Method and arrangement for recovering energy during extrusion

Abstract

The invention relates to a method for producing plastic granules (6), wherein a raw material (2) containing plastic is fed to an extruder (4), extruded in the plasticised state and granulated downstream of the extruder (4) in the flow direction, and wherein the material exiting the extruder (4) is brought into contact with a cold, liquid coolant (7), as a result of which the material is cooled and the coolant (7) is heated, and wherein the heated coolant (8) is cooled and then, as a cold coolant (7) again, is brought into contact with material exiting the extruder (4). According to the invention, the heated coolant (8) gives off heat to a heat pump (9) and is thereby cooled, a cold heat carrier (16) is likewise fed into the heat pump (9) and heated there, and the heated heat carrier (10) is cooled such that the heat given off by the heat carrier (10) in this respect is used as process heat or heating heat. The invention further relates to a system (1) for carrying out the method.

Description

[0001] "Method and arrangement for energy recovery during extrusion"

[0002] Description:

[0003] The invention relates to a method according to the preamble of claim 1.

[0004] Such a process is known in practice. In a granulator downstream of the extruder, the extruded material strand is granulated, for example, in the form of strand granulation, water-jet granulation, or underwater granulation. In addition to dividing the material strand emerging from the extruder into a multitude of pieces known as granules, the plastic material is also cooled in the granulator area, typically using a liquid coolant, although in practice, water is usually used as the coolant.

[0005] A model plant for the production of plastic granules, for example, has the following key data: an extruder produces 1 t of plastic material per hour. This material leaves the extruder at a temperature of approximately 200°C. It is cooled to approximately 40°C using a cooling liquid to reach its packaging temperature, at which the material can be packaged, for example, in big bags or octabins, and at which the individual granules cannot stick together due to their inherent residual heat. In the plant used for extrusion and granulation, water is used as the cooling liquid and circulated, for example, a water quantity of approximately 500 to 2,000 l.In order to cool the water, which is heated by the extruded plastic material, to a sufficiently low temperature that enables the desired cooling of the extruded material, a cooling capacity of approximately 100 to 140 kW must be installed in this model plant.

[0006] In addition, the raw material to be extruded is usually freed of adhering water before entering the extruder to prevent the formation of vapor bubbles as much as possible. In practice, it is observed that the delivered raw material can have a residual moisture content of approximately 10%. Drying is often carried out using a thermal dryer. Each liter of water to be removed requires an installed drying capacity of approximately 1 kW.

[0007] The production of plastic granules essentially consists of the two process steps of extrusion and subsequent granulation. In addition to the process energy required for these core processes, the production of plastic granules requires considerable additional energy for secondary processes, namely drying the raw material and cooling the extruded and granulated material.

[0008] The invention is based on the object of improving a generic process such that the production of plastic granulate can be carried out with the highest possible efficiency in relation to the required installed capacity and with the lowest possible environmental impact. Furthermore, the invention is based on the object of specifying a plant suitable for carrying out this process.

[0009] This object is achieved by a method according to claim 1 and by a system according to claim 13.

[0010] In other words, the invention proposes feeding the coolant, which has been heated in the granulator, to a heat pump. For this purpose, in one embodiment of the method, the coolant can be fed directly to the heat pump so that it can transfer its heat content directly to the heat pump. Alternatively, in another embodiment of the method, two heat transfer medium circuits can be used so that the coolant flowing through the granulator and guided in a first circuit can transfer its heat content indirectly to the heat pump, for example by connecting the heat pump to a heat exchanger that is integrated into the first circuit. The second circuit, namely the heat transfer medium flowing through the heat pump, can thus be kept free of loads that could potentially burden the first circuit, such as dirt loads or particles resulting from the granulation process.

[0011] Using a heat pump, a comparatively high temperature level of a heat transfer medium heated in the heat pump can be achieved with comparatively low energy expenditure, which would otherwise require a higher use of separate energy sources, for example through the combustion of finite raw materials such as gas or oil. Compared to the temperature of the heated coolant, the significantly higher temperature level of the heat transfer medium enables the heat to be used as process heat or heating heat. The process heat can, for example, be used in the proposed process itself, e.g. for drying the raw material used. The heat can be used in the form of convection and / or radiant heat, for example by heating the housing walls, rotor axis, paddle arms and / or paddles of the dryer, or by blowing hot air into the dryer.If a liquid heat transfer medium is used, the heat from the warm heat transfer medium can be used to directly heat dryer components, for example, by directing the liquid heat transfer medium into the housing walls, the rotor shaft, the paddle arms, and / or the paddles of the dryer. Alternatively, the heat from the warm heat transfer medium can be used to indirectly heat dryer components by transferring the heat contained in the liquid heat transfer medium to a gas, e.g., using an air / water heat exchanger, and then directing this gas into the respective dryer components to heat them.

[0012] In particular, if virgin plastic material is not used as the raw material for producing the granulate, but rather if the raw material contains recycled materials, namely recycled plastic, but also fibrous components, e.g., plant fibers, the resulting granulate has a lower economic value than granulate produced from virgin plastic material. The proposed process can reduce production costs because the process heat generated in the core processes can be used for secondary processes such as drying or cooling.

[0013] The proposed manufacturing process offers two advantages: environmental impact is reduced in that the waste heat from the extruded material is not simply released into the environment, for example, by passing the coolant through a cooler through which ambient air flows, and the coolant is cooled by releasing its heat into the ambient air. Rather, the heat absorbed by the coolant is harnessed by the proposed heat pump, as explained in more detail below. A second economic advantage can arise from the fact that the use of the heat contained in the coolant can eliminate or reduce the need for separately generated process heat elsewhere.

[0014] It is also possible to utilize the heat generated in the corresponding temperature zones, particularly in the hot zones of the extruder, melt filter, or similar components of the system, which is released in the form of radiant heat. Radiant heat can also be used to heat a heat transfer medium, for example, to generate warm air, which can also be used for drying moist precursor material.

[0015] The coolant, e.g., water, leaves the granulator at a temperature of 40°C, for example. This is because the circulating water serves to cool the plastic granulate produced in the granulator to the packaging temperature of 40°C. The coolant flow is therefore adjusted so as not to exceed this temperature. The water is fed into the heat pump at a temperature of 40°C in order to generate a temperature level of 60°C or more, advantageously 80 to 90°C, using the heat pump. To achieve this high temperature level, the heat pump can be designed with multiple stages, i.e., comprise two or more heat pump stages connected in series.

[0016] The medium used in granulation, for example water, can be conducted in its own closed circuit, whereby the thermal energy contained in this medium is transferred to a second medium by means of a heat exchanger, which is conducted in a second circuit and flows through the heat pump. This separation of the heat transfer media into two separate circuits ensures that the unit that cools the second cooling circuit is not contaminated with fine particles that originate from the granulation and are generated during the granulation process. To also minimize the particle load on the heat exchanger in the first cooling circuit, a filter can be installed upstream of the heat exchanger in the granulation water. This ensures better heat transfer efficiency, even over a longer period.

[0017] In the heat pump, a heat transfer medium is heated to the aforementioned temperature level of 60 °C or more, preferably 80 to 90 °C. The heat transfer medium can be a gas or a liquid, particularly air or water, as economically available heat transfer media that are safe in terms of their chemical and health properties. The heat transfer medium heated by the heat pump can transport the heat to where it can be used either for the production of the plastic granulate or for other purposes, replacing heat energy that would otherwise have to be provided separately.

[0018] 1 . For example, the heat can be used to thermally dry the raw material before it enters the extruder,

[0019] 2. the heat can be used to preheat the raw material before it enters the extruder, so that the amount of energy required to plasticize the material in the extruder can be reduced,

[0020] 3. The heat can be used to olfactorily clean the plastic material by denaturing organic deposits. This can be achieved, for example, at the beginning of the process by treating the provided raw material with a hot wash using caustic soda and / or other suitable agents before the raw material enters the extruder. Alternatively, or in addition, the heat can be used at the end of the process to heat the produced granules and maintain them at or above a certain minimum temperature for a specific period of time, e.g., several hours (e.g., 8-30 hours), so that volatile contaminants and odors can diffuse out, thus providing the granules with a final olfactory cleaning.

[0021] 4. the heat can be used to heat a building, for example the building in which the system is located, so that the need for external energy sources such as gas or oil for heating the building can be reduced,

[0022] 5. the heat can be used to heat nearby buildings, for example production halls or administrative buildings located on the same property as the building in which the system is located, so that heat losses of the heat transfer medium can be kept as low as possible by keeping the pipes short,

[0023] 6. or the heat can be used for other processes not related to the production of plastic granules, which are carried out in the same building or a closely adjacent building. In this case, the energy advantage of the proposed process lies not in direct energy savings during the production of plastic granules, but in indirect energy savings, namely elsewhere. A positive environmental aspect of the production of plastic granules also arises in this case from the fact that the waste heat generated during this production process, namely during the cooling of the extruded plastic material immediately downstream of the extruder and in the granulator, is not released into the environment as a heat load, but is utilized for other purposes.

[0024] The invention is explained in more detail below using a purely schematic representation in which some components - for example pumps - are not shown for reasons of clarity.

[0025] The diagram illustrates a process and a plant 1 for producing plastic granulate. Recycled material, which contains plastic components as well as plant fiber components, is fed into a dryer 3 as raw material 2—indicated by an arrow—to reduce the moisture content of the raw material 2. The dryer 3 has a double-walled housing 17 and, inside, a rotating shaft 18, also referred to as the rotor axis. This shaft is equipped with several paddles 19 to loosen, circulate, and ventilate the recycled material in the dryer 3 from as many sides as possible.

[0026] The dried raw material 2 is fed from the dryer 3 into an extruder 4, plasticized, and extruded into a strand of material, which is then granulated—i.e., divided into a multitude of small particles—in a granulator 5 downstream of the extruder 4. The plastic granulate 6 thus produced—also indicated by an arrow—is packaged in large containers and, similar to virgin plastic granulate, can be used and further processed as a raw material in the plastics processing industry.

[0027] A liquid coolant, water in the illustrated embodiment, is circulated through the extruder 4 and the granulator 5. In the diagram, the cold coolant is marked 7, which is fed at its low temperature to the extruder 4 and the granulator 5. In the area of ​​the extruder 4, for example, a drive motor, a gear unit, a vacuum degassing system or the like can be cooled. After the cold coolant 7 has heated up and cooled the plastic material and, if applicable, system components, it exits the extruder 4 and the granulator 5 and is marked 8 in the diagram as a warm coolant. It flows from the extruder 4 and the granulator 5 into a first heat exchanger 20, where it gives off heat, and is then fed back to the extruder 4 and the granulator 5 as the cold coolant 7.A heat pump 9, in which a liquid heat transfer medium is heated, is connected to the first heat exchanger 20 via connecting lines 21. In the illustrated embodiment, water serves as the heat transfer medium flowing through the heat pump 9.

[0028] The connecting lines 21, on the one hand, effect heat transfer from the first heat exchanger 20 to the heat pump 9; on the other hand, they create two separate heat transfer circuits, one of which flows through the first heat exchanger 20 and the other through the heat pump 9. This prevents residual particles from the granulation process, which may be contained in the warm coolant 8 of the first circuit, from entering the heat pump 9 or even the second circuit. A filter can be arranged in the first circuit, particularly in its line carrying the warm coolant 8, to remove the aforementioned residual particles from the granulation process from the first circuit. The residual particles can, for example, be added to the raw material 2.

[0029] However, in deviation from the illustrated embodiment, it may also be provided to feed the warm coolant 8 directly into the heat pump 9, for example if, due to appropriate filtration or due to the granulation process used, the problem of particle contamination and the resulting reduced efficiency of the heat pump 9 is not to be expected.

[0030] The heat transfer medium is heated in the heat pump 9, with the warm heat transfer medium being designated 10 in the diagram. In a first line, the warm heat transfer medium 10 is guided into a second heat exchanger 11, which is designed as an air / water heat exchanger. Cold air 12 flows into the second heat exchanger 11, is heated there, and the hot air 14 thus generated is blown into the interior of the dryer 3 by a fan 15 to thermally dry the raw material 2 located in the dryer 3. To remove the moisture, a portion of the moisture-laden and now cooled hot air 14 can be blown outside; however, air drying can also be provided, for example in the form of a condensate dryer or the like.

[0031] The cold air 12 can be drawn in as fresh air from outside into the second heat exchanger 11. However, the diagram shows that the cold air 12 is drawn in from the dryer 3, resulting in an intensive air flow through the raw material 2 in the dryer 3, which improves the drying performance. The energy balance is also improved because the hot air 14 is circulated and contains residual heat when it is drawn in, now cooled, as cold air 12 from the dryer 3. The cold air 12 is passed through a condensate separator (not shown in the drawing) before it reaches the second heat exchanger 11. Furthermore, a mixed mode of operation is possible, in which partly fresh air and partly circulated air drawn in from the dryer 3 is used as cold air 12.

[0032] In a second line, the warm heat transfer medium 10 from the heat pump 9 is fed into the double-walled housing 17 of the dryer 3. The shaft 18 and the paddles 19 can also be designed to allow flow through them, so that they can be heated by the warm heat transfer medium 10 and support the drying process of the raw material 2 through the emitted heat radiation.

[0033] Heat is extracted from the warm heat transfer medium 10 in the second heat exchanger 11 as well as in the double-walled housing 17 and, if applicable, in the shaft 18 and the paddles 19, so that the heat transfer medium flows back to the heat pump 9 as a cold heat transfer medium 16 to be heated up again there.

[0034] Reference symbol:

[0035] Plant Raw material Dryer Extruder Granulator Plastic granulate Cold coolant Warm coolant Heat pump

[0036] Warm heat carrier second heat exchanger cold air

[0037] Hot air

[0038] Fan Cold heat carrier housing

[0039] Shaft paddle first heat exchanger connecting line

Claims

Claims:

1. A method for producing plastic granules (6), wherein a plastic-containing raw material (2) is fed to an extruder (4), extruded in the plasticized state and granulated downstream of the extruder (4), and wherein the material exiting the extruder (4) is brought into contact with a liquid, cold coolant (7), thereby cooling the material and heating the coolant (7), and wherein the warm coolant (8) is cooled and subsequently brought back into contact with material exiting the extruder (4) as a cold coolant (7), characterized in that the warm coolant (8) releases heat to a heat pump (9) and is thereby cooled, a cold heat transfer medium (16) is also fed into the heat pump (9) and heated there, and the warm heat transfer medium (10) is cooled in such a way that the heat released by the heat transfer medium (10) is used as process or heating heat.

2. Method according to claim 1, characterized in that the warm coolant (8) is fed into a heat pump (9) and cooled there.

3. Method according to claim 1, characterized in that the warm coolant (8) is fed into a first heat exchanger (20), and that the heat thereby gained in the first heat exchanger (20) is transferred via connecting lines (21) to the heat pump (9), such that that two separate circuits flow through the first heat exchanger (20) on the one hand and the heat pump (9) on the other.

4. Method according to one of the preceding claims, characterized in that the cold heat transfer medium (16) is heated to a temperature of at least 60°C in the heat pump (9).

5. Method according to claim 4, characterized in that the cold heat transfer medium (16) in the heat pump (9) is heated to a temperature between 80 and 90°C.

6. Method according to one of the preceding claims, characterized in that the cold heat transfer medium (16) is heated to successively increasing temperature levels in several successively connected heat pump stages.

7. Method according to one of the preceding claims, characterized in that a liquid heat transfer medium (10, 16) is used and the heat of the warm heat transfer medium (10) is used in an air / water heat exchanger (11) to heat air.

8. Method according to claim 7, characterized in that the raw material (2) is thermally dried by means of warm air before it is fed into the extruder (4).

9. Method according to claim 7, characterized in that the raw material (2) is preheated by means of the warm air. is before it enters the extruder (4).

10. Method according to claim 7, characterized in that the raw material (2) is olfactorily cleaned by means of warm air by thermally denaturing organic adhesions before the raw material (2) enters the extruder (4).

11. Method according to claim 7, characterized in that the produced plastic granules (6) are olfactorily cleaned by means of warm air by thermally denaturing organic adhesions by keeping the plastic granules (6) at or above a certain minimum temperature for a certain period of time of at least one hour.

12. Method according to one of claims 1 to 6, characterized in that a liquid heat transfer medium (10, 16) is used and the heat of the warm heat transfer medium (10) is used to heat components of a dryer (3) in which the raw material (2) is dried before it enters the extruder (4).

13. Plant (1 ) for carrying out the method according to one of the preceding claims, comprising an extruder (4), a granulator (5) downstream of the extruder (4), and a circuit arrangement in which a coolant (7, 8) first flows through the extruder (4) and / or the granulator (5) and subsequently through a cooling device, wherein the cooling device comprises a heat pump (9) through which a heat transfer medium (10, 16) flows.

14. System according to claim 13, characterized in that the system has two separate circuits such that the cooling device has a first heat exchanger (20) through which the warm coolant (8) flows, and that the first heat exchanger (20) is thermally connected to the heat pump (9) via connecting lines (21) for the transfer of the heat gained in the first heat exchanger (20), wherein the coolant (7, 8) and the heat transfer medium (10, 16) are each guided in a separate circuit.

15. System according to claim 13 or 14, characterized in that the system (1 ) contains a liquid heat transfer medium (10, 16) which is circulated through the heat pump (9) and an air / water heat exchanger (11 ).

16. System according to claim 15, characterized in that a warm air line runs from the heat exchanger (11) to a dryer (3) which is located upstream of the extruder (4).

17. System according to claim 16, characterized in that the warm air duct opens into the interior of the dryer (3).

18. System according to one of claims 13 to 17, characterized in that a line carries the heat transfer medium (10, 16) from the heat pump (9) to the dryer (3) in such a manner, that one or more components of the dryer (3) can be heated by means of the heat transfer medium (10, 16). System according to one of claims 13 to 18, characterized in that the heat pump (11) is connected to a building heating system.

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