odors and / or pose a safety risk, such as in the case of improper storage of the plastic granules. The plastic granules are fed into a conveying line via a feed unit during and after degassing. The rising leakage air generated in the feed unit is collected in a leakage gas collection tank located above the feed unit to prevent a drop in the conveying capacity of the plastic granules. Such a leakage gas collection tank is known from EP 0 075 899 A2. It has been found that the placement of a leakage gas collection tank on a degassing silo can cause leakage air to enter the leakage gas collection tank.This allows so much plastic granulate to enter the leakage gas collection tank that its function is impaired or completely lost. This, in turn, affects the conveying capacity of the conveyor system. GB 2 271 114 A discloses a method for removing non-polymerized monomers. DE 10 2004 044 586 A1 discloses a device for tempering bulk material. AT 505 872 B1 discloses a material separator for conveying systems. The object of the present invention is to improve the feeding of plastic granules from a degassing silo into a conveying line and, in particular, to enable the reliable and fault-free removal of leakage gas from the feed unit. Specifically, it aims to prevent leakage air from escaping. This problem is solved according to the invention by a conveying system with the features specified in claim 1 and by a method with the features specified in claim 8. According to the invention, it has been found that a leakage gas collection tank can be dispensed with if leakage air from a feed unit, in particular a rotary valve, is supplied as degassing air to a connected degassing silo via a leakage air discharge unit. In particular, the function of the conveying system is not impaired by the absence of a leakage gas collection tank and is even improved. The leakage air from the rotary valve consists in particular of displacement air, which is displaced into the empty rotary valve chamber by incoming plastic granules, air lost through gaps on the sides opening and closing to the leakage air, and suction air. Suction air is defined as the air that is drawn from the conveying line into the empty chamber of the rotary valve.The side facing the leakage air is defined as the side perpendicular to the rotary axis of the rotor, located between the discharge and inlet chutes in the direction of rotation of the rotor. Similarly, the side facing away from the leakage air is located between the inlet and discharge chutes in the direction of rotation of the rotor. The degassing silo has at least one air connection. The plastic granules are stored and degassed in the degassing silo. The plastic granules used are primarily polyolefin granules, specifically granules made of polyolefin elastomer (POE), ethylene-vinyl acetate copolymer (EVA), peroxide-treated polypropylene (PP), and / or low-density polyethylene (LDPE, VLDPE). These polyolefin granules are elastic and / or soft. To prevent bridging above the discharge of the degassing silo, the granules are continuously circulated and / or conveyed in a closed loop.During degassing, degassing air is blown into the degassing silo to dilute and remove monomers that escape from the plastic granules. Degassing air is supplied for a degassing period of, for example, one to sixty hours, both while and after the silo is filled. The degassing duration depends primarily on the product temperature and the temperature of the degassing air during the process. The warmer the product, the faster the residual monomers escape. The degassing temperature is typically between 20 °C and 80 °C. The volumetric flow rate of the degassing air is, for example, between 1 and 50 Nm³ / h per 1,000 kg of plastic granules in the degassing silo. The standard volumetric flow rate is specified according to DIN 1343 at a temperature of 0 °C and a pressure of 1.01325 bar. The degassing air is fed into the degassing silo at a feed pressure between 30 and 300 mbar. The feed unit is connected to the degassing silo via a gravity drain. The gravity drain is typically vertically oriented, meaning it has an angle of inclination of 90° relative to the horizontal. It is also conceivable that the gravity drain could be arranged at a different angle of inclination relative to the horizontal, with the angle of inclination being, in particular, at least 30°, at least 45°, at least 60°, at least 75°, at least 80°, and at least 85°. The essential point is that the angle of inclination of the gravity drain is large enough to ensure the automatic, gravity-driven conveyance of the plastic granules along the drain.Because the downpipe provides a direct connection between the feed unit and the degassing silo—that is, the downpipe is connected directly to the degassing silo at one end and directly to the feed unit at the other—the pipe diameter for the downpipe can be flexibly determined to suit the conveying conditions. In particular, it is not necessary to match the downpipe diameter to the leakage gas collection tank. The inner diameter of the downpipe can be smaller than, equal to, or larger than the connection diameter of the feed unit. The plastic granules can travel from the degassing silo to the feed unit via the downpipe. The feed unit serves to feed the plastic granules into a conveying line. The feed unit also serves to separate the conveying air from the degassing air.This separation is particularly necessary because the conveying pressure at which the plastic granules are transported in the conveying line is greater than the gas pressure in the silo cone and the downpipe. For the sake of simplicity, this pressure will be referred to as "silo pressure" in the following, although a uniform, and especially homogeneous, silo pressure is not present within the silo. As the plastic granules flow through the degassing silo, a pressure drop develops, which depends on the gas velocity and thus on the amount of gas being fed in. A comparatively high gas pressure results in the silo cone due to the bulk of the plastic granules, with the gas pressure decreasing upwards towards the so-called silo head. By deliberately omitting a leakage gas collection tank, stray gas flows are avoided.Impairment of the conveying system's functionality by a leakage gas collection tank filled with plastic granules is ruled out. Reliable removal of leakage air directly from the feed unit is ensured via the leakage air extraction unit. The leakage air extraction unit primarily serves as a housing vent for the rotary valve. A container with a storage volume between 20 m³ and 2,000 m³, particularly between 50 m³ and 1,000 m³, serves as the degassing silo. A retaining element at the air inlet of the degassing silo reliably prevents plastic granules from unintentionally escaping the silo via the air inlet and, in particular, from clogging the degassing air line. The retaining element is located inside the degassing silo and is typically made of a screen fabric or wire mesh. A separate, and in particular centrally located, air supply unit for supplying at least one air connection with degassing air improves the provision of degassing air. In particular, the degassing of the plastic granules in the degassing silo is essentially independent of the leakage air used for degassing. This ensures continuous and independent degassing even when no leakage air occurs, for example, when the plastic granules are not intended to be fed into the conveying line. A degassing air silo distributor ensures a controlled extraction of degassing air from the degassing air line as well as the supply of degassing air from the degassing air line into the degassing silo. A leakage air extraction unit according to claim 2 enables targeted, and in particular variable, extraction of leakage air from the feed unit. An internal leakage air extraction line is arranged within the downpipe and, in particular, integrated into the downpipe. Specifically, the downpipe itself can be used as the internal leakage air extraction line, with plastic granules from the degassing silo falling downwards along the downpipe into the feed unit due to gravity. A portion of the leakage air, in particular displacement air and leakage air due to gap losses at the discharge end, from the feed unit flows against gravity through the downpipe as the internal leakage air extraction line, towards the plastic granules in the degassing silo. A lower discharge opening of the degassing silo serves as the air connection for supplying the degassing air to the internal leakage air extraction line.Additionally or alternatively, at least one external leakage air discharge line can be provided, located outside the downpipe. The external leakage air discharge line connects the feed unit directly to the air connection on the degassing silo. Plastic granules, which are ejected from the feed unit along with the leakage air by the pressure release, can be conveyed back into the degassing silo via the internal and / or external leakage air discharge line. Because the plastic granules can return to the degassing silo unimpeded via the leakage air discharge lines, it is prevented that the plastic granules unintentionally fall back into the feed unit and potentially clog it and / or the leakage air discharge line. It is essential that the degassing air line is kept clear of the plastic granules.A hat screen or perforated plate can be arranged at the air connection on the degassing silo to prevent the plastic granules from the degassing silo from entering the degassing air line. An external leakage air discharge line from a rotary valve is known from EP 1 832 404 A1 for conveying and degassing powder material using nitrogen. The air connection on the degassing silo for the external leakage air discharge line is located, in particular, on an outer cylindrical wall of the degassing silo and / or on a lower conical section of the degassing silo. The leakage air can be supplied to several injection points, distributed horizontally and / or vertically. The injection points are air connections. A distributed arrangement of the injection points in the area of ​​the conical section of the degassing silo is particularly advantageous. Multiple air connections according to claim 3 enable a targeted supply of leakage air as degassing air. Vertically spaced air connections according to claim 4 enable the supply of degassing air at different height levels of the degassing silo. Additionally or alternatively, a uniform distribution of the degassing air along the circumference of the degassing silo can be achieved by arranging the injection points at intervals in a horizontal plane. A control unit according to claim 5 enables the controlled generation of degassing air for the at least one degassing silo. A temperature control unit according to claim 6 enables targeted control of the degassing rate. The warmer the temperature of the degassing air and / or the conveying gas, the higher the degassing rate. An upper limit to the degassing rate is reached, depending on the type of plastic granules, for example, when oxidation processes occur. This can happen, for instance, at degassing temperatures of at least 80 °C. The process for conveying plastic granules essentially offers the advantages of the conveying system described here. Leakage air is returned to the degassing silo, for example, partially via the downpipe, primarily as degassing air. Additionally, degassing air can be supplied as leakage air via air connections located, for example, in the lower conical area of ​​the degassing silo and / or on an outer cylindrical wall of the degassing silo. Alternatively, an external leakage air discharge line can also be connected to the silo head, particularly in the area of ​​an upper end face of the degassing silo. A controlled supply of degassing air according to claim 9 enables targeted control of the degassing process in the degassing silo. Taking into account the degassing state in the degassing silo according to claim 10 for a time-varying and / or spatially varying and / or flow-rate-varying feed enables specific control of a degassing process, particularly one that is locally inhomogeneous and varies over time. In particular, this makes it possible to ensure a minimum quantity of degassing air in the degassing silo in order to comply with a lower explosive limit (LEL), which is to be selected as less than 20% to 50%, and in particular 25% to 30%, for degassing silos. This reduces the safety risk for conveying plastic granules, especially during degassing.To increase safety, if a monitored limit value of the minimum quantity of degassing air in the degassing silo is reached or exceeded, the degassing silo can be flooded with nitrogen or another inert gas to prevent extrusion and / or fire. A temperature control system according to claim 11 enables the targeted temperature control of degassing air and / or conveying gas. This temperature control can include both heating and cooling. A method according to claim 12 enables the advantageous conveying of plastic granules. The plastic granules have a particle size distribution, i.e., a grain size distribution, between 1.0 mm and 5.0 mm. The plastic granules have a particle diameter equivalent to that of a sphere of the same volume with a diameter of 1.0 mm to 5.0 mm. In particular, the grain size distribution is so homogeneous that the maximum deviation of the particle diameters, i.e., the grain sizes, is at most 1.0 mm. An embodiment of the invention is explained in more detail below with reference to the drawing. In this drawing: Fig. 1 shows a schematic overall representation of a conveying system according to the invention, and Fig. 2 shows an enlarged view of a degassing silo with a feed unit and a leakage air extraction unit. A conveying system shown in Figures 1 and 2, represented as a whole by 1, serves for degassing and conveying plastic granules. The conveying system 1 has several degassing silos 2, five in the illustrated embodiment. The degassing silos 2 can be identical and, in particular, have identical capacities. Fewer, but at least one, or more than five degassing silos, in particular up to 30 or more, can also be provided. The degassing silos 2 can also be of different designs, in particular with different capacities. The degassing silos 2 of the conveying system 1 shown differ, in particular, in the design of a leakage air extraction unit, which will be explained in more detail below. Each degassing silo 2 is connected to a plastic granule production unit (not shown). In the plastic granule production unit, a plastic strand is extruded, granulated, and dried in a drying unit. The resulting plastic granules are then conveyed to the degassing silos 2. The degassing silos 2 have a longitudinal axis 3 that is oriented vertically. Perpendicular to the silo's longitudinal axis 3, the degassing silo 2 has a circular cross-sectional area. The degassing silo 2 has a lower conical section 4 with a discharge opening 5, which is arranged, in particular, concentrically to the silo's longitudinal axis 3 and which can be closed by means of a manually operated closure element 6. The closure element 6 is, in particular, designed as a slide or flap. The closure element 6 can also be pneumatically operated. A discharge pipe 7 is connected to the discharge opening 5, directly connecting the degassing silo 2 to a rotary valve 8. The rotary valve 8 is rotatable by means of a drive 9. The rotary valve 8 is a feed unit. The rotary valve 8 serves to feed the plastic granules into a conveying line 10. The conveying line 10 is pressurized with conveying gas, in particular conveying air. The conveying air is supplied at a conveying pressure pF. The conveying pressure pF is generated by means of a conveying pressure unit 11. Along the conveying line 10, the plastic granules are conveyed to one or more destination locations 12. In degassing silo 2, the plastic granules are stored at a silo pressure pS, which is generated by the injection of degassing air. The plastic granules are degassed in degassing silo 2 using degassing air. Several air connections 13 are provided on the outer cylindrical wall and in the conical section 4 of the degassing silo to supply degassing air. The conveying pressure pF is greater than the silo pressure pS. The conveying pressure pF ranges from 0.3 barg to 4 barg. The silo pressure pS in the degassing silo is approximately between 30 mbar and 300 mbar. A separate air supply unit 14, connected to the air connections 13 via a degassing air line 15, serves to supply the air connections 13 with degassing air. At least one degassing air silo distributor 16 is connected to each degassing air line 15, by means of which the degassing air is supplied to the degassing silo 2 and / or the downpipe 7 in a controlled manner. The system includes the allocation of degassing air to the individual degassing silo 2 and the distribution of the degassing air to different levels within the degassing silo 2 to direct the degassing air to the respective air connections, also referred to as injection points. To prevent backflow of plastic granules into the degassing air, a retention element 25, such as check valves, hat screens, wire mesh, perforated plates, and / or screen fabric, may be provided. Screen fabric and wire mesh are particularly advantageous due to their lower pressure losses and simplified assembly and disassembly. These retention elements 25 are specifically integrated into and / or on the air connection 13. If the degassing air also contains leakage air, the retention elements 25 are not provided, as plastic granules may also be present in the leakage air. The air supply unit 14 can be configured as a single screw compressor, a rotary lobe blower (a so-called "Roots blower"), or a compressor. If the air supply unit 14 is connected to several degassing silos 2, it can include multiple screw compressors, rotary lobe blowers, and / or compressors, which are interconnected to form a network. Depending on the current air demand, the individual components can be switched on or off. A compressor, in particular a flow compressor or a forced-action compressor, in particular a fan, a turbo compressor, or a piston compressor, can additionally have its gas delivery volume continuously adjusted, in particular controlled, in particular regulated, by means of a frequency converter. It is also conceivable that the compressors are each assigned to a degassing silo 2. The air supply unit 14 has an integrated temperature control unit 17, which serves to temper the degassing air. The temperature control unit 17, or a further temperature control unit, can be provided for tempering the conveying gas. In particular, the further temperature control unit is connected to or integrated into the conveying pressure unit 11. A leakage air discharge unit 18 is connected to the rotary valve 8, which has an internal leakage air discharge line 20 and / or one or more external leakage air discharge lines 19. In the degassing silo 2 shown on the left in Fig. 1, the internal leakage air discharge line 20 is integrated into the downpipe 7. In particular, the downpipe 7 is simultaneously the internal leakage air discharge line 20. The external leakage air discharge line 19 allows leakage air from the rotary valve 8 to be supplied to the air connections 13. The supply of leakage air via the air connections 13 serves to support the actual degassing process using degassing air from the central air supply unit 14. The air connections 13 can be supplied with degassing air from the central air supply unit 14 via the degassing air line 15. The degassing air causes the actual degassing of the plastic granules in the degassing silo 2. The additional leakage air can advantageously be used as degassing air. The volume flow rate of the leakage air is small compared to the volume flow rate of the degassing air. The ratio of the volume flow rates of leakage air to degassing air is between 1% and 50%, particularly between 3% and 40%, and especially between 5% and 30%. Leakage air, which can be used as degassing air, is generated, particularly during product conveying.Leakage air increases plant safety. The air connections 13 are arranged vertically spaced along the longitudinal axis 3 of the silo. Additionally, several air connections 13 are provided in a plane perpendicular to the longitudinal axis 3 of the silo, particularly at equal intervals around the outer circumference of the degassing silo 2. In the degassing silo 2 shown in Fig. 1 in the second position from the left, an external leakage air discharge line 21 is provided to supply leakage air from the rotary valve 8 to the degassing silo 2 in an upper area. A central control unit 22 enables the controlled generation of degassing air into the degassing silos. For this purpose, the control unit 22 is in signal communication with the air supply unit 14 and the degassing silos 2. For illustrative purposes, Fig. 1 shows only one signal connection from the control unit 22 to one degassing silo 2. This signal connection can be wired or wireless. The control unit 22 is also in signal communication with the degassing air silo distributors 16 to enable the time-varying, spatially variable, and / or flow-rate-varying injection of degassing air into the respective degassing silo 2 via valves (not shown), frequency converters, and / or compressors that are switched on or off. It has proven advantageous if the targeted injection of degassing air occurs depending on the degassing state in the degassing silo 2. For example, the amount of degassing air can be increased over time and later decreased again. For this purpose, the supply of degassing air at individual air connections 13, which are grouped together in a feed ring at different levels perpendicular to the longitudinal axis of the silo, can be switched on or off.It is advantageous to switch an upper feed ring on or off, since the gas volume in a lower section of degassing silo 2 is distributed across the cross-section of the silo. Sensors (not shown) can be installed in degassing silo 2 as input variables for controlling the degassing air volume, particularly for measuring temperature, pressure, and / or the composition of volatile components. The degassing duration is of particular importance. Furthermore, it is possible to perform a low-emission gas (LEG) measurement in the exhaust air from degassing silo 2. It is advantageous to use sensors to measure the air volume in the degassing silo, a sensor to measure the CO2 content, and / or a temperature sensor in the exhaust air. The measurement results can be provided to the control unit 22 as input data. The degassing of the plastic granules in the degassing silo 2 of the conveying system 1 is explained in more detail below with reference to Fig. 2. Degassing air, supplied primarily by the central air supply unit 14 via the degassing air line 15, is used to pressurize the plastic granules in the degassing silo 2. The degassing air is represented by the flow arrows 23 in Fig. 2. The degassing air 23 is used to degas the plastic granules, particularly during and after filling the degassing silo 2. The separated monomers are then discharged from the degassing silo via an exhaust air line at the top and purified. During the operation of the conveying system 1, plastic granules from the degassing silo 2 are discharged by gravity via the drop line 7 into the rotary valve 8 and from there fed into the conveying line 10. The conveying line 10 is pressurized by the conveying pressure unit 11 with conveying air at a conveying pressure pF, which is greater than the silo pressure pS. Leakage air is generated in the rotary valve 8, which is indicated by the flow arrows 24 in Fig. 2. The leakage air 24 flows as additional degassing air, partially via the internal leakage air discharge line 20, i.e., against the product flow direction in the drop line 7, upwards through the discharge opening 5 into the degassing silo 2. The discharge opening 5 forms an air connection for the supply of degassing air. The proportion of leakage air 24 that flows upwards through the downpipe 7 corresponds to approximately 20% to 50% of the leakage air.The remaining portion of the leakage air 24 flows via the external leakage air discharge line 19 outside the downpipe 7 to the degassing silo 2 and in this variant is fed directly to the degassing silo 2 at the various air connections 13, in particular in a targeted, individual or distributed manner, in particular in a controlled manner. Instead of directly supplying the leakage air 24 to the air connections 13 in the cone area or on the cylindrical surface of the degassing silo 2, the leakage air 24 can also be returned to the top area of ​​the degassing silo 2 via the external leakage air discharge line 21. In this configuration, the external leakage air line 21 is connected, in particular, in an area on an upper end face of the degassing silo 2.

Claims

Conveying system for plastic granules comprising: a. several degassing silos (2) for storing and degassing the plastic granules, b. an air connection (5, 13) at each degassing silo (2) for supplying degassing air (23, 24), c. a feed unit (8) connected to the respective degassing silo (2) by means of a drop line (7) for feeding the plastic granules into a conveying line (10), d. a conveying pressure unit (11) for generating a conveying pressure (pF) in the conveying line (10) that is greater than a silo pressure (pS), e. a leakage air discharge unit (18) for discharging leakage air from the feed unit (8) as degassing air (24) into the respective degassing silo (2), characterized by: a retention element (25) arranged at the air connection (13) to prevent the plastic granules from flowing back into the degassing air (23),g.a separate air supply unit (14) for supplying at least one air connection (5, 13) with degassing air (23, 24), h. a degassing air silo distributor (16) for the controlled supply of degassing air from a degassing air line (15) connected to the air supply unit (14) into the respective degassing silo. Conveyor system according to claim 1, characterized in that the leakage air removal unit (18) has an inner leakage air removal line (20) inside the drop pipe (7) and / or at least one outer leakage air removal line (19, 21) outside the drop pipe (7). Conveyor system according to one of the preceding claims, characterized by several air connections (5, 13) which are connected to a leakage air removal unit (18). Conveyor system according to claim 3, characterized in that at least two air connections (13) are arranged vertically and / or horizontally spaced apart from each other. Conveyor system according to one of the preceding claims, characterized by a control unit (22) for the controlled generation of degassing air (23) into the degassing silo (2). Conveyor system according to one of the preceding claims, characterized by a temperature control unit (17) for temperature control of the degassing air (23, 24) and / or the conveying gas. Conveyor system according to one of the preceding claims, characterized in that the retention element (25) is arranged inside the degassing silo (2) and / or the retention element (25) is designed as a screen fabric or wire mesh. A method for conveying plastic granules comprising the process steps of: - providing the plastic granules in one of several degassing silos (2), - feeding plastic granules into a conveying line (10) by means of a feed unit (8), wherein the feed unit (8) is connected to the degassing silo (2) via a drop line (7), - generating a conveying pressure (pF) in the conveying line (10) by means of a conveying pressure unit (11), wherein the conveying pressure (pF) is greater than the silo pressure (pS), - removing leakage air (24) from the feed unit (8) by means of a leakage air removal unit (18), - supplying the leakage air as degassing air (24) into the degassing silo (2) by means of an air connection (5, 13) on the degassing silo (2), wherein at least one air connection (5, 13) is supplied with degassing air (23, 24) by means of a separate air supply unit (14) is charged,wherein the degassing air is supplied in a controlled manner from a degassing air line (15) connected to the air supply unit (14) into the degassing silos by means of a degassing air silo distributor (16), - preventing backflow of the plastic granules into the degassing air (23) by means of a retention element (25) arranged at an air connection (13) of the degassing silo (2). Method according to claim 8, characterized in that the degassing air (23) is fed into the at least one degassing silo (2) in a controlled manner by means of a control unit (22). Method according to claim 8 or 9, characterized in that the feed-in is variable over time and / or spatially and / or in terms of quantity flow depending on the degassing state in the degassing silo (2). Method according to one of claims 8 to 10, characterized in that the degassing air (23, 24) and / or the conveying gas is tempered by means of a tempering unit (17). Method according to one of claims 8 to 11, characterized in that the plastic granules have a particle size distribution between 1.0 mm and 5.0 mm, wherein in particular a maximum deviation of the particle sizes is 1 mm.

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