Method and apparatus for drying plastic granules

Abstract

Method for drying plastic granules, encompassing the process steps - Pre-drying of the plastic granules in a pre-drying unit (5), - Conveying the pre-dried plastic granules by means of a conveying / heating unit (9) into a post-drying unit (22), wherein the plastic granules are mixed with heating water for heating, - Post-drying of the pre-dried plastic granules, heated only on the surface, in the post-drying unit (22), characterized in that - the plastic granules have a temperature of no more than 40 °C before heating, - a heating time is less than 60 s, so that only the surface of the plastic granules in the conveying / heating unit (9) is heated.

Description

[0001] The invention relates to a method and a device for drying plastic granules.

[0002] In the production of plastic granules, for example in a granulation process, the plastic is cooled with water, either as a strand or already pelletized, which prevents the granules from sticking together. Depending on the temperature of the granules, the subsequent drying process, typically in a centrifugal dryer, is insufficient. The residual moisture content of the granules is elevated. A typically desired maximum residual moisture content can only be achieved with complex drying processes, such as in large fluidized bed dryers. Alternatively, the granulation step would have to be carried out with a higher cooling water temperature, which reaches its limits, especially with polymers that have a low softening point.

[0003] Methods for treating plastic granules are known from DE 20 2004 017 275 U1, DE 19 05 677 A and US 2003 / 0109640 A1.

[0004] DE 10 2012 011 641 A1 discloses a method and a device for drying and crystallizing granules.

[0005] US 2008 / 0185758 A1 discloses a granule treatment unit.

[0006] The object of the present invention is to simplify the drying of plastic granules.

[0007] This problem is solved according to the invention by a method having the features specified in claim 1.

[0008] According to the invention, it was discovered that a drying process is more effective when plastic granules are heated and, in particular, when they have an elevated temperature, at least on the surface. The inherent heat of the plastic granules makes it possible to achieve a lower residual moisture content. As the temperature on the granule surface increases, the vapor pressure of the water rises, resulting in faster evaporation. The rising temperature also reduces the viscosity of the water, leading to improved mechanical dewatering performance. In a pre-drying unit, a mixture of water and plastic granules is pre-dried, i.e., dewatered, before heating, so that the residual surface moisture content, based on the mass, is less than 10%. The pre-dried, i.e., dewatered, plastic granules have a reduced water content.This reduces the energy required for the subsequent heating of the plastic granules using a conveying / heating unit. The conveying / heating unit transports the pre-dried, heated plastic granules to a post-drying unit, where they are dried further as described above. The inventive method ensures that the plastic granules do not clump together and also guarantees high dewatering performance. The heating of the pre-dried plastic granules and the subsequent post-drying are essential. In particular, it is possible to use relatively simple drying units for pre- and / or post-drying. Specifically, the use of complex, large fluidized bed dryers is unnecessary. Nevertheless, the inventive method is also suitable for fluidized bed dryers.The conveying / heating unit and the post-drying unit can be arranged very compactly and, in particular, directly adjacent to each other. The space required for such a device is minimal. Because the plastic granules are only in contact with the heating water for a short time, the risk of the plastic granules absorbing water, at least partially, is reduced. During post-drying, the surface water is also spun off along with any adhering plastic dust. The heating of the plastic granules, and especially their surface, according to the invention increases the vapor pressure of the adhering surface water and thus improves the drying of the plastic granules in the airflow. As a positive side effect, the plastic granules are immediately cooled again by the evaporation of the surface water.By only superficially heating the plastic granules and cooling them during the final drying phase, the risk of clumping due to warm granules after drying is significantly reduced. An additional cooling process is usually unnecessary.

[0009] Gravity conveying is particularly advantageous, as it allows the conveying / heating unit to be located directly or almost directly upstream of the drying unit.

[0010] In this process, the plastic granules are mixed with a heating medium for heating. This enables efficient heat transfer. Heating water is used as the heating medium. Depending on the process being carried out, and especially on the type of plastic granules being heated, the heating water has a temperature between 25°C and 95°C, and particularly between 40°C and 70°C.

[0011] The mixing with warm heating water to heat the plastic granules is carried out primarily gravimetrically. This prevents the granules from being subjected to an additional mechanical and / or pneumatic conveying step. This also prevents additional abrasion of the plastic granules.

[0012] Before heating, and especially before the plastic granules are conveyed into the pre-drying unit, they have a temperature of no more than 40°C, particularly no more than 30°C, and particularly no more than 20°C. This process reduces the risk of the granules sticking together.

[0013] A process in which the plastic granules are produced in a granulation process, particularly underwater pelletizing, enables straightforward and direct hydraulic conveying of the plastic granules, especially in cold water. Production is efficient and reliable. In particular, direct hydraulic conveying of the produced plastic granules to the pre-drying unit is possible. The pelletizing process can be carried out inline, with a pelletizing unit directly connected to the pre-drying unit via a conveying line. Alternatively, the plastic granules produced in the pelletizing process can be stored, for example, in a buffer tank connected to the pre-drying unit. This buffer tank can be fed directly from the pelletizing unit and / or from external sources.It is particularly conceivable that the plastic granules are produced at a first location by granulation and transported by means of transport vehicles or a transport pipeline to the buffer container located at a second location. The first and second locations can be several kilometers apart, in particular several hundred kilometers.

[0014] A process in which the plastic granules, after pre-drying in the pre-drying unit, have a residual surface moisture content of less than 10% relative to their mass, in particular less than 5% and in particular less than 1%, ensures that the subsequent heating for post-drying is energy-efficient.

[0015] A process in which the plastic granules, with a softening temperature below 60°C, particularly below 50°C and especially below 40°C, are briefly and, in particular, superficially heated before post-drying, ensures a lower temperature of the plastic granules at the process outlet, thus reducing clumping. Ethylene vinyl acetate (EVA) with a vinyl acetate content of more than 28% is suitable for this process, for example. Alternative polymers include, for example, thermoplastic elastomers (TPE), in particular olefin-based thermoplastic elastomers (TPE-O), cross-linked olefin-based thermoplastic elastomers (TPE-V), urethane-based thermoplastic elastomers (TPE-U), thermoplastic polyester elastomers (TPE-E), styrene block copolymers such as SBS, SEBS, SEPS, SEEPS and MDS (TPE-S) and / or thermoplastic copolyamides (TPE-A), as well as polyolefin elastomers (POE).

[0016] The inventive method heats the plastic granules from the granulation process in a comparatively short time, since the heat transfer from the heating water to the plastic granules occurs within a few seconds for the usual granule size of no more than 5 mm in diameter. The heating time is less than 60 s, in particular less than 40 s, and especially less than 20 s. The rapid heating of the plastic granules with the heating medium ensures that only the surface of the plastic granules heats up. In particular, complete heating of the plastic granules is neither necessary nor desirable.

[0017] A process in which the plastic granules, after post-drying in the post-drying unit, have a residual surface moisture content relative to their mass of less than 0.1%, in particular less than 0.05% and especially less than 0.01%, is well suited for further processing.

[0018] Another object of the present invention is to improve a device for drying plastic granules.

[0019] This problem is solved according to the invention by a device for drying plastic granules with the features specified in claim 8.

[0020] According to the invention, a conveying / heating unit is provided between a pre-drying unit and a post-drying unit for plastic granules. This conveying / heating unit transports pre-dried plastic granules into the post-drying unit and heats them. The conveying / heating unit according to the invention is very compact. The advantages resulting from this design essentially correspond to the advantages of the method, to which reference is hereby made.

[0021] A device in which the pre-drying unit is designed either for static pre-drying, in particular as a dewatering screen, or for mechanical pre-drying, in particular as a screening machine or centrifugal dryer, especially without a fan, enables uncomplicated and cost-effective pre-drying. Because a reduced dewatering capacity is sufficient for pre-drying, complex pre-drying units are unnecessary.

[0022] In the device according to the invention, the conveying / heating unit has a drop line that connects the pre-drying unit with the post-drying unit arranged below it, and enables uncomplicated, in particular gravimetric and especially exclusively gravimetric, conveying of the pre-dried plastic granules. Additional conveying elements for pneumatic and / or mechanical conveying processes are unnecessary.

[0023] A device in which the downpipe is arranged at an angle to the vertical ensures reliable gravimetric conveyance. The angle of inclination is between 0° and 90°, and particularly between 30° and 45°.

[0024] A device with at least one mixing element arranged in the downpipe ensures a homogeneous mixture of the plastic granules with the heating medium, in particular heating water. Guide plates can be used as mixing elements.

[0025] A device with at least one drive connected to the downpipe enables targeted assistance in conveying the plastic granules. The drive can be designed, in particular, as a vibratory drive, which is located outside the downpipe and connected to it to transmit vibration to the downpipe for conveying the plastic granules. By means of this vibration in the downpipe, the residence time of the plastic granules, and thus their heating, can be precisely controlled.

[0026] A device in which the post-drying unit is designed as a centrifugal dryer enables effective and uncomplicated, i.e., cost-effective, post-drying.

[0027] Exemplary embodiments of the invention are explained below with reference to the drawings. These show: Fig. 1 a schematic representation of a device for drying plastic granules, Fig. 2 to 7 Exemplary embodiments of a conveying / heating unit in the device according to Fig. 1.

[0028] One in Fig. 1 The device marked as a whole with 1 is used for drying plastic granules.

[0029] The device 1 comprises an extruder 2 for producing plastic extrudate and an associated granulation unit 3 for granulating the plastic extrudate. The granulation unit 3 is connected via a conveying line 4 to a pre-drying unit 5. The pre-drying unit 5 is designed as a static separator with a dewatering screen 6. Separated water can be discharged from the pre-drying unit 5 via a water discharge line 7. The pre-drying unit 5 is connected via a product line 8 to a conveying / heating unit 9, which is described below based on Fig. 2 will be explained in more detail.

[0030] The conveying / heating unit 9 is connected via a supply line 10 to a water separator 11, which is designed as an inclined perforated plate. The water separator 11 is optional and can therefore be omitted. The water separator 11 is connected to a water circuit line 12, to which a filter 13, a buffer 14, a circulation pump 15, and a heat exchanger 16 are connected. The water circuit line 12 opens into the conveying / heating unit 9 at an end opposite the water separator 11, in particular adjacent to the product line 8. In the conveying / heating unit 9, plastic dust is washed off, most of which is carried away from the water separator 11 with the heating water and filtered out in the filter 13.

[0031] The conveying / heating unit 9 is connected to a post-drying unit 22 via the feed line 10 and the optional water separator 11. The post-drying unit 22 is designed as a centrifugal dryer. A discharge pipe 23 for discharging plastic granules is connected to the post-drying unit 22. The centrifugal dryer has a rotary drive 24, which enables the rotation required for centrifugal drying. The drying process is supported by an additional airflow 25. The airflow 25 can be drawn in at the top of the post-drying unit 22. The airflow can be extracted from the post-drying unit 22 by the blower 26.

[0032] The following will be based on the Fig. 2. The conveying / heating unit 9 is explained in more detail. The conveying / heating unit 9 is designed as an inclined pipe. At an upper, in Fig. The water circuit line 12 is connected at the end shown on the left. Adjacent to the water circuit line 12, the product line 8 is connected via a vertical nozzle 17. At a lower, in Fig. At the end shown on the right, the outlet of the inclined pipe is connected to the supply line 10. A vent line 21 is arranged in the area of ​​the outlet to the supply line 10. A vibratory motor 18 is connected to the pipe on one underside. The vibratory motor 18 enables vibration excitation of the inclined pipe transversely and, in particular, perpendicularly to the pipe's longitudinal axis 19. The pipe's longitudinal axis 19 is arranged at an angle n to the vertical. The angle of inclination is preferably between 30° and 45°. The pipe of the conveying / heating unit 9 is a downpipe 20.

[0033] The following describes a process for drying plastic granules. Extrudate produced in extruder 2 is granulated in the granulation unit 3 and conveyed hydraulically via conveying line 4 to the pre-drying unit 5. The solids concentration in the mixture of plastic granules and water is between 3% and 35% before pre-drying unit 5. In pre-drying unit 5, water is separated and discharged via water discharge line 7. The pre-dried plastic granules have a residual surface moisture content of less than 10% by mass. The pre-dried plastic granules have a temperature of less than 40°C and are fed by gravity via product line 8 to the conveying / heating unit 9.

[0034] In the conveying / heating unit 9, the plastic granules enter the downpipe 20 via the nozzle 17. The plastic granules are carried along by heated water, which is supplied to the downpipe 20 via the water circuit line 12. Within a few seconds, the heated water raises the temperature of the plastic granules from an initial temperature of less than 40°C to an elevated temperature, which can be adjusted depending on the application. According to the illustrated embodiment, the elevated temperature is at least 70°C. This temperature of at least 70°C is present at least on the surface of the plastic granules. The heated water thus serves to heat and convey the plastic granules. Simultaneously, the heated water also serves to wash the plastic granules.

[0035] Rising air or gases can be selectively vented via the vent line 21. This ensures an uninterrupted heating and washing process in the conveying and heating unit.

[0036] The vibration motor 18 can be used to apply a vibration to the downpipe 20. This allows the conveying speed of the mixture of plastic granules and heating water to be precisely controlled. This, in turn, allows the residence time of the plastic granule and water mixture, and thus the elevated temperature when the plastic granules leave the conveying / heating unit and enter the feed line 10, to be precisely controlled.

[0037] The mixture of heating water and plastic granules is fed to the water separator 11. The separated water is filtered from the water separator 11 via the water circulation line 12 in the filter 13, temporarily stored in the buffer tank 14, circulated and heated by means of the circulation pump 15 and the heat exchanger 16, and fed back to the conveying / heating unit 9 as heating water.

[0038] The heated plastic granules are fed from the water separator 11 to the post-drying unit 22. The post-dried plastic granules are discharged via the discharge pipe 23 for further use. The post-dried plastic granules have a residual surface moisture content of less than 0.1% by mass.

[0039] Fig. Figure 3 shows a further embodiment of a conveying / heating unit. 9. Components corresponding to those described above with reference to the Fig. The elements already explained in sections 1 and 2 bear the same reference numerals and will not be discussed again in detail. The essential difference compared to the previous embodiment is that, instead of an inclined heating pipe, a static plastic granule heating water mixer 28 is provided, which comprises an inclined pipe. Static mixing elements 29 are provided on the inside of the pipe 20, which can, for example, be attached to an inner wall of the pipe, in particular by welding. The mixing elements 29 are arranged along the longitudinal axis 19 of the pipe at different inclinations. This ensures improved, and in particular homogeneous, mixing of the plastic granules with the heating water.

[0040] More intensive and therefore faster heating of the granules is possible. This means that the plastic-granule-heating water mixer can have a smaller size. The washing process is intensified. The cleaning result is improved.

[0041] The Fig. Figure 4 shows another embodiment of a conveying / heating unit 9, which is essentially the same as the embodiment of Fig. 3 corresponds to components that correspond to those mentioned above with reference to the Fig. Items 1 to 3, which have already been explained, bear the same reference numbers and will not be discussed again in detail. The main difference is that pipe 20 is vertically oriented. The plastic granules are fed to pipe 20 via the product line 8 at the end. The heating water is supplied via the water circuit line 12 at one side wall. The vent line 21 is located at the upper end where the return line 8 is connected to pipe 20.

[0042] The plastic granulate heating water mixers 28 according to Fig. 3 and Fig. Four of the 4 connections lead directly into the drying unit 22. This results in a very short residence time for the plastic granules in the heating water, specifically less than 60 seconds and less than 10 seconds. This ensures that the granules absorb a reduced amount of water and, in particular, no water that would subsequently be difficult and costly to remove.

[0043] The Fig. Figure 5 shows a non-inventive embodiment of a conveying / heating unit. 9. Components corresponding to those described above with reference to the Fig. The elements already explained in sections 1 to 4 bear the same reference numbers and will not be discussed again in detail. The essential difference compared to the previous embodiments is that the plastic granules enter a mixing tank 30 of the conveying / heating unit 9 directly via the product line 8 and the heating water via the water circuit line 12. The plastic granules fall into the mixing tank 30. The heating water is supplied via line 12. If the plastic granules have a higher density than water, they sink with the flow of heating water and are thereby heated immediately and rapidly. The volume of the mixing tank 30 and the flow rate of the heating water directly influence the residence time of the plastic granules in the heating water and thus the heating temperature to be set.A siphon 31 is provided on the underside of the mixing container 30 to prevent the mixing container 30 from emptying completely via the supply line 10.

[0044] The Fig. Figure 6 shows a non-inventive embodiment of a conveying / heating unit 9, which is essentially the embodiment according to Fig. 5 corresponds to components that correspond to those mentioned above with reference to the Fig. Items 1 to 5, which have already been explained, bear the same reference numbers and will not be discussed again in detail. The essential difference is that an agitator 32 is provided inside the mixing vessel 30 to improve the mixing of the plastic granules in the heating water. In particular, if the density of the plastic granules is lower than that of water, the agitator 32 ensures that the granules are sufficiently dispersed in the heating water flow.

[0045] The Fig. Figure 7 shows a non-inventive embodiment of a conveying / heating unit. 9. Components corresponding to those described above with reference to the Fig. Items 1 to 6, which have already been explained, bear the same reference numbers and will not be discussed again in detail. The essential difference compared to the previous embodiments, in particular the one according to Fig. 5. The feature consists of the siphon 31 being positioned upstream of the mixing tank 30. This ensures that the plastic granules, which have a lower density than water, are carried along by the flow of heating water and can thus enter the mixing tank 30. With this arrangement, an agitator is unnecessary for plastic granules that have a lower density than water.

Claims

[1] Method for drying plastic granules comprising the process steps - Pre-drying of the plastic granules in a pre-drying unit (5), - Conveying the pre-dried plastic granules by means of a conveying / heating unit (9) into a post-drying unit (22), wherein the plastic granules are mixed with heating water for heating, - Post-drying of the pre-dried plastic granules, which are only heated on the surface, in the post-drying unit (22), characterized by , that - the plastic granules have a temperature of no more than 40 °C before heating, - a heating time is less than 60 s, so that only the surface of the plastic granules in the conveying / heating unit (9) is heated. [2] Method according to claim 1, characterized by, that the plastic granules have a temperature of no more than 30 °C and in particular no more than 20 °C before heating, in particular before being conveyed into the pre-drying unit (5). [3] Method according to any of the preceding claims, characterized by that the plastic granules, especially inline, are produced in a granulation process. [4] Method according to any one of the preceding claims, characterized by , that after pre-drying in the pre-drying unit (5) the plastic granules have a surface residual moisture content of less than 10%, in particular less than 5% and in particular less than 1%, relative to their mass. [5] Method according to any one of the preceding claims, characterized by that the plastic granules have a softening temperature (T g ) exhibits a temperature that is less than 60 °C, in particular less than 50 °C and in particular less than 40 °C. [6] Method according to any one of the preceding claims, characterized by that the heating time is less than 40 s and in particular less than 20 s. [7] Method according to any one of the preceding claims, characterized by , that after post-drying in the post-drying unit (22) the plastic granules have a surface residual moisture content of less than 0.1%, in particular less than 0.05% and in particular less than 0.01%, relative to their mass. [8] Device (1) for drying plastic granules comprising, a. a pre-drying unit (5) for pre-drying the plastic granules, b. a conveying / heating unit (9) for conveying the dewatered plastic granules into a post-drying unit (22) and for heating only the surface of the plastic granules, c. the post-drying unit (22) for post-drying the pre-dried plastic granules, which have only been heated on the surface, d. a water circulation line (12) which is connected to the pumping / heating unit (9) for the supply of heating water, characterized by , that the conveying / heating unit (9) is a downpipe (20) which connects the pre-drying unit (5) with the post-drying unit (22) arranged below it. [9] Device (1) according to claim 8, characterized by that the pre-drying unit (5) is designed for static pre-drying, in particular as a dewatering screen (6), or for mechanical pre-drying, in particular as a screening machine or centrifugal dryer. [10] Device (1) according to claim 8 or 9, characterized by , that the downpipe (20) is arranged with an angle of inclination (n) relative to the vertical, wherein in particular: 0° < n < 90° and in particular 30° < n < 45°. [11] Device (1) according to any one of claims 8 to 10, characterized by at least one mixing element (29) arranged in the downpipe (20). [12] Device (1) according to any one of claims 8 to 11, characterized by at least one drive (18) connected to the drop pipe (20). [13] Device (1) according to any one of claims 8 to 12, characterized by that the drying unit (22) is designed as a centrifugal dryer.

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